Improved performance in n-channel organic thin film transistors by nanoscale interface modification

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Letter

Improved performance in n-channel organic thin ﬁlm

transistors by nanoscale interface modiﬁcation

Chih-Wei Chu

a,b,*

, Chao-Feng Sung

c

, Yuh-Zheng Lee

c

, Kevin Cheng

c

a

Research Center for Applied Sciences, Academia Sinica, Taipei 11529, Taiwan

b

Department of Photonics, National Chiao-Tung University, Hsinchu 300, Taiwan

c

Industrial Technology Research Institute, Hsinchu 300, Taiwan

Received 16 July 2007; received in revised form 22 November 2007; accepted 27 November 2007 Available online 26 December 2007

Abstract

We demonstrate that the electrical properties of n-channel thin ﬁlm transistors can be enhanced by inserting a nanoscale interfacial layer, namely, cesium carbonate (Cs2CO3), between organic semiconductor and source/drain electrodes. Devices

with the Cs2CO3/Al electrode showed a reduction of contact resistance, not only with respect to Al, but also compared to

Ca. The improvement is attributed to the reduction in the energy barrier of electron injection and the prevention of unfa-vorable chemical interaction between the organic layer and the metal electrode. High field-effect mobility of 0.045 cm2/V s and on/off current ratios of 106_{were obtained in the [6,6]-phenyl C60 butyric acid methyl ester-based organic thin film}

transistors using the Cs2CO3/Al electrodes at a gate bias of 40 V.

PACS: 73.40.Cg

Keywords: n-Type; Organic thin ﬁlm transistors; Nanoscale interface modiﬁcation

In recent years, there has been a worldwide inter-est in developing organic thin film transistors (OTFTs) due to their potential application in display drivers, radio frequency identification tags, and smart cards[1–3]. Great progress has been achieved so far in p-type OTFTs, whose electronic properties have already reached the level of hydrogenated amorphous silicon (a-Si:H). For example, field effect mobilities greater than 1 cm2/V s and high on/off

current ratio (>106) have been obtained in penta-cene-TFTs[4]. However, the development of n-type OTFTs with comparable performance remains a key issue in terms of meeting the requirements for prac-tical applications. Although a large number of stud-ies have focused on improving the intrinsic electrical properties of n-type materials[5,6]and their applica-tions,[7,8]devices still exhibit limited life span. This could be attributed to the instability of single com-ponent p-electron materials, which can easily undergo surface oxidation/deoxidation and chemi-cal interaction with metal electrodes. Moreover, low work function metals, used to reduce energy bar-riers and promote electron injection[9,10], are

* _{Corresponding author. Address: Research Center for Applied}

Sciences, Academia Sinica, Taipei 11529, Taiwan. Tel.: +886 2 2789 8000.

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

Organic Electronics 9 (2008) 262–266

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cult to process because of their susceptibility to atmospheric moisture and oxygen. These factors henceforth indicate that the property of organic– metal electrode interface play an important role in obtaining high performance n-type OTFTs; how-ever, engineering of the interface has not received much attention in n-type OTFTs[11].

An alternative approach for replacing the reac-tive metal as an electrode is to insert a thin layer of alkali metal halides [12] and carboxylates [13], in various attempts at improving the charge injec-tion from an Al cathode to an emitting layer for organic light emitting diodes (OLED). Similarly, the source/drain (S/D) contacts in the OTFTs have signiﬁcant inﬂuence on device operation. For exam-ple, their contribution to the contact resistance arise from mismatching work functions, and/or interac-tion between the metal electrodes and the organic semiconductor [14,15]. Recently, it has been reported that Cs2CO3 is one of the best electron

injection materials among a wide range of metal electrodes, and can be used as an electron injection layer for OLED by thermal and solution deposition [13,16]. In this letter, we present the conclusion that the performance of the [6,6]-phenyl C60 butyric acid methyl ester (PCBM)-based n-type OTFTs with an bi-layer S/D electrodes is greatly improved over the bare Al, and calcium electrodes. The perfor-mance enhancement of our devices is achieved by using a nanoscale interfacial modiﬁcation layer made of Cs2CO3. The presence of a Cs2CO3 layer

at the organic/Al interface signiﬁcantly reduces the contact barrier and provides protection against dif-fusion and chemical interaction between organic layer and metal electrodes.

The devices were fabricated on ITO-coated glass substrates (10–20 X/sq sheet resistance). ITO-coated glass substrate was used as the gate electrode. After routine solvent cleaning, the substrates were treated with UV-ozone for 15 min. The cleaned ITO sub-strates were then covered with 750-nm-thick poly-mer dielectric insulator, prepared by spin-coating a solution of poly-4-vinylphenol (11 wt%) and poly(melamineco-formaldehyde) (4 wt%) in propyl-ene glycol monomethyl ether acetate (PGMEA). The substrate was then prebaked at 100°C for 5 min, followed by baking at 200°C for 20 min, to cross-link the polymer. The thickness of the PVP ﬁlms was 680 nm. The resulting capacitance per unit area of the ﬁlm, Ci, was 5.47 nF/cm2. The

semicon-ductor layer, consisting of a 50-nm-thick layer of PCBM (purchased from Solenne B.V.), was spun

over the cross-linked PVP from a 1 wt% ratio chlo-roform solution in a nitrogen environment inside a glove box. Prior to S/D electrode deposition, the device was thermally annealed on a hot plate at 70°C for a period of 20 min. Finally, Cs2CO3

(Sigma–Aldrich, 99% purity) was deposited by a Knudsen cell (k-cell), a molecular beam epitaxy sin-gle-filament effusion cell from Veeco/Applied Epi, and Al was thermally evaporated onto the PCBM film through a shadow mask to form the S/D elec-trodes. The thicknesses of Cs2CO3and Al films were

1 nm and 80 nm, respectively. A schematic cross-section of our top-contact OTFTs is presented in the inset of Fig. 1a with a channel length of 170 lm and width of 2 mm. For comparison, Ca, Al, and Au were investigated as alternative to the Cs2CO3/Al as S/D contacts. All thermal

evapora-tions were done under a pressure of less than

Fig. 1. (a) Typical transfer characteristics of the OTFT with Cs2CO3/Al electrodes at a constant drain voltage of 40 V. Inset:

schematic structure of a top-contact OTFT. (b) Source/drain current–voltage characteristics of the OTFT with Cs2CO3/Al

electrodes. Inset: source/drain current–voltage characteristics of the OTFT with Au electrode.

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6 10 6torr and the ﬁlm thickness was monitored with a quartz oscillator. The electrical measure-ments of the devices were performed in a nitrogen environment inside a glove box using a Keithley 4200 semiconductor parameter analyzer and HP 4980A Precision LCR meter.

Typical transfer and output curve characteristics of the PCBM OTFTs, with Cs2CO3/Al S/D

elec-trodes, are shown in Fig. 1a and b, respectively. The device exhibited typical n-channel characteris-tics with good linear/saturation behavior, without any apparent negative drain current resulting from gate leakage at VDS= 0 V and signiﬁcant low gate

leakage (<4 10 9A) even at VG= 40 V. Strong

field-effect modulation of the channel conductance was observed, with on/off current ratios (Ion/Ioff)

as high as 106 (measured between gate voltage, VG= 10–40). The ﬁeld-eﬀect mobility (l) and

threshold voltage (VT) were extracted from the

mea-sured transfer curve by comparing it with the stan-dard transistor’s current–voltage equation in the saturation regime: IDS,sat= (WCi/2L)l(VG VT)

[2,17] where IDS,sat is the saturated drain current.

The l and the VT of the OTFT were found to be

4.45 10 2

cm2/V s and 2.3 V, respectively. It has been proposed that PCBM has much lower electron aﬃnity compared to C60 [18]. In addition,

the energy level of lowest occupied molecular orbital (LUMO) for PCBM is 3.7 eV [19]. Therefore, the electron injection current can be limited by the selec-tion of electrode materials. As shown in the ideal-ized transfer characteristics for diﬀerent materials for the S/D electrodes, Fig. 2, the slope for Cs2CO3/Al devices results in higher l and lower

VT, not only with respect to Al (U = 4.1 eV), but

also compared to Ca (U = 2.8 eV). The device with

Ca and Al electrodes had similar Ion/Ioﬀ compared

to the device with Cs2CO3/Al electrode, while the

decreased slope of the transfer characteristics corre-sponded to the decrease in field-effect mobilities. For further demonstrated the electron current is contact-limited. We have fabricated the devices with Au (U = 5.0 eV) as S/D electrodes. The output curve characteristic for the PCBM OTFTs with Au is shown in the inset ofFig. 1b. Due to the large energy level mismatch between Au work function and LUMO level of PCBM, the injection of elec-trons from Au to PCBM is a difficult process. The mobility for the PCBM OTFTs with Au was deter-mined from the transfer curve characteristics (satu-ration region) to be 0.0081, which is smaller by quintuple of magnitude than that of the PCBM OTFTs with Cs2CO3/Al as electrodes. Thus, the

increase of the field effect mobilities with decreasing work-function suggests that the contact effect lowers the extrinsic field-effect mobility than the intrinsic value. The summary of parameters for the devices made in this study is given inTable 1.

Although the work-function of Ca is lower than the LUMO level of PCBM, with no energy barrier, and with a thin layer Cs2CO3between PCBM and

electrode, mobility is more than doubled compared to that obtained with the Ca electrodes. It was reported that the lowering of the vacuum level of organic semiconductors was observed with a thin layer of Cs2CO3by an ultraviolet photoelectron

spec-trometer[20,21]. The energy oﬀset between the Fermi level of the electrode and the LUMO of semiconduc-tors at interface is greatly reduced. Therefore, the improvement of the device performance is attributed to Cs2CO3lowering the barrier for electron injection

from Al to the LUMO level of PCBM. Moreover, earlier studies on Cs2CO3 indicated that Cs2CO3

decomposes into cesium oxide during thermal evapo-ration[22]. With a metal oxide as a buﬀer layer, it will consume most of the interface of metal and leave the

Fig. 2. Idealized ISD–VG(solid) and (ISD)1/2–VG(open) plots for

diﬀerent materials as the source/drain contacts at VD=40 V.

Cs2CO3/Al (triangles); Ca/Al (circle); Al (square).

Table 1

Summary of the performance of the PCBM OTFTs with diﬀerent source/drain electrodes materials

S/D electrode l(cm2_/ V 1_s) Vt (V) S (V/ decade) On/oﬀ ratio Au 0.0081 4.87 1.81 103 Al 0.0120 3.15 1.80 106 Ca/Al 0.0227 0.74 1.45 106 Cs2CO3/Al 0.0445 2.30 1.39 106

The values are an average of ten diﬀerent devices with practical standard deviation.

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organic semiconductor intact [23]; therefore, Cs2O

could serve as a protective agent for PCBM against metal-induced degradation and a better interface (absence of reactive Ca), hence further minimizing contact resistance.Fig. 3shows the contact resistance (RC), obtained according to the method in Ref.[24],

(extracted at |VSD| = 4 V and in linear regime) as a

function of the VG with diﬀerent materials as S/D

electrodes. The RC of Ca electrode decreased from

8 108

to 3 108

Xas VGvaried from 10 to 40 V.

In contrast, the RCof Cs2CO3/Al electrode was

insen-sitive with VG, and is decreased more than twofold

compared to the Ca electrode. The change of poten-tial drop at the organic–metal electrode interface due to the variation of RCwill change the distribution

of the electric ﬁeld in the vertical direction, which inﬂuences the VT(the higher the contact resistance,

the larger the VT) [25]. Hence the decrease of RC

due to the introduction of the Cs2CO3in our OTFTs

will lead to the reduction of VT, as can be seen in

Table 1. This result demonstrates that the introduc-tion of the Cs2CO3in our OTFTs played an

impor-tant role in the enhancement of the device performance.

Recently, Li et al. reported that Cs2CO3 is

decomposed to metallic Cs during thermal evapora-tion, as measured with the quartz crystal microbal-ance [26]. However, contrary to the observation of Li et al., we found that the 230-nm-thick Cs2CO3

ﬁlm has a capacitance of about 6.9 nF/cm2 at 1 kHz with good insulator properties. In addition, the pressure in the vacuum chamber had increased rapidly as the k-cell temperature raised above 690°C which we believe was due to the formation of Cs2O and CO2 [23,27]. Since Cs2CO3 layer was

deposited by thermal evaporation, the small amount of metallic Cs might also be introduced into the ﬁlm during thermal evaporation. However, the propor-tion of the Cs within the composite ﬁlm is quite low, and Cs would absorb the residual oxygen in vacuum to form Cs2O; the electrical conductivity

of the composite ﬁlm is still in the insulating region. Hence, as-deposited Cs2CO3ﬁlm is a dielectric with

good insulating properties. With the presence of a thin Cs2CO3layer, the energy barrier for electrons

from Al to PCBM could be greatly lowered by the larger potential drop, resulting in increasing the injection of electrons via tunneling. As the Cs2CO3

becomes thicker, it leads to slowing down of tunnel-ing probability. As can be seen inFig. 4, the transfer characteristics of the devices are dependent on the thickness of Cs2CO3 layer. The device with 1 nm

thick Cs2CO3 exhibits the highest mobility. With

the further incremental thickness of Cs2CO3layer,

mobility has gradually decreased.

In conclusion, we have demonstrated that the performance of PCBM-based n-channel OTFTs can be improved by inserting a thin Cs2CO3 ﬁlm

between the PCBM and S/D electrode. The current and the field-effect mobility were significantly improved, when compared to Ca as S/D electrodes. The improvement in device performance is due to improved electron injection at the interface, result-ing from the narrowed and lowered tunnelresult-ing bar-rier by the insertion of the Cs2CO3 layer. In

addition, it also serves as a protective agent against unfavorable chemical reaction between the organic layer and the metal electrode. Further improvement of the device performance can be achieved by opti-mizing fabrication conditions.

Fig. 3. Contact resistance vs. gate voltage for diﬀerent materials as the source/drain contacts.

Fig. 4. (ISD)1/2–VG plots of OTFTs with a variety of Cs2CO3

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Acknowledgements

Financial support from Academia Sinica, the Na-tional Science Council, ROC (NSC-95-2218-E-001-003) and the Ministry of Economic Aﬀairs, ROC is deeply appreciated.

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