Dynamical co-existence of excitons and free carriers in perovskite   probed by density-resolved fluorescent spectroscopic method
  Using transient fluorescent spectra at time-zero, we develop a density-resolved fluorescent spectroscopic method for investigating photoproducts in CH3NH3PbI3 perovskite and related photophysics. The density dependent dynamical co-existence of excitons and free carriers over a wide density range is experimentally observed for the first time. The exciton binding energy (EB) and the effective mass of electron-hole pair can be estimated based on such co-existence. No ionic polarization is found contributing to photophysical behavior. It also solves the conflict between the large experimentally measured EB and the small predicted values. The spectroscopic method also helps to detect the true free carrier density under continuous illumination without the interference of ionic conductivity. Our methods and results profoundly enrich the study and understanding of the photophysics in perovskite materials for photovoltaic applications. 
Wei Wang, Yu Li, Xiangyuan Wang, Yanping Lv, Shufeng Wang, Kai Wang,   Yantao Shi, Lixin Xiao, Zhijian Chen, and Qihuang Gong
cond-mat.mtrl-sci physics.chem-ph