Chalcogenide Sol-Gel Assembly: Controlling The Kinetics Of Nanocrystal Aggregation And Film Formation For Applications In Optoelectronics

The dissertation work is focused on 1) optimization of gelation conditions in order to tune the properties of the resultant gel structures with the aim of addressing application-specific needs; 2) incorporation of gel materials into device architectures for evaluation of intrinsic properties and assessment of suitability for optoelectronic applications. Sol-gel assembly of CdSe/ZnS core/shell NCs typically results in opaque gel materials, which is one of the major obstacles for their use in optoelectronic applications such as LEDs. Macroscopic properties (i.e. transparency) of CdSe/ZnS gel structures were tuned by controlling the aggregation and gelation kinetics by means of oxidant concentration. The mechanism and kinetics of aggregation of 11-mercaptoundecanoic acid (MUA)-capped CdSe/ZnS NCs, as a function of oxidant concentration (tetranitromethane, (TNM)), were studied by means of time resolved dynamic light scattering (TRDLS). The structural characteristics of the resultant gels were probed by optical transmission, transmission electron microscopy (TEM), and small-angle X-ray scattering (SAXS). Transparent gel structures were achieved for small NCs (~4.5 nm) at relatively low NC concentration (~4 × 10-7 M) and oxidant concentration (~4-10 μL) enabling the tuning of macroscopic properties for application-specific needs in optoelectronics. In order to incorporate chalcogenide gel materials into device architectures, it is essential to fabricate gel materials in film form. The synthesis of transparent micron-thick xerogel films of CdSe/ZnS NCs was achieved by employing similar gelation conditions that resulted in transparent gel structures. Films were deposited by immersing a glass substrate horizontally in the pre-oxidized sols, gelling and drying under ambient conditions. CdSe/ZnS xerogel films exhibited strong photoluminescence and high conductivities (~10-3 S*cm-1). However, the films were rough, and attempts to make crack-free thinner films by this approach were not successful. High quality thin (ca 70 nm) sol-gel NC films were deposited by spin coating sols of thioglycolic acid-capped CdSe and CdSe/ZnS NCs onto substrates and dipping the films into an oxidizing solution (TNM in acetone). Optical, structural and electrical properties of the sol-gel films were evaluated and the films were found to exhibit 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 NCs. This suggests that the sol-gel approach may yield NC thin films suitable for a range of optoelectronic applications. Composite films of poly(3-hexylthiophene) (P3HT) with either CdSe and CdSe/ZnS aerogels, with were evaluated for hybrid photovoltaic devices by analyzing the variation of morphology and photocurrent generation with the composition of the hybrid film. Compared to films composed of ligand capped, physically aggregated NCs and P3HT, aerogel/P3HT films showed increased photocurrent and charge generation attributed to the interconnected NC network. These data suggest the use of chalcogenide aerogel materials for hybrid photovoltaic applications is feasible