Tail state formation in solar cell materials: First principles analyses of zincblende, chalcopyrite, kesterite and hybrid perovskite crystals

TL;DR

This study uses first-principles calculations to analyze tail state formation in various solar cell materials, revealing that cation disordering significantly contributes to tail states in some compounds, while hybrid perovskites show minimal tailing.

Contribution

Developed a general theoretical scheme comparing experimental Urbach energies with DFT-derived absorption edges to analyze tail state formation in solar cell materials.

Findings

Tail states are negligible in CdTe, CISe, CGSe, and hybrid perovskites.

Large Urbach energies in CZTSe and CZTS are due to cation disordering.

Hybrid perovskite CH3NH3PbI3 shows sharp absorption edges consistent with minimal tail states.

Abstract

Tail state formation in solar cell absorbers leads to a detrimental effect on solar cell performance. Nevertheless, the characterization of the band tailing in experimental semiconductor crystals is generally difficult. In this article, to determine the tail state generation in various solar cell materials, we have developed a quite general theoretical scheme in which the experimental Urbach energy is compared with the absorption edge energy derived from density functional theory (DFT) calculation. For this purpose, the absorption spectra of solar cell materials, including CdTe, CuInSe2 (CISe), CuGaSe2 (CGSe), Cu2ZnSnSe4 (CZTSe), Cu2ZnSnS4 (CZTS) and hybrid perovskites, have been calculated by DFT particularly using very-high-density k meshes. As a result, we find that the tail state formation is negligible in CdTe, CISe, CGSe and hybrid perovskite polycrystals. However, coevaporated CZTSe and CZTS layers exhibit very large Urbach energies, which are far larger than the theoretical counterparts. Based on DFT analysis results, we conclude that the quite large tail state formation observed in the CZTSe and CZTS originates from extensive cation disordering. In particular, even a slight cation substitution is found to generate unusual band fluctuation in CZT(S)Se. In contrast, CH3NH3PbI3 hybrid perovskite shows the sharpest absorption edge theoretically, which agrees with experiment.