Spectroscopic signatures of photocharging due to hot-carrier transfer in solutions of semiconductor nanocrystals under low-intensity ultraviolet excitation

We show that excitation of solutions of well-passivated PbSe semiconductor nanocrystals (NCs) with ultraviolet (3.1 eV) photons can produce long-lived charge-separated states in which the NC core is left with a nonzero net charge. Since this process is not observed for lower-energy (1.5 eV) excitation, we ascribe it to hot-carrier transfer to some trap site outside the NC. Photocharging leads to bleaching of steady-state absorption, partial quenching of emission, and additional fast time scales in carrier dynamics due to Auger decay of charged single- and multiexciton states. The degree of photocharging, f, saturates at a level that varies from 5 to 15% depending on the sample. The buildup of the population of charged NCs is extremely slow indicating very long, tens of seconds, lifetimes of these charge-separated states. Based on these time scales and the measured onset of saturation of f at excitation rates around 0.05-1 photon per NC per ms, we determine that the probability of charging following a photon absorption event is of the order of 10^-4 to 10^-3. The results of these studies have important implications for the understanding of photophysical properties of NCs, especially in the case of time-resolved measurements of carrier multiplication.