Molecular engineering of new electron acceptor for highly efficient solution processable organic solar cells using state-of-the-art polymer donor PffBT4T-2OD

Two molecularly engineered fullerene-based electron acceptors, coded as BITh-C60 and BITh-C70, were designed, synthesized and fully characterized. The molecules have LUMO energy levels at −3.62 and-3.65 eV; and HOMO energy levels at −5.62 and −5.60 eV, respectively as confirmed by cyclic voltammetry. The acceptor BITh-C70 shows comparatively broader and stronger absorption with respect to BITh-C60. A narrow band gap polymer poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3′′′-di(2-octyldodecyl) 2,2′;5′,2″;5″,2′′′-quaterthiophen-5,5′′′-diyl)] (PffBT4T-2OD) was used to construct the solution processed bulk-heterojunction solar cells. Under optimized fabrication and evaluation conditions, BITh-C60 and BITh-C70 showed a highest power conversion efficiency (PCE) of 8.1 % and 8.8 %, respectively, which are comparatively higher than the PCEs obtained from the respective reference solar cells. The PffBT4T-2OD:BITh-C70 based BHJ solar cell shows a nearly flat EQE spectra of 75 % between 380 and 700 nm.