Influence of the N-heterocycle substituent of the dithieno[3,2-b:2′,3′-d]pyrrole (DTP) spacer as well as sensitizer adsorption time on the photovoltaic properties of arylamine organic dyes

Dithieno[3,2-b:2',3'-d]pyrrole (DTP) compounds with strong electron-donating ability are promising spacers for photosensitizers. A series of new arylamine organic dyes bearing N-heterocycle-substituted DTP spacers has been designed and synthesized for iodine-free dye-sensitized solar cells (DSCs). The absorption, electrochemical, and photovoltaic properties for all the sensitizers have been systematically investigated. It was found that the incorporation of a dihexylaniline or hexylcarbazole unit, instead of a benzene unit, can notably retard the charge recombination at the titania-electrolyte interface, thus improving the cell photocurrent and photovoltage. Furthermore, we found a significant efficiency enhancement upon prolonging the adsorption time during the dye uptake from solution. When the dye adsorption was performed for 36 hours, the photovoltaic performance parameters measured under standard reporting conditions improved, in comparison to cells stained for 12 hours. The effects of adsorption time, correlated dye load amounts and electron lifetime were also scrutinized in terms of light-harvesting, interfacial kinetic parameters, as well as their joint contribution to the photovoltaic performance. Profiting from the favorable influence that the 36 hours dye uptake exerts on photovoltaic performance, we have realized a highly efficient iodine-free DSC displaying a power conversion efficiency of 8.20% measured at 100 mW cm(-2) simulated AM1.5 conditions.