Strong addition effect of charge-bridging polymer in polymer: Fullerene solar cells with low fullerene content

We report a strong influence of charge-bridging polymer addition on the performance of polymer:fullerene solar cells with a low fullerene content, which were fabricated with the bulk heterojunction (BHJ) films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM). Poly[(4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b:4, 5-b′]dithiophene)-2,6-diyl-alt-(N-2-ethylhexylthieno[3,4-c]pyrrole-4, 6-dione)-2,6-diyl]] (PBDTTPD) was employed as a charge-bridging polymer by considering its energy band structure, while the PC₆₁BM content was fixed at 33.3 wt.% (P3HT:PC₆₁BM = 1:0.5 by weight). Results showed that the power conversion efficiency (PCE) of solar cells was significantly improved by more than 7-fold by adding 20 wt.% PBDTTPD; however, it slightly decreased with further addition of PBDTTPD. This huge PCE enhancement has been assigned to the charge-bridging role of PBDTTPD between P3HT and PC ₆₁BM domains, which is closely related to the pronounced nanostructure changes in the BHJ films, including the increased size of PC ₆₁BM aggregates (∼8.5 nm).