Specificity and Limits of Organic-Based Electronic Devices - ACS

Chapter 28

Specificity and Limits of Organic-Based Electronic Devices

Downloaded by NORTH CAROLINA STATE UNIV on October 8, 2012 | http://pubs.acs.org Publication Date: September 1, 1997 | doi: 10.1021/bk-1997-0672.ch028

Francis Garnier Laboratoire des Matériaux Moléculaires, Centre National de la Recherche Scientifique, 2 rue Henri-Dunant, 94320 Thiais, France

Various organic conjugated materials have been up to now proposed as active semiconducting layers in thin film transistors, based on conjugated polymers or on shorter conjugated oligomers and small π-electron rich molecules. Mode of operation of these devices shows that a high carrier mobility together with a low conductivity are required for their figure of merit. Experimental results from the literature show that as-grown conjugated polymers and other amorphous materials exhibit a low carrier mobility, of the order of 10 to 10 cm V s . All attempts to increase the mobility through slight doping of the organic semiconductor have failed due to a from the variable range hopping mechanism which describes charge transport in these materials. On the other hand, conjugated oligomers are well defined materials offering various physical and chemical ways for a control of the structural organization of thin films made out of them. It is thus shown that carrier mobility is directly related to the long range structural order in these films, i.e. to the decrease of grain boundaries, leading to values close to 10 cm V s which is comparable to that of amorphous hydrogenated silicon. Additionally, conductivity in thin films of conjugated oligomers is mainly determined by the purity of the materials, allowing values lower than 10 Scm . This independent control of mobility and conductivity allows the realization of oligomer-based thin film transistors showing characteristics close to those of classical a-Si:H based ones. -3

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© 1997 American Chemical Society

In Photonic and Optoelectronic Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

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Organic semiconductors have been studied in the literature since the early 1950s (1), but they raised a steady increasing interest since conjugated materials emerged in the literature in the late 1970s. As-synthesized, these materials possess a semiconducting state, with a conductivity in the range of 10" to 10" Scnr , which is generally p-type, although ion implantation, such as L i in polyacetylene, also allows realization of ntype semiconduction. Although the first conjugated polymer synthesized, polyacetylene, showed low stability in air and very poor processability, new classes of environmentally stable conjugated polymers were soon proposed, such as polythiophenes, polyaniline, and poly(phenylenevinylene) (2,3). Their processability was largely improved by the use of soluble polymer precursors, or by the grafting of solubilizing alkyl or alkoxy groups. But, due to the very low control of the polymerization reaction, these conjugated polymers mainly exist in an amorphous state, showing large distribution of conjugation lengths, together with significant concentration of chemical impurities and structural defects. Later, in the mid 1980s, conjugated oligomers were proposed as a new class of better defined materials, which raised a steady increasing interest both as model compounds for their parent polymers, and also for their promissing electrical and optoelectrical properties in their own right (41 9

6

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+

These semiconductors have been studied through the characterization of various devices, such as thin film transistors, TFTs, and light emitting diodes, LEDs, with a long term goal toward the development of a new area of organic-based electronics (516). Among these electronic devices, TFTs operate in the most simple way, involving only charge injection and transport in a thin semiconductor layer. These devices are thus well adapted for analyzing and discussing the basic charge transport properties of these organic semiconductors. MODE OF OPERATION OF TFTs Various types of substrates have been proposed for the construction of TFTs, with the first ones involving a highly doped silicon wafer acting as the gate electrode, on which an S1O2 layer formed the insulator, as shown in Figure 1 (6-9). Other materials have been later used as substrate, such as glass, on which a gate electrode (Ag, Al) was vacuum evaporated, followed by the deposition of a thin insulating layer. The source, S, and drain, D, metal electrodes were then patterned on the insulator, using either conventional lithography techniques, or vacuum evaporation through a mask. These electrodes are generally made out of gold, in order to build an ohmic contact with the organic p-type semiconductor, whereas silver appears better fitted in the case of an n-



422

PHOTONIC AND OPTOELECTRONIC P O L Y M E R S

Figure 1. Schematic view of an organic-based Thin Film Transistor.



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type semiconductor. The organic semiconductor was deposited in a last step, leading to a bottom gated planar device structure. In the case of most conjugated polymers, due to their low processability, thin films were obtained through the electropolymerization of a monomer, e.g., thiophene, or through the deposition of a soluble polymer precursor. Soluble conjugated polymers, obtained by the grafting of long alkyl chains, e.g., poly(3-alkyl)thiophenes, could be easily spin-casted (8,17). Another interesting route for the construction of TFTs has been based on the use of the Langmuir-Blodgett technique (11) In the case of conjugated oligomers, phthalocyanines and other small molecules, the most convenient and rational route toward highly pure and defect free semiconducting films involved vacuum deposition from the semiconductor powder in a tungsten boat, heated to the semiconductor melting point (10,12). The potentially interesting feature of an organic-based device originates from the fact that all its elements, including substrate (polycarbonate, polyimide,...), insulator (polymethylmethacrylate, PMMA, or polyimide), semiconductor, and even electrodes (conducting ink) can be realized out of organic materials, which require only low processing temperatures, thus opening interesting perspectives for flexible organic devices (13,18,19). These insulated-gate thin film transistors operate as unipolar devices, in which the majority positive charge carriers, in the case of a p-type semiconductor, are attracted to the semiconductor-insulator interface, through negative polarization of the gate electrode, forming a conducting channel through which current flows between source and drain electrodes. The geometrical parameters defining a TFT are the channel width, W, the channel length, L, and the capacitance per unit area of the insulator, Ci. Two main regimes are observed when plotting the drain current as function of drain voltage, Vd, at constant gate voltage, Vg. A linear regime, observed for low drain voltage, followed by a saturation regime, when the drain voltage exceeds the gate voltage. The variation of drain current, Id, with applied gate voltage Vg and drain voltage Vd , is given by equation (1) (20): 2

2

Id = ( W / L ^ Q [(V - V ) - (1/2) V ] g

T

d

(1)

where μ is the field-effect carrier mobility, and V T , the threshold voltage. For higher values of Vd, the saturation regime is described by equation (2) Id,sat. = ( W / 2 L ^ C i ( V - V ) g

2

T

(2)

Whereas an accumulation regime, equations (1, 2), has been clearly identified under negative gate bias for p-type organic semiconductors, the occurence of a In Photonic and Optoelectronic Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

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depletion regime under positive gate bias has been seldom characterized. The thickness of this depletion layer, W c, varies following equation (3): S

1 2

Wsc = (e/Ci)[(l + 2Q2(V - VfbViqNes) / - 1]

(3)

g

where e is the dielectric constant of the semiconductor, Vfb the flat band potential and Ν the dopant concentration. Interestingly, the thickness of the depletion layer, W c. increases up to the total thickness of the semiconductor layer, d, which is reached at a "pinch-off voltage Vp given by equation (4): s


S

V = (qNd2/2£ e )(l + 2C /Ci) p

0

s

(4)

S

where Cs is the dielectric capacitance of the semiconducting layer. Equation (4) is particularly interesting, as it shows that the dopant concentration, N, can be obtained by the determination of the depletion pinch-off voltage Vp. Gate Voltage V (V) g

Drain Voltage V ( V ) d

Figure 2. Experimental output Id = f(Vd) obtained with organic TFT (on glass substrate) with polymethylmethacrylate insulating layer (Ci = 10 nF), sexithiophene semiconducting layer (25 nm thick), and gold source and drain electrodes (W = 5 mm, L = 50 μπι). Experimental output curves, Id = f(Vd), obtained with sexithiophene-based TFTs in the accumulation regime, as shown in Figure 2, confirm the relevance of organic-based TFTs (15). Besides the obtained drain current, a critical characteristic of a TFT device concerns the dynamic range, or IonAoff ratio, expressed by equation (5), which must be as high as possible, and exceed some 10 for practical applications: 7

Ion/Ioff = l + C i V 0 x / 2 a d ) d


(5)

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These equations clearly define the relevance of using a semiconductor in TFT devices. Equations (1,2) show that a high carrier mobility value μ is indeed needed for reaching a high current (ca. μΑ) output in the "on" position of the device, as well as for obtaining short switching times. However, equation (5) also shows that a high fieldeffect mobility is not sufficient, and that a simultaneous low value of the conductivity σ is also required for reaching a high dynamic ratio Ion/Ioff. In fact, when considering amorphous hydrogenated silicon, it must be remembered that its mobility reaches 1 c m V s , and that its conductivity is very low, around 10" Scnr , which ensures a high dynamic Ion/Ioff ratio, larger than 10 , for a-Si:H based devices. 2



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CONJUGATED MATERIALS USED IN ORGANIC-BASED TFTs. Conjugated materials can be grouped into two main classes: (1) macromolecular and amorphous, i.e. conjugated polymers; and (2) molecular, i.e. conjugated oligomers and other π-electron rich molecules. The most studied conjugated polymers include the following: Polyacetylene

dT\l

Polythiophcnc

Pdy(3-alkylthiophene)

Conjugated oligomers, e.g., pentacene and oligothiophenes, have been more recently used as active layers in TFTs, either unsubstituted or alkyl substituted, as side or end groups (9,10,12,1321). Unsubstituted oligothiophenes, from terthiophene 3T to octithiophene 8T, and α,ω-dialkyl substituted oligothiophenes, from dialkylterthiophene to dialkyloctithiophene, have been deposited by vacuum evaporation. Other classes of π-electron rich molecules have been studied during the

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H

Cn 2n+l


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last ten years, such as scandium, lutetium and thulium diphthalocyanines, nickel and zinc phthalocyanines (23), fullerene (24), and tetracyanoquinodimethane, TCNQ (14), which were deposited by vacuum evaporation. These organic semiconductors present charateristics distinct from inorganic covalent semiconductors. Inorganic semiconductors, e.g., silicon, have a three dimensional architecture, in which atoms are held together by strong covalent bonds, with energies on the order of 76 kcal mole" such as in the case of Si—Si bonds. Semiconductivity appears then as a collective property, which develops with the constitution of the 3D architecture of the material. Owing to the strong interatomic bonds, the width of the conduction and valence bands is large, from which one may expect significant carrier mobility values. These materials are highly sensitive to chemical impurities and to surface states, owing to the presence of dangling bonds at their surface. On the other hand, organic molecular semiconductors are made out of molecules, which are only held together by weak van der Waal forces of about 10 kcal mole . The electronic properties of the solid are already present in the individual molecules, as shown for instance by the similarity of the absorption spectra of individual molecules and of their assembly in the solid state. These features indicate that charge transport in molecular solids operates mainly through individual states. Furthermore, the width of the valence and conduction bands is small, which suggests that carrier mobility should be much lower in these solids. Finally, these organic molecular materials are less sensitive to chemical impurities, undergoing much fewer substitutions, and the essential absence of dangling bonds prevents them from being highly sensitive to surface states. These considerations show that organic molecular materials cannot be expected to present as high carrier mobilities as those of their inorganic counterparts, which reach 10 cm V" s- for monocrystalline silicon. (20) On the other hand, it has been shown that carrier mobility in monocrystalline condensed aromatic hydrocarbons reaches values in the range of 1 to 10 cm V~ s at room temperature (25). Thus, owing to the ease of realizing highly structured films of organic molecular compounds, one can reasonably hope that organic semiconducting films will be able to reach mobility values in the range of 10" to 1 crr^V-V , close to those shown by a-Si:H, opening a potentially interesting field of organic-based devices.


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DEVICE CHARACTERISTICS. Electrical characteristics obtained in the literature will be discussed in terms of the two classes of conjugated materials defined previously. The main data obtained with conjugated polymers and other amorphous materials are listed in Table I. These semiconductors generally behave as p-type, unless quoted as otherwise. The range of values, given for some compounds, indicates that various experimental attempts have In Photonic and Optoelectronic Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

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been carried out for improving their electrical properties. The lowest values of conductivity and mobility refers to as-deposited semiconducting films, which indicate that as-prepared semiconductors possess very low mobilities, in the range of 10 cm V- s to K H c n ^ V - U . This is believed to be a result of poor efficiency of charge hopping in highly disordered materials, originating from self-localization and defects. In order to improve the field-effect mobility of these organic semiconductors, their doping level has been intentionally increased, either electrochemically in the case of electropoiymerized polythiophene (6), or chemically as in the case of poly (3alkylthiophene) (8), poly(DOT)3 (17), Ceo (24), and TCNQ (14) . Some other attempts have been based on the "annealing" of the semiconducting film, often realized under oxygen atmosphere (23). -8


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Table I. Electrical characteristics of conjugated polymers and amorphous materials Material

Polyacetylene Polyacetylene Polythiophene Polyalkylthiophene Poly(DOT)3 Polythienylenevinylene Lu, Tm diphthalocyanine Fullerene (n-type) TCNQ (n-type)

Conductivity (S/cm)

Polymer. Precursor Electropoly. Spin coating

10-5

10-5

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1

ίο10-6 to io-5 10-8 to 10-5 ΙΟ" to ΙΟ"

10-6 to 10-5 10-8 to 10-5 Spin coating ΙΟ" to ΙΟ" Precursor 10-6 to ΙΟ" 8

Ref.

Mobility (cn^v^s- )

Deposition Technique

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22

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23

4

10-5 to ΙΟ"

Vac. evap.

10- to 10-3

ΙΟ" to ΙΟ"

Vac. evap. Vac. evap.

10-8 to 10-5 10- to 10"

10-5 to ΙΟ"

4

4

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2

lO-^tolO"

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5 7 6 8 17

4

24 14

The results listed in Table I confirm that a significant increase of field-effect mobility is indeed obtained by intentional doping by almost two orders of magnitude. This behavior also establishes the existence of a relationship between intentional doping and carrier mobility (6,8,1723). However, the data on Table I also indicate that simultaneously a very large enhancement is observed in the conductivity, which is increased by a factor of 10 to 10 . This behavior can be easily understood on the basis of the mechanism of charge transport in these materials. Conductivity, which is given by the relation σ = Nfqμ where Nf is the density of free carriers, is well known to increase with the doping level, according to a σ ~ Ν relationship. It follows that a direct relationship between mobility and conductivity, μ ~ σ Λ can be expected, 2

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Specificity and Limits of Organic-Based Electronic Devices - ACS

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