有機共軛高分子與小分子的結構變化對分子的堆疊行為與有機太陽能電池的主動層形貌之影響

Abstract

由於有機半導體具由質量輕、可撓曲等特性，使得有機電子元件在近年來受到廣泛的討論與研究。其中，有機太陽能電池更是被認為極具應用潛力的一塊領域。有機太陽能電池不同於一般矽晶圓電池，其元件製作可結合濕試圖塗佈技術，如3D列印，網印或噴墨等，利用此類快速且大面積的製程，可望大幅降低太陽能電池的成本，有利太陽能電池的普及。在目前有機太陽能電池中，以塊材異質介面(BHJ)為主動層所製作的元件具有最佳的效率。塊材異質介面(BHJ)乃是將P型與N型有機半導體混和製成的薄膜，此薄膜中的形貌因素(如結晶度，兩相分布等)對整體元件效率有著決定性的影響力。然而在早期新穎有機半導體的設計與開發的過程中，人們主要著重在分子能隙與能階的調整，對於分子的溶解度，結晶性乃至於材料間的互溶性則較少著墨，但這些物理性質對於塊材異質介面的形貌或是元件的製作流程，卻有著很大的影響力。在本篇論文中，我們主在研究分子結構的變化對於這些物理性質的影響，以及在元件的製作與效能上差異。 首先，我們以benzotrithiophene (BTT) and benzothiadiazole(BT)的共聚高分子(PBTTBT)為研究對象。我們將4-alkyl thiophene作為BTT與BT兩單元之間的共軛間隔，合成新的高分子(PBTT4BT)。此新的高分子較原來的高分子具有更佳的結晶性，同時在效率的表現上也從原來2.3%提升到5.6%。另一方面，為了提電池的電壓，我們也利用benzooxadiazole (BO)取代BT合成新的高分子(PBTT4BO)，但此高分子的溶解度不好，限制了高分子的分子量與其元件效率。為此，我們調整BTT單元上的碳鏈結構，選擇長度相同但體積不同的兩種溶解基。我們發現碳鏈的體積對於分子的結晶性與吸收特性有著很顯著的影響，同時也會影響高分子與碳球的互溶性，乃至於最後的元件效率。最後，我們利用PBTT4BO-C13C8此高分子所製作的太陽能元件效率可達6.2%。 除了共軛高分子外，我們的另一個研究對象為寡聚噻吩(oligothiophene)為主體的共軛小分子，此類小分子寡聚噻吩為分子骨幹同時也是推電子單元，然後在分子兩側尾端接上強力的拉電子基團。在本篇論文中，我們使用三種不同結構但推電子能力相似的單元作為此類分子中心單元(bithiophene, benzotrithiophene, and benzodithiophene)，我們發現中心推電子基團的幾何結構與平面性對分子的結晶特性有很顯著的影響。高平面性與對稱性的材料可提升整體分子的結晶驅動力，也會降低材料與碳球的互溶性，有助於提升小分子太陽能電池的效率。此外，我們選擇以bithiophene為中心單元的小分子作為接下來的研究對象，我們在bithiophene的單元上加入三種不同長度的溶解基(butyl, octyl, and dodecyl)合成一列具有不同溶解度的小分子。我們發現隨著側鏈組成的不同，分子的堆疊晶型也會隨之變化，藉由碳鏈的多寡或長度的變化，我們可以調控分子具有不同的排列特性。另外，在BHJ薄膜中的結晶特性也深受分子的側鏈結構影響，我們發現整體側鏈較一致的分子，易於在BHJ薄膜中形成高度結晶度的晶體，因而具有較佳的元件效率。

Solution-processed organic solar cells (OSCs) prepared through roll-to-roll or inject printing and used within inexpensive, lightweight, flexible devices are being considered as next-generation energy sources. Bulk heterojunctions (BHJ), in which a conjugated polymer (molecule) as a p-type material is blended with a fullerene derivative (e.g., PC61BM) as an n-type material, are the most successful active layer structures for OSCs. In this system, the morphology of BHJ layer play an important role for the high efficient OSCs due to it controls the efficiency of photon-to-current. At early stage, the design and synthesis of novel conjugated polymers and molecules mainly focused on tuning electronic properties, including bang-gaps and energy levels. However, other physical properties, such as solubility, miscibility, and crystallinity, are also important for OSCs, because they could affect the morphology of BHJ layer and the processed condition; therefore, the structure-property relationships of those conjugated materials are worth to be studied. In this thesis, we aimed the investigation at the effects of conjugated-backbone frameworks and alky-chain architectures on molecular stacking characteristics and corresponding processing conditions of devices. In the first part, we insert an extra 4-alkyl thiophene unit as conjugated spacer between benzotrithiophene (BTT) and benzothiadiazole(BT) to synthesize new donor-acceptor conjugated polymer, PBTT4BT. We found this polymer exhibited semi-crystalline characteristic, while the original polymer, PBTTBT, is amorphous. When we used this polymer in bulk heterojunction photovoltaic device applications, the device of PBTT4BT/PC61BM showed an efficiency of 5.6 % which is higher than the device of PBTTBT/PC61BM. On the other hand, we also use benzooxadiazole (BO) to synthesize another polymer, PTT4BO, which has a configuration of PBTT4BT but lower HOMO level. Because PBTT4BO exhibited unsatisfactory solubility limiting the molecular weight, we changed the alkyl-chain bulk on BTT unit to get two high molecular weight polymers, PBTT4BO-C13C11 and PBTT4BO-C13C8. The bulk of the alkyl side chains of these polymers impacted their solubility and molecular interactions, as well as their absorptions properties, crystallinities, and BHJ morphologies. The best efficiency was obtained from the device containing annealed PBTT4BO-C13C8 and PC71BM (w:w/1:2) active layer that had been maintained at 150 °C for 15 min with a power conversion efficiency of 6.2%. Then for the second part, we studied structure-property relationship of small molecules with a configuration of acceptor-donor-acceptor. At first, we synthesized three new small molecules－ TB3t-BT, TB3t-BTT, and TB3t-BDT－ that feature different central cores, including bithiophene, benzotrithiophene, and benzodithiophene units, respectively. The molecular structures of these cores significantly affected the melting and crystallization behaviors as well as formation of crystalline domains in blend films with PC61BM, which leads different efficiencies and processing conditions. Then we further synthesized three new molecules based on the structure of TB3T-BT by attaching different lengths of alkyl chains on central bithiophene unit. Those four molecules displayed two diffraction peaks of the (100) plane in their GIWAXS patterns, indicating polymorphism. Interestingly, the relative intensity of these two peaks changed when we modified the length of the central side chain, suggesting that it also affected the preferred stacking of the small molecules. In addition, the crystallinity within BHJ layer was also affected by side-chain arrangement, the molecules with identical molecules show comparatively better crystalline characteristic resulting higher performance.