Thin-layer solar cell

Remarkable efforts have been directed to photocatalytic fuel production from sunshine and water for creating energy resources, and now UV light cleavage of water for fuel production is possible. However, conversion of solar visible light to fuels is still under way, and further concentrated researches in this direction are strongly desired. Conversion of solar energy into electricity has been attained by a sensitized TiOz thin layer solar cell to reach nearly 10% efficiency, comparable to that obtained by the amorphous silicone solid cell. This is an important achievement in photocataiysis. 

Copper Sulfide—Cadmium Sulfide. This thin-film solar cell was used in early aerospace experiments dating back to 1955. The Cu S band gap is ca 1.2 eV. Various methods of fabricating thin-film solar cells from Cu S/CdS materials exist. The most common method is based on a simple process of serially overcoating a metal substrate, eg, copper (16). The substrate first is coated with zinc which serves as an ohmic contact between the copper and a 30-p.m thick, vapor-deposited layer of polycrystaUine CdS. A layer is then formed on the CdS base by dipping the unit into hot cuprous chloride, followed by heat-treating it in air. A heterojunction then exists between the CdS and Cu S layers. 

Yet another alternative is the thin-film solar cell. This cannot use silicon, because the transmission of solar radiation through silicon is high enough to require relatively thick silicon layers. One current favourite is the Cu(Ga, InjSci thin-film solar cell, with an efficiency up to 17% in small experimental cells. This material has a very high light absorption and the total thickness of the active layer (on a glass substrate) is only 2 pm. 

Fig. 3.18 Schematic outline and ideal band diagram of an extremely thin absorber solar cell. The n-Ti02 crystallites are clustered together to form a relatively open, network-like morphology, accommodating a thin layer of CdTe absorber, with p-ZnTe at the back contact. (Reprinted from [270], Copyright 2009, with permission from Elsevier)...

Thin-film solar cell devices based on CIGS have already demonstrated an efficiency of 19.52%.40 The direct energy gap of CIGS results in a large optical absorption coefficient, which, in turn, permits the use of thin (-1 pm) layers of active material. CIGS solar cells are also known for their long-term stability.76... 

Abou-Ras, D. Kostorz, G. Romeo, A. Rudmann, D. Tiwari, A. N. 2005. Structural and chemical investigations of CBD- and PVD-CdS buffer layers and interfaces in Cu(In,Ga)Se2-based thin film solar cells. Thin Solid Films 480 181 118-123. 

Nakada, T. Mizutani, M. Hagiwara, Y. Kunioka, A. 2001. High-efficiency Cu(In,Ga)Se2 thin-film solar cells with a CBD-ZnS buffer layer. Solar Energy Mater. Solar Cells 67 255-260. 

Australia, and scaled up by BP Solar in Spain, the heterojunction with intrinsic thin layer (HIT) cells developed by Sanyo by replacing the diffused P-doped emitter with an amorphous silicon layer and the back contact cells developed by Stanford University for use in concentrator technology and now converted to a large area for flat plate use. All three use single-crystalline silicon, while the majority of screen-printed cells use multicrystalline silicon wafers. 

The issue of Schottky barrier formation to ZnO is not treated in this chapter as such contacts are not of big importance in thin-film solar cells. This is related to the fact that in thin film solar cells metals are only used to contact highly-doped films. For degenerately doped semiconductors, the barrier heights become very small because of the large space charge associated with depletion layers in such materials. 

Fig. 4.2. Structure and energy band diagram of a Cu(In,Ga)Se2 (CIGS) thin-film solar cell. The ZnO window layer typically consists of a combination of a nominally undoped ZnO and a highly doped ZnO layer...

The results presented in this section further illustrate that there is a considerable dependence of the band alignment at the CdS/ZnO interface on the details of its preparation. An important factor is the local structure of the ZnO film. There is considerable local disorder when the films are deposited at room temperature in pure Ar, deposition conditions that are often used in thin film solar cells. It is recalled that the disorder is only on a local scale and does not affect the long range order of the films, as obvious from clear X-ray diffraction patterns recorded from such films (see discussion in Sect. 4.2.3.3). Growth of sputter deposited ZnO on CdS always results in an amorphous nucleation layer at the interface. The amorphous nucleation layer affects the valence band offset. 

In principle, a Cu(In,Ga)Se2 thin-film solar cell should be possible without the use of so-called buffer layers like CdS. The necessary p-n junction might be provided by the p-type Cu(In,Ca)Se2 absorber and the n-type TCO. Such a cell structure is also advantageous as it requires less production steps. Consequently, there has been considerable effort to prepare Cu(In,Ca)Se2 thin-film solar cells without a chalcogenide buffer layer (see Chap. 9 of this book and [120]). Conversion efficiencies above 16% have yet been achieved [121]. 

I112S3 or I112S3 containing compounds are possible alternatives for the CdS buffer layer in Cu(In,Ga)Se2 thin-film solar cells [120,145-148], The In2S3 layers are prepared by various techniques as chemical bath deposition [145], thermal evaporation [146], atomic layer deposition (ALD) [147], and magnetron sputtering [148], Energy conversion efficiencies above 16% have been... 

A particular buffer layer experiment, carried out by Nguyen et al. [154, 155], is shown in Fig. 4.34. Two different combinations of chemical bath deposited CdS and Inx(OH,S)y buffer layers were used to fabricate Cu(In,Ga)Se2 thin-film solar cells. The experiment was defined in order to identify the interface that leads to poor efficiencies if single Inx(OH,S)y buffer layers are used. The type A arrangement of the two buffer layers with a Cu(In,Ga)Se2/CdS and an Inx(OH,S)y/ZnO interface results in poor efficiencies, while type B arrangement with a Cu(In,Ga)Se2/Inx(OH,S)y and a CdS/ZnO interface results in a high efficiency. This observation strongly suggests that the interface between Inx(OH,S)y and ZnO limits the efficiency. 

Finally, the last paragraph of this chapter summarizes the work so far published on thin film solar cells using CVD-deposited ZnO layers as transparent contacts. 

Characteristics Required for CVD ZnO Layers Incorporated within Thin Film Solar Cells... 

The present paragraph investigates in more detail the various ways in which CVD ZnO layers have been incorporated into thin film solar cells. It, thus, attempts to define a framework that can help the solar cell designer to choose the most appropriate CVD deposition parameters for producing his ZnO layers. 

The properties of ZnO layers that have an importance for their application within the context of thin film solar cells are (a) transparency,... 

Thin ZnO films can be used either as a transparent and conductive window layer, or as a buffer layer, within CuInS2 (CIS) and Cu(In,Ga)Se2 (CIGS) thin film solar cell devices (see Chaps. 4 and 9). In both cases, the ZnO layers... 

Here we describe the layer structure for single junction as well as for tandem solar cells consisting of a-Si H and pc-Si l I. Further, this section will deal with the stability of silicon thin film solar cells and the possibility to reduce degradation by special design. 

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