Over the last decade metal halide perovskites have attracted a tremendous amount of attention owing to their favorable intrinsic optoelectronic properties, such as high absorption coefficients, suitable carrier mobilities, remarkable defect tolerance, as well as ease of fabrication. The rapid progress in the development of single-junction perovskite solar cells achieved a certified power conversion efficiency (PCE) of 26.1%. These high PCEs are generally found in FA-rich MHPs combined with a small amount of Cs and MA. Although these state-of-the-art triple cation, mixed halide perovskites have been extensively studied, an in-depth fundamental understanding of how the phase behavior in Cs_0.05FA_0.85MA_0.10Pb(I_0.97Br_0.03)₃ (CsMAFA) affects the optoelectronic properties is still lacking. Here temperature-dependent XRD, photoluminescence combined with electrodeless microwave photoconductivity measurements (TRMC) were carried out. The refined unit cell parameters a and c in combination with the thermal expansion coefficients derived from XRD patterns reveal that CsMAFA undergoes an alfa-beta phase transition at ~ 280 K and another transition to the gamma-phase at ~ 180 K. From the analyses of the TRMC measurements we show 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 we find a considerable increase in deep trap states most likely related to the temperature-induced compressive microstrain leading to a huge imbalance in charge carrier diffusion lengths between electrons and holes.

To increase the open-circuit voltage in solar cells based, we investigated the charge carrier dynamics in bi- and tri-layers using TRMC. From the results of the bilayers, we find almost balanced mobilities for electrons and holes in CsMAFA, and carrier extraction is nearly quantitative. For the n-i-p and p-i-n triple layers, both carriers are extracted at low laser intensities independent of the configuration, which is based on the small, rapidly decaying TRMC signal. An important remaining open question in such systems is the fate of light induced carriers after collection by the TL. Here we demonstrate long-lived charge separation over the n-i-p and p-i-n structures from the fact that on applying bias illumination (BI) the photoconductance signal is much higher than without BI. In the former case charge extraction is retarded, which is attributed to the electric field that is build up from the extracted electrons and holes oppressing further charge collection. Finally, we want to see how charge collection and recombination at the interface affects the quasi Fermi level splitting (QFLS) which is a measure of the possible attainable open circuit voltage. For those bilayers showing short-lived charge separation such as in CsMAFA/C60, implying electron collection is followed by rapid interfacial recombination, the corresponding QFLS is reduced with respect to the pristine CsMAFA layer. Most importantly for the other bilayer combinations including in C60/CsMAFA long-lived charge separation is observed which translates in an increase of the QFLS with respect to the neat CsMAFA layer.