The power conversion efficiency of perovskite-based optoelectronics seen a remarkable surge, largely driven by sophisticated surface treatment strategies (interface engineering approaches). These approaches can also extend the operational lifetime of perovskite-based optoelectronic devices. Among these, the use of bulky organic cations, such as phenethylammonium (PEA+), has become a cornerstone for enhancing environmental stability by forming thin passivation layers. However, these organic spacers can inherently act as tunneling barriers, necessitating equal attention to the kinetics of carrier transfer across the passivated interface toward the charge-transport layers (CTL). Furthermore, the inherent excitonic nature of the 2D system formed between the perovskite and CTL is often overlooked. As these materials are also in their excited state during light irradiation, it is imperative to understand the delicate photophysical processes at the 2D perovskite/CTL interfaces. 

In this work, we aimed to reveal the correlation between the electron transport layer (ETL) thickness and the electron extraction rate at the PEA-modified interfaces. As a model system, we employed C60 fullerene layers of controlled thicknesses (4 and 8 monolayers) thermally evaporated onto PEA2PbI4 surfaces. To monitor exciton dynamics on the ultrafast timescale, pump-probe transient absorption spectroscopy (TAS) was utilized, tracking the evolution of excited-state populations from the sub-picosecond regime to several nanoseconds. A systematic investigation of the excitation density dependence was performed for each configuration. Our results show that below a certain pump energy threshold, no significant difference in the decay kinetics is observed between the various samples, indicating a regime dominated by intrinsic relaxation pathways present in PEA2PbI4. However, above a certain threshold, we observed a clear dependence of the charge-transfer rate on the C60 overlayer thickness. 4 monolayers of C60 did not yield a significant acceleration of excited-state recovery. A distinct transition was observed in the 8 monolayer configuration, which exhibited a pronounced acceleration that can be attributed to charge-transfer. 

This behavior suggests that a critical C60 thickness is required to establish sufficient electronic coupling of the density of states to overcome the barrier imposed by the PEA spacer. These findings provide important design principles for optimizing CTL/passivator architectures, ensuring that stability gains do not compromise charge-extraction efficiency.