A comprehensive study was carried out to understand the influence of organic cations on high-performance CH3NH3PbI3 based photovoltaics. Scanning electron microscope, X-ray diffractometer, Hall measurement, absorbance spectrum, photoluminescence (PL) spectrum and nanosecond time-resolved PL measurements were used to explore the structural, electrical, optical and excitonic characteristics of CH3NH3PbI3 thin films fabricated under different thermal annealing temperatures from 60 °C to 140 °C. The decrease in the open-circuit voltage (VOC) with an increase in the thermal annealing temperature can be explained as due to the reduced work function of the CH3NH3PbI3 thin film. The short-circuit current density (JSC) of the CH3NH3PbI3 based photovoltaics is dependent on the efficiencies of light absorption and carrier collection, which results in an optimized JSC when the thermal annealing temperature is 120 °C. The atomistic interaction between the organic cations and Pb-I framework strongly influences the absorbance of CH3NH3PbI3 thin films, as confirmed by the libration of CH3NH3 cations shown by Raman scattering spectroscopy. In addition, the experimental results indicate that the power conversion efficiency can be further improved when the absorption strength of the CH3NH3PbI3 thin film and the energy-level alignment of each photovoltaic layer are simultaneously fulfilled.