D-π-A-π-D type thiazolo[5,4-d]thiazole-core organic chromophore and graphene modified PEDOT:PSS buffer layer for efficient bulk heterojunction organic solar cells

•SMOSC is fabricated with graphene (Gr) modified PEDOT:PSS buffer layer.•A new thiazolo[5,4-d]thiazole-core organic chromophore with furan spacer (TP-FTzF-TP) is designed as HTM.•PEDOT-PSS-Gr as a buffer layer has significantly improved the PCE.•The PCE of ∼3.63% is achieved by Au/TP-FTzF-TP:PC60BM (1:2, w/w)/PEDOT:PSS-Gr/ITO.