Organic photovoltaics (OPV) is a promising technology for renewable energy sources due to the advantages of organic materials, such as flexibility, low cost, and tunable optical properties. While significant progress has been made, with record power conversion efficiencies (PCEs) reaching 19%, these still lag behind inorganic counterparts like silicon (up to 26.7%) and GaAs (29.1%). This is largely attributed to higher open-circuit voltage (Voc) losses in organic solar cells (OSCs), typically around 0.55 eV for state-of-the-art PM6:Y6 systems. The development of non-fullerene acceptors (NFAs) has significantly reduced Voc loss by bringing charge-transfer (CT) state energies closer to the strongly absorbing (and emitting) local-exciton (LE) states. This contrasts with fullerene-based OSCs, which suffer from large Voc losses due to a much lower CT-state energy compared to the LE-state energy. Furthermore, NFAs have demonstrated a reduction in non-radiative decay. First-generation NFAs like IT-4F derivatives achieved over 15% PCE, while the emergence of Y6 and its derivatives has pushed PCEs above 18%. The success of Y6 is linked to its banana shape and porous 3D packing, resulting in desirable optoelectronic properties. The evolution of acceptor molecular symmetries shows a trend towards reduced symmetry: from spherical fullerenes to ITIC molecules with inversion symmetry to the chiral Y6. The next logical step is the development of asymmetric acceptors, a strategy gaining increasing traction. Efforts to design and synthesize asymmetric acceptors by tuning side chains, central cores, and terminal groups have already yielded devices exceeding 18% efficiency. However, a fundamental understanding of how asymmetric molecules impact molecular conformations, energetics, and optoelectronic properties is crucial for the development of next-generation high-performance OSCs. This work aims to provide such an understanding, focusing on how asymmetric NFAs influence device performance.

The literature extensively documents the advancement of organic solar cells (OSCs), particularly with the introduction of non-fullerene acceptors (NFAs). Studies highlight the importance of reducing voltage losses, a key factor limiting efficiency. The work of Hou et al. (2018) and Cheng et al. (2018) provide comprehensive reviews of the field, emphasizing the significant role of NFAs in improving efficiency. The impact of molecular design and morphology control has also been investigated, with Cui et al. (2021) demonstrating a single-junction cell with 19% efficiency. The Y6 acceptor and its derivatives have emerged as particularly successful, exhibiting high performance due to unique packing properties and low exciton binding energies (Yuan et al., 2019; Zhang et al., 2020). Research exploring the relationship between molecular symmetry and device performance has shown a trend toward the use of increasingly asymmetric acceptors (Li et al., 2019; Chen et al., 2021; Chen et al., 2020; Gao et al., 2020; Luo et al., 2020). These studies emphasize the need to understand how asymmetric molecules influence molecular packing, charge transport, and exciton dynamics. However, there remains a gap in our understanding of the precise mechanisms by which asymmetric acceptors enhance device performance at a fundamental level, a gap this research addresses.

This study focuses on the design, synthesis, and characterization of asymmetric non-fullerene acceptors (NFAs) and their application in organic solar cells (OSCs). The researchers synthesized four acceptors: BO-4F, BO-4Cl, BO-5Cl, and BO-6Cl, with BO-5Cl being the key asymmetric molecule. The synthesis involved Knoevenagel condensation, attaching different end-groups to a BTP core. The optical properties of these acceptors were characterized using UV-vis absorption and fluorescence spectroscopies, revealing the impact of the different end groups on absorption and emission wavelengths. Cyclic voltammetry (CV) and ultraviolet photoelectron spectroscopy (UPS) were used to determine the energy levels of the acceptors. Single-crystal X-ray diffraction was employed to analyze the molecular packing in the solid state, revealing differences in stacking patterns based on the acceptor's structure. Grazing-incidence wide-angle X-ray scattering (GIWAXS) and grazing-incidence small-angle X-ray scattering (GISAXS) were used to examine the morphology of both pristine acceptor films and blend films. Femtosecond transient absorption (TA) spectroscopy investigated interfacial exciton dissociation dynamics, providing insights into the hole transfer process from acceptor to donor. Time-resolved photoluminescence (TRPL) spectroscopy measured exciton lifetimes. Electroluminescence (EL) measurements, including temperature-dependent studies, helped characterize the charge-transfer states. Finally, organic solar cells were fabricated using a conventional inverted device architecture (ITO/PEDOT:PSS/active layer/PFN-Br/Ag), and their photovoltaic performance was evaluated using current density-voltage (J-V) curves and external quantum efficiency (EQE) measurements. Density Functional Theory (DFT) calculations were also performed to complement the experimental findings.

The asymmetric acceptor BO-5Cl, when blended with the PM6 donor, demonstrated a record-high electroluminescence external quantum efficiency (EQEEL) of 0.1%, resulting in a low non-radiative voltage loss (0.178 eV) and a PCE exceeding 15%. Incorporating BO-5Cl as a third component into a PM6:BO-4Cl blend led to a certified PCE of 18.2%, one of the highest reported certified values. Single-crystal analysis and GIWAXS/GISAXS revealed different molecular packing patterns for the various acceptors, with BO-5Cl exhibiting efficient 3D charge transport networks. TA spectroscopy showed ultrafast interfacial exciton dissociation, although exciton diffusion contributed significantly to the overall dissociation process. The hole transfer kinetics followed a trend consistent with the Marcus electron transfer model. DFT calculations showed that the asymmetric nature of BO-5Cl resulted in a dual interfacial electronic manifold, with LE-CT energy differences varying depending on the interacting end groups. This dual manifold contributed to faster exciton dissociation and reduced non-radiative voltage losses. Temperature-dependent EL measurements supported the small LE-CT energy difference in PM6:BO-5Cl blends. The photovoltaic properties of devices using different acceptors showed that BO-5Cl-based devices had higher Voc values due to the combined effects of reduced non-radiative and non-ideal radiative losses. The ternary blend (PM6:BO-4Cl:BO-5Cl) displayed a significant improvement in EQEEL and a reduced non-radiative loss, leading to the high PCE of 18.56% (certified as 18.2%).

The results demonstrate that the use of asymmetric acceptors, such as BO-5Cl, leads to a superior balance between charge generation and recombination in OSCs. The diverse D:A interfacial conformations arising from the asymmetric structure optimize blend interfacial energetics, promoting both efficient charge generation and suppression of non-radiative recombination. The high luminescence efficiency observed in the BO-5Cl-based devices is a direct consequence of this balanced interplay. The success of the ternary blend further highlights the potential of leveraging the unique properties of asymmetric acceptors to enhance device performance. The combined experimental and theoretical data strongly support the hypothesis that the dual nature of interfacial structural and electronic characteristics is the key to achieving high-performance OSCs with low voltage losses. This work provides valuable insights into the design principles for next-generation organic solar cells, underscoring the importance of considering both molecular structure and packing behavior for optimizing device efficiency.

This study successfully demonstrates the effectiveness of asymmetric non-fullerene acceptors, specifically BO-5Cl, in achieving highly efficient and luminescent organic solar cells. The record-high certified PCE of 18.2% achieved using a ternary blend highlights the potential of this approach. The combination of experimental and theoretical investigations clarifies the underlying mechanisms, emphasizing the importance of balanced charge generation and recombination through diverse D:A interfacial conformations. Future research could explore further modifications of asymmetric acceptors to further improve device performance and stability, while also investigating other material combinations to expand the applicability of this strategy.

While the study demonstrates significant advancements in OSC efficiency, some limitations exist. The research focused on a specific set of materials and device architectures; the results may not be directly transferable to all OSC systems. Further investigations are needed to explore the long-term stability of these devices under various environmental conditions. The DFT calculations provide valuable insights but have inherent approximations, and the precise extent of energetic disorder influencing charge transport needs further exploration. Finally, the study primarily examined the performance under laboratory conditions; real-world performance may differ depending on environmental factors.