Revealing the spacing effect of neighboring side‐chains in modulating molecular aggregation and orientation of M‐series acceptors

Controlling the aggregation of small‐molecule acceptors (SMAs) is essential to obtain an optimal morphology and to improve the photovoltaic performance of polymer solar cells (PSCs). However, reducing intermolecular aggregation of SMAs is usually accompanied by the disruption of compact molecular packing thereby leading to their decreased electron mobilities. Here, two novel M‐series SMAs (MD1T and MD2T) based on ladder‐type heterononacenes with neighboring side‐chains separated by one or two thiophene rings are designed and synthesized. It is found that shortening the spacing of the neighboring side‐chains of the SMAs can greatly alleviate the intermolecular aggregation and alter the molecular orientation from bimodal edge‐on/face‐on to predominant face‐on while maintaining the compact molecular packing. As a result, a more favorable morphology with smaller domain sizes is formed for the MD1T‐based blend films, which greatly improves the charge generation and charge transport for the corresponding PSCs. The best‐performing MD1T‐based device affords an efficiency of 12.43%, over seven times higher than that of the MD2T‐based device. This work reveals the importance of the spacing between the neighboring side‐chains in modulating the molecular aggregation and active layer morphology, and the obtained structure‐performance relationships shall provide important guidance for designing highly efficient SMAs.


INTRODUCTION
The recent rise of small-molecule nonfullerene acceptors (SMAs) has offered polymer solar cells (PSCs) with more than 19% efficiencies. [1][2][3][4][5][6] Compared to the traditional fullerene-derived electron acceptors (e.g., PC 61 BM and PC 71 BM), SMAs possess easily tailored molecular structures, and they exhibit several unique merits such as large absorption coefficients, tunable optical bandgaps and frontier orbital energy levels in a wide range, and small exciton binding energies, which allow the corresponding solar cell devices to have larger open-circuit voltages (V oc s), higher shortcircuit current densities (J sc s), and smaller energy losses comparable to those of fullerene-based solar cells. [7][8][9][10][11][12] This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2023 The Authors. Aggregate published by SCUT, AIEI, and John Wiley & Sons Australia, Ltd. Therefore, SMA-based PSCs can potentially achieve much higher power conversion efficiencies (PCEs) than their fullerene-based counterparts. [13][14][15][16][17][18][19] In addition, SMAs with improved chemical stability can be made through proper molecular tailoring, thereby extending device lifetime of the resulting PSCs. [20][21][22][23][24][25] Molecular aggregation behavior (i.e., molecular crystallinity) plays a key role in determining photovoltaic performance of SMAs. The acceptor molecules with excessive aggregation tend to form large-sized phase separation in their blend films, which would reduce exciton dissociation rates and increase charge recombination probability, while those with low crystallinity tend to have excessive miscibility and inferior electron transport. [2][3]26] Therefore, it is necessary to engineer the acceptor molecules elaborately to balance the delicate relationship between crystallinity and miscibility of SMAs for an optimal active layer morphology. For example, conventional perylene diimide (PDI)-based electron acceptors exhibit good electron transport properties due to their high crystallinity. [2,[27][28][29] However, the unique large and planar π-conjugated backbone of PDI molecules makes themselves tend to aggregate into large-sized crystals in blend films, which limits their photovoltaic performance with efficiencies less than 3%. [27][28] When the overaggregation effect of PDIs was solved, and their miscibility with polymer donors was improved, the efficiencies of PDIbased electron acceptors can be increased to over 11%. [29] Another example is the benchmark electron acceptor of 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]s-indaceno[1,2-b:5,6-b′]dithiophene (ITIC) developed by Zhan et al. which consists of a linearly fused acceptor-donoracceptor (A-D-A) aromatic backbone with four side-chains located on the two sp 3 -hybridized-carbon atoms. [30] The four side-chains stuck out from the backbone plane of ITIC molecules can suppress their over-aggregation. As a result, ITIC-based active layer exhibited an optimal nanoscale phase separation and an excellent PCE of more than 11%. [31] However, these out-of-plane side-chains will inevitably interfere with the compact intermolecular π-π interactions which prevents the further efficiency increase for the ITIC-based PSCs. [32][33][34][35] Therefore, novel molecular design strategies are needed to further enhance the performance of these fused-ring-based A-D-A-type SMAs.
One feasible solution to obtain SMAs with both high crystallinity and good miscibility is to use sp 2 -hybridized nitrogen atoms as bridging groups to construct SMAs without sp 3 -hybridized-carbon in the ladder-type skeletons. [36] To maximize the potential of such SMAs, it is necessary to restrain their over-aggregation caused by the large and planar π-conjugated backbones, similar to the problem encountered in the PDI-based acceptors. Very recently, based on nitrogen-bridged ladder-type heteroheptacene cores, we developed a new family of SMAs (M-series acceptors). [37][38][39][40] Despite being free of the out-of-plane side-chains on the sp 3 -hybridized-carbons, the M-series acceptors still possess good miscibility with polymer donors, resulting in an ideal nanoscale phase-separated blend morphology. An interesting structural feature of the M-series acceptors is the presence of two pairs of neighboring side-chains on the conjugated backbone, and the steric hindrance created by the two neighboring branched chains should be the main reason for preventing the excessive molecular aggregation. [41] The unique "neighboring side-chain" strategy motivates us to further explore the spacing effect of neighboring side-chains on the molecular aggregation and orientation of these nonfullerene acceptors to gain a deeper and more comprehensive understanding of the underlying mechanism in the molecular design strategy, which shall provide useful guidance for further development of novel high-performance nonfullerene acceptors.

Synthesis and characterization
The synthetic route of MD1T and MD2T are shown in Scheme 1. The starting tetrabrominated benzodithiophene 1 and tetrabrominated benzodithienothiophene 6 were obtained with reference to our previously reported method. [38] In the first step, tetrabrominated compounds 1 and 6 were reacted with the corresponding organozinc derivatives (2 and 7) by Negishi cross-coupling reactions with Pd(dppf)Cl 2 as catalyst to afford intermediates 3 and 8 in 51% and 76% yields, respectively. Organozinc species 2 and 7 were obtained from 2,3-dibromothienothiophene and 2,3-dibromothiophene by lithiation with n-BuLi and reaction with zinc dichloride, respectively. The tandem Buchwald-Hartwig amination of 3 and 8 with 2-hexyldecan-1-amine in the presence of sodium tert-butoxide as base and Pd(dba) 2 /dppf as catalytic system gave 4 and 9 in yields of 33% and 65%, respectively. Then, the dialdehydes 5 and 10 were synthesized by Vilsmeier-Haack formylation of 4 and 9 using N,N-dimethylformamide (DMF)/POCl 3 in 92% and 91% yields, respectively. Finally, the target molecules MD1T and MD2T were obtained by the Knoevenagel condensation of the IC2F group with the corresponding aldehyde intermediates in 78% and 86% yields, respectively. The detailed synthetic procedures are shown in the Supporting Information. We note that the two acceptors exhibit different solubilities, although their conjugated skeletons have the same number and type of aromatic rings and carry the identical alkyl side-chains. At room temperature, the solubility of MD1T in chloroform exceeds 15 mg mL −1 , while that of MD2T is only 2 mg mL −1 . However, MD2T still met the basic requirements for solution processing because its solubility increases to more than 8 mg mL −1 at an elevated temperature of 50 • C.
The results indicate that the spacing between neighboring The difference in solubility should be due to the different steric hindrance between their neighboring side-chains. The sidechains with smaller spacings can provide greater intramolecular steric hindrance, which leads to a slightly twisted molecule backbone, thus increasing its solubility. The low solubility of MD2T would lead to excessive aggregation during the film formation process and the formation of over-sized domains in blend films. The thermal properties of MD1T and MD2T were characterized by thermogravimetric analysis and differential scanning calorimetry (DSC) measurements. Both acceptors exhibit good thermal stability with decomposition temperatures (5% weight loss) of 320 and 336 • C for MD1T and MD2T, respectively ( Figure S1a). The DSC curves in Figure S1b show that MD1T has a weak re-crystallization peak at 138 • C during cooling and an endothermic peak at 164 • C during heating corresponding to the melting process. However, there is no phase transition peak related to melting or crystallizing from 25 to 300 • C for MD2T. The results suggest that varying the neighboring side-chain spacing has an effect on the thermal properties of the resulting acceptors.

Optical and electrochemical properties and density functional theory calculations
The absorption spectra of MD1T and MD2T were examined in both dilute chloroform solutions and thin films, and the  relevant data are shown in Figure S2a and Figure 2A. The detailed optical data are summarized in Table 1. In solution, MD1T displays strong absorption in the wavelength range of 600-850 nm with a molar extinction coefficient (ε) of 1.92 × 10 5 M −1 cm −1 at the absorption peak (λ max ) of 765 nm, while MD2T exhibits a hypsochromic absorption with an absorption peak at 730 nm and a slightly lower ε of 1.82 × 10 5 M −1 cm −1 . The red-shifted absorption of MD1T could be attributed to the increased intramolecular charge transfer (ICT) when the nitrogen-bridged rings are shifted from the outer positions to central positions. One possible reason for the origin of this discrepancy is that the shift of the pyrrole ring with a high dipole moment (1.8 Debye) affects the distribution of the π-electron cloud on the acceptor backbone, which in turn influences the transition dipole and oscillation strength. [42] In thin films, both SMAs exhibit considerable red-shifted and broadened absorption spectra relative to their respective solutions due to the enhanced intermolecular interaction in the solid state. The optical bandgaps (E g opt ) estimated from the absorption edges of the thin films are 1.32 and 1.41 eV for MD1T and MD2T, respectively.
The electrochemical properties of MD1T and MD2T were investigated by cyclic voltammetry measurements. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels were estimated according to the onsets of the oxidation and reduction curves. All redox potentials were internally calibrated using the ferrocene/ferrocenium (Fc/Fc + ) redox couple (4.80 eV below the vacuum level). As depicted in Figure S2b, the onset reduction (φ red )/oxidation (φ ox ) potentials for MD1T and MD2T were −0.80/0.71 and −0.86/0.73 V, respectively. The derived LUMO/HOMO energy levels were thus estimated as −4.02/−5.53 eV for MD1T and −3.96/−5.55 eV for MD2T using the equation of E LUMO/HOMO = −(4.82 + φ red /φ ox ) eV. [43] Clearly, MD1T exhibits a slightly higher HOMO level but a lower LUMO level compared to MD2T ( Figure 2B), which should be ascribed to the stronger electron-donating ability of the donor core in the former. Given that the V oc value of a PSC is related to the offset between the LUMO level of an electron acceptor and the HOMO level of an electron donor, the MD1T-based PSC might exhibit a lower V oc than that of the MD2T-based counterpart.
To understand the geometric and electronic structures of MD1T and MD2T, theoretical calculations were performed with density functional theory (DFT) at the B3LYP/6-31G** level, where the long alkyl chains were replaced by shorter branched ones for simplicity. The calculated results are displayed in Figure 2C. It can be seen that the two end groups are distributed on both sides of the long axes of the two molecules due to the presence of O⋅⋅⋅S intramolecular noncovalent interaction. The distances between the O atoms of IC2F and their neighboring S atoms were calculated to be 2.68 Å in MD1T and 2.71 Å in MD2T. In addition, the distances between neighboring side-chains on the molecular backbone change from 3.10 Å for MD1T to 6.72 Å for MD2T as the number of thiophene rings between the central benzene ring and pyrrole ring increases. Therefore, a greater steric hindrance of neighboring branched alkyl chains within the MD1T molecule is expected, which may result in distortion of the conjugated backbone plane. Side views show that MD1T has a slightly curved backbone with a dihedral angle of 9.86 • between the two end groups along the backbone. In contrast, MD2T shows a better planarity with a smaller dihedral angle of 3.51 • . The more curved backbone facilitates the alleviation of excessive molecular aggregation, thus enhancing the solubility of MD1T. Note that the slightly enhanced curvature of the MD1T molecular backbone does not disrupt its ordered molecular packing, as evidenced by the following crystallographic packing motif and the grazing-incidence wide-angle X-ray scatte (GIWAXS) results.
The distributions of the frontier orbitals and surface electrostatic potentials (ESPs) are quite similar for the two molecules ( Figure 2C and Figure S3). In both cases, the electron densities of HOMO are predominantly localized at the central donor unit, whereas those of LUMO are distributed along the whole molecular backbone, suggesting very efficient ICT for these A-D-A-type molecular systems. For this reason, a small change in the donor core also affects the LUMO energy levels of the target acceptors. We also note that electron density was distributed along the molecular orbitals that reside on the pyrrole nitrogen, suggesting that the lone pair electron on the nitrogen can delocalize along the π system to extend the conjugation. This is not observed on the sp 3 -hybridized carbon of the commonly used cyclopentadiene core. [36] The calculated HOMO level of MD1T is 0.01 eV higher than that of MD2T, while its LUMO level is 0.05 eV lower than that of MD2T, which collectively contributes to the smaller optical bandgap of MD1T. Overall, the calculated results are consistent with the above experimental observations. For both acceptors, the groups with strong electronegative atoms (such as oxygen, nitrogen, and fluorine) have negative ESP while most of their conjugated surface have positive ESP. The largely distributed positive ESP will direct the intermolecular electric field from the acceptor to the polymer donor, which is favorable for the exciton dissociation and charge extraction. [44]

Aggregation and crystalline structure
Single crystals of MD1T were then cultivated to gain detailed insight into its molecular geometry and molecular packing pattern that is closely related to its charge transport property. MD1T single crystals for diffraction study were grown by evaporative diffusion of ethanol into toluene solution and the detailed metrical parameters of the structure are listed in Table S1. We also tried to grow MD2T single crystals with diffraction quality by using various solvents and different solvent diffusion methods, but we did not succeed. It can be attributed to the fact that the poor solubility of MD2T results in small-sized single crystals or clusters. The MD1T single crystal exhibits a triclinic crystal system with a P-1 space group. As shown in Figure 3A, MD1T has a linear molecular structure with a length of about 32.92 Å. The S⋅⋅⋅O interaction distance and the torsional angle between the donor core and end groups connected via vinyl were calculated to be 2.69 Å and 2.08 • , respectively. In addition, the distance between the neighboring side-chains on one side of the MD1T molecular backbone was determined to be 3.12 Å. These data are in good agreement with the theoretical calculations showed in the previous section. The unit cell and packing motif of MD1T are presented in Figure 3B and 3C, respectively. MD1T possesses one molecular conformation in each of its unit cell. According to the ratio of total molecular volume to unit cell volume, the crystal packing coefficient for MD1T can be determined to be 62.93%, which is similar to that of simple aromatic molecules such as pentacene (∼70%). [45] This implies that there is a highly condensed molecular assembly in the MD1T molecules, which favors an increase in the dielectric constant and suppresses non-geminate charge recombination. [46] The MD1T molecules in the single crystal are packed in a face-to-face, slip-stacking fashion, and connected by head-totail π-π interactions. Such parallel molecular stacking allows each MD1T molecule to interact with four adjacent molecules via π-π interactions, resulting in two kinds of J-aggregated dimers, one of which is the A-to-A type, and the other is the A-to-D type. Specifically, the A-to-A type dimer is formed by the intermolecular π-π interaction between adjacent IC2F end groups at a distance of 3.41 Å, while the A-to-D dimer is generated by the intermolecular π-π interaction (distance ≈ 3.36 Å) between the IC2F unit and the adjacent outermost thienothiophene unit of the donor core. As shown in Figure 3C, the two types of J-aggregated dimers in the MD1T crystal are regularly arranged together to form a "2D brickwork" packing structure. By taking the c-axis perspective, the "2D brickwork" layers assemble into a lamella structure via the side-chain interactions. Thus, MD1T molecules can provide 2D electron transport channels for efficient electron hopping. These results suggest that the introduction of neighboring side-chains with appropriate spacings induces the formation of J-type aggregates and a continuous intermolecular charge transport channel within the MD1T molecules.
The aggregation behaviors of the acceptor molecules in thin films were then investigated by atomic force microscopy (AFM). Figure 4A,B displays the AFM height images of the neat MD1T and MD2T films. MD1T showed a uniform and smooth surface with a small root-mean-square roughness (R q ) of 1.16 nm. For the neat MD2T film, a rougher surface with a R q up to 9.68 nm was observed. We further measured the dimensions of the bumps to make a more distinct comparison, and line profiles across the AFM height images are shown in Figure 4C. The full-width at half-maximum of the peak was used to estimate the diameter of the bump, and the average values obtained for the MD1T and MD2T neat films are 63 and 281 nm, respectively. The results indicate that MD2T is more prone to aggregation compared to MD1T, which should be attributed to its larger spacing of neighboring side-chains The two-dimensional (2D) GIWAXS diffraction patterns and the corresponding one-dimensional (1D) line-cut profiles of the neat MD1T and MD2T films are presented in Figure 4D-F. The results of quantitative analysis are summarized in Table S3. The 2D GIWAXS texture of MD1T showed many strong diffraction spots, indicating highly ordered molecular packing structures. The MD1T crystallites adopt a preferred face-on orientation, as evidenced by the obvious (010) π-π diffraction peak at q z = 1.84 Å −1 in the out-of-plane (OOP) direction (q z axis) and the strong (100) lamellar diffraction peak at q xy = 0.33 Å −1 in the in-plane (IP) direction (q xy axis). The π-π stacking distance (d π ) and TA B L E 2 Photovoltaic properties of polymer solar cells (PSCs) based on PBDB-T: Acceptor blends and space-charge-limited current (SCLC) carrier mobilities of the corresponding active layers. lamellar stacking distance (d l ) for MD1T were calculated to be 3.41 Å and 19.03 Å, respectively, which are consistent with the results observed in its single crystal. MD2T also exhibits a strong tendency to self-assemble and it crystallizes readily, as the strong lamellar (100), (200), (300), and (010) diffraction peaks are clearly visible. Nevertheless, the π-π stacking diffraction and the lamellar stacking diffraction appeared along both the q xy and q z axes, indicating that edgeon and face-on crystallites co-exist in the MD2T film. As found in the 1D line-cuts of the MD2T film, the (100) peak in the IP direction and the (010) peak in the OOP direction are located at 0.28 and 1.78 Å −1 , corresponding to d l and d π of 22.19 and 3.53 Å, respectively. These results indicate that the molecular packing of MD1T is tighter than MD2T, although MD2T is expected to possess a more planar molecular backbone than MD1T according to the DFT calculations. This might be due to the high aggregation and low solubility of MD2T. It can be speculated that due to the low solubility, the MD2T molecules crystallize out rapidly from its solution, which prevents the molecular backbone and crystallites from being ordered, resulting in a loose aggregation structure and a bimodal texture. However, with a higher solubility than MD2T, MD1T would have sufficient time to self-assemble, thereby providing more compact molecular packing. The differences in molecular packing and orientation of the two acceptor films are closely correlated with their carrier transport and photovoltaic properties, which will be discussed later.

Photovoltaic properties
The photovoltaic properties of the two SMAs were evaluated by fabricating bulk-heterojunction solar cells with a conventional device structure of ITO/PEDOT:PSS/active layer/PDIN/Ag, where PDIN is 2,9-bis(3-(dimethylamino) propyl) anthra[2,1,9-def:6,5,10-d′e′f′] diisoquinoline-1,3,8,10 (2H,9H)-tetraone. Here, we chose the wide bandgap polymer of PBDB-T as an electron donor material mainly because it has complementary absorption and matched energy levels with the two acceptors (Figure 2A,B). Different processing conditions were used to optimize the photovoltaic performance of solar cells, such as the active layer composition (i.e., donor:acceptor weight ratio), the amount of additive, and the thermal annealing temperature. The optimized device conditions are described in the Supporting Information (Table S2). Figure 5A depicts the current density-voltage (J-V) curves of the best-performing devices, and Table 2 summarizes the corresponding photovoltaic parameters.
As shown in Figure 5B and Table 2, the MD2T-based devices showed a maximum PCE of 1.53%, with a V oc of 0.82 V, a J sc of 5.42 mA cm −2 and a FF of 34.33%. In contrast, for the optimal MD1T-based device, the V oc was found to drop to 0.76 V, which is attributed to the down-shifted LUMO energy level of the MD1T compared with that of MD2T. Nevertheless, the MD1T-based device still showed a much higher PCE of 12.43% mainly due to the simultaneous increase in J sc (24.17 mA cm −2 ) and FF (68.00%) values. Obviously, shortening the spacing of neighboring side-chains on the conjugated backbones of the acceptors can significantly improve its photovoltaic properties. The greater J sc and FF values of the MD1T-based devices should be related to their higher carrier mobilities induced by the improved active layer morphology. External quantum efficiency (EQE) spectra were obtained to check the accuracy of our photocurrent measurements, and the relevant results are illustrated in Figure 5C. Both devices displayed a broad photo-response range from 300 to 900 nm, indicating that the polymer donor and two acceptors contribute simultaneously to the J sc generation. Compared to the MD2T-based device, the MD1T-based device showed a wider-range light-harvesting and a higher photon-to-current conversion efficiency (ranging from 300 to 940 nm with a maximal value of 79%), thus yielding a higher J sc value. The integrated J sc values from the EQE spectra are 23.21 and 5.31 mA cm −2 for the MD1T-and MD2T-based devices, respectively, which are in good agreement with the J sc values from the J-V curves within 4.0% deviation.
To better understand the variations in device parameters of these two PSCs, we first examined the charge generation properties by measuring the dependence of the photocurrent density (J ph ) on the effective voltage (V eff ) for the corresponding devices. The definition of each parameter and the related calculation method are described in the Supporting Information. As shown in Figure 5D, the J ph of the MD1Tbased device is saturated (J sat ) at a relatively low V eff of 1.0 V. However, for the MD2T-based cases, the J ph shows a strong dependence on the applied voltage, indicating a low charge generation efficiency. For a quantitative comparison, we calculated the exciton dissociation probability (P diss ) by using the equation P diss = J ph /J sat . [47] Under the short-circuit condition, the MD1T-based device exhibited a P diss value of 93.49%, which is much higher than that for the MD2Tbased device (77.39%). The higher P diss value means the more efficient charge generation in the MD1T-based device.
To study the charge recombination behavior in the device, the dependence of J sc on the light intensity (P light ) was measured. As plotted in Figure 5E, by fitting the curve according to the power law equation J sc ∝ P light α , [48] the α of the MD1T-based device was calculated to be 0.95, which is higher than that of the MD2T-based device (0.76). Generally, the α value is an evaluation indicator for the extent of bimolecular charge recombination, and a higher α value close to 1 indicates weaker bimolecular recombination in the device. It is clear that the bimolecular recombination in the MD1T-based device is less than that in the MD2T-based device.
The charge transport properties in the device perpendicular to the substrate direction, including hole (μ h ) and electron (μ e ) mobilities, were evaluated by the space-chargelimited current (SCLC) method. The results are illustrated in Figure 5F and Figure S4. It was found that the single carrier devices based on the two PBDB-T:SMA blends demonstrated similar hole mobilities with 3.71 × 10 −4 cm 2 V −1 s −1 for the PBDB-T:MD1T blend and 1.91 × 10 −4 cm 2 V −1 s −1 for the PBDB-T:MD2T blend. However, their electron mobilities are quite different. The PBDB-T:MD1T blend showed an electron mobility of 1.85 × 10 −4 cm 2 V −1 s −1 which is much higher than that for the PBDB-T:MD2T blend (9.75 × 10 −6 cm 2 V −1 s −1 ) probably due to the more favorable backbone orientation for vertical electron transport in the PBDB-T:MD1T blend. The higher μ e of PBDB-T:MD1T blend also results in its more balanced charge transport. As presented in Table 2, the μ h /μ e ratios for PBDB-T:MD1T and PBDB-T:MD2T blends were calculated to be 2.01 and 19.59, respectively. Overall, more efficient exciton dissociation together with the higher and more balanced charge transport and less charge recombination result in higher J sc and FF values for the MD1T-based PSCs than the MD2T-based ones.

Film morphology analysis
AFM measurements were performed to investigate the surface morphology of bulk-heterojunction blend films. As illustrated in Figure 6, both blend films presented fibrillar structures, which has been shown to be favorable for charge transport. The PBDB-T:MD1T blend film displayed a uniform and smooth surface with a small R q of 0.98 nm. When the acceptor was replaced with MD2T, the R q of the resulting PBDB-T:MD2T blend film increased to 2.29 nm. The increase in roughness should be attributed to the strong aggregation of MD2T. In the AFM phase images ( Figure 6C,D), both blend films exhibited clear phase separation, but the domain sizes in PBDB-T:MD2T blend are obviously larger than those in PBDB-T:MD1T blend. The larger domains would reduce the D/A interfaces and thus the exciton dissociation rate, which partially explains the lower EQE and J sc values for the PBDB-T:MD2T devices.
To further study the crystallinity and molecular orientation of the blend films, GIWAXS characterization was performed, and the results are displayed in Figure 7. For both acceptor blends, the scattering characteristics generated from the acceptors dominate the GIWAXS patterns, mainly due to the better crystallinity of the two acceptors than PBDB-T. As a result, the two blend films showed similar molecular orientation to their corresponding pure acceptor films. For example, the PBDB-T:MD1T blend also exhibited a predominantly face-on orientation, while PBDB-T:MD2T blend film still presented a bimodal orientation of face-on and edge-on. In the OOP direction, the π-π stacking diffraction peaks of both blend films were observed at q z = 1.76 Å −1 , corresponding to a d π of 3.57 Å, which is 0.16 Å and 0.04 Å longer compared to the pure MD1T and MD2T films, respectively. In addition, the well-defined diffraction spots in the pure MD1T films disappeared after the addition of the polymer donor of PBDB-T, implying good miscibility between PBDB-T and MD1T. In contrast, the lamellar (100), (200), and (300) diffraction peaks observed in the pure MD2T film are still clearly visible in its blend film. These results suggest that the introduction of PBDB-T does not affect the molecular orientation of the acceptors but inevitably interferes their ordered molecular packing. Besides, the crystal coherence lengths (CCLs) calculated from the (010) and (100) diffraction peaks of the PBDB-T:MD2T blend are both larger than those of the PBDB-T:MD1T blend (Table  S3), indicating a higher crystallinity of the former.
To gain a quantitative comparison of the molecular orientation distributions in two blend films, a pole figure analysis was carried out. Figure 7D shows the pole figures extracted from the lamellar (100) diffraction peaks of the two blend films. The (100) peak intensity was recorded as a function of the polar angle and then corrected with a weighting factor sin (χ), where χ is the polar angle measured with respect to the q z axis. [49] Here, we define the areas integrated with χ ranges of 0-45 • and 45-90 • as the portions of edge-on and face-on crystallites, respectively. [50] As shown in Figure 7E, the proportion of face-on crystallites for the PBDB-T:MD1T blend film was determined to be 74%, which is higher than that for the PBDB-T:MD2T blend film (53%). The different molecular orientation behaviors of the two blends should be related to their intermolecular π-π interactions. It has been reported that molecules with high propensity to aggregate are more likely to adopt an edge-on orientation, while those with relatively low aggregation property prefer a face-on orientation. [51] The larger proportion of face-on crystallites in PBDB-T:MD1T blend film helps to increase vertical charge transport channels, thus improving the carrier transport and the final photovoltaic performance. This finding is in good agreement with the SCLC results and explains the higher J sc and FF values obtained in the PBDB-T:MD1T devices.

CONCLUSION
In summary, we have designed and synthesized two isomeric SMAs, MD1T and MD2T, by shifting the position of the pyrrole rings in the ladder-type heterononacene cores. Based on the two SMAs, we exhaustively investigated the spacing effects of neighboring side-chains on their intermolecular aggregation, morphology, charge transport, and photovoltaic properties. Shortening the spacing of neighboring side-chains on the ladder-type conjugation backbone of SMAs (from MD2T to MD1T) alleviates excessive molecular aggregation thereby greatly improving the solubility while maintaining the highly ordered molecular stacking, as evidenced by the crystallographic analysis and GIWAXS results. As the molecular aggregation weakens, an improved active layer morphology with relatively small-sized phase separation is formed and, more importantly, the preferential molecular orientation of the acceptor is altered from bimodal edge-on/face-on to face-on. These unique characteristics promote exciton dissociation, enhance charge transport, and reduce charge recombination, resulting in higher J sc and FF values of the MD1T-based devices. As a result, the PSCs based on PBDB-T:MD1T exhibited a PCE of up to 12.43%, which is more than seven times higher than that of the bestperforming device based on PBDB-T:MD2T (1.53%). This work reveals the important role of the spacing of neighboring side-chains in modulating the molecular aggregation and orientation properties, which provides important insights for the future rational design of high-performance SMAs based on sp 3 -hybridized-carbon-free ladder-type building blocks.

A C K N O W L E D G M E N T S
This work was supported by the National Natural Science Foundation of China (grant numbers: 52130306, 22075287, 22101285) and the Program of Youth Innovation Promotion Association CAS (grant numbers: 2021299).

C O N F L I C T O F I N T E R E S T
The authors declare no conflict of interest.

D ATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are available from the corresponding author upon reasonable request.