ARTICLE pubs.acs.org/JPCC
Enhanced Stability of Organic Field-Effect Transistors with Blend Pentacene/Anthradithiophene Films Chia-Hsin Wang,† Shuo-De Jian,‡ Sheng-Wen Chan,‡ Ching-Shun Ku,† Peng-Yi Huang,§ Ming-Chou Chen,§ and Yaw-Wen Yang*,†,‡ †
National Synchrotron Radiation Research Center, Hsinchu, Taiwan 30076 Department of Chemistry, National Tsing-Hua University, Hsinchu, Taiwan 30013 § Department of Chemistry, National Central University, Jhongli, Taiwan 32001 ‡
bS Supporting Information ABSTRACT: By taking advantage of the similarity between pentacene (PEN) and anthradithiophene (ADT) in molecular dimension and charge transport property, we have produced organic field-effect transistors (OFETs) with active layers consisting of well-blended PEN/ADT films. The blend-films were characterized by atomic force microscopy, X-ray diffraction, and soft X-ray spectroscopies. It is found that the blend-films containing no more than 10% of ADT exhibit a single-phase structure, large crystallinity, and improved oxidation resistance, as compared to PEN. The best performance achieved with 90% PEN-OFET gives a mobility of 0.37 cm2 V1 s1 and an on/off current ratio of 107. More importantly, this device provides a 3-fold improvement in operational stability as well as extended environmental stability. After the repetitive scanning between on and off states of OFET in ambient 940 times, the mobility decreases only to 0.33 cm2 V1 s1. In comparison, the mobility of PEN-OFET decreases from 0.46 to 0.22 cm2 V1 s1. After 3-month storage in ambient, the mobility of the optimal device decreases to 0.1 cm2 V1 s1, whereas PEN-OFET almost loses its mobility.
’ INTRODUCTION Organic π-conjugated materials have received great interest in recent years due to greatly improved device performance and the anticipated low-cost and flexible engineering approaches to the device fabrication.13 Among viable active-layer materials used in organic field-effect transistors (OFETs), pentacene (PEN) has established itself as one of the front-running materials exhibiting both high field-effect mobility and high on/off current ratio. Unfortunately, the electrical performance of PEN thin-film transistors (TFTs) is known to degrade over time due to a gradual oxidation of conjugated molecules in the presence of oxygencontaining species such as oxygen, water vapor, and ozone.48 The oxygen bonding to the PEN molecule destroys the delocalized π-bonding, impedes the charge transport, and decreases the carrier mobility accordingly. The reaction of PEN with molecular oxygen and/or water under visible light is believed to proceed gradually unless illuminated by UV radiation. Moreover, a simple photoaddition mechanism of oxygen attachment to the reactive central ring of the PEN, forming endoperoxide or 6,13pentacenequinone, cannot fully account for the complex reaction products observed with infrared spectroscopy7,8 and mass spectrometry.9 It is interesting to note that in the latter study, a concomitant presence of humidity, oxygen, and light is claimed to be essential in initiating a significant oxidation, resulting in a series of hydroxyl- and carbonyl-bonded PEN. r 2011 American Chemical Society
One strategy to improve the stability of p-type organic semiconducting materials is to synthesize π-conjugated molecules with a relatively large ionization potential so as to retard the photooxidation.1016 As one example, Klauk et al. have reported that di(phenylvinyl)anthracene, as compared to PEN, not only has similar mobility but better air-stability, thanks to an increase of ionization potential (5.4 vs 5.0 eV).15 In this regard, the recent development of dinaphtho[2,3-b:20 ,30 -f]thieno[3,2-b]-thiophene (DNTT) by Takimiya and co-workers marks an important achievement in which an air-stable, single-crystal OFET with a mobility of 8.3 cm2 V1 s1 has been demonstrated.1719 Anthradithiophene (ADT) is structurally analogous to PEN, and the molecular parameters governing charge transport phenomenon have been determined experimentally and theoretically to be similar for both.20 Additionally, ADT possesses an attractive feature of larger optical band gap as compared to PEN where air-stability is concerned (2.45 eV (in chloroform)21 vs 1.86 eV (thin film),22 determined via λmax in UVvis spectrum). Unfortunately, only a meager mobility of less than 0.1 cm2 V1 s1 is reported for ADT OFET, 23 presumably limited by the difficulty of realizing a large crystalline growth of ADT because of Received: September 1, 2011 Revised: December 9, 2011 Published: December 09, 2011 1225
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the coexistence of anti and syn isomers of ADT. Blending of two organic films of complementary nature is often practiced to produce novel materials with desirable properties, for example, solar cells,24 photovoltaic cells,25 ambipolar transistors,26 and OFET with a switchable operation mode.27 If a uniform crystallinity of the blend-film is of the utmost importance, the size of molecular constituents ought to be a priori matched; otherwise, a likely separation into component phases can take place.28,29 In this study, we take advantage of the similarity between ADT and PEN in molecular dimension and carrier transport property to grow blend-films of different compositions by a coevaporation method. For the optimal OFET device fabricated from a blendfilm of 90% PEN, a mobility of 0.38 cm2 V1 s1 and an on/off current ratio of over 107 have been achieved. The present result demonstrates the feasibility of using the blend-film technique to grow high mobility OFETs, against the odds of loading up to 10% of electrically inactive species of ADT in the films. More importantly, the blend-film OFET gains improved stability in tests of cyclic scanning and 3 month ambient storage, as compared to PEN-OFET.
’ EXPERIMENTAL SECTION The n-type heavily doped Si(100) wafers capped with 300 nmthick, thermally grown SiO2 served as gate dielectrics. The wafers were cleaned by hot piranha solution, an acid mixture of H2SO4 and H2O2 at a volume ratio of 7:3, heated through a water bath for 2 h to remove organic contaminants, and then thoroughly rinsed with deionized water and dried with nitrogen gas before use. The clean SiO2 wafers were terminated with octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM) by immersing the wafers in 1 wt % OTS in toluene solution for 1 h to complete the SAM growth. Afterward, the wafers were ultrasonicated successively in toluene and isopropanol for 10 min in each solvent to remove the possible physisorbed OTS. Afterward, the wafers were blown dry with nitrogen and then kept in a vacuum chamber for storage. ADT and PEN were obtained from Luminescence Technology Corp., Taiwan, and Aldrich, respectively. The blend-films of the desired ADT to PEN ratio were produced by coevaporation at room temperature with the flux rates monitored by a quartz crystal microbalance. The combined flux rate was varied between 1.0 and 2.8 nm/min, depending on the desired film composition. The total thickness of the blend-films was maintained at 150 nm. The composition ratio was also verified with X-ray photoemission spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. While both ADT and PEN contribute to XPS C 1s signal, only ADT yields the S 2p signal, with its C 1s to S 2p intensity ratio close to the expected elemental ratio in ADT. Taking together the C 1s and S 2p spectra, the PEN percentage in the blend-film can be determined. Ionization potentials of the films were determined by ultraviolet photoemission spectroscopy (UPS). The film morphology was characterized by an atomic force microscope (AFM) operated in contact mode (Digital instruments, NanoScope E). X-ray diffraction (XRD) measurements were carried out at the wiggler beamline (BL-17B1) of the National Synchrotron Radiation Research Center (NSRRC). UPS, XPS, and NEXAFS spectroscopies were performed in the Wide-Range beamline (BL24A) of NSRRC. The XPS binding energy scale was referenced to the bulk Au 4f7/2 core level located at 84.00 eV relative to the Fermi level.
Figure 1. AFM images of the blend PEN/ADT films grown on OTS/ SiO2 at 298 K with the film composition expressed in PEN percentage: (A) PEN, (B) 99% PEN, (C) 90% PEN, (D) 75% PEN, (E) 50% PEN, and (F) pure ADT. All of the images are sized at 5 5 μm2.
The fabrication of top-contact, bottom gate OFETs was complete with forming 50 nm thick Au electrodes on top of PEN/ADT films via vacuum-depositing gold through a shadow mask patterned with a fixed channel length of 80 μm and channel width of 800 μm. The output and transfer characteristics of the OFETs were measured in ambient condition using a Keithley 2636A dual-channel source-meter. For the experiments of assessing the oxidized fraction of molecules in the thin films subject to ambient oxidation condition, the thin films that had been vacuum-deposited in a separate UHV chamber were exposed to 300 Torr of oxygen for the designated length of time. Afterward, the films were transferred to the UHV surface analysis chamber for XPS measurements. During oxygen exposure, the films were simultaneously illuminated with lighting provided by a 20 W halogen lamp (Sunnex, Inc.) through a Pyrex 7056 glass window in the vacuum chamber. The window has a short wavelength transmission cutoff at 300 nm, and no effort was made to quantify the intensity of light incident on the films.
’ RESULTS AND DISCUSSION Figure 1 shows a set of AFM images taken for various blend PEN/ADT thin films grown on OTS/SiO2 at 298 K. Granular morphology dominates all of the AFM images, but average grain size, as enumerated by manual counting, varies considerably over different films. For example, PEN/ADT (75% PEN) and ADT films are typified with smaller grains of about 200 and 190 nm, respectively. In comparison, relatively larger grains are found for the blend PEN/ADT films of 90% PEN and 99% PEN 1226
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Figure 2. θ2θ X-ray diffraction patterns for the blend PEN/ADT films of 150 nm in thickness grown on OTS/SiO2 at 298 K with the film composition expressed in PEN percentage.
compositions, with typical dimensions of about 520 and 430 nm, respectively. By comparison, PEN film has a typical grain size of 320 nm. Figure 2 shows θ2θ out-of-plane XRD patterns for the different blend PEN/ADT films grown on OTS/SiO2 at 298 K, with the intensity scale presented in log scale to illuminate the smaller diffraction features. Diffraction peaks observed with X-ray of wavelength of 1.54981 Å here are all derived from (00l) reflections, and thus discussion will be focused on (001) reflection for brevity. Pure PEN film grown at 298 K here consists of bulk and thin-film phases with the corresponding d-spacings calculated as 1.44 and 1.545 nm, respectively, in agreement with earlier reports.30,31 For the blend-films of 99% PEN (Figure 2B)
and 90% PEN (Figure 2C), the stronger diffraction intensity indicates a better crystallinity relative to that of pure PEN along the substrate normal. The improved in-plane (along the substrate surface) crystallinity for 99% and 90% PEN films is revealed by in-plane XRD data (not shown) where a small intensity increase is found for all of the major diffraction peaks such as (110), (200), and (210). There is no doubt that a homogeneous mixing leading to enlarged crystallites indeed occurs for the blend-films of 99% and 90% PEN compositions. The d001-spacings for 99% and 90% PEN films are 1.545 and 1.532 nm, respectively. It is also interesting to note the disappearance of the bulk-phase diffraction peak of PEN in these two blend-films, indicating a single-phase nature of these two blend-films. 1227
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Table 1. 2θ Angle of (001) Peak and d001-Spacing for the 150 nm-Thick Blend-Filmsa blend-films designated by PEN %
2θ angle (deg)
d001-spacing (nm)
100 99
5.77 (6.17) 5.75
1.545 (1.44) 1.540
90
5.80
1.532
75
5.90 (6.04)
1.506 (1.471)
50
6.00
1.481
6.27 (6.39)
1.417 (1.390)
0 a
The numbers in parentheses refer to the bulk phase of the blend-films.
As the PEN percentage in the blend-films is decreased, the film crystallinity is generally degraded except in the case of 50% PEN film. The corresponding d-spacing also decreases with the increasing percentage of ADT, reflecting the slightly smaller molecular dimension of ADT. For the blend-film of 75% PEN composition (Figure 2D), the crystallinity is considerably degraded, inferred from a much reduced diffraction intensity. Concurrently, smaller grains are also found in the AFM image (Figure 1D). The d001-spacing is determined to be 1.506 nm, and the smaller shoulder feature is attributed to the bulk phase. For the blend-film of 50% PEN (Figure 2E), only a single phase with a d001-spacing of 1.481 nm is observed. The ADT film (Figure 2F) diffracts weakly, with its (001) intensity about 2 orders of magnitude lower than that of 99% PEN film. The calculated d001 is 1.417 nm using 2θ = 6.27°, the same as the reported value of 1.415 nm.23 The weak shoulder feature at 2θ = 6.39° yields a d001 = 1.39 nm and is again attributed to the bulk phase ADT, similar to the bulk phase of PEN film. It is interesting to note the presence of several prominent, broad peaks located between first- and second-order peaks in pure PEN, 50% PEN, and ADT films (Figure 2A,E,F). These peaks are weak with the intensity varying between 1/500 and 1/10 000 of the respective (001) peak and are not observable in the usual linear-scale intensity plot. The exact origin for these peaks is not clear at present. The result from the θ2θ X-ray diffraction analysis is compiled in Table 1. It can be seen that d001-spacings of the blend-films vary between those of ADT and PEN. A closer examination by plotting d001-spacing with respect to PEN percentage indeed shows that a rather linear relation exists (plot no shown), in line with the expectation based on Veegard’s law that states how the lattice constant of an alloy varies linearly with the concentration of constituent elements. The electrical properties of the blend-films were assessed by measuring their OFETs characteristics. The field-effect mobility (μ) is extracted from the saturation regime using the relationship: IDS ¼
W Ci μðVGS VT Þ2 2L
ð1Þ
where IDS is the sourcedrain saturation current, W and L are channel width and length, respectively, Ci is the capacitance per unit area of gate dielectric (=10.8 nF/cm2),32 VGS is the gate voltage relative to source electrode, and VT is the threshold voltage. The latter can be estimated as the x intercept of the linear section of the plot of (IDS)1/2 versus VGS. By repeating electrical measurements of the devices fabricated from different blend-films, a plot showing how the mobility of blend-film OFETs varies with PEN percentages in the films can be constructed, as presented in Figure 3. It is clear that an increasing PEN composition in the
Figure 3. Averaged hole mobility for the top-contact OFETs fabricated from the blend PEN/ADT films of different PEN composition. All of the films were 150 nm thick and deposited at 298 K except in the case of 25% PEN where a 50 nm thick film was prepared at 70 °C. Mobility data were obtained by averaging over more than one-half dozen devices.
Table 2. Electrical Parameters Obtained from Blend-Film OFETs blend-films designated
mobility
threshold
current on/off
by PEN %
(cm2 V1 s1)
voltage (VT)
ratio
100 99
0.44 0.35
28 16
107 107
90
0.37
18
107
75
0.08
8
105
50
0.03
5
105
25
0.004
20
105
0
0.008
6
103
blend-films generally leads to a mobility increase. ADT-OFET exhibits a meager hole mobility of 0.01 cm2 V1 s1, in agreement with earlier report.23 This disappointing performance is related to the difficulty of realizing high-crystallinity film due to the coexistence of two isomers. In comparison, the averaged mobility of the PEN-OFET reaches 0.44 cm2 V1 s1, comparable to what has been reported.15,3335 The mobilities of 99% PEN and 90% PEN devices averaged over eight different devices are 0.35 and 0.37 cm2 V1 s1, respectively, matching up to the PEN device. If it were not for the combined effect of good transport property of ADT and better crystallinity for the 90% PEN blend-film, the OFET formed by the blend-film loaded with 10% ADT cannot be on a par with the PEN-OFET. The electrical performance of the OFETs fabricated from different blend-films is summarized in Table 2. To evaluate the performance stability of blend-film OFETs in ambient condition, two tests were performed. One is to test whether and how the electrical performance might change over the number of continuous, repetitive scans (cycling test). The device parameters of concerns include the mobility, threshold voltage, and on/off current ratio. Another is to appraise how the OFETs, stored in ambient environment for a set period, might change their electrical performance, that is, environmental stability test. The cycling test was performed similarly to what was done to obtain transfer curves of the transistor: VDS maintained 1228
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Figure 4. Degradation of mobility with the number of repetitive scans (operational stability) for the OFETs fabricated from various blendfilms: (A) PEN, (B) 99% PEN, (C) 90% PEN, (D) 75% PEN, (E) 50% PEN, and (F) ADT. The repetitive scan was performed in ambient condition by keeping VDS at 50 V while stepping VGS from 20 to 50 V at a 0.5 V increment. The duration of each scan was 30 s, and, after the scan, another 30 s was allocated as a break. Three single exponential decay curves superimposed on the data for blend-films (A)(C) are used to model the mobility decrease.
at 50 V, while the VGS swept from 20 V (transistor-off state) to 50 V (transistor-on state) at a voltage increment of 0.5 V. The time to complete one cycle was 60 s, with 30 s expended on scanning and 30 s on the break, and each blend-film OFET was subjected to nearly 1000 cycles. Figure 4 shows how the mobilities of various blend-film OFETs change with the number of onoff scan. For the PENOFET, the mobility increases from an initial value of 0.46 to 0.78 cm2 V1 s1 after six cycles, but decreases monotonically afterward and reaches 0.22 after 1000 cycles. For the 99% PENOFET, its mobility increases from 0.37 to 0.49 cm2 V1 s1 during the first 80 scans and decreases gradually to 0.26 after 1000 scans. A similar trend of the mobility change with the increasing number of the scan is also found for the 90% PENOFET. Its mobility increases from 0.37 to 0.51 cm2 V1 s1 after the first 120 scans, and additional scans see a slow falloff of the mobility to 0.33 cm2 V1 s1. The corresponding threshold voltages all exhibit a gradual shift toward more positive values as the number of scans is increased. The VT changes for three higher-mobility devices are from 25 to 10 V for PEN, 16 to 1 V for 99% PEN, and 15 to 6 V for 90% PEN. This positive shift of VT is likely to be associated with the formation of oxygeninduced acceptor-like states in the films. To further quantify how the mobility decreases with the increasing number of scans, a single exponential-decay formula is used to fit the decay portion of the mobility. The number of scans needed to have the scaled mobility decrease to 1/e (=36.8%) of its initial value is 250 for PEN-, ∼500 for 99% PEN-, and ∼800 for 90% PEN-OFET. In other words, 90% PEN-OFET seems to be 3 times more robust in terms of operational stability, as opposed to PEN-OFET. For the low mobility devices (
