Exploring the Effects of the Guanidinium:Methylammonium Ratio on the Photophysical Dynamics of ⟨n⟩ = 5 ACI Perovskites

Quasi-two-dimensional (quasi-2D) lead halide perovskites with alternating cations in the interlayer (ACI) space represent a promising type of material for optoelectronics. Similar to the Ruddlesden–Popper and Dion–Jacobson types of perovskites, domains with different thicknesses (n) and bandgaps are formed within a single film. This work focuses on ⟨n⟩ = 5 ACI perovskites based on guanidinium (GA⁺) and methylammonium (MA⁺) cations and investigates the influence of the GA:MA ratio in the interlayer space on the photophysical processes after photoexcitation. Using a combination of time-resolved photoluminescence (TRPL) and femtosecond transient absorption (TA) spectroscopy, hot carrier cooling, the occurrence and directionality of energy or charge transfer between the different domains, and the exciton and charge carrier dynamics are studied and modeled using target analysis. After the thermalization of hot carriers and excitons, exciton transfer from low-n to high-low-n domains occurs within 10 ps, after which they dissociate into free charges. From there, charge transfer into the intermediate-n domains occurs in about 22–54 ps. In the layers with excess GA, this process possibly occurs in an undesirable competition with self-trapped exciton (STE) formation. From the intermediate-n domains, charges are transferred into the high-n domains in 95–159 ps, which process occurs the fastest in the GA-MA layer. Finally, charge carriers decay intrinsically on the nanosecond scale with the longest lifetimes for the GA-MA and GA-2MA systems, which is beneficial for PV applications.