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

Abstract

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 MAPbI3 (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 MAPbI3. Density functional theory calculations predict that deep energy-levels of MAPbI3 exhibit electronically delocalized character, with significant overlap with the localized valence band charge of the spiro-OMeTAD molecules lying on the surface of MAPbI3. Consequently, hot holes are easily extracted from the deep energy-levels of MAPbI3 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.