In the quest for more efficient and environmentally friendly solar cells, a team of researchers from the University of Cambridge, the University of Barcelona, and the Catalan Institute of Nanoscience and Nanotechnology have been investigating a promising class of materials known as pnictogen chalcohalides (MChX). These materials, which combine strong light absorption with low-temperature synthesis, could potentially revolutionize the solar energy industry.
The researchers, led by Dr. Cibrán López and Prof. Aron Walsh from the University of Cambridge, have been exploring the defect chemistry of bismuth-based chalcohalides to understand why their efficiency remains below 10%, far from their theoretical potential. Their findings, published in the journal Nature Communications, reveal a complex defect landscape that significantly impacts the performance of these solar cells.
The team’s comprehensive first-principles investigation uncovered that chalcogen vacancies, which are defects where atoms are missing from the crystal structure, are particularly problematic. These vacancies act as deep nonradiative recombination centers, meaning they capture electrons and holes (the positively charged counterparts of electrons) and convert their energy into heat rather than electricity. 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.
Interestingly, the researchers found that sulfur vacancies in BiSI and BiSBr are comparatively benign, presenting smaller capture coefficients due to weaker electron-phonon coupling. However, despite its high nonradiative charge-carrier recombination rate, BiSeI presents the best conversion efficiency among all four compounds due to its most suitable bandgap for outdoor photovoltaic applications.
The study identifies defect chemistry as a critical bottleneck in MChX solar cells and proposes chalcogen-rich synthesis conditions and targeted anion substitutions as effective strategies for mitigating detrimental vacancies. This research provides valuable insights for the energy sector, particularly for companies and researchers developing next-generation solar cells. By addressing these defect issues, the efficiency of pnictogen chalcohalide solar cells could be significantly improved, bringing us closer to a future powered by clean, renewable energy.
Source: López, C., Kavanagh, S.R., Benítez, P. et al. Defect-limited efficiency of pnictogen chalcohalide solar cells. Nat Commun 15, 1121 (2024).
This article is based on research available at arXiv.