Effect of structural non-planarity and π-conjugated unit of novel bicarbazole derivatives for dye-sensitized solar cells

Novel bicarbazole-based organic dyes (hereafter denoted as BEC, BECS1, and BECS2) having nonplanar structures were designed and synthesized; for comparison, the corresponding dye (EC) with a planar structure was also synthesized. The effects of length of the π-conjugated unit on the photophysical, electrochemical, and photovoltaic properties of the dyes were investigated. The results showed that the nonplanar molecular structure helps in preventing charge recombination. In addition, more extended π-conjugated systems should lead to long-wavelength absorption, broadening of the absorption profile, and enhancement of the molar extinction coefficient with respect to those of D-π-A sensitizers. These factors collectively contribute to improved light harvesting. The maximum power conversion efficiency reached 6.4% in the case of BECS2 under an opencircuit voltage of 0.653 V and short-circuit photocurrent density of 15.45 mA cm^-2, which could be due to the relatively high absorption ability of this dye.