Control of active semiconducting layer packing
in organic thin
film transistors through synthetic
tailoring of dielectric materials
†
Ranjodh Singh,aJagan Singh Meena,*abYu-Cheng Chang,aChung-Shu Wua and Fu-Hsiang Ko*a
Apart from the development of new dielectric and semiconductor materials, the semiconductor–dielectric interface study is also very important for the optimum performance of organic thinfilm transistors (OTFTs). Herein, we have reported the detailed synthesis of a whole new family of dielectric materials which are 1,3,5,7-tetrabromoadamantane; 1,3,5,7-tetrachloroadamanatane; 1,3,5,7-tetraiodoadamantane and 1,3,5,7-tetrauraciladamantane (AdUr4). The unique ability of these molecules to undergo supramolecular thinfilm formation at low temperature, was analysed for their potential use as an insulator in organic electronic devices. Owing to the good leakage current density property shown by AdUr4 dielectric material it was further employed as a gate dielectric in regioregular poly(3-hexylthiophene), (P3HT) based OTFT. This OTFT device which was fabricated on aflexible PI plastic substrate has shown a good on/off current ratio (e.g., 2.18 104) and high mobility (e.g., 0.15 cm2V1s1). The semiconductor–dielectric interface study, has revealed that the AdUr4gate dielectric layer has guided the P3HT molecular chain domains to undergo edge-on orientation, which is the charge transport direction in OTFTs. In this process, the grazing incidence X-ray diffraction (GI-XRD) analysis and AFM study was also employed.
1.
Introduction
Organic thinlm transistors (OTFTs) have been emerging as a new class of organic electronic devices, with a potential to replace the traditional silicon based transistors.1 They are
expected to play a major role in future for the development of low cost electronic devices which will be tough, light weight and exible.2They have been the focus of research for the
develop-ment of exible electronics because of their cost-effective production, solution based processability and compatibility with variety ofexible substrates.3 Instead of so much merits
associated with them, there are lot of challenges needs to be met before their commercialization for practical uses such as in the eld of displays, sensors, invertors and radio frequency identication tags.4The major hurdle coming in the
commer-cialization of OTFTs is mainly due to their low output current and high operating voltage, as resulting from low charge carrier mobility of the active semiconducting layer employed respec-tively in them.5 In general, OTFTs are composed of three
components namely an electrode, semiconducting layer and a dielectric insulating layer separating the active semiconducting material from a gate electrode. Thus the performance of OTFTs can be improved either by employing an organic semiconduct-ing with better charge carrier mobility or with the use of dielectric material having low leakage current and high dielec-tric constant.6In past various research groups have successfully
achieved the high charge carrier mobility,mFEz 10 cm2V1s1 by employing new organic semiconductor materials in OTFTs.7
However, the dielectric layer which also plays an important role in working of OTFTs, has been less explored as compared to semiconductor materials.8The mobility and threshold voltage
of OTFTs in the linear regime are related by the equation,9
IDS¼WCLimFEðVGVthÞVDS (1) where, IDS and VDS are the current and voltage bias between source and drain, respectively, VGdenotes the gate voltage at which the current starts to rise. Further, W and L denotes the source–drain width and length, respectively; mFE is the eld effect mobility; Ciis the capacitance per unit area of the insu-lator, and Vthis the threshold voltage. Thus, the proper choice of the dielectric material having low leakage current and high capacitance is essential for the low operating voltage OTFTs.10
Theeld induced charge carriers are conned to the thin region i.e. interface between semiconductor and dielectric layer in OTFTs.11Hence, in combination with the development of new aDepartment of Materials Science and Engineering, National Chiao Tung University,
Hsinchu, Taiwan. E-mail:[email protected]; [email protected]; Fax: +886-35744689; Tel: +886-35712121 ext. 55803
bDepartment of Electronics Engineering, National Chiao Tung University, Hsinchu,
Taiwan
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra02077d
Cite this: RSC Adv., 2014, 4, 29383
Received 10th March 2014 Accepted 19th June 2014 DOI: 10.1039/c4ra02077d www.rsc.org/advances
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dielectric and semiconductor materials, the semiconductor– dielectric interface study is also pivotal for the optimum performance of OTFTs.12Kim et al. have also reported,12athe
effect of dielectric surface modication on the eld-effect mobility of regioregular poly(3-hexylthiophene) (P3HT) based transistor. When the silicon dioxide SiO2 insulator substrate was modied with –OH terminated self-assembled monolayers (SAM),mFEwas 0.08 cm2V1s1. However, upon SiO2surface
modication with –NH2 terminated SAM, the mobility
increased by several orders of magnitude of 0.28 cm2V1s1for the same P3HT semiconductor. This was due to the difference in orientation (perpendicular and parallel) of P3HT chains with respect to the modied SiO2insulator substrate which lead to the difference in behaviour of device performance. Recently, Jenn-Chang Hwang et al. have reported13the use of silkbroin
as a dielectric on theexible poly(ethylene terephthalate) (PET) plastic substrate for the fabrication of pentacene OTFTs. The planner structure of the silkbroin dielectric has allowed the pentacene molecules to self-assemble in the planner form which resulted in a very highmFEvalue of 23.2 cm2V1s1in the saturation regime and a low operating voltage of3 V. Hence, the proper choice of dielectric material with a good compati-bility with the active semiconducting layer is also essential for the optimum performance of OTFTs.
Organic and polymeric materials have been extensively explored in recent years for their potential use as a gate dielectric in OTFTs. Nowadays, they are widely pursued over traditional SiO2dielectric because of their low cost and their ability to form high quality thin lms from them can be prepared by simple processing methods. These are spin coating, ink-jet printing and sol–gel process.14Some of the most
exten-sively explored polymers in thiseld are poly(methyl methac-rylate) (PMMA), poly(styrene) (PS), poly(4-vinyl phenol) (PVP), poly(acrylic acid) (PAA), and poly(peruorobutenyl vinyl ether) (CYTOP).15These polymer gate dielectrics are known to have low
leakage current which leads to the performance enhancement of OTFTs. However, due to their solubility in common organic solvents these polymer dielectrics get washed away during the deposition of organic semiconducting material by dip coating or spin coating methods.16Some research groups have achieved
the dielectric thinlm of thickness <50 nm with high break-down voltages by thermally crosslinking of the gate dielectric material.17However, the curing temperature as needed for this
process is higher than the glass transition temperature of ex-ible substrate.18This leads to the poor performance of device.
Therefore, there is a strong desire to design new organic dielectric materials which should be processable into thinlm at low temperature from their solution in specic solvents.
Diamond is also a good insulator material,19however its high
cost, restricts its use as a gate dielectric in low cost OTFTs. On
the other hand, the adamantane, (chemical formula
C(4n+6)H(4n+12), where n¼ 0, 1, 2, 3.), which is the smallest cage structure of diamond, is cheap and can be easily obtained from oil and natural gas condensates aer purication.20 This
structurally rigid, thermally stable (pyrolysis temperature >660 C) and easy to be chemically modied21 diamondoid
molecule has recently been reviewed by Schwertfeger et al.22The
spatial arrangement of carbon atoms in adamantane is similar to that in diamond lattice.23It has been broadly studied for its
numerous applications in biology and chemistry.24
Motivated by so much merits associated with adamantane, we modied adamantane surface structure, chemically to synthesize its four derivatives namely; 1,3,5,7-tetra-bromoadamantane (AdBr4); 1,3,5,7-tetrachloroadamanatane (AdCl4); 1,3,5,7-tetraiodoadamantane (AdI4) and 1,3,5,7-tet-rauraciladamantane (AdUr4). The solution of these functional-ized molecules has the ability to undergo smooth thin lm formation at low temperature through supramolecular self-assembly (STF) (Scheme 1). The halogen–halogen interactions, van der Waals force of attractions and electrostatic force of attractions have played an important role in supramolecular polymeric thin lms formation of AdBr4, AdCl4, AdI4. This unique ability of these small nanodiamond molecules to undergo thinlm formation, with network structure similar to that of diamond lattice has prompted our group to explore their potential application as an insulator in organic electronics. This was analysed by employing these materials as insulating layer in MIM devices, as fabricated on exible polyimide (PI) plastic substrate. The scanning electron microscope (SEM) study was employed to study morphology of organic dielectric thinlms and further its effect was explored on MIM device performances as based on them. The difference in morphology of AdBr4, AdCl4, AdI4STF has prevailed because of the different type of intermolecular forces involved in their formation. This was determinant in MIM device performance based on them. To further explore the effect of intermolecular forces on dielectric STF and its subsequent effect on the electrical performance of device we synthesised another unique derivative of ada-mantane, i.e. AdUr4. The uracil substituents on the surface of AdUr4molecules has allowed them to adsorb on the gold coated
Scheme 1 Schematic representation of the AdUr4 insulator supra-molecular thinfilm formation on flexible PI plastic substrate: (a) flexible PI coated with Au/Cr layer is placed on a rigid support (solid block) in a beaker containing the solvent (THF) (b) injecting the drops of AdUr4 solution on this PI plastic substrate after 3–4 hours (c) the small rigid AdUr4 molecules self-assemble through U/U intermolecular hydrogen bonding into the form of a thinfilm. This last process takes 5–6 hours.
exible PI plastic substrate. Moreover, with Fourier transform infrared (FTIR) spectroscopy, it has been found that these AdUr4 molecules have undergone lateral interaction with each other through uracil–uracil (U/U) intermolecular hydrogen bonding that has led to the formation of their STF. Because of this the MIM devices as fabricated from AdUr4 STF showed better insulator properties as compared to that of AdBr4, AdCl4, AdI4 STF. The thermal stability of AdUr4STF based MIM devices, was further analysed by heating them to various temperature and then measuring the leakage current at that corresponding temperature. The variable temperature FTIR spectroscopy was performed to nd out the effect of heat on intermolecular hydrogen bonding involved in AdUr4dielectric thinlm. Aer this detailed analysis study of AdUr4 dielectric thinlm, we employed it as a gate dielectric material in the P3HT based OTFTs with bottom gate and top contact. Through semi-conductor–dielectric interface study, it has been found, that the unique rigid structure of AdUr4 has guided the regioregular P3HT chain domains to crystallize in edge-on orientation. The GI-XRD and atomic force microscopy study (AFM) was per-formed to successfully explore this phenomenon. The edge-on conformation has increased the charge carrier transfer to in-plane direction of P3HT chains which has led to the success of AdUr4as a gate dielectric in P3HT based OTFTs with good on/off current ratio.
2.
Experimental section
2.1 Reagents and materials
1-Bromoadamantane (99%), bromine (99.5%) and iodomethane was procured from Sigma Aldrich and used as received. Aluminum chloride and aluminum bromide anhydrous, powder of 99.999% trace metal basis was supplied by Alfa Aesar. The dichloromethane, chloroform and tetrahydrofuran (THF) obtained from Alfa Aesar were dried and distilled from sodium/ benzophenone prior to their use. Chromium shots (Cr, 99.999%, Admat Inc.) 3–5 mm in size, aluminium shots (Al, 99.999%, Admat Inc.) 3–5 mm in size, and gold shots (Au, 99.999%, Admat Inc.) 1–2 mm in size, were purchased from Gredmann Taiwan Ltd. The organic semiconductor regiore-gular poly(3-hexylthiophene) (P3HT catalog number 698997 electronic grade, purchased from Aldrich, average molecular weight Mn54 000–75 000, >98% head-to-tail regioregular) was adopted for this research. DuPont Kapton Polyimidelm, 38 mm in thickness and from PV9100 series was used as substrate to fabricate the device.25All other reagents and solvents were
obtained from commercial suppliers and used as such, unless specied. It should be noted that all experiments were per-formed under dry nitrogen atmosphere and in standard fume hood.
2.2 Dielectric thinlm preparation and fabrication of the MIM and OTFT devices
Scheme 1 presents the general method26for the preparation of
the respective dielectric thinlms of AdBr4, AdCl4, AdI4, and AdUr4. In a typical thin lm preparation experiment AdUr4
(0.05 mg) was dissolved in (10 mL) of THF under static condi-tions at 50C for 30 minutes. It wasltered with micro-syringe before drop casting on PI substrate. The dielectric thin lm from tetrahedral molecules of adamantane derivatives were produced by their slowly self-assembling from a solution (0.05 mg mL1) under a certain solvent pressure in a closed jar, as depicted in Scheme 1. It should be noted, that the solvent in the bottom of the jar was very crucial for the formation of well-dened smooth thin lm from adamantane derivatives, which could play two important roles: (1) slow evaporation of the solvent guaranteed the small tetrahedral molecules had suffi-cient time to adjust themselves and come closer together with strong intermolecular interactions so that they can further self-assemble into smooth thin lm on the substrate; (2) certain solvent vapour pressure in the upper space of the jar could guarantee the free movement of these molecules and then induced them to assemble together leading to the growth of polymer likelm on the substrate. Apart from AdUr4which have good solubility in THF the other halogen derivatives of ada-mantane as reported here, have good solubility in all organic solvents such as toluene, chloroform, chlorobenzene. However, the good solubility of all adamantane derivatives in THF and low vapor pressure of THF has prompted us tonally choose THF as a solvent of choice for the respective STF preparation of all the adamantane derivatives in this work.
A DuPont Kapton plastic PI plastic substrate with a thickness of 38mm was used as the exible substrate for the fabrication of MIM capacitor and OTFT devices. The PI plastic substrate was ultrasonically cleaned using ethanol (Fluka; water content: <0.1%) for 30 min and then rinsed with DI water. A high-pres-sure stream of N2gas was then used to remove the water and any remaining particles from the PI surface. Aer being cleaned, the PI plastic substrate was annealed at 200C for 1 h under vacuum to achieve relative thermal stability and to enhance the adhesion strength. Next, Cr (20 nm thick) and Au (80 nm thick) were sequentially deposited onto the PI plastic substrate using a thermal coater. The Cr layer was used as the adhesion layer between the PI plastic substrate and the Au thin lm.27Au was deposited as a gate electrode over the Cr layer on
the PI plastic substrate. This PI was then placed in a jar con-taining the THF solvent and the jar was covered for 6–8 h to maintain a suitable vapour pressure inside it as shown in Scheme 1. Finally, theltered THF solution of AdUr4was drop casted over this PI plastic substrate using a syringe in such a way that it should cover the whole PI surface. Aer, the complete evaporation of solvent from the PI plastic substrate as evapo-rated; it was heated at 40C for 10 min to remove any solvent trapped inside the dielectric lm, formed by self-assembly of molecules. At the end of this experiment, Al lms with a thickness of 300 nm were patterned as the top electrode using a shadow mask and a thermal coater.
The electrical insulating properties of the halogen and uracil adamantane derivatives respective thin lms have been opti-mized for OTFT on the PI plastic substrate. For OTFT device fabrication Cr and Au, which had thickness of 20 and 80 nm, respectively, were sequentially deposited through a shadow mask using a thermal coater to function as gate electrodes.
Then, gate dielectric AdUr4 thin lm was prepared on it, as described in Scheme 1. The P3HTlm was then deposited as a channel layer using a spin casting method, at room temperature and annealed at 40C for 30 minutes. Finally, source (S) and drain (D) Al electrodes with a thickness of 100 nm were deposited onto the Al/P3HT/AdUr4/Au/Cr/PI through a shadow mask, which yielded the top-contact electrode OTFTs. The channel length (L) and width (W) were 82mm and 2000 mm, respectively.
2.3 Thinlm characterizations and electrical measurements The surface morphology of the AdBr4, AdCl4, AdI4and AdUr4 respective self-assembled thinlms on the PI plastic substrate was evaluated using scanning electron microscope (SEM, JEOL). The thicknesses of these lms were estimated by optical ellipsometry techniques. The surface morphology of the P3HT lm that was spin casted over insulator substrate on the PI plastic substrate was evaluated using atomic force microscopy (AFM, Digital Instruments Nanoscope, D-5000) with a scan size of 2mm 2 mm and a scan rate of 1 Hz. The FTIR spectrum was recorded using the FTIR spectrometer (model 580, Perkin-Elmer) with a resolution of 4.00 cm1. An infrared spectrum was recorded under nitrogen atmosphere in the range of 500– 4000 cm1to determine the functional groups in the molecular structure. The samples were prepared on PI substrate (1 1 cm2) for FTIR measurement. Further, variable temperature FTIR experiments were performed on AdUr4 STF. The AdUr4 thinlm was prepared on PI plastic substrate (1 1 cm2) by following the procedure as shown in Scheme 1. This PI plastic substrate was then in situ heated from 40–120C under nitrogen
ow (20 mL min1) in a FTIR instrument equipped with temperature controller function. Corresponding to each temperature, FTIR spectrums were recorded for this AdUr4STF. The XRD pattern for the organic P3HTlm was recorded using a Rigaku D/max-IIIB diffractometer with Cu Ka radiation (l ¼ 1.5406 ˚A). The sample for XRD study was prepared by spin casting the THF solution of P3HT on the AdUr4dielectric STF with PI as the substrate.
MIM Capacitor. To investigate, the electrical characteriza-tion of all the four adamantane derivatives dielectric thinlms, the capacitance–voltage (C–V) and the leakage current–voltage (I–V) were measured using a HP-4284A C–V analyser and Agi-lent-4156 probe station, respectively. The real part of the dielectric constant (k) was calculated using the following formula
k ¼ Ct
3oAm (2)
in which C is capacitance, t is the thickness of the dielectriclm (measured by ellipsometry technique), Amthe capacitance area, and 3o the permittivity of vacuum. The leakage current was measured in direct current on the capacitors in the range 0– 100 V with a holding time of 100 ms (Agilent 4156). A quasi-static capacitance versus voltage curve was taken at a sweep rate of 0.25 V s1using a Keithley 595 quasi-static C–V meter. We examined an additional property for AdUr4 dielectric based
MIM device, to check its stability against the temperature. This was done by heating the MIM device to various temperatures and measuring the leakage current at that respective temperature.
OTFT device. The electrical characteristics of the transistors were measured in a nitrogen glove box on an Agilent-4156 probe station. The transistor parameters, such as charge carrier mobility, were calculated using the standard formalism of eld-effect transistors in the linear and saturation regimes respec-tively. The P3HT based OTFTs exhibits output characteristics with pinch off and current saturation. The electrical character-istics of the P3HT based OTFTs are very stable during operation. ThemFEvalue in the linear region (mFE, lin) is obtained from the transfer characteristics using eqn (1). The Civalue for the P3HT thinlm transistor was measured using MIM structure fabri-cated next to the P3HT based OTFT device.
3.
Results and discussion
The adamantane, a tricyclic saturated hydrocarbon (tricyclo [3.3.1.1]decane) is the smallest“nanodiamond” molecule.26Its
bulky molecular volume, rigidity and highly symmetrical tetra-hedral shape makes it an ideal building block for nano-fabrication.21,24Our research group have took the advantage of
this unique rigid structure of adamantane for its application, as an insulator inexible organic electronic devices, rst in MIM devices and thennally in OTFTs. The ability of adamantane molecule to undergo substitution reaction readily at its 1,3,5,7 position allowed us to prepare its four derivatives.20eThese are
AdBr4, AdCl4, AdI4, AdUr4, and was synthesized by following the Scheme 2. The STF of these compounds were prepared on exible PI plastic substrate, as illustrated in Scheme 1 for their application as an insulator in MIM and OTFTs devices. It has been found that the supramolecular interactions, has guided these individual small rigid molecules to self-assemble28 in
form of their rigid andexible thin lms. The STF of AdBr4, AdCl4, AdI4molecules have occurred through halogen–halogen interactions i.e. Br/Br, Cl/Cl, I/I respectively.29–31However,
the AdUr4molecules have knitted up together in the form of STF through U/U intermolecular hydrogen bonding.32The Fig. 1
Scheme 2 Synthetic route27 of dielectric materials based on
adamantane (AdBr4, AdCl4, AdI4, AdUr4).
shows that in AdUr4 STF each AdUr4 molecule is involved in
intermolecular hydrogen bonding with nearby AdUr4
molecules.
Furthermore, through FTIR study we also conrmed the presence of some free–NH (not involved in U/U intermolec-ular hydrogen bonding) of uracil in AdUr4STF.
The difference in the respective STF of AdBr4, AdCl4, AdI4, AdUr4is because of the different type of intermolecular forces involved in their formation. Due to this they have exhibited the different leakage current density, when employed as an insu-lating layer, in MIM devices. The I–V characteristics of these respective MIM devices are shown in ESI (Fig. S5–S8†). In Fig. 2, we have compared the leakage current density (capacitor area¼ 0.01 mm2) of these MIM devices and corresponding results has been shown in Table 1. The leakage current density of AdUr4, MIM device is 2.28 109A cm2while the leakage current density of MIM devices based on AdCl4, AdBr4, AdI4insulating materials are 1.74 106 A cm2, 2.74 107 A cm2 and 6.01 108A cm2respectively. From this it can be concluded that the AdUr4STF is an excellent insulator material followed by that of AdI4, AdBr4, and AdCl4. The good leakage current density of AdUr4 STF has motivated us to carry our further study to explore its potential use as a gate dielectric in OTFTs. Moreover, we have also measured the capacitance for these lms and found the low dielectric constants, which are 2.1 for AdCl4, 2.09 for AdBr4 and 2.4 for AdI4. But, the better capacitance (209.0 nF cm2) was measured for AdUr4 STF with better dielectric constant (2.6) than the other threelms. Moreover, the thicknesses of AdCl4, AdBr4, AdI4and AdUr4lms as esti-mated by ellipsometry are 10.2, 10.7, 11.5 and 11.0 nm respectively.
It has been found that the difference in morphology of STF formed by these respective materials on PI plastic substrate has led to this difference in their electrical performance of MIM devices based on them.33The AdBr
4, AdCl4, AdI4are well known to form plastic crystals through X/X interactions, as do other symmetrical adamantane derivatives. Pedireddi et al., have also reported30that X/X (where X ¼ Cl, Br, I) interactions can be
represented as specic attractive forces. The attractive nature of X/X contacts depend on atomic polarization of respective halogen atoms, which varies as follow.34
Cl/Cl < Br/Br < I/I
The atomic size of chlorine, bromine and iodine atoms are 0.99 ˚A, 1.15 ˚A and 1.40 ˚A respectively.35 The I/I attractive Fig. 1 U/U intermolecular hydrogen bonding in AdUr4molecules
leading to their STF formation on theflexible PI plastic substrate. Free –NH (not involved in U/U intermolecular hydrogen bonding) are shown in yellow square box.
Fig. 3 SEM image of (a) AdUr4STF when prepared from THF solution of AdUr4without following Scheme 1, (b) supramolecular thin film formed from THF solution of AdUr4by following the Scheme 1.
Table 1 Summary of all the characteristic properties of AdCl4, AdBr4, AdI4and AdUr4when incorporated as insulator layer in MIM devices
Dielectric material Thickness (nm) Dielectric constant Capacitance density (nF cm2) Leakage current density (A cm2) AdCl4 10.2 2.1 182.0 1.74 106 AdBr4 10.7 2.09 172.0 2.74 107 AdI4 11.5 2.4 185.4 6.01 108 AdUr4 11.0 2.6 209.0 2.28 109
Fig. 2 Comparison of leakage current behaviour of AdCl4, AdBr4, AdI4 and AdUr4.
contacts appeared because of electron cloud distortion of iodine atoms further leading to the formation of partial positive and negative charge on them. Thus, the I/I contacts that have prevailed in AdI4 STF are because of electrostatic forces of attractions among iodine atoms.30Owing, to the larger atomic
size, this distortion of electron clouds is much more pronounced in case of iodine atoms as compared to that of bromine and chlorine atoms. The X/X (where X ¼ Cl, Br) that are present respectively in AdCl4, AdBr4STF are because of van der Waals (vdW) interactions (attractive). Each molecule of AdBr4has been interacted with the nearby two or three mole-cules of AdBr4 through these Br/Br vdW interactions.36The vdW interactions are known to be of weaker strength as compared to that of electrostatic forces of attractions and intermolecular hydrogen bondings.37 Hence, due to weak
intermolecular forces, AdBr4 molecules in its STF are not packed as closely as that of AdI4molecules in AdI4STF. Further, the smaller atomic size of chlorine atoms has resulted into more weaker X/X (X ¼ Cl) vdW interactions among AdCl4 molecules resulting into the more loose packing of the AdCl4 molecules in their STF.31,34 The effect of this intermolecular
interactions on molecular packing on surface morphology of STF was further analysed by SEM. It has been found that the AdI4STF has smaller cracks followed by that of AdBr4and AdCl4 (SEM images of these STF are shown in ESI S1–S3†). Due to this difference in surface quality of AdX4(X¼ Cl, Br, I) STF, the MIM devices based on them has showed the difference in their electrical performance.38The halogen–halogen interactions are
the weaker intermolecular forces of interactions as compared to that of intermolecular hydrogen bonds.39 Therefore, when
AdUr4STF were analysed by SEM, they appeared to be smooth with no pinholes or cracks, as shown in Fig. 3b. The SEM image in Fig. 3a shows the AdUr4 STF when prepared from its low concentration solution on PI plastic substrate (without following Scheme 1) appeared as cluster of AdUr4molecules. In this case, the THF solution of AdUr4 was drop casted on PI plastic substrate and allowed to dry it in open atmosphere.
We found that, for preparation of AdUr4 STF on PI plastic substrate by Scheme 1, has played a signicant role in the formation of its smooth STF. In this method, the slow evapo-ration of solvent gives sufficient time to AdUr4 molecules to move closer to each other and then self-assemble in the form of their STF through U/U intermolecular hydrogen bonds.32The
AdUr4molecules undergoes adsorption on gold coated surface layer (bottom electrode for MIM and OTFTs devices) on PI plastic substrate through uracil groups. This adsorption holds the individual AdUr4molecules strongly against the gold coated PI plastic substrate.40These molecules further interacts laterally
with each other through intermolecular hydrogen bonding between uracil groups as shown in Fig. 1. In this process,–NH and–CO present in imide functional group of uracil acts as the donor and acceptor respectively, leading to the formation of rigid STF of AdUr4on gold coated PI plastic substrate.
The role played by U/U intermolecular hydrogen bonds on stability of the AdUr4STF was further characterized by variable temperature FTIR spectrometer.41The AdUr
4STF on PI plastic
substrate was heated to various temperatures and
corresponding to each temperature the leakage current property of MIM capacitor device was analysed as shown in Fig. 4.
It has been found that MIM devices, incorporated with AdUr4 STF as an insulator has shown good electrical performance up to 100C. This can be seen in terms of its high leakage current density, 3.30 103A cm2at 120C as calculated from its I–V characteristics curve in Fig. 4a. This has been attributed to breakage of U/U intermolecular hydrogen bonds at 120 C,
which has prevailed in AdUr4STF. The variation in FT-IR spectra of AdUr4 thin lm at 120C, in Fig. 4b shows this change. Hence, the excellent electrical performance shown by MIM devices incorporated with AdUr4STF is due to the U/U inter-molecular hydrogen bonds in them. The FTIR spectra of AdUr4 STF in Fig. 4b, shows the characteristic peaks corresponding to those of the free amide NH groups at 3475 cm1and that of–NH involved in U/U interactions at 3188 cm1.41cThus, the U/U
intermolecular hydrogen bonds has played a signicant role in success of the AdUr4STF as new dielectric layer inexible MIM device at high temperature. The success of AdUr4 STF based MIM devices, have prompted us to incorporate it as a gate dielectric layer in OTFTs.
In order to study the potential of the AdUr4STF as a dielectric layer, we fabricated a P3HT based OTFTs with a bottom gate and top contact having a channel width of 2000mm and a channel length of 82mm (Fig. 5a). The Fig. 5b shows the photographic optical image of OTFTs as fabricated on exible PI plastic substrate with its corresponding conguration shown in Fig. 5a. The Fig. 5c, shows the transfer characteristics for OTFTs. The Fig. 5d, presents the drain current–drain voltage (IDS–VDS) output curve as obtained from our OTFT with a 11.0 nm thick AdUr4dielectric layer, which is prepared according to Scheme 1. The device demonstrated, desirable OTFT characteristics at an operating voltage lower than 45 V. Maximum saturation current of 0.26mA was achieved. The observed OTFT charac-teristics conrmed closely to conventional transistor models in both the linear and saturation regimes with IDS increasing linearly at low drainage voltage, and clear saturation behaviour at high drain voltage. According to the drain current–gate
Fig. 4 (a) I–V characteristic property under different temperature of flexible 5 5 cm2MIM device incorporated with AdUr
4insulator layer (b) corresponding variable temperature FTIR spectra of the AdUr4STF as prepared onflexible PI plastic substrate.
voltage (IDS–VGS) transfer curve of Fig. 5c, the OTFT with the AdUr4 dielectric layer displayed an average eld mobility of 0.15 cm2V1s1at45 V, threshold voltages (Vth) of3 V, an on/off current ratio of 2.18 104. Regioregular, P3HT is a material of choice for research OTFTs because of its higheld effect mobility, stability, and solution processability. The orientation of P3HT molecular chains i.e. parallel (face-on structure) or perpendicular (edge-on structure) to the insulator substrate is an important phenomenon which need to be explored for the optimum performance of OTFTs. This phenomenon is much dependent on the chemical nature of the gate insulator material as it is the penultimate layer on which semiconducting layer is spin-casted. To study the structural ordering of spin coatedlms of P3HT on the insulator surface (AdUr4), we used synchrotron grazing-incidence X-ray diffrac-tion (GI-XRD). The grazing incidence angle of 0.18 was employed and in order to isolate the scattering data from the P3HT polymer lm only, the scan from a bare insulator substrate was used to remove background.42The GI-XRD result
is shown in Fig. 6a which demonstrates that the nanocrystalline regioregular P3HT molecular domains has mainly preferred to crystallize in edge-on orientation over the AdUr4gate insulator Fig. 5 (a) Schematic diagram of bottom gate OTFT device that
features AdUr4as a gate insulator and P3HT as a semiconducting layer (b) photograph of the OTFT devices on a 5 5 cm2area on aflexible PI plastic substrate and plots for OTFT; (c) transfer characteristic (IDS– VGS), when VDS¼ 2 V, and (d) output characteristic (IDS–VDS), where VGSranges from 0 to15 V at 3 V step.
Fig. 6 (a) Out of plane grazing-incidence angle X-ray diffraction intensities as a function of the scattering angle 2q for regioregular P3HT thin films crystallized against insulator material AdUr4on silicon wafer. (b) The insets show schematically the crystallography of the P3HT nano-crystallites with respect to the insulator material, AdUr4(c) tapping phase mode AFM image of a regioregular P3HTfilm against the AdUr4insulator onflexible PI substrates.
as compared to their possible face-on orientation. These two different chain orientations of nanocrystalline regioregular P3HT domains with respect to the AdUr4STF are evident from the different intensity distribution of the (100) reections due to the lamellar layer structure (16.4 ˚A) and the (010) reections due to interchain stacking (3.8 ˚A) through out-of-plane and in-plane geometric mode.12a,bThe intense (100) reections in the XRD
spectra with respect to the (010) reections shows that on AdUr4 STF the P3HT molecular chain domains has preferred to crys-tallize in edge on orientations, as shown in Fig. 6b. The GI-XRD has revealed this type of nanocrystalline structural order that has prevailed throughout the P3HTlm, while the AFM obser-vations was further done to provide images of the top surface. Fig. 6c, shows AFM topographic image of regioregular P3HT thin lms on insulator substrate (AdUr4). The wormlike morphology of P3HT molecular chains in AFM image, has arisen because of their perpendicular orientations over insu-lator substrate (AdUr4).12a This edge on orientation of P3HT chains has led to their enhancedp–p stacking in the direction of charge transfer in OTFT.
The presence of uracil moiety at the 1,3,5,7 position of the adamantane, in AdUr4STF has played an important role in the appearance of edge on orientation of P3HT molecular chain domains. The AdUr4 moleculesrst undergoes adsorption on gold coated PI plastic substrate and then form AdUr4 STF through U/U intermolecular hydrogen bonds. The FTIR spectra of AdUr4STF in Fig. 4b, shows that, their exist two type of –NH protons in uracil substructure. The one which are involved in intermolecular hydrogen bonding shows their characteristic peak at 3188 cm1 while the other, free –NH protons which are not involved in intermolecular hydrogen bonding appears at 3475 cm1. The free polar–NH protons have undergonep–H interaction (attractive) with thienyl backbone of the regioregular P3HT molecular chains. However, the lone pair of electrons on other type of (–NH)c c protons in AdUr4STF were involved in repulsion force with the p-electron cloud of the P3HT molecular chains. Therefore, the P3HT molecular chain domains experiences two opposite force of interactions, over the AdUr4gate insulator thinlm. These two counter forces i.e. force of attraction and repulsion makes the regioregular P3HT molecular chains to pack into the edge-on orientation, i.e. perpendicular to the insulator substrate.12a,b
Fig. 7 shows this whole mechanism of formation of edge-on orientation of P3HT molecular chains over AdUr4STF dielectric layer. Further, the adamantane is a rigid tetrahedral molecule21
hence during molecular packing of the AdUr4molecules in the form of STF there may exist some voids in them. In these voids the hexane chains on the backbone of P3HT stucks, which further helps the regioregular P3HT molecular chains to crys-tallize in edge-on orientations. These all factors has led to the success of our P3HT based OTFT device as fabricated with new gate insulator layer material AdUr4.
4.
Conclusion
The recent advances inexible OTFTs have highlighted the use of low cost technology and materials to replace the most commonly used semiconductor and dielectric materials. We have successfully synthesized new dielectric materials AdCl4, AdBr4, AdI4and AdUr4through easy to follow synthesis process. Their potential to act as a dielectric material was analysed by fabrication of MIM devices respectively from them. The comparisons of I–V characteristics study of these respective MIM devices have showed that AdUr4 is a better insulator material for plastic electronics. The ability of AdUr4molecules to undergo supramolecular thinlm formation through inter-molecular hydrogen bonding has been successfully explored here. Its use as a gate electrode in electronic devices, through fabrication of a new and fullyexible P3HT, a p-type OTFT on theexible PI plastic substrate has also been demonstrated in this work. This P3HT based OTFT device incorporated with AdUr4 as a gate electrode showed good on/off current ratio (2.18 104) and high charge mobility (0.15 cm2V1s1). The detailed semiconductor–dielectric interface study, lead us to conclude that the P3HT molecular chain domains has under-gone edge-on crystallization (perpendicular to the insulator substrate), i.e. the charge transport direction in OTFTs. This study has explored a new efficient pathway for development of exible OTFTs to their full potential on commercial scale, by exploiting the self-assembling ability of small molecules into thinlms as a dielectric material in them.
Acknowledgements
The authors are grateful to the National Device Laboratories for their support in the device fabrication, the National Science Council of Taiwan for nancially supporting this research under the contract NSC 101-2113-M-009-MY3.
Notes and references
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