Enhancing the photovoltaic performance of dithienobenzodithiophene based polymer donors through backbone fluorination and radical polymer additives

Abstract

Unveiling the corresponding relationship between the molecular architecture of organic semiconductor materials and the morphology within the active layer of the organic solar cells (OSCs) is essential for the innovation of novel optical materials and the refinement of device optimization strategies to overcome the bottlenecks in efficiency. In this work, three dithieno[3,2-b]benzo[1,2-b;4,5-b’]dithiophene(DTBDT)-alt-benzothiadiazole (BT) based polymer donors (PDTBDT-F-BT, PDTBDT-F-FBT, and PDTBDT-F-2FBT) with varying fluorine atom content within their acceptor segments were designed and synthesized, and the photovoltaic performances of these polymers when blended with Y6 were meticulously investigated. Incorporating fluorine atoms into the BT segment was observed to not only incrementally increase the optical bandgap, lower the HOMO energy level, and bolster the self-aggregation of the polymer, but also effectively reduce the surface energy of the resulting polymer, thereby altering the donor-acceptor (D-A) interfacial spacing and the phase separation in the blend films as illustrated by molecular dynamic simulations and morphology characterization. As a result, the OSCs fabricated using PDTBDT-F-FBT, which incorporates a single fluorine substitution in the BT unit, demonstrated the highest power conversion efficiency (PCE) of 9.92 %. More importantly, this blend film's morphology is likely to be more conducive to the incorporation of the radical polymer additive, GDTA. This has resulted in a notable reduction in voltage loss, ultimately achieving a higher PCE of 11.55 % for the PDTBDT-F-FBT:Y6 based OSCs. This research uncovered a synergistic impact of backbone fluorination and the incorporation of radical polymer additives, which contributed to reducing the energy loss and enhancing the efficiency of OSCs from DTBDT-based polymer donors.