Efficient all-polymer solar cells based on a narrow-bandgap polymer acceptor

The synthesis of novel polymer acceptors by polymerizing narrow-bandgap small molecule acceptors (SMAs) is an important strategy in the development of all-polymer solar cells (all-PSCs). Herein, a novel narrow-bandgap polymerized SMA (PSMA) PG1, based on a Y6-like building block of 2,2′-((2Z,2′Z)-((12,13-bis(2-butyloctyl)-3,9-dioctyl-12,13-dihydro-[1,2,5]thiadiazo-lo[3,4-e]thieno[2′′,3′′:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indo-le-2,10-diyl)bis(methanylylidene))bis(3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile copolymerized with a thienyl substituted benzo-[1,2-b:4,5-b′]dithiophene (BDT-T) unit, was designed and synthesized. The PSMA PG1 shows a broad and strong absorption in the region of 550–900 nm with a narrow optical bandgap (Eoptg) of 1.42 eV, a high maximum extinction coefficient of 1.19 × 105 cm−1 and a high electron mobility of 6.49 × 10−4 cm2 V−1 s−1, benefiting from the narrow bandgap, strong absorption and high electron mobility of the SMA building blocks. Furthermore, PG1 shows an expected up-shift in the lowest unoccupied molecular orbital energy level of −3.81 eV compared to the SMA building block, which is beneficial for achieving a higher open-circuit voltage (Voc) in all-PSCs. Upon processing with 1-chloronaphthalene (CN) as a solvent additive, the optimal all-PSC based on a blend of PBDB-T:PG1 achieved an outstanding PCE of 11.5% with a high Voc of 0.94 V, a short-circuit current density (Jsc) of 17.8 mA cm−2, a fill factor (FF) of 0.69 and a low energy loss of 0.48 eV. These Voc and PCE values are among the highest of the all-PSCs reported to date. These results indicate that PG1 is a promising polymer acceptor for highly efficient all-PSCs.