One of the most pressing issues of our time is to move away from fossil fuels to stop or limit global warming. One important technology which is used to replace energy from fossil fuels are photovoltaic solar cells. This thesis deals with a material class which can be used as a solar cell absorber called kesterites.
All commercially relevant solar cell absorber materials have certain drawbacks like high material usage and utilizing scarce or poisonous elements. Kesterite solar cells rely on abundant and relatively environmentally friendly elements. Furthermore kesterites have a high absorption coefficient for visible light which allows to use them in so called thin-film solar cells which are very efficient in terms of material usage. Kesterite solar cells are not yet matured to a level where they can compete commercially. Their biggest issue is a low conversion efficiency of the incoming solar energy. It stagnated at 13 % for the last 7 years while beyond 20 % conversion efficiency are required to be successful in the commercial market. One of the causes of the low conversion efficiency is that kesterites can easily get disordered, in particular Cu and Zn atoms which share a plane in the unit cell can interchange easily. Partly or fully replacing Cu and/or Zn with another cation can reduce the tendency to disorder. In this thesis we investigate unsubstituted kesterite (Cu2ZnSnS4) as well as various substituted variants.
In thin-films compressive stress can occur for various reasons, e.g. if a reactant is sputtered in the synthesis. We use first principle methods to scan for high pressure phase transitions for ordered and disordered kesterite and the following substituted variants: Cu2FeSnS4, Cu2MnSnS4, Ag2ZnSnS4 and Ag2CdSnS4. We investigate how the electronic structure changes through the structural phase transition. Cu2CdSnS4 has a different crystal structure than Cu2ZnSnS4. Using first principle methods we determine the maximum amount of Cd which can be used in Cu2ZnSnS4 to substitute Zn without changing the crystal structure.
For all investigated materials we predict a high pressure transition to a compressed rocksalt structure. The electronic structure of this high pressure phase is in all cases metallic which renders the materials useless as a solar cell absorber. The transition pressure for Cu2ZnSnS4 is predicted at 16~GPa, in excellent agreement with experimental results. The transition pressure for the substituted variants Cu2FeSnS4, Cu2MnSnS4 and Ag2ZnSnS4 are lower but still well off the pressures we expect in thin-film synthesis. Only the transition pressure for Ag2CdSnS4 which is 4.7 GPa is close to pressures which can occur in thin-films. Therefore we advice to monitore the structure if this material is used as a solar cell absorber. For the solid solution series Cu2Cd(x)Zn(1-x)SnS4 we predict the kesterite-type structure to be most stable up to Cd(x)=0.51. Experimentally it is most stable up to Cd(x)=0.40. We show that the difference may be due to a small disorder in the experimental samples.
