| Summary: | 博士 === 國立臺灣大學 === 物理研究所 === 101 === In this thesis, we report the design, fabrication, and characterization of graphene-based nanoscale semiconductors and optoelectronic devices. Because of its Dirac-fermionic energy dispersion, two-dimensional graphene possesses many unique properties, like high electron mobility, high optical transparency, low sheet resistance, great flexibility and surface plasmon effect. Accroding to our research, several intriguing phenomena and highly efficient devices have been discovered. The highlight of our research is briefly described as follows.
1. Vertically Aligned Inorganic Nanorods/Organic Hybrid Solar Cells with Bending Enhanced Performance: An Advanced Alternative for Flexible Solar Cells
A new flexible organic/inorganic hybrid photovoltaic (HPV) device with bending enhanced performance has been demonstrated. The layered structure of the HPV consists of polyethylene terephthalate (PET)/graphene/vertically aligned ZnO nanorods/poly (3-hexylthiophene): phenyl-C61-butyric acid methyl ester (P3HT: PCBM)/Ag. Unlike all previous reports for flexible organic photovoltaic devices, it is found that the power conversion efficiency and the short-circuit current density of the device can be enhanced with increasing bending angles. The highest enhancement of power conversion efficiency can reach up to 44.8 % compared with its flat counterpart. While the device returns to the original condition, the power conversion efficiency recovers to its initial value. This unique property can be attributed to the enhanced light trapping effect due to the geometric structure of ZnO nanorods as well as the outstanding optical transparency, conductivity, and mechanical flexibility characteristics of two–dimensional graphene crystal.
2. Enhancement of laser action in ZnO nanorods assisted by surface plasmon resonance of reduced graphene oxide nanoflakes
We report the discovery of an enhancement of the random laser action in a nanocomposite comprising reduced graphene oxide nanoflakes and ZnO nanorods. We show that both emission intensity and lasing threshold exhibit an obvious improvement. Based on our theoretical calculations, the mechanism underlying the enhanced stimulated emission can be attributed to coupling between the optical transition and the surface plasmon resonance of the reduced graphene oxide nanoflakes, induced by the ZnO nanorod surface roughness. The approach we describe here will be very useful for the future development of high-efficiency optoelectronic devices and offers an alternative route for application of reduced graphene oxide.
3. All Carbon-Based Photodetector with Ultrahigh Responsivity: A feasible eminent integration of zero and two dimensional graphene
A photodetector with ultrahigh sensitivity based on the composite made with all carbon-based materials consisting of graphene quantum dots (QDs), two dimensional graphene crystal, and conducting carbon paste electrodes has been demonstrated. Under light illumination, remarkably, photocurrent responsivity up to 4 × 107 AW -1 can be obtained. The underlying mechanisms are dominated by several important factor, simultaneously. Firstly, the ultrahigh sensitivity can be attributed to the spatial separation of photogenerated electrons and holes due to the charge transfer caused by the appropriate band alignment across the interface between graphene QDs and graphene. Besides, the well-matched bonding network arising between graphene QDs and graphene under coherent atomic interface, the large absorptivity of graphene QDs, and the excellent continuity of electronic structure for the carriers transport on the graphene sheet also play significant roles. Our result therefore demonstrates a highly efficient all carbon-based photodetector, which serves as an excellent example for the integration of unique physical properities of nano-carbons with different dimensionalities. Together with the associated mechanism, it can pave a new route for the further development of all carbon, cheap, non-toxic and highly efficient optoelectronic devices.
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