Semiconducting Polymers Consisting of Anthracene and Benzotriazole Units for Organic Solar Cells

An alternating copolymer composed of 2,6-dibromo-9,10-bis(2-ethylhexyloxy)anthracene and benzotriazole units, poly(An-alt-BTz), was synthesized, through a Suzuki cross-coupling polymerization, for use in photovoltaic devices as a p-type electron donor. For the reduction of the bandgap energy of benzotriazole units, 5,5'-dibromo-2,2'-bithiophene, or 2,5-dibromothieno [3,2-b] thiophene units were introduced into the polymer. Poly(anthracene-co-benzotriazole-co-bithiophene(thienothiophene))s were synthesized using the same polymerization reactions. The measured optical bandgap energy of poly(anthracene-alt-benzotriazole) was 2.62 eV. As the contents of the flat comonomer units in the ter-polymers increased, the bandgap energies of the resulting polymers decreased up to 1.95 eV. The energy levels of the HOMO and the LUMO of the copolymers were determined from the cyclic voltammetry. Photovoltaic devices were fabricated with the polymers as electron donors and PC71 BM as an electron acceptor. One of the fabricated devices showed the maximum PCE of 0.74% with 0.57 V of VOC, 2.59 mA/cm2 of JSC, and 0.48 of FF under AM 1.5G (100 mW/cm2) condition.