Tuning of the photovoltaic parameters of molecular donors by covalent bridging

The synthesis of donor-acceptor molecules involving triarylamines and dicyanovinyl blocks is described. Optical and electrochemical results show that rigidification of the acceptor part of the molecule by a covalent bridge leads to a ca. 0.20 eV increase of the band gap due to a parallel increase of the lowest unoccupied molecular orbital level. A preliminary evaluation of these compounds as donor materials in organic solar cells shows that although this structural modification reduces the light-harvesting properties of the donor molecule, it nevertheless induces an increase of the efficiency of the resulting solar cells due to a simultaneous improvement of the open-circuit voltage and fill factor. Covalent bridging of the dicyanovinyl group with the adjacent thiophene ring of small push-pull molecular donors based on triarylamines leads to a significant improvement of the efficiency of the resulting organic solar cells due to the simultaneous increase of the fill factor and open-circuit voltage.