Thin film solar cells aim to reduce the cost of single-crystal silicon technology, as they use less than 1% of the raw material (silicon) compared to wafer based solar cells. Thin film technology basically consists in coating a glass or ceramic substrate with a thin film of semiconductor, such as cadmium telluride (CdTe), copper indium gallium selenide (CIGS), amorphous silicon (a-Si) or nanocrystalline silicon. These materials are all strong light absorbers and only need to be about 1 micron thick, so materials costs are significantly reduced.
Amorphous silicon, which has no crystalline structure, is the most well developed of the thin film technologies. Fig. 1 shows how amorphous silicon solar panels look like. This material (a-Si) has a higher band gap (1.7 eV) than crystalline silicon (1.1 eV), which means it is more efficient to absorb the visible part of the solar spectrum, but it fails to collect an important part of the spectrum: the infrared. As nanocrystalline Si has about the same band gap as c-Si, the two materials can be combined by depositing two diodes on top of each other. The top cell in a-Si absorbs the visible light and leaves the infrared part of the spectrum for the bottom cell in nanocrystalline Si. In general, using two or more layers of different materials with different band gaps helps improving efficiency of solar cells. The higher band gap material is on the surface, absorbing high-energy photons, whereas lower-energy photons are absorbed by the lower band gap material underneath.
Fig. 1 Amorphous silicon solar panel. (Taken fron Ref.).
Copper-Indium-Gallium-Selenide(CIGS) Solar cells
CIGS solar cells are the low cost and high efficiency thin film solar cells. From the National Renewable Energy Laboratory(NREL) and the Zentrun für Sonnenenergie und Wasserstoff Forschung(ZSW), the efficiency of CIGS solar cells are greater than 20%. CIGS is a direct bandgap that can be varied from 1.0 eV to 1.65 eV by the Ga/(In+Ga) ratio. The bandgab of CIGS correspond to the spectum of the visible light. Therefore the CIGS solar cells can absorb photons nearly 100% from the visible light. Commonly, the CIGS solar cells compose of 5 thinfilm layers on soda-lime glass substrate as show in Fig.1.
Fig.1 the CIGS solar cell structure(Taken from Ref.)
The CIGS solar cells structure
1. Molybdenum(Mo) layer: This is the first layer that is deposited by sputtering on the soda-lime glass substrate. Mo layer is the back contact of the CIGS solar cells.
2. Copper Indium Gallium Selenide(CIGS) layer: this layer is p-type and absorber that is composited on Mo layer by evaporation. This is the main layer of the CIGS solar cells.
3. Cadmium Sulfide(CdS) layer: this layer is n-type that is deposited by chemical bath deposition on CIGS layer. This is a buffer layer.
4. Zinc Oxide doped with aluminium(ZnO:Al): This is a window layer of the CIGS solar cells. This layer is deposited on CdS layer by sputtering.
5. Aluminium Nickel(Al:Ni) grids: this layer is the front contact of the CIGS solar cells. this layer is the grids on ZnO:Al layer.