Fluorinated benzoselenadiazole-based low-band-gap polymers for high efficiency inverted single and tandem organic photovoltaic cells

We designed and synthesized two low-band-gap conjugated copolymers with alternating difluorinated benzoselenadiazole (DFDTBSe) and ethylhexyloxy (EH)- or octyldodecyloxy (OD)-substituted benzo[1,2-b:4,5-b′]dithiophene (BDT) building blocks. PEHBDT-DFDTBSe and PODBDT-DFDTBSe have optical band gap energies of 1.66 and 1.69 eV, respectively, and HOMO energy levels of -5.44 and -5.43 eV, respectively. The different alkyloxy side chains in the polymers affect the molecular packing and ordering in active-layer films blended with [6,6]-phenyl-C₇₁ butyric acid methyl ester (PC₇₁BM). The PEHBDT-DFDTBSe:PC₇₁BM film comprises predominantly "face-on" crystal structures with short π-π stacking distances (3.69 Å) while PODBDT-DFDTBSe:PC₇₁BM has mostly "edge-on" structures according to two-dimensional grazing-incidence X-ray diffraction analysis. Bulk heterojunction solar cells were fabricated with an inverted structure of ITO/ethoxylated polyethylenimine/polymer:PC ₇₁BM/MoO₃/Ag. The device fabricated using the PEHBDT-DFDTBSe:PC₇₁BM active layer shows a maximum power conversion efficiency (PCE) of up to 5.74%, which is the highest value reported for OPVs containing benzoselenadiazole and BDT-derivative polymers. A tandem solar cell was also fabricated using PEHBDT-DFDTBSe:PC₇₁BM and poly(3-hexylthiophene):indene-C₆₀-bisadduct as the top and bottom cell components, respectively; its maximum PCE was 7.15%.