富勒烯吡咯烷的分子結構對於高分子太陽能電池之光伏特性影響探討

Abstract

本論文分成兩個研究主題，第一個研究主題是探究碳六十的化學結構對於元件性能的影響。我們針對一系列在苯環上連結不同烷基基團的碳六十衍生物進行有系統性探討，烷基基團分別為乙基、異丙基與第三丁基。研究結果顯示碳六十衍生物上之烷基基團推電子強度的會對於碳六十衍生物的溶解度、自身聚集行為以及與P3HT的相容性會造成很大的影響。從粉末XRD與溶解度的結果顯示當烷基分支愈多時，推電子能力隨之上升，會導致碳六十衍生物的堆疊能力上升，進而影響其溶解度下降。電子遷移率與光電子光譜的結果則顯示，碳六十衍生物卻因有序的堆疊，使得碳球與碳球間距離較遠，電子遷移率隨之變慢。但在薄膜中卻發現，因有序的堆疊導致提升其LUMO能階，有助於開環電壓的上升。另外TEM的結果發現，當在苯環上導入烷基基團時，可增進碳六十衍生物與P3HT之相容性，增加異質接面面積，提升光電轉換效率，雖然當烷基取代基為異丙基時，碳六十衍生物出現了針狀的聚集，降低了異質接面面基，可是其開環電壓卻不因其異質接面的減少而下降，且當取代基為第三丁基時，可生成奈米尺度下的雙連續互穿網絡結構，並因有序的堆疊導致其開環電壓又進一步提升至670 mV，最高效率為3.47%。 第二個研究主題是為了改善PCBM在可見光吸收較為不佳的缺點，我們應用一個在可見光中具有較多吸收的碳六十衍生物-TTC60作為元件中的電子受體，期望能利用TTC60的2,2':5',2'-terthiophene (TT)產生更多激子進一步提升高分子太陽能電池的短路電流。研究結果顯示，TTC60確實可較PCBM在可見光有較多的吸收，且當TT吸收可見光產生激子後，激子可快速的導入C60上形成自由電子與電洞，減少了激子再結合的機會。將其製備成太陽能電池時，在使用添加少量TCB與o-DCB(體積比為1:10)之共溶劑時，其效率可達2.14 %。我們進一步與PCBM做比較，雖然其效率不及PCBM，但仍可從UV-vis光譜圖發現TTC60在350~400 nm吸收較高，且IPCE在350~400 nm的光致電子的轉換亦較PCBM優異。

This thesis consists of two parts concerning the use of new C60 derivatives as electron acceptor of polymer solar cells. In the first part, the effect of the alkyl moieties, including ethyl, isopropyl, and tert-butyl groups, of N-methyl-2-(4- alkylbenzenyl)-3,4-fulleropyrrolidine on the performance of poly(3-hexylthiophene) (P3HT)-based solar devices was investigated. XRD experiments showed that as the number of branch of alkyl group increased that raised its electron donating ability, the crystallinity of these molecules increased, thus resulting in the drop of solubility. Moreover, the ordered structure of fulleropyrrolidine lengthened the inter distance of fullerene balls that visibly reduced the electron mobility. In the thin film, however, the crystalline structure boosted the LUMO energy level of C60 derivatives and enhanced open circuit voltage. On the other hand, the TEM images demonstrated the presence of alkyl groups greatly improved the compatibility of C60 derivatives and P3HT, increasing the interfacial area of bulk heterojunction and the power conversion efficiency of devices. As a tert-butyl group was employed, the blend of P3HT and fulleropyrrolidine formed a morphology of nano-scaled bi-continuous interpenetrating network and the device based on this blending film exhibited an open circuit voltage of 670 mV and an power conversion efficiency of 3.47%. In the second part, to overcome the low absorptivity of PCBM in visible region, a 2,2':5',2'-terthiophene (TT) substituted fullerene, abbreviated as TTC60, was utilized as acceptor to blend with P3HT to fabricate polymer solar cells. Photoluminence (PL) measurements indicated the photo-excited electrons generated in terthiophene units can be efficiently transferred to the neighbor C60 cage to produce free electron/hole pairs. The cells of P3HT/TTC60 processed from a solvent mixture of TCB and o-DCB at a volume ratio of 1:10 gave the best power conversion efficiency of 2.14%. Therefore, similar to the UV-vis absorption spectrum, the P3HT/TTC60 device exhibited higher IPCE than P3HT/PCBM device in the wavelength region of 350 to 400 nm.