Charge injection and transport in films of CdSe nanocrystals

We have studied charge injection and charge transport in thin disordered fi lms of CdSe nanocrystals between metal electrodes. Current-voltage characte ristics of these devices are investigated as a function of electrode materi al, nanocrystal size, and temperature. We measure the photocurrent response of these structures and find that the photocurrent action spectra follow t he 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 t o be poor electron injectors, consistent with the estimated conduction and valence band levels of the nanocrystals. We observe that the current-voltag e characteristics exhibit a history and time dependence which is characteri stic of persistent photoconductivity, with current at constant bias decayin g with time according to a stretched exponential form. We propose a model b ased on space-charge limited current dominated by mobile electrons which sl owly fill deep traps. Numerical simulations show that the model is able to describe the observed time dependence. We also find that the conductivity i s strongly temperature dependent, and is qualitatively consistent with an a ctivated hopping process at temperatures above 180 K. We use the data and s imulations to estimate the electron mobilities to be in the range of simila r to 10(-4)-10(-6) cm(2) V-1 s(-1) and the trap densities to be approximate ly 2 x 10(16) cm(-3). (C) 2000 American Institute of Physics. [S0021-8979(0 0)01703-5].