高分子太陽能電池元件光吸收增益之探討

Abstract

有機太陽能電池元件具有低成本、可彎曲性、容易製程、半透明性，以及材料容易取得等優勢，使其受到廣泛的重視，而有效的光吸收將有助於提升有機太陽能電池的元件特性。本博士論文著重於有機高分子太陽能電池元件光吸收增益之探討，其主動層材料為poly(3-hexyl thiophene) (P3HT)及[6,6]-phenyl C61-butyric acid methyl ester (PCBM)。

首先，我們開發出高效能的半透明有機高分子太陽能電池元件，此可應用在堆疊式或串接式的元件結構，使其可吸收更廣泛的太陽放射光譜，進而提升元件的功率轉換效率，此外，我們利用此透明電極的結構，發展出高效率的上照光有機太陽能電池元件，並且完成以金屬薄片為基板的可彎曲太陽能電池元件。另外，我們利用銦錫氧化物作為光學間隙層，藉由光學干涉效應調整元件內光場的分佈，在適當的銦錫氧化物厚度條件下，可以產生有利於提升元件光電流的激子分佈狀態可以減少激子在電極界面的消散效應，並增加有效激子的數目，進而提升倒置式有機太陽能電池的元件功率轉換效率。同時，我們利用了表面電漿子的特殊光學特性來提升有機太陽能電池的元件效率，在陽極緩衝層內加入金奈米粒子激發侷域性表面電漿共振，此現象可在金屬奈米粒子周圍產生局部電場增益，使得元件內的激子產生率及激子分離率上升，進而增加有機太陽能電池元件的光電流及填充係數。最後，我們探討了電荷轉移能態的光學特性，此能態存在於P型/N型異質界面結構中，具有吸收長波長的光子的特性，有助於延伸有機太陽能電池元件對太陽光譜的吸收，據此，我們也成功發展出以近紅外光雷射驅動之有機太陽能電池元件，可以有效地將980 nm雷射光轉換成電能，由於生物組織對於980 nm近紅外光具有高透光性，可將此元件置於人體組織內作為具生物醫療功能性的奈米元件之無線電源，同時此應用也為有機太陽能電池領域開創一個新的研究方向。

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.