Synthesis and characterization of III-V semiconductor nanowires and fabrication of colloidal nanorod solar cells

Nanowires have attracted intensive research efforts due to their one-dimensional quantum confinement and their ability to serve as the building blocks for and functional components of future semiconductor devices. Widespread use of nanowires in bottom-up device fabrication will require a general method for the controllable synthesis of nanowires with regards to shape, size, composition, and interfacial properties. Gallium arsenide and gallium phosphide nanowires as small as 8 nm in diameter were synthesized in supercritical hexane and seeded by alkanethiol-stabilized 7 nm gold nanocrystals. The wires are single crystal with a zinc-blende structure and grow exclusively in the <111> direction. The importance of precursor degradation kinetics was explored. Multiple lamellar {111} twins are observed in GaAs, GaP and InAs nanowires synthesized by supercritical fluid-liquid-solid (SFLS) and solution-liquid-solid (SLS) approaches. All of these nanowires have zinc blende (cubic) crystal structure and were grown in the <111> direction. The twins cross-section the nanowires to give them a “bamboo”-like appearance in TEM images. In contrast, Si and Ge nanowires with <111> growth direction do not exhibit {111} twins, even though this is a common twin plane with relatively low twin energy in diamond cubic Ge and Si. However, Si and Ge nanowires with <112> growth directions typically have several {111} twins extending down the length of the nanowires. A semi-quantitative model that explains the observed twinning in III-V and IV nanowires is presented. Heterojunction solar cells were fabricated from colloidal solutions of CdTe and CdSe nanorods by sequential spin casting onto ITO coated glass substrates. A broad range of factors impacting the success of the solar cell fabrication were explored; including the method of nanorod synthesis, choice of capping ligand, method of active layer application, and use of hydrazine treatment. It is necessary to maximize the stability of the nanords in solution to ensure even application into thin films. However, electrical properties of the nanorod films are improved by using weaker stabilizing agents. The devices exhibit good diode behavior.