Improvement of intramolecular charge transfer within a donor–acceptor blend by doping novel synthesized benzothiadiazole small molecules in solid state

[Display omitted] •Large shifts of emission spectra indicated CT complex formation.•Photoinduced energy/electron transfer from BT1 dye to fullerene occurred.•BT1 dopant favored CT possibility in P3HT:BT1:PC61BM ternary blend. Three electron-deficient small molecules based on 2,1,3-benzothiadiazole (BTD) units namely, 4,7-bis(3-methoxyphenyl)-2,1,3-benzothiadiazole (BT1), (3-{7-[3-(dimethylamino)phenyl]-2,1,3-benzothiadiazole-4-yl}phenyl)dimethylamine (BT2) and 3,3′-(2,1,3-benzothiadiazole-4,7-dyl)dianiline (BT3) were synthesized and their photophysical properties were investigated systematically to understand their potential usage in ternary organic solar cells (OSCs) as additive material to enhance the cell efficiency. All these molecules show broad absorption bands in 350–750nm on glass substrate and their optical band gaps were calculated to be around 2.50–2.80eV. BTD fluorescence dynamics were measured in polymer:BT1:fullerene blends with varying emission wavelengths of active layer. Fluorescence emission and time resolved measurements indicated photoinduced energy shift from BT1 dye to fullerene and also from polymer to BT1 dye upon excitation of the active layer.