We present a general synthesis for a family of n-type transparent conducting oxide nanocrystals through doping with aliovalent cations. These monodisperse nanocrystals exhibit localized surface plasmon resonances tunable in the mid-and near-infrared with increasing dopant concentration. We employ a battery of electrical measurements to demonstrate that the plasmonic resonance in isolated particles is consistent with the electronic properties of oxide nanocrystal thin films. Hall and Seebeck measurements show that the particles form degenerately doped ntype solids with free electron concentrations in the range of 10 19 to 10 21 cm −3 . These heavily doped oxide nanocrystals are used as the building blocks of conductive, n-type thin films with high visible light transparency. ■ INTRODUCTIONWidespread adoption of liquid-crystal and light-emitting diode displays has encouraged an extensive search both for new transparent conducting materials and new methods for fabricating transparent electrodes. A small number of doped oxides dominate the commercial market for transparent conducting electrodes, led by indium tin oxide (ITO). 1 Despite its utility, the rarity of indium makes ITO potentially expensive, and the most common deposition methods for ITO remain energy-intensive. Many replacement materials have been suggested, including patterned metals, 2 metal nanowires, 3 carbon nanotubes, 4,5 graphene, 6 other oxides, 7,8 and conductive polymers. 9 At the same time, solution-processing methods, in contrast to evaporation or sputtering, are increasingly used to fabricate transparent conducting thin films. 4,5,10−13 Separately, the set of nanocrystalline materials exhibiting localized surface plasmon resonances (LSPRs) has substantially diversified from metallic nanoparticles to include heavily doped chalcogenides, 14−18 phosphides, 19 nitrides, 20 oxides, 21−25 and silicon nanostructures. 26,27 LSPRs arise from the collective oscillation of the free carriers of an individual particle, with the frequency of the plasmon resonance related to several properties of the material, especially the carrier concentration. Transparent conducting oxide nanocrystals (NCs) contribute to both research efforts: wide band gap, high carrier density (>10 18 cm −3 ) oxides show near-infrared (NIR) LSPRs, and they can be deposited into conductive thin films with high visible light transparency using solution-casting methods.In this paper, we explore the controlled synthesis and the optical, structural, and electrical characterization of a family of n-type oxide NCs and their solution-cast thin films. Subtle differences in the kinetics of precursor decomposition make the direct synthesis of controllably doped colloidal nanocrystals a continuing challenge. 28 Nonaqueous, high-temperature syntheses of doped oxide NCs have been reported for a number of oxides using isovalent dopants, 29 aliovalent dopants, 21,22,24 interstitial dopants, 30 and vacancy-doping of the NC stoichiometry. 23 Particularly successful efforts to make highly uniform ternary...
Passivating surface defects and controlling the carrier concentration and mobility in quantum dot (QD) thin films is prerequisite to designing electronic and optoelectronic devices. We investigate the effect of introducing indium in CdSe QD thin films on the dark mobility and the photogenerated carrier mobility and lifetime using field-effect transistor (FET) and time-resolved microwave conductivity (TRMC) measurements. We evaporate indium films ranging from 1 to 11 nm in thickness on top of approximately 40 nm thick thiocyanate-capped CdSe QD thin films and anneal the QD films at 300 °C to densify and drive diffusion of indium through the films. As the amount of indium increases, the FET and TRMC mobilities and the TRMC lifetime increase. The increase in mobility and lifetime is consistent with increased indium passivating midgap and band-tail trap states and doping the films, shifting the Fermi energy closer to and into the conduction band.
Advanced architectures are required to further improve the performance of colloidal PbS heterojunction quantum dot solar cells. Here, we introduce a CdI-treated CdSe quantum dot buffer layer at the junction between ZnO nanoparticles and PbS quantum dots in the solar cells. We exploit the surface- and size-tunable electronic properties of the CdSe quantum dots to optimize its carrier concentration and energy band alignment in the heterojunction. We combine optical, electrical, and analytical measurements to show that the CdSe quantum dot buffer layer suppresses interface recombination and contributes additional photogenerated carriers, increasing the open-circuit voltage and short-circuit current of PbS quantum dot solar cells, leading to a 25% increase in solar power conversion efficiency.
In this work, we report the effects on CdSe nanocrystal (NC) surface chemistry of acetone and methanol when used as the antisolvents for NC washing and as the solvents for ligand exchange of NC solids with ammonium thiocyanate (NH 4 SCN). We find that NCs washed with methanol have significantly fewer remaining organic ligands and lower photoluminescence quantum yield than those washed with acetone. When used as the ligand exchange solvent, methanol leaves more organic ligands and introduces fewer bound thiocyanates on the NC surface than when acetone is used. We demonstrate the effect of these different surface chemistries on NC solid optoelectronic properties through photoconductivity measurements, showing a greater photocurrent in NC solids with greater organic ligand coverage. We also show that NC washing with methanol or ligand exchange with NH 4 SCN in methanol removes a significant number of surface Cd atoms from the NCs, creating Cd vacancies that act as traps for recombination. Independent of the wash and exchange process, the NC surface may be repaired by introducing CdCl 2 to the NC surface, enhancing the measured photocurrent.
thickness have the potential to be tuned by varying the solvent, NC concentration, substrate surface treatment, and withdrawal speed. Dip-coating has been utilized previously for deposition of high-quality NC-based electronics, [26][27][28][29][30] but the work to date that explores dip-coating as a method for control over NC superlattice assembly is limited. [ 31,32 ] Here, we demonstrate that dipcoating can be used to deposit ordered, polycrystalline superlattices of a variety of NC materials over 4 in (100 mm) wafer scales. We show that fi lm thickness can be controlled by varying the withdrawal speed to deposit fi lms from sub-monolayer to several multilayers. The combination of transmission electron microscopy (TEM), scanning electron microscopy (SEM), and grazing incidence small-angle X-ray scattering (GISAXS) shows that the ordered superlattice structure observed at the nanoscale by microscopy extends over the entire dip-coated area. Finally, we present the fi rst demonstration that NC assembly via dipcoating can be extended to form several binary NC superlattices (BNSLs) including AlB 2 , NaZn 13 , CaCu 5 , and MgZn 2 structures, exhibiting the versatility of dip-coating for depositing a variety of NC materials and superlattice structures.Lead selenide (PbSe) [ 33,34 ] was chosen as the pilot material for optimizing the dip-coating conditions because of its particularly narrow size distribution. Different dispersing solvents and substrate treatments commonly used in NC deposition were explored (see Section S1.1 and S1.2 of the Supporting Information). We focused primarily on assemblies deposited from toluene dispersions, which were found to favorably wet n-octyltriethoxysilane (OTS)-treated surfaces, facilitating longrange, planar order. Additionally, we studied how dip-coating withdrawal speed affects the fi lm thickness and coverage on the substrate. Figure 1 shows a series of PbSe NC thin fi lms deposited by dip-coating from a NC solution in toluene onto OTS-treated substrates at withdrawal rates of 1, 5, 15, 25, 50, and 100 mm min -1 . The 100 mm min -1 withdrawal rate is rapid in comparison to typical dip-coating processes. The fi lm color shows the optical uniformity and the color change refl ects thickness-dependent interference as the fi lms become thicker at increasing withdrawal speeds (Figure 1 a), which is quantifi ed by a shift in the energy of the fi lm refl ectance spectrum (Figure 1 b). The proportional relationship of withdrawal speed to fi lm thickness is likely due to higher withdrawal speeds wicking up a thicker solvent fi lm along the substrate surface. The slowest withdrawal speed of 1 mm min -1 , is an exception to this trend and results in a fi lm thicker than that formed at 5 mm min -1 . At the slowest withdrawal rates, the solvent evaporates across the substrate faster than the withdrawal speed as illustrated in Figure 1 c. Atomic force microscopy (AFM) ( Figure S1, Supporting Information) measurements were performed to analyze Monodisperse nanocrystals (NCs) provide an opportuni...
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