Although thin-film PV cells have a long history of development and market presence, they are not as widely adopted as crystalline silicon PV cells. On one hand, thin-film cells have several significant advantages such as having a flexible substrate, being very light in weight, and reaching a high conversion efficiency. On the other hand, they are much more costly to manufacture than crystalline silicon cells. Consequently, low cost-performance ratio remains a major constraint to the market growth of the thin-film technology.
However, there is good news recently. This July, researchers at the Pennsylvania State University (PSU) in the US presented a concept of a thin-film cell that theoretically has a conversion efficiency rate of 34.45% on account of its tandem cell design. With this level of performance, the thin-film technology could become more cost effective and gain greater market acceptance.
The tandem thin-film cell created by the PSU team comprises a layer of copper-indium-gallium-selenide (CIGS) and a layer of copper-zinc-tin-sulfide/selenide (CZTSSe). Both materials are used in thin-film cells and sold on the market. The conversion efficiency rate of CIGS is around 20% on average, while the conversion efficiency rate of CZTSSe reaches just about 11%. Nevertheless, the PSU team has managed to combine them together to achieve an efficiency rate that is higher than their sum.
Akhlesh Lakhtakia, who is a professor of engineering at PSU and participated in the design of the tandem thin-film cell, said efficiency rate has to be above 30% in order to make a difference.
To reach the 34% level, which is higher than the sum of the rates of CIGS and CZTSSe (i.e., when each of the two materials converts light on its own), the PSU team has developed a computational model that can optimize both the electrical and optical aspects of the tandem cell design. Basically, a tandem PV cell is made up of two PV cells of different materials stacking one on top of the other. While the top layer absorbs the light of one portion of the frequency spectrum, the bottom layer absorbs the light of a different portion. Their joint performance leads to a higher overall efficiency. Presently, commercially available tandem cells are perovskite-silicon cells that boast higher efficiency rates compared with conventional crystalline silicon cells.
The computational model was used to test the feasibility of various material combinations. In the end, the PSU team found that CIGS and CZTSSe complement each other and together can form the ideal tandem thin-film cell. Lakhtakia pointed out that the lattice structure of the two materials are similar, so stacking via epitaxy is attainable. At the same time, CIGS and CZTSSe together boost the overall efficiency by targeting different sections of the spectrum.
(Source: The Pennsylvania State University.)
The CIGS-CZTSSe cell is only a concept at the moment. The team has yet to actually build a prototype based on this design, which could theoretically achieve a conversion efficiency rate of 34.45%. The next step is therefore to put this idea into practice so as to see if this tandem thin-film cell can advance from the starting point of the development process.
Besides making a CIGS-CZTSSe cell and testing it in a laboratory to gauge its performance, the team is also looking into alternative material combinations. Perhaps experiments undertaken by scientists at other institutions could help improve the tandem cell design and the material mix. Details of the modeling by the PSU team have been published in the journal Applied Physics Letters.
(Credit for the photo at the top of the article goes to Pixabay.)