The number of component elements increased steadily in the 60-year development of photovoltaic semiconductors, i.e., from silicon in 1950s, to GaAs and CdTe in 1960s, CuInSe2 in 1970s, Cu(In,Ga)Se2 in 1980s, Cu2ZnSnS4 in 1990s and more recently Cu2ZnSn(S,Se)4 and CH3NH3PbI3. The increased number of elements makes the material properties more flexible, however, it also causes the dramatic increase of possible point defects in the lattice, which can significantly influence the optical and electrical properties and thus the photovoltaic performance of these multinary compound semiconductors. Whether they can work as ideal solar cell absorber material depends on the behavior of their intrinsic defects. Through ab initio calculations, we can predict the dominant defects in new semiconductors and determine whether there are high concentration of deep-level defects that may act as electron-hole recombination centers. Then the semiconductors free of detrimental defects can be identified, which will accelerate the discovery of new photovoltaic semiconductors with high energy-conversion efficiency. I will discuss such ab initio screening in four classes of semiconductor systems, including the quaternary I2-II-IV-VI4 (Cu2ZnSnS4, Cu2ZnSnSe4), the ternary I-V-VI2 (CuSbS2 and CuSbSe2), the binary Sb2Se3 as well as the halide perovskites (CsSnI3, CH3NH3SnI3 and CH3NH3PbI3), which were all proposed as the candidate photovoltaic materials with high efficiency. Based on the calculated formation energies (concentration) and transition energy levels of possible defects, I will discuss the influence of the chemical component and growth conditions on the defect formation/ionization and thus the electrical and optical properties of the samples, which will help us understand the related experiments and also judge whether the defects impose any intrinsic limit to the efficiency of these photovoltaic semiconductors.
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