Photoconductivity in CdSe quantum dot solids

We report measurements of photoconductivity and electric field induced phot oluminescence quenching in three-dimensional close-packed solids of colloid al CdSe quantum dots. Our measurements suggest that photoexcited, quantum c onfined excitons are ionized by the applied electric field with a rate that depends on both the size and surface passivation of the quantum dots. Sepa ration of electron-hole pairs confined to the core of the quantum dot requi res significantly more energy than separation of carriers trapped at the su rface and occurs through tunneling processes. We present a simple resonant tunneling model for the initial charge separation step that qualitatively r eproduces 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 recombi nation pathways increasingly compete with charge separation.