Charge injection and transport in films of CdSe nanocrystals

We have studied charge injection and charge transport in thin disordered films of CdSe nanocrystals between metal electrodes. Current–voltage characteristics of these devices are investigated as a function of electrode material, nanocrystal size, and temperature. We measure the photocurrent response of these structures and find that the photocurrent action spectra follow the quantum-confined absorption spectra of the nanoparticles. For dissimilar top and bottom electrodes, we find that the devices are highly rectifying. High work function materials such as gold and indium-tin oxide are found to be poor electron injectors, consistent with the estimated conduction and valence band levels of the nanocrystals. We observe that the current–voltage characteristics exhibit a history and time dependence which is characteristic of persistent photoconductivity, with current at constant bias decaying with time according to a stretched exponential form. We propose a model based on space-charge limited current dominated by mobile electrons which slowly fill deep traps. Numerical simulations show that the model is able to describe the observed time dependence. We also find that the conductivity is strongly temperature dependent, and is qualitatively consistent with an activated hopping process at temperatures above 180 K. We use the data and simulations to estimate the electron mobilities to be in the range of $∼10−4–10−6$ $cm2 V−1 s−1$ and the trap densities to be approximately $2×1016 cm−3.$