Structural variations to a donor polymer with low energy losses

Two regioregular narrow bandgap conjugated polymers with a D′-A-D-A repeat unit architecture, namely PIFCF and PSFCF, were designed and synthesized. Both polymers contain strictly organized fluorobenzo[c][1,2,5]thiadiazole (FBT) orientations and different solubilizing side chains for solution processing. Compared to the previously reported asymmetric pyridyl-[2,1,3]thiadiazole (PT) based regioregular polymer, namely PIPCP, PIFCF and PSFCF exhibit wider bandgaps, tighter π-π stacking, and improved hole mobilities. When incorporated into solar cells with fullerene acceptors, the E_loss = E_g - eV_oc values of PIFCF and PSFCF devices are increased compared to solar cells based on PIPCP. Determination of E_ct in these solar cells reveals that, relative to PIPCP, PIFCF solar cells lose more energy from E_g - E_ct, and PSFCF solar cells lose more energy from both E_g - E_ct and E_ct - eV_oc. The close structural relationship between PIPCP and PIFCF provides an excellent framework to establish molecular features that impact the relationship between E_g and E_ct. Theoretical calculations predict that E_loss of PIFCF:PC₆₁BM would be higher than in the case of PIPCP:PC₆₁BM, due to greater E_g - E_ct. These findings provide insight into the design of high performance, low voltage loss photovoltaic polymeric materials with desirable optoelectronic properties.