Influence of organic cations on high-performance CH₃NH₃PbI₃ based photovoltaics

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 (V_OC) 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 (J_SC) of the CH₃NH₃PbI₃ based photovoltaics is dependent on the efficiencies of light absorption and carrier collection, which results in an optimized J_SC 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.