| Summary: | 博士 === 國立臺灣大學 === 材料科學與工程學研究所 === 97 === This thesis aims to explore an alternative for silicon based solar cell. The hybrid materials, which are a combination of conjugated polymer and inorganic nanomaterial, provide numerous promising device properties such as effective carrier transport, strong light absorption and flexibility. Compared with conventional silicon based solar cell, this hybrid material can provide a low cost, environmental friendly, light weight and easy to process possibility. Though the performance of polymer based solar cell is still too low for large scale application, it is still possible to increase device conversion efficiency by improving carrier transport and extending light harvesting range.
In chapter 3, we focus on studying organic-inorganic hybrid bulk heterojunction solar cell based on conjugated polymer P3HT and TiO2 nanocrystal. Our result show the optimal device performance can be achieved by introducing 50 weight percent TiO2 nanorod into P3HT matrix. By TiO2 surface modification, the optimal device performance has a power conversion efficiency of 2.2%. Compared with CdSe/conjugated polymer hybrid, this material system not only provides comparable device efficiency, but also develops a nontoxic, environmental friendly solar cell.
In Chapter 4, we demonstrate enhanced the performance of polymer solar cell based on poly(3-hexylthiophene)(P3HT)/ZnO nanorods array heterojunction hybrid. By infiltrating P3HT polymer chain along ZnO nanorods array nanostructure, carrier mobility has been found a increase from 8.2×10-5 cm2/Vs to 7.7×10-4 cm2/Vs, companied with polymer chain were aligned perpendicularly to substrate surface. The optical anisotropic measurement revealed that chain orientation of P3HT prefers align along ZnO (l0Ī0) surface. Our experiments also showed that device performance can be further improved by surface modified ZnO nanorod surface.
A novel approach to improve polymer solar cell using electric field assisting process was proposed in chapter 5. Our results showed better device performance can be achieved by carefully applied electric field during thin film process. Atomic force microscopy measurement showed higher polymer chain organization properties of blend film. By changing the natural orientation of polymer order, the electrical properties, including device performance, carrier mobility in vertical direction can both be enhanced. The optical anisotropic measurement also showed the optical anisotropic ratio is as a function of the magnitude of electric field.
A solution process single wall carbon nanotube (SWCNT) thin film as a transparent electrode for organic solar cell application was studied in chapter 6. By chemical modified SWCNT thin films using nitric acid and thionyl chloride treatments, a significant decrease of sheet resistance can be achieved. Photovoltaic devices based on P3HT and PCBM fabricated on surface functionalized SWCNT electrode shows a promising device conversion efficiency of 1.87% can be performed. The variation of open circuit voltage (Voc) in P3HT and PCBM bulk heterojunction organic photovoltaic with functionalized transparent SWCNT networks indicated that the change of surface potential of SWCNT thin films resulted in correlated change in short circuit current density and open circuit voltage of the photovoltaic devices.
In previous chapters, we have proposed several approaches to improve device performance. In chapter 7, we use new material for organic IR harvesting solar cells application based on P3HT/FeS2 blend. The devices exhibited high photo-electric current conversion efficiency in infrared region (>700 nm).where the external quantum efficiency was 6.5% at wavelength 650nm and 1% at 700 nm. The photoresponsed measurement also indicated that onset of photogenerated edge was about 900nm, which is contributed by FeS2 NCs. These results also pointed out that FeS2 NCs: P3HT hybrid can provide a low cost, environment friendly and easy process organic solar cell.
Finally, polymer solar cells that have been constructed by hybrid materials are very promising. This thesis mainly studied polymer solar cells and has provided some approaches to improve device performance. Our findings showed carrier transport properties and excitons dynamics are both directly influenced by photoactive layer morphology. In the future, we believe device performance can be further improve by optimized morphology of polymer based heterojunction solar cell with a good percolation of both phases to the respective electrode.
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