Structural, optical and excitonic properties of urea grading doped CH₃NH₃PbI₃ thin films and their application in inverted-type perovskite solar cells

—The structural, optical and excitonic characteristics of urea doped CH₃NH₃PbI₃ (MAPbI₃) multi-crystalline thin films were investigated by using the X-ray diffraction patterns, atomic-force microscopic images, absorbance spectra, photoluminescence (PL) spectra and Raman scattering spectra. The surface-sensitive Raman scattering spectra show that the urea small molecules are mainly distributed in the top region of the MAPbI₃ thin films and thereby effectively passivating the electron-poor defects (interfacial MA cations) of MAPbI₃ thin films. Besides, the thermal annealing temperature and the concentration of urea additive both strongly influence the formation of MAPbI₃ thin films, which dominates the photovoltaic performance. The use of 5-wt% urea can increase the open-circuit voltage (V_OC) and short-circuit current density (J_SC) of the MAPbI₃ solar cells from 0.88 V to 19.75 mA/cm² to 0.94 V and 22.97 mA/cm², respectively. In addition, the reduced current hysteresis in the J-V curves of the MAPbI₃ solar cells can be explained as due to the effective defect passivation in the top region of the MAPbI₃ thin film by the oxygen-donors of the urea small molecules.