Carrier Recombination in Passivated Polycrystalline Perovskite Thin Films

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Organic-inorganic halide perovskites (OHPs) are the rising stars of photovoltaic and optoelectronic research [1-3]. With power conversion efficiencies surging from 3.8% to 22.7% [4,5] in the last 8 years, OHP solar cells will begin to compete with silicon-based technology in the coming years. However, high defect densities within perovskite materials lead to the parasitic loss of charge carriers via non-radiative recombination (NRR), limiting the efficiencies of state-of-the-art devices. Passivation treatments can be employed to render these defects inert. Such treatments include the chemical doping of perovskite solutions before processing, and the atmospheric post-treatment of films after fabrication [6-9]. The aim of my PhD project is to contribute to a better understanding of the mechanisms that drive NRR in polycrystalline perovskite films, and to uncover the physics behind the passivation treatments which are employed to eliminate defects. Utilising a combination of fast spectroscopy techniques (time-resolved photoluminescence spectroscopy, transient absorption spectroscopy, time-resolved photoemission electron microscopy...) my goal is to characterise the behaviour of carriers under the influence of various passivation methods, to reveal pathways towards more optimal treatments and enhanced device performance. I intend to focus on understanding the role of surface defects - shown to be the most dominant limit on efficiency in OHP thin films [10,11] - using existing surface recombination velocity models [12], as well as through the construction of a comprehensive model of hole-trapping in triple-cation perovskite.References: [1] Stranks, S. D. & Snaith, H. J. Metal-halide perovskites for photovoltaic and light-emitting devices. Nature Nanotechnology 10, 391-402 (2015). [2] Bhatt, M. D. & Lee, J. S. Current progress and scientific challenges in the advancement of organic-inorganic lead halide perovskite solar cells. New Journal of Chemistry 41, 10508-10527 (2017). [3] Tang, H., He, S. & Peng, C. A Short Progress Report on High-Efficiency Perovskite Solar Cells. Nanoscale Research Letters 12, Article 410 (2017). [4] Kojima, A., Teshima, K., Shirai, Y. and Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 131, 6050-6051 (2009). [5] Green, M. A. et al. Solar cell efficiency tables (version 51). Progress in Photovoltaics: Research and Applications 26, 3-12 (2018).[6] Abdi-Jalebi, M. et al. Maximising and stabilizing luminescence from halide perovskites with potassium passivation. Nature 555, 497-501 (2018). [7] Tang, Z. et al. Hysterisis-free perovskite solar calls made of potassium-doped organometal halide perovskite. Scientific Reports 7, 12183 (2017). [8] Zheng, X. et al. Defect passivation in hybrid perovskite solar cells using quaternary ammonium halide anions and cations. Nature Energy 2, 17102 (2017) [9] Li, H. et al. Enhancing Efficiency of Perovskite Solar Cells via Surface Passivation with Graphene Oxide Interlayer. ACS Appl. Mater. Interfaces 9, 38967-38976 (2017).[10] Yang, Y. et al. Top and bottom surfaces limit carrier lifetime in lead iodide perovskite films. Nature Energy 2, Article 16207 (2017).[11] Zarazua, I. et al. Surface Recombination and Collection Efficiency in Perovskite Solar Cells from Impedance Analysis. J. Phys. Chem. Lett. 7, 5105-5113 (2016).[12] Yablonovitch, E., Allara, D. L., Chang, C. C., Gmitter, T. & Bright, T. B. Unusually Low Surface-Recombination Velocity on Silicon and Germanium Surfaces. Phys. Rev. Lett. 57, 249-252 (1986).