Thin-film solar cells are a new type of photovoltaic device that can alleviate the energy crisis. The first generation of solar cells were monocrystalline and polycrystalline silicon cells. The second generation of solar cells used materials with high light absorption coefficients, and the cell thickness did not need to be too thick to absorb sunlight, hence the name thin-film solar cells.
Based on the different light-absorbing materials, common thin-film solar cells can be classified into: cadmium telluride (CdTe), copper indium gallium selenide (CIGS), dye-sensitized solar cells (DSSC), and organic polymer (OPV) solar cells. Currently, research is still underway to reduce the cost of fabrication processes for each type of cell; therefore, the fabrication method for each type is not unique. Below, we will introduce some technologies used in module production:
1. Cadmium telluride (CdTe) solar cells
The production of cadmium telluride solar cells begins with glass. SnO2 and CdSnO4 are prepared as transparent conductive layers (TCO) using sputtering. CdS is used as a buffer layer via chemical bath deposition (CBD). The main component, the CdTe light-absorbing layer, is deposited using a close-spaced sublimation (CSS) method, which involves heating and sublimating the CdTe layer before depositing it onto the substrate. Finally, the carbon electrode is brushed on. In addition, the cell requires some heat treatment.
2. Copper indium gallium selenide (CIGS) solar cells
The technology employed in copper indium gallium selenide (CIGS) solar cells begins with glass. Mo is deposited using magnetron sputtering. CIGS, the light-absorbing layer, has several fabrication processes, the most common being: a) co-evaporation, where several elements (Cu, In, Ga, Se) are evaporated onto the substrate. German companies and NREL frequently use this method. b) ink method, which uses chemical methods to prepare CIGS nanocrystals, then disperses them in a solvent and prints them onto the substrate like ink. Nano Solar in the United States uses this technology. c) metal selenization, where copper indium gallium metal is sputtered or electroplated into a metal film, then reacted with Se vapor or H2Se. Honda Soltec and Showa Oil use this method. CdS is also deposited using chemical bath deposition (CBD). ZnO and Ag can both be prepared using sputtering methods.
3. Dye-sensitized solar cells (DSSC)
This type of battery has been extensively studied in laboratories, but due to stability and packaging issues, the production of components is still limited. Since the production of components is still relatively small, the specific manufacturing process will not be introduced here.
4. Organic polymer solar cells (OPV)
Polymer solar cells are a type of thin-film solar cell that is mostly studied in laboratories, with relatively few commercially available components. Organic solar cells also have two structures: forward and inverted. The image above shows an inverted one. Each layer in this type can be fabricated using solution printing, making OPV (Optical Printed Vehicle) a relatively low-cost fabrication technology, requiring only roll-to-roll printing, similar to printing newspaper. However, due to OPV's low efficiency, poor material stability, and expensive encapsulation materials, it has not yet gained a significant market share.
Thin-film solar cells can be manufactured using various inexpensive materials such as ceramics, graphite, and metal sheets as substrates, forming a thin film only a few μm thick that can generate voltage. Besides being planar, thin-film solar cells are also flexible enough to be made into non-planar structures, expanding their applications to include integration with buildings or becoming part of a building structure.
