Synthesis and characterization of NFA-based polymers for solar cells with improved thermal stability

Abstract

Among the latest advances in organic solar cells, all-polymer solar cells based on non-fullerene acceptors (NFAs) are emerging as a promising way to improve device stability. However, the synthesis of new electron-accepting polymers suitable for the active layer remains relatively unexplored. Current efforts primarily focus on maximizing photovoltaic conversion efficiency using PY-IT, a polymer derived from Y6. By contrast, there is a lack of fundamental research into controlling polymerization processes and the effect of the intrinsic optoelectronic properties of NFA-based polymers on their reactivity and stability when subjected to thermal stress and light soaking. To address this, we present the synthesis of a series of NFA-based polymers that incorporate thiophene, indacenodithiophene or a thienothiophene analogue. We systematically optimized Stille polymerisation by evaluating a range of phosphine ligands and correlating their performance with Tollman electronic and steric parameters, an approach that has rarely been explored in the literature. The resulting polymers exhibit improved macromolecular control, good solubility in o-xylene and optical properties suited to integration into the active layer of solar cells. Comprehensive spectroscopic and morphological characterisation (UV-vis, AFM and GIWAXS) of pristine polymer films confirms their amorphous nature in the solid state. The thermal and photochemical stability of the three new polymers was evaluated in devices under ISOS-D-2 (thermal ageing) and ISOS-L-1 (light soaking) protocols. After 1000 hours of thermal stress, all devices retained over 90% of their initial efficiency and they also demonstrated outstanding photostability over 300 hours under 1 Sun illumination. Some materials showed no degradation under these conditions, highlighting the potential of all-polymer solar cells to overcome long-standing stability challenges in organic photovoltaics.