Molecular engineering of indoline based organic sensitizers for highly efficient dye-sensitized solar cells

Abstract

The increasing electron-donating ability of the donor part is focused to further optimize the light-harvesting capability. Our strategy is to introduce an additional donor group into the indoline unit in the donor part to form a donor–donor structure (D–D moiety). Three different units (carbazole, fluorene and 4-methylphenyl groups) with different degrees of electron-donating capability are incorporated, thus constructing the specific donor–donor–π–acceptor (D–D–π–A) system (C-CA, F-CA and I-3) and giving a systematic view of the absorption evolution. Through molecular engineering, their light-harvesting capabilities, energy levels and photovoltaic performances were studied. As expected, utilizing strong electron-donating carbazole unit as additional donor, the IPCE spectrum of DSSC based on C-CA is successfully broadened to NIR region on the premise of suitable LUMO level, with an extraordinarily high plateau in visible region till around 700 nm. In the system of C-CA and F-CA, the introduction of n-pentyl group in donor part of carbazole and fluorene unit has little effect on preventing the molecular π-aggregation due to the good co-planarity of π-linker (vinyl thiophene), suggesting that the most effective way to prevent π-aggregation is still the incorporation of long alkyl groups into planar π-linker segment. However, the introducing long alkyl group can effectively prevent the electron recombination between electrons in conduction band (CB) of TiO2 and I3− ions. Along with the preferable light-harvesting capability, C-CA presents excellent IPCE performance with a short-circuit photocurrent (Jsc) of 18.53 mA cm−2, an open-circuit photovoltage (Voc) of 649 mV, a fill factor of 0.71, corresponding to a power conversion efficiency (η) of 8.49%. The internal relations between chemical structure and conversion efficiency provide a strategy for developing highly efficient organic sensitizers working in whole visible region with high photovoltaic performance.