Intrinsic point defects and intergrowths in layered bismuth triiodide

Defect-tolerant semiconductors have the ability to retain the electronic properties of their pristine form even in the presence of defects. Currently, the presence of antibonding states at the valence band edges induced by a lone pair of $6{s}^{2}$ or $5{s}^{2}$ electrons is used as a descriptor to predict defect-tolerant semiconductors. Based on this descriptor, bismuth triiodide ($\mathrm{Bi}{\mathrm{I}}_{3}$) has been proposed as a defect-tolerant semiconductor with promise for photovoltaic applications. However, clear demonstration of the defect tolerance of $\mathrm{Bi}{\mathrm{I}}_{3}$ including a comprehensive study of the type of defects and their effect on the electronic structure has not been reported so far. Here, we present an atomic-scale landscape of point defects and intergrowths in $\mathrm{Bi}{\mathrm{I}}_{3}$ using a combination of density-functional-theory calculations and aberration-corrected scanning transmission electron microscope imaging. We show that $\mathrm{Bi}{\mathrm{I}}_{3}$ is not a defect-tolerant semiconductor as intrinsic point defects have low formation energy and show transition levels that are deep within the band gap and can act as nonradiative recombination centers. We show that Bi-rich growth conditions lead to higher carrier concentration over I-rich conditions. We also show the presence of intergrowths that are made up of a bilayer of bismuth atoms sandwiched within $\mathrm{Bi}{\mathrm{I}}_{3}$ sheets with a missing layer of iodine atoms. These intergrowths result in metallic behavior within the semiconducting matrix of $\mathrm{Bi}{\mathrm{I}}_{3}$. We propose that atomic-scale control of the intergrowths can be beneficial to avoid carrier trapping and to enhance photon absorbance. Overall, this work highlights the need to go beyond heuristic descriptors based on band-edge characteristics to predict defect-tolerant semiconductors.