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

Four novel D-A-II-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 (epsilon). QL2 and QL3 with the sevenmembered azacycle, i.e. with N-phenyl-iminostilbene (ISB) and N-phenyl-iminodibenzyl (IDB) as donors, respectively, show the red-shifted absorption spectra and higher epsilon 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 cosensitizing 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.