  In semiconducting solar-cell absorbers, high absorption coefficient (alpha)
near the band-edge region 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 (DFT) calculation, focusing on the
band-edge oscillator strength of seven practical solar cell absorbers (GaAs,
InP, CdTe, CuInSe2, CuGaSe2, Cu2ZnSnSe4, and Cu2ZnSnS4) with zincblende,
chalcopyrite and kesterite structures. We find that all these crystals exhibit
the giant oscillator strength near the band gap region, revealing the fact that
alpha in the band gap region is enhanced significantly by the anomalous high
oscillator strength. In high-energy optical transitions, however, the
oscillator strength reduces sharply and the absorption properties are
determined primarily by the joint density-of-state contribution. Based on DFT
results, we show that the giant oscillator strength in the band edge region
originates from a unique tetrahedral-bonding structure, with a negligible
effect of constituent atoms.
