Synthesis of a halogenated low bandgap polymeric donor for semi-transparent and near-infrared organic solar cells

Semi-transparent organic solar cells (ST-OSCs) have garnered significant interest because of their potential in aesthetic and space-saving solar energy systems such as multi-colored semitransparent building‐integrated photovoltaic or grow light transparent agrivoltaic systems. As visibly semitransparent photoactive materials, the low bandgap (LBG) donor polymer and acceptor present new opportunities for the realization of ST-OSCs because they can facilitate photovoltaic generation of electricity from near-infrared (NIR) light without significant absorption of visible light. However, while various LBG non-fullerene acceptors have been recently developed to realize highly efficient ST-OSCs, there are only a few reports on LBG donor polymers that achieve efficient photo-induced charge generation from NIR light as well as allow the propagation of visible light. In this study, LBG donor polymers consisting of BD-F and BD-Cl as the halogenated derivatives of poly{2,6′-4,8-di(5-ethylhexylthienyl)benzo[1,2-b; 3,4-b]dithiophene-alt-5-dibutyloctyl-3,6-bis(5-bromothiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4-dione} (BD-H) were synthesized. The BD-F:Y6 and BD-Cl:Y6 OSCs showed higher open-circuit voltages and fill factors than BD-H:Y6 due to their downshifted energy level and efficient charge extraction characteristics. Consequently, the BD-Cl:Y6 OSCs achieved a power conversion efficiency (PCE) of 5.62%. Furthermore, with the introduction of a metal oxide/metal/metal oxide transparent electrode, the BD-Cl:Y6 ST-OSC demonstrated a high average visible transmittance of 35.1% and PCE of 3.69%. This approach contributes to enhancing the potential of ST-OSCs.