An Ultrafast Investigation of the Surface and Bulk Passivation Effects of Phenylethylammonium in CH ₃ NH ₃ PbI ₃ Thin Films

The incorporation of Ruddlesden–Popper (RP)/3D perovskite heterostructures into photovoltaic cells has been shown to increase both the efficiency and stability of the devices. Here, a series of methylammonium lead triiodide (MAPbI 3 ) thin films treated with varying 2‐phenylethylammonium (PEA) concentrations are investigated with static and ultrafast spectroscopic techniques to reveal the mechanisms of the observed performance benefits. Transient absorption spectroscopy is employed to elucidate the effect of a surface RP layer on the excited state of the MAPbI 3 films and reveal that several different RP structures are formed and participate in the charge‐carrier dynamics. The passivation effects of PEA are investigated with optical pump–terahertz probe (OPTP) experiments using a variety of excitation conditions to simultaneously probe the surface and bulk recombination dynamics. Fitting models to the OPTP data for each excitation scheme allows the material parameters that govern the ultrafast dynamics to be quantified. It is found that as the PEA concentration increases, the surface recombination velocity exhibits a monotonic decrease, suggesting the RP layer is effective at passivating surface traps. Furthermore, the bulk monomolecular recombination rate is also found to decrease with the addition of PEA, indicating that the benefits of this passivation approach are not limited to the upper surface of the MAPbI 3 films.