Nanocrystal Photovoltaics: The Case of Cu2S-CdS

In this dissertation, fine control over the morphology and composition of a nanostructured semiconductor thin film is demonstrated at a degree not previously possible. While such control is beneficial in a variety of optical and electrical devices, this research was performed from the perspective of photovoltaics.A new architecture is presented for low cost and high conversion efficiency photovoltaics, utilizing self-assembled nanocrystals. A method is introduced for vertically aligning nanorods using a controlled-evaporation self-assembly technique, enabling the uniform alignment of nanorods on a square-centimeter substrate. Such meso-scale assembly and optimization required the development of a morphology quantification technique. The technique presented here utilizes grazing incidence x-ray diffraction to assemble a quantitative pole figure, or nanorod orientation histogram, and can be used with any thin film. After nanorod assembly, an asymmetric electrical junction (required for photovoltaic operation) is achieved in each rod using a cation exchange technique, which can be performed reliably and reversibly on-chip. This results in a film comprising a massively parallel array of single-crystal nanodiodes. Electrical measurements of these films show rectified behavior and reveal a photocurrent upon illumination of the aligned and cation-exchanged films. Finally, the stability of the Cu2S phase is investigated as a function of nanocrystal size. The low chalcocite to high chalcocite solid-solid crystallographic phase transition is found to occur at temperatures depressed 60 Kelvin below the bulk phase transition temperature.The research summarized in this dissertation describes a set of synthetic and analytical techniques that enable nanoscopic control of morphology and composition in device-scale semiconductor thin films. Such control may be leveraged to precisely manipulate the flow of charge carriers in optoelectronic devices. Due to the simple wet chemical proc