5. Fabrication techniques

The deposition of semiconductors is the determining step of the OFET fabrication. And it will decide the performance of the devices significantly. Here, we will introduce some important techniques commonly used in the OFET fabrication.

#### 5.1. Vacuum evaporation

This technique allows for deposition and purification of small molecule organic semiconductors. The process is performed in an ultrahigh vacuum environment. The organic semiconductor material is placed in a metal boat and heated by Joule effects or electron gun, and the substrate is placed above the boat to allow growth and formation of the organic materials. In principle, high molecular weight organic semiconductors cannot be deposited by this way, because they are too heavy to evaporate and tend to decompose at high temperatures. The main advantages of the vacuum evaporation are the facile control of the purity and thickness of the deposited film. Meanwhile, that highly ordered crystalline thin films can be realized by controlling the deposition rate and the temperature of the substrate. Its major deficiency is that it requires complicated instruments. This is different with the solution processing technique which is simple and low-cost.

### 5.2. Liquid deposition

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

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

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 unprec-

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

The deposition of semiconductors is the determining step of the OFET fabrication. And it will decide the performance of the devices significantly. Here, we will introduce some important

This technique allows for deposition and purification of small molecule organic semiconductors. The process is performed in an ultrahigh vacuum environment. The organic semiconductor material is placed in a metal boat and heated by Joule effects or electron gun, and the substrate is placed above the boat to allow growth and formation of the organic materials. In principle, high molecular weight organic semiconductors cannot be deposited by this way,

<sup>1</sup> as well as good ambient stability compared to unsubstituted

<sup>1</sup> (up to 0.85 cm2 V<sup>1</sup> s

1

) and on/

as high as 0.11 cm<sup>2</sup> V<sup>1</sup> s

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

groups and alkyl chains, respectively.

5. Fabrication techniques

5.1. Vacuum evaporation

edented high performance, with a mobility of >0.1 cm2 V<sup>1</sup> s

techniques commonly used in the OFET fabrication.

all-printed polymeric complementary inverters (with gain 25–65) [54].

Liquid deposition process is an important part of most OFET fabrication process, either to deposit the active layers or to manipulate layers deposited through other means. Many organic semiconductor materials have been engineered to be soluble or dispersible in solution, which gives a possibility toward the device fabrication. Many strategies have been applied to the deposition of organic semiconductors for utilization in OFETs. The common deposition methods include spin-coating, drop-casting, dip-coating, spray-coating, and rollcoating techniques (Figure 4) [55].

Printing comprises a family of techniques and can simultaneously deposit and pattern a target material. This technique mainly contains ejected drop printing, contact stamp printing, indirect and offset printing methods, and capillary stylus dispensing. The comparison of these techniques in terms of advantages and disadvantages has been reported by some comprehensive reviews [55, 56]. Among them, piezo inkjet printing has dominated OFET fabrication printing techniques due to its excellent compatibility and the availability of sophisticated print heads to the development community. In this technique, some parameters like ink viscosity, ink surface tension, and substrate surface energy are crucial for ejection and deposition of the droplets. It is necessary to control the droplet spreading and drying to avoid "coffee ring" effect and form

Figure 4. A schematic summary of the solution-based deposition techniques discussed. Reproduced with permission from Ref. [55]. Copyright 2014, The Royal Society of Chemistry.

precisely patterned arrays. Some method such as combining the antisolvent crystallization and inkjet printing has been used to produce highly crystalline organic semiconducting thin films. By this approach, thin-film transistors with average carrier mobilities as high as 16.4 cm2 V<sup>1</sup> s 1 have been achieved based on single crystalline thin films of 2,7-dioctyl[1]benzothieno[3, 2-b][1] benzothiophene (C8-BTBT) [57].

#### 5.3. Thin-film alignment

Depositing of crystalline organic semiconductors with controlled in-plane orientation is one important issue for high-performance OFETs. It is generally accepted that charge transport in organic materials occurs via the hopping mechanism, which depends on the degree of orbital overlap between the molecules. Since charge carriers are preferentially transported along the π-π stacking direction in organic semiconductors, macroscopically aligned organic films have potentially higher mobilities and provide more unusual properties, such as optically and electrically anisotropic characteristics. Therefore, many deposition techniques have been investigated for patterning and alignment of organic semiconductors [56]. The techniques mainly contain (1) mechanical forces alignment, such as friction-transfer, nanoimprinting, and the Langmuir-Blodgett (LB) technique; (2) depositing the organic semiconductors directly on the alignment layers prepared by different methods, such as rubbing and photoirradiation; (3) growing the organic semiconductors on inorganic single crystals; (4) using magnetic or electric field-induced alignment; (5) using solution-processed technique to align organic semiconductors on isotropic substrates.

Among the solution processing techniques, the traditional techniques such as spin-coating and drop-casting cannot control the thin-film orientation. Therefore, some methods have been used to overcome this issue. For example, zone-casting offers a route to control the orientation of the deposited layers. In this process, a continuously supplied hot solution is deposited by means of a nozzle onto a moving, thermally controlled support. Under appropriate rates of solution supply and solvent evaporation, a stationary gradient of concentration is formed within the meniscus, which gives rise to directional crystallization [58]. Dip-coating is another technique to give a better thin-film alignment in solution-processed devices [59, 60]. This process can be controlled by the substrate lift rate, solvent evaporation, and capillary flow. Solvent choice is especially important because of its effect on the rate of solvent evaporation. The drying speed which influences the thin-film morphologies can be quantitatively controlled during the dipcoating process by adjusting the substrate lifting rate.

Solution-sheared deposition is a recently developed approach that can deposit highly crystalline and aligned thin films on isotropic substrates [61]. This method is related to doctor blading, which employs a blade to distribute a viscous solution over a substrate. A small volume of a diluted organic solution is sandwiched between two preheated silicon substrates, which move relatively to each other at a controlled speed. The top wafer acts as the shearing tool and is treated to be hydrophobic and the bottom wafer acts as the device substrate. The motion of the wafers exposes a liquid front that quickly evaporates to form a seeding film comprising multiple crystal grains. These crystals act as nucleation sites and allow the remaining molecules in solution to grow along the direction of the shearing direction (Figure 5) [61].

precisely patterned arrays. Some method such as combining the antisolvent crystallization and inkjet printing has been used to produce highly crystalline organic semiconducting thin films. By this approach, thin-film transistors with average carrier mobilities as high as 16.4 cm2 V<sup>1</sup> s

have been achieved based on single crystalline thin films of 2,7-dioctyl[1]benzothieno[3, 2-b][1]

Depositing of crystalline organic semiconductors with controlled in-plane orientation is one important issue for high-performance OFETs. It is generally accepted that charge transport in organic materials occurs via the hopping mechanism, which depends on the degree of orbital overlap between the molecules. Since charge carriers are preferentially transported along the π-π stacking direction in organic semiconductors, macroscopically aligned organic films have potentially higher mobilities and provide more unusual properties, such as optically and electrically anisotropic characteristics. Therefore, many deposition techniques have been investigated for patterning and alignment of organic semiconductors [56]. The techniques mainly contain (1) mechanical forces alignment, such as friction-transfer, nanoimprinting, and the Langmuir-Blodgett (LB) technique; (2) depositing the organic semiconductors directly on the alignment layers prepared by different methods, such as rubbing and photoirradiation; (3) growing the organic semiconductors on inorganic single crystals; (4) using magnetic or electric field-induced alignment; (5) using solution-processed technique to align organic semiconduc-

Among the solution processing techniques, the traditional techniques such as spin-coating and drop-casting cannot control the thin-film orientation. Therefore, some methods have been used to overcome this issue. For example, zone-casting offers a route to control the orientation of the deposited layers. In this process, a continuously supplied hot solution is deposited by means of a nozzle onto a moving, thermally controlled support. Under appropriate rates of solution supply and solvent evaporation, a stationary gradient of concentration is formed within the meniscus, which gives rise to directional crystallization [58]. Dip-coating is another technique to give a better thin-film alignment in solution-processed devices [59, 60]. This process can be controlled by the substrate lift rate, solvent evaporation, and capillary flow. Solvent choice is especially important because of its effect on the rate of solvent evaporation. The drying speed which influences the thin-film morphologies can be quantitatively controlled during the dip-

Solution-sheared deposition is a recently developed approach that can deposit highly crystalline and aligned thin films on isotropic substrates [61]. This method is related to doctor blading, which employs a blade to distribute a viscous solution over a substrate. A small volume of a diluted organic solution is sandwiched between two preheated silicon substrates, which move relatively to each other at a controlled speed. The top wafer acts as the shearing tool and is treated to be hydrophobic and the bottom wafer acts as the device substrate. The motion of the wafers exposes a liquid front that quickly evaporates to form a seeding film comprising multiple crystal grains. These crystals act as nucleation sites and allow the remaining molecules in solution to grow along the direction of the shearing direction (Figure 5) [61].

benzothiophene (C8-BTBT) [57].

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

5.3. Thin-film alignment

tors on isotropic substrates.

coating process by adjusting the substrate lifting rate.

1

Figure 5. Schematic diagram of the solution-shearing method (a) and (b) cross-polarized optical microscope images of solution-sheared TIPS-pentacene thin films, formed with shearing speeds of 0.4 mm/s. Adapted with permission from Ref. [62]. Copyright 2011, Nature Publishing Group.

It has been reported that by using this method, metastable molecular packing motifs (or latticestrained crystal structure) could be formed, which can alter the intermolecular π-π stacking distance and enhance the charge transport properties [62].

Slot-die coating is also a promising technique to control the thin-film alignment and selfassembly process for OFET applications. It has been proved to be a simplistic and manufacturable approach to fabricate large area high-performance field-effect transistors. This technique saves raw materials and controls film uniformity reliably, accurately, and reproducibly. The slot die coating is scalable to large areas and, therefore, applicable for the fabrication of large area low-cost electronics. We first applied this technique in the OFET fabrication [63]. Figure 6 schematically illustrates this process. A temperature-controlled vacuum suction plate was used to fix and preheat the substrates to a certain temperature. A small volume of organic solution is deposited onto the modified substrate surface by a slot die with different coating gaps ranging

Figure 6. Schematic of slot-die coating and the AFM image of the film with molecular structure superimposed. (a), (b) and (c) indicate the crystal axises of the crystal structure. Adapted with permission [63]. Copyright 2013, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

from 15 to 90 µm. The depositing speed is controlled within the range of 0.1–19.9 mm/s. The pre-exposed seeding film can act as nucleation sites and allow the remaining molecules in the solution to grow along the coating direction [63].
