Reducing V OC loss via structure compatible and high lowest unoccupied molecular orbital nonfullerene acceptors for over 17%-efficiency ternary organic photovoltaics

Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Hong Kong, China Department of Chemistry and Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration & Reconstruction, Hong Kong University of Science and Technology (HKUST), Hong Kong, China Department of Physics, Chinese University of Hong Kong, Hong Kong, China Department of Materials Science and Engineering, College of Engineering, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Peking University, Beijing, China College of New Materials and New Energies, Shenzhen Technology University, Shenzhen, China

Active-layer morphology has a close relationship with exciton dissociation, charge transport, and charge collection, and thus strongly influences the performance of OPVs. 21 Historically, optimizing active-layer morphology is as vital as the design of photoactive materials and interfacial materials. Several device engineering techniques have been reported to modulate morphology, such as thermal annealing, solvent vapor annealing, and solvent additives. However, the BHJ morphology, including the aspects of phase separation, molecular packing, crystallinity, and domain size, is very often sensitive to the previous strategies. In particular, in high-efficiency binary OPV systems, the technique of delicately tuning the active-layer morphology requires further study toward higher device performance.
The ternary blend strategy can effectively improve the performance of binary-blend OPVs, including efficiency and stability, without involving complicated tandem or multi-junction structures. 22,23 The third component in ternary OPVs can exert one or multiple benefits, including broadening/strengthening absorption spectra, facilitating charge generation/transfer/transport/collection, reducing carrier recombination, and optimizing/freezing the active-layer morphology. [24][25][26][27][28][29][30] For instance, Yan and coworkers incorporated IXIC-4Cl with an absorption edge of 1000 nm into PM7:ITC-2Cl binary system, to broaden the absorption of active layer. In another case, Zhan and coworkers incorporated 4,4 0 -Biphenol (BPO) into F-containing donor/fullerene blend. The hydrogen bond formed between F-containing donor and BPO forms the dense donor network, and freezes the film morphology, and thus improves the thermal stability of OSCs based on F-containing donors. Recently, Yang and coworkers reported the ternary blend of PTB7-Th/ PC 71 BM/IDIC with a more controlled structure via sequential solution deposition. IDIC near the cathode can improve the electron extraction and block holes to cathodes, delivering 14% to 21% PCE improvement to those conventional ternary blends. The utilization of the third component of a chemical structure assembling the host materials is also feasible to regulate morphology. Early in 2015, by comparing successful and failed multiple-donor systems, we found it was the harmonious co-existence of two structural compatible PTB7 and PBDTT-SeDPP that endowed the PTB7:PBDTT-SeDPP: PC 71 BM based ternary OPVs with an improved PCE of 8.7%. 28 FREAs generally present good planarity, when bulky side chains are not incorporated. Hence, the demand for structure compatibility can be easily meet in dual-FREAs based OPVs. Previously, we designed a nearinfrared absorbing FREA (IHIC), which features thienothiophene based fused hexacylic donor core, and exhibits an open-circuit voltage (V OC ) of 0.752 V and a PCE of 10.6% along with PTB7-Th. 31 Inspired by the success of PTB7:PBDTT-SeDPP:PC 71 BM ternary OPVs, we designed, synthesized, incorporated IHIC-N into PTB7-Th: IHIC binary system, which has naphthalene based fused-hexacylic donor core, assembles IHIC and guarantees the structure compatibility of IHIC-N and IHIC. 32 Moreover, IHIC-N has a 0.13-eV higher lowest unoccupied molecular orbital (LUMO) energy level than IHIC, which potentially improves V OC. PTB7-Th:IHIC: IHIC-N based ternary devices successfully delivered a higher PCE of 11.9% and an improved V OC of 0.785 V in relative to the PTB7-Th: IHIC control group. The utilization of polymeric donors with deep HOMO as third component can improve V OC and efficiency. 33,34 Our very recent work demonstrates the 5% incorporation of widebandgap polymer donor (PBT[E]BTz) with a deeper HOMO level than host PM6 can improve PCE from 15.43% to 16.26%. 35 Early in 2017, Zou group reported the first A-DA'D-A pentacylic NFA (BZIC), which obtained a efficiency of 6.3% along with polymeric donor HFQx-T. 36 The A-DA'D-A type molecules have advantages over the A-D-A molecules. [37][38][39][40] First, the electron-deficient unit in the fused -ring core can increase D-A interactions, and enhance intermolecular and intramolecular interactions, which improves molecular aggregation and charge transport. Second, the strongly electron-donating pyrrole unit can elevate the HOMO level and reduce the Eg of the acceptors. Third, the alkyl chains attached to the nitrogen atoms of pyrrole can introduce steric hindrance and prevent excessive aggregation. On the basis of BZIC, Zou group recently designed another A-DA'D-A structure NFA (Y6), which achieved a very high PCE of 15.7% in both conventional and inverted device structures when blended with the polymeric donor PM6. 41,42 OSCs based on PM6: Y6 exhibited high electroluminescence quantum efficiency and low nonradiative recombination loss. Hence, Y6 and its derivatives are extensively reported and used in recent research work. [43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58] For instance, other D-A copolymer donors, such as PTQ10, P2F-EHP, D16, W1, and D18, have been paired with Y6 to achieve excellent PCEs of 15% to 18%. [59][60][61][62][63] Besides, several research groups employed fullerene derivatives and Y6 to construct dual-acceptors ternary OPVs. Hou and coworkers found that the loading of PC 61 BM into PM6: Y6 blend not only enhanced electron mobility but also increased the electroluminescence quantum efficiency, which simultaneously balanced charge transport and reduced nonradiative energy loss (E loss ). Consequently, the addition of PC 61 BM into PM6: Y6 improved the PCE from 15.3% to 16.2%, attributed to the simultaneous enhancement of V OC , short-circuit current density (J SC ), and fill factor (FF). Parallelly, the PM6:Y6: PC 71 BM based device was demonstrated with PCEs of 16.67% for rigidsubstrate devices and 14.06% for flexible ITO-free devices. 64 FREAs are also incorporated as the third component. As an example, Zhang and coworkers successfully achieved a 16.27% efficiency in ternary OPVs with PM6:Y6: IT-4F blend containing 20% IT-4F, which is structurally compatible with Y6, and simultaneously enhanced J SC of 25.40 mA cm −2 , V OC of 0.84 V and FF of 75.9% 65 were achieved. However, carbon disulfide (CS 2 ) vapor of about 40 seconds in the active layer treatment process is difficult to control. CS 2 is an enzyme inhibitor with cytotoxic effects that can disrupt the normal metabolism of cells, interfere with lipoprotein metabolism and cause vascular disease, neuropathy, and damage to organs. Besides, CS 2 is inflammable with a very low flashing point of −30 C and a low boiling point of 46.5 C, and the upper limit of exposure to CS 2 in the air of the workshop is only 20 ppm. Accordingly, CS 2 vapor treatment prohibits the ambient fabrication of OPVs for future commercialization. Moreover, the chosen third component (IT-4F) is a narrow-bandgap acceptor with low-lying LUMO, and thus more obvious V OC improvement is expected along with NFAs of high-lying LUMO. Herein, we not only improved the device processing of PM6:Y6: IT-4F 66 based OPVs by using nonflammable chloroform solvent, but also designed the fascinating PM6:Y6: ITIC-Th 67 ternary blend, due to ITIC-Th's high LUMO of −3.93 eV. Relative to the PM6: Y6 control with a 16.7% efficiency, with a V OC of 0.84 V, a J SC of 25.5 mA cm −2 and a FF of 77.4%. The PM6:Y6: ITIC-Th ternary OPVs realized a higher PCE of 17.2% as a result of simultaneous improvement of V OC (0.86 V), FF (78.2%), and J SC (25.6 mA cm −2 ). ITIC-Th is found to increase and balance the charge mobilities. Increased crystalline coherence length and smaller d-spacing of π-π peaks are observed with the third component incorporation, indicating enhanced crystallinity and thus improved morphology of the blend. To the best of our knowledge, 17.2% is one of the highest PCEs for OPVs. The dualacceptor ternary OPV device shows clearly smaller E loss than both binary OPV systems. These results delineate the feasibility of constructing dual-NFA based OPVs to fulfill the potential of the exciting pool of OPV materials reported.

| RESULTS AND DISCUSSION
Scheme 1A,B displays the chemical structures and the energy levels of PM6, ITIC-Th, and IT-4F and Y6. PM6 exhibits a highest occupied molecular orbital (HOMO) of −3.50 eV and a LUMO of −5.56 eV. 38 The energy levels of PM6: ITIC-Th, PM6: IT-4F, and PM6: Y6 systems are well aligned to split photo-generated excitons. The HOMO and LUMO of ITIC-Th are situated between those of PM6 and Y6, while IT-4F exhibits the lowest HOMO and LUMO. As shown in Figure 1A 68 These OPVs are fabricated using the same process parameters for direct comparison, which is summarized in Supporting Information. Table 1 summarizes the best and average device characteristics based on PM6: acceptors (1:1.2 w/w) blended films. The current density-voltage (J-V) curves of the best devices under the illumination of an AM 1.5G solar simulator, 100 mW cm −2 are shown in Figure 1D [PL] and absorption spectra as shown in Figure S1) Such a high J SC is ascribed to the wide and low-energy absorption at approximately 320 to 1100 nm of Y6. The PM6: IT-4F device exhibited a moderate PCE of 12.0%, with a V OC of 0.88 V, a J SC of 18.6 mA cm −2 , a FF of 73.3%, and E loss of 0.72 eV. ITIC-Th exhibited more high-lying LUMO than IT-4F and Y6, and OPVs based on PM6: ITIC-Th exhibited a higher V OC of 0.98 V and a higher E loss of 0.69 eV than those of PM6:Y6. Nonetheless, this device only gave undesirable J SC of 16.0 mA cm −2 and FF of 69.5%, leading to the lowest PCE of 10.9%. Hence, the success of the Y6 molecule is indeed attributed to the optimization of the trade-off between V OC and J SC .
Device data of ternary OPVs based on PM6: Y6: IT-4F and PM6: Y6: ITIC-Th (the weight ratio of D:A is 1:1.2) are also summarized in Table 1, and shown in Figure 1D, E. The PM6:Y6: IT-4F ternary devices achieved the optimal PCE of 17.0% at the weight ratio of 1:1.12:0.08, ascribed to the slightly increased V OC of 0.85 V and J SC of 26.0 mA cm −2 . As a comparison, the 6.7% loading of ITIC-Th simultaneously improved V OC from 0.84 to 0.86 V, J SC from 25.5 to 25.6 mA cm −2 , and FF from 77.4% to 78.2%, which synergistically contributes to its high PCE of 17.2%, which is one of the highest PCE for OPVs. The more obvious V OC enhancement is attributed to the 0.22 eV higher LUMO of ITIC-Th than IT-4F. The PM6: Y6: ITIC-Th based OSCs presents E loss of 0.53 to 0.54 eV, smaller than their binary-blend counterparts.
The external quantum efficiency (EQE) spectra of the binary and ternary blends are shown in Figure 1F,G. The binary blends based on PM6: ITIC-Th and PM6: IT-4F both exhibit narrow photo-response from 300 to 850 nm, which resembles their absorption spectra. As a comparison, the EQE based on D:A blends incorporating Y6 The photocurrent density (J ph ) vs the effective voltage (V eff ) is measured to quantify the effect of IT-4F and ITIC-Th in the charge generation and extraction properties ( Figure 1H,I and Table S1). 69 At a high V eff of 3 V, J ph is assumed to become fully saturated, being independent of voltage and temperature. The saturated photocurrent density (named J sat ) is given by the equation of J sat = qG max L, where q is the electric charge, L refers to the thickness of the active layer, and G max represents the maximum generation rate of excitons in OPVs, which is in principle dominated by absorption. The optimized PM6: Y6, PM6: Y6: IT-4F (1:1.12:0.08) and PM6: Y6: ITIC-Th (1:1.12:0.08) present J sat s of 27.2, 27.6, and 27.0 mA cm −2 , significantly higher than that of the PM6: IT-4F (20.4 mA cm −2 ) and PM6: ITIC-Th (18.0 mA cm −2 ), due to the strong panchromatic absorption from 300 to 1000 nm of Y6. The exciton dissociation efficiency (η diss ) and charge collection efficiency (η coll ) are calculated under the short circuit and maximum power output conditions, according to the equations of η diss = J SC /J sat and η coll = J max /J sat , respectively. Compared with the PM6: Y6 control, the addition of IT-4F not only slightly increases the J sat , but also elevates η diss from 93.8% to 94.2% and η coll from 85.3% to 86.6%. Besides, although the addition of ITIC-Th has negligible contribution to J sat , it effectively enhances η diss from 93.8% to 94.8% and η coll from 85.3% to 87.4%.
Charge mobility is also fitted from the dark current density-voltage curves using the space-charge-limited current (SCLC) model ( Figure S2, Table 2, and Table S2). Electron-only devices employ the architecture of ITO/ZnO/active layer/PNDIT-F3N/Ag, while hole-only devices use the ITO/MoO x /active layers/MoO x /Ag. As for pure acceptor film, the pristine electron mobility (μ e ) declines from 6.21 × 10 −4 cm 2 V −1 second −1 for Y6, 5.75 × 10 −4 cm 2 V −1 second −1 for IT-4F, to 5.05 × 10 −4 cm 2 V −1 second −1 for ITIC-Th. This order of magnitude illustrates that all these nonfullerene   To decipher the role of IT-4F and ITIC-Th, the PL spectra of Y6, IT-4F, and ITIC-Th pure films, and Y6: IT-4F and Y6: ITIC-Th blend films (excited at 710 nm) are further measured and plotted in Figure 2. The PL spectra of IT-4F and ITIC-Th, peaking at 808 and 773 nm, strongly overlaps the absorption of Y6 and affords the possibility of energy transfer from IT-4F/ITIC-Th to Y6. The PL intensity of IT-4F pristine film is approximately as much as thrice that of ITIC-Th pristine film. In the Y6: IT-4F blend film, the emission of IT-4F is entirely quenched by Y6 while that of Y6 shows some enhancement, indicating effective energy transfer from IT-4F to Y6, which is in accordance with the slight J sat increase from 27.2 (PM6: Y6) to 27.6 mA cm −2 (PM6:Y6: IT-4F). By comparison, the emission of ITIC-Th is also effectively quenched while that of Y6 shows almost negligible enhancement in Y6: ITIC-Th blend, in line with the similar J sat of PM6:Y6: ITIC-Th and PM6: Y6 systems.
Atomic force microscope (AFM) is one of the most widely used tools for surface morphology characterization in the OPV field. It provides the basic information of surface morphology, mainly the surface topography with high spatial resolution within 10 nm. The AFM Height images of binary blends and ternary blends show smooth and uniform surface morphology, which guarantee their good contact with the PNDIT-F3N interlayer (Figures S3  and S4). With the increased ratio of IT-4F, the root mean square roughness (R q ) increased from 1.03 nm for PM6: Y6, 1.40 nm for PM6: Y6: IT-4F, and 2.41 nm for PM6: IT-4F. Likewise, with the increased ratio of ITIC-Th, RMS increased from 1.03 nm for PM6: Y6, 1.22 nm for PM6:Y6: ITIC-Th, and 1.79 nm for PM6: ITIC-Th. AFM can only probe the surface information of the active layer, while the inward donor/acceptor configuration is pivotal for device performance. Grazing incidence wideangle X-ray scattering (GIWAXS) is therefore adopted to investigate crystalline structures on the molecular scale, including information of the distance of the lamellar layers, π-π stacking, and crystalline coherence length (CCL). [70][71][72] The 2D diffraction patterns and 1D line-cuts are depicted in Figures 3A-G, S5, and S6. The PM6 neat film exhibits a π-π peak at q z ≈ 1.70 Å −1 (d ≈ 3.70 Å) along the out-of-plane (OOP) direction and bimodal lamellar peaks in both in-plane (IP) and OOP directions at q r ≈ 0.281 Å −1 (d ≈ 22.4 Å) and q z ≈ 0.303 Å −1 (d ≈ 20.7 Å). The Y6 neat film exhibits a strong π-π stacking peak at q z ≈ 1.76 Å −1 (d ≈ 3.57 Å), indicating a preferential face-on orientation of Y6. It is noteworthy that the co-existence of two diffraction peaks at q r ≈ 0.285 Å −1 (d ≈ 22.0 Å) and q r ≈ 0.420 Å −1 (d ≈ 15.0 Å) in Y6 film drops a hint of the concomitance of two distinct structure F I G U R E 2 A, PL spectra of Y6 and IT-4F pure films and Y6: IT-4F blend film; and B, PL spectra of Y6 and ITIC-Th pure films and Y6: ITIC-Th blend film. (excited at 710 nm) orders. Since one of the IP diffraction peaks is assigned to the lamellar peak, the other presumably originate from the backbone ordering shaped by the π-π stacking of end groups. 38 This unusual phenomenon is also observed in the pristine film of ITIC-Th, which mainly exhibited two diffraction peaks at q r ≈ 0.352 Å − 1 and q r ≈ 0.466 Å −1 , and π-π peak at q z ≈ 1.77 Å −1 . The IT-4F pristine film exhibited lamellar peaks at q r ≈ 0.354 Å − 1, with an extremely weak π-π peak. Among all the blend films, the PM6: Y6 blend takes on preferential face-on packing by exhibiting a strong OOP (010) peaks at q z ≈ 1.73 Å −1 (d ≈ 3.63 Å) and bimodal (100) peaks at q r ≈ 0.293 Å −1 (d ≈ 21.4 Å) and q z ≈ 0.296 Å −1 (d ≈ 21.2 Å). The PM6: IT-4F blend presented bimodal lamellar peaks at q r ≈ 0.295 Å −1 (d ≈ 21.3 Å) and q z ≈ 0.315 Å −1 (d ≈ 19.9 Å), and π-π peak at q z ≈ 1.68 Å −1 (d ≈ 3.74 Å), indicating its relatively incompact π-π stacking. Interestingly, as Table S3 shows, the slight doping of IT-4F into PM6: Y6 blend increased the CCL of the OOP π-π peak of PM6: Y6 blend from 23.3 to 25.2 Å. (CCL is calculated based on the equation: CCL = 1.8 × π/FWHM, where the FWHM is short for the full width at half-maximum.) The PM6: ITIC-Th blend presented bimodal lamellar peaks at q r ≈ 0.291 Å −1 (d ≈ 21.6 Å) and q z ≈ 0.320 Å −1 (d ≈ 19.6 Å), and π-π peak at q z ≈ 1.78 Å −1 (d ≈ 3.53 Å), indicating its tighter π-π stacking than PM6: Y6 blend. The addition of ITIC-Th into PM6: Y6 blend also increased the CCL of the OOP π-π peak from 23.3 to 24.1 Å, and decreased the π-π d-spacing from 3.63 (q z ≈ 1.73 Å −1 ) to 3.59 Å (q z ≈ 1.75 Å −1 ).
Grazing incidence small-angle X-ray scattering (GISAXS) is supplemented to investigate nanoscale phase separation in thin films. Figure 3H,I presented GISAXS intensity profiles with best fittings along the in-plane direction of PM6: Y6, PM6:Y6: ITIC-Th, and PM6: ITIC-Th blends. The data were fitted with fractal-like network models and Debye-Anderson-Brumberger (DAB) models, to account for the scattering contribution from acceptor domains and the intermixing amorphous phase, respectively ( Table 2). The pure acceptor domain size gradually decreased from 24.4 nm for PM6: Y6, to 22.6 nm for PM6: Y6: IT-4F (1:1.12:0.08) and 17.9 nm for PM6: IT-4F, and increased monotonically from PM6: Y6 (24.4 nm), PM6: Y6: ITIC-Th (28.9 nm) to PM6: ITIC-Th with an excessively large acceptor domain size of 49.3 nm. Comparing PM6: Y6 and PM6:Y6: ITIC-Th blends, the increased acceptor domain size in the latter blend potentially favors electron transport, and can partially account for the increased electron mobilities from 3.96 × 10 −4 to 4.54 × 10 −4 cm 2 V −1 second −1 . Besides, it is understandable that PM6:Y6: IT-4F exhibited the higher electron mobilities than the PM6: Y6 control, which is attributed to the increased CCL of oop π-π peak and the decreased electron energy disorder. [73][74][75] All the blend films exhibited reasonable phase segregation scales with acceptor domain size around 20 nm, except for the PM6: ITIC-Th blend with an acceptor domain size of approximately 49.3 nm. The excessively large phase separation in PM6: ITIC-Th devices is unfavorable for charge separation,

| CONCLUSION
To summarize, we improved the device processing and performance of PM6:Y6: IT-4F based OPVs, and further incorporated ITIC-Th with higher LUMO into the PM6: Y6 blend. The PM6:Y6: ITIC-Th device afforded a PCE of 17.2%, which outcompeted PM6: Y6 control device and became one of the highest PCE for OPVs. PM6:Y6 system demonstrated small E loss but low voltage; by contrast, PM6:ITIC-Th system demonstrated high voltage but large E loss . The resulting PM6:Y6: ITIC-Th OSCs interestingly demonstrated the lowest E loss along with over 17% PCE. The incorporation of ITIC-Th and IT-4F can balance the charge mobilities, and thereby retain and improve FF. Increased crystalline coherence length and smaller d-spacing of π-π peaks are observed with the third component incorporation, indicating enhanced crystallinity and thus improved active-layer morphology. It is also found that the energy transfer from IT-4F to Y6 is more evident than that from ITIC-Th to Y6. Our findings demonstrate the practicability of "new use for an old drug" strategy to pursue breakthroughs of OPVs. 75

SUPPORTING INFORMATION
Additional supporting information may be found online in the Supporting Information section at the end of this article.