Suppressing Trap States via Donor Orientation Engineering in Non-Halogenated Layer-by-Layer Organic Solar Cells

This study investigates the origin of reduced-recombination-induced voltage loss in organic solar cells (OSCs) processed from non-halogenated solvents using a layer-by-layer (LbL) architecture with a PM6-incorporated D18 donor system. Transient photovoltage and photocurrent measurements, along with grazing incidence wide-angle X-ray scattering analysis, reveal that PM6 promotes a favorable face-on molecular orientation and enhances structural ordering within the D18 donor layer. This improved ordering effectively mitigates carrier trapping through tail-state-localized energetic disorder, which intensifies bimolecular recombination and contributes to notable voltage losses. Moreover, the PM6-induced molecular alignment of D18 modulates the morphology of the overlying eC9 acceptor layer, leading to a further reduction in trap state density. Transient absorption spectroscopy verifies that the reduction in trap-state energy levels limits charge carrier recombination and prolongs carrier lifetimes, thereby improving photovoltaic performance. These findings identify donor molecular orientation as a critical morphological parameter governing energetic disorder in LbL OSCs. Overall, the proposed strategy offers a viable route for overcoming the limitations of OSCs processed using environmentally benign non-halogenated solvents.