OTFTs have gained much importance in the field of research as the most promising organic electronic devices for flexible displays, storage, radio frequency identification circuitry and sensors because of their characteristics such as large-area, cheaper, high mechanical flexibility as well as low-temperature processing. Figure.1 illustrates the Schematic diagram of the OTFT using tri-layer source-drain electrodes. As dielectric layers, OTFTs with silicon oxide (SiO2)/ Poly-Ether-Imide – Epoxy Polymer (PEI-EP)/ Poly-Oxy-Methylene Homopolymer (POM-H) are formed and designed as a tri-layer dielectric structure. Materials such as n-type Fluorinated Copper Phthalo–Cyanine (F16CuPc) or p-type pentacene organic semiconductor are employed in the design of OTFTs to validate the tri-layer structure.
3.1 Design of OTFT using tri-layer polymers
A highly n-doped silicon wafer (0.02 cm) with a thickness of 0.3 m serves as the gate electrode and the first dielectric layer, which is thermally oxidized by the SiO2 layer. To simulate the other two dielectric layers, PEI-EP is spin-coated by using ethanol solution and baked for 60 minutes at 120°C on a hot plate. The anisole solution is then used to spin coat POM-H on the SiO2/PEI-EP substrate, which is then baked on a hot plate for 60 minutes at 120°C. The thickness of PEI-EP and POM-H is about 0.8 µm and 0.3 µm, which is determined by the surface profile. Following that, a 0.04 m pentacene semiconductor is deposited on the tri-layer in a vacuum at a pressure of \(5\times {10}^{-4}\)Pa. Then, the deposition rate of substrate is approximately 0.02 nm/s kept at room temperature. At last, a 0.8 µm copper (Cu) is used for the source/drain electrodes, which are thermally evaporated through a shadow mask. The channel length (L) and width (W) of the source-drain electrodes were 2.6 µm and 2.5 µm. A Keithley 4200CS semiconductor analyzer has been used to evaluate the electrical characteristics of all OTFTs under atmospheric conditions.
3.2 Tri-layer dielectric polymers
Tri-layered dielectric polymers are used in the OTFT for the insulator surface property. There are two crucial points to consider, one is the smoothness of the insulator surface, and another one is the surface hydrophobicity. Many studies have shown that a surface roughness not only produces surface traps but also distributes charges and slows charge flow. As a result, strong hysteresis effects and high work voltage are frequently observed. According to reports, hydrophilic surfaces with a high concentration of –OH groups, including bare silicon oxide (SiO2) can easily trap electrons and result in poor OTFT performance. To overcome such insulator surface problems, hydroxyl-free polymer and solution processing dielectrics such as PEI-EP and POM-H have been proposed because they can provide better surface properties such as being more hydrophobic and finer.
(a)PEI-EP
Thin-film of the tri-layer is prepared by spin coating the mixture of EP and PEI in chloroform for 2 hours at a 70 ͦ c low-temperature annealing process. The amine groups of PEI react with the epoxy groups in EP at this temperature and form hydroxyl groups with additional secondary and tertiary amine groups. Figure 2 illustrates the Mixture of PEI-EP [18] film with the curing reaction.
The epoxy groups in EP react with the freshly generated hydroxyl groups. A crosslinking network containing both amine groups and ether connections is produced as a result of this function. The PEI-EP dielectric allows the device to operate in air for 2 months and in high-humidity environments ranging from 20–100% without substantial performance deterioration. It also has low hysteresis transfer characteristics as well as an excellent electrical performance with a 100% operating rate. These exceptional benefits make PEI-EP an attractive dielectric choice for solution-processed flexible OTFTs. Parameters of PEI-EP polymer is illustrated at Table 1.
Table 1. Parameters of PEI-EP polymer
PEI-EP parameters
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Processability
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Low
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Initial decomposing temperature (td)
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340 ͦ C
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Processing temperature
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70 ͦ
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Glass transition temperature (tg)
|
230 ͦ
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Tensile strength (Mpa)
|
89.2
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Elongation
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4.7%
|
The PEI-EP dielectric has a low process temperature of 70°C while also having a glass transition temperature (230°C) and high initial decomposition temperature (340°C) with the humidity-resistant dielectric characteristics and frequency-independent capacitance.
(b)POM-H
POM is an abbreviation for Poly-Oxy-Methylene, which is a thermoplastic polymer with a high molecular weight that is extensively employed in a variety of manufacturing applications, and it is called acetal polyformaldehyde and polyacetal. The formaldehyde POM copolymer is made up of –CH2O- repeating units. Generally, POM polymers have outstanding mechanical qualities such as reduced friction, high tensile strength, and higher strain resistance with improved stiffness and toughness. Moreover, POM has excellent scratch resistance, low moisture absorption and resistant to a wide range of organic solvents, weak acids and strong bases. Furthermore, based on the chemical characteristics of the POM, which is unstable in acidic (pH 4), high-temperature settings and then the polymer degrades. As a result, POM is regularly blended with cyclic ethers, including ethylene oxide, in order to disrupt the chemical structure and increase the polymer's endurance. The chemical formula for the POM-H is given below
[-H2-C-O-] n (1)
POM occurs in two forms: homopolymers (POM-H) (POM-Hs) and copolymers (POM-Cs). The fundamental difference between POM-H and POM-C is the melting point of these two types of POM. POM-C has a melting point of 160–175°C, while POM-H has a melting point of 172–184°C, and these parameters are illustrated in Table 2.
Table 2. Parameters of POM-H polymer
POM-H parameters
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Processability
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Low
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Melting Point
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172-184 °C
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Processing temperature
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194-244°C.
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Elastic modulus (MPa)
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4623.
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Glass transition temperature (tg)
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-85°C.
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Tensile strength
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70 MPa.
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Elongation
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25%.
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POM-H is made through anion exchange polymers of formaldehyde, which results in good crystallization and excellent strength and stiffness. POM-H has higher physical and mechanical qualities than POM-C in general. POM-H are suitable for applications requiring fatigue resistance as well as a low coefficient of thermal expansion.
Thus the designed model is simulated for electrical characteristics such as carrier mobility (\(\mu\)), Threshold voltage (VT), Capacitance (C), and ON/OFF ratio by comparing tri-layer dielectric OTFT with existing Bi-layer dielectric OTFT. The electrical characteristics for this designed model is analyzed using Comsol multiphysics version 6.0. These validated outcomes will be discussed in the upcoming section.
(c)Transfer electrical characteristics for tri-layer OTFT
An inherently undoped organic semiconductor has no charge carriers at zero gate voltage. Charged particles are injected into the organic substance by drawing the source and drain electrodes near the dielectric. A p-type/n-type conducting channel is produced when a positive/negative gate voltage is applied to the semiconductor. If the source/drain metal work function is near to the OSC's HOMO-LUMO level, then using the electrodes providing a voltage can extract the positive/negative charges between the source and drain. The bulk of carriers in pentacene are holes, making it a p-type semiconductor. When a negative gate voltage is applied, an electric field is formed across the dielectric, resulting in a hole accumulation area at the interface of the dielectric semiconductor. Current flows through the accumulation layer between the contacts when a voltage is applied at the source-drain terminals. In the linear region, \({V}_{D}\) << \({V}_{G}\) − \({V}_{T}\) the channel is continuous and \({I}_{D}\) is given by $${I}_{D,lin}=\frac{{W}_{C}\mu {C}_{j}}{{L}_{C}}\left[\left({V}_{G}-{V}_{T}\right){V}_{D}-\frac{{V}_{{D}^{2}}}{2}\right]$$ 2
Further increasing \({V}_{D}>{V}_{G}-{V}_{T}\) will change the electric field of drain contact to zero. As a result, a pinch-off area forms around the drain contact. The saturation zone occurs above this pinch-off point, and the drain current is no longer controlled by the drain voltage but only by the gate voltage. Drain current varies quadratically in this region:
$${I}_{D,sat}=\frac{{W}_{C}\mu {C}_{j}}{{2L}_{C}}{\left({V}_{G}-{V}_{T}\right)}^{2}$$ 3
Equations (1) and (2) represents the formula used for calculating the mobility in OTFTs. For transistor properties, the mobility and conductivity of the organic semiconductor layers have a reliable operation and important attention to the transistor demands in greater mobility. The field-effect mobility, which evaluates how easily charge carriers can migrate in the device, is defined as the average charge carrier drift velocity per unit applied electric field. The mobility of various structures can be assessed using their transfer characteristics. Trans-conductance, which is defined as mobility in the linear region:
$${g}_{m}=\frac{{\partial I}_{D,lin}}{\partial {V}_{G}}$$ 4
Mobility of linear region (\({\mu }_{lin})\) is validated by associating the small and constant VD $${\mu }_{lin}=\frac{L{g}_{m}}{{W}_{C}{C}_{j}{V}_{D}}$$ 5
Mobility of saturation region (\({\mu }_{sat})\)is validated by using the Eq. (5) $${\mu }_{sat}=\frac{2{L}_{C}}{{W}_{C}{C}_{j}} {\left(\frac{\partial \sqrt{{I}_{Dsat}}}{\partial {V}_{G}}\right)}^{2}$$ 6
Where, \({W}_{C}\)and \({L}_{C}\) is the width and length of the channel, respectively,\({C}_{j}\) is the capacitance of the gate insulator per unit area, \(\frac{\partial \sqrt{{I}_{Dsat}}}{\partial {V}_{G}}\) is the slope of the curve, and Ion/Ioff is the proportion of accumulation mode current to depletion mode current. Ion is the drain current above the saturation threshold voltage VT, while Ioff is the drain current below the saturation threshold voltage. The threshold voltage is the minimum gate voltage at which holes begin to collect at the insulator semiconductor contact or at which the OTFT begins to conduct. The gate insulator capacitance or the thickness of the organic layer affects the threshold voltage of OTFTs. Devices with smaller channel lengths and thicker film sizes have lower threshold voltages. Ion/Ioff was calculated using the transfer parameters of numerous OTFT structures.