CHARACTERIZATION OF THE SIZE-QUANTIZED ELECTRONIC AND OPTICAL PROPERTIES OF CdSe NANOCRYSTALS FOR APPLICATIONS IN PHOTOCATALYSIS, SOLAR CELLS AND DIFFRACTION GRATINGS

Abstract

This dissertation presents novel applications of ligand-capped II-VI semiconductor nanocrystals (i.e. CdSe and CdTe).Hybrid polymer-nanocrystal thin films were prepared using a bottom-up electrochemical crosslinking method, where thiophene-functionalized CdSe NCs were wired to electron-rich 3,4-dioxy-substituded thiophene polymers. Both nanocomposite and effective monolayer (EML) films were achieved by controlling monomer feed ratios during the crosslinking steps. These hybrid thin films showed enhanced photoelectrochemical current efficiencies with a variety of solution acceptor molecules compared to polymer control films, which was due to sensitization by the CdSe NCs. The electronic structure of the polymer played a critical role in the potential (doping) dependent hole capture efficiency from photoexcited CdSe NCs. Furthermore, photocurrent efficiencies were correlated with nanocrystal size, which was a direct product of frontier orbital energy shifting due to quantum confinement effects.All-inorganic CdTe-CdSe nanocrystal solar cells were fabricated by a facile layer-by-layer procedure. A low-temperature sintering strategy was utilized to electronically couple the nanocrystal thin films, which maintained the individual electronic properties of the nanocrystals. The electrical characteristics of these solar cells displayed predictable trends in open circuit voltage with varying CdSe NC diameter.Novel CdSe NC diffraction gratings were prepared by a facile microcontact molding procedure. These transmission gratings showed exceptionally high diffraction efficiencies that were dependent on optimum grating morphologies and the refractive index contrast provided by the nanocrystals, which was size-dependent. These films also showed promise as coupling gratings for internal reflection elements.