A Study of Solution-processed Small Molecules and Fullerene Derivatives for Bulk Heterojunction Solar Cells

本論文主要研究方向是設計與發展溶液式製程有機小分子半導體材料與富勒烯衍生物的相互作用，進而應用於有機太陽能電池的主動層中；同時，本研究探討此有機材料與電子受子材料的作用、成膜後的形貌差異並進一步結合熱處理與三元混摻以提升與改善元件的光電轉換效率。 本文的第一部分(第三章)以2-octylthiene-2-yl-substituted benzodithiophene (BDT)為核心合成A-D-A形式的共軛有機小分子材料，並比較不同末端基cyanoacetate (CNR)、dicyanovinyl (CN)所得之材料結構，搭配富勒烯衍生物PCBM組成有機太陽能電池之主動層。在BDT6T的最佳比例1:0.40下，可以得到元件的短路電流為9.08 mA/cm2、開環電壓為0.90 V與元件效率5.42%；其中所得之填充因子為0.66更是目前所報導BDT為核心的小分子材料中所得最高，亦說明此小分子材料具有很好結晶度與共軛堆疊。 本文之第二部分(第四章)，為了能進一步提高元件效率，本研究將主動層的電子受體PCBM以ICBA取代，因ICBA擁有較高的最低未占分子軌域 (lowest unoccupied molecular orbital)，在搭配小分子材料上可以有效提升元件之開環電壓進而能提升整體元件效率；而實驗結果發現搭配ICBA後元件的短路電流較低，是因為來自此二材料的表面能較接近，以致在成膜時沒有辦法得到適當的相分離。為進一步解決此問題，將利用熱處理來增加主動層的相分離，並得到短路電流為8.07 mA/cm2、開環電壓為1.04 V與填充因子0.48，而元件表現從2.68%提升至4.03%；此外，利用同步輻射小角X射線散射與廣角X射線散射來分析此二主動層系統與熱處理後小分子與富勒烯衍生物的分布。 第三部分(第五章)，將利用三元混摻的方法來提升異質相接面太陽能電池的元件效率，以全小分子材料來作為太陽能電池之主動層，其中包含小分子授子材料與富勒烯衍生物PC71BM、ICBA，在最佳比例的ICBA下(15%)，可以得到光電轉換效率為6.36%、短路電流為12.00 mA/cm2、開環電壓0.93 V與填充因子0.57，其短路電流的提升來自ICBA能有效地幫助授子與受子的混摻，進而得到適當的主動層形貌與較佳的電荷轉移，也說明此三元混摻異質相接面可以保持元件製備的簡易與有效提升元件效率。The dissertation is mainly focused on designing and developing the novel solution-processed small molecule donors and different fullerene derivative acceptors for organic solar cell (OSC) application. Besides, the interaction between the donor and acceptor and the morphological difference after the film formation of active layer were studied. This study further combines other techniques, including the thermal treatment and ternary blend, to further improve the cell performance of OSCs. In the first part (Chapter 3), two novel, symmetrical, and linear A–D–A-type π-conjugated donor molecules (BDT6T, BDTCN) were strategically designed and convergently synthesized, each containing a planar electron-rich 2-octylthiene-2-yl–substituted benzodithiophene (BDT) unit as the core, flanked by octylthiophene units and end-capped with electron-deficient cyanoacetate (CNR) or dicyanovinyl (CN) units. Both of these materials were thoroughly characterized, and the effects of the end groups (CNR, CN) on their optical, electrochemical, morphological, and photovoltaic properties were investigated. Solution-processed bulk heterojunction organic solar cells were fabricated by incorporating BDT6T and BDTCN. Among these tested devices, the one containing BDT6T and PCBM in a 1:0.40 ratio (w/w) exhibited the highest power conversion efficiency (5.42%) with a short-circuit current density (JSC) of 9.08 mA/cm2, an open circuit voltage (VOC) of 0.90 V, and an impressive fill factor (FF) of 0.66 under AM 1.5G irradiation (100 mW/cm2). The FFs of these solution-processed small molecule organic solar cells (SMOSCs) are outstanding when compared with those recently reported for benzodithiophene (BDT)-based SMOSCs, due to the high crystallinity and excellent stacking properties of the BDT-based compounds. In the second part (Chapter 4), the conventional bulk heterojunction (BHJ) solar cells incorporating the small molecule donor (2Z,2´E)-dioctyl 3,3´-(5´´,5´´´´´-(4,8-bis(5-octylthiophen-2-yl)benzo[1,2-b:5,4-b´]dithiophene-2,6-diyl)bis(3,4´,4´´-trioctyl-[2,2´:5´,2´´-terthiophene]-5´´,5-diyl))bis(2-cyanoacrylate) (BDT6T) and [6, 6]-Phenyl-C61-butyric acid methyl ester (PCBM) acceptor archive the power conversion efficiency (PCE) of 5.26%. Replacing PCBM with indene-C60 bisadduct (ICBA) which possesses a higher lowest unoccupied molecular orbital (LUMO) level enhanced an open-circuit voltage (VOC) from 0.90 V to 0.97 V, but decreased the short-circuit current density (JSC) and fill factor (FF), leading to a low cell efficiency of 2.68%. It was found that BDT6T:ICBA film reveals a well-mixed morphology due to lower surface energy difference (∆γ) than BDT6T:PCBM, which is attributed to less thermodynamic driving force for phase separation. After the thermal annealing, the morphology of BDT6T:ICBA displayed the suitable phase separation for effective charge transport rather than the serve aggregation from BDT6T:PCBM and showed the enhanced PCE of 4.03% with VOC of 1.04 V, JSC of 8.07 mA cm-2, and FF of 0.48. It is therefore concluded that the value of ∆γ not only provides the useful information for synthetic strategies, but also manifests a promising strategy to improve the ICBA-based small molecules solar cells. Ternary bulk heterojunctions (BHJs) are platforms that can improve the power conversion efficiencies of organic solar cells. In Chapter 5, we report an all-small-molecule ternary BHJ solar cell incorporating [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) and ICBA as mixed acceptors and the conjugated small molecule BDT6T as a donor. When incorporating a 15% content of ICBA relative to PC71BM, the ternary BHJ solar cell reached a power conversion efficiency of 6.36% with a short-circuit current density (JSC) of 12.00 mA/cm2, an open-circuit voltage (VOC) of 0.93 V, and a fill factor of 0.57. The enhancement in efficiency, relative to that of the binary system, resulted mainly from the increased value of JSC, attributable to not only the better intermixing for the donor and acceptor that improved charge transfer but also the more suitable morphology for efficient dissociation of excitons and more effective charge extraction. Our results suggest that there is great potential for exceeding the efficiencies of binary solar cells through adding a third component, without sacrificing the simplicity of the fabrication process