Highly Photoconductive CdSe Quantum-Dot Films: Influence of Capping Molecules and Film Preparation Procedure

Highly photoconductive films of CdSe nanocrystals have been prepared by exchanging the original bulky ligands with 1,2-ethanedithiol (EDT) and 1,2-ethanediamine (EDA). Different methods to achieve this exchange, layer-by-layer (LbL) deposition and soaking of drop-casted films, have been compared in detail. Introduction of EDT and EDA by the soaking method results in a broadening of the optical absorption due to disorder in the film. In contrast, the width of the absorption features is unaffected in the LbL films, while the position of the first optical absorption peak is red-shifted by tens of millielectronvolts. The photoluminescence is completely quenched for the LbL films. These findings are characteristic for strong and homogeneous electronic coupling between the quantum dots (QDs) in the LbL films. The photoconductivity of these films was studied with the time-resolved microwave conductivity (TRMC) technique. With this electrodeless technique effects of electrode injection on charge transport are avoided, so that information about the intrinsic mobility of charge carriers is obtained. We find that in simple drop-casted films the conductivity is mainly imaginary and dominated by the polarizability of photogenerated excitons. When the orginal ligands are exchanged by soaking or by the LbL procedure, the conductivity becomes real and dominated by interparticle transport of free charge carriers. It is found that the product of the exciton dissociation yield and the charge carrier mobility is 4 × 10−3 cm2/(V s) in the LbL grown films with EDA capping molecules. This implies that a surprisingly high fraction of free carriers is generated or, alternatively, that the carrier mobility is higher than all previously reported mobility values for layers of CdSe QDs.