organic tfts: polymers … · the electrons in the p-orbitals are “delocalized” and determine...

Organic TFTs: Polymers

Feng Liu*, Sunzida Ferdous and Alejandro L. BrisenoPolymer Science and Engineering, Conte Research Center, University of Massachusetts, Amherst, MA, USA

Abstract

Polymer semiconductor field-effect transistors are expected to be next-generation devices for use asdrivers in displays and a variety of other technological applications. They are of particular interest becauseof their low-cost fabrication, solution processability, mechanical flexibility, and large-area fabricationcapabilities. An overview on the development of polymer-based organic field-effect transistors (OFETs)is discussed with emphasis on device configurations, choice of materials, and device engineering.

List of Abbreviations

mCP Microcontact printingBBL Poly(benzobisimidazobenophenanthroline)BC/TG Bottom contact/top gateBC/BG Bottom contact/bottom gateBTS BenzyltrichlorosilaneCMOS Complementary metal-oxide-semiconductorDMSO Dimethyl sulfoxideF8T2 Poly(9,90-dioctylfluorene-co-bithiophene)FTS (Tridecafluoro-1,1,2,2-tetrahydrooctyl) trichlorosilaneHMDS HexamethyldisilazaneHOMO Highest occupied molecular orbitalITO Indium tin oxideLUMO Lowest unoccupied molecular orbitalMIMIC Micro-molding in capillariesMTP Metal transfer printingnTP Nano-transfer printingOFET Organic field-effect transistorOTS OctadecyltrichlorosilaneP3HT Poly(3-hexylthiophene)PBTTT Poly(2,5-bis(3-alkylthiophene-2-yl)-thieno[3,2-b]thiophene)PCBM [6,6]-Phenyl C61-butyric acid methyl esterPDMS PolydimethylsiloxanePEDOT/PSS Poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acidPMMA Poly(methylmethacrylate)PQT-12 Poly(3,3000-didodecylquaterthiophene)PS-PMMA-PS Poly(styrene-block-methylmethacrylate-block-styrene)PVP PolyvinylphenolSAM Self-assembled monolayer

*Email: [email protected]

Handbook of Visual Display TechnologyDOI 10.1007/978-3-642-35947-7_51-2# Springer-Verlag Berlin Heidelberg 2015

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SEM Scanning electron micrographTC/BG Top contact/bottom gateVTS 7-Octenyltrichlorosilane

Introduction

Conjugated polymer semiconductors are of high interest due to their broad-range applications in light-emitting diodes, thin-film transistors, and photovoltaics (van Mullekom et al. 2001). For example, inpolythiophene, the backbone consists of alternating single and double carbon–carbon bonds. Thesecarbon atoms are sp2 hybridized where an overlap of p-orbitals creates a pathway for charge transport.The electrons in the p-orbitals are “delocalized” and determine the intrinsic properties of a polymer suchas light absorption, energy levels of frontier orbitals, and charge transport properties (van Mullekomet al. 2001). The chemical and physical properties of a conjugated polymer can be well controlled andfinely tuned by synthetic chemistry, which yields a broad library of materials that can meet specificrequirements for different applications. To achieve next-generation all-plastic electronic devices, onemust have access to high-performance, air-stable semiconductor materials. Polymer semiconductors areexcellent candidates as they possess good solubility, roll-to-roll processability, mechanical flexibility, and,in certain materials, environmental stability. Organic transistors, the very basic elements of circuits, havebeen the focus of intensive investigation over the past two decades (Facchetti 2007). Both p- and n-typepolymers are well developed and complementary logic devices have been recently demonstrated(Facchetti 2007; Bao and Locklin 2007). In recent years, device engineering has improved the perfor-mance of OFETs to a point where their carrier mobilities have surpassed that of amorphous silicon (seeChap. 5.2.1 and (Bao and Locklin 2007)). With the advent of modern technology such as ink-jet printers,polymer semiconductors are now processed over large areas in patterned arrays and employed in printedelectronic applications by several start-up companies.

Device Configuration

The operation of an OFET includes three components (Fig. 1a): a thin semiconductor active layer, adielectric layer, and three electrodes (Zaumseil and Sirringhaus 2007). Two of these electrodes, source anddrain, are in ohmic contact with the active layer and form the conducting channel. The third electrode,gate, modulates the charge carrier density in the conduction channel, and the dielectric layer separates theactive layer from the gate. The source electrode is grounded and serves as the charge-injecting electrode,and the drain serves as the charge-extracting electrode. Voltage is applied to the gate (Vg) and to the drain(Vds). An OFETessentially works as a parallel plate capacitor. For a p-channel device, applying a negativeVg induces positive charges at the dielectric–semiconductor interface and vice versa for an n-channeldevice. The number of induced charges depends on the applied Vg, dielectric constant of the insulator (k)and its thickness.

Figure 1b–d shows different working regimes of an OFET and their corresponding I–V characteristics.When a relatively low Vds is applied compared to Vg, the current flow follows Ohm’s law and isproportional to both Vds and Vg. This is known as the linear regime. As Vds is increased, a point isreached when Vds equals the effective gate voltage (Vg � Vth). This point is called the “pinch-off” point(Fig. 1c), where a depletion region is formed near the drain electrode. The potential drop between thesource and this point becomes constant and the current begins to saturate (Zaumseil and Sirringhaus2007). Any further increase in Vds only increases the depletion width, but does not increase the drain


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current significantly. This is the saturation regime (Fig. 1d). Figure 2a illustrates the output characteristicsof a typical transistor, and Fig. 2b, c shows the transfer characteristics for both linear and saturationregimes, respectively. The mobility in these two regimes can be calculated from the following equations:

IDlin ¼ W

LmCi VG � VTð ÞVD (1)

IDsat ¼ W

2LmCi VG � VTð Þ2 (2)

where W, L, Ci, and m are the channel width, channel length, insulator capacitance, and charge carriermobility, respectively. These equations are based on the assumptions of the gradual channel approxima-tion (electric field normal to the channel created by the Vg is much higher than the electric field parallel tothe channel created by Vds) and constant mobility (Zaumseil and Sirringhaus 2007). Gradual channelapproximation is only valid when the channel length is much larger than the insulator thickness

Source

a

b

c

d

Insulator

SemiconductorDrain

Channel

Channel

Pinch-off point

Pinch-off point

Vg

Id

Vds

Id

Vds

Id

Vds

Vg>VTh

Vg>VTh

Vg>VTh

Vd<<Vg−Vth

Vd,sat=Vg−Vth

Vd>Vd,sat

V(x)=Vg−VTh

Vd=Vg−VTh

Vd

Gate

W

L

Fig. 1 (a) Schematic of a field-effect transistor, (b–d) different operation regimes of an OFET with their corresponding I–Vcurves: (b) linear region, (c) onset of saturation, and (d) saturated region (Reprinted with permission from Zaumseil andSirringhaus (2007) # 2007 Chemical Reviews)


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(L > 10 � dinsulator) (Zaumseil and Sirringhaus 2007; Chua et al. 2004). When L is close to or smallerthan insulator thickness, electric field due to source–drain voltage could dominate the charge distributionin the channel causing the breakdown of the first assumption.

OFETs can have different device structures with respect to the substrate. Figure 3 shows a bottomcontact/top gate (BC/TG), bottom contact/bottom gate (BC/BG), and a top contact/bottom gate (TC/BG).For fabrication reasons, bottom-gate structure is more suitable since organic semiconductors are veryfragile compared to their inorganic counterparts. The devices in Fig. 3a, c are in staggered configuration,whereas the device in Fig. 1b is in the coplanar configuration. The conducting channel in the staggereddevices is separated from the source and drain electrodes by the semiconducting layer, whereas, in thecoplanar device, the edges of the source and drain are in direct contact with the channel. This leads todirect charge injection into the conducting channel in the BC/BG structure. Several parameters such as thesemiconductor–dielectric interface and the interface between the semiconductor and source–drain elec-trodes (contact resistance) will affect the performance of an OFET (Ito et al. 2009; Salleo et al. 2002; Bocket al. 2006).

p-Type Polymer Semiconductors

For polymer OFETs, p-type semiconductor polymers are well developed, and a variety of these polymersare summarized in Fig. 4. Poly(thiophene) (1) was the first polymer to be used in field-effect transistors(Tsumura et al. 1986). Poly(thiophene) has a long chain of thiophene units, and the alternating single- anddouble-bond structure effectively extends the conjugation of this material and provides the conductionchannel for charge to transport. A great deal of work has been done to chemically modify poly(thiophene)to increase solubility and improve the crystallinity. Regiorandom poly(3-hexylthiophene) (P3HT) (2) isregarded as the first solution-processable OFET with mobilities of 10�4�10�5 cm2/Vs (Assadi

BC/TG BC/BG TC/BGS D

G

a b c

D

G

S

S

G

D

Fig. 3 OFET configurations. (a) Bottom contact/top gate, (b) bottom contact/bottom gate, and (c) top contact/bottom gate. S,D, and G stand for source, drain, and gate, respectively (Reprinted with permission from Zaumseil and Sirringhaus (2007)# 2007 Chemical Reviews)

Source-drain voltage (V)

Saturation

Line

ar

Gate voltage (V)

Von VTh

Sub

thre

shol

d

Gate voltage (V)

Saturation regimeLinear regimea b c

Dra

in c

urre

nt (

A)

Dra

in c

urre

nt (

A)

Dra

in c

urre

nt1/

2

log

(Dra

in c

urre

nt (

A))

log

(Dra

in c

urre

nt (

A))

Fig. 2 (a) Typical output characteristics with linear and saturation regimes. (b) Transfer characteristics in linear (Vd < < Vg)regime both as semilog and linear plots. Onset voltage (Von) and subthreshold swing are obtained from the semilog plot. (c)Transfer characteristics in saturation (Vd > Vg � Vth) regime. Threshold voltage (Vth) is obtained from the square-root plot(Reprinted with permission from Zaumseil and Sirringhaus (2007) # 2007 Chemical Reviews)


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et al. 1988). Increasing the regioregularity of P3HT decreases its bandgap and increases its crystallinity.The mobility of regioregular P3HT reached 0.05�0.1 cm2/Vs with on/off ratios of 106 (Bao et al. 1996).Molecular weight and process conditions also have a profound effect on device performance (Baoet al. 1996; Sirringhaus et al. 1999; Kline et al. 2005; Lim et al. 2010; Chang et al. 2004; Hoshinoet al. 2004; Majewski et al. 2006). A major drawback for P3HT is the poor stability. P3HT thin-filmdevices are very sensitive to oxygen and moisture (Bao and Locklin 2007; Hoshino et al. 2004). If thedevice is fabricated and tested in air, the oxidative doping of P3HT thin films will increase the off currentand thus decrease the on/off ratio. This effect can be minimized by increasing the ionization potential orthe highest occupied molecular orbital (HOMO). For poly(thiophene)s, the HOMO could be tuned bychemically modifying the polymer structure or by introducing rotational degrees of freedom or geometrictwists (Bao and Locklin 2007; Ong et al. 2004a; Wu et al. 2005; McCulloch et al. 2005). Poly(3,3000-dialkylquaterthiophene) (PQT-12) (3) is a well-studied air-stable polymer semiconductor basedon this design (Ong et al. 2004b). The rotational freedom of unsubstituted thienylene moieties in PQT-12reduces the conjugation length, and its ionization potential is 0.1 eV higher than that of regioregularP3HT. Bottom-gate, top-contact devices fabricated in air show mobilities up to 0.14 cm2/Vs with on/offratios of 107 (Ong et al. 2004b).

Fused ring thiophene and thioacene building blocks are also proven to increase the stability ofthiophene copolymers (Heeney et al. 2005; McCulloch et al. 2006; Umeda et al. 2009; Li et al. 2006,2008; Fong et al. 2008; He et al. 2009). Poly(2,5-bis(3-alkylthiophene-2-yl)-thieno[3,2-b]thiophene)(PBTTT) (4) is a good example. The ionization potential for PBTTT is 0.3 eV higher than that ofregioregular P3HT (McCulloch et al. 2006). Mobilities as large as 0.6 cm2/Vs and on/off ratios largerthan 106 can be obtained on annealed devices under nitrogen atmosphere (Fig. 5a). In low humidity(�4 %), the devices show good stability with almost no change in performance. The alkyl-substitutedthienothiophene polymer (5) is another well-studied material. The incorporation of the rigidthienothiophene moiety into the polymer increases the ionization potential and results in optimal packingof polymer chains (He et al. 2009). The best device performance exhibited a mobility of 0.33 cm2/Vs withon/off ratios exceeding 105. The device with octyltrichlorosilane-modified gate dielectrics showed nosignificant degradation in output characteristics over nine months when stored at 30 % relative humidity

n

OC12H25

s

s

s

s

s

s

s

s s ss

ss

s

R

R

s

s

n

ss s s s

sN

sss

ss

1 2

5 6

8

11 12

3

R

n

o o

n

m = 1, 2, 3

4

7

13

9 10

s s s ss

n n

n n

nn

n

nC6H13

C10H21 C12H25 C5H13

C12H25

C16H33C16H33

C8H17

C12H25

C5H13C5H13

C8H13C12H25

C12H25C12H25

C8H13

C10H21m

C12H25 C12H25

s

s

s

nR

R = C12H25C14H29C16H33C18H37C20H41

C12H25

R = C10H21

C14H29

C16H33

N

N s

s

s

si

s s ss

C8H13 C8H17 C8H17

C12H25

C12H25O

ss

N

Ns

N

Fig. 4 Polymer semiconductors that exhibit p-type characteristics


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(He et al. 2009). Introducing electron-withdrawing groups to the polymer backbone is another effectivemethod to increase the air stability of thiophene-based polymers (Facchetti 2007; Bao and Locklin 2007;Murphy et al. 2005; Osaka et al. 2007; Kim et al. 2009). A typical example is the introduction of adidodecylbithiazole unit to the backbone of poly(didodecylquaterthiophene) (6). OFET devices showedmuch better improved stability in air with mobilities of 0.33 cm2/Vs (Kim et al. 2009).

Additional examples of high-mobility, high-stability semiconductor polymers (Pan et al. 2006,2007a, b; Rieger et al. 2010; Osaka et al. 2010; Sirringhaus et al. 2000a; Zhang et al. 2007; Liuet al. 2008, 2009; Usta et al. 2006; Lu et al. 2008; Guo et al. 2009; Beaujuge et al. 2009) include poly(4,8-didodecylbenzo[1,2-b:4,5-b0]dithiophene) (7) developed by Ong and coworkers. This particularmaterial yielded mobilities of 0.25 cm2/Vs with on/off ratios of 106 (Pan et al. 2007a, b).A naphthodithiophene polymer (8) increased the ionization potential and enhanced the interchain packingto yield a mobility of 0.54 cm2/Vs with on/off ratios of 107 (Osaka et al. 2010). A hole mobility of0.11 cm2/Vs was measured for a benzothiadiazole–cyclopentadithiophene copolymer (9). The fusedcyclopentadithiophene ring reduced the reorganization energy and enhanced the mobility (Zhanget al. 2007). McCullough and coworkers synthesized n-alkyldithieno[3,2-b:20,30-d]pyrrole (10) to yieldas-cast OFET devices with a mobility of 0.21 cm2/Vs. However, thermal annealing largely reduced thecarrier mobility (Liu et al. 2008, 2009). Marks and coworkers incorporated dithienosilole (11) anddibenzosilole (12) units into thiophene polymers to obtain air-stable transistor devices with mobilitiesof 0.06 and 0.08 cm2/Vs, respectively (Usta et al. 2006; Lu et al. 2008). Jenekhe andWatson incorporated

1 × 10−2a

b c

I sd

(A)

I SD (

µA)

I SD (

A)

8 × 10−3

6 × 10−3

0

154

60V

50V

40V

30V

20V

2

010

0 6

5

00 20 40 60

−10

−6−2

−20

−20 0 20 40

PS

PTBSD2200PMMA

CYTOP

60

−30 −40Vd (V)

VSD (V) VSG (V)

Vg = −60V

Vg = −45V

Vg = −15V

Vg = 0V

Vg = −30V

−50 −60

4 × 10−3

10−4

10−6

10−8

10−10

10−12

2 × 10−3

0 × 100

Fig. 5 (a) Field-effect transistor output characteristics of polymer (4) under N2 atm (W = 10,000 mm and L = 20 mm). (b)Output characteristics of polymer (18) with a PMMA-based dielectric insulator. The inset shows the linear I–V characteristicsat low voltages. (c) Overlay of transfer characteristics of (18) with various polymer dielectric layers (Reprinted with permissionfrom McCulloch et al. (2006) # 2006 Nature Materials, and Yan et al. (2009) # 2009 Nature)


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phthalimide units into thiophene polymers (13), and an air-stable mobility of 0.28 cm2/Vs was measured(Guo et al. 2009).

n-Type and Ambipolar Polymer Semiconductors

Compared to p-type polymers, the development of n-type and ambipolar polymer semiconductors inOFET applications has remained far behind. Figure 6 shows a series of n-type polymer semiconductorsrecently employed in OFET and ambipolar applications (Usta et al. 2009; Chua et al. 2005; Babel andJenekhe 2003; Briseno et al. 2008; Letizia et al. 2008; H€uttner et al. 2008; Zhan et al. 2007; Chenet al. 2009, 2010; Yan et al. 2009; Yoon et al. 2006; Cheng et al. 2009; Steckler et al. 2009; Kimet al. 2010). The design of air-stable n-type polymers requires low-lying LUMO energies which enhancedevice stability by energetically stabilizing the induced electrons during charge transport (Ustaet al. 2009). Moreover, the semiconductor–dielectric interface is of particular importance since traps atthe interface could largely reduce the current through the conducting channel (Chua et al. 2005). Poly(benzobisimidazobenophenanthroline) (BBL) (14) is the first few well-studied n-type polymers with highelectron mobility up to 0.1 cm2/Vs (Babel and Jenekhe 2003). The rich nitrogen and oxygen heteroatomsin BBL gave rise to good electron-accepting properties and a low electron affinity of 4.0–4.4 eV (Babeland Jenekhe 2003; Briseno et al. 2008). Marks and coworkers developed n-alkyl-2,20-bithiophene-3,30-dicarboximide (15) that showed n-channel electron mobilities up to 0.01 cm2/Vs (Letiziaet al. 2008). It was recently discovered that perylene bisimide building blocks work well in n-typematerials. For example, Marder and coworkers synthesized a low-bandgap dithienothiophene (16)

N

N

N OO

N

O

C10H21

C10H21

C8H17

C8H17

OO

O

N

14

17 18

19

22

R = C12H25

= C8H13

n

n/2

N

N

N

O O

N

N

C10H21

C8H17

O

N On

O

O

C10H21

C10H21C10H21

C8H17

SS

N

N OO

C10H21

C10H21

C12H25

C12H25

OC12H25

OC12H25C12H25O

OO

16

n

S S

S

S S

N

S S

S

Se Se Se n

R

S

SS

N

2120

C4H9

C2H5

C12H25O

OC12H25

O

N On

O

O

C4H9

C2H5

SS

N

C12H25 C12H25R

NC

NC

CN

CN

S

S

O OC6H17

C10H2115

n

S

S

n

S

nNN

SNN

N

Fig. 6 Polymer semiconductors that exhibit n-type characteristics


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copolymer with an electron mobility of 1.3 � 10�2 cm2/Vs under inert gas (Zhan et al. 2007). Facchettiand coworkers reported perylenedicarboximide and naphthalenedicarboximide-based polymers (17) witha mobility of 0.002 cm2/Vs under vacuum, while the naphthalenedicarboximide polymer (18) showed amobility of 0.06 cm2/Vs (Chen et al. 2009). Their subsequent work showed that the electron mobility ofnaphthalenedicarboximide polymer could reach up to 0.85 cm2/Vs under ambient conditions in combi-nation with Au contacts and various polymer dielectrics. The device performance of this polymer isshown in Fig. 5b, c (Yan et al. 2009).

For ambipolar polymer OFETs, the device structure and material design are still not as well developedas those of small-molecule devices (see Chaps. 5.3.1 and 5.3.2 and (Bao and Locklin 2007; Zaumseil andSirringhaus 2007; Yoon et al. 2006). It is difficult to efficiently inject both electrons and holes from oneelectrode such as gold due to the injection barriers for at least one type of carrier. Sirringhaus andcoworkers demonstrated that the electron and hole contact resistance in ambipolar OFETs could becontrolled by using thiol-based self-assembled monolayers (SAMs). By employing specific SAMs, theinjection of both electrons and holes can be achieved (Cheng et al. 2009). Marks and coworkers finelytuned the HOMO–LUMO energy levels of a family of ladder-type polymers which resulted in ambipolartransport (19) (Usta et al. 2009). Several additional polymers such as donor–acceptor polymers (20),naphthalenedicarboximide copolymer (21), and polyselenophene (22) also showed interesting ambipolartransport properties (Steckler et al. 2009; Chen et al. 2010; Kim et al. 2010).

Complementary Circuits and Inverters

In order for p- and n-channel polymer OFETs and complementary inverters to be useful in real-worldelectronic applications, they must exhibit long-term air stability and endure the effects of atmosphericcontaminants such as oxygen and moisture (Bao and Locklin 2007; Hoshino et al. 2004). Althoughseveral reports have demonstrated complex all-polymer circuits, the devices from these reports wereexclusively fabricated from p-type polymer semiconductors in unipolar operation (Sirringhauset al. 2000b; Krumm et al. 2004; Brown et al. 1995). The advantage of having a truly complementarysystem is low static power dissipation, better noise margins, a more robust operation, and the ease of usingthem to design highly sophisticated circuits (Zaumseil and Sirringhaus 2007). A conducting substrate(i.e., ITO, gold, etc.) serves as the gate electrode and functions as the input node (Vin). The supply voltage(Vdd) is provided by the source of the load transistor, and the source of the driver is grounded. Outputvoltage (Vout) is given by the drain electrodes from both load and driver. A circuit diagram describing aninverter is shown in Fig. 7a. Two approaches are usually taken to make an organic inverter: using a p- andn-type material or using ambipolar materials on a common substrate. Ambipolar polymer (21) mentionedabove has been used (Fig. 7b) as a complementary inverter by electrically connecting it to both load anddriver mode on a common substrate with a voltage gain of 30 (Kim et al. 2010). Their results illustrate theimportance of utilizing high-mobility ambipolar transistors for use in complementary circuits. Ambipolaroutput curves and static transfer characteristics of this donor–acceptor copolymer are shown in Fig. 7c, d,respectively. Depending on the polarity of the supply voltage, it is possible to observe well-definedvoltage-transfer characteristics in the first and third quadrant of the output versus input diagram (Fig. 7d).Complementary inverters were also previously demonstrated using small-molecule organic semiconduc-tors (Briseno et al. 2007). The ideal transfer characteristics should show symmetrical gate thresholdswitching (Vm) at nearly half of the supply voltage (Vm = Vdd/2). The symmetry is a result of equallymatched mobilities and threshold voltages of the p- and n-channel transistors of the inverter. This pointstresses the importance of having polymers that exhibit similar mobilities and electrical characteristics.


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The switching thresholds of the inverters can also be graphically estimated at the intersection of thetransition region from the transfer curves as shown in Fig. 7d.

Other examples of polymeric complementary inverters include solution-processable P3HT and n-typenaphthalenedicarboximide polymer (18) with gains as high as 65 (Yan et al. 2009). Ambipolar transistors,based on [6, 6]-phenyl C61-butyric acid methyl ester (PCBM) and poly[2-methoxy-5-(30,70-dimethyl-octyloxy)]-p-phenylene vinylene blends, have also been reported to show CMOS inverter behavior withhigh gain (Meijer et al. 2003).

Vsupply

Vds (V)

Vdd (V)

Vin (V)

Vou

t(V

)

I ds(

µA)

I ds(

µA)Vin

Vout

load

−80

−0.0

−0.5

−1.0

−1.5

−2.0

−2.5

−60 −40 −20 0 20

PNIBT

40 60 80

Vdc

Gain

VinVout

−80

−80

−100

−100−100

100

−50

50

−60

−60

−40

−40

−20

−20

0

0

00

10

20

30

30

20

10

00

Gain

20

20

40

40

60

60

80

±100±90±80±70±60

80

100

100

0

5

10

15

20

2580 V

60 V

40 V

driver

H9C4

C2H5

C2H5

C4H9

H25C12

OC12H25

O

O

O

O

n

O

S S

N

N

a

b

c

d

Fig. 7 Chemical structure of copolymer along with its device characteristics. (a) Circuit diagram of an inverter, (b) chemicalstructure of PNIBT ambipolar semiconductor, (c) ambipolar output characteristics showing good gate modulation, and (d)static inverter transfer characteristics and the corresponding voltage gain (Reprinted with permission from Kim et al. (2010)# 2010 Advanced Materials)


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Dielectrics and Interfaces

Polymer transistors intrinsically suffer from low mobility and stability issues in ambient conditions. Oneof the main challenges is to achieve high mobility with a low operating voltage. Molecular orderingthrough self-assembly is a key factor for high mobility in polymer OFETs. Apart from the moleculardesign of semiconductor materials, the interface between the dielectric and active layers plays animportant role in device performance. An ideal dielectric film needs to have low trap density at thesemiconductor–dielectric interface, low surface roughness, low impurity concentration, and compatibilitywith organic semiconductors. Both inorganic and organic materials have been used as dielectric layers.For instance, SiO2 and SiNx are commonly used inorganic insulators with thicknesses ranging from 100 to300 nm, and polyvinylphenol (PVP) and poly(methylmethacrylate) (PMMA) are widely used solution-processable organic dielectrics among many others.

Efforts have been applied to achieve a high drain current with a low operating voltage by increasing theinsulator capacitance (Ci). The use of high-k dielectric, polymer–TiO2 composites, and ion-gel electro-lytes is just one of a few examples (Wang et al. 2004; Lee et al. 2003a; Tate et al. 2000; Maliakalet al. 2005; Panzer and Frisbie 2008; Cho et al. 2008). OFETs with high dielectric capacitance allowhigher charge injection into the semiconductor layer at a given gate voltage, and therefore, the device canturn on at lower voltage. Frisbie and coworkers demonstrated low operating voltage (Fig. 8b, d) printedOFETs with a triblock copolymer ion-gel as a gate dielectric for P3HT, PQT-12, and poly-9,90 dioctyl-fluorene-co-bithiophene (F8T2) semiconductors (Cho et al. 2008). The ion-gel polymer dielectric layer(poly(styrene-block-methylmethacrylate-block-styrene) (PS-PMMA-PS) dissolved into an ionic liquid,1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI])), has a much higherspecific capacitance (1 to �20 mF cm�2 depending on the frequency) compared to other solution-processable polymer dielectrics (�0.02 mF cm�2). This high capacitance is attributed to the formationof electrical double layers at the electrolyte–electrode interface. One of the benefits of ion-gel dielectric isthat due to their high polarizability, the gate electrode registration does not have to be extremely preciseand can be physically offset instead of placing directly on top (for top gate) or bottom (for bottom gate) ofthe source–drain channel (Fig. 8c). OFETs with both aligned and offset gate electrodes show very similarcurrent–voltage characteristics (Fig. 8b, d) showing the importance of ion-gel as a promising dielectricmaterial for printed electronics.

Many research groups have also investigated the effects of chemical modification of the dielectric layerprior to the deposition of the semiconductor layers (Ito et al. 2009; Salleo et al. 2002; Yoon et al. 2006).Salleo reported that a self-assembled monolayer of octadecyltrichlorosilane (OTS) on the oxide layeryielded improved mobilities of 0.015 cm2/Vs for F8T2 among the other silanes studied(7-octenyltrichlorosilane (VTS), (tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane (FTS), andbenzyltrichlorosilane (BTS)) and hexamethyldisilazane (HMDS) (Salleo et al. 2002). They have foundthat the carrier mobility in polymer OFETs does not solely depend on the dielectric contamination (i.e.,silanol groups) by showing the non-monotonic dependence of mobility on dielectric surface energy(Table 1). FTS-treated surfaces show the lowest surface energy, while BTS show the highest; however,highest mobility was observed on OTS-treated surfaces and not on FTS-treated substrates. Mobility isrelated to specific SAM structure in addition to the surface energy. Longer alkane chains in OTS orbenzene rings in BTS can interact with similar features in F8T2, enhancing the performance of the OFET.Low-voltage operating polymer transistors have also been demonstrated using only SAM as the gatedielectric (Park et al. 2005) as well as spin-coated polymer dielectrics (Yoon et al. 2005) for P3HT.Solution-processable polymer dielectrics are of particular interests because of their low fabrication costcompared to thermally grown inorganic oxides and their compatibility toward all-polymer printed


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electronics. For top-gate architecture, dielectric and its processing solvent limit the device performance,while for a bottom-gate architecture, dielectric–semiconductor interfaces play important roles.

The interface between source–drain and semiconductor is also important for better charge injection intothe active layer. These interfaces require low contact resistance which occurs from parasitic resistance andthe energy barrier between electrode and HOMO level of a p-type semiconductor (LUMO for n-type).

800 µm

20 µm

PEDOT:PSS

a b

c d

lon gel

lon gel

G

DS

lon gel

P3HT

P3HT

P3HT

Gate

Drain

Polyimide substrate

Polyimide substrate

Reverse

Source

Forward

VG (V)

Au Au

10−2

10−3

10−4

10−5

10−6

10−7

10−8

−3−I

D (

A)

−2 −1 0 1 2

10−9

Reverse

Forward

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10−2

10−3

10−4

10−5

10−6

10−7

10−8

−3

−ID (

A)

−2 −1 0 1 2

10−9

Fig. 8 (a) Optical image of an aerosol-printed ion-gel top-gated OFET array on a plastic substrate with channel lengths of20 mm andwidths of 1,400 mm, (b) schematic cross section of a top-gated ion-gel OFETwith aligned gate electrode and transfercurve for P3HT-based transistor showing very low operating voltage with Vd = �1 V, (c) a schematic cross section of amisaligned gate electrode relative to the channel area by 60 mm and optical image of the corresponding device, and (d) transfercharacteristics of the displaced gate OFETwith 20 mm channel length and 1,400 mm channel width (Reprinted with permissionfrom Cho et al. (2008) # 2008 Nature Materials)


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Au and Pt are commonly used electrodes due to their environmental stability. Treatment of Au contactswith a SAM of alkanethiol and aromatic thiols has been investigated for small-molecule OFETs. Aromaticthiol treatment showed better performance with good charge injection and high on/off ratios because of itshigher conductivity relative to aliphatic thiols (Bock et al. 2006). Organic conductive materials such aspoly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid (PEDOT/PSS), a high workfunction water-based organic ink, have also been used as electrodes. Efforts have been made to improvetheir conductivity and reduce contact resistance by adding high dielectric solvent or Ag nanoparticles(Lim et al. 2006; Sholin et al. 2008). Addition of dimethyl sulfoxide (DMSO) solvent to this ink hasshown to improve the conductivity as well as the interfacial contact between P3HT and the printed ink.These results demonstrate the significance of device engineering in terms of the governing parametersmentioned above.

Device Patterning

For commercial application of OFETs, they must be fabricated into large-area devices at relatively lowcosts through patterning on cheaper substrates. Patterning OFETs reduces the off current by preventingcross talk between devices and improves transconductance. In this section, we discuss the fabrication ofOFETs by printing methods rather than small-scale drop-cast or spin-coat methods. Solution process-ability of organic materials is the key factor for device pattering by printing methods. Large-areapatterning has been achieved by many printing techniques such as microcontact printing (mCP), ink-jetprinting, screen printing, and gravure printing. Microcontact printing (mCP) uses a patterned polymerstamp (i.e., polydimethylsiloxane (PDMS)) to selectively transfer a patterned organothiol or organosilaneonto a substrate (i.e., Si/SiO2/Au or Si/SiO2). Subsequently, three different ways can be followed forpatterning: etching (Wilbur et al. 1994; Xia et al. 1996a, b), electroless plating (Lee et al. 2003b; Huanget al. 1997; Zschieschang et al. 2003), and area-selective electropolymerization (Gorman et al. 1995).Other soft-lithographic techniques based on mCP include metal transfer printing (MTP), micro-molding incapillaries (MIMIC), and nano-transfer printing (nTP). Patterning in the range of 1–10 mm is achievablewith these techniques (Parashkov et al. 2005).

Table 1 Contact angle measurements of water, xylene, and F8T2 solution on SiO2 substrates with different treatments as wellas mobility and on/off values of OFETs on substrates with different treatments (Reprinted with permission from Salleoet al. (2002) # 2002 Applied Physics Letters)

Contact angle Spun-cast films Drop-cast films

Water Xylene

0.5 wt%F8T2in xylene

Mobility(cm2/Vs) Ion/Ioff

Mobility(cm2/Vs) Ion/Ioff

No treatment <10� <10� <10� (7 � 3) � 10�4 104 (1.7 � 0.3) � 10�4 400

HMDS 77� � 3� <10� <10� (9 � 2) � 10�4 104 NA NA

VTS 87� � 3� <10� <10� (7 � 1) � 10�4 104 (7 � 1) � 10�4 3 � 103

FTS 100� � 3� 57� � 3� 50� � 3� (3.5 � 0.5) � 10�3 105 (2 � 1) � 10�4 103

BTS 49� � 3� <10� <10� (5 � 1) � 10�3 104 (3 � 1) � 10�3 104

OTS 95� � 3� 25� � 3� 28� � 3� (1.5 � 0.5) � 10�2 104 (5.1 � 0.5) � 10�3 104


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Ink-jet printing has earned considerable attention for pattering of electrodes and semiconductingmaterials (Sirringhaus et al. 2000b; Arias et al. 2004; Paul et al. 2003). This is an attractive method dueto its low cost and compatibility with flexible substrates and roll-to-roll printing methods. Two commonlyused printhead technology are piezoelectric and acoustic (Parashkov et al. 2005). The governing param-eters of this method are the ink viscosity and surface energy of solid–liquid, liquid–vapor, and solid–vaporinterfaces. Sirringhaus reported the first use of ink-jet printing in OFET fabrication by patterningelectrodes (Sirringhaus et al. 2000b). Due to the low resolution (20–50 mm) of the ink-jet printing, thespreading of PEDOT/PSS was controlled by creating hydrophilic and hydrophobic regions withpolyimide stripes. Different printing techniques can be combined to fabricate different layers of anOFET; however, it is much more cost effective if one single method can be used for all layers. Subtractiveand additive ink-jet printing has been used to fabricate all layers of a polymer OFET using PQT-12 as theactive layer (Arias et al. 2004).

Salleo and Arias have used jet-printed wax mask lithography for producing lateral contrast in surfaceenergy by OTS (Fig. 9) (Salleo and Arias 2007). They have demonstrated vertical and lateral phaseseparation of PQT-12 and PMMA from a single solution by spin coating on a pre-patterned substrate.PQT-12 is selectively phase-separated on the OTS region (Fig. 9c, d) with PMMA layer on top.Subsequently, they used this technique to OTS pattern the channel region of a predefined transistorarray with isolated devices (Fig. 9e). Top PMMA layer improved device stability (no change in transfercharacteristics upon exposure to air for 48 h) by acting as an encapsulation layer. Figure 9f compares thedevice performance of the self-assembled semiconductor/insulator on an OTS-patterned andnon-patterned substrate. The off current is reduced by several orders of magnitude for the patternedOFETs.

Even though there are many suitable printing methods, each has their advantages and disadvantagesregarding registration, process temperature, and compatibility with the electronic material. Due to thedifference in achievable resolution, different techniques can be used to process different layers of anOFET. Overall, the key is to have solution-processable and environmentally stable electronic materialsthat can be deposited and patterned by printing techniques more cost effectively without compromisingdevice performance.

Conclusion and Future Outlook

A fundamental challenge facing p- and n-type polymer semiconductors is to achieve high mobilities andlong-term air stability in OFET devices. Electron-transporting polymer semiconductors are essential fordeveloping n-channel transistors and all-polymer complementary integrated circuits for logic, memory,and other complex functions. Fortunately, there have been some major breakthroughs in synthesis anddevice engineering to demonstrate that the future of polymer semiconductors looks promising. Mobilitiesas large as 1.0 cm2/Vs for p-type polymers and 0.85 cm2/Vs for n-type polymers have been successfullyreported. Understanding the mechanism of non-ideal behaviors of OFETs such as bias stress (shifting ofthreshold voltage) will further assist in designing optimal organic electronic circuits. Likewise, designingmaterials that are environmentally stable, along with care in fabrication process and proper encapsulationlayer, is essential for commercialization of polymer OFETs. It is clear that polymer semiconductors willdefinitely have a place in the future of consumer electronics, especially with the advent of high-throughput, roll-to-roll processing methods such as ink-jet printing.


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The outlook for polymer OFET research will be twofold. First, new air-stable, high-mobility polymersof both p- type and n-type materials still need to be improved. Second, in device engineering, optimizingthe dielectric–semiconductor interface to achieve low-threshold, high-stability devices is absolutelyimperative. The integration of OFET elements into high-level complementary logic devices on plasticsubstrates by patterning methods is another interesting direction in device engineering.

300 µma d

e

f

b

c

10 µm

Sio2 PQT-12

wax etch mask

pxl contact

gate line

data

line

300 µm

300 µm 10−7

10−9

10−11

10−13

−30 −20 −10 0VG (V)

Patterned

Cur

rent

(A

)

UnPatterned

10

Fig. 9 Patterning of PQT-12/PMMA blend on thermally grown SiO2 surface and patterning of transistor arrays. (a) Opticalimage of jet-printed waxmask to pattern OTS. (b) Spin-coated dry film on patterned OTSwhere underlying PQT-12 selectivelyself-assembles on OTS region. One of the patterned areas is highlighted in gray. (c) Optical image of patterned PQT-12 afterremoving the top PMMA layer with hot toluene. Inset shows a scanning electron micrograph (SEM), where dark arearepresents PQT-12. (d) Magnified SEM showing sharp boundaries between PQT-12 and SiO2 surface. (e) Patterning ofOTS in the channel region of OFET array. (f) Transfer characteristics comparing patterned and non-patterned deviceperformance (Reprinted with permission from Salleo and Arias (2007) # 2007 Advanced Materials)


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Acknowledgments The authors acknowledge partial support by the NSF Materials Research Scienceand Engineering Center on Polymers (DMR-0820506), the Center for Hierarchal Manufacturing (CMMI-0531171), and the Energy Frontier Research Center funded by the US Department of Energy(DE-SC000108).

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organic tfts: polymers … · the electrons in the p-orbitals are “delocalized” and determine...

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