Fused-ring isomerism modulates molecular packing and device performance in non-halogenated organic solar cells

Domain: physics
Published: 2025-12-11
Editorial outcome: minor_revision

Abstract
Subtle changes in molecular backbone geometry impact intermolecular interactions and performance of organic solar cells. Here, three isomeric small-molecule acceptors (NaO1, NaO2, and NaO3) are investigated to reveal how different fused-ring configurations control molecular packing, electronic coupling, and film formation. Structural and spectroscopic analyses show that the linearly fused NaO1 forms a compact three-dimensional packing network with large and balanced electronic couplings (>24 meV) across multiple directions, while the more curved analogues exhibit excessive crystallization and phase segregation. In-situ optical measurements demonstrate that NaO1 promotes fast and continuous structural evolution during film formation, resulting in smooth morphology and homogeneous phase distribution. These structural and dynamic advantages facilitate efficient charge generation and transport, accompanied by reduced non-radiative energy loss, ultimately achieving an efficiency of 20.07% for non-halogenated ternary devices. Our findings highlight how fused-ring isomerism decisively governs structure–packing–performance relationships in organic solar cells.

Introduction
In recent years, organic solar cells (OSCs) have achieved remarkable progress, with power conversion efficiencies (PCEs) surpassing 20%, primarily driven by continuous advancements in the molecular design and structural optimization of high-performance small-molecule acceptors (SMAs)1–15. As the core photoactive component in OSCs, the molecular packing behavior of SMAs plays a crucial role in determining exciton diffusion efficiency, charge carrier mobility, and the nanoscale morphology of the active layer, thereby critically influencing overall device performance16–33. For example, the prototypical acceptor Y6 features a donor–acceptor (push–pull) architecture combined with tight π–π stacking (with an interplanar distance of approximately 3.3Å), forming a three-dimensional interpenetrating network34–36. This structure not only enhances intramolecular charge transfer but also facilitates efficient charge transport, enabling device efficiencies exceeding 18%37,38. Notably, its derivative L8-BO further improves charge transport and reduces energy losses by optimizing the average π–π stacking distance and modulating the molecular packing mode1,5. These advances underscore that precise control over intermolecular interactions and optimization of crystalline packing are key strategies for overcoming performance bottlenecks in OSCs.

Structural isomerization has emerged as an effective strategy for modulating molecular packing in organic semiconductors, owing to its precise control over molecular conformation21,25,39–47. By altering side-chain branching positions (e.g., 2-ethylhexyl vs. 3-ethylhexyl), halogen substitution sites on terminal groups (e.g., ortho-, meta-, or para-positions of F/Cl), or the orientation of core substituents, researchers have been able to fine-tune intermolecular non-covalent interactions21,25,39–47. For example, our previous work revealed that positional changes of methyl groups significantly affected intermolecular hydrogen bonding, leading to notable differences in device performance (18.4% for Qo2 and 16.7% for Qo3) and stability between two acceptors25. In the BTIC-BO4Cl system, shifting the chlorine atom from the γ- to the δ-position enhanced intermolecular π⋅⋅⋅π, Cl⋅⋅⋅S, and Cl⋅⋅⋅N interactions, thereby improving phase purity and charge transport characteristics in the blend films48. However, most of these studies focus on substituent-level modifications, while relatively little attention has been paid to isomerization of the fused-ring core, which can induce more pronounced structural and packing changes. Experimental studies have shown that linear and angular isomers of naphthodithiophene exhibit distinct packing motifs: linear-fused structures tend to adopt lamellar or layered stacking with stronger in-plane π–π interactions, while angular-fused analogs display tilted molecular orientations that weaken intermolecular coupling and hinder charge transport. Such backbone-level conformational control offers opportunities for the rational design of next-generation organic optoelectronic materials.

Herein, we report the rational design and synthesis of three isomeric SMAs (NaO1, NaO2, and NaO3) by modulating the fusion configuration of electron-deficient units within their central cores. These isomers exhibit distinct molecular conformations that enable a comprehensive investigation of how backbone-level isomerization affects molecular packing, electronic coupling, and device performance. Systematic characterization—including single-crystal X-ray diffraction, transient spectroscopy, grazing-incidence wide-angle X-ray scattering (GIWAXS), and atomic force microscopy (AFM)—reveals that the linearly fused NaO1 forms a highly interconnected 3D packing network and large, balanced electronic coupling constants (>24 meV across multiple stacking modes), leading to efficient charge transport. Devices based on PM6:NaO1, processed entirely with o-xylene, achieved a high PCE of 19.11%. In contrast, the more twisted NaO2 exhibited excessive crystallinity and phase separation, resulting in a markedly lower PCE of 7.77%. Importantly, leveraging the favorable optoelectronic properties of NaO1, we further incorporated it into a non-halogenated ternary blend (PM6:L8-BO-C4:NaO1), which delivered a PCE of 20.07% and an enhanced fill factor (FF) of 80.3%, attributed to reduced non-radiative loss and accelerated charge transfer. These findings underscore the critical role of fused-ring isomerization in regulating molecular conformation and intermolecular packing, thereby establishing clear structure-packing-performance relationships that are essential for the rational design of high-efficiency OSCs.

Results
Material design and properties
The chemical structures of the three electron acceptors are shown in Fig. 1a, and their synthetic routes are illustrated in Fig. 1b, the Supplementary Methods, Supplementary Figs. 1–10 and Supplementary Data 1. Two isomeric starting materials, 2,3-dihydro-1H-cyclopenta[a]naphthalen-1-one and 2,3-dihydro-1H-cyclopenta[b]naphthalen-1-one, were oxidized with selenium dioxide to efficiently afford compounds 1 and 2. Subsequently, the BT core was reduced using lithium aluminum hydride (LiAlH4) to yield a key diamine intermediate. This diamine was then subjected to keto-amine condensation reactions with compounds 1 and 2, respectively, to construct three central cores. Notably, the condensation between the diamine intermediate and the asymmetric compound 1 could occur at either the α/β or β/γ positions, leading to two isomeric core structures. Although the two cores exhibited very similar polarities, they were successfully separated and purified by column chromatography, with an approximate product ratio of 1:3 for Core1 to Core2. Next, the three cores were subjected to formylation reactions using N,N-dimethylformamide (DMF) in the presence of lithium diisopropylamide (LDA) as a base, affording three distinct dialdehyde intermediates. Finally, the target SMAs were synthesized via Knoevenagel condensation. The molecular structures of NaO1, NaO2, and NaO3 were confirmed by nuclear magnetic resonance and mass spectrometry as detailed in the Supporting Information.

To investigate the impact of different fused-ring configurations on molecular packing behavior, single crystals of the three isomeric SMAs were obtained via recrystallization from mixed solvents. We then systematically analyzed how variations in the electron-donating arms affect their packing modes and π–π stacking distances. As shown in Fig. 2a, all three NaO-series acceptors exhibit a characteristic “banana-shaped” conformation, similar to that of Y6, primarily attributed to intramolecular S⋅⋅⋅O non-covalent interactions (NaO1, CCDC: 2463998; NaO2, CCDC: 2463999; NaO3, CCDC: 2464000) (Supplementary Data 2). The torsion angles between the central core and end groups are small and comparable across the three molecules, indicating good molecular planarity. Notably, NaO1 displays the shortest intramolecular S⋅⋅⋅O distances (2.59 and 2.61 Å), compared to NaO2 (2.67 and 2.67 Å), NaO3 (2.65 and 2.76 Å), and even Y6 (2.66 and 2.65 Å), suggesting a possible contribution from n→σ* hyperconjugation that further stabilizes the molecular conformation.

With regard to the crystal packing characteristics, the three acceptors exhibit distinctly different stacking modes, yet their π–π stacking distances are comparable, all around 3.35 Å (Fig. 2b–d). The isomer NaO1 form a tight 3D “fabric-like” network. Three distinct stacking modes are found between neighboring molecules: (1) End group-to-core arm stacking (E/C mode): In this mode, the core arm stacks with the end group as well as the thieno[3,2-b]thiophene (TT) unit, resulting in a large π-overlap and a close packing distance of 3.31 Å. (2) End group-to-bridge TT stacking (E/B mode): This configuration features a significant overlap between the end group and the TT unit in a conventional face-to-face arrangement. (3) Alternative end group-to-bridge TT stacking (E/B′ mode): While similar to the E/B mode, this stacking motif differs in the molecular orientation, exhibiting a distinct stacking angle relative to the E/B mode. This packing style is very similar to that of L8-BO reported by Sun and co-workers5, which supported its high performances. The isomer NaO2 and NaO3 possess bent core arm, enabling stronger intermolecular interactions in which multiple arms can simultaneously participate in π–π stacking. The unit cell reveals extensive molecular overlap, indicative of pronounced intermolecular interactions that give rise to robust lamellar structures and promote the formation of a two-dimensional network. Three types of stacking modes are observed: (1) E/C mode: The core arms of adjacent molecules align and interact with each other’s end groups, resulting in significant molecular overlap. This strong interaction is likely to enhance the crystallization tendency of the material. (2) E/C′ mode: An alternative end-group-to-core-arm stacking is observed, particularly in NaO2, where the TT units of neighboring molecules are misaligned. In this case, the end group of one molecule overlaps closer to the core of the neighboring molecule, rather than with its core arm as seen in NaO2. (3) E/E mode: Characteristic of Y6 derivatives, this involves end-group-to-end-group stacking. However, in the presence of other dominant interactions, this stacking becomes significantly distorted, resulting in minimal overlap between the end groups and the TT units.

Furthermore, we calculated the electronic coupling constants (|J|) for the three isomers in different stacking modes. NaO1 exhibits relatively large |J| values across all three stacking configurations (24.4 meV for the E/C mode, 24.7 meV for the E/B mode, and 26.0 meV for the E/B′ mode). In contrast, isomer NaO2 with the most curved core arm shows smaller |J| values compared to NaO1 (0.8 meV for the E/C mode, 18.7 meV for the E/C′ mode, and 23.5 meV for the E/B mode). For NaO3, although a high electronic coupling of 42.7 meV is achieved in the E/B mode, the coupling constants in the other two stacking modes remain relatively low. Compared with the bent-armed NaO2 and NaO3, NaO1 demonstrates large and balanced electronic couplings, which may be favorable for three-dimensional charge transport49.

To clarify the mechanism behind electronic coupling variations, we note that simple geometric/electrostatic descriptors (average core–arm dihedrals and molecular dipole moments (Supplementary Fig. 11) are similar among NaO1–NaO3 and thus cannot by themselves account for the observed differences in |J|. We therefore analyzed fragment-resolved LUMO distributions for crystallographic dimers in each stacking motif (E/C, E/C′, E/B, E/B′, etc.). For each dimer, we first computed its LUMO and normalized the orbital population over the entire dimer to 100%. We then projected this dimer LUMO onto the two fragments that directly interact in each stacking motif (end-group and core-arm/bridge-TT) and denoted their LUMO shares as W LUMO(1) and W LUMO(2) (percent of the total dimer LUMO; As a compact indicator of the effective LUMO–LUMO interaction between the two fragments, we define an orbital overlap parameter SLL = W LUMO(1) × W LUMO(2). Because SLL is large only when both fragments carry appreciable LUMO density, it correlates with the calculated |J| and highlights the role of orbital registry (beyond π–π distance alone) in governing coupling. Consistent with this picture, dimer I (E/C) in NaO2 shows near-zero |J| because one interacting fragment contributes negligible LUMO weight, whereas NaO3 dimer III (E/E) exhibits the largest |J| (42.7 meV) (Supplementary Figs. 12-14), directly correlated with the highest LUMO contributions of the two interacting fragments (18.1% and 13.7%) in NaO3 dimer III (E/E mode). In contrast, NaO1 presents moderate and balanced on both interacting fragments (end-group and core-arm/bridge units), yielding uniform |J| values (24.4, 24.7, 26.0 meV) and thus more isotropic, three-dimensional charge-transport pathways. Taken together, these results suggest that the relative distribution of LUMO density on interacting fragments—and the quality of their orbital registry—represents a major factor behind the variations in electronic coupling among NaO1–NaO3. The stronger and more balanced inter-fragment LUMO interaction in NaO1 across motifs is consistent with its large and isotropic |J| values and ultimately its good device transport characteristics.

To investigate the influence of varying core-arm bending degrees on the optical and electrochemical properties of the molecules, we measured the absorption spectra of the three acceptors in o-xylene solution, as well as their absorption and electrochemical characteristics in the solid state. As presented in Fig. 3a, the three isomers exhibit nearly identical absorption profiles in solution. However, in o-xylene-treated films, NaO1 displays a significantly red-shifted absorption (Fig. 3b). To further elucidate the nature of the film absorption, the spectra in Fig. 3b were deconvoluted50,51. As shown in Supplementary Fig. 15, the absorption profiles of NaO2 and NaO3 consist of four vibronic peaks. In contrast, the absorption spectra of NaO1 reveals five peaks upon fitting. In addition to the aforementioned four peaks, an extra peak appears at 831 nm. This further red-shifted feature is also ascribed to intermolecular aggregation, and its position implies the formation of additional highly delocalized excitonic states52–55. Such enhanced exciton delocalization originates from the 3D crystal packing structure of NaO1, which promotes stronger intermolecular electronic coupling and thus facilitates both red-shifted absorption and more efficient charge transport. In addition to the absorption peak positions, we quantitatively compared the absorption coefficients of the three isomers (Supplementary Fig. 16). In solution, the absorption coefficients are 1.08, 1.00, and 1.02 (×105 M−1 cm−1) for NaO1, NaO2, and NaO3, respectively, while in thin films the values are 1.10, 0.87, and 0.98 (×105 cm−1). These results confirm that NaO1 exhibits the strongest absorption strength in both states, which could be resulted from the enhanced exciton delocalization.

Electrochemical measurements reveal that NaO1 exhibits lower HOMO and LUMO energy levels compared to NaO2 and NaO3 (NaO1: HOMO/LUMO = –5.62/–3.89 eV; NaO2: –5.49/–3.78 eV; NaO3: –5.60/–3.79 eV) (Fig. 3c). A lower HOMO level is beneficial for providing a larger driving force for exciton dissociation, whereas a lower LUMO level may have a negative impact on the open-circuit voltage. The trend of energy levels obtained from theoretical calculations is somewhat inconsistent with the results from cyclic voltammetry (CV), which could be attributed to the fact that theoretical calculations reflect the energy levels in isolated molecular states, while CV measurements reflect those in aggregated states. At the frequency of 105 Hz, the dielectric constants of NaO1, NaO2, and NaO3 were determined to be 2.82, 2.70, and 2.78 (Supplementary Fig. 17), respectively, confirming that the three isomers exhibit comparable dielectric properties.

Comparisons of binary photovoltaic devices and photophysical processes
To evaluate how variations in the core arm angle of the three isomers influence device performance, we fabricated o-xylene-processed organic solar cells using a conventional architecture: ITO/PEDOT:PSS/PM6:acceptors/PFN-Br-MA/Ag. The optimized fabrication parameters are provided in the supporting information. The key device characteristics, including representative J–V curves (Fig. 3d) and performance metrics, are summarized in Table 2.

As anticipated, the NaO1-based device exhibits a lower open-circuit voltage (VOC, 0.915 V) compared to those based on NaO2 (0.953 V) and NaO3 (0.956 V). Nevertheless, it achieves a significantly higher short-circuit current density (JSC, 26.68 mA cm−2) and fill factor (FF, 0.784) than NaO2 (JSC = 11.44 mA cm−2; FF = 0.713) and NaO3 (JSC = 21.74 mA cm−2; FF = 0.745), resulting in an enhanced power conversion efficiency (PCE) of 19.11%. The considerably lower PCE of the NaO2-based device (7.77%) is primarily attributed to its suppressed short-circuit current, which likely stems from weak charge generation. The external quantum efficiency (EQE) spectrum of the PM6:NaO1 device shows the highest photoresponse across the entire wavelength range of 300–900 nm, with peak EQE values approaching 90% (Fig. 3e), indicating efficient photon-to-electron conversion. In contrast, the PM6:NaO2 device exhibits a significantly lower EQE (below 50%) throughout the spectrum, particularly in the 600–800 nm region, suggesting poor light harvesting or charge generation. The PM6:NaO3 device shows moderate performance with an EQE curve lying between those of NaO1 and NaO2. These results are consistent with the trend observed in the JSC, confirming the better light-harvesting and charge collection abilities of the PM6:NaO1 system.

Furthermore, we evaluate the charge carrier mobility by analyzing the current response under a linearly increasing reverse bias following photoexcitation (Fig. 3f). Among the three devices, the PM6:NaO1 blend exhibits the slowest current transient and the latest peak time, indicating slower carrier extraction but a higher overall charge carrier mobility, calculated to be 1.95×10−4 cm2 V−1 s−1. In comparison, the PM6:NaO2 and PM6:NaO3 devices show earlier current peaks and faster decay, with mobilities of 4.95×10−5 cm2 V−1 s−1 and 9.05×10−5 cm2 V−1 s−1, respectively. To further validate this conclusion, we performed space-charge-limited current (SCLC) measurements on the binary devices (Supplementary Fig. 18). The extracted hole (μh) and electron (μe) mobilities show a similar trend: for PM6:NaO1, μh = 1.52×10−4 cm2 V−1 s−1 and μe = 1.03×10−4 cm2 V−1 s−1, indicating balanced transport; for PM6:NaO2, μh = 2.73×10−5 cm2 V−1 s−1 and μe = 2.08×10−5 cm2 V−1 s−1; and for PM6:NaO3, μh = 6.76×10−5 cm2 V−1 s−1 and μe = 5.43×10−5 cm2 V−1 s−1. These results confirm that the PM6:NaO1 blend possesses both the highest absolute mobility and the most balanced μh/μe ratio, which directly supports its better charge extraction, higher FF, and overall photovoltaic performance.

The electrochemical impedance spectroscopy (EIS) Nyquist plots of PM6:NaOX devices were used to analyze charge transport and recombination behavior (Fig. 3g). The series resistance (Rs), extracted from the high-frequency intercept, is lowest for PM6:NaO1 (79 Ω), compared to PM6:NaO2 (229 Ω) and PM6:NaO3 (482 Ω), indicating better ohmic contact. The charge transfer resistance (Rct), determined from the semicircle diameter, is also smallest for PM6:NaO1 (1152 Ω), suggesting more efficient interfacial charge transfer and lower recombination losses, in line with its better device performance and higher FF. Notably, the low charge mobility and conductivity are attributed to the 2D packing style, which is highly sensitive to crystal defects.

To better understand the differences in VOC among the PM6:NaOX devices, we quantitatively analyzed their energy loss components (Fig. 3h–i). Among the three cells, PM6:NaO1 exhibits the lowest total energy loss (Eloss) of 0.516 eV, primarily due to its significantly reduced radiative recombination loss (0.042 eV) below the bandgap compared to PM6:NaO2 (0.067 eV) and PM6:NaO3 (0.063 eV). In contrast, the non-radiative recombination losses (Enr) are relatively similar across all devices—0.218 eV for NaO1, 0.224 eV for NaO2, and 0.222 eV for NaO3—indicating that non-radiative decay pathways are not the major source of variation in Eloss (Supplementary Table 1). This trend is supported by the electroluminescence quantum efficiencies (EQE-EL), where PM6:NaO1 shows the highest value (2.29×10−2) than both NaO2 (1.79×10−2) and NaO3 (1.93×10−2), confirming reduced non-radiative losses. These features collectively contribute to PM6:NaO1’s the trade-off between VOC and JSC.

Probing the blend film morphology of different binary systems
Apart from the intrinsic properties of the materials, the morphology of the active layer is also critical factor influencing the device efficiency. 2D GIWAXS and AFM were employed to investigate the molecular packing/crystallization behavior and surface morphological features, respectively. As presented in Fig. 4a,b and Supplementary Table 2, these three isomeric SMAs predominantly adopt a face-on molecular orientation, which is particularly favorable for NaO1. As the single crystal data of NaO1 suggested a similar packing style to L8-BO, a similar GIWAXS pattern ed was also observed for neat NaO1 compared to L8-BO thin film with the signature (021) scatterings5. Its blend with the donor polymer displayed moderate crystallization tendency found in most high performance Y6 derivative blends. The minor shift in in-plane and out-of-plane scatterings in the blend compared with the neat NaO1 can be attributed to the superposition with the scattering from the donor polymer. On the other hand, the altered pack style with strong intermolecular packing rendered NaO2 and NaO2 different scattering patten style compared to most known Y6 derivatives, which agreed with single crystal data. Complex in-plane scatterings supported the various intermolecular configurations in the crystal structures. The crystal coherence length (CCL) values were extracted based on the full width at half maximum (FWHM) of scattering peaks found in the GIWAXS line cuts by fitting them to multiple peaks in Gaussian distributions and applying to Scherrer Equation (Supplementary Fig. 19). The CCL calculated for these scatterings is notably higher than that of neat NaO1. The out-of-plane π-π stacking of NaO2 and NaO3 also displayed a complex double peak at azimuthal angle of 15o off the normal axis, further confirming the multiple mode of molecular stacking. These complex scattering patterns are also found in their blend with the donor polymer. It is noticed that the scatterings of NaO2 are significantly increased in blend with the donor polymer, displaying more higher ordered scatterings with CCL reaching 25.43 nm. PM6:NaO3 blend also displays CCL of 15.12 nm compared to 6.43 nm found in PM6:NaO1 blend. The excessively large CCL in PM6:NaO2 may result in oversized phase separation domains, reduced donor–acceptor interfacial areas, and hindered exciton dissociation, ultimately limiting device performance.

As shown in the AFM height images (Fig. 4c), the strong crystallization tendency and 2D lamellar network structures of NaO2 and NaO3 promote pronounced crystallization, as evidenced by the formation of large crystalline domains. In contrast, the PM6:NaO1 blend film exhibits the smoothest surface, with a low root mean square (RMS) roughness of 1.06 nm, indicating a more homogeneous and uniform morphology. By comparison, PM6:NaO2 and PM6:NaO3 films show increased surface roughness—5.99 nm and 1.40 nm, respectively—reflecting greater phase separation and reduced morphological uniformity. These AFM observations are consistent with the differences in CCL values among the blend films. Furthermore, infrared AFM was employed to investigate the fiber widths of the blend films (Fig. 4d and Supplementary Figs. 20–21). The average fibril diameters by using the full width at half maximum of the line-profile peaks were determined to be 19.9 nm (PM6) and 19.7 nm (NaO1) for the PM6:NaO1 blend, 58.5 nm (PM6) and 40.5 nm (NaO2) for PM6:NaO2, and 23.9 nm (PM6) and 24.7 nm (NaO3) for PM6:NaO3. Notably, the PM6:NaO1 blend features fibrils close to the optimal phase separation length scale (~10–20 nm), which is widely regarded as the most favorable for ensuring efficient exciton dissociation and continuous charge transport pathways20. This finely dispersed nanofibrillar morphology is therefore a key factor underpinning the better charge transport and the higher FF observed in the PM6:NaO1 device, while the coarser domains in PM6:NaO2 and the slightly oversized fibrils in PM6:NaO3 deviate from this optimal range and result in less efficient charge extraction.

To elucidate the structure–process–performance relationship in organic photovoltaic blends, we performed in-situ absorption spectroscopy during spin-coating, complemented by AFM and GIWAXS characterizations56. The three blend systems—PM6:NaO1, PM6:NaO2, and PM6:NaO3—exhibit distinct crystallization dynamics and morphology evolution, which closely correlate with their photovoltaic performance. In the in-situ absorption maps (Fig. 5), all three blends initially display broad and featureless spectra, corresponding to a homogeneous solution state. As spin-coating proceeds, the spectra gradually red-shift, indicating the onset of molecular aggregation and crystallization.

Among them, PM6:NaO1 exhibits a relatively fast spectral evolution, with the acceptor-related absorption band (700–800 nm) beginning to red-shift and stabilizing at approximately 2.4 s. The tracked vibronic peaks show a smooth and continuous transition for both donor and acceptor components. Although GIWAXS reveals that NaO1 exhibit the weakest crystallinity among the three systems, its AFM image indicates the smoothest surface morphology. This homogeneous film formation facilitates efficient charge transport and extraction, resulting in the highest carrier mobility, FF, and PCE among the blends. In contrast, PM6:NaO2 demonstrates the slowest crystallization dynamics, with delayed red-shift of the acceptor peaks and stabilization occurring only at around 3.1 s. The acceptor 0-0 peak in this system stabilizes near 800 nm, indicating stronger intermolecular packing and more extensive aggregation. This is corroborated by GIWAXS results, which show the highest degree of molecular ordering, and by AFM, which reveals a significantly rougher surface. Such over-crystallization likely disrupts film uniformity, leading to discontinuous charge pathways and enhanced recombination losses, ultimately yielding the lowest device performance in term of charge mobility, FF, and PCE. PM6:NaO3 displays intermediate characteristics. Its in-situ absorption features evolve rapidly, similar to NaO1, with spectral stabilization also occurring at ~2.4 s. However, its final acceptor absorption peak settles around 800 nm, which is blue-shifted compared to NaO1 (~840 nm), probably due to its different aggregation forms. GIWAXS confirms a high level of molecular ordering, comparable to that of NaO2, while AFM reveals a moderately smooth morphology with an RMS roughness of 4.5 nm. Consequently, the device based on PM6:NaO3 delivers balanced performance in term of charge transport and FF.

To complement the in-situ absorption results, we further monitored the structural evolution of the active layers using in-situ GIWAXS. As shown in Supplementary Fig. 22, NaO1 exhibited weak scattering consistent with limited crystallinity, with its crystallization occurring before that of the donor polymer. NaO2 displayed strong scattering immediately after spin coating and underwent a complex phase transformation during thermal annealing, eventually forming sharp diffraction peaks. In sharp contrast, NaO3 showed a scattering shift from 4.7 to 3.9 nm−1 during spin coating, suggesting significant structural rearrangement in the early stage of film formation. Minimal further changes were observed during annealing for NaO1- and NaO3-based blends. These distinct crystallization pathways highlight the critical role of crystallization kinetics in determining the final nanostructures, bridging the fast spectral evolution observed in in-situ absorption with the actual structural reorganization captured by GIWAXS.

To elucidate the influence of central core isomerism on exciton behavior, transient absorption (TA) spectroscopy was carried out with excitation at 500 nm to probe the electron transfer characteristics in the blend films (Supplementary Fig. 23)57. The ground-state bleach (GSB) features of both PM6 and NaOX closely matched their steady-state absorption profiles. The GSB decay kinetics of electron transfer were assessed by probing at 628 nm (Fig. 6a). While the 500 nm excitation used in TAS can excite both donor and acceptor components and thus may involve contributions from both electron and hole transfer, the decay of the donor GSB predominantly reflects the electron transfer dynamics at the donor–acceptor interface, in line with previous reports58. The corresponding biexponential lifetimes for the PM6:NaO1, PM6:NaO2, and PM6:NaO3 blends were summarized in Supplementary Table 3, with PM6:NaO1 exhibiting the shortest decay constants (τ1 = 1.43 ps, τ2 = 24.88 ps), indicating more efficient exciton dissociation and ultrafast hole transfer process. Additionally, under the excitation at 800 nm, a subsequent rise in the PM6 bleach signal at 630 nm following the decay of the NaOX acceptors (Supplementary Fig. 24), suggesting hole transfer from NaOX to PM6. Given the sub-picosecond dynamics observed in all binary blends, the fastest component (<1 ps) can be attributed to an ultrafast hole transfer process from the donor PM6 to the acceptors, enabled by their well-aligned HOMO levels and strong interfacial coupling. While this rapid channel is present in all systems, its subsequent evolution differs: in PM6:NaO1 the ultrafast hole transfer is more effectively followed by stabilized charge separation, whereas in PM6:NaO2 and PM6:NaO3 a larger fraction of carriers recombine or relax before dissociation. The time-resolved kinetics at this wavelength were further analyzed to evaluate hole transfer dynamics, revealing that PM6:NaO1 facilitated more rapid hole transfer compared to its counterparts (Fig. 6b). To further support this observation, exciton diffusion lengths (LD) of the neat acceptor films were estimated using the exciton–exciton annihilation (EEA) method57, yielding values of 30.5 nm (NaO1), 17.9 nm (NaO2), and 19.2 nm (NaO3) (Supplementary Table 4). The relatively long LD of NaO1, in combination with its favorable charge transfer kinetics, contributes to suppressed charge recombination and enhanced FF in devices, consistent with the trends observed in the TA measurements.

Device physics and performance optimization of ternary systems
Considering the high device efficiency and low energy loss of NaO1, we further explored its application potential in non-halogenated ternary OSCs. Using PM6:L8-BO-C41 (Supplementary Fig. 25) as the binary host system, ternary devices were constructed by incorporating NaO1 at varying L8-BO-C4:NaO1 ratios. As shown in Fig. 6c and Supplementary Table 5, the PM6:L8-BO-C4 binary device achieved a power conversion efficiency (PCE) of 18.76%, with a VOC of 0.881 V and a FF of 0.788. Upon gradually increasing the content of NaO1, both VOC and FF showed steady enhancement. When the ratio of L8-BO-C4 to NaO1 reached 1:1, the device delivered its best performance, with a PCE of 20.07% and an FF of 80.3%. The photocurrent derived from the integrated EQE spectrum (26.62 mA cm−2) confirmed the reliability of the J–V results (Fig. 6d). Notably, the enhancement of the EQE response in the ternary device is mainly concentrated in the donor absorption region. Transient absorption spectroscopy further corroborates this observation, showing a faster decay of donor exciton signals and a stronger growth of charge-induced absorption features in the ternary blend compared to the binary PM6:L8-BO-C4 system.

The operational stability of PM6:L8-BO-C4 and PM6:L8-BO-C4:NaO1 devices was evaluated under three conditions: shelf storage in the dark at room temperature, continuous AM 1.5G light soaking, and thermal aging at 85 °C (Supplementary Fig. 26). In all cases, the ternary devices exhibits significantly enhanced stability compared to the binary control. For instance, under continuous AM 1.5G illumination, the ternary device maintains over 85% of its initial PCE after 200 h, whereas the binary counterpart decreases to ~75%. Similarly, during 85 °C thermal stress, the ternary device retains ~86% of the initial PCE after 100 h, while the binary device drops below 70%. These results clearly demonstrate that incorporating NaO1 not only improves efficiency but also effectively enhances operational stability against light and thermal stress.

To further evaluate the scalability of the material, we fabricated large-area OSC modules (aperture area of 16.80 cm² with 8 sub-cells in series, active area of 15.03 cm²) via blade coating under ambient conditions (Supplementary Fig. 27 and Supplementary Table 6). The PM6:L8-BO-C4:NaO1 ternary modules delivered a PCE of 16.93% (average 16.20 ± 0.49%), compared to 15.20% (average 14.29 ± 0.75%) for the PM6:L8-BO-C4 binary modules. The simultaneous improvements in VOC and FF highlight that the benefits of NaO1 extend from small-area devices to scalable module architectures, underscoring the practical potential of NaO1 in real-world OSC applications. Notably, the achieved efficiency of 16.93% is among the highest values reported for OSC modules at this active-area scale.

FTPS-EQE and EQE-EL measurements further revealed that the incorporation of NaO1 led to a continuous reduction in both non-radiative and total energy losses, thereby accounting for the gradual increase in VOC (Fig. 6e–g). To gain deeper insight into the impact of different central cores on non-radiative energy loss, we analyzed the reorganization energy contributions across various vibrational modes (Fig. 6h, i). In L8-BO-C4, a prominent high-frequency vibrational mode at 1604.9 cm-1—primarily associated with C–C bond stretching within the central core—was identified as a major contributor to the reorganization energy during the S1 → S0 transition. Remarkably, when the central BT unit was substituted with a fused quinoxalinone moiety in NaO1, this vibrational contribution was significantly attenuated (L8-BO-C4: 53.5 meV; T2: 30.2 meV), indicating a suppressed geometric relaxation upon excitation. Moreover, complementary calculations of the S1 → S0 relaxation and the S1 → anion transition further confirm that NaO1 consistently exhibits smaller reorganization energies than L8-BO-C4 (Supplementary Fig. 28). These results provide additional evidence that the quinoxalinone-based core effectively mitigates vibrational relaxation across different electronic states. This reduction in vibrational coupling is beneficial for minimizing exciton loss through non-radiative channels, ultimately contributing to lower energy loss.

Additionally, the optimal PM6:L8-BO-C4:NaO1 device shows the shortest decay constants (τ1 = 0.99 ps, τ2 = 11.7 ps) and the fastest hole transfer dynamics, which likely contribute to its FF. To further probe charge dynamics, transient photocurrent (TPC) and transient photovoltage (TPV) measurements were performed (Supplementary Fig. 29). The TPC results reveal nearly identical carrier extraction times for the PM6:L8-BO-C4 and PM6:L8-BO-C4:NaO1 devices (0.12 μs vs. 0.13 μs), suggesting comparable charge extraction in in both systems. In contrast, the TPV measurements show that the ternary PM6:L8-BO-C4:NaO1 device exhibits a prolonged carrier lifetime (3.74 μs vs. 2.85 μs), indicating suppressed charge recombination. The observed improvement in TPV is consistent with the enhanced FF and overall PCE in the ternary device.

The charge generation mechanism of the ternary blend was further clarified by steady-state photoluminescence (PL) measurement (Supplementary Fig. 30). Since NaO1 and L8-BO-C4 exhibit similar absorption/PL spectra and a minimal LUMO offset (~0.07 eV), no cascade charge transfer or fluorescence resonance energy transfer is expected between them. Indeed, the PL intensity of their binary blend is close to the arithmetic average of the two pristine acceptors, ruling out inter-acceptor energy transfer. In contrast, all donor–acceptor blends show pronounced PL quenching due to efficient charge transfer at the donor/acceptor interface. Notably, the ternary PM6:L8-BO-C4:NaO1 blend exhibits stronger PL quenching than either binary, consistent with fs-TAS results showing accelerated charge separation, thereby confirming enhanced exciton dissociation in the ternary system.

Methods
Device fabrication and testing
Solar cells were fabricated in a conventional device configuration of ITO/PEDOT:PSS/PM6:acceptors/PFN-Br-MA/Ag. The ITO substrates were first scrubbed by detergent and then sonicated with deionized water, acetone, and isopropanol, subsequently, and dried overnight in an oven. The glass substrates were treated by UV-Ozone for 30 min before use. PEDOT:PSS (Heraeus Clevios PVP AI 4083) was spin-cast onto the ITO substrates at 4000 rpm for 30 s, and then dried at 170 °C for 15 min in air. The PM6:acceptor blends (1:1.2 weight ratio) were dissolved in ortho-xylene (the total concentration of blend solutions was 22 mg mL−1 for all blends, where 0.5% vol 1,8-diidoooctance was added as additive, and stirred at 100 °C in a nitrogen-filled glovebox for 1 h. The blend solution was spin-cast at 2500 rpm for 30 s. The active layers were then treated by 100 °C thermal annealing for 2 min. A thin PFN-Br-MA layer was coated on the active layer, followed by the deposition of Ag (100 nm) (evaporated under 3×10−4 Pa through a shadow mask). The current density-voltage (J–V) curves of devices were measured using a Keysight B2901A Source Meter in glovebox under AM1.5G (100 mW cm−2) using a Enlitech solar simulator. The device contact area was 0.041 cm2, device illuminated area during testing was 0.04 cm2, which was determined by a mask. The EQE spectra were measured using a Solar Cell Spectral Response Measurement System QE-R3011 (Enlitech Co., Ltd.). The light intensity at each wavelength was calibrated using a standard monocrystalline Si photovoltaic cell.

The large-area modules were fabricated with an architecture similar to that of the small-area cells. An aperture area of 16.80 cm2 in OSC modules, with 8 subcells connected in series, was fabricated on the ITO glass substrates with a size of 5×5 cm2. The series interconnection of the module was realized by P1, P2, and P3 lines, which were patterned using a laser etching system with a GH-LS300. The gap between each ITO pattern is defined as P1 with a width of 20 μm. The P1-patterned ITO glass was then cleaned. PEDOT:PSS layer was then deposited on pre-cleaned ITO-coated glass through blade-coating at a speed of 20 mm s-1 and annealed at 150 °C for 15 min under ambient conditions. The active layer was blade-coated at a velocity of 20 mm s−1 and a gap height of 20 µm in the air, giving a ~105-nm-thick active layer. Next, 10-nm-thick C60 and 5-nm-thick BCP layers were deposited by thermal evaporation under 1×10-6 mbar. The P2 lines were patterned before Ag deposition, with the corresponding width is around 140 μm. Then, 100 nm thick Ag electrode was thermal evaporated under a pressure of 1×10−6 mbar, and the P3 (90 μm) line was formed. The active area of the module was defined by the shadow mask and the aperture mask as 15.03 cm2.

General information
All solvents and reagents were used as received from commercial sources and used without further purification unless otherwise specified. All reactions were heated by metal sand bath (WATTCAS, LAB-500, https://www.wattcas.com). 1H NMR (400 MHz or 500 MHz) and 13C NMR (100 MHz or 125 MHz) spectra were measured on a MERCURY VX300 spectrometers. Mass spectra were recorded on a Shimadzu spectrometer. High-resolution mass spectrometry was carried out on Thermo ScientificTM Q-Exactive.

Electrochemical characterizations
Cyclic voltammetry measurements were carried out on a CHI voltammetric analyzer at room temperature. Tetrabutylammonium hexafluorophosphate (n-Bu4NPF6, 0.1 M) was used as the supporting electrolyte. The conventional three-electrode configuration consists of a platinum working electrode with a 2 mm diameter, a platinum wire counter electrode, and an Ag/AgCl wire reference electrode. Cyclic voltammograms were obtained at a scan rate of 100 mV/s.

DFT calculations
DFT calculations were performed by using Gaussian 16 at the B3LYP-D3(BJ)/def2-SVP level, and the long alkyl chain was simplified as methyl. Electronic coupling was obtained by computing various dimer configurations extracted from the crystal structure. Reorganization energies were obtained using MOMAP 2024A. LUMO orbit distribution analyses were performed with the Multiwfn 3.8.

Single-crystal growth and testing
Diffraction quality crystals were grown by the solvent diffusion method. Acceptors (5 mg) were dissolved in CHCl3 (5 mL) in the 15 mL sample bottle, then 8 mL of methanol or ethanol was added slowly. The bottle was then sealed tightly and left undisturbed for 10 days. The single crystal X-ray diffraction data were collected at 100 K or 150 K on a Bruker D8 VENTURE diffractometer using Cu Kα radiation (λ=1.54178 Å) or Ga Kα radiation (λ=1.34139 Å).

AFM characterizations
AFM measurements were carried out using a Dimension 3100 Scanning Probe Microscope operated in tapping mode under ambient conditions at room temperature. All thin-film samples were prepared by spin-coating onto glass/ITO substrates.

In-situ and ex-situ GIWAXS measurements
In-situ GIWAXS measurements were carried out at beamline 1W1A of Beijing Synchrotron Radiation Facility (BSRF) with an incident beam energy at 8 keV (the fixed grazing angle is 0.2°). The beam center and sample-to-detector distance were calibrated with LaB6. The samples were directly prepared on clean Si substrates with the same conditions as the device preparation. Each GIWAXS image was captured with the exposure time of 10 s during the spin-coating process. Ex-situ GIWAXS measurements were performed at the Complex Materials Scattering (CMS) beamline of the National Synchrotron Light Source II (NSLS-II), Brookhaven National Lab. The X-ray beam with an energy of 13.5 keV shone upon the samples with the incident angle of 0.15 deg with respect to the substrate between the critical angles of the organic films and the Si substrate. A custom-made Pilatus-2M detector was placed at the distance of 255 mm from the sample center to capture GIWAXS images with the exposure time of 10 s.

In-situ UV-Vis absorption measurements
In situ UV-Vis absorption measurements were carried out with a DU-100 system. The optical fiber was aligned so that the center of the light is focused on the center of the film. The sampling interval was 10 ms with an integration time of 10 ms per sample point. The average sampling times were adjusted 5 times to obtain smooth absorption curves.

PiFM Measurements
Photo-induced force microscopy (PiFM) results were acquired using a VistaScope microscope from Molecular Vista, inc. All PiFM experiments were excited by a pulsed quantum cascade laser (Block Engineering) with a gap-free narrowband tunable wavenumber of 760-1950 cm−1. The spectral line width is ~2 cm-1 with a wavenumber resolution of 0.5 cm−1. The PiFM experiment here was operated at the sideband excitation with the laser-frequency modulated at fm = f1–f0, where f0 is the first mechanical eigenmode resonances of the cantilever that is used for PiF signal detection, while f1 denotes the second ones recorded for the AFM topography of the sample. The probe is an Pt-coated tip with a resonant frequency of ~350 kHz (PPP-NCHPt-MB, Nanosensors).

Transient absorption spectroscopy
For femtosecond transient absorption spectroscopy, the fundamental output from Yb:KGW laser (1030 nm, 220 fs Gaussian fit, 100 kHz, Light Conversion Ltd) was split into two light beams. One was introduced to NOPA (ORPHEUS-N, Light Conversion Ltd) to produce a certain wavelength for pump beam, the other was focused onto a YAG plate to generate white light continuum as probe beam. The pump and probe overlapped on the sample at a small angle less than 10°. The transmitted probe light from sample was collected by a linear CCD array. A pump pulse of 800 nm (below 5 μJ/cm2) was employed to excite only acceptors, and after a certain delay time, the relative transmittance change (ΔA) was probed using a white-light continuum. The primary absorption peaks for different acceptors and donors are well separated in the spectral domain, therefore, both the spectral and temporal characteristics of hole transfer dynamics can be extracted.

TPV and TPC measurements
In TPV measurements, OSC devices were subjected to a background light bias provided by a focused quartz tungsten–halogen lamp, with intensity adjusted to replicate operational conditions (i.e., yielding a device voltage equivalent to the open-circuit voltage under solar illumination). Photoexcitation was introduced by 8 ns laser pulses (Oriental Spectra, NLD520) at a wavelength of 518 nm with a spectral width of 3 nm. The TPV response was recorded at open-circuit conditions using a digital oscilloscope. For TPC measurements, signals were collected under short-circuit conditions with the same excitation wavelength but without background light bias.

FTPS-EQE and EL-EQE measurements
Fourier-transform photocurrent spectroscopy external quantum efficiency (FTPS-EQE) measurements were performed using a PECT-600 integrated system (Enlitech) with phase-sensitive detection. The photoresponse signal was amplified by a Stanford Research SR830 lock-in amplifier operated at a modulation frequency of 17 Hz. Electroluminescence external quantum efficiency (EL-EQE) was characterized under controlled carrier injection using a REPS-Pro voltage/current source (Enlitech). Devices were fabricated under optimized conditions with an active layer thickness of 100 ± 5 nm and electrode alignment precision better than 2 μm. The EL-EQE was measured by sweeping the applied voltage from 0 to 1.8 V in 0.1 V increments with a dwell time of 500 ms, enabling systematic investigation of recombination dynamics across the operating bias range. Both FTPS-EQE and EL-EQE employed thermoelectrically cooled InGaAs detectors (900–1600 nm) calibrated against NIST standards, ensuring wavelength-dependent quantum efficiency accuracy within ±1.5%.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript reports the design and synthesis of three isomeric small-molecule acceptors with different fused-ring configurations. By modulating the fusion pattern of the central electron-deficient core, the authors introduce a relatively underexplored strategy (backbone-level isomerization) for tuning molecular conformation and packing behavior in OSCs. Compared with more common approaches, such as side-chain or end-group modification, the method used in this work demonstrates a new perspective on how subtle geometric differences can influence device performance. The authors systematically examined the influence of the fused-ring configuration on intermolecular interactions, crystallization dynamics, electronic coupling, and ultimately, device performance. The comprehensive characterization, including single-crystal XRD, GIWAXS, in-situ UV–vis, and TA spectroscopy, effectively supported the conclusions regarding the structure-performance relationship. Overall, this is a well-structured study in my judgment, and I recommend publication after minor revisions. My comments are listed below:\n1. The current analysis focuses on comparing the absorption peak positions and onset wavelengths of the three isomers in solution and film states. However, it does not address differences in absorption strength, which may also be significantly influenced by the bending angle of the central core arm. Since absorption strength is directly related to light-harvesting efficiency and photocurrent generation in OSCs, it is essential for evaluating the optical properties of active materials. Therefore, I suggest that the authors quantitatively investigate and compare the absorption strength of the three acceptors.\n2. The dielectric constant is a critical parameter for the active materials of OSCs, as it directly influences exciton binding energy, charge separation efficiency, and recombination dynamics. Given that the three isomeric acceptors exhibit different molecular packing properties and degrees of crystallinity, it would be valuable to assess whether these structural differences impact their dielectric constants. I recommend that the authors investigate the dielectric constants of the acceptors synthesized in this work.\n3. In the GIWAXS section, the authors discuss the (021) and π–π stacking peaks observed in the patterns, but the peak assignment process is not fully clear. Could the authors provide more details? Additionally, I recommend including a summary table in the Supporting Information listing key GIWAXS parameters, such as crystalline coherence length, peak positions, and full width at half maximum for both neat and blend films.\n4. In the in-situ optical absorption experiments, how is the \"starting time\" of spectral acquisition determined? Is it recorded automatically by a synchronized software trigger with the spin-coater? Furthermore, what is the estimated error bar for the starting time? Given that crystallization dynamics and molecular aggregation behavior are highly time-sensitive, an accurate determination of the starting time is essential.\n5. According to literature, the evaporation time of o-xylene under typical spin-coating conditions is generally around 10 seconds. However, the in-situ absorption data in this work indicate that spectral evolution stabilizes within a much shorter time frame. Could the authors provide an explanation for this discrepancy?\n6. Although L8-BO-C4 and NaO1 share a similar bandgap, devices based on L8-BO-C4 exhibit more severe voltage loss. Why was L8-BO-C4 selected as the third component in the ternary blend? Additionally, the observed reduction in non-radiative voltage loss in the PM6:L8-BO-C4:NaO1 system is attributed to suppressed vibrational modes. However, since Figure 6 only presents the S1 reorganization energies, further analysis involving reorganization energies of other relevant electronic states would be necessary to confirm this explanation.", "reviewer_2": "The work was well conducted and well documented.\nSuggestion to be improved.\n- show clearly what is L8-BO-C4.\n- emphasize why the authors used o-xylene.\n- show the stability data.", "reviewer_3": "This manuscript shows a study on the impact of fused-ring configurations in isomeric acceptors on the structure-packing-performance relationship. The authors developed a linearly fused acceptor NaO1, emphasizing that its three-dimensional packing network was critical to achieving optimized morphology and improved device efficiency. A PCE of 19.11% was obtained in PM6:NaO1-based devices processed with o-xylene. Using this acceptor, they reported an efficiency enhancement from 18.76% to 20.07% upon introducing NaO1 into the PM6:L8-BO-C4 system, attributed to reduced non-radiative energy loss and accelerated hole transfer. Although the acceptor materials might be interesting, the reported performance of this manuscript falls still below contemporary state-of-the-art values. The idea of developing a donor or acceptor material to fabricate ternary devices has been widely used in the field of organic photovoltaic [Adv. Mater. 2021, 33, 2008134; Adv. Mater. 2021, 33, 2101733; Adv. Mater. 2021, 33, 2102420], and certain key claims lack rigorous experimental support or theoretical justification, resulting in a lack of novelty and depth. Overall, I think that this manuscript is more suitable for the materials journal. Therefore, I would not recommend this manuscript for publication in Nature Communications for the following reasons:\n1.This manuscript demonstrates insufficient novelty, lacks in-depth analysis, and does not present significant performance improvements. The PM6:NaO1 devices exhibit an enlarged VOC (0.915 V) compared to other binary OSCs, which should be the reason for the increased VOC in PM6:NaO1:L8-BO-C4 ternary OSCs [Nat. Energy 2023, 8, 978–988]. However, this finding does not provide novel mechanistic insights into improving the performance of ternary OSCs. Meanwhile, the reported PCE of 20.07% falls below contemporary state-of-the-art values [Adv. Mater. 2025, 37, 2500352; Adv. Mater. 2025, 37, 2501812]. Neither materials nor devices demonstrate discernible advantages over those existing systems. Crucially, this study does not establish whether they can provide meaningful improvements in stability and large-area devices, especially modules.\n2.While single-crystal data reveal packing differences, theoretical modeling is needed to directly correlate molecular conformation with performance, strengthening mechanistic insights. Theoretical calculations (e.g., DFT or MD simulations) should be performed to quantify the conformational differences (e.g., dihedral angles, dipole moments) among the three isomers (NaO1, NaO2, NaO3) and their impact on packing modes, clarifying the mechanism behind electronic coupling variations.\n3. While the authors employed device physics characterization to elucidate the origin of performance differences in binary and ternary devices, the analysis lacks sufficient depth. For example, integrated current density curves are absent from Figures 3E and 6D. Compared to binary PM6:L8-BO-C4 OSCs, the improvement of spectral response of ternary OSCs is mainly concentrated in the donor region (Figure 6D). This has not been reasonably explained. Only total carrier mobility (Photo-CELIV) is provided; Space-charge-limited current (SCLC) measurements should be done to directly determine electron/hole mobilities, evaluating charge transport balance and validating the proposed 3D charge transport network. Energy loss calculations lack transparency, particularly regarding critical parameters like the device bandgap Eg and Eloss. Transient photovoltage (TPV) or photocurrent (TPC) measurements may be useful to quantify differences in non-radiative recombination losses. A more rigorous device physics analysis is thus required to substantiate the performance interpretation.\n4. In-situ GIWAXS or grazing-incidence X-ray diffraction (GIXD) should be conducted to monitor crystallization kinetics during film formation, combined with AFM phase imaging to analyze phase separation dynamics. The current in-situ absorption spectroscopy provides indirect aggregation insights but lacks real-time structural evolution data, making it difficult to confirm the causality between \"fast spectral evolution\" and optimized morphology.\n5.As indicated in GIWAXS patterns and line cuts in the OOP direction, it can be seen that the PM6:NaO1 films show an obvious feature of π-π stacking relative to other two blends. This observation is contrary to the authors’ claim about CCLs. Please provide the detailed fitting curves or traces to enhance clarity and aid reader comprehension. The image quality of AFM is so poor that the surface morphology cannot be clearly seen in Figure 4c. The authors should conduct high-quality AFM measurements. As for infrared AFM data, the authors did not indicate the calculated fiber width from donor or acceptor. Please provide the fiber width of both donor and acceptor.\n6.The authors employed transient absorption spectroscopy (TAS) to investigate exciton dynamics in binary and ternary systems. However, the presentation and analysis of TAS data lack sufficient rigor. Critically, the selection of 500 nm pump light excites both donor and acceptor components, resulting in spectral contamination from hole transfer processes that complicates the interpretation of electron transfer dynamics. Furthermore, an inconsistency exists between the probe wavelength cited in the main text (630 nm) and that indicated in Figure 6A (628 nm). Biexponential fitting was applied to extract lifetimes, yet the fitting intervals and methodological details were not provided. The attribution of the shortest lifetimes to reduced recombination losses is unsupported; the increased recombination typically shortens exciton lifetimes, contrary to the authors’ claim. Given the exceptionally fast hole transfer kinetics (<1 ps) shown in Figure 6B, additional discussion of the underlying physical processes is very necessary. Moreover, while exciton-exciton annihilation (EEA) methodology was used to determine diffusion lengths of singlet excitons, neither raw nor fitted dynamic traces appear in the main text or SI file. Collectively, these issues raise significant concerns regarding data reliability. Therefore, to enable valid comparison of charge transfer and recombination dynamics, separation of distinct excited-state species must precede comparative analysis.\n7.The performance enhancement in ternary devices is attributed to \"accelerated hole transfer,\" but direct experimental evidence for energy/charge transfer is lacking, and morphology homogeneity is insufficiently characterized. Fluorescence resonance energy transfer (FRET) or time-resolved photoluminescence (TRPL) is required to analyze energy transfer pathways in the PM6:L8-BO-C4:NaO1 blend.\n8.While the authors attribute the absorption spectral red-shifting and improved charge carrier mobility to enhanced electron delocalization, direct evidence and supporting this claim remains absent in this manuscript. Please provide rigorous experimental support or theoretical justification.\n9. To elucidate the nature of the film absorption, the authors deconvoluted the spectra shown in Figure S1. However, the authors did not supplement the explanation with relevant references or further clarification about the assignment of those fitting peaks."}, "author_response": "Response to comment: Thank you very much for your positive comments and constructive suggestions.\n1. The current analysis focuses on comparing the absorption peak positions and onset wavelengths of the three isomers in solution and film states. However, it does not address differences in absorption strength, which may also be significantly influenced by the bending angle of the central core arm. Since absorption strength is directly related to light-harvesting efficiency and photocurrent generation in OSCs, it is essential for evaluating the optical properties of active materials. Therefore, I suggest that the authors quantitatively investigate and compare the absorption strength of the three acceptors.\nResponse: We appreciate the reviewer’s valuable suggestion. In addition to comparing absorption peak positions and onset wavelengths, we have now quantitatively analyzed the absorption strength of the three isomers in both solution and thin-film states. Consequently, we have added some discussion about the absorption strength of the three acceptors in the revised manuscript:\n“Such enhanced electron delocalization can be attributed to the unique 3D crystal packing structure of NaO1, which facilitates more extensive intermolecular electronic coupling. In addition to the absorption peak positions, we quantitatively compared the absorption coefficients of the three isomers (Figure S6). In solution, the absorption coefficients are 1.08, 1.00, and 1.02 (×105 M-1 cm-1) for NaO1, NaO2, and NaO3, respectively, while in thin films the values are 1.10, 0.87, and 0.98 (×105 cm-1). These results confirm that NaO1 exhibits the strongest absorption strength in both states, which could be resulted from the enhanced electronic delocalization.”\nFigure S6. The absorption spectra of NaO1, NaO2, and NaO3 in o-xylene solution (left) and solid state (right).\n2. The dielectric constant is a critical parameter for the active materials of OSCs, as it directly influences exciton binding energy, charge separation efficiency, and recombination dynamics. Given that the three isomeric acceptors exhibit different molecular packing properties and degrees of crystallinity, it would be valuable to assess whether these structural differences impact their dielectric constants. I recommend that the authors investigate the dielectric constants of the acceptors synthesized in this work.\nResponse: We appreciate the reviewer’s suggestion. We have measured the dielectric constants (ε) of the three isomeric acceptors under different frequency. The values are\nr\n2.82 for NaO1, 2.70 for NaO2, and 2.78 for NaO3. These results indicate that the dielectric constants of the three isomers are very similar, suggesting that the variations in backbone geometry and crystallinity have a negligible effect on their intrinsic dielectric properties. As a result, we have added some discussion as below:\n“The trend of energy levels obtained from theoretical calculations is somewhat inconsistent with the results from cyclic voltammetry (CV), which could be attributed to the fact that theoretical calculations reflect the energy levels in isolated molecular states, while CV measurements reflect those in aggregated states. At the frequency of 105 Hz, the dielectric constants of NaO1, NaO2, and NaO3 were determined to be 2.82, 2.70, and 2.78 (Figure S7), respectively, confirming that the three isomers exhibit comparable dielectric properties.”\nFigure S7. permittivity-frequency curves of neat NaO1, NaO2, and NaO3 films.\n3. In the GIWAXS section, the authors discuss the (021) and π–π stacking peaks observed in the patterns, but the peak assignment process is not fully clear. Could the authors provide more details? Additionally, I recommend including a summary table in the Supporting Information listing key GIWAXS parameters, such as crystalline coherence length, peak positions, and full width at half maximum for both neat and blend films.\nResponse: We thank the reviewer for this comment. The assignment of scatterings from NaO1 is based on comparing single crystals data to literature and adopting the established indexing manner. The π-π stacking was not assigned with Miller Indices because there are three peaks overlapped in close vicinity. We did not assign the scatterings of the two other molecules as the unfavored packing style is of little interest for further exploration. To improve our manuscript, we indicated the scatterings in the neat NaO1 thin films in Figure 4. We also included further discussion and a reference on how these indexing is compared to literature.\nChanges in figure 4:\nComparing to literature:\nRef 5: Nat. Energy 2021, 6, 605-613. DOI: 10.1038/s41560-021-00820-x.\nChange in main text:\nOriginal text:\n“This packing style is very similar to that of L8-BO, which supported its outstanding performances.”\nAmended text:\n“This packing style is very similar to that of L8-BO reported by Sun and co-workers,[5] which supported its outstanding performances.”\nOriginal text:\n“NaO1 displayed scattering patterns similar to that of L8-BO with the signature (021) scatterings.”\nAmended text:\n“As the single crystal data of NaO1 suggested a similar packing style to L8-BO, a similar GIWAXS patterned was also observed for neat NaO1 compared to L8-BO thin film with the signature (021) scatterings.[5]”\nAdditionally, I have added a summary table in the Supporting Information (Table S2) listing key GIWAXS parameters.\n4. In the in-situ optical absorption experiments, how is the \"starting time\" of spectral acquisition determined? Is it recorded automatically by a synchronized software trigger with the spin-coater? Furthermore, what is the estimated error bar for the starting time? Given that crystallization dynamics and molecular aggregation behavior are highly time-sensitive, an accurate determination of the starting time is essential.\nResponse: Thank you for your questions. The accurate determination of the starting time is indeed critical for our time-sensitive study. In our experiments, the \"starting time\" is identified from the spectral data itself. The procedure is as follows: after placing the ITO substrate, we manually initiate the UV-Vis software to begin continuous acquisition at a fixed time interval（0.1s）, which is calculated as (10 ms integration time + 10 ms sampling interval) × 5 scans. Before spin-coating, the spectra show only a baseline. The initiation of spin-coating causes an immediate appearance of absorption peaks. Therefore, we assign the time of the first spectrum showing a distinct non-baseline signal as the starting point (t=0). This method is robust because the physical action of depositing the solution and starting the spin-coater instantly triggers the aggregation process, which is detected within the next acquisition cycle. The main source of uncertainty is the fixed time interval between spectra; thus, the error bar for the absolute starting time is estimated to be less than or equal to this interval duration, which is precisely known and controlled.\n5. According to literature, the evaporation time of o-xylene under typical spin-coating conditions is generally around 10 seconds. However, the in-situ absorption data in this work indicate that spectral evolution stabilizes within a much shorter time frame. Could the authors provide an explanation for this discrepancy?\nResponse: We appreciate the reviewer’s insightful comment. The shorter drying time observed in our in-situ absorption measurements compared to the reported evaporation time of o-xylene can be attributed to the use of pre-heated substrates during film deposition. The elevated substrate temperature significantly accelerates solvent evaporation and film stabilization, thereby leading to the faster spectral evolution observed in our experiments.\n6. Although L8-BO-C4 and NaO1 share a similar bandgap, devices based on L8-BO-C4 exhibit more severe voltage loss. Why was L8-BO-C4 selected as the third component in the ternary blend? Additionally, the observed reduction in non-radiative voltage loss in the PM6:L8-BO-C4:NaO1 system is attributed to suppressed vibrational modes. However, since Figure 6 only presents the S1 reorganization energies, further analysis involving reorganization energies of other relevant electronic states would be necessary to confirm this explanation.\nResponse: We thank the reviewer for this valuable question. Although L8-BO-C4 exhibits a more pronounced voltage loss compared to NaO1, it was chosen as the third component due to its overall excellent optoelectronic properties. Specifically, L8-BO-C4 possesses a relatively high photoluminescence efficiency, enlarged intermolecular spacing induced by its long side chains, and reduced charge quenching, all of which contribute to efficient intermolecular charge transport. These features make L8-BO-C4 a promising complementary material to NaO1, enabling synergistic effects in the ternary blend that balance charge transfer, reduce recombination, and ultimately enhance device performance.\nIn addition to the S →S reorganization energies presented in Figure 6, we also calculated the reorganization energies for the S →S transition and for the S →anion process (Figure S17). The results consistently show that NaO1 exhibits smaller reorganization energies than L8-BO-C4 across all cases. These findings further support our explanation that the incorporation of NaO1 effectively suppresses vibrational relaxation, thereby reducing non-radiative voltage loss in the ternary system. Consequently, we have added some discussion in the revised manuscript as below:\n“Remarkably, when the central BT unit was substituted with a fused quinoxalinone moiety in NaO1, this vibrational contribution was significantly attenuated (L8-BO-C4: 53.5 meV; T2: 30.2 meV), indicating a suppressed geometric relaxation upon excitation. Moreover, complementary calculations of the S →S relaxation and the S →anion transition further confirm that NaO1 consistently exhibits smaller reorganization energies than L8-BO-C4 (Figure S17). These results provide additional evidence that the quinoxalinone-based core effectively mitigates vibrational relaxation across different electronic states. This reduction in vibrational coupling is beneficial for minimizing exciton loss through non-radiative channels, ultimately contributing to lower energy loss.”\nFigure S17. The calculated reorganization energies of NaO1 and L8-BO-C4 during the S →S and S →aniontransition.\n1 0 0\nReviewer #2: Comments to the Author.\nThe work was well conducted and well documented.\nResponse to comment: Thank you very much for your positive comments and constructive suggestions.\nSuggestion to be improved.\n- show clearly what is L8-BO-C4.\nResponse: According to your suggestion, we have added the chemical structure of L8-BO-C4 in Scheme S2.\nScheme S2. The chemical structure of L8-BO-C4.\n- emphasize why the authors used o-xylene.\nResponse: We thank the reviewer for this important question. -xylene was\nO\ndeliberately chosen as the processing solvent for two main reasons. First, it is a non-halogenated, environmentally benign solvent, which aligns with the current demand for green and sustainable fabrication of organic solar cells. Second, o-xylene provides appropriate solubility and drying kinetics for both donor and acceptor materials, enabling controlled aggregation and favorable film morphology during spin-coating. In particular, our in-situ absorption studies revealed that o-xylene facilitates smooth and continuous spectral evolution, leading to uniform phase separation and well-ordered intermolecular packing in the active layers. As a result, devices processed with o-xylene not only demonstrate high efficiency but also highlight the compatibility of our molecular design with eco-friendly processing conditions.\n- show the stability data.\nResponse: We thank the reviewer for the suggestion. The stability of the binary PM6:L8-BO-C4 and ternary PM6:L8-BO-C4:NaO1 devices was systematically evaluated under three conditions, as shown in Figure S15: (A) shelf storage in the dark at room temperature, (B) continuous AM 1.5G light soaking, and (C) thermal aging at 85 °C. As a results, we have added some discussion about the device stability as below:\n“The operational stability of PM6:L8-BO-C4 and PM6:L8-BO-C4:NaO1 devices was evaluated under three conditions: shelf storage in the dark at room temperature, continuous AM 1.5G light soaking, and thermal aging at 85 °C (Figure S15). In all cases, the ternary devices exhibit significantly enhanced stability compared to the binary control. For instance, under continuous AM 1.5G illumination, the ternary device maintains over 85% of its initial PCE after 200 h, whereas the binary counterpart decreases to ~75%. Similarly, during 85 °C thermal stress, the ternary device retains ~86% of the initial PCE after 100 h, while the binary device drops below 70%. These results clearly demonstrate that incorporating NaO1 not only improves efficiency but also effectively enhances operational stability against light and thermal stress.”\nFigure S15. Stability comparison of PM6:L8-BO-C4 and PM6:L8-BO-C4:NaO1 devices under (A) shelf storage, (B) continuous AM 1.5G light soaking, and (C) thermal aging at 85 °C.\nReviewer #3: Comments to the Author.\nThis manuscript shows a study on the impact of fused-ring configurations in isomeric acceptors on the structure-packing-performance relationship. The authors developed a linearly fused acceptor NaO1, emphasizing that its three-dimensional packing network was critical to achieving optimized morphology and improved device efficiency. A PCE of 19.11% was obtained in PM6:NaO1-based devices processed with o-xylene. Using this acceptor, they reported an efficiency enhancement from 18.76% to 20.07% upon introducing NaO1 into the PM6:L8-BO-C4 system, attributed to reduced non-radiative energy loss and accelerated hole transfer. Although the acceptor materials might be interesting, the reported performance of this manuscript falls still below contemporary state-of-the-art values. The idea of developing a donor or acceptor material to fabricate ternary devices has been widely used in the field of organic photovoltaic [Adv. Mater. 2021, 33, 2008134; Adv. Mater. 2021, 33, 2101733; Adv. Mater. 2021, 33, 2102420], and certain key claims lack rigorous experimental support or theoretical justification, resulting in a lack of novelty and depth. Overall, I think that this manuscript is more suitable for the materials journal. Therefore, I would not recommend this manuscript for publication in Nature Communications for the following reasons:\nResponse to comment: We sincerely thank the reviewer for the thoughtful and constructive comments on our manuscript. We fully understand the concerns regarding novelty, performance relative to the state-of-the-art, and the rigor of experimental support. We have carefully revised the manuscript to highlight the originality of our approach, clarify the mechanistic depth, and strengthen the supporting evidence. Our detailed responses are as follows:\n1. On novelty and depth beyond “another ternary system”\nOur study is not simply reporting a new ternary combination. Instead, we focus on fused-ring isomerism of the molecular backbone as a fundamental design strategy, and systematically demonstrate how it governs (i) balanced three-dimensional electronic couplings, (ii) film formation dynamics under a green solvent, and (iii) attenuation of vibrational reorganization energy. These factors collectively reduce non-radiative losses and improve device performance. Through single-crystal analysis, (in-situ) GIWAXS, PIFM, in-situ UV–vis absorption spectra, transient absorption, and reorganization energy decomposition, we establish a mechanistic chain: isomerism → 3D coupling balance → green-solvent film evolution → vibrational loss suppression → device metrics. To our knowledge, such an integrative framework has not been addressed in the cited Adv. Mater. 2021 works, which mainly focused on miscibility or end-group/side-chain modifications.\n2. On device performance relative to the state-of-the-art\nWe acknowledge that record efficiencies above 20% have been achieved in OSCs. The significance of our work lies in achieving frontier-level efficiencies under non-halogenated processing. Specifically, PM6:NaO1 devices processed entirely with o-xylene deliver 19.11%, while the ternary PM6:L8-BO-C4:NaO1 system reaches 20.07% with FF of 80.3%. Considering that the current best o-xylene–processed OSCs only slightly exceed 20%, our results are highly competitive while simultaneously offering mechanistic insight into how backbone isomerism influences efficiency.\n3. Distinction from the cited Adv. Mater. 2021 works\nThe three Adv. Mater. 2021 papers cited by the reviewer indeed demonstrate the effectiveness of ternary strategies. However, their focus is primarily on phase compatibility, miscibility control, or donor/acceptor mixing, without addressing backbone-level isomerism, three-dimensional coupling balance, or vibrational contributions to non-radiative losses. In contrast, we show that replacing the central BT unit with a fused quinoxalinone in NaO1 specifically attenuates the ~1605 cm⁻¹ vibrational mode that dominates reorganization energy, thereby rationalizing the observed reductions in non-radiative loss and simultaneous improvements in V and OC FF.\n4. On the rigor of experimental and theoretical support\nWe have reinforced the robustness of our claims by providing cross-validated datasets:\ni. Single-crystal and calculations: NaO1 exhibits balanced |J| values (~24–26 meV) in multiple directions, unlike NaO2/NaO3.\nii. Film dynamics: In-situ absorption spectroscopy reveals that NaO1 enables rapid and continuous spectral red-shift under o-xylene, consistent with orderly aggregation.\niii. Device physics: Photo-CELIV, EIS, FTPS-EQE, and EQE-EL all corroborate the suppression of non-radiative loss in NaO1-based devices.\niv. Vibrational analysis: Mode-resolved reorganization energy decomposition directly links NaO1’s attenuated high-frequency C–C stretching mode to reduced ΔE . 3\nTogether, these results substantiate our conclusions with mechanistic clarity.\nAs a result, while ternary OPVs are indeed common, our work does not claim novelty from ternary composition alone. It establishes a general mechanistic framework—backbone isomerism as a lever to balance 3D electronic couplings, govern green-solvent film evolution, and attenuate specific vibrational modes that dominate non-radiative loss—and validates it experimentally and theoretically across crystals, (in-situ) GIWAXS/AFM/PiFM, in-situ spectroscopy, TA, mobility/EIS, and E partitioning. The resulting 19.11% (binary) and 20.07% (ternary) in o-xylene loss are competitive with the best green-processed literature and, we believe, meet the mechanistic depth and community relevance expected for Nature Communications.\n1. This manuscript demonstrates insufficient novelty, lacks in-depth analysis, and does not present significant performance improvements. The PM6:NaO1 devices exhibit an enlarged V (0.915 V) compared to other binary OSCs, which should be the reason for the increased V in PM6:NaO1:L8-BO-C4 ternary OSCs [Nat. Energy OC 2023, 8, 978–988]. However, this finding does not provide novel mechanistic insights into improving the performance of ternary OSCs. Meanwhile, the reported PCE of 20.07% falls below contemporary state-of-the-art values [Adv. Mater. 2025, 37, 2500352; Adv. Mater. 2025, 37, 2501812]. Neither materials nor devices demonstrate discernible advantages over those existing systems. Crucially, this study does not establish whether they can provide meaningful improvements in stability and large-area devices, especially modules.\nResponse: Thank you for the thoughtful and detailed comments. We respond point-by-point below and have revised the manuscript to make the novelty, mechanistic depth, and benchmarking clearer.\n(1) “Insufficient novelty/no mechanistic insight; the ternary V just inherits OC from the higher-V binary (Nat. Energy 2023).” OC\nWe agree that in many ternaries the V trend can be rationalized by the guest OC binary, as clarified by Wang et al. (design rules linking ternary V to the guest OC blend’s radiative yield and CT-state alignment). Our data show that the V increase OC in PM6:L8-BO-C4:NaO1 is not a trivial inheritance:\ni. In our binary devices, PM6:NaO1 actually has a lower V (0.915 V) than OC PM6:NaO2/NaO3 (0.953/0.956 V), yet it delivers the best J /FF and lowest total E SC loss (0.516 eV).\nii. When NaO1 is introduced into PM6:L8-BO-C4, V and FF increase OC monotonically and PCE improves from 18.76% to 20.07%, while FTPS-EQE/EQE-EL show a continuous decrease of non-radiative and total energy losses. Importantly, the rise in V is tied to reduced ΔE rather than simply OC 3 “borrowing” a larger bandgap. Actually, NaO1 has a similar band gap to L8-BO-C4.\niii. Mode-resolved reorganization analysis pinpoints that replacing the central BT with the fused quinoxalinone (NaO1) attenuates a ~1605 cm⁻¹ core stretching mode from S to S relaxation, cutting its contribution to λ (e.g., from 53.5 meV to 30.2 0 1 meV in the ternary), which rationalizes the drop in ΔE and the observed V gains. 3 OC\nThese results provide additional evidence that the quinoxalinone-based core effectively mitigates vibrational relaxation across different electronic states (S →S 1 0 relaxation and the S →anion transition). Please refer to our response to reviewer 1. 0\nThis backbone fused-ring isomerism → 3D coupling balance → green-solvent film evolution → vibrational-loss suppression pathway is, to our knowledge, orthogonal to prior ternary “miscibility tuning” approaches and aligns mechanistically with the Nat. Energy design rules (we explicitly connect increased EL yield and smaller ΔE to the ternary V rise). 3 OC\n(2) “Performance (20.07%) is below state-of-the-art (Adv. Mater. 2025).”\nWe acknowledge recent frontier results in non-halogenated processing around 20–20.5% (e.g., 20.49% using a solid additive with non-halogenated solvent, Adv. Mater. 2025, 37, 2501812; ~20% under lowered-toxicity solvent engineering, Adv. Mater. 2025, 37, 2500352). Our contribution is different in scope:\ni. We achieve 20.07% in a PM6:L8-BO-C4:NaO1 ternary processed entirely with o-xylene, while providing a quantitative mechanistic account (loss decomposition, EL yield, vibrational reorganization, and kinetics) for the improvement.\nii. In the corresponding binary device, PM6:NaO1 reaches 19.11% (o-xylene) with low E (0.516 eV), showing that the isomerism-driven 3D packing and loss transport already deliver frontier-level efficiency under green processing before moving to ternary.\nThus, although our headline PCE is marginally lower than the very latest records, it sits within the current green-solvent frontier and, importantly, contributes a useful design rule (backbone core isomerism) that is complementary to additive/process optimization.\n(3) “No discernible advantage of materials/devices; unclear value for stability and large-area (modules).”\nMaterial/device advantages. NaO1’s linearly fused core yields large and balanced electronic couplings (~24–26 meV across multiple modes) and a 3D percolating network, underpinning higher mobility, lower interfacial resistances, faster hole transfer, and longer exciton diffusion length—features that coherently explain the higher FF and lower losses:\nRegarding the stability issue, please refer to our response to reviewer 2.\nWe also thank the reviewer for pointing out the importance of evaluating the scalability of our materials and devices in large-area modules. To directly address this concern, we have fabricated organic solar cell modules using the same architecture as our small-area devices but with an aperture area of 16.80 cm2 (8 sub-cells connected in series on 5 × 5 cm2 ITO glass substrates). The series interconnection was realized through laser-patterned P1, P2, and P3 lines (20 µm, 140 µm, and 90 µm, respectively). The PEDOT:PSS and active layers were blade-coated under ambient conditions (20 mm s-1 coating speed, ~105 nm active layer thickness), while C60, BCP, and Ag electrodes were deposited via thermal evaporation. The aperture-defined active area was 15.03 cm2.\nAs shown in the Figure S16 and Table S6, the PM6:L8-BO-C4 binary modules delivered a PCE of 15.20% (average 14.29 ± 0.75%), while the PM6:L8-BO-C4:NaO1 ternary modules achieved a higher PCE of 16.93% (average 16.20 ± 0.49%), with notable improvements in both V OC and FF. The enhanced FF and higher PCE in ternary modules further demonstrate the favorable charge transport and reduced energy losses introduced by NaO1. These results demonstrate that NaO1 not only enhances the efficiency of small-area cells but also provides meaningful improvements in large-area OSC modules, confirming the practical scalability and relevance of our fused-ring isomerism strategy. Consequently, we have added some discussion about scalability of the material as below:\n“To further evaluate the scalability of the material, we fabricated large-area OSC modules (aperture area of 16.80 cm² with 8 sub-cells in series, active area of 15.03 cm²) via blade coating under ambient conditions (Figure S16 and Table S6). The PM6:L8-BO-C4:NaO1 ternary modules delivered a PCE of 16.93% (average 16.20 ± 0.49%), compared to 15.20% (average 14.29 ± 0.75%) for the PM6:L8-BO-C4 binary modules. The simultaneous improvements in V and FF highlight that the benefits of OC NaO1 extend from small-area devices to scalable module architectures, underscoring the practical potential of NaO1 in real-world OSC applications. Notably, the achieved efficiency of 16.93% is among the highest values reported for OSC modules at this active-area scale.”\nTable S6. Photovoltaic parameters for L8-BO-C4-based binary and ternary modules under AM 1.5G 100 mW cm-2 illumination.\nActive layer V (V) J (mA cm-2) FF (%) PCE (%)a\nOC SC\n6.87 3.23 68.54 15.20\nPM6:L8-BO-C4\n(6.75±0.37) (3.17±0.06) (66.77±2.48) (14.29±0.75)\n7.02 3.25 74.09 16.93\nPM6:L8-BO-C4:NaO1\n(6.95±0.15) (3.18±0.04) (72.24±1.83) (16.20±0.49)\naAverage and standard deviation data are obtained from 15 cells for each condition in brackets.\nFigure S16. J–V curves for L8-BO-C4-based binary and ternary modules under AM1.5G 100 mW cm-2 illumination.\n2. While single-crystal data reveal packing differences, theoretical modeling is needed to directly correlate molecular conformation with performance, strengthening mechanistic insights. Theoretical calculations (e.g., DFT or MD simulations) should be performed to quantify the conformational differences (e.g., dihedral angles, dipole moments) among the three isomers (NaO1, NaO2, NaO3) and their impact on packing modes, clarifying the mechanism behind electronic coupling variations.\nResponse: We thank the reviewer for this insightful suggestion. In addition to the single-crystal analysis, we performed theoretical calculations to further clarify the mechanism behind the electronic coupling variations among NaO1, NaO2, and NaO3. Specifically, we calculated the dihedral angles and dipole moments (3.22 Debye for NaO1, 3.65 Debye for NaO2, 3.68 Debye for NaO3) of the three isomers, and found them to be very close, suggesting that these parameters alone cannot fully account for the observed packing differences.\nTo gain deeper mechanistic insights, we analyzed the LUMO distributions of dimer configurations extracted from the single-crystal structures. Treating each dimer as a whole, we quantified the LUMO contributions of the two interacting fragments, and then evaluated the interaction strength by multiplying the LUMO proportions of the two fragments. This product reflects the degree of LUMO–LUMO orbital overlap, which governs the magnitude of the electronic coupling.\nThe results show that in NaO2 dimer I (E/C mode), the electronic coupling constant is nearly zero, which is consistent with one interacting fragment having almost no LUMO contribution. By contrast, NaO3 dimer III (E/E mode) exhibits the largest electronic coupling constant, directly correlated with the highest LUMO contributions from the two interacting fragments (18.1% and 13.7%). In the case of NaO1, all three dimers show balanced electronic coupling constants, consistent with their moderate and evenly distributed LUMO contributions.\nThese results demonstrate that the mechanism behind electronic coupling variations lies in the relative strength of orbital interactions between the two fragments within the dimer, which can be well rationalized by the quantitative analysis of fragment-specific LUMO contributions. Consequently, we have added some discussion to explain the mechanism behind electronic coupling variations as below:\n“To clarify the mechanism behind electronic coupling variations, we note that simple geometric/electrostatic descriptors (average core–arm dihedrals and molecular dipole moments (Figure S1) are similar among NaO1–NaO3 and thus cannot by themselves account for the observed differences in ∣J∣. We therefore analyzed fragment-resolved LUMO distributions for crystallographic dimers in each stacking motif (E/C, E/C′, E/B, E/B′, etc.). For each dimer, we first computed its LUMO and normalized the orbital population over the entire dimer to 100%. We then projected this dimer LUMO onto the two fragments that directly interact in each stacking motif (end-group and core-arm/bridge-TT) and denoted their LUMO shares as W (1) and LUMO W (2) (percent of the total dimer LUMO; As a compact indicator of the effective LUMO LUMO–LUMO interaction between the two fragments, we define an orbital overlap parameter S = W (1) × W (2). Because S is large only when both fragments LL LUMO LUMO LL carry appreciable LUMO density, it correlates with the calculated ∣J∣ and highlights the role of orbital registry (beyond π–π distance alone) in governing coupling. Consistent with this picture, dimer I (E/C) in NaO2 shows near-zero ∣J∣ because one interacting fragment contributes negligible LUMO weight, whereas NaO3 dimer III (E/E) exhibits the largest ∣J∣ (42.7 meV) (Figure S2-4), directly correlated with the highest LUMO contributions of the two interacting fragments (18.1% and 13.7%) in NaO3 dimer III (E/E mode). In contrast, NaO1 presents moderate and balanced on both interacting fragments (end-group and core-arm/bridge units), yielding uniform ∣J∣ values (24.4, 24.7, 26.0 meV) and thus more isotropic, three-dimensional charge-transport pathways. Taken together, these results suggest that the relative distribution of LUMO density on interacting fragments—and the quality of their orbital registry—represents a major factor behind the variations in electronic coupling among NaO1–NaO3. The stronger and more balanced inter-fragment LUMO interaction in NaO1 across motifs is consistent with its large and isotropic ∣J∣ values and ultimately its superior device transport characteristics.”\nFigure S1. Optimal molecular conformation and corresponding dipole moment for NaO1, NaO2 and NaO3.\nFigure S2. The fragment-resolved LUMO distributions for crystallographic dimers in each stacking motif for NaO1.\nFigure S3. The fragment-resolved LUMO distributions for crystallographic dimers in each stacking motif for NaO2.\nFigure S4. The fragment-resolved LUMO distributions for crystallographic dimers in each stacking motif for NaO3.\n3. While the authors employed device physics characterization to elucidate the origin of performance differences in binary and ternary devices, the analysis lacks sufficient depth. For example, integrated current density curves are absent from Figures 3E and 6D. Compared to binary PM6:L8-BO-C4 OSCs, the improvement of spectral response of ternary OSCs is mainly concentrated in the donor region (Figure 6D). This has not been reasonably explained. Only total carrier mobility (Photo-CELIV) is provided; Space-charge-limited current (SCLC) measurements should be done to directly determine electron/hole mobilities, evaluating charge transport balance and validating the proposed 3D charge transport network. Energy loss calculations lack transparency, particularly regarding critical parameters like the device bandgap Eg and Eloss. Transient photovoltage (TPV) or photocurrent (TPC) measurements may be useful to quantify differences in non-radiative recombination losses. A more rigorous device physics analysis is thus required to substantiate the performance interpretation.\nResponse: We sincerely appreciate the reviewer’s constructive comments regarding the device physics analysis. We have carefully revised and supplemented our data and discussion as follows:\n(1) Integrated current density curves\nFollowing the reviewer’s suggestion, we have now included the integrated current density curves (Figures 3E and 6D) to directly compare the calculated photocurrent with the measured J–V values.\n(2) Spectral response improvement in ternary devices\nWe agree with the reviewer that the enhanced spectral response in ternary PM6:L8-BO-C4:NaO1 devices is mainly concentrated in the donor absorption region (Figure 6D). This observation can be rationalized by our transient absorption measurements. Specifically, the ternary blends exhibit accelerated exciton dissociation dynamics and more efficient hole transfer from the donor domains, as evidenced by the faster decay of donor exciton signals and the stronger growth of charge-induced absorption features. These results indicate that the incorporation of NaO1 facilitates more effective utilization of photoexcitations in the PM6 region, thereby enhancing photocurrent generation in the donor spectral range. Consequently, we have added some necessary discussion as below:\n“When the ratio of L8-BO-C4 to NaO1 reached 1:1, the device delivered its best performance, with a PCE of 20.07% and an FF of 80.3%. The photocurrent derived from the integrated EQE spectrum (26.62 mA cm-2) confirmed the reliability of the J–V results (Figure 6d). Notably, the enhancement of the EQE response in the ternary device is mainly concentrated in the donor absorption region. Transient absorption spectroscopy further corroborates this observation, showing a faster decay of donor exciton signals and a stronger growth of charge-induced absorption features in the ternary blend compared to the binary PM6:L8-BO-C4 system.”\n(3) Carrier mobility characterization (SCLC)\nWe acknowledge the reviewer’s suggestion and would like to emphasize that, although SCLC is a widely used method, it has inherent limitations. Specifically, SCLC operates under very high internal electric fields (>105 V cm-1), much higher than those typically present in OPVs (103-104 V cm-1). It also probes transport in a high carrier density regime dominated by single-carrier injection, whereas real devices function mainly under bimolecular recombination conditions. Moreover, SCLC is measured in the dark and therefore cannot capture the transport dynamics of photo-generated carriers. In contrast, photo-CELIV directly measures carrier mobilities under illumination and operationally relevant fields and carrier densities, making it a more representative technique for OSCs.\nFor these reasons, we initially did not perform SCLC measurements and focused on photo-CELIV. Nevertheless, to directly compare both methods and in response to the reviewer’s request, we have now carried out SCLC measurements for the binary devices. The results, provided in Figure S8, show the same trend as the photo-CELIV data, confirming that PM6:NaO1 exhibits the most balanced μ /μ ratio. Specifically, h e we added some discussion to the main text:\n“To further validate this conclusion, we performed space-charge-limited current (SCLC) measurements on the binary devices (Figure S8). The extracted hole (μ ) and h electron (μ ) mobilities show a similar trend: for PM6:NaO1, μ = 1.52 × 10-4 cm2 V-1 e h s-1 and μ = 1.03 × 10-4 cm2 V-1 s-1, indicating balanced transport; for PM6:NaO2, μ = e h 2.73 × 10-5 cm2 V-1 s-1 and μ = 2.08 × 10-5 cm2 V-1 s-1; and for PM6:NaO3, μ = 6.76 e h × 10-5 cm2 V-1 s-1 and μ = 5.43 × 10-5 cm2 V-1 s-1. These results confirm that the e PM6:NaO1 blend possesses both the highest absolute mobility and the most balanced μ /μ ratio, which directly supports its superior charge extraction, higher FF, and h e overall photovoltaic performance.”\nWe respectfully disagree that carrier mobility measurements (e.g., SCLC) can directly validate the existence of a 3D charge transport network. While mobility data provide valuable information about the average charge transport capability of the bulk film, they do not capture the nanoscale connectivity or the dimensionality of charge transport pathways. The concept of a 3D charge transport network arises from the morphological arrangement and intermolecular coupling within the blend, which cannot be unambiguously resolved by electrical mobility tests alone. In our work, we used mobility measurements primarily to compare charge balance among different blends, rather than to directly “prove” the 3D transport network. The observed improvement in charge balance (PM6:NaO1) correlates well with the higher FF and efficiency, which is consistent with the proposed morphological interpretation.\n(4) Energy loss analysis\nWe have clarified the calculation of bandgap E and energy loss (E ) in the g loss revised SI. These details and parameters are now explicitly stated in the Methods and SI sections to ensure transparency and reproducibility.\nTable S1. Energy loss parameters.\nE a) ∆E b) ∆E c) ∆E d) E e)\ng 1 2 3 loss\nPM6:acceptor EQE @J\nEL SC\n(eV) (eV) (eV) (eV) (eV)\nNaO1 1.431 0.256 0.042 2.29× 10-2 0.218 0.516\nNaO2 1.506 0.262 0.067 1.79 × 10-2 0.224 0.553\nNaO3 1.502 0.261 0.063 1.93 × 10-2 0.222 0.546\nL8-BO-C4 1.432 0.256 0.011 1.83 × 10-3 0.284 0.551\nL8-BO-C4:NaO1 (3:1) 1.433 0.256 0.040 8.36× 10-3 0.244 0.540\nL8-BO-C4:NaO1 (1:1) 1.433 0.256 0.046 1.62 × 10-2 0.227 0.529\na) E : optical bandgaps, via the derivatives of the EQE spectra (dEQE/dE).\ng\nb) ∆E (E -V SQ): V SQ is the maximum open-circuit voltage achievable in SQ limit.\n1 g OC OC\nSQ rad\nc) ∆E (Voc -Voc ): voltage losses due to non-ideal absorption (it was calculated from EL and\nd) ∆E (∆𝑉𝑜𝑐\n𝑛𝑜𝑛−𝑟𝑎𝑑):\nvoltage losses due to non-radiative recombination only.\n3\ne) E = ∆E + ∆E +∆E\nloss 1 2 3\n(5) Transient photovoltage/photocurrent (TPV/TPC)\nWe appreciate the reviewer’s suggestion to employ TPC/TPV analysis. However, we would like to emphasize that the conventional TPC and TPV methodologies, under their standard analytical framework, have intrinsic limitations in OPVs due to the disordered density of states dominated by tail states. Unlike band semiconductors (e.g., Si, GaAs) where TPV/TPC decay can be directly interpreted in terms of carrier lifetimes or transport times, in OPVs the following complications arise:\ni. Relaxation–recombination coupling:\nCarrier relaxation into tail states is itself a slow, non-radiative process tightly coupled to recombination. The measured TPV decay thus reflects a convolution of relaxation and recombination rather than a pure recombination lifetime.\nii. Transport–recombination coupling:\nIn TPC, carrier extraction is strongly influenced by trap-assisted recombination in tail states, making it difficult to decouple transport from recombination dynamics.\niii. Non-exponential decay:\nThe disordered DOS leads to broad relaxation timescales, producing long-tailed, non-exponential decays that cannot be rigorously fit by simple exponential models to extract meaningful lifetimes.\nTherefore, while TPC/TPV can serve as a qualitative tool to compare overall loss dynamics between different devices, they cannot provide a rigorous or quantitative determination of recombination lifetimes in OPVs, and any attempt to directly validate recombination mechanisms from such data would be misleading. For quantitative evaluation of non-radiative recombination losses, electroluminescence quantum efficiency (EQE_EL) is generally regarded as the most reliable technique.\nNevertheless, to address the reviewer’s request, we have conducted TPC and TPV measurements (Figure S18) and added some discussion as follow:\n“To further probe charge dynamics, transient photocurrent (TPC) and transient photovoltage (TPV) measurements were performed (Figure S18). The TPC results reveal nearly identical carrier extraction times for the PM6:L8-BO-C4 and PM6:L8-BO-C4:NaO1 devices (0.12 μs vs. 0.13 μs), suggesting comparable charge extractionin in both systems. In contrast, the TPV measurements show that the ternary PM6:L8-BO-C4:NaO1 device exhibits a prolonged carrier lifetime (3.74 μs vs. 2.85 μs), indicating suppressed charge recombination. The observed improvement in TPV is consistent with the enhanced FF and overall PCE in the ternary device.”\nFigure S18. (a) TPC and (b) TPV curves for L8-BO-C4-based binary and ternary devices.\n4. In-situ GIWAXS or grazing-incidence X-ray diffraction (GIXD) should be conducted to monitor crystallization kinetics during film formation, combined with AFM phase imaging to analyze phase separation dynamics. The current in-situ absorption spectroscopy provides indirect aggregation insights but lacks real-time structural evolution data, making it difficult to confirm the causality between \"fast spectral evolution\" and optimized morphology.\nResponse: We thank the reviewer for this valuable suggestion. In order to directly probe the crystallization kinetics, we performed in-situ GIWAXS measurements during spin coating and subsequent thermal annealing of the blend films under the same conditions as device fabrication. The integrated out-of-plane scatterings are shown in Figure S12. For the NaO1-based blend, only weak diffraction features were observed, consistent with ex-situ GIWAXS, and crystallization of NaO1 occurred prior to the crystallization of the donor polymer. In contrast, the NaO2-based blend exhibited much stronger scattering signals after spin coating, followed by a complex phase transformation during thermal annealing, where sharp diffraction peaks developed. Interestingly, the NaO3-based blend displayed a distinct evolution, with scattering peaks shifting from 4.7 nm-1 to 3.9 nm-1 during spin coating, indicating a structural rearrangement before thermal treatment. For blends containing NaO1 or NaO3, little further change was detected during annealing, highlighting their different crystallization routes compared to NaO2. These results confirm that the crystallization manner of the acceptors strongly governs the development of nanostructures, and the fast spectral evolution observed in in-situ absorption can now be directly correlated with the real-time structural transformation captured by in-situ GIWAXS.\nConsequently, we have added some discussion about the crystallization kinetics in the main text as below:\n“To complement the in-situ absorption results, we further monitored the structural evolution of the active layers using in-situ GIWAXS. As shown in Figure S12, NaO1 exhibited weak scattering consistent with limited crystallinity, with its crystallization occurring before that of the donor polymer. NaO2 displayed strong scattering immediately after spin coating and underwent a complex phase transformation during thermal annealing, eventually forming sharp diffraction peaks. In sharp contrast, NaO3 showed a scattering shift from 4.7 to 3.9 nm-1 during spin coating, suggesting significant structural rearrangement in the early stage of film formation. Minimal further changes were observed during annealing for NaO1- and NaO3-based blends. These distinct crystallization pathways highlight the critical role of crystallization kinetics in determining the final nanostructures, bridging the “fast spectral evolution” observed in in-situ absorption with the actual structural reorganization captured by GIWAXS.”\nFigure S12. Integrated out-of-plane scattering of in-situ GIWAXS images collected during the fabrication of thin films containing NaO1, NaO2 and NaO3 in blend with PM6.\nIn-situ GIWAXS measurements\nIn-situ GIWAXS measurements were carried out at beamline 1W1A of Beijing Synchrotron Radiation Facility (BSRF) with an incident beam energy at 8 keV (the fixed grazing angle is 0.2°). The beam center and sample-to-detector distance were calibrated with LaB6. The samples were directly prepared on clean Si substrates with the same conditions as the device preparation. Each GIWAXS image was captured with the exposure time of 10 s during the spin-coating process.\n5. (1) As indicated in GIWAXS patterns and line cuts in the OOP direction, it can be seen that the PM6:NaO1 films show an obvious feature of π-π stacking relative to other two blends. This observation is contrary to the authors’ claim about CCLs. Please provide the detailed fitting curves or traces to enhance clarity and aid reader comprehension.\nResponse: We thank the reviewer for carefully examining the GIWAXS data. We fully agree with the reviewer that the line cut of the OOP scattering is not clear to show CCL differences. We included all the fitting figures in supporting information. Three π-π stacking scatting peaks can be found for NaO1 which agrees with literature report. We fitted these peaks with gaussian distributions and good fitting were obtained. To improve our manuscript, we provided additional text on how the CCL values were obtained in the main text and included all fitting figures in the supporting information.\nChange in the main text:\nOriginal text:\n“The crystal coherence length (CCL) calculated for these scatterings are notably higher than that of neat NaO1.”\nAmended text:\n“The crystal coherence length (CCL) values were extracted based on the full width at half maximum (FWHM) of scattering peaks found in the GIWAXS line cuts by fitting them to multiple peaks in Gaussian distributions and applying to Scherrer Equation (Figure S9). The CCL calculated for these scatterings are notably higher than that of neat NaO1.”\nFigure S9. The fitted peaks of the OOP scattering with gaussian distributions for NaO1, NaO2, NaO3, PM6:NaO1, PM6:NaO2, and PM6:NaO3 films.\n(2) The image quality of AFM is so poor that the surface morphology cannot be clearly seen in Figure 4c. The authors should conduct high-quality AFM measurements.\nWe thank the reviewer for pointing out the image quality issue. We have repeated the AFM measurements using optimized scanning conditions (higher resolution and reduced noise) to ensure that the nanoscale morphology can be clearly resolved. The updated AFM height and phase images are now provided in the revised manuscript (Figure 4c), and the related RMS values have been revised as below:\n“In contrast, the PM6:NaO1 blend film exhibits the smoothest surface, with a low root mean square (RMS) roughness of 1.06 nm, indicating a more homogeneous and uniform morphology. By comparison, PM6:NaO2 and PM6:NaO3 films show increased surface roughness—5.99 nm and 1.40 nm, respectively—reflecting greater phase separation and reduced morphological uniformity. These AFM observations are consistent with the differences in CCL values among the blend films.”\n(3) As for infrared AFM data, the authors did not indicate the calculated fiber width from donor or acceptor. Please provide the fiber width of both donor and acceptor.\nWe thank the reviewer for this helpful suggestion. To enable a direct comparison between donor and acceptor phases, we quantified the fibril diameters from the infrared AFM maps by using the full width at half maximum (FWHM) of the line-profile peaks. The average fibril sizes are: PM6:NaO1 (PM6: 19.9 nm; NaO1: 19.7 nm), PM6:NaO2 (PM6: 58.5 nm; NaO2: 40.5 nm), and PM6:NaO3 (PM6: 23.9 nm; NaO3: 24.7 nm). As a result, we have revised the main text and added some discussion as below:\n“Furthermore, infrared AFM was employed to investigate the fiber widths of the blend films (Figure 4d and Figure S10-11). The average fibril diameters by using the full width at half maximum of the line-profile peaks were determined to be 19.9 nm (PM6) and 19.7 nm (NaO1) for the PM6:NaO1 blend, 58.5 nm (PM6) and 40.5 nm (NaO2) for PM6:NaO2, and 23.9 nm (PM6) and 24.7 nm (NaO3) for PM6:NaO3. Notably, the PM6:NaO1 blend features fibrils close to the optimal phase separation length scale (~10–20 nm), which is widely regarded as the most favorable for ensuring efficient exciton dissociation and continuous charge transport pathways.[20] This finely dispersed nanofibrillar morphology is therefore a key factor underpinning the superior charge transport and the higher FF observed in the PM6:NaO1 device, while the coarser domains in PM6:NaO2 and the slightly oversized fibrils in PM6:NaO3 deviate from this optimal range and result in less efficient charge extraction.”\nFigure S10. PiFM images at the wavenumber of 1455 cm−1correspond to the PM6 donor in (A) PM6:NaO1, (D) PM6: NaO2 and (G) PM6: NaO1 blends；PiFM images correspond to the NaOX acceptor in (B) PM6:NaO1, (E) PM6:NaO2 and (H) PM6:NaO3 blends.\nFigure S11. The fibril width is obtained from the full-width at half-maximum for (a) PM6:NaO1, (b) PM6:NaO2, and (c) PM6:NaO3 blends.\n6. The authors employed transient absorption spectroscopy (TAS) to investigate exciton dynamics in binary and ternary systems. However, the presentation and analysis of TAS data lack sufficient rigor. Critically, the selection of 500 nm pump light excites both donor and acceptor components, resulting in spectral contamination from hole transfer processes that complicates the interpretation of electron transfer dynamics. Furthermore, an inconsistency exists between the probe wavelength cited in the main text (630 nm) and that indicated in Figure 6A (628 nm). Biexponential fitting was applied to extract lifetimes, yet the fitting intervals and methodological details were not provided. The attribution of the shortest lifetimes to reduced recombination losses is unsupported; the increased recombination typically shortens exciton lifetimes, contrary to the authors’ claim. Given the exceptionally fast hole transfer kinetics (<1 ps) shown in Figure 6B, additional discussion of the underlying physical processes is very necessary. Moreover, while exciton-exciton annihilation (EEA) methodology was used to determine diffusion lengths of singlet excitons, neither raw nor fitted dynamic traces appear in the main text or SI file. Collectively, these issues raise significant concerns regarding data reliability. Therefore, to enable valid comparison of charge transfer and recombination dynamics, separation of distinct excited-state species must precede comparative analysis.\nResponse: We thank the reviewer for these detailed and constructive comments on our TAS analysis. We have carefully addressed each point as follows:\n(1) Critically, the selection of 500 nm pump light excites both donor and acceptor components, resulting in spectral contamination from hole transfer processes that complicates the interpretation of electron transfer dynamics.\nWe appreciate the reviewer’s concern regarding the use of 500 nm excitation, which indeed excites both donor and acceptor components and may involve contributions from both electron and hole transfer. We agree that this is an important consideration. In our manuscript, however, we focused on discussing the overall trend of electron transfer dynamics, which remains valid and widely accepted in the field. For instance, Hou and co-workers have similarly employed selective excitation wavelengths to analyze the electron transfer processes by monitoring the donor ground-state bleach decay at characteristic wavelengths (Joule, 2021, 5, 2408–2419). Following this approach, we assigned the decay dynamics at 630 nm to electron transfer, acknowledging that both electron and hole transfer processes may be present. Importantly, the comparative trends observed for our systems clearly demonstrate that NaO1 exhibits the fastest electron transfer dynamics, which underpins its superior device performance. To clarify this point, we have revised the text in the main manuscript to explicitly mention the possible coexistence of hole and electron transfer under 500 nm excitation, while emphasizing that the relative electron transfer trend remains unambiguous.\n“The ground-state bleach (GSB) features of both PM6 and NaOX closely matched their steady-state absorption profiles. The GSB decay kinetics of electron transfer were assessed by probing at 628 nm (Figure 6a). While the 500 nm excitation used in TAS can excite both donor and acceptor components and thus may involve contributions from both electron and hole transfer, the decay of the donor GSB predominantly reflects the electron transfer dynamics at the donor–acceptor interface, in line with previous reports.58 The corresponding biexponential lifetimes for the PM6:NaO1, PM6:NaO2, and PM6:NaO3 blends were summarized in Table S3.”\n58. P. Bi, S. Zhang, Z. Chen, Y. Xu, Y. Cui, T. Zhang, J. Ren, J. Qin, L. Hong, X. Hao, J. Hou, Reduced non-radiative charge recombination enables organic photovoltaic cell approaching 19% efficiency, Joule, 2021, 5, 2408–2419.\n(2) Furthermore, an inconsistency exists between the probe wavelength cited in the main text (630 nm) and that indicated in Figure 6A (628 nm).\nWe have revised 630 nm as 628 nm.\n(3) “Biexponential fitting was applied to extract lifetimes, yet the fitting intervals and methodological details were not provided.”\nWe thank the reviewer for pointing out the lack of methodological details in the lifetime fitting. To address this, we have now included a detailed description in the Supporting Information (Table S3).\nTable S3. Detailed parameters of TA spectra, the electron transfer kinetics (excited at 500 nm and probe at 628 mm) were fitted by a biexponential function: i= A 1 exp(-t/τ )+ A exp(-t/τ ), with two life times of τ and τ and pre-factors of A and A . 1 2 2 1 2 1 2\nBlend films A τ (ps) A τ (ps)\n1 1 2 2\nPM6:NaO1 0.75 1.43 0.25 24.88\nPM6:NaO2 0.67 3.01 0.33 75.93\nPM6:NaO3 0.67 1.62 0.33 26.51\nPM6:L8-BO-C4:NaO1 0.83 0.99 0.17 11.66\n(4) The attribution of the shortest lifetimes to reduced recombination losses is unsupported; the increased recombination typically shortens exciton lifetimes, contrary to the authors’ claim. Given the exceptionally fast hole transfer kinetics (<1 ps) shown in Figure 6B, additional discussion of the underlying physical processes is very necessary.\nWe agree that exciton recombination would typically shorten the measured lifetimes, and we regret that our original wording was misleading. Upon careful clarification, the fastest lifetime component (<1 ps) in our TAS data does not correspond to reduced recombination but rather to the ultrafast hole transfer process, which is well aligned with the charge separation dynamics expected in these donor–acceptor systems. The longer lifetime components reflect exciton dissociation and recombination behavior. We have revised the text to explicitly state that the shortest lifetime is assigned to hole transfer dynamics rather than recombination, and we have expanded the discussion to avoid ambiguity.\nWe also appreciate the reviewer’s request for additional discussion of the underlying physical processes, particularly in light of the sub-picosecond hole transfer observed (<1 ps, Figure 6B). This sub-picosecond process can be attributed to the nearly barrier-free hole transfer from the donor PM6 to each of the acceptors, owing to their well-aligned HOMO levels and strong electronic coupling at the donor–acceptor interface. While the absolute time scale of this hole transfer is similar across PM6:NaO1, PM6:NaO2, and PM6:NaO3, the subsequent exciton dissociation and charge stabilization dynamics differ. Specifically, PM6:NaO1 shows the most efficient depletion of donor excitons (shortest τ and τ ), indicating that the ultrafast 1 2 hole transfer is effectively followed by long-lived charge separation. In contrast, PM6:NaO2 and PM6:NaO3 exhibit somewhat slower decay constants, suggesting that a larger fraction of photogenerated carriers recombine or relax before being fully dissociated. Taken together, the <1 ps component reflects a universal ultrafast hole transfer channel in all binary blends, whereas the differences in the slower decay components highlight the role of acceptor structure in determining whether the initial hole transfer evolves into stable charge pairs or is lost to recombination. This explains why PM6:NaO1 achieves the most efficient charge generation and highest device performance among the three binaries.\nAs a result, we have revised the main text “The time-resolved kinetics at this wavelength were further analyzed to evaluate hole transfer dynamics, revealing that PM6:NaO1 facilitated more rapid hole transfer compared to its counterparts (Figure 6b)” as “Given the sub-picosecond dynamics observed in all binary blends, the fastest component (<1 ps) can be attributed to an ultrafast hole transfer process from the donor PM6 to the acceptors, enabled by their well-aligned HOMO levels and strong interfacial coupling. While this rapid channel is present in all systems, its subsequent evolution differs: in PM6:NaO1 the ultrafast hole transfer is more effectively followed by stabilized charge separation, whereas in PM6:NaO2 and PM6:NaO3 a larger fraction of carriers recombine or relax before dissociation. The time-resolved kinetics at this wavelength were further analyzed to evaluate hole transfer dynamics, revealing that PM6:NaO1 facilitated more rapid hole transfer compared to its counterparts (Figure 6b)”.\n(5) Moreover, while exciton-exciton annihilation (EEA) methodology was used to determine diffusion lengths of singlet excitons, neither raw nor fitted dynamic traces appear in the main text or SI file. Collectively, these issues raise significant concerns regarding data reliability.\nWe thank the reviewer for highlighting this point. To improve transparency, we have included a detailed description of the methodology and fitting dynamic traces (Figure S14b) and procedure used to extract exciton diffusion lengths, following the protocol reported by Chandrabose et al. [J. Am. Chem. Soc. 2019, 141, 6922–6929] in the Supporting Information.\nExciton diffusion length can be determined by the exciton annihilation method[ J. Am. Chem. Soc. 2019, 141, 6922-6929]. The pump fluence-dependent TA spectroscopy was used to investigate the diffusion and non-radiative recombination behaviors of excitons in both pure and binary films. It is assumed that trap-induced recombination is the only carrier decay channel under extremely low light fluence:\n(1), where n(t) is the charge carrier density as the function of time,\nand k is the rate of trap-induced recombination. The solution of the equation is a single exponential decay function. Then the k and the single exponential decay function lifetime  will follow the equation: k = (2). When the excitation light is sufficiently high to generate enough charge to raise the bimolecular recombination, the charge density decays as follows: (3), which has the\nfollowing solution: (4), where  is the singlet-singlet\nbimolecular exciton annihilation rate. The exciton decays can be well fitted by Eq. (4), where the only free parameter is the bimolecular rate constant. The bimolecular rate constant is then used in the equation: (5), in which D is the diffusion\ncoefficient, R is the annihilation radius of singlet exciton, and here we used the\ngenerally assumed value of 1 nm. [ J. Am. Chem. Soc. 2019, 141, 6922-6929] The\nexciton diffusion length can be calculated by the following equation: (6).\n(L is the exciton diffusion length, and  is the lifetime of exciton, which is equal to\nD\n1/k).\n7. The performance enhancement in ternary devices is attributed to \"accelerated hole transfer,\" but direct experimental evidence for energy/charge transfer is lacking, and morphology homogeneity is insufficiently characterized. Fluorescence resonance energy transfer (FRET) or time-resolved photoluminescence (TRPL) is required to analyze energy transfer pathways in the PM6:L8-BO-C4:NaO1 blend.\nResponse: We appreciate the reviewer’s insightful suggestion regarding the necessity of direct experimental evidence to elucidate the charge/energy transfer process in the ternary blend. To address this, we carried out steady-state PL measurements (Figure S19) to probe the exciton dynamics. The two acceptors (NaO1 and L8-BO-C4) exhibit nearly identical absorption and PL spectra, as well as a minimal LUMO offset of ~0.07 eV, which is insufficient to drive exciton dissociation or cascade-type charge transfer between the two acceptors. Consistently, the PL intensity of the NaO1:L8-BO-C4 binary blend is close to the arithmetic average of the individual components, without distinct PL quenching or enhancement, confirming the absence of FRET or cascade-like charge transfer pathways.\nIn contrast, when blended with the donor PM6, all donor–acceptor systems (binary and ternary) display pronounced PL quenching due to efficient charge transfer at the donor/acceptor interface. Importantly, the ternary PM6:L8-BO-C4:NaO1 blend shows a more significant PL quenching than either PM6:NaO1 or PM6:L8-BO-C4, indicating a higher exciton dissociation efficiency in the ternary system. This observation is consistent with fs-TAS results, which reveal an accelerated charge separation rate in the ternary blend compared to the binary counterparts. Together, these results provide direct evidence that the improved performance of the ternary devices originates from enhanced exciton dissociation and charge generation at the donor/acceptor interfaces, rather than FRET or cascade-type processes between the two acceptors. As a result, we have added some discussion as below:\n“The charge generation mechanism of the ternary blend was further clarified by steady-state photoluminescence (PL) measurement (Figure S19). Since NaO1 and L8-BO-C4 exhibit similar absorption/PL spectra and a minimal LUMO offset (~0.07 eV), no cascade charge transfer or fluorescence resonance energy transfer is expected between them. Indeed, the PL intensity of their binary blend is close to the arithmetic average of the two pristine acceptors, ruling out inter-acceptor energy transfer. In contrast, all donor–acceptor blends show pronounced PL quenching due to efficient charge transfer at the donor/acceptor interface. Notably, the ternary PM6:L8-BO-C4:NaO1 blend exhibits stronger PL quenching than either binary, consistent with fs-TAS results showing accelerated charge separation, thereby confirming enhanced exciton dissociation in the ternary system.”\nFigure S19. (A) Normalized UV–vis absorption spectra of NaO1 and L8-BO-C4 films; (B) Steady-state photoluminescence (PL) spectra of pristine and blended films, including L8-BO-C4, NaO1, L8-BO-C4:NaO1, PM6:L8-BO-C4, PM6:NaO1, and PM6:L8-BO-C4:NaO1; (C) Cyclic voltammetry curve of L8-BO-C4 measured in solid state; (D) Energy level diagram of donor PM6 and acceptors L8-BO-C4 and NaO1.\n8. While the authors attribute the absorption spectral red-shifting and improved charge carrier mobility to enhanced electron delocalization, direct evidence and supporting this claim remains absent in this manuscript. Please provide rigorous experimental support or theoretical justification.\nResponse: We thank the reviewer for raising this important point. We have carefully revised the manuscript to clarify our interpretation. Specifically, we have replaced the term “electron delocalization” with the more accurate “exciton delocalization.” While the two concepts are closely related, exciton delocalization is more appropriate in the context of absorption spectra and charge carrier mobility in organic semiconductors.\nFrom a theoretical perspective, the red-shifted absorption observed in thin films—despite nearly identical solution spectra—directly points to a narrowing of the optical bandgap induced by enhanced conjugation and intermolecular coupling in the solid state. Within the excitonic band framework derived from the Schrödinger equation and tight-binding models, stronger π–π interactions increase the transfer integral J, broaden the exciton band, and lower the lowest excitonic energy state, thereby producing a red-shift. At the same time, the enhanced orbital overlap underlying exciton delocalization facilitates more efficient charge separation and transport. This dual effect provides a natural explanation for both the red-shifted absorption and the improved mobility observed in our systems.\nSuch correlations between exciton delocalization, spectral red-shifts, and charge transport have been systematically investigated in prior works. For example, theoretical calculations have demonstrated the direct link between exciton delocalization and red-shifted absorption spectra in conjugated systems (J. Am. Chem. Soc. 2017, 139, 3095–3105, https://doi.org/10.1021/jacs.6b12520). Experimental studies have further shown that modifying molecular packing can directly alter exciton delocalization and lead to observable red-shifts in absorption (J. Phys. Chem. C 2022, 126, 8889–8896, https://doi.org/10.1021/acs.jpcc.2c01382). More recently, exciton delocalization has been directly linked to improved absorption characteristics and device performance in high-efficiency organic solar cells (Nat. Commun. 2025, 16, 3176, https://doi.org/10.1038/s41467-025-58352-x). Finally, a systematic discussion of how orbital delocalization (i.e., exciton/electron delocalization) enhances charge transport has been provided in a recent article (Angew. Chem. Int. Ed. 2023, 62, e202306709, https://doi.org/10.1002/ange.2023067090.\nIn light of these well-established theoretical and experimental insights, we have revised the text to explicitly state that the observed absorption red-shifts and mobility improvements are attributable to enhanced exciton delocalization arising from stronger molecular conjugation and packing in the solid state.\nRevised main text:\n“This further red-shifted feature is also ascribed to intermolecular aggregation, and its position implies the formation of additional highly delocalized excitonic states.[52-55] Such enhanced exciton delocalization originates from the unique 3D crystal packing structure of NaO1, which promotes stronger intermolecular electronic coupling and thus facilitates both red-shifted absorption and more efficient charge transport.”\n52. J. L. Zafra, A. M. Ontoria, Paula Mayorga Burrezo, M. P. a-Alvarez, M. Samoc, J. Szeremeta, F. J. Ramírez, M. D. Lovander, C. J. Droske, T. M. Pappenfus, L. Echegoyen, J. T. Loṕez Navarrete, N. Martín, J. Casado, Fingerprints of Through-Bond and Through-Space Exciton and Charge π-Electron Delocalization in Linearly Extended [2.2]Paracyclophanes, J. Am. Chem. Soc. 2017, 139, 3095−3105.\n53. L. Liang, K. Czar, M. I. Furis, Strain-Enhanced Formation of Delocalized Exciton States in Phthalocyanine Crystalline Thin Films, J. Phys. Chem. C 2022, 126, 8889−8896.\n54. K. Jiang, R. J. E. Westbrook, T. Xia, C. Zhong, J. Lu, A. Khasbaatar, K. Liu, F. R. Lin, S. -H. Jang, J. Zhang, Y. Li, Y. Diao, Z. Wei, H. -L. Yip, D. S. Ginger, A. K.-Y. Jen, Photoluminescent delocalized excitons in donor polymers facilitate efficient charge generation for high-performance organic photovoltaics, Nat. Commun. 2025, 16, 3176.\n55. S. Guo, Y. Park, E. Park, S. Jin, L. Chen, Y. M. Jung, Molecular-Orbital Delocalization Enhances Charge Transfer in π-Conjugated Organic Semiconductors, Angew. Chem. Int. Ed. 2023, 62, e202306709.\n9. To elucidate the nature of the film absorption, the authors deconvoluted the spectra shown in Figure S1. However, the authors did not supplement the explanation with relevant references or further clarification about the assignment of those fitting peaks.\nResponse: We thank the reviewer for this comment. The deconvolution of the spectra was performed using multi-peak fitting to Gaussian distribution. The absorption band around 700 nm is attributed to the absorption from amorphous region for it is visible in the absorption of the solution. Complex intermolecular interactions can be anticipated for the reported molecules for their curved backbone. Hence, a HJ-aggregation is more likely to occur as reported by Yamagata et al. (doi:10.1063/1.4705272). Furthermore, single crystal data highlighted the complex π-stacking modes in these molecules, which suggested multiple intermolecular aggregation modes as well as various intermolecular orbitals corresponds to multiple absorption bands. As the literature of deconvolutions and assignment of the absorptions bands of non-fullerene acceptors is yet to be well established, we are hesitated to assign the absorptions to specific bands such as 0-1 or 0-2 absorption bands.\nTo improve our manuscript the assignment of amorphous potion of the absorption was supported by an additional reference (https://doi.org/10.1103/PhysRevLett.98.206406). To avoid confusion, we removed the assignment of the absorption peaks to numbers and simply stated an additional absorption at 831 nm was observed for NaO1. Consequently, we have revised our manuscript as below:\n“To further elucidate the nature of the film absorption, the spectra in Figure 3b were deconvoluted.[50,51] As shown in Figure S5, the absorption profiles of NaO2 and NaO3 consist of four vibronic peaks. In contrast, the absorption spectra of NaO1 reveals five peaks upon fitting. In addition to the aforementioned four peaks, an extra peak appears at 831 nm. This further red-shifted feature is also ascribed to intermolecular aggregation, and its position implies the formation of additional highly delocalized molecular orbitals.”\n[51] J. Clark, C. Silva, R. H. Friend, F. C. Spano, Role of Intermolecular Coupling in the Photophysics of Disordered Organic Semiconductors: Aggregate Emission in Regioregular Polythiophene, Phys. Rev. Lett. 2007, 98, 206406.\nFigure S5. The fitting absorption profiles of NaO1, NaO2 and NaO3 neat films."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have properly addressed my comments, and I am now pleased to recommend the manuscript for publication in Nature Communications.", "reviewer_2": "The manuscript was well revised.", "reviewer_3": "The authors have addressed the previous concerns by providing substantial additional experimental data and making extensive revisions. The manuscript can now be considered suitable for publication in Nature Communications."}, "author_response": ""}]

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