Ultrafast time-resolved mid-IR spectroscopy is one of the most powerful techniques that allow us to understand the evolution of structural changes in real time by probing the vibrational marker modes of the molecules in the excited states. For instance, mid-IR spectroscopy has been used to fully study the excited state dynamics in metal complexes, dyes-sensitized solar cells, and many electron donor-acceptor systems. The hybrid organic-inorganic perovskite is one of the most promising materials for light conversion applications including solar cells, however, the mechanism of carrier transfer at the molecular level at their interface with the transporting layers is still unclear.

Here, we present the study of the injection dynamics of hot and cooled carriers at the interface between organic-inorganic hybrid perovskite FAPbBr₃ and organic electron acceptor IEICO-4F using femtosecond mid-IR spectroscopy, by following the ν(C≡N) and ν(C=O) vibrational modes of IEICO-4F, and ν(C=N) vibrational mode of FAPbBr₃ after selective excitation of perovskite. This material system has been selected due to the well-isolated vibrational modes of ν(C≡N) and ν(C=O) as well as suitable energy level alignment of the donor and acceptor units, and investigated by using femtosecond (fs) mid-infrared (IR), transient absorption (TA) and fluorescence spectroscopy. The visible fs-TA measurements reveal the generation of hot carriers and their transition to free carriers in the pure FAPbBr₃ film. Meanwhile, the fs-mid-IR unfolds the efficient hot carrier injection with a transfer time of 200 fs from FAPbBr₃ to IEICO-4F. The time-resolved mid-IR experiments demonstrated the ultrafast formation of two positive peaks at 2115 cm^-1 and 2233 cm^-1 which could be attributed to C≡N symmetrical and asymmetrical vibrational modes of IEICO-4F in the anion form, providing crystal clear evidence for the electron transfer process between the donor and acceptor units. Moreover, the PL lifetime measurements reveal an approximately tenfold decrease in the donor lifetime confirming the efficient electron injection. In addition, the density functional theory (DFT) calculations for the FAPbBr₃/IEICO-4F interface support the efficient electron injection which affects the C≡N bond character. This work offers new insights into the electron injection process at the FAPbBr₃/IEICO-4F interface, which is important for developing efficient optoelectronic devices.