Temperature-Dependent Interplay between Structural and Charge Carrier Dynamics in CsMAFA-Based Perovskites

State-of-the-art triple cation, mixed halide perovskites are extensively studied in perovskite solar cells, showing very promising performance and stability. However, an in-depth fundamental understanding of how the phase behavior in Cs0.05FA0.85MA0.10Pb(I0.97Br0.03)3 (CsMAFA) affects the optoelectronic properties is still lacking. The refined unit cell parameters a and c in combination with the thermal expansion coefficients derived from X-ray diffraction patterns reveal that CsMAFA undergoes an alpha-beta phase transition at approximate to 280 K and another transition to the gamma-phase at approximate to 180 K. From the analyses of the electrodeless microwave photoconductivity measurements it is shown that shallow traps only in the gamma-phase negatively affect the charge carrier dynamics. Most importantly, CsMAFA exhibits the lowest amount of microstrain in the beta-phase at around 240 K, corresponding to the lowest amount of trap density, which translates into the longest charge carrier diffusion length for electrons and holes. Below 200 K a considerable increase in deep trap states is found most likely related to the temperature-induced compressive microstrain leading to a huge imbalance in charge carrier diffusion lengths between electrons and holes. This work provides valuable insight into how temperature-dependent changes in structure affect the charge carrier dynamics in FA-rich perovskites. Analysis of temperature-dependent X-ray diffraction measurements on formamidinium-rich perovskites shows phase transitions at around 280 and 180 K with a gradual change from tensile to compressive microstrain. Based on additional photoconductivity measurements, the lowest concentration of traps corresponds to the smallest amount of microstrain in the beta-phase at around 240 K.image