Solar cell materials – an overview
The search for new materials in solar cells is one of the biggest and most hard-fought topics that challenges scientists around the world, including the physicists and engineers at HZB. Headlines praising ever higher degrees of efficiency and ever thinner absorber layers are published with quite a reliable regularity. As this is such a vivid and innovative sector, it is sometimes hard to maintain an overview about the materials and details the scientists are dealing with. So this is the outcome of my try to bring a little order in the amazing and promising field of solar cell materials.
At the beginning, there was the silicon solar cell made with the crystalline wafer technology. In 1954 it was produced first with an efficiency of roundabout 5 % and has been improved until today. Now it can reach efficiencies of up to 25%, which is quite remarkable and useful already. Although these traditional silicon cells are well established and new thin film silicon cells are being studied, this material isn't the only peak of the solar sector’s enormous possibilities which are still to be exploited completely.
The most promising alternative to the silicon cells are the ones made of chalcopyrites. These thin film cells can be more than 100 times thinner than their old brothers made of crystalline silicon: roughly just one micrometer. This leads to enormous material and money savings. The name chalcopyrite summarizes the many materials which are either made of a copper-indium-sulfide/selenide mix (CIS), or consist of copper-indium-gallium-sulfide/selenide (CIGS). They can be placed on a substrate made of glass, which is easy and affordable to produce. By using plastics instead of solid glass, you can get very flexible solar modules that you could bring into any shape you like. Researchers around the world are working on improving the efficiency of these cells. By now they have reached 20,9% in a solar cell of comparable small size (~1 cm²). The challenge remains to get such good results for bigger solar modules that could actually be used in everyday life.
As great as all of this sounds, chalcopyrites come with one disadvantage: the needed elements indium and gallium are very rare and therefore hard to get. But there is hope: In the so-called kesterites, zinc and iron are used instead of indium and gallium. This makes kesterite a more affordable and sustainable option than chalcopyrite. Since kesterites are part of a relatively young research field, the efficiency of these cells has only reached around 10%. But of course, scientists are continuously working on increasing this number.
And then there are the perovskites. They are the rising star of the industry: Research on these calcium titanates has just started in 2006, and until now an efficiency of 15% has been reached. In the scene of solar cells, this is a remarkably fast career! What’s promising about this type is the flexibility of the material and the easy and cheap way to produce it. But – as you might have expected – even here there is a stinger: today’s perovskites contain lead, which is toxic. So the search is on for some better options to that.
As you see, the options for future solar cells are fascinating and diverse. Next to the few materials I have just named, there are even more, which shall only be mentioned briefly here: graphene, III-V- and II-VI-compound-semiconductors, organic materials… The list goes on – and so does the development of cheaper and more efficient ways to produce clean energy from sunlight!