Cadmium Sulfide and Titanium Dioxide Thin Film Nanostructures for Nanocrystal-Based Solar Cells

New solar cell architectures leverage nanostructured materials in attempting to achieve high light-to-electricity conversion efficiencies using low-cost materials and processes. One such example is a dye sensitized solar cell, wherein light is absorbed by an organic dye sensitizer (rather than by a semiconductor, as in a traditional solar cell), and the photogenerated charge transports out of the device through a nanostructured percolating titanium dioxide (TiO2) network. Because organic dyes have a limited spectral absorption range, they are not readily suited to capture all incident solar energy. Inorganic semiconductor quantum dots represent an alterative solar cell sensitizer with potential advantages because their spectral light absorption can be controlled by their size and composition. In principle, one can design a device having a spectral absorbance range well-matched to the incident solar spectrum by using an array of differently sized quantum dots – thereby providing a pathway to higher performance efficiencies. We use solution-phase chemistry to synthesize cadmium sulfide (CdS) quantum dots with precise diameter control over the range of 2 to 10 nanometers, and corresponding control of peak optical absorption from 320nm to 365nm. We produce CdS particles using a reverse micelle method using a surfactant (AOT in heptane) allowing further integration into thin film devices using solution processing. We have characterized the optical properties of thin films of both CdS and TiO2 nanocrystal using ultraviolet-visible spectroscopy in order to determine their absorbance. We have measured the nanocrystal film morphologies (size, structure, and thickness) using scanning electron microscopy and profilometry in order to understand the effects of different methods of film deposition (spin coating versus doctor-blading). Spin coating of both CdS and TiO2 nanocrystals yields uniform, three-dimensional nanocrystalline thin films. We have fabricated nanocrystal thin film devices by sandwiching nanocrystalline films of either CdS or TiO2 between a transparent indium-tin oxide electrical contact and an aluminum contact deposited by thermal evaporation. In both CdS and TiO2 nanocrystal devices, the device current increases with applied voltage. Under simulated solar illumination, the conductance of both CdS and TiO2 devices increases, consistent with excitation of photogenerated carriers in the semiconductor nanocrystal film network.

The Journal of Young Investigators is not affiliated with the US Department of Energy. This paper was written by a student intern with the Department of Energy and does not constitute a declarative position of either the Department of Energy or the Journal of Young Investigators.
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One of the founding fathers of JYI, Brian Su, became the youngest person to co-PI a grant from the NSF. The purpose of the grant was to fund the start-up costs for JYI.
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