Why are we still researching new solar materials?

Credit: L.M. Peter, Phil. Trans. R. Soc. A 369 (2011) 1840-1856.

There are a variety of reasons. Most commercial solar technologies are currently based off silicon or cadmium telluride (CdTe). CdTe is relatively new to the market and has been instrumental to lowering the cost of cells since it can be processed using inexpensive technologies (it's more defect-tolerant) and only requires a very thin layer of material (~2 millionths of a meter, thinner than a human hair). Silicon, itself is very abundant and is very well understood thanks to the microelectronics industry. Unfortunately, though, silicon requires more material to make a good solar cell and that material needs to be of higher purity and quality. Silicon technologies, however, can capitalize on the existing knowledge band and manufacturing advances by the microelectronics industry to lower costs. My adviser always says: "Never bet against silicon".

That being said, these technologies have their limitations. Silicon, while raw material is cheap, getting it to the required purities for good devices is an expensive and energy-intensive process. Also, it can be expensive to get rid of defects that harm performance. For every solar cell, there is a trade-off between cost and performance. Record silicon cells are close to the theoretical maximum efficiency possible, although these record cells are costly to manufacture. CdTe requires special encapsulation because Cd (cadmium) and Te (tellurium) are toxic if released from the CdTe compound (cells based on these materials are great safe ways to dispose of Cd waste when refining zinc ores and Te waste when refining copper), raising the cost. CdTe developments have been instrumental in lowering the cost of solar cells and modules, although it is unclear how long that will continue. The Department of Energy expects a Te shortage ~2025, which will raise costs. As you can see from the chart above, Cd and Te are fairly scarce and current reserves are not enough to supply our current energy needs completely.

A material  known as CIGS (Cu(In,Ga)Se2) that has been researched for decades is just now being commercialized. These cells look to fulfill entirely new niches in the solar market. The cells are lightweight and flexible. The constituent elements have fewer scarcity concerns compared to CdTe, although competition with other industries (touch screen devices use In, as well), will likely limit production capacity (and raise costs). This is another potentially really inexpensive technology with good performance comparable to CdTe. People in these research groups actively research this material, as well as CdTe to further improve performance.

The material that we research, Cu2ZnSn(S,Se)4, is composed of earth-abundant and inexpensive elements, making it unlikely that the scarcity concerns that hinder the other technologies would limit it. Research focuses on improving the efficiency of cells using CZTS as an absorber layer. It also seems amazingly amenable to cheap fabrication methods. However, CZTS device performance isn't yet good enough to compete commercially. The record cell efficiency is 12.6 %, lower than module efficiencies of CIGS and CdTe commercial modules. We research these materials in order to improve efficiencies and make them competitive. Even if they don't pan out, we also learn about the broader materials system. For example, in my quest to learn about the CZTS material, I'm also learning about Cu2SnS3 and Cu4Sn7S16, which may also be of engineering importance, to name a few. Much of the understanding of CIGS helped in understanding CZTS.

Perovskite solar cells came out of nowhere and are achieving amazingly high efficiencies (although their lifetimes and tendency to degrade leave more to be desired). We don't know what the next technology will be, but it is always good to gain a better fundamental understanding of the world around us so that we have a knowledge base that can actually be applied intelligently.