4. Organic semiconductor materials

The mobilities of organic semiconductors have achieved significant progress in OFETs from the initially reported 10�<sup>5</sup> cm2 V�<sup>1</sup> s �<sup>1</sup> for polythiophene in 1986 [35] to 10 cm2 V�<sup>1</sup> s �<sup>1</sup> for present diketopyrrolopyrrole (DPP)-based polymers [36]. The high mobility of organic semiconductors over conventional amorphous silicon indicates large potential application of organic electronic devices. The remarkable progress of organic semiconductors provides a road for organic electronic industry. Generally, for high-performance organic semiconductors, some critical factors, such as molecular structure, molecular packing, electronic structure, energy alignment, and purity, play important roles. Among them, tuning the molecular packing is especially important for high-performance semiconductors since the charge carrier transport is along the molecular π orbitals. Hence, the overlap degree of neighboring molecular orbitals significantly determines the charge carrier mobility. Molecular packing with strong intermolecular interactions is favorable for efficient charge transport and high field-effect mobility. The electronic structure and energy levels are crucial for the materials and device stability. In order to obtain the high-performance and stable organic semiconductors, structural modification with electron donors and acceptors are necessary. Except the above talked aspects, the film morphologies such as grain boundaries also could affect the charge carrier transport. The grain boundaries and disordered domains could hamper the efficient intermolecular charge hopping between them. Hence, increasing the crystal grain size and film uniformity could efficiently improve the charge transport and mobility. In this section, we introduce some feature compounds with mobility of/over amorphous silicon and/or with high stability.

#### 4.1. P-type semiconductors

along the channel and the extracted value only represents a mean value. Therefore, it is often more rational to extract the mobility in the linear regime, in which the density of charge is more uniform. This is usually done through the transconductance gm, which follows from the

¼ W

This equation assumes that the mobility is gate voltage independent. However, the mobility is actually gate voltage dependent. In this case, an extra term ∂μ/∂VG should be involved in Eq. (5), so that this method is only applicable when the mobility varies slowly with the gate voltage [30]. Moreover, this method is very sensitive to the charge injection limitation and

The current on/off ratio is another important FET parameter that can be extracted from the transfer characteristics. It is the ratio of the drain current in the on-state (at a particular gate voltage) and the drain current in the off-state (Ion/Ioff). For best performing behavior of the transistor, this value should be as large as possible. When neglected the contact resistance effects at the source-drain electrodes, the on-current mainly depends on the mobility of the semiconductor and the capacitance of the gate dielectric. The off-current is mainly determined by gate leakage current. It can be increased for unpatterned gate electrodes and semiconductor layers due to the conduction pathways at the substrate interface and the bulk conductivity of the semiconductor. Moreover, the unintentional doping could also increase the off-current [31].

Threshold voltage originates from several effects and strongly depends on the organic semiconductor and dielectric used. Generally speaking, the threshold voltage could be caused by interface states, charge traps, built-in dipoles, impurities, and so on [31, 32]. And it can be reduced by increasing the gate capacitance, which induces more charges at lower applied voltages. In many cases, the threshold voltage is not always constant for a given device. The Vth tends to increase when organic transistors are operated under an extended time scale. This is called bias stress behavior, and it has a significant effect on the applicability of organic transistors in electric circuits and real applications. And thus is presently under intense investigation [33, 34]. A current hysteresis could be caused by the shift of the threshold voltage on the time scale of current-voltage measurements. Large stable threshold shifts, for example, induced by polariza-

The mobilities of organic semiconductors have achieved significant progress in OFETs from

�<sup>1</sup> for polythiophene in 1986 [35] to 10 cm2 V�<sup>1</sup> s

�<sup>1</sup> for

tion of a ferroelectric gate dielectric, can be used in organic memory devices.

4. Organic semiconductor materials

the initially reported 10�<sup>5</sup> cm2 V�<sup>1</sup> s

<sup>L</sup> CiµVD <sup>ð</sup>11<sup>Þ</sup>

gm <sup>¼</sup> <sup>∂</sup>ID ∂VG

first derivative of Eq. (3) with respect to the gate voltage [30].

132 Different Types of Field-Effect Transistors - Theory and Applications

retrieval at source and drain electrodes.

3.2.2. Current on/off ratio

3.2.3. Threshold voltage

In the last two decades, p-type semiconductor materials have achieved much progress because of their simple design and synthetic approach. P-type organic semiconductors mainly contain acene, heteroacene, thiophenes, as well as their correlated oligomers and polymers, and twodimensional (2D) disk-like molecules. Several comprehensive reviews have given detailed information about these compounds [32, 36]. Among them, the polycyclic aromatic hydrocarbons are most representative of the class of compounds due to their unique features. Some representative p-type semiconductors are shown in Chart 1.

Pentacene (1), as the benchmark of organic semiconductors, was first reported in 1970s, but the numerous OFET applications were only conducted recently [37, 38]. With strong intermolecular interactions and herringbone packing motif, pentacene exhibits efficient charge transport. Hence, polycrystalline thin film of pentacene (1) and tetracene (2) showed surprisingly high mobility approaching 0.1 cm<sup>2</sup> V<sup>1</sup> s <sup>1</sup> [37] and 3.0 cm2 V<sup>1</sup> s <sup>1</sup> [39], respectively. The substituted tetracene derivative rubrene (3) showed the highest charge carriers mobility with 20 cm<sup>2</sup> V<sup>1</sup> s <sup>1</sup> for single crystal device in the FET configuration [40]. This implies that the conjugated acene is a good building block for the p-type semiconductors. Later on, phthalocyanines (4) and more core-extended hexa-peri-benzocoronenes (HBC) (5) containing twodimensional (2D) aromatic core were reported and showed typically discotic columnar liquid crystalline phases. As a result, the HBC showed enhanced mobility along the column due to the solid-state organization. Moreover, HBC-based OFETs by zone casting method exhibited a high mobility up to 0.01 cm<sup>2</sup> V<sup>1</sup> s <sup>1</sup> [41]. The chemistry based on acene has paved the way for designing efficient p-type semiconducting materials.

Chart 1. Chemical structures of some p-type semiconducting materials.

The sulfur containing heteroacenes and their derivatives constitute another large group of p-type aromatic hydrocarbons, as shown in Chart 1. The thienoacenes and their derivatives were also synthesized and investigated as semiconductors for p-type materials. The asymmetric oligoacene, such as the tetraceno-thiophene (6), was also synthesized and showed similar mobility (0.3 cm<sup>2</sup> V<sup>1</sup> s 1 ) compared to their centrosymmetric counterparts processed in the same conditions [42]. The tetrathienoacene (7) with aryl groups had a higher mobility up to 0.14 cm<sup>2</sup> V<sup>1</sup> s <sup>1</sup> by vapor deposition [43]. The sulfur-sulfur interaction in the packing motif was believed to enhance the charge carrier transport. The introduction of sulfur and other heteroatoms induced different energy alignments and crystal packing, which promotes the development of p-type materials.

Among the p-type polymers, poly(3-hexylthiophene) (P3HT) (8) has been studied extensively and showed high mobility due to its good crystalline properties and well-ordered lamella structure which facilitates the efficient charge transport [44]. And it has been widely used as electron donor in organic solar cells. DPP-based polymers, such as PDDPT-TT (9), have been shown as high-performance semiconductor materials with hole mobility over 10 cm2 V<sup>1</sup> s 1 . Moreover, the device exhibited excellent shelf life and operating stabilities under ambient conditions. Finally, exceptionally high-gain inverters and functional ring oscillator devices on flexible substrates have been demonstrated [45, 46].

#### 4.2. N-type semiconductors

Although the p-type semiconductor materials have achieved much progress, the development of n-type organic semiconductors still lags behind that of p-type organic semiconductors due to low device performance, ambient instability, and complex synthesis. Owing to their important roles in organic electronics, such as p-n junctions, bipolar transistors, and complementary circuits, it is desirable to develop stable n-type semiconductor materials with high charge carrier mobility for organic field-effect transistors.

To date, n-type organic semiconductors with high mobility are relatively rare and significantly lagging behind p-type semiconductors, and most of the n-type materials are still air unstable in ambient conditions due to its high lowest unoccupied molecular orbital (LUMO) energy level. De Leeuw et al. reasoned that the air unstable problem is due to redox reaction with oxygen and water [47]. Based on this result, we can calculate the LUMO energy level and it should be lower than 3.97 eV in order to be stable toward water and oxygen. N-type organic semiconductors mainly contain halogen or cyano-substituted n-type semiconductors that could be converted from p-type materials, perylene derivatives, naphthalene derivatives, fullerenebased materials, and so on (Chart 2).

The important n-type semiconductor material perfluoropentacene (11) was first reported by Sakamoto et al. [48]. This molecule adopted similar crystal packing to pentacene, and transistors fabricated from vacuum-deposited films showed a high mobility up to 0.11 cm2 V<sup>1</sup> s <sup>1</sup> and an on/off ratio of 105 . It was thought that attaching fluorine atoms could lower the LUMO energy level of this compound. However, the LUMO energy level is not low enough to make the OFET device stable in the ambient condition. Similarly, the 2,5,8,11,14,17-hexafluoro-hexa-perihexabenzocoronene was synthesized by Kikuzawa et al. from hexakis(4-fluorophenyl) benzene [49]. This fluorinated compound was also suitable for the fabrication of n-channel transistors due to the decreased LUMO energy level, showing a mobility of 1.6 <sup>10</sup><sup>2</sup> cm2 <sup>V</sup><sup>1</sup> s <sup>1</sup> and an on/off ratio of 10<sup>4</sup> . Based on these results, they showed that the halogen substitution is a proper way to obtain n-type semiconductors.

Chart 2. Chemical structures of some n-type semiconducting materials.

The sulfur containing heteroacenes and their derivatives constitute another large group of p-type aromatic hydrocarbons, as shown in Chart 1. The thienoacenes and their derivatives were also synthesized and investigated as semiconductors for p-type materials. The asymmetric oligoacene, such as the tetraceno-thiophene (6), was also synthesized and showed

in the same conditions [42]. The tetrathienoacene (7) with aryl groups had a higher mobility

motif was believed to enhance the charge carrier transport. The introduction of sulfur and other heteroatoms induced different energy alignments and crystal packing, which promotes

Among the p-type polymers, poly(3-hexylthiophene) (P3HT) (8) has been studied extensively and showed high mobility due to its good crystalline properties and well-ordered lamella structure which facilitates the efficient charge transport [44]. And it has been widely used as electron donor in organic solar cells. DPP-based polymers, such as PDDPT-TT (9), have been shown as high-performance semiconductor materials with hole mobility over 10 cm2 V<sup>1</sup> s

Moreover, the device exhibited excellent shelf life and operating stabilities under ambient conditions. Finally, exceptionally high-gain inverters and functional ring oscillator devices on

Although the p-type semiconductor materials have achieved much progress, the development of n-type organic semiconductors still lags behind that of p-type organic semiconductors due

) compared to their centrosymmetric counterparts processed

1 .

<sup>1</sup> by vapor deposition [43]. The sulfur-sulfur interaction in the packing

similar mobility (0.3 cm<sup>2</sup> V<sup>1</sup> s

4.2. N-type semiconductors

the development of p-type materials.

flexible substrates have been demonstrated [45, 46].

up to 0.14 cm<sup>2</sup> V<sup>1</sup> s

1

Chart 1. Chemical structures of some p-type semiconducting materials.

134 Different Types of Field-Effect Transistors - Theory and Applications

Naphthalene diimide (12) and perylene diimide (13) derivatives are two of the most studied n-type materials used in OFETs. Simple naphthalene and perylene diimides can be prepared from bisanhydrides and primary amines. Generally, the aromatic diimide in transistors shows an n-type character due to imide functionalization. Then, cyano or halogen was introduced to improve the air stability. Naphthalene diimide substituted with electron-withdrawing CN groups at the core position was reported by Jones et al. [50]. This molecule showed a mobility as high as 0.11 cm<sup>2</sup> V<sup>1</sup> s <sup>1</sup> as well as good ambient stability compared to unsubstituted compound. Cyano-substituted perylene diimide was also reported by the same group [51]. The good air stability was also observed, which indicates that cyano substituent is another efficient way to lower the LUMO energy level and achieve stable n-type materials. Later on, the core-expand NDI bearing two 2-(1,3-dithiol-2-ylidene)malonitrile moieties at the core (14) needs to be mentioned due to its good solution processability and good air stability [52].

Based on these results, it could be concluded that there is an efficient way to achieve stable n-type materials by combining imide functionalization and cyano or halogen substitutions. Electron-deficient aromatic diimides, such as ovalene diimide (ODI-CN), have attracted increasing attention as promising n-type semiconductors for OFETs (15) [53]. The materials of this class showed not only highly planar conjugated backbone but also easily tunable electronic properties through core and imide-nitrogen substituents with electron withdrawing groups and alkyl chains, respectively.

Similar to organic small molecules, the high-performance n-type polymers reported so far are much scarcer than that of p-type polymers. However, in order to achieve complementary circuits and plastic electronics, developing high-performance polymeric semiconductor with good air stability is essential. According to the previous works, the most promising results for n-type polymers is naphthalene-based polymer (P(NDI2OD-T2)), which exhibited an unprecedented high performance, with a mobility of >0.1 cm2 V<sup>1</sup> s <sup>1</sup> (up to 0.85 cm2 V<sup>1</sup> s 1 ) and on/ off ratio of 106 and excellent air stability in ambient conditions. Furthermore, the semiconductors could be processed by gravure, flexographic, and inkjet printing technique, and achieve all-printed polymeric complementary inverters (with gain 25–65) [54].
