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Naphthalene Tetracarboxydiimide-Based n‑Type Polymers with Removable Solubility via Thermally Cleavable Side Chains Sabina Hillebrandt,*,†,‡,○ Torben Adermann,§,○ Milan Alt,‡,∥,⊥,○ Janusz Schinke,‡,#,○ Tobias Glaser,†,‡ Eric Mankel,‡,¶ Gerardo Hernandez-Sosa,‡,∥ Wolfram Jaegermann,‡,¶ Uli Lemmer,‡,∥,+ Annemarie Pucci,†,‡,△ Wolfgang Kowalsky,†,‡,# Klaus Müllen,◊ Robert Lovrincic,‡,# and Manuel Hamburger*,§,‡ †
Kirchhoff-Institut für Physik, Universität Heidelberg, Im Neuenheimer Feld 227, 69120 Heidelberg, Germany InnovationLab GmbH, Speyerer Str. 4, Heidelberg, Germany § Organisch-Chemisches Institut, Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany ∥ Lichttechnisches Institut, Karlsruher Institut für Technologie, Engesserstrasse 13, 76131 Karlsruhe, Germany ⊥ Merck KGaA, Mainzer Straße 41, 64579 Darmstadt, Germany # Institut für Hochfrequenztechnik, Technische Universität Braunschweig, Schleinitzstr. 22, 38106 Braunschweig, Germany ¶ Material & Geowissenschaften, Technische Universität Darmstadt, Jovanka-Bontschits-Str. 2, 64287 Darmstadt, Germany + Institut für Mikrostrukturtechnik, Karlsruher Institut für Technologie, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany △ Centre for Advanced Materials, Universität Heidelberg, Im Neuenheimer Feld 225, 69120 Heidelberg, Germany ◊ Max-Planck Institut für Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany ‡
S Supporting Information *
ABSTRACT: Multilayer solution-processed devices in organic electronics show the tendency of intermixing of subsequently deposited layers. Here, we synthesize naphthalene tetracarboxydiimide (NDI)-based n-type semiconducting polymers with thermally cleavable side chains which upon removal render the polymer insoluble. Infrared and photoelectron spectroscopy were performed to investigate the pyrolysis process. Characterization of organic field-effect transistors provides insight into charge transport. After the pyrolysis homogeneous films could be produced which are insoluble in the primary solvent. By varying curing temperature and time we show that these process parameters govern the amount of side chains in the film and influence the device performance.
KEYWORDS: n-type polymers, thermal cleavage of side chains, photoelectron spectroscopy, infrared spectroscopy, organic field effect transistors
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linking4,5 or removal of solubilizing groups triggered by chemical,6 thermal,7−15 or photochemical16,17 stimuli or a mixture thereof. In this work, we focus on noncatalyzed solubility removal in polymeric n-channel semiconductors by an external stimulus of heat. Pioneering work on this approach has been done by Fréchet8 and Krebs.9,12 However, they found that carboxylic acid functionalities remaining on the polymer backbone after a first thermal cleavage below 250 °C significantly alter electronic performance.10 Temperatures above 250 °C are considered to
INTRODUCTION π-Conjugated polymers have attracted considerable attention over the past 20 years due to the advantageous combination of their optoelectronic properties and solution processability.1 One crucial fabrication step for the functioning of organic electronic devices is the stacking of multiple layers, ideally in a solution-based printing process on flexible substrates. Stacked deposition of multiple thin layers from solution is a nontrivial challenge for device fabrication. Two primary possibilities exist to prevent unintentional layer mixing at interfaces: solvents can be chosen orthogonally,2 limiting the options for material combinations severely, or the solubility needs to be reduced after deposition to enable deposition of consecutive layers from the same solvent.3 The latter can both be achieved by cross© 2016 American Chemical Society
Received: November 11, 2015 Accepted: February 1, 2016 Published: February 1, 2016 4940
DOI: 10.1021/acsami.5b10901 ACS Appl. Mater. Interfaces 2016, 8, 4940−4945
Research Article
ACS Applied Materials & Interfaces be incompatible with common flexible substrates18 and most high performance semiconducting polymers;19 however, there were nice results achieved with thiophene oligomers.20 Zambounis7 and Ma13 presented a solution-processed organic solar cell based on small-molecule quinacridone using a similar approach. Despite the variety of reports, there has been only very little application of thermally driven side-chain removal to air-stable n-channel semiconductors. Luscombe et al. recently reported a curable n-channel material by presenting a copolymer of tert-butoxycarbonyl-protected diaminobenzenes copolymerized with NDI. Acid-catalyzed cleavage followed by condensation to a ladderized, BBL-resembling structure achieved field-effect electron mobilities in the range of 10−3 cm2/(V s).21 The elegancy of this approach is only hampered by the additional acid treatment step that is obviously not favorable from the processing point of view where a one-step approach would be preferable. Inspired by the work of Fréchet and Zambounis, we developed a synthetic strategy to replace the typical aliphatic side chain attached to the NDI nitrogen by a thermally cleavable group. The two strategies used in this paper are (i) the introduction of tertiary carbonate attached to the NDI core with a short aliphatic linker and (ii) a tertiary carbamate linked directly to the imide nitrogen. Both side groups are expected to undergo thermal decomposition liberating gaseous CO2 as well as a volatile alkene to give a primary alcohol or the free imide, respectively. We recently presented the feasibility of this approach and the control of the scission temperature for polythiophenes.22 Especially, the possible intermolecular H-bonding motif of the free imide is expected to dramatically reduce the solubility of the resulting compounds. We study the mechanism of the pyrolysis process in the polymer films in detail via X-ray photoelectron spectroscopy (XPS) and infrared transmission spectroscopy (IRS), giving insight into chemical composition. Additionally, the impact of subsequent solvent washing is investigated to test the stability and insolubility of the films. Charge transport properties in our novel materials were investigated by employing them as active layers in organic field-effect transistors (OFETs) and relating the performance to morphological properties. We prove that after pyrolysis the films are insoluble in the primary solvent chlorobenzene (CB). Moreover, OFET performance is not negatively affected by the solvent treatment under optimized processing conditions. Varying the thermal load, i.e., curing temperature and time, led to an optimization of the pyrolysis process and a reduction of the cleavage temperature which is important for the application on temperature-sensitive, flexible substrates.
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>97%, N0536) were purchased from TCI Europe and were used as received. Acetic anhydride was purchased from Sigma-Aldrich and stored under argon. Sample Preparation. Thin-Film Preparation. Prior to spincoating all 12 × 12 mm2 silicon substrates were cleaned for 15 min with acetone and 2-propanol in an ultrasonic bath and dried with gaseous nitrogen. To remove all remaining organic contaminants the substrates were treated with oxygen plasma for 5 min. The prepared samples were transferred through controlled clean room conditions into the inert atmosphere of a glovebox. All polymer semiconductors were spin coated from chlorobenzene and thermally treated in N2 atmosphere and immediately transferred to the respective measurement technique. Device Preparation. OFETs were prepared in BCTG architecture on glass substrates. Gold source−drain electrodes of ∼60 nm thickness were thermally evaporated and structured via a shadow mask. As a gate dielectric, poly(chloro-p-xylylene) (ParyleneC) layers of ∼300 nm were deposited using chemical vapor deposition. Silver top gate electrodes of ∼100 nm thickness were also prepared by shadow-maskstructured thermal evaporation. The OFETs feature 1000 μm channel width and 10, 20, and 50 μm channel length. Characterization. PES. The PES experiments were performed at the integrated UHV system “Clustertool” at the InnovationLab in Heidelberg. A Phi VersaProbe II spectrometer was used equipped with a monochromatized Al Kα X-ray source (1486.6 eV) as the excitation source for X-ray photoelectron spectroscopy (XPS). The base pressure of the measuring chamber during all experiments was kept in the 10−10 mbar regime. All spectra are referenced in binding energy with respect to the Fermi edge and the core level lines of in situ cleaned metal samples. To determine the exact core level binding energies all measured emission lines were fitted; thus, a mixed Gaussian− Lorentzian profile was applied after Shirley background subtraction. To get a detailed picture of the sample survey spectra, the following core levels were measured: C 1s, O 1s, S 2p, N 1s, and Si 2p. IRS. IR transmission spectra of the prepared thin films were measured with an FT-IR spectrometer Vertex 80v (Bruker) and are an average over 200 scans with a resolution of 4 cm−1. A nitrogen-cooled mercury cadmium telluride (MCT) detector was used, and the measurements were performed under vacuum conditions at 3 mbar. All spectra show rel. transmission as the polymer films on silicon substrates were measured relative to the cleaned bare silicon wafer. The spectroscopy software OPUS (Bruker) was used to integrate over the peak area of the stretching vibration assigned to the side chain C O. OFET Devices. Device characterization was carried out in ambient atmosphere in a three-probe setup using an Agilent 4155C semiconductor parameter analyzer. All device characteristics presented were measured in the dark. Channel resistances were determined via the “transfer line method”.23,24 AFM. For the AFM measurements an ambient AFM DualScopeTM DS 95 Series form DME was used. All measurements were performed in tapping mode using highly doped silicon cantilevers from NanoWorld (Arrow NCR) with tip radii of less than 10 nm and resonance frequencies around 285 kHz. The DME Scan Tool was used in order to process and evaluate the recorded AFM images.
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EXPERIMENTAL DETAILS
Materials. All reactions requiring exclusion of oxygen and moisture were carried out in dry glassware under a dry and oxygen-free argon or nitrogen atmosphere. Solvents were purchased from commercial suppliers such as Sigma-Aldrich Co. LLC, abcr & Co. KG (1,2dichlorobenzene), or VWR International GmbH (toluene) and distilled prior to use, if necessary. The majority of absolute and dry solvents (THF, DCM, toluene, Et2O, and ACN) was dried by a MB SPS-800 using drying columns or after drying over anhydrous molecular sieves and storage under argon (DMF, DMSO, pyridine). Chemicals were purchased from commercial laboratory suppliers Sigma-Aldrich Co. LLC, abcr GmbH & Co. KG, and TCI Europe N.V. Reagents were used without further purification unless otherwise noted. Naphthalene-1,4,5,8-tetracarboxylic dianhydride (NTCDA, >95%, N0369) and 1,4,5,8-naphthalene tetracarboxdiimide (NDI,
RESULTS AND DISCUSSION Scheme 1 shows the three functionalized monomers and their respective polymerization. The structural differences in the monomers were designed to find a first indication of an optimal ratio of cleavable side groups and persistent alkyl chains to allow some residual flexibility. In the following these polymers are referenced as polymers 1−3 with polymer 1 having the largest number of residual side groups after cleavage and polymer 3 the lowest. Two independent synthetic routes were developed in order to enable the synthesis of the carbonateand carbamate-bearing monomer, respectively. Starting from bromination of NDA5−8, polymers 1 and 3 are obtained after 4941
DOI: 10.1021/acsami.5b10901 ACS Appl. Mater. Interfaces 2016, 8, 4940−4945
Research Article
ACS Applied Materials & Interfaces Scheme 1. Reaction Scheme of the NDI-Based Polymersa
curing temperatures between 180 and 230 °C and lead to a weight loss of 24.3% (calc. 26.3%), 28.9% (calc. 32.3%), and 42.0% (calc. 48.5%) for polymers 1, 2, and 3, respectively (see SI). The weight loss being lower than the calculated value can be explained by residual fragments which do not fully evaporate during the TGA measurement. Figure 1a shows the O 1s emission lines of the surfacesensitive XP measurements of the three polymers before and after pyrolysis. In order to understand and describe the pyrolysis process in detail, the measured O 1s core levels were fitted using a Lorentzian line shape. Before the pyrolysis all three polymer films exhibit three different oxygen species (black, gray, light gray) according to their structure (for details see SI). After the pyrolysis the number of oxygen species should be reduced as the side chains cleave off and exit the films including two oxygen species each. In the case of the carbonatebased polymers 1 and 3 only two singlets are expected to fit the measured spectra after pyrolysis (double-bonded oxygen in the core and single-bonded oxygen on the side chain). For polymer 1 this is the case; however, the peak ratios indicate that some side chain residuals are present on the surface of the film. Polymer 3 yet employs three singlets with different ratios compared to the untreated film, which corresponds to residuals of the used solubilizing side groups. In the carbamate-based cured polymer 2 film almost no residual side chains are detectable, and only a very small shoulder (ca. 534 eV) is still present. This indicates that the pyrolysis process is most complete in the case of polymer 2. Figure 1b shows the IR spectra of the films in the range 1650−1830 cm−1 where the CO stretching vibrations are located.26 The absorption band that corresponds to the CO stretching vibration in the side chain is marked with a dashed line. In the case of polymer 2 this absorption band vanishes after pyrolysis, indicating the complete removal of side chains as IRS is probing the bulk properties of the film. In the case of polymers 1 and 3, the pyrolysis leads to an intensity decrease of about 50% for the marked CO stretching mode. This shows that after the pyrolysis process a significant fraction of the side chains is still present in these films, in accordance to the XPS results. However, after washing the films in CB this CO stretching mode completely vanishes also for polymers 1 and 3, showing that residual side chains were washed out. Furthermore, washing the films in CB after pyrolysis does not change the intensity of the CO stretching vibrations that are located on the core of the polymers (marked with * in Figure 1b). This
a
In red the thermally cleavable side chains are marked. For details see SI.
seven linear steps of synthesis. Polymer 2, based on a carbamate monomer, is synthesized in six linear steps. Key steps in both synthesis routes are the isolation of the pure 2,6-functionlized NDI core. For the carbonate monomers, brominated NDA is reacted with 6-hydroxyhexylamine in the presence of propionic acid, followed by extensive column chromatography to give the desired regioisomer. The carbamate monomer is enabled by esterification of brominated NDA,9 forming a well-soluble naphthalene intermediate which is readily functionalized. After hydrolysis of the ester groups the imide functionality is formed using acetic anhydride followed by ammonium acetate. Polymerization is achieved at room temperature using stoichiometric amounts of Ni(cod)2 in tetrahydrofuran until the solubility limit is reached. Moderate chain lengths with an average of 40 repeat units in polymer 1, 15 repeat units in polymer 2, and 10 repeat units in polymer 3 were obtained. The higher degree of polymerization of polymer 1 is explained by an improved monomer solubility arising from the combination of long, branched alkyl side chains attached to the NDI cores and hexyl groups on the thiophenes, the latter also increasing the solubility of the polymer by additional torsion in the dithiophene fragments in the growing chain. It has been shown that this approach is still compatible with charge transport along the conjugated backbone.25 The use of alkyl-substituted thiophenes was also necessary to ensure a sufficient solubility of all intermediates in the monomer synthesis (cf. Supporting Information). Side chains split off at
Figure 1. Analysis of polymer films before and after pyrolysis of polymer 1 at 230 °C, polymer 2 at 180 °C, and polymer 3 at 200 °C: (a) Measured XP spectra of the O 1s emission line with background removed (black points) and fitted Lorentzian oscillators (shaded areas) corresponding to different oxygen species in the sample. (b) IR transmission spectra of the polymer films on a Si substrate referenced to the spectrum of the bare substrate. In blue the spectra of the films after the additional washing step with CB are shown. The dashed line marks the CO stretching vibration in the side chain. The asterisk marks the CO stretching vibrations that are located on the core of the polymers. 4942
DOI: 10.1021/acsami.5b10901 ACS Appl. Mater. Interfaces 2016, 8, 4940−4945
Research Article
ACS Applied Materials & Interfaces
Figure 2. (a) Transfer curves of top gate bottom contact OFET devices for the three different polymers before and after pyrolysis and additionally after solvent washing for polymer 3. (b) Channel resistance of OFET devices calculated via the transfer line method.23,24 (c) Integrated area of the side chain CO mode in the IR spectra (see SI) as a measure of the amount of side chains in the film.
Figure 3. Atomic force measurements of polymer 3 based films (a) before the pyrolysis, (b) after the pyrolysis at 200 °C for 3 min and CB, and (c) after the pyrolysis at 220 °C for 20 min and CB.
demonstrates that the amount of molecules in the films does not change, thus a polymer film insoluble in CB is formed after the pyrolysis. In Figure 2a the OFET characterization of devices with a top gate bottom contact architecture (for details see SI) is summarized. The source−drain current has an on/off ratio of 103−104. Devices utilizing NDI-based polymers without cleavable side chains in a similar device architecture show on/off ratios in the same range.27,28 For all polymers the mobility increases after the pyrolysis, possibly via a shorter interchain distance. Further diffraction-based experiments that are beyond the scope of this work will have to clarify the structural changes upon pyrolysis. Overall, polymer 3 shows the best OFET characteristics (μ = 2.4 × 10−4 cm2/(V s)), outperforming the other polymers by 1 order of magnitude (see SI for all mobility data). Devices built of a polymer with a similar core (P(NDI2OD-T2) or N2200) usually exhibit higher mobilities of (0.04−0.85) cm2/(V s),19 yet Zhang et al. showed that in the case of small molecules with an NDI core the choice of the side chains can change the mobility by several orders of magnitude (10−3 < μ < 3 cm2/(V s)).29 Comparisons between polymers and small molecules are generally difficult to make. Yet this example demonstrates how delicately the charge transport depends on the shape and configuration of the alkyl side chains, which are electrically passive but influence the final film morphology in small molecules and polymers alike. A further optimization process for the side chains remaining attached after pyrolysis or the substituents in the core could therefore lead to improved mobilities.
However, we focus here on the working principle of our approach and test the polymer 3 based OFET devices in detail for solvent resistivity, as they show the most promising results. The thermal load of the pyrolysis process was chosen to be 200 °C and 3 min according to thermogravimetric analysis (see SI). The additional washing step with CB after pyrolysis in the device fabrication leads to a loss in performance indicated by a decreased on/off ratio in Figure 2a. The mobility also decreases to values below the ones of untreated films (
