Light Harvesting Schemes for Improving the Performance of Polymer Solar Cells

• Main Authors: Wu, Jyh-Lih, 吳志力
• Other Authors: Chen, Fang-Chung
• Format: Others
• Language: en_US
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/97490393969200320990

博士 === 國立交通大學 === 光電工程學系 === 100 === Organic photovoltaic devices (OPVs) are attracting a great deal of attention because of their low cost fabrication, mechanical flexibility, ease of processing, semi-transparency, and abundant availability. Efficient harvesting of sunlight in the photoactive layer is critical for achieving higher-performance OPVs. The work in this dissertation aims to design light harvesting schemes for improving the device performance of OPVs based on a blend of poly(3-hexyl thiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM). We have developed transparent electrode contacts for the semi-transparent OPVs attempting to implement stacked or tandem devices where more solar radiation can be absorbed by the multiple active layers. Based on the transparent top electrode, we realized efficient top-illuminating OPVs and flexible OPVs fabricated on metal foils. Furthermore, we utilized indium tin oxide (ITO), a transparent conducting oxide, as an optical spacer to improve the performance of inverted OPVs. The resulting optical interference effect led to spatial redistribution of the optical electric field in the devices. The incorporation of an ITO optical spacer at an appropriate thickness brought about a favorable distribution profile of exciton generation rate in the active region, thus increasing the number of “effective” photon-generated excitons due to the reduced level of exciton quenching near the electrodes. Meanwhile, the unique optical properties of surface plasmons were also exploited to increase the power conversion efficiency of OPVs. The addition of gold nanoparticles (Au NPs) into the anodic buffer layer increased the rate of exciton generation and the probability of exciton dissociation, thereby enhancing the photocurrent and the fill factor. We attribute the improvement in device performance to the local enhancement of the electromagnetic field originating from the excitation of the localized surface plasmon resonance (LSPR). Finally, we investigated the intrinsic nature of charge transfer (CT) states existing in polymer/fullerene blends, which absorbs long-wavelength photons. Based on the unique optical properties, we realized the near-infrared laser-driven (NIRLD) OPVs which convert 980-nm light into electrical power for the biomedical applications. Because of the high transparency of biological tissues in the NIR wavelength regime, these NIRLD OPVs might be a promising wireless electrical source for powering biologically functional nanodevices placed underneath the human body. Meanwhile, this application also initiated a new direction for OPVs.