Abstract
How photoexcitations evolve into Coulomb-bound electron and hole pairs, called excitons, and unbound charge carriers is a key cross-cutting issue in photovoltaics and optoelectronics. Until now, the initial quantum dynamics following photoexcitation remains elusive in the hybrid perovskite system. Here we reveal excitonic Rydberg states with distinct formation pathways by observing the multiple resonant, internal quantum transitions using ultrafast terahertz quasi-particle transport. Nonequilibrium emergent states evolve with a complex co-existence of excitons, carriers and phonons, where a delayed buildup of excitons under on- and off-resonant pumping conditions allows us to distinguish between the loss of electronic coherence and hot state cooling processes. The nearly ∼1 ps dephasing time, efficient electron scattering with discrete terahertz phonons and intermediate binding energy of ∼13.5 meV in perovskites are distinct from conventional photovoltaic semiconductors. In addition to providing implications for coherent energy conversion, these are potentially relevant to the development of light-harvesting and electron-transport devices.
Highlights
How photoexcitations evolve into Coulomb-bound electron and hole pairs, called excitons, and unbound charge carriers is a key cross-cutting issue in photovoltaics and optoelectronics
The initial pathways for the photoconversion into exciton populations rely on two dynamic processes mediated by carrier–carrier and –phonon interactions[1,2]: quantum dynamics leading to conversion of electronic coherence into incoherent populations following polarization dephasing, and cooling of hot states via phonon-assisted energy loss and momentum transfer
The previous approaches provide no insight into the exciton populations and dynamics, since they measure linear responses determined by the optical polarization and not the populations1. (See Supplementary Note 1)
Summary
How photoexcitations evolve into Coulomb-bound electron and hole pairs, called excitons, and unbound charge carriers is a key cross-cutting issue in photovoltaics and optoelectronics. We report the direct observation of resonant, internal quantum transitions between excitonic Rydberg states and their distinct formation pathways in a model perovskite CH3NH3PbI3 (or MAPbI3) by measuring the sub-ps resolved, full THz response functions This precisely yields a binding energy 13.5 meV that appears exclusively below the tetragonal-to-orthorhombic structural phase transition at TS 1⁄4 160 K. Nonequilibrium emergent states evolve with a complex co-existence of excitons, carriers and phonons, where a delayed buildup of excitons under various pumping conditions allows us to distinguish between the loss of electronic coherence and hot state cooling processes Such initial coherent and cooling dynamics of excitons further elucidate the nearly B1 ps dephasing time and strong interaction between electron and low-frequency phonons in perovskites that are distinct from conventional semiconductors. In addition to providing implications for coherent energy conversion, these fundamental insights are potentially relevant to the development of light-harvesting and electron-transport devices
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