結合分子動力學與量子力學探討高效率高分子太陽能電池給體材料其分子鏈構形與光電性質之關聯性

Abstract

在本研究中，我們利用分子動力學與量子力學模擬方法來探討PffBT4T-2OD共軛高分子在有序排列及無序溶液狀態下分子鏈構型與光電性質之關聯性。首先我們先使用Gaussian量子計算軟體，來建構一PffBT4T-2OD單體分子，並對其做幾何最佳化觀察其構型並計算吸收光譜，並發現結果與實驗值相比有較大的紅位移，因此我們認為以Gaussian所直接做出的幾何最佳化構型，相較實際的構型來的可能過於平面。為了得到高分子鏈在溶液下真正的構型，我們便利用分子動力學的模擬加入真實溶劑來觀察高分子鏈在溶劑中情況，由於分子鏈間缺乏pi-pi作用力，分子鏈運動較為自由，均呈現出蜷曲的構形，我們經由統計各單元間的扭轉角度來得到的幾個最大機率單體，可觀察到主鏈單元間扭轉角度約為180±40˚，再將這些統計出的單體進行吸收光譜計算，並觀察到與實驗值的高度一致性，分別在400nm及550nm附近都出現了兩個峰值，吸收範圍也大約落在400nm~600nm，證明我們結合分子動力學所做出的構型模擬與真實情況是非常相似的。在有序排列下，我們同樣利用分子動力學來模擬高分子鏈在緊密堆疊時的構型，由於pi-pi作用力的影響高分子鏈呈現規整的有序排列，相對於無序溶液狀態下的結果構型來的較平面，吸收光譜的結果也呈現紅位移的現象，重要的是如同實驗吸收光譜一樣在700nm也出現了一個由聚集的pi-pi作用力產生的峰，吸收範圍也介於400nm~700nm，並藉由我們的模擬結果與之前本實驗室所作的P3HT作比較，藉此發現PffBT4T-2OD高分子在堆疊時與P3HT無太大差異，這可能是PffBT4T-2OD高分子與P3HT同樣有高結晶度，且有高電洞遷移率的原因，也是造就這個分子如此高效率的原因。

We employed all-atom molecular dynamics simulation and quantum mechanical methods to explore the relationship between molecular conformation and Photoelectric properties of PffBT4T-2OD polymer in ordered state and disordered state. First, we used Gaussian to build a monomer model and optimized the model then we calculated the UV spectrum. In comparison with experimental data we found that there’s a big red-shift of our result. We thought that the structure which was optimized by Gaussian was too coplanar. In order to obtain the true configuration of the polymer chain in the solution, we used the molecular dynamics simulation to add the real solvent to observe the polymer chain in the solvent. Due to the lack of pi-pi interaction, the molecular chain showed a cured configuration. We can obtain the maximum probability of monomer by counting the torsional angle between the units. We founded the values were approximately 180±40˚, and then we calculated the UV spectrum. The spectrum was shown in two peaks just like the experimental data. In the ordered state, the polymer chain experience the pi-pi interaction and result in more planarity for main chain. Thus, it shows significantly red-shift. In comparison with experimental data there’s also a new peak at 700nm of our result. Overall, our simulated spectrum for PffBT4T-2OD in disordered state and ordered state showed the excellent agreement with experimental data. Comparing with our previous study about P3HT, we found that there is no significant difference in order state. This is why PffBT4T-2OD has high crystallinity as P3HT and high mobility. And it may be the reason why the polymer has high performance.