Low-cost, nontoxic, and earth-abundant photovoltaic materials are long-sought targets in the solar cell research community. Perovskite-inspired materials have emerged as promising candidates for this goal, with researchers employing materials design strategies including structural, dimensional, and compositional transformations to avoid the use of rare and toxic elemental constituents, while attempting to maintain high optoelectronic performance. These strategies have recently been invoked to propose Ti-based vacancy-ordered halide perovskites (A2TiX6; A = CH3NH3, Cs, Rb, or K; X = I, Br, or Cl) for photovoltaic operation, following the initial promise of Cs2SnX6 compounds. Theoretical investigations of these materials, however, consistently overestimate their band gaps, a fundamental property for photovoltaic applications. Here, we reveal strong excitonic effects as the origin of this discrepancy between theory and experiment, a consequence of both low structural dimensionality and band localization. These findings have vital implications for the optoelectronic application of these compounds while also highlighting the importance of frontier-orbital character for chemical substitution in materials design strategies.
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The authors thank Alex M. Ganose for useful discussions regarding the electronic structure of ABX vacancy-ordered perovskites. S.R.K. acknowledges the EPSRC Centre for Doctoral Training in the Advanced Characterisation of Materials (CDT-ACM) (EP/S023259/1) for funding a Ph.D. studentship. C.N.S. is grateful to the Department of Chemistry at UCL and the Ramsay Memorial Fellowship Trust for the funding of a Ramsay fellowship. D.O.S. acknowledges support from the EPSRC (EP/N01572 X/1) and from the European Research Council, ERC (Grant 758345). The authors acknowledge the use of the UCL Kathleen High Performance Computing Facility (Kathleen@UCL), the Imperial College Research Computing Service, and associated support services in the completion of this work. Via membership of the UK’s HEC Materials Chemistry Consortium, which is funded by the EPSRC (EP/L000202, EP/R029431, and EP/T022213), this work used the ARCHER2 UK National Supercomputing Service ( www.archer2.ac.uk ) and the UK Materials and Molecular Modelling (MMM) Hub (Thomas EP/P020194 and Young EP/T022213). The authors acknowledge financial support from the European Research Council (ERC) under the European Unions Horizon 2020 research and innovation program (Grant Agreement 725165) as well as from the European Unions Horizon 2020 research and innovation program under Marie Skodowska-Curie Grant Agreement 713729. This project has received funding also from the Spanish State Research Agency, through the Severo Ochoa Center of Excellence (CEX2019-000910-S), the CERCA Programme/Generalitat de Catalunya and Fundacio Mir-Puig. The authors also acknowledge funding by the Fundacio Joan Ribas Araquistain (FJRA). This project was funded also by EQC2019-005797-P (AEI/FEDER UE). 2 6
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