功能性高分子複合材料於光電元件應用 ：型態、製程與元件工程

Abstract

溶劑製程為主的高分子光電元件廣泛採用複合材料型態作為主動層以提升元件效能。同時，利用分子摻雜的方式去改變複合材料的特性是目前相當受矚目的研究方向。本篇研究提出一系列的功能性高分子複合材料於光電元件應用。首先，本論文第一部分提出利用簡單方式實現奈米級介面修飾工程。利用混合材料之間自發性的垂直相分離，使摻雜在主動層內部的聚乙二醇自組裝形成在主動層表面上形成陰極端修飾層。由於聚乙二醇相對於常被使用的低功函數金屬(例如：鈣)更為穩定，因此所製作的有機太陽能電池具有較高的元件操作穩定度。此外，透過一系列針對熱力學以及動力學的角度上探討，我們發現基板的表面張力以及聚乙二醇的分子量扮演非常重要的角色在於引發自發性垂直形貌改變。 此外，不單只是針對太陽能元件，類似的方法也成功運用在高分子發光二極體，進而降低元件操作電壓以及提升能量發光效率。更重要的，我們發現不同類型的電荷捕捉機制特性會因摻入聚乙二醇產生有不同的元件提升效果。 一般高分子太陽能電池，多半的複合材料都是以一個正型的共軛高分子以及負型的碳六十衍生物作為基底。本研究中，我們額外加入另一個功能性碳六十衍生物(Methano-PCBM)於混合材料系統。由於Methano-PCBM具有較高的最低佔有分子軌域(LUMO)，因此加入之後主體的能階會成為階梯式配位且同時提供較大的激子分離能階差異(energy offset for exciton dissociation)，進而有效提升有機太陽能電池的能量轉換效率，更進一步實現高效能半穿透式高分子太陽能電池。 另一方面，透過摻雜進近紅外光染料方式，我們引入一個功能性電荷載子捕捉點在主動層內，此近紅外光染料不單是在電性上促使光增益性現象的發生，並且在光學上可以造成更廣的光吸收範圍。透過此方式，可以實現低操作電壓、高光感度以及寬譜反應的有機光偵測器。

Polymer optoelectronics fabricated through solution-processable methods commonly adopts polymer blends as the photoactive layer for improving their device performance. This dissertation proposed a series of functional polymer blends for various opto-electronical applications. The first part of this dissertation is to conceptually introduce a simple approach to realize nano-scaled interfacial modification for organic photovoltaic devices (OPVs). Through spontaneous vertical-phase-separation, a self-assembled poly(ethylene glycol) (PEG) buffer layer can be formed on the top-surface of the active layer, thereby naturally acting as a cathode buffer layer. The stable PEG materials can replace traditionally low-work-function metals, such as calcium (Ca), to improve the device stability. Furthermore, a series of systematical studies aiming for understanding the mechanism behind the formation of such self-assembled polymer buffer layer were performed from both thermodynamics and kinetics point of views. We found out that the surface energy of the substrate and the molecule weight of PEGs play essential roles in triggering the vertical-type morphological change. Moreover, not only for OPVs devices, this approach can also be employed to fabricate high-performance sky-blue, red and white polymer light-emitting diodes (PLEDs). The operating-voltage and the power efficiency have been improved after incorporating PEG. More importantly, we found that the charge-trapping effect lead to different enhancement mechanism between these PLEDs emitting different colors after the addition of PEG. Typical photoactive layer of polymer solar cells commonly consisted of one p-type conjugated polymer as the electron donor and one n-type electron acceptor. We additionally blended one functional fullerene derivatives, 1,2-dihydromethano-phenyl-C61-butyric acid methyl (Methano-PCBM), featuring a higher lowest unoccupied molecular orbital (LUMO) respect to that of PCBM, into the photoactive layer. Incorporating this Methano-PCBM, a larger energy offset between the highest occupied molecular orbital (HOMO) of the polymer donor and the LUMO of the acceptor can increase the open-circuit voltage (Voc); the resulting cascade energy level structure possibly also facilitated the charge transport in the devices. Further manipulation of this optimized photoactive layer could be employed to demonstrate efficient semi-transparent OPVs exhibiting high transparency. Finally, high-performance organic photodetectors featuring photomultiple effect have been demonstrated. The incorporation of near-infrared dyes not only resulted in electron-trapping behavior but their absorption was also optically complemented to the original film. As a result, high external quantum efficiencies (>7000%), high responsivities (32.4 A/W) and broadband response (300 nm to 1050 nm) have been achieved simtaneously at an extremely low operating voltage (–1.5 V).