The ability to accurately model the behaviour of crystalline defects would constitute a significant step towards improving photovoltaic efficiencies and semiconductor doping control, accelerating materials discovery and design. In this work, we apply relativistic hybrid Density Functional Theory (DFT) to accurately describe the atomistic behaviour of the cadmium vacancy (VCd) in cadmium telluride (CdTe).1 In doing so, we resolve several longstanding discrepancies in the extensive literature on this species. CdTe is a champion thin-film absorber for which defects, through facilitation of non-radiative recombination, significantly impact photovoltaic (PV) performance, contributing to a reduction in efficiency from an ideal detailed-balance limit of 32% to a current record of 22.1%. Despite over 70 years of research, many of the relevant defects are still not well understood, with the definitive identification of the atomistic origins of experimentally observed defect levels remaining elusive.1–3 In this work, through identification of a tellurium dimer structure for the neutral Cd vacancy, we obtain a single negative-U defect level for VCd at 0.35 eV above the VBM, finally reconciling theoretical predictions with experimental observations. Moreover, we reproduce the polaronic, optical and magnetic behaviour of VCd-1 in excellent agreement with previous Electron Paramagnetic Resonance (EPR) characterisation.4 We find that VCd facilitates rapid charge-carrier recombination, reducing maximum power-conversion efficiency by over 5% for untreated CdTe—a consequence of tellurium dimerisation, metastable structural arrangements, and anharmonic potential energy surfaces. The origins of previous discrepancies between theory and experiment, namely incomplete mapping of the defect potential energy surface (PES) and approximation models, are highlighted and discussed. Importantly, these results demonstrate the necessity to include the effects of both metastability and anharmonicity for the accurate calculation of both charge-carrier recombination rates in emerging photovoltaic materials and the efficiency limits imposed by both native and extrinsic defect species.

Rapid recombination by cadmium vacancies in CdTe

ACS Fall 2021

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