以分子設計與溶液製程優化提升有機太陽能電池性能之研究

Abstract

有機太陽能電池(Organic Solar Cells, OSCs)因其輕量化、機械柔性佳，以及可相容於低成本溶液製程等特性，被視為極具潛力的新興光電技術。在多種 OSC 架構中，目前主要發展方向可分為兩類：其一為採用聚合物與非富勒烯小分子受體(non-fullerene acceptors, NFAs)組成的混合型系統(polymer:NFA)；其二則為全聚合物太陽能電池(All-Polymer Solar Cells, All-PSCs)，即同時以共軛高分子作為電子施體與受體材料。前者已達成優異的光電轉換效率(PCE)，但仍面臨施體材料分子設計與形貌穩定性等挑戰；而後者雖具備更優異的熱穩定性、機械強度與長期穩定性，卻因兩種聚合物間相容性差與形貌控制困難，導致元件表現及可重複性受限。 本論文針對上述兩類系統所面臨之材料與製程挑戰，分別提出兩項互補且具策略性的解決方案：(1) 在 polymer:NFA 系統中，透過分子工程手法設計並優化聚合物施體的結構與光電特性；(2) 在 All-PSC 系統中，導入綠色共溶劑搭配揮發性固態添加劑的製程策略，以調控主動層的微觀形貌並實現理想的電荷傳輸行為。 在材料分子設計方面，本研究合成六種以異靛青(isoindigo)為主體的共軛高分子電子給體，並系統性探討其側鏈對稱性與主鏈氟化對光電與結晶性質的影響。這些材料分為兩大系列：未氟化的PII2T系列(P1-P3)與氟化的PII2TF系列(P4-P6)，其中側鏈分別採用對稱(雙DT或雙SiO-C8)或非對稱(DT/SiO-C8)設計。分析結果指出，氟化能有效提升分子的共平面性與π-π堆疊能力，而非對稱側鏈設計則有助於形成有利的face-on堆疊取向與更緊密的鏈間堆積。基於非對稱側鏈的P2與P5在各自系列中表現出最高的功率轉換效率(PCE)，並在短路電流(Jsc)、填充因子(FF)與復合抑制方面皆有顯著改善。此外，氟化設計亦能減緩側鏈結構變異對形貌與元件效能的影響，提升元件穩定性與製程耐受度。本部分結果突顯了分子結構工程在優化OSC材料性能中的關鍵角色。 接續材料設計成果，本研究進一步發展綠色溶劑製程策略，應用於高性能PM6:PY-IT全高分子主-受體系統，以優化其活性層形貌並兼顧環境永續性。鑑於傳統使用的氯仿(CF)對環境與健康具潛在危害，本研究選用三種較高沸點的環保輔助溶劑—THF、2-MeTHF與3-MeTHF—與CF混合，以調控薄膜形成過程中的乾燥速率。再搭配具揮發性的固體添加劑，得以在不殘留的前提下，有效調控聚合物聚集、相分離行為與奈米結構形成。紫外可見光譜與變溫吸收測試證實，混合溶劑系統能促進分子間有序堆積並穩定溶液態聚集。另外由同步輻射之結果則指出，這些製程條件有助於降低相區尺寸、增進垂直結晶性並提升主/受體的混合相容性。CF+THF與CF+2-MeTHF製程所製備的元件其PCE皆超過17%，顯著優於使用純CF或CF+3-MeTHF之對照組。此類元件不僅展現更佳的電荷傳輸平衡與抑制復合能力，其薄膜亦具備較高的內建電壓、更低的陷阱密度與能態整序，進一步提升激子解離效率、電荷抽取能力與開路電壓(Voc)。此外，此優化策略同樣適用於雙成份(binary)與三成份(ternary)系統，顯示出其高度通用性與潛力。 綜合上述研究成果，本論文從材料分子設計到製程控制，建立了一套完整策略以系統性優化有機太陽能電池之奈米形貌、光電特性與載子動態行為。本研究不僅提供深入理解OSC中結構-形貌-性能之關聯性，也為未來發展具備高效率與環境永續性的製程技術奠定重要基礎。

Organic solar cells (OSCs) have emerged as a promising photovoltaic technology due to their lightweight nature, mechanical flexibility, and compatibility with low-cost solution processing. Among various OSC architectures, two major directions have demonstrated significant potential: polymer:non-fullerene acceptor (polymer:NFA) systems, and all-polymer solar cells (All-PSCs), which utilize conjugated polymers as both donor and acceptor. While polymer:NFA systems have achieved excellent power conversion efficiencies (PCEs), ongoing challenges remain in optimizing donor molecular design and reducing morphology sensitivity. All-PSCs, on the other hand, offer enhanced thermal stability, mechanical robustness, and long-term device durability, but suffer from more complex morphology control and limited miscibility between two polymer components. To address these challenges across both material systems, this thesis integrates two synergistic strategies: (1) molecular engineering of donor polymers in polymer:NFA systems to enhance structural and electronic properties, and (2) solvent-processing optimization in All-PSCs using green co-solvents and solid additives to achieve ideal phase morphology and balanced charge transport. For molecular engineering of conjugated polymers, six isoindigo-based donor polymers were synthesized and systematically investigated to evaluate the effects of side-chain symmetry and backbone fluorination on their optoelectronic and morphological properties. The polymers were divided into two series—PII2T (non-fluorinated, P1–P3) and PII2TF (fluorinated, P4–P6)—with variations in symmetric or asymmetric side-chain substitution using alkyl (DT) and siloxane (SiO–C8) branches. Optical absorption, cyclic voltammetry, and GIWAXS analyses revealed that backbone fluorination significantly improves molecular planarity and facilitates stronger π–π stacking, while asymmetric side chains induce favorable face-on orientation and tighter interchain packing. Devices based on asymmetric polymers (P2 and P5) exhibited the highest power conversion efficiencies (PCEs) in each series, with enhanced short-circuit current (Jsc), fill factor (FF), and reduced recombination losses. Furthermore, fluorination was shown to mitigate the influence of side-chain structure, improving morphology robustness and reducing performance variation. These findings underscore the importance of rational molecular engineering in optimizing OSC materials for enhanced device performance. Building upon these insights into material design, a green solvent engineering strategy was developed to further optimize the morphology of All-PSCs based on a high-performance PM6:PY-IT donor–acceptor blend. Recognizing the environmental and health concerns associated with commonly used solvents like chloroform (CF), three eco-friendly co-solvents—tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeTHF), and 3-methyltetrahydrofuran (3-MeTHF)—were blended with CF to modulate the drying kinetics during film formation. In combination with a volatile solid additive (DTT), this strategy enabled precise control over domain size, polymer aggregation, and phase separation dynamics. Optical absorption and temperature-dependent UV-Vis measurements confirmed that the mixed solvent systems promoted more ordered molecular packing and stronger solution-state aggregation. GIWAXS and GISAXS analyses further showed that these processing conditions resulted in reduced domain sizes, enhanced vertical crystallinity, and improved donor–acceptor miscibility. Devices fabricated with CF+THF and CF+2-MeTHF blends achieved PCEs over 17%, significantly outperforming devices processed with only CF or with CF+3-MeTHF. These high-performing devices exhibited balanced charge transport and suppressed recombination losses. Additionally, the optimized films exhibited higher built-in potential, lower trap densities, and reduced energetic disorder, all of which contributed to enhanced exciton dissociation, more efficient charge extraction, and higher Voc values. These improvements were consistently observed across binary and ternary device systems, confirming the generality and robustness of the proposed solvent engineering approach. Together, the results provide a comprehensive framework for tuning the nanoscale morphology, optoelectronic properties, and charge transport characteristics of OSCs. This study not only contributes valuable insights into structure—morphology—performance relationships in OSCs, but also offers practical guidelines for scaling up organic photovoltaics with eco-conscious manufacturing protocols.