Proceedings of International Conference Asia-Pacific Hybrid and Organic Photovoltaics 2018 (AP-HOPV18)
DOI: https://doi.org/10.29363/nanoge.ap-hopv.2018.031
Publication date: 27th October 2017
Lead halide perovskites-based photovoltaic devices are attracting much interest for their high photoconversion efficiency, which may be attributed to the remarkable carrier properties in this class of materials, e.g. long carrier lifetime and long carrier diffusion length. These properties should be ascribed to the efficient charge separation; electrons and holes are well separated so that the recombination is suppressed.
However, the mechanism of such efficient charge separation is still under debate. Especially, role of the molecular cation component of the perovskites is a controversial problem in this research field.
In this work, through first-principles molecular dynamics simulations for four types of lead iodide perovskites (MAPI, FAPI, GAPI, and CsPI), we demonstrated that the structural fluctuation of the inorganic (lead iodide) part of the perovskites decreases the spatial overlap between the valence band maxima (VBM) and the conduction band minima (CBM), i.e. enhances the charge separation.
Also, we discussed the mechanism of the structural fluctuation-induced charge separation. First, based on a simple model (a one-dimensional tight-binding model), we proposed that the charge separation is induced by fluctuation of electrostatic potential. Second, we confirmed the above discussion by first-principles. We calculated the electrostatic part of the energy of holes and electrons, and found that the electrostatic stabilization of the carriers determines the spatial overlap between VBM and CBM. Third, we demonstrated that the fluctuation of electrostatic potential mainly comes from the structural fluctuation of the inorganic part.
Considering the above, we propose a possible mechanism of the charge separation in this kind of material; the structural fluctuation of the inorganic part induces the fluctuation of electrostatic potential inside the materials, and the fluctuation of the electrostatic potential causes the charge separation.
Our results indicate that the charge separation is governed by the inorganic part and the organic cations may not be the dominant factor, suggesting that all-inorganic lead halide perovskites-based photovoltaic devices might be able to rival the organic-inorganic lead halide perovskites-based ones in performance.
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