Abstract
In high performance perovskite based solar cells, CH3NH3PbI3 is the key material. We carried out a study on charge diffusion in spin-coated CH3NH3PbI3 perovskite thin film by transient fluorescent spectroscopy. A thickness-dependent fluorescent lifetime was found. By coating the film with an electron or hole transfer layer, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) or 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) respectively, we observed the charge transfer directly through the fluorescence quenching. One-dimensional diffusion model was applied to obtain long charge diffusion distances in thick films, which is ~1.7 μm for electrons and up to ~6.3 μm for holes. Short diffusion distance of few hundreds of nanosecond was also observed in thin films. This thickness dependent charge diffusion explained the formerly reported short charge diffusion distance (~100 nm) in films and resolved its confliction to thick working layer (300–500 nm) in real devices. This study presents direct support to the high performance perovskite solar cells and will benefit the devices’ design.
Highlights
Substantial attention has been drawn to the inorganic-organic perovskite-based solar cells, which currently achieve a certified high light conversion efficiency of 20.1%1
In reports by Yamada, the lifetime under low excitation light intensity can be 140 ns[21]. It is even longer than 100 μ s under low excitation intensity[19]
Concentration of PbI2 (M) Thickness τS an important parameter when calculating the charge diffusion distance by one-dimensional diffusion model[11,12]. It seems that all these conflicts need a better explanation
Summary
Substantial attention has been drawn to the inorganic-organic perovskite-based solar cells, which currently achieve a certified high light conversion efficiency of 20.1%1. Many high efficient perovskite solar cells based on CH3NH3PbI3 were made with perovskite layers thicker than this distance[13,14,15] It is investigated by impedance spectroscopy, photoinduced time-resolved microwave conductance (TRMC) and electron beam-induced current (EBIC) method, which hint a much longer charge transfer distance within perovskite layer[16,17,18]. It seems that all these conflicts need a better explanation To clarify these conflicts, we performed a study of directly observing the charge transfer in perovskite with various thicknesses and with an electron/hole transfer layer, by means of time-resolved transient fluorescence. We performed a study of directly observing the charge transfer in perovskite with various thicknesses and with an electron/hole transfer layer, by means of time-resolved transient fluorescence It shows that the charge diffusion in CH3NH3PbI3 is of distance at micrometer scale, which obviously longer than film thickness. The results show that hole diffusion is faster than electron within perovskite thin film
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