Controlling the energy-loss pathways in perovskite nanocrystal based solar cells: an ultrafast spectroscopic...

The unique optoelectronic properties of metal halide perovskites (MHPs) have led to an unprecedented success in perovskite solar cells (PSCs) with efficiency exceeding 25% within a short span of time. The PSCs, like other photovoltaic technologies, however, suffer from efficiency loss exceeding 50% primarily due to high-energy carrier thermalization and in-band transparency for the sub-band-gap low-energy photons. Addressing these energy loss pathways in MHPs is key to enhancing the light conversion efficiency of PSCs beyond their theoretical limiting values. Ultrafast optical spectroscopy is a powerful tool in this context for characterizing these loss mechanisms, understanding the factors that influence ultrafast dynamics of the carriers and exploring the means of harnessing excess energy by generating multiple carriers, a process particularly relevant for nanocrystal-based solar cells. Despite the critical role of hot-carrier and multi-exciton harvesting in improving the performance of the operational devices, research in this area has largely been limited to optical studies on a few model hybrid material systems. Moreover, in-band losses in PSCs, which is also a major contributing factor to efficiency loss, can potentially be mitigated by developing effective strategies for up-converting the sub-band-gap photons. This review highlights recent advancement in optical studies of these phenomena in MHP nanocrystals (NCs) and proposes a roadmap for their practical implementation in PSCs.