Effect of Isomerization of Linking Units on the Photovoltaic Performance of PSMA-Type Polymer Acceptors in All-Polymer Solar Cells

Isomerization of functional groups in photovoltaic materials plays a vital role in molecular packing and their photovoltaic performance. In this work, two narrow-band gap polymer acceptors, PY5-TTz-FT and PY5-TTz-CT, are synthesized by copolymerization of the small-molecule acceptor (SMA) Y5 as the main backbone and thiophene-thiazolothiazole (TTz)-thiophene with isomerized ethylhexyl substitution on the thiophene pi-bridges as the linking unit. The polymerized SMA (PSMA) PY5-TTz-CT with the ethylhexyl substituents close to the TTz unit exhibits red-shifted absorption spectra and a relatively higher lying lowest unoccupied molecular orbital (LUMO) energy level, in comparison with PY5-TTz-FT with the ethylhexyl substituents far from the TTz unit. However, PY5-TTz-CT exhibits poorer photovoltaic performance than that of PY5-TTz-FT in the all-polymer solar cells (all-PSCs) with PBDB-T as the polymer donor, due to the poor morphology of the PBDB-T:PY5-TTz-CT active layer, while the optimized all-PSC based on PBDB-T:PY5-TTz-FT shows a higher power conversion efficiency of 12.73%. The results imply that isomerization of polymer acceptors is an effective strategy to tune the molecular packing of the photovoltaic materials and photovoltaic performance of the all-PSCs.