title: Exciton versus free carrier photogeneration in organometal trihalide perovskites probed by broadband ultrafast polarization memory dynamics.
abstract: We studied the ultrafast transient response of photoexcitations in two hybrid organic-inorganic perovskite films used for high efficiency photovoltaic cells, namely, CH(3)NH(3)PbI(3) and CH(3)NH(3)PbI(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(3)NH(3)PbI(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(3)NH(3)PbI(1.1)Br(1.9). Surprisingly, we also discovered in CH(3)NH(3)PbI(3), but not in CH(3)NH(3)PbI(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(3)NH(3)PbI(3), D≈0.01  cm(2) s(-1).
authors.affiliation: School of Electronic and Optical Engineering, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, China.|Department of Physics and Astronomy, University of Utah, 115 South 1400 East, Salt Lake City, Utah 84112, USA.
authors.forename: ChuanXiang
authors.initials: C
authors.lastname: Sheng
authors.affiliation: Department of Physics and Astronomy, University of Utah, 115 South 1400 East, Salt Lake City, Utah 84112, USA.
authors.forename: Chuang
authors.initials: C
authors.lastname: Zhang
authors.affiliation: Department of Physics and Astronomy, University of Utah, 115 South 1400 East, Salt Lake City, Utah 84112, USA.
authors.forename: Yaxin
authors.initials: Y
authors.lastname: Zhai
authors.affiliation: Department of Physics, University of Texas at Dallas, Richardson, Texas 75080, USA.
authors.forename: Kamil
authors.initials: K
authors.lastname: Mielczarek
authors.affiliation: Institute of Functional Nano and Soft Materials (FUNSOM), Soochow University, Suzhou 215123, China.
authors.forename: Weiwei
authors.initials: W
authors.lastname: Wang
authors.affiliation: Institute of Functional Nano and Soft Materials (FUNSOM), Soochow University, Suzhou 215123, China.
authors.forename: Wanli
authors.initials: W
authors.lastname: Ma
authors.affiliation: Department of Physics, University of Texas at Dallas, Richardson, Texas 75080, USA.
authors.forename: Anvar
authors.initials: A
authors.lastname: Zakhidov
authors.affiliation: Department of Physics and Astronomy, University of Utah, 115 South 1400 East, Salt Lake City, Utah 84112, USA.
authors.forename: Z Valy
authors.initials: ZV
authors.lastname: Vardeny