Herein, a series of random donor copolymers were reported, by incorporating different contents of electron-rich 4,8-bis(9,9-bis(2-ethylhexyl)-9H-fluoren-2-yl)benzo[1,2-b:4,5-b']dithiophene (BDTF) units into PBDB-T (poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b’]dithiophene))-alt-(5,5-(1’,3’-di-2-thienyl-5’,7’-bis(2ethylhexyl)benzo[1’,2’-c:4’,5’-c’]dithiophene-4,8-dione))]) polymer to fine-tune the highest occupied molecular orbital (HOMO) energy level. As we expected, downshifted HOMO of corresponding copolymers was observed with increased the ratios of BDTF unit on PBDB-T polymer, causing by the synergy of weak electron-donating ability of the fluorene side chains and the enhanced dihedral angles. Notably, compared with PBDB-T, the HOMO energy level of P91 containing 10% BDTF was shifted down by 0.07 eV. Among them, the devices based on P55:ITIC reached a high open-circuit voltage (V_OC) of 1.00 V with incorporation of 50% of BDTF units. The result indicated that the BDTF unit, as an excellent building unit in donor materials, could play a crucial role to realize high-V_OC polymer solar cells.

在此，通过引入不同含量的富电子的4,8-双（9,9-双（2-乙基己基）-9H-芴-2-基）苯并[1,2- b：4,5-b']二噻吩（BDTF）单元变成PBDB-T（聚[（2,6-（4,8-双（5-（2-乙基己基）噻吩-2-基）-苯并[1] ，2-b：4,5-b']二噻吩））-alt-（5,5-（1'，3'-二-2-噻吩基-5'，7'-双（2乙基己基）苯并[1' ，2'-c：4'，5'-c'] dithiophene-4,8-​​dione））））聚合物微调最高占据分子轨道（HOMO）的能级。正如我们预期的那样，观察到相应共聚物的HOMO下降，同时PBDB-T聚合物上BDTF单元的比例增加，这是由于芴侧链的弱供电子能力和增强的二面角的协同作用所致。值得注意的是，与PBDB-T相比，含10％BDTF的P91的HOMO能级降低了0.07 eV。其中，V _OC）为1.00 V，并结合了50％的BDTF单元。结果表明，BDTF单元作为供体材料中的优良构建单元，对于实现高V _OC聚合物太阳能电池可能起着至关重要的作用。