Quinoxaline based A-A type polymer donors for non-fullerene polymer solar cells

In this study, we designed and synthesized two polymers, PDFBT-Qx and PDTBT-Qx, using the quinoxaline unit containing a long alkyl side chain as an unit and DTBT with or without fluorine atom as the other unit, respectively. The optical and electrochemical properties of the above two polymer donors were studied, and the optical bandgaps of PDTBT-Qx and PDFBT-Qx were 1.59 eV and 1.69 eV, respectively. The optical and electrochemical properties of the above two polymer donors were studied, and the HOMO and LUMO energy levels of PDTBT-Qx and PDFBT-Qx were obtained, and the energy levels of L8-BO were compared to explore the feasibility of the study. Polymer solar cells (PSCs) were prepared by blending with the non-fullerene acceptor material L8-BO. Finally, it was found that the power conversion efficiency (PCE) based on PDFBT-Qx:L8-BO and PDTBT-Qx:L8-BO was 7.2 % and 3.5 %, respectively. The systematic analysis of the optical properties, electronic energy level, charge transport, photovoltaic performance, charge dissociation efficiency and surface morphology of the material shows that the F atom can not only reduce the HOMO energy level of the donor material to improve the V_OC of the device due to its huge electronegativity. Furthermore, when the fluorine atom is introduced into the polymer skeleton, it can form weak intermolecular forces (F—S) with the sulfur atom in DTBT. This fluorination strategy can enhance the rigidity of the polymer and is known as “the molecular lock effect”. This phenomenon is conducive to adjusting the morphology of the active layer and ultimately improving the efficiency of the device.