Proceedings of International Conference on Perovskite Thin Film Photovoltaics and Perovskite Photonics and Optoelectronics (NIPHO20)
DOI: https://doi.org/10.29363/nanoge.nipho.2020.069
Publication date: 25th November 2019
Tin perovskites are valuable materials for less toxic perovskites solar cells. Their efficiencies in devices are however strongly limited by self p-doping phenomena; and the oxidation of Sn(II) to Sn(IV) is responsible of the lattice instability in air and of increased carrier recombination. Both processes limit the efficiencies of tin-perovskites to less than 10%, although in principle tin-iodide perovskites could exhibit excellent optoelectronic properties with a reduced band gap compared to lead-iodide ones.
Based on state of the art density functional theory simulations we investigate the defects chemistry of the MASnI3 perovskite. Our analysis shows that the large stability of tin vacancies (density ~1020) is responsible of the severe p-doping exhibited by this material, which is directly related to the high ionization potential of MASnI3. By comparing the electronic structure of analogous tin- and lead-iodide perovskites we show that the higher band edge energies of MASnI3 compared to MAPbI3 lead to the emergence of deep electron traps associated to under-coordinated tin defects (e.g. interstitial tin) and the suppression of deep transitions active in MAPbI3 associated to under-coordinated iodine defects, such as interstitial iodine and lead vacancies.
We also show how a highly defective tin-iodide perovskite spontaneously transforms into oxidized Sn(IV) phases.
Thus stabilizing tin-iodide perovskites may really seem a desperate case, due to intrinsic instability related to band edge energetics. We finally illustrate possible strategies aimed at mitigating MASnI3 instability through tailored substitution of tin and iodine with isovalent and aliovalent metals and with different halides.
The authors acknowledge support from the Ministero Istruzione dell’Università e della Ricerca (MIUR) and the University of Perugia through the program “Dipartimenti di Eccellenza 2018-2022” (grant AMIS) and from the European 531 Union’s Horizon 2020 research and innovation programme 532 under Grant Agreement No 764047 of the Espresso project.
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