A novel optical band gap of 1.57 eV isoindigo (IID)-containing polymer of PBDP-IID was synthesized and the HOMO of the polymer descends to −5.44 eV due to strong electron-deficient isoindigo unit. Bulk heterojunction solar cells made from PBDP-IID exhibited a high Voc of 0.95 V, which shows the highest value among the PSCs based on the narrow bandgap polymer donors with Eg < 1.6 eV.
![Graphical abstract: Poly(benzo[2,1-b:3,4-b′]dithiophene-alt-isoindigo): a low bandgap polymer showing a high open circuit voltage in polymer solar cells](http://hg.y866.cn/compound/lib/scimg/usr/1/C4RA12171F.jpg)
