Very high oscillator strength in the band-edge light absorption of zincblende, chalcopyrite, kesterite, and hybrid perovskite solar cell materials

In semiconducting solar cell absorbers, high absorption coefficient (a) near the band-edge is critical to maximize the photocurrent generation and collection. Nevertheless, despite the importance of the band-edge absorption characteristics, the quantitative analysis of the band-edge optical transitions has not been performed. In this study, we have implemented systematic density-functional theory calculation, focusing on the band-edge oscillator strength (f) of eight practical solar cell absorbers (GaAs, InP, CdTe, CuInSe2, CuGaSe2, Cu2ZnSnSe4, Cu2ZnSnS4, and CH3 NH3 PbI3) with zincblende, chalcopyrite, kesterite, and hybrid perovskite structures. We find that all the crystals exhibit very high f in the band-gap (E-g) region, compared with f in the higher-energy region, revealing the fact that a in the E-g region is enhanced significantly by the high f . We have further established that the magnitude of f can be interpreted from the charge distributions involved in the optical transition. In contrast to general understanding that the band-edge light absorption is derived from the joint density of state, our result reveals the critical role of f in the determination of absolute a values near E-g in quite general solar cell materials.