Engineering of push-pull thiophene dyes to enhance light absorption and modulate charge recombination in mesoscopic solar cells

The elaborate selection of diverse π-conjugated segments which bridge the electron donors and acceptors in organic push-pull dyes can not only tune the molecular energy-levels but also impact the interfacial energetics and kinetics of dye-sensitized solar cells (DSCs). In this paper, a series of triphenylamine-cyanoacrylic acid photosensitizers is reported with TT, EDOT-BT, EDOT-CPDT, and CPDT-EDOT (herein TT, EDOT, BT, and CPDT denote terthiophene, ethylenedioxythiophene, bithiophene, and cyclopentadithiophene, respectively) as the π-linkers, and the dye-structure correlated photocurrent and photovoltage features of DSCs based on a cobalt electrolyte are scrutinized via analyzing light absorption and multichannel charge transfer kinetics. Both stepwise incorporation of more electron-rich blocks and rational modulation of connection order of dissimilar segments can result in a negative movement of ground-state redox potential and a red-shift of the absorption peak. While these styles of reducing energy-gap do not exert too much influence on the electron injection from photoexcited dye molecules to titania, the dyestuff employing the EDOT-BT linker presents a faster interfacial charge recombination and a slower dye regeneration, accounting for its inferior cell efficiency of 5.3% compared to that of 9.4% at the AM1.5G conditions achieved by the CPDT-EDOT dye. Four new triphenylamine-cyanoacrylic acid photosensitizers with different thiophene conjugated linkers are synthesized and their important photocurrent and photovoltage features in mesoscopic titania solar cells based on a cobalt electrolyte are detailed via measuring transient photophysical and electrical signals. The best C250 dye presents an excellent power conversion efficiency of 9.4% at the AM1.5G conditions.