The operational and environmental stability of the devices was studied by the bias stress and hysteresis measurements. Effect of polymer gate dielectric material on the performance of the organic field-effect transistors”, 3rd International Conference on Physics at Surfaces and Interfaces (PSI 2014), Feb. Puri, India.
Introduction
Properties of organic semiconductors
The development of small bandgap π-conjugated polymers with high mobility and efficiency are the most challenging topics in polymer chemistry.9 The semiconducting properties of organic molecules allow the realization of a range of electronic and optoelectronic devices, such as organic field-effect transistors (OFETs ), 10 organic light-emitting diodes (OLEDs)11 and organic solar cells.12 Organic semiconductor lasers (OSLs), on the other hand, are relatively new members of the laser and organic family. The energy of the molecule is higher, when the * orbital is occupied and not when the orbital is occupied.
Different types of organic semiconductors
- p- type organic semiconductors
- n- type organic semiconductors
Molecular structures of some of the p-type organic semiconductor molecules used in the literature to fabricate p-type OFETs. The molecular structures of some of the p-type organic semiconductor molecules are shown in Figure 1.4.
Charge transport mechanisms in organic semiconductors
- Band like transport in crystalline semiconductors
- Variable-range hopping (VRH) model…
- Multiple traps and release (MTR) model
Similarly, reduction in S was also observed with film thickness as shown in Figure 3.10 (c). This may be due to the slow polarization of dipoles in the inner dielectric layer of the OFET.
Working principles of OFETs
Architectures of the organic field-effect transistors
- Bottom Gate - Top Contact
- Bottom Gate-Bottom Contact
- Top Gate -Top Contact
- Top Gate -Bottom Contact
On the other hand, since photolithographic patterning of S and D contacts is not possible due to the damage of the organic layer by organic solvents. The gate insulator may not be compatible with sputtering or atomic layer deposition, due to damage to the organic material.
Stability of the OFETs
In amorphous silicon transistors, the bias-stress effect is generally explained by the creation of dangling bonds during the operation of the device. Schematic representation of (a) clockwise hysteresis (b) counterclockwise hysteresis (c) bias stress effect and (d) anomalous bias stress effect.
Fabrication of Pentacene organic field-effect transistors containing Sio2 nanoparticle thin film as the gate dielectric. Mobile ionic impurities in poly(vinyl alcohol) gate dielectrics: Possible source of the hysteresis in organic field-effect transistors.
Experimental Details
Organic semiconductors
Most of the experimental work was done using a conventional p-type material using copper phthalocyanine (CuPc), while in another chapter n-type perylene derivs. Experimental Details 31 The goal of fabricating monomeric semiconductor OFETs was to study the stability of the materials and to improve the transistor performance under ambient conditions through new device engineering.
Polymer dielectric materials
Effect of the inner layer in the three-layer dielectric gate system on the performance of the OFETs. We observed a substantial improvement in the performance and parameters of the devices under ambient conditions. The area of hysteresis was larger in the presence of oxygen compared to the device measured under vacuum.
Substrates
Gate electrode
The heavily doped Si acts as the gate electrode and the amorphous SiO2 layer (typical thickness: 300 nm) acts as the gate insulator. In the case of glass substrates and transparent flexible substrates, the patterned ∼200 nm thick Al film acted as a gate electrode.
Thermal evaporation system for metal electrodes deposition
The surface morphology of CuPc deposited on bilayer dielectric material is shown in Figure 3.11 (b). Effect of inner layer in tri-layer gate dielectric system on the performance of the OFETs 69. Effect of inner layer in tri-layer gate dielectric system on the performance of OFETs 73 4.2.3.
Anodic oxidation of aluminum
Spin coating technique
With a suitable speed and concentration, the solution will be spread uniformly and cover the entire surface of the substrate. However, it is essential that the solvent used in the subsequent spin coating should not dissolve the previously coated film. a) Schematic diagram of the spin-coating process (b) Photo of the APEX spin-coater was used in this thesis work.
Spin coating of polymer gate dielectric materials
PMMA has been extensively investigated as a gate polymer dielectric in organic field-effect transistors due to its good film-forming ability and hydrophobicity.
Thermal annealing…
Profilometer for thickness measurement
The influence of the inner layer in the three-layer gate dielectric system on the performance of OFETs 77 4.3.3. The effect of the inner layer in a three-layer dielectric system with a gate on the operation of OFETs 81. The effect of the inner layer in a three-layer dielectric system with a gate on the operation of OFETs 85.
Growth of organic semiconducting molecules
Microscopic characterization techniques
- Atomic force microscopy
- Electron microscopy
The reflected beam from the top of the cantilever is captured by a position-sensitive photodiode. However, as soon as the tip 'feels' a force, a deflection of the cantilever will occur. The field emission scanning electron microscopy (FESEM) is one of the most widely used imaging tools.
Electrical characterization techniques
- Semiconductor characterization system
- Probe station
- Leakage current and capacitance measurement
These gated three-terminal measurements were performed by connecting two of the SMUs to the drain and gate and the source grounded. The difference between the forward and reverse swings gives a first indication of the electrical stability of the transistor. A gate bias was applied for an extended period while the source was grounded and the drain potential was held at 0V.
Introduction
Since then, the induced charges remain largely confined to the first few monolayers at the dielectric-semiconductor interfaces.1-3 However, a fraction of the charges are captured by the interface states. These properties of the chromophore are not significantly affected by the chemical modification of imide groups. However, these properties are drastically changed upon the functionality of the PDI core with electron acceptor or donor groups.
Experimental Section
- Cyclic voltammetry and theoretical modeling
- Device fabrication
The molecular energy levels, optimized geometries and electron density isocontours of the HOMO and LUMO are shown in Figure 3.1(b) and (c), respectively. To compare the results from these calculations, the electrochemical properties of PTCDI-Br2-C18 were investigated by cyclic voltammetry (CV) technique. The experimental band gap calculated from the UV-Visible absorption maximum is 2.36 eV, which is comparable to the theoretical value. a) UV-visible absorption spectrum of the PTCDI-Br2-C18 molecule was collected using chloroform as solvent.
Results and Discussion
- Morphology of Anodized alumina and PMMA layers
- Leakage current and capacitance measurement
- Morphology of PTCDI-Br 2 -C 18 molecule
- Electrical characterization of the OFET devices …
- Gate-field independent mobility
- Variation of device parameters and trap density
Higher capacitance of the dielectric layer with a minimal leakage current is desirable for fabrication of an OFET. This field is called the gate-field (E=VGS/d, where d is the thickness of the total dielectric layer) depends on the voltage applied at the gate and the thickness of the dielectric layer. We have observed a systematic reduction of VTh and NTrap as we decrease the thickness of the Al2O3 layer.
Device fabrication using CuPc as the active material
The high dipole moment of the water molecules causes positive charges in the CuPc film due to the strong electronegativity. The polarization of the dielectric material increased with the temperature, may be due to the increased polarization of –OH functional groups in the polar PVA dielectric layer. The response of the device to the ethanol vapors was reflected in the hysteresis and IDS of the transfer characteristic curves.
Stability of the OFET devices: Bias-stress effect
Summary and conclusions
The double-layer dielectric gating system for the fabrication of OFETs 65 essentially controls the roughness of the film. Moreover, we observed that the mobility of the carrier does not depend on the thickness of the dielectric layer. Our measurements also demonstrated a way to determine the thickness of the dielectric layers for better processing conditions of OFETs.
Therefore, the charge carrier scattering by the trapping states does not affect the carrier mobility in the linear region and the saturation region of the OFET operation. Solution processed organic field effect transistors with low leakage and self-pattern registration based on a patterned dielectric barrier. Temperature dependence of the field effect mobility of sexithiophene - Determination of trap density.
Effect of inner layer in tri-layer gate dielectric system on the
Experimental details
- Preparation of gate electrode and anodization
- Spin-coating of polymer dielectric Layers
- CuPc molecule and source/drain electrodes deposition
- Surface morphology of polymer dielectric materials
- Surface morphology of CuPc deposited on dielectric system
- Electrical Characterizations
- Leakage Current and capacitance measurement
- Device characterization under vacuum conditions
- Device characterization under ambient humidity conditions 81
- Mechanism of anomalous bias-stress effect
The effect of the inner layer in the three-layer gate dielectric system on the performance of OFETs 83 and thick layer SiO2. The effect of the inner layer in the three-layer gate dielectric system on the performance of OFET dipoles 89 causes additional charges in the accumulation region. The effect of the inner layer in the three-layer gate dielectric system on the performance of OFETs 93 4.3.4.6.
Conclusions
As a result, the –OH functional groups polarize much faster in the presence of water compared to vacuum conditions, where there were no water molecules in the device. Therefore, the induced charges in the accumulation layer increase further, eliminating the anomalous effects that arise due to the slow polarization in the absence of water molecules and leading to the increase of the drain current. This phenomenological model suggests that the permeated water molecules increased the hole concentration at the interface and also act as traps.
Low Operating Voltage Organic Field Effect Transistors and High K Cross-Linked Cyanoethylated Pullulan Polymer Gate Dielectrics. Influence of Polymer Gate Dielectrics on N-Channel Conduction of Pentacene-Based Organic Field Effect Transistors. Low voltage organic field effect transistors (Ofets) with solution processed metal oxide as gate dielectric.
Effect of humidity and ambient gases on the hysteresis and
- Experimental section
- Results and Discussions
- Capacitance measurement of MIM and MISM structures
- Characterization of the devices under different RH
- Effect of relative humidity on the device parameters
- Floating gate measurement under RH conditions
- Effect of relative humidity on Bias-stress …
- Effect of different environmental conditions on hysteresis
- Conclusions
- References
In this chapter, we discuss the effect of different relative humidity conditions on the hysteresis and bias stress of the B device fabricated in the previous chapter. These act as charge traps and lead to the faster decay of IDS under bias stress. In this chapter we studied the effect of different relative humidity conditions on the device bias stress and hysteresis.
Effect of measurement temperature and white light on
Experimental section
For the characterization of the same set of devices under illumination, SCHOTT 150W halogen lamp equipped with an optical fiber was used as a white light source. Illumination of the device was performed directly on the active material and the incident optical power of the lamp was recorded at sample position with a silicon photodiode (Newport 818UV) of ∼7mW/cm2. The variation of hysteresis and bias stress was investigated with white light under vacuum and moisture conditions.
Results and Discussions
- Temperature dependent hysteresis under vacuum
- Temperature dependent hysteresis under ambient humidity
- Mechanism of hysteresis variation with temperature
- Variation of capacitance with the measurement temperature
- Temperature Dependence of hole mobility
- Calculation of activation energy
- Temperature dependence of Threshold voltage (V Th )
- Temperature Dependence of Subthreshold voltage(S)
- Temperature dependence of bias-stress effect
- Mechanism of hysteresis and bias-stress variation
- Effect of white light on hysteresis and bias-stress
- Effect of light on hysteresis
- Effect of light on Bias-stress
At low temperature (150K), the device exhibited both ACH and CH in the same transfer characteristic curve as shown in Figure 6.2 (a). As shown in Figure 6.3 (a), the dipoles are randomly oriented and frozen in the PVA film at low temperature. Entrapment will also increase due to the surrounding water molecules as shown in Figure 6.10 (e & f).
Conclusions
On the other hand, τ2, which has a high value in humidity, exhibits a negligible value in vacuum, as shown in Table 6.4. It shows that under vacuum the generation and recombination processes are the dominant mechanisms, and under humidity the hole capture and release processes are the dominant process, because the other time constants are negligible. Tuning parameters of photo current decay and rise parts under light on-off conditions.
Highly mobile copper-phthalocyanine field-effect transistors with tetratetracontane passive layer and metal-organic contacts. Temperature-dependent charge transfer in organic field-effect transistors by variation of both carrier concentration and electric field. High-performance flexible ultraviolet photoconductors based on solution-processed ultrathin ZnO/Au nanoparticle composite films.
Introduction
The lower thickness with greater roughness of the active layer ensures efficient absorption of analyte molecules. In this chapter, we studied the performance and stability of the CuPc/PMMA/PVA/Al2O3/Al device under long-term operation, device recovery, and bias stress effect. We also report the effect of various chemical analytes on the hysteresis, VTh, and charge carrier mobility of the devices.
Experimental section
Results and Discussion
- Capacitance and leakage current under vacuum and humidity
- Electrical characterization under vacuum
- Device parameter extraction under vacuum
- Bias-stress effect: Abnormal threshold voltage shift
- Electrical characterization under ambient
- Device parameter extraction under ambient humidity
- Bias-stress effect under ambient humidity
- Recovery of the device under bias-stress
- Drain voltage dependent transfer characteristics
- Gate voltage dependent bias-stress
- Bias-stress dependent hysteresis
- Effect of switching of V GS
- Long-term stability of the device
- Comparison of B device with SiO 2 device
The inset in Figure 7.2 (c) shows the zoomed-in view of the set of transfer characteristic curves to identify the shift. Recovery of the device under vacuum and ambient conditions after 30 min of bias voltage with VGS=VDS=–7V. To estimate the stability of the device, bias stress-dependent transfer characteristics were measured under vacuum and ambient conditions.
Effect of dipole moment on hysteresis and humidity sensor fabrication
- Experimental section
- Results and discussion
- Under saturated acetone vapors
- Under saturated hexane vapors
- Under saturated ethanol vapors
- Under saturated methanol vapors
- Under saturated chloroform vapors
- Under saturated diethyl ether vapors
- Under saturated Dichloromethane vapors
- Under saturated Acetonitrile vapors
- Under saturated water vapors
- Variation of I DS with analyte exposure time
- Variation of V Th with analyte exposure time
- Variation of Mobility with analyte exposure time
- Variation of device response at 15 min exposure time
- Bias stress effect under different environments
- Humidity sensor testing
Conclusions
Fabrication and Characterization of Flexible OFETs
Experimental section
Results and discussion
- Surface morphology
- Leakage current and Capacitance measurement
- Electrical characterization of the FOFETs under vacuum
- Bias-stress measurement of the FOFETs under vacuum
- Bending test measurements of the FOFET under vacuum
