Molecular engineering of the fused azacycle donors in the D-A-π-A metal-free organic dyes for efficient dye-sensitized solar cells

Four novel D-A-π-A metal-free organic sensitizers QL1 – 4 with the fused azacycle units as donors have been designed and synthesized to investigate the relationship between both the sizes and conjugations of azacycle and performances of dye-sensitized solar cells systematically. QL1 with the five-membered azacycle, i.e., with carbazole ( Cz ) as donor, exhibits the smallest molar extinction coefficients ( ε ). QL2 and QL3 with the seven-membered azacycle, i.e. with N -phenyl-iminostilbene ( ISB ) and N -phenyl-iminodibenzyl ( IDB ) as donors, respectively, show the red-shifted absorption spectra and higher ε compared to QL1 . However, QL2 and QL3 with an only distinction in a single or double bond, display distinctive PCEs. QL4 with the six-membered azacycle, i.e., 9,9,10-triphenyl-9,10-dihydroacridine ( TDD ), exhibits a blue-shifted absorption spectrum due to a big dihedral angle , resulting in a lower photovoltaic performance . Especially, QL3 with the ISB donor is conducive to achieving a PCE of 6.22%, which is higher than QL2 , due to its better planar structure and more conjugation. When co-adsorbed with CDCA, QL3 exhibits the highest PCE of 7.14% among all dyes, and up to 8.26% by co-sensitizing with dye WL5 , which surpasses that of the reference dye N719 (7.75%). The results indicate that the size and conjugation of the fused azacycle donors may affect the photovoltaic performance significantly, and the ISB unit is demonstrated as a promising donor for efficient organic dyes.