運用分子動力學建構高效率太陽能電池系統於堆疊結構下之粗粒化模型勢能函數並探討其與分子鏈構形之關聯性

Abstract

本研究以厚膜型高效率太陽能電池PffBT4T:PC61BM為系統，並採用全原子模型，建構其於粗粒化模型下之分子內/分子間勢能函數，分析在無序/有序狀態下之分子鏈特性及構形，從中探討此高效率材料之影響因素。首先在分子間作用力部分中，我們發現環與環之作用力中以BT環與硫環(A-E)之間的吸引力為最強，表示此材料相較於P3HT而言，於主鏈上加入BT環更能幫助此材料於分子間的作用力提升，有助於π-π作用力提升；於各分子和PC61BM之作用力中，發現PC61BM與硫環之吸引力是大於與BT環之吸引力，表示PC61BM會頃向與主鏈上之硫環互相吸引，原因來自於BT環末端之氟取代基導致其極性差異所影響，因此加入BT環並沒有提升高分子與PC61BM之間的作用力，因此於混摻下與PC61BM之間能有適當之作用力，且因分子間作用力提升，推斷於混摻下高分子仍有良好的結晶性質，有助於電洞傳遞率提升。 接著我們初步分析此無序系統下之PffBT4T分子鏈，觀察到其持續長度約為125Å，表示此D-A型共聚合高分子之因內部電荷轉移 (internal charge transfer, ICT)作用，更加穩定重複單元間的雙鍵，使其能夠擁有較長的共軛長度，有助於提升電洞傳遞率。接著我們探討PffBT4T於有序堆疊狀態下之分子鏈特性與構形，發現此材料於π-π方向之作用力是相當強的，並且觀察到此材料於聚合度為8之分子鏈長度156Å並沒有明顯之分子鏈轉折，相較於P3HT系統於分子鏈長度大於80Å即有明顯之分子鏈轉折，因此於域的尺寸大時仍有良好的結晶性質，使電洞傳遞率能大幅提高；在分子鏈構形中，統計其扭轉角度發現主鏈上之BT環與硫環間因立體障礙及作用力差異，導致其偏離共平面並有兩個峰值存在，而在硫環之間的扭轉角度中則呈現相當共平面，側鏈部分則發現其與主鏈硫環平面之間是有存在約33°之扭轉，藉此降低與主鏈之間的立體障礙，整體而言此材料仍然保持相當好的緊密堆疊，由此得知分支點設置於第二個碳上是能有效減緩與主鏈上之立體障礙。整體而言，相較於P3HT材料之單一環類分子結構，其主鏈上加入BT環形成D-A型共聚合物，使其擁有較長的共軛長度，並且提升分子間吸引力，導致其於有序狀態有很強的π-π吸引力，因此有高結晶性質及電洞傳遞率，並且與PC61BM混摻下能有適當的作用力，使其於混摻下亦能維持高結晶特性，因此能應用於厚膜型元件，使能量轉換效率大幅提高。

In this study, the thick-film high-efficiency solar cell PffBT4T:PC61BM was used as the system, and the all-atomic model was used to construct the intramolecular/intermolecular potential energy function under the coarse-grained model, and the molecules in the disordered/ordered state were analyzed. Chain characteristics and configuration, from which to explore the influencing factors of this high efficiency material. First, in the intermolecular force part, we found that the interaction between the ring and the ring is the strongest between the BT ring and the sulfur ring , indicating that the material is added to the main chain compared to P3HT. The BT ring can help the material to increase the force between the molecules and contribute to the π-π force. In the force of each molecule and PC61BM, it is found that the attraction of PC61BM and sulfur ring is greater than that of the BT ring. Force, indicating that PC61BM will attract the sulfur ring on the main chain, because the fluorine substituent at the end of the BT ring causes the difference in polarity, so the addition of the BT ring does not enhance the force between the polymer and PC61BM. Therefore, under the mixing and PC61BM can have a suitable force, and due to the increase of the intermolecular force, it is concluded that the polymer still has good crystal properties, which contributes to the increase of the hole transmission rate. Then we initially analyzed the PffBT4T molecular chain under this disordered system and observed that its sustained length is about 125 Å, indicating that the D-A type copolymerized polymer is more stable due to internal charge transfer (ICT). More stable double-bonds between repeating units allows it to have a longer conjugate length, which helps to increase the hole transfer rate. Next, we investigate the molecular chain characteristics and configuration of PffBT4T in the ordered-state. It is found that the force of this material in the π-π direction is quite strong, and the molecular length of the material is observed to be 156Å. There is no obvious molecular chain twisting. Compared with the P3HT system, the molecular chain length is more than 80 Å, there is a significant molecular chain twisting. Therefore, when the size of the domain is large, there is still good crystallization property, so that the hole transmission rate can be greatly improved; In the molecular chain configuration, the torsional angle is found to find the steric hindrance and the force difference between the BT ring and the sulfur ring in the main chain, which leads to the twist the coplanar plane and two peaks exist. But the torsional angle between the sulfur rings are quite coplanar. The side chain part is found to have a twist of about 33° with the main chain sulfur ring plane, thereby reducing the steric hindrance between the main chain and the material remains good stacking. So that the branch point set on the second carbon can effectively slow down the steric hindrance on the main chain. On the whole, compared with the single aromatic ring molecular structure of P3HT material, the BT ring is added to the main chain to form a D-A type copolymer, which has a longer conjugate length and enhances the intermolecular attraction, resulting in the ordered state has a strong π-π attraction, so it has high crystallinity and hole transfer rate, and it can have a suitable force when mixed with PC61BM, so that it can maintain high crystallization characteristics under mixing. Therefore, it can be applied to a thick-film PSCs device, and the energy conversion efficiency is greatly improved.