CHAPTER 5. Thin-film Photovoltaics Based on Earth-abundant Materials

At some stage in the near future, the rapid expansion of photovoltaic solar energy conversion based on thin films of semiconductors such as cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS) could become subject to constraints arising from materials availability and security. For this reason, the development of alternative PV technologies based on earth-abundant elements has become a research priority. This chapter deals with a range of compound semiconductors that could be used to replace CdTe or CIGS as the light-absorbing layer in thin film solar cells. Because this is a rapidly expanding field, the authors have chosen to place the main emphasis on important fundamental aspects and emerging issues rather than just on device performance. The highly promising kesterite copper zinc tin sulfide/selenide (CZTS(Se)) is discussed in detail in order to illustrate the importance of phase equilibria and thermodynamics when considering the quaternary systems that are alternatives to CIGS. The chapter continues with an in depth discussion of the current state of understanding of the electronic properties of CZTS(Se) before reviewing the different synthetic methods that are being used to prepare kesterite layers for devices. The approach taken by the authors clearly demonstrates the central importance of thermodynamics and kinetics in understanding the formation and thermal stability of CZTS(Se) layers. After a short section on the most important opto-electronic properties of absorber layers, the chapter concludes with a comprehensive survey of a range of other potential absorber materials such as pyrite (FeS2), tin sulfide (SnS), copper tin sulfide (Cu2SnS3) and the copper bismuth/antimony sulfide family