Band gap and molecular energy level control of perylene diimide-based donor–acceptor copolymers for all-polymer solar cells

Four types of perylene diimide-based electron acceptor materials, namely, poly[9,9-dioctylfluorene-2,7-diyl-alt-N,N′-di(2-ethylhexyl)-3,4,9,10-perylene diimide-1,7-diyl] (PF-PDI), poly[9,9-dioctylfluorene-2,7-diyl-alt-1,7-dithien-2-yl-N,N′-di(2-ethylhexyl)-3,4,9,10-perylene diimide-5′,5′′-diyl] (PF-DTPDI), poly{N-[1-(2-ethylhexyl)-3-ethylheptanyl]-dithieno[3,2-b:2′,3′-d]pyrrole-2,6-diyl-alt-N,N′-di(2-ethylhexyl)-3,4,9,10-perylene diimide-1,7-diyl} (PDTP-PDI) and poly{N-[1-(2-ethylhexyl)-3-ethylheptanyl]-dithieno[3,2-b:2′,3′-d]pyrrole-2,6-diyl-alt-1,7-dithien-2-yl-N,N′-di(2-ethylhexyl)-3,4,9,10-perylene diimide-5′,5′′-diyl} (PDTP-DTPDI), have been synthesized. By changing the donor segment from fluorene to dithienopyrrole and/or introducing a thiophene unit as a spacer, the band gap and energy levels of the resulting polymers could be tuned in a wide range. PDTP-DTPDI exhibited the narrowest band gap of 1.24 eV, and the absorption edge extended to 1 μm. All-polymer solar cells based on these electron acceptors, blended with different electron donor polymers, namely, a polythiophene derivative (P1) and a low band gap polymer (P2), were also investigated. P1:PDTP-PDI blends exhibited the highest power conversion efficiency of 0.93% under the illumination of AM 1.5 (100 mW cm−2). The monochromatic photocurrent response of the photovoltaic device based on P2:PDTP-PDI blends extended to the near-infrared region up to 1 μm.