Thermodynamic regulation of CH3NH3PbI3 crystal growth and its effect on photovoltaic performance of perovskite solar cells

We report a theoretical analysis on the crystallization of CH3NH3PbI3 and the control of grain sizes by varying the two-step reaction temperature from -10 degrees C to 50 degrees C based on the present analysis. The thermodynamic equation for CH3NH3PbI3 crystallization is derived by considering the change in Gibbs free energy and the equilibrium concentration of the reaction between the PbI2 film and CH3NH3I solution. The photovoltaic performance of a perovskite solar cell is found to depend on the reaction temperature, which is critical in determining the crystal size of perovskite. The reaction temperature was varied between -10 degrees C and 50 degrees C, and the optimal temperature was found to be around 20 degrees C in our two-step process. The performance enhancement controlled by the grain size with the increase of reaction temperature could be compensated by the generation of defects for a large crystal perovskite layer device.