CA1187970A - Photovoltaic device having incident radiation directing means for total internal reflection - Google Patents
Photovoltaic device having incident radiation directing means for total internal reflectionInfo
- Publication number
- CA1187970A CA1187970A CA000421646A CA421646A CA1187970A CA 1187970 A CA1187970 A CA 1187970A CA 000421646 A CA000421646 A CA 000421646A CA 421646 A CA421646 A CA 421646A CA 1187970 A CA1187970 A CA 1187970A
- Authority
- CA
- Canada
- Prior art keywords
- layer
- amorphous silicon
- incident radiation
- doped
- radiation
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
- 230000005855 radiation Effects 0.000 title claims abstract description 62
- 239000000956 alloy Substances 0.000 claims abstract description 75
- 229910045601 alloy Inorganic materials 0.000 claims abstract description 74
- 239000000463 material Substances 0.000 claims abstract description 67
- 229910021417 amorphous silicon Inorganic materials 0.000 claims abstract description 63
- 230000000737 periodic effect Effects 0.000 claims abstract description 29
- 239000004065 semiconductor Substances 0.000 claims abstract description 26
- 239000002800 charge carrier Substances 0.000 claims description 13
- GWEVSGVZZGPLCZ-UHFFFAOYSA-N Titan oxide Chemical compound O=[Ti]=O GWEVSGVZZGPLCZ-UHFFFAOYSA-N 0.000 claims description 12
- XOLBLPGZBRYERU-UHFFFAOYSA-N tin dioxide Chemical compound O=[Sn]=O XOLBLPGZBRYERU-UHFFFAOYSA-N 0.000 claims description 12
- 229910052782 aluminium Inorganic materials 0.000 claims description 11
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 claims description 11
- 229910001887 tin oxide Inorganic materials 0.000 claims description 11
- 210000003298 dental enamel Anatomy 0.000 claims description 6
- 239000004408 titanium dioxide Substances 0.000 claims description 6
- 229910010293 ceramic material Inorganic materials 0.000 claims description 5
- PFNQVRZLDWYSCW-UHFFFAOYSA-N (fluoren-9-ylideneamino) n-naphthalen-1-ylcarbamate Chemical compound C12=CC=CC=C2C2=CC=CC=C2C1=NOC(=O)NC1=CC=CC2=CC=CC=C12 PFNQVRZLDWYSCW-UHFFFAOYSA-N 0.000 claims description 4
- BUGBHKTXTAQXES-UHFFFAOYSA-N Selenium Chemical compound [Se] BUGBHKTXTAQXES-UHFFFAOYSA-N 0.000 claims description 4
- 229910052711 selenium Inorganic materials 0.000 claims description 4
- 239000011669 selenium Substances 0.000 claims description 4
- 239000005083 Zinc sulfide Substances 0.000 claims description 3
- HBMJWWWQQXIZIP-UHFFFAOYSA-N silicon carbide Chemical compound [Si+]#[C-] HBMJWWWQQXIZIP-UHFFFAOYSA-N 0.000 claims description 3
- 229910010271 silicon carbide Inorganic materials 0.000 claims description 3
- 239000007787 solid Substances 0.000 claims description 3
- DRDVZXDWVBGGMH-UHFFFAOYSA-N zinc;sulfide Chemical compound [S-2].[Zn+2] DRDVZXDWVBGGMH-UHFFFAOYSA-N 0.000 claims description 3
- 210000005056 cell body Anatomy 0.000 claims 2
- PXGOKWXKJXAPGV-UHFFFAOYSA-N Fluorine Chemical compound FF PXGOKWXKJXAPGV-UHFFFAOYSA-N 0.000 abstract description 14
- 229910052731 fluorine Inorganic materials 0.000 abstract description 14
- 239000011737 fluorine Substances 0.000 abstract description 14
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- 239000010410 layer Substances 0.000 description 170
- 239000000758 substrate Substances 0.000 description 30
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- 239000001257 hydrogen Substances 0.000 description 20
- 229910052739 hydrogen Inorganic materials 0.000 description 20
- 229910052751 metal Inorganic materials 0.000 description 20
- 239000002184 metal Substances 0.000 description 20
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- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 description 17
- 229910052710 silicon Inorganic materials 0.000 description 17
- 239000010703 silicon Substances 0.000 description 17
- 238000000151 deposition Methods 0.000 description 16
- 239000007789 gas Substances 0.000 description 13
- BLRPTPMANUNPDV-UHFFFAOYSA-N Silane Chemical compound [SiH4] BLRPTPMANUNPDV-UHFFFAOYSA-N 0.000 description 12
- 230000008021 deposition Effects 0.000 description 12
- 229910000077 silane Inorganic materials 0.000 description 12
- 239000013078 crystal Substances 0.000 description 11
- 235000010210 aluminium Nutrition 0.000 description 10
- 239000004020 conductor Substances 0.000 description 9
- 238000010521 absorption reaction Methods 0.000 description 8
- ABTOQLMXBSRXSM-UHFFFAOYSA-N silicon tetrafluoride Chemical compound F[Si](F)(F)F ABTOQLMXBSRXSM-UHFFFAOYSA-N 0.000 description 8
- 230000003247 decreasing effect Effects 0.000 description 7
- 230000007547 defect Effects 0.000 description 7
- 239000002019 doping agent Substances 0.000 description 7
- 239000008393 encapsulating agent Substances 0.000 description 7
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- GNPVGFCGXDBREM-UHFFFAOYSA-N germanium atom Chemical compound [Ge] GNPVGFCGXDBREM-UHFFFAOYSA-N 0.000 description 7
- 239000002178 crystalline material Substances 0.000 description 6
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- 229940071182 stannate Drugs 0.000 description 5
- 239000007858 starting material Substances 0.000 description 5
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 4
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- 238000005215 recombination Methods 0.000 description 4
- RYGMFSIKBFXOCR-UHFFFAOYSA-N Copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 description 3
- XYFCBTPGUUZFHI-UHFFFAOYSA-N Phosphine Chemical compound P XYFCBTPGUUZFHI-UHFFFAOYSA-N 0.000 description 3
- 238000005275 alloying Methods 0.000 description 3
- 230000004888 barrier function Effects 0.000 description 3
- 239000000919 ceramic Substances 0.000 description 3
- 239000011248 coating agent Substances 0.000 description 3
- 238000000576 coating method Methods 0.000 description 3
- 229910052802 copper Inorganic materials 0.000 description 3
- 239000010949 copper Substances 0.000 description 3
- ZOCHARZZJNPSEU-UHFFFAOYSA-N diboron Chemical compound B#B ZOCHARZZJNPSEU-UHFFFAOYSA-N 0.000 description 3
- PCHJSUWPFVWCPO-UHFFFAOYSA-N gold Chemical compound [Au] PCHJSUWPFVWCPO-UHFFFAOYSA-N 0.000 description 3
- 229910052737 gold Inorganic materials 0.000 description 3
- 239000010931 gold Substances 0.000 description 3
- 238000010348 incorporation Methods 0.000 description 3
- 150000002500 ions Chemical class 0.000 description 3
- 230000031700 light absorption Effects 0.000 description 3
- 230000003287 optical effect Effects 0.000 description 3
- 239000000523 sample Substances 0.000 description 3
- 238000001228 spectrum Methods 0.000 description 3
- 238000007740 vapor deposition Methods 0.000 description 3
- NCGICGYLBXGBGN-UHFFFAOYSA-N 3-morpholin-4-yl-1-oxa-3-azonia-2-azanidacyclopent-3-en-5-imine;hydrochloride Chemical compound Cl.[N-]1OC(=N)C=[N+]1N1CCOCC1 NCGICGYLBXGBGN-UHFFFAOYSA-N 0.000 description 2
- XKRFYHLGVUSROY-UHFFFAOYSA-N Argon Chemical compound [Ar] XKRFYHLGVUSROY-UHFFFAOYSA-N 0.000 description 2
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 2
- 229910001188 F alloy Inorganic materials 0.000 description 2
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 2
- RTAQQCXQSZGOHL-UHFFFAOYSA-N Titanium Chemical compound [Ti] RTAQQCXQSZGOHL-UHFFFAOYSA-N 0.000 description 2
- XLOMVQKBTHCTTD-UHFFFAOYSA-N Zinc monoxide Chemical compound [Zn]=O XLOMVQKBTHCTTD-UHFFFAOYSA-N 0.000 description 2
- 238000000149 argon plasma sintering Methods 0.000 description 2
- 230000005540 biological transmission Effects 0.000 description 2
- CXKCTMHTOKXKQT-UHFFFAOYSA-N cadmium oxide Inorganic materials [Cd]=O CXKCTMHTOKXKQT-UHFFFAOYSA-N 0.000 description 2
- CFEAAQFZALKQPA-UHFFFAOYSA-N cadmium(2+);oxygen(2-) Chemical compound [O-2].[Cd+2] CFEAAQFZALKQPA-UHFFFAOYSA-N 0.000 description 2
- 229910052799 carbon Inorganic materials 0.000 description 2
- 229910052729 chemical element Inorganic materials 0.000 description 2
- -1 copper and aluminum Chemical class 0.000 description 2
- BERDEBHAJNAUOM-UHFFFAOYSA-N copper(I) oxide Inorganic materials [Cu]O[Cu] BERDEBHAJNAUOM-UHFFFAOYSA-N 0.000 description 2
- KRFJLUBVMFXRPN-UHFFFAOYSA-N cuprous oxide Chemical compound [O-2].[Cu+].[Cu+] KRFJLUBVMFXRPN-UHFFFAOYSA-N 0.000 description 2
- 229940112669 cuprous oxide Drugs 0.000 description 2
- 238000000354 decomposition reaction Methods 0.000 description 2
- 150000002431 hydrogen Chemical class 0.000 description 2
- RHZWSUVWRRXEJF-UHFFFAOYSA-N indium tin Chemical compound [In].[Sn] RHZWSUVWRRXEJF-UHFFFAOYSA-N 0.000 description 2
- 238000004519 manufacturing process Methods 0.000 description 2
- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 description 2
- 229910052757 nitrogen Inorganic materials 0.000 description 2
- 230000009257 reactivity Effects 0.000 description 2
- 238000005488 sandblasting Methods 0.000 description 2
- 229910052709 silver Inorganic materials 0.000 description 2
- 239000004332 silver Substances 0.000 description 2
- 239000002356 single layer Substances 0.000 description 2
- 239000000126 substance Substances 0.000 description 2
- 239000010936 titanium Substances 0.000 description 2
- 229910052719 titanium Inorganic materials 0.000 description 2
- 239000012780 transparent material Substances 0.000 description 2
- ZOXJGFHDIHLPTG-UHFFFAOYSA-N Boron Chemical compound [B] ZOXJGFHDIHLPTG-UHFFFAOYSA-N 0.000 description 1
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 1
- OAICVXFJPJFONN-UHFFFAOYSA-N Phosphorus Chemical compound [P] OAICVXFJPJFONN-UHFFFAOYSA-N 0.000 description 1
- 241001274197 Scatophagus argus Species 0.000 description 1
- 208000036366 Sensation of pressure Diseases 0.000 description 1
- 229910000676 Si alloy Inorganic materials 0.000 description 1
- BQCADISMDOOEFD-UHFFFAOYSA-N Silver Chemical compound [Ag] BQCADISMDOOEFD-UHFFFAOYSA-N 0.000 description 1
- UCKMPCXJQFINFW-UHFFFAOYSA-N Sulphide Chemical compound [S-2] UCKMPCXJQFINFW-UHFFFAOYSA-N 0.000 description 1
- VDRSDNINOSAWIV-UHFFFAOYSA-N [F].[Si] Chemical compound [F].[Si] VDRSDNINOSAWIV-UHFFFAOYSA-N 0.000 description 1
- 230000004075 alteration Effects 0.000 description 1
- PNEYBMLMFCGWSK-UHFFFAOYSA-N aluminium oxide Inorganic materials [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 description 1
- 229910052787 antimony Inorganic materials 0.000 description 1
- WATWJIUSRGPENY-UHFFFAOYSA-N antimony atom Chemical compound [Sb] WATWJIUSRGPENY-UHFFFAOYSA-N 0.000 description 1
- 238000013459 approach Methods 0.000 description 1
- 229910052786 argon Inorganic materials 0.000 description 1
- 229910052785 arsenic Inorganic materials 0.000 description 1
- RQNWIZPPADIBDY-UHFFFAOYSA-N arsenic atom Chemical compound [As] RQNWIZPPADIBDY-UHFFFAOYSA-N 0.000 description 1
- 125000004429 atom Chemical group 0.000 description 1
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 1
- 229910052788 barium Inorganic materials 0.000 description 1
- DSAJWYNOEDNPEQ-UHFFFAOYSA-N barium atom Chemical compound [Ba] DSAJWYNOEDNPEQ-UHFFFAOYSA-N 0.000 description 1
- 230000009286 beneficial effect Effects 0.000 description 1
- 229910052797 bismuth Inorganic materials 0.000 description 1
- JCXGWMGPZLAOME-UHFFFAOYSA-N bismuth atom Chemical compound [Bi] JCXGWMGPZLAOME-UHFFFAOYSA-N 0.000 description 1
- 230000000903 blocking effect Effects 0.000 description 1
- 229910052796 boron Inorganic materials 0.000 description 1
- BDOSMKKIYDKNTQ-UHFFFAOYSA-N cadmium atom Chemical compound [Cd] BDOSMKKIYDKNTQ-UHFFFAOYSA-N 0.000 description 1
- 230000002301 combined effect Effects 0.000 description 1
- 238000004891 communication Methods 0.000 description 1
- 230000003138 coordinated effect Effects 0.000 description 1
- 230000000875 corresponding effect Effects 0.000 description 1
- CVSVTCORWBXHQV-UHFFFAOYSA-N creatine Chemical compound NC(=[NH2+])N(C)CC([O-])=O CVSVTCORWBXHQV-UHFFFAOYSA-N 0.000 description 1
- 230000002939 deleterious effect Effects 0.000 description 1
- 238000005137 deposition process Methods 0.000 description 1
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- 229910000078 germane Inorganic materials 0.000 description 1
- QUZPNFFHZPRKJD-UHFFFAOYSA-N germane Chemical compound [GeH4] QUZPNFFHZPRKJD-UHFFFAOYSA-N 0.000 description 1
- 229910052986 germanium hydride Inorganic materials 0.000 description 1
- 238000009499 grossing Methods 0.000 description 1
- 238000010438 heat treatment Methods 0.000 description 1
- 239000012535 impurity Substances 0.000 description 1
- 229910052738 indium Inorganic materials 0.000 description 1
- APFVFJFRJDLVQX-UHFFFAOYSA-N indium atom Chemical compound [In] APFVFJFRJDLVQX-UHFFFAOYSA-N 0.000 description 1
- 230000001939 inductive effect Effects 0.000 description 1
- 239000012212 insulator Substances 0.000 description 1
- 239000007788 liquid Substances 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- KRTSDMXIXPKRQR-AATRIKPKSA-N monocrotophos Chemical compound CNC(=O)\C=C(/C)OP(=O)(OC)OC KRTSDMXIXPKRQR-AATRIKPKSA-N 0.000 description 1
- 238000006386 neutralization reaction Methods 0.000 description 1
- 229910052760 oxygen Inorganic materials 0.000 description 1
- 239000001301 oxygen Substances 0.000 description 1
- RVTZCBVAJQQJTK-UHFFFAOYSA-N oxygen(2-);zirconium(4+) Chemical compound [O-2].[O-2].[Zr+4] RVTZCBVAJQQJTK-UHFFFAOYSA-N 0.000 description 1
- 239000002245 particle Substances 0.000 description 1
- 229910052698 phosphorus Inorganic materials 0.000 description 1
- 239000011574 phosphorus Substances 0.000 description 1
- 229910000073 phosphorus hydride Inorganic materials 0.000 description 1
- 229910021420 polycrystalline silicon Inorganic materials 0.000 description 1
- 238000002310 reflectometry Methods 0.000 description 1
- 230000001105 regulatory effect Effects 0.000 description 1
- 239000004576 sand Substances 0.000 description 1
- 238000000926 separation method Methods 0.000 description 1
- 241000894007 species Species 0.000 description 1
- 238000005507 spraying Methods 0.000 description 1
- 238000004544 sputter deposition Methods 0.000 description 1
- 125000005402 stannate group Chemical group 0.000 description 1
- 229910052716 thallium Inorganic materials 0.000 description 1
- BKVIYDNLLOSFOA-UHFFFAOYSA-N thallium Chemical compound [Tl] BKVIYDNLLOSFOA-UHFFFAOYSA-N 0.000 description 1
- 239000011701 zinc Substances 0.000 description 1
- 239000011787 zinc oxide Substances 0.000 description 1
- 229910001928 zirconium oxide Inorganic materials 0.000 description 1
Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
- H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
- H01L31/20—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof such devices or parts thereof comprising amorphous semiconductor materials
- H01L31/202—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof such devices or parts thereof comprising amorphous semiconductor materials including only elements of Group IV of the Periodic System
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
- H01L31/02—Details
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
- H01L31/04—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
- H01L31/04—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices
- H01L31/054—Optical elements directly associated or integrated with the PV cell, e.g. light-reflecting means or light-concentrating means
- H01L31/056—Optical elements directly associated or integrated with the PV cell, e.g. light-reflecting means or light-concentrating means the light-reflecting means being of the back surface reflector [BSR] type
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
- H01L31/04—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices
- H01L31/06—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices characterised by at least one potential-jump barrier or surface barrier
- H01L31/075—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices characterised by at least one potential-jump barrier or surface barrier the potential barriers being only of the PIN type
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E10/00—Energy generation through renewable energy sources
- Y02E10/50—Photovoltaic [PV] energy
- Y02E10/52—PV systems with concentrators
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E10/00—Energy generation through renewable energy sources
- Y02E10/50—Photovoltaic [PV] energy
- Y02E10/548—Amorphous silicon PV cells
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
- Y02P70/50—Manufacturing or production processes characterised by the final manufactured product
Abstract
ABSTRACT OF THE DISCLOSURE
There is disclosed new and improved photo-voltaic devices which provide increased short cir-cuit currents and efficiencies over that previ-ously obtainable from prior devices. The dis-closed devices include incident radiation direct-ing means for directing at least a portion of the incident light through the active region or re-gions thereof at angles sufficient to substan-tially confine the directed radiation in the devices. This allows substantially total utiliza-tion of photogenerated electron-hole pairs. Fur-ther, because the light is directed through the active region or regions at such angles, the active regions can be made thinner to also in-crease collection efficiencies.
The incident radiation directors can be ran-dom surface or bulk reflectors to provide random scattering of the light, or periodic surface or bulk reflector to provide selective scattering of the light.
While the present invention is applicable to photovoltaic devices formed from any type of semi-conductor material, as for example, crystalline, polycrystalline, or amorphous semiconductor alloys or any combination thereof, disclosure herein is primarily directed to photovoltaic devices formed from amorphous silicon alloys preferably incor-porating fluorine as a density of states reducing element. The disclosure is also directed to, without limitation, photovoltaic devices of the p-i-n configuration, both as single cells and multiple cells arranged in tandem.
There is disclosed new and improved photo-voltaic devices which provide increased short cir-cuit currents and efficiencies over that previ-ously obtainable from prior devices. The dis-closed devices include incident radiation direct-ing means for directing at least a portion of the incident light through the active region or re-gions thereof at angles sufficient to substan-tially confine the directed radiation in the devices. This allows substantially total utiliza-tion of photogenerated electron-hole pairs. Fur-ther, because the light is directed through the active region or regions at such angles, the active regions can be made thinner to also in-crease collection efficiencies.
The incident radiation directors can be ran-dom surface or bulk reflectors to provide random scattering of the light, or periodic surface or bulk reflector to provide selective scattering of the light.
While the present invention is applicable to photovoltaic devices formed from any type of semi-conductor material, as for example, crystalline, polycrystalline, or amorphous semiconductor alloys or any combination thereof, disclosure herein is primarily directed to photovoltaic devices formed from amorphous silicon alloys preferably incor-porating fluorine as a density of states reducing element. The disclosure is also directed to, without limitation, photovoltaic devices of the p-i-n configuration, both as single cells and multiple cells arranged in tandem.
Description
3~
This invention relates to improved photo-voltaic devices which provide enhanced short cir-cuit currents and efficiencies. The present inven-tion has particular applicability to photo-voltaic de~ices ~ormed from :Layers of amorphoussemiconductor alloys. The photovoltaic devices of the present invention include incident radia-tion directing means for directing either a portion or substantially all of the incident radiation through the active region or regions, wherein the charge carriers are created, at angles sufficient to cause the clirected radiation to be substan-tially confined within the devices. mhis provides multiple reflections of the directed light in the active regions of the devices in which they are employed. One advantage of -this approach is that increased photon absorption and charge carrier generation in the active regions is possible, pro-vidina increased short circuit currents. Another advantage is that since the directed light passes through the active region of the improved devices at an angle, the active region or regions can be made thinner to reduce charge carrier recombina-tion while maintaining efficient charge carrier
This invention relates to improved photo-voltaic devices which provide enhanced short cir-cuit currents and efficiencies. The present inven-tion has particular applicability to photo-voltaic de~ices ~ormed from :Layers of amorphoussemiconductor alloys. The photovoltaic devices of the present invention include incident radia-tion directing means for directing either a portion or substantially all of the incident radiation through the active region or regions, wherein the charge carriers are created, at angles sufficient to cause the clirected radiation to be substan-tially confined within the devices. mhis provides multiple reflections of the directed light in the active regions of the devices in which they are employed. One advantage of -this approach is that increased photon absorption and charge carrier generation in the active regions is possible, pro-vidina increased short circuit currents. Another advantage is that since the directed light passes through the active region of the improved devices at an angle, the active region or regions can be made thinner to reduce charge carrier recombina-tion while maintaining efficient charge carrier
2~ generation. The invention while not being limited 'r3' ~
to any particular device configuration~ has its most important application in making improved amorphous silicon alloy photovoltaic devices of the p-1-n configuration, either as single cells or multiple cells comprising a plurality of single cell units.
Silicon is the basis of the huge crystalline semiconductor industry and is the material which has produced expensive high e~ficiency (18 per-cent) crystalline solar cells for space applica-tions. For terrestrial applications, the crys-talline solar cells typically have much lower efEiciencies on the order of 12 percent or less.
When crystalline semiconductor technology reached a commercial state, i-t became the foundation of the present huge semiconductor device manufactur~
ing industry. This was due to the ability of the scientist to ~row substantially defect-free germanium and, particularly, silicon crystals, and then turn them into e~trinsic materials with p-type and n-type conductivity regions therein.
This was accomplished by diffusing into such crys talline material parts per million of donor (n) or acceptor (p) dopant materials introduced as sub-stitutional impurities into the substantially pure crystalline materials, to increase their electri-cal conductivity and to control their being either of a p or n conduction type. The fabrica-tion pro-cesses for making p-n junction crystals lnvolve extremely complex, time consuming, and expensive proceduresO Thus, these crystalline materials use~ul ln solar cells and current control devices are produced under very carefully controlled con-ditions by growing individual single silicon or germanium crystals, and when p-n junctions are requlred, by doping such single crystals with extremely small and critical amounts of dopan-ts.
These crystal growiny processes produce such relatively small crystals that solar cells require the assembly of many single crystals to encompass the desired area of only a single solar ce~1 panelO The amount of energy necessary to make a solar cell in this process, the limitation caused by the size limitations of the silicon crystal, and the necessity to cut up and assemble such a crystalline material have all resulted in an impossible economic barrier to the large scale use oE crystalline semiconc~uctor solar cells Eor energy conversion. Further, crystalline silicon ~5 has an indirect optical edge which results in poor
to any particular device configuration~ has its most important application in making improved amorphous silicon alloy photovoltaic devices of the p-1-n configuration, either as single cells or multiple cells comprising a plurality of single cell units.
Silicon is the basis of the huge crystalline semiconductor industry and is the material which has produced expensive high e~ficiency (18 per-cent) crystalline solar cells for space applica-tions. For terrestrial applications, the crys-talline solar cells typically have much lower efEiciencies on the order of 12 percent or less.
When crystalline semiconductor technology reached a commercial state, i-t became the foundation of the present huge semiconductor device manufactur~
ing industry. This was due to the ability of the scientist to ~row substantially defect-free germanium and, particularly, silicon crystals, and then turn them into e~trinsic materials with p-type and n-type conductivity regions therein.
This was accomplished by diffusing into such crys talline material parts per million of donor (n) or acceptor (p) dopant materials introduced as sub-stitutional impurities into the substantially pure crystalline materials, to increase their electri-cal conductivity and to control their being either of a p or n conduction type. The fabrica-tion pro-cesses for making p-n junction crystals lnvolve extremely complex, time consuming, and expensive proceduresO Thus, these crystalline materials use~ul ln solar cells and current control devices are produced under very carefully controlled con-ditions by growing individual single silicon or germanium crystals, and when p-n junctions are requlred, by doping such single crystals with extremely small and critical amounts of dopan-ts.
These crystal growiny processes produce such relatively small crystals that solar cells require the assembly of many single crystals to encompass the desired area of only a single solar ce~1 panelO The amount of energy necessary to make a solar cell in this process, the limitation caused by the size limitations of the silicon crystal, and the necessity to cut up and assemble such a crystalline material have all resulted in an impossible economic barrier to the large scale use oE crystalline semiconc~uctor solar cells Eor energy conversion. Further, crystalline silicon ~5 has an indirect optical edge which results in poor
-3-~ 3'~
light absorption in the material. Because of the poor light absorption, crystalline solar cells have to be at least 50 microns thick to absorb the incident sunlight. Even if the sinyle crystal material is replaced by polycrystalline silicon with cheaper production processes, the indirect optical edge is still maintalned; hence the material thickness is not reduced. The poly-crystalline materials also contain grain bounda ries and other defect problems, which are ordi-narily deleterious.
In summary, crystal silicon devices have fixed parameters ~Jhich are not variable as desired, require large amounts of material~ are only producible in relatively small areas and are expensive and time consuming to produceO The use of devices based upon amorphous silicon alloys can eliminate these crystal silicon disadvantages. An amorphous silicon alloy has an optical absorption edge haviny properties similar to a direct gap semiconductor and only a material thickness of one micron or less is necessary to absorb the same amount of sunlight as the 50 micron thick crystal-line silicon~ Furthert amorphous silicon alloys ~5 can be made faster, easier and in larger areas -than can crystalline silicon.
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Accordingly, a considerable effort has been rnade to develop processes for readily depositing amorphous semiconductor alloys or films, each of which can encompass relatively large areas, if desired, limited only by khe size of the deposi-tion equipment, and which could be readily dopedto form p-type and n-type materials where p-n junction devices are to be made therefrom equiva-lent to those produced by their crystalline coun-terparts~ For many years such work was substan-tially unproductive. Amorphous silicon or german-ium (Group IV) films are normally four-Fold coor-dinated and were found to have microvoids and dangling bonds and other defects which produce a high density of localized states in the energy gap thereof. The presence of a high density of local~
ized states in the energy gap of amorphous silicon semiconductor films results in a low degree of photocoilductivity and short carrier lifetime, making s~lch films unsuitable for photoresponsive applications. Additiona]ly, such films cannot be successfully doped or otherwise modified to shift the Fermi level close to the conduction or valence bands, mal~ing them unsuitable for making p-n ~unc-tions for solar cell and current control device ~5 applications.
In an attempt to minimize the acorementioned problems involved with amorphous silicon (origi-nally thought to be elemental), ~J.E. Spear and P G. Le Comber of Carnegie Laboratory of Physics, University of Dundee, in Dundee, Scotlancl, did some work on "Substitutional Doping of l~morphous Silicon~l, as reported in a paper publishecl in Solid State Communications, Vol. 17, pp. 1193-1196, 1975, toward the end of redueing the local-ized states in the energy gap in amorphous silicon-to make the same approximate more closely intrin-sic crys-talline silicon and of substitu-tionally doping the amorphous materials with sui.table clas-sic dopants, as in doping crystalline rnaterials, to make them extrinsie and of p or n concluction types.
The reduetion oE the localized states was accomplished by glow discharge deposition of amor-phous silicon films wherein a gas of silane (SiH4) was passed through a reaction tube where the gas was decomposed by an r.f. glol,~ discharge and deposited on a substrate at a substrate tempera ture of about 500-600K (227-327C). The material so deposited on the substrate was an intrinsic 5 amorphous material consisting of silicon and hydrogenO To produce a doped amorphous rnaterial a gas o:E phosphine (PH3) for n-type conduc-tion or a gas of diborane (B2H6) for p-type conduction were premixed with the silane gas and passed through the glow discharge reaction tube under the same operating conditions. The gaseous concentration of the dopants used was between about 5 x 10-6 and 10-~ parts per volume. The material so deposited was shown to be extrinsic and of n or p conduction type.
~ hile it was not known by these researchers, lt is now known by the work of others that the hydrogen in the silane combines at an optimum temperature with many of the dangling bonds of the silicon during the glow discharge deposition, to substantially reduce the density of the localized states in the energy gap toward the end of making the electronic properties of the amorphous mate-rial approximate more nearly those of the corres-ponding crystalline material.
The incorporation of hydrogen in the aboveme-thod however has limitations hased upon the fixed ratio of hydrogen to silicon in silane, and various Si:H bonding configurations which intro-duce new antibonding states. Therefore, there are bcl~ic linlitatiorls in reducing the density of l.ocalizedst:ates in -these materials.
(-,reatly improved amorphous si.licon alloys having significantly reduced concen-trations o:E localized sta-tes in the energy gaps -thereof and high quali-ty electronic ~roperties have been prepared by glow discharge as fully described in U.S. Patent ~o. 4,226,898, Amorphous Semic~nduc-tors Equivalen-t to Crystalline Semiconductors, StanEord R.
Ovshinsky and Arun Madan which issued October 7, 1980, and by vapor de~osition as fully described in U.S. Patent No. 4,217,374, Stanford R. Ovshinsky and Masa-tsugu Izu, which issued on August 12, 1980, under the same -title.
As disclosed in these pa-ten-ts, fluorine is introduced into the amorphous silicon semiconductor allo~ to substantial.ly reduce the density o:E localized sta-tes -therein. ~ctiva-ted :Eluorirle especially readily bonds to silicon in the amorphous body to substantially decrease the density oE localized defec-t s-tates -therein, because the small size, high reactivity and specifici-ty of chemical bonding of -the ~0 :E].uorine atoms enables them to achieve a more defect free amorphous silicon alloy. The fluroine cr/
bonds to the dangling bonds of the silicon and forms what is believed to be a predomlnan-tly ionic s-table bond with flexible bonding angles, which results in a more stable and more efficient com-pensation or alteration than is formed by hydrogenand other compensat`ing or altering agentsO
Fluorine also combines in a preferable manner with silicon and hydrogen, utlliziny the hydrogen in a more clesirable manner, since hydrogen has several bonding options. Without fluorine, hydrogen may not bond in a desirable manner in the material, causing extra defect status in the band gap as well as in the material itself. Therefore, Eluorine is considered to be a more efficient com-~
pensating or altering element than hydrogen whenemployed alone or with hydrogen because of its high reactivity, specificity in chemical bonding, and high electronegativity.
As an example, compensation may be achieved with fluorine alone or in combination with hydro-gen with the addition of these element(s) in very small quantities (eOg., fractions of one atomic percent)O However, the amounts of fluorine and hydrogen most desirably used are much greater than `d~;3 such small percentages so as to form a silicon-hydrogen-fluorine alloy, Such alloying amounts of fluorine and hydrogen may, for example, be in the range of 1 to 5 percent or yreaterO It is believed that the alloy so formed has a lower den-sity o defect states in the energy gap than that achieved by the mere neutralization of dangling bonds and similar defect states. Such larger amount of fluorine, in particular, is believed to participate substantially in a new structural con-figuration of an amorphous silicon-containing material and facilitates the addition of other alloying materials, such as germanium. Fluorine, in addition to its other characteristics mentioned herein, is believed to be an organizer of local structure in the silicon-containing alloy through inductive and ionic effects. It is believed that Eluorine also influences the bonding of hydrogen by acting in a beneficial way to decrease the den-2.0 sity of defect states which hydrogen contribu-tes while acting as a density of states reducing ele-ment. The ionic role that Eluorine plays in such an alloy is believed to be an important Eactor in terms of the nearest neighbor relationships.
~'L~ J~3"~3 Amorphous silicon alloys containing fluorine have thus demonstrated greatly improved character-istics for photovoltaic applications as compared to amorphous silicon alloys containing just hydro-gen alone as a density of states reducing ele-ment. ~owever, in order to realize the full advantage of these amorphous silicon alloys con-taining fluorine when used to form the ac-tive regions of photovoltaic devices, it is necessary to assure that the greatest possible portion-of the available photons are absorbed therein for efficiently generating electron-hole pairs.
The foregoing is important in, for example, pho-tovoltaic devices of the p-i-n configuration.
1~ Devices of this type have p and n-type doped layers on opposite sides of an active intrinsic layer, wherein the electron-hole pairs are gener-ateæ. They establish a potential gradient across the device to facilitate the separation of the electrons and holes and also form contact layers to facilitate the collection of the electrons and holes as electrical current.
Not all of the available photons are absorbed by the active regions in a single pass there-through. While almost all of the shorter wave-length photons are absorbed during the first pass, ~11--a large por-tion of the longer wavelength photons, for example, photons having wavelengths of ~r angstroms or greater, are not so absorbedO The loss of these unabsorbed photons places a limit on the short circuit currents which can be producedO
To preclude the loss of these ~onger wavelen~th photons, back reflectors, formed from conductive metals have been employed to reflect the unused or unabsorbed light back into the active regions of the devices, The p and n-type layers are conductive and, at least in the case of the p-type layer, can have a wide band gap to clecrease photon absorption. A
back reflector is therefore extremely aclvantageous when used in conjunction with a p~type layer having a wide band gap forming the top layer of such a device. Back reflectors are also advan tageous when the wide band gap p layer forms the bottom layer of the device. In either case, back reflecting layers serve to reflect unused light back into the intrinsic region of the device to permit further utilization of the solar energy for generating additional electron-hole pairs. A bac~
reflecting layer permits a greater portion of the available photons to pass into the active intrin-sic layer and to be absorbed therein.
Unfortunatelyl the best back reflectors of the prlor art have been capable of reflecting only about ~0 percent of the unused light back into the devices in which they are employed. Metals such as copper and aluminum, because they are highly reflecting, have been suggested as possible back reflector materials. However, these rnetals can diffuse into the semiconductor of the devices in which they are employed and, in doing so, adverse-ly effect the photoresponsive characteris-tics of the devices. As a result, other less reflec-tive metals have been employed as back reflectors.
Such less reflective metals include molybden~rn and chromiumO Although these metals do not diffuse into the semiconductor of the devices, they cannot achieve the reflectance of the more highly reflec-tive metals~ This is particularly true when the less reflective rnetals interface with a material such as amorphous silicon alloys which have a high index of refraction. Furthermore, the back re-flectors of the prior art reflect the unused light back into the active regions in the same direction as the original direction of incidences (assuming normal incidence). Hence, after being reflected, the light which is not absorbed durins the second R ~.~
pass is permitted to escape. Hence, not all the light is absorbed. Also, since the ligh-t passes normal to the active regions, the active regions ~ust be of sufficient thickness to permit ei-fi~
cient absorption. However, because the minority carrier diffusion length is finite, the ac-tive region cannot be made arbitrarily thicko If, to achieve substantial absorption, the ac-tive recjion thickness is increased much beyoncd the diffusion length, recombination effects will predominate malcing it difficult to efficiently collect the photogenerated charge carriers as elect-rical cur~
rentO Hence, there is a need for better photo-voltaic devices which not only provide greater utiliza-tion of the incident light, bu-t also more efficient collection of the charge carriers created in the active region or regions of the devices.
~e have found that the above disadvantages may be overcome by providing means for directing a-t least a portion of the incident radiation through the active region or regions at an angle which is sufficient to confine the directed light within the devices to substantially increase absorption. Further, because the radiation is c1irected through the active region at~an angle, the present invention permits the active regions to be made thinner and, therefore, recombination effects are reduced. The incident radia-tion directing means of the present invention provide mu:Ltiple passes of light within the active reyions of the devices in which they are employed to enable substantially total absorption while assur-ing more compLete collection of the electron-hole pairs.
The radiation directors can be either random or periodic reflecting or transmitting struc-tures~ The random and periodic reflecting struc-tures can be either surface or bulk .reflectors.
:L5 Over each of the foregoing reflectors~ a coating of a transparent conductor, such as a transparent eonductive oxide, ean be deposited.
When these refleetors are utilized as the sub-strates for the devices, the transparent conduc-tive oxide serves as a contact layer.
Applicants herein have discovered new and improved photovoltaic devices ~hich provide both increased light utilization for creatin~ electron-hole pairs and more efficient collection of the charge carriers. Basically, the present invention )'Y~
provides means for direc-ting at least a portion of the incident radiation through the active region or regions at an angle which is sufficient to con-:Eine the directed light within the devices -to sub-stantiall~ increase absorptionO Further, thepresent inventlon permits the active regions to be made thinner to reduce recombination effects. The radi.ation directors of the present invention can be utilized in any form of photovoltaic cell, and find particular application in thin Eilm solar cells in both single cell photovoltaic devices of the p-i-n configuration, and multiple cell struc-tures having a plurality of single cell units.
The present invention provides new and :L5 improved photovoltaic devices having incident radiation directing means for directing at least a portion of the incident radiation through the active region or regions, wherein the charge car-riers are created, at an angle sufficien-t to sub-stantially confine the directed radiation withinthe photovoltaic devices. For normal radiation incidence, the radiation directing means directs the radiation through the active region or regions at angles at least greater than the angle (the g ~
critical angle) whose sine is the inde~ of refrac-tion of air divided by the index of refraction of the material which forms the active region or reyions, The incident radiation directing means of the present invention provide multiple passes of light within the active regions of the devices in which they are employed to enable substantially total absorption while assuring more complete collection of the electron-hole pairs.
The radiation directors can be either random or periodic reflecting or transmitting struc-tures. The random and periodic reflecting struc-tures can be either surface or bulk reflectors.
For e~ample, the random surface reflector can be a lS roughened reflective surface of aluminu~, gold, silver, copper~ or other highly reflective mate-rial. The periodic surface reflector can be a reflective diffraction grating and preferably a blazed diffraction grating. The grating spacing can be optimized for reflecting light of predeter-mined wavelengths and the grating shapes and heights can be optimized for selecting -the order and reflectance order magnitudes as desired to achieve internal reflection at desired l~aterial interfaces.
The random bulk reflector can be, for exam-ple, a body of ceramic material such as titanium dioxide, zinc selenide, alumina, ~inc sulphide, selenium~ and silicon carbicle, or a body of enamel ma-terialD The grains and randomly distri~uted facets of the polycrystalline components of these materials provide random reflections from their bulk~ The bulk periodic reflector can be, for example, a hologram.
Over each of the foregoiny reflectors a coat ing of a transparent conductor such as a trans-paren-t conductive oxide can be deposited~ When these reElectors are utilized as the subs-trates Eor the devices, the transparent conductive oxides :15 serve as a contact layer. The transparent conduc-tive oxide can be indiu~ tin oxide, cadr~lium stan-nate, or doped tin oxide, for example.
By direc-ting the light through the active region or regions at an angle greater than the critical angle for an air-active region material interface, the directed light will be internally reflected and substantially confined within the devices. The radiation directors of the present invention t:herefore enable substantially total absorption of light for the yeneration of electron-hole pairs within the devices~
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The present invention is particularly appli-cable in photovoltaic devices of p-i-n configura--tion. Such devices include an intrinsic active semiconductor re~ion wherein photogenerated electron hole pairs are created and doped regions of opposite conductivity disposed on opposite res-pective sides of the intrinsic region. The active intrinsic region is preferably an amorphous sili-con alloy body or layer containing fluorine as a density of states reducing element. The doped regions also preferably include an amorphous sili-con wide ~and gap p-type alloy layer forming either the top or bottom semiconductor layer of the clevice. In either case, the amorphous semi-conductor regions are preferably deposi-ted onto the radia-tion reflectors with the layer of trans-parent conductor disposed between the radiation director and the bottom doped layer. Alterna-tively, in accordance with the present invention, ~0 a transparent radiation director can be provided on the top doped layer~ Such a transparent radia--tion director can be, for example, a transmission diffraction grating.
Substantially all of the shorter wavelength 5 photons are absorbed in the active intrinsic ~ 7~
regions during the first pass therethrough while only a portion of the photons having wavelengths longer -than about 6000A are so absorbec1. The periodic refleetors ean be optimized for these longer wavelengths to optimize the directing of the longer wavelength photonsO To that end, the angle of diffraction provided by a diffraetion grating can be determined by the relationship:
~iff = Sin-l m ~
nd Where: d is the grating spaclng;
is the minimum photon vacuum wavelength to be diffractecl;
n is the index of refraction of the medium in which the grating cliffraets the radiation into; and m is the di:Efraetion order.
The baek refleetor systems of the present invention can also be utilizecl in multiple cell devices such as tandem eells.
The preferred embodiment of this inven-tion will now be deseribed by wav of example with referenee to the drawings aeeompanying this speei-Eieation in whieh:
Fig. 1 is a c7ia~rammatie representation of aglow discharge deposition system which may be utilized for ma]cing the photovoltaie c?evices of the invention;
~ iy. 2 is a sectional view of a portion of -the system of Fig. 1 taken along the lines of 2-2 therein;
FigO 3 is a sectional view in schematic form, S of a photovoltaic device embodying the present invention which illustrates the general principles and advantages of the present invention;
Fig. ~ is a sectional view of a of p-i n photovoltaic device including a random surface reflector embodying the present invention;
Fig. 5 is a sectional view of a p i~n photo-voltaic device including a random bulk reflector ernbodying the present invention;
Fig. 6 is a sectional view of a p-i~n photo-voltaic device including a periodic surface reflector embodying the present invention;
FigO 7 is a sectional view of a p-i-n pho-to voltaic device includirlg a transparent incident light director embodylng the present invention;
Fig~ 8 is a sectional view of a p-i-n photo-voltaic device including a periodic bulk reflector embodying the present invention;
Fig. 9 is a sectional view of a multiple cell incorporating a plurality of p-i-n photovoltaic cell units arranged in tandem configuration which includes a radlation directing means embodying the present invention;
FigO 10 is a sectional view of another tandem device inclucling a periodic surface reflec-tor embodying the present invention; and FigO 11 is a sectional view of a p~i-n photo-voltaic device structured in accordance with a still further embodiment of the present invention.
Referring now more particularly to Fig~ 1, there is shown a glow discharge deposition system 10 including a housing 12. The housing 12 encloses a vacuum chamber 1'l and includes an inlet chamber 16 and an outlet chamber 18. A cathode backi.ng member 20 is mounted in the vacuum chamber 14 through an insulator 22.
The backing member 20 includes an insulatinq sleeve 24 circumferentially enclosing the backing member 20. A dark space shield 26 is spaced from and circumferentially surrounds the sleeve 24. A
substrate 28 is secured to an inner end 30 of the backing member 20 by a holder 32. The holder 32 can be screwed or otherwise conventionally secured to the backing member 20 in electrical contact therewith.
~22 The cathode backing member 20 includes a well 34 into which is inserted an electrical heater 36 for heating the backing memher 20 and hence the substrate 28. The cathode backing member 20 also includes a temperature responsive probe 38 for measuring the temperature of the backing member 20. The temperature probe 1~ is utilized to con-trol -the energization of the heater 36 to maintain the backing member 20 and the substrate 28 at any desired temperature.
The system 10 also includes an electrode 40 which extends from the housing 12 into the vacuum chamber 14 spaced from the cathode backing member 200 The electrode 40 includes a shield 42 sur-rounding the electrode 40 and which in turn car-ries a substra-te 44 mounted thereon. The elec-trode 40 includes a well 46 into which is inserted an electrode heater 48. The electrode 40 also includes a temperature responsive ~robe 50 for measuring the temperature of the electrode 40 and hence the substrate 44. The probe 50 is utilized to control the energization of the heater 48 to maintain the electrode 40 and the substrate 44 at any desired temperature, independently oE the mem-ber 20.
A glow discharge plasma is developed in a space 52 between the substrates 28 and 44 by the power generated from a regulated R,F~, A.C~ or D~C. power source coupled to the cathode backing member 20 across the space 52 to the electrode 40 ~hich is coupled to ground. The vacuum chamber 14 is evacuated to the desired pressure by a vacuum pump 54 coupled to the chamber 14 through a parti-cle trap 56. A pressure gauge 58 is coupled to the vacuum system and is utilized to control -the pump 54 to maintain the system 10 at the desired pressureO
The inlet chamber 16 of the housing 12 ~re-ferably is provided with a plurality of conduits 60 for introducing materials into the system 10 to be mixed therein and to be deposited in -the cham-ber 14 in the glow discharge plasma space 52 upon the substrates 28 and 44. If desired, the inlet chamber 16 can be located at a remote location and the gases can be premixed prior to being fed into the chamber 14, The gaseous materials are fed into the conduits 60 through a filter or other purifying device 62 at a rate controllecl by a valve 640 ~ ^~hen a material initially is not in a gaseous form, but instead is in a liquid or solid form, it can be placed into a sealed container 66 as indi-cated at 68. The material 68 -then is heated by a heater 70 to increase the vapor pressure thereof in the con-tainer 66. A suitable gas, such as argon, is fed through a dip tube 72 into the mate-rial 68 so as to entrap the vapors of the material 68 and convey the vapors through a filter 62' and a valve 64' into the conduits 60 and hence into the system 10.
The inlet chamber 16 and the outlet chamber 18 preferably are provided with screen means 74 to conEine the p].asma in the chamher 14 ancl princi-pally between the substrates 28 and 44.
The materials fed through the conduits 60 aremixed in the inlet chamber 16 and then fecl into the glow discharge space 52 to maintain the plasma and deposit the alloy on the substrates with -the incorporation of silicon, fluorine, oxygen and the other desired alterant elemen~cs, such as hydrogen, andjor dopants or other desired materials.
In operation, and for depositing layers of intrinsic amorphous silicon alloys, the system 10 J~
is first pumped down to a desired deposition pres-sure, such as less than 20 mtorr prlor to deposi-ti.on. Starting materials or reaction gases such as silicon tetrafluoride (SiE`4) and molecular hydrogen (H2) and/or silane are Eed into the inlet chamber 16 through separate conduits 60 and are then mixed in the inlet chamber. The gas mixture is fed in-to the vacuum chamber to maintain a par-tia]: pressure therein of about o6 torr. A plasma is generated in the space 52 between the sub-strates 28 and 44 using either or both a DC volt-aye of greater than 6C0 volts or radio frequency power of about 10 to 15 watts operating at a fre-quency of 13.56 ~Hz or other desired frequency.
In addition to the intrinsic amorphous sili-con alloys deposited in the manner as described above, the devices of the present invention as illustrated in the various embodiments to be des-cribed hereinafter also utilize doped amorphous silicon alloys including wide band gap p amorphous silicon alloys. These doped alloy layers can be p, p+, n, or n+ type in conductivity and can be formed by in-troducing an appropriate dopant into ~he vacuum chamber along with the intrinsic start-ing material such as silane (SiH4) or the silicon tetrafluoride (SiY4) starting material ancl/or hyclrogen and/or silane.
E`or n or p doped layers, the material can be cloped with 5 to 100 ppm of dopant materials as it is deposited. For n+ or p~ doped layers, the material is doped with 100 ppm to over 1 percent of dopank material as it is deposited. The n clopants can be phosphorus, arsenic, antimony, or bismuth. Preferably, the n doped layers are deposited by the glow discharge decomposition of at least silicon tetrafluoride (SiF~) and phos-phine (P~3). Hydrogen and/or silane gas (SiH~) may also be added to this mixture.
The p dopants can be boron, aluminum, qal-lium, indium, or thallium. Preferably, the p--type layers are deposited by the glow discharge decom-position of at least silane and diborane (B2H6) or silicon tetrafluoride and diborane. To tne sili-con tetrafluoride and diborane, hydrogen and/or silane can also be added.
In addition to the foregoing, and in accor-dance with the present invention, the p-type layers are formed from amorphous silicon alloys con-taining at least one band gap increasing ele-mentO For example, carbon and/or nitrogen can be A 3~
incorporated into the p-type alloys to increase the band gaps thereof. A wide band gap amorphous silicon alloy can be formed Eor example by a gas mi.cture of silicon tetrafluoride (SiF4), silane ~SiH4), diborane (B2H6), ancl methane (CH~). This results in a p-type amorphous silicon alloy having a wide band gap.
The doped layers of the devices are deposited at various temperatures depending upon the mate-rial to be deposited and the substrate used. Foraluminum substrates~ the upper temperature should not be above about 600C and for stainless steel it could be above about 1000C. For the intrinsic and doped alloys initially compensated with hydro-gen, as for example those deposited from silanegas starting material, the substrate temperature should be less than about 400C and preferably between 250C and 350C.
Other materials and alloying elements may also be added to the intrinsic and doped layers to achieve optimized current generation. These other materials and elements will be described herein-after in connection with the device configurations embodying the present invention illustrated in Figs. 4 through 10.
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P~eferring now to Fig. 3, it illustrates in yenerally schematic sectional view a photovoltaic device 8Q which is referred to herein to facili~
tate a general understanding of the features and advantages of the present invention. The device 80 can be o~ any configuration for a photovoltaic device and can be, for example 7 a p-i-n device, p-n deviee, or Sehottky barrier deviee, Eor exam-ple, The deviee includes a body 82 of semicondue-tor material whieh ean be erystalline, polycrys-talline, or amorphous semiconduetor material or any combination of these~ As will be dLsclosed in reference to Figs. 4 throucJ~ 10, the body 82 of semieonductor material is preEerably but not limited to an amorphous silieon alloy including at least one active reyion wherein photogenerated electron~holes are created.
The body 82 of semiconductor material is dis-posed on radiation direeting means ~4 whic:h ean be conduetive or eoated with a eonductive material, such as a transparent eonduetive oxide to form a bottom contaet for the device 80. Overlying the semiconductor body 82 is a layer 86 of conduc-tive material such as a transparent conductive oxide 5 (TCO). The TCO can be, for example, indium tin ~ J3 ~
oxide, cadmium stannate, or doped tin oxide.Deposited onto the conductive layer 86 is a grid pattern 880 The grid 88 can be a plurali-tv of orthogonally related lines of a conductive metal and cover about 5 to 10 percent of the surface area of the layer 86. The layer 86 and grid 8g serve as the top contact for the device. Depos-ited over the grid 88 and conductive layer 86 is an antireElection (AR) layer 30. Layers of this type will be described in greater detail subse-quently. A layer of glass encapsulant may be used in place of the AR layer 90 as well.
As can be observecl in Fig. 3, the radiation directing means 84, semiconductor body 82, conduc-tive layer 86, and AR coating 90 are all substan-tially planar and define substantially parallel interfaces 94, 96, and 98. The radiation incident surEace 92 of the device 80 and the interfaces 94, 96, and 98 are arranged to receive incident light represented by the dashed ray line 100 substan tially normal thereto.
With prior art back reflectors, the photons of ray 100 not absorbed in the semiconductor body 82 during the second or reflected pass there through are free to escape from the front surface of the device. This results because the ray 100 is reflected along its initial line of inciclence to the device.
In accordance with the present invenl:ion, the ray 100 will not escape from the device because the incident radiation directing means 84 directs the ray through the semiconductor material at an angle sufficient to cause the ray to be substan-tially conined within the cdevice 80. More speci-fically, when the ray 100 impinges upon the radia~tion directing means 84, it is reflected therefrom at an angle ~1 which is greater than the critical angle Eor an tnterface between the material form-ing semiconcluctor body 82 and air. For example, if the body 82 is an amorphous silicon alloy having an index of refraction (n) of 3.5, the ~c is 16.6. This angle can be calculated using Snell's law, where, for total reflection, ~c is given by the relationship:
~c - Sin~l nl n2 Where: nl is the lower index of refrac-tion; and n2 is the higher index of - refraction.
Here, n, is equal -to 1 for air and n2 is equal to 3.5 for amorphous silicon. Hence, nl divided by n2 is equal to 236 and the angle whose sine is ~236 is 1~.6.
Any ray directed throuyh the semiconduc-tor boc3y ~2 (assuming it is amorphous silicon) at an angle of 1606 or greater to the norma3 will be internally reflected within the device at least at the inci~ent surface 92. ~Jowever, the internal re.Election can occur earlier, for example at interface 9S or interface 96. For the internal reflection to occur at interface 98 where the antire:Election (AR) or ylass layer 90 can have an index of refraction of about lo 45 the ray would have to be directed away from the normal by an angle ~2 whieh is equal to or greater than 2~o 5.
Similarly, for the internal reflection to occur at interface 96 where the TCO material can have an index of refraetion of 2.0, the eritical ancJle ~3 would be 34.8. As will be diselosed hereinafter, the incident radiation directincJ means 84 ean, in accordance with the present invention take many different forms for direeting at least a portion of the incident radiation through the active region or regions of photovoltaie devlces a-t angles sufficient to substantially confine ~he directed radiation within the devices~ The inci-dent racli.a-tion directing means can be a random reflector or a periodic refl.ector. ~ith a r~ndom reflector, not all incident radiation is confined 5 but internal reflection can take place at any one of the interfaces or surfaces previously dis-cussed. ~lith a periodic reflector, the angles of direction can be controlled so that nearly all of -the light reaching thls forrn of incident radiat.ion directing means can be confined. ~dditionally, the angle of direction can be controlled so that a speci:Eic interface where internal reflection takes place can be selected. The radiation which is directed by the radiation directing means 84 is primarily light in the red spectrum or longer wavelengths since the shorter wavelenyths are more readily absorbed during the first pass through the amorphous silicon alloy material. However, as will be seen in relation to Fig. 7, the incident 2~ radiation can be directed through the active region in accordance with the present invention c]uring its first pass into a device.
Referring now to Fig. 4, it illustrates in sectional view a p-i-n device 110 including a ran-5 dom surface reflector 111 embodying the present-33-invention. The random surEace reflector 111includes a substrate 112 which may be glassO The glass 112 has an upper surface which is randomly roughened by, for example, sandblasting to form an upper roughened surface 113. Sandblastiny ls a ~ell known process in which very fine particle grains of an abrasive are projected at high velo-city against the surface to be roughened. The substrate 112 is of a width and length as desired.
In accordance with the present invention, a layer 114 of highly reflective metal is cleposited upon the roughened glass surface 113. The layer 114 is deposited by vapor deposition, which is a relatively fast deposition process. The layer 114 preferably is a hi~hly reflecting metal such as silver~ aluminum, gold, copper or any other highly reflecting material. Deposited over the layer 114 is a layer 115 of a transparent conductor such as a transparent conductive oxide (TCO). The trans-parent conductor must at least be transparent forthe photons having wavelengths which are not ini-tially absorbed during the first pass through the deviceO The TCO layer 115 can be depositecl in a vapor deposition environment and, for example, may -3~-be multiple layers 115a and 115b of indium tin oxide (ITO), cadmium stannate (Cd2SnO4~, cadmium oxide (CdO), cadmium sulphicle (CdS~, zinc oxide (Zn~)~ cuprous oxide (Cu2O), barium plumbate (Ba2R~O4), or tin oxide (SnO2) or a single layer of any of the foregoing. The TCO layer or layers 115 serves as a back contact for the device 110 and also serves as a smoothing layer to provide a substantially more planar surface upon which the semiconductor can be depositedO The TCO layer or layers also serves as a diffusion barrier to pre-vent diffusion of the highly conductive metal forming layer 114 into the semiconductor material of the device. The glass substrate 112, the layer 11'l of highly reflective metal, and the layer 115 of transparent conductor Eorm a random surface reflector in accordance wi-th the present inven-tion. Because the layer 114 is randomly rough-ened, at least a portion of the incident light striking the reflector 111 will be directed through the device at an angle sufficient to cause the directed light to be confined within the device as previously described~
The random surface reflector 111 is then placed in the glow discharge deposition environ-ment. A first doped wide band gap p-type amor-phous silicon alloy layér 116 is deposited on the layer 115 in accordance with the presen-t inven-tion. The layer 116 as shown is p_L in conduc~
tivity. The p+ region is as thin as possible on the order of 50 to 500 angstroms in thickness which is sufficient for the p+ region to make yood ohmic contact with the transparent conductive oxide layer 115. The p+ region also serves to establish a potential gradient across the device to facilitate the collection of photo induced electron-hole pairs as electrlcal currentO The p+
region 115 can be deposited from any of the yas mixtures previously referred to for the deposition of such material in accorclance with the presen-t invention~
A body of intrinsic amorphous silicon alloy 118 is next deposited over the wide band gap p-type layer 116. The intrinsic body 118 is rela-tively thick, on the order of 4500A, and is depos-ited from silicon tetrafluoride and hydrogen and/or silane. The intrinsic body preferably con-tains the amorphous silicon alloy compensated with -3~-~3'~
fluorine where the majority of the electron-hole pairs are generated. The short circuit current of the clevice is enhanced by the combined effects of the back reflector of the present invention and the wide band gap of the p-type amorphous silicon alloy layer 116.
Deposited on the intrinsic body 11~ is a fur-ther doped layer 120 which is of opposi-te conduc-tivity with respect to the first doped layer 1160 It comprises an n+ conductivity amorphous silicon alloy. The n+ layer 120 is deposi-ted frorn any of the gas mixtures previously referred to for the deposition of such material. The n+ layer 120 is deposited to a thickness between 50 and 500 angstroms and serves as a contact layer.
Another transparent conductive oxide (TCO) layer 122 is then deposited over the n+ layer 120, The I'CO layer 122 can also be deposited in a vapor deposition environment and, for example, may be indium tin oxide (ITO), cadmium stannate (Cd2SnO~), or doped tin oxide (SnO2).
On the surface of the TCO layer 122 is depos-i-ted a grid electrode 124 made of a metal having good electrical conductivity. The grid may com-prise orthogonally related lines of conductive material occupying only a minor portion of the area of -the metallic regionv the rest of which is to be exposed to solar eneryy. For example t the grid 124 may occupy only about from 5 to 10~ of the entire area of the TCO layer 122. The grid electrode 124 uniformly collects current from the TCO layer 122 to assure a good low series resis-tance for the device.
To complete the device 110, an anti-reflection (AP~) layer or glass encapsulant 126 is applied over the grid electrode 124 and the areas of the TCO layer 122 bet~een the grid electrode areas. The AR layer or glass 126 has a solar radiation incident surface 12~ upon which impinyes the solar radiation. If the layer 126 is an ~R
layer, it may have a thickness on the order of magnitude of the wavelength of the maximum energy point of the solar radiation spectrum, divided by four times the index of refraction of the anti 20 reflection layer 126. A suitable AR layer 126 would be zirconium oxide of about 500A in thick-ness with an index of refraction of 2.1. If the layer 126 is an encapsulant the thickness of TCO
layer 122 can be selected to allow it to also act 5 as an antireflection layer for the device 110.
-3~-t~
As an alternative embodiment, the random sur-face reflector 111 can comprise a sheet of stain-less steel or other metal in place of the glass 1i2. The roughened surface can be provided by sputtering a highly conductive metal, such as aluminum, over the stainless steel sheetO Alumi-num of relative~y large grain size can be so sput-tered to form a randomly roughened surface~ Over the aluminum, a TCO layer, like layer 115 may be deposited.
Nearly all of the photons of the incident light having shorter wavelengths will be absorbed by the active intrinsic layer l 18. As a result, the major portion of the photons which are not absorbed and which reach the random surEace reflector 111 will have longer wavelengths~ about 6000A and longer. This incident radiation strik-ing the reflector 111 will be randomly scattered and at least some of these rays will be directed through the intrinsic region 118 at angles suffi-cient to cause them to be internally reflected at one oE the interfaces of layers 118 and l20, layers 120 and 122, layers 122 and 126, or at -the interface oE layer 126 and the atmosphere above.
5 The rays of incident light which are so directed wil:L be substantially confined within -the devi.ce 110 The bancl gap of the intxinsic layer 118 can he adjus-ted for a particular photoresponse charac-teristic with the incorporation of band gap decreasing elements.
As a fur-ther alternative, the band gap of the intrinsic body 118 can be graded so as to be gradually increasing from the p-~ layer 11~ to n+ layer 120. For example, as the intrinsic layer 118 is deposited, one or more band gap decreasing elements such as germanium, tin~ or lead can be incorporated into the alloys in gradually decreasing concentration~ Germane gas (GeH4) for example c~n be introduced into the glow discharge deposi-tion chamber from a relatively high concentration at first and gradually diminished thereafter as the in-trinsic layer is deposi-ted to a point where such introduc-tion is terminated. The resulting cr/
'~'' ~'' `' ~ J~3'~
intrinsic body will thus have a band gap decreas-ing element, such as germanium, therein in gradu-ally decreasing concentrations from the p+ layer 116 towards the n-~ layer 1200 Referring now to Fig. 5, a p-i n photovoltaic cell 130 is there illustrated which incl~des a random bulk reflector 132 embodying the present invention. The cell 130 includes a p-type layer 138, an intrinsic layer 140, and a n-type layer 142. The layers 138, 140, and 142 can be Eormed from the amorphous silicon alloys as previously described with respect to the device 110 o~ Fig.
light absorption in the material. Because of the poor light absorption, crystalline solar cells have to be at least 50 microns thick to absorb the incident sunlight. Even if the sinyle crystal material is replaced by polycrystalline silicon with cheaper production processes, the indirect optical edge is still maintalned; hence the material thickness is not reduced. The poly-crystalline materials also contain grain bounda ries and other defect problems, which are ordi-narily deleterious.
In summary, crystal silicon devices have fixed parameters ~Jhich are not variable as desired, require large amounts of material~ are only producible in relatively small areas and are expensive and time consuming to produceO The use of devices based upon amorphous silicon alloys can eliminate these crystal silicon disadvantages. An amorphous silicon alloy has an optical absorption edge haviny properties similar to a direct gap semiconductor and only a material thickness of one micron or less is necessary to absorb the same amount of sunlight as the 50 micron thick crystal-line silicon~ Furthert amorphous silicon alloys ~5 can be made faster, easier and in larger areas -than can crystalline silicon.
--d~
Accordingly, a considerable effort has been rnade to develop processes for readily depositing amorphous semiconductor alloys or films, each of which can encompass relatively large areas, if desired, limited only by khe size of the deposi-tion equipment, and which could be readily dopedto form p-type and n-type materials where p-n junction devices are to be made therefrom equiva-lent to those produced by their crystalline coun-terparts~ For many years such work was substan-tially unproductive. Amorphous silicon or german-ium (Group IV) films are normally four-Fold coor-dinated and were found to have microvoids and dangling bonds and other defects which produce a high density of localized states in the energy gap thereof. The presence of a high density of local~
ized states in the energy gap of amorphous silicon semiconductor films results in a low degree of photocoilductivity and short carrier lifetime, making s~lch films unsuitable for photoresponsive applications. Additiona]ly, such films cannot be successfully doped or otherwise modified to shift the Fermi level close to the conduction or valence bands, mal~ing them unsuitable for making p-n ~unc-tions for solar cell and current control device ~5 applications.
In an attempt to minimize the acorementioned problems involved with amorphous silicon (origi-nally thought to be elemental), ~J.E. Spear and P G. Le Comber of Carnegie Laboratory of Physics, University of Dundee, in Dundee, Scotlancl, did some work on "Substitutional Doping of l~morphous Silicon~l, as reported in a paper publishecl in Solid State Communications, Vol. 17, pp. 1193-1196, 1975, toward the end of redueing the local-ized states in the energy gap in amorphous silicon-to make the same approximate more closely intrin-sic crys-talline silicon and of substitu-tionally doping the amorphous materials with sui.table clas-sic dopants, as in doping crystalline rnaterials, to make them extrinsie and of p or n concluction types.
The reduetion oE the localized states was accomplished by glow discharge deposition of amor-phous silicon films wherein a gas of silane (SiH4) was passed through a reaction tube where the gas was decomposed by an r.f. glol,~ discharge and deposited on a substrate at a substrate tempera ture of about 500-600K (227-327C). The material so deposited on the substrate was an intrinsic 5 amorphous material consisting of silicon and hydrogenO To produce a doped amorphous rnaterial a gas o:E phosphine (PH3) for n-type conduc-tion or a gas of diborane (B2H6) for p-type conduction were premixed with the silane gas and passed through the glow discharge reaction tube under the same operating conditions. The gaseous concentration of the dopants used was between about 5 x 10-6 and 10-~ parts per volume. The material so deposited was shown to be extrinsic and of n or p conduction type.
~ hile it was not known by these researchers, lt is now known by the work of others that the hydrogen in the silane combines at an optimum temperature with many of the dangling bonds of the silicon during the glow discharge deposition, to substantially reduce the density of the localized states in the energy gap toward the end of making the electronic properties of the amorphous mate-rial approximate more nearly those of the corres-ponding crystalline material.
The incorporation of hydrogen in the aboveme-thod however has limitations hased upon the fixed ratio of hydrogen to silicon in silane, and various Si:H bonding configurations which intro-duce new antibonding states. Therefore, there are bcl~ic linlitatiorls in reducing the density of l.ocalizedst:ates in -these materials.
(-,reatly improved amorphous si.licon alloys having significantly reduced concen-trations o:E localized sta-tes in the energy gaps -thereof and high quali-ty electronic ~roperties have been prepared by glow discharge as fully described in U.S. Patent ~o. 4,226,898, Amorphous Semic~nduc-tors Equivalen-t to Crystalline Semiconductors, StanEord R.
Ovshinsky and Arun Madan which issued October 7, 1980, and by vapor de~osition as fully described in U.S. Patent No. 4,217,374, Stanford R. Ovshinsky and Masa-tsugu Izu, which issued on August 12, 1980, under the same -title.
As disclosed in these pa-ten-ts, fluorine is introduced into the amorphous silicon semiconductor allo~ to substantial.ly reduce the density o:E localized sta-tes -therein. ~ctiva-ted :Eluorirle especially readily bonds to silicon in the amorphous body to substantially decrease the density oE localized defec-t s-tates -therein, because the small size, high reactivity and specifici-ty of chemical bonding of -the ~0 :E].uorine atoms enables them to achieve a more defect free amorphous silicon alloy. The fluroine cr/
bonds to the dangling bonds of the silicon and forms what is believed to be a predomlnan-tly ionic s-table bond with flexible bonding angles, which results in a more stable and more efficient com-pensation or alteration than is formed by hydrogenand other compensat`ing or altering agentsO
Fluorine also combines in a preferable manner with silicon and hydrogen, utlliziny the hydrogen in a more clesirable manner, since hydrogen has several bonding options. Without fluorine, hydrogen may not bond in a desirable manner in the material, causing extra defect status in the band gap as well as in the material itself. Therefore, Eluorine is considered to be a more efficient com-~
pensating or altering element than hydrogen whenemployed alone or with hydrogen because of its high reactivity, specificity in chemical bonding, and high electronegativity.
As an example, compensation may be achieved with fluorine alone or in combination with hydro-gen with the addition of these element(s) in very small quantities (eOg., fractions of one atomic percent)O However, the amounts of fluorine and hydrogen most desirably used are much greater than `d~;3 such small percentages so as to form a silicon-hydrogen-fluorine alloy, Such alloying amounts of fluorine and hydrogen may, for example, be in the range of 1 to 5 percent or yreaterO It is believed that the alloy so formed has a lower den-sity o defect states in the energy gap than that achieved by the mere neutralization of dangling bonds and similar defect states. Such larger amount of fluorine, in particular, is believed to participate substantially in a new structural con-figuration of an amorphous silicon-containing material and facilitates the addition of other alloying materials, such as germanium. Fluorine, in addition to its other characteristics mentioned herein, is believed to be an organizer of local structure in the silicon-containing alloy through inductive and ionic effects. It is believed that Eluorine also influences the bonding of hydrogen by acting in a beneficial way to decrease the den-2.0 sity of defect states which hydrogen contribu-tes while acting as a density of states reducing ele-ment. The ionic role that Eluorine plays in such an alloy is believed to be an important Eactor in terms of the nearest neighbor relationships.
~'L~ J~3"~3 Amorphous silicon alloys containing fluorine have thus demonstrated greatly improved character-istics for photovoltaic applications as compared to amorphous silicon alloys containing just hydro-gen alone as a density of states reducing ele-ment. ~owever, in order to realize the full advantage of these amorphous silicon alloys con-taining fluorine when used to form the ac-tive regions of photovoltaic devices, it is necessary to assure that the greatest possible portion-of the available photons are absorbed therein for efficiently generating electron-hole pairs.
The foregoing is important in, for example, pho-tovoltaic devices of the p-i-n configuration.
1~ Devices of this type have p and n-type doped layers on opposite sides of an active intrinsic layer, wherein the electron-hole pairs are gener-ateæ. They establish a potential gradient across the device to facilitate the separation of the electrons and holes and also form contact layers to facilitate the collection of the electrons and holes as electrical current.
Not all of the available photons are absorbed by the active regions in a single pass there-through. While almost all of the shorter wave-length photons are absorbed during the first pass, ~11--a large por-tion of the longer wavelength photons, for example, photons having wavelengths of ~r angstroms or greater, are not so absorbedO The loss of these unabsorbed photons places a limit on the short circuit currents which can be producedO
To preclude the loss of these ~onger wavelen~th photons, back reflectors, formed from conductive metals have been employed to reflect the unused or unabsorbed light back into the active regions of the devices, The p and n-type layers are conductive and, at least in the case of the p-type layer, can have a wide band gap to clecrease photon absorption. A
back reflector is therefore extremely aclvantageous when used in conjunction with a p~type layer having a wide band gap forming the top layer of such a device. Back reflectors are also advan tageous when the wide band gap p layer forms the bottom layer of the device. In either case, back reflecting layers serve to reflect unused light back into the intrinsic region of the device to permit further utilization of the solar energy for generating additional electron-hole pairs. A bac~
reflecting layer permits a greater portion of the available photons to pass into the active intrin-sic layer and to be absorbed therein.
Unfortunatelyl the best back reflectors of the prlor art have been capable of reflecting only about ~0 percent of the unused light back into the devices in which they are employed. Metals such as copper and aluminum, because they are highly reflecting, have been suggested as possible back reflector materials. However, these rnetals can diffuse into the semiconductor of the devices in which they are employed and, in doing so, adverse-ly effect the photoresponsive characteris-tics of the devices. As a result, other less reflec-tive metals have been employed as back reflectors.
Such less reflective metals include molybden~rn and chromiumO Although these metals do not diffuse into the semiconductor of the devices, they cannot achieve the reflectance of the more highly reflec-tive metals~ This is particularly true when the less reflective rnetals interface with a material such as amorphous silicon alloys which have a high index of refraction. Furthermore, the back re-flectors of the prior art reflect the unused light back into the active regions in the same direction as the original direction of incidences (assuming normal incidence). Hence, after being reflected, the light which is not absorbed durins the second R ~.~
pass is permitted to escape. Hence, not all the light is absorbed. Also, since the ligh-t passes normal to the active regions, the active regions ~ust be of sufficient thickness to permit ei-fi~
cient absorption. However, because the minority carrier diffusion length is finite, the ac-tive region cannot be made arbitrarily thicko If, to achieve substantial absorption, the ac-tive recjion thickness is increased much beyoncd the diffusion length, recombination effects will predominate malcing it difficult to efficiently collect the photogenerated charge carriers as elect-rical cur~
rentO Hence, there is a need for better photo-voltaic devices which not only provide greater utiliza-tion of the incident light, bu-t also more efficient collection of the charge carriers created in the active region or regions of the devices.
~e have found that the above disadvantages may be overcome by providing means for directing a-t least a portion of the incident radiation through the active region or regions at an angle which is sufficient to confine the directed light within the devices to substantially increase absorption. Further, because the radiation is c1irected through the active region at~an angle, the present invention permits the active regions to be made thinner and, therefore, recombination effects are reduced. The incident radia-tion directing means of the present invention provide mu:Ltiple passes of light within the active reyions of the devices in which they are employed to enable substantially total absorption while assur-ing more compLete collection of the electron-hole pairs.
The radiation directors can be either random or periodic reflecting or transmitting struc-tures~ The random and periodic reflecting struc-tures can be either surface or bulk .reflectors.
:L5 Over each of the foregoing reflectors~ a coating of a transparent conductor, such as a transparent eonductive oxide, ean be deposited.
When these refleetors are utilized as the sub-strates for the devices, the transparent conduc-tive oxide serves as a contact layer.
Applicants herein have discovered new and improved photovoltaic devices ~hich provide both increased light utilization for creatin~ electron-hole pairs and more efficient collection of the charge carriers. Basically, the present invention )'Y~
provides means for direc-ting at least a portion of the incident radiation through the active region or regions at an angle which is sufficient to con-:Eine the directed light within the devices -to sub-stantiall~ increase absorptionO Further, thepresent inventlon permits the active regions to be made thinner to reduce recombination effects. The radi.ation directors of the present invention can be utilized in any form of photovoltaic cell, and find particular application in thin Eilm solar cells in both single cell photovoltaic devices of the p-i-n configuration, and multiple cell struc-tures having a plurality of single cell units.
The present invention provides new and :L5 improved photovoltaic devices having incident radiation directing means for directing at least a portion of the incident radiation through the active region or regions, wherein the charge car-riers are created, at an angle sufficien-t to sub-stantially confine the directed radiation withinthe photovoltaic devices. For normal radiation incidence, the radiation directing means directs the radiation through the active region or regions at angles at least greater than the angle (the g ~
critical angle) whose sine is the inde~ of refrac-tion of air divided by the index of refraction of the material which forms the active region or reyions, The incident radiation directing means of the present invention provide multiple passes of light within the active regions of the devices in which they are employed to enable substantially total absorption while assuring more complete collection of the electron-hole pairs.
The radiation directors can be either random or periodic reflecting or transmitting struc-tures. The random and periodic reflecting struc-tures can be either surface or bulk reflectors.
For e~ample, the random surface reflector can be a lS roughened reflective surface of aluminu~, gold, silver, copper~ or other highly reflective mate-rial. The periodic surface reflector can be a reflective diffraction grating and preferably a blazed diffraction grating. The grating spacing can be optimized for reflecting light of predeter-mined wavelengths and the grating shapes and heights can be optimized for selecting -the order and reflectance order magnitudes as desired to achieve internal reflection at desired l~aterial interfaces.
The random bulk reflector can be, for exam-ple, a body of ceramic material such as titanium dioxide, zinc selenide, alumina, ~inc sulphide, selenium~ and silicon carbicle, or a body of enamel ma-terialD The grains and randomly distri~uted facets of the polycrystalline components of these materials provide random reflections from their bulk~ The bulk periodic reflector can be, for example, a hologram.
Over each of the foregoiny reflectors a coat ing of a transparent conductor such as a trans-paren-t conductive oxide can be deposited~ When these reElectors are utilized as the subs-trates Eor the devices, the transparent conductive oxides :15 serve as a contact layer. The transparent conduc-tive oxide can be indiu~ tin oxide, cadr~lium stan-nate, or doped tin oxide, for example.
By direc-ting the light through the active region or regions at an angle greater than the critical angle for an air-active region material interface, the directed light will be internally reflected and substantially confined within the devices. The radiation directors of the present invention t:herefore enable substantially total absorption of light for the yeneration of electron-hole pairs within the devices~
q3'~
The present invention is particularly appli-cable in photovoltaic devices of p-i-n configura--tion. Such devices include an intrinsic active semiconductor re~ion wherein photogenerated electron hole pairs are created and doped regions of opposite conductivity disposed on opposite res-pective sides of the intrinsic region. The active intrinsic region is preferably an amorphous sili-con alloy body or layer containing fluorine as a density of states reducing element. The doped regions also preferably include an amorphous sili-con wide ~and gap p-type alloy layer forming either the top or bottom semiconductor layer of the clevice. In either case, the amorphous semi-conductor regions are preferably deposi-ted onto the radia-tion reflectors with the layer of trans-parent conductor disposed between the radiation director and the bottom doped layer. Alterna-tively, in accordance with the present invention, ~0 a transparent radiation director can be provided on the top doped layer~ Such a transparent radia--tion director can be, for example, a transmission diffraction grating.
Substantially all of the shorter wavelength 5 photons are absorbed in the active intrinsic ~ 7~
regions during the first pass therethrough while only a portion of the photons having wavelengths longer -than about 6000A are so absorbec1. The periodic refleetors ean be optimized for these longer wavelengths to optimize the directing of the longer wavelength photonsO To that end, the angle of diffraction provided by a diffraetion grating can be determined by the relationship:
~iff = Sin-l m ~
nd Where: d is the grating spaclng;
is the minimum photon vacuum wavelength to be diffractecl;
n is the index of refraction of the medium in which the grating cliffraets the radiation into; and m is the di:Efraetion order.
The baek refleetor systems of the present invention can also be utilizecl in multiple cell devices such as tandem eells.
The preferred embodiment of this inven-tion will now be deseribed by wav of example with referenee to the drawings aeeompanying this speei-Eieation in whieh:
Fig. 1 is a c7ia~rammatie representation of aglow discharge deposition system which may be utilized for ma]cing the photovoltaie c?evices of the invention;
~ iy. 2 is a sectional view of a portion of -the system of Fig. 1 taken along the lines of 2-2 therein;
FigO 3 is a sectional view in schematic form, S of a photovoltaic device embodying the present invention which illustrates the general principles and advantages of the present invention;
Fig. ~ is a sectional view of a of p-i n photovoltaic device including a random surface reflector embodying the present invention;
Fig. 5 is a sectional view of a p i~n photo-voltaic device including a random bulk reflector ernbodying the present invention;
Fig. 6 is a sectional view of a p-i~n photo-voltaic device including a periodic surface reflector embodying the present invention;
FigO 7 is a sectional view of a p-i-n pho-to voltaic device includirlg a transparent incident light director embodylng the present invention;
Fig~ 8 is a sectional view of a p-i-n photo-voltaic device including a periodic bulk reflector embodying the present invention;
Fig. 9 is a sectional view of a multiple cell incorporating a plurality of p-i-n photovoltaic cell units arranged in tandem configuration which includes a radlation directing means embodying the present invention;
FigO 10 is a sectional view of another tandem device inclucling a periodic surface reflec-tor embodying the present invention; and FigO 11 is a sectional view of a p~i-n photo-voltaic device structured in accordance with a still further embodiment of the present invention.
Referring now more particularly to Fig~ 1, there is shown a glow discharge deposition system 10 including a housing 12. The housing 12 encloses a vacuum chamber 1'l and includes an inlet chamber 16 and an outlet chamber 18. A cathode backi.ng member 20 is mounted in the vacuum chamber 14 through an insulator 22.
The backing member 20 includes an insulatinq sleeve 24 circumferentially enclosing the backing member 20. A dark space shield 26 is spaced from and circumferentially surrounds the sleeve 24. A
substrate 28 is secured to an inner end 30 of the backing member 20 by a holder 32. The holder 32 can be screwed or otherwise conventionally secured to the backing member 20 in electrical contact therewith.
~22 The cathode backing member 20 includes a well 34 into which is inserted an electrical heater 36 for heating the backing memher 20 and hence the substrate 28. The cathode backing member 20 also includes a temperature responsive probe 38 for measuring the temperature of the backing member 20. The temperature probe 1~ is utilized to con-trol -the energization of the heater 36 to maintain the backing member 20 and the substrate 28 at any desired temperature.
The system 10 also includes an electrode 40 which extends from the housing 12 into the vacuum chamber 14 spaced from the cathode backing member 200 The electrode 40 includes a shield 42 sur-rounding the electrode 40 and which in turn car-ries a substra-te 44 mounted thereon. The elec-trode 40 includes a well 46 into which is inserted an electrode heater 48. The electrode 40 also includes a temperature responsive ~robe 50 for measuring the temperature of the electrode 40 and hence the substrate 44. The probe 50 is utilized to control the energization of the heater 48 to maintain the electrode 40 and the substrate 44 at any desired temperature, independently oE the mem-ber 20.
A glow discharge plasma is developed in a space 52 between the substrates 28 and 44 by the power generated from a regulated R,F~, A.C~ or D~C. power source coupled to the cathode backing member 20 across the space 52 to the electrode 40 ~hich is coupled to ground. The vacuum chamber 14 is evacuated to the desired pressure by a vacuum pump 54 coupled to the chamber 14 through a parti-cle trap 56. A pressure gauge 58 is coupled to the vacuum system and is utilized to control -the pump 54 to maintain the system 10 at the desired pressureO
The inlet chamber 16 of the housing 12 ~re-ferably is provided with a plurality of conduits 60 for introducing materials into the system 10 to be mixed therein and to be deposited in -the cham-ber 14 in the glow discharge plasma space 52 upon the substrates 28 and 44. If desired, the inlet chamber 16 can be located at a remote location and the gases can be premixed prior to being fed into the chamber 14, The gaseous materials are fed into the conduits 60 through a filter or other purifying device 62 at a rate controllecl by a valve 640 ~ ^~hen a material initially is not in a gaseous form, but instead is in a liquid or solid form, it can be placed into a sealed container 66 as indi-cated at 68. The material 68 -then is heated by a heater 70 to increase the vapor pressure thereof in the con-tainer 66. A suitable gas, such as argon, is fed through a dip tube 72 into the mate-rial 68 so as to entrap the vapors of the material 68 and convey the vapors through a filter 62' and a valve 64' into the conduits 60 and hence into the system 10.
The inlet chamber 16 and the outlet chamber 18 preferably are provided with screen means 74 to conEine the p].asma in the chamher 14 ancl princi-pally between the substrates 28 and 44.
The materials fed through the conduits 60 aremixed in the inlet chamber 16 and then fecl into the glow discharge space 52 to maintain the plasma and deposit the alloy on the substrates with -the incorporation of silicon, fluorine, oxygen and the other desired alterant elemen~cs, such as hydrogen, andjor dopants or other desired materials.
In operation, and for depositing layers of intrinsic amorphous silicon alloys, the system 10 J~
is first pumped down to a desired deposition pres-sure, such as less than 20 mtorr prlor to deposi-ti.on. Starting materials or reaction gases such as silicon tetrafluoride (SiE`4) and molecular hydrogen (H2) and/or silane are Eed into the inlet chamber 16 through separate conduits 60 and are then mixed in the inlet chamber. The gas mixture is fed in-to the vacuum chamber to maintain a par-tia]: pressure therein of about o6 torr. A plasma is generated in the space 52 between the sub-strates 28 and 44 using either or both a DC volt-aye of greater than 6C0 volts or radio frequency power of about 10 to 15 watts operating at a fre-quency of 13.56 ~Hz or other desired frequency.
In addition to the intrinsic amorphous sili-con alloys deposited in the manner as described above, the devices of the present invention as illustrated in the various embodiments to be des-cribed hereinafter also utilize doped amorphous silicon alloys including wide band gap p amorphous silicon alloys. These doped alloy layers can be p, p+, n, or n+ type in conductivity and can be formed by in-troducing an appropriate dopant into ~he vacuum chamber along with the intrinsic start-ing material such as silane (SiH4) or the silicon tetrafluoride (SiY4) starting material ancl/or hyclrogen and/or silane.
E`or n or p doped layers, the material can be cloped with 5 to 100 ppm of dopant materials as it is deposited. For n+ or p~ doped layers, the material is doped with 100 ppm to over 1 percent of dopank material as it is deposited. The n clopants can be phosphorus, arsenic, antimony, or bismuth. Preferably, the n doped layers are deposited by the glow discharge decomposition of at least silicon tetrafluoride (SiF~) and phos-phine (P~3). Hydrogen and/or silane gas (SiH~) may also be added to this mixture.
The p dopants can be boron, aluminum, qal-lium, indium, or thallium. Preferably, the p--type layers are deposited by the glow discharge decom-position of at least silane and diborane (B2H6) or silicon tetrafluoride and diborane. To tne sili-con tetrafluoride and diborane, hydrogen and/or silane can also be added.
In addition to the foregoing, and in accor-dance with the present invention, the p-type layers are formed from amorphous silicon alloys con-taining at least one band gap increasing ele-mentO For example, carbon and/or nitrogen can be A 3~
incorporated into the p-type alloys to increase the band gaps thereof. A wide band gap amorphous silicon alloy can be formed Eor example by a gas mi.cture of silicon tetrafluoride (SiF4), silane ~SiH4), diborane (B2H6), ancl methane (CH~). This results in a p-type amorphous silicon alloy having a wide band gap.
The doped layers of the devices are deposited at various temperatures depending upon the mate-rial to be deposited and the substrate used. Foraluminum substrates~ the upper temperature should not be above about 600C and for stainless steel it could be above about 1000C. For the intrinsic and doped alloys initially compensated with hydro-gen, as for example those deposited from silanegas starting material, the substrate temperature should be less than about 400C and preferably between 250C and 350C.
Other materials and alloying elements may also be added to the intrinsic and doped layers to achieve optimized current generation. These other materials and elements will be described herein-after in connection with the device configurations embodying the present invention illustrated in Figs. 4 through 10.
~ 3~
P~eferring now to Fig. 3, it illustrates in yenerally schematic sectional view a photovoltaic device 8Q which is referred to herein to facili~
tate a general understanding of the features and advantages of the present invention. The device 80 can be o~ any configuration for a photovoltaic device and can be, for example 7 a p-i-n device, p-n deviee, or Sehottky barrier deviee, Eor exam-ple, The deviee includes a body 82 of semicondue-tor material whieh ean be erystalline, polycrys-talline, or amorphous semiconduetor material or any combination of these~ As will be dLsclosed in reference to Figs. 4 throucJ~ 10, the body 82 of semieonductor material is preEerably but not limited to an amorphous silieon alloy including at least one active reyion wherein photogenerated electron~holes are created.
The body 82 of semiconductor material is dis-posed on radiation direeting means ~4 whic:h ean be conduetive or eoated with a eonductive material, such as a transparent eonduetive oxide to form a bottom contaet for the device 80. Overlying the semiconductor body 82 is a layer 86 of conduc-tive material such as a transparent conductive oxide 5 (TCO). The TCO can be, for example, indium tin ~ J3 ~
oxide, cadmium stannate, or doped tin oxide.Deposited onto the conductive layer 86 is a grid pattern 880 The grid 88 can be a plurali-tv of orthogonally related lines of a conductive metal and cover about 5 to 10 percent of the surface area of the layer 86. The layer 86 and grid 8g serve as the top contact for the device. Depos-ited over the grid 88 and conductive layer 86 is an antireElection (AR) layer 30. Layers of this type will be described in greater detail subse-quently. A layer of glass encapsulant may be used in place of the AR layer 90 as well.
As can be observecl in Fig. 3, the radiation directing means 84, semiconductor body 82, conduc-tive layer 86, and AR coating 90 are all substan-tially planar and define substantially parallel interfaces 94, 96, and 98. The radiation incident surEace 92 of the device 80 and the interfaces 94, 96, and 98 are arranged to receive incident light represented by the dashed ray line 100 substan tially normal thereto.
With prior art back reflectors, the photons of ray 100 not absorbed in the semiconductor body 82 during the second or reflected pass there through are free to escape from the front surface of the device. This results because the ray 100 is reflected along its initial line of inciclence to the device.
In accordance with the present invenl:ion, the ray 100 will not escape from the device because the incident radiation directing means 84 directs the ray through the semiconductor material at an angle sufficient to cause the ray to be substan-tially conined within the cdevice 80. More speci-fically, when the ray 100 impinges upon the radia~tion directing means 84, it is reflected therefrom at an angle ~1 which is greater than the critical angle Eor an tnterface between the material form-ing semiconcluctor body 82 and air. For example, if the body 82 is an amorphous silicon alloy having an index of refraction (n) of 3.5, the ~c is 16.6. This angle can be calculated using Snell's law, where, for total reflection, ~c is given by the relationship:
~c - Sin~l nl n2 Where: nl is the lower index of refrac-tion; and n2 is the higher index of - refraction.
Here, n, is equal -to 1 for air and n2 is equal to 3.5 for amorphous silicon. Hence, nl divided by n2 is equal to 236 and the angle whose sine is ~236 is 1~.6.
Any ray directed throuyh the semiconduc-tor boc3y ~2 (assuming it is amorphous silicon) at an angle of 1606 or greater to the norma3 will be internally reflected within the device at least at the inci~ent surface 92. ~Jowever, the internal re.Election can occur earlier, for example at interface 9S or interface 96. For the internal reflection to occur at interface 98 where the antire:Election (AR) or ylass layer 90 can have an index of refraction of about lo 45 the ray would have to be directed away from the normal by an angle ~2 whieh is equal to or greater than 2~o 5.
Similarly, for the internal reflection to occur at interface 96 where the TCO material can have an index of refraetion of 2.0, the eritical ancJle ~3 would be 34.8. As will be diselosed hereinafter, the incident radiation directincJ means 84 ean, in accordance with the present invention take many different forms for direeting at least a portion of the incident radiation through the active region or regions of photovoltaie devlces a-t angles sufficient to substantially confine ~he directed radiation within the devices~ The inci-dent racli.a-tion directing means can be a random reflector or a periodic refl.ector. ~ith a r~ndom reflector, not all incident radiation is confined 5 but internal reflection can take place at any one of the interfaces or surfaces previously dis-cussed. ~lith a periodic reflector, the angles of direction can be controlled so that nearly all of -the light reaching thls forrn of incident radiat.ion directing means can be confined. ~dditionally, the angle of direction can be controlled so that a speci:Eic interface where internal reflection takes place can be selected. The radiation which is directed by the radiation directing means 84 is primarily light in the red spectrum or longer wavelengths since the shorter wavelenyths are more readily absorbed during the first pass through the amorphous silicon alloy material. However, as will be seen in relation to Fig. 7, the incident 2~ radiation can be directed through the active region in accordance with the present invention c]uring its first pass into a device.
Referring now to Fig. 4, it illustrates in sectional view a p-i-n device 110 including a ran-5 dom surface reflector 111 embodying the present-33-invention. The random surEace reflector 111includes a substrate 112 which may be glassO The glass 112 has an upper surface which is randomly roughened by, for example, sandblasting to form an upper roughened surface 113. Sandblastiny ls a ~ell known process in which very fine particle grains of an abrasive are projected at high velo-city against the surface to be roughened. The substrate 112 is of a width and length as desired.
In accordance with the present invention, a layer 114 of highly reflective metal is cleposited upon the roughened glass surface 113. The layer 114 is deposited by vapor deposition, which is a relatively fast deposition process. The layer 114 preferably is a hi~hly reflecting metal such as silver~ aluminum, gold, copper or any other highly reflecting material. Deposited over the layer 114 is a layer 115 of a transparent conductor such as a transparent conductive oxide (TCO). The trans-parent conductor must at least be transparent forthe photons having wavelengths which are not ini-tially absorbed during the first pass through the deviceO The TCO layer 115 can be depositecl in a vapor deposition environment and, for example, may -3~-be multiple layers 115a and 115b of indium tin oxide (ITO), cadmium stannate (Cd2SnO4~, cadmium oxide (CdO), cadmium sulphicle (CdS~, zinc oxide (Zn~)~ cuprous oxide (Cu2O), barium plumbate (Ba2R~O4), or tin oxide (SnO2) or a single layer of any of the foregoing. The TCO layer or layers 115 serves as a back contact for the device 110 and also serves as a smoothing layer to provide a substantially more planar surface upon which the semiconductor can be depositedO The TCO layer or layers also serves as a diffusion barrier to pre-vent diffusion of the highly conductive metal forming layer 114 into the semiconductor material of the device. The glass substrate 112, the layer 11'l of highly reflective metal, and the layer 115 of transparent conductor Eorm a random surface reflector in accordance wi-th the present inven-tion. Because the layer 114 is randomly rough-ened, at least a portion of the incident light striking the reflector 111 will be directed through the device at an angle sufficient to cause the directed light to be confined within the device as previously described~
The random surface reflector 111 is then placed in the glow discharge deposition environ-ment. A first doped wide band gap p-type amor-phous silicon alloy layér 116 is deposited on the layer 115 in accordance with the presen-t inven-tion. The layer 116 as shown is p_L in conduc~
tivity. The p+ region is as thin as possible on the order of 50 to 500 angstroms in thickness which is sufficient for the p+ region to make yood ohmic contact with the transparent conductive oxide layer 115. The p+ region also serves to establish a potential gradient across the device to facilitate the collection of photo induced electron-hole pairs as electrlcal currentO The p+
region 115 can be deposited from any of the yas mixtures previously referred to for the deposition of such material in accorclance with the presen-t invention~
A body of intrinsic amorphous silicon alloy 118 is next deposited over the wide band gap p-type layer 116. The intrinsic body 118 is rela-tively thick, on the order of 4500A, and is depos-ited from silicon tetrafluoride and hydrogen and/or silane. The intrinsic body preferably con-tains the amorphous silicon alloy compensated with -3~-~3'~
fluorine where the majority of the electron-hole pairs are generated. The short circuit current of the clevice is enhanced by the combined effects of the back reflector of the present invention and the wide band gap of the p-type amorphous silicon alloy layer 116.
Deposited on the intrinsic body 11~ is a fur-ther doped layer 120 which is of opposi-te conduc-tivity with respect to the first doped layer 1160 It comprises an n+ conductivity amorphous silicon alloy. The n+ layer 120 is deposi-ted frorn any of the gas mixtures previously referred to for the deposition of such material. The n+ layer 120 is deposited to a thickness between 50 and 500 angstroms and serves as a contact layer.
Another transparent conductive oxide (TCO) layer 122 is then deposited over the n+ layer 120, The I'CO layer 122 can also be deposited in a vapor deposition environment and, for example, may be indium tin oxide (ITO), cadmium stannate (Cd2SnO~), or doped tin oxide (SnO2).
On the surface of the TCO layer 122 is depos-i-ted a grid electrode 124 made of a metal having good electrical conductivity. The grid may com-prise orthogonally related lines of conductive material occupying only a minor portion of the area of -the metallic regionv the rest of which is to be exposed to solar eneryy. For example t the grid 124 may occupy only about from 5 to 10~ of the entire area of the TCO layer 122. The grid electrode 124 uniformly collects current from the TCO layer 122 to assure a good low series resis-tance for the device.
To complete the device 110, an anti-reflection (AP~) layer or glass encapsulant 126 is applied over the grid electrode 124 and the areas of the TCO layer 122 bet~een the grid electrode areas. The AR layer or glass 126 has a solar radiation incident surface 12~ upon which impinyes the solar radiation. If the layer 126 is an ~R
layer, it may have a thickness on the order of magnitude of the wavelength of the maximum energy point of the solar radiation spectrum, divided by four times the index of refraction of the anti 20 reflection layer 126. A suitable AR layer 126 would be zirconium oxide of about 500A in thick-ness with an index of refraction of 2.1. If the layer 126 is an encapsulant the thickness of TCO
layer 122 can be selected to allow it to also act 5 as an antireflection layer for the device 110.
-3~-t~
As an alternative embodiment, the random sur-face reflector 111 can comprise a sheet of stain-less steel or other metal in place of the glass 1i2. The roughened surface can be provided by sputtering a highly conductive metal, such as aluminum, over the stainless steel sheetO Alumi-num of relative~y large grain size can be so sput-tered to form a randomly roughened surface~ Over the aluminum, a TCO layer, like layer 115 may be deposited.
Nearly all of the photons of the incident light having shorter wavelengths will be absorbed by the active intrinsic layer l 18. As a result, the major portion of the photons which are not absorbed and which reach the random surEace reflector 111 will have longer wavelengths~ about 6000A and longer. This incident radiation strik-ing the reflector 111 will be randomly scattered and at least some of these rays will be directed through the intrinsic region 118 at angles suffi-cient to cause them to be internally reflected at one oE the interfaces of layers 118 and l20, layers 120 and 122, layers 122 and 126, or at -the interface oE layer 126 and the atmosphere above.
5 The rays of incident light which are so directed wil:L be substantially confined within -the devi.ce 110 The bancl gap of the intxinsic layer 118 can he adjus-ted for a particular photoresponse charac-teristic with the incorporation of band gap decreasing elements.
As a fur-ther alternative, the band gap of the intrinsic body 118 can be graded so as to be gradually increasing from the p-~ layer 11~ to n+ layer 120. For example, as the intrinsic layer 118 is deposited, one or more band gap decreasing elements such as germanium, tin~ or lead can be incorporated into the alloys in gradually decreasing concentration~ Germane gas (GeH4) for example c~n be introduced into the glow discharge deposi-tion chamber from a relatively high concentration at first and gradually diminished thereafter as the in-trinsic layer is deposi-ted to a point where such introduc-tion is terminated. The resulting cr/
'~'' ~'' `' ~ J~3'~
intrinsic body will thus have a band gap decreas-ing element, such as germanium, therein in gradu-ally decreasing concentrations from the p+ layer 116 towards the n-~ layer 1200 Referring now to Fig. 5, a p-i n photovoltaic cell 130 is there illustrated which incl~des a random bulk reflector 132 embodying the present invention. The cell 130 includes a p-type layer 138, an intrinsic layer 140, and a n-type layer 142. The layers 138, 140, and 142 can be Eormed from the amorphous silicon alloys as previously described with respect to the device 110 o~ Fig.
4. Also as in the device 110 of Fig. 4, the device 130 includes a layer 14~ of transparent conductive oxide, a collection grid 146, and an antir:eflection layer or glass encapsulant 148.
The random bulk reflector 132 includes a sheet or substantially planar member 134 oE a ceramic or enamel material. Such materials have a high index of refraction, for example greater than 1.45, are not light absorptive and have grains and randomly distributed facets of polycrystalline components in their bulk which randomly scatter incident light in all directions. The ceramic or enamel may contain, for example, titanium dioxide, zinc selenide, zinc sulphide, selenium, or silicon carbide~ The sheet 134 can also be formed by the co-deposition of tin oxide and titanium dioxideO
Because the random scat:tering of the light from the sheet 134 is a bulk effect, the surface thereof can be polished or otherwise made very smooth. This is advantageous because it presents a smooth surface for the deposition of the semi-conductor material. Even though ceramics and enamels can be made electrically conductive to some extent, a layer 136 of transparent conductive oxide (TC0) can be provided between the sheet 134 and p~type amorphous silicon alloy layer 138 to form a bottom contact for the device 130.
Referring now to Fig. 6, there is illustrated a p-i~n photovoltaic device 150 which includes a periodic surface reflector 152 embodying the pres-ent invention. The cell 150 includes a layer 158 of p-type amorphous silicon alloy, a layer 160 of intrinsic amorphous silicon alloy, and a layer 162 of n-type amorphous silicon alloy. The layers 158, 160, and 162 can be formed from the alloys and processes as previously described. The device also includes a TC0 layer 164, a collection grid 166, and an AR layer or glass encapsulant 168.
~ 97~
The periodic surface reflector 152 comprises a reflective diffraction grating 154 which can be formed from a conductive metal such as aluminum to form a back contact for the cell 150 and an over-lying layer 156 of transparent conduc-tive oxide.
The pattern of the diffraction grating can take any periodic form in cross-section such as a sinu-soid, square-wave, or the likeO As illustrated, and as a preferred embodiment, the grating 154 is a blazed ~rating. Gratings of this kind are pre-ferred because the æero (0) order reflections, those nor~al to the grating, are minimized.
As previously mentioned, periodic reflectors are advantageous because the angles of diffraction can be selected by proper design of the grating.
This effectively enables selection of the inter-face where internal reflection will occur. In the device 150, it is desirable that the internal reflection occur at or below the interface of 20 layers 16~ and 164 so that the collection grid ls prevented from blocking a portion of the inter nally reflected light.
In designing a diffraction grating the fol-lowing expression can be used:
~Diff = Sin -1 m ~
n tl~
~here; n is the inclex of refraction of the mediu~ that the grating diffracts light i.nto;
~ is the wavelength oE light in a vacuum d is the grating spacing; and m is the order of diffrackion, The height (h) of the cliffraction grating is also a variable .which allows adjustment of the lntensity of the light diffracted in the various diffract-ion orders~ Cenerally, to enhance the intensity of the first order of diffracted rays, h should he about a wavelength in height at the fre-quency of interest.
First order diffraction is also enhanced when d is about equal to a wavelength at the frequency of interest. Here, because most of the shorter wavelength photons are absorbed in the active intrinsic region 160 during their first pass, the longer wavelength photons of about 6600A and longer are of interest.
~5 With d being equal to 6600A, m being equal to 1 for first order diffraction, and with the grating 154 being coated with a layer 156 of transparent conductive oxide such as inclium tin oxide having an index of refraction (n) of 2.1, .he above expression can be solved for ~Diffo iff = Sin -1 6600A
6600A x 2.1 ~ 28.4 This angle of 28.4 within the TCO layer 156, by Snell Ig law, is sufficient to direct the rays through the amorphous silicon alloy layers 158~
160, and 162 at an angle greater than the critical angle for an interface of arnorphous silicon with air to provide internal reflection at leas~ at the interface of layer 168 and the air. Of course, designing a diffraction grating for higher order diffraction will provide a greater angle to achieve internal reflection before this interface.
~ eferring now to Fig. 7, there is illustrated a p-i n photovoltaic device 170 which includes an incident light directing means 172 disposed between the n-type amorphous silicon alloy layer 174 and the TCO layer 175. The incident light directing means 172 comprises a transmission dif-fraction grating 178 arranged to direct all of the incident light through the intrinsic region 180 at an angle. However, since nearly all of the shorter wavelength light will be absorbed in the intrinsic region 180 during the first pass, the diffraction gratiny 178 can be optimized for the
The random bulk reflector 132 includes a sheet or substantially planar member 134 oE a ceramic or enamel material. Such materials have a high index of refraction, for example greater than 1.45, are not light absorptive and have grains and randomly distributed facets of polycrystalline components in their bulk which randomly scatter incident light in all directions. The ceramic or enamel may contain, for example, titanium dioxide, zinc selenide, zinc sulphide, selenium, or silicon carbide~ The sheet 134 can also be formed by the co-deposition of tin oxide and titanium dioxideO
Because the random scat:tering of the light from the sheet 134 is a bulk effect, the surface thereof can be polished or otherwise made very smooth. This is advantageous because it presents a smooth surface for the deposition of the semi-conductor material. Even though ceramics and enamels can be made electrically conductive to some extent, a layer 136 of transparent conductive oxide (TC0) can be provided between the sheet 134 and p~type amorphous silicon alloy layer 138 to form a bottom contact for the device 130.
Referring now to Fig. 6, there is illustrated a p-i~n photovoltaic device 150 which includes a periodic surface reflector 152 embodying the pres-ent invention. The cell 150 includes a layer 158 of p-type amorphous silicon alloy, a layer 160 of intrinsic amorphous silicon alloy, and a layer 162 of n-type amorphous silicon alloy. The layers 158, 160, and 162 can be formed from the alloys and processes as previously described. The device also includes a TC0 layer 164, a collection grid 166, and an AR layer or glass encapsulant 168.
~ 97~
The periodic surface reflector 152 comprises a reflective diffraction grating 154 which can be formed from a conductive metal such as aluminum to form a back contact for the cell 150 and an over-lying layer 156 of transparent conduc-tive oxide.
The pattern of the diffraction grating can take any periodic form in cross-section such as a sinu-soid, square-wave, or the likeO As illustrated, and as a preferred embodiment, the grating 154 is a blazed ~rating. Gratings of this kind are pre-ferred because the æero (0) order reflections, those nor~al to the grating, are minimized.
As previously mentioned, periodic reflectors are advantageous because the angles of diffraction can be selected by proper design of the grating.
This effectively enables selection of the inter-face where internal reflection will occur. In the device 150, it is desirable that the internal reflection occur at or below the interface of 20 layers 16~ and 164 so that the collection grid ls prevented from blocking a portion of the inter nally reflected light.
In designing a diffraction grating the fol-lowing expression can be used:
~Diff = Sin -1 m ~
n tl~
~here; n is the inclex of refraction of the mediu~ that the grating diffracts light i.nto;
~ is the wavelength oE light in a vacuum d is the grating spacing; and m is the order of diffrackion, The height (h) of the cliffraction grating is also a variable .which allows adjustment of the lntensity of the light diffracted in the various diffract-ion orders~ Cenerally, to enhance the intensity of the first order of diffracted rays, h should he about a wavelength in height at the fre-quency of interest.
First order diffraction is also enhanced when d is about equal to a wavelength at the frequency of interest. Here, because most of the shorter wavelength photons are absorbed in the active intrinsic region 160 during their first pass, the longer wavelength photons of about 6600A and longer are of interest.
~5 With d being equal to 6600A, m being equal to 1 for first order diffraction, and with the grating 154 being coated with a layer 156 of transparent conductive oxide such as inclium tin oxide having an index of refraction (n) of 2.1, .he above expression can be solved for ~Diffo iff = Sin -1 6600A
6600A x 2.1 ~ 28.4 This angle of 28.4 within the TCO layer 156, by Snell Ig law, is sufficient to direct the rays through the amorphous silicon alloy layers 158~
160, and 162 at an angle greater than the critical angle for an interface of arnorphous silicon with air to provide internal reflection at leas~ at the interface of layer 168 and the air. Of course, designing a diffraction grating for higher order diffraction will provide a greater angle to achieve internal reflection before this interface.
~ eferring now to Fig. 7, there is illustrated a p-i n photovoltaic device 170 which includes an incident light directing means 172 disposed between the n-type amorphous silicon alloy layer 174 and the TCO layer 175. The incident light directing means 172 comprises a transmission dif-fraction grating 178 arranged to direct all of the incident light through the intrinsic region 180 at an angle. However, since nearly all of the shorter wavelength light will be absorbed in the intrinsic region 180 during the first pass, the diffraction gratiny 178 can be optimized for the
5 longer wavelengths as previously described. Here ~ 7~ ~
however, a sinusoidal diffraction grating is illustrated, but it could of course be any of the other types previously mentioned.
Like the previous p-i-n cells described, the cell 170 further inGludes a p-type layer 182 of arnorphous silicon alloy, a collection grid 184, and a layer 186 antireflection material or glass encapsulant. The various layers are deposited on a substrate 171 of glass, stainless steel, or other suitable subs-trate material. Deposited over the substrate 171 is a layer 173 of highly conduc-tive and thus highly reflective metal, and a TCO
layer 175. The reflective metal layer 173 and TCO
layer 175 form a back reflector to reflect unused light back into the intrinsic region 180.
Alternatively, the transparent director can be glass having a roughened surface formed by sandblasting for example. The various amorphous silicon alloy layers can then be deposited cnto the roughened surface followed by the deposition of a specular back reflector. In this form of device, the incident radiation is first directed through the glass substrate. The glass substrate forms a random radiation director disposed on the side of the active region upon ~hich the light first impinges.
~ 3`~
Referring now to Fig. 8, there is illustrated another p-i-n photovoltaic cell 190 which includes a periodic bulk re~lector 192 embodying the pres-ent invention. Because the device 190 is other-S wise identical to the cells o~ Figs. 4 through 6,only the periodic bulk reflector will be described in detail~
The periodic bulk reflector 192 is disposed upon a substrate 194 of glass, stainless steel, or other suitable substrate material. The periodic bulk reflector 192 takes the form of a hologram comprising a plurality of thin planar members or lines 196 of reflective material, such as alumi-num, embedded in a medium 198 of transparent mate-rial. Here the transparent material is a trans-parent conductive oxide, such as indium tin oxide, to provide both a suitable medium for the lines 195, and a bottom contact for the device 190.
The lines 196 are disposed at an angle, are spaced apart, and are substantially parallel. The diffraction of light by a hologram can be pre~
dicted by the same expression previously de~ined for a diffraction grating. Here, the spacing (d) is the spacing between the lines 196.
-~7-Because the diffraction of the light occurs in the bulk of the hologram, the upper surface thereof can be polished or otherwise made smooth.
This presents a smooth surface upon which the amorphous silicon alloy layer can be deposited.
Referring now to Fig. 9, a multiple cell devlce 200 is there illustrated in sectional view which is arranged in tandem configuration and which includes a random surface reflector embody-ing the present invention. The device 200 com~prises two single cell units 202 and 204 arranged in series relation. As can be appreciated, plural single cell uni-ts of more than two can be uti-lized.
The device 200 includes a random surface reflector 206 including a sand blasted glass layer 203 which is coated with a layer 205 of metal having good reflectivity such as aluminum, for example. Deposited on the metal layer 205 is a layer of transparent conductive oxide 207 which can be a first layer of indium tin oxide 207a and a second layer of doped tin oxide 207b or a single layer of indium tin o~ide. The layer 207 of the transparent conductive oxide can be depositecl as previously described.
-4~-The flrst cell unit 202 includes a first doped p+ amorphous silicon alloy layer 208 deposited on the transparent conductive oxide layer 207~ The p+ layer is preferably a wide band gap p-type amorphous silicon alloy in accordance with the present invention. It can be deposited from any of the previously mentioned starting materials for depositing such material.
Deposited on the wide band gap p+ layer 208 is a first intrinsic amorphous silicon alloy body 210. The first intrinsic alloy body 210 is pre-Eerably an amorphous silicon-fluorine alloy.
Deposited on the intrinsic layer 210 is a further doped amorphous silicon alloy layer 212.
It is opposite in conductivity with respect to the conductivity of the first doped layer 208 and thus is an n+ layer.
The second unit cell 204 is essentially iden-tical and includes a first doped p+ layer 214, an intrinsic body 216 and a further doped n~ layer 218. The device 200 is completed with a TCO layer 220, a grid electrode 222, and an antireflection layer or glass encapsulant 224.
The band yaps of the intrinsic layers are preferably adjusted so that the band gap of layer -4~-3t~
216 is greater than the band gap of layer 210. To that end~ the alloy forming layer 216 can include one or more band gap increasing elements such as nitrogen and carbon. The intrinsic alloy forming the intrinsic layer 210 can include one or more band gap decreasing elements such as germanium, tin, or lead.
It can be noted frorn the figure that the intrinsic layer 210 of the cell is thicker than the intrinsic layer 216. This allows the entire usable spectrum of the solar energy to be utilized for generating electron-hole pairs.
Although a tandem cell embodiment has been shown and described herein, the unit cells can also be isolated from one another with oxide layers for example to form a stac~ed multiple cell. Each cell could include a pair of collec-tion electrodes to facilitate the series connec--tion of the cells with external wiring.
As a further alternative, and as mentioned with respect to the single cells previously des-cribed, one or more of the intrinsic bodies of the unit cells can include alloys having graded band gapsO Any one or more of the band gap increasing or decreasing elements previously mentioned can be incorpora-ted into the intr nsic alloys for -this purpose.
Referring now to Fig. 10, there is illustrated a tandem p-i-n photovoltaic cell 230 which is subs-tantially identical -to -the tandem cell 200 of Fig. 9 except that -the cell 230 includes a periodic surface reElector 232.
Therefore, this cell will be described in de-tail only wi-th respect to the reElector 232.
Like the embodiment of Fig. 6, the periodic surface reflec-tor 232 takes the form of a reflective diffrac-tion grating 234. Although the gra-ting 23~1 can be a sinusoidal~ square-wave, or other periodic configura-tion, the grating 234 here again as illustrated is a blazed grating. The grating can be ~ormed Erom a soft metal, such as aluminurn. It is coated with a layer 236 oE transparent conduc-tive oxide, such ~s indium tin oxide, cadmium stannate, or doped -tin oxide, upon which the amorphous silicon alloys can be deposi-ted. The diffraction gra-ting 234 operates in the same manner as previously described in relation to Fig 6 cr/~
~ q3~'~
F~eferring now to Fig. 11, it illustrates another single cell p-i-n photovoltaic device 240 embodying the present invention. Here, a tran-sparent substrate 242 formed from glass~ for exam-ple, has a TCO layer 250 and p-type, intrinsic, and n-type amorphous silicon alloy layers 244, 246, and 248 respectively successively deposited thereon. Over the n-type layer 248 is provided a layer 252 of a conductive, light diffusant paint.
The layer 252 can be formed from aluminum or gold paint, for example. Such paints are conductive and, when applied by wiping or spraying, or the like, will form a random light scattering inter-face between the layers 2~8 and 252. Alterna-tively, the layer 252 can comprise a first layer of a transparent conductor such as a transparent conductive oxide, and a second layer of a noncon-ductive, but light scattering paint such as a flat white paint having a high titanium content.
The device of Fig. 11 is configured to receive the incident light radiation through the glass substrate. The incident light not absorbed during the first pass through the device will be randomly scattered by the layer 252. At least some of the scattered light rays will be directed through the amorphous silicon layers 2~, 246, and 24~ at angles sufficient to cause these rays to be internal reflected and substantially confined within the device 240.
As can be appreciated from the foregoing~ the present invention provides new and improved photo-voltaic cells which provide enhanced short circuit currents and efficiencies. The incident radiation directors herein disclosed provide a means by which at least a portion of the incident light can be directed through the active region or regions of the cells, at angles sufficient to cause inter-nal reflection within the cells and thus substan tially total confinement of the light therein.
Because the light is permitted to make ~ultiple passes through the active region or regions, the active regions can be made thinner than previously allowed. This enables more efficient collection of the photogenerated charge carriers while at the same time more of the light is absorbedO
For each embodiment of the invention des-cribed herein, the alloy layers other than the intrinsic alloy layers can be other than amorphous layers, such as polycrystalline layers. (By the 5 term "amorphous" is meant an alloy or material which has long range disorder, although it mayhave short or intermediate order or even contain at times some crystalline inclusions.) Modifications and variations of the present invention are ~ossible in light of the above teachings. It is therefore, to be understood that within the scope of the appended claims the inven-tion may be practiced otherwise than as specif-ically described~
however, a sinusoidal diffraction grating is illustrated, but it could of course be any of the other types previously mentioned.
Like the previous p-i-n cells described, the cell 170 further inGludes a p-type layer 182 of arnorphous silicon alloy, a collection grid 184, and a layer 186 antireflection material or glass encapsulant. The various layers are deposited on a substrate 171 of glass, stainless steel, or other suitable subs-trate material. Deposited over the substrate 171 is a layer 173 of highly conduc-tive and thus highly reflective metal, and a TCO
layer 175. The reflective metal layer 173 and TCO
layer 175 form a back reflector to reflect unused light back into the intrinsic region 180.
Alternatively, the transparent director can be glass having a roughened surface formed by sandblasting for example. The various amorphous silicon alloy layers can then be deposited cnto the roughened surface followed by the deposition of a specular back reflector. In this form of device, the incident radiation is first directed through the glass substrate. The glass substrate forms a random radiation director disposed on the side of the active region upon ~hich the light first impinges.
~ 3`~
Referring now to Fig. 8, there is illustrated another p-i-n photovoltaic cell 190 which includes a periodic bulk re~lector 192 embodying the pres-ent invention. Because the device 190 is other-S wise identical to the cells o~ Figs. 4 through 6,only the periodic bulk reflector will be described in detail~
The periodic bulk reflector 192 is disposed upon a substrate 194 of glass, stainless steel, or other suitable substrate material. The periodic bulk reflector 192 takes the form of a hologram comprising a plurality of thin planar members or lines 196 of reflective material, such as alumi-num, embedded in a medium 198 of transparent mate-rial. Here the transparent material is a trans-parent conductive oxide, such as indium tin oxide, to provide both a suitable medium for the lines 195, and a bottom contact for the device 190.
The lines 196 are disposed at an angle, are spaced apart, and are substantially parallel. The diffraction of light by a hologram can be pre~
dicted by the same expression previously de~ined for a diffraction grating. Here, the spacing (d) is the spacing between the lines 196.
-~7-Because the diffraction of the light occurs in the bulk of the hologram, the upper surface thereof can be polished or otherwise made smooth.
This presents a smooth surface upon which the amorphous silicon alloy layer can be deposited.
Referring now to Fig. 9, a multiple cell devlce 200 is there illustrated in sectional view which is arranged in tandem configuration and which includes a random surface reflector embody-ing the present invention. The device 200 com~prises two single cell units 202 and 204 arranged in series relation. As can be appreciated, plural single cell uni-ts of more than two can be uti-lized.
The device 200 includes a random surface reflector 206 including a sand blasted glass layer 203 which is coated with a layer 205 of metal having good reflectivity such as aluminum, for example. Deposited on the metal layer 205 is a layer of transparent conductive oxide 207 which can be a first layer of indium tin oxide 207a and a second layer of doped tin oxide 207b or a single layer of indium tin o~ide. The layer 207 of the transparent conductive oxide can be depositecl as previously described.
-4~-The flrst cell unit 202 includes a first doped p+ amorphous silicon alloy layer 208 deposited on the transparent conductive oxide layer 207~ The p+ layer is preferably a wide band gap p-type amorphous silicon alloy in accordance with the present invention. It can be deposited from any of the previously mentioned starting materials for depositing such material.
Deposited on the wide band gap p+ layer 208 is a first intrinsic amorphous silicon alloy body 210. The first intrinsic alloy body 210 is pre-Eerably an amorphous silicon-fluorine alloy.
Deposited on the intrinsic layer 210 is a further doped amorphous silicon alloy layer 212.
It is opposite in conductivity with respect to the conductivity of the first doped layer 208 and thus is an n+ layer.
The second unit cell 204 is essentially iden-tical and includes a first doped p+ layer 214, an intrinsic body 216 and a further doped n~ layer 218. The device 200 is completed with a TCO layer 220, a grid electrode 222, and an antireflection layer or glass encapsulant 224.
The band yaps of the intrinsic layers are preferably adjusted so that the band gap of layer -4~-3t~
216 is greater than the band gap of layer 210. To that end~ the alloy forming layer 216 can include one or more band gap increasing elements such as nitrogen and carbon. The intrinsic alloy forming the intrinsic layer 210 can include one or more band gap decreasing elements such as germanium, tin, or lead.
It can be noted frorn the figure that the intrinsic layer 210 of the cell is thicker than the intrinsic layer 216. This allows the entire usable spectrum of the solar energy to be utilized for generating electron-hole pairs.
Although a tandem cell embodiment has been shown and described herein, the unit cells can also be isolated from one another with oxide layers for example to form a stac~ed multiple cell. Each cell could include a pair of collec-tion electrodes to facilitate the series connec--tion of the cells with external wiring.
As a further alternative, and as mentioned with respect to the single cells previously des-cribed, one or more of the intrinsic bodies of the unit cells can include alloys having graded band gapsO Any one or more of the band gap increasing or decreasing elements previously mentioned can be incorpora-ted into the intr nsic alloys for -this purpose.
Referring now to Fig. 10, there is illustrated a tandem p-i-n photovoltaic cell 230 which is subs-tantially identical -to -the tandem cell 200 of Fig. 9 except that -the cell 230 includes a periodic surface reElector 232.
Therefore, this cell will be described in de-tail only wi-th respect to the reElector 232.
Like the embodiment of Fig. 6, the periodic surface reflec-tor 232 takes the form of a reflective diffrac-tion grating 234. Although the gra-ting 23~1 can be a sinusoidal~ square-wave, or other periodic configura-tion, the grating 234 here again as illustrated is a blazed grating. The grating can be ~ormed Erom a soft metal, such as aluminurn. It is coated with a layer 236 oE transparent conduc-tive oxide, such ~s indium tin oxide, cadmium stannate, or doped -tin oxide, upon which the amorphous silicon alloys can be deposi-ted. The diffraction gra-ting 234 operates in the same manner as previously described in relation to Fig 6 cr/~
~ q3~'~
F~eferring now to Fig. 11, it illustrates another single cell p-i-n photovoltaic device 240 embodying the present invention. Here, a tran-sparent substrate 242 formed from glass~ for exam-ple, has a TCO layer 250 and p-type, intrinsic, and n-type amorphous silicon alloy layers 244, 246, and 248 respectively successively deposited thereon. Over the n-type layer 248 is provided a layer 252 of a conductive, light diffusant paint.
The layer 252 can be formed from aluminum or gold paint, for example. Such paints are conductive and, when applied by wiping or spraying, or the like, will form a random light scattering inter-face between the layers 2~8 and 252. Alterna-tively, the layer 252 can comprise a first layer of a transparent conductor such as a transparent conductive oxide, and a second layer of a noncon-ductive, but light scattering paint such as a flat white paint having a high titanium content.
The device of Fig. 11 is configured to receive the incident light radiation through the glass substrate. The incident light not absorbed during the first pass through the device will be randomly scattered by the layer 252. At least some of the scattered light rays will be directed through the amorphous silicon layers 2~, 246, and 24~ at angles sufficient to cause these rays to be internal reflected and substantially confined within the device 240.
As can be appreciated from the foregoing~ the present invention provides new and improved photo-voltaic cells which provide enhanced short circuit currents and efficiencies. The incident radiation directors herein disclosed provide a means by which at least a portion of the incident light can be directed through the active region or regions of the cells, at angles sufficient to cause inter-nal reflection within the cells and thus substan tially total confinement of the light therein.
Because the light is permitted to make ~ultiple passes through the active region or regions, the active regions can be made thinner than previously allowed. This enables more efficient collection of the photogenerated charge carriers while at the same time more of the light is absorbedO
For each embodiment of the invention des-cribed herein, the alloy layers other than the intrinsic alloy layers can be other than amorphous layers, such as polycrystalline layers. (By the 5 term "amorphous" is meant an alloy or material which has long range disorder, although it mayhave short or intermediate order or even contain at times some crystalline inclusions.) Modifications and variations of the present invention are ~ossible in light of the above teachings. It is therefore, to be understood that within the scope of the appended claims the inven-tion may be practiced otherwise than as specif-ically described~
Claims (19)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. In a photovoltaic device formed from semiconductor material including at least one active region upon which incident radiation can impinge to produce charge carriers the improvement comprising a random bulk reflector for directing at least a portion of said incident radiation through said at least one active region at an angle sufficient to cause said directed radiation to be substantially confined within said photovoltaic device.
2. A device as defined in claim 1 wherein said random bulk reflector is disposed adjacent said active region on the side thereof opposite the side upon which the radiation first impinges.
3. A device as defined in claim 2 wherein said random bulk reflector comprises a planar member formed from a material having an index of refraction greater than 1.45 and which is non-absorbing of light which impinges thereon.
4. A device as defined in claim 2 wherein said random bulk reflector comprises a planar member formed from a ceramic material.
5. A device as defined in claim 4 wherein said ceramic material is formed from one of the group consisting of titanium dioxide, zinc selenide, zinc sulphide, selenium, and silicon carbide.
6. A device as defined in claim 2 wherein said random bulk reflector comprises a planar member coated with an enamel material.
7. A device as defined in claim 2 wherein said random hulk reflector comprises a layer of co-deposited tin oxide and titanium dioxide.
8. In a photovoltaic device formed from semiconductor material including at least one active region upon which incident radiation can impinge to produce charge carriers, the improvement comprising a periodic bulk reflector for directing at least a portion of said incident radiation through said at least one active region at an angle sufficient to cause said directed radiation to be substantially confined within said photovoltaic device, said reflector being disposed adjacent said active region on the side thereof opposite the side upon which the incident radiation first impinges.
9. A device as defined in claim 8 wherein said periodic bulk reflector comprises a hologram.
10. A device as defined in claim 9 wherein said hologram comprises a plurality of relatively thin reflective planar members disposed within a solid transparent medium, said planar members being disposed in spaced apart parallel relation and at an angle to the incident radiation.
11. A device as defined in claim 10 wherein said reflective planar members are formed from aluminum.
12. A device as defined in claim 10 wherein said transparent medium comprises a transparent conductive oxide.
13. A photovoltaic device formed from multiple layers of amorphous silicon alloys, said device comprising:
a cell body including a first doped amorphous silicon alloy layer; a body of intrinsic amorphous silicon alloy deposited on said first doped layer upon which incident radiation can impinge to produce charge carriers, a further doped amorphous silicon alloy layer deposited on said intrinsic body and being of opposite conductivity with respect to said first doped amorphous silicon alloy layer, and a random bulk reflector for directing at least a portion of said incident radiation through said body of intrinsic amorphous silicon alloy at an angle sufficient to cause said directed radiation to be substantially confined within said photovoltaic device; one of said doped layers forming the bottom most layer of said device upon which said incident radiation last impinges, and said random bulk reflector being disposed beneath said one doped layer.
a cell body including a first doped amorphous silicon alloy layer; a body of intrinsic amorphous silicon alloy deposited on said first doped layer upon which incident radiation can impinge to produce charge carriers, a further doped amorphous silicon alloy layer deposited on said intrinsic body and being of opposite conductivity with respect to said first doped amorphous silicon alloy layer, and a random bulk reflector for directing at least a portion of said incident radiation through said body of intrinsic amorphous silicon alloy at an angle sufficient to cause said directed radiation to be substantially confined within said photovoltaic device; one of said doped layers forming the bottom most layer of said device upon which said incident radiation last impinges, and said random bulk reflector being disposed beneath said one doped layer.
14. A device as defined in claim 13 wherein said random bulk reflector comprises a body of ceramic material.
15. A device as defined in claim 14 wherein said ceramic material is titanium dioxide, zinc selenide, zinc sulphide, selenium, or silicon carbide.
16. A device as defined in claim 13 wherein said random bulk-reflector comprises a body of enamel material.
17. A device as defined in claim 13 wherein said random bulk reflector comprises a body of co-deposited tin oxide and titanium dioxide.
18. A photovoltaic device formed from multiple layers of amorphous silicon alloys, said device comprising:
a cell body including a first doped amorphous silicon alloy layer; a body of intrinsic amorphous silicon alloy deposited on said first doped layer upon which incident radiation can impinge to produce charge carriers; a further doped amorphous silicon alloy layer deposited on said intrinsic body and being of opposite conductivity with respect to said first doped amorphous silicon alloy layer, and a periodic bulk reflector for directing at least a portion of said incident radiation through said body of intrinsic amorphous silicon alloy at an angle sufficient to cause said directed radiation to be substantially confined within said photovoltaic device; one of said doped layers forming the bottom most layer of said device upon which said incident radiation last impinges, and said periodic bulk reflector being disposed beneath said one doped layer.
a cell body including a first doped amorphous silicon alloy layer; a body of intrinsic amorphous silicon alloy deposited on said first doped layer upon which incident radiation can impinge to produce charge carriers; a further doped amorphous silicon alloy layer deposited on said intrinsic body and being of opposite conductivity with respect to said first doped amorphous silicon alloy layer, and a periodic bulk reflector for directing at least a portion of said incident radiation through said body of intrinsic amorphous silicon alloy at an angle sufficient to cause said directed radiation to be substantially confined within said photovoltaic device; one of said doped layers forming the bottom most layer of said device upon which said incident radiation last impinges, and said periodic bulk reflector being disposed beneath said one doped layer.
19. A device as defined in claim 18 wherein said periodic bulk reflector comprises a hologram.
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
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US06/354,285 US4419533A (en) | 1982-03-03 | 1982-03-03 | Photovoltaic device having incident radiation directing means for total internal reflection |
US354,285 | 1982-03-03 |
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CA1187970A true CA1187970A (en) | 1985-05-28 |
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Application Number | Title | Priority Date | Filing Date |
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CA000421646A Expired CA1187970A (en) | 1982-03-03 | 1983-02-15 | Photovoltaic device having incident radiation directing means for total internal reflection |
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US (1) | US4419533A (en) |
JP (1) | JPS58159383A (en) |
KR (1) | KR840004309A (en) |
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BR (1) | BR8300902A (en) |
CA (1) | CA1187970A (en) |
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-
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- 1982-03-03 US US06/354,285 patent/US4419533A/en not_active Expired - Lifetime
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1983
- 1983-01-31 IL IL67794A patent/IL67794A/en unknown
- 1983-02-04 ZA ZA83748A patent/ZA83748B/en unknown
- 1983-02-14 GB GB08304033A patent/GB2116364B/en not_active Expired
- 1983-02-14 IE IE294/83A patent/IE54408B1/en not_active IP Right Cessation
- 1983-02-15 CA CA000421646A patent/CA1187970A/en not_active Expired
- 1983-02-16 AU AU11494/83A patent/AU543213B2/en not_active Ceased
- 1983-02-16 FR FR8302480A patent/FR2522880A1/en not_active Withdrawn
- 1983-02-17 NL NL8300603A patent/NL8300603A/en not_active Application Discontinuation
- 1983-02-18 IT IT47739/83A patent/IT1167617B/en active
- 1983-02-21 PH PH28546A patent/PH19299A/en unknown
- 1983-02-22 DE DE19833306148 patent/DE3306148A1/en not_active Withdrawn
- 1983-02-24 BR BR8300902A patent/BR8300902A/en unknown
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- 1983-03-02 GR GR70656A patent/GR78799B/el unknown
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- 1983-03-03 KR KR1019830000861A patent/KR840004309A/en not_active Application Discontinuation
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ES520247A0 (en) | 1984-03-16 |
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JPS58159383A (en) | 1983-09-21 |
FR2522880A1 (en) | 1983-09-09 |
SE8301051D0 (en) | 1983-02-25 |
ZA83748B (en) | 1983-11-30 |
AU1149483A (en) | 1983-09-08 |
SE454225B (en) | 1988-04-11 |
EG15060A (en) | 1985-12-31 |
IT8347739A0 (en) | 1983-02-18 |
NL8300603A (en) | 1983-10-03 |
IN157618B (en) | 1986-05-03 |
PH19299A (en) | 1986-03-05 |
US4419533A (en) | 1983-12-06 |
GB2116364A (en) | 1983-09-21 |
IL67794A0 (en) | 1983-05-15 |
AU543213B2 (en) | 1985-04-04 |
GR78799B (en) | 1984-10-02 |
IE54408B1 (en) | 1989-09-27 |
IL67794A (en) | 1986-01-31 |
SE8301051L (en) | 1983-09-04 |
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