Introduction

Over the past decade, organic solar cells (OSCs) have garnered remarkable progress on power conversion efficiencies (PCEs)1,2,3,4, owing to the continuous innovation of non-fullerene acceptors (NFAs)5,6,7,8. However, despite these significant advancements, the long-term operational stability of OSCs remains a primary challenge that hinders their potential large-scale commercial application9,10,11, with NFA degradation being a key limiting factor12,13,14,15. Only by overcoming these challenges can OSCs achieve the necessary reliability for widespread commercialization.

In typical NFA architectures, an electron-rich core (denoted as D or DA’D) connects two electron-deficient terminal groups (A) through exocyclic vinyl linkages, forming the widely adopted A-D-A or A-DA’D-A configurations16,17,18,19. While this molecular design contributes to broad absorption20,21, high charge mobility22,23, and favorable energy-level alignment24,25,26, the exocyclic double bonds within these structures introduce inherent vulnerabilities24,27. During long-term operation under real-world conditions, external environmental factors such as oxygen and moisture can permeate into the active layer28,29, triggering multiple degradation pathways30,31, including photoisomerization (causing undesirable molecular configuration changes)32,33,34,35, photooxidation (leading to irreversible chemical modifications)36,37,38,39, nucleophilic attack (resulting in bond cleavage)40,41 and so on. Such degradation mechanisms disrupt the molecular integrity of NFAs, ultimately causing severe deterioration or even complete failure of device performance. Moreover, even in the absence of external oxygen and moisture, the exocyclic vinyl linkages in NFAs exhibit intrinsic instability due to their susceptibility to [2 + 2] cycloaddition reactions under light exposure or heat stress, followed by cycloreversion42. This dynamic process can lead to the redistribution of terminal groups, altering the original molecular structure and electronic properties, thereby compromising the photovoltaic performance43. Addressing these stability issues requires innovative strategies, such as developing more robust vinyl groups or exploring alternative molecular frameworks with enhanced resistance to both environmental and intrinsic degradation pathways. However, the existing strategies usually raise a trade-off between efficiency and stability.

In this contribution, a terminal group 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)fluoroacetonitrile (denoted as IFC) is designed and prepared by using fluoroacetonitrile group to replace the malononitrile group in the widely used 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (denoted as IC), as shown in Fig. 1a. The introduction of fluoroacetonitrile groups brings out isomeric issue of E/Z configurations. After meticulous optimization, the E isomer wherein the fluorine on the fluoroacetonitrile group is adjacent to the exocyclic vinyl linkage can be chemoselectively synthesized. As anticipated, an intramolecular F···H interaction is formed in the acceptor E-2IFC-F with the E configuration, stabilizing the vinyl group and thereby promoting its chemical and photo-stability. Meanwhile, the F···H interaction locks the molecular conformation, resulting in a more planar skeleton, which in turn improves molecular packing and film morphology. When incorporating the low-cost polymer donor P3HT, the relevant acceptor E-2IFC-F realizes a PCE of 11.77%, as the record efficiency among P3HT-based OSCs. Furthermore, through chemical modification involving asymmetric and chlorination strategies, the asymmetric acceptor E-IFC-IC-Cl with improved energy levels and absorption is developed, consequently delivering a PCE of 19.31% in the resulting device incorporating the polymer donor PM6. Benefiting from the F···H interaction, the redistribution of terminal groups is completely suppressed in the case of E-IFC-IC-Cl, endowing the OSC device with better stability.

Fig. 1: Design concept and synthetic routes of fluoroacetonitrile-containing acceptors.
figure 1

a Chemical structures of typical terminal groups in literature and the fluoroacetonitrile-containing terminal groups developed in this work. b Synthetic routes of acceptors E-2IFC-F and Z-2IFC-F. Method Ⅱ can chemoselectively synthesize E-2IFC-F.

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Results

Chemoselective synthesis

Initially, a typical Knoevenagel condensation between fluoroacetonitrile and 5,6-difluoro-1H-indene-1,3(2H)-dione was used to directly prepare fluoroacetonitrile-modified indanone through a one-step reaction, but failed. Then, a feasible strategy to synthesize IFC involving cyano group substitution can react in gem-difluoroolefin derivatives. The target intermediate 3-(difluoromethylene)-5,6-difluoro-2,3-dihydro-1H-inden-1-one (I2F-F) was obtained with a high yield using a reported Witting reagent (Ph3P+CF2COO−)44. After successfully obtaining the intermediate I2F-F, a cyanation reaction is conducted to synthesize IFC-F. Notably, there are two fluorine atoms with similar reactivities in I2F-F, probably leading to two isomers, named E-IFC-F and Z-IFC-F. As shown in Fig. 1b, Method I, and Table S1, the cyanation reaction is firstly performed according to the reported conditions using trimethylsilyl cyanide (TMSCN) and Cs2CO3 in MeCN solution45, a major Z configuration product Z-IFC-F is obtained with a low yield accompanied by a moderate E/Z ratio. To improve the regioselectivity and reaction efficiency, a variety of bases, including K2CO3, t-BuOK and CsF, are examined, and all give a dominated Z-conformational product. It is speculated that the alkali metal ions may have a weak interaction with I2F-F, which enhances the electron-deficient properties of the aryl group within I2F-F, and then causes an electron repulsion with the cyano group (Fig. S1), resulting in the formation of the Z isomer46. Notably, the monovalent cations, particularly cesium ion, can readily coordinate with DMSO to form a solvent shell47. Hence, DMSO may diminish the effect of alkali metal ions on the electronic properties of aryl groups, thereby favoring the formation of the E isomer. Based on this consideration, a small-sized base CsF associated with DMSO can achieve a dominant E-IFC-F product with a moderate E/Z ratio in a decent yield. Moreover, the polar solvent DMF also attains IFC-F in high yield, while the E/Z ratio is slightly lower than DMSO. Briefly, fluoroacetonitrile-containing terminal group IFC-F can be regioselectively synthesized with decent configuration selectivity through the above fine optimizations, in which using t-BuOK and MeCN as reaction conditions is favorable to obtain Z-IFC-F with the E/Z ratio of 16.1:100, while the use of CsF and DMSO facilitates the production of E-IFC-F with the E/Z ratio of 100:21.7. After separating the E/Z isomers by multiple column chromatography, Knoevenagel condensations with BTP−2CHO are performed to afford the isomerically pure acceptors, E-2IFC-F and Z-2IFC-F. The chemical structures of the intermediates and target acceptors are confirmed by 1H, 13C and 19F nuclear magnetic resonance (NMR), as well as matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectroscopy (MS) (Figs. S2–S14). Both acceptors exhibit good solubility in common organic solvents (e.g., chloroform and tetrahydrofuran) (Table S2). Significantly, the NMR monitoring of both isomers in solution during prolonged heating reveals no observable interconversion or decomposition (Figs. S15–S18), demonstrating that both E-2IFC-F and Z-2IFC-F possess substantial intrinsic stability.

The tedious purification of E/Z isomers significantly raises the synthetic complexity and is evidently inapplicable in large-scale manufacturing. Considering the molecular conformation, the fluorine atom adjacent to the exocyclic vinyl group might form an intramolecular F···H interaction to stabilize this fluorine atom by reducing its elimination capability. On the contrary, such F···H interaction is absent in the other fluorine atom. Thus, the synthetic route is modified by alternating the sequence of the key cyanation reaction and the Knoevenagel condensation as displayed in Fig. 1b, Method II. The intermediate 2I2F-F is readily prepared via the Knoevenagel condensation of I2F-F and BTP-2CHO. Then, the regioselective cyanation is successfully achieved with 2I2F-F in the mixture solvent of DMSO and toluene using TMSCN and K3PO4, and thus affording exclusively E-2IFC-F with a decent yield. As a result, E-2IFC-F can be selectively prepared through taking advantage of the different reactivity of the fluorine atoms with/without intramolecular F···H interactions. Compared with Method I, Method II avoids the tedious separation process of isomers, and thus is more compatible with large-scale production.

F···H interactions and solid-state packing behaviors

To study the effect of isomerism on molecular conformation, theoretical calculations based on density functional theory (DFT) are conducted. Both E-2IFC-F and Z-2IFC-F exhibit relatively planar geometry (Fig. S19). In E-2IFC-F, the distance between the fluorine atom of the terminal group and the hydrogen atom of the exocyclic double bond is only 2.09 Å, indicating the existence of F···H interaction. Furthermore, the E and Z isomers exhibit significantly distinct electrostatic potential distributions on the segments of exocyclic double bonds and terminal groups (Fig. S20), consequently leading to different dipole moments for E-2IFC-F (3.93 D) and Z-2IFC-F (0.94 D), respectively. The enlarged dipole moment in E-2IFC-F is believed to facilitate exciton dissociation and optimize the molecular packing48.

To verify the F···H interaction in E-2IFC-F, a thorough investigation of NMR spectra is conducted. As shown in Figs. 2a, S21 and S22, the proton on exocyclic vinyl linkage of E-2IFC-F is observed as a doublet peak at 8.44 ppm, whereas the same proton of Z-2IFC-F is located at 8.57 ppm as a singlet peak. Moreover, the fluorine adjacent to the exocyclic vinyl group in E-2IFC-F also splits into a double peak at −130.76 ppm. These results indicate that there is a strong interaction between the fluorine atom of the terminal group and the hydrogen atom of the exocyclic vinyl bond in E-2IFC-F. To further validate this F···H interaction, 2D NMR 1H-19F nuclear overhauser effect spectroscopy (NOESY) is conducted in the E-2IFC-F solution. As depicted in Fig. 2b, a pronounced cross-correlation resonance signal of Fa···Ha is observed, wherein Fa is assigned to the fluorine atom in the fluoroacetonitrile group and Ha belongs to the hydrogen atom of the exocyclic vinyl linkage. Such a signal is absent in Z-2IFC-F (Fig. S23). These results provide compelling evidence for the strong intramolecular F···H interaction in E-2IFC-F.

Fig. 2: F···H interactions and molecular packing behaviors of isomeric acceptors.
figure 2

a 1H and 19F NMR spectra of E-2IFC-F and Z-2IFC-F. b 1H-19F NOESY spectrum of E-2IFC-F. c Single crystals of E-2IFC-F and Z-2IFC-F. d 1D line-cut profiles of grazing-incidence wide-angle X-ray scattering (GIWAXS) and e atomic force microscopy (AFM) height images of E-2IFC-F and Z-2IFC-F.

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Moreover, single crystals of E-2IFC-F and Z-2IFC-F are cultivated using the solvent diffusion method to further unravel the molecular conformation and intermolecular packing (Figs. S24, S25 and Table S3). As shown in Fig. 2c, the intramolecular distances (dF···H) of E-2IFC-F are 2.16 and 2.18 Å, indicating the existence of F···H interactions. The dihedral angles between the core unit and the terminal groups in the E/Z isomers are found to be 4.14/2.91° for E-2IFC-F and 6.56/4.12° for Z-2IFC-F, respectively, suggesting that the F···H interaction can lock the conformation of E-2IFC-F to afford a more planar skeleton and facilitate intermolecular stacking. As for the intermolecular packing, both E-2IFC-F and Z-2IFC-F show a three-dimensional interpenetrating network structure (Fig. S26). In the E-2IFC-F crystal, multiple intermolecular noncovalent interactions, such as F···π, F···N ≡ C and S···N interactions are observed, and three types of π-π stacking are shown in Fig. S27, involving end to end (3.33 Å), core to core (3.34 Å) and end to core (3.33 Å). Although Z-2IFC-F also has three types of π-π interactions, the type of end to core is not observed and the π-π distances are obviously large, as 3.46, 3.46 and 3.34 Å. Compared with Z-2IFC-F, the smaller stacking distances of E-2IFC-F suggest the more compact π-π stacking. These terminal groups exert a significant impact on molecular geometry, demonstrating that the E/Z configuration influences aggregation behavior, wherein the E-2IFC-F crystal exhibits tighter and more ordered packing.

To gain deep insight into the impact of the F···H interaction on crystallization behaviors, the grazing-incidence wide-angle X-ray scattering (GIWAXS) measurement is conducted. As depicted in Figs. 2d, S28 and Table S4, the E-2IFC-F displays multiple diffraction peaks in the out-of-plane (OOP) direction at 1.86, 1.43, 0.53 and 0.21 Å−1, with corresponding spacing distance (d-spacing) of 3.37, 4.39, 11.85 and 29.92 Å, respectively, indicating the strong crystallinity at E-2IFC-F film. The diffraction peak of E-2IFC-F at 1.86 Å−1 is assigned to π-π stacking (010) diffraction peak, and the crystal coherence lengths (CCL) are calculated to be 42.84 Å, suggesting the neat E-2IFC-F films present a predominant face-on packing. In contrast, the Z-2IFC-F only displays a weak (010) diffraction peak located at 1.84 Å−1 corresponding to a larger π-π stacking distance of 3.41 Å and a smaller CCL value of 36.02 Å in OOP direction. These results reveal that the molecular packing of E-2IFC-F is tighter and more ordered compared with Z-2IFC-F, owing to the existence of the F···H interaction and the large dipole moment. Moreover, the morphology of Z-2IFC-F and E-2IFC-F neat films is investigated using atomic force microscopy (AFM). As shown in Figs. 2e and S29, the neat films of these two isomers display evidently varied surfaces with significantly different root-mean-square surface roughness (Rq). The E-2IFC-F film exhibits an island-like morphology with large-size domains, as demonstrated by a huge Rq value of 8.10 nm. As for Z-2IFC-F, the film is more uniform, accompanied by a relatively small Rq value of 1.28 nm. Such a significant distinction between these two isomers indicates the F···H interaction in E-2IFC-F improves molecular packing and enhances crystallinity even in a spin-coated film. The favorable aggregation of E-2IFC-F is expected to facilitate charge transport, suppress charge recombination, and thus improve the photovoltaic properties in OSCs.

Optoelectrical properties and stability of isomeric acceptors

The optical properties of E-2IFC-F and Z-2IFC-F in solution and as film are investigated, and the detailed parameters are summarized in Table S5. As depicted in Fig. 3a, the two isomers display similar absorption spectra in the range from 500 to 700 nm with the close maximum absorption peaks (λmaxsol). Significantly, E-2IFC-F shows a higher molar extinction coefficient (1.51 × 105 M−1 cm−1) relative to Z-2IFC-F (1.34 × 105 M−1 cm−1), suggesting the effect of conformational locking is still effective in dilute solutions. The λmaxfilm values as film show red-shifts of 64 nm for both E-2IFC-F and Z-2IFC-F compared with their solutions (Fig. 3b). Furthermore, E-2IFC-F exhibits a higher 0-0/0-1 intensity ratio (A0-0/A0-1) in both solution and film, indicating the stronger intermolecular π-π interaction and tighter stacking in E-2IFC-F that are consistent with the findings from single crystal. The electrochemical properties of two isomers are further characterized through cyclic voltammetry. The LUMO/HOMO energy levels of E-2IFC-F and Z-2IFC-F are calculated to be −3.71/−5.53 and −3.75/−5.56 eV, respectively (Figs. 3c and S30). The elevated energy levels, especially LUMO, are attributed to the moderate electron-withdrawing ability of the fluoroacetonitrile group.

Fig. 3: Optoelectronic properties, stability properties of acceptors.
figure 3

a Absorption spectra in chloroform solution, and b normalized absorption spectra as films of Z-2IFC-F, E-2IFC-F, Z-IFC-IC-Cl and E-IFC-IC-Cl. c Energy levels diagrams of P3HT, PM6 and acceptors. Absorption changes over time of d Z-2IFC-F and e E-2IFC-F solutions with 200-equivalent nucleophilic (Nu) reagent in THF solutions. Nu reagent is ethanolamine. f Corresponding maximum absorption peak intensity ratios of the fresh solution (I0) and the aged solution (It). Absorption spectra of g Z-2IFC-F and h E-2IFC-F films under 65 W white-light LED irradiation. i Corresponding maximum absorption peak intensity ratios (It/I0) of the fresh film and the aged film.

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The stability of E-2IFC-F and Z-2IFC-F, in particular the exocyclic double bond, is further studied. The reported acceptor BTP-eC9-4F (Fig. S31), featuring the typical terminal group IC-F, is used as a counterpart to clarify the superiority of the fluoroacetonitrile-modified acceptors. The exocyclic double bond is easily attacked by nucleophile reagents, such as amine derivatives, which are widely used as interface materials in OSCs. Thus, the chemical stability of BTP-eC9-4F, E-2IFC-F and Z-2IFC-F is investigated. After the addition of 200-equivalent ethanolamine into the dilute acceptor solutions (~10−5 M) in THF, the change of absorption spectra is recorded to analyze the degree of degradation for these acceptors, since the decomposition of the double bond would destroy the conjugation and then decrease the absorption. The absorption intensity of BTP-eC9-4F solution rapidly declines within 2.5 h, accompanied by a noticeable color change from cyan to orange, indicating that the occurrence of nucleophile reaction would produce corresponding Michael adducts (Figs. S32 and S33). As shown in Fig. 3d–f, compared with BTP-eC9-4F, fluoroacetonitrile-modified acceptors exhibit better chemical stability. The absorption of Z-2IFC-F disintegrates completely after 8 h (Fig. 3d), which might be attributed to the weak electron deficiency of IFC-F resulting in a relatively low reactivity of exocyclic double bond. Notably, the degradation product that fluorine atom is substituted by ethanolamine is also detected in the aged Z-2IFC-F solution (Fig. S34). Moreover, E-2IFC-F still retains 55.2% of its initial absorption intensity even after 24 h (Fig. 3e), with only Michael adducts detected (Fig. S35), demonstrating its enhanced chemical stability. As shown in Fig. 2a, the F···H interaction in E-2IFC-F shifts the hydrogen atom of the exocyclic vinyl linkage to a higher field (8.44 ppm) compared to Z-2IFC-F (8.57 ppm), indicating reduced reactivity of this double bond in Michael addition. Additionally, this interaction suppresses the activity of the fluorine atom in nucleophilic attack. Together, these synergistic effects contribute to the improved stability of E-2IFC-F. To further study the stability of these acceptors in photochemical reactions, the neat films of BTP-eC9-4F, E-2IFC-F and Z-2IFC-F are exposed to 65 W white-light LED irradiation. As shown in Figs. 3g–i and S36, the absorption intensity of BTP-eC9-4F film reduces rapidly and degrades completely after 48 h, while Z-2IFC-F remains 48.8% of its initial intensity, followed by complete photodegradation occurring after 72 h. Notably, the E-2IFC-F neat film exhibits excellent photostability, remaining 47.3% of its initial absorption intensity even after 120 h. As expected, the F···H interaction between the fluoroacetonitrile-containing terminal group and the exocyclic vinyl group significantly improves the chemical and photo-stability of acceptor.

Photovoltaic performance of P3HT-based OSCs

OSC devices are fabricated to investigate the photovoltaic properties of E-2IFC-F and Z-2IFC-F. The energy levels of the two acceptors are closely aligned with donor material PM6 (−3.71/−5.54 eV), which may result in insufficient driving force for charge transfer. Fortunately, their energy levels match well with the low-cost polymer P3HT49,50,51. Thus, P3HT is used as a donor for the devices based on E-2IFC-F and Z-2IFC-F (Fig. S37). Notably, both E-2IFC-F and Z-2IFC-F exhibit excellent solubility in non-halogenated non-aromatic solvents, such as tetrahydrofuran and 2-methyltetrahydrofuran, which are more environmentally friendly compared with the typically used chloroform or chlorobenzene. After the optimization of processing conditions as summarized in Tables S6–S8, the optimal devices are fabricated by P3HT:acceptors (1:1.5, w/w) solutions in mixed solvents (tetrahydrofuran:2-methyltetrahydrofuran, 0.9:0.1), with 4 mg/mL 1,3,5-trichlorobenzene (TCB) as solid additive, followed by thermal annealing at 135 °C for 5 min. The corresponding current density-voltage (J-V) curves are shown in Fig. 4a and the relevant parameters are listed in Table 1. The device based on P3HT:Z-2IFC-F offers a rather low PCE of 5.85% with an open-circuit voltage (Voc) of 0.868 V, a short circuit-current density (Jsc) of 12.38 mA cm−2, and a fill factor (FF) of 54.41%. Noted that the P3HT:E-2IFC-F-based device exhibits a high PCE of 11.77% with the simultaneous improvements in Voc (0.912 V), Jsc (18.36 mA cm−2), and FF (70.27%). The PCE of 11.77% for binary device reaches the highest value for the P3HT-based OSCs, surpassing all previously reported values (Fig. 4c). The integrated photocurrent density (Jsccal) values, derived from the corresponding EQE spectra, are 11.73 and 17.83 mA cm−2 for the Z-2IFC-F and E-2IFC-F-based devices, respectively, which are in consistence with the Jsc results obtained from the J-V curves (within 5% error) (Fig. 4b). Additionally, the EQE values of E-2IFC-based binary device are significantly improved relative to the Z isomer, suggesting more efficient charge transfer and carrier collection.

Fig. 4: Photovoltaic performance of P3HT:symmetric acceptors-based devices.
figure 4

a J-V curves and b corresponding EQE spectra of the optimal OSCs based on P3HT:E-2IFC-F and P3HT:Z-2IFC-F. The Jsccal represents integrated photocurrent density. c Statistical data of PCE and FF of P3HT-based OSCs in the literature and this work. d Photocurrent density versus Veff of the optimal OSCs. The ηdiss and ηcoll represent the probability of exciton dissociation and charge collection, respectively. e Electron and hole mobilities of P3HT:E-2IFC-F and P3HT:Z-2IFC-F blend films calculated from space charge limited current (SCLC) mode. f Irradiation stability of the optimal OSC devices based on P3HT:E-2IFC-F and P3HT:Z-2IFC-F.

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Table 1 Photovoltaic parameters of OSCs under AM 1.5 G illumination (100 mW cm−2)
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Considering the approximate energy levels of the isomers yet distinct Voc in P3HT-based devices, the electroluminescence external quantum efficiency (EQEEL) of devices is performed to analyze the energy losses (Eloss). As depicted in Fig. S38 and Table S9, the radiative recombination losses (∆E1 for above-bandgap and ∆E2 for below-bandgap radiative losses) show negligible variation. Notably, the P3HT:E-2IFC-F device exhibits a significantly lower non-radiative recombination loss (∆E3 = 0.299 eV) than that of Z-isomer-based device (0.342 eV). This pronounced difference directly correlates with the higher crystallinity of E-2IFC-F, which effectively suppresses energy disorder and minimizes voltage loss in photovoltaic devices. Furthermore, the exciton and carrier dynamic behaviors are investigated to reveal the effect of the isomerization. The values of the exciton dissociation probability (ηdiss) and charge collection probability (ηcoll) are calculated to be 82.34%/71.63%, and 93.39%/82.94% for the devices based on Z-2IFC-F and E-2IFC-F, respectively (Fig. 4d), indicating that E-2IFC-F-based device exhibits higher exciton dissociation and charge collection compared with Z-isomer-based device. To rationalize the high values of ηdiss/ηcoll, the charge mobilities of blend films are measured using the space-charge-limited current (SCLC) method for the P3HT:acceptor blend films. The determined electron/hole mobility (μe/μh) values are calculated to be 6.21/4.71 × 10−4 and 3.54/1.74 × 10−4 cm2 V−1 s−1 for P3HT:E-2IFC-F and P3HT:Z-2IFC-F, respectively (Figs. 4e and S39). Accordingly, the E-2IFC-F-based device shows a much higher and more balanced μe/μh ratio of 1.32 than the Z-2IFC-F counterpart (2.03), indicating that the device based on E-2IFC-F can effectively boost the charge transport, and thereby leading to the enhanced Jsc and FF. The charge recombination behaviors in optimized devices are evaluated through the dependences of Voc and Jsc on light intensity (Plight). The n values are determined to be 1.26 kT/q and 1.18 kT/q, while the corresponding α values are evaluated as 0.972 and 0.984, for the P3HT:Z-2IFC-F and P3HT:E-2IFC-F-based devices, respectively (Fig. S40). These results indicate that E-2IFC-F-based devices have more suppressed trap-assisted and bimolecular recombination, resulting in enhancements of Jsc and FF. The device stability of OSCs is also assessed, and the encapsulated devices are worked under continuous one sun illumination in air. The device based on E-2IFC-F exhibits significantly improved light stability, retaining 80.41% of its initial efficiency after 340 h of continuous illumination, compared with only 33.38% for the Z-2IFC-F-based device (Fig. 4f). These results demonstrated the F···H interaction in the E-conformational acceptor enhances the long-term stability of the device, which is also consistent with the aforementioned acceptor stability test.

The morphology of the active layer determines device performance. GIWAXS measurements demonstrate that both E-2IFC-F and Z-2IFC-F acceptors induce a transition of P3HT from edge-on to face-on orientation (Fig. S41 and Tables S10, S11). Notably, the P3HT:E-2IFC-F blend film displays tighter π-π stacking and enhanced crystallinity than P3HT:Z-2IFC-F blend film. This discrepancy arises primarily from the enhanced molecular planarity and dipole moment of E-2IFC-F, thereby facilitating the formation of a more ordered micro-phase-separated structure to ensure efficient charge transport. AFM measurements further reveal the differences in morphology between the blend films (Fig. S42). The E-2IFC-F-based blend film exhibits a dense granular morphology (Rq = 3.98 nm), which is conducive to exciton dissociation and charge transport. In contrast, the P3HT:Z-2IFC-F blend film exhibits smaller grain sizes (Rq = 2.78 nm) and poorer crystalline order. The inferior packing order and increased amorphous regions in the P3HT:Z-2IFC-F blend film result in compromised charge transport, accounting for the diminished device performance.

Asymmetric NFAs with fluoroacetonitrile groups

Notably, the F···H interaction in E-2IFC-F can enhance the stability of acceptor and OSC device, as well as excellent photovoltaic performance when pairing with the low-cost donor P3HT. However, due to the moderate electron-withdrawing ability of the fluoroacetonitrile group, the energy level of E-2IFC-F is mismatched with the state-of-the-art donor PM6 (Fig. 3c). Thus, the asymmetric strategy by introducing two different terminal groups, IFC and IC, into acceptors is used to tune energy levels, while chlorination on terminal groups is also employed to further broaden absorption. As shown in Fig. 5a, the asymmetric acceptor E-IFC-IC-Cl is prepared with exclusive regioselectivity through Method Ⅱ, and the Z isomer Z-IFC-IC-Cl is also synthesized as reference (Figs. S43–S63). The E-IFC-IC-Cl and Z-IFC-IC-Cl exhibit similar absorption ranges and absorption peaks, and the λmaxsol values are recorded at 729 nm for two isomers in solution (Fig. 3a). Whereas, E-IFC-IC-Cl displays a larger A0-0/A0-1 in solution, indicating the stronger intermolecular π-π interaction and tighter stacking in E-isomer. Furthermore, E-IFC-IC-Cl presents a higher molar absorption coefficient (2.42 × 105 M−1 cm−1) relative to Z-IFC-IC-Cl (2.25 × 105 M−1 cm−1). As depicted in Fig. 3b, the λmaxfilm values of 799 nm for neat E-IFC-IC-Cl film are observed to be red-shifted by 4 nm compared with Z-IFC-IC-Cl (795 nm). As shown in Figs. 3c and S64, the calculated LUMO/HOMO energy levels of E-IFC-IC-Cl and Z-IFC-IC-Cl are estimated to be −3.91/−5.65 eV, and −3.89/−5.64 eV, respectively. As expected, compared with the symmetric acceptors, asymmetric and the chlorination strategies not only successfully reduced the energy levels of E-IFC-IC-Cl to match well with PM6, but also the absorption redshift is conducive to achieving high Jsc of the devices.

Fig. 5: Chemical structures and terminal-group redistribution of asymmetric NFAs.
figure 5

a Chemical structures of E-IFC-IC-Cl and Z-IFC-IC-Cl. b Schematic diagram of terminal-group redistribution (TR). A1 and A2 represent heterogeneous electron-deficient terminal groups, D represents electron-rich core. 19F NMR spectra of c E-IFC-IC-Cl and d Z-IFC-IC-Cl fresh (0 h) and aged (48 h) solutions in CDCl3. e 19F NMR spectra of PM6:E-IFC-IC-Cl and PM6:Z-IFC-IC-Cl blend films after 14 days storage.

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As for the F···H interaction, NMR and single crystal measurements (Figs. S65–S68 and Table S12) confirm it only exists in E-IFC-IC-Cl, similar to the case of the symmetric acceptor. Benefited from the intramolecular F···H interactions, E-IFC-IC-Cl exhibits better chemical and photochemical stability relative to Z-IFC-IC-Cl (Fig. S69). These findings further provide definitive evidence for the crucial role of F···H interaction in enhancing the material stability. However, an easily overlooked problem is that the terminal-group redistribution would adversely affect the stability of OSC devices based on asymmetric NFAs (Fig. 5b)42,43. We speculate that the F···H interaction in E-IFC-IC-Cl can anchor the exocyclic double bond and then suppress the terminal-group redistribution. To verify this effect, the fresh and aged asymmetric solutions are conducted to 1H and 19F NMR measurements. In the 48-h-aged E-IFC-IC-Cl solution, no obvious change of either 1H NMR (Fig. S70) or 19F NMR (Figs. 5c and S71) spectra is observed, suggesting no product of terminal-group redistribution (TR). Comparatively, in aged solution of Z-IFC-IC-Cl, fresh fluorine signals attributed to TR product appear after only 2 h, and these signal intensity and integration areas are significantly increased over time (Figs. 5d, S72 and S73), indicating that undesirable redistribution of terminal groups occurred in Z-IFC-IC-Cl, resulting in the formation of symmetric products Z-2IFC-Cl and BTP-eC9. Meanwhile, the 1H NMR spectra of Z-IFC-IC-Cl at 0 h and 48 h are carefully analyzed, and the characteristic proton signal at 9.18 ppm in the aged solution indicates the generation of terminal-group-redistribution product BTP-eC9 (Fig. S74). These results demonstrate that the F···H interaction in E-IFC-IC-Cl can significantly inhibit the redistribution of terminal groups in the asymmetric acceptor solution. To verify the stability in the solid state, the potential terminal-group redistribution in blend (with PM6) films is studied. As shown in Figs. 5d and S75, the 19F signal of TR product only appears in the 14-day-aged PM6:Z-IFC-IC-Cl blend film, whereas the PM6:E-IFC-IC-Cl blend film remains stable. These results further verify that the F···H interaction is still effective to suppress the redistribution of terminal groups in the asymmetric NFA within solid-state films.

To characterize the morphology and packing order of asymmetric NFAs, AFM and GIWAXS measurements are conducted to E-IFC-IC-Cl and Z-IFC-IC-Cl neat films. Both acceptors exhibit particle-aggregated morphologies but show distinct surface roughness (Fig. S76). The E-IFC-IC-Cl shows a larger Rq value compared to its Z-counterpart, indicating enhanced crystallinity and more pronounced aggregation. GIWAXS results demonstrate that E-IFC-IC-Cl film exhibits tighter π-π stacking and stronger crystallinity than Z-IFC-IC-Cl one, although two asymmetric acceptors adopt face-on molecular orientations (Fig. S77 and Table S13). These structural advantages originate from F···H interaction that enhances molecular planarity, thereby facilitating exciton dissociation and charge transport.

The photovoltaic properties, including efficiency and stability, are further measured to confirm the superiority of the designed acceptors. The detailed optimized processes are summarized in Tables S14–S16. The J-V curves of optimized OSCs based on PM6:NFAs are plotted in Fig. 6a, and the corresponding device parameters are summarized in Table 1. Taking the reported acceptor BTP-eC9 as the symmetric counterpart, the PM6:Z-IFC-IC-Cl-based device performs an obviously higher Voc of 0.911 V, but the Jsc and FF are significantly decreased, resulting in an inferior PCE of 13.74%. Significantly, the E-isomer-based device achieves a champion PCE of 19.31%, attributed to the synergistical improvements of a Voc of 0.917 V, a Jsc of 26.41 mA cm−2, as well as a surprising FF of 79.72%. The significant disparity of PCEs based on two isomers is mainly due to the differences in Jsc and FF values, which may originate from the intramolecular F···H interaction leading to different packing behaviors. As shown in the EQE spectra (Fig. 6b), the integrated Jsc values obtained from EQE measurement for BTP-eC9, Z-IFC-IC-Cl- and E-IFC-IC-Cl-based devices are 26.78, 22.01 and 25.38 mA cm−2, respectively. Furthermore, the E-IFC-IC-Cl-based device displays a stronger photo-response ability ranging from 350 to 880 nm than the Z-IFC-IC-Cl device, which may be benefited from intramolecular F···H interaction and more ordered molecular packing. Compared with the BTP-eC9-based device, the asymmetric acceptors with fluoroacetonitrile exhibit significantly higher Voc values. Thus, the Eloss of the corresponding devices is determined to understand the Voc differences, and the detailed parameters of Eloss are shown in Table S17. As shown in Fig. 6c, three optimized devices exhibit the similar ΔE1 values of 0.264, 0.265 and 0.261 eV. The ΔE2 of PM6:E-IFC-IC-Cl device is lower than the PM6:Z-IFC-IC-Cl and PM6:BTP-eC9-based device, indicating faster charge transfer. More importantly, the E-isomer-based device displays the lowest ΔE3 value of 0.217 eV than that of Z-isomer (0.222 eV) and BTP-eC9 (0.238 eV) based devices (Fig. S78). Briefly, E-IFC-IC-Cl-based device shows a reduction in non-radiative energy loss and lower charge separation driving force, guaranteeing the small Eloss and high Voc.

Fig. 6: Photovoltaic performance of PM6:asymmetric acceptors-based devices.
figure 6

a J-V curves, b corresponding EQE spectra, c energy loss analysis and d photocurrent density versus Veff of the optimal OSCs based on PM6:asymmetric acceptors. The Jsccal represents integrated photocurrent density. The ηdiss and ηcoll represent the probability of exciton dissociation and charge collection, respectively. ∆E1 and ∆E2 are related to radiative recombination above and below the bandgap, respectively. ∆E3 is non-radiative recombination loss. e Electron and hole mobilities of PM6:asymmetric acceptors-based blend films calculated from space charge limited current (SCLC) mode. f Irradiation stability of the optimized OSCs based on PM6:asymmetric acceptors. Herein, the reported acceptor BTP-eC9 with a symmetric configuration is used as reference.

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To further investigate the disparities in the Jsc values of the PM6-based devices, charge dissociation, transport, and recombination properties are comprehensively analyzed. The PM6:Z-IFC-IC-Cl-based device shows the ηdiss/ηcoll values about 91.39%/68.97%, whereas the E-IFC-IC-Cl (97.23%/88.18%) and BTP-eC9 (96.46%/88.78%) based devices perform higher ηdiss/ηcoll values, contributing to the higher Jsc and FF (Fig. 6d). This highlights better efficiency in exciton dissociation and charge collection in E-IFC-IC-Cl-based device conducts to efficient charge transport. Thus, the μe/μh of PM6:E-IFC-IC-Cl, PM6:Z-IFC-IC-Cl and PM6:BTP-eC9-based blend films are measured by the SCLC method. The calculated μe/μh are 6.10/4.46 × 10−4, 8.49/7.57 × 10−4 and 8.06/7.12 × 10−4 cm2 s−1 V−1 for Z-IFC-IC-Cl, E-IFC-IC-Cl and BTP-eC9-based devices, respectively, and the E-IFC-IC-Cl-based device exhibits a more balanced μe/μh ratio of 1.12 (Figs. 6e and S79). These findings collectively suggest that the high and balanced charge carrier mobility can facilitate efficient charge transport in the blend film and retain high FF and Jsc values in E-IFC-IC-Cl-based devices. To investigate the impact of the suppressive redistribution of terminal groups through intramolecular F···H interaction on the photostability of OSCs. The normalized PCE decay curves for devices based on PM6:E-IFC-IC-Cl, PM6:Z-IFC-IC-Cl and PM6:BTP-eC9 are depicted in Fig. 6f. The results revealed a clear correlation between suppressive redistribution of terminal groups and device stability. The device based on E-IFC-IC-Cl exhibits higher stability, remaining 82.07% of its initial PCE after 340 h of continuous light exposure. In contrast, the Z-IFC-IC-Cl and BTP-eC9-based devices maintain 46.71% and 59.60% of its initial PCE. The enhanced stability of the device based on PM6:E-IFC-IC-Cl could be attributed to intramolecular F···H interaction to inhibit terminal-group redistribution.

To elucidate the impact of F···H interactions in asymmetric NFAs on the active layer microstructure and crystalline properties, the GIWAXS and AFM of PM6:E-IFC-IC-Cl and PM6:Z-IFC-IC-Cl blend films are also measured (Figs. S80, S81 and Table S18). Notably, the PM6:E-IFC-IC-Cl blend film exhibits tighter π-π stacking, more ordered packing, and well-defined fibrous network morphology with enlarged roughness relative to the Z-isomer counterpart. The optimized morphology in the E-isomer blend facilitates more efficient exciton dissociation and charge transport, ultimately contributing to the improved device performance.

Discussion

In summary, a fluoroacetonitrile-containing terminal group IFC is developed to mitigate the instability of exocyclic vinyl linkages in NFAs. When replacing the widely used malononitrile group with the fluoroacetonitrile group, an isomeric issue is raised to cause E/Z configurations. Initially, the terminal groups are synthesized with moderate regioselectivity by the conventional method, and then tedious purification is essential to separate these two isomeric intermediates for the subsequent preparation of the isomerically pure acceptors. Then, a synthetic strategy is developed by taking advantage of the intramolecular F···H interaction to avoid the annoying isomerism. As a result, cyano group substitution occurs exclusively at the preferred fluorine atom, while the other fluorine atom is restricted by the F···H interaction, thus providing isomerically pure E configuration. Due to the moderate electron-withdrawal ability of the terminal group IFC, the symmetric acceptors with two IFC groups possess matchable energy levels with the low-cost polymer donor P3HT, while the E/Z isomers exhibit distinct photovoltaic performance. The F···H interaction enhances molecular planarity and dipole moment of the isomer E-2IFC-F, and thus improves the molecular packing and film morphology, eventually achieving a PCE of 11.77% which is the highest efficiency among P3HT-based OSCs. More importantly, the F···H interaction reduces the reactivity of the exocyclic bond, thereby improving the chem and photo-stability of E-2IFC-F and OSC devices. Furthermore, chlorination and the asymmetric strategy are further engaged in regulating the absorption and energy levels. Thus, the asymmetric acceptor with E configuration realizes a PCE of 19.31% in the device incorporating the PM6 donor. As expected, the F···H interaction completely suppresses the terminal-group redistribution, consequently delivering better stability of OSC devices. Briefly, this work not only offers insights into the chemoselective synthesis of organic semiconductors through the utilization of noncovalent interactions but also provides a paradigm of high-efficiency, low-cost and stable OSCs.

Methods

Synthesis of the symmetric acceptor E-2IFC-F

Method I: A dried flask was charged with BTP-2CHO (108 mg, 0.100 mmol), E-IFC-F (49.1 mg, 0.220 mmol), acetic anhydride (0.100 mL), BF3·Et2O (62.0 μL, 0.500 mmol) and toluene (10.0 mL). The reaction mixture was stirred at room temperature until completion. It was then poured into MeOH (50.0 mL), yielding a blue suspension. The precipitate was collected by filtration and further purified by silica gel column chromatography using a mixture of dichloromethane and hexane (1:2, v/v) as the eluent to afford the pure product E-2IFC-F as a brown solid (yield 89.1%, 133 mg).

Method Ⅱ: Under an argon atmosphere, a dried Schleck tube was charged with 2I2F-F (148 mg, 0.100 mmol), K3PO4 (6.40 mg, 0.030 mmol) and anhydrous DMSO (30.0 mL) and toluene (6.00 mL). TMSCN (59.6 mg, 0.600 mmol) was then added dropwise to the mixture. The reaction was stirred at 80 °C and monitored by TLC until complete consumption of 2I2F-F was observed. After completion, the mixture was cooled to room temperature, quenched with water and extracted with chloroform. The combined organic layers were washed with brine and dried over anhydrous Na2SO4, and concentrated under reduced pressure. The crude product was purified by flash column chromatography on silica gel using a mixture of dichloromethane and hexane (1:2, v/v) as the eluent. The E-2IFC-F was afforded as a brown solid (yield 65.9%, 98.5 mg), and the isomeric Z-2IFC-F was not obtained.

1H NMR (400 MHz, chloroform-d) δ (ppm) 8.44 (d, J = 7.5 Hz, 2H), 8.16–8.12 (m, 2H), 7.72 (t, J = 7.7 Hz, 2H), 4.85–4.67 (m, 4H), 3.10 (t, J = 7.8 Hz, 4H), 2.20–2.07 (m, 2H), 1.86 (t, J = 7.7 Hz, 4H), 1.50–0.87 (m, 56H), 0.86–0.59 (m, 18H). 13C NMR (101 MHz, chloroform-d) δ (ppm) 186.60, 156.03, 155.88, 154.92, 154.55, 154.39, 153.45, 153.31, 151.97, 149.72, 147.65, 144.28, 137.64, 136.55, 136.46, 136.44, 134.03, 134.00, 133.97, 133.44, 132.37, 132.26, 130.74, 130.58, 130.42, 128.44, 128.41, 126.72, 124.28, 117.16, 116.73, 116.54, 116.35, 114.25, 113.80, 113.05, 112.50, 112.32, 55.73, 39.20, 32.01, 31.71, 31.04, 30.63, 30.51, 30.45, 29.93, 29.85, 29.71, 29.62, 29.59, 29.55, 29.50, 29.44, 28.17, 28.00, 25.55, 25.36, 23.00, 22.96, 22.80, 22.62, 22.60, 14.24, 14.14, 14.12, 13.90, 13.86. 19F NMR (376 MHz, chloroform-d) δ (ppm) −124.09 (2F), −129.45 (2F), −130.76 (2F). MS (MALDI-TOF) m/z: calculated for C84H94F6N6O2S5 [M+], 1492.595; found, 1492.577.

Synthesis of the symmetric acceptor Z-2IFC-F

Z-2IFC-F refers to the synthesis of E-2IFC-F (Method I) and replaces E-IFC-F with Z-IFC-F (49.1 mg, 0.220 mmol). The pure product Z-2IFC-F was obtained as a brown solid (yield 88.3%, 131 mg). 1H NMR (400 MHz, chloroform-d) δ (ppm) 8.57 (s, 2H), 7.86 (m, 2H), 7.72 (t, J = 7.7 Hz, 2H), 4.87–4.65 (m, 4H), 3.18 (t, J = 7.7 Hz, 4H), 2.20–2.07 (m, 2H), 1.87 (m, 4H), 1.55–0.85 (m, 56H), 0.85–0.47 (m, 18H). 13C NMR (101 MHz, chloroform-d) δ (ppm) 185.86, 156.01, 155.87, 154.24, 153.44, 153.30, 151.82, 151.67, 149.09, 147.64, 135.03, 135.02, 134.69, 133.54, 132.67, 130.06, 128.54, 125.21, 121.11, 114.09, 113.57, 113.10, 112.86, 112.67, 112.52, 112.31, 55.70, 39.19, 31.99, 31.70, 30.78, 30.60, 30.49, 30.43, 29.91, 29.64, 29.60, 29.58, 29.54, 29.49, 29.42, 28.16, 27.98, 25.51, 25.32, 22.99, 22.94, 22.80, 22.61, 22.59, 14.23, 14.13, 14.11, 13.90, 13.86. 19F NMR (376 MHz, chloroform-d) δ (ppm) −124.64 (2F), −128.58 (2F), −130.68 (2F). MS (MALDI-TOF) m/z: calculated for C84H94F6N6O2S5 [M+], 1492.595; found, 1492.590.

Synthesis of the asymmetric acceptor E-IFC-IC-Cl

The synthesis of E-IFC-IC-Cl refers to the synthesis of E-2IFC-F (Method Ⅱ). Under an argon atmosphere, a dried Schleck tube was charged with I2F-IC-Cl (156 mg, 0.100 mmol), K3PO4 (6.40 mg, 0.030 mmol) and anhydrous DMSO (30.0 mL) and toluene (6.00 mL). TMSCN (29.8 mg, 0.600 mmol) was then added dropwise to the mixture. The reaction was stirred at 80 °C until 2I2F-F disappeared. After completion, the mixture was cooled to room temperature, and then quenched with water and extracted with chloroform. The organic layers were washed with brine, dried over anhydrous Na2SO4, and concentrated under reduced pressure. The residue was purified by flash column chromatography on silica gel using a mixture of dichloromethane and hexane (1:2, v/v) as the eluent to obtain pure product E-IFC-IC-Cl as a brown solid (yield 58.3%, 91.3 mg). 1H NMR (400 MHz, chloroform-d) δ (ppm) 9.18 (s, 1H), 8.80 (s, 1H), 8.47 (d, J = 2.5 Hz, 1H), 8.40 (s, 1H), 7.99 (s, 1H), 7.95 (s, 1H), 4.81–4.71 (m, 4H), 3.26–3.22 (m, 2H), 3.13–3.09 (m, 2H), 2.14–2.08 (m, 2H), 1.92–1.82 (m, 4H), 1.49–0.86 (m, 56H), 0.85–0.50 (m, 18H). 13C NMR (101 MHz, chloroform-d) δ (ppm) 186.61, 186.32, 158.91, 154.36, 149.81, 147.73, 147.59, 145.17, 144.79, 139.52, 139.36, 139.16, 138.91, 138.67, 138.23, 137.36, 136.93, 136.90, 136.84, 136.22, 136.02, 135.87, 135.06, 133.56, 132.99, 132.89, 132.15, 131.63, 130.92, 130.58, 130.43, 129.23, 128.52, 127.00, 125.28, 125.01, 119.64, 117.53, 115.30, 114.83, 114.25, 114.03, 112.93, 68.44, 55.80, 39.28, 32.01, 31.75, 31.39, 31.05, 30.61, 30.50, 29.98, 29.95, 29.74, 29.61, 29.45, 28.15, 28.00, 25.59, 25.42, 22.98, 22.94, 22.81, 22.65, 22.62, 14.26, 14.19, 14.17, 13.92, 13.88. 19F NMR (376 MHz, chloroform-d) δ (ppm) −129.30 (1F). MS (MALDI-TOF) m/z: calculated for C84H94Cl4FN7O2S5 [M+], 1563.481; found, 1563.313.

Synthesis of the asymmetric acceptor Z-IFC-IC-Cl

A dried flask was charged with BTP-2CHO (108 mg, 0.100 mmol), Z-IFC-Cl (31.0 mg, 0.120 mmol), acetic anhydride (0.100 mL) and toluene (10.0 mL). Then BF3·Et2O (62.0 μL, 0.500 mmol) was added to the solution. The reaction was stirred at room temperature until complete consumption of BTP-2CHO was observed. Subsequently, IC-Cl (26.2 mg, 0.100 mmol) was introduced, and stirring continued for an additional 30 min. The mixture was poured into MeOH to afford a dark suspension. The precipitate was collected by filtration and further purified by flash column chromatography on silica gel using a mixture of dichloromethane and hexane (1:2, v/v) as the eluant to give the pure product Z-IFC-IC-Cl as a brown solid (yield 64.5%, 101 mg). 1H NMR (400 MHz, chloroform-d) δ (ppm) 9.18 (s, 1H), 8.79 (s, 1H), 8.60 (s, 1H), 8.14 (s, 1H), 7.99 (s, 1H), 7.95 (s, 1H), 4.77–4.75 (m, 4H), 3.26–3.16 (m, 4H), 2.15–2.11 (m, 2H), 1.91–1.85 (m, 4H), 1.51–0.88 (m, 56H), 0.84–0.62 (m, 18H). 13C NMR (101 MHz, chloroform-d) δ (ppm) 186.35, 185.90, 158.99, 154.36, 149.25, 147.73, 147.62, 147.29, 145.60, 145.19, 144.77, 139.57, 139.38, 139.20, 139.09, 138.92, 137.40, 136.24, 135.96, 133.59, 133.42, 133.07, 127.03, 125.62, 124.84, 124.52, 123.62, 119.75, 119.17, 118.99, 113.96, 113.00, 77.48, 77.16, 76.84, 68.53, 55.77, 39.28, 35.13, 34.59, 32.01, 31.99, 31.74, 31.65, 31.40, 30.80, 30.59, 30.46, 30.28, 29.99, 29.92, 29.85, 29.61, 29.49, 29.45, 29.43, 28.14, 27.97, 25.56, 25.37, 22.96, 22.92, 22.81, 22.64, 22.61, 14.26, 14.17, 13.91, 13.87. 19F NMR (376 MHz, chloroform-d) δ (ppm) −128.58 (1F). MS (MALDI-TOF) m/z: calculated for C84H94Cl4FN7O2S5 [M+], 1563.481; found, 1563.730.

Single crystal

The needle-like diffraction quality crystals were obtained through the gas-liquid diffusion method. E-2IFC-F, Z-2IFC-F and E-IFC-IC-Cl were dissolved severally in CHCl3 at a concentration of 1.00 mg mL−1, and then placed in a 2.00 mL vial, which was then placed in a 20.0 mL brown vial containing 5.00 mL methanol. These systems were tightly sealed and left standing for a few days (3–5 days) until needle-like crystals emerged.

Reporting summary

Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.