In addition, the electro-optical properties of the organic semiconductors used for OFET fabrication can be easily designed or tuned due to the versatility of organic synthesis. Due to these reasons, OFETs have been found to be one of the most popular research topics in recent decades and are expected to have wide applications in the future from flexible displays to high-performance disposable sensors.
Organic Thin Film Transistors (OTFTs)
Organic Electro-Chemical Transistors (OECTs)
However, to practically achieve these goals, more attention should be paid to reducing the operating voltage and electrical stability of the fabricated OFET device. Therefore, considering the demand of today's technology, reducing the operating voltage and increasing the stability of the OFET-based sensing device has become one of the most interesting research topics due to their promising future use as a replacement for their inorganic counterparts.
Organic Field Effect Transistors (OFETs)
On the other hand, since photolithographic patterning of the source and drain contacts is not possible due to solvent damage to the organic layer, they must be deposited on top of the organic semiconductor usually through a shadow mask, limiting the lithographic resolution. First, the gate dielectric and gate electrode must be deposited and patterned on top of the organic semiconductor layer, and this process must preserve the organic material.
Working Principle of OFETs
When VDS 0V and VGS= 0V (Figure 1.3e), negatively charged electrons, which were initially present in the n-type semiconductor, begin to move towards the positively biased drain contact. VDS 0V and VGS 0V, (Figure 1.3f) mobile charges are induced into the semiconductor layer through the dielectric and the transistor is turned to the “ON” state.
Parameters of OFETs
The device parameters can be derived from the equations derived for the drain current in the linear and saturation regions. 𝐼𝐷𝑆 = 𝜇𝑖
Charge Transport Mechanism in OFETs
- Hopping Model
- Band Theory Model
- Small Polaron or Energy Transfer Model
- Multiple Trap and Release Models
- Grain Boundary Models
The key to the translation process is whether there are electrons in the conduction band or not. In general, another factor, the ratio of free to trapped carriers, which depends on the gate voltage (𝜑), should be considered in the grain boundary model to determine the mobility of the actual field-effect device.
Stability of OFETs
Hysteresis
Therefore, in some cases most of the traps within the grain and at the interface can be filled. As a result, a maximum mobility can be observed at an initial gate voltage and decreases with increasing gate voltage until the carrier concentration 𝑝 reaches the critical concentration.
Bias-stress effect
Sensors
A transducer is a component that translates changes in a sensing unit into measurable signals, usually electrical signals. Since the FET itself will act as a converter, the detection unit will be integrated with the converter itself, greatly simplifying the sensor structure.
Thesis Synopsis
Two different dielectric bilayer configurations, namely alumina/poly (vinyl alcohol) (Al2O3/PVA) and alumina/poly (methyl methacrylate) (Al2O3/PMMA) were used to reduce the device operating voltage. In Chapter 7, we use the tri-layer dielectric configuration mentioned in the previous chapter for bio-sensing applications.
Thin-film morphology control in naphthalene-diimide-based semiconductors: high-mobility n-type semiconductor for organic thin-film transistors. Low Voltage Organic Thin Film Transistors with High-K Nanocomposite Gate Dielectrics for Flexible Electronics and Optothermal Sensors.
Experiments
Materials
NDI-OD2 exhibited typical n-type OFET performance with a maximum observed electron mobility of 1.0 cm2 V−1 s−1 under vacuum. We observed that these NDI-OD2 devices exhibit a controlled loss of performance followed by complete failure when continuously exposed to a high humidity environment for several days.
Characterization Details
Results and Discussion
- Theoretical Study
- Band Gap Analysis
- Thermal Studies
- Thin Film Microstructure
- OFET Device Fabrication and Characterization
The crystalline nature of NDI-OD2 was confirmed using high power (18 kW) X-ray diffractometer. The crystalline nature of NDI-OD2 is also confirmed from SAED results as shown in Figure 2.5b.
Conclusion
Thermally evaporated films of NDI-OD2 molecules on PVA were found to have a very good layer structure. Therefore, higher mobility values are observed for the NDI-OD2 active layer and the PVA dielectric.
Critical role of alkyl chain branching of organic semiconductors in enabling solution-processed organic N-channel thin-film transistors with mobilities up to 3.50 cm2 V−1 s−1. Fully engineered flexible organic thin film transistor arrays with high mobility and exceptional uniformity.
Experiments
Materials
In this chapter, we have discussed the effect of inorganic-organic double layer dielectric system for low duty n-type OFETs. Both of this two-layer dielectric system are expected to have significant influence on the dielectric-semiconductor interface.
Characterization Details
Results and Discussion
- Band Gap Analysis
- Thin Film Microstructure
- Device Fabrication Method
- Device Characterizations
Figure 3.6 and Figure 3.7 demonstrate the OFET characteristics curve of NDI-OD2 and NDI-CY2 molecule on double layer Al2O3/PVA and Al2O3/PMMA dielectric system respectively. Drain and Transfer characteristics curves of [(a) and (b)] NDI-OD2 and [(c) and (d)] NDI-CY2 molecule with Al2O3/PMMA double layer dielectric system.
Conclusion
Thus, the Al2O3/PMMA bilayer dielectric system demonstrates a drastic improvement in device performance due to better capacitive coupling between the gate and the channel through the dielectric layer to improve field-effect carrier mobility and threshold voltage compared to the PVA dielectric system.
In this study, before fabricating the PS-OFET device, a systematic analysis of the molecular growth nature of ZnPc deposited at different substrate temperatures (Ts) was performed. Device bias stress analysis revealed that the stress effect is extremely small in the presence of light (decay of 𝐼𝐷𝑆 20% after 30 min) compared to the dark, with characteristic carrier relaxation time 𝜏′104 sec.
Experiments
Materials
Characterization Details
Device Fabrication
The capacitance density (Cox25.8 nF.cm-2) and leakage current density (J) of this double layer Al2O3/PMMA gate insulator were calculated separately from the metal-insulator-metal (MIM) parallel plate capacitor structure using copper as upper electrodes (Figure 4.3). It was found that under the applied field, the leakage current density J between the gate and drain contacts through the dielectric layers was very low (10-8 A.cm-2) compared to the source-drain contacts (10-5 A.cm -2), which confirms that the dielectric layers exhibit excellent capacitive behavior and are suitable for the fabrication of OFETs.
Results and Discussion
Morphology and Optical Analysis
The calculated d-spacing of all thin films deposited at different Ts, along with their respective peak positions, are shown and summarized in Figure 4.7 and Table 4.1, respectively. However, in the case of FWHM (Figure 4.9c and Figure 4.9d), a systematic increase in behavior was observed from RT to 120°C.
Operation Mechanism of PS-OFETs
𝑉𝐷𝑆< 0𝑉 and 𝑉𝐺𝑆< 0 𝑉, (Figure 4.10c) mobile charges are induced in the ZnPc semiconductor thin film by the double layer dielectric system and the transistor becomes in its "ON" state. Again due to the absence of mobile charges, however, the drain current here is less, but more than the similar situation in the dark state (Figure 4.10b).
Device Characteristics
From Figure 4.11c it was observed that the photocurrent of ZnPc-PS-OFET at 𝑉𝐺𝑆= -8V continuously increases to the power of 0.1458 Wm-2 and with further increase in power (𝑃𝑖𝑛= 0.2957 Wm-2), it started to decrease, indicating the maximum limitation of the photocurrent generation of ZnPc-based PS-OFET. All the device parameters of the ZnPc-based PS-OFETs under dark and various optical power illumination are summarized in Table 4.2.
Conclusion
This inexpensive, low bias stress and electrically stable device is expected to have potential applications in optoelectronic devices and energy efficient sensors. On the other hand, multiple transfer characteristic scans and bias stress results indicated that these OFETs exhibit better operational stability, indicating potential applications of these transistors in electronic devices and sensors.
Highly sensitive organic thin film phototransistors: effect of light source wavelength on device performance. High-performance ZnPc thin film-based photosensitive organic field effect transistors: influence of multilayer dielectric systems and thin film growth structure.
Experiments
Materials
Moreover, it was observed that the operating voltage of each of the fabricated devices was extremely low (-10 V) due to the influence of high-k inorganic Al2O3. The electrical stability of each of the fabricated devices was also investigated by bias stress analysis under both light and dark conditions in vacuum.
Characterization Details
To our knowledge, the photo-response values (R~2679.40 A.W-1), reported here with the Al2O3/PVA/PMMA three-layer dielectric configuration, are the highest among all thin-film-based PS-OFETs extremely low functioning. voltage of -10 V. The optical power of the illuminated light was measured by a Tenmars TM-207 power meter and a Newport optical power/energy meter (Model 842-PE).
Device Fabrication
The capacitance density (Ci) of the Al2O3/PMMA and Al2O3/PVA/PMMA gate insulators were independently calculated from the MIM structure of the parallel-plate capacitor using copper as the top electrode (Figure 5.2). Further, Cu source drain electrodes were thermally deposited at RT (~80 nm) to account for the three terminal properties of PS-OFETs.
Results and Discussion
Device Characterizations
However, in the case of the Al2O3/PMMA two-layer dielectric system, although the devices were very stable and hysteresis-free, the drain current was less and the charge transport through the channel was poor. Therefore, high-k polar PVA dielectric was inserted between PMMA and Al2O3 layers, so that the polar PVA dielectric could induce more charges into the channel, and furthermore, the non-polar PMMA dielectric would prevent the direct interaction between the OH groups of polar PVA dielectric layers with the active layer mol.
Voltage (V)
As a result, the improvement in drain current with superior device stability could be achieved simultaneously.
Area=0.48 mm2
Operational Mechanism
Figure.5.4 Schematic representation of the operation mechanism involved in ZnPc-based p-channel OFETs (a-f) with Al2O3/PMMA bilayer and (g-l) Al2O3/PVA/PMMA trilayer device configurations respectively. Consequently, drain current becomes more at this condition compared to two-layer dielectric configuration (Figure 5.4e).
Conclusion
Al2O3/PVA/PMMA trilayer dielectric configuration, is the highest reported value among all the thin film based PS-OFETs with remarkably low operating voltage of -7 V. This study confirmed that the Al2O3/PVA/PMMA trilayer dielectric configuration is a highly efficient, ITO- and Si/SiO2-free system, which increases the photoresponsiveness of PS-OFETs to significantly high level not previously observed in any thin-film-based OFETs and which is expected to have applications in futuristic low-cost and portable optoelectronic devices .
High-performance, low operating voltage n-type organic field effect transistor based on an inorganic-organic double-layer dielectric system. Herein, we reported a very simple, highly stable, cost-effective and compatible with flexible substrate, inorganic-organic hybrid three-layer dielectric system for fabricating an n-channel organic field-effect transistor with ultra-low operating voltage.
Experiments
- Materials
- Characterization Details
Laurell and Spin 150 spin coaters were used to deposit TiO2 sol-gel and PMMA dielectric thin films. The TEM image of TiO2 dielectric nanoparticles was characterized by Tecnai G2 F20 S-twin JEOL 2100 transmission electron microscope.
Results and Discussion
- Synthesis and Growth Structure of TiO 2 Sol-Gel
- Thin Film Growth Structure of Active Layer Materials
- Device Fabrication and Characterization
Layer-by-layer AFM images of OFET based on PDI-C8 are shown in Figure 6.3. From Figure 6.4 (a) it was observed that the total capacitance density (Cox) of the three-layer dielectric layer increases systematically with increasing r.p.m.
Conclusion
Furthermore, this low-cost dielectric architecture was found compatible with flexible PET substrate and exhibits ultra-low operating voltage. This n-channel OFET device with ultra-low operating voltage and three-layer dielectric Al2O3/TiO2/PMMA system is expected to have diverse applications in portable organic electronics applications in the future.
Cross-linked high dielectric constant polymer dielectric for low voltage organic field effect transistor memory devices. Rapid detection of gram positive and gram negative bacteria using n-type organic field effect transistor.
Experiments
- Materials
- Characterization Details
- Device Fabrication Method
- Bacterial Culture
- Bacterial Cell Wall Lysis
A and to prevent direct contact of the second high-k inorganic dielectric layer TiO2 with the gate electrode. Due to this polymer dielectric layer, the top surface roughness of the TiO2 layers decreased significantly.
Results and Discussion
- Operation Mechanism
- Device Characterizations
As a result, a higher drain current is achieved even in the absence of the external gate bias compared to the bare device [Figure 7.2e]. Therefore, the outer layer of the cell wall of gram-positive bacteria is very well arranged compared to the gram-negative one.
Conclusion
A step-by-step structural standardization of the device was performed before the sensing application. The average electron mobility (μe) and threshold voltage (VTh) of the bare PDI-C8-OFET were observed to be 0.3 cm2V-1s-1 and 0.2 V, respectively.
In the next step, the use of inorganic-organic bilayer dielectric contained low operating voltage OFETs was demonstrated. In the next step again, the operating voltage of the OFET was successfully reduced with a new combination of hybrid three-layer dielectric system.
Journals
Proceedings
Patents
Book Chapter
Journals (not included in the Thesis)
Collective effect of hybrid metal nanoparticles and double cathode interfacial layers for high-performance organic bulk heterojunction solar cells. Influence of differently shaped plasmonic AuNPs on the energy conversion efficiency of the double cathode interface layer. rrP3HT: PCBM-based organic bulk heterojunction solar cell.
Proceedings (not included in the Thesis)
