Thienopyrazine-based low-bandgap polymers for flexible polymer solar cells

International audienceThe optical gaps of the low-bandgap PPVs (PM-20, PM-19, PM-18) are decreased down to 1.6-1.7 eV compared with that of MDMO-PPV (2.2 eV). The best lateral hole mobility was determined to be 2.1 × 10 cm/V s (PM-18) in field effect transistors and exceeds that of MDMO-PPV (poly-[ 2-methoxy-5-(3'.7'-dimethyloctyloxy)-1.4-phenylenevinylene], 8.5 × 10 cm/V s). This allows to reduce the PCBM ([6.6]-phenyl-C-butanoic acid methyl ester) content in solar cell devices down to 1:2 w/w giving a better than for MDMO-PPV:[60]-PCBM cells (PM-19:[60]-PCBM 2.32% on ITO-PET, 2.86% on ITO glass). The charge transfer to PCBM as acceptor occurs quite normally and shows an effective charge separation using light-induced spin resonance spectroscopy (LESR). The [70]-PCBM signals are shifted to lower field related to those of [60]-PCBM and overlap more with the polaron signal of PM-19. The LESR -factor components of [70]-PCBM are reported for the first time. The external quantum efficiency peak values achieve up to 42% at ~350-400 nm and 26% at ~640 nm (PM-19:[60]-PCBM)