High absorption coefficient π-conjugation-extended donor-acceptor copolymers for ternary-blend solar cells

Abstract The effects of one-dimensional (1D) and two-dimensional (2D) π-conjugation extensions of donor–acceptor (D–A) copolymers on their intrinsic and photovoltaic properties in ternary-blend organic solar cells (OSCs) were investigated systematically by using one polymer donor and two acceptors (PC71BM, ITIC). A series of wide bandgap D–A copolymers (PBT-OTTs) were synthesized based on the benzo[1,2-b:4,5-b′]dithiophene (BDT) unit and the thieno[3,4-c]pyrrole-4,6(5H)-dione (TPD) unit, which extend in the backbone (1D) and side-group directions (2D) respectively from the PBT-OTT backbone. These copolymers are PBT-OTT (the parent polymer), PDTBT-OTT (with a 1D backbone extension consisting of dithieno[2,3-d; 2′,3′-d′]benzo[1,2-b; 4,5-b′]dithiophene), PTTBT-OTT (with a 2D side group extension consisting of 2-alkyl thieno[3,2-b]thiophene (TT)), and PBTBT-OTT (with a 2D side group extension consisting of 2-alkyl benzo[b]thiophene (BT)). Light absorption is significantly increased in the case of PBTBT-OTT due to its extended 2D π-conjugation length and high crystallinity. This increase results from the extended delocalization length and the increased absorption coefficient α of PBTBT-OTT. The wide bandgap polymer PBTBT-OTT has the highest α of the series, so the complementary absorption of the low bandgap ITIC means that the ternary-blend OSCs based on PBTBT-OTT exhibit the highest JSC = 16.61 mA cm−2 and PCE = 8.61%. We believe that these findings provide systematic guidelines for the π-conjugation extension of conjugated polymers and thus the design of high-efficiency ternary-blend OSCs.