Pulsed laser processing of poly(3,3‴-didodecyl quarter thiophene) semiconductor for organic thin film transistors

Accepted Manuscript Pulsed laser processing of poly(3,3’’’-didodecyl quarter thiophene) semiconductor for organic thin film transistors C. Constantinescu, L. Rapp, P. Rotaru, P. Delaporte, A.P. Alloncle PII: DOI: Reference:

S0301-0104(15)00028-2 http://dx.doi.org/10.1016/j.chemphys.2015.02.004 CHEMPH 9255

To appear in:

Chemical Physics

Received Date: Accepted Date:

28 November 2014 4 February 2015

Please cite this article as: C. Constantinescu, L. Rapp, P. Rotaru, P. Delaporte, A.P. Alloncle, Pulsed laser processing of poly(3,3’’’-didodecyl quarter thiophene) semiconductor for organic thin film transistors, Chemical Physics (2015), doi: http://dx.doi.org/10.1016/j.chemphys.2015.02.004

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Pulsed laser processing of poly(3,3'''-didodecyl quarter thiophene) semiconductor for organic thin film transistors *C. 1

Constantinescu1,2, L. Rapp1, P. Rotaru3, P. Delaporte1, A.P. Alloncle1

Aix-Marseille Université / CNRS, Laboratoire LP3 (UMR CNRS 7341), Marseille, F-13288, France 2 INFLPR - National Institute for Laser, Plasma and Radiation Physics, Magurele, 409 Atomistilor blvd., Bucharest, RO-077125, Romania 3 University of Craiova, Faculty of Physics, 13 A.I. Cuza St., Craiova, RO-200585, Romania

Abstract We report on the growth of thin solid layers of poly(3,3’’’ didodecylquaterthiophene) (PQT-12) by matrix-assisted pulsed laser evaporation (MAPLE), on silicon and quartz substrates. The effects of PQT12 solubilization in toluene, anisole, 1,2-dichlorobenzene, and a mixture of chlorobenzene and 1,2dichlorobenzene, are discussed with respect to the MAPLE technique. Different film thicknesses have been grown, and their morphology and optical properties are presented. Thermal analysis studies have been realized to understand and explain the laser-induced photo-thermal effects on the organic semiconductor. Subsequently, micrometric-sized pixels of PQT-12 have been printed by laser-induced forward transfer (LIFT), with the goal to fabricate organic thin-film transistors (OTFT) devices. The influence of the donor films thickness and morphology, in LIFT experiments, is discussed. Electrical characterizations supplement this study, the resulting printed transistors are fully functional and provide field-effect mobility up to 5 × 10-3 cm2 · V-1 · s-1 together with current modulation of 106. Keywords: Organic compounds; polymers; semiconductors; thin film; atomic force microscopy; thermogravimetric analysis.

1. Introduction Conductive polymers are organic compounds that intrinsically conduct electricity. Ionic polymers, or polymer electrolytes, exhibit lower conductivity compared to conjugated polymers. Certain conjugated polymers also possess interesting optical and magnetic properties, owing to the delocalization of electrons in a continuously overlapped π-orbital along the catenae / backbone, and may have metallic conductivity or can be semiconductors [1-3]. Although investigations on conductive polymers have been described in the mid-19th century [4], the first reports on organic electronics date since the 1970s [5, 6]. In 2000, the Nobel Prize in Chemistry was awarded to Alan J. Heeger, Alan MacDiarmid, and Hideki Shirakawa, for the "discovery and development of conductive polymers". Since then, development and synthesis of new materials are a topic of intensive investigations, and the production of organic electronic components is an alternative to inorganic devices for *

Authors for correspondence: [email protected]; [email protected]; [email protected]; [email protected].

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applications based on cost-efficient processes. However, due to the backbone rigidity intrinsically

associated

with

the

delocalized

conjugated

structure,

most

unfunctionalized conjugated polymers are intractable (i.e. insoluble, infusible and brittle) and thus unsuitable for thermoforming. Some are even unstable when exposed to air, limiting their industrial applications to just a few. One of the key frontiers in technology is to gain control over matter at small scales. This has prompted the development of novel technologies that are suitable for a wide variety of materials, in both liquid and solid form, that can be used with rigid as well as with flexible substrates. The most common route to process conductive polymers is related to their solubility and/or dispersive properties, in order to be further transformed as thin films by various conventional methods. The main manufacturing technologies used today are mass printing processes, such as serigraphy or inkjet printing. The drawback is that such techniques require the use of soluble materials (1) and a step of annealing (2), which may prevent the use of some low-cost flexible substrates. In fact, these issues actually impose serious limitations in using printing technologies, for instance when designing special inks that fulfil the strict requirements in terms of rheology, physicochemical stability, and costs. These tasks can be extremely complicated or even impossible to fulfil. Nevertheless, the performances of organic thin film transistors (OTFTs) are continuously improved, as these devices will be one of the basic components of future plastic electronic devices [7-9]. OTFTs have attracted intense research interest in recent years because of their potential in fabricating cost-efficient integrated circuits and electronic devices, where the use of the current inorganic, silicon-based technology can be costly or unsuitable. The study and the implementation of new methods for electronic components manufacture, particularly on flexible supports, represent an important stage in developing plastic microelectronics. Laser-based processes [10] offer versatile alternatives for the deposition of thin films, in developing organic devices that operate on flexible supports. Lasers offer a highly directional and localized source of energy, which facilitate material modifications at precise locations without physical contact. Specific technological requirements can be fulfilled by simply controlling the laser parameters, such as beam size, energy, wavelength, and pulse duration. The most common laser-based 2

techniques are the pulsed laser deposition (PLD), the matrix-assisted pulsed laser evaporation (MAPLE) and the laser-induced forward transfer (LIFT) [11-20]. However, the PLD technique, i.e. the direct laser irradiation and ablation of materials, cannot be considered here as it usually induces photochemical rearrangement in organic materials (referred to as photo-Fries rearrangement) and/or photo-induced pyrolysis and oxidation, depending on the laser wavelength and fluence. Thus, the MAPLE technique [11] was considered for the growth of PQT-12 thin films, together with LIFT printing [12] for the fabrication of organic electronic components. Notably, since LIFT does not require the use of nozzles, it can print a wide variety of materials, ranging from high viscous inks to materials in solid, rigid form. Therefore, laser methods overcome the main limitations of conventional techniques, thus offering an adequate approach for the heterogeneous integration demanded in the fabrication of the next generation of organic miniaturized devices. Polythiophenes have been intensely studied over the last three decades, with a significant amount of papers being published on these compounds particularly in the last 5 years. Characterized in detail for their chemical and physical properties, it was demonstrated that self-assembled π-conjugated thiophene polymers show superior electronic and optical functionalities [16-19, 21-26], compared to similar conductive organics. Among such conjugated polythiophene compounds, the regioregular poly(3,3’’‘-didodecyl quarter thiophene) (PQT-12) [27] was shown to be suitable for laser processing and/or printing [19], as it possesses high ionization potential and a good stability against doping phenomena [21-23]. PQT-12 is a solution-processable organic semiconductor, intensively studied by spin-coating [24-26], with relatively high charge carrier mobility. B.S. Ong et al. reported on PQT-12 transistors with mobility as high as 0.2 cm2 · V-1 · s-1 and On/Off ratio of 108 [26]. In this paper, we present results on PQT-12 thin films grown by MAPLE technique, and on the printing of micrometer-size pixels by LIFT technique, putting an emphasis on their morphology and electrical properties.

2. Experimental MAPLE is a deposition technique that involves dissolving or suspending the guest material (in this case, the PQT-12) in a volatile solvent (e.g. organic solvents, 3

water, or mixtures of various choice solvents), freezing this mixture to create a solid target and using a low fluence pulsed laser to evaporate the target material inside a special design cryogenic deposition system [11, 13-15]. The solvent evaporates when the matrix is irradiated by laser light, whereas the guest molecules are collected on a substrate. As the film is not actually in contact with any solvent during and after the deposition, the MAPLE technique can be used for the combined growth of several thin films / multilayered structures, particularly when no common solvent exists. Since for the MAPLE technique one usually needs low concentration solutions, and corroborated with the PQT-12 behaviour observed during the solubility tests for spin coating experiments, we have decided to use and compare the suitability of toluene (melting point: −95 °C), anisole (melting point: −37 °C), 1,2-dichlorobenzene (melting point: −17 °C), and a mixture of chlorobenzene and 1,2-dichlorobenzene in 1:1 ratio (melting point: ≈ −20 °C) for thin film growth. PQT-12 polymer was supplied by American Dye Source Inc. in powder and used without any further purification. 1.5 wt. % PQT-12 solutions in the four different solvents, respectively, were frozen in liquid nitrogen on a large copper target holder [13-15], and kept in solid state throughout the MAPLE processing. A neodymium doped yttrium aluminium garnet laser source (Nd:YAG, “Surelite II” from “Continuum Company”: 266 nm wavelength, 7 ns pulse duration, 10 Hz repetition rate) was used for all depositions. The pressure inside the deposition chamber (~10-4 mbar) was kept constant using a “Pfeiffer-Balzers TPU 170” turbomolecular pump (170 l · s-1 volume flow rate). The target was irradiated with 12 000 – 100 000 laser pulses, in order to investigate the dependence of morphology, optical properties and further laser processing with respect to the thin film thickness. The substrates, i.e. Si wafers (polished on both sides for spectroscopy use, cut as 1x1 cm2 squares) and quartz slides (“suprasil”, pre-cut size of 2x2 cm2), were kept at room temperature during the thin film growth. Prior to the deposition, the substrates were cleaned by ultrasonic bath for a minimum of 15 min, using acetone and isopropanol as cleaning mediums, and then dried in nitrogen gas flow. The target was in rotation during MAPLE, while the target – substrate distance was set at 3 cm. Thermal analysis (TA) measurements, i.e. thermogravimetry (TG), differential thermogravimetry (DTG), differential thermal analysis (DTA), and differential 4

scanning calorimetry (DSC), were carried out in dynamic air atmosphere under nonisothermal linear regimes using a horizontal “Diamond TG/DTA Differential/Thermogravimetric Analyzer”, from “PerkinElmer Instruments”. The morphology and roughness were analyzed by atomic force microscopy (AFM), using a “XE-100” setup produced by Park Systems. These investigations were carried out in non-contact mode using a silicon nitride tip (10 nm radius of curvature). Scanning electron microscopy (SEM) investigations were performed on a “JEOL JSM-6390” setup (3-5 kV electron acceleration voltage). Optical transmission measurements were realized in a 250 to 800 nm spectral range (step of 2 nm) on a Woolam Vertical Variable Angle Spectroscopic Ellipsometer (V-VASE), equipped with a high-pressure Xe discharge lamp incorporated in an HS-190 monochromator. Finally, PQT-12 pixels were printed under the action of a laser pulse by LIFT, from a MAPLE grown donor thin film to a receiver substrate. The laser irradiates the sample from behind as if pushing the material towards the receiving substrate: only the material area that is irradiated by the laser beam will be transferred, which means that the laser beam defines the lateral shape of the deposition [18]. We used a picosecond laser (Nd:YAG, “Leopard S10/20” from Continuum company, 355 nm wavelength, 50 ps pulse duration, 10 Hz repetition rate). A square mask of (2 x 2 mm2) was placed in the beam line to select a smaller, homogeneous part of it. Further on, this was projected on the donor thin film using a converging lens, with the goal to obtain square pixels (350 x 350 μm2). The irradiation and deposition areas have been monitored through a camera, which enabled accurate alignment of the donor and receiver substrate. Positioning was controlled by x, y and z motorized translation stages. The ambient pressure inside the LIFT printing setup was kept at 90 mbar in order to minimize shockwave effects. This aspect has been widely studied by the LIFT community and revealed as a key parameter for the improvement of pixels transfer and printing, especially for organic materials [16]. The Si wafers, covered with 300 nm thick silicon dioxide (from Vegatec, France; Ci = 12 nF · cm-2), play the role of gate and dielectric of the transistors. The quality of laser-printed structures was directly assessed by SEM and 3D optical surface metrology system (Leica “DCM 3D”).

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3. Results and Discussion 3.1 Thermal behaviour of PQT-12 TG and DTG are important techniques for studying changes in the physical and chemical properties of materials, measured as a function of increasing temperature, e.g. vaporization, sublimation, absorption, adsorption, and desorption (second-order phase transitions), but also chemisorptions, desolvation and/or dehydration, decomposition, and solid-gas reactions such as oxidation or reduction. On the other hand, DTA and DSC are means to investigate exothermic or endothermic changes in the samples, which can be detected relative to an inert reference. Thus, a DTA curve provides data on the transformations that have occurred, e.g. glass transitions, crystallization, melting and sublimation, while a DSC curve provides information on the difference in the amount of heat required to increase the temperature of a sample and reference, measured as a function of temperature. The thermal behaviour of the PQT-12 semiconductor was investigated in aluminium crucibles between room temperature and 600°C, in dynamic air atmosphere (150 cm3 · min-1), and at a heating rate of 10 K · min−1 [28-31]. As presented in Figure 1, the compound is stable up to ≈270°C, after which it starts to decompose by oxidation. One may distinguish several temperature intervals in the thermograms, each corresponding to a specific mass loss. The first step of decomposition takes place between 270°C and 401°C, leading to a weight loss of 6.7% and a weak endothermic effect (∆H = -1.056 kJ · g-1). Next, four exothermic effects take place between 401°C and 535°C. The total mass loss here is 91.2%, while the total decomposition enthalpy is also important (∆H = -14.824 kJ · g1).

Further on, heating up to 600°C leads to almost total decomposition, as the residue

that remains represents less than 1.6%. Figure 2 and Figure 3 provide further details on the mass loss, together with the accompanying thermal effects, respectively. To conclude, the decomposition of PQT-12 leads to the formation of over 98% gaseous products.

3.2 MAPLE of PQT-12 thin films Direct laser irradiation and ablation of organic materials may lead to photochemical rearrangement, and/or photo-induced pyrolysis and oxidation. The formation

of

various

oxygenated

species,

conjugated

polyenes,

or

other 6

photochemical by-products due to ring-opening reactions [32], is possible. On the other hand, decomposition/degradation of the thin film structure induced through mechanical impact during the LIFT procedure may also affect the integrity of the compound and thus the optical and/or electrical properties, regardless if the printing is realized at ambient or low pressure [16], with or without the use of a dynamic release layer [20]. Chemical stability of the compound, with respect to the solvent, is also an important aspect during laser irradiation as solute-solvent interactions are to be reduced to a minimum, if not completely avoided [10-15, 32-35]. In this work, PQT-12 thin films were grown starting from PQT-12 solutions in toluene, anisole, 1,2-dichlorobenzene, and a mixture of chlorobenzene and 1,2dichlorobenzene (1:1 ratio), selected due to their optimal solubilization of PQT-12. The FTIR investigations of the MAPLE grown thin films revealed that the halogenated solvents induce chemical damage to the molecules when irradiated at the tested fluences (0.100 – 1 J/cm2), compared to the corresponding FTIR fingerprint in the PQT-12 reference samples (i.e. drop-cast films). This is due to the photodecomposition of the solvent in the laser-evaporated target material (i.e. halogenated solvent / mixture + PQT-12) that discharges chlorine atoms, which further interact chemically with the polymer, thus rendering the films unexploitable [13]. Further on, when comparing the effect of toluene and anisole we observed that the latter exhibited a lower protective effect during MAPLE. As a result of these preliminary tests, and based on previous observations [11-15], we continued the experiments by using a 1.5 % PQT-12 solution in toluene, and at 266 nm laser wavelength. The optimal laser fluence was determined to range between 0.300 and 0.400 J/cm2, leading to a very low deposition rate (≈0.020 – 0.025 nm/second). In order to investigate the influence of these laser-based thin film growing and printing techniques, upon the quality of the PQT-12 films, several donor thicknesses have been fabricated by using a 2 mm2 laser spot, i.e. 30 nm, 80 nm, 100 nm, and 150 nm, corresponding to 12 000, 30 000, 42 000 and 78 000 laser pulses, respectively. The films have been subsequently used as donors in LIFT printing of micrometric-sized pixels, for thin-film transistor device fabrication.

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3.3 Morphology and optical properties When grown on quartz substrates, PQT-12 thin films exhibit strong absorption between 350 nm and 550 nm. As seen in Figure 4, a peak around 590 nm is believed to be associated with crystalline PQT-12 [36-39], proving that MAPLE is a suitable technique in achieving highly ordered films. A significant decrease in absorption is encountered at higher wavelengths (above 600 nm), typical for polythiophenederivatives [36-42]. A maximum transparency of ≈80% was determined for 80 nm thick films, ≈75% for 100 nm films, and ≈70% for 150 nm films, respectively. SEM and AFM analysis of the MAPLE grown thin films revealed smooth surfaces, with a few droplets randomly distributed. Different surface sizes and areas were scanned on the samples, but only the 50 x 50 µm2 are presented here. In Figure 5, we present AFM images on MAPLE grown PQT-12 thin films (300 mJ/cm2, 266 nm laser wavelength) on quartz substrates, of different thicknesses: 30 nm (a), 150 nm (b). For 150 nm films, we determined a roughness of about 7 – 9 nm when grown on Si substrates, and ≈11 nm for the quartz grown ones. Such characteristics of the MAPLE grown PQT-12 thin film samples make them suitable for organic electronic applications, where adequate electrode/semiconductor and dielectric/semiconductor interfacial zones are needed.

3.4 LIFT of PQT-12 pixels In LIFT (Figure 6a), the thickness of the donor layer is a decisive parameter that requires careful control in order to achieve optimal transfer and printing of pixels. As previously shown, we prepared several donor films for LIFT printing, in order to assess the quality of the pixels with respect to their thickness. Thus, four different donor thin film thicknesses, i.e. 30 nm, 80 nm, 100 nm, and 150 nm, have been printed at a fixed laser fluence of 0.300 J/cm2. Optical microscopy images of printed pixels are presented in Figure 6(b-e). If the donor film is too thin (typically below 50 nm), the pixel structure is not maintained during the transfer. Even if the transfer is realized at a controlled pressure (90 mbar), the films break and only fragments are collected on the receiver substrate (Figure 6b). Thus, for a 30 nm thick film, the transferred PQT-12 structure consists only of fragments. The debris are not limited to the 350 x 350 µm2 irradiated zone, but are ejected outside this area also. 8

One may also notice the change in colour of the silicon wafer in a few areas (from pink to white), suggesting laser-induced modifications of the silicon through the SiO2 layer. As the film is too thin, it has a very low optical absorption and thus the laser light may go through it and further interact and damage the silicon substrate (silicon is highly absorbent at 355 nm). Increasing film thickness to 80 nm leads to the printing of complete pixels. However, holes are visible from burst bubbles due to gas formation by photochemical decomposition (Figure 6c). The presence of a few splashes is also noticed, from the edges of the pixels outwards, also indicating local thermal modifications of the PQT-12. When further increasing the film thickness, one observes an improvement and, at 100 nm, the pixels are less damaged but a few small bubbles are still present (Figure 6d). However, when increasing the thickness to 150 nm the printed pixels are continuous, intact in their structure (Figure 6e). Once the adequate film thickness has been determined (100 – 150 nm), then the laser fluence is to be controlled. The best printing conditions were found to range between 0.210 J/cm2 and 0.380 J/cm2, dependent to the actual donor thickness. Again, too low laser fluence resulted in partial or no transfer of the pixels. The threshold fluence for 150 nm thick films is found at ~0.280 J/cm2. A maximum fluence of 0.380 J/cm2 has been found prior to observing extensive damage on the pixel’s structure, i.e. burns and/or bubbles, holes and cracks in the surface and/or its structure.

3.5 Electrical properties of the LIFT printed pixels The electrical properties of PQT-12 printed pixels have been investigated as follows. 50 nm thick gold lines have been thermally deposited on top of the pixels, through a titanium mask (channel width: W = 350 µm, and length: L = 80 µm) at 7×10−7 mbar and room temperature (Edwards AUTO306 evaporator). These further served as source-drain electrodes in top-contact (TC) configuration. Current-voltage characteristics are presented in Figure 7, measured with a pico-amperemeter DC voltage source (Hewlett-Packard 4140B). The source-drain current (ID) in the saturation regime is governed by the following equation: (ID)sat = (W/2L) Ciµ (VG-Vt)2

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where Ci is the capacitance per unit area of the gate insulator layer, VG is the gate voltage, Vt is the threshold voltage, and µ is the field-effect mobility. The on/off ratio values have been determined from the current ID at VG = -100 V to the current ID at VG = +100 V, under a constant drain-source voltage VD = -30 V, by randomly measuring individual printed OTFTs. The voltage applied to the gate is in the range of 0 to -100V, with step of -20V. We determined field-effect mobilities of 5 × 10-3 cm2 · V-1 · s-1, together with current modulation of 106. Such results are somewhat lower when compared to literature data, but still very promising. Thus, by controlling the laser printing technique [18, 43], the electrode/semiconductor interface [19], as well as the dielectric material [44-47], one can further improve the electrical response of such transistors.

4. Conclusion MAPLE and LIFT processed PQT-12 thin films and pixels have been successfully demonstrated to work as functional semiconductor layers in organic thin film transistors. The influence of the thin films thickness in LIFT experiments is discussed, and the morphology and optical properties are emphasized. The effects of four different solvents are presented and discussed with respect to the MAPLE technique. Thermal analysis studies of PQT-12 have been corroborated with laser processing in order to better understand and avoid laser-induced thermal effects that may lead to the damage of the semiconductor. The resulting printed transistors are fully functional and provide field-effect mobility up to 5 × 10-3 cm2 · V-1 · s-1 together with current modulation of 106.

Acknowledgements The authors thank Dr. Christine Videlot-Ackermann for her help in measuring the electrical properties of the LIFT printed pixels. This work was partially supported (i.e. the MAPLE experiments) by a grant of the Romanian National Authority for Scientific Research, CNCS–UEFISCDI, project number PN-II-RU-TE2011-3-0301 – acronym “TE-12”.

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Figure captions

Figure 1. Thermal analysis of a PQT-12 sample (0.828 mg) in atmospheric conditions (air), when heating by 10K/min and up to 600°C. Insert: the PQT-12 molecule.

Figure 2. Mass loss due to oxidative degradation of the PQT-12 semiconducting compound. Insert image: schematic of the MAPLE setup.

Figure 3. Thermal effects that accompany the oxidative degradation of the PQT-12 semiconducting compound.

Figure 4. Optical characterization of the films (transmission spectra); the green circle to the right highlights the peak associated with a highly crystalline PQT-12.

Figure 5. AFM images of PQT-12 MAPLE grown thin films (300 mJ/cm2, 266 nm laser wavelength) on quartz substrates, with different thicknesses: 30 nm (a), 150 nm (b). The roughness for 150 nm films is typically about 7 to 9 nm when grown on Si substrates, and ≈11 nm on quartz grown ones.

Figure 6. Schematic of the LIFT printing technique (a), and the influence of thickness on PQT-12 pixels printed by LIFT (300 mJ/cm2 laser fluence), starting from a MAPLE grown thin film donor (pixel size is 350 µm x 350 µm): 30 nm (b), 80 nm (c), 100 nm (d), 150 nm (e). All pixels have been printed in a low pressure chamber (90 mbar).

Figure 7. Output characteristics of PQT-12 based transistor pixels, printed by LIFT at 300 mJ/cm2 laser fluence on Si/SiO2 substrates, with 50 nm thick Au source/drain electrodes, and schematic of the thin film transistor configuration.

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Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

a.)

b.)

Figure 6

a.)

b.)

c.)

d.)

e.)

Figure 7

*Graphical Abstract (pictogram)

Graphical abstract: pictogram

Highlights




poly(3,3’’’ didodecylquaterthiophene) / “PQT-12” thin films are synthesized




Thermogravimetry is employed to assess the laser-induced photothermal effects




Laser-based technology is used to print PQT-12 pixels for OTFT devices




The structure and electrical properties of the transistors are discussed

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