Fullerene derivative integration controls morphological behaviour and recombination losses in non-fullerene acceptor-based organic solar cells

Abstract

The complex and varied relationship found in intermolecular interactions within the photo-active layers plays a decisive role in determining the photovoltaic energy conversion and overall device performance of organic solar cells (OSCs). Among different approaches, the ternary blend strategy serves as an effective technique to control the morphology within the active layer in OSCs. In this work, PM6:L8-BO is used as the main host system (binary) while the fullerene molecules PC₆₁BM and PCBC6 are introduced to form ternary OSCs. The results highlight the important role of fullerenes in enhancing the performance of binary non-fullerene acceptor-based cells by suppressing trap-assisted recombination and optimizing the active layer morphology. The improved film phase microstructure, enabled by fullerene derivatives with higher lowest unoccupied molecular orbital (LUMO) energy levels in comparison to the host acceptor (L8-BO), facilitates more efficient charge collection and reduced non-radiative recombination. This results in an increase in the fill factor (FF) and open circuit voltage (V_oc) in the ternary OSCs. Consequently, power conversion efficiencies (PCEs) of binary OSCs were increased from 17.28% to 18.10% and 18.38% for the PC₆₁BM- and PCBC6-based ternary OSCs, respectively. Furthermore, the addition of the fullerene molecules in the active layer provided the devices with enhanced long-term photo and thermal stability. The ternary OSCs demonstrated degradation pathways distinct from those of binary cells (ISOS-L1-I and ISOS-D2-I protocols), as identified through in situ ultraviolet-visible (UV-Vis) absorption and Raman spectroscopy. Molecular dynamics (MD) simulations, for the first time, reveal the significant role of fullerene molecules as morphology regulators in non-fullerene acceptor (NFA)-based systems. Their presence ensures improved dispersion of blend components and promotes more uniform and isotropic thermal and mechanical behaviour. Finally, mini-modules with active areas of 3.8 cm² were fabricated, achieving PCEs of 12.90%, 13.32%, and 13.70% for the binary and ternary cells using PC₆₁BM-and PCBC6-based ternary cells, respectively. Our results demonstrate that regulation of the morphology of the photo-active layer in OSCs through fullerene incorporation reduces the non-radiative energy loss pathways, enabling high-efficiency, stable and scalable OSCs.