Studies On Single Walled Carbon Nanotube Photovoltaic Device

Due to unique quasi-one-dimensional nanostructure, excellent electrical and photovoltaic characteristics, single-walled carbon nanotubes(SWNTs) are widely used to fabricate high performance electrical and photovoltaic devices, and structure next generation circuit and optical system with nanoscale size. Compared with the traditional silicon, SWNTs have exceptionally high carrier mobility, low capacitance, and strong optical response(direct band gap). In terms of photonics, the SWNTs are used as photo-sensitive materials, modified materials and electrode materials to fabricate most kinds of photovoltaic devices including inorganic devices, organic devices and dye sensitized devices. In this thesis, a novel SWNT photovoltaic device based on asymmetric contacts was probed including the fabrication process, electrical and photovoltaic characteristics.Raman spectrum and absorption were used to probe the SWNT material’s non-tubular carbon content, structure(chirality) of the produced nanotubes, and structural defects, which determined the diameter range and the band structure. The fabrication process was probed included electron beam lithography, metal sputtering and lift-off to obtain the optimal technological parameter. The SWNT devices were fabricated with individual SWNT used as the semiconducting material. Pd and Al were used as the electrodes materials.The metal/SWNT characteristics were probed by structuring the symmetrical contact SWNT devices, in which Pd and Al were used as the electrodes material respectively. Pd/SWNT/Pd devices showed the characteristics of p-type field effect transistors: the Ids increased when the positive gate-voltage increased. While the Al/SWNT/Al devices showed the characteristics of n-type effect transistors: the Ids increased when the negative gate-voltage increased. In general, the Pd device showed a more large Ids indicated that the Pd/SWNT had better contact characteristics.The two metals were fabricated on the two ends of an individual SWNT, the asymmetrical Schottky contacts were formed. The devices exhibited excellent rectification characteristics as an diodes, which had a Iforward/Ireverse ratio of 103. When the gate voltage applied, the source-drain current showed the p-type conduction characteristic. The on-off ratio of forward current was nearly 103 by controlling the gate voltage. Moreover, different devices with different SWNT diameter were compared to probe the band gap effect. The results clearly showed that the one with larger diameter had larger source-drain current under the same bias voltage, which is mainly due to the different barriers height between Al Fermi level and top of valance band of SWNT caused by the different band gap energy.Optical measurements were carried out by monochromatic light(1550nm). The power conversion efficiency can reach 0.45%. The quantum efficiency can be over 7%. Moreover, the SWNT diameter also has important influence on the photovoltaic performances: the power conversion efficiency and quantum efficiency increased exponentially as the diameter of SWNT decreased, which corresponded to the stimulated results reported before. Those results have a great significance to promote the photovoltaic performance of the same type devices or other similar structures.