D-A copolymers containing lactam moieties for polymer solar cellsElectronic supplementary information (ESI) available: Material preparation and characterization, solar cell fabrication and characterization. See DOI: 10.1039/c5py01064k

Two D-A copolymers P1 and P2 were developed by copolymerizing a new lactam acceptor unit, [7,7′-bidithieno[3,2- b :2′,3′- d ]pyridine]-5,5′(4 H ,4′ H )-dione (BDTP), with benzo[1,2- b :4,5- b ′]dithiophene (BDT) or selenophene. The copolymers have good solubility, thermal stability and deep HOMO levels. P2 :PC 71 BM solar cells gave a PCE of 6.76%, with a V oc of 0.91 V, a J sc of 9.96 mA cm −2 and an outstanding FF of 74.5%. D-A copolymers P1 and P2 containing a new lactam acceptor unit BDTP and BDT or selenophene as a donor unit gave PCEs of 5.05% and 6.76%, respectively.