Geometrical effect of stilbene on the performance of organic dye-sensitized solar cells

Abstract

New metal-free organic donor–bridge–acceptor dyes comprising a triphenylamine moiety as the electron donor (D), a cyanoacrylic acid moiety as the electron acceptor (A), and a conjugated chromophore between D and A as a spacer (S) were synthesized for the application of dye-sensitized solar cells (DSSCs). Two types of S were designed, one contains a normal stilbene backbone (dye 1) and the other contains a 2-phenylindene moiety (dye 2) with the central double bond locked in a transoid geometry. The performance of DSSCs made with these two types of dyes was compared, and it was found that the inhibition of trans/cis isomerization of the central double bond was advantageous to the efficiency of DSSCs. The phenomenon was investigated by a comparison of their photophysical properties, and rationalized by quantum mechanical calculations by using density functional theory (DFT) at the B3LYP/6-31G(d,p) level. The electronic transitions were analyzed by time-dependent DFT, and the results were consistent with the experimental observations. The DSSCs were fabricated using both types of materials, and the performance was recorded under AM 1.5G irradiation (100 mW cm⁻²) condition. The overall conversion efficiencies of dye 2 (5.14–5.67%) were higher than those of dye 1 (4.52–4.98%). Both the short-circuit photocurrent density (J_sc) and the open-circuit voltage (V_oc) were improved in 2 with respect to those of 1.