Ag-Bi Charge Redistribution Creates Deep Traps in Defective Cs2AgBiBr6: Machine Learning Analysis of Density Functional Theory

Dongyu Liu; Carlos Mora Perez; Andrey S Vasenko; Oleg V Prezhdo

doi:10.1021/acs.jpclett.2c00869

Ag-Bi Charge Redistribution Creates Deep Traps in Defective Cs₂AgBiBr₆: Machine Learning Analysis of Density Functional Theory

J Phys Chem Lett. 2022 Apr 28;13(16):3645-3651. doi: 10.1021/acs.jpclett.2c00869. Epub 2022 Apr 18.

Authors

Dongyu Liu¹, Carlos Mora Perez², Andrey S Vasenko^{1

3}, Oleg V Prezhdo^{2

4}

Affiliations

¹ HSE University, 101000 Moscow, Russia.
² Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States.
³ I.E. Tamm Department of Theoretical Physics, P.N. Lebedev Physical Institute, Russian Academy of Sciences, 119991 Moscow, Russia.
⁴ Department of Physics & Astronomy, University of Southern California, Los Angeles, California 90089, United States.

PMID: 35435697
DOI: 10.1021/acs.jpclett.2c00869

Abstract

Lead-free double perovskites hold promise for stable and environmentally benign solar cells; however, they exhibit low efficiencies because defects act as charge recombination centers. Identifying trap-assisted loss mechanisms and developing defect passivation strategies constitute an urgent goal. Applying unsupervised machine learning to density functional theory and nonadiabatic molecular dynamics, we demonstrate that negatively charged Br vacancies in Cs₂AgBiBr₆ create deep hole traps through charge redistribution between the adjacent Ag and Bi atoms. Vacancy electrons are first accepted by Bi and then shared with Ag, as the trap transforms from shallow to deep. Subsequent charge losses are promoted by Ag and Bi motions perpendicular to rather than along the Ag-Bi axis, as can be expected. In contrast, charge recombination in pristine Cs₂AgBiBr₆ correlates most with displacements of Cs atoms and Br-Br-Br angles. Doping with In to replace Ag at the vacancy maintains the electrons at Bi and keeps the trap shallow.