Why are Hot Holes Easier to Extract than Hot Electrons from Methylammonium Lead Iodide Perovskite?

Charge-carriers photoexcited above a semiconductor's bandgap rapidly thermalize to the band-edge. The cooling of these difficult to collect "hot" carriers caps the available photon energy that solar cells-including efficient perovskite solar cells-may utilize. Here, the dynamics and efficiency of hot carrier extraction from MAPbI(3) (MA = methylammonium) perovskite by spiro-OMeTAD (a hole-transporting layer) and TiO2 (an electron-transporting layer) are investigated and explained using both ultrafast electronic spectroscopy and theoretical modeling. Time-resolved spectroscopy reveals a quasi-equilibrium distribution of hot carriers forming upon excess-energy excitation of the perovskite-a distribution largely unaffected by the presence of TiO2. In contrast, the quasi-equilibrium distribution of hot carriers is virtually nonexistent when spiro-OMeTAD is present, which is indicative of efficient hot hole extraction at the interface of MAPbI(3). Density functional theory calculations predict that deep energy-levels of MAPbI(3) exhibit electronically delocalized character, with significant overlap with the localized valence band charge of the spiro-OMeTAD molecules lying on the surface of MAPbI(3). Consequently, hot holes are easily extracted from the deep energy-levels of MAPbI(3) by spiro-OMeTAD. These findings uncover the origins of efficient hot hole extraction in perovskites and offer a practical blueprint for optimizing solar cell interlayers to enable hot carrier utilization.