Giant oscillator strength in the band-edge light absorption of zincblende, chalcopyrite and kesterite solar cell materials

TL;DR

This study uses density functional theory to analyze the band-edge optical transitions of various solar cell materials, revealing giant oscillator strength that enhances absorption near the band gap, crucial for efficient photocurrent generation.

Contribution

It provides the first systematic DFT analysis of band-edge oscillator strength in seven practical solar cell absorbers, highlighting the origin of giant oscillator strength in these materials.

Findings

All studied materials exhibit giant oscillator strength near the band gap.

Giant oscillator strength significantly enhances absorption in the band-edge region.

High-energy transitions are dominated by joint density-of-states, not oscillator strength.

Abstract

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.