Volatile additive for selective tuning of non-fullerene acceptor morphology in the active layer of organic solar cells

Abstract

2-(2-Methylphenyl)benzo[b]thiophene (MPBT) is synthesized and applied as volatile additive to PM6/Y6 active layer of organic solar cells, where PM6 is donor polymer, Y6 is non-fullerene acceptor. Contrary to usual solvent additives, MPBT is not removed from the active layer during spin-coating. However, due to volatility of MPBT, annealing at 120 °C allows for its complete removal from PM6/Y6 composite. Moreover, its morphology is markedly altered by MPBT, as it is evidenced by optical spectroscopy and atomic force microscopy. For a thin PM6/Y6 active layer (about 70 nm) cast from chlorobenzene, the optimal concentration of MPBT is 5 wt %, relative to the solvent. Use of this additive leads to improvement of short circuit current J_SC and fill factor (FF), and consequently, to power conversion efficiency (PCE) increase. External quantum efficiency (EQE) is improved within the whole range of PM6:Y6 optical absorption (350–850 nm). Optical, X-ray diffraction, and atomic-force microscopy data confirm ordering of Y6 domains during MPBT volatilization, while morphology of PM6 domains is almost not altered. Space-charge limited current measurement evidences that electron mobility increases several times upon volatilization of MPBT from PM6:Y6 film, while hole mobility remains essentially unchanged. Surface free energy measurement confirms preferential miscibility of MPBT with Y6 component of the active layer rather than with PM6 component. Storage and operational stability of the solar cells processed with MPBT markedly increase. GIWAXS pattern of pristine Y6 film processed with MPBT show unusually high orientation ordering of Y6 crystallines with respect to the substrate. The novel approach based on intermediate donor:acceptor:additive ternary blend gives additional degree of control of active layer properties by selective tuning of the acceptor morphology.