Article pubs.acs.org/Macromolecules
Impact of Dimerization on Phase Separation and Crystallinity in Bulk Heterojunction Films Containing Non-Fullerene Acceptors Dani M. Stoltzfus, Andrew J. Clulow, Hui Jin, Paul L. Burn,* and Ian R. Gentle Centre for Organic Photonics & Electronics, School of Chemistry and Molecular Biosciences, The University of Queensland, St. Lucia, QLD 4072, Australia S Supporting Information *
ABSTRACT: We report the synthesis of monomeric and dimeric dicyanovinylbenzothiadiazole-based non-fullerene acceptors and investigate the phase behavior of blend systems comprising poly(3-n-hexylthiophene) (P3HT) and the new materials. Differential scanning calorimetry (DSC) thermograms of blends with different compositions revealed that no eutectic composition existed for the blends, in contrast to previously reported P3HT/ non-fullerene acceptor combinations. Analysis of the DSC data shows that the thermal properties of the P3HT and the dimeric non-fullerene acceptor are almost independent of blend composition. However, the thermal properties of blends containing the monomeric acceptor were found to depend strongly on the blend composition, with increasing concentrations of the monomeric acceptor depressing the observed melting point of the polymer. These results are complemented by specular X-ray diffraction data, which reveals stark differences in the crystallinity of the two blend systems. In the case of the dimeric acceptor, the polymer and non-fullerene molecules crystallize in a form similar to that of the neat compounds, whereas in the case of the monomeric acceptor the crystallinity of both molecules is substantially altered by the other. We then correlate the structure of the materials with the organic solar cell performance.
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INTRODUCTION
The most common method for evaluating new materials and their blends is to fabricate and test organic solar cells to empirically determine the optimum blend ratio and annealing conditions with other measurements backfilling the explanation. This is both a time-consuming and material intensive process. While the photophysical and electronic properties of the materials can be relatively straightforward to determine, perhaps the most complex aspect of organic solar cells is the film morphology (structure) and methods to control it. The film morphology can be controlled by the structure of the materials in the processing solution14−16 as well as post deposition treatments such as thermal or solvent annealing.17−30 The present consensus is that optimized two-component bulk heterojunction films incorporating polymer donors are threephase systems comprising pure donor, pure acceptor, and intermixed phases.31−34 Forming this morphology in a film involves optimizing a number of deposition parameters including the donor/acceptor blend ratio and processing protocols with the proof of the optimized structure being the best performing solar cell. There have now been a small number of reports that aim to determine the optimized blend ratio in a predictive manner based on the thermal properties of the blended materials.14,35−37 In these cases, two component blends containing poly(3-alkylthiophene)s (P3ATs) and electron
Photocurrent generation in a typical organic solar cell has traditionally been thought to occur predominantly through photoexcitation of the donor and subsequent oxidation by the acceptor (photoinduced electron transfer, channel I).1 More recently, it has been acknowledged that photoexcitation of the acceptor can also play a significant role in photocurrent generation, especially in acceptor-rich devices.2−11 The photoinduced hole transfer (channel II) process occurs when the electron acceptor is photoexcited followed by reduction by the donor. In many cases it is difficult to disentangle the two mechanisms due to the absorption overlap of the two materials, but the recent development of non-fullerene acceptors that absorb light at longer wavelengths than typical donor materials has enabled unambiguous determination of the channel II mechanism.1 A further driver for the development of nonfullerene acceptor materials is that there are now reports of bulk heterojunction (BHJ) solar cells comprising donor/non-fullerene acceptor active layers that have efficiencies approaching the best fullerene acceptor-based devices.12,13 This, coupled with the fact that the non-fullerene acceptors can have large extinction coefficients, can be designed to absorb at specific wavelengths and/or have different spacial dimensionalities, means that they have the potential to have a significant impact on the organic solar cell field. Given these factors, it is timely to begin constructing a framework of understanding of the factors that affect the performance of non-fullerene acceptors in devices. © XXXX American Chemical Society
Received: May 11, 2016 Revised: May 18, 2016
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DOI: 10.1021/acs.macromol.6b00984 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 1. Acceptor materials used in this study. electrolyte solution of 0.1 M tetra-n-butylammonium perchlorate in argon-purged anhydrous tetrahydrofuran, which was distilled from lithium aluminum hydride. Glassy carbon working, platinum counter, and silver/silver nitrate in acetonitrile reference electrodes were used, and the E1/2 values are quoted relative to the ferrocenium/ferrocene couple.38 Electrodes were polished between measurements with a polishing pad wetted with methanol. PESA measurements were recorded with a Riken Keiki AC-2 PESA spectrometer with a power setting of 5−10 nW and a power number of 0.5. 7-(4,4-Di-n-hexyl-4H-silolo[3,2-b:4,5-b′]dithiophen-2-yl)benzo[c][1,2,5]thiadiazole-4-carbaldehyde (5). A solution of 4,4-di-n-hexyl4H-silolo[3,2-b:4,5-b′]dithiophene39 (3) (0.17 g, 0.47 mmol) in anhydrous tetrahydrofuran (14 mL) was cooled in an acetone/dry ice bath under an atmosphere of argon. A solution of n-butyllithium in hexanes (0.38 mL, 1.6 M) was added dropwise, and the resulting solution was stirred at −78 °C for 2 h, during which time the solution became yellow. Tri(n-butyl)tin chloride (0.18 mL, 0.66 mmol) was added, the solution was allowed to warm to ambient temperature and was then stirred in the absence of light for 3 days. The solution was diluted with water (50 mL), and the aqueous solution was extracted with diethyl ether (2 × 100 mL). The combined organic extracts were dried over anhydrous sodium sulfate, filtered, and the solvent was removed in vacuo. The crude stannane 4 was used without further purification. 4 and 7-bromobenzo[c][1,2,5]thiadiazole-4-carbaldehyde40 (0.14 g, 0.56 mmol) in anhydrous tetrahydrofuran (7 mL) were subjected to two freeze−pump−thaw cycles, backfilling with argon. Bis(triphenylphosphine)palladium(II) dichloride (0.16 g, 0.02 mmol) was added to the solution, and the mixture was then heated in an oil bath held at 65 °C for 4 h. The solution was allowed to cool, and the solvent was removed in vacuo. The crude material was purified by column chromatography over silica using a dichloromethane/light petroleum (40−60 °C) mixture (2:1) as the eluent, to afford 5 as a red gum (0.16 g, 65%). IR (solid) υ/̅ cm−1: 1683 (CO); UV−vis (dichloromethane): λmax (ε) cm3 mol−1 cm−1 = 501 (21 000). 1H NMR (500 MHz, CDCl3) δ: 0.84 (6H, m, CH3), 0.98 (4H, m, CH2), 1.20−1.28 (8H, m, CH2), 1.28−1.35 (4H, m, CH2), 1.39−1.46 (4H, m, CH2), 7.12 (1H, d, J = 4.5 Hz, TH H), 7.33 (1H, d, J = 4.5 Hz, TH H), 7.96 (1H, d, J = 7.5 Hz, BT H), 8.19 (1H, d, J = 7.5 Hz, BT H), 8.31 (1H, s, TH H), 10.68 (1H, s, CHO). 13C NMR (75 MHz, CDCl3) δ: 11.8, 14.1, 22.5, 24.1, 31.4, 32.8, 122.9, 124.7, 127.3, 129.9, 132.9, 133.2, 133.7, 139.1, 143.5, 143.9, 148.7, 152.2, 153.8, 154.0, 188.4. LRMS (MALDI-TOF-MS) for C27H32N2OS3Si+ (M)+ Calcd: 524.1 (100%), 525.1 (39%), 526.1 (24%), 527.1 (7%). Found: 524.0 (88%), 525.0 (100%), 526.0 (44%), 527.0 (10%). HRMS (ESIMS) for C27H33N2OS3Si+ (M + H)+ Calcd: 525.1519 (100%). Found: 525.1504 (100%). 2-[(7-(4,4-Di-n-hexyl-4H-silolo[3,2-b:4,5-b′]dithiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)methylene]malononitrile (1). A solution of 5 (0.35 g, 0.67 mmol) in toluene (27 mL) and pyridine (1.5 mL) was heated in an oil bath held at 65 °C under an atmosphere of argon. Malononitrile (1.77 g, 26.8 mmol) was added, and the solution was heated for 30 min. The solution was allowed to cool, and the solvents were removed in vacuo. The crude material was dissolved in dichloromethane (50 mL) and was washed with water (2 × 300 mL). The solution was then dried over anhydrous sodium sulfate, filtered, and the solvent was removed in vacuo to give 1 as a purple powder (0.21 g, −1 : 55%); MP = 92 °C; Tg = 17 °C; Td(5%) = 364 °C. IR (solid) υ/cm ̅ 3 −1 −1 2221 (CN). UV−vis (dichloromethane): λmax (ε) cm mol cm = 338 (10 000), 369 (14 100), 390 (sh, 5300), 582 (34 600). 1H NMR (400 MHz, CDCl3) δ: 0.86 (6H, m, CH3), 0.99−1.03 (4H, m, CH2), 1.23−
acceptors such as the [6,6]-phenyl-C61(or C71)-butyric acid methyl esters (PCBMs)35 or the small molecule non-fullerene acceptors 2-[(7-(9,9-di-n-propyl-9H-fluoren-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)methylene]malononitrile (K12)14 or 2[(7-(4,4-di-n-propyl-4H-silolo[3,2-b:4,5-b′]dithiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)methylene]malononitrile (YF25)14 were found to form a eutectic composition, and the blend ratio that gave the best device performance was hypoeutectic with respect to the polymer. The results indicated that the acceptor concentration should be sufficiently high to form seed crystals that act as percolation pathways for electron extraction, which are dispersed between crystalline polymer domains and the finely intermixed eutectic phase in a three-phase blend system. In this paper we build on the predictive concept and explore the thermal properties and crystallinity of blends of poly(3-nhexylthiophene) (P3HT) with the non-fullerene acceptors 1 or 2 (Figure 1). We show that the blends do not exhibit eutectic phases, and the morphology of the film is strongly dependent on whether the monomer or dimer is used. Diffractometry measurements showed that dimerization of chromophore 1 to give dimer 2 profoundly altered the crystalline properties of the non-fullerene acceptors and consequently those of the blends with P3HT, leading to only moderate solar cell performance with both acceptors but for opposite reasons.
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MATERIALS AND METHODS
Materials Synthesis. All reagents were purchased from commercial sources and were used as received unless otherwise stated. Tetrahydrofuran, N,N-dimethylformamide, and toluene were dried on an LC Systems solvent purification system prior to use. Solvents for chromatography were distilled prior to use. Column chromatography was performed with Davisil LC60A 40−63 μm silica. Thin layer chromatography (TLC) was performed using aluminum backed silica 60 F254 plates. 1H and 13C NMR were performed using Bruker Avance 300, 400, or 500 MHz spectrometers in deuterated chloroform referenced to 7.26 ppm for 1H and 77.0 ppm for 13C; TH H = thiophenyl H; BT H = benzothiadiazolyl H; VIN H = vinyl H. Coupling constants are given to the nearest 0.5 Hz. UV−vis spectrophotometry was performed using a Cary 5000 UV−vis spectrophotometer on either thin films on glass substrates or a solution of the material in spectroscopic grade dichloromethane. FT-IR spectroscopy was performed on solid samples using a PerkinElmer Spectrum 100 FT-IR spectrometer with an ATR attachment. Melting points (MPs) were measured in glass capillaries on a Büchi B-545 melting point apparatus and are uncorrected. Microanalyses were performed using a Carlo Erba NA 1500 elemental analyzer. Mass spectra were acquired on a Voyager DE STR MALDI-TOF or a Bruker HCT 3D Ion Trap mass spectrometer (ESI-TOF). Thermal transitions were determined by using a PerkinElmer Diamond differential scanning calorimeter. Thermal gravimetric analysis was undertaken using a PerkinElmer STA 6000 simultaneous thermal analyzer. Thermal decomposition temperatures [Td(5%)] were reported as the temperature corresponding to a 5% mass loss. Cyclic voltammetry measurements were carried out on a BASi Epsilon voltammetric analyzer at room temperature in an B
DOI: 10.1021/acs.macromol.6b00984 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
−1 67%); MP = 292 °C; Td(5%) = 333 °C. IR (solid) υ/cm ̅ −1 : 2222 (CN). 3 −1 UV−vis (dichloromethane): λmax (ε) cm mol cm = 418 (15 000), 684 (48 500). 1H NMR (400 MHz, CDCl3) δ: 0.86 (12H, m, CH3), 1.01−1.05 (8H, m, CH2), 1.24−1.30 (16H, m, CH2), 1.31−1.38 (8H, m, CH2), 1.42−1.49 (8H, m, CH2), 7.24 (2H, s, TH H), 7.97 (2H, d, J = 8.0 Hz, BT H), 8.35 (2H, s, TH H), 8.75 (2H, d, J = 8.0 Hz, BT H), 8.77 (2H, d, J = 0.5 Hz, VIN H). LRMS (MALDI-TOF-MS) for C60H62N8S6Si2+ (M)+ Calcd: 1142.3 (100%), 1143.3 (86%), 1144.3 (69%), 1145.3 (38%), 1146.3 (28%), 1147.3 (8%). Found: 1142.9 (94%), 1143.9 (100%), 1144.9 (87%), 1145.9 (59%), 1146.9 (27%), 1147.9 (13%). Anal. Calcd for C60H62N8S6Si2: C, 63.0; H, 5.5; N, 9.8. Found: C, 62.6; H, 5.5; N, 9.7. Sample Preparation. Poly(3-n-hexylthiophene) (P3HT) (Mw = 94 100 g mol−1, polydispersity = 1.9, regioregularity ∼96%) was purchased from Merck Chemicals, United Kingdom. Films for thermal analysis and specular X-ray diffraction were prepared by dissolving P3HT and the relevant electron acceptor in 1,2-dichlorobenzene (DCB) at 50 °C overnight, followed by drop casting of the solution onto glass slides and solvent evaporation under ambient conditions. P3HT/ acceptor ratios are given by weight throughout. Thermal Analysis. Differential scanning calorimetry (DSC) was conducted under a nitrogen atmosphere at a scan rate of 10 °C min−1 with a PerkinElmer Diamond DSC. For analysis of the neat materials and dried blend films, 3−5 mg of the material was sealed into aluminum crucibles. Liquidus transitions correspond to the end melting temperatures of the neat materials and blends. Specular X-ray Diffraction Measurements. Specular X-ray diffraction (XRD) measurements were performed on a Bruker D8 Advance diffractometer using Cu Kα radiation (λ = 1.54 Å). X-rays were focused onto the samples using a Göbel mirror and collimated with a presample slit (0.2 mm). Diffracted X-rays were collimated using Soller slits and detected by a YAP:Ce scintillation detector. Measurements were performed over a range of scattering angles (2θ) from 2.0° to 30.0° to give a Qz range of 0.14−2.11 Å−1 [Q = (4π/λ) sin θ]. Data points were recorded in continuous scan mode every 0.02° and the accumulation time at each point was 2 s. Three X-ray diffractograms were recorded at different locations on the films and averaged to produce the final spectra. Overlapping peaks were deconvoluted using the IGORPro Multipeak Fitting function to determine peak areas. The peak shapes of the neat compounds were modeled initially, and the shapes of these peaks were applied to model the overlapping peaks of the blends. Thermal annealing for specular XRD measurements was performed within a class 1000 cleanroom on a Heidolph MR Hei-Standard hot plate monitored with a surface thermometer. Device Fabrication. For preparation of the P3HT/acceptor 1 or 2 devices, solutions of P3HT (15 mg mL−1) and acceptor 1 or 2 (15 mg mL−1) were prepared separately and different volumes of the solutions were mixed for different ratios. The solvent used in all cases was DCB at room temperature. Patterned 2.5 cm × 2.5 cm ITO-coated glass substrates were purchased from Xinyan, China. The ITO-coated glass substrates were first cleaned with detergent (Alconox), ultrasonicated in deionized water, acetone, and 2-propanol, and subsequently dried with a nitrogen flow. PEDOT:PSS (CLEVIOS P VP Al 4083) was filtered through a 0.45 μm PVDF filter and then spin-coated (5000 rpm) to achieve thicknesses of ∼30 nm on the ITO-coated substrate. Substrates were dried at 150 °C for 10 min in air prior to spin-coating of the photoactive layer. The photoactive film thicknesses were around 60−80 nm. Finally, Ca (15 nm) and Al (100 nm) were deposited through a shadow mask to define the active area of the devices (0.2 cm2). All device fabrication steps were performed under inert conditions (
