Correlation between polymer architecture, mesoscale structure and photovoltaic performance in side-chain-modified poly(p-arylene-ethynylene)-alt-poly(p-arylene-vinylene): PCBM bulk-heterojunction solar cells

Recent investigations have shown that an anthracene containing poly(p-arylene-ethynylene)-alt-poly(p-arylene-vinylene) statistical copolymer consisting of a well defined conjugated backbone, along which linear and branched alkoxy side chains are attached in a random manner, yields, compared to its counterparts with regular side chain substitution, an improved performance in polymer [6,6]:-phenyl-C61-butyric acid methyl ester (PCBM) bulk-heterojunction solar cells. The microscopic origin for the improved power conversion efficiency (η ≈ 3.8%) of the statistical copolymer - which is the best in its material class - has not been resolved. We conducted grazing incidence wide-angle x-ray scattering investigations in order to correlate the nanomorphology of the active layers to the photovoltaic performance of the device. A comparison of the results obtained for the statistical copolymer to those obtained for the corresponding regular copolymers shows that the improved performance of the former may be attributed to a combination of the following structural characteristics: 1. well, ordered stacked domains that promote backbone planarization, 2. partly face-on alignment of domains relative to the electrodes for an improved active layer-electrode charge transfer, and 3. a more isotropic domain orientation throughout the active layer that ensures that the backbone alignment direction has components perpendicular and parallel to the electrodes in order to compromise between light absorption and efficient intra-chain charge transport. The regular copolymers exhibiting inferior performance lack either sufficient stacking order or face-on alignment of the domains. None of them shows isotropic domain orientation.