以分子動力學模擬有機太陽能電池中二氧化鈦表面含氰基丙烯酸基之共軛染料分子排列型態及其對聚3-已基噻吩吸附位向的影響

Abstract

本模擬研究利用全原子模型的分子動力學模擬，探討不同噻吩環數的染料分子(W1、W2、W3)及改變分子鏈上環的種類，並將其接枝於二氧化鈦基板表面後，觀察染料分子鏈間的排列型態對於P3HT(聚3-己烷噻吩)分子鏈構形及吸附位向的影響。由染料的排列型態發現，50%與100%接枝密度的系統中，由W1到W3染料系統中隨著接枝鏈上環數增加，鏈上環彼此間π-π堆疊吸附明顯，且接枝鏈上環與環的堆疊方式多以平行錯位的吸引位向為主，本模擬所架構之50%接枝密度基板較符合真實實驗系統的情形，故本文便可將實驗及模擬系統做比較。在50%接枝密度下，因接枝鏈局部聚集，使排列型態稀疏，並且鏈間作用力小，使染料上的環能保持較佳的共平面性，因此提供良好共軛系統使電子能有效傳遞，其中又以W3染料最為優異，實驗中也得知其電流密度最高。而W4系統中，因鏈上苯併噻二唑環使接枝鏈無法保持共平面性，故電流密度不如W3染料系統高，但加入此環使分子鏈上有D/A系統，可降低能隙，使開路電壓上升，最後整體元件光電轉換率以W4染料系統最高。當將P3HT放置於染料系統時，會因P3HT受到染料環數的增加而導致其容易穿插於染料接枝層中，由模擬系統可觀察到在W2及W3系統中P3HT很靠近二氧化鈦基板，而使電子電洞再結合的機會增加，因此推測可藉由提高接枝密度，以減少此現象產生。但由100%接枝系統中可發現，雖能減少P3HT側鏈插入，但因染料接枝密度較高，而使接枝鏈彼此作用力大，染料共平面性差，不利電荷於鏈上傳遞；此外，也會因為P3HT的側鏈不易插入，使主鏈與染料接枝層間距離較遠，而使電荷傳遞性較差。因此當P3HT烷基側鏈長度減少，例如我們分析在無烷基側鏈的導電高分子PT(Polythiophene)情況下，在100%接枝密度下因染料間彼此過分聚集，不利於PT插入，鏈上之噻吩環因著π-π作用力與接枝鏈上的環以T-shaped吸引，而平躺於接枝層表面，並維持良好的共平面型態，使電荷於導電高分子鏈上傳遞較佳。因此可推估當減少P3AT(poly 3-alkyl thiophene)側鏈碳數，可提升主鏈共平面性而使電荷能有效於導電高分子與接枝層中傳遞。

We employ the all-atom molecular dynamic simulation and the theoretical calculation to study the different thiophene ring numbers of dye (W1、W2、W3) and change the ring type on molecular chain (W4).The characteristic of their packing orientation may affect the conformation and the adsorption orientation of P3HT. Among the packing morphologies of these dyes, we found that the π-π interaction between the chains become strong when the numbers of ring increase. Parallel displaced and T-shaped stacking are the major geometry among packing orientation of thiophene rings. In this study, the simulation of 50% grafting density is closer to real experiment, so we can compare the experiment and the simulation system. In the 50% grafted density, the arrangement of dyes is sparse and the interaction between chains is small due to the dyes are partial aggregation, so the rings can maintain a better coplanarity and therefore provide a good conjugation system to enable electronic effective delivery; and from the simulation result, we can find the W3 system have the best coplanarity of all the system. In the W4 system, The coplanarity of molecular chain is worse due to the benzothiadiazole ring, so the current density is not as higher as W3 dye system, but the band gap will be reduced because of the molecular chain added D/A system to make the VOC increase. Therefore, the IPCE of W4 system is the highest one of all the systems. When placed the P3HT into the dye system, it’s easier to insert in the dye-layer surface due to the ring numbers increased of dye. Our simulations indicate that P3HT is closer to titanium dioxide substrate, this result lead to electron-hole pair recombination; In order to improve this phenomenon, we raise the grafting density of dyes, however, P3HT is difficult to insert into the dye-layer at higher grafting density cause the long distance between P3HT and dye-layer. The coplanarity of chain is also poor due to high grafting density,so we attempted to reduce the alkyl side chain length of P3HT to improve the charge transfer ability. According to above metion, PT (Polythiophene) was used to adsorb on the high grafting density surface because there exit no alkyl side chain. We found that PT is not only easier to close dye –layer, but PT maintains a coplanar structure. Therefore the ability of charge transfer between polymer and dye-layer were better. The present result represents when reducing the side chain length of P3AT(poly 3-alkyl thiophene) will promote the coplanarity of the main chain and provide a route to transfer charge.