Femtosecond Charge‐Injection Dynamics at Hybrid Perovskite Interfaces

With power conversion efficiency (PCE) exceeding 22%, perovskite solar cells (PSCs) have thrilled the photovoltaic research. However, interface behavior is still not understood and a hot subject of research: different processes occur over a hierarchy of timescales, from femtoseconds to seconds, making the perovskite interface physics intriguing. Here, by femtosecond transient absorption spectroscopy with spectral coverage extended to the crucial IR region, we interrogate the ultrafast interface- specific processes at standard as well as newly molecularly-engineered perovskite interfaces used in state of the art PSCs. We demonstrate that ultrafast interfacial charge injection happens with a time constant of 100 fs, resulting in hot transfer from energetic charges and setting the time-scale for the first step involved in the complex charge transfer process. This also stems true for 20% efficient devices measured under real operation, where the femtosecond injection is followed by a slower picosecond component. Our findings provide compelling evidence of the femtosecond interfacial charge injection step demonstrating a robust method for a straightforward identification of the interfacial non-equilibrium processes at ultrafast timescale.