Incorporating a stable fluorenone unit into D–A–π–A organic dyes for dye-sensitized solar cells

Abstract

Two new organic dyes based on a donor–acceptor–π–bridge–acceptor/anchor (D–A–π–A) configuration, HIQF1 and HIQF2, were synthesized, characterized, and successfully employed in dye-sensitized solar cells. The incorporation of the weakly electron-withdrawing fluorenone group as an additional acceptor enhanced the dyes' properties for solar energy conversion in several ways. First, the absorption spectra of the dyes were broadened, covering a large range of sunlight. Second, the absorption spectra observed upon dye adsorption on TiO₂ were only slightly blue-shifted as compared to the spectra of the dyes in solution. Third, the orbital energy levels and electron distributions of the D–A–π–A backbone were ideal for highly efficient electron transport and injection. The incorporation of long electron-donating alkoxy groups in the donor unit of HIQF1 further increased the dye's short-circuit photocurrent density (J_sc), open-circuit voltage (V_oc), and overall conversion efficiency (η), but had little negative effect on the fill factor (FF). Solar cells sensitized with HIQF1 showed excellent current–voltage characteristics; the J_sc, V_oc, and FF values were 12.26 mA cm⁻², 0.70 V, and 0.73, corresponding to a η value of 6.26%.