# Defect-Limited Efficiency of Pnictogen Chalcohalide Solar Cells

## TL;DR

This paper investigates why pnictogen chalcohalide solar cells underperform, finding that chalcogen vacancies significantly reduce efficiency.

## Contribution

The study provides a first-principles analysis of defect chemistry in Bi-based chalcohalides, identifying key performance-limiting defects.

## Key findings

- Chalcogen vacancies act as deep nonradiative recombination centers, reducing theoretical maximum efficiencies by 6-10%.
- BiSeI shows the best efficiency due to its optimal bandgap despite high recombination rates.
- Chalcogen-rich synthesis and anion substitutions are proposed to mitigate harmful vacancies.

## 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 propose chalcogen-rich synthesis conditions and targeted anion
substitutions as effective strategies for mitigation of detrimental
vacancies.