Rigidifying the π-Linker to Enhance Light Absorption of Organic Dye-Sensitized Solar Cells and Influences on Charge Transfer Dynamics

The usage of coplanar it-conjugated segments represents a feasible strategy on reducing the energy gap of organic push pull dyes for mesoscopic titania solar cells. In this paper, we report two new dyes coded as C254 and C255 with the respective 1,4-di(thiophen-2-yl)benzene and indacenodithiophene pi-linkers, in combination with the electron-releasing triphenylamine and electron-withdrawing cyanoacrylic acid units. The energy-gap reduction stemming from the rigidity of the g-linker is accompanied by a negative shift of the ground-state redox potential, which however does not affect the yield of hole injection from the oxidized state of dye molecules to a cobalt redox electrolyte. On the other side, we have identified from femtosecond transient absorption measurements a diminished rate of electron injection from the relaxed, low-energy excited state of C255 to titania, albeit a comparable rate of electron injection from the high-energy excited states of these two dyes. The bulkier C255 dye with four hexyl side chains tethered on the two sp(3) carbons of the fused indacenodithiophene unit can form a more compact self-assembling monolayer on titania, considerably attenuating the charge recombination of photoinjected electrons in titania with the cobalt electrolyte and thus enhancing the cell photovoltage and efficiency.