Defects Chemistry and Charge Traps in MA(Pb,Sn)I3 Perovskites: A Computational Perspective
Daniele Meggiolaro a b
a D3-CompuNet, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova
b Computational Laboratory for Hybrid/Organic Photovoltaics (CLHYO), CNR-ISTM, Via Elce di Sotto 8, 06123 Perugia, Italy
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV19)
Roma, Italy, 2019 May 12th - 15th
Organizers: Prashant Kamat, Filippo De Angelis and Aldo Di Carlo
Invited Speaker Session, Daniele Meggiolaro, presentation 165
Publication date: 11th February 2019

Lead halide perovskites are promising materials for photovoltaics, exceeding 22% of efficiency in solar cells devices.[1] Recently, mixed Pb-Sn and lead-free perovskites (MASnI3) have gain great interest due to the reduced toxicity compared to full-lead perovskites and the possibility of tuning the band gap to lower values. While the  partial substitution of Pb with Sn in mixed Pb-Sn perovskites has been demonstrated a successfull strategy to decrease the toxicity of these materials with limited impact on the overall efficiency (~17%),[2] the use of full tin MASnI3 perovskites is still limited due to self-doping phenomena which have detrimental effects on the charge carriers lifetimes and on the efficiencies in solar cell devices (<6%).[3]  

In this presentation a comparative study of native defects in MAPbI3 and MASnI3 perovskites based on state of the art Density Functional Theory (DFT) is presented. The nature of deep charge traps in these materials and the associated defects chemistry is investigated by the analysis of the defects formation energies in different conditions of growth and of the associated thermodynamic ionization levels.[4] This study aims to illustrate the most relevant similarities and differences in the defects chemistry of these materials. Starting from the analysis of MAPbI3 defects chemistry, the effects of partial Pb substitution by Sn will be discussed, by focussing on the potential holes self-trapping processes that Sn-doping can induce in the perovskite. Thus, the discussion moves to show results concerning the defects chemistry of full tin MASnI3 perovskite. Our analysis reveals that while in the full-lead perovskite iodine play a prominent role in determining the defects chemistry by modulating the relative trapping activity, in full-tin perovskites the different electronic structure and the high propensity of Sn to oxidation lead to a different defects chemistry scenario. Furthermore, a possible origin of the p self-doping in Sn-perovskites related to the high stability of tin vacancies in this system is discussed.

The research leading to these results received funding from the European Union’s Horizon 2020 research and innovation
program under Grant Agreement No. 763977 of the PerTPV project.

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