Improvement of short circuit current density by intermolecular interaction between polymer backbones for polymer solar cells

Benzodithiophene- and quinoxaline (Qu)-based derivatives were used to obtain two kinds of D-p-A-type polymers, poly(diethylhexyloxy benzo dithiophene-dioctyloxy dithiophene diphenyl Qu) (PBDTAQ) and poly(diethylhexyloxy benzo dithiophene-dioctyloxy dithiophene dibenzophenazine (PBDTAFQ), through a Stille coupling reaction. Phenyl (Qu) and fused-phenyl (phenazine, Pz) were introduced in locations 2 and 3, respectively, of Qu derivatives. For the polymer introduced with Pz, the tilt angle (theta(3)) between the 2,3-carbon and the bonded phenyl side chains of Qu decreased from 44.06 degrees to 0 degrees. As a result, PBDTAFQ displayed a higher absorbance compared with PBDTAQ at the UV-vis absorption spectra in the film state. Also, the intra/intermolecular charge transfer (ICT) peak intensity of 500-700 nm increased relative to the solution peak. Furthermore, the results of the X-ray diffraction measurement suggest that the d(pi)-spacing distance for PBDTAFQ (d(pi)=3.89 angstrom) was smaller than that for PBDTAQ (d(pi)=4.36 angstrom), exhibiting a stronger intermolecular interaction. PBDTAQ showed an edge-on dominant orientation, whereas the PBDTAFQ thin films showed an increase in the face-on structure for crystallinity. As a consequence, PBDTAFQ showed an improved short current density (J(SC)) in organic photovoltaic cells. The PBDTAFQ:PC70BM blend-based devices that were fabricated exhibited a power conversion efficiency of 3.0% at a 1:5 ratio and reached 3.4% when treated with additive and methanol.