Halide double perovskites have gained significant attention, owing to their composition of low-toxicity elements, stability in air and long charge-carrier lifetimes. However, most double perovskites, including Cs2AgBiBr6, have wide bandgaps, which limits photoconversion efficiencies. The bandgap can be reduced through alloying with Sb3+, but Sb-rich alloys are difficult to synthesize due to the high formation energy of Cs2AgSbBr6, which itself has a wide bandgap. We develop a solution-based route to synthesize phase-pure Cs2Ag(SbxBi1-x)Br6 thin films, with the mixing parameter x continuously varying over the entire composition range. We reveal that the mixed alloys (x between 0.5 and 0.9) demonstrate smaller bandgaps than the pure Sb- and Bi-based compounds. The reduction in the bandgap of Cs2AgBiBr6 achieved through alloying (170 meV) is larger than if the mixed alloys had obeyed Vegard's law (70 meV). Through in-depth computations, we propose that bandgap lowering arises from the type II band alignment between Cs2AgBiBr6 and Cs2AgSbBr6. The energy mismatch between the Bi and Sb s and p atomic orbitals, coupled with their non-linear mixing, results in the alloys adopting a smaller bandgap than the pure compounds. Our work demonstrates an approach to achieve bandgap reduction and highlights that bandgap bowing may be found in other double perovskite alloys by pairing together materials forming a type II band alignment. This journal is
Bibliographical noteFunding Information:
Z. L. would like to thank Cambridge Trust and Chinese Scholarship Council for nancial support. S. R. K. acknowledges funding from the EPSRC Centre for Doctoral Training in Advanced Characterisation of Materials (CDT-ACM) (EP/ S023259/1), the use of the UCL Grace High Performance Computing Facility (Grace@UCL), the Imperial College Research Computing Service (DOI: 10.14469/hpc/2232), and associated support services in the completion of this work. Via membership of the UK's HEC Materials Chemistry Consortium, which is funded by EPSRC (EP/L000202), this work also used the UK Materials and Molecular Modelling (MMM) Hub for computational resources, which is partially funded by the EPSRC (EP/P020194). R. L. Z. H. acknowledges support from the Royal Academy of Engineering under the Research Fellowship Programme (No. RF\201718\1701), the Isaac Newton Trust (Minute 19.07(d)), and the Kim and Juliana Silverman Research Fellowship at Downing College, Cambridge. M. A.-J. thanks Cambridge Materials Limited, Wolfson College, University of Cambridge and EPSRC (grant no. EP/M005143/1) for their funding and technical support. For the RBS experiments, the authors acknowledge the RADIATE project under the Grant Agreement 824096 from the EU Research and Innovation programme HORIZON 2020. The X-ray photoelectron (XPS) and UV photoelectron (UPS) data collection was performed at the EPSRC National Facility for XPS (“HarwellXPS”), operated by Cardiff University and UCL, under Contract No. PR16195.
All Science Journal Classification (ASJC) codes
- Renewable Energy, Sustainability and the Environment
- Materials Science(all)