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Excellent Long-Term Stability of Power Conversion Efficiency in Non-Fullerene-Based Polymer Solar Cells Bearing Tricyanovinylene-Functionalized n-Type Small Molecules Eun Yi Ko, Gi Eun Park, Ji Hyung Lee, Hyung Jong Kim, Dae Hee Lee, Hyungju Ahn, Mohammad Afsar Uddin, Han Young Woo, Min Ju Cho, and Dong Hoon Choi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15707 • Publication Date (Web): 24 Feb 2017 Downloaded from http://pubs.acs.org on February 24, 2017

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ACS Applied Materials & Interfaces

Excellent Long-Term Stability of Power Conversion Efficiency in Non-Fullerene-Based Polymer Solar Cells Bearing Tricyanovinylene-Functionalized nType Small Molecules Eun Yi Ko†, ‡, Gi Eun Park†, ‡, Ji Hyung Lee†, Hyung Jong Kim†, Dae Hee Lee†, Hyungju Ahn∥, Mohammad Afsar Uddin§, Han Young Woo†, Min Ju Cho*,† and Dong Hoon Choi*,†

†

Department of Chemistry, Research Institute for Natural Sciences, Korea University, 5 Anam-

dong, Sungbuk-gu, Seoul 136-701, Korea §Department

of Nanofusion Engineering, Departement of Cogno-Mechatronics Engineering,

Pusan National University, Miryang 627-706, Korea ∥Department

of Life Science & Chemical Materials, Pohang Accelerator Laboratory, POSTECH,

80 Jigok-ro, Nam-gu Pohang 790-834, Korea

KEYWORDS: tricyanovinylene, n-type small molecules, non-fullerene solar cell, bulk heterojunction organic solar cell, stability *Corresponding authors: [email protected], [email protected] 1

ACS Paragon Plus Environment


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ABSTRACT New small molecules having modified acceptor strength and π-conjugation length and containing dicyanovinylene (DCV) and tricyanovinylene (TCV) as a strongly electronaccepting unit with indacenodithiophene, namely, IDT(DCV)2, IDT(TCV)2, and IDTT(TCV)2, were synthesized and studied in terms of their applicability to polymer solar cells with PTB7-Th as an electron-donating polymer. Intriguingly, the blended films containing IDT(TCV)2 and IDTT(TCV)2 exhibited superior shelf-life stabilities of more than 1000 h without any reduction in the initial power conversion efficiency. The low-lying lowest unoccupied molecular orbital energy levels and robust internal morphologies of small TCV-containing molecules could afford excellent shelf-life stability.

INTRODUCTION Polymer solar cells (PSCs) have been extensively investigated owing to their advantages of low cost, light weight, and flexibility, and are considered promising alternatives for efficient utilization of solar energy based on a bulk heterojunction (BHJ) structure.1–4 In conventional BHJ PSCs, fullerene derivatives such as [6,6]-phenyl-C61/C71-butyric acid methyl esters (PC61BM/PC71BM) are commonly used as electron acceptors.5 Although they enable record-high efficiencies in PSCs incorporating both p-type donor polymers and small molecules, the intrinsic properties of fullerene derivatives are difficult to tune, and fullerene-based PSCs exhibit inferior stability under ambient conditions. Moreover, synthesis of fullerene derivatives is difficult and costly, and it is hard to modify their chemical structures for efficient fabrication of BHJ PSCs.6,7 Thus, considerable efforts have been made to advance non-fullerene acceptors, which are 2


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currently undergoing rapid development owing to their versatile molecular structure and adjustable intrinsic properties.8,9 In addition, it is possible to tune the absorption spectral region and realize appropriate lowest unoccupied molecular orbital (LUMO) energy levels of acceptor molecules in PSCs.8 Various accepting motifs such as naphthalene diimide, perylene diimide, and dicyanovinylene (DCV) derivatives are most frequently exploited owing to their valuable functionality and the resultant performance in non-fullerene-based PSCs.1,8,10–14 Acceptor-donor-acceptor-type non-fullerene acceptors with DCV moieties have recently been reported to display outstandingly improved efficiencies.15,16 It was found that the accepting strength of the entire molecule can be modulated using combinations of various donating units and the DCV moiety, producing various optical and electrochemical behaviors. Accordingly, fullerene-free PSCs using DCV-containing small molecules showed high performance. For example, Beaujuge et al. synthesized three small molecules with different donor units containing 2-(benzo[c][1,2,5]thiadiazol-4-ylmethylene)malononitrile moieties, achieving facile control of the intrinsic properties by varying the core units and realizing an outstanding power conversion efficiency (PCE) above 5%.11 Additionally, Zhan et al. designed several small molecules based on 2-(3-oxo-2,3dihydroinden-1-ylidene)malononitrile groups, such as ITIC analogues, and demonstrated a design principle for restricting planarity and self-aggregation. The ITIC-based PSC demonstrated a high PCE above 11%, suggesting that ITIC analogues represent a new way to obtain highperformance non-fullerene PSCs.1,13,17 Despite the high efficiencies of small molecules, several shortcomings that prevent further 3



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improvement of their photovoltaic performance need to be overcome. The main vulnerability of PSCs is their limited stability in the presence of oxygen, moisture, thermal stress, etc.18–24 There are a few previous studies on the thermal stability of non-fullerene-based PSCs.22–24 For instance, non-fullerene based PSCs containing PBDB-T:ITIC blend films maintained their PCE well upon thermal treatment, whereas a PBDB-T:PC71BM-based device showed relatively poor thermal stability.23 Therefore, the structure of ITIC can be recognized as effective for improving the thermal stability of devices. Another important factor limiting the stability of PSC performance is known to be oxygen intrusion into the active layer.25–27 To overcome this shortcoming, new n-type small molecules should be designed and synthesized to have the desired molecular energy levels. In particular, low-lying LUMO energy levels (below −4.0 eV) are required to realize good oxidative stability.28,29 Therefore, a strong accepting moiety bearing a cyano unit is the key design requirement for obtaining the energy levels needed to retard oxidation. In this study, we present the design and syntheses of new small molecules containing strongly electron-accepting tricyanovinylene (TCV) units. TCV has a relatively compact molecular structure and can be attached by simple synthesis to the conjugated indacenodithiophene (IDT) core having hexylphenyl side groups. The structure is very favorable for suppression of strong self-aggregation owing to the shorter length of the TCV unit and the unique three-dimensional molecular structure.9,13,30 We synthesized three small molecules, IDT(DCV)2, IDT(TCV)2, and IDTT(TCV)2, with modified acceptor strength and π-conjugation length. These molecules exhibited various absorption regions from the visible to the near4


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infrared and cascading LUMO energy levels. Remarkably, IDT(TCV)2 and IDTT(TCV)2 exhibited superior shelf-life stabilities of more than 1000 h while maintaining the initial PCE, whereas IDT(DCV)2- and PC71BM-based devices retained only 44 and 61% of their initial PCEs, respectively, after the designated test time. The excellent long-term stability resulted from the low-lying LUMO energy levels and robust internal morphologies, demonstrating that TCV is a promising new acceptor unit for highly efficient non-fullerene PSCs.

EXPERIMENTAL SECTION Materials and Synthesis. All materials and solvents used were purchased from the Alfa Aesar Company, Sigma-Aldrich Chemical Company, and TCI, and were used without further purification. PTB7-Th was purchased from 1-Materials (Mn = 40,000 Da). IDT, IDT-CHO, and IDT-2Br were synthesized according to previously described methods.31,32

Synthesis of IDT(DCV)2: A mixture of IDT-CHO (0.35 g, 0.36 mmol), malononitrile (0.60 g, 0.91 mmol), a trace amount of ammonium acetate, and chloroform (20 mL) was placed in a 250-mL two-necked round bottom flask with a condenser and heated to reflux at 60 °C after degassing with nitrogen. Methanol was poured into the mixture, and the precipitate was filtered while hot. The obtained orange-red powder was purified by silica gel column chromatography using hexane and dichloromethane (1:2) as an eluent to yield the final product as orange-red solid (0.32 g, 83.1% 5



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yield). 1H NMR (500 MHz, CDCl3): δ 7.76 (s, 2H), 7.61 (s, 2H). 7.57 (s, 2H), 7.07–7.11 (m, 16H), 2.55–2.58 (m, 8H), 1.55–1.59 (m, 8H), 1.27–1.34 (m, 24H), 0.86–0.89 (t, J = 7.5 Hz, 12H). 13

C NMR (125 MHz, CDCl3): δ 157.78, 155.90, 153.20, 150.91, 142.60, 139.87, 138.83, 136.03,

134.03, 128.84, 127.49, 119.67, 114.22, 113.66, 82.08, 63.10, 35.52, 31.69, 31.29, 29.07, 22.58, 14.09. MALDI-TOF m/z: calcd for C72H74N4S2, 1058.54; found: 1058.49. Anal. calcd for C72H74N4S2: C 81.62, H 7.04, N 5.29, S 6.05; found: C 81.73, H 6.98, N 5.15, S 6.14.

Synthesis of IDT(TCV)2: IDT (0.50 g, 0.55 mmol) was dissolved in anhydrous THF (50 mL) in a dry two-necked round bottom flask, and the mixture was degassed with nitrogen for 30 min. n-BuLi (0.46 mL, 1.20 mmol) was dropwise added at –78 °C. After 2 h of stirring at –78 °C, tetracyanoethylene (0.15 g, 1.17 mmol) was added, and the mixture was warmed to room temperature. The reaction was quenched by the addition of diluted HCl, and the organic layer was extracted with dichloromethane, washed with brine, and dried over anhydrous Na2SO4. After evaporating the solvent, the residue was purified by silica gel column chromatography using dichloromethane as an eluent to yield the final product as dark blue powder (0.18 g, 34.6% yield). 1H NMR (500 MHz, CDCl3): δ 7.91 (s, 2H), 7.66 (s, 2H), 7.12–7.14 (d, J = 10 Hz, 8H), 7.06–7.08 (d, J = 10 Hz, 8H), 2.57–2.60 (t, J = 7.5 Hz, 8H), 1.55–1.59 (m, 8H), 1.29–1.33 (m, 24H), 0.86–0.89 (t, J = 7.5 Hz, 12H). 13C NMR (125 MHz, CDCl3): δ 159.13, 156.97, 155.55, 143.06, 139.01, 137.88, 136.31, 134.68, 132.65, 129.08, 127.43, 120.40, 112.78, 112.55, 111.98, 81.65, 63.41, 35.53, 31.68, 31.28, 29.06, 22.58, 14.09. MALDI-TOF m/z: calcd for C74H72N6S2, 1108.53; 6


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found: 1108.91. Anal. calcd for C74H72N6S2: C 80.10, H 6.54, N 7.57, S 5.78; found: C 79.80, H 6.60, N 7.43, S 6.17. Synthesis of IDT-2T: A two-necked round bottom flask was charged with IDT-2Br (0.60 g, 0.56 mmol), 2(tributylstannyl)thiophene (0.50 g, 1.34 mmol), and dry toluene. After sufficient degassing with nitrogen, Pd2(dba)3 (5 mol.%) and P(o-tol)3 (10 mol.%) were added. The mixture was heated to 100 °C, stirred overnight, and subsequently cooled down to room temperature, treated with water, and extracted with dichloromethane. The organic layer was dried over anhydrous Na2SO4. After evaporating the solvent, the residue was purified by silica gel column chromatography using hexane and dichloromethane (1:1) as an eluent to yield the final product as yellow-green powder (0.52 g, 0.49 mmol, 87.5% yield). 1H NMR (500 MHz, CDCl3): δ 7.37 (s, 2H), 7.16–7.19 (m, 10H), 7.14–7.15 (dd, J= 5 Hz, 2H), 7.06–7.08 (m, 10H), 6.97–6.99 (m, 2H), 2.54–2.57 (t, J = 7.5 Hz, 8H), 1.57–1.61 (m, 8H), 1.26–1.36 (m, 24H), 0.85–0.88 (t, J = 7.5 Hz, 12H).

Synthesis of IDTT(TCV)2: A dry two-necked round bottom flask was charged with IDT-T (0.50 g, 0.47 mmol) dissolved in anhydrous THF (50 mL), and the synthesis was carried out similarly to the case of IDTT(TCV)2 using n-BuLi (0.4 mL, 1.04 mmol) and tetracyanoethylene (0.15 g, 1.17 mmol). The reaction was quenched as for IDT(TCV)2. The residue was purified by silica gel column chromatography using dichloromethane as an eluent to yield the final product as dark green powder (0.32 g, 0.25 7



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mmol, 53.5% yield). 1H NMR (500 MHz, CDCl3): δ 7.95–7.96 (d, J = 5 Hz, 2H), 7.49 (s, 2H), 7.41 (s, 2H), 7.32–7.33 (d, J = 5 Hz 2H), 7.13–7.15 (d, J = 10 Hz, 8H), 7.10–7.12 (d, J = 10 Hz, 8H), 2,56–2.59 (t, J = 7.5 Hz, 8H), 1.58–1.62 (m, 8H), 1.26–1.37 (m, 24H), 0.86–0.89 (t, J = 7.5 Hz, 12H).

13

C NMR (125 MHz, CDCl3): δ 158.78, 154.83, 152.81, 146.60, 142.35, 141.50,

140.39, 137.55, 136.64, 131.94, 131.21, 128.74, 127.70, 124.91, 124.57, 118.44, 112.72, 112.30, 80.42, 63.26, 35.57, 31.70, 31.33, 29.12, 22.59, 14.10. MALDI-TOF m/z: calcd for C82H76N6S4, 1272.50; found: 1271.32. Anal. calcd for C82H76N6S4: C 77.32, H 6.01, N 6.60, S 10.07; found: C 77.42, H 6.03, N 6.49, S 10.06

Instrumetation. A Bruker 500 MHz spectrometer (AscendTM 500, Bruker) was used to record 1

H and

13

C NMR spectra of all compounds. A Thermo Scientific Flash 2000 (Thermo Fisher

Scientific) elemental analyzer was used to determine the content of C, H, N and S. Matrixassisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry (LRF20, a Bruker Daltonics) was used to determine the molecular mass of each compound. The absorption spectra of small molecules in chloroform solutions and thin films were recorded using a UV-Vis absorption spectrometer (Agilent 8453, photodiode array, λ = 190–1100 nm). Electrochemical properties were characterized by cyclic voltammetry (CV, eDAQ EA161), with the electrolyte solution prepared by dissolving tetrabutylammonium hexafluorophosphate (Bu4NPF6) in dry dichloromethane. A Pt wire and Ag/AgCl were used as the counter and reference electrodes, respectively. The small molecules were added to the electrolyte solution for CV measurements. 8


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Grazing incidence wide-angle X-ray scattering (GIWAXS) measurements were conducted at the 9A (U-SAXS) beamline (energy = 11.05 keV, pixel size = 79.59 µm, 1.12199 Å, 2θ = 0–20°) of the Pohang Accelerator Laboratory (PAL). Samples were prepared by spincoating small molecule solutions onto a SiO2 wafer. The surface morphologies of films were explored using an atomic force microscope (XE-100, Advanced Scanning Probe Microscope, PSIA) with a silicon cantilever. The samples were fabricated as in the case of GIWAXS. Optical microscopy (OM) imaging was performed on a KSM-BA3 (T) microscope (Samwon). Transmission electron microscopy (TEM) imaging was performed using a Tecnai G2F30 transmission electron microscope (FEI Inc.; accelerating voltage = 300 kV). The corresponding samples were prepared by coating the blend solution on a carbon-coated copper grid.

Fabrication of Thin Film Transistors To fabricate bottom-gate top-contact (BGTC) TFTs, the n-doped Si/SiO2 wafer substrate was washed as described in literature.33 After treatment with n-octyltrichlorosilane (OTS) to form the self-assembled monolayer, 10 mg mL–1 chloroform solutions of IDT(DCV)2, IDT(TCV)2, and IDTT(TCV)2 were spin-coated at 2000 rpm for 40 s to fabricate the devices. The thin films on the OTS monolayer were annealed at 100 °C for 10 min. The source and drain Au electrodes (70 nm) (channel width of 1500 µm and length of 100 µm) were deposited by thermal evaporation under high vacuum. The TFTs were characterized using a Keithley 4200 SCS analyzer under vacuum. The field-effect carrier mobility (µ) was calculated using a previously reported equation.33 9



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Fabrication of Inverted Solar Cells Inverted solar cells were fabricated on an indium-tin-oxide (ITO) glass anode. The ITO-coated glass was successively washed with acetone, water, and isopropanol (10 min each). The ZnO layer was spin-coated on the washed ITO glass at 3000 rpm for 40 s after a 20-min UV-ozone pre-treatment and then annealed at 165 °C for 1 h. The blended solutions of PTB7-Th and small molecules were spin-coated onto the ZnO layer to generate the photoactive layer. IDT(DCV)2 was blended with PTB7-Th in a 1:1.2 weight ratio to produce a 2 wt.% o-dichlorobenzene solution. TCV-functionalized small molecules were blended in a 1:1.5 weight ratio to produce 2 wt.% chlorobenzene solutions. The blended solutions were stirred overnight at 60 °C. Finally, MoO3 (10 nm) and Ag (100 nm) were deposited on the photoactive layer by thermal evaporation. The current density-voltage (J-V) characteristics were measured by a Keithley 2400 source meter under AM 1.5 G illumination at 100 mW cm–2 (Oriel, 1000 W). The incident light intensity was measured by a calibrated broadband optical power meter (Spectra Physics, Model 404). EQE spectra were recorded by a K3100 EQX instrument with a K240 XE300 lamp source.

Fabrication of space-charge-limited current devices The hole and electron carrier mobilities were determined using the space-charge-limited current (SCLC) method.34 The hole-only device has the device configuration of ITO/poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/active layer/Au (100 nm), and the electron-only devices were fabricated with a configuration of ITO/ZnO/active layer/LiF (1 nm)/Al (100 nm). After the solution was spin-coated on the PEDOT:PSS or ZnO layer, the blend 10


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film was dried, and LiF (1 nm)/Al (100 nm) or Au (100 nm) was deposited on top of the blend film under vacuum.

RESULTS AND DISCUSSION

Scheme 1. Procedure and conditions for synthesis of the small molecules: i. Lithium diisopropylamide (LDA), DMF, THF, –78 °C; ii. malononitrile, AcONH4, chloroform, reflux; iii. n-BuLi, tetracyanoethylene, THF, –78 °C; iv. N-bromosuccinimide (NBS), chloroform; v. 2(tributylstannyl)thiophene, Pd2(dba)3, P(o-tol)3, toluene, 100 °C. Chemical structure of PTB7Th as donor polymer (in dashed line).

Design and synthesis

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To investigate the effect of the acceptor strength and conjugation length on the optoelectronic characteristics, three new small molecules were synthesized. The core IDT unit, featuring bulky alkyl side chains designed to alleviate aggregation, was prepared using previously described methods.31 IDT(DCV)2 was synthesized from IDT-CHO and malononitrile in the presence of a trace amount of catalyst via Knoevenagel condensation, whereas IDT(TCV)2 and IDTT(TCV)2 were synthesized by the same method from IDT and IDT-2T (Scheme 1). All the small molecules exhibited good solubility in common organic solvents such as chlorobenzene (CB), odichlorobenzene (o-DCB), and chloroform at room temperature. Differential scanning calorimetry measurements were performed to examine the thermal properties of the small molecules (Figure S1); none of them displayed any specific transitions until 250 °C. The electron distributions and molecular geometries were calculated at the B3LYP/631G level of theory to compare the contribution of the molecular structure to facile charge transport (Figure S2). To facilitate the calculations, the hexylphenyl side chain was replaced with a methylphenyl group. The bulky alkyl side chains in small molecules induce distorted geometries, resulting in steric hindrance. The three small molecules clearly show a delocalized electron distribution in both the highest occupied molecular orbitals (HOMOs) and LUMOs because of their highly coplanar backbones. These highly planar structures facilitate π-electron delocalization and possibly improve the charge mobility. Furthermore, the bulky alkyl side chains can be expected to suppress strong molecular aggregation.32

12


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Figure 1. UV–vis absorption spectrum and photographs of small molecule solutions. (a) Molar extinction coefficient (ε) in solution and (b) absorption spectra of thin films (c) Cyclic voltammograms in solution. (d) Energy levels of PTB7-Th, n-type small molecules, and PC71BM. Table 1. Optical and electrochemical properties of small molecules. Small molecules

Absorption (nm)

λonsetb (nm)

Egopt b (eV)

Eoxd (V)

Eredd (V)

HOMO (eV)

LUMO (eV)

Solution (ε)

Film

IDT(DCV)2

498, 532 (1.06)a

501, 538

574

2.16 (2.24)c

1.44

-

–5.81d

–3.65f

IDT(TCV)2

561, 602 (1.96)a

569, 616

668

1.86 (1.95)c

-

–0.19

–6.04e

–4.18d

IDTT(TCV)2

691 (1.64)a

713

817

1.52 (1.59)c

1.20

–0.31

–5.58d

–4.06d

a

Molar extinction coefficient (ε) in solution (× 105 cm–1 L mol–1); b Film; c Egopt determined by the intersection between the absorption and PL spectra. d Values obtained from cyclic voltammograms; e HOMO (eV) = LUMO (eV) – Egopt (eV); f LUMO (eV) = HOMO (eV) + Egopt (eV).

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Optical and electrochemical properties The absorption spectra of IDT(DCV)2, IDT(TCV)2, and IDTT(TCV)2 in chloroform solutions and thin films are displayed in Figure 1a and b, respectively, and the corresponding parameters are listed in Table 1. In particular, the molar extinction coefficients (ε) obtained for diluted chloroform solutions equaled 1.06 × 105, 1.96 × 105, and 1.64 × 105 cm−1 L mol−1 for IDT(DCV)2, IDT(TCV)2, and IDTT(TCV)2, respectively. The molecules containing TCV units exhibited higher ε values, showing better light-harvesting ability than IDT(DCV)2 for solar emission in the intense wavelength range. The absorption maxima (λmax) in solution occur at 532, 602, and 691 nm, respectively. IDT(DCV)2 and IDT(TCV)2 display distinct shoulder peaks (i.e., the 0-1 absorption peak) preceding λmax in solution because of their intrinsic optical properties. In the film phase, IDT(DCV)2 and IDT(TCV)2 maintain their intrinsic absorption properties, showing 0-0 and 0-1 vibronic transitions owing to the rigidity of the core. In addition, a relatively small spectral shift is ascribed to weak intermolecular interaction in the pristine films. On the other hand, IDTT(TCV)2 shows a relatively broad absorption spectrum without obvious vibronic transitions, suggesting diverse molecular arrangements and packing behaviors owing to reduced crystallinity due to the thienyl bridges. IDT(TCV)2 and IDTT(TCV)2 show red-shifted absorption spectra compared to IDT(DCV)2 owing to augmentation by an additional cyano group and a π-extended thienyl bridge. Detailed X-ray diffraction analyses of the molecular packing and arrangement are discussed further and are consistent with the absorption spectral analysis of the thin films. Cyclic voltammetry (CV) measurements were employed to investigate the electrochemical 14


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properties of the n-type small molecules in dichloromethane solutions (Figure 1c). IDT(DCV)2 shows distinct oxidation behavior arising from the weaker electron-accepting nature of DCV. In contrast, IDT(TCV)2 and IDTT(TCV)2 exhibit distinct reduction behavior, showing reduction potentials of −0.19 and −0.31 V, respectively. IDTT(TCV)2 also shows oxidation behavior, which suggests that it is ambipolar, whereas IDT(DCV)2 and IDT(TCV)2 exhibit only p- and n-type characteristics, respectively. The LUMO/HOMO energy levels were determined from the redox potentials and optical band gap energy (Egopt) values [e.g., −3.65/−5.81 eV for IDT(DCV)2, −4.18/−6.04 eV for IDT(TCV)2, and −4.06/−5.57 eV for IDTT(TCV)2] (Figure 1d). As DCV is a weaker acceptor than TCV, it increases the LUMO energy level.35 On the other hand, the introduction of a π-bridge results in a much higher HOMO energy with a narrower band gap. Owing to the strong electron-withdrawing nature of TCV, IDT(TCV)2 and IDTT(TCV)2 have low-lying LUMO energies (below −4.0 eV). The LUMO of PTB7-Th lies at −3.59 eV, and its HOMO is located at −5.20 eV.13 Thus, the energy levels of the small molecules match those of PTB7-Th well, as shown in Figure 1d, which is a prerequisite for efficient electron and hole transfer. To investigate the electrochemical stability in the reduction region, CV measurements of IDT(TCV)2 and IDTT(TCV)2 were repeatedly conducted in ambient atmosphere (Figure S3). For proper comparison, the reduction potential of PC71BM in solution was also repeatedly measured as a control sample. Figure S3d shows the differences between the initial and final reduction onset values. The cyclic voltammograms of PC71BM displayed evident changes. The onset potential of PC71BM shifted gradually during repeated measurements. IDT(TCV)2 and IDTT(TCV)2 showed smaller shifts of the onset potential than PC71BM, exhibiting highly stable 15



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initial reduction onset values that implied good electrochemical stability.

Morphological properties To better understand the effects of the molecular structure on the morphology, the molecular packing behavior of the small molecules was investigated by two-dimensional grazing incidence wide-angle X-ray scattering (2D-GIWAXS) and atomic force microscopy (AFM) to probe the crystallinity and surface morphology of pristine and thermally annealed films (Figures S4 and S5). The GIWAXS patterns of pristine films of all the small molecules indicated amorphous structures. After annealing at 100 °C, IDT(DCV)2 and IDT(TCV)2 showed scattered diffraction patterns, implying improved crystallinity. The surface morphologies of IDT(DCV)2 and IDT(TCV)2, as observed by AFM imaging, were also affected by molecular aggregation, which led to the formation of large grains, in agreement with the results of the GIWAXS and absorption spectral analyses. The planarity of the small molecules may be the primary reason for their individual molecular arrangement after thermal treatment. In IDTT(TCV)2, the additional thienyl bridge increased the flexibility of the molecular structure and the disorder of the blend structure, making the molecule amorphous.13,14,29

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Figure 2. Transfer curves for (a, c) pristine films and (b, d) films annealed at 100 °C. (a, b) IDT(TCV)2 and (c, d) IDTT(TCV)2. VDS = 80 V.

To investigate the effect of the acceptor strength and the length of π-conjugation in the acceptor molecule on the charge transport properties, thin-film transistor (TFT) devices were fabricated using IDT(DCV)2, IDT(TCV)2, and IDTT(TCV)2. The transfer characteristics are illustrated in Figure 2, and the corresponding TFT characteristics are listed in Table S1. All the small molecules except IDT(DCV)2 exhibited n-channel characteristics, showing maximum electron mobilities (µe) of 9.58 × 10−3 and 6.56 × 10−5 cm2 V−1 s−1 for IDT(TCV)2 and 17



IDTT(TCV)2, respectively. The highest mobility value was observed for IDT(TCV)2, which possesses good crystallinity as a result of thermal treatment, as well as the lowest-lying LUMO. For IDTT(TCV)2, no increment in the mobility was observed after thermal treatment owing to its amorphous character. Moreover, IDTT(TCV)2 displayed ambipolar characteristics in both the pristine and thermally annealed films, which is consistent with the CV observations.

0

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Figure 3. (a) J–V characteristics and (b) EQE spectra of PSCs bearing small molecules.

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Photovoltaic performance Inverted structures with ITO/ZnO/PTB7-Th:small molecule/MoO3/Ag configurations were fabricated to investigate the effect of the small molecular structure on the photovoltaic properties. Cells with optimized blend ratios [PTB7-Th:IDT(DCV)2 = 1:1.2 w/w; PTB7-Th:IDT(TCV)2 = 1:1.5 w/w; PTB7-Th:IDTT(TCV)2 = 1:1.5 w/w] were cast from o-DCB and CB. The BHJ PSC devices demonstrated PCE values of 3.63, 2.84, and 3.57% for IDT(DCV)2, IDT(TCV)2, and IDTT(TCV)2, respectively. The corresponding current density–voltage (J–V) curves are shown in Figure 3a, and the detailed photovoltaic parameters are summarized in Table S2. The opencircuit voltage (VOC) of IDT(DCV)2 was much higher than those of IDT(TCV)2 and IDTT(TCV)2 because of its higher LUMO energy level, which leads to a larger gap with respect to the HOMO energy level of PTB7-Th. On the other hand, the PSCs based on IDT(TCV)2 and IDTT(TCV)2 showed higher short-circuit currents (JSC) of 11.02 and 11.98 mA cm−2, respectively. Consequently, the introduction of the cyano group into the molecular structure leads to a low VOC because of the smaller energy gap between the HOMO of the electron donor and the LUMO of the acceptor. On the other hand, charge recombination between the TCV-bearing acceptor and PTB7-Th could be suppressed because of the lower-lying LUMO levels.36,37 The photovoltaic parameters of devices fabricated under different conditions are shown in Figure S6 and listed in Table S2. The PSCs prepared from o-DCB exhibited an improved fill factor (FF) but displayed reduced JSC values in the cases of IDT(TCV)2 and IDTT(TCV)2. Figure 3b shows the external quantum efficiency (EQE) spectra of the photovoltaic cells. The broad spectra show that both the non-fullerene acceptors and the donor material contribute 19



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to the enhanced EQE responses. In brief, the photoresponse of the blend film was quite consistent with the corresponding UV–vis absorption spectra (Figure S7), indicating that the small acceptor molecules and PTB7-Th made effective contributions. Both the IDT(TCV)2- and IDTT(TCV)2-based devices exhibited higher EQE values than the device with IDT(DCV)2, indicating more efficient exciton dissociation and charge transport in the BHJs. Moreover, the light-intensity-dependent JSC values (Figure S8) were also measured to estimate the extent of charge recombination in the fabricated devices. The power-law dependence of Jsc upon the incident light intensity can generally be expressed as Jsc ∝ Pα, where P is the light intensity, and α is the exponential factor.38 It is known that when the carrier recombination losses are small, the FF is relatively high.10 The IDT(DCV)2-, IDT(TCV)2-, and IDTT(TCV)2-based PSCs exhibited α values of 0.96, 0.98, and 0.97, respectively. The high α values suggested relatively weak bimolecular recombination in these devices.39 In addition, the SCLC method was employed to measure the hole and electron mobilities. As shown in Figure S9 and Table S3, the hole mobilities (μh) of the PTB7-Th:IDT(DCV)2, PTB7-Th:IDT(TCV)2, and PTB7-Th:IDTT(TCV)2 films are 1.41 × 10−5, 1.12 × 10−5, and 1.15 × 10−5 cm2 V−1 s−1, respectively. The hole mobilities of the devices are comparable. Using the electron-only device, the electron mobility in the PTB7-Th:IDT(TCV)2 film was measured to be 2.45 × 10−6 cm2 V−1 s−1, which is higher than those of the PTB7-Th:IDT(DCV)2 (1.58 × 10−6 cm2 V−1 s−1) and PTB7-Th:IDTT(TCV)2 films (1.69 × 10−6 cm2 V−1 s−1). As a result, more balanced bulk charge transport in the PTB7-Th:IDT(TCV)2 film can be expected to support the relatively high FF (= 0.53%). 20


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The surface morphologies of the BHJ layers were investigated by AFM (Figure S10a–f). The PTB7-Th:IDT(DCV)2 blend film showed the coarsest surface, with a root mean square roughness of 2.30 nm, whereas the IDT(TCV)2 and IDTT(TCV)2 films exhibited smoother surfaces. The internal morphologies of the active blend films were also characterized by transmission electron microscopy (TEM) (Figure S10g–i). TEM imaging of the PTB7Th:IDT(DCV)2 blend film showed separated microphase domains. The IDT(TCV)2 and IDTT(TCV)2 blends, which have a uniform internal nanostructure, are more favorable for exciton generation and dissociation, resulting in increased JSC and FF values.

Figure 4. Evaluation of PSC performance stability under ambient conditions for 1000 h. IDT(DCV)2 (black), IDT(TCV)2 (red), IDTT(TCV)2 (blue), and PC71BM (gray). Plots of normalized (a) PCE, (b) VOC, (c) JSC, and (d) FF vs. elapsed time.

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Stability of PSC performance Oxygen is well known to be one of the main factors reducing the stability of PSCs. Thus, oxidative stability is essential for achieving better long-term performance of PSC materials. To investigate the air stability of the PTB7-Th:IDT(DCV)2, -IDT(TCV)2, and -IDTT(TCV)2 blends, the photovoltaic performance of the devices was periodically monitored. For comparison, a PTB7-Th:PC71BM blend was prepared as a control device employing the same inverted device architecture used for IDT(TCV)2 and IDTT(TCV)2 without an additive. The devices were stored in the dark under ambient conditions without encapsulation, and their performance was monitored for 1000 h (Figure 4).

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Figure 5. EQE spectra of pristine PSCs (solid circles) (i) and PSCs aged for 1000 h (open circles) (ii) for (a) PBT7-Th:IDT(DCV)2, (b) PBT7-Th:IDT(TCV)2, (c) PBT7-Th:IDTT(TCV)2, and (d) PBT7-Th:PC71BM.

After the initial measurement, the devices containing IDT(DCV)2 and PC71BM showed an obvious performance decline, asymptotically approaching PCE retention values of 44 and 61%, respectively, after 1000 h. On the other hand, the IDT(TCV)2- and IDTT(TCV)2-based devices displayed highly stable performance, and these devices even outperformed during the airexposure time. The devices based on IDT(TCV)2 and IDTT(TCV)2 demonstrated small FF drops, and JSC increased owing to the modulated work function of the silver electrode, which can 23



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enhance charge transport upon exposure to oxygen.25 In addition, Figure 5b and c explain the stability of the EQE spectra with elapsed time, which is ascribed to the invariant light absorption and photoinduced charge collection efficiency. As a result, the properties of the PTB7Th:IDT(TCV)2 and PTB7-Th:IDTT(TCV)2 blends were found to be unaffected by oxygen penetration. In contrast, as shown in Figure 4b–d, all the parameters (VOC, JSC, and the FF) were found to contribute to the deterioration of the PTB7-Th:IDT(DCV)2 device efficiency. The decline of VOC, which is determined by the LUMO energy level of the corresponding acceptor, implies that oxidized IDT(DCV)2 cannot form an effective p-n junction with PTB7-Th. In addition, the EQE spectrum of IDT(DCV)2 also shows degeneration in the entire absorption range after 1000 h, owing to plausible oxygen penetration into the blend matrix (Figure 5a).

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pristine and aged IDT(TCV)2 and IDTT(TCV)2 devices showed similar roughness of the active layers. The PC71BM-based device exhibited a significant FF drop due to several distinct surface defects and an increased RS, resulting in a decrease in JSC. In contrast, IDT(TCV)2 and IDTT(TCV)2 exhibited stable performance even after 1000 h of storage in air. It might be conjectured that it was difficult to change the initial molecular packing of IDT(TCV)2 and IDTT(TCV)2 in PTB7-Th because of the geometrically hindered structures (Figure 7). Both IDT(TCV)2 and IDTT(TCV)2 have bulky side chains and a planar backbone, as shown in Figure S2, and so cannot migrate through the polymer chain network and form molecular aggregates. However, in PC71BM, the pseudo-globular structure easily induces self-aggregation, in contrast to IDT(TCV)2 and IDTT(TCV)2. With time, migration of PC71BM molecules and molecular rearrangement might occur in the active layer, leading to the observed significant surface change in the PTB7-Th:PC71BM blend film. PC71BM molecules are thought to move slowly in the BHJ system, eventually forming a rough surface and inducing the decrease in the FF during the stability measurement.

Figure 7. Schematic diagram of the expected morphology change in the BHJ blend film after 1000 h. (a) PTB7-Th:PC71BM and (b) PTB7-Th:IDT(TCV)2 or IDTT(TCV)2. 26


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In particular, to examine the universal contribution of TCV-based acceptors to the device stability, PTB7-Th, PPDT2FBT, and PIIDBT:IDT(TCV)2 blends (Figure S14b) and PTB7-Th:ntype small molecule blends (Figure S15b) were applied as the photoactive layer. IDT(TCV)2, IDTT(TCV)2, PC71BM, IDT-IC, and CP-V were exploited as n-type small molecules, and these molecules, except for IDT(TCV)2 and IDTT(TCV)2, exhibited high-lying LUMO energy levels (above −4.0 eV). The polymers and small molecules used here have already been reported to show high efficiencies.10,34,40,41 As a result, the PSCs based on IDT(TCV)2 exhibited good performance stability, irrespective of the donor polymer type (Figure S14a). However, the choice of n-type small molecule strongly influences the stability of the device efficiency, as shown in Figure S15a. These results showed that IDT(TCV)2 and IDTT(TCV)2 provide device stability regardless of the donor polymer. The photovoltaic parameters of the devices are listed in Table S4. The incorporation of the cyano unit into DCV gave rise to low-lying LUMO energy levels (below −4.0 eV), resulting in stable electron transport, which promoted device performance.28,29 The electrochemical and morphological stabilities of IDT(TCV)2 and IDTT(TCV)2 shown in Figures S3 and 6 also support the superior oxidative stability of the PCE in the corresponding PSCs.

CONCLUSIONS Three small molecules containing cyanovinylene electron-accepting units were synthesized, 27



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and the performance of PSCs based on PTB7-Th was thoroughly investigated, including the long-term stability of the PCE. Dicyanovinyl and tricyanovinyl units were attached to the ends of the conjugated core to modify its acceptor strength and π-conjugation length. In particular, IDT(TCV)2 and IDTT(TCV)2 exhibited superior shelf-life stabilities irrespective of the donor polymer type, resulting in a stable initial PCE. It was unambiguously demonstrated that small TCV-containing molecules display excellent long-term stability owing to their low-lying LUMO energy levels, good electrochemical stability, and robust internal morphologies in a blend matrix with donor polymers.

♦ ASSOCIARED CONTENT Supporting Informantion DSC spectra, DFT calculation, CV, GIWAXS patterns and spectra, AFM images of small molecules, TFTs data, PSC data, TEM images of blend films, stability measurement This material is available free of charge via the Internet at http://pubs.acs.org.

♦ AUTHOR INFORMATION Corresponding Authors *Corresponding authors:

Dr. Min Ju Cho, Prof. Dong Hoon Choi, 28


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E-mail: [email protected], and [email protected]

Author Contributions ‡

E.Y, Ko. and G. E. Park.: These authors contributed equally.

Funding Sources No funding source is stated.

Notes Any additional relevant notes should be placed here.

ACKNOWLEDGEMENTS The authors acknowledge the financial support from the National Research Foundation of Korea (NRF2012R1A2A1A01008797) and the Key Research Institute Program (NRF20100020209). We are grateful to the Pohang Accelerator Laboratory (Pohang, Korea) for allowing us to conduct the grazing incidence X-ray diffraction measurements, and also thank KBSI for allowing us to use their HRTEM instrument.

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