Photovoltaic properties of low band gap polymer based on phenanthrene and diketopyrrolopyrrole

A new low band gap polymer based on 9,10-dialkoxyphenanthrene and diketopyrrolopyrrole, poly[9,10-bis-(2-octyl-dodecyloxy)-phenanthrene-alt-2,5-dioctyl-3,6-di-thiophen-2-yl-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione] (PN40DPP) has been synthesized for photoactive polymer in polymer solar cells. The HOMO and LUMO energy level calculated from the onset oxidation and reduction potential of the cyclic voltammogram of PN40DPP were −5.13 and −3.33 eV, respectively. The optical band gap calculated from the absorption edge of the UV–vis spectrum of PN40DPP was 1.65 eV, which is smaller than that the electrochemical band gap (1.80 eV). The field effect transistor based on PN40DPP showed p-type behavior with a hole mobility of 1.15 × 10−3 cm2 V−1 s−1 in the saturation regime. The polymer provides a low HOMO level for obtaining solar cells with a high open circuit voltage of 0.68–0.70 V. The best bulk heterojunction solar cell with the blend of a 1:2 ratio of PN40DPP and PCBM showed a Jsc of −3.08 mA cm−2, a fill factor of 56.0%, and a power conversion efficiency of 1.19%.