Subtle Effect of Alkyl Substituted π-Bridges on Dibenzo[a,c]phenazine Based Polymer Donors towards Enhanced Photovoltaic Performance

Selection of the strategically substituted alkyl chains has a significant effect to modulate the physical properties of conjugated polymers, electro-optical characteristics, and active layer morphology of the corresponding polymer solar cells (PSCs). Herein, we systematically synthesized three dibenzo[a,c]phenazine based D-pi-A donor polymers named PBP-C0, PBP-C8, and PBP-C6 with different alkyl substitutions on thiophene pi-bridges, without alkyl, 2-ethylhexyl and n-hexyl groups, respectively. The absence of the alkyl chain (PBP-C0) on the pi-bridge caused poor solubility and unfavorable miscibility with the Y5 acceptor, leading to the lower photovoltaic performance. The bulky alkyl chain of 2-ethylhexyl on the pi-bridge group caused the twisting of PBP-C8 conjugated backbone, which limits the charge transport and also compromises the photovoltaic performance. In contrast, the PBP-C6-with flexible linear alkyl chains has almost planar curvature geometry resulting in the small uniform domain size and appropriate phase separation in the blend film morphology. These favorable properties enhanced the exciton generation to dissociation, charge carrier mobility, and also lowered the charge recombination. Among three polymers, PBP-C6-based devices exhibit the best PCE of 11.60%. From these results, thiophene pi-bridge alkyl substitution demonstrated an important strategy to adjust energy level, absorption, and phase separation morphology to enhance the photovoltaic performance of the PSCs.

Automated summary

It was found that the selection of appropriate substituents has a significant impact on the physical properties, electro-optical characteristics, and active layer morphology of conjugated polymers. In this study, three D-pi-A donor polymers with different alkyl substitutions on thiophene pi-bridges were synthesized and compared. It was observed that the absence of alkyl chain and the presence of a bulky alkyl chain led to poor photovoltaic performance. On the other hand, polymers with flexible linear alkyl chains showed enhanced photovoltaic performance.