A–D–A'–D–A type nonfused ring electron acceptors for efficient organic solar cells via synergistic molecular packing and orientation control

Nonfused ring electron acceptors (NFREAs) are promising candidates for future commercialization of organic solar cells (OSCs) due to their simple synthesis. Still, the power conversion efficiencies (PCEs) of NFREA‐based OSCs have large room for improvement. In this work, by merging end group halogenation and side chain engineering, we developed four A–D–A'–D–A type NFREAs, which we refer to as EH‐4F, C4‐4F, EH‐4Cl, and C4‐4Cl. Single crystal X‐ray diffraction revealed that multiple intermolecular S···F interactions between cyclopentadithiophene and 5,6‐difluoro‐3‐(dicyanomethylene)indanone could cause an unfavorable dimer formation, leading to ineffective π–π stackings in EH‐4F and C4‐4F, whereas no such dimer was found in EH‐4Cl and C4‐4Cl after replacing with 5,6‐dichloro‐3‐(dicyanomethylene)indanone. Moreover, although the shorter n‐butyl side chain resulted in a closer molecular packing in C4‐4Cl, EH‐4Cl (2‐ethylhexyl substitution) with proper crystallinity exhibited enhanced face‐on orientation in thin film, which is favorable for vertical charge transport and further reducing charge recombination. As a result, a PCE of 13.0% is obtained for EH‐4Cl‐based OSC with a fill factor of 0.70. This work highlights the importance of molecular packing and orientation control toward future high‐performance A–D–A'–D–A type NFREAs.

[25] In 2017, Liu et al. exploited a noncovalently conformational locking design strategy based on FREA by inserting conformational control units between end groups and central core to form O⋅⋅⋅H−C and S⋅⋅⋅O C intramolecular interactions. [26]Soon afterwards, Li et al. developed a A-D-D'-D-A type NFREA, namely, DP-PCIC, where a nearly planar geometry was realized through noncovalent F⋅⋅⋅H−C intramolecular interactions, yielding a PCE of 10.14%. [27]Encouragingly, the recent PCEs of A-D-D'-D-A type NFREA-based OSCs have exceeded 16%, [28,29] further narrowing the PCE gap between NFREAs and the state-of-the-art FREAs.
[32][33] In 2020, our group reported two A-D-A'-D-A type NFREAs possessing benzothiazole (BT) as electron-deficient central A' core and extended the absorption range up to 946 nm. [31]Later in 2021, Zhang et al. synthesized a A-D-A'-D-A type NFREA based on the central A' core of alkoxy-substituted benzotriazole, namely, NoCA-5, and the photovoltaic device obtained a certified PCE of 14.5%. [34]37] On the one hand, ffBTz, with moderate electron-withdrawing ability, endows shallow-lying lowest unoccupied molecular orbital (LUMO) energy levels of NFREAs, which is beneficial for realizing higher open circuit voltage (V oc ).On the other hand, the single crystal of ffBTz-based model compound revealed that the noncovalent S⋅⋅⋅F intramolecular interactions could be introduced to maintain the molecular backbone planarity, thus promoting more compact and ordered stacking patterns in solid state. [36]Moreover, the side chains on central ffBTz core provide the opportunity to readily adjust material solubility, crystallinity, and even molecular orientation. [38][50] It is found that structural modifications of small molecular acceptors could profoundly impact molecular packing arrangement.For example, Li et al. systematically studied the effect of different side chains on the crystal packing of Y-series FREAs. [11]They found that the branched 2-butyloctyl-modified L8-BO could form a molecular packing arrangement with small ellipse-shaped voids, whereas the linear undecyl-modified Y6 formed orthorhombic vacancies.Thus, a higher packing coefficient and more π-π packing motifs of L8-BO lead to a higher electron mobility than Y6.[53][54] Ma et al. reported a NFREA single crystal of A4T-16 and found that a three-dimensional interpenetrating network could be formed due to the compact π-π stacking between adjacent end groups. [55]Recently, Zheng et al. investigated the chalcogen atom effect on single crystals of A-D-D'-D-A type NFREA. [52]They revealed the sulfur atom-based TT-S-2F not only has more planar molecular skeleton but also the largest π-π stacking area between end group and thiophene bridge unit, which facilitates TT-S-2F with more efficient electron transport.Compared with other small molecular acceptors, studies regarding the crystalline behaviors of A-D-A'-D-A type NFREAs have been rarely reported, which inevitably limits the understanding of this kind of emerging acceptor materials.
Based on the above discussion, we designed and synthesized four A-D-A'-D-A type NFREAs, namely, EH-4F, C4-4F, EH-4Cl, and C4-4Cl (Figure 1A) via end group halogenation and side chain engineering.Furthermore, we successfully obtained the single crystals of this kind of NFREAs for the first time.According to single crystal X-ray diffraction results, multiple intermolecular S⋅⋅⋅F interactions between cyclopentadithiophene (CPDT) and 5,6-difluoro-3-(dicyanomethylene)indanone (IC-2F) were observed in EH-4F and C4-4F, thus two molecules in the same plane can form a dimer in single crystals.The branched side chains on dimer protrude out from molecular backbone and increase the intermolecular steric hindrance, which resulted in ineffective intermolecular π-π stackings.On the contrary, EH-4Cl and C4-4Cl with 5,6-dichloro-3-(dicyanomethylene)indanone (IC-2Cl) end group, their molecular packing arrangements have been improved with more packing motifs and become more ordered due to the lack of above-mentioned dimer formation.Furthermore, the modification of side chain is found to play a crucial role in tuning molecular crystallinity and orientation.Although C4-4Cl (n-butyl substitution) possessed the strongest crystallinity and tightest molecular packing arrangement, thin films based on EH-4Cl (2-ethylhexyl substitution) exhibited favorable face-on orientation, which is beneficial for vertical charge transport and suppressing charge recombination.Thus, by using the prevailing polymer PM6 as donor material, the EH-4Cl-based OSC delivered a PCE of 13.0%, which is higher than the other three acceptors.This work not only unveils the structure-performance relationship of A-D-A'-D-A type NFREAs from crystallography and morphology perspectives but also highlights the importance of molecular packing arrangement and orientation control toward the future high-performance A-D-A'-D-A type NFREA molecular design.

RESULTS AND DISCUSSION
Figure 1A displays the chemical structures of EH-4F, C4-4F, EH-4Cl, and C4-4Cl.The synthetic routes (Scheme S1) and experimental methods are described in Figures S1-S18.Thermogravimetric analysis suggests all NFREAs own excellent thermal stabilities with decomposition temperature (T d , 5% weight loss) over 350 • C (Figure S19 and Table S1).From differential scanning calorimetry results (Figure S20 and Table S1), EH-4F and C4-4F exhibit similar melting temperatures (T m s) of 275 and 278 • C, while the T m is increased to 296 • C for EH-4Cl.Notably, no melting peak was observed for C4-4Cl, probably due to the decomposition of C4-4Cl before its melting.This indicates that end group halogenation and side chain engineering could synergistically affect intermolecular interactions and control molecular ordering.In particular, the stronger crystallinity is accompanied by the IC-2Cl end group as well as the short linear n-butyl side chain.The normalized ultraviolet-visible-near-infrared (UVvis-NIR) absorption spectra are shown in Figure 1B and Figure S21.In chloroform solutions, side chains have a negligible impact but the absorption peaks of EH-4Cl and C4-4Cl are both red-shifted by approximately 10 nm compared with EH-4F and C4-4F, which can be ascribed to the slightly stronger electron-withdrawing ability of IC-2Cl end group.From solution to solid state, the onsets of absorption are all red-shifted, corresponding to the optical bandgaps (E g opt s) of 1.47 eV for EH-4F, 1.45 eV for C4-4F, 1.44 eV for EH-4Cl, and 1.43 eV for C4-4Cl, respectively.The higher and steeper A 0-0 absorption peak suggests the more ordered molecular packing in the solid state. [56]Notably, the highest A 0-0 peak of C4-4Cl implies the strongest intermolecular interactions in thin film.Electrochemical square wave voltammetry (SWV) measurements (Figure S22) were performed to characterize the highest occupied molecular orbital (HOMO) and LUMO energy levels.The energy levels of donor material PM6 (Figure S23) and four acceptors are depicted in Figure 1C.Compared with EH-4F and C4-4F, the chlorinated EH-4Cl and C4-4Cl exhibited slightly lower LUMO energy levels, while the side chain engineering has less effect on energy levels.
Single crystal with high purity and low density of defects can provide an ideal platform to study the intrinsic physical properties of materials.Here, four diffraction-quality single crystals of EH-4F, C4-4F, EH-4Cl, and C4-4Cl were successfully cultivated.The growth methods and crystal data (Table S2) are shown in the Supporting Information.First, the single crystalline structure differences were compared (Figure S24).Multiple intramolecular noncovalent interactions can be observed, such as the intramolecular S⋅⋅⋅O═C conformational locks.Notably, the central ffBTz core played a significant role.Taking EH-4Cl as an example, the distances of S⋅⋅⋅F (2.79 Å), F⋅⋅⋅H−C (2.30 Å), N⋅⋅⋅H−C (2.35 Å), and S⋅⋅⋅N (2.83 Å) between ffBTz core and CPDT units are all smaller than the sum of van der Waals radii (R w s) of S⋅⋅⋅F (3.27 Å), F⋅⋅⋅H−C (2.67 Å), N⋅⋅⋅H−C (2.75 Å), and S⋅⋅⋅N (3.35 Å).These conformational locks have endowed the small torsion angles (less than 6 • ), thus giving the essential molecular backbone planarity and good molecular rigidity of NFREAs.Moreover, EH-4F is axisymmetric with a C-shape molecular geometry, whereas C4-4F, EH-4Cl, and C4-4Cl present centrosymmetric S-shape molecular geometries.To understand the different molecular geometries, we first conducted potential energy scan (PES) of each NFREA to estimate the thermodynamic stabilities of C-and S-shape comformational geometries (Figure S25).Probably due to the similar molecular backbone, the scan curves of NFREAs exhibited little difference.Moreover, the energies of C-and S-shape conformational geometry were comparable.Then, by analyzing the intermolecular packing of EH-4F, it is found that the multiple intermolecular noncovalent hydrogen bond interactions could contribute to C-shape molecular geometry.As shown in Figure S26, the branched side chains on the CPDT units could interact with the adjacent molecules.Several noncovalent hydrogen bonds, such as S⋅⋅⋅H−C, N⋅⋅⋅H−C, and C⋅⋅⋅H−C, are formed.Such up and down pulling interactions finally lead EH-4F to an axisymmetric C-shape molecular geometry.
Having revealed the crystalline structures, we turn our attention to the intermolecular packing arrangement.EH-4F and C4-4F were first analyzed.For clarity, four molecules were intercepted as A (orange), B (violet), C (blue), and D (red) into one unit (Figure 2A,B).In EH-4F, molecules A and B are antiparallelly located in the upper layer, while molecules C and D are clustered in the lower layer (Figure 2C).Two π-π stacking interactions (3.38 and 3.41 Å) between end group and ffBTz core are observed in a unit, and a two-dimensional stacked structure is enabled by its C-shape configuration (Figure S27a,b).Similarly, in C4-4F, two π-π stackings between end group and ffBTz core are also observed but with reduced π-π distances of 3.35 Å (Figure 2D), implying the stronger intermolecular interactions after shortening the side chain length.And a two-dimensional brickwork molecular packing patterns are formed (Figure S27c,d  For example, in EH-4F, the fluorine atoms on the IC-2F have four strong intermolecular interactions with the sulfur atoms on CPDT units, as the intermolecular distances of 2.97 and 3.00 Å.By anchoring the shorter n-butyl substitution, C4-4F adopts a similar dimer.However, due to the S-shape molecular geometry, the dimer is smaller and with more interactions.The presented unique dimer in EH-4F and C4-4F, where two molecules are in the same plane and eight branched 2-ethylhexyl side chains on four CPDT units can protrude out from the molecular backbone, could lead to the large intermolecular distances between the dimer and the neighboring molecules (Figure S28).Such molecular packing arrangements would cause the dislocation of vertically stacked molecules and prohibit the sufficient formation of π-π stackings, which is inferior to charge transport and detrimental to device performance.Since the smaller dimer in C4-4F, the less steric hindrance could mitigate this side effect to some extent.
In EH-4Cl and C4-4Cl, likewise, four molecules were intercepted into one unit (Figure 3A,B).However, no abovementioned dimer was observed due to the absence of S⋅⋅⋅F intermolecular interactions observed in EH-4F and C4-4F, and the dislocation of vertically stacked molecules is suppressed.In EH-4Cl, two pairs and four π-π stacking interactions (3.45 and 3.49 Å) between end group and ffBTz core are found (Figure 3C).After shortening side chain length, the intermolecular distances in C4-4Cl become closer, and the shortest π-π stacking distance is measured as only 3.27 Å (Figure 3D), which represents one of the tightest molecular packing systems among small molecular acceptors. [41]Notably, another two π-π stacking motifs between end groups (3.40 Å) are found in C4-4Cl due to the improved intermolecular stacking arrangement.The more packing motifs could provide more charge-hopping channels and is advantageous to enhance device performance. [11]Both acceptors exhibited more ordered two-dimensional brickwork molecular packing pattern (Figure S27e Furthermore, grazing incidence wide angle X-ray scattering (GIWAXS) was employed to explore the molecular packing and orientation of neat films. [57]The corresponding two-dimensional scattering patterns and one-dimensional line-cut profiles are shown in Figure 4A,B, and the detailed parameters are summarized in Table S3.For the EH-4F, C4-4F, and EH-4Cl neat films, the notable (010) diffraction peaks are recorded at 1. F I G U R E 4 (A) Two-dimensional grazing incidence wide angle X-ray scattering (GIWAXS) patterns and (B) one-dimensional GIWAXS line-cut profiles of neat films of the nonfused ring electron acceptors (NFREAs).For the line-cut profiles, solid line depicts out-of-plane and dash line depicts in-plane.
The photovoltaic performance of the NFREAs was evaluated using conventional device architecture of ITO/PEDOT:PSS/active layer/PFN-Br/Ag.Considering the absorption spectra and energy level matching (Figure 1B,C), the prevailing polymer PM6 was chosen as electron donor to fabricate OSCs. [59]The devices were fully optimized as displayed in Tables S4-S8.The current density-voltage (J-V) characteristics of the optimal devices are shown in Figure 5A and the photovoltaic parameters are summarized in Table 1.EH-4F offered a high V oc of 0.87 V but a modest PCE of 8.8% due to the relatively low J sc (16.5 mA cm −2 ) and FF (0.61).While both J sc and FF were increased to 19.2 mA cm −2 and 0.62 in the n-butyl-modified C4-4F-based device, getting a higher PCE of 10.3%.On the other hand, NFREAs based on IC-2Cl end group achieved better performance.As for C4-4Cl, though the reduced V oc (0.83 V), a decent J sc (21.4 mA cm −2 ) and an FF (0.66) resulted in 11.7% efficiency.Yet, it is worth noting that EH-4Cl delivered a champion PCE of 13.0%, as the result of both high J sc of 21.4 mA cm −2 and FF of 0.70.The external quantum efficiency (EQE) spectra (Figure 5B) of all optimized devices showed the photo-response in the wavelength range of 300-850 nm, which is consistent with the blend film absoprtion spectra (Figure S30).Especially, the EQE values of devices F I G U R E 5 (A) J-V curves, (B) external quantum efficiency (EQE) spectra, (C) J ph as a function of V eff , (D) J sc as a function of P light , (F) V oc as a function of P light , and (E) detailed energy loss of the optimized organic solar cells (OSCs) based on the nonfused ring electron acceptors (NFREAs).

Active layer
V oc a (V) J sc a (mA cm −2 ) based on EH-4Cl and C4-4Cl are over 70% (within the range of 550-810 nm), indicating the efficient photon-to-electron conversion processes.The integrated J sc from EQE spectra agreed well with the J sc values from J-V curves within 5% mismatch.Moreover, we also tested the storage and thermal stabilities of devices in a dark, nitrogen-filled glovebox at room temperature and under continuous thermal stress at 65 • C (Figures S31 and S32).It is noted that the device performance during storage showed degradation mainly due to the decline of V oc and FF value, while the J sc value were kept the same as the initial one even after 10 days.This result implies that the excellent stability of active layer, [60] yet more studies should be done in the future on the stability of interlayer and device structure.Next, we studied the charge transfer process through photoluminescence (PL) quenching experiments (Figures S33-S34 and Table S9).The PM6 emissions were nearly quenched in four blend films with quenching efficiencies over 96%, suggesting the efficient electron transfer process from donor to acceptor.However, the quenching efficiencies of acceptor in blend films varied from each other, as EH-4F (74.6%),C4-4F (78.3%),EH-4Cl (88.9%), and C4-4Cl (81.9%), respectively.Additionally, we performed time-resolved photoluminescence (TRPL) test of neat and blend films (Figure S35 and Table S10).It is shown that end group chlorination and decreased alkyl chain length could shorten the fluorescence lifetimes of acceptors, as 1.15/1.11ns for EH-4F/C4-4F and 1.09/0.94ns for EH-4Cl/C4-4Cl.Furthermore, the quantum yield of the formation of the charge-transfer state (η CT ) can be also extracted from the PL lifetimes. [61]As displayed in Table S10, the EH-4Clbased blend film obtained the highest η CT value (50.61%) than other films.The above results indicate that PM6:EH-4Cl as active layer possesses the most efficient charge transfer process.Then, to evaluate the exciton dissociation probability P(E, T) in solar cells, the photocurrent density (J ph ) versus effective voltage (V eff ) curves were characterized (Figure 5C).The P(E, T) were calculated to be 92.1%,93.7%, 96.0%, and 95.1% for devices based on EH-4F, C4-4F, EH-4Cl, and C4-4Cl, respectively.The hole mobility (μ h ) and electron mobility (μ e ) of active layers were measured from single-carrier devices by SCLC method (Figure S36).Among the four blends, PM6:EH-4Cl and PM6:C4-4Cl afford the higher charge carrier mobilities and the corresponding values (μ h /μ e ) come out to be 2.45 × 10 −3 /2.01 × 10 −3 and 2.09 × 10 −3 /1.57× 10 −3 cm 2 V −1 s −1 , respectively, which are one magnitude higher than the blends of PM6:EH-4F and PM6:C4-4F, as summarized in Table S9.The efficient charge transport in PM6:EH-4Cl and PM6:C4-4Cl blend films endowed the corresponding devices with the high J sc values.Moreover, the more balanced charge transport in PM6:EH-4Cl blend is beneficial to prohibiting charge accumulation and recombination, thus obtaining the highest FF.
Furthermore, the charge recombination behaviors of four devices were investigated through the dependence of J sc and V oc on light intensity (P light ) experiments. [62]Following the relation J sc ∝ P light α , bimolecular recombination could be negligible in device when the α tends to the limit 1.The high α values were achieved in PM6:EH-4Cl (0.995) and PM6:C4-4Cl (0.980), suggesting the suppressed bimolecular recombination in devices (Figure 5D).Besides, one could judge the dominant recombination type occurring in devices from the relation V oc ∝ nkT/qln(P light ), where k is the Boltzmann constant, T is the Kelvin temperature, and q is the elementary charge, as bimolecular recombination (if n is close to 1) or trap-assisted recombination (if n is close to 2).As presented in Figure 5E, the PM6:EH-4Cl device presents the lowest n value of 1.14, indicating that both monomolecular recombination and trap-assisted recombination have been suppressed.On the contrary, the device of PM6:EH-4F suffered from the highest degree of trap-assisted recombination (n = 1.55) and thus obtained the lowest J sc and FF.
To understand the varied V oc values, we conducted the energy loss analysis of the devices according to the previously reported methods (Figure 5F and Figure S37). [63]he total energy loss (∆E loss ) can be divided into the following three parts, that is, ∆E 1 is an inevitable energy loss above optical bandgap, and the four devices exhibited similar ∆E 1 values around 0.27 eV (Table S11).∆E 2 is an energy loss below optical bandgap and can be measured by Fourier-transform photocurrent spectroscopy-EQE.It is noted the lowest ∆E 2 value (0.044 eV) was obtained in C4-4Cl-based device.Usually, ∆E 2 value is related with their energetic disorder at the tailstate absorption and the degree of energetic disorder can be quantified with the Urbach energy (E U ) following the Urbach rule. [64]Thus, the values of E U s were calculated to be 29.1, 29.2, 28.8, and 28.0 meV for PM6:EH-4F, PM6:C4-4F, PM6:EH-4Cl, and PM6:C4-4Cl, respectively, implying IC-2Cl can regulate the ordered molecular packing arrangement and attenuate energetic disorder compared with IC-2F.∆E 3 is a nonradiative recombination energy loss, which can be obtained by external quantum efficiency of electroluminescence (EQE EL ) and calculated from ∆E 3 = −kTln(EQE EL ), where k is the Boltzmann constant and T is the Kelvin temperature.Devices based on EH-4Cl and C4-4Cl (0.256 and 0.259 eV) exhibited lower ∆E 3 values than those of EH-4F and C4-4F (0.271 and 0.264 eV), which demonstrates that introducing chlorine atoms into end group could enhance the electroluminescence efficiencies and reduce the nonradiative energy loss.
The morphology of active layer is of vital importance to device performance.GIWAXS was further employed to investigate the crystallization characteristics of four blend films (Figure 6A and Table S12).For comparison, the neat film of PM6 was also measured to distinguish the diffraction signals.A notable (100) diffraction peak at q = 0.33 Å −1 in OOP direction was recorded, suggesting a preferential edge-on orientation of PM6 (Figure S38).Upon mixing with acceptor, PM6:EH-4F and PM6:C4-4F blend films showed the (100) diffraction peaks in OOP direction at 0.33 Å −1 , which can be ascribed to the diffraction signals of PM6.Additionally, the (010) diffraction peaks in OOP direction (1.74 and 1.76 Å −1 , respectively) should belong to the acceptors.However, the intensities of (010) diffraction peaks have been relatively weakened compared with the neat acceptor films.This might be related to their molecular packing arrangements of acceptors.As we mentioned in single crystal section, the introduction of IC-2F end group can cause unfavorable dimer formation and increase intermolecular steric hindrance.Thus, the less fixed acceptor packing arrangements of EH-4F and C4-4F are more susceptible after introducing PM6, which makes it hard to keep the predominant face-on orientation in blend films.On the other hand, owing to the intrinsic edge-on and face-on orientations of neat C4-4Cl film, PM6:C4-4Cl blend film displayed a weakened (010) diffraction peak (q = 1.76 Å −1 with d = 3.57Å) in OOP direction and suppressed face-on orientation.Among them, PM6:EH-4Cl exhibited a well-defined (010) diffraction peak (q = 1.74 Å −1 ) in OOP direction with the large CCL value of 23.94 Å, implying the stacking characteristics of EH-4Cl has been maintained to maximum extent.Meanwhile, it is noteworthy that the intensity of (100) diffraction peak (q = 0.33 Å −1 ) in OOP direction was gentler compared with other blend films (Figure 6B), indicating the less proportion of edge-on orientation from PM6.The preferential face-on orientation of PM6:EH-4Cl blend film guarantees more efficient vertical charge transport, which is conducive for reducing charge recombination, thus contributing to the highest FF value of the corresponding device.Combining the results of single crystals and GIWAXS experiments, it is further verified that end group halogenation and side chain engineering are effective to synergistically modulate molecular packing arrangement and orientation of A-D-A'-D-A type NFREAs.
Next, the film morphology was investigated by atomic force microscopy (AFM), transmission electron microscopy (TEM), and contact angle characterization measurements.In Figure 6C, blend films based on EH-4Cl and C4-4Cl give smoother and more uniform morphologies with root-meansquare roughness of 3.48 and 2.77 nm, whereas blend films based on EH-4F and C4-4F exhibit larger values of 4.89 and 3.98 nm.The better surface contact with the interfacial layers guarantees the effective charge extraction, thus affording the higher current and FF value.The corresponding phase images were further confirmed by TEM characterization (Figure 6D).PM6:EH-4F blend film showed a larger phase separation with unclear boundaries between donor and acceptor domains.In contrast, more suitable phase separations with fibrillary networks were presented in other three blend films.To understand the origin of the phase separation, we analyzed the thermodynamic miscibility between PM6 and four NFREAs by calculating the Flory-Huggins interaction parameters (χ) and the corresponding miscibility parameters between donor and acceptor are displayed in Figure S39 and summarized in Table S13.The χ values were obtained from the empirical equation , where K is a positive constant, and γ D and γ A represent to the surface energy of polymer and the corresponding NFREA neat films, respectively.The χ value was decreased in the order of PM6:EH-4F (0.73 K) > PM6:C4-4F (0.64 K) > PM6:EH-4Cl (0.63 K) > PM6:C4-4Cl (0.61 K).The smaller χ value represents the better mixing between donor and acceptor.Thus, EH-4F shows the poorest miscibility with PM6, which is in good accordance with the largest surface roughness and phase separation in AFM and TEM result.Overall, the above results provide new insights from crystallography and morphology perspectives, and expand on the structure-performance relationship of A-D-A'-D-A type NFREAs.

CONCLUSION
In summary, by merging end group halogenation and side chain engineering, this work designed and synthesized four A-D-A'-D-A type NFREAs and systematically investigated their structure-performance relationship.It should be noticed that, for the first time, four single crystals of A-D-A'-D-A type NFREAs were successfully obtained and studied.
According to the results, IC-2F end groups could induce multiple noncovalent intermolecular S⋅⋅⋅F interactions with CPDT units and finally lead to a dimer formation in the same plane, whereas no such dimer is observed in IC-2Cl-based EH-4Cl and C4-4Cl single crystals.The large branched side chains of dimer could increase the intermolecular steric hindrance and suppress the effective intermolecular π-π stacking, which further limited the charge hopping channel.Besides end group halogenation, it is found alkyl side chain engineering could not only enhance the molecular crystallinity, but also affect the molecular orientation.Although the closer molecular packing presented in C4-4Cl (n-butyl substitution), EH-4Cl (2-ethylhexyl substitution) with proper crystallinity favors to π-π interact with PM6 and thus stacks in the more face-on manner on the substrate surface, which is favorable for the vertical charge transport and the suppression of charge recombination in OSCs.As a result, the highest PCE of 13.0% was obtained in device based on PM6:EH-4Cl.Overall, the above results demonstrate the importance of molecular packing arrangement as well as orientation control in A-D-A'-D-A type NFREAs.Meanwhile, the well-established structureperformance relationship presented in this work provides new insights for designing high-performance NFREAs in the follow-up research.Despite the development of central core and end group, the option of electron-donating building block is limited and cyclopentadithiophene (CPDT) has been used in most cases. [21]Based on our results, the alkyl chains on sp 3 -hybridized carbon of CPDT could more or less cause some unfavorable steric hindrance during molecular packing.Therefore, to design a sp 3 -hybridized carbon free electrondonating unit is fairly important to optimize the solid state packing arrangement.
).Interestingly, given the sum of the R w s of S⋅⋅⋅F (3.27 Å) and O⋅⋅⋅H−C (2.72 Å), a dimer induced by multiple noncovalent bonds are formed between molecule C and molecule D in both EH-4F and C4-4F (Figure 2E,F).

F
I G U R E 2 (A-D) Single crystal stacking modes of EH-4F and C4-4F: (A and B) top view; (C and D) side view.(E and F) Dimer formation induced by multiple noncovalent bonds in (E) EH-4F and (F) C4-4F (the alkyl chains were omitted for clarity).
,g), which is beneficial to reducing Urbach energy and voltage loss.Collectively, the crystalline behaviors of A-D-A'-D-A type NFREAs were unveiled for the first time by single crystal analysis.The above results indicate that the molecular packing arrangements of A-D-A'-D-A type NFREAs, including stacking distances, packing motifs and intermolecular interactions, can be successfully fine-tuned by end group halogenation and side chain engineering.

F
I G U R E 6 (A) Two-dimensional grazing incidence wide angle X-ray scattering (GIWAXS) patterns and (B) one-dimensional GIWAXS line-cut profiles of blend films.For the line-cut profiles, solid line depicts out-of-plane and dash line depicts in-plane.(C) Atomic force microscopy (AFM) height images and (D) transmission electron microscopy (TEM) images of blend films.