Photoconductivity in CdSe quantum dot solids

We report measurements of photoconductivity and electric field induced photoluminescence quenching in three-dimensional close-packed solids of colloidal CdSe quantum dots. Our measurements suggest that photoexcited, quantum confined excitons are ionized by the applied electric field with a rate that depends on both the size and surface passivation of the quantum dots. Separation of electron-hole pairs confined to the core of the quantum dot requires significantly more energy than separation of carriers trapped at the surface and occurs through tunneling processes. We present a simple resonant tunneling model for the initial charge separation step that qualitatively reproduces both the size and surface dependence of the photoconductivity as a function of applied field. We show that the charge generation efficiency increases with increasing temperature as nonradiative and radiative recombination pathways increasingly compete with charge separation.