Charge Recombination Losses in Perovskite Solar Cells
Hideo Ohkita a
a Kyoto University, Japan, Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245, Japan
Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics
Proceedings of Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics (IPEROP20)
Tsukuba-shi, Japan, 2020 January 20th - 22nd
Organizers: Michio Kondo and Takurou Murakami
Invited Speaker, Hideo Ohkita, presentation 012
Publication date: 14th October 2019

Metal halide perovskite solar cells have attracted increasing attention as one of the most promising photovoltaic devices.  Currently, the power conversion efficiency (PCE) of lead-based perovskite solar cells has exceeded 25%.  Although it is approaching to the theoretical limit, there is still room for further improvement in PCE.  In most cases, charge recombinations are still one of the main loss processes in perovskite solar cells.  In this talk, I will focus on voltage losses due to various charge recombinations in perovskite solar cells.  First, I will talk about the origin of the initial improvement in open-circuit voltage (VOC) and fill factor (FF) of lead-based perovskite solar cells after storage in the ambient atmosphere.  We estimated trap density in perovskite solar cells with different storage durations by analyzing the intensity dependence of VOC with direct and SRH recombination model as reported previously [1].  As a result, we found that trap density is decreased with increasing storage time and hence VOC is also improved.  In order to address the origin of the decrease in trap density, we measured external quantum efficiency (EQE) spectra over the wide wavelength above and below the bandgap energy Eg.  Consequently, EQE signals were observed at wavelengths even below Eg for the device before the storage, suggesting that there are some defects in sub-bandgap states.  After the storage, the EQE signals below Eg were effectively reduced.  A similar reduction in the EQE at sub-bandgap was observed for the device with surface passivation treatment.  We therefore conclude that the initial improvement in VOC is due to the decreasing defects in sub-bandgap states, which are probably located at surface or boundary of perovskite grains.  We further discuss the origin of voltage losses in tin-based perovskite solar cells [2].





This study was partly supported by a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO), and by JST ALCA Grant Number JPMJAL1603, Japan.

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