New acceptor-pended conjugated polymers based on 3,6- and 2,7-carbazole for polymer solar cells

A series of new acceptor-pended conjugated polymers P27CZSDCN, P27CZSDTA, P36CZSDCN and P36CZSDTA, which are composed of carbazole and triphenylamine alternatingly arranged main chain, electron-withdrawing pendant groups (malononitrile and 1,3-diethyl-2-thiobarbituric acid) and thiophene π-bridge, are designed and synthesized. Both 2,7-carbazole and 3,6-carbazole are used as the building blocks to tune conjugation length and charge transfer properties of the resulting polymers. The relationships between chemical structure and optoelectronic properties of the polymers were systematically investigated. It was found that the 3,6-carbazole-derived polymers exhibited blue-shifted absorption at the short wavelength region and lower hole mobility compared to their 2,7-carbazole-derived analogues because of the conjugation breaks induced by 3,6-carbazole units. Moreover, the 3,6-carbazole-derived polymers exhibited much enhanced and slightly red-shifted intramolecular charge transfer absorption at the long wavelength region, as well as lower oxidation potential with respect to their 2,7-carbazole-derived analogues, possibly being caused by the more efficient electron-donating capability of nitrogen atoms on 3,6-carbazole units. The photovoltaic properties of the resulting polymers are studied by blending the polymers with (6,6)-phenyl-C71-butyric acid methyl ester. The highest power conversion efficiencies of 2.47% and 1.69% are achieved for 2,7-carbazole-derived polymers and 3,6-carbazole-derived polymers, respectively.