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Ultra-thin and graded sliver electrodes for use in transparent pentacene field-effect transistors
Organic Electronics 15 (2014) 1990–1997
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Ultra-thin and graded sliver electrodes for use in transparent pentacene field-effect transistors Shun-Wei Liu ⇑, Tsung-Hao Su, Ya-Ze Li Department of Electronic Engineering, Ming Chi University of Technology, New Taipei City 24301, Taiwan, Republic of China
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Article history: Received 25 January 2014 Received in revised form 30 May 2014 Accepted 30 May 2014 Available online 14 June 2014 Keywords: Organic field-effect transistors Ultra-thin electrodes Graded structure Ag Transparent device Pentacene
a b s t r a c t The authors report the fabrication of efficient and transparent pentacene field-effect transistors (FETs) using a graded structure of ultra-thin silver (Ag) source and drain (S–D) electrodes. The S–D electrodes were prepared by thermal evaporation with a controlled deposition rate to form Ag layer with a graded structure, leading to a reduced injection barrier and smoothing the contact surface between the electrode and the pentacene channel. The sheet resistance of such Ag electrode was found to be as low as 9 X/sq. In addition, a hole-only behavior of device with Ag electrode characterized by current– voltage measurement and conductive atomic-force microscopy shows the injection property of high current flowing as compared with device using Au electrode, resulting in an efficient injection condition existing at the interface of the graded Ag/pentacene. Device characterization indicates the transparent pentacene FET with a graded ultra-thin Ag electrode and organic capping layer of N,N0 -di(1-naphthyl)-N,N0 -diphenylbenzidine exhibits a high transmission rate of 75% in the range of visible light from 400 to 550 nm, a threshold voltage of 6.0 V, an on–off drain current ratio of 8.4 105, and a field-effect mobility of 1.71 cm2/V s, thus significantly outperforming pentacene FETs with multilayer oxide electrodes or other transparent thin metal layers. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Transparent organic electronic devices have attracted considerable for use in display [1], smart circuit [2], sensor [3], and solar energy [4]. Previous studies have demonstrated that an oxide/thin metal/oxide trilayer structure can be used to replace a transparent indium tin oxide (ITO) anode [5], bilayer cathode (metal type) [6], and graphene electrodes [7]. For example, Yook et al. reported that tungsten oxide (WO3)/Ag/WO3 (WAW) cathode electrodes in the transparent organic light-emitting diodes (OLEDs) show high transmission and low sheet resistance [8]. Most importantly, these electrodes were fabricated using only thermal evaporation at room temperature to ⇑ Corresponding author. Tel.: +886 2 29089899; fax: +886 2 29041914. E-mail address: [email protected] (S.-W. Liu). http://dx.doi.org/10.1016/j.orgel.2014.05.037 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.
deposit the WAW, thus avoiding high temperature process damage to the active layer [9]. Such a WAW electrode has potential to serve as an anode electrode, as compared opposed to other types of transparent conductive films, such as ITO [10], indium zinc tin oxide [11], indium zinc oxide [12], aluminum zinc oxide [13], zinc oxide [14], nickel oxide [15], carbon nanotube [16], and graphene [17]. More recently, Zhang et al. demonstrated a transparent WAW source and drain (S–D) electrodes with a transparency rate exceeding 80% in visible light in the pentacene field-effect transistors (FETs), but this electrode exhibits a low carrier mobility of 0.08 cm2/V s [18]. This issue is attributed to high contact resistance because a thick WO3 is inserted between the Ag and the pentacene, and this must be resolved to improve the electrical characteristics of transparent devices. On the other hand, the thin bilayer structures of Ba/Ag [19], Ca/Ag [20], and LiF/Yb:Ag
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[21] have also been proposed to replace a transparent electrodes in OLEDs. Although these show high transmission of 70%, Ba, Ca, and LiF layers are still unsuitable for use as injection electrode to their low workfunction to reduce the hole injection property. To solve the problem of energy mismatching between the metal electrode and organic channel, our previous work demonstrated an efficient bottom-contact pentacene FET fabricated at room-temperature with Ag/self-assembled monolayers electrodes, which effectively change the workfunction from 4.2 to 5.8 eV [22]. These results reveal that the injection efficiency is highly correlated to the metal/organic interfacial property. In this work, we have developed a highly transparency and high performance pentacene FET based on ultra-thin graded Ag electrodes with top-contact structure. It exhibits better electrical characteristics than thick Au or Ag S–D electrodes. Using
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the optimized thickness of 15 nm Ag and 35 nm capping layer of N,N0 -di(1-naphthyl)-N,N0 -diphenylbenzidine (NPB) produces an efficient pentacene FET with a threshold voltage of 6.0 V, on–off drain current ratio of 8.4 105, field-effect mobility of 1.71 cm2/V s, and totally device transmission of 75% in visible light. In addition, electrical characteristics and conductive atomic-force microscopy (AFM) were used to address the injection property at the interface of Ag/pentacene and Au/pentacene.
2. Experiments Fig. 1 shows a schematic cross section of pentacene transistors with Au, graded Ag, ultra-thin graded Ag/NPB S–D electrodes based on a top-contact structure. Our device was fabricated on a bare glass substrate coated with
Fig. 1. Schematic cross section of top-contact pentacene-based FETs with Au, graded Ag, and ultra-thin graded Ag with capping NPB electrodes.
Fig. 2. (a and b) Source-drain current–voltage and (c and d) the transfer characteristics of the pentacene FETs with Au and graded Ag electrodes.
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Table 1 Device performances of the pentacene FETs with Au and graded Ag electrodes. Electrode
l (cm2/V s)
VT (V)
Ion/off (A)
Au Graded Ag
0.34 0.86
4.1 5.0
0.6 105 3.8 105
Park System XE-70 with a conductive mode using conductive Pt coated tip cantilever, where the tip radius is between 100 and 200 nm, and the substrate applied the 5 V to evaluate the charge injection property. All measurements herein were carried out in an air environment. 3. Results and discussion
a 150 nm indium tin oxide (ITO; 15 X/sq) layer, which served as a gate electrode. The substrate was cleaned by ultrasonic bath in successive solutions of acetone and isoproponal, etched in a dilute solution of sulphuric acid, and finally dried by 5 N nitrogen blow. The cross-linked poly-4vinylphenol dielectric layer was prepared from a solution of PVP and poly(melamine-co-formaldehyde) in propylene glycol monomethl ether acetate (PGMEA) deposited via spin-coater, and baked in a vacuum tube (2 106 Torr) using two steps annealing processes of 200 °C for 1 h and then 100 °C for 24 h. The final thickness of the polymer dielectric layer was 200 nm with a unit capacitance of 16 nF/cm2. A 50 nm high purity pentacene layer was then thermally deposited onto the patterned substrate. All materials including the S–D metal, semiconducting material, and insulator layers were purchased from Sigma–Aldrich. The S–D electrodes were patterned through a shadow mask to form the contact film, defining a channel length (L) and width (W) of 50 and 500 lm, respectively. The performance of different electrodes and thicknesses was compared. The Au electrode has a thickness of 50 nm and a deposition rate fixed at 0.5 Å/s. For the Ag electrode, two different thicknesses were used, 50 and 15 nm. To form the graded structure, the Ag thin films were deposited in two steps. First, 5 nm was deposited at 0.5 Å/s, with the remainder deposited at 5.0 Å/s. Therefore, we designated the ‘‘graded Ag’’ and ‘‘ultra-thin graded Ag’’ as Ag with respective thicknesses of 50 and 15 nm. The electrical properties were performed at a source meter (Keithley 2636A) with home-made probe station. A more detailed description of the setup was provided in our previous work [23]. All the performances are recorded on an average of 10 devices during one fabrication process. The AFM image of the Au (15 nm) or graded Ag (15 nm) coating on 50 nm pentacene/ n-type Si wafer was analyzed by a
The output and transfer characteristics of pentacene FET with Au and graded Ag S–D electrodes are shown in Fig. 2. Note that our pentacene FETs based on a PVP insulator exhibit leakage when the drain voltage equals zero. We attribute this to mobile ions in the PVP resulting, as reported previously [24,25], from the presence of the hydroxyl group in PVP. Due to the PVP insulator, the leakage currents are identical for all devices. The field-effect mobility of pentacene FET in the saturation regime is obtained from the following equation:
ID ¼
W C i lsat ðV G V T Þ2 2L
ð1Þ
where ID, Ci, lsat, VT, VG, L, and W are the drain current, the capacitance per unit area of gate dielectric, the hole mobility, the threshold voltage, the gate voltage, the channel length, and the channel width, respectively. According to such an equation, the field-effect mobility of pentacene FET is respectively calculated at 0.34 and 0.86 cm2/V s for Au and graded Ag electrodes, which represents an improvement of about 60% at a gate voltage (VG) of 5 V and drain-source voltage (VDS) of 10 V (see Fig. 2a and b). Obviously, the FET with graded Ag electrodes exhibits excellent electrical characteristics, which is much higher than that of constant deposition rate of Au or Ag electrodes based on pentacene channel [22,26]. Therefore, we expect that the graded Ag can be used to reduce a contact barrier and provide tunneling injection process at the electrode/ pentacene interface. Note that the injection barrier at Ag/ pentacene is 1.3 eV higher than that at the Au/pentacene interface of 0.8 eV in recent measurement [22]. This result suggests that the high-energy barrier may hinder the device’s charge injection efficiency [27]. However, our experimental results clearly show that using the graded Ag layer in the S–D electrodes significantly enhances l as
Fig. 3. AFM images of pentacene channel with heat radiation effect of (a) Au and (b) graded Ag electrodes.
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Fig. 4. Current–voltage characteristics the FET device using Au and graded Ag electrodes.
compared with the Au layer. This may indicate that the injection efficiency is not dominated by the workfunction difference between Ag and pentacene. Wang et al. reported
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that the device with Cu contact electrode exhibits lower contact resistance, although the workfunction difference at the interface of Cu/pentacene is 0.95 eV [28]. Therefore, the contact resistance is possibly determined by the tunneling process if the contact interface has a high workfunction difference between Ag and pentacene. To explore other possible reasons, the relationship between device performance and the graded Ag layer will be explained by AFM and electrical characteristics in the latter. Table 1 summarizes the electrical properties (l, VT, and on/off ratio) of the pentacene FET measured in ambient. In addition, the VT slightly changes from 4.1 to 5.0 V and on/off ratio significantly improves from 0.6 105 to 3.8 105, as shown in Fig. 2(c) and (d), when graded Ag layer was used in our pentacene FETs. In general, FET performances are highly correlated to the channel’s film morphology and the contact property at the pentacene/metal interface [29–32]. In our case, the carrier mobility of the graded Ag electrode is much higher than that of the Au electrode. Therefore, we first investigate the morphological change to the pentacene channel. Fig. 3(a) and (b) respectively show the pentacene channel after the deposition of the Au and graded Ag electrodes.
Fig. 5. (a and b) AFM images of pentacene/Au and pentacene/graded Ag. (c and d) are the corresponding current images. Note that the applied voltage of substrate was 5 V for conductive AFM measurements.
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Surprisingly, the morphology of the pentacene channel was significantly changed, exhibiting smaller grain size for the Au-deposited FET than the graded Ag-deposited one. Note that the large grain size is expected to improve carrier transportation in the pentacene channel. We speculate that the morphological change may be caused by the different levels of heat radiation during the deposition of the Au and Ag metals. Similar results are mentioned in the literature [33]. Fig. 4 shows the IDS–VDS curves in the devices with the Au or graded Ag layers. Note that we do not apply any voltage of VG to the pentacene FET during device measurement. Therefore, such measurement method is similar to the hole-only device to evaluate the carrier injection and transport properties in the pentacene FET device. In our electrical measurement, the IDS in the device with graded Ag electrodes is one order of magnitude higher than that in the device with Au electrodes. This indicates that graded Ag electrodes show very well injection property, which can be used to alternate Au electrodes for efficient device fabrication. In addition, we believe that the injection barrier of the graded Ag layer may be moved towards the lower injection barrier as compared with Au layer. In addition, the electrical characteristics of hole-only measurements are consistent with performance of the FETs (see Fig. 2a and b). In order to further understand the issue of contact property, the surface morphology and current image at interface of electrode/pentacene are found to be strongly related to the efficiency of charge injection in device. Fig. 5 shows the conductive AFM images of the Au and graded Ag layers on the pentacene thin film. The root mean square (rms) roughnesses are measured to be 6.6 and 4.3 nm for Au and graded Ag on the pentacene, as shown in Fig. 5(a) and (b), respectively. From AFM measurement, we found that these thin metal films (Au and graded Ag) have very different surface morphology on pentacene surface. The deposition of the graded Ag layer on pentacene thin film significantly reduces the grain size as compared to that of the Au electrode (see Fig. 5a and b). This can be attributed to the evaporated graded Ag layer just requiring lower heat energy than that of Au during the deposition process defining S–D contact electrodes, while changing deposition rate from slow to fast might be helpful for achieving good metal/organic contact interface to enhance the charge injection, as shown in Fig. 5(c) and (d). Note that such a fabrication process is usually used to improve the carrier injection in organic semiconducting materials due to reducing the voided thin-film and smoothing the local contact surface at the organic/metal interface [34]. It has also been shown that the film morphology, metal diffusion, and large contact area improve the contact property and/or induce the tunneling mechanism [35]. Therefore, this result demonstrates that the graded Ag electrodes may possibly become a good choice to alternate the expensive Au electrode in the pentacene FET. Fig. 6 shows the output and transfer characteristics of the transparent pentacene FET without and with capping N,N0 -di(1-naphthyl)-N,N0 -diphenylbenzidine (NPB) layer (35 nm) atop ultra-thin graded Ag (15 nm) electrodes. Both devices also show high performance p-channel
characteristics. Compared with previous studies in the field of transparent organic FETs [18], the device with ultra-thin graded Ag electrodes performed efficiently with a l of 1.42 cm2/V s, VT of 5.9 V, and on-off ratio of 2.8 105. This result demonstrates that the total thickness of 15 nm Ag is sufficient to form the well contact structure with sheet resistance of 15 X/sq and provide high
Fig. 6. (a) Source-drain current–voltage, (b) on–off ratio, and (c) threshold voltage characteristics of the transparent pentacene FETs using ultrathin graded Ag without (w/o) and with (w/) capping NPB.
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Fig. 7. Transmission spectra of the transparent pentacene FETs using ultra-thin graded Ag without (w/o) and with (w/) capping NPB. Inset: the photo of 144 devices array of these transparent pentacene FETs positioned above the logo of ‘‘Formosa Plastic Group’’.
transparency at near 50–65% in the range of visible light as shown in Fig. 7. Note that the sheet resistance of such ultra-thin graded Ag is slightly smaller than that of graded Ag electrode (sheet resistance of 1 X/sq), resulting in the good conduction property for the ultra-thin contact pad. More interestingly, it can be further observed that the ultra-thin graded Ag electrode adding 35 nm capping layer of NPB significantly increases the l, VT, and on/off ratio to be 1.71 cm2/V s, 6.0 V, 8.4 105 as compared with only the graded Ag structure (see Fig. 6a–c). This result may suggest that depositing the NPB layer on the Ag electrode can improve the electrical conductivity of thin metal film, which has low sheet resistance of 9 X/sq. The increased
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mobility obtained by depositing NPB atop pentacene is attributed to the double-channel effect due to the p-type property of NPB [36,37]. On the other hand, the pentacene FET is possibly sensitive to environmental factors, such as air, water, and oxygen. For this reason, we rule out the possibility that ambient factor may contribute the more hole carrier in the NPB/pentacene channel. In addition, we found that, at high voltages, the drain current of pentacene FET using Ag electrodes (i.e., the graded or ultra-thin Ag) exhibited different tendencies in the transfer curves of Figs. 2(d) and 6(c). We concluded that this is caused by the different levels of contact resistance or by the properties of the metal pads, which are capable of limiting the number of carriers injected into our device. As shown in Fig. 7, the Ag/NPB electrode structure exhibits high transparency of 75% from 400 to 550 nm due to an optical effect in multilayer structure [38,39], while the inset of Fig. 7 is the photograph of the transparent pentacene FETs with ultra-thin graded Ag/NPB electrodes in this work. Note that the group logo could be easily seen through the layout of our transparent device. On the other hand, many groups reported that the transmission and morphology of Ag could be considerably changed for thin film at varied thickness or deposited at annealed substrate [40,41]. We believed that it is potentially interesting to investigate the differences between ordinary Ag and graded Ag as used here. Fig. 8(a) shows the transmission spectra for different metal thin films. Note that the Ag and graded Ag film were prepared both at a constant rate (15 nm at 0.5 Å/s) and at variable rates (first 5 nm at 0.5 Å/s, residual 10 nm at 5.0 Å/s). A high degree of transparency (70%) in the visible light range was obtained for the graded Ag, while the Ag thin film exhibited a monotonically decreased transmission from
Fig. 8. (a) Transmission spectra of 15 nm Ag, graded Ag, and Au thin films. (b–d) AFM images of 15 nm Ag, graded Ag, and Au, respectively.
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70% to 50% in this range. In the field of transparent organic electronics, transparent electrodes are usually fabricated with multilayer structures using Ag as the middle layer [42–44]. However, the thickness of the Ag should be decreased to a certain level to ensure sufficient transparency [45]. Note that the 15 nm thickness studied here is in the range of highest transparency (typically from 10 to 15 nm). Therefore, we demonstrate a simple fabrication process to obtain a highly transparent metal using a single Ag layer. Given identical thicknesses, the transparency of Au thin film is lower than that of both Ag and graded Ag. Fig. 8(b)–(d) shows the AFM images of Ag, graded Ag, and Au thin films. The disconnection is apparently more severe in the constant-rate-deposited Ag than in the variable-rate-deposited Ag, resulting in the electrode’s high conductivity and the FET’s excellent electrical properties. Therefore, these results demonstrate that the structure of the ultra-thin graded Ag electrode makes it a good choice to replace Au or other metal oxide multilayer electrodes for the fabrication of efficient and transparent pentacene FETs. 4. Conclusion In summary, high performance pentacene FET using novel structure of graded Ag layer as S–D electrodes has been demonstrated. The field-effect mobility 1.71 cm2/ V s for the transparent device based ultra-thin graded Ag layer is much better than other organic FET with metaloxide multilayer or other thin metal electrodes. Moreover, the large workfunction difference at the interface of the Ag/pentacene may induce the tunneling injection effect to dominate the injection property in the pentacene FET, which plays an important role in improving the contact barrier of the metal/organic interface. Acknowledgments The authors acknowledge the financial support from the National Science Council (Grant Nos. NSC 103-ET-E011-004-ET, 102-2511-S-131-002, 102-3113-E-001-001, 102-2627-E-002-002, 102-2221-E-011-142, 102-2221-E131-030-MY2, and 102-2221-E-131-026-MY2). In addition, the corresponding author (Dr. S.-W. Liu) would like to give special thanks to Mr. H.-H. Wu, Syskey Technology Corporation (Taiwan), for the assistance in fabrication system designed. References [1] J. Lee, S. Hofmann, M. Thomschke, M. Furno, Y. Hyun Kim, B. Lüssem, K. Leo, Highly efficient bi-directional organic light-emitting diodes by strong micro-cavity effects, Appl. Phys. Lett. 99 (2011) 073303. [2] C.-S. Chuang, S.-T. Tsai, Y.-S. Lin, F.-C. Chen, H.-P.D. Shieh, Photocurrent suppression of transparent organic thin film transistors, Jpn. J. Appl. Phys. 46 (2007) L1197–L1199. [3] T.Q. Trung, N.T. Tien, Y.G. Seol, N.-E. Lee, Transparent and flexible organic field-effect transistor for multi-modal sensing, Org. Electron. 13 (2012) 533–540. [4] W. Jose da Silva, H.P. Kim, A. Rashid bin Mohd Yusoff, J. Jang, Transparent flexible organic solar cells with 6.87% efficiency manufactured by an all-solution process, Nanoscale 5 (2013) 9324–9329.
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Ultra-thin and graded sliver electrodes for use in transparent pentacene field-effect transistors Shun-Wei Liu ⇑, Tsung-Hao Su, Ya-Ze Li Department of Electronic Engineering, Ming Chi University of Technology, New Taipei City 24301, Taiwan, Republic of China
a r t i c l e
i n f o
Article history: Received 25 January 2014 Received in revised form 30 May 2014 Accepted 30 May 2014 Available online 14 June 2014 Keywords: Organic field-effect transistors Ultra-thin electrodes Graded structure Ag Transparent device Pentacene
a b s t r a c t The authors report the fabrication of efficient and transparent pentacene field-effect transistors (FETs) using a graded structure of ultra-thin silver (Ag) source and drain (S–D) electrodes. The S–D electrodes were prepared by thermal evaporation with a controlled deposition rate to form Ag layer with a graded structure, leading to a reduced injection barrier and smoothing the contact surface between the electrode and the pentacene channel. The sheet resistance of such Ag electrode was found to be as low as 9 X/sq. In addition, a hole-only behavior of device with Ag electrode characterized by current– voltage measurement and conductive atomic-force microscopy shows the injection property of high current flowing as compared with device using Au electrode, resulting in an efficient injection condition existing at the interface of the graded Ag/pentacene. Device characterization indicates the transparent pentacene FET with a graded ultra-thin Ag electrode and organic capping layer of N,N0 -di(1-naphthyl)-N,N0 -diphenylbenzidine exhibits a high transmission rate of 75% in the range of visible light from 400 to 550 nm, a threshold voltage of 6.0 V, an on–off drain current ratio of 8.4 105, and a field-effect mobility of 1.71 cm2/V s, thus significantly outperforming pentacene FETs with multilayer oxide electrodes or other transparent thin metal layers. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Transparent organic electronic devices have attracted considerable for use in display [1], smart circuit [2], sensor [3], and solar energy [4]. Previous studies have demonstrated that an oxide/thin metal/oxide trilayer structure can be used to replace a transparent indium tin oxide (ITO) anode [5], bilayer cathode (metal type) [6], and graphene electrodes [7]. For example, Yook et al. reported that tungsten oxide (WO3)/Ag/WO3 (WAW) cathode electrodes in the transparent organic light-emitting diodes (OLEDs) show high transmission and low sheet resistance [8]. Most importantly, these electrodes were fabricated using only thermal evaporation at room temperature to ⇑ Corresponding author. Tel.: +886 2 29089899; fax: +886 2 29041914. E-mail address: [email protected] (S.-W. Liu). http://dx.doi.org/10.1016/j.orgel.2014.05.037 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.
deposit the WAW, thus avoiding high temperature process damage to the active layer [9]. Such a WAW electrode has potential to serve as an anode electrode, as compared opposed to other types of transparent conductive films, such as ITO [10], indium zinc tin oxide [11], indium zinc oxide [12], aluminum zinc oxide [13], zinc oxide [14], nickel oxide [15], carbon nanotube [16], and graphene [17]. More recently, Zhang et al. demonstrated a transparent WAW source and drain (S–D) electrodes with a transparency rate exceeding 80% in visible light in the pentacene field-effect transistors (FETs), but this electrode exhibits a low carrier mobility of 0.08 cm2/V s [18]. This issue is attributed to high contact resistance because a thick WO3 is inserted between the Ag and the pentacene, and this must be resolved to improve the electrical characteristics of transparent devices. On the other hand, the thin bilayer structures of Ba/Ag [19], Ca/Ag [20], and LiF/Yb:Ag
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[21] have also been proposed to replace a transparent electrodes in OLEDs. Although these show high transmission of 70%, Ba, Ca, and LiF layers are still unsuitable for use as injection electrode to their low workfunction to reduce the hole injection property. To solve the problem of energy mismatching between the metal electrode and organic channel, our previous work demonstrated an efficient bottom-contact pentacene FET fabricated at room-temperature with Ag/self-assembled monolayers electrodes, which effectively change the workfunction from 4.2 to 5.8 eV [22]. These results reveal that the injection efficiency is highly correlated to the metal/organic interfacial property. In this work, we have developed a highly transparency and high performance pentacene FET based on ultra-thin graded Ag electrodes with top-contact structure. It exhibits better electrical characteristics than thick Au or Ag S–D electrodes. Using
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the optimized thickness of 15 nm Ag and 35 nm capping layer of N,N0 -di(1-naphthyl)-N,N0 -diphenylbenzidine (NPB) produces an efficient pentacene FET with a threshold voltage of 6.0 V, on–off drain current ratio of 8.4 105, field-effect mobility of 1.71 cm2/V s, and totally device transmission of 75% in visible light. In addition, electrical characteristics and conductive atomic-force microscopy (AFM) were used to address the injection property at the interface of Ag/pentacene and Au/pentacene.
2. Experiments Fig. 1 shows a schematic cross section of pentacene transistors with Au, graded Ag, ultra-thin graded Ag/NPB S–D electrodes based on a top-contact structure. Our device was fabricated on a bare glass substrate coated with
Fig. 1. Schematic cross section of top-contact pentacene-based FETs with Au, graded Ag, and ultra-thin graded Ag with capping NPB electrodes.
Fig. 2. (a and b) Source-drain current–voltage and (c and d) the transfer characteristics of the pentacene FETs with Au and graded Ag electrodes.
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Table 1 Device performances of the pentacene FETs with Au and graded Ag electrodes. Electrode
l (cm2/V s)
VT (V)
Ion/off (A)
Au Graded Ag
0.34 0.86
4.1 5.0
0.6 105 3.8 105
Park System XE-70 with a conductive mode using conductive Pt coated tip cantilever, where the tip radius is between 100 and 200 nm, and the substrate applied the 5 V to evaluate the charge injection property. All measurements herein were carried out in an air environment. 3. Results and discussion
a 150 nm indium tin oxide (ITO; 15 X/sq) layer, which served as a gate electrode. The substrate was cleaned by ultrasonic bath in successive solutions of acetone and isoproponal, etched in a dilute solution of sulphuric acid, and finally dried by 5 N nitrogen blow. The cross-linked poly-4vinylphenol dielectric layer was prepared from a solution of PVP and poly(melamine-co-formaldehyde) in propylene glycol monomethl ether acetate (PGMEA) deposited via spin-coater, and baked in a vacuum tube (2 106 Torr) using two steps annealing processes of 200 °C for 1 h and then 100 °C for 24 h. The final thickness of the polymer dielectric layer was 200 nm with a unit capacitance of 16 nF/cm2. A 50 nm high purity pentacene layer was then thermally deposited onto the patterned substrate. All materials including the S–D metal, semiconducting material, and insulator layers were purchased from Sigma–Aldrich. The S–D electrodes were patterned through a shadow mask to form the contact film, defining a channel length (L) and width (W) of 50 and 500 lm, respectively. The performance of different electrodes and thicknesses was compared. The Au electrode has a thickness of 50 nm and a deposition rate fixed at 0.5 Å/s. For the Ag electrode, two different thicknesses were used, 50 and 15 nm. To form the graded structure, the Ag thin films were deposited in two steps. First, 5 nm was deposited at 0.5 Å/s, with the remainder deposited at 5.0 Å/s. Therefore, we designated the ‘‘graded Ag’’ and ‘‘ultra-thin graded Ag’’ as Ag with respective thicknesses of 50 and 15 nm. The electrical properties were performed at a source meter (Keithley 2636A) with home-made probe station. A more detailed description of the setup was provided in our previous work [23]. All the performances are recorded on an average of 10 devices during one fabrication process. The AFM image of the Au (15 nm) or graded Ag (15 nm) coating on 50 nm pentacene/ n-type Si wafer was analyzed by a
The output and transfer characteristics of pentacene FET with Au and graded Ag S–D electrodes are shown in Fig. 2. Note that our pentacene FETs based on a PVP insulator exhibit leakage when the drain voltage equals zero. We attribute this to mobile ions in the PVP resulting, as reported previously [24,25], from the presence of the hydroxyl group in PVP. Due to the PVP insulator, the leakage currents are identical for all devices. The field-effect mobility of pentacene FET in the saturation regime is obtained from the following equation:
ID ¼
W C i lsat ðV G V T Þ2 2L
ð1Þ
where ID, Ci, lsat, VT, VG, L, and W are the drain current, the capacitance per unit area of gate dielectric, the hole mobility, the threshold voltage, the gate voltage, the channel length, and the channel width, respectively. According to such an equation, the field-effect mobility of pentacene FET is respectively calculated at 0.34 and 0.86 cm2/V s for Au and graded Ag electrodes, which represents an improvement of about 60% at a gate voltage (VG) of 5 V and drain-source voltage (VDS) of 10 V (see Fig. 2a and b). Obviously, the FET with graded Ag electrodes exhibits excellent electrical characteristics, which is much higher than that of constant deposition rate of Au or Ag electrodes based on pentacene channel [22,26]. Therefore, we expect that the graded Ag can be used to reduce a contact barrier and provide tunneling injection process at the electrode/ pentacene interface. Note that the injection barrier at Ag/ pentacene is 1.3 eV higher than that at the Au/pentacene interface of 0.8 eV in recent measurement [22]. This result suggests that the high-energy barrier may hinder the device’s charge injection efficiency [27]. However, our experimental results clearly show that using the graded Ag layer in the S–D electrodes significantly enhances l as
Fig. 3. AFM images of pentacene channel with heat radiation effect of (a) Au and (b) graded Ag electrodes.
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Fig. 4. Current–voltage characteristics the FET device using Au and graded Ag electrodes.
compared with the Au layer. This may indicate that the injection efficiency is not dominated by the workfunction difference between Ag and pentacene. Wang et al. reported
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that the device with Cu contact electrode exhibits lower contact resistance, although the workfunction difference at the interface of Cu/pentacene is 0.95 eV [28]. Therefore, the contact resistance is possibly determined by the tunneling process if the contact interface has a high workfunction difference between Ag and pentacene. To explore other possible reasons, the relationship between device performance and the graded Ag layer will be explained by AFM and electrical characteristics in the latter. Table 1 summarizes the electrical properties (l, VT, and on/off ratio) of the pentacene FET measured in ambient. In addition, the VT slightly changes from 4.1 to 5.0 V and on/off ratio significantly improves from 0.6 105 to 3.8 105, as shown in Fig. 2(c) and (d), when graded Ag layer was used in our pentacene FETs. In general, FET performances are highly correlated to the channel’s film morphology and the contact property at the pentacene/metal interface [29–32]. In our case, the carrier mobility of the graded Ag electrode is much higher than that of the Au electrode. Therefore, we first investigate the morphological change to the pentacene channel. Fig. 3(a) and (b) respectively show the pentacene channel after the deposition of the Au and graded Ag electrodes.
Fig. 5. (a and b) AFM images of pentacene/Au and pentacene/graded Ag. (c and d) are the corresponding current images. Note that the applied voltage of substrate was 5 V for conductive AFM measurements.
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Surprisingly, the morphology of the pentacene channel was significantly changed, exhibiting smaller grain size for the Au-deposited FET than the graded Ag-deposited one. Note that the large grain size is expected to improve carrier transportation in the pentacene channel. We speculate that the morphological change may be caused by the different levels of heat radiation during the deposition of the Au and Ag metals. Similar results are mentioned in the literature [33]. Fig. 4 shows the IDS–VDS curves in the devices with the Au or graded Ag layers. Note that we do not apply any voltage of VG to the pentacene FET during device measurement. Therefore, such measurement method is similar to the hole-only device to evaluate the carrier injection and transport properties in the pentacene FET device. In our electrical measurement, the IDS in the device with graded Ag electrodes is one order of magnitude higher than that in the device with Au electrodes. This indicates that graded Ag electrodes show very well injection property, which can be used to alternate Au electrodes for efficient device fabrication. In addition, we believe that the injection barrier of the graded Ag layer may be moved towards the lower injection barrier as compared with Au layer. In addition, the electrical characteristics of hole-only measurements are consistent with performance of the FETs (see Fig. 2a and b). In order to further understand the issue of contact property, the surface morphology and current image at interface of electrode/pentacene are found to be strongly related to the efficiency of charge injection in device. Fig. 5 shows the conductive AFM images of the Au and graded Ag layers on the pentacene thin film. The root mean square (rms) roughnesses are measured to be 6.6 and 4.3 nm for Au and graded Ag on the pentacene, as shown in Fig. 5(a) and (b), respectively. From AFM measurement, we found that these thin metal films (Au and graded Ag) have very different surface morphology on pentacene surface. The deposition of the graded Ag layer on pentacene thin film significantly reduces the grain size as compared to that of the Au electrode (see Fig. 5a and b). This can be attributed to the evaporated graded Ag layer just requiring lower heat energy than that of Au during the deposition process defining S–D contact electrodes, while changing deposition rate from slow to fast might be helpful for achieving good metal/organic contact interface to enhance the charge injection, as shown in Fig. 5(c) and (d). Note that such a fabrication process is usually used to improve the carrier injection in organic semiconducting materials due to reducing the voided thin-film and smoothing the local contact surface at the organic/metal interface [34]. It has also been shown that the film morphology, metal diffusion, and large contact area improve the contact property and/or induce the tunneling mechanism [35]. Therefore, this result demonstrates that the graded Ag electrodes may possibly become a good choice to alternate the expensive Au electrode in the pentacene FET. Fig. 6 shows the output and transfer characteristics of the transparent pentacene FET without and with capping N,N0 -di(1-naphthyl)-N,N0 -diphenylbenzidine (NPB) layer (35 nm) atop ultra-thin graded Ag (15 nm) electrodes. Both devices also show high performance p-channel
characteristics. Compared with previous studies in the field of transparent organic FETs [18], the device with ultra-thin graded Ag electrodes performed efficiently with a l of 1.42 cm2/V s, VT of 5.9 V, and on-off ratio of 2.8 105. This result demonstrates that the total thickness of 15 nm Ag is sufficient to form the well contact structure with sheet resistance of 15 X/sq and provide high
Fig. 6. (a) Source-drain current–voltage, (b) on–off ratio, and (c) threshold voltage characteristics of the transparent pentacene FETs using ultrathin graded Ag without (w/o) and with (w/) capping NPB.
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Fig. 7. Transmission spectra of the transparent pentacene FETs using ultra-thin graded Ag without (w/o) and with (w/) capping NPB. Inset: the photo of 144 devices array of these transparent pentacene FETs positioned above the logo of ‘‘Formosa Plastic Group’’.
transparency at near 50–65% in the range of visible light as shown in Fig. 7. Note that the sheet resistance of such ultra-thin graded Ag is slightly smaller than that of graded Ag electrode (sheet resistance of 1 X/sq), resulting in the good conduction property for the ultra-thin contact pad. More interestingly, it can be further observed that the ultra-thin graded Ag electrode adding 35 nm capping layer of NPB significantly increases the l, VT, and on/off ratio to be 1.71 cm2/V s, 6.0 V, 8.4 105 as compared with only the graded Ag structure (see Fig. 6a–c). This result may suggest that depositing the NPB layer on the Ag electrode can improve the electrical conductivity of thin metal film, which has low sheet resistance of 9 X/sq. The increased
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mobility obtained by depositing NPB atop pentacene is attributed to the double-channel effect due to the p-type property of NPB [36,37]. On the other hand, the pentacene FET is possibly sensitive to environmental factors, such as air, water, and oxygen. For this reason, we rule out the possibility that ambient factor may contribute the more hole carrier in the NPB/pentacene channel. In addition, we found that, at high voltages, the drain current of pentacene FET using Ag electrodes (i.e., the graded or ultra-thin Ag) exhibited different tendencies in the transfer curves of Figs. 2(d) and 6(c). We concluded that this is caused by the different levels of contact resistance or by the properties of the metal pads, which are capable of limiting the number of carriers injected into our device. As shown in Fig. 7, the Ag/NPB electrode structure exhibits high transparency of 75% from 400 to 550 nm due to an optical effect in multilayer structure [38,39], while the inset of Fig. 7 is the photograph of the transparent pentacene FETs with ultra-thin graded Ag/NPB electrodes in this work. Note that the group logo could be easily seen through the layout of our transparent device. On the other hand, many groups reported that the transmission and morphology of Ag could be considerably changed for thin film at varied thickness or deposited at annealed substrate [40,41]. We believed that it is potentially interesting to investigate the differences between ordinary Ag and graded Ag as used here. Fig. 8(a) shows the transmission spectra for different metal thin films. Note that the Ag and graded Ag film were prepared both at a constant rate (15 nm at 0.5 Å/s) and at variable rates (first 5 nm at 0.5 Å/s, residual 10 nm at 5.0 Å/s). A high degree of transparency (70%) in the visible light range was obtained for the graded Ag, while the Ag thin film exhibited a monotonically decreased transmission from
Fig. 8. (a) Transmission spectra of 15 nm Ag, graded Ag, and Au thin films. (b–d) AFM images of 15 nm Ag, graded Ag, and Au, respectively.
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70% to 50% in this range. In the field of transparent organic electronics, transparent electrodes are usually fabricated with multilayer structures using Ag as the middle layer [42–44]. However, the thickness of the Ag should be decreased to a certain level to ensure sufficient transparency [45]. Note that the 15 nm thickness studied here is in the range of highest transparency (typically from 10 to 15 nm). Therefore, we demonstrate a simple fabrication process to obtain a highly transparent metal using a single Ag layer. Given identical thicknesses, the transparency of Au thin film is lower than that of both Ag and graded Ag. Fig. 8(b)–(d) shows the AFM images of Ag, graded Ag, and Au thin films. The disconnection is apparently more severe in the constant-rate-deposited Ag than in the variable-rate-deposited Ag, resulting in the electrode’s high conductivity and the FET’s excellent electrical properties. Therefore, these results demonstrate that the structure of the ultra-thin graded Ag electrode makes it a good choice to replace Au or other metal oxide multilayer electrodes for the fabrication of efficient and transparent pentacene FETs. 4. Conclusion In summary, high performance pentacene FET using novel structure of graded Ag layer as S–D electrodes has been demonstrated. The field-effect mobility 1.71 cm2/ V s for the transparent device based ultra-thin graded Ag layer is much better than other organic FET with metaloxide multilayer or other thin metal electrodes. Moreover, the large workfunction difference at the interface of the Ag/pentacene may induce the tunneling injection effect to dominate the injection property in the pentacene FET, which plays an important role in improving the contact barrier of the metal/organic interface. Acknowledgments The authors acknowledge the financial support from the National Science Council (Grant Nos. NSC 103-ET-E011-004-ET, 102-2511-S-131-002, 102-3113-E-001-001, 102-2627-E-002-002, 102-2221-E-011-142, 102-2221-E131-030-MY2, and 102-2221-E-131-026-MY2). In addition, the corresponding author (Dr. S.-W. Liu) would like to give special thanks to Mr. H.-H. Wu, Syskey Technology Corporation (Taiwan), for the assistance in fabrication system designed. References [1] J. Lee, S. Hofmann, M. Thomschke, M. Furno, Y. Hyun Kim, B. Lüssem, K. Leo, Highly efficient bi-directional organic light-emitting diodes by strong micro-cavity effects, Appl. Phys. Lett. 99 (2011) 073303. [2] C.-S. Chuang, S.-T. Tsai, Y.-S. Lin, F.-C. Chen, H.-P.D. Shieh, Photocurrent suppression of transparent organic thin film transistors, Jpn. J. Appl. Phys. 46 (2007) L1197–L1199. [3] T.Q. Trung, N.T. Tien, Y.G. Seol, N.-E. Lee, Transparent and flexible organic field-effect transistor for multi-modal sensing, Org. Electron. 13 (2012) 533–540. [4] W. Jose da Silva, H.P. Kim, A. Rashid bin Mohd Yusoff, J. Jang, Transparent flexible organic solar cells with 6.87% efficiency manufactured by an all-solution process, Nanoscale 5 (2013) 9324–9329.
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