Self-assembly and charge carrier transport of sublimated dialkyl substituted quinacridones

Accepted Manuscript Self-assembly and charge Carrier transport of sublimated dialkyl substituted quinacridones Tomasz Marszalek, Izabela Krygier, Adam Pron, Zbigniew Wrobel, Paul M.W. Blom, Irena Kulszewicz-Bajer, Wojciech Pisula PII:

S1566-1199(18)30574-3

DOI:

https://doi.org/10.1016/j.orgel.2018.11.004

Reference:

ORGELE 4967

To appear in:

Organic Electronics

Received Date: 19 July 2018 Revised Date:

9 October 2018

Accepted Date: 5 November 2018

Please cite this article as: T. Marszalek, I. Krygier, A. Pron, Z. Wrobel, P.M.W. Blom, I. KulszewiczBajer, W. Pisula, Self-assembly and charge Carrier transport of sublimated dialkyl substituted quinacridones, Organic Electronics (2018), doi: https://doi.org/10.1016/j.orgel.2018.11.004. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Self-assembly and charge carrier transport of sublimated dialkyl substituted quinacridones Tomasz Marszalek,a,b Izabela Krygier,a,b Adam Pron,c Zbigniew Wrobel,d Paul M. W. Blom,a Irena Kulszewicz-Bajer,c,* and Wojciech Pisulaa,b,* a

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Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany. E-mail: [email protected]

b

Department of Molecular Physics, Faculty of Chemistry, Lodz University of Technology, Zeromskiego 116, 90-924 Lodz, Poland

c

Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland. E-mail: [email protected]

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d

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Institute of Organic Chemistry, Polish Academy of Science, Kasprzaka 44/52, 01-224 Warsaw, Poland

Abstract

Quinacridone, an industrial pigment, has recently shown a high charge carriers mobility in field-effect transistors. In search for new cheap organic semiconductors of improved vacuum processability we have synthesized three dialkyl derivatives of quinacridone, namely N,N’-

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dialkylquinacridones (alkyl = butyl, octyl, dodecyl), abbreviated as QA-C4, QA-C8 and QAC12. The alkylation of quinacridone results in a significant decrease of its melting temperature which drops from 390 °C for quinacridone to 261 °C, 177 °C and 134 °C for QAC4, QA-C8 and QA-C12, respectively, while retaining the onset of thermal decomposition

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above 390 °C. The elimination of the hydrogen bonding network between the carbonyl groups and amine hydrogens through alkylation not only lowers the melting temperature, but also

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induces supramolecular ordering in contrast to unsubstituted quinacridone. Detailed morphological and structural investigations of the vacuum deposited thin films have revealed that the length of the alkyl substituent is crucial for the molecular self-organization. Compound QA-C4 forms poorly ordered films, whereas QA-C8 and QA-C12 grow into a spherulitic dense morphology with increasing domain size at higher deposition temperatures. The more pronounced morphology is related to the lower melting point of the compounds and strong molecular diffusion during deposition. The poorly ordered films of QA-C4 do not show any field-effect response, what is consistent with previous reports. In contrast, transistors with QA-C8 or QA-C12 as active layers exhibit hole transport. Optimization of the deposition temperature, in which nucleation and crystal growth are properly balanced,

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ACCEPTED MANUSCRIPT resulted in OA-C8-based transistors with a hole mobility of 0.3 cm²/Vs, i.e. higher than in devices with unsubstituted quinacridone.

1. Introduction Low molecular weight organic molecules offer advantages as compared to their high

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molecular (polymeric) counterparts: i) they can be more easily purified and processed into technologically useful forms and ii) they show no dispersion of their molecular mass, thus they more easily form ordered supramolecular structures that are crucial for electrical transport properties. As a result, they can serve as active layers in organic

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field-effect transistors (OFETs) exhibiting high charge carrier mobilities.1

Among the variety of low molecular weight semiconductors, heteroacenes deserve a special interest as candidates for application in OFETs.2 Depending on the number and

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position of heteroatoms in the molecule, they can show either unipolar or bipolar electrical transport properties.2e More recently, applications going beyond OFETs have been demonstrated, for example as components of organic light-emitting diodes (OLEDs) and other devices.2c, 2f-k

Quinacridone can be considered as an azapentacene derivative with broken

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conjugation. This compound is used on industrial scale as the magenta toner in inkjet printers and household paints and is available for a few cent per kilogram.3 Surprisingly, it has been demonstrated that, despite its limited conjugation, quinacridone can be used as an organic semiconductor in OLEDs, solar cells and FETs

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exhibiting a hole mobility up to 0.2 cm²/Vs.4 This is, among others, due to the fact that its molecules readily form ordered supramolecular aggregations due to intermolecular

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hydrogen-bonding between their carbonyl groups and amine hydrogens. Depending on the crystallization conditions various biomimetic three-dimensional arrangements of nanocrystals can be obtained.5 The above presented findings clearly indicate that hydrogen-bonding-type intermolecular interactions are responsible for molecular packing in thin films of quinacridone and finally for the formation of pathways which facilitate charge carrier transport.6 The importance of intermolecular hydrogen bonding in the formation of ordered supramolecular structures was also observed for other organic semiconductors.7 Unsubstituted conjugated molecules are, in general, very difficult to solubilize. As a result their purification is difficult as well as their deposition as thin films in electronic devices. Attaching alkyl groups is a common procedure of improving vacuum 2

ACCEPTED MANUSCRIPT processability of organic semiconductors. In order to improve the film morphology and molecular ordering, a substrate temperature close to melting point is necessary to initiate diffusion of the molecules. Since QA has a high melting temperature of 390 °C limiting the optimization of the film morphology and thus device performance. For an enhanced processing, a significant lowering of the melting point is required.

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For these reasons we have undertaken the task of synthesizing dialkyl N,N’dialkylquinacridones (Scheme 1). Since they can be prepared from quinacridone in one step, they can be considered as promising low cost organic semiconductors with improved processability. A challenge to overcome is that the elimination of the

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intermolecular hydrogen bonding network by replacing the amine hydrogens by alkyl groups might lead to poor structural organization. Indeed, earlier reports indicated that dialkyl derivatives of quinacridone showed no field-effect response in the OFET

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configuration.8 However, ordered supramolecular organization can exist in this type of compounds as revealed by detailed single crystal analysis of dibutyl quinacridone.9 This structure is characterized by strong face-to-face π-stacking with an intermolecular distance of 3.48 Å and the formation of one-dimensional molecular columns. Intra- and inter-columnar weak hydrogen bonding network is evidenced. In these stacks, the

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interactions involve carbonyl and CH2 groups of packed molecules, while molecules in neighbouring columns establish hydrogen bonding between oxygen atoms of the carbonyl group and hydrogen atoms of the outer aromatic ring. The preferential molecular orientation of the stacking is that the nitrogen atoms in one molecule acting

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as electron donor approach the electron acceptor carbonyl groups in the stacked partner. The electron-deficient atoms prefer to stack with both the electron-rich and electron-deficient

atoms

of

the

partner

molecule,

maximizing

electrostatic

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complementarity.10

Inspired by these structural findings we have studied the effect of alkyl substituent length and processing conditions on the supramolecular organization and electrical transport properties in thin films of N,N’-dialkylquinacridones. We have found that optimizing the processing conditions it is possible to obtain thin films of spherulitic crystallinity which exhibit charge carriers mobility in the FET configuration of 0.3 cm2/Vs i.e. exceeding the value reported for unsubstituted quinacridone.

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Scheme 1. Chemical structure of dialkyl substituted quinacridones.

2. Experimental

Thermogravimetric analysis (TGA) was carried out on a Mettler 500 at a heating rate

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of 10 °C/min under nitrogen flow, differential scanning calorimetry (DSC) was performed on a Mettler DSC 30 at a heating/cooling rate of 10 °C/min under nitrogen flow.

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Optical images of the films were taken using a Hitachi KP-D50 Color Digital chargecoupled device camera on a Zeiss microscope equipped with polarizing filters. Veeco Dimension 3100 Atomic Force Microscope was used to inspect the microstructure and the thickness of the sublimed films. All images were obtained in the tapping mode with Olympus silicone cantilevers at 320 kHz resonance frequency. Film thickness analysis was performed by extracting profile of the grains in the height mode

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in Gwyddion 2.47 software.

To investigate the molecular ordering in the sublimed films, GIWAXS measurements were performed at the DELTA Synchrotron using beamline BL09 with a photon

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energy of 10 keV (λ = 1.239 Å). The beam size was 1.0 mm × 0.2 mm (width x height), and samples were irradiated just below the critical angle for total reflection with respect to the incoming X-ray beam (∼0.1°). The scattering intensity was detected

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on a 2-D image plate (MAR-345) with a pixel size of 150 µm (2300 × 2300 pixels), and the detector was placed 381 mm from the sample center. The raw detector image needs to be converted into reciprocal-space. This was done by using a calibration standard (silver behenate), which has rings at known 2Θ positions. Scattering data are expressed as a function of the scattering vector: q=4π/λ sin(Θ), where Θ is a half the scattering angle and λ=1.239 Å is the wavelength of the incident radiation. Here qxy (qz) is a component of the scattering vector in-plane (out-of-plane) to the sample surface. All X-ray scattering measurements were performed under vacuum (~1 mbar) to reduce air scattering and beam damage to the sample. All GIWAXS data processing

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analysis

was

performed

by

using

the

software

package

Datasqueeze

(http://www.datasqueezesoftware.com). OFET devices were fabricated in a bottom-gate top-contact (BGTC) configuration on Si/SiO2 substrates. The substrates were cleaned sequentially with acetone and isopropanol,

and

then

modified

by

UV

treatment

and

subsequently

by

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octadecyltrichlorosilane (OTS) deposition by immersing into 4% OTS/toluene solution, on a hot plate at 60 °C for 60 min. The aim of the modification was to create a self-assembled monolayer (SAM), which reduces the surface energy of the inorganic dielectric and ensures a three-dimensional growth of the film. The organic

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semiconductor films were deposited by vacuum-evaporation at a rate of 0.1 Å/min at different substrate temperatures using a TecTra Mini-Coater high vacuum coating system. The film thickness was about 50 nm controlled using a quartz crystal

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microbalance monitor and adjusted by atomic force microscopy. The QA-C8 film deposited at 25 °C was additionally annealed at 170 °C for one hour. Transistors were measured by using a Keithley 2634B source meter in a glove box under nitrogen atmosphere. Mobilities were calculated from the transfer characteristics in the saturation regime, using:

μ

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=

2

−

Where Id is the drain current, W the channel width, L the channel length, Ci the capacitance of gate dielectric, Vg the gate voltage and Vth the threshold voltage. The

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reported mobilities are averaged values over 10 transistors. The OFET devices had a channel width of 1 mm and a channel length of 30 µm. The SiO2 dielectric layer was

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300 nm thick with dielectric constant of 3.9.

3. Results and discussion 3.1. Thermal properties

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Fig. 1. a) DSC (2nd heating) and b) TGA scans of QA-C4, QA-C8 and QA-C12. In view of the fact that the optimization of the thermal processing conditions was one of the

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goals of this work we performed detailed differential scanning calorimetry (DSC) and thermogravimetry (TGA) investigations of the synthesized compounds (see Scheme 1 for their chemical formulae). In the temperature range from -20 °C to 310 °C the DSC curves reveal one endothermic peak, ascribed to melting of the studied compounds. The determined

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melting temperatures are significantly lower than that of quinacridone (390 °C) and decrease with the alkyl substituent length from 261 °C for QA-C4, to 177 °C for QA-C8 and 134 °C for QA-C12 (see Fig. 1a), which is a common feature in alkyl substituted conjugated

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molecules.2h Additionally, the exothermic peak at around 75 °C for QA-C12 is attributed to the rearrangement of the longer flexible side chains what has been also observed for discotic liquid crystals. The length of the alkyl substituents also affects the onset of the thermal decomposition temperature which increases from 305 °C for QA-C4 to 320 °C for QA-C8 and 350 °C for QA-C12 (Fig. 1b). The determined thermal stability of all three molecules perfectly justifies the use of thermal processing techniques. The highest applied substrate temperatures of 170 °C is more than 100 °C lower than the onset of the thermal decomposition temperature for the least thermally stable compound (QA-C4).

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ACCEPTED MANUSCRIPT 3.2. Impact of substrate temperature on the morphology and charge carrier transport of QA-C8 thin films Based on the DSC data, three substrate temperatures for deposition of QA-C8 were chosen: 25 °C, 120 °C and 170 °C with large temperature differences to ensure a high variation in the grown film morphologies. The highest selected temperature is only few degrees lower than

temperature, no film was formed on the substrate.

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the melting temperature of 177 °C for this compound. During sublimation above the melting

Thin films of QA-C8 were deposited on OTS modified SiO2 substrates. Their polarized optical microscopy (POM) images clearly show a pronounced effect of the substrate

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temperature on the morphology of the resulting films. Deposition at 25 °C leads to a film with randomly distributed birefringent crystallization nuclei and small spherulitic domains of ca.

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65 µm in average size, embedded in a black amorphous matrix (Fig. 2a). This amorphous part remains black also during the rotation of the sample with respect to the cross-polarizers. In contrast, the film deposited at 120 °C consists of much larger spherulites with an average size of ca. 265 µm (Fig. 2b). However, the number of crystallization nuclei seems to be similar as for the film formed at 25 °C and grain boundaries between the spherulites are clearly visible. The spherulites exhibit a typical Maltese cross following the direction of the

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cross-polarizers. This gradual optical extinction indicates that the molecules are arranged radially from the nucleation centre. Increasing the substrate temperature to 170 °C results in even larger spherulitic domains (Fig. 2c) exceeding by far the square millimetre area of the

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polarized microscope. In order to induce crystallization of the amorphous parts of the film deposited at 25 °C, the sample was annealed at 170 °C. Indeed, upon annealing the amorphous matrix shrinks and the existing crystalline domains remarkably grow with small

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grains appearing in between them. The POM study of the film morphology shows that the deposition temperature has a significant effect on the crystal formation and growth. As commonly known, at high temperatures a slow nucleation occurs with a relatively high fill rate, resulting in the formation of only a few but large crystals. Decreasing the temperature causes an acceleration of the nucleation process which may become faster than the crystal growth.1 The film deposited at 25 °C, and then annealed at 170 °C, does not show the same morphology as the one directly deposited at 170 °C (compare Fig. 2d and 2c). In particular, during annealing, the crystalline domains remarkably grow, but are still significantly smaller than those formed in the film deposited at 170 °C. This is caused by a relatively large number

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ACCEPTED MANUSCRIPT of small spherulitic domains, already formed at 25 °C, whose growth is impeded by the

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growth of the neighbouring, closely situated spherulites.

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Fig. 2. Polarized optical microscopy images of QA-C8 films deposited at a) 25 °C, b) 120 °C, c) 170 °C and d) 25 °C and subsequently annealed at 170 °C.

Additional investigations of the morphology of N,N’-dialkylquinacridone films were carried out using atomic force microscopy (AFM) in tapping mode (Fig. 3). The height image of the amorphous part of the QA-C8 film deposited at 25 °C reveals its fine granular topography

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(Fig. 3a), with grains of a few hundred nanometers in size clearly visible. The film deposited at 120 °C presents a different image of a tribe-like morphology. It seems that the ca. 60 nm thick film consists of grains that randomly grow and coalesce with each other (Fig. 3b). It is

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not possible to evaluate the average grain size or their geometric shape since individual grains are not distinguishable. This type of image also does not allow an unequivocal determination of the film growth mechanism. The morphology of the ca. 80 nm thick QA-C8 film deposited at 170 °C is similar to that found for the film deposited at 120 °C, showing however, much larger domains and, as a consequence, smaller contribution of interdomain boundaries (compare Fig. 3c and 3b). This might indicate that at higher temperatures a smaller number of grain nuclei are created and the growth of a particular grain is less impeded by the presence of neighbouring grains. This is consistent with the POM studies which revealed the largest spherulites in films deposited at 170 °C. The QA-C8 film deposited at 25 °C and then annealed at 170 °C undergoes recrystallization at the annealing temperature which 8

ACCEPTED MANUSCRIPT significantly changes its morphology, leading to a significant increase of the domain size (compare Fig. 3d and 3a). The AFM image displays a similar type of morphology as observed for the films deposited of 120 °C and 170 °C, however, the individual domains are larger. The organization of the QA-C8 molecules in the films was studied by grazing incidence wide-angle X-ray scattering (GIWAXS) (Fig. 4). The diffraction pattern recorded for the film

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deposited at 25 °C shows only reflections in the small-angle region indicative of low structural order (Fig. 4a). These reflections correspond to a hexagonal unit cell with the parameter of ahex = 2.65 nm of the columnar structures that are formed by the molecules. The position of the peaks corresponding to the out-of-plane arrangement is characteristic of an

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edge-on arrangement of the molecules on the surface. However, the broad azimuthal distribution of the 100 reflection indicates certain misalignment of the domains towards the

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surface. Moreover, the missing π-stacking reflection and the amorphous halo imply a poor packing of the molecules in the stacks. This analysis is again consistent with the predominantly amorphous microstructure observed in the POM images. Films deposited at 120 °C and 170 °C exhibit significantly improved structural order as evidenced by the presence of the spot-like reflections (Fig. 4b and 9b). In both cases, the same supramolecular organization is found. The presence of only one small-angle reflection corresponding to the d-

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spacing of 1.38 nm does not allow a precise determination of the unit cell of QA-C8 crystallized at 120 °C and 170 °C. However, the in-plane π-stacking reflection at qxy = 1.73 Å-

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confirms close packing of the molecules with an intermolecular distance of 0.36 nm.

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Fig 3. AFM height images of QA-C8 films deposited at a) 25 °C, b) 120 °C, c) 170 °C, and d) 25 °C with subsequent annealing at 170 °C.

Fig. 4. GIWAXS patterns of QA-C8 films deposited at a) 25 °C and b) 170 °C. 10

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The charge carrier transport of the QA-C8 films was investigated using FET devices in the top-contact bottom-gate configuration. As evidenced by the obtained transfer and outputs characteristics all studied QA-C8 films show a typical p-type behavior. The maximum and average charge carrier mobility, threshold voltage (Vth) and on/off ratio derived from the

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transfer characteristics are summarized in Table 1. The transistors fabricated using films deposited at 25 °C exhibit a maximum hole mobility of 4x10-4 cm2/Vs, a threshold voltage of -32 V and on/off ratio of 3x103 (Fig. 5 and 6). Structurally better ordered films, deposited at higher temperatures result in hole mobilities improved by almost three orders of magnitude

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(3x10-1 cm2/Vs and 2x10-1 cm2/Vs for films deposited at 120 °C and 170 °C, respectively (Fig. 5 and 6). In addition they show a two times lower threshold voltage and nearly three

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orders of magnitude higher on/off ratio (see the data collected in Table 1). The improved transistor performance is attributed to higher crystallinity, larger grain size and smaller grain boundaries contribution in films deposited at higher substrate temperatures.2 From these data, it seems evident that the domains size in films formed at 120 °C is already sufficient to span the transistor channel between the source and drain electrodes. In contrast, small grain size and large grain boundaries in the film fabricated at 25 °C lead to significant worsening of the

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charge carrier transport in the channel and yield poor transistor parameters. The transistors with QA-C8 films fabricated at 25 °C and subsequently annealed at 170 °C exhibit improved parameters as compared to the case of non-annealed ones. These parameters are, however, significantly lower than those measured for transistors in which films deposited at 120 °C and

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170 °C served as active layers (see Table 1). The charge carrier mobility drops to 7x10-2 cm2/Vs and the threshold voltage significantly increases to -47 V probably due to a higher

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contact resistance as implied by the non-linear output curves in the low voltage regime (Fig. 6c). The main conclusion drawn from these device data is that the selection of the deposition temperature is the dominant factor in obtaining transistors of optimized operation parameters. Thermal annealing of films deposited at low temperatures can improve the transport properties, but not to the level of those obtained for films deposited at the optimum temperature. This is probably associated with the fact that low temperature deposition leads to small ordered domains whose presence preconditions the crystal growth at the annealing temperature. In consequence, the resulting structural organization may not be optimal for the transport of charge carriers.

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Fig. 5. Transfer characteristics of transistors based on QA-C8 films deposited at 25 °C (with and without subsequent annealing at 170 °C), 120 °C, and 170 °C.

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Fig. 6. Output characteristics of field-effect transistors with QA-C8 films as active layers deposited at a) 120 °C, b) 170 °C, and c) 25 °C and subsequently annealed at 170 °C. Output cures for the film deposited at 25 °C was not measureable.

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Table 1. Maximum and average charge carrier mobility, threshold voltage and on/off ratio for QA-C8 transistors prepared at different temperatures.

[°C]

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Substrate temperature

µ h max. 2

µ h av.

Vth [V]

on/off [-]

2

[cm /Vs]

[cm /Vs]

4x10-4

(1.0 ± 0.9)x10-4

-32

3x103

25 and annealed at 170

7x10-2

(4.1 ± 2.4)x10-2

-47

2x105

120

3x10-1

(1.8 ± 0.8)x10-1

-18

2x106

170

2x10-1

(1.2 ± 0.5)x10-1

-14

1x106

25

3.3. Influence of the alkyl chain length on the properties of quinacridone derivatives 12

ACCEPTED MANUSCRIPT The length of alkyl substituents in conjugated molecules not only influences their vacuum processing, but also affects their self-organization. This is especially important in the case of dialkylated quinacridone, where the alkylation eliminates the hydrogen bonding network between the carbonyl groups and amine hydrogens and the main driving force for self-organization are π-π interactions and interdigitation of the alkyl

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substituents. The latter is strongly dependent on the alkyl length. For this reason, we have undertaken detailed comparative studies of the morphological, structural and electrical transport properties of dialkylated quinacridones differing in the substituent

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length.

Fig. 7. POM images of film of a) QA-C4, b) QA-C8, and c) QA-C12. The compounds are deposited at 120 °C.

In Fig. 7, POM images of the QA-C4, QA-C8 and QA-C12 films deposited at 120 °C are compared. In the image of the QA-C4 film small crystallites/grains can be 13

ACCEPTED MANUSCRIPT distinguished, uniformly distributed over the whole area (Fig. 7a). They are much smaller than the spherulites observed for the QA-C8 film deposited at the same temperature (Fig. 7b). Under the same deposition conditions, QA-C12 readily crystallizes yielding crystals whose size exceeds the area of the POM image such that only a small part of the spherulite is visible (Fig. 7c). The POM study indicates that in

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the case of alkylated quinacridone, an increase of the substituent length results in a decrease of the number of seeds and an increase of the crystal size. The tendency has two origins: i) easier formation of ordered structures due to more efficient interdigitation in the case of derivatives with longer substituents; ii) lowering of their

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melting temperature since, as shown in the first part of this paper, the number of seeds decreases and the size of spherulites increases when the substrate temperature

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approaches the melting temperature.

Fig. 8. AFM height images of films of a) QA-C4 and b) QA-C12 deposited at 120 °C.

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ACCEPTED MANUSCRIPT The differences in their morphologies are also clearly observed in the tapping mode AFM images of the QA-C4, QA-C8 and QA-C12 films (compare Fig. 8a,b and 3b). Compound QA-C4 forms 5-10 µm long and ca. 120 nm thick rod-like crystallites in the deposited film. This morphology is similar to that reported for unsubstituted quinacridone films which also comprise rod-shaped crystallites, however, of a smaller

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length of 100-200 nm.3 The discrepancy to the theoretical film thickness arises from the microbalance in the vacuum sublimation chamber which records an average value. The films of QA-C8 and QA-C12 exhibit similar images which are, however, strikingly different than that of QA-C4. As already stated (vide supra), it can be

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characterized as a tribe-like morphology with grains that randomly grow and coalesce.

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The domains are larger for the ca. 60 nm thick QA-C12 film (Fig. 8b).

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Fig. 9. GIWAXS patterns of films of a) QA-C4, b) QA-C8, and c) QA-C12. The compounds are deposited at a substrate temperature of 120 °C.

The impact of the substituent length on the molecular organization of alkylated quinacridones is also reflected in the GIWAXS results which show clear differences between the three compounds (Fig. 9). In the case of the QA-C4 film a hexagonal organization of columnar structures ahex = 2.35 nm is derived from the small-angle reflections (Fig. 9a). This spacing is smaller than the corresponding value found for the QA-C8 film deposited at 25 °C and reflects the difference in the substituent size. The azimuthal smearing out of these reflections indicates a random orientation of the 16

ACCEPTED MANUSCRIPT domains towards the surface. The packing of the molecules in the stacks is poor as evidenced from the low intensity in-plane π-stacking reflection at qxy = 1.73 Å-1 (πstacking distance of 0.37 nm). This finding is in line with the weakly developed film microstructure displayed in the POM and AFM images. In comparison to QA-C4, the structural order is increased in films of QA-C8 and QA-C12 whose diffraction patterns

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exhibit distinct reflections (Fig. 9b,c). The diffractograms of both films show an identical molecular order and organization. The d-spacing of the out-of-plane reflection increases from 1.38 nm for QA-C8 to 1.69 nm for QA-C12 manifesting the increase of the substituent length. A similar π-stacking distance of 0.36 nm is found for QA-C12

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as for the other two compounds.

The FET devices in the top-contact bottom-gate configuration were prepared to measure the electrical properties of QA-C4, QA-C8 and QA-C12 films deposited at

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120 °C. The maximum and average charge carrier mobility, and Vth and on/off ratio are summarized in Table 2. The field-effect response of the QA-C4 film is too poor to calculate the charge carrier mobility. As already observed for the QA-C8 film (Fig. 5), an unipolar hole transport is found from the transfer and output characteristic of the QA-C12 based transistor (Fig. S1). The maximum hole mobility of 3x10-2 cm2/Vs

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derived from the transfer characteristic is about one order of magnitude lower than the value obtained for the QA-C8 based transistor fabricated under the same conditions. The other device parameters are also worse (see Table 2).

Compound

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Table 2. Maximum and average holes mobility, threshold voltage and on/off ratio for transistors based on QA-C4, QA-C8 and QA-C12 deposited at 120 °C. µ h max. [cm2/Vs]

µ h av. [cm2/Vs]

Vth [V]

on/off [-]

no field-effect

QA-C8

3x10-1

(1.8 ± 0.8)x10-1

-18

2x106

QA-C12

3x10-2

(2.3 ± 0.6)x10-2

-24

1x104

AC C

QA-C4

Conclusions In search for cheap organic semiconductors of improved vacuum processability we have synthesized and studied three N,N’-dialkylquinacridones (alkyl = butyl, octyl dodecyl). These compounds were obtained in one-step from quinacridone, a cheap pigment widely used in industry. The compounds show significantly lower melting temperature than unsubstituted quinacridone. Despite the fact that alkylation of quinacridone eliminates the hydrogen 17

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network

between

the

carbonyl

groups

and

amine

hydrogens,

N,N’-

dialkylquinacridones with longer alkyl substituents (QA-C8 and QA-C12) are still capable of forming highly ordered structures through π-π stacking and alkyl chain interdigitation. The enhanced molecular diffusion due to the lowered melting point of the compounds and optimization of the deposition temperature leads to thin films consisting of spherulitic crystals

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densely covering the substrate surface. Apart from N,N’-dibutylquinacridone (QA-C4) which does not show any field-effect due to its poor structural organization in thin films, the remaining two quinacridone derivatives, QA-C8 and QA-C12, form highly ordered films which can be used as active layers in p-channel FETs. Maximum hole mobilities up to 0.3

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cm²/Vs with a good on/off ratio of 106 are obtained for QA-C8. It is expected that an optimization of the morphology and the device performance is still possible by adjusting the side chain geometry and the processing conditions. For instance, the processing conditions are

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not yet optimized for QA-C4 and QA-C12, but only adapted from QA-C8. A modification of the chemical structure could include the incorporation of minor branching points or heteroatoms in the side chains to improve the diffusion of the molecules during sublimation and at the same time to ensure a close molecular packing.

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Acknowledgements

This work was partially prepared as part of the „Self-standing, flexible and solution processable organic field effect transistors for complementary inverter applications” project that is carried out within the First Team programme of the Foundation for

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Polish Science co-financed by the European Union under the European Regional Development Fund (First TEAM/2017-3/26). I.K. and W.P. acknowledge the National

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Science Centre, Poland, through the grant UMO-2015/18/E/ST3/00322. I.K.B. acknowledge the financial support of NCN through the grant No 2015/17/B/ST5/00179 whereas Z.W. and A.P. through the grant 2015/17B/ST4/03837. The authors also acknowledge beamline 9 of the DELTA electron storage ring in Dortmund for providing synchrotron radiation and technical support for the GIWAXS measurements.

References [1] a) L. Yu, X. Li, J. Smith, S. Tierney, R. Sweeney, B. K. C. Kjellander, G. H. Gelinck, T. D. Anthopoulos, N. Stingelin, J. Mater. Chem. 22 (2012) 9458; b) Y. Yuan, G. Giri, A. L. Ayzner, A. P. Zoombelt, S. C. B. Mannsfeld, J. Chen, D. Nordlund, M. F. Toney, J. Huang, 18

ACCEPTED MANUSCRIPT Zhenan Bao, Nat. Commun. 5 (2014) 3005; c) H. Jeong, S. Han, S. Baek, S. H. Kim, H. S. Lee, ACS Appl. Mater. Interfaces 8 (2016) 24753. [2] a) J. KIm, AR. Han, J.H. Seo, J.H. Oh, C. Yang, Chem. Mater. 24 (2012) 3464; b) T. Mori, T. Nishimura, T. Yamamoto, I. Doi, E. Miyazaki, I. Osaka, K. Takimiya, J. Am. Chem. Soc. 135 (2013) 13900; c) U.H.F. Bunz, Acc. Chem. Res. 48 (2015) 1676; d) J. I. Park, W. J.

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J. Phys. Chem. C 119 (2015) 10700; i) Y. Min, C. Dou, H. Tian, Y. Geng, J. Liu, L. Wang, Angew. Chem. 57 (2018) 2000; j) D. Cortizo-Lacalle, A. Pertegas, L. Martinez-Sarti, M. Melle-Franco, H. J. Bolink, A. Mateo-Alonso, J. Mater. Chem. C 3 (2015) 9170; k) J. Li, F. Yan, J. Gao, P. LI, W. W. Xiong, Y. Zhao, X. W. Sun, Q. Zhang, Dyes and Pigments 112 (2015) 93.

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[3] H. Zollinger, Color Chemistry: Syntheses, Properties, and Applications of Organic Dyes and pigments, 3rd ed. Wiley-VCH, Weinheim, 2003. [4] a) H. Bi, K. Ye, Y. Zhao, Y. Yang, Y. Liu, Y. Wang, Org. Electron. 11 (2010) 1180; b) T. Zhou, T. Jia, B. Kang, F. Li, M. Fahlman, Y. Wang, Adv. Energy Mater. 1 (2011) 431; c) E.

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[5] M. Sytnyk, M. Jakesova, M. Litvinukova, O. Mashkov, D. Kriegner, J. Stangl, J. Nebesarova, F. W. Fecher, W. Schöfberger, N. S. Sariciftci, R. Schindl, W. Heiss, E. D. Glowacki, Nat. Commun. 8 (2017) 91. [6] E. D. Glowacki, L. Leonat, M. Irimia-Vladu, R. Schwödiauer, M. Ullah, H. Sitter, S. Bauer, N. S. Sariciftci, Appl. Phys. Lett. 101 (2012) 023305. [7] a) E. D. Glowacki, M. Irimia-Vladu, S. Bauer, N. S. Sariciftci, J. Mater. Chem. B 1 (2013) 3742; b) H. T. Black, D. F. Perepichka, Angew. Chem. Int. Ed. 53 (2014) 2138; c) W. L. Hao, S. F. Zou, J. H. Gao, H. R. Zhang, R. Chen, H. X. Li, W. P. Hu, Org. Electron. 53 (2018) 57; d) H. C. Zhang, R. N. Deng, J. Wang, X. Li, Y. M. Chen, K. W. Liu, C. J. Taubert, S. Z. D.

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M. Che, J. Phys. Chem. B 109 (2005) 8008. [10] H. Sun, K. Ye, C. Wang, H. Qi, F. Li, Y. Wang, J. Phys. Chem. A 110 (2006)

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10750.

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Electronic Supporting Information Self-assembly and charge carrier transport of

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sublimated dialkyl substituted quinacridones

a

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Tomasz Marszalek,a,b Izabela Krygier,a,b Adam Proń,c Zbigniew Wrobel,d Paul Blom,a Irena Kulszewicz-Bajer,c,* and Wojciech Pisula,a,b,* Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany.

b

Department of Molecular Physics, Faculty of Chemistry, Lodz University of Technology, Zeromskiego 116, 90-924 Lodz, Poland.

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Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland.

d

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Institute of Organic Chemistry, Polish Academy of Science, Kasprzaka 44/52, 01-224 Warsaw, Poland.

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*To whom correspondence should be addressed.

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Email: [email protected], [email protected]

Synthesis

Characterization techniques 1

H NMR spectra were recorded on a Varian Mercury (500 MHz) spectrometer and referenced

with respect to TMS and solvents. IR spectra were monitored on a Bio-RAD FTS-165 spectrometer using KBr pellets technique. Mass spectra were measured by EI method on an AMD 604 mass spectrometer. All synthesized compounds studied were subject to C, H, N elemental combustion analysis.

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ACCEPTED MANUSCRIPT Reagents Quinacridone, QA was purchased from TCI. Butyl bromide, octyl bromide, dodecyl bromide, cesium carbonate anhydrous, N-methylpyrrolidinone, NMP anhydrous were purchased from Aldrich. All glassware was oven dried, assembled hot, and cooled under a dry argon stream before

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use. All reactions were performed under dry argon. N,N’-dioctylquinacridone, QA-C8

0.499 g (1.6 mmol) of QA and 4.2 g (12.8 mmol) of Cs2CO3 were dispersed in 4 ml of anhydrous NMP under an argon atmosphere. The mixture was heated at 100°C followed by

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the drop wise addition of 2.47 g (12.8 mmol) of n-octyl bromide. Then it was heated for 24 h. After consecutive cooling, the mixture was poured into 400 ml of water. The formed

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precipitate was filtered and washed with water. It was then dissolved in CHCl3 and dried over Na2SO4. The crude product was purified by chromatography on silica gel eluting with CHCl3. Thus obtained product was crystallized from CHCl3/diethyl ether to give 0.557 g (1.04 mmol, 65% yield) of red powder. 1H NMR (500 MHz, CDCl3) ä, 8.77 (s, 2H), 8.58 (dd, J=1.5, 8 Hz, 2H), 7.76 (td, J=1.5, 8 Hz, 2H), 7.52 (d, J=8 Hz, 2H), 7.28 (d, J=8 Hz, 2H), 4.51 (t, J=8 Hz, 4H), 2.04-1.98 (m, 4H), 1.65-1.59 (m, 4H), 1.50-1.45 (m, 4H), 1.38-1.30 (m, 12H), 0.90 (t,

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J=7 Hz, 6H). IR (cm-1): 3073, 2976, 2926, 2858, 1636, 1590, 1492, 1473, 1388, 1262, 1204, 753. Anal. Calcd. for C36H44N2O2: C, 80.56; H, 8.26; N, 5.22; O, 5.96. Found: C, 80.82; H,

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8.13; N, 5.05. M/z=536.2.

N,N’-didodecylquinacridone, QA-C12 The same procedure as above was used for the alkylation of QA with dodecyl bromide.

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After crystallization from CHCl3/ diethyl ether mixture 0.725 g (1.12 mmol, 70% yield) of red powder was obtained. 1H NMR (500 MHz, CDCl3) δ, 8.76 (s, 2H), 8.58 (dd, J=1.5, 8 Hz, 2H), 7.75 (td, J=1.5, 8 Hz, 2H), 7.50 (d, J=8 Hz, 2H), 7.28 (d, J=8 Hz, 2H), 4.50 (t, J=8 Hz, 4H), 2.02-1.96 (m, 4H), 1.66-1.59 (m, 4H), 1.50-1.46 (m, 4H), 1.38-1.30 (m, 28H), 0.86 (t, J=7 Hz, 6H). Anal. Calcd. for C44H60N2O2: C, 81.43; H, 9.32; N, 4.32; O, 4.93. Found: C, 81.66; H, 9.45; N, 4.02. M/z=648.5. N,N’-dibutylquinacridone, QA-C4 The same procedure as above was used for the alkylation of QA with butyl bromide. After the crystallization from CHCl3/ diethyl ether mixture 0.36 g (0.85 mmol, 53% yield) of 22

ACCEPTED MANUSCRIPT red powder was obtained. 1H NMR (500 MHz, CDCl3) δ, 8.78 (s, 2H), 8.58 (dd, J=1.5, 8 Hz, 2H), 7.74 (td, J=1.5, 8 Hz, 2H), 7.50 (d, J=8 Hz, 2H), 7.28 (d, J=8 Hz, 2H), 4.52 (t, J=8 Hz, 4H), 2.04-1.98 (m, 4H), 1.66-1.62 (m, 4H), 0.90 (t, J=7 Hz, 6H). Anal. Calcd. for C28H28N2O2: C, 79.22; H, 6.65; N, 6.60; O, 7.53. Found: C, 79.55; H, 6.67; N, 6.52.

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M/z=424.2.

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Field-effect transistor results

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Figure S1. a) Transfer and b) output characteristics of a transistor with QA-C12 film deposited at 120 °C.

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Highlights Self-assembly and charge carrier transport of sublimated films of dialkyl quinacridones were investigated Length of the alkyl substituents is crucial for the supramolecular self-organization and thin film morphology

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Optimization of the deposition conditions results in a balanced nucleation and crystal growth leading to charge carrier mobility of 0.3 cm²/Vs which is higher than for unsubstituted quinacridone