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

The introduction of additional electron donor and elongation of pi-conjugated bridge represents a feasible strategy to enhance light harvesting in both breadth and intensity of donor-(pi conjugated spacer)-acceptor (D-pi-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 pi-linker between phenothiazine donor and cyanoacrylic acid electron-withdrawing group, were synthesized by modifying the simple D-pi-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 similar to 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. (C) 2016 Elsevier Ltd. All rights reserved.