Bicontinuous donor and acceptor fibril networks enable 19.2% efficiency pseudo‐bulk heterojunction organic solar cells

Realizing bicontinuous fibrillar charge transport networks in the photoactive layer has been considered a promising method to achieve high‐efficiency organic solar cells (OSCs); however, this has been rarely achieved due to the interference of molecular organization of donor and acceptor components during solution casting. In this contribution, the fibrillization of polymer donor PM6 and small molecular nonfullerene acceptor L8‐BO is realized with the assistance of conjugated polymer D18‐Cl. Atomic force microscopy and photo‐induced force microscopy reveal that PM6 and D18‐Cl co‐assemble into long and slender fibrils within wide blending ratios due to their high compatibility; in contrast, the fibrillization of L8‐BO can be encouraged with the incorporation of 1% D18‐Cl. By utilizing a pseudo‐bulk heterojunction (p‐BHJ) active layer fabricated by layer‐by‐layer deposition, the optimized PM6+20% D18‐Cl/L8‐BO+1% D18‐Cl OSCs obtain bicontinuous fibril networks, leading to enhanced exciton dissociation and charge transport processes and superior power conversion efficiency of 19.2% (certified 18.91%) compared to 18.8% of the PM6:D18‐Cl:L8‐BO ternary BHJ OSCs.

organic materials, [11][12][13][14] the PCEs of OSCs are still inferior compared with their inorganic counterparts such as Si-and GaAs-based photovoltaics. [15,16][19][20] As discussed in previous works, [21][22][23][24] fibrillation refers to the self-assembly of materials growing in a specific direction to obtain one-dimensional structure and has been demonstrated to require different organization dynamics for materials with different structures.For example, with long backbones, conjugated polymers could easily assemble through main chains to form fibrils [25,26] or ribbons. [27]On the other side, small molecules such as small molecular nonfullerene acceptors (NFAs) only exhibit weak van der Waals force among their adjacent molecules, [28,29] and hence enhanced main-chain interactions (e.g., between the electron-withdrawing end groups) [30] or reduced sidechain interactions [31] are generally required to promote their growth along the conjugated plane.While it is not challenging to realize fibrillation for neat organic components, this situation can be very complicated when binary or ternary components are mixed to construct photoactive layer for OSCs, as their self-assembly can compete with each other. [32,33]Although very recent works have demonstrated that the fibrillization of polymer donors or NFAs in mixed films can be enhanced via the co-crystallization of the conjugated polymers having high compatibility (i.e., D18-Cl and PM6) [25,34] or NFAs with similar molecular packing forms (i.e., BTP-ThMeCl and L8-BO), [35] it is still difficult to construct fibrils of donor and acceptor components simultaneously to realize bicontinuous charge transport networks.
[38][39] On the one hand, this structure allows graded component distribution for efficient charge transport and collection along the vertical direction of OSCs. [36,40]On the other hand, it also provides high feasibility to optimize the nanoscale morphology of donor and acceptor components independently. [41,42]In this work, the fibrillization of state-of-the-art PM6 donor and L8-BO acceptor was realized by controlling the content of D18-Cl in each layer.D18-Cl is highly compatible with PM6 and could induce the fibrillization of PM6 through co-crystallization, [25] which is revealed by atomic force microscopy (AFM) and photo-induced force microscopy (PiFM) techniques.A small content of D18-Cl also acts as the initiator to induce the fibrillization of L8-BO.By adopting a p-BHJ structure, the optimized PM6+20% D18-Cl/L8-BO+1% D18-Cl film exhibits fine and bicontinuous donor and acceptor networks, translating to increased absorption coefficient and improved exciton dissociation and charge transport efficiencies, leading to a superior PCE of 19.2% (certified 18.9%) compared to 18.8% of the PM6:D18-Cl:L8-BO ternary BHJ OSCs.

| Fibrillization of PM6 and L8-BO
The chemical structures and energy levels of PM6, L8-BO, and D18-Cl are shown in Figure 1A and Supporting Information S1: Figure S1.From their AFM images, fibrillar domains are apparent in the D18-Cl film (Supporting Information S1: Figure S2a), confirming the superior selfassembly ability of D18-Cl, while PM6 displays strongly aggregated nanoparticles on its film surface (Figure 1B) and L8-BO exhibits a flat film surface with less texture (Figure 1E), suggesting their very different aggregation behaviors compared to D18-Cl.To explore the effect of D18-Cl on the molecular assembly of PM6 and L8-BO, binary films with different compositions of PM6:D18-Cl and L8-BO:D18-Cl were first probed by AFM.As shown in Supporting Information S1: Figure S2b,c and Figure 1C,D, we found that decent fibril domains can be well observed in the PM6 film upon the addition of D18-Cl in a wide range (from 5 to 30 wt%).With a similar chemical structure to PM6 and the low surface energy of D18-Cl, it locates in the PM6 phase and co-assembles with PM6 into nanoscale fibrils during solution casting, as concluded from our previous work. [25]Distinctly, for L8-BO, incorporating only 0.5 wt% of the D18-Cl can induce long L8-BO aggregates with a length of 100-200 nm (Supporting Information S1: Figure S2d), while 1 wt% addition of D18-Cl leads to the formation of L8-BO fibril network (Figure 1F).However, with 3 wt% addition of D18-Cl, the fibrillization of L8-BO has been depressed and results in more amorphous domains (Figure 1G).The above results suggest that although D18-Cl could induce the fibrillization of both PM6 and L8-BO, different fundamentals might be behind it.
To clarify the origin of the formation of the above fibrillar domains, PiFM was then performed to probe the characteristic photochemical responses of PM6 (1579 cm −1 ), D18-Cl (1458 cm −1 ), and L8-BO (1507 cm −1 ) (see Supporting Information S1: Figure S4) in their blend films.As shown in Figure 2A, PM6+20 wt% D18-Cl film exhibits clear PM6 fibrils with D18-Cl dispersed uniformly across the probed area (Figure 2B), implying that D18-Cl as a minor component in the PM6:D18-Cl blend would not form standing along fibrils but either co-crystallized into PM6 fibrils or being amorphous.This can be further confirmed by their absorption spectra (Supporting Information S1: Figure S5a), where the two characteristic peaks of PM6 both maintain but the peak of D18-Cl at 544 nm disappears in the PM6:D18-Cl blend.Two-dimensional (2D) GIWAXS patterns shown in Supporting Information S1: Figures S6a-c and S7a suggest that the addition of 20 wt% D18-Cl has only enhanced the (100) and (010) diffraction peaks compared to those of PM6 but did not affect the nature of molecular packing.
In contrast, when L8-BO was added with 0.5 or 1 wt% D18-Cl, its absorption characteristic peak showed clear redshift from 810 to 822 nm (Supporting Information S1: Figure S5b), together with enhanced absorption coefficient and largely red-shifted π-π stacking diffraction peaks from 1.72 to 1.76 Å −1 (Supporting Information S1: Figures S6d,e and S7b) with enhanced diffraction intensity.This confirms that the presence of a tiny amount of D18-Cl has modified the molecular packing of L8-BO toward more compact.L8-BO fibrils are also clear in its PiFM image in Figure 2D, and 1 wt% D18-Cl in L8-BO would not lead to standing along the D18-Cl domain (Figure 2E).Due to its superior self-assembly ability, D18-Cl will assemble first during solution casting and then act as the initiator to trigger the fibrillization of L8-BO.Additionally, when further increasing the D18-Cl content to 3 wt%, the red-shift in the absorption spectrum disappears and the π-π stacking diffraction peak of D18-Cl shifted to lower q z value (associating with a larger π-π stacking distance), implying that 3 wt% D18-Cl has retarded the assembly of L8-BO (Supporting Information S1: Figures S6f and S7b

| Photovoltaic performance
We further sequentially deposited the PM6+20% D18-Cl and L8-BO+1% D18-Cl solutions to cast the p-BHJ photoactive layer.Compared to its PM6:L8-BO binary or PM6:L8-BO:20% D18-Cl ternary counterparts, we find this finely optimized PM6+20% D18-Cl/L8-BO+1% D18-Cl p-BHJ layer enables a higher absorption coefficient (Figure 3A),  which might be attributed to the enhanced molecular packing as discussed above.Then, these three photoactive layers were assembled in OSC devices having the architecture of ITO/PEDOT:PSS/active layer/PDINN/ Ag with the photovoltaic performance shown in Figure 3B and Table 1.PM6:L8-BO binary BHJ device shows a PCE of 18.4% with an open-circuit voltage (V OC ) of 0.868 V, short current density (J SC ) of 26.8 mA cm −2 , and fill factor (FF) of 79.1%, consistent to previous work. [43]When D18-Cl was introduced into the PM6:L8-BO BHJ active layer as the third component (Supporting Information S1: Table S1), negligible improvement was observed in J SC and FF while V OC was increased to 0.879 V (attributing to the lower nonradiative V OC loss of D18-Cl), leading to a maximum PCE of 18.8%.In contrast, with the PM6+20% D18-Cl/ L8-BO+1%D18-Cl p-BHJ active layer, the corresponding OSC exhibits superior PCE of 19.2%, which not only maintains a high V OC of 0.881 V but also obtains J SC and FF to 27.3 mA cm −2 and 80.0%, respectively.This champion device obtained a certified PCE of 18.91% at the National Photovoltaic Product Quality Inspection & Testing Center in China (Supporting Information S1: Figure S8), which is greater than the previous report for the PM6/L8-BO binary device.Additionally, we also found the photovoltaic parameters of our p-BHJ structured OSCs exhibit a significant reliance on the D18-Cl concentrations in each layer.As presented in Supporting Information S1: Table S2, with the increasing D18-Cl content in PM6, the device efficiency fluctuates within a narrow range from 18.9% to 19.2%, whereas it is significantly affected by the content in L8-BO with an enhancement from 18.2% to 18.7% at the optimum.This finding aligns with the observed AFM images, suggesting that the morphology of L8-BO plays a crucial role in determining device performance.To illustrate the photoresponsivity of the above OSCs, EQE measurements were performed (Figure 3C), and all OSCs show less than 5% difference between the J SC values obtained from J-V and EQE tests, validating the reliability of our testing.

| Bicontinuous fibril networks of the photoactive layer
Contact angle measurements were performed to investigate the compatibility of the above materials (Supporting Information S1: Figure S9 and Table S4), and enhanced compatibility (reduced surface energy) between the PM6 and L8-BO phases upon the addition of D18-Cl is observed, suggesting that L8-BO can be incorporated into the donor layer more effectively.We then examine morphologies of the above PM6:L8-BO, PM6:L8-BO:20% D18-Cl, and PM6+20% D18-Cl/L8-BO+1% D18-Cl films by GIWAXS and AFM.As shown in Figure 4, compared with the relatively amorphous PM6:L8-BO BHJ film, slender fibrils are observed in the PM6:L8-BO:20% D18-Cl ternary film, and more dense fibrils can be further developed in the PM6+20% D18-Cl/L8-BO+1% D18-Cl p-BHJ film.The improved fibril network is associated with enhanced (010) and (021) diffraction peaks of L8-BO (Figure 4D-F and Supporting Information S1: Figure S10), denoting the enhanced fibril morphology might be attributed to L8-BO. [35]As such, these optimized fibril networks also translate to extended exciton lifetime and improved exciton diffusion (Figure 3D and Supporting Information S1: Table S3) for both PM6 and L8-BO, contribute to enhanced hole and electron mobilities (Supporting Information S1: Figure S11 and Table 2), and finally result in improved exciton dissociation and charge collection efficiency (Figure 3E and Table 2) toward superior FF and J SC as we presented above.
The component distributions of PM6, D18-Cl, and L8-BO in their BHJ-and p-BHJ-based films were further revealed by PiFM.As shown in Supporting Information S1: Figure S12, PM6:L8-BO film presents a relatively amorphous morphology with well-mixed PM6 and L8-BO domains.Although PM6:L8-BO:20% D18-Cl film shows clear PM6 fibril domains (Figure 5A), spherical clusters with a domain size of around 50 nm are received for the L8-BO phase (Figure 5B), and no distinct texture can be attributed to the D18-Cl phase.On the other hand, for the p-BHJ structured PM6+20% D18-Cl/L8-BO+1% D18-Cl film, not only PM6 shows long and slender fibril domains (Figure 5D), clear L8-BO fibrils can also be identified (Figure 5E), leading to bicontinuous fibril network within the active layer (Figure 5J), which is beneficial to efficient charge transport and collection as we discussed above.

| CONCLUSIONS
In summary, we have realized the fibrillization of polymer PM6 and small molecular NFA L8-BO upon the assistance of polymer D18-Cl, which has a superior self-assemble ability.Through a comprehensive characterization via X-ray diffraction techniques and PiFM measurements, we found D18-Cl can co-crystallize with PM6 toward nanoscale fibrils within wide blending ratios, that is, with the D18-Cl content ranging from 5 to 30 wt%.Distinctly, D18-Cl also leads to the formation of L8-BO fibrils, but within a tiny amount (ca. 1 wt%) of D18-Cl incorporation.By sequentially depositing PM6+20%D18-Cl and L8-BO+1%D18-Cl, bicontinuous fibrillar networks with slender and condensed donor and acceptor fibrils were observed, which contributed to enhanced exciton dissociation, charge transport, and collection in the p-BHJ structured device and enabled a maximum efficiency of 19.2% (certified 18.91%).

| Materials
PM6 (M w = 40.9kDa), D18-Cl (M w = 71.1 kDa), and PDINN were purchased from Solarmer Materials Inc. L8-BO was purchased from Nanjing Zhiyan Technology Co., Ltd.PEDOT:PSS (4083) was purchased from the Clevios TM .ZnO precursor solution was prepared according to a previous literature report. [44]Unless otherwise stated, all chemicals and solvents were of reagent grade and used as received.

| Device fabrication
All OSCs were fabricated with a conventional structure of ITO/PEDOT:PSS/active layer/PDINN/Ag.The prepatterned indium tin oxide (ITO) glass substrates (sheet resistance ca. 15 Ω per square) were sequentially sonicated in deionized water, ethanol, and isopropanol for 15 min each and then dried at 150°C on a hotplate.These cleaned substrates were further treated with ultraviolet/ozone for 30 min.Then, PEDOT:PSS solution was spin-coated onto precleaned ITO glass at a speed of 5000 rpm and annealed at 150°C for 15 min in air, with a final thickness of 20 nm.For BHJ device, the donor PM6 and acceptor L8-BO were dissolved together in chloroform (CF) solution (1:1.2, 15.4 mg mL −1 ) with 0.25 vol% 1,8-diiodooctane (DIO) and then coated onto the PEDOT:PSS layer.For ternary BHJ device, PM6, D18-Cl, and L8-BO were dissolved together in CF (0.8:0.2:1.2, 13.2 mg mL −1 ) with 0.25 vol% DIO and coated in the same way.For ternary p-BHJ device, PM6 and L8-BO solutions were first prepared with concentrations of 6.5 and 8 mg mL −1 in CF, respectively, 20 and 1 wt% contents of D18-Cl were added into preprepared PM6 and L8-BO solutions, and 0.25 vol% DIO was introduced into L8-BO solution before spin coating.The PM6 solution was deposited onto the top of the PEDOT:PSS layer, and then the L8-BO solution was deposited on the top of the PM6 layer to form the photoactive layer.All final active layers were annealed at 80°C for 3 min, with a final thickness of ca. 100 nm.Then, PDINN (1.5 mg mL −1 ) in methanol solution was cast on the photoactive layer at a speed of 3000 rpm for 60 s to form the electron transport layer.Afterward, devices were transferred to the evaporation chamber and 100 nm Ag layers were thermally evaporated under a high vacuum through the shadow mask to form anode.Each single-substrate device consists of four individual pixels that each can operate separately; the active area of each pixel is 6.625 mm 2 and the aperture mask area is 4 mm 2 , defined by the overlapping of the anode and cathode.

| Characterizations
Film absorption spectra were measured by ultravioletvisible spectrophotometer (Hitachi).Film thickness was measured using a spectroscopic ellipsometer (J. A. Woollam) and was fitted with a Cauchy model using Complete Ease software.Device current density-voltage (J-V) measurement was performed under AM 1.5 G (100 mW cm −2 ) using a Newport 3A solar simulator in air at room temperature.The light intensity was calibrated using a standard silicon reference cell (with a KG2 filter) certified by the National Renewable Energy Laboratory.J-V characteristics were measured using a programmed software developed by Ossila Ltd. and a source meter unit (2612B; Keithley).External quantum efficiency (EQE) was measured using an EQE system (Zolix) equipped with a standard Si diode.The morphologies of active layer films were characterized using AFM (NT-MDT), AFM probes (ETALON Series HA_NC; Scansens GmbH, Ostec Group), and PiFM (Molecular Vista).Synchrotron grazing-incidence wide-angle X-ray scattering (GIWAXS) measurement was conducted using the beamline BL14B1 at the Shanghai Synchrotron Radiation Facility in China.Time-resolved photoluminescence (TRPL) measurements were carried out using the Fluorescence lifetime spectrometer (Picoquant), with an excitation wavelength of 518 nm.Contact angles were measured using a water contact angle measurement system (Attension Theta Lite), and the surface energy was calculated using the equation of state.

| Space-charge-limited current (SCLC) mobility measurements
The electron-only devices were fabricated with ITO/ ZnO/Active layer/PDINN/Ag structures and hole-only devices were fabricated with ITO/PEDOT:PSS/active layer/MoO 3 /Ag structures.The thickness of the active layer is ca.120 nm.Mobilities were obtained by fitting the current-voltage curves in the range from 1 to 4 V of the space charge limited range, where the SCLC is described following the equation below: where ε 0 is the permittivity of free space, ε r is the relative permittivity of the material, μ 0 is the hole or electron mobility, V is the applied voltage, and V bi is the built-in voltage.

| PiFM measurements
PiFM measurement is a scanning probe microscopy technique that combines the high-resolution AFM with infrared laser spectroscopy to obtain morphological information.The operational principle of PiFM is shown in Supporting Information S1: Figure S3: a pulse laser at a specific wavelength was first used to excite the sample and the tip, where dipoles can be formed on the sample and the tip.Then, the dipole interactions between the tip and sample can lead to a response of the van der Waals force gradient during sample thermal expanding, resulting in photo-induced force detected by the instrument. [7,45,46]n our work, the PiFM images were generated at a resolution of 256 × 256 pixels and a size of 1 × 1 µm, with wavenumbers of 1579, 1507, and 1458 cm −1 to map PM6, L8-BO, and D18-Cl, respectively.