Facile Reconstruction of Extended Defects in Antimony Selenide Demonstrated by First-Principles Calculations and Electron Microscopy

Antimony selenide (Sb₂Se₃) and sulfoselenides have rapidly emerged as promising materials for application as solar absorbers in thin film photovoltaic and photoelectrochemical cells with device efficiencies now in excess of 10% [1-3]. These materials exhibit a highly anisotropic crystal structure (consisting of (Sb₄Se₆)_n ribbons oriented along the [001] direction) with weaker bonding between ribbons [4]. In polycrystalline films most extended defects (such as grain boundaries and surfaces) are observed to cut across these (Sb₄Se₆)_n ribbons and as a result break bonds. The conventional picture that emerges from studies of many other compound semiconductors is that dangling bonds introduced at such extended defects give rise to deep gap states that contribute to non-radiative recombination, reducing material performance relative to single crystals [5].<br/><br/>By employing density functional theory (DFT) calculations to model the structure and properties of a wide range of extended defects our previous work predicted that (in the absence of point defects) surfaces and grain boundaries in Sb₂Se₃ and Sb₂S₃ are unusually free of deep gap states associated with dangling bonds [6,7]. The reason is that extended defects undergo a facile reconstruction where undercoordinated atoms reconfigure to restore their coordination and eliminate associated defect states within the band gap (an effect we termed ‘self-healing’). While the absence of deep gap states is consistent with recent experimental results obtained by deep level transient spectroscopy [8] and Kelvin probe force microscopy [9], directly probing the atomic structure and electronic properties of extended defects remains challenging.<br/><br/>Associated with the reconstruction of extended defects DFT calculations predict significant long-range strain fields. In fact, the relative softness of these materials, with significant space between ribbons, likely facilitates the significant atomic rearrangements needed to eliminate dangling bonds at extended defects. This prediction is one that is more amenable to experimental investigation. In a recent study we have mapped the strain field within (Sb₄Se₆)_n ribbons terminating at using scanning transmission electron microscopy [10]. Strains of up to ~4% extending approximately 2 nm into the grain interior are observed for a (041) grain boundary with results in good agreement with the DFT predictions. This provides further evidence to support our prediction that there are significant atomic reconstructions that take place at extended defects that bestow Sb₂Se₃ and related materials with a high level of grain-boundary-defect tolerance.<br/><br/><b>References</b><br/>[1] Y. Zhao et al, Adv. Energy Mater. 12, 2103015 (2022).<br/>[2] Z. Duan et al, Adv. Energy Mater. 34, 2202969 (2022).<br/>[3] P.J. Dale and M.A. Scarpulla, Sol. Energy Mater. Sol. Cells, 251, 112097 (2023).<br/>[4] Y. Zhou et al, Nat. Photonics 9, 409 (2015).<br/>[5] J.M. Burst et al, Nat. Energy 1, 16015 (2016).<br/>[6] R.E. Williams et al, ACS Appl. Mater. & Inter. 12, 21730 (2020).<br/>[7] K.P. McKenna, Adv. Electron. Mater. 7, 2000908 (2021).<br/>[8] T.D.C. Hobson et al Appl. Phys. Lett. 116, 261101 (2020).<br/>[9] A. Vashishtha et al J. Alloys Compd. 948, 169714 (2023).<br/>[10] R.A. Lomas-Zapata et al, Phys. Rev. X Energy 3, 013006 (2024).