We present a photocurrent study of CdSe quantum dot films exhibiting unity internal quantum efficiency as a result of post-deposition treatments. While the photocurrent of untreated films is highly voltage dependent at all voltages, the treated films depend strongly on voltage at low voltage, linearly with voltage above a voltage threshold, and finally saturate at high voltage. The voltage dependence of the treated films can be reproduced with a model assuming blocking contacts and a field dependent exciton ionization efficiency that saturates to unity. The increase in exciton ionization efficiency is a result of increased surface passivation and decreased QD spacing.

Synthesis of monodisperse samples of CdSe nanorods with CdTe tips is achieved using the mechanism of rod nucleated growth to form CdSe/CdTe nanobarbells. This synthesis produces a nanocrystal displaying "type-II" behavior with a morphology that is particularly well suited for internal exciton separation and carrier transport.

We report electrical transport measurements of arrays of PbSe nanocrystals forming the channels of field effect transistors. We measure the current in these devices as a function of source-drain voltage, gate voltage and temperature. Annealing is necessary to observe measurable current after which a simple model of hopping between intrinsic localized states describes the transport properties of the nanocrystal solid. We find that the majority carriers are holes, which are thermally released from acceptor states. At low source-drain voltages, the activation energy for the conductivity is given by the energy required to generate holes plus the activation over barriers resulting from site disorder. At high source-drain voltages the activation energy is given by the former only. The thermal activation energy of the zero-bias conductance indicates that the Fermi energy is close to the highest-occupied valence level, the 1S h state, and this is confirmed by field-effect measurements, which give a density of states of approximately eight per nanocrystal as expected from the degeneracy of the 1S h state.PACS numbers:Colloidal semiconductor nanocrystals can be made to self assemble into a close-packed array, creating a novel material known as a nanocrystal (NC) solid. 1,2,3,4,5,6 Electrons in an NC solid made from semiconductor NCs, in contrast to metallic ones, have long-range Coulomb interactions; 7 therefore, the motion of electrons is expected to be highly correlated as long as the number of electrons per NC is not too small. The ability to tune both the energy levels of the individual NCs and the electronic coupling between NCs makes these solids a promising test bed for investigating many-body physics. It has been demonstrated that the charge density in semiconductor NC solids can be modulated, 8 which makes the system suitable for observing the predicted characteristics of electronic correlations as a function of charge density. 9We study PbSe NC solids because they have higher conductances than the well-studied solids composed of II-VI NCs. 10,11,12,13,14,15 PbSe NCs display a narrower dispersion in electronic energy levels than the II-VI NCs as well as a higher degeneracy, 16 which increases the density of states available for conduction. 8,17 It is possible that charge carriers in PbSe NC solids, which are holes, are generated by thermal excitation of electrons into midgap states that arise from dangling bonds on the surface. Bulk PbSe has midgap states closer to the band edge than bulk CdSe, and thus we expect a higher density of charge carriers in PbSe NCs. Furthermore, PbSe NCs have attracted much attention because of their interband transitions in the IR and multiple exciton generation, 18 and hence potential for application in novel optoelectronic devices. 19,20,21 Such applications involve electronic transport through NC solids, thus necessitating a fundamental understanding of the conduction properties.Experimental studies of charge transport in PbSe NC solids have revealed varied behavior.It has been rep...

We present a study of photoconductivity in close-packed films of CdSe/ZnS core/shell nanocrystals, from which the majority of the organic ligands are removed. The ZnS inorganic shells separate the CdSe cores, and passivate the surface of the CdSe nanocrystals, slowing the nonradiative exciton decay rate. These films retain the size-dependent quantum-confined electronic properties of the nanocrystals, including the tunability of their band gap. We demonstrate that replacing the organic ligands, which form the tunnel barrier between nanocrystals in films of CdSe nanocrystals, with the inorganic ZnS shell results in photoconduction with near unity internal quantum efficiency at room temperature.

We report the influence of trap states on charge transport through films of mixed CdTe and CdSe nanocrystals ͑NCs͒ between lateral electrodes, through layered films of CdTe and CdSe NCs in a layered geometry, and through films of CdTe/CdSe nanobarbells in a layered geometry. We find that an electron trapping state on the surface of the CdTe NCs dominates the conduction in all devices studied. X-ray photoelectron spectroscopy and thermal activation studies implicate unpassivated or oxidized Te as the electron-trapping site.

This paper describes the electrical characteristics of junctions composed of three-dimensional arrays of colloidal CdSe quantum dots (QDs) with tin-doped indium oxide (ITO)/poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) and eutectic Ga-In (EGaIn) electrodes. It focuses on a comparison of junctions containing QDs of one size to those of arrays containing QDs of multiple sizes. This comparison makes it possible to estimate the relative contributions of transport across various interfaces (e.g., between the QDs and between the QDs and the electrodes) to the observed electrical characteristics of the junction and to evaluate the dependence of these contributions on the locations of various sizes of QDs within the junction. The junctions were diodes, and their turn-on voltage depended on the size of the QDs next to the PEDOT:PSS. We describe this dependence using a Marcus model to estimate the barrier for charge transfer induced by the difference in energies between the orbitals of the QDs and the valence band of PEDOT:PSS.

This paper describes a study of the generation and flow of photocurrent through junctions containing three-dimensional arrays of colloidal CdSe quantum dots (QDs) of either a single size or multiple sizes. The electrodes were indium tin oxide (ITO) covered with a thin layer of poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) and a eutectic alloy of Ga and In (EGaIn). We measured the current-voltage characteristics of the junctions in the dark and under illumination, with various sources and wavelengths of excitation, and their photocurrent action spectra. Size-selective photoexcitation of the arrays of multiple sizes of QDs helped to determine (i) the location of the interface at which photoinduced separation of charge occurred, (ii) whether the energy absorbed by the QDs was redistributed before separation of charge, and (iii) the dependence of the photovoltage on the locations of various sizes of QDs within the junction. This research is a step toward the use of QDs for harvesting light and for transporting energy and charge in devices-for example, solar cells and photodetectors-that operate at zero bias.

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