2, Diamond Light Source, Didcot, , United Kingdom
3, University of Bath, Bath, , United Kingdom
4, Imperial College London, London, , United Kingdom
5, Yonsei University, Seoul, , Korea (the Republic of)
Bismuth-based solar absorbers are of interest due to similarities in the chemical properties of bismuth halides and the exceptionally efficient lead halide hybrid perovskites. Both Pb2+ and Bi3+ possess a similar soft polarisability and form a wide range of compounds with rich structural diversity, such as BiX6 clusters, 1D ribbons, and layered perovskite type structures. Whilst both are composed of earth-abundant materials and experience the same beneficial relativistic effects acting to increase the width of the conduction band, bismuth is non-toxic and non-bioaccumulating, meaning the impact of environmental contamination is greatly reduced.
Here, we use hybrid density functional theory, with the addition of spin orbit coupling (SOC), to examine a range of bismuth containing V-VI-VII candidate photovoltaic (PV) absorbers.[2-5] We show that BiSI and BiSeI possess electronic structures suitable for photovoltaic applications. Furthermore, we calculate band alignments against commonly used hole transporting and buffer layers, which indicate band misalignments are likely to be the source of the poor efficiencies reported for devices containing these materials. Based on this, we have suggested alternative device architectures expected to result in improved power conversion efficiencies. Lastly, we explore the defect properties of BiSI and suggest ideal growth conditions for optimised film properties.
 A. M. Ganose, C. N. Savory and D. O. Scanlon, Chem. Commun. 53, 20–44 (2017)
 K. T. Butler, J. M. Frost, and A. Walsh, Energy Environ. Sci. 8, 838 (2015)
 A. M. Ganose, K. T. Butler, A. Walsh, and D. O. Scanlon, J. Mater. Chem. A 4, 2060 (2016).
 A. M. Ganose, M. Cuff, K. T. Butler, A. Walsh and D. O. Scanlon, Chem. Mater. 28, 1980 (2016)
 D. S. Bhachu et. al., Chem. Sci. DOI: 10.1039/C6SC00389C (2016)