Co-sensitization of Dithiafulvenyl-Phenothiazine Based Organic Dyes with N719 for Efficient Dye-Sensitized Solar Cells

Abstract The introduction of additional electron donor and elongation of π-conjugated bridge represents a feasible strategy to enhance light harvesting in both breadth and intensity of donor-(π conjugated spacer)-acceptor (D-π-A) dyes suitable for dye-sensitized solar cells (DSSCs). Here, two novel metal-free organic dyes, in which dithiafulvenyl unit was introduced into phenothiazine donor as an additional electron donor, cyanoacrylic acid as the electron acceptor, and without ( PTZ-2 ) or with ( PTZ-3 ) a benzene as the π-linker between phenothiazine donor and cyanoacrylic acid electron-withdrawing group, were synthesized by modifying the simple D-π-A dye ( PTZ-1 ) and successfully utilized in DSSCs. Their photophysical, electrochemical, photovoltaic properties and theoretical calculations were further investigated. Relative to the reference dye PTZ-1 , the power conversion efficiencies (PCEs) increased significantly from 4.04% to 5.81% ( PTZ-2 ) and 5.22% ( PTZ-3 ), which came mainly from the enhancement of short-circuit current ( J sc ) due to the broader molecular absorption spectra and higher molar extinction coefficients. What's more, it is worth noting that the dyes of PTZ-2 and PTZ-3 have strong absorption, which can compensate for that of N719 in the low wavelength region, especially in the region of ∼300–500 nm. Therefore, a co-sensitization approach has been used for achieving enhanced performances in DSSCs wherein N719 was co-sensitized with PTZ-2 and PTZ-3 , respectively. Consequently, the co-sensitized DSSC ( PTZ–2  +  N719 ) gave the highest efficiency of 8.12%, exhibiting an improvement of 16.50% compared to the device sensitized with N719 alone (PCE = 6.97%) under the same conditions.