Heterojunctions and Schottky Diodes on Semiconductor Nanowires for Solar Cell Applications

Photovoltaic devices are receiving growing interest in both industry and research institutions due to the great demand for clean and renewable energy. Among all types of solar cells, cadmium sulfide (CdS) – cadmium telluride (CdTe) and cadmium sulfide (CdS) - copper indium diselenide (CuInSe2 or CIS) heterojunctions based thin film solar cells are of great interest due to their high efficiency and low cost. Further improvement in power conversion efficiency over the traditional device structure can be achieved by tuning the optical and electric properties of the light absorption layer as well as the window layer, utilizing nano template-assisted patterning and fabrication. In this dissertation, simulation and calculation of photocurrent generation in nanowires (NW) based heterojunction structure indicated that an estimated 25% improvement in power conversion efficiency can be expected in nano CdS – CdTe solar cells. Two novel device configurations for CdTe solar cells were developed where the traditional thin film CdS window layer was replaced by nanowires of CdS, embedded in aluminum oxide matrix or free standing. Nanostructured devices of the two designs were fabricated and a power conversion efficiency value of 6.5% was achieved. Porous anodic aluminum oxide (AAO) was used as the template for device fabrication. A technology for removing the residual aluminum oxide barrier layer between indium tin oxide (ITO) substrate and AAO pores was developed. Causes and remedies for the non-uniform barrier layer were investigated, and barrier-free AAO on ITO substrate were obtained. Also, vertically aligned nanowire arrays of CIS of controllable diameter and length were produced by simultaneously electrodepositing Cu, In and Se from an acid bath into the AAO pores formed on top of an aluminum sheet. Ohmic contact to CIS was formed by depositing a 100 nm thick gold layer on top and thus a Schottky diode device of the Au/CIS nanowires/Al configuration was obtained. Material properties of all these nanowires were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), absorption measurement. Current-voltage (I-V), capacitance-voltage (C-V) and low-temperature measurements were performed for all types of devices and the results were analyzed to advance the understanding of electron transport in these nano-structured devices