Benzodifuran-alt-thienothiophene based low band gap copolymers: substituent effects on their molecular energy levels and photovoltaic properties
Benzodifuran-alt-thienothiophene based low band gap copolymers: substituent effects on their molecular energy levels and photovoltaic properties
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苯并二呋喃-替代噻吩并噻吩基低带隙共聚物：取代基对其分子能级和光伏性能的影响
DOI：
10.1039/c3py00074e
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发表时间：
2013-04
影响因子：
4.6
通讯作者：
Jianhui Hou
中科院分区：
文献类型：
--
作者：
Lijun Huo;Zhaojun Li;Xia Guo;Yue Wu;Maojie Zhang;Long Ye;Shaoqing Zhang;Jianhui Hou
设计并合成了一系列含苯并二呋喃-alt-噻吩并[3，4-B]噻吩（BDF-alt-TT）的低带隙光伏共聚物。通过在噻吩并[3，4-B]噻吩单元上引入羰基、酯基、氟基、硫酰基和氰基等具有不同吸电子能力的官能团，对共聚物的分子能级进行了微调。所有这些共聚物的特征在于通过紫外-可见光谱和电化学循环伏安法（CV），以评估这些缺电子官能团的影响。根据它们的电化学行为，这些官能团的缺电子能力可按羰基<氟+酯<氟+羰基<磺酰基<氰基+酯的顺序排列。发现硫酰基和氰基取代的共聚物与其他三种共聚物相比，吸收光谱出现蓝移，表明硫酰基和氰基可能对共轭骨架产生更强的空间位阻。这些共聚物的光伏性能也进行了研究，通过制作聚合物太阳能电池（PSC）设备。有趣的是，尽管共聚物具有不同的分子结构，但所有器件的最佳D/A（聚合物/PC 71 BM，wt/wt）比为1：1.5。 使用1，8-二碘辛烷（DIO）来进一步改善器件的光伏性能。基于PBDFTT-CF/PC 71 BM的器件在开路电压（Voc）为0.63 V的情况下实现了5.23%的冠军效率。基于氰基取代的共聚物PBDFT-ECN的器件显示出0.87 V的相当高的Voc，但是短路电流密度（Jsc）显著低于基于PBDFT-CF的器件，并且似乎PBDFT-ECN和PC 71 BM的LUMO能级之间的偏移太小而不能实现有效的电荷分离。总之，通过引入缺电子基团，可以有效地调节具有交替芳香-醌型（A-Q）结构的共聚物BDF-alt-TT的分子能级。此外，根据PBDFTT-S和PBDFTT-CNF的光伏结果，-3.70 eV的LUMO能级可以达到聚合物/PC 71 BM共混物获得有效电荷分离的阈值。
A series of low band gap photovoltaic copolymers containing benzodifuran-alt-thieno[3,4-b]thiophene (BDF-alt-TT) were designed and synthesized. Functional groups with different electron-withdrawing abilities such as carbonyl, ester, fluorine, sulfuryl and cyano were introduced to the thieno[3,4-b]thiophene unit to finely tune the molecular energy levels of these copolymers. All these copolymers are characterized by UV-Vis spectroscopy and electrochemical cyclic voltammetry (CV) to evaluate the effects of these electron-deficient functional groups. According to their electrochemical behaviours, the electron-deficient abilities of these functional groups can be arranged in the order carbonyl < fluorine + ester < fluorine + carbonyl < sulfuryl < cyano + ester. It was found the sulfuryl group and cyano substituted copolymers show blue-shift in absorption spectra compared to the other three copolymers, indicating that sulfuryl and cyano groups may cause stronger steric hindrance to the conjugated backbones. The photovoltaic properties of these copolymers were also investigated by making polymer solar cell (PSC) devices. Interestingly, although the copolymers have different molecular structures, the optimal D/A (polymer/PC71BM, wt/wt) ratios of all of the devices are 1 : 1.5. 1,8-Diiodooctane (DIO) was used to further improve the photovoltaic performance of the devices. The champion efficiency of 5.23% with an open circuit voltage (Voc) of 0.63 V was achieved from PBDFTT-CF/PC71BM based devices. The device based on the cyano-substituted copolymer, PBDFTT-ECN, shows a rather high Voc of 0.87 V, but the short current density (Jsc) is significant lower than the device based on PBDFTT-CF, and its seems that the offset between the LUMO levels of PBDFTT-ECN and PC71BM is too small to achieve efficient charge separation. In conclusion, the molecular energy level of the copolymers with the alternating aromatic–quinoid (A–Q) structure, BDF-alt-TT, can be tuned effectively by introducing electron-deficient groups. Furthermore, according to the photovoltaic results of PBDFTT-S and PBDFTT-CNF, a LUMO level of −3.70 eV may reach the threshold of getting efficient charge separation of the polymer/PC71BM blend.
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