**3. Printable materials for PE**

The printable materials are selected depending on the type of substrate, the type of ink, the type of printing technology and final PE application.

The conductive inks are gathering increasingly attention over the past two decades, and are revolutionizing the industry. Elected due to their attributes, such as, conductivity, suitability for printing substrates, its processing simplicity and mechanical flexibility, but also due to its ability to assign new properties, capabilities and complex functionalities. These emerging inks are penetrating the market with an opportunity to reach \$400 m by 2027, according to IDTechEX report "Condutive Ink Market 2017-2027". A large variety of materials, organic and inorganic, conductors and semiconductors, have been explored for electronics applications. The most common types of inks are water, oil or solvents based. The general form of the ink consist of a mixture of compounds (pigments or dyes, resins, solvents, fillers, humectant and additives), in liquid or solid state, with specific proprieties adapted to the printing technology characteristic, such as viscosity, surface tension, etc., to be easily printed in a large variety of substrates. What makes conductive inks electrically conductive is the fact that it contains in the composition conductive nanoscale particles. The incorporation of conductive polymers [48], carbon (C) [49] or metallic particles (e.g., silver (Ag) [8, 36], copper (Cu) [50], and gold (Au) [51]) are the most common selections. **Table 5** shows the resistivity of the bulk metal particles and the sintered metal ink form. Commonly, the metallic nanoparticles are stabilized in ink solutions by organic ligand shells, i.e., the nanoparticles are encapsulated with an organic material, called a capping agent, to form a uniform and stable dispersion, preventing particles agglomeration. This capping agent can be removed after printing through curing or sintering to allow physical contact between nanoparticles, forming continuous connectivity, i.e., a percolation path for electrical conductivity. Thus, sintering consist on welding the particles to each other below their melting point [2], and this particle welding could be achieved by exposure of the printed pattern to laser sintering [52], to microwave radiation [53], by applying an electrical voltage [54], by a chemical agent at room temperature (RT sintering) [55], or, the most conventional approach, by heating (thermal sintering) [33, 56]. In the case of thermal sintering, the temperature (typically between 100 to 400°C) must be below the softening temperature of the substrate. The presence of a few nanometers organic layer between the conductive particles is enough to block the movement of electrons from one particle to the other [33], thus reducing electrical conductivity. If this happens, the removal of this organic layer is required at high temperatures. For this reason, the sintering temperature of the nanoparticle based inks has extreme importance in plastic electronic applications, where materials, such as polyethylene terephthalate [56] and polycarbonate [56], are widely used as substrates, but have low T<sup>g</sup> (98 and 148°C, respectively). The electrical conductivity of a printed nanoparticles based ink layer also depends on the shape and size of the nanoparticles. The amount of sintering temperature and time required depend upon how easy the organic encapsulation breaks, the particle dimensions and upon the thickness of the ink film. The smaller the particle size (2–10 nm) the lower the temperature required to sinter the particles, the short is the process and a higher electrical conductivity is achieved. Typically, the nanoparticle loading inks is higher than 20 wt%. Metal nanoparticles hold the highest electrical conductivity, although, the use of the above categorized precious metals hardly fits in the so called low cost PE.

of alternating single and double carbon bonds. It's this alternation of bonds that produces conjugated π-bonds, resulting in an intrinsically conductive material [58]. Polyacetylene (PAc), Polyaniline (PAni), polypyrrole (PPy), polyphenylene vinylene (PPV), polythiophene

Printing Technologies on Flexible Substrates for Printed Electronics

http://dx.doi.org/10.5772/intechopen.76161

59

Within the intrinsically conductive materials, the regioregular PTh has a tremendous potential for applications in flexible organics electronics because of its low cost and specific properties, such us, solubility (thanks to the three-substituents alkyl-chain in the PTh core [59]), spectroscopic and electronic properties, low-temperature process [60], highly ordered structure and semi-crystallinity state in its solid states [61], regioregular compatibility to large-area fabrications and industrial mass production technologies. In the chemical structure of the regioregular PThs, the backbone of the polymer is formed by thiophene rings and a chemical side-chain group can be attached on each thiophene ring along the polymer (**Figure 7**). An

Within the PTh and its derivatives, the poly(3-hexylthiophne) (P3HT) and poly (3,4-ethylenedioxythiophene) (PEDOT) [62] are the most well-known. The P3HT is a reference material in organic electronic, physics and chemistry to which any new p-type or donor conjugate molecule should be compared and evaluated. The PEDOT is the most widely used [63] intrinsically conductive polymer. PEDOT stands out for its high transparency [64] when deposited in thin oxidized films, high electrical conductivity [64], very high chemical stability in the oxidized state, processability and simplicity of production [65]. All these features make them suitable for several printing technologies, such as, spin coating [66], screen printing [67] and inkjet printing [68]. These unique properties make intrinsically conductive polymers excellent for various applications, such as, electrochromic devices [69], sensors [60], biosensors [68], actuators [70], capacitors [70], and photovoltaic cells [70], thin film diode [71]; organic thin film transistors (OTFTs) [72], photodiodes, Organic Field-Effect Transistor (OFETs) [52], organic light-emitting diodes (OLEDs) [73], etc., with a growing interest in PE due to its rela-

end-group or a secondary copolymer chain can be added to each end of the PTh.

(PTh) are intrinsically conductive polymers (**Figure 6**).

**Figure 7.** Schematic diagram of a regioregular polythiophene based polymers.

**Figure 6.** Chemical structure of conductive polymers.

tively low cost [74].

Conductive polymers are classified into two different categories: extrinsically or intrinsically conductive polymers. The extrinsically conductive polymers normally involve a blend of conductive or nonconductive polymers, and a highly conductive additive (e.g., metallic particles) suspended in the polymer matrix [57], meaning that they are extrinsically enhanced to be conductive. Relatively to the intrinsically conductive polymer, they consist simply in a network


\* Dependent on sintering temperature and time-higher temperature.

**Table 5.** Metal resistivity [33].

**Figure 6.** Chemical structure of conductive polymers.

additives), in liquid or solid state, with specific proprieties adapted to the printing technology characteristic, such as viscosity, surface tension, etc., to be easily printed in a large variety of substrates. What makes conductive inks electrically conductive is the fact that it contains in the composition conductive nanoscale particles. The incorporation of conductive polymers [48], carbon (C) [49] or metallic particles (e.g., silver (Ag) [8, 36], copper (Cu) [50], and gold (Au) [51]) are the most common selections. **Table 5** shows the resistivity of the bulk metal particles and the sintered metal ink form. Commonly, the metallic nanoparticles are stabilized in ink solutions by organic ligand shells, i.e., the nanoparticles are encapsulated with an organic material, called a capping agent, to form a uniform and stable dispersion, preventing particles agglomeration. This capping agent can be removed after printing through curing or sintering to allow physical contact between nanoparticles, forming continuous connectivity, i.e., a percolation path for electrical conductivity. Thus, sintering consist on welding the particles to each other below their melting point [2], and this particle welding could be achieved by exposure of the printed pattern to laser sintering [52], to microwave radiation [53], by applying an electrical voltage [54], by a chemical agent at room temperature (RT sintering) [55], or, the most conventional approach, by heating (thermal sintering) [33, 56]. In the case of thermal sintering, the temperature (typically between 100 to 400°C) must be below the softening temperature of the substrate. The presence of a few nanometers organic layer between the conductive particles is enough to block the movement of electrons from one particle to the other [33], thus reducing electrical conductivity. If this happens, the removal of this organic layer is required at high temperatures. For this reason, the sintering temperature of the nanoparticle based inks has extreme importance in plastic electronic applications, where materials, such as polyethylene terephthalate [56] and polycarbonate [56], are widely used as substrates, but

(98 and 148°C, respectively). The electrical conductivity of a printed nanoparticles

based ink layer also depends on the shape and size of the nanoparticles. The amount of sintering temperature and time required depend upon how easy the organic encapsulation breaks, the particle dimensions and upon the thickness of the ink film. The smaller the particle size (2–10 nm) the lower the temperature required to sinter the particles, the short is the process and a higher electrical conductivity is achieved. Typically, the nanoparticle loading inks is higher than 20 wt%. Metal nanoparticles hold the highest electrical conductivity, although, the use of the above categorized precious metals hardly fits in the so called low cost PE.

Conductive polymers are classified into two different categories: extrinsically or intrinsically conductive polymers. The extrinsically conductive polymers normally involve a blend of conductive or nonconductive polymers, and a highly conductive additive (e.g., metallic particles) suspended in the polymer matrix [57], meaning that they are extrinsically enhanced to be conductive. Relatively to the intrinsically conductive polymer, they consist simply in a network

**Metal Ag Cu Au** Pure state (Ω.m) 1.59 × 10−8 1.68 × 10−8 2.44 × 10−8

Printed ink (μΩ.cm)\* 10–50 5–7 8

Dependent on sintering temperature and time-higher temperature.

have low T<sup>g</sup>

58 Flexible Electronics

\*

**Table 5.** Metal resistivity [33].

of alternating single and double carbon bonds. It's this alternation of bonds that produces conjugated π-bonds, resulting in an intrinsically conductive material [58]. Polyacetylene (PAc), Polyaniline (PAni), polypyrrole (PPy), polyphenylene vinylene (PPV), polythiophene (PTh) are intrinsically conductive polymers (**Figure 6**).

Within the intrinsically conductive materials, the regioregular PTh has a tremendous potential for applications in flexible organics electronics because of its low cost and specific properties, such us, solubility (thanks to the three-substituents alkyl-chain in the PTh core [59]), spectroscopic and electronic properties, low-temperature process [60], highly ordered structure and semi-crystallinity state in its solid states [61], regioregular compatibility to large-area fabrications and industrial mass production technologies. In the chemical structure of the regioregular PThs, the backbone of the polymer is formed by thiophene rings and a chemical side-chain group can be attached on each thiophene ring along the polymer (**Figure 7**). An end-group or a secondary copolymer chain can be added to each end of the PTh.

Within the PTh and its derivatives, the poly(3-hexylthiophne) (P3HT) and poly (3,4-ethylenedioxythiophene) (PEDOT) [62] are the most well-known. The P3HT is a reference material in organic electronic, physics and chemistry to which any new p-type or donor conjugate molecule should be compared and evaluated. The PEDOT is the most widely used [63] intrinsically conductive polymer. PEDOT stands out for its high transparency [64] when deposited in thin oxidized films, high electrical conductivity [64], very high chemical stability in the oxidized state, processability and simplicity of production [65]. All these features make them suitable for several printing technologies, such as, spin coating [66], screen printing [67] and inkjet printing [68]. These unique properties make intrinsically conductive polymers excellent for various applications, such as, electrochromic devices [69], sensors [60], biosensors [68], actuators [70], capacitors [70], and photovoltaic cells [70], thin film diode [71]; organic thin film transistors (OTFTs) [72], photodiodes, Organic Field-Effect Transistor (OFETs) [52], organic light-emitting diodes (OLEDs) [73], etc., with a growing interest in PE due to its relatively low cost [74].

**Figure 7.** Schematic diagram of a regioregular polythiophene based polymers.
