High-performance solution-processed polymer space-charge-limited transistor

We demonstrate a polymer non-field-effect transistor in a vertical architecture with an Al grid embedded in a polymer sandwiched between another two electrodes. The Al grid containing high density of self-assembled submicron openings is fabricated by a non-lithography method. This device modulates the space-charge-limited current of a unipolar polymer diode with the Al grid. The operating voltage of the device is as low as 4 V, the on/off ratio is higher than one hundred, and the current gain is 10⁴. The current density is higher than 1 mA/cm².     

Effective protection of sequential solution-processed polymer/fullerene bilayer solar cell against charge recombination and degradation

Both charge recombination and degradation in sequential solution processed polymer/fullerene bilayer organic photovoltaics (OPV) are effectively reduced by the insertion of a TiO₂ inter-layer between the bilayer and Al electrode. The polymer/fullerene bilayer composed of a poly(3-hexylthiophene) (P3HT) bottom-layer and a [6,6] phenyl C61-butyric acid methyl ester (PCBM) top-layer shows significant change in morphology due to the substantial inter-penetration of P3HT and PCBM during the thermal annealing process. Consequently, the bilayer surface becomes P3HT rich resulting in significant charge recombination at the bilayer/Al interface of the bilayer OPV. The charge recombination rate of the bilayer OPV is reduced by one order of magnitude upon the insertion of a TiO₂ nanoparticle inter-layer between the bilayer and the Al electrode after the thermal annealing process. In contrast, when the thermal annealing process is conducted after insertion of the inter-layer, the effect of the TiO₂ inter-layer becomes insignificant. The V_OC and efficiency of the bilayer OPV is greatly enhanced from 0.37 to 0.66 V and 1.2% to 3.7%, respectively by utilizing the properly constructed TiO₂ inter-layer in the bilayer OPV. Additionally, insertion of the TiO₂ inter-layer significantly improves the stability of the bilayer OPV. The bilayer OPV with a TiO₂ inter-layer maintains 51% of its initial PCE after storage under dark ambient conditions for 700 h without encapsulation, whereas the bilayer OPV without a TiO₂ inter-layer did not show any solar cell performance after 200 h under the same conditions.     

Molecular subtractive surface-energy engineering of crosslinked Poly(4-vinylphenol) insulators for a solution-processed organic thin-film transistor

We propose a feasible method of manipulating the surface energy of crosslinked poly(4-vinylphenol) (c-PVP) thin films through the surface compositional modification assisted by an etchant. A physical picture of the surface-energy manipulation of c-PVP thin films based on the surface-selective molecular subtractive approach is clarified by investigating the chemical composition of the c-PVP film surfaces. We reveal that the molecular detachment by solvation on the surface leads to a reduction in the surface PVP density, thereby decreasing residual hydroxyl groups on the surface. In particular, it is found that the surface energy of a c-PVP thin film can be controlled by exploiting the thermal-treatment-time dependence of the soluble-PVP density. Organic thin-film transistors (TFTs) are fabricated via a solution process for demonstrating the applicability of our surface-energy-engineered c-PVP film as a gate insulator. The TFTs with the engineered c-PVP gate insulators exhibit improved electrical characteristics, compared to those with ordinary c-PVP gate insulators.     

High triplet,bipolar polymeric hosts for highly efficient solution-processed blue phosphorescent polymer light-emitting diodes

Two polymeric hosts PCzTPP and PCzTPPO with twisted geometrical configurations for blue phosphorescent polymer light-emitting diodes (PhPLEDs) were designed and synthesized by incorporating electron-accepting carbazole units with electron-donating TPP/TPPO groups. This molecular design endows PCzTPP and PCzTPPO with high glass transition temperatures of 204 °C and 215 °C, high triplet energies of 2.72 eV and bipolar features. In addition, the HOMO and LUMO of these polymers matched well with the HOMO of the hole-transport layer and the Fermi level of cathode compared with PVK, which facilitated the injection of holes and electrons. PCzTPP- and PCzTPPO-based single-emissive-layer blue PhPLEDs were fabricated with simplified device configuration by solution process using FIrpic as a dopant. These devices exhibited lower turn on voltages (<8 V) than PVK-based devices (12 V). The maximum luminances of PCzTPP- and PCzTPPO-based devices were twofold and threefold that of PVK-based devices, and the maximum current efficiencies were nearly threefold and ninefold, respectively. Moreover, PCzTPPO-based solution processed blue PhPLEDs with improved configuration showed maximum current efficiency and external quantum efficiency of 14.5 cd/A and 6.6%, respectively.     

A PNPN tunnel field-effect transistor with high-k gate and Iow-k fringe dielectrics

A PNPN tunnel field effect transistor(TFET) with a high-k gate dielectric and a low-k fringe dielectric is introduced.The effects of the gate and fringe electric fields on the TFET’s performance were investigated through two-dimensional simulations.The results showed that a high gate dielectric constant is preferable for enhancing the gate control over the channel,while a low fringe dielectric constant is useful to increase the band-to-band tunneling probability.The TFET device with the proposed structure has good switching characteristics,enhanced on-state current,and high process tolerance.It is suitable for low-power applications and could become a potential substitute in next-generation complementary metal-oxide-semiconductor technology.     

All-solution processed polymer light-emitting diodes with air stable metal-oxide electrodes

We present an all-solution processed polymer light-emitting diode (PLED) using spincoated zinc oxide (ZnO) and vanadium pentoxide (V₂O₅) as electron and hole injecting contact, respectively. We compare the performance of these devices to the standard PLED design using PEDOT:PSS as anode and Ba/Al as cathode. We show that the all-solution processed PLEDs have an equal performance as compared to the standard design directly after fabrication. However, the ambient stability of the PLEDs with spincoated transition metal oxide electrodes is exceptionally good in comparison to the standard design.     

Reliability of SiO2 and high-k gate insulators: A nanoscale study with conductive atomic force microscopy

In this work, a conductive atomic force microscope (C-AFM) is used to study the reliability (degradation and breakdown, BD) of SiO₂ and high-k dielectrics. The effect of a current limit on the post-BD SiO₂ electrical properties at the nanoscale is discussed. In particular, the impact of a current limit imposed during the stress on the post-BD oxide conductivity at the position where BD has been triggered, the area affected by the BD event and the structural damage induced in the broken down region will be investigated. A purposely developed C-AFM with enhanced electrical performance (ECAFM) is also presented, which has been used for the electrical characterization of HfO₂/SiO₂ gate stacks. The conduction of the fresh (without stress), electrically stressed and broken down stacks have been analyzed.     

Inverted polymer solar cells with a solution-processed cesium halide interlayer

We demonstrate the utility of a low-cost cesium iodide interlayer spun from an aqueous or 2-ethoxyethanol solution on ITO in inverted polymer solar cells of the structure ITO/CsI/P3HT:PCBM/MoO₃/Al, where P3HT is poly(3-hexylthiophene) and PCBM is [6,6]-phenyl-C₆₀-butyric acid methyl ester. The power conversion efficiency (PCE) of optimized cells was ～3.4%, comparable to that we obtained for inverted cells with Cs carbonate. The thickness of the CsI film was adjusted by varying the solution concentration. The concentration affected the surface morphology of P3HT:PCBM and the density of fractal-like aggregates (possibly related to the presence of Cs and film fabrication conditions) formed near the anode, as revealed by scanning electron microscopy. Auger analysis indicated a P3HT-rich surface. Optimization of the cells was achieved also by varying the thickness of the MoO₃ and the drying/annealing conditions of the active layer, as was evident from the current–voltage characteristics, external quantum efficiency spectra, and PCE. The cells with the CsI interlayer were compared additionally to cells with CsCl or CsF interlayers (with a PCE of up to ～2.7%), which were inferior to the comparable cells with Cs₂CO₃ or CsI. The surface concentrations of Cs and the halide on ITO were monitored using X-ray photoelectron spectroscopy. The iodine level was low with the Cs:I ratio exceeding 8:1. In contrast, the Cs:Cl ratio was ～1.4:1 and the Cs:F ratio was ～1:1; the Cs₂CO₃ decomposed partially, as expected. Therefore, for CsI, as is the case for Cs₂CO₃ but not for CsF, Cs–O bonds are formed at the surface. Such bonds on ITO are important in modifying the ITO work function, improving the cell performance. The results indicate that spin coating solutions of the high polarity CsI is a promising and easy approach to introduce Cs–O on ITO in inverted structures for increased electron extraction from PCBM and possibly hole extraction from the P3HT-rich surface at the anode.     

A bilayer graphene nanoribbon field-effect transistor with a dual-material gate

This paper introduces dual-material gate (DMG) configuration on a bilayer graphene nanoribbon field-effect transistor (BLGNRFET). Its device characteristics based on nonequilibrium Green׳s function (NEGF) are explored and compared with a conventional single-material gate BLGNRFET. Results reveal that an on-off ratio of up to 10 is achievable as a consequence of both higher saturation and lower leakage currents. The advantages of our proposed DMG structure mainly lie in higher carrier transport efficiency by means of enhancing initial acceleration of incoming carriers in the channel region and the suppression of short channel effects. Drain-induced barrier lowering, subthreshold swing and hot electron effect as the key short channel parameters have been improved in the DMG-based BLGNRFET.     

Defect modification in ZnInSnO transistor with solution-processed Al2O3 dielectric by annealing

The properties of solution-processed Al₂O₃ thin films annealed at different temperatures were thoroughly studied through thermogravimetry–differential thermal analysis, UV–vis-NIR spectrophotometer measurements, scanning electron microscopy, X-ray diffraction, atomic force microscopy and a series of electrical measurements. The solution-processed ZnInSnO thin films transistors (TFTs) with the prepared Al₂O₃ dielectric were annealed at different temperatures. The TFTs annealed at 600 °C have displayed excellent electrical performance such as the field-effect mobility of 116.9 cm² V⁻¹ s⁻¹ and a subthreshold slope of 93.3 mV/dec. The performance of TFT device could be controlled by adjusting the annealing temperature. The results of two-dimensional device simulations demonstrate that the improvement of device performance are closely related with the reduction of interface defects between channel and dielectric and subgap density of stats (DOS) in the channel layer.     

White emission polymer light-emitting devices with efficient electron injection from alcohol/water-soluble polymer/Al bilayer cathode

We demonstrate highly efficient white emission polymer light-emitting diodes (WPLEDs) from multilayer structure formed by solution processed technique, in which alcohol/water-soluble polymer, poly [(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) was incorporated as electron-injection layer and Al as cathode. It was found that the device performance was very sensitive to the solvents from solution of which the PFN electron-injection layer was cast. Devices with electron-injection layer cast from methanol solution show degraded performance while the best device performance was obtained when mixed solvent of water and methanol with ratio of 1:3 was used. We attribute the variation in device performance to washing out the electron transport material in the emissive layer due to rinse effect. As a result of alleviative loss of electron transport material in the emissive layer, the optimized device with a peak luminous efficiency of 18.5 cd A^?1 for forward-viewing was achieved, which is comparable to that of the device with same emissive layer but with low work-function metal Ba cathode (16.6 cd A^?1). White emission color with Commission International de I’Eclairage coordinates of (0.321, 0.345) at current 10 mA cm^?2 was observed.     

Large plate-like organic crystals from direct spin-coating for solution-processed field-effect transistor arrays with high uniformity

Solution-processed organic crystals are important in field-effect transistors because of their highly ordered molecular packing and ease of device fabrication. For practical applications, the patterning of organic crystal transistor arrays is critical. However, uniformity, which concerns the variation in electrical performance among devices fabricated simultaneously on the same substrate, is a common consideration in the commercial applications of the solution-processed organic crystal transistor arrays. Here, a simple approach for fabricating field-effect transistor arrays based on organic plate-like crystals is reported. Through this method, a direct spin-coating process from a mixture solution of organic semiconductor and polymer dielectric can produce organic plate-like crystals. The grain size of the crystals is observed to be hundreds of micrometers. By controlling the concentrations of the active materials, the transistor arrays exhibit high uniformity and good device performance. The results presented in this work promise that this approach is a comparable technology to hydrogenated amorphous silicon-based FETs and is a great candidate for practical applications in electronic devices.     

A novel field effect transistor with dielectric polymer gel

A novel organic-field effect transistor (OFET) has been fabricated. This device is original in the sense that it can be produced in ambient conditions with facile and cost-effective methods. Experimental results surprisingly revealed a high mobility value at the order of 0.38 cm²/Vs, and the gate voltage is also found to be lower than 1 V. The device exhibited excellent transistor characteristics at low voltages. Threshold voltage is around 0.26 V with 10³ on/off ratio. The device design is based on high effective capacitance value of a polymer gel, 1 μF, which is sandwiched between glass substrates on which source and drain electrodes were constructed.     

AlGaN/GaN metal-oxide semiconductor heterostructure field-effect transistor with photo-chemical-vapor deposition SiO<Subscript>2</Subscript> gate oxide

High-quality SiO₂ was successfully deposited onto GaN by photo-chemicalvapor deposition (photo-CVD) using a D₂ lamp as the excitation source. The AlGaN/GaN metal-oxide semiconductor, heterostructure field-effect transistors (MOSHFETs) were also fabricated with photo-CVD oxide as the insulating layer. Compared with AlGaN/GaN metal-semiconductor HFETs (MESHFETs) with similar structure, we found that we could reduce the gate-leakage current by more than four orders of magnitude by inserting the photo-CVD oxide layer in between the AlGaN/GaN and the gate metal. With a 2-μm gate, it was found that the saturated I_ds, maximum g_m, and gate-voltage swing (GVS) of the fabricated nitride-based MOSHFET were 512 mA/mm, 90.7 mS/mm, and 6 V, respectively.     

Aqueous solution-processed MoO3 as an effective interfacial layer in polymer/fullerene based organic solar cells

The authors demonstrate an effective anode interfacial layer based on aqueous solution-processed MoO₃ (sMoO₃) in poly (3-hexylthiophene) (P3HT) and indene-C60 bisadduct (ICBA) based bulk-heterojunction organic solar cells (PSCs). Various sMoO₃ concentration (0.03–0.25 wt%) was obtained by dissolving MoO₃ powder into deionized water directly with weak solubility. The characteristics of sMoO₃ films evaluated by atomic force microscope (AFM) and scanning electron microscope (SEM) suggest that the sMoO₃ films continuously cover the entire indium tin oxide (ITO) surface. The sMoO₃ based PSCs exhibit comparable power conversion efficiency with poly (3,4-ethylenedioxythiophene)–polystyrenesulfonic acid (PEDOT:PSS) based devices. However, even more importantly, the stability of sMoO₃ based devices have been greatly improved in air under continual light-illumination at 52 mW/cm². Further evaluations on Mo valence states and work function of sMoO₃ films by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) demonstrate that the aqueous solution-processed MoO₃ could act as an better anode interfacial layer than the conventional PEDOT:PSS.     

High-k polymer dielectrics with different cross-linked networks for nonvolatile transistor memory device

Nonvolatile OFET memory devices using different pPFPA/bPEI cross-linked polymers as the dielectric layer are fabricated. The influence of bPEI content on the electrical property and memory performance of devices are systematically investigated. The results demonstrate that the introduction of bPEI into pPFPA can significantly enhance the capacitance and dielectric constant of the pPFPA/bPEI cross-linked polymer dielectrics, but it also causes a slight increase in the leakage current density. Besides, the excess bPEI induces more morphology defects of the semiconductor film, leading to an apparent decrement in charge mobility. Transistors with the 119:250 pPFPA/bPEI dielectric layer exhibit the highest on/off current ratio (~10⁷ at V_g = − 20V) and a relatively low hole mobility of 0.38 cm² V⁻¹s⁻¹. Moreover, the corresponding memory devices show good reliability in information record with a data retention time over 10⁵ s, indicating that an appropriate amount of bPEI is crucial for improving the stability of the memory devices.     

Effective improvement of high-k Hf-silicate/silicon interface with thermal nitridation

The electrical properties of polysilicon gate MOS capacitors with hafnium silicate (HfSiO) dielectric, with and without NH/sub 3/ nitridation, were investigated. The results show that with NH/sub 3/ nitridation prior to deposition of HfSiO can effectively tune the flatband voltage close to that of conventional oxide and significantly improve the leakage properties over SiO/sub 2/ (three orders reduction). Furthermore, the excellent interface quality has been evidenced by the result of immunity against soft breakdown with NH/sub 3/ nitridation.     

An NMOS input merged bipolar/sidewall-MOS transistor with a bypasssidewall MOS transistor (NBiBMOS transistor)

The concept of merging a vertical n-p-n bipolar and two sidewall NMOS transistors into an NMOS input merged bipolar/sidewall-MOS transistor with a bypass sidewall NMOS transistor structure (NBiBMOS transistor) is described. The output current of this structure, unlike that of NBiMOS transistors, is significant even when the output voltage (V_CE or V_DE) is less than the turn-on voltage of the n-p-n bipolar transistor (V_BE=~0.8 V). This structure, when used in BiCMOS logic gates, will allow the output voltage to swing all the way to 0.0 V rather than to 0.8 V. The feasibility of this concept was demonstrated by fabricating and DC characterizing the NBiBMOS transistor structures, which occupy ~1.2 times the area of a single n-p-n bipolar transistor. The NBiBMOS transistor has a higher drive capability than that of a structure consisting of an NBiMOS and a separate bypass transistor, because the body-source junction of the bypass NMOS transistor is forward biased     

High efficiency, solution-processed, red phosphorescent organic light-emitting diodes from a polymer doped with iridium complexes

High efficiency, solution-processed, red emissive phosphorescent organic light-emitting diodes (PhOLEDs) have been developed. The PhOLEDs utilize bis[9-ethyl-3-(4-phenylquinolin-2-yl)-9H-carbazolato-N,C²′]iridium picolinate (Et-Cvz-PhQ)₂Ir(pic) and bis[9-(2-(2-methoxyethoxy)ethyl)-3-(4-phenylquinolin-2-yl)-9H-carbazolato-N,C²′]iridium picolinate (EO-Cvz-PhQ)₂Ir(pic) in a nonconjugated polymer host of PVK that contains the electron transport material of OXD-7 and the hole transport material TPD. The electroluminescence (EL) spectra of the PhOLEDs parallel those of (Et-Cvz-PhQ)₂Ir(pic) and (EO-Cvz-PhQ)₂Ir(pic) with maxima at 608 nm and a CIE (Commission International de l’Eclairage) coordinate of (0.62, 0.38). The red emitting PhOLEDs, based on ITO/PEDOT:PSS/PVK:OXD-7:TPD:Ir complex/cathode configuration, have a maximum external quantum efficiency of 10.6% and a luminance efficiency of 17.5 cd/A. The efficiency is significantly higher than those obtained using common solution-processed red emissive PhOLEDs.     

Improved efficiency and stability of inverted polymer solar cells with a solution-processed BPhen interlayer and polystyrene beads

We demonstrate improved power conversion efficiency (PCE) and strongly enhanced stability of inverted organic solar cells (OSCs) with Cs halides by solution casting BPhen (4,7-di(phenyl)-1,10-phenanthroline) on the halide layer and ∼100 nm polystyrene beads (PSB) on the blank side of the OSC’s substrate. The PCE of ITO/CsCl/P3HT:PCBM/MoO₃/Al (where P3HT is poly 3-hexylthiophene and PCBM is [6,6]-phenyl-C₆₀-butyric acid methyl ester) improves by up to 46%, from 2.5% to ∼3.7%, by adding a solution-processed BPhen layer between the CsCl and the active layer. For such cells with CsI (PCE ∼3.3–3.4%) the increase was only 6–9%, to 3.5–3.7%. The PCE of cells devoid of the halides but with BPhen was ∼3.3%. The cells were optimized by varying the BPhen concentration in a chlorobenzene solution. The results are consistent with reduced charge recombination at the ITO interface in the presence of the hole blocking BPhen interlayer. The use of hole blocking BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline), as a substitute for BPhen, also showed an enhancement (though lower due to its lower electron mobility), verifying the effect of these materials as hole blocking interlayers. Interestingly, the stability of such non-encapsulated devices with CsCl/BPhen or CsI/BPhen improved significantly. For example, the PCE of unencapsulated cells with CsCl/BPhen kept in the dark under ambient conditions dropped by less than 2% after more than 3 weeks; the PCE of similar cells devoid of the BPhen layer dropped by ∼60% during the same period. The PCE of the cell with CsCl/BPhen dropped by ∼16% after 2 months. High humidity, as expected, resulted in faster deterioration in cell performance. The PCE, however, was restored to within ∼10% of the original value for 2 week old cells by solution–application of a PSB layer on the blank side of the cell’s glass substrate. These beads direct and scatter the light to enhance absorption in the active layer. The results demonstrate that a simple approach such as casting a film of ∼100 nm diameter PSB from an aqueous suspension on the blank side of the OSC substrate can improve long-term performance, and that spin coating BPhen is a low-cost and easy approach to reduce charge recombination at the cathode in inverted structures for increased PCE and stability.