Improving the all-polymer solar cell performance by adding a narrow bandgap polymer as the second donor.

Ternary all-polymer solar cells are fabricated using an N2200 acceptor and two donor polymers (PF2 and PM2) with complementary absorption. The major donor PF2 is a relatively wide bandgap polymer that contributes the most photon absorption in the UV-vis region while the second donor PM2 improves the light harvesting due to its strong absorption in the near-IR region. By carefully tuning the ratio of two donor polymers, the best ratio of 9 : 1 : 5 (PF2 : PM2 : N2200) is achieved and shows a PCE of 6.90%, which is better than two binary devices. This work demonstrates an effective strategy of utilizing a narrow bandgap donor polymer as the second donor to improve the performance of all-polymer solar cells.