溶液製程低能隙高分子倒置結構太陽能電池形貌及結構改良

Abstract

能源議題在近年來受到廣泛重視，石油的枯竭與環保議題使得許多新興替代能源被積極開發，其中太陽光能發電技術因取之不竭且不會造成環境多餘負擔而成為新興產業，近來該技術中，由於導電高分子太陽電池較佳的成本效益、質輕可撓應用廣等特性而廣受注目，故本研究主要針對高分子太陽電池技術結構改良以提升光電轉換效率。 本論文以低成本之溶液製程高分子太陽電池為基礎，加以近來光電轉換效率也因為低能隙材料的使用而快速上升，並採用高元件穩定度的倒置結構，研究著於重低能隙材料倒置結構太陽能電池的形貌控制與結構優化，其中重點有利用混合溶劑與添加劑控制低能隙PBDTTT-C-T:PC71BM材料系統的主動層形貌，使主動層內部施體受體分部被控制，達到優化元件效率的效果，元件光電轉換效率可達4.61%。而在倒置結構中，無毒且可以簡單製程大量生產的氧化鋅被廣泛做為元件陰極中介層，在倒置結構中非常重要，目前氧化鋅薄膜製做方式中，應用在有機太陽電池的製做，以溶膠-凝膠法(sol-gel)製程最為普遍，因該製程為簡單的溶液製程，且不需昂貴真空製程設備而成本低廉。本研究發現可藉由控制sol-gel法溶液製程氧化鋅層的成膜，來控制氧化鋅層與有機主動層的介面形貌，使低能隙導電高分子主動層與氧化鋅達到良好的接觸，增加光吸收產生載子導出機率，使元件效率提升到5.56%，其中因為該層形貌造成的效率差異可達40%，影響甚鉅，而使用此法改良形貌增進效率，其方法不僅簡單，也可被應用於多數的倒置低能隙材料有機太陽電池系統。 研究中也提出利用氧化鋅奈米柱結構來解決高分子太陽電池之激子擴散長度短與載子移動率過低這兩項問題，利用水熱法在氧化鋅種子層上生長奈米柱結構，製做成伸入主動層的載子傳輸路逕，元件的光電流因為有效的載子傳輸效率大幅提升，並提出退火輔助主動層 分層塗佈法，使元件的主動層垂直分層形貌更佳，解決奈米柱元件填充因子過低的問題，使元件的填充因子上升到50%，併聯電阻效能也被提升。而藉由控制氧化鋅奈米柱形貌，包括柱直徑與柱間間隙大小，使得奈米柱間有更多空隙可讓主動層滲入，形成指狀交叉結構，同時改善元件的奈米柱清洗步驟、退火程序及利用主動層分層塗佈手續，可以進一步提升元件的效能，大幅提高元件短路電流。經過退火輔助主動層分層塗佈法之低能隙高分子混成氧化鋅奈米柱元件效率可達7.05%，是奈米結構太陽電池的一大突破，使單層高效率之奈米結構太陽電池深具潛力。 我們期待利用主動層形貌控制法，氧化鋅奈米柱平台搭配分層塗佈法，造出通用於多種材料的高光電轉換效率有機太陽電池結構平台及製程技術，讓低成本、適合大面積製造的高分子太陽電池技術更往商用化目標邁進一步。

The development of renewable energy technologies has become an important issue because of the energy crisis. Among them the photovoltaic technology stands out due to the abundance of sun light and less contamination to the environment. Especially the organic photovoltaic (OPV) based on polymer–fullerene composite system, the cost effectiveness of solution process and mechanical flexibility makes it received much attention. In this dissertation, it is mainly focus on solution processed polymer solar cell systems, and because of the high stability, we employ the inverted device configuration to fabricated polymer solar cells. In chapter three, the mechanism of blended solvent with solvent additive method is discussed, as the way to control the photoactive layer morphology of the new kind low band gap polymer and fullerene system PCDTTT-C-T:PC[70]BM inverted solar cell. This method easily changes photoactive layer solution’s viscosity and fullerene derivative’s cluster size, and devices’ power conversion efficiency(PCE) can be improved to 4.61%. It is simple, costless, and can be applied to plenty of polymer solar cell system to optimize device performance. We also found the interface morphology between a ZnO electron transporting layer and a low band gap polymer photoactive layer affects the power conversion efficiency (PCE) enormously for inverted structure solar cells. By simply changing the concentration and spin speed of ZnO sol-gel, the ZnO morphology can be controlled, and thus the best morphology to enhance device’s PCE can be shaped. The cell conversion efficiency can be improved to more than 40% PCE enhancement by applying this method to create the ZnO layer with lower peak-to-valley difference, but higher roughness surface density. Such feature improves the morphology of the photoactive layer and the carrier transportation. The best cell PCE in this work is 5.56% for a low band gap material PBDTTT-C-T:PC[70]BM inverted structure solar cell. This solution process optimization method can be further applied to various low band gap polymer solar cells as a way to accelerate the commercial development of organic solar cell systems. In the second part of this research, the ZnO nano rod structure is also introduce to help the carrier transport, which is relatively low in organic material because of short exciton diffusion length and carrier mobility. The ZnO nano rod fabricated bt hydrothermal method successfully increasing device shour circuit current, which because of the nanostructure forming a finger-like cross morphology with the photoactive layer, increasing the probability of effective carrier transport. By photoactive layered coating method with annealing process, the low devices’ FF and Rsh can be solved. At the mean while, improving the cleaning process of ZnO nano rod and tuning annealing temperature further increasing device PCE to 7.05%, which is a breakthrough of ZnO nano rod polymer solar cell. We hope to create a high efficiency universal platform with ZnO nano rod for different low band gap polymer solar cell, combining with blended solvent with solvent additive method, To reach the goal of cells commercialization.