Conjugated and Nonconjugated Substitution Effect on Photovoltaic Properties of Benzodifuran-Based Photovoltaic Polymers

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• 作者：Lijun Huo ; Long Ye ; Yue Wu ; Zhaojun Li ; Xia Guo ; Maojie Zhang ; Shaoqing Zhang ; Jianhui Hou
• 刊名：Macromolecules
• 出版年：2012
• 出版时间：September 11, 2012
• 年：2012
• 卷：45
• 期：17
• 页码：6923-6929
• 全文大小：455K
• 年卷期：v.45,no.17(September 11, 2012)
• ISSN：1520-5835

文摘

In order to investigate the influence of two-dimensional (2D) conjugated structure on photovoltaic properties of benzo[1,2-b:4,5-b鈥瞉difuran (BDF)-based polymers, two low band gap photovoltaic polymers, named PBDFTT-CF-O and PBDFTT-CF-T, were designed and synthesized. These two polymers have the same backbones and different side groups. Although these two polymers show similar optical band gaps (ca. 1.5 eV), the polymer with alkylthienyl side groups, PBDFTT-CF-T, exhibits stronger absorption in long wavelength direction than the polymer with alkoxyl side groups, PBDFTT-CF-O. Meanwhile, PBDFTT-CF-T exhibits a HOMO level of 鈭?.21 eV, which is 0.23 eV lower than that of PBDFTT-CF-O due to weaker electron-donating ability of alkylthienyl side groups than that of aloxyl side groups. The hole mobility of the blend of PBDFTT-CF-T/PC₇₁BM (1:1.5, w/w) is 0.128 cm² V^鈥? s^鈥?, which is 1 order of magnitude higher than that of the blend of PBDFTT-CF-O/PC₇₁BM. Density functional theory (DFT) model shows thiophene pendants on dithienyl-BDF are more coplanar than it on dithienyl-BDT. These results indicate that the 2D-conjugated structure is helpful for molecular structure design of the BDF-based polymers in enhancing the intermolecular 蟺鈥撓€ stacking and improving charge transport property. Furthermore, the photovoltaic devices based on these two polymers show similar short circuit density and fill factor values, while the open circuit voltage of the PBDFTT-CF-T-based device is 0.78 V, which is 0.15 V higher than that of the PBDFTT-CF-O-based device. Therefore, the efficiencies of the devices based PBDFTT-CF-T/PC₇₁BM and PBDFTT-CF-O/PC₇₁BM are 6.26% and 5.22%, respectively. The results in this work demonstrate that the weak electron-donating ability of alkylthienyl side groups can be seen as an effective strategy to improve photovoltaic properties of the BDF-based polymers and the 2D-conjugated molecular structure is favorable to improve hole mobility.