Multi-junction ZnO Nanowires for Enhanced Surface Area and Light Trapping Solar Cells and Room Temperature Gas Sensing
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Multi-junction ZnO Nanowires for Enhanced Surface Area and Light Trapping Solar Cells and Room Temperature Gas Sensing M. Kevin, W. L. Ong and G. W. Ho Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117576. Abstract A maskless method of employing polymer growth inhibitor layers is used to modulate the conflicting parameters of density and alignment of multi-junction nanowires via large-scale low temperature chemical route. This low temperature chemical route is shown to synthesize multijunction nanostructures without compromising the crystal quality at the interfaces. The final morphology of an optimized multi-junctions nanowire arrays can be demonstrated on various substrates due to substrate independence and low temperature processing. Here, we also followup on device demonstrations whereby p-n junction are created by exposure of secondary nanowires to ammonia plasma, converting them to p-type characteristics and also the density modulated multi-junction nanowires were tuned to infiltrate nanoparticles to create a hybrid hierarchically-structured nanowire/nanoparticles solar cell. The fabrication of hierarchicallystructured nanowire/nanoparticles composites presents an advantageous structure, one that allow nanoparticles to provide large surface areas for the dye adsorption, whilst the nanowires can enhance the light harvesting, electron transport rate, and also the mechanical properties of the films. This work can be of great scientific and commercial interest since the technique employed is of low temperature (< 90 °C) and economical for large-scale solution processing, much valued in today’s flexible display and photovoltaic industries. In addition, ZnO nanostructures for gas sensing will be presented. INTRODUCTION For fabrication of nanodevice, the density and alignment modulation of nanowires on various substrates is very important since it is directly related to how the nanowires interact with each other optically, electronically, chemically or mechanically. So far, the most common technique used to control the density and alignment of nanowires is top down lithography and etching or adjusting the thickness of the metal catalyst layer [1-3]. Although they can offer precise control, stringent requirements such as high reaction temperature, single crystalline substrate, complex processing and expensive equipment are needed. As for the case of the low temperature chemical route, the density control of nanowires has always been achieved via tuning of seed layer thickness [4-5]. However, the change in the density is limited and usually at the cost of the nanowires’ alignment. In regard to the multi-junction synthesis, one-dimensional (1D) doped nanowires have mainly been synthesized by vapor phase techniques which relies mainly on the vapor-liquid-solid (VLS) mechanism in which the alternate growth of different segments can be achieved by modulation of the vapor-phase reactants during growth of the wires. Similarly, stringent requirements such as high reaction tempe
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