Optoelectronic properties of quantum dot (QD) films are limited by (1) poor interfacial chemistry and (2) non-radiative recombination due to surface traps. To address these performance issues, sol-gel methods are applied to fabricate thin films of CdSe and core(shell) CdSe(ZnS) QDs. Highangle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging with chemical analysis confirms that the surface of the QDs in the sol-gel thin films are chalcogen-rich, consistent with an oxidative-induced gelation mechanism in which connectivity is achieved by formation of dichalcogenide covalent linkages between particles. The ligand removal and assembly process is probed by thermogravimetric, spectroscopic and microscopic studies. Further enhancement of inter-particle coupling via mild thermal annealing, which removes residual ligands and reinforces QD connectivity, results in QD sol-gel thin films with superior charge transport properties, as shown by a dramatic enhancement of electrochemical photocurrent under white light illumination relative to thin films composed of ligand-capped QDs. A more than 2-fold enhancement in photocurrent, and a further increase in photovoltage can be achieved by passivation of surface defects via overcoating with a thin ZnS shell. The ability to tune interfacial and surface characteristics for the optimization of photophysical properties suggests that the solgel approach may enable formation of QD thin films suitable for a range of optoelectronic applications. KeywordsQuantum dots; Sol-gel methods; Photocurrent; surface passivation; ligand exchange Thin films of semiconductor quantum dots (QDs) are promising materials for electronic and optoelectronic device applications including field-effect transistors (FETs), 1,2 photodetectors, 3,4 light emitting diodes (LEDs) 5,6 and solar cells. 7,8 The size-and shapetunable optical and electronic properties of QDs, along with their solution processibility, permit great flexibility in device design, enabling the use of low cost fabrication methods such as solution coating 9,10 or printing. 11 The charge transport properties of QD thin films play a major role in device performance and depend on the extent of electronic coupling sbrock@chem.wayne.edu, TEL: (313) FAX: (313) 12 Unfortunately, bulky organic ligands used in the synthesis of QDs reduce the inter-particle coupling, and consequently have a destructive effect on the charge transport properties of QD films.Reduction of the inter-QD spacing is one of the main strategies being investigated to improve the electronic communication between QDs in thin films. This can be done by removing the bulky organic surfactants by thermal annealing [13][14] or by exchanging them with smaller ligands, either in the solution phase before depositing the QDss as thin films (nitrosonium tetrafluoroborate, 15 molecular metal chalcogenides, 16-18 chalcogenide anions, 19-20 thiocyanate 21 ) or in the solid phase after deposition (hydrazine, 22,13 NaOH, 23-24 thiols, 10, 25-27 amines,...

Nanocrystal (NC) Cu 2 ZnSnS 4 (CZTS) solar cells, composed of a nontoxic and earth abundant absorber material, have great potential in low-cost solar energy harvesting. However, CZTS NC films typically must be thermally annealed at elevated temperatures and under harsh environments to produce high-efficiency devices. The efficiencies of unannealed CZTS NC solar cells have been hampered by low open circuit potentials (V oc , <325 mV) and low short circuit current densities (J sc , <2 mA), primarily because of the incomplete passivation of the crystal surface. Although great progress has been made in understanding the surface chemistry of II−VI and IV−VI semiconductor NCs, the surface chemistry of complex quaternary CZTS NCs is largely unexplored. Here, for the first time, we report a comprehensive study of the surface chemistry of CZTS NCs focusing on depositing ligand-passivated, uniform NC thin films to address the issue of large V oc deficit and low current collection efficiency typically observed for CZTS NC solar cells. The ligand exchange reactions were rationally designed to target each metal ion on the surface [using both organic L-type ligands such as ethylenediamine and inorganic X-type ligands (I − and S 2− )] and to passivate anionic chalcogen sites with inorganic Z-type ligands (ZnCl 2 ). Herein, we show that CZTS/CdS heterojunction NC solar cells made of uniformly passivated CZTS NCs demonstrate a >180 mV improvement in V oc . Furthermore, the influence of device configuration on the collection efficiency of photogenerated carriers in the CZTS NC absorber layer is presented, and the implications of both surface and internal defects in CZTS NCs for photovoltaic performance are discussed.

Transparent CdSe(ZnS) sol-gel materials have potential uses in optoelectronic applications such as light emitting diodes (LEDs) due to their strong luminescence properties and the potential for charge transport through the prewired nanocrystal (NC) network of the gel. However, typical syntheses of metal chalcogenide gels yield materials with poor transparency. In this work, the mechanism and kinetics of aggregation of two sizes of CdSe(ZnS) core(shell) NCs, initiated by removal of surface thiolate ligands using tetranitromethane (TNM) as an oxidant, were studied by means of time-resolved dynamic light scattering (TRDLS); the characteristics of the resultant gels were probed by optical absorption, transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS). At low concentrations of NCs (ca. 4 × 10−7 M), the smaller, green-emitting NCs aggregate faster than the larger, orange-emitting NCs, for a specific oxidant concentration. The kinetics of aggregation have a significant impact on the macroscopic properties (i.e. transparency) of the resultant gels, with the transparency of the gels decreasing with the increase of oxidant concentration due the formation of larger clusters at the gel point and a shift away from a reaction limited cluster aggregation (RLCA) mechanism. This is further confirmed by the analyses of the gel structures by SAXS and TEM. Likewise, the larger orange-emitting particles also produce larger aggregates at the gel point, leading to lower transparency. The ability to control the transparency of chalcogenide gels will enable their properties to be tuned in order to address application-specific needs in optoelectronics.

Synthesis of efficient photocatalysts based on CdS nanomaterials for oxidative decomposition of organic effluents typically focuses on (a) enhancement of surface area of the catalysts and (b) promotion of the separation of photogenerated electron-hole pairs. CdS aerogel, which are synthesized by simple sol–gel assembly of discrete nanocrystals (NCs) into a porous network followed by supercritical drying, could provide higher surface area for photocatalytic reactions along with facile charge separation due to direct contact between NCs via covalent bonding. We evaluated the efficiency of CdS aerogel materials for degradation of organic dyes using methylene blue (MB) and methyl orange (MO) as test cases. CdS aerogel materials exhibited remarkable photocatalytic activity for dye degradation compared to typical, ligand-capped CdS NCs. The catalytic efficiency of CdS aerogels was further improved by decreasing the chain-length and extent of surface organics, leading to higher, and more hydrophilic, accessible surface area. The use of porous, chalcogenide-based solid state architectures for photocatalysis enables easy separation of catalyst while ensuring a high-interfacial surface area for analyte reactivity and visible light activation.