Bandgap Lowering in Mixed Alloys of Cs2 Ag(Sbx Bi1-x )Br6 Double Perovskite Thin Films Seán R. Kavanagh, Zewei Li, Robert G. Palgrave, Daniel W. Davies, Richard H. Friend, David O. Scanlon, Aron Walsh, Robert Hoye et al. SCAN LON MATE RIALS TH EORY GRO UP 

Lead-Halide Perovskites - Groundbreaking, but… Cubic CsPbBr3 Chemical Formula: AIBIIX3 Defect Tolerant1 Low-Temperature, Solution-based Synthesis High-Efficiency PV Devices (Record: 25.2%)2 Poor Stability Toxic (Bioavailable Lead) 1. Huang, Y.-T., Kavanagh, S. R., Scanlon, D. O., Walsh, A. & Hoye, R. L. Z. Perovskite-Inspired Materials for Photovoltaics -- From Design to Devices. arXiv:2008.08959 (2020). 2. NREL. National Renewable Energy Laboratory Photovoltaic Research: Best Research-Cell Efficiency Chart (2020). 2 

Cs2 AgBiBr6 Chemical Formula: AIBIBIIIX3 Non-Toxic Air-Stable1 Long Charge-Carrier Lifetimes1,2 Large Bandgaps (> 2 eV) Eg, ideal = ~2 eV (Indoor PV); 1.7-1.9 eV (Tandem top-cells) 1. Huang, Y.-T., Kavanagh, S. R., Scanlon, D. O., Walsh, A. & Hoye, R. L. Z. Perovskite-Inspired Materials for Photovoltaics -- From Design to Devices. arXiv:2008.08959 (2020). 2. Hoye, R. L. Z. et al. Fundamental Carrier Lifetime Exceeding 1 µs in Cs2 AgBiBr6 Double Perovskite. Adv. Mater. Interfaces 5, 2–9 (2018). Double Perovskites – A Promising Alternative 3 

Can We Lower Their Bandgaps? How? Cs2 AgSbBr6 Cs2 AgBiBr6 4 Group 15 

Cs2 AgSbBr6 Cs2 AgBiBr6 5 Can We Lower Their Bandgaps? How? Group 15 

Cs2 AgSbBr6 Cs2 AgBiBr6 6 Can We Lower Their Bandgaps? How? Group 15 

Cs2 AgSbBr6 Cs2 AgBiBr6 7 Can We Lower Their Bandgaps? How? Group 15 

Likely Origins of Bandgap Bowing (In Semiconductor Alloys) o Volume deformation potential effects. o Local structural distortion (due to broken symmetry). o Chemical effects due to intermixing of atomic orbitals. 8 1. Goyal, A. et al. Origin of Pronounced Nonlinear Band Gap Behavior in Lead-Tin Hybrid Perovskite Alloys. Chem. Mater. 30, 3920–3928 (2018). 

9 1. Li YH, Walsh A, Chen S, Yin WJ, Yang JH, Li J, et al. Revised ab initio natural band offsets of all group IV, II-VI, and III-V semiconductors. Appl Phys Lett. 2009;94(21). Electronic Band Alignment WMD-group/MacroDensity 

10 Electronic Band Alignment Group 15 DFT Functional: HSE06 + Spin-Orbit Coupling 

11 Electronic Band Alignment Group 15 Cs2 AgSbBr6 Cs2 AgBiBr6 

12 Electronic Band Alignment Group 15 Cs2 AgSbBr6 Cs2 AgBiBr6 

13 Alloy Electronic Structure 

14 Alloy Electronic Structure 

15 Conclusions & Outlook o Type II ‘staggered’ band alignment & chemical similarity leads to extreme bandgap bowing in the Cs2 AgSbx Bi1-x Br6 system. o Novel strategy to lower the bandgaps of other similar materials (viz. double perovskites) for photo- voltaic/catalytic applications. 

16 Conclusions & Outlook o Novel strategy to lower the bandgaps of other similar materials (viz. double perovskites) for photo- voltaic/catalytic applications. Eg, ideal (Indoor PV) Eg (Typical Double Perovskite) 

17 Conclusions & Outlook o Novel strategy to lower the bandgaps of other similar materials (viz. double perovskites) for photo- voltaic/catalytic applications. Eg, ideal (Indoor PV) Eg (Double Perovskite Alloy) 

18 Conclusions & Outlook o Novel strategy to lower the bandgaps of other similar materials (viz. double perovskites) for photo- voltaic/catalytic applications. 

19 Conclusions & Outlook o Novel strategy to lower the bandgaps of other similar materials (viz. double perovskites) for photo- voltaic/catalytic applications. 

Acknowledgements @Kavanagh_Sean_ kavanase [email protected] arXiv:2007.00388 20 SCAN LON MATE RIALS TH EORY GRO UP