First Principle Hybrid Density Functional Theory Study of Halide Perovskite Surfaces and Interfaces Eric Welch, Young-Kwang Jung, Luisa Scolfaro, Aron Walsh and Alex Zakhidov Texas State University Department of Physics Zakhidov Optoelectronics Group https://zakhidov.wp.txstate.edu In collaboration with the Material’s Design Group 1

Inorganic halide perovskites – Promising emissive materials Texas State University Department of Physics ITO PEDOT:PSS/Spiro-OMETAD Perovskite TPBi Electrode ❌ Expensive ❌ Toxic/acidic ❌ Many defects/cavities ✅ High maximum brightness ~ 4.6×10! 𝑐𝑑 𝑚"# ✅ Promising EQE ~ 13% ✅ Solution processable ✅ Room temperature ✅ Inexpensive precursors Cs Pb Br 2

CuI – A good solution for cost and stability Texas State University Department of Physics ✅ Large bandgap: 𝐸$ = 3.1 𝑒𝑉 ✅ High RT conductivity: 𝜎 = 156 𝑆 𝑐𝑚"% ✅ High hole mobility: 𝜇 > 40 𝑐𝑚# 𝑉"% 𝑠"# ✅ Non-toxic ✅ Inexpensive precursors ✅ Less expensive than Spiro-OMETAD ✅ Non-porous/dense ✅ Comparable lattice constant to CsPbBr3 : CuI = 6.05 Å, CsPbBr3 = 5.83 Å Cu I 1. C. Yang, M. Kneiβ, M. Lorenz, and M. Grundmann, Proc. Natl. Acad. Sci. U. S. A. 113, 12929 (2016). 2. D. Chen, Y. Wang, Z. Lin, J. Huang, X. Chen, D. Pan, and F. Huang, Cryst. Growth Des. 10, 2057 (2010). 3. C Swartz et. al. Phys. Status Solidi A, 218, 2000721 (2021) 3

Why not CuBr?? ❌ CuBr is used for lasing ❌ 𝜎&'() is two times smaller than 𝜎&'* ❌ CuBr is $3/g more expensive than CuI Texas State University Department of Physics 1. D. Astadjov, et. al. IEEE J. Quant. Elec. 30, 1994. 4

Bulk properties o 𝐾!"# > 𝐾!$%&'(! o 𝑎!"# ~ 𝑎!$%&'(! Texas State University Department of Physics CuI CsPbBr3 5

Surface properties o𝐸!"#$ 𝑡 = 𝐸%& + 𝐸#'& (𝑡) oCu terminated and CsBr terminated show lowest surface energies o Also need interface energy and bonding information to determine favorable interface termination Texas State University Department of Physics 1. E Welch et. al. AIP Advances. 10, 085023 (2020). 6

Interface properties o 𝐸!"#$ 𝑡 = % &' [𝐸()*+ !"#$ 𝑡 − 𝐸()*+ ,(-+./! 𝑡 − 𝐸()*+ ,01 𝑡 ] o Interface energy reveals PbBr2 terminated CsPbBr3 on Cu terminated CuI to be the lowest energy Texas State University Department of Physics 1. E Welch et. al. AIP Advances. 10, 085023 (2020). 7

Interface properties o Charge density difference isosurface plots corroborate this o Yellow (red) isosurfaces indicated positive/accumulation (negative/depletion) regions Texas State University Department of Physics 1. E Welch et. al. AIP Advances. 10, 085023 (2020). 8

Interface properties o As the thickness is increased, the interface barrier potential changes sign o Type I to type II potential barrier transition o 𝑉𝐵𝑂 𝐶𝐵𝑂 = Δ𝐸! Δ𝐸" + Δ𝑉 Texas State University Department of Physics 1. E Welch et. al. AIP Advances. 10, 085023 (2020). 9

Conclusions o CuI may be used to template cubic CsPbBr3 growth o A stable interface exists where Cu is bonded to Br o Thickness is a tunable parameter o Our group recently showed CuI functions well as an HTL with MAPI for PV applications: C Swartz et. al. Phys. Status Solidi A, 218, 2000721 (2021) Texas State University Department of Physics 10

Acknolwedgements Texas State University Department of Physics Zakhidov Optoelectronics Group https://zakhidov.wp.txstate.edu In collaboration with the Material’s Design Group 11 Alex Zakhidov Luisa Scolfaro Young-Kwang Jung Aron Walsh @EricWelch31415 Funding: NSF #1927020 (Dr. Anna Bardy-Estevez) ONR N000141912576 (Dr. Paul Armistead) NRF MSIT 2018r1c1b6008728 This work has been published in: AIP Advances 10, 085023 (2020); https://doi.org/10.1063/5.0018925

Thank you!! Questions?? Texas State University Department of Physics

Extra slide 1 • Costs: Sigma Aldritch • CsBr: $43.30/10 g (21.65/5 g) • PbBr2: $53.20/5 g • Spiro-OMETAD: $1280/5 g • PEDOT:PSS: $42.30/5 g • TPBi: $391/500 mg (3910/5 g) • CuI: $6.55/5 g Texas State University Department of Physics 13 • Other LEDs • AlGaInN green LEDs: ~15% EQE

Extra slide 2 • DOS and PDOS Texas State University Department of Physics I-p to Cu-s transition Br-p to Pb-p transition 𝐸$ = 0 𝑒𝑉 14 𝐸2 = 3.1 𝑒𝑉 𝐸2 = 2.2 𝑒𝑉

Extra slide 3 • DOS and PDOS Texas State University Department of Physics Hybridized p-orbitals of Pb, I, Br, Cs, and Cu to Pb-p transition 𝐸$ = 0 𝑒𝑉 15

Extra slide 4 • Formulas: • 𝐸#$%& 𝑡 = 𝐸'( + 𝐸%)( 𝑡 = * +, 𝐸#(-. $/%)( 𝑡* + 𝐸#(-. $/%)( 𝑡0 − ( 𝑁#(-. -123(𝑡* + 𝑁#(-. -123 𝑡0 ∗ 4!"#$ 5!"#$ %&'( ) + * 0, 𝐸#(-. %)( 𝑡 − 𝐸#(-. $/%)( 𝑡 • 𝐸6/1& 𝑡 = * 0, 𝐸#(-. 6/1& 𝑡 − 𝐸#(-. "#7.8%) 𝑡 − 𝐸#(-. "$9 𝑡 • 𝑉𝐵𝑂 = Δ𝐸: + Δ𝑉 = 𝐸!8; "#7.8%) − 𝐸!8; "$9 + Δ𝑉 • 𝐸!8; = 𝑒!8; <= − 𝐸7.*# .$(> − 𝐸7.*# #$%& − 𝑉 :-' (Cu1s used for CuI) Texas State University Department of Physics 16