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A Nonfullerene Small Molecule Acceptor with 3D Interlocking Geometry Enabling Efficient Organic Solar Cells Jaewon Lee, Ranbir Singh, Dong Hun Sin, Heung Gyu Kim, Kyu Chan Song, and Kilwon Cho* Organic solar cells (OSCs) based on electron donor–acceptor (D–A) bulk-heterojunction (BHJ) thin films have attracted extensive attention due to their advantages of low cost, light weight, mechanical flexibility, solution processability, and rapid energy payback time.[1–4] Fullerene derivatives have played a central role as electron acceptors in BHJ OSCs due to their high electron affinity and electron mobility, isotropic charge transport, and favorable nanoscale network forming behaviors. The power conversion efficiencies (PCEs) of OSCs exceed 10.8%.[5,6] Despite the widespread use of fullerene acceptors, these acceptors tend to exhibit several drawbacks, including poor photochemical stability in air, weak absorption of visible light, limited energy level variability, and high production costs.[7–9] These limitations have driven researchers to develop nonfullerene acceptors including small molecules and polymers[10–36] designed to replicate the favorable electronic properties of fullerenes while overcoming their aforementioned deficiencies. Thus far, the PCEs of nonfullerene OSCs have exceeded 6%.[10–13,24,34,36] Over the past several years, considerable efforts have been dedicated to the synthesis of new nonfullerene small molecule acceptors that incorporate electron-withdrawing building blocks, such as perylene diimide (PDI),[10–22] naphthalene diimide,[23] tetraazabenzodifluoranthene diimide,[24] bis(naphthalene imide)diphenylanthrazoline,[25] diketopyrrolopyrrole,[26] azadipyrromethene complexes,[27] phthalimide,[28] fluoranthene-fused imide,[29] quinacridone,[30] decacyclene triimide,[31] dicynodistyrylbenzene,[32] 3-ethylrhodanine,[33] and 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile.[34] Among these, PDI is the most commonly used building block for OSCs based on nonfullerene small molecule acceptors due to its good chemical, thermal, and light stabilities, strong electron-withdrawing ability, and high electron mobility;[10] however, monomeric PDI possesses a highly planar conformation and thus strong intermolecular π–π stackings,[37,38] leading to large crystalline domain formation and severe donor–acceptor phase separation in BHJ films.[39,40] Therefore, one of the most important design principles for the development of efficient PDI-based nonfullerene small molecule acceptors for use in OSCs is the restriction of self-aggregation among planar PDI Dr. J. Lee, Dr. R. Singh, D. H. Sin, Dr. H. G. Kim, K. C. Song, Prof. K. Cho Department of Chemical Engineering Pohang University of Science and Technology Pohang 790-784, South Korea E-mail: [email protected]

DOI: 10.1002/adma.201504010

Adv. Mater. 2016, 28, 69–76

molecules to facilitate nanoscale phase domain formation. Controlling the molecular geometries of PDI-based small molecule acceptors prepared from planar 2D π-conjugated structures to form nonplanar/twisted 3D conformations presents one of the most promising approaches to preventing the formation of excessively large domains and facilitating the isotropic charge transport needed for efficient charge separation and transport in BHJ OSCs.[10–21] Moreover, these 3D molecular geometries consisting of a plurality of PDI building blocks and extended π-conjugated frameworks promote charge delocalization and provide a large density of states at the lowest unoccupied molecular orbital (LUMO).[10] Here, we report a novel 3D small molecule acceptor, SF-PDI4, based on a spiro-bifluorene (SF) core connected with tetra-PDI building blocks that enables the construction of nonfullerene OSCs. SF-PDI4 was designed to transform planar PDI subunits into a 3D molecular conformation with a highly twisted spiro-bifluorene core (Figure 1) that could suppress molecular aggregation and facilitate excitation energy transfer among PDI subunits. Interestingly, SF-PDI4 exhibited a 3D interlocking geometry in which four PDI subunits depended on the central SF-core and were 3D interdigitated, thereby preventing excessive rotation among the PDIs and reinforcing conformational uniformity among the SF-PDI4 molecules. Two difluorobenzothiadiazole (2FBT)-based donor polymers, poly[(5,6-difluoro2,1,3-benzothiadiazole-4,7-diyl)-alt-(3′,3″′-di(2-decyltetradecyl)2,2′;5′,2″;5″,2″′-quaterthiophene-5,5″′-diyl)], P4T2FBT, and poly[(5,6-difluorobenzo-2,1,3-thiadiazole-4,7-diyl)-alt-((3,3′-di(2decyltetradecyl)-di(thiophene-2-yl)-2,2′-(E)-2-(2-(thiophen-2-yl)vinyl)thiophene)-5,5′-diyl], PV4T2FBT (Figure 2), were selected in our study because their electronic and optical properties matched those of the SF-PDI4 acceptor, a requirement of OSCs.[11] The combination of SF-PDI4 with PV4T2FBT, a novel donor polymer, yielded a high short circuit current density (JSC) of 12.02 mA cm–2 with a reasonably high open circuit voltage (VOC) and fill factor (FF), leading to a PCE of 5.98%. Importantly, the PV4T2FBT:SF-PDI4 blend film exhibited a wellmixed interpenetrating BHJ morphology with highly crystalline polymer domains and nanoscale donor–acceptor phase separation, indicating that SF-PDI4 formed an excellent 3D charge transport network for the fabrication of efficient OSCs. The chemical structure of SF-PDI4 is shown in Figure 1a. The 3D molecular conformation of SF-PDI4 was investigated by density functional theory (DFT) calculations using B3LYP/6-31G* basis[22] applied to a model molecule bearing methyl-trimmed alkyl chains for simplicity of computation. Figure 1b–d and Figure S1a–c (Supporting Information) illustrate the optimized ground-state geometries of SF-PDI4. The

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Figure 1. a) Molecular structure of SF-PDI4 nonfullerene acceptor. b) Front view, c) top view, and d) side view of the optimized geometry of SF-PDI4 based model molecule with methyl-trimmed alkyl chains using DFT calculation (B3LYP/6-31G*).

PDI planes had an interplanar angle of 56° to the fluorene plane of the SF core and oriented in the opposite direction each other (Figure 1b and Figure S1a (Supporting Information)). In the top view, the two fluorene planes of the SF-core were tilted at an angle of 90°, and the attached tetra-PDI subunits on the SF-core were oriented along the four cardinal directions and evenly distributed in space (Figure 1c and Figure S1b of Supporting Information). Interestingly, each PDI plane assumed a nonplanar structure with an 18° twisted angle due to the intramolecular steric hindrance (Figure 1d and Figure S1c

of Supporting Information). The twisted PDI plane significantly impacted its aggregation properties in the solid state, which, in turn, influenced the optoelectronic properties of the material.[12,41] The synthetic routes to SF-PDI4 are shown in Scheme S1 (Supporting Information). SF-PDI4 was synthesized via Pd(PPh3)4-catalyzed Suzuki coupling reactions between the monobrominated PDI (1Br-PDI1) and 2,2′,7,7′-tetrakis(4,4,5,5tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9′-spirobi[ fluorene] (4Bpin-SF). SF-PDI4 was readily soluble in common organic

Figure 2. a) Chemical structures of donor polymers investigated in this work. b) UV–vis absorption spectra. c) Energy level diagram of the inverted device structure with active materials. d) J–V curves and e) EQE spectra of P4T2FBT or PV4T2FBT:SF-PDI4 BHJ nonfullerene organic solar cells.

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according to the modified literature procedure.[11,43,44] The fluorination of the benzothiadiazole unit increases the crystallinity and hole transporting properties of the polymer as well as lowers the highest occupied molecular orbital (HOMO) level of the polymer, thereby enabling the achievement of a high VOC in the OSCs.[45–47] These features render the 2FBT-based polymers one of the most promising classes of donor materials yet identified for use in OSC applications. The novel PV4T2FBT compound was designed and synthesized to improve the intrinsic properties of the polymer, such as the light absorption and aggregation properties, relative to those of P4T2FBT by introducing a rigid π-extended thienylenevinylene (TVT) unit.[48] The thin film UV–vis absorption spectra of P4T2FBT, PV4T2FBT, and SF-PDI4 are shown in Figure 2b. The absorption maxima (λmax), which corresponded to the absorption coefficients (αmax), and the optical band gap (Egopt) of the films, and the solution absorption data are summarized in Table S1 (Supporting Information). SF-PDI4 displayed an intense UV absorption band centered at 531 nm (αmax = 4.8 × 104 cm−1) due to the π–π* transition of the chromophore, which complemented the absorption bands of P4T2FBT and PV4T2FBT. The absorption spectra of the two copolymers contain two spectral features, high-energy bands attributed to the localized π–π* transition and lower-energy bands ascribed to the intramolecular charge transfer (ICT) transition.[49] PV4T2FBT exhibited better absorption properties compared to P4T2FBT, probably due to the enhanced ICT transitions resulting from the stronger electron-donating properties of the TVT unit compared to the bithienyl (2T) unit.[48] The molecular orbital distributions calculated using DFT methods (Figure S13, Supporting Information) indicated that the dimer V4T2FBT model molecule (diV4T2FBT) presented longer and more delocalized molecular orbital isosurfaces compared to di-4T2FBT, further supporting the aforementioned issues associated with the light absorption behavior. The electrochemical properties of SF-PDI4, P4T2FBT, and PV4T2FBT were characterized by cyclic voltammetry (Figure S14, Supporting Information) and are summarized in Table S1 (Supporting Information). The onsets of oxidation and reduction potentials versus FeCp2+/0 (0.43 V) are 1.17 and −1.02 V, respectively. Thus, the HOMO and LUMO energies are estimated to be −5.97 and −3.78 eV from the onset oxidation and reduction potentials, respectively, assuming the absolute energy level of FeCp2+/0 to be 4.8 eV below vacuum.[50] The energy diagrams of the three photoactive materials, the interlayers, and the electrodes used in our study of the photovoltaic characteristics are shown in Figure 2c. The LUMO level of SF-PDI4 provided LUMO offsets sufficient for efficient exciton dissociation when blended with two donor polymers. BHJ OSC devices were fabricated using P4T2FBT and PV4T2FBT as the donor and SF-PDI4 as the acceptor. The inverted OSC devices with a configuration of glass/indium tin oxide (ITO) (110 nm)/zinc oxide (ZnO) (40 nm)/polymer:SFPDI4 (85–90 nm)/V2O5 (2 nm)/Ag (80 nm) were fabricated and characterized.[51] The current density–voltage (J–V) characteristics of the devices, measured under AM 1.5 G solar illumination at 1 sun (100 mW cm−2), are shown in Figure 2d and are summarized in Table 1. The optimal performances of the polymer:SF-PDI4-based devices were achieved in 1:0.8 (w/w) blend compositions spin-coated from a CB solution containing

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solvents such as dichloromethane, chloroform, tetrahydrofuran (THF), and chlorobenzene (CB) at room temperature due to the solubilizing alkyl substituents. Interestingly, SF-PDI4 displayed indistinct chemical shifts in the aromatic region in the 1H nuclear magnetic resonance (NMR) spectra at room temperature (Figure S4a, Supporting Information), indicating that the SF-core and the attached PDI subunits did not freely rotate in solution at room temperature. As confirmed by the DFT results (Figure 1 and Figure S1 of Supporting Information), the four PDI subunits depending from the central SF-core were threedimensionally interdigitated to stabilize the molecular geometry, which suppressed the free rotation of the SF-core and PDIs. This 3D molecular geometry reinforced the structural rigidity and conformational uniformity of the SF-PDI4 molecule, leading to a 3D interlocking structure. A similar interlocking conformation was reported recently in a 3D PDI-based small molecule acceptor using a compact tetrahedron core.[14] We could obtain more clear chemical shifts of SF-PDI4 in the aromatic region in the 1H NMR spectra at elevated temperature (120 °C); the high boiling point solvent of o-dichlorobenzene-d4 was used for the high-temperature 1H NMR spectra measurement to increase the rotational freedom of the subunits in the SF-PDI4 molecule at elevated temperature (Figure S4b, Supporting Information).[42] To verify the synthesis of the target molecule (SF-PDI4), we performed the matrix-assisted laser desorption ionization time of flight mass spectroscopic measurements (Figure S5, Supporting Information) and elemental analysis (Figure S6, Supporting Information). SF-PDI4 exhibited good thermal stability with a decomposition temperature (Td, 5% weight loss) of 398 °C under a nitrogen atmosphere, as measured by thermogravimetric analysis (Figure S9, Supporting Information). Differential scanning calorimetry (DSC, Figure S10, Supporting Information) analysis indicated that neither melting nor recrystallization peaks were observed in the SF-PDI4 sample, suggesting that the 3D structural conformation of the tetra-PDIs substituted on the SF-core suppressed the overwhelming aggregation of planar PDI subunits. The two donor polymers, P4T2FBT and PV4T2FBT (Figure 2a), exhibited an endothermic transition for melting (Tm = 250 and 245 °C, respectively) during heating and an exothermic transition for aggregation (Tc = 235 and 191 °C, respectively) during cooling (Figure S10, Supporting Information). Grazing incidence wide angle X-ray scattering (GIWAXS) studies revealed that the SF-PDI4 film was amorphous (Figure S11b, Supporting Information), whereas the P4T2FBT and PV4T2FBT films were highly crystalline (Figure S12, Supporting Information). Atomic force microscopy (AFM) images indicated that SF-PDI4 could be solution-processed to form very smooth surface features in the thin film, whereas PDI1, which is a monomeric PDI used as the subunits of the SF-PDI4 molecule, displayed large surface features (Figure S11, Supporting Information). Since it is highly required to prevent excessive aggregation/crystallization of PDI molecules for the formation of nanoscale BHJ morphology, SF-PDI4 is a suitable candidate as a nonfullerene acceptor for use in BHJ OSCs. The molecular structures of the donor polymers (P4T2FBT and PV4T2FBT) are presented in Figure 2a, and their synthetic routes are illustrated in Scheme S2 (Supporting Information). P4T2FBT, a previously reported polymer, was synthesized

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www.MaterialsViews.com Table 1. Photovoltaic performances of the solar cells based on P4T2FBT:SF-PDI4 and PV4T2FBT:SF-PDI4 under standard AM 1.5G illumination. D:Aa) P4T2FBT:SF-PDI4

Additive

VOC [V]

JSC [mA cm–2]

FF [%]

PCEave(max) [%]

µh [cm2 V–1 s–1]

µe [cm2 V–1 s–1]



4.42 (4.54)

1.93 × 10–4

1.45 × 10–5

5.11 (5.27)

3.26 ×

1.52 × 10–5

0.92 ± 0.01

9.93 ± 0.13

49.2 ± 1.65

b)

0.93 ± 0.001

11.03 ± 0.18

51.1 ± 1.36



0.90 ± 0.01

10.98 ± 0.16

50.6 ± 1.84

4.96 (5.08)

1.28 × 10–4

1.39 × 10–5

DIOb)

0.90 ± 0.005

12.02 ± 0.09

54.2 ± 1.63

5.82 (5.98)

2.46 × 10–4

1.93 × 10–5

DIO PV4T2FBT:SF-PDI4

10–4

a) The optimized donor:acceptor ratios were 1:0.8 (w/w%); b)The active layers were deposited from solution in chlorobenzene with DIO (3 vol%) and (2 vol%) for P4T2FBT:SF-PDI4 and PV4T2FBT:SF-PDI4, respectively. The averages are from over 12 devices.

3 vol% (for P4T2FBT:SF-PDI4) and 2 vol% (for PV4T2FBT:SFPDI4) 1,8-diiodooctane (DIO) additive (Figure S16 and Table S2 of Supporting Information). The resulting PV4T2FBT:SF-PDI4 BHJ solar cells showed a high PCE of 5.98% with an open circuit voltage (VOC) of 0.90 V, a short circuit current density (JSC) of 12.02 mA cm−2, and an FF of 54.2. Although both polymer:SF-PDI4-based solar cells showed high VOC values, the P4T2FBT:SF-PDI4-based cells exhibited a higher VOC (≈0.93 V) compared to the PV4T2FBT:SF-PDI4-based solar cells (≈0.90 V) due to the deeper HOMO level of P4T2FBT.[52] These results indicated that P4T2FBT:SF-PDI4 is the better-performing material combination in terms of providing favorable energy level matching properties. The replacement of P4T2FBT with PV4T2FBT, however, increased the PCE from 5.27% to 5.98%. In addition to stronger light absorption property of PV4T2FBT (Figure 2b), favorable molecular packing and orientation, distinct BHJ morphology, and efficient charge dissociation and collection in the PV4T2FBT:SF-PDI4 blends may have contributed to the higher JSC and FF in the PV4T2FBT:SF-PDI4based solar cell devices.[15,42,48] This suggests that the better-performing material combination depends not only on the energy level matching between the donor polymer and small molecule acceptor, but also on the intrinsic polymer properties and BHJ film morphology.[11,42,48] The external quantum efficiency (EQE) spectra of the optimal P4T2FBT:SF-PDI4 and PV4T2FBT:SF-PDI4 BHJ solar cells are shown in Figure 2e. It can be seen that the EQE is in the range of 38%–55% between 400 and 600 nm, indicating that SF-PDI4 contributed significantly to the photocurrent generation in the BHJ solar cells. This is another attractive feature of nonfullerene small molecule acceptor-based OSCs, which can readily cover different regions of the solar spectrum with donor polymers.[10–18] The short circuit current densities calculated from the EQE spectra agreed well with the trends in the direct J–V measurements (Table S3, Supporting Information). The morphologies of the P4T2FBT:SF-PDI4 and PV4T2FBT:SF-PDI4 blend films were characterized by transmission electron microscopy (TEM) to better understand the BHJ morphologies in the devices.[53] As shown in Figure 3, TEM images showed nanofibrillar structures in both the P4T2FBT:SF-PDI4 and PV4T2FBT:SF-PDI4 blend systems. These fibrils arose from the highly crystalline features of both the P4T2FBT and PV4T2FBT polymers, as confirmed by DSC (Figure S10, Supporting Information) and GIWAXS (Figure S12, Supporting Information) measurements. Both blend systems prepared with DIO (w/DIO) yielded TEM

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images that revealed more distinct and dense nanofibrillar domains compared with the corresponding systems prepared without DIO (w/o DIO). These results were corroborated by the enhanced intermolecular π–π* transition peaks observed in the UV–vis spectra of the blend films (Figure S18, Supporting Information). The PV4T2FBT:SF-PDI4 w/DIO, however, yielded TEM images that displayed smaller fibrils and a wellmixed BHJ morphology compared to the P4T2FBT:SF-PDI4 w/DIO. The differences in the domain sizes can be partially attributed to the distinct aggregation properties of P4T2FBT and PV4T2FBT.[42,46–48,54] The bi-continuous interpenetrating networks of the PV4T2FBT:SF-PDI4 blends prepared w/DIO produced efficient exciton diffusion, dissociation, and charge transport.[55–57] AFM was employed to investigate the surface morphologies of the photoactive layers (Figures S19 and S20, Supporting Information). The surface roughness values of both blend systems dramatically decreased w/DIO treatment (Figure S19, Supporting Information); however, the surface of the PV4T2FBT:SF-PDI4 film prepared w/DIO was more smooth and homogeneous (root-mean square (rms) surface roughness values ≈ 0.87 nm) than that of the P4T2FBT:SF-PDI4 film prepared w/DIO (rms ≈ 2.64 nm). Moreover, the PV4T2FBT:SFPDI4 film prepared w/DIO showed much smaller domains compared to the P4T2FBT:SF-PDI4 film prepared w/DIO. As the solubility improved, the amorphous properties of SF-PDI4, which formed a highly twisted molecular structure originating from the SF-core, contributed to the small domain sizes in the PV4T2FBT:SF-PDI4 blend films. The AFM phase images revealed that the PV4T2FBT:SF-PDI4 blend films have smaller and denser nanofibrils compared to the P4T2FBT:SF-PDI4 blend films (Figure S20, Supporting Information), consistent with the TEM results. The correlation between the high PCE and BHJ morphological properties of the OSC devices was explored by collecting photoluminescence (PL) intensity measurements from the blend films and comparing these results with those obtained from the relevant neat polymer films (Figure 3c,f). The PL quenching efficiencies in the P4T2FBT:SF-PDI4 w/DIO and the PV4T2FBT:SF-PDI4 w/DIO blend films, as compared with the respective neat polymer films, were 90% and 95%, respectively, indicating that better exciton dissociation at donor–acceptor interfaces occurred in the PV4T2FBT:SF-PDI4 blend than in the P4T2FBT:SF-PDI4 blend.[58] We also investigated the PL quenching efficiency of SF-PDI4 (excited at 531 nm), revealing almost complete quenching (>95%) in the PV4T2FBT:SFPDI4 blend films (Figure S21, Supporting Information), which

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COMMUNICATION Figure 3. TEM images of a) P4T2FBT:SF-PDI4 without DIO, b) P4T2FBT:SF-PDI4 with DIO (3 vol%), d) PV4T2FBT:SF-PDI4 without DIO, and e) PV4T2FBT:SF-PDI4 with DIO (2 vol%), and corresponding PL quenching spectra (right, panels (c) and (f), excited at 632 nm for P4T2FBT and 640 nm for PV4T2FBT, respectively).

suggests efficient charge transfer driven by SF-PDI4 as well as PV4T2FBT. Considering the strong visible light absorption of SF-PDI4, efficient dissociations of excitons generated from SF-PDI4 significantly contributed to the incident photon to current conversion. The charge transport properties of the P4T2FBT:SF-PDI4 and PV4T2FBT:SF-PDI4 blend systems were investigated using the space charge-limited current (SCLC) method.[59] The resulting hole and electron mobilities are shown in Figure S22 (Supporting Information) and are summarized in Table 1. The both blend systems clearly exhibited significant enhancements in the carrier mobilities upon DIO treatment; however, among the optimized BHJ devices prepared w/DIO, the PV4T2FBT:SFPDI4 blend displayed more balanced hole/electron mobility compared to the P4T2FBT:SF-PDI4 blend, which contributed to the higher FF measured in the PV4T2FBT:SF-PDI4-based OSCs (Table S4, Supporting Information). The enhanced hole and electron mobilities of the SCLC devices upon DIO treatment clearly indicated that SF-PDI4 formed interpenetrating BHJ networks in conjunction with the donor polymers, yielding multidimensional interconnectivity that could be attributed to the 3D interlocking geometry, thereby enabling good rigidity and uniformity in the molecular conformation. To understand the dramatic performance increase of the SF-PDI4-based SCLC and OSC devices w/DIO treatment, the crystalline properties of both the polymer:SF-PDI4 blend films were investigated using GIWAXS measurements (Figure 4). The diffraction patterns collected from both polymer:SF-PDI4 blend films revealed dramatic changes in the molecular orientations of P4T2FBT and PV4T2FBT. Both polymer:SF-PDI4 blend films w/o DIO showed dominant (h00) reflections in the out-ofplane (qz) direction, indicating that most of the polymer crystals

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adopted an edge-on orientation on the substrate;[60] however, the polymer:SF-PDI4 blend films w/DIO revealed both (100) and (010) diffraction peaks in the out-of-plane (qz) direction, indicating that the polymer crystals assumed both edge-on and face-on orientations (mixed orientation).[47,48,61] The (010) diffraction peaks of P4T2FBT and PV4T2FBT in the blend films w/DIO in the out-of-plane direction yielded π–π stacking distances of 3.61 Å (at qz = 1.74 Å–1) and 3.64 Å (at qz = 1.73 Å–1), respectively. Intense (100) diffraction peaks at qxy = 0.25 Å–1 and 0.26 Å–1 corresponding to the lamellar interchain distances of 25.1 and 23.7 Å for P4T2FBT and PV4T2FBT, respectively, were observed. These results indicate that P4T2FBT and PV4T2FBT in the blend films have a strong tendency to take face-on orientations upon DIO treatment, which is beneficial for the vertical charge transportation in the solar cell devices.[40,47,48] Therefore, the molecular orientation changes observed in P4T2FBT and PV4T2FBT upon w/DIO treatment increased the SCLC hole mobilities and JSC. Because the π–π stacking distances (3.61 and 3.64 Å at qz) of P4T2FBT and PV4T2FBT, respectively were not significantly different, the higher PCEs of the PV4T2FBT:SF-PDI4-based cells than those of the P4T2FBT:SFPDI4-based cells could be attributed to the well-mixed interpenetrating BHJ morphology, as confirmed by the TEM, AFM, and PL results. In conclusion, we have presented the synthesis and photovoltaic properties of a new nonfullerene small molecule acceptor material consisting of SF-core and tetra-PDI subunits that constructed a 3D geometry. Two donor polymers, P4T2FBT and PV4T2FBT, were synthesized and combined with SF-PDI4 to achieve complementary light absorption and energy level matching. We demonstrated SF-PDI4-based OSCs with PCEs up to 5.98%, enabled by the PV4T2FBT:SF-PDI4 combination.

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Figure 4. GIWAXS images of a) P4T2FBT:SF-PDI4 (1:0.8 w/w) and b) PV4T2FBT:SF-PDI4 (1:0.8 w/w) blend films prepared from CB without or with DIO (3 vol% for P4T2FBT:SF-PDI4 and 2 vol% for PV4T2FBT:SF-PDI4, respectively). The asterisks indicate si-wafer diffuse rings. c) qz and d) qxy scans of GIWAXS from both blend films.

SF-PDI4, with great solubility and 3D interlocking geometry, yielded smooth and homogeneous blend thin films that favored the interpenetrating BHJ morphology. Our comparative study revealed that replacing P4T2FBT with PV4T2FBT significantly increased the solar cell efficiency by increasing light absorption and providing a more favorable BHJ morphology when blended with SF-PDI4. The proof-of-concept device for a new 3D PDI-based nonfullerene acceptor offered a high photovoltaic performance and this achievement represents an important strategy for designing 3D electron-accepting materials to achieve efficient photovoltaic performances.

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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgments J.L. and R.S. contributed equally to this work. This work was supported by a grant (code No. 2011-0031628) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Science, ICT and Future Planning, Korea. The authors thank the

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Received: August 17, 2015 Revised: September 17, 2015 Published online: November 5, 2015

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Adv. Mater. 2016, 28, 69–76

A Nonfullerene Small Molecule Acceptor with 3D Interlocking Geometry Enabling Efficient Organic Solar Cells.

A new 3D nonfullerene small-molecule acceptor is reported. The 3D interlocking geometry of the small-molecule acceptor enables uniform molecular confo...
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