Silver nanowire-polymer composite electrode for high performance solution-processed thin-film transistors

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Silver nanowire-polymer composite electrode for high performance

solution-processed thin-ﬁlm transistors

Liang-Hsiang Chen

a

, Pang Lin

a

, Ming-Chou Chen

b

, Peng-Yi Huang

b

, Choongik Kim

c,⇑

,

Jia-Chong Ho

d

, Cheng-Chung Lee

d

a

Department of Materials Science and Engineering, National Chiao-Tung University, Hsinchu, Taiwan, ROC

b_{Department of Chemistry, National Central University, Chung-Li, Taiwan, ROC}

c_{Department of Chemical and Biomolecular Engineering, Sogang University, 1 Shinsoo-Dong, Mapo-gu, Seoul 121-742, Republic of Korea} d

Process Technology Division, Display Technology Center, Industrial Technology Research Institute, Hsinchu, Taiwan, ROC

a r t i c l e

i n f o

Article history: Received 25 March 2012

Received in revised form 4 May 2012 Accepted 12 May 2012

Available online 15 June 2012

Keywords:

Organic thin-ﬁlm transistor (OTFT) Conductive electrode

Silver nanowire Anthradithiophene

a b s t r a c t

Silver nanowires (AgNWs)/poly-(3,4-ethylenedioxythiophene/polystyrene sulphonate) (PEDOT:PSS) composite films as conductive electrode for OTFTs were prepared, and their optical and electrical properties were investigated. The conductive composite films used in this study afforded low sheet resistance of <140X/sq and transmittance as high as 70% in the visible region. For the composite film with 0.1 wt.% of AgNWs, contact resistance as low as 2.7 104_X_{cm was obtained, as examined by Transfer length model (TLM)}

anal-ysis, and work function of the corresponding film was 5.0 eV. Furthermore, the composite films were employed as source and drain electrodes for top-gate/bottom-contact organic thin-film transistors (OTFTs) based on solution-processed 5,11-bistriethylsilylethynyl anthradithiophene (TES-ADT) as organic semiconductor, and the resulting device showed high electrical performance with carrier mobility as high as 0.21 cm2_{/ V s.}

1. Introduction

Organic thin-film transistors (OTFTs) have extensively been studied for low cost, lightweight, flexible electronics such as smart cards, identification tags, active-matrix dis-plays, and sensors [1]. Hence, the development of solu-tion-processed components — semiconductors, insulators, and conductors — in OTFTs at low cost and in a large scale, are of great interest[2]. There have been great efforts to develop and improve the electrical properties of solution-processed organic semiconductors, some of which perform comparably to amorphous silicon [3,4]. Furthermore, polymers or polymer-based composite films with high capacitance and low leakage current density have been investigated as high performance solution-processed insulators [5]. Solution-processed conductors, however,

have received relatively little attention, despite their criti-cal roles in OTFTs. The high performance conductor mate-rials need to meet several electrical and material requirements – (i) high electrical conductivity, (ii) proper energy match with organic semiconductor, and (iii) processibility at low temperature that is compatible with ﬂexible substrates.

Conductive materials that have mostly been employed in OTFTs are metals such as gold, due to their high conduc-tivity. However, physical damage onto the underlying organic layers caused by vacuum deposition could deterio-rate the device performance. Solution-processed conduc-tive materials such as poly-(3,4-ethylenedioxythiophene/ polystyrene sulphonate) (PEDOT:PSS) and doped polyani-line have been employed in OTFTs, but they suffer from low electrical conductivity[6]. Recently, other candidates for conductive materials such as carbon nanotubes, metal grids, and metallic nanowires have also been investigated

[7]. Among them, random silver nanowire (AgNW)

http://dx.doi.org/10.1016/j.orgel.2012.05.040 ⇑Corresponding author.

E-mail address:[email protected](C. Kim).

Contents lists available atSciVerse ScienceDirect

Organic Electronics

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networks are promising candidates because of their attrac-tive electrical, thermal, and optical properties[8]. Although AgNW-based conductive ﬁlms found some applications in electronic devices, these ﬁlms have never been employed in solution-processed OTFTs, to the best of our knowledge

[8]. To this end, we report conductive composite ﬁlms based on AgNW and PEDOT:PSS with varying AgNW con-tents, and their electrical, optical, and material properties in terms of sheet resistance (Rs), contact resistance (Rc), work function, and optical transparency were investigated. Furthermore, the potential of AgNW-based composite con-ductors for electronic devices was evaluated for top-gate/ bottom-contact OTFTs by employing composite conductors as source/drain contacts, 5,11-bistriethylsilylethynyl anth-radithiophene (TES-ADT)[4,9], a representative high per-formance solution-processed p-type organic material, as semiconductors, and Parylene C as gate insulator (Fig. 1).

2. Experimental section 2.1. Materials

All chemicals were purchased from Aldrich, including the PEDOT:PSS (1.3 wt.% dispersion in H2O), and were used without further puriﬁcation. AgNWs were prepared using a synthetic procedure in the literature[10,11]. Organic semi-conductor, TES-ADT, was synthesized following the syn-thetic procedure in the literature [4], and the synthetic product was puriﬁed further by chromatography (silica gel) and recrystallization.

2.2. Preparation and characterization of AgNW-PEDOT:PSS composite ﬁlms

For composite conductive ﬁlms, a desired amount of AgNWs stabilized with trisodium citrate in water (0.1 wt.%, 0.5 wt.%, and 1 wt.%) was added to the PEDOT:PSS solution (10 mL). For pristine AgNW ﬁlms (0.1 wt.%, without PEDOT:PSS), deionized water (10 mL) was used instead of PEDOT:PSS solution. The solution was sonicated for 30 min using a high intensity ultrasonic probe to prevent local inhomogeneity before spin-coating.

To form the composite films, the mixtures (AgNWs and PEDOT:PSS) were spin-coated (1500 rpm) on a glass substrate, and the resulting films were annealed at 100, 150, 200, and 250 °C for 2 h on a hotplate. The thickness of the composite films was 500 nm, as estimated via a Dektak 3030 profilometer. The sheet resistances of the composite films were measured using a 4-point probe technique (Keithley 2400 source meter). Optical transmit-tance of the films were measured on quartz slides from 300 nm to 800 nm (with an increment of 5 nm) using a Perkin–Elmer Lambda 750 UV/Vis/NIR spectrophotometer. The surface morphologies of the films were analyzed using atomic force microscopy (AFM, Digital Instruments Nano-scope III) and scanning electron microscopy (SEM, Hitachi S-4700). The work function of the films was analyzed by ultraviolet photoemission spectroscopy (UPS) with line source energy of He I (21.21 eV).

2.3. Device fabrication and characterization

To fabricate the top-gate/bottom-contact OTFT, conduc-tive composite ﬁlms on glass substrates were patterned as source/drain contacts, by focused laser ablation using a UV laser beam (k = 365 nm). The channel width of the result-ing electrodes was 500

l

m, and the channel lengths were 20

l

m, 30

l

m, 40

l

m, and 50

l

m (various channel lengths were employed to extract the contact resistances of the de-vices). Organic semiconductor (TES-ADT) was spin-coated onto the substrates from a 20 mL toluene solution (1 wt.%) at 500 rpm for 2 s and 1000 rpm for 28 s. The resulting film was thermally annealed at 90 °C for 2 min in a N2glove box. The film thickness of the semiconducting film was 90 nm, as measured by AFM. To improve the crystallinity of the active layer, TES-ADT films were an-nealed under chloroform solvent-vapor (at a vapor pres-sure of 12 kPa) for 5 min in a glass chamber. For an insulating layer, Parylene-C was deposited by vapor phase polymerization system (chemical vapor polymerization reactor consisted of a vaporizer, pyrolysis and polymeriza-tion chambers, a cold trap, and a vacuum system)[12]. The film thickness of the insulator was 900 nm with an areal capacitance value of 2.7 nF/cm2_{. For gate electrode, a} 50 nm-thick Au layer was thermally evaporated through a shadow mask. The electrical characteristics of the devices were measured under darkness with a semiconductor parameter analyzer (HP 4155A) at room temperature in a glove box (H2O < 10 ppm).

3. Results and discussion

To investigate the electrical conductivity of the AgNW-PEDOT:PSS composite films in comparison to that of pris-tine PEDOT:PSS film and random AgNW networks (prispris-tine AgNW films), sheet resistances (X/sq) of the films were measured as a function of film annealing temperature (Fig. 2A). In general, sheet resistances of the measured films gradually decreased as the annealing temperature in-creased, then reached a nearly constant values at annealing temperature >200 °C. Furthermore, sheet resistances of the composite films, regardless of the AgNW contents, were

Fig. 1. Top-gate/bottom-contact TFT structure, and chemical structures of TES-ADT and Parylene-C employed in this study.

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much lower than that of pristine PEDOT:PSS film. For example, at annealing temperature of 200 °C, sheet resis-tance of the PEDOT:PSS film was 3 103_X_{/sq, while} the composite films exhibited higher electrical conductiv-ity with sheet resistance values of 9–140X/sq, comparable to that of AgNW film (5X/sq) at the same annealing tem-perature. High electrical conductivity of the composite films achieved at relatively low annealing temperature could be attributed to the formation of a 3D conductive network via random AgNW networks in polymer matrices (vide infra,Fig. S1), which shows the promise of the pres-ent composite films used on plastic substrates for flexible electronics.

Optical properties of the composite ﬁlms with varying AgNW contents were investigated by UV–Vis spectroscopy

[13]. Fig. 2B shows optical transmittance spectra of the conductive films on glass substrates at a range of 300–800 nm. As shown, optical transparency of the con-ductive composite films were lower than that of pristine AgNW films. Average optical transmittance of the pristine AgNW films were 86% in the measured region, compara-ble to that of AgNW films reported as transparent conduc-tors in the literatures[14,15]. Furthermore, as the content of AgNWs increased in the composite films, the optical transparency decreased. The average optical transmittance

of the conductive composite films with 0.1 wt.%, 0.5 wt.%, and 1 wt.% of AgNW contents were 70%, 62%, and 45%, respectively[16]. Low optical transparency of the compos-ite films with high AgNW contents might be attributed to the formation of AgNW aggregates, which could deterio-rate optical properties of the films (Fig. S1) [17].

To test the potential of the conductive composite ﬁlms for electronic devices, top-gate/bottom-contact OTFTs were fabricated by employing the composite ﬁlms as source/ drain contacts, TES-ADT as organic semiconductor, and Parylene-C as organic insulator (Fig. 1).Fig. 3andTable 1

collect the typical TES-ADT TFT characteristics with varying AgNW contents. Note that the saturation mobility (

l

sat) and the threshold voltage (VT), an average value of ten device measurements, were calculated at VDS= 50 V. Overall, TES-ADT TFTs employing the composite ﬁlms as conductors showed much higher device performance (carrier mobility of 0.03–0.21 cm2_{/V s) compared to the device with pristine} AgNW ﬁlms as contacts (carrier mobility of 0.008 cm2_{/V s).} Especially, the TES-ADT TFTs constructed with 0.1 wt.%

AgNW-PEDOT:PSS composite ﬁlms showed highest

electrical performance with carrier mobility as high as 0.21 ± 0.05 cm2/V s, demonstrating the promise of the conductive composite films as conductors for OTFTs.Figs. 3B andS2show output characteristics of the TES-ADT TFTs with 0.1 wt.% AgNW-PEDOT:PSS composite film and pris-tine AgNW film as conductors, respectively. The TFT with electrodes of 0.1 wt.% AgNW-PEDOT:PSS composite films exhibited relatively reasonable output characteristics with pinch-off and current saturation behaviors (Fig. 3B). On the other hand, the TFT with electrodes of pristine AgNW films showed more current crowding in the low drain volt-age region (linear region) compared to that with electrodes of composite films, which could be attributed to the high contact resistance between the AgNW electrodes and the TES-ADT channel layer (Fig. S2, vide infra).

It has been reported that the performance of TES-ADT TFTs is highly dependent upon the manner in which elec-trical contacts are made to the organic semiconductors, due to weak van der Waals interactions between TES-ADT molecules in the solid state[18]. For example, TES-ADT TFTs employing laminated gold source/drain elec-trodes via elastomeric stamps showed higher device per-formance compared to those with gold electrodes fabricated via thermal evaporation [18]. To investigate the nature of the electrical contacts between electrodes and TES-ADT active layer, contact resistances and work functions of the conductive ﬁlms were examined.

First, for contact resistance calculation, TLM analysis was used, in which the total resistance (Rtot) of a device was ﬁtted as a function of the channel length (L)[19,20]. Then, the contact resistance (Rc) was obtained at the inter-section of the measured device resistance in the limit of L ? 0 at different gate voltage (VG), as indicated by Eq. 1:

Rtot¼ Rcþ

L W½

l

CiðVG VthÞ

ð1Þ

where

l

and VTare the intrinsic ﬁeld-effect mobility and the threshold voltage, respectively. Fig. 4A shows Rtot of TES-ADT TFTs with pristine AgNW ﬁlms as contacts,

Fig. 2. (A) Sheet resistances of the conductive composite ﬁlms as a function of annealing temperature. (B) Optical transmittance spectra of the conductive ﬁlms on glass substrates.

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normalized by the channel width, plotted as a function of channel length L at different VG. The obtained contact resis-tances of the conductive films employed in this study at different gate voltage are shown inFig. 4B. As shown, con-tact resistances normalized with the channel width (RcW), of the films decreased as the gate voltage increased, in con-sistent with the previous study in the literature[21]. For example, contact resistances of the pristine AgNW films were 2.0 108_X_{cm and 5.4 10}6_X_{cm at a gate voltage} of 10 V and 50 V, respectively. Furthermore, the con-ductive composite films employed in this study afforded much lower contact resistances than pristine AgNW films. For AgNW-PEDOT:PSS composite films with 0.1 wt.%, 0.5 wt.%, and 1 wt.% of AgNW contents, the corresponding contact resistances at a gate voltage of 50 V were

2.7 104

Xcm, 6.7 104

Xcm, and 1.5 106

Xcm, respectively. Higher contact resistances of the composite ﬁlms with larger contents of AgNWs might be due to de-fected semiconductor–electrode interface originated from large surface roughnesses (Fig. S1) [17,22].

Secondly, work functions of the conductive films were determined by ultraviolet photoemission spectra (UPS, He(I): 21.2 eV), via calculation of the UPS spectra width (Fig. 5). Measured work functions of the pristine AgNW film was 4.2 eV, while those of AgNW-PEDOT:PSS com-posite films with 0.1 wt.%, 0.5 wt.%, and 1 wt.% of AgNW contents were 5.0 eV, 4.8 eV, and 4.5 eV, respectively. Work functions of the conductive composite films decreased as the contents of AgNWs in the film increased. The lowest contact resistance and highest work function of

Fig. 3. Electrical characteristics of top-gate/bottom-contact TES-ADT TFTs. (A) Transfer characteristics of TFTs employing the conductive composite films (with varying AgNW contents) and pristine AgNW films as source/drain contacts. (B) Output characteristics of TFTs employing the 0.1 wt.% AgNW-PEDOT:PSS composite films as source/drain contacts.

Table 1

Charge carrier mobility (lsat,cm2/V s), current on/off ratios (Ion/Ioff), threshold voltage (VT, V), subthreshold swing (S.S., V/decade), and contact resistance (Rc,

MXcm) data for TES-ADT TFTs fabricated with pristine AgNW ﬁlms and AgNW-PEDOT:PSS composite ﬁlms with varying AgNW contents as source/drain contacts.

Samples lsat Ion/Ioff VT S.S. Rc

AgNWs 0.008 ± 0.005 5.0 105 15 ± 5.0 1.5 ± 0.8 5.4 ± 2.1 1 wt.% AgNWs/PEDOT:PSS 0.03 ± 0.02 1.5 106 7.0 ± 4.0 1.4 ± 0.6 1.5 ± 0.8 0.5 wt.% AgNWs/PEDOT:PSS 0.15 ± 0.03 3.0 106 10 ± 3.0 1.2 ± 0.3 0.067 ± 0.023 0.1 wt.% AgNWs/PEDOT:PSS 0.21 ± 0.05 5.0 106 8.0 ± 2.0 1.3 ± 0.5 0.027 ± 0.012

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the 0.1 wt.% AgNW-PEDOT:PSS composite ﬁlm could afford better energy level match with better charge carrier injec-tion into the active layer, resulting in better device perfor-mance (vide supra)[23].

4. Conclusions

This study reported TES-ADT TFTs using spin-coated conductive composite films based on AgNWs and PED-OT:PSS as source and drain electrodes. The composite films with low content of AgNW (0.1 wt.%) showed low sheet resistance of 140X/sq and decent optical transmittance of 70%. Furthermore, low contact resistance and high work function of the corresponding composite film afforded good electrical contacts and energy-level alignment to an active layer, resulting in high performance top-gate/bot-tom-contact TES-ADT TFTs with carrier mobility up to 0.21 cm2_{/V s. These results demonstrated the viability of} using spin-coated conductive composite films as source and drain electrodes for large area and flexible electronic applications.

Acknowledgements

This work was supported by the Display Technology Center for the Industrial Technology Research Institute of Taiwan, R.O.C. (Contract No. B301AR1L11), by National Science Council, Taiwan, Republic of China (Grant Nos. NSC100-2628-M-008-004 and NSC100-2627-E-006-001), and by the National Research Foundation of Korea (2011-0007730), and by a Grant (2011-0031628) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Education, Science and Technology, Korea.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, athttp://dx.doi.org/10.1016/ j.orgel.2012.05.040.

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