Organic solar cells (OSCs) have seen significant advancements due to the development of light-harvesting materials, particularly non-fullerene acceptors (NFAs). However, to achieve commercial viability, further improvements in PCE and stability are crucial. Improving OSC efficiency requires enhancing the open-circuit voltage (Voc) while maintaining efficient exciton dissociation and charge transport. This necessitates suppressing Voc loss (the difference between the bandgap and eVoc, also known as energy loss, Eloss) and achieving ideal nanoscale morphology. Minimizing Voc loss involves suppressing charge transfer (CT) state absorption and improving electroluminescence quantum efficiency (EQEEL). Eloss depends not only on the intrinsic properties of donor (D) and acceptor (A) molecules but also on the D/A blend's nanomorphology (distribution, aggregation, and packing of D/A molecules). Therefore, surface energy and crystallinity of new molecules are important design considerations. A common approach to enhance efficiency is introducing a third component into the benchmark binary blend, a technique refined from fullerene-based OSCs to NFA-based systems. The ideal third component should not disrupt the host binary blend's morphology. NFAs with strategically positioned halogen substitutions offer the potential for similar molecular orientation, excellent miscibility, adjustable aggregation, and tunable photoelectronic properties. This study focuses on optimizing the halogen substitution position in the terminal groups of NFAs to achieve efficiency enhancement.

The literature extensively discusses the challenges and strategies for improving OSC efficiency. Studies have highlighted the importance of minimizing voltage losses by manipulating the CT state and improving EQEEL (references 4-10). Research also emphasizes the role of nanoscale morphology and the need for balanced charge transport (references 7-11, 18). The use of ternary blends has emerged as a promising technique to optimize morphology and further enhance efficiency (references 7-11, 18). Previous work has demonstrated the effectiveness of halogen substitution in modifying molecular properties and improving OSC performance (reference 19). However, a systematic design rule for molecules simultaneously optimizing all the required properties for high-efficiency OSCs has been lacking.

The molecular design strategy started with optimizing the halogen substitution position in the terminal groups of the NFA, BTP-eC9. Theoretical calculations guided the synthesis of o-BTP-eC9, an isomer of BTP-eC9 with altered chlorine substitutions on the IC-2Cl end groups. Quantum chemistry calculations were performed to evaluate the dipole moments, LUMO, and HOMO levels of various IC-2Cl isomers. o-o-IC-2Cl was selected due to its lowest dipole moment and highest LUMO level. o-BTP-eC9 was synthesized and characterized using various techniques (1H NMR, 13C NMR, GCMS, single crystal analysis, MALDI-TOF MS). UV-vis absorption spectroscopy, dielectric constant measurements (using fabricated capacitors), and electrochemical cyclic voltammetry (CV) were used to characterize the optical and electrochemical properties of o-BTP-eC9 and BTP-eC9. Grazing incidence wide-angle X-ray scattering (GIWAXS) was employed to analyze the molecular orientation and crystallinity. Contact angle measurements were used to determine surface energy and assess miscibility between materials. Conventional OSC devices with the structure ITO/PEDOT:PSS/active layer/PFN-Br/Ag were fabricated using PM6 as the donor. The effects of different ratios of o-BTP-eC9 and BTP-eC9 in ternary blends were investigated. Device performance was evaluated using J-V curves, EQE measurements, and electroluminescence (EL) spectroscopy. Space charge limited current (SCLC) was used to measure charge mobilities. Transient photocurrent (TPC) and transient photovoltage (TPV) measurements were used to study charge dynamics. Atomic force microscopy (AFM) and GIWAXS/GISAXS were used to characterize the nanomorphology and molecular packing of the blend films.

Theoretical calculations predicted and experimental results confirmed that o-BTP-eC9 possesses a higher LUMO level and dielectric constant than BTP-eC9. The PM6:o-BTP-eC9 binary device exhibited a comparable PCE of 18.7% to the PM6:BTP-eC9 device (18.9%), but with a significantly lower energy loss (41 meV reduction). The ternary blend device, PM6:BTP-eC9:o-BTP-eC9 (with a 15 wt% o-BTP-eC9), achieved a record PCE of 19.9% (19.5% certified). Voc loss analysis revealed that o-BTP-eC9 effectively suppressed both radiative and non-radiative recombination. The upshifted CT state in the PM6:o-BTP-eC9 device contributed to the reduced radiative recombination loss. The higher dielectric constant of o-BTP-eC9 and balanced charge transport in the ternary system contributed to the lower non-radiative recombination loss. AFM and GIWAXS/GISAXS data showed that o-BTP-eC9 optimized the phase separation morphology of the PM6:BTP-eC9 blend, leading to improved charge transport and collection. The ternary device showed enhanced operational stability compared to the binary devices.

This study successfully demonstrated the effectiveness of a rational molecular design strategy in achieving high-efficiency OSCs. The optimization of the halogen substitution position in the IC-2Cl group of the NFA molecule led to a significant reduction in energy loss, primarily by suppressing radiative and non-radiative recombination. The synergistic effects of the higher LUMO level, higher dielectric constant, and weaker crystallinity of o-BTP-eC9, combined with its excellent miscibility with BTP-eC9, resulted in the record-breaking PCE of 19.9% in the ternary device. The findings highlight the importance of considering both molecular properties and device morphology in the design of high-performance OSCs. The successful incorporation of o-BTP-eC9 as a guest acceptor into a ternary system provides a promising strategy for fine-tuning the optoelectronic properties and morphology of the active layer, paving the way for further improvements in OSC performance.

This research successfully designed and synthesized o-BTP-eC9, a non-fullerene acceptor, leading to a significant improvement in OSC efficiency. The rational design, guided by theoretical calculations, resulted in a record PCE of 19.9% (19.5% certified) in a ternary OSC system, surpassing previous single-junction OSC records. Future work could explore further modifications of the molecular structure to further enhance performance and stability, and investigate other ternary and multi-component systems to expand the applicability of this design strategy.

While this study demonstrates a significant advancement in OSC efficiency, some limitations exist. The study primarily focuses on a specific ternary blend system. The generalizability of this design strategy to other donor-acceptor combinations requires further investigation. Long-term stability tests over extended periods are necessary to fully assess the durability of the ternary devices under various environmental conditions. The precise mechanisms underlying the improved operational stability of the ternary device need further detailed studies.