Defect-Limited Efficiency of Pnictogen Chalcohalide Solar Cells

TL;DR

This study uses first-principles calculations to analyze defect chemistry in pnictogen chalcohalide solar cells, revealing defect-related efficiency limits and proposing strategies to improve device performance.

Contribution

It provides a comprehensive defect chemistry analysis of Bi-based chalcohalides, identifying dominant defect types and their impact on efficiency, which was previously not well understood.

Findings

Chalcogen vacancies act as deep nonradiative recombination centers.

High defect concentrations reduce maximum efficiencies by up to 10%.

BiSeI shows the best efficiency despite high nonradiative recombination rates.

Abstract

Pnictogen chalcohalides (MChX) have recently emerged as promising nontoxic and environmentally friendly photovoltaic absorbers, combining strong light absorption coefficients with favorable low-temperature synthesis conditions. Despite these advantages and reported optimized morphologies, device efficiencies remain below 10%, far from their ideal radiative limit. To uncover the origin of these performance losses, we present a systematic and fully consistent first-principles investigation of the defect chemistry across the Bi-based chalcohalide family. Our results reveal a complex defect landscape dominated by chalcogen vacancies of low formation energy, which act as deep nonradiative recombination centers. Despite their moderate charge-carrier capture coefficients, the high equilibrium concentrations of these defects reduce the theoretical maximum efficiencies by 6% in BiSeI and by 10% in BiSeBr. In contrast, sulfur vacancies in BiSI and BiSBr are comparatively benign, presenting smaller capture coefficients due to weaker electron-phonon coupling. Interestingly, despite its huge nonradiative charge-carrier recombination rate, BiSeI presents the best conversion efficiency among all four compounds owing to its most suitable bandgap for outdoor photovoltaic applications. Our findings identify defect chemistry as a critical bottleneck in MChX solar cells and proposes chalcogen-rich synthesis conditions and targeted anion substitutions as effective strategies for mitigation of detrimental vacancies.