The integrated adjustment of chlorine substitution and two-dimensional side chain of low band gap polymers in organic solar cells

A series of conjugated T2 polymers (PBBF1-T2 and PBBCl1-T2), and T3 polymers (PBBF1-T3, PBBCl1-T3 and PBBCl2-T3) were synthesized using chlorinated/fluorinated benzothiadiazole (BT) and the two-dimensional benzo[1,2-b:4,5-b′]dithiophene (BDT) units as the building blocks. When compared to the fluorinated polymer, the performance of the polymer photovoltaic devices showed that these chlorinated polymers gave extended optical absorption spectrum, and lower highest occupied molecular orbital (HOMO) energy levels. The introduction of chlorine atoms increases the twist angle between the polymer backbones, and led to a lower HOMO energy level and resulted in the increase of open circuit voltage (Voc) up to 0.84 V in PBBCl2-T3 based devices with a two chlorine substitution. However, the device based on PBBCl1-T3 with only one chlorine atom exhibited the best power conversion efficiency (PCE) which was as high as 6.87% with a Voc of 0.73 V, and this was about 10% higher than that of its fluorinated analogs. This result indicated that the introduction of chlorine atoms into polymers is not only a simple route to synthesize a large amount of material and which avoids the tedious synthesis steps in widely used fluorinated polymers, but it is also a feasible and effective strategy to fine tune the energy level of polymer solar cell with optimized PCE. Furthermore, it is worth noting that the introduction of longer π-conjugation side chains could minimize the influence of chlorine substitution by reducing the twist angle between the polymer backbones, which would reduce the gap of Voc between the chlorinated polymers and their fluorinated analogs.