Solid films of colloidal quantum dots show promise in the manufacture of photodetectors and solar cells. These devices require high yields of photogenerated charges and high carrier mobilities, which are difficult to achieve in quantum-dot films owing to a strong electron-hole interaction and quantum confinement. Here, we show that the quantum yield of photogenerated charges in strongly coupled PbSe quantum-dot films is unity over a large temperature range. At high photoexcitation density, a transition takes place from hopping between localized states to band-like transport. These strongly coupled quantum-dot films have electrical properties that approach those of crystalline bulk semiconductors, while retaining the size tunability and cheap processing properties of colloidal quantum dots.

The assembly of quantum dots is an essential step toward many of their potential applications. To form conductive solids from colloidal quantum dots, ligand exchange is required. Here we study the influence of ligand replacement on the photoconductivity of PbSe quantum-dot solids, using the time-resolved microwave conductivity technique. Bifunctional replacing ligands with amine, thiol, or carboxylic acid anchor groups of various lengths are used to assemble quantum solids via a layer-by-layer dip-coating method. We find that when the ligand lengths are the same, the charge carrier mobility is higher in quantum-dot solids with amine ligands, while in quantum-dot solids with thiol ligands the charge carrier lifetime is longer. If the anchor group is the same, the charge carrier mobility is ligand length dependent. The results show that the diffusion length of charge carriers can reach several hundred nanometers.

PbSe quantum-dot solids are of great interest for low cost and efficient photodetectors and solar cells. We have prepared PbSe quantum-dot solids with high charge carrier mobilities using layer-by-layer dip-coating with 1,2-ethanediamine as substitute capping ligands. Here we present a time and energy resolved transient absorption spectroscopy study on the kinetics of photogenerated charge carriers, focusing on 0-5 ps after photoexcitation. We compare the observed carrier kinetics to those for quantum dots in dispersion and show that the intraband carrier cooling is significantly faster in quantum-dot solids. In addition we find that carriers diffuse from higher to lower energy sites in the quantum-dot solid within several picoseconds.

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 cm 2 /(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.

Thermal annealing of thin films of CdSe/CdS core/shell quantum dots induces superordering of the nanocrystals and a significant reduction of the interparticle spacing. This results in a drastic enhancement of the quantum yield for charge carrier photogeneration and the charge carrier mobility. The mobile electrons have a mobility as high as 0.1 cm(2)/(V x s), which represents an increase of 4 orders of magnitude over non-annealed QD films and exceeds existing literature data on the electron mobility in CdSe quantum dot films. The lifetime of mobile electrons is longer than that of the exciton. A fraction of the mobile electrons gets trapped at levels below the conduction band of the CdSe nanocrystals. These electrons slowly diffuse over 50-300 nm on longer times up to 20 micros and undergo transfer to a TiO2 substrate. The yield for electron injection in TiO2 from both mobile and trapped electrons is found to be >16%.