Assessing the defect tolerance of kesterite-inspired solar absorbers

TL;DR

This study analyzes defect properties in kesterite-inspired solar absorbers, revealing how specific defect clusters influence band tailing and providing criteria for designing more defect-tolerant photovoltaic materials.

Contribution

It offers a comprehensive dataset and identifies defect types affecting band tailing, guiding the discovery of more efficient, defect-tolerant kesterite-inspired solar absorbers.

Findings

Substitutions on the Zn site reduce band tailing but do not eliminate deep defects.

Deep Urbach tails are linked to band gap narrowing by specific defect clusters.

Shallow Gaussian tails are related to polymorph energy differences and cation disorder.

Abstract

Various thin-film I$_2$-II-IV-VI$_4$ photovoltaic absorbers derived from kesterite Cu$_2$ZnSn(S,Se)$_4$ have been synthesized, characterized, and theoretically investigated in the past few years. The availability of this homogeneous materials dataset is an opportunity to examine trends in their defect properties and identify criteria to find new defect-tolerant materials in this vast chemical space. We find that substitutions on the Zn site lead to a smooth decrease in band tailing as the ionic radius of the substituting cation increases. Unfortunately, this substitution strategy does not ensure the suppression of deeper defects and non-radiative recombination. Trends across the full dataset suggest that Gaussian and Urbach band tails in kesterite-inspired semiconductors are two separate phenomena caused by two different antisite defect types. Deep Urbach tails are correlated with the calculated band gap narrowing caused by the (2I$_\mathrm{II}$+IV$_\mathrm{II}$) defect cluster. Shallow Gaussian tails are correlated with the energy difference between the kesterite and stannite polymorphs, which points to the role of (I$_\mathrm{II}$+II$_\mathrm{I}$) defect clusters involving Group IB and Group IIB atoms swapping across \textit{different} cation planes. This finding can explain why \textit{in-plane} cation disorder and band tailing are uncorrelated in kesterites. Our results provide quantitative criteria for discovering new kesterite-inspired photovoltaic materials with low band tailing.