| Summary: | 博士 === 國立中興大學 === 電機工程學系所 === 101 === Polymer solar cells (PSCs) have attracted great interest as potential alternatives for inorganic-based solar cells due to their possibility of low-cost fabrication, light-weight, simple process, and mechanical flexibility. Efficiency of the polymer solar cell can be improved by various methods. The incorporation of metal nanostructures to increase the photocurrent by the presence of surface plasmon and also another possible way to improve the device is electrical annealing through the electrode.
In this thesis, a gold (Au) nanomesh layer was manufactured on an ITO-coated glass substrate at room temperature. The Au nanomesh was used to induce surface plasmon resonance (SPR) to enhance the photocurrent of a polymer solar cell. The Au nanomesh was manufactured by lift-off process on closely packed PS nanospheres (diameter ~50 nm; density ~1010/cm2). The PS nanospheres were fabricated by modified block copolymer nano-patterning on ITO. A transmittance–reflection– absorbance spectrum was used to explore the induced surface plasmon. An extinction peak was observed at ~580 nm indicate the possibility of Au nanomesh induced surface plasmon resonance. The short-circuit current density of the polymer solar cell was enhanced from 7.02 to 14.2 mA/cm2 by the addition of Au nanomesh. Consequently, the power conversion efficiency enhanced from 1.9% to 3.2%. By the normalized input photon-to-current conversion efficiency (IPCE) measurement, enhanced photocurrent conversion efficiency at approximately 580 nm was observed that coincided with the extinction spectrum, indicating that the surface plasmon enhanced the photocurrent.
Moreover, we deposit different thickness of gold nanodot to examined the optical properties of Au nanodots, and analyze the effect of surface plasmon and plasmonic nano cavities. While the deposition of gold thickness increases, the diameter of the gold nanodot also increases. The localized surface plasmon resonance peak is red shifted from 534 nm to 611 nm when the diameter of the Au nanodot increased. The photoactive layer poly(3-hexylthiophene) : 6,6-phenyl-C61-butyric acid methyl ester film and 300 nm a-Si with Au nanodots have a higher absorbance as compared to the without Au nanodots. Scattered light efficiently couple the incident light into waveguide mode to the a-Si film dramatically increasing the optical path and absorption of the light inside the film. Combine effect of local field enhancement from LSP and guided modes are the reason for the strong enhancement in higher wavelength.
Also, we demonstrated the effect of electrical annealing treatment under different reverse bias on the performance of bulk heterojunction photovoltaic cells based on P3HT:PCBM. After electrical annealing at -6 V, the polymer solar cell exhibits an 7.77 mA/cm2 short circuit current density, an 0.53 fill factor, and an 0.63 V open circuit Voltage. A corresponding efficiency of 2.59 % was achieved. In comparison, solar cell without electrical annealing exhibits power conversion efficiency of 2.37 %. This enhanced efficiency is attributed to the modified orientation of the polymer chains inside the photoactive layer that increases the mobility of charge carriers. X-ray photoelectron spectroscopy (XPS) was used to investigate the metal-organic interfaces of a P3HT:PCBM bulk-heterojunction (BHJ) organic solar cell under a high electrical field. This high electrical field was built by applying reverse bias to the solar cell. In addition, after electrical annealing, the Al cathode will penetrate further into the active layer increases the contact area and reduces the contact resistance. This Al penetration was confirmed by the depth profile of atomic concentration in the X-ray photoemission spectroscopy. In addition, the reverse current density was quite low, consumed only a small amount of energy (0.36 mJ/cm2) during stressing. The intermixing and atomic concentration gradient of Al and C shown by an XPS depth profile confirmed the penetration of Al into the polymer layer under a large electrical field, in which improved the contact properties.
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