Semi-crystalline photovoltaic polymers with siloxane-terminated hybrid side-chains

Abstract

Three types of semi-crystalline photovoltaic polymers were synthesized by incorporating a siloxane-terminated organic/inorganic hybrid side-chain and changing the number of fluorine substituents. A branch point away from a polymer main backbone in the siloxane-containing side-chains and the intra- and/or interchain noncovalent coulombic interactions enhance a chain planarity and facile interchain organization. The resulting polymers formed strongly agglomerated films with high roughness, suggesting strong intermolecular interactions. The optical band gap of ca. 1.7 eV was measured for all polymers with a pronounced shoulder peak due to tight pi-pi stacking. With increasing the fluorine substituents, the frontier energy levels decreased and preferential face-on orientation was observed. The siloxane-terminated side-chains and fluorine substitution promoted the intermolecular packing, showing well resolved lamellar scatterings up to (300) for this series of polymers in the grazing incidence wide angle X-ray scattering measurements. The PPsiDTBT, PPsiDTFBT and PPsiDT2FBT devices showed a power conversion efficiency of 3.16%, 4.40% and 5.65%, respectively, by blending with PC71BM. Langevin-type bimolecular charge recombination was similar for three polymeric solar cells. The main loss in the photocurrent generation for PPsiDTBT:PC71BM was interpreted to originate from the trap assisted charge recombination by measuring light-intensity dependent short-circuit current density (J (sc)) and open-circuit voltage (V-oc). Our results provide a new insight into the rational selection of solubilizing substituents for optimizing crystalline interchain packing with appropriate miscibility with PC71BM for further optimizing polymer solar cells.