How photoexcitations evolve in time into Coulomb-bound electron and hole pairs, called excitons, and unbound charge carriers is a key cross-cutting issue in photovoltaic and optoelectronic technologies. Until now, the initial quantum dynamics following photoexcitation remains elusive in the organometal halide 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, unbound 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 terahertz transport with rather long dephasing time and scattering processes due to discrete terahertz phonons in perovskites are distinct from conventional photovoltaic materials. In addition to providing implications for ultrafast coherent transport, these results break ground for a perovskite-based device paradigm for terahertz and coherent optoelectronics.
We reported for the first time that key nanocrystal quantum dot (NQD) optical properties - quantum yield, photobleaching and blinking - can be rendered independent of NQD surface chemistry and environment by growth of a very thick, defect-free inorganic shell (Chen, et al. J. Am. Chem. Soc. 2008). Here, we show the precise shell-thickness
dependence of these effects. We demonstrate that 'giant-shell' NQDs can be largely non-blinking for observation times as long as 54 minutes and that on-time fractions are independent of experimental time-resolution from 1-200 ms. These effects are primarily demonstrated on (CdSe)CdS (core)shell NQDs, but we also show that alloyed shells comprising CdxZn1-xS and terminated with a non-cytotoxic ZnS layer exhibit similar properties. The mechanism for suppressed
blinking and dramatically enhanced stability is attributed to both effective isolation of the NQD core excitonic
wavefunction from the NQD surface, as well as a quasi-Type II electronic structure. The unusual electronic structure
provides for effective spatial separation of the electron and hole into the shell and core, respectively, and, thereby, for
reduced efficiencies in non-radiative Auger recombination.
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