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Nature Communications volume 17, Article number: 712 (2026)
4849
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Non-radiative losses present a significant hurdle limiting the performance of organic solar cells (OSCs). Selenium substitution in non-fullerene acceptors (NFAs) is an effective approach for tuning intermolecular stacking and energy levels in state-of-the-art near-infrared absorbing OSCs. However, the inter-system crossing (ISC) induced by selenium may enhance the formation of triplet excitons in NFAs, aggravating non-radiative losses in OSCs. Herein, we have structurally engineered selenium-containing NFAs through introducing achiral N-alkyl substituents with varied branching sites at C2 or C3 position onto the pyrrole moiety of NFA core. We find that the C2-branched ones in our material system exhibit more intimate molecular packing, improved luminescence efficiency and reduced triplet exciton generation in both neat and blend films, reflected as improved external quantum efficiencies and open-circuit voltages in higher-performance devices. We utilize the best-performing NFA as a ternary component into the benchmark D18:L8-BO absorber to realize a high efficiency of 20.4% (certified as 19.88%). This work shows how precisely regulating the microstructure of NFAs by side-chain modification can reduce the non-radiative losses of OSCs.
Organic solar cells (OSCs) exhibit distinct advantages over other photovoltaic technologies, attributed to the facilely tunable bandgap of photovoltaic materials, solution processability, and potential application in flexible device1,2,3,4,5. In the past few years, the rapid progression in OSCs has been primarily driven by the advancement of narrow-bandgap non-fullerene acceptors (NFAs), which enable efficient conversion of near-infrared (NIR) photons to electricity6,7,8,9,10,11. Recently, OSCs have achieved a power conversion efficiency (PCE) exceeding 20%, rendering them viable candidates for commercial applications12,13,14,15,16. However, a significant hurdle impeding OSC performance is the substantial non-radiative energy losses17,18,19,20. To further promote the OSC performance, rigid molecules that can form a closely-packed and ordered structure21,22 have been pursued to mitigate the vibration of environmental molecules and suppress the electron–phonon coupling23,24,25, rendering the pathways of non-radiative relaxations less accessible. Moreover, the generated lowest-energy triplet excitons (T₁) in OSCs have been widely reported to mediate non-radiative relaxations26,27,28. Efforts have been made for mitigating T1 formation, including modulation of interfacial singlet-triplet energetics and engineering the degree of donor-acceptor mixings in OSCs29,30,31.
In the design of organic photovoltaic materials, the incorporation of Se is an efficient approach to tune the energy gap and molecular interactions32,33,34,35. Since Se is more polarizable, its lone electron pairs enable a strong intermolecular Se-Se interaction, which promotes the formation of charge-transporting networks between molecules36,37. Several studies on introducing Se into NFAs have revealed the advantages of red-shifted photoresponse, closer molecular packing, and reduced energetic disorder38,39,40,41,42. These not only help realize high photocurrent, but also minimize the photovoltage-photocurrent trade-off in narrow-bandgap OSCs. Moreover, the near-infrared absorption of Se-containing NFAs renders them highly promising for application in tandem solar cells43. However, the incorporation of Se atom may promote spin-orbital couplings (SOCs) in the NFAs and thus enhance the formation of T1 excitons from singlet (S1) excitons through inter-system crossing (ISC), opening up a pathway that could lead to additional non-radiative losses in OSCs44,45.
Side chain engineering has been applied as an effective strategy to tune the intermolecular interaction and crystallinity of NFAs46,47,48. Regulation of the length and branching position of the alkyl side chain can alter its steric hindrance, thereby influencing the molecular stacking behavior. Recent work of Sun et al.49 on the modification of state-of-the-art L8-BO has reflected the potential of delicate side chain modification for tuning molecular crystallinity and improving photoluminescence quantum yield (PLQY). To suppress the non-radiative relaxations in Se-containing NFAs, we here attempted to optimize molecular stacking and crystallinity through side chain engineering. We incorporated 3 Se atoms into the central core and introduced achiral N-alkyl substituents with varied lengths and branching positions on the pyrrole moiety to tune the steric hindrance. Ultimately, a series of NFAs, namely S9SBH-F, S9S3B-F, S9SHO-F, and S9SHN-F, were successfully synthesized (Fig. 1a) with the synthetic route depicted in Supplementary Fig. 1.
a Chemical structure of the NFAs. b Normalized absorption spectra of the NFAs in the thin films. c Energy levels of polymer donor and the NFAs. d 3D interpenetrating network packing structures of S9SBH-F, S9S3B-F, S9SHO-F, and S9SHN-F. e The π-π stacking distance averaged from the stacked molecules in one crystal lattice of S9SBH-F, S9S3B-F, S9SHO-F, and S9SHN-F.
Crystallography, morphology, and spectroscopy analyses, as well as device fabrication, were utilized for establishing a comprehensive structure-property-performance relationship for these NFAs. Among the four NFAs, we found that the C2-branched ones display more compact stacking than those with C3-branched N-alkyl chain, which facilitates intermolecular charge transport and improves luminescence efficiencies in both neat and blend films. The C2-branched NFAs also exhibit reduced ISC from S1 to T1 excitons, alleviating the T1 formation in neat and blend films. These characteristics improve exciton separation and suppress non-radiative loss pathways, reflected as higher external quantum efficiencies (EQEs) and open-circuit voltages (Vocs) in devices. As a result, S9SHO-F, substituted with a long branching N-alkyl chains at C2 position, delivered the highest PCE of 19.2% in a binary OSC device with D18, showing a Voc of 0.835 V and short-circuit current density ( Jsc) of 29.68 mA cm−2. Furthermore, a high efficiency of 20.4% (certified as 19.88%) with Voc of 0.884 V is achieved in the ternary OSCs of D18:L8-BO:S9SHO-F. This work provides an efficient strategy to optimize the molecular packing, improve luminescence efficiency, and alleviate the formation of T1, thereby suppressing non-radiative losses.
Figure 1a shows the molecular structures of the NFAs synthesized in this study. The steric hindrance of side chains is modulated by both the branching position and the chain length. The UV-Vis absorption spectra of these NFAs in thin films and solutions are shown in Fig. 1b and Supplementary Fig. 2. From the absorption profiles in solution, all NFAs exhibit a maximum absorption peak at ~760 nm. The absorption spectra of films are much red-shifted to the near-infrared region, among which S9SBH-F and S9SHO-F, featuring branched N-alkyl chains at C2 position (noted as 2nd-branched NFAs), exhibit more red-shifted absorption peaks alongside blue-shifted absorption onsets when compared to the other two NFAs that possess N-alkyl chains branched at C3 position (noted as 3rd-branched NFAs), as detailed in Supplementary Table 1. The shapes of absorption spectra of films are also different, where the C2-branched NFAs have a more pronounced peak with lower-intensity shoulders corresponding to a more Frenkel-like nature, while the C3-branched NFAs have a broader peak with higher-intensity shoulders corresponding to a more charge-transfer (CT)-like nature, according to the previous literature50. The electrochemical properties of the NFAs were investigated by cyclic voltammetry (Supplementary Fig. 3). As shown in Fig. 1c, the highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energy levels are determined as −5.66/−3.83 and −5.68/−3.82 eV for S9SBH-F and S9SHO-F, −5.63/−3.86 and −5.64/−3.86 eV for S9S3B-F and S9SHN-F. The up-shifted LUMO energy levels of S9SBH-F and S9SHO-F are potentially advantageous for achieving a high Voc in their corresponding devices.
To investigate the molecular self-assembly, single-crystal X-ray diffraction experiment was conducted. Single crystals of S9SBH-F, S9S3B-F, S9SHO-F, and S9SHN-F were grown through a ternary solvent system, and the diffraction data were collected (Supplementary Tables 2–5). Besides, the ORTEP view of all single-crystal structures is presented in Supplementary Figs. 4–7. As shown in Fig. 1d and Supplementary Figs. 8–10, S9SBH-F with the shortest N-alkyl chain forms an ordered 3D honeycomb network, similar to the previously reported Y633. Compared with S9SBH-F, S9SHO-F, with longer branched N-alkyl chains, exhibits a long-range ordered packing in the oblique direction across the crystal lattice (Supplementary Fig. 11), forming columns composed of π-stacked NFA molecules with a small average π-π stacking distance (3.38 Å for S9SBH-F, and 3.20 Å for S9SHO-F). The transition of crystal packing structure from a 3D honeycomb network (S9SBH-F) into a network of rather independent columns (S9SHO-F) is caused by the long N-alkyl chains occupying the interval between the adjacent columns in S9SHO-F single crystal, and results in a mismatch between the two parallel molecules. When changing the branching site from C2-position to C3-position, although the branching site of N-alkyls moves farther from the π-system, they still occupy the interval between the adjacent molecular columns thus the packing patterns of S9S3B-F and S9SHN-F are similar to that of S9SHO-F (Supplementary Fig. 12). We also infer that the distance between the neighboring columns, which is associated with the size of N-alkyl substituents, may offer the flexibility for NFA molecules to pack in a tighter manner. Therefore, we observe a reverse trend between the distance in the neighboring columns (10.8 Å < 15.3 Å < 19.5 Å) and π-π stacking distance (3.52 Å > 3.50 Å > 3.20 Å) in S9S3B-F, S9SHN-F, and S9SHO-F (Fig. 1e and Supplementary Table 6).
Subsequently, the grazing-incidence wide-angle X-ray scattering (GIWAXS) measurement was performed to further verify the π-π stacking in the C2-branching S9SHO-F and C3-branching S9SHN-F in thin films. As shown in Supplementary Fig. 13, S9SHO-F shows a (010) diffraction peak at 1.76 Å−1 corresponding with a d-spacing of 3.57 Å in the out-of-plane (OOP) direction. When changing the branching site to C3-position, S9SHN-F exhibits a (010) diffraction peak at 1.73 Å−1 with a d-spacing of 3.63 Å in the OOP direction, indicating a slightly more intimate packing in C2-position S9SHO-F. These results are consistent with our conclusions obtained from the single crystal analysis. The tight packing of S9SBH-F and S9SHO-F may reduce the vibration of environmental molecules and suppress electron–phonon coupling23,51,52. It is found that the C2-branched S9SBH-F and S9SHO-F exhibit significantly higher PLQYs (2.1% and 2.8%) than the C3-branched S9S3B-F and S9SHN-F (1.2% and 1.4%) in films (Supplementary Fig. 14). According to the energy-gap law, there exists an inverse correlation between the PLQY and static disorder23,53,54. When increasing the degree of energetic disorder, it will broaden the energy levels and make more low-energy electronic states accessible. As a result, these additional low-energy electronic states facilitate non-radiative recombination, ultimately resulting in the suppressed PLQY. Compared with the C2-branching NFAs, the C3-branching NFAs exhibit a rather blue-shifted absorption peak (Fig. 1b) and the corresponding devices display a more obvious tailing in the EQE spectra, indicating a larger energetic disorder49. As a result, the more intimate packing and lower energetic disorder in S9SHO-F should enable a higher PLQY.
In addition, computed intermolecular electronic couplings by density functional theory (Supplementary Fig. 15) show that S9SHO-F possesses a higher number of electron couplings induced by the large π-core torsion angle, potentially leading to more efficient 3D charge transport. The space-charge-limited-current (SCLC) mobility measurement (Supplementary Fig. 16) also supports our findings here, showing the highest electron mobility of 3.3 × 10−4 cm2/V·s for S9SHO-F, and 3.2 × 10−4, 2.8 × 10−4, and 3.0 × 10−4 cm2/V·s for S9SBH-F, S9S3B-F, and S9SHN-F (Supplementary Table 7), respectively, agreeing well with their packing properties.
The photovoltaic performance based on these NFAs was investigated by fabricating OSC devices with an architecture of ITO/2PACz/D18:NFAs/PDINN/Ag. As shown in Fig. 2a and Table 1, the D18:S9SBH-F-based devices exhibit a PCE of 18.9% with a Voc of 0.833 V and a Jsc of 29.65 mA cm−2, while the devices based on D18:S9SHO-F deliver a highest PCE of 19.2% with a Jsc of 29.68 mA cm−2 and a Voc of 0.835 V. The D18:S9SHN-F-based cell exhibits a lower PCE of 18.5% with a low Voc of 0.824 V, while the cell containing S9S3B-F possess an even lower Voc of 0.800 V and a fill factor (FF) of 70.30%, leading to a lowest PCE of 16.6%. Utilizing the near-infrared absorption of Se-containing NFAs, we incorporated the best-performing S9SHO-F into L8-BO with a weight ratio of 15% for a ternary device. The D18:L8-BO:S9SHO-F based cell displays a high Voc of 0.884 V and a high Jsc of 28.85 mA cm−2, achieving a PCE of 20.4% (Supplementary Fig. 17), which is among the best values for the OSCs. The ternary device was also sent to an independent laboratory and certified with a PCE of 19.88% (Supplementary Fig. 18). Furthermore, the long-term storage stability in a nitrogen-filled glove box and thermal stability annealed at 70 °C of the S9SHO-F based binary and ternary devices were performed (Supplementary Fig. 19). Compared with the binary devices based on S9SHO-F, the ternary D18:L8-BO:S9SHO-F-based OSCs exhibit improved stability, indicating the enhanced molecular packing helps stabilize the morphology in the ternary active layer.
a J–V curves of the optimized D18:NFAs OSCs. b EQE spectra of the optimized D18:NFAs OSCs. c Photocurrent-effective voltage plots. d The dependence of Jsc on light intensity. e The dependence of Voc on light intensity. f EQEEL curves.
It was found that the devices based on C2-branched NFAs deliver a high PCE and Voc among the cells based on the four Se-containing NFAs (Table 1). According to the EQE spectra of the devices (Fig. 2b), despite all the NFAs demonstrating similar Jscs of ~29 mA cm−2, the C3-branched NFAs exhibit weaker photoresponse in the range of 610–900 nm and broader band tails compared to their counterparts with C2-branched N-alkyl chains (Fig. 2b). These broader band tails, analogous to the red-shifted absorption onsets in neat films (Fig. 1b), suggest the larger ratios of below-gap states, which are probably associated with voltage losses55. Furthermore, we explored the impact of S9SHO-F on the photoresponse characteristics in the ternary device. As shown in Supplementary Fig. 17b, the D18:L8-BO exhibits an EQE tail at around 900 nm; however, the devices based on D19:S9SHO-F have a strong photoresponse in the range of 900–1000 nm. When S9SHO-F is integrated into the binary material system of D18:L8-BO, it effectively enhances the light-harvesting capability in the range of 830–930 nm, significantly contributing to the improved Jsc observed in the ternary device.
Further in-depth electrical characterizations were conducted to evaluate the exciton separation and charge recombination in the devices. The photocurrent density (Jph) plotted against the effective voltage (Veff) was conducted to access the exciton dissociation probability (Pdiss) and charge collection probability (Pcoll) in the devices. As illustrated in Fig. 2c, the D18:S9SBH-F and D18:S9SHO-F (C2-branched) based devices possess a larger Pdiss values of 93.6% and 93.7%, as well as larger Pcoll values of 89.4% and 90.2% than the other two devices (C3-branched), which will be beneficial for efficient exciton dissociation and charge generation. To investigate the charge recombination process in the cells, the dependence of Jsc and Voc on light intensity was performed (Fig. 2d, e). The slopes of the Jsc versus light intensity curve (α) are 0.972 for the S9SHO-F- and S9SBH-F-based devices, which are higher than those of D18:S9S3B-F (0.970) and D18:S9SHN-F (0.968), indicating suppressed bimolecular recombination in the devices based on C2-branched NFAs. From the plot of Voc versus light intensity, the slope βkT/q was calculated to elaborate the recombination mechanism. The β values of D18:S9SHO-F, D18:S9SBH-F, D18:S9S3B-F, and D18:S9SHN-F are 1.03, 1.03, 1.10, and 1.04, respectively, suggesting less trap-assisted recombination in the C2-branched S9SHO-F- and S9SBH-F-based devices.
Energy loss (Eloss) was analyzed to study the differences of Voc in the devices. The Eg value was derived from the normalized absorption and photoluminescence (PL) emission spectra (Supplementary Fig. 20). As depicted in Supplementary Table 8, the Eloss values for devices based on D18:S9SBH-F, D18:S9S3B-F, D18:S9SHO-F, and D18:S9SHN-F are 0.536 eV, 0.559 eV, 0.531 ev, and 0.534 eV, respectively. Furthermore, electroluminescence external quantum efficiency (EQEEL) was measured to verify the non-radiative energy losses in the devices. As shown in Fig. 2f, the devices with C2-branched NFAs exhibit larger EQEELs than those with C3-branched NFAs, benefiting from the higher PLQYs in their neat NFA films (Supplementary Fig. 14)56. The non-radiative Voc losses were calculated using the equation: ({Delta V}_{{{{rm{oc}}}},,{{{rm{non}}}}-{{{rm{rad}}}}}=-{kT}{{mathrm{ln}}}({{{{rm{EQE}}}}}_{{{{rm{EL}}}}})), giving 0.180, 0.191, 0.175, and 0.185 eV for D18:S9SBH-F, D18:S9S3B-F, D18:S9SHO-F, and D18:S9SHN-F cells, respectively. These phenomena illustrate that the side-chain modulation is a useful strategy for Se-containing NFAs to improve the photoluminescence efficiencies not only in neat NFA films but also in blends, suppressing non-radiative losses in devices.
To investigate the morphologies of blend films, atomic force microscopy (AFM) and GIWAXS measurements were performed. As depicted in Fig. 3a, the blend films composed of C2-branched NFAs exhibit well-mixed morphology with clear fibrous network compared to the other two blend films. The calculated surface root-mean-square roughness (Rq) in D18:S9SBH-F and D18:S9SHO-F are 1.16 nm and 1.01 nm, respectively, which are much smaller than those of D18:S9S3B-F (1.43 nm) and D18:S9SHN-F (1.36 nm).
a AFM height images of D18:S9SBH-F, D18:S9S3B-F, D18:S9SHO-F and D18:S9SHN-F blend films. b 2D GIWAXS patterns of D18:S9SBH-F, D18:S9S3B-F, D18:S9SHO-F and D18:S9SHN-F blends. c The corresponding IP and OOP line-cut profiles of the 2D GIWAXS based on the blend films.
The 2D GIWAXS patterns and the corresponding line-cut profiles are shown in Fig. 3b, c. It is found that all the blends exhibit a π−π diffraction peak at ~1.71 Å−1 in the OOP direction and a lamellar (100) peak at ~0.31 Å−1 in the in-plane (IP) direction, indicating a face-on orientation, which is beneficial for the charge transport. The crystal coherence length (CCL) of the (010) diffraction peak in the OOP direction was also calculated (Supplementary Table 9), and the values are 32.0 and 34.0 Å for blend films based on C2-branched S9SBH-F and S9SHO-F, respectively, larger than those of D18:S9S3B-F (30.8 Å) and D18:S9SHN-F (31.2 Å). This indicates a better morphology compatibility between S9SHO-F and D18, which can form a high degree of molecular order and further improve the charge transport. Furthermore, we conducted the contact angle measurements to verify the miscibility between the polymer donor and small molecular acceptor. As shown in Supplementary Fig. 21 and Supplementary Table 10, the Flory–Huggins interaction parameter (χ) was calculated by the surface energy, and the χ parameters of D18:S9SBH-F, D18:S9S3B-F, D18:S9SHO-F, and D18:S9SHN-F are 0.20K, 0.51K, 0.15K, and 0.40K, respectively. This indicates a high miscibility of D18:S9SHO-F blend with a more homogeneous morphology, which would benefit the charge transport and exciton dissociation in the active layer.
Subsequently, the GIWAXS measurements of D18:L8-BO and D18:L8-BO:S9SHO-F blend films were employed to investigate the molecular packing and crystallization behavior. As shown in Supplementary Fig. 22, the D18:L8-BO:S9SHO-F blend exhibits a π−π stacking distance of 3.62 Å in the OOP direction, which is slightly smaller than that of D18:L8-BO blend (3.64 Å). Additionally, the CCLs of the (010) diffraction peak in the OOP direction for the D18:L8-BO and D18:L8-BO:S9SHO-F blends are 30.2 and 34.9 Å, respectively. These results indicate the incorporation of S9SHO-F beneficially regulates the crystallinity of the blend films, which may lead to more efficient charge-transporting properties.
To elucidate how the side chain engineering influences the excited-state kinetics in the related materials, femtosecond-resolved transient absorption spectroscopy (TAS) was employed to investigate the neat NFA films as well as the corresponding blend films. The broadband TAS color maps of neat NFAs films pumped at 850 nm are presented in Fig. 4a, with the corresponding data shown in Supplementary Fig. 23. After photoexcitation, a negative ground-state bleaching signal of acceptor (GSBA) emerges around 850 nm, accompanied by the appearance of the excited-state absorption (ESA) signal of local excitons (LEs) near 950 nm. Then, a new ESA band around 1600 nm corresponding to the delocalized singlet excitons (DSEs) generates together with the decay of the LE in hundreds of femtoseconds30,57. Then, as the DSE signal vanishes in ~100 ps, a new ESA signal near 1450 nm gradually emerges, reaching its peak intensity in ~1000 ps. This signal, characterized by an exceptionally prolonged lifetime, can be unequivocally attributed to T₁ excitons arising from the ISC of singlet excitons, as corroborated by our prior investigations on analogous Se-containing molecules30.
a TAS color maps of S9SBH-F, S9S3B-F, S9SHO-F and S9SHN-F neat films, respectively. The assignments of signals are as follows: GSBA: 850–900 nm; LE: 930–950 nm; DSE: 1600 nm; T1: 1450 nm. Normalized TAS spectra probed at b 0.5 ps and c 1000 ps in different NFA films. The data in (b, c) are normalized to the minima of GSBAs, respectively. d The temporal progressions of GSBA peaks in different NFA films. e The TAS color map of D18:S9SHO-F blend film. GSBA: 850–860 nm; LE: 930 nm; DSE: 1600 nm; T1: 1450 nm; GSBD: 590 nm; CS: 630 and 800 nm, and 900–950 nm. f Normalized TAS spectra probed at 1000 ps in different blend films. g Normalized TAS dynamics probed around 1450 nm (T1) in different blend films. The data in (f, g) are normalized to the minima of GSBAs, respectively.
For the comparison of different NFA films, all the data are normalized to the minima of GSBAs, respectively (Supplementary Fig. 24a). As depicted in the normalized TAS spectra probed at 0.5 ps (Fig. 4b), the C2-branched S9SBH-F and S9SHO-F films exhibit weaker DSE signals but stronger LE signals compared to their C3-branched counterparts, which is also supported by the dynamics of DSEs in Supplementary Fig. 24b. Such differences in exciton delocalization indicate that the C2-branched side chains in S9SBH-F and S9SHO-F facilitates the formation of more Frenkel-like excitons rather than CT-like excitons. This finding aligns with our prior analysis of the absorption spectra in Fig. 1b, as supported by relevant literature50. More importantly, these films exhibit great differences in generation of T1 excitons, at the time delay of 1000 ps (Fig. 4c). The C2-branched S9SBH-F and S9SHO-F films exhibit markedly reduced T1 signals compared to the S9S3B-F and S9SHN-F films, as corroborated by the T1 dynamics presented in Supplementary Fig. 24c.
In addition, it is observed that the GSBA peaks of the NFA films continuously red shift at different time scales along with the evolution of photoexcited excitons (Fig. 4a). Through conducting an in-depth analysis of the red-shift processes in NFA films, we found that they can be categorized into three stages, as depicted by the temporal progression of the GSBA peak position in Fig. 4d. The first stage occurs within ~0.3 ps, predominantly involving the cooling process of photogenerated excitons and the conversion from LE to DSE57. In the last stage (50–1000 ps), the primary process is the generation of T1 excitons. The second stage is the most noteworthy one, where the S9S3B-F and S9SHN-F films exhibit a distinct red shift of GSBA, while the S9SBH-F and S9SHO-F films show negligible red shift. In view of that the T1 excitons are generated over a relatively longer timescale (as fitted in Supplementary Fig. 24d), the second stage cannot be ascribed to the ISC process. Instead, it is more likely to be attributed to the further interaction of DSEs with surrounding environments, leading to the formation of lower-energy states, which is consistent with the larger Strokes shifts in S9S3B-F and S9SHN-F films (Supplementary Fig. 23). The loss of energy during this process may cause the broader band tails of EQE spectra (Fig. 2b) and lower Vocs (Table 1) observed in S9SHN-F- and S9S3B-F-based devices. Moreover, the low-energy property and separated electron-hole distribution of DSEs probably facilitate the ISC to T1 excitons, according to the literatures58,59. Combining the results of TAS and absorption spectra, we conclude that the C2-branched NFAs generate more Frenkel-like excitons with higher energy, suppressing the ISC process to T1 excitons. In contrast, the C3-branched NFAs generate more CT-like (delocalized) excitons with lower energy, facilitating the formation of T1 excitons.
We further investigated the T1 formation in the blend films based on the Se-containing NFAs. As shown by the TAS data in Fig. 4e and Supplementary Fig. 25, the blend films were pumped at 850 nm to selectively excite the NFAs. All the blend films exhibit a DSE-mediated hole transfer process close to the previous reports57, where the GSB signal of D18 (GSBD at 590 nm) and the ESA signal of CS (630 and 800 nm) emerge as the GSBA (850 nm) and the ESA of DSE (1600 nm) decay in ~100 ps. In the longer time scales (after 100 ps), the ESA signal of T1 excitons (1450 nm) emerges and gradually rises to its maximum with the additional contribution of non-geminate recombination31. To compare the T1 excitons generated in different blend films, we normalized the TAS spectra and dynamics to the minima of GSBAs, respectively (Fig. 4f, g). Similar to the cases of neat NFA films (Fig. 4c), we still observed suppressed formation of T1 excitons in the C2-branched NFA-based blends compared to those in the C3-branched-based blends. The D18:S9S3B-F film exhibits the most intense T1 formation, while the D18:S9SHO-F film exhibits the least T1 formation.
We also note that T1 excitons could form through different pathways (i.e., ISC and non-geminate recombination) in these Se-containing NFAs. To provide a profound understanding into the mechanism of T1 formation in different material systems, we further investigated two Se-free NFAs, T9TBO-F (C2-branched N-alkyl chain on the pyrrole unit) and T9TBN-F (C3-branched N-alkyl chain on the pyrrole unit) (Supplementary Fig. 26). As shown in Supplementary Fig. 27, in the absence of heavy-atom-mediated ISC, there is negligible generation of T1 excitons in neat NFA films. In the Se-free blend films (Supplementary Fig. 28), relatively fewer T1 excitons are formed compared to the Se-containing blend films, with the majority arising from non-geminate recombination of free carries at D-A interfaces29. In contrast to the case of Se-containing NFAs, the Se-free NFA with C2-branched N-alkyl chains is associated with an increased generation of T1 excitons in blend films and a higher non-radiative Voc loss in device (0.228 and 0.220 eV for devices based on C2-branched D18:T9TBO-F and C3-branched D18:T9TBN-F, respectively) (Supplementary Fig. 29).
In addition, the charge carrier dynamics in the binary D18:L8-BO and ternary D18:L8-BO:S9SHO-F blends were investigated by TAS measurements. As shown in Supplementary Fig. 30, compared with D18:L8-BO film, strong GSBD and CS signals are observed in D18:L8-BO:S9SHO-F blend after hole transfer, suggesting more efficient charge separation in the ternary film, which should benefit the improved Jsc in the corresponding devices.
Due to the low-energy nature of T1 excitons, they are generally unable to dissociate into free charges and tend to relax via non-radiative pathways29. Therefore, the ISC process that S1 transfers to T1 in NFAs usually compete with the hole-transfer pathway in blend films. In Fig. 2b, we have observed that the C2-branched NFA-based devices exhibit lower EQEs than C3-branched NFA-based devices in the NFA-absorption region, which can be attributed to the lower hole-transfer efficiencies induced by faster ISC processes in C3-branched NFAs (Supplementary Fig. 24d). Moreover, the increased formation of non-radiative T1 excitons in the blend film indicates that a great proportion of excitons/carriers are prone to forming T₁ excitons, subsequently relaxing to the ground state through non-radiative channels. Our findings reveal that the material properties including steric hindrance, packing distance, exciton delocalization, T₁ formation in both neat and blend films, as well as device performance, are intricately interconnected, helping us establish a clear structure-property-performance relationship among these NFAs.
In conclusion, a series of Se-substituted NFAs were designed and synthesized by introducing different achiral N-alkyl substituents onto the pyrrole moiety in NFA core. Among these new NFAs, S9SHO-F, substituted with a long branching N-alkyl chains at C2 position, exhibits dense molecular packing and efficient intermolecular charge transport. The different N-alkyl substituents affect exciton properties, leading to improved luminescence efficiencies and suppressed T1 formation in neat and blend films of S9SHO-F. As a result, the D18:S9SHO-F binary devices achieved a high efficiency of 19.2% with extraordinary Jsc of 29.68 mA cm−2 and high Voc of 0.835 V. Moreover, a record efficiency of 20.4% (certified as 19.88%) is realized in the ternary OSCs. Our work exploits a new alkyl chain regulation strategy for Se-containing NFAs to modulate molecular packing, PLQY and T1-related non-radiative relaxation pathways, providing an efficient approach to further improve the performance of low-bandgap OSCs.
All chemicals and reagents were purchased from commercial resources and used without further purification. Polymer donor D18 was synthesized according to the previous report60. The synthetic procedures of S9SBH-F, S9S3B-F, S9SHO-F, S9SHN-F, and the corresponding structural characterization are presented in Supplementary Figs. 1 and 31–82.
The 1H NMR spectrometry was conducted on a Bruker 600 MHz AVANCE III spectrometer using CDCl3. High-resolution mass spectrometry was performed by a Q Exactive Focus Hybrid Quadrupole-Orbitrap Mass Spectrometer (QE Orbitrap MS, Thermo Fisher Scientific). UV-Vis absorption spectra were collected by a Ultra-Violet Visible Scanning Spectrophotometer (Shimadzu 1700). Cyclic voltammetry (CV) measurements were performed utilizing an Electrochemical Analyzer System (CH 1660C) in an anhydrous acetonitrile solution containing tetra-n-butylammonium hexafluoro-phosphate (Bu4NPF6) a concentration of 0.1 M under a scan rate of 50 mV s−1. A conventional three-electrode cell was applied with glassy carbon, platinum wire, and Ag/AgCl serving as the working electrode, counter-electrode, and reference electrode, respectively. Furthermore, FeCp₂/FeCp₂⁺ was utilized as an internal reference system. AFM images were investigated by a Bruker Multimode & Microscope. For GIWAXS measurement, the films were prepared on the silicon substrate.
The single crystals were grown using a ternary solvent diffusion method. A solution containing ~0.5 mg S9SBH-F, S9S3B-F, S9SHO-F or S9SHN-F dissolved in 0.3 mL CHCl3, was meticulously transferred into a clean NMR tube. Around 0.2 mL CH2Cl2 was slowly layered on top of the CHCl3 solution, followed by layering of acetone to fill the NMR tube. The CH2Cl2 layer serves as a buffer to establish a solubility gradient, thereby modulating the crystal growth kinetics and preventing rapid solid precipitation. The NMR tube was then sealed and left to stand for a few days (7–14 days) until the solution’s color faded.
The OSCs were fabricated with a structure of ITO/2PACz/D18:NFAs/PDINN/Ag. The ITO/glass substrates underwent ultrasonic cleaning using detergent, deionized water, acetone, and isopropanol. After being dried in an oven at 110 °C overnight, the ITO glasses were subjected to plasma treatment for 25 min. A thin layer of 2PACz was spin-cast onto the ITO substrates at 5000 rpm for 25 s, then annealed at 70 °C for 5 min in air. The weight ratio of D18:NFAs blends was 1:1.2 and the optimal weight ratio of D18:L8-BO:S9SHO-F was 1:1.05:0.15. The mixture was dissolved in chloroform (the total concentration of blend solutions were 7 mg mL−1 for all blends), with the addition of 0.5% CN as additive, and stirred 2 h on a hotplate at 55 °C in a nitrogen-filled glove box. The solution were spin-cast on the 2PACz layer at 2000 rpm for 20 s and accompanied by a thermal annealing step at 90 °C for 5 min., followed by a thin layer of PDINN coated on top of the active layer. Finally, a layer of Ag (~90 nm) was thermally evaporated at 5 × 10−5 Pa through a shadow mask at a rate between 2.5 and 5 Å s−1. The J–V curves of the devices were obtained using a Keithley 2400 source meter under room temperature in an N2-filled glove box, with a scan step of 0.02 V and a dwell time of 10 ms. The simulated sunlight was calibrated using an AM 1.5G solar simulator (Enlitech, SS-F5, Taiwan), which was measured with an Si diode applied with KG-2 filter calibrated by the National Renewable Energy Laboratory. The EQE curves were measured by an EQE measurement system manufactured by EnLiTech (Taiwan).
The TAS measurements were conducted using a Yb:KGW laser (Pharos, Light Conversion). The wavelength of fundamental output is at ~1030 nm. We used a second harmonic noncollinear optical parametric amplifier (2HNOPA, Light Conversion) to generate the pump pulses at 850 nm. The probe beam was supercontinuum by focusing a small fraction of the fundamental 1030 beam to a sapphire plate for visible detection or an yttrium aluminum garnet (YAG) plate for near-infrared (NIR) detection. The experiments were conducted with a commercial TAS system (TA100, TIME TECH SPECTRA). The time delay between the pump and probe pulses was controlled by a delay line. The pulse-to-pulse spectral analysis was conducted at 10 kHz. The signal-to-noise ratio (ΔO.D.) was better than 1 × 10−5 after averaging over 5000 pump-on and pump-off shots for each data point. The pump densities in TAS experiments was set at ~2 μJ cm−2 unless otherwise specified. The samples for TAS measurement were spin-coated on quartz substrates. The samples were kept in a nitrogen atmosphere during the measurement to prevent photo-degradation.
The EQEEL data were collected using an in-house-built system, comprising a Hamamatsu silicon photodiode 1010B, a Keithley 2400 Source Meter for voltages supply and current injection recording, and a Keithley 485 Picoammeter for measuring the emitted light intensity.
Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.
The data generated in this study are provided in the Supplementary Information/Source data file. The X-ray crystallographic coordinates for structures reported in this study have been deposited at the Cambridge Crystallographic Data Centre, under deposition numbers CCDC 2468250 (S9SBH-F), 2468257 (S9S3B-F), 2468259 (S9SHO-F), and 2468262 (S9SHN-F). These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Source data are provided with this paper.
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F.Q. thanks the support from the National Natural Science Foundation of China (22409108), the Shandong Province Natural Science Foundation of (ZR2024QE324), the Taishan Scholars Program (NO.tsqnz20240815), and the Natural Science Foundation of Qingdao City (24-4-4-zrjj-22-jch). H.L acknowledges the support from the National Natural Science Foundation of China (52403234) and the Shandong Province Natural Science Foundation of (ZR2024QB194). R.Z. thanks the support from the Jiangsu Specially Appointed Professorship (SR21400224), and the Collaborative Innovation Center of Suzhou Nano Science & Technology, the 111 Project, Suzhou Key Laboratory of Surface and Interface of Intelligent Matter (SZS2022011). H.Z. thanks the support from the Shandong Province Natural Science Foundation (2023HWYQ-085) and the Taishan Scholars Program (NO.tsqnz20221137). Z.B. acknowledges the support of the Taishan Scholars Program (NO.tstp20221121). D.L. thanks the financial support from the Research Grants Council of the Hong Kong through a Collaborative Research Equipment Grant (C1015-21EF). A.K.Y.J. thanks the sponsorship of the Lee Shau-Kee Chair Professor (Materials Science), and the support from the APRC Grants (9380086, 9610419, 9610440, 9610492, 9610508) of the City University of Hong Kong, the MHKJFS Grant (MHP/054/23), TCFS grant (GHP/121/22SZ) and MRP Grant (MRP/040/21X) from the Innovation and Technology Commission of Hong Kong, the Green Tech Fund (202020164) from the Environment and Ecology Bureau of Hong Kong, and the GRF grants (11304424, 11307621, 11316422) and CRS grants (CRS_CityU104/23, CRS_HKUST203/23) from the Research Grants Council of Hong Kong. This work was partially financially supported by City University of Hong Kong (9610739) for the project “Fostering Innovation for Resilience and Sustainable Transformation,” officially endorsed by the United Nations Educational, Scientific and Cultural Organization (UNESCO) under the International Decade of Sciences for Sustainable Development (2024–2033).
These authors contributed equally: Feng Qi, Qian Li.
College of Materials Science and Engineering, Qingdao University, Qingdao, China
Feng Qi, Hao Lu & Zhishan Bo
College of Textiles & Clothing, Qingdao University, Qingdao, China
Feng Qi, Hao Lu, Shuzhe Liu & Zhishan Bo
Department of Materials Science and Engineering, City University of Hong Kong, Kowloon, Hong Kong, China
Qian Li, Dangyuan Lei, Francis R. Lin & Alex K.-Y. Jen
Hong Kong Institute for Clean Energy, City University of Hong Kong, Kowloon, Hong Kong, China
Qian Li, Dangyuan Lei, Francis R. Lin & Alex K.-Y. Jen
Department of Physics, Chemistry and Biology (IFM), Linköping University, Linköping, SE-58183, Sweden
Rui Zhang
State Key Laboratory of Bioinspired Interfacial Materials Science, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou, 215123, China
Rui Zhang
Department of Chemistry, University of Washington, Seattle, WA, USA
Werner Kaminsky
School of Chemistry and Chemical Engineering, Linyi University, Linyi, Shandong, China
Yaoyao Wei
State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an, China
Qunping Fan
School of Public Health, Qingdao University, Qingdao, China
Hongna Zhang
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F.Q., Z.B., and A.K.-Y.J. conceived the idea. F.Q. and F.R.L. synthesized the materials and grew the single crystals. Q.L. and D.L. conducted the TAS measurements and corresponding analysis. H.L. fabricated and characterized the devices and performed the certification. S.L. conducted the SCLC and AFM measurements. R.Z. and Q.F. performed GIWAXS measurements and corresponding analysis. Y.W. conducted calculations on the intermolecular electronic couplings. F.Q. and F.R.L. analyzed the intermolecular interactions in single crystals. W.K. resolved the single-crystal structures. H.Z. performed the characterization of the chemical structure. F.Q., Q.L., and F. R.L. drafted the manuscript with input from all authors. Z.B. and A.K.-Y.J. supervised this project.
Correspondence to Hao Lu, Francis R. Lin, Zhishan Bo or Alex K.-Y. Jen.
The authors declare no competing interests.
Nature Communications thanks Xiaozhang Zhu and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. A peer review file is available.
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Qi, F., Li, Q., Lu, H. et al. Alleviating non-radiative losses in organic solar cells through side-chain regulation of low-bandgap non-fullerene acceptors. Nat Commun 17, 712 (2026). https://doi.org/10.1038/s41467-025-67351-x
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