Effect of Sb 2 Se 3 grain orientation on device efficiency, and evidence of the self-healing mechanism through structural relaxation Roy Lomas-Zapata 1 , LJ Philips 2 , JD Major 2 , BG Mendis 1 1 Durham University, UK, 2 University of Liverpool, UK Due to the highly anisotropic nature of the Sb 2 Se 3 structure, carrier transport along the [001] ribbon direction is the most efficient since it occurs along covalent bonded atoms. For transport to occur along any other direction it would be necessary for electrons to “hop” between adjacent ribbons, which is inefficient. In practice however, growth along (211), and (221) plane normals is predominant in Sb 2 Se 3 thin-films. Given that these directions are not parallel to [001], grain boundaries for these growth textures produce dangling bonds that can act as recombination centres, and therefore have a direct impact on device efficiency. However, recent density-functional theory (DFT) work has shown that significant structural relaxation can also take place, thereby removing the harmful defect electronic states at Sb 2 Se 3 grain boundaries. 1 In this study, an orientation-dependant solar cell simulation methodology was developed to analyse the effect of grain orientation on device efficiency. Utilising the one-dimensional nature of ribbon transport the 2D simulation area is divided into multiple 1D ribbons, for which the individual contributions to current are calculated. The boundary conditions for transport along a given ribbon is determined by the space charge region (SCR) and the ribbon termination at either the back contact (BC) or a grain boundary. Therefore, there are two transport scenarios, namely SCR-BC and SCR-GB. The relative fraction of each will depend on the ribbon orientation, film thickness and grain size. For 1 μm diameter Sb2Se3 grains in a 3 μm thick film oriented along the ideal [001] direction only the SCR-BC transport mechanism can occur. However, for orientations with more than 20° deviation from this orientation, the SCR-GB transport mechanism takes the lead, and as a result of this, the efficiency drops rapidly from 19.69% to 6.81% for the non-passivated grain boundary. The efficiency can be improved to 9.28% by passivating the grain boundary (recombination velocity of 103 cm/sec); the passivation from the self-healing mechanism is expected to occur for all orientations above 20°. Although efficiency does not decrease as fast, high angles are undesirable because of their low efficiency. Li et al. have shown that growing Sb2Se3 thin-films with the ideal [001] orientation is a complex process, that is still under development. 2 However, for the commonly occurring non-ideal orientations, the possibility of grain boundary passivation through deep structural relaxation could offer a new approach to increasing Sb 2 Se 3 device efficiencies. In this work, a complementary strain analysis study on atomic resolution TEM experimental data is presented; the observed variation in the inter-ribbon distance is consistent with the published DFT theoretical work. References 1. McKenna, K. P. Adv. Electron. Mater. 7, 2000908 (2021). 2. Li, Z., et al. Nat. Commun. 10,125 (2019).
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