Exciton versus Free Carrier Photogeneration in Organometal Trihalide Perovskites Probed by Broadband Ultrafast Polarization Memory Dynamics

摘要

We studied the ultrafast transient response of photoexcitations in two hybrid organic-inorganic perovskite films used for high efficiency photovoltaic cells, namely, CH_3NH_3PbI_3 and CH_3NH_3PbI_(1.1)Br_(1.9) using polarized broadband pump-probe spectroscopy in the spectral range of 0.3-2.7 eV with 300 fs time resolution. For CH_3NH_3PbI_3 with above-gap excitation we found both photogenerated carriers and excitons, but only carriers are photogenerated with below-gap excitation. In contrast, mainly excitons are photogenerated in CH_3NH_3PbI_(1.1)Br_(1.9). Surprisingly, we also discovered in CH_3NH_3PbI_3, but not in CH_3NH_3PbI_(1.1)Br_(1.9), transient photoinduced polarization memory for both excitons and photocarriers, which is also reflected in the steady state photoluminescence. From the polarization memory dynamics we obtained the excitons diffusion constant in CH_3NH_3PbI_3, D ≈ 0.01 cm~2 s~(-1).