Bulky dendritic triarylamine-based organic dyes for efficient co-adsorbent-free dye-sensitized solar cells

Three new organic photosensitizers (HK1-HK3) incorporating functionalized bulky trimer of triarylamine derivatives as the electron donor units, 3,4-ethylenedioxythiophene moiety as the conjugated spacer, and cyanoacrylic acid as the electron acceptor, have been synthesized and the effects of different bulky electron donor substituents on the photovoltaic performances of dye-sensitized solar cells (DSSCs) are investigated accordingly. The DSSC based on HK1 with tris(triphenylamine) electron donor in the absence of the co-adsorbent chenodeoxycholic acid (CDCA) shows the best photovoltaic performance under standard global AM 1.5 sunlight: a short-circuit current density (J_sc) of 10.41 mA cm^-2, an open-circuit voltage of 0.76 V and a fill factor of 0.65, corresponding to a maximum overall power conversion efficiency (PCE) of 5.11%. However, in the presence of co-adsorbent CDCA, the PCEs of DSSCs based on these photosensitizers, decrease significantly by about 9.7%, which is contrary to the conventional concept that CDCA can improve the efficiency by inhibition of dye aggregation and intermolecular electron recombination. The results demonstrate that the synthesized new organic dyes with bulky dendritic triarylamine and its derivatives as electron-donation units can efficiently suppress dye aggregation without the need of the co-adsorbent and effectively reduce the intramolecular electron recombination in DSSCs.