Effects of structure and electronic properties of D-π-A organic dyes on photovoltaic performance of dye-sensitized solar cells

Abstract Herein, we examine the performance of dye-sensitized solar cells containing five D-π-A organic dyes designed by systematic modification of π-bridge size and geometric structure. Each dye has a simple push-pull structure with a triarylamino group as an electron donor, bithiophene-4,4-dimethyl-4H-cyclopenta[1,2-b:5,4-b’]dithiophene (M11), 4,4-dimethyl-4H-cyclopenta[1,2-b:5,4-b’]dithiophene-thiophene (M12), thiophene-4,4-dimethyl-4H-cyclopenta[1,2-b:5,4-b’]dithiophene (M13), 4,4-dimethyl-4H-cyclopenta[1,2-b:5,4-b’]dithiophene-benzene (M14), and 4,4-dimethyl-4H-cyclopenta[1,2-b:5,4-b’]dithiophene (M15) units as π-bridges, and cyanoacrylic acid as an electron acceptor/anchor. The extension of the π-bridge linkage favors wide-range absorption but, because of the concomitant molecular volume increase, hinders the efficient adsorption of dyes on the TiO2 film surface. Hence, higher loadings are achieved for smaller dye molecules, resulting in (i) a shift of the TiO2 conduction band edge to more negative values, (ii) a greater photocurrent, and (iii) suppressed charge recombination between the photoanode and the redox couple in the electrolyte. Consequently, under one-sun equivalent illumination (AM 1.5 G, 100 mW/cm2), the highest photovoltage, photocurrent, and conversion efficiency (η = 7.19%) are observed for M15, which has the smallest molecular volume among M series dyes.