DE19713215A1 - Solar cell with textured transparent conductive oxide layer - Google Patents

Abstract

In a solar cell with a textured transparent conductive oxide (TCO) layer, preferably of ZnO, the TCO layer structure is so dense that it cannot be penetrated by an etchant. Also claimed is a TCO layer which can be textured and which has a dense structure. Further claimed is a method of producing a textured TCO layer involving deposition of a TCO layer with a dense structure on a substrate, the layer preferably being etched during or after deposition.

Description

The invention relates to a solar cell with textured TCO layer according to the preamble of claim 1. Furthermore, the invention relates to a method for Production of such a TCO layer for one Solar cell according to the preamble of claim 5.

As state of the art, transparent, conductive metal oxides such as ITO, SnO ₂ and ZnO are used as a so-called TCO layer (from the English "Transparent Conductive Oxide") as a substrate and functional layer in various types of solar cells. In general, such TCO layers are highly transparent - with more than 80% transmission - and comparatively low-resistance, the so-called layer resistance R, for example, having a value below 10 Ω.

Particularly for use in silicon thin-film solar cells, but also for other types of solar cells, texturing of the TCO surface as microscopic roughness is desirable in order to lengthen the optical path of the incident solar radiation in the active solar cell layer by scattering light To increase the light absorption and thus the efficiency of the solar cell. In combination with an effective rear reflector, a so-called "light trap effect" (referred to as "light trapping") can be achieved for radiation that is poorly absorbed without this measure. As an example of known solar cell 1, the layer structure is shown a thin film tandem cell amor PHEM silicon in Fig..

For thin-film solar cells based on amorphous or microcrystalline silicon, textured tin oxide (SnO ₂ ) has predominantly been used as TCO. The use of zinc oxide (ZnO) instead of tin oxide is often sought in order to use the better transparency and plasma stability of the ZnO compared to SnO ₂ . So far, various methods for the preparation of textured ZnO layers have been presented. All previous preparation processes for the production of rough ZnO layers have in common that the generation of the texturing takes place during the deposition. This is achieved, for example, by CVD processes, in particular organometallic CVD or atmospheric pressure CVD, or by cathode sputtering with water vapor or at high temperatures. Although these layers show useful optical properties with regard to transparency or light scattering, they were for use in solar cells because of poor electrical properties, in particular high resistance or an unfavorable, sharp-edged surface morphology, which is never reflected in the solar cell in high failure probabilities, not yet suitable. So far, there is no simple manufacturing process that leads to good layer properties.

The object of the invention is therefore a solar cell to create with textured TCO layer and a ver drive to produce such a TCO layer for to provide such a solar cell, this Disadvantages are avoided.

The task is solved by a solar cell according to the Set of features according to claim 1. The task is further solved by a method according to Ge togetherness of the features according to claim 5. Further expedient ßige or advantageous embodiments or variants can be found in each of these claims related subclaims.

It was recognized a solar cell as well as a process for the production of highly conductive, transparent TCO layers with controlled texturing and good To provide surface morphology, a ver improved textured TCO layer surface obtuse angles reached without sharp-edged structures becomes.  

It was recognized in the case of ZnO that it was related to the etching behavior of ZnO different forms of ZnO layer there are. A criterion for characterizing the respective shape of the TCO layer formed can by the etching behavior should be justified. The characterization Different shapes can also be based on the crystal litges. Controlling the structure of the Surface texturing is based on the type of etching zens. It was found within the scope of the invention that by forming a sufficiently thick TCO layer and egg Chemicals that run simultaneously or afterwards Etching process of the surface of these TCO layers desired texturing is formed.

It was also found that with a suitable loading creation of those initially formed by deposition TCO layer during the etching process a controlled texture ZnO retention while maintaining good electrical egg properties is generated. This was achieved with shifts coherent, "compact" layer structure. The "Compact" layers are characterized by a dense Crystal structure.

It has been recognized that the different forms of TCO-Schich and to form the light scatter favorable etching to be carried out so are related that a TCO layer then is usable for texturing if it can is compact.  

It was recognized that such a compact TCO layer has a dense crystal structure. Here is the Ge add so dense that in a subsequent Etching to form a textured TCO layer surface the etchant onto the surface of the TCO layer attacks attacks and the desired Tex turation is formed. The etching rate is ver comparatively low, since comparatively little TCO layer surface is available.

In this context it was recognized that in the opposite in the case of a TCO layer with insufficient dense structure, the etchant not only on the TCO layer surface leads to etching, but also in reach the lower areas of the TCO layer, which due to the insufficiently compact structure are accessible to the etchant. Such an etching on the one hand has a relatively high etching rate. On the other hand, the type of etching is fictional according to the type of etching so different that a morpho Logic of the TCO layer surface is obtained, the one does not have a beneficial effect on the incidence of light.

The inventive method for producing textu rier TCO, especially ZnO layers at least least the following steps:  

• 1. Immediate or indirect deposition of TCO material on a substrate via one or more suitable interlayers. ZnO is preferably used as the material. The layer thickness d in _{front of} the layer formed by deposition can be so large that at least d _after = ρ _after / R is retained after the subsequent etching, P _after the specific resistance of the ZnO and R the desired surface resistance of the ZnO layer after the etching are. The output layer must be compact and can have a specific resistance ρ _before <1 × 10 ^-2 Ωcm before etching;
• 2. etching of the formed by deposition of the TCO layer, in particular a reduced layer thickness d _to. The texturing of the TCO layer can be controlled via the thickness and properties of the TCO layer formed by deposition and via the etching parameters.

Instead of using the method according to the invention initially to form the TCO layer by deposition and then such an etching can also be etched during the De position.

The invention is further based on figures and Embodiment explained in more detail. It shows:  

FIG. 1 is known as prior art tandem cells on the basis of a-Si: H; (a) pinpin structure, (b) inverted structure;

FIG. 2 shows scanning electron fracture edge receiving a compact, at low operating pressure (for example 2 mTorr) deposited ZnO layer having already briefly etched surface;

Fig. 3 scanning electron microscope image of a compact ZnO layer after long etching, with a visibly textured TCO layer surface and a very dense structure of connected crystallites in plan view at two different enlargements;

Fig. 4 specific resistance as a function of the layer thickness for smooth (before etching) and textured (after etching) ZnO layers, the specific resistance of the textured layer below 1. 10 ^-3 ohmcm remains;

FIG. 5 shows transmission of a textured by etching the ZnO layer deposited on Corning 7059 glass substrate, as a function of wavelength where the entire vertical diffuse transmission and are shown separately;

Fig. 6 Haze factor (Haze) as a function of the wavelength for a different length etched ZnO layer;

Fig. 7 SEM image of a ZnO layer (type 3, see table), which was deposited at room temperature and a working pressure of 30 mTorr; the layer shows clearly distinguishable acicular crystallites; such a layer cannot be textured by etching but instead provides a smooth layer surface which does not favor light scattering;

Fig. 8 quantum efficiency as a function of the wavelength of two codeposed solar cells, which were deposited on the one hand on textured, rough ZnO and on the other hand on smooth ZnO;

Fig. 9 quantum efficiency as a function of the wavelength of two codeposed solar cells, which were deposited on textured ZnO or commercial ASAHI U TCO;

Fig. 10 quantum efficiency as a function of the wavelength of an inverted (nip) solar cell before and after etching the front TCO layer in hydrochloric acid.

Embodiment

In the “compact” layer according to the invention, it is difficult to distinguish a single crystallite using SEM fracture edge images (for magnifications between 30,000 and 60,000) (see FIG. 2). These "compact" ZnO layers can be anisotropically etched (see FIG. 3). A defined texture can be set via concentration, temperature, etching time or applied voltage. The textured ZnO layers are highly conductive and transparent even after the etching (FIG. 4 and FIG. 5). The proportion of the diffuse to the total transmission (the so-called haze, as a measure of the scattering power) can be set within wide limits ( FIG. 6).

To distinguish them from the TCO layers according to the invention with a "compact" structure, non-compact ZnO layers show that, for example, in the scanning electron microscope (SEM) at 30,000 to 60,000 times magnification, a needle-shaped structure with clearly visible grain boundaries, as in FIG. 7 shown.

These layers become very much through the etching process removed quickly and evenly without a Tex turation arises. In this case, it penetrates Etchant unfavorably between the acicular Structure in the depth of the TCO layer and prevent changes the targeted texturing of the TCO layer not only the surface of the layer, but also Due to the structure, the inside of the TCO layer is also etched becomes, which leads to an even removal of the TCO-Ma terials leads.

Suitable compact TCO layers according to the invention were formed by rf magnetron sputtering of Al-doped ZnO targets (2 wt.% Al ₂ O ₃ in ZnO) in an argon atmosphere at a working pressure below 10 mTorr, in particular at 2 mTorr, by deposition . For this purpose, substrate temperatures in the range from room temperature (RT) to 330 ° C were selected. The layer thickness of the layer formed by deposition is then, for example, 1 µm.

Table 1 lists three different, etchable ZnO layers. The compact ZnO layers according to the invention (type 1 and type 2) from FIG. 2 can be textured by etching and, as a result, show a textured TCO layer surface with crater formations which favor light scattering. The acicular, non-compact structure (type 3, see FIG. 7) was evenly removed by the etching attack without the formation of a texture.

ZnO layers with different etchings create

ZnO layers with different etchings create

These TCO layers according to the invention were etched in 0.5% hydrochloric acid solution at room temperature between 0 and 100 s. The results were as follows:

• 1. The layers are highly conductive even after the etching process (ρ _after <1. 10 ^-3 Ωcm, see FIG. 4);
• 2. The layers are highly transparent even after the etching. Fig. 5 shows the example of a 20 seconds the etched layer of type 2, the high transparency. In addition to vertical transmission, total transmission is also shown. Also included is the haze factor, which is a measure of the spreading power. The total transmission in the wavelength range between 400 and 800 nm is clearly over 80%. The haze factor at 500 nm is 20%.
• 3. The scattering power of the films can be controlled via the texture. In the simplest case, the roughness of the layers increases with increasing etching time. As an example, FIG. 6 shows the wavelength-dependent haze for a type 2 ZnO layer that has been etched for different lengths. The haze increases with increasing etching time and in the case of an etching of 50 seconds reaches a value of 50% at 500 nm.

It was also found that films made at higher Substrate temperature (<200 ° C) were deposited, were removed more slowly than those at low low temperature.

A layer which is particularly suitable for the application results from the use of multilayer starting layers which have been produced either from ZnO layers which can be etched at different speeds or from composite layers of ZnO and other TCO materials, in particular SnO ₂ . The first deposited, more slowly etchable ZnO layer or the SnO ₂ intermediate layer act as an etch stop layer and prevent complete removal of the entire layer package, which can occur in the event of locally inhomogeneous etching attack by a single-layer ZnO layer.

Pin solar cells made of amorphous silicon were deposited on some of these layers (smooth and textured). The thickness of the intrinsic absorber layer was 400 nm. The back contact of the pin solar cells was ZnO / Ag. In FIG. 8, the quantum efficiency is as a proportion of the incident photons run of the sun Spek, which contribute to the photocurrent of the solar cell for a particular wavelength, as shown by two solar cells, which were deposited under identical conditions (co-deposition). The substrate was type 2 smooth ZnO in one case, and type 2 textured ZnO in the other case (etched for 20 seconds). The light coupling is significantly improved through the texturing in the entire wavelength range.

In a further experiment, solar cells were deposited under identical conditions on a textured ZnO layer (type 2, etched for 20 s) and on commercial SnO ₂ -TCO (ASAHI U) from Asahi Glass. In Fig. 9 it is shown that the current efficiency of the solar cell on the textured ZnO is almost in the entire wavelength range higher than on the comparison cell deposed on Asahi U. In the example, the current gain is 1 mA / cm ² .

Rough ZnO can also be built into the rear contact. As a result, the light is scatteredly reflected at the back contact, the light path in the solar cell increases and the light can be caught by multiple reflections in the solar cell ("light trapping"). In the case of a solar cell structure known from FIG. 1 as prior art, the etching of the back contact TCO can take place after the deposition of the TCO layer on the solar cell; in the case of the “inverted” structure, such a solar cell can be formed on the previously textured TCO layer by deposition.

The invention is also used on the top TCO layer of an “inverted” solar cell. In this case, the top TCO is etched after the TCO layer has been deposited on the solar cell. In FIG. 10, the quantum efficiency as a function of wavelength for an inverted pin solar cell before and after etching is shown top TCO layer. The current yield has increased significantly through the etching process.

With stacked solar cells (also multiple or cascades the textured TCO layer can also be arranged between sub-cells.  

The texture of the TCO layer can be used for optimal Light trapping tailored to the respective solar cell type will. For example, using this method textured ZnO layers also for solar cells a-SiGe: H, microcrystalline silicon (µc-Si), crystalline nem silicon and copper indium diselenide and related Chalcopyrite compounds for improved light input be used.

Known wet and dry etching are used as the etching method procedures in question, including electrochemical Processes such as anodic etching or reak tives ion etching.

Claims (9)

1. Solar cell with a textured TCO layer, characterized in that the structure of the TCO layer is so dense that an etchant cannot penetrate into the TCO layer. 2. Solar cell identified according to the preceding claim characterized by ZnO as a material for education the TCO layer. 3. Solar cell according to one of the preceding claims, characterized in that the TCO layer by means of one or more additional layers ten is indirectly connected to the substrate. 4. Textured TCO layer, labeled through a dense structure. 5. Process for producing a textured TCO layer, where TCO material by deposition on egg Nem substrate is formed, characterized  records that a TCO layer with dense Ge add is selected. 6. The method according to the preceding claim, characterized characterized in that during or after the Deposition the TCO layer formed is etched. 7. The method according to the preceding claim, characterized characterized in that ZnO as material for Formation of the TCO layer is selected. 8. The method according to any one of the preceding claims, characterized in that the TCO layer by deposition indirectly on the substrate is formed. 9. The method according to any one of the preceding claims, characterized in that the medium ble formation of the TCO layer on the substrate between one or more others Layers are formed.