By using optical pump-terahertz probe (OPTP) experiments, we study the free charge carrier dynamics in photoexcited drop-cast CH 3 NH 3 PbI 3 -based perovskite thin film at room temperature. Compared with the pump photon energy at 1.55 eV, the measured OPTP signal following excitation of 3.1 eV shows an additional fast decay channel of the photoconductivity. Our experimental results demonstrate that effective carrier lifetime can be strongly modulated by surface recombination. In addition, the Drude-Smith-like transient terahertz photoconductivity spectra suggest that photogenerated free carriers experience backscattering at grain boundaries in our solutionprocessed perovskite films studied here.Over the past few years, organic/inorganic hybrid perovskite absorbers have emerged as novel functional materials for thin-film solar cells reaching efficiencies of over 15 % [1,2]. Notably, owing to a combination of strong optical absorption with particularly high charge diffusion lengths, the organic-lead iodide CH 3 NH 3 PbI 3 enables devices showing strong photovoltaic performance based on simple planar heterojunction architecture [3,4].Moving forward, understanding the dynamics of nonequilibrium photoexcited carriers in CH 3 NH 3 PbI 3 is of particular interest in developing high-performance optoelectronic and photovoltaic solar cells. Past efforts are based on probing directly the parameters of charge carrier dynamics, such as carrier lifetime, mobility, and diffusion length, by using the time-resolved photoluminescence (TRPL) [5-9] and optical pump-probe spectroscopy [10][11][12][13][14]. Elucidating the role of excitons, free carriers and especially carrier trap-state in these organic/inorganic materials would provide a deeper understanding of the mechanisms that give rise to the high performance of hybrid perovskite-based devices [15,16].Ultrafast optical pump-terahertz probe (OPTP) spectroscopy as a time-resolved, contactless conductivity probe method has previously been successfully employed to investigate the dynamics of photoexcited excitons and free charge carriers with subpicosecond resolution [17]. OPTP signal can be directly converted into the time-and frequency-resolved complex-valued sheet conductivity spectrum b r s x; t p À Á ¼ b r  d, where d is the thickness of the conductive layer. Photoinduced terahertz-frequency-integrated sheet conductivity is a product of mobility and carrier concentration, r s

We use transient terahertz photoconductivity measurements to demonstrate that upon optical excitation of CH3NH3PbI3 perovskite, the hole transfer from CH3NH3PbI3 into the organic hole-transporting material (HTM) Spiro-OMeTAD occurs on a sub-picosecond timescale. Second-order recombination is the dominant decay pathway at higher photo-excitation fluences as observed in neat CH3NH3PbI3 films. In contrast, under similar experimental conditions, second-order recombination weakly contributes the relatively slow recombination between the electrons in the perovskite and the injected holes in HTM, as a loss mechanism at the CH3NH3PbI3/Spiro-OMeTAD interface. Our results offer insights into the intrinsic photophysics of CH3NH3PbI3-based perovskites with direct implications for photovoltaic devices and optoelectronic applications.

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