Defect-Limited Efficiency of Pnictogen Chalcohalide Solar Cells

TL;DR

This study reveals that defect chemistry, dominated by chalcogen vacancies in BiChX pnictogen chalcohalides, sets a fundamental efficiency limit for these materials. Using a consistent first-principles workflow (DFT with PBE+D3, SOC, and HSE+SOC, plus ShakeNBreak and Freysoldt corrections), the authors map defect formation energies, charge-transition levels, and defect-limited carrier capture across BiSI, BiSeI, BiSBr, and BiSeBr, under Bi-poor and chalcogen-poor synthesis conditions. They find selenium vacancies induce deep nonradiative centers with strong electron–phonon coupling, especially in the selenides, while sulfur vacancies are comparatively benign in the sulfides; antisite defects contribute far less to nonradiative losses due to small capture coefficients. By computing defect-limited efficiencies, the work shows significant reductions in the selenides (up to ~10 percentage points) relative to radiative limits, and minor losses in the sulfides, highlighting synthesis strategies (chalcogen-rich or Bi-poor growth) and targeted anion substitutions as viable routes to mitigate trap-assisted recombination. Overall, the results establish defect engineering as a central design principle for improving MChX solar cell performance and offer a transferable framework for evaluating defects in emerging photovoltaic materials.

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.

Figures (7)

• Figure 1: MChX photovoltaic absorbers.a. MChX orthorhombic crystal structure (space group Pnma) characterized by columnar motifs held together by weak dispersion forces. Pnictogen, chalcogen and halide atoms are represented by violet, yellow and dark pink spheres, respectively. b. A systematic study of MChX point defects disentangles general trends. Four different Bi-based chalcohalides and all possible intrinsic point defects have been systematically analyzed in this study. The van der Waals radius of each species is marked for reference.
• Figure 2: Point-defect chemistry in BiSI.a. Charge-state transition levels of the most prominent (i.e., lowest energy) point defects determined for BiSI. b. Defect concentrations of BiSI considering an annealing temperature of $550$ K. The chemical stability region is delimited by S-poor ($\mu$Bi, $\mu$S, $\mu$I) = (0, -0.58, -0.75) eV and Bi-poor conditions ($\mu$Bi, $\mu$S, $\mu$I) = (-0.87, 0, -0.46) eV.
• Figure 3: Point-defect chemistry in BiSeI.a. Charge-state transition levels of the most prominent (i.e., lowest energy) point defects determined for BiSeI. b. Defect concentrations of BiSeI considering an annealing temperature of $550$ K. The chemical stability region is delimited by Se-poor ($\mu$Bi, $\mu$Se, $\mu$I) = (0, -0.65, -0.75) eV and Bi-poor conditions ($\mu$Bi, $\mu$Se, $\mu$I) = (-0.97, 0, -0.42) eV.
• Figure 4: Point-defect chemistry in BiSBr.a. Charge-state transition levels of the most prominent (i.e., lowest energy) point defects determined for BiSBr. b. Defect concentrations of BiSBr considering an annealing temperature of $550$ K. The chemical stability region is delimited by S-poor ($\mu$Bi, $\mu$S, $\mu$Br) = (0, -0.60, -0.97) eV and Bi-poor conditions ($\mu$Bi, $\mu$S, $\mu$Br) = (-0.90, 0, -0.67) eV.
• Figure 5: Point-defect chemistry in BiSeBr.a. Charge-state transition levels of the most prominent (i.e., lowest energy) point defects determined for BiSeBr. b. Defect concentrations of BiSeBr considering an annealing temperature of $550$ K. The chemical stability region is delimited by Se-poor ($\mu$Bi, $\mu$Se, $\mu$Br) = (0, -0.68, -0.97) eV and Bi-poor conditions ($\mu$Bi, $\mu$Se, $\mu$Br) = (-1.03, 0, -0.63) eV.