Accurate capacitance-voltage characterization of organic thin films with current injection

To deal with the invalidation of commonly employed series model and parallel model in capacitance-voltage(C-V)characterization of organic thin films when current injection is significant,a three-element equivalent circuit model is proposed.On this basis,the expression of real capacitance in consideration of current injection is theoretically derived by small-signal analysis method.The validity of the proposed equivalent circuit and theoretical expression are verified by a simulating circuit consisting of a capacitor,a diode,and a resistor.Moreover,the accurate C-V characteristic of an organic thin film device is obtained via theoretical correction of the experimental measuring result,and the real capacitance is 35.7%higher than the directly measured capacitance at 5-V bias in the parallel mode.This work strongly demonstrates the necessity to consider current injection in C-V measurement and provides a strategy for accurate C-V characterization experimentally.

Performance improvement in pentacene organic thin film transistors by inserting a C_(60) ultrathin layer

The contact effect on the performances of organic thin film transistors is studied here. A C60 ultrathin layer is inserted between Al source-drain electrode and pentacene to reduce the contact resistance. By a 3 nm C60 modification, the injection barrier is lowered and the contact resistance is reduced. Thus, the field-effect mobility increases from 0.12 to 0.52 cm2/（V.s）. It means that inserting a C60 ultra thin layer is a good method to improve the organic thin film transistor （OTFT） performance. The output curve is simulated by using a charge drift model. Considering the contact effect, the field effect mobility is improved to 1.15 cm2/（V-s）. It indicates that further reducing the contact resistance of OTFTs should be carried out.

Organic Thin Film Electroluminescent Passive Matrix Display

Long life green emitting matrix display based on organic light emitting diode is reported. The pixel number is 96×60, equivalent pixel size 0.4×0.4 mm 2, and the pixel gap 0.1 mm. An image with no crosstalk between pixels is obtained. The average luminance of these pixels at duty cycle of 1/64 is 100 cd/m 2, and the power consumption is 0.6 W. The dark room contrast of 1∶100 is achieved without using a polarization filter.

Contact-Size-Dependent Cutoff Frequency of Bottom-Contact Organic Thin Film Transistors

The contact-size-dependent characteristic of cutoff frequency fT in bottom-contact organic thin film transistors （OTFTs） is studied. The effects of electrode thickness, field-effect mobility, channel length and gate-source voltage on the contact length （source and drain electrodes＇ length） related contact resistance of bottom-contact OTFTs are performed with a modified transmission line model. It is found that the contact resistance increases dramatically when the contact length is scaled down to 20O nm. With the help of the contact length related contact resistance, contact-size-dependent fT Of bottom-contact OTFTs is studied and it is found that fr increases with the decrease of the contact length in bottom-contact OTFTs.

Top Contact Pentacene Organic Thin Film Field Effect Transistors

Using pentacene as an active material, the organic thin film transistors were fabricated on Si3N4/p-Si substrates by using RF-magnetron sputtered amorphous aluminium as the gate electrode contact, and using highly doped Si as the gate electrode and substrate with plasma-enhanced chemical vapor deposited （PECVD） silicon nitride as gate dielectric. Pentacene thin films were deposited by thermal evaporation on dielectrics as the active layer, then RF-magnetron sputtered amorphous aluminium was used as the source and drain contacts. Measurement results show that field effect mobility and threshold voltage are 0.043 cm2/（V·s） and 12.6 V, respectively, and on-off current ratio is nearly 1×103.

Organic Electroluminescent Thin Film Using 8-hydroxyquinoline Zinc as Emitting Layer

Organic electroluminescent thin film using Znq 2 (Znq 2) as the emitting layer material with structure of glass/ITO/Znq 2/Al (cell)was fabricated. The V I curve, V B curve and electroluminescent spectra of the cell were measured. Meanwhile the fluorescent spectra, excited spectra and absorption spectra of Znq 2 with powder and film states were also measured.

Temperature Dependence of Electrical Properties of Organic Thin Film Transistors Based on pn Heterojuction and Their Applications in Temperature Sensors

Organic thin film transistors based on an F16CuPc/α6T pn heterojunction have been fabricated and analyzed to investigate the temperature dependence of electrical properties and apply in temperature sensors. The mobility follows a thermally activated hopping process. At temperatures over 200 K, the value of thermal activation energy (EA) is 40. 1 meV, similar to that of the single-layer device. At temperatures ranging from 100 to 200 K, we have a second regime with a much lower EA of 16.3 meV, where the charge transport is dominated by shallow traps. Similarly, at temperatures above 200 K, threshold voltage (VT) increases linearly with decreasing temperature, and the variations of VT of 0.185 V/K is larger than the variation of VT (~0.020 V/K) in the single layer devices. This result is due to the interface dipolar charges. At temperatures ranging from 100 K to 200 K, we have a second regime with much lower variations of 0.090 V/K. By studying gate voltage (VG)-dependence temperature variation factor (k), the maximum value of k (~0.11 dec/K) could be obtained at VG = 5 V. Furthermore, the pn heterojunction device could be characterized as a temperature sensor well working at low operating voltages.

Properties of C_(60) thin film transistor based on polystyrene

This paper reports that the n-type organic thin-fihn transistors have been fabricated by using C60 as the active layer and polystyrene as the dielectric. The properties of insulator and the growth characteristic of C60 film were carefully investigated. By choosing different source/drain electrodes, a device with good performance can be obtained. The highest electron field effect mobility about 1.15 cm2/（V. s） could reach when Barium was introduced as electrodes. Moreover, the C60 transistor shows a negligible ＇hysteresis effect＇ contributed to the hydroxyl-free of insulator. The result suggests that polymer dielectrics are promising in applications among n-type organic transistors.

Capacitance-frequency Spectrum Characterization of Organics/Metal Schottky Diodes

An organics/metal Schottky diode is fabricated using 3, 4： 9, 10-perylenetetracarboxylic- dianhydride（PTCDA） thin film sandwiched between ITO and Au by simple thermal evaporation technique. The current-voltage（I-V） characteristics are investigated at room temperature in open air. The results show the rectification ratio is in excess of 100. From the capacitance-frequency（C-f） and capacitance-voltage（C-V） measurements, the Schottky barrier height between 0.2-0.3 eV is obtained according to standard Schottky theory.

Organic Electroluminescence Using TPD as the Emitting Layer

Organic electroluminescent thin film using TPD as an emitting layer with structure of Glass/ITO/TPD/Al is fabricated. Its V-I curve, V-B curve and electroluminescent spectra are measured and analysed.The blue emission with luminance of 0.74 cd/m 2 and luminous efficiency of 1.35×10 -3 lm/W is achieved at DC voltage of 24 V.

Enabling Solution Growth of Insoluble Organic Materials in Common Solvents

Copper phthalocyanine (CuPc) amorphous film was successfully deposited on a silicone substrate by physical vapor deposition. When the film was in contact with a common solvent such as aniline, 1-propanol and toluene, the CuPc solid film was partially dissolved followed by nucleation and crystal growth in the solution. Based on these experimental results, we propose a novel method for preparation of the organic thin film by combination of dry and wet processes.

Impact of source and drain contact thickness on the performance of organic thin film transistors

This paper analyzes the impact of source（ts） and drain（td） contact thicknesses on top contact（TC） and bottom contact（BC） organic thin film transistors（OTFTs） with a gate in the bottom, using a benchmarked industry standard Atlas 2-D numerical device simulator. The parameters including drive current（Ids）, mobility（μ）, threshold voltage（Vt）and current on-off ratio（ION/IOFF） are analyzed from the device physics point of view on different electrode thicknesses, ranging from infinitesimal to 50 nm, for both top and bottom contact structures. Observations demonstrate that the performance of the BC structure is more affected by scaling of ts=din comparison to its counterpart. In the linear region, the mobility is almost constant at all the values of ts=dfor both structures. However,an increment of 18% and 83% in saturation region mobility is found for TC andBC structures, respectively with scaling down ts=dfrom 50–0 nm. Besides this, the current on-off ratio increases more sharply in the BC structure.This analysis simplifies a number of issues related to the design and fabrication of organic material based devices and circuits.

Electronic Structure Effect of Polythiophene Derivatives and Nano N-Zn-Ag/TiO2 on Performance of Organic Thin Film Solar Cell

Nanocrystal N-Zn-Ag/TiO2 powders were prepared with N-Zn/TiO2 by photo deposition method. A series of pure polymers P3HT[poly（3-hexylthiophene）], P3OT[poly（3-octylthiophene）], P3DT[poly（3-decylthiophene）] and P3DDT[poly（3-dodecylthiophene）], was synthesized, which were used to synthesize p-n type semiconductor mate- rials P3HT/N-Zn-Ag-TiO2, P3OT/N-Zn-Ag-TiO2, P3DT/N-Zn-Ag-TiO2 and P3DDT/N-Zn-Ag-TiO2 by in situ che- mical method. X-Ray diffraction（XRD） and infrared（IR） spectroscopy showed the structure of the polymers and complexes. Ultraviolet-visible（UV-Vis） spectra and cyclic voltammograms（CV） showed the optical and electronic performance of the polymers and complexes. Two new single and double organic thin film heterojunction solar cells were prepared with the above mentioned synthesized powders as raw materials. Current-voltage（I-V） measurements indicate that the conversion efficiency of the single organic thin film heterojunction solar cell is higher than that of the double organic thin film heterojunction solar cells. Single organic thin film heterojunction solar cells based on P3DT/N-Zn-Ag-TiO2 can get a photoelectric conversion efficiency of 0.0408%. The performance of electronic trans- form between electron donor and acceptor on organic thin film solar cells was researched.

n-Channel Organic Transistors Processed from Halogen-Free Solvents： Solvent Effect on Thin-Film Morphology and Charge Transport

Non-chlorinated solvents are highly preferable for organic electronic processing due to their environmentally friendly characteristics. Four different halogen-free solvents, tetrafuran, toluene, meta-xylene and 1,2,4-trimethyl- benzene, were selected to fabricate n-channel organic thin film transistors （OTFTs） based on 3-hexylundecyl sub- stituted naphthalene diimides fused with （1,3-dithiol-2-ylidene）malononitrile groups （NDI3HU-DTYM2）. The OTFTs based on NDI3HU-DTYM2 showed electron mobility of up to 1.37 cm2-V-1·s -1 under ambient condition. This is among the highest device performance for n-channel OTFTs processed from halogen-free solvents. The dif- ferent thin-film morphologies, from featureless low crystalline morphology to well-aligned nanofibres, have a great effect on the device performance. These results might shed some light on solvent selection and the resulting solution process for organic electronic devices.

Asymmetric side-chain engineering of organic semiconductor for ultrasensitive gas sensing

Molecular structure of organic semiconductor plays a critical role in determining the performance and functionality of organic electronic devices,by optimizing the electrical,optical and physicochemical properties.Substituted alkyl chains are fundamental units in tailering the solubility and assemblability,among which the asymmetric properties have been reported as key element for controlling the packing motifs and intrinsic charge transport.Here,we expanded the scope of molecular asymmetry dependent sensing features based on a new series of naphthalene diimides(NDI)-based derivatives substituted with a same branching alkyl chain but various linear-shaped alkyl chains(Cn-).A clear molecular stacking change,from head-to-head bilayer to head-to-tail monolayer packing model,is observed based on the features of anisotropic molecular interactions with the change in the chain length.Most importantly,a unique LUMO level shift of 0.17 eV is validated for NDI-PhC4,providing a record sensitivity up to 150%to 0.01 ppb ammonia,due to the desired molecular reactivity and device amplification properties.These results indicate that asymmetric side-chain engineering opens a route for breath healthcare.

All-solution-processed PIN architecture for ultra-sensitive and ultra-flexible organic thin film photodetectors

An ideal organic thin film photodetectors(OTFPs) should adopt a hierarchical, multilayer p-type/blend-type/n-type(PIN) structure,with each layer having a specific purpose which could greatly improve the exciton dissociation while guarantee efficient charge transport. However, for the traditional layer-by-layer solution fabrication procedure, the solvent used can induce organic material mixing and molecular disordering between each layer. Hence, such architecture for OTFPs can now only be formed via thermal evaporation. In this paper, a contact-film-transfer method is demonstrated to all-solution processing organic PIN OTFPs on flexible substrates. The fabricated PIN OTFPs exhibit high photoresponse and high stability under continuous mechanical bending. Hence,the method we described here should represent an important step in the development of OTFPs in the future.

Control of polymorphism in solution-processed organic thin film transistors by self-assembled monolayers

Polymorphism of organic semiconductor films is of key importance for the performance of organic thin film transistors(OTFTs).Herein,we demonstrate that the polymorphism of solution-processed organic semiconductors in thin film transistors can be controlled by finely tuning the surface nanostructures of substrates with self-assembled monolayers(SAMs).It is found that the SAMs of 12-cyclohexyldodecylphosphonic acid(CDPA)and 12-phenyldodecylphosphonic acid(Ph DPA)induce different polymorphs in the dip-coated films of 2-dodecyl[1]benzothieno[3,2-b][1]benzothiophene(BTBT-C12).The film of BTBT-C12 on CDPA exhibits field effect mobility as high as 28.1 cm2 V-1 s-1 for holes,which is higher than that of BTBT-C12 on Ph DPA by three times.The high mobility of BTBT-C12 on CDPA is attributable to the highly oriented films of BTBT-C12 with a reduced in-plane lattice and high molecular alignment.

Pendant Group Effect of Polymeric Dielectrics on the Performance of Organic Thin Film Transistors

Polymer dielectric is superior to its inorganic counterparts due to not only the low cost and intrinsic flexibility,but also the readily tunable dielectric constant,surface charge trap density,charge ejection and releasing ability and dipole moment,and all these properties play decisive roles in regulating the characteristic and performances of organic thin film transistors(OTFT).However,systematical studies on the relationship between structure and properties of polymeric dielectrics are rare.To this end,a series of polymeric dielectrics with well-defined linkages(ester or amide bonds)and predesigned pendant groups(alkyl-and aromatic-groups)are synthesized in high yields.Detailed studies show that the polyamide dielectrics exhibit higher dielectric constant,surface charge trapping density,and better charge storage capability than corresponding polyester dielectrics.Further,increasing theπelectron delocalization of the pendant groups generally benefits the charge storage property and transistor memory behavior.Theoretical calculation reveals that the hydrogen bonding between the linkage groups and the energy alignment between polymeric dielectric and semiconductor are responsible for the observed performance differences of OTFT with different polymeric dielectrics.These results may shine light on the design of polymeric dielectrics for OTFTs with different applications.

Organic thin film transistors with a SiO_2/SiN_x/SiO_2 composite insulator layer

We have investigated a SiO2/SiNx/SiO2composite insulation layer structured gate dielectric for an organic thin film transistor(OTFT) with the purpose of improving the performance of the SiO2gate insulator. The SiO2/SiNx/SiO2composite insulation layer was prepared by magnetron sputtering.Compared with the same thickness of a SiO2insulation layer device,the SiO2/SiNx/SiO2composite insulation layer is an effective method of fabricating OTFT with improved electric characteristics and decreased leakage current.Electrical parameters such as carrier mobility by field effect measurement have been calculated.The performances of different insulating layer devices have been studied,and the results demonstrate that when the insulation layer thickness increases,the off-state current decreases.

OTFT with Bilayer Gate Insulator and Modificative Electrode

An organic thin-film transistor （OTFT） with an OTS/SiO2 bilayer gate insulator and a MoO3/AI electrode configuration between gate insulator and source/drain electrodes has been investigated. A thermally grown SiO2 layer is used as the OTFT gate dielectric and copper phthalocyanine（CuPc） is used as an active layer. This OTS/SiO2 bilayer gate insulator configuration increases the field-effect mobility, reduces the threshold voltage, and improves the on/off ratio simultaneously. The device with a MoO3/Al electrode has shown similar Ids compared to the device with an Au electrode at the same gate voltage. Our results indicate that using a double-layer of electrodes and a double-layer of insulators is an effective way to improve OTFT performance.