Influence of source/drain electrodes on external quantum efficiency of ambipolar organic light-emitting transistors

The influence of source/drain (S/D) electrodes on the external quantum efficiency (EQE) of ambipolar organic light-emitting transistors (OLETs) based on fluorene-type polymer films is investigated. The electrical properties and the maximum EQE value of the device with indium tin oxide (ITO) S/D electrodes are almost the same as those of the device with Ag S/D electrodes. A relatively high EQE of 1% is achieved regardless of the emission site for the OLET with ITO. In contrast, the EQE of the OLET with Ag is low when the emission occurs close to the S/D electrodes. The maximum EQE of the device with Ag is obtained when the emission is observed in the middle of the channel. It is found that the exciton quenching by Ag electrodes significantly influences the low EQE of the OLET with Ag electrodes. The achievement of high EQE regardless of the emission site is attributable to both better carrier injection and lower exciton quenching at the interface of S/D electrodes for the OLET with ITO.     

Simultaneous Tenfold Brightness Enhancement and Emitted‐Light Spectral Tunability in Transparent Ambipolar Organic Light‐Emitting Transistor by Integration of High‐k Photonic Crystal

In organic light‐emitting transistors, the structural properties such as the in‐plane geometry and the lateral charge injection are the key elements that enable the monolithic integration of multiple electronic, optoelectronic, and photonic functions within the same device. Here, the realization of highly integrated multifunctional optoelectronic organic device is reported by introducing a high‐capacitance photonic crystal as a gate dielectric into a transparent single‐layer ambipolar organic light‐emitting transistor (OLET). By engineering the photonic crystal multistack and bandgap, it is showed that the integration of the photonic structure has a twofold effect on the optoelectronic performance of the device, i.e., i) to modulate the spectral profile and outcoupling of the emitted light and ii) to enhance the transistor source–drain current by a 25‐fold factor. Consequently, the photonic‐crystal‐integrated OLET shows an order of magnitude higher emitted power and brightness with respect to the corresponding polymer‐dielectric device, while presenting as‐designed electroluminescence spectral and spatial distribution. The results validate the efficacy of the proposed approach that is expected to unravel the technological potential for the realization of highly integrated optoelectronic smart systems based on organic light‐emitting transistors.     

The color change in polychromatic organic light-emitting field-effect transistors: Optical filtering versus reemission

In this contribution the color conversion process of a polychromatic organic light-emitting field-effect transistor (OLET) is revisited on the basis of an analytic device model. The device of interest consists of a color conversion layer out of rubrene on top of a monochromatic light-emitting transistor based on poly(9,9-di-n-octyl-fluorene-alt-benzothiadiazole) (F8BT). The model describes the relation of color coordinate and emission intensity – set by the applied drain and gate biases – linking the optoelectronic response of the employed monochromatic OLET to the optical processes occurring in the color conversion layer. The model shows that the color shift is rather due to partial absorption of the F8BT emission by rubrene than, as was claimed earlier, due to a color conversion process by absorption and reemission in the conversion layer. In addition to the earlier publication, it will be demonstrated that such a device allows for an independent electrical tunability of emission intensity and color coordinate within the color span of the F8BT and the rubrene spectrum being a unique feature of such a polychromatic light-emitting field-effect transistor.     

Mapping charge carrier density in organic thin-film transistors by time-resolved photoluminescence lifetime studies

The device performance of organic transistors is strongly influenced by the charge carrier distribution. A range of factors effect this distribution, including injection barriers at the metal-semiconductor interface, the morphology of the organic film, and charge traps at the dielectric/organic interface or at grain boundaries. In our comprehensive experimental and analytical work we demonstrate a method to characterize the charge carrier density in organic thin-film transistors using time-resolved photoluminescence spectroscopy. We developed a numerical model that describes the electrical and optical responses consistently. We determined the densities of free and trapped holes at the interface between the organic layer and the SiO₂ gate dielectric by comparison to electrical measurements. Furthermore by applying fluorescence lifetime imaging microscopy we determine the local charge carrier distribution between source and drain electrodes of the transistor for different biasing conditions. We observe the expected hole density gradient from source to drain electrode.     

Efficient and bright polymer light emitting field effect transistors

Light emitting field effect transistors (LEFETs) are emerging as a multi-functional class of optoelectronic devices. LEFETs can simultaneously execute light emission and the standard logic functions of a transistor in a single architecture. However, current LEFET architectures deliver either high brightness or high efficiency but not both concurrently, thus limiting their use in technological applications. Here we show an LEFET device strategy that simultaneously improves brightness and efficiency. The key step change in LEFET performance arises from the bottom gate top-contact device architecture in which the source/drain electrodes are semitransparent and the active channel contains a bi-layer comprising of a high mobility charge-transporting polymer, and a yellow–green emissive polymer. A record external quantum efficiency (EQE) of 2.1% at 1000 cd/m² is demonstrated for polymer based bilayer LEFETs.     

Integration of silk protein in organic and light-emitting transistors

We present the integration of a natural protein into electronic and optoelectronic devices by using silk fibroin as a thin film dielectric in an organic thin film field-effect transistor (OFET) ad an organic light emitting transistor device (OLET) structures. Both n- (perylene) and p-type (thiophene) silk-based OFETs are demonstrated. The measured electrical characteristics are in agreement with high-efficiency standard organic transistors, namely charge mobility of the order of 10(-2) cm(2)/Vs and on/off ratio of 10(4). The silk-based optolectronic element is an advanced unipolar n-type OLET that yields a light emission of 100nW.     

High-κ related reliability issues in advanced non-volatile memories

In the last decade, important technology solutions have been proposed to scale down Flash memory devices beyond the 30 nm node. The most important innovations are the introduction of charge trapping layer and high-κ materials in both bottom and top dielectric stacks. Such innovations allow reducing both the bottom dielectric thickness and the Program/Erase (P/E) voltages, while maintaining the P/E performances without degrading (theoretically) the memory device reliability. Theoretical advantages and reliability issues of these important innovations will be reviewed by addressing physical mechanisms responsible of reliability degradation. In particular, the reliability consequences of the discrete charge storage and of the high-κ band-gap engineered barriers bottom and top dielectric stacks will be carefully analyzed, relating high-κ material properties to memory device performances and reliability.     

Unlocking the full potential of light emitting field-effect transistors by engineering charge injection layers

Light emitting field-effect transistors (LEFETs) are a class of next generation devices which combine the switching properties of field-effect transistors (FETs) with light emitting capabilities of organic light-emitting diodes (OLEDs) in a single device architecture. Current LEFET architectures suffer from inefficient charge injection of electrons and holes from the source and drain electrodes, leading to unbalanced charge transport and hence poor device performance. Here we report a simple fabrication method for LEFETs that delivers asymmetric source and drain electrodes comprised of low and high work function materials. The interdigitated low and high work function source–drain electrodes consist of combinations of organic materials, salts, metal oxides and metals. Using this method we were able to obtain a maximum EQE of up to 1.2% in a single layer device with Super Yellow as the active material.     

Experimental and numerical analysis of channel-length-dependent electrical properties in bottom-gate,bottom-contact organic thin-film transistors with Schottky contact

Channel length dependence of field-effect mobility and source/drain parasitic resistance in pentacene thin-film transistors with a bottom-gate, bottom-contact configuration was investigated. Schottky barrier effect such as nonlinear behaviors in transistor output characteristics appeared and became more prominent for shorter channel length less than 10 μm, raising some concerns for a simple utilization of conventional parameter extraction methods. Therefore the gate-voltage-dependent hole mobility and the source/drain parasitic resistance in the pentacene transistors were evaluated with the aid of device simulation accounting for Schottky contact with a thermionic field emission model. The hole mobility in the channel region shows smaller values with shorter channel length even after removing the influence of Schottky barrier, suggesting that some disordered semiconductor layers with low carrier mobility exist near the contact electrode. This experimental data analysis with the simulation enables us to discuss and understand in detail the operation mechanism of bottom-gate, bottom-contact transistors by considering properly each process of charge carrier injection, carrier flow near the contact region, and actual channel transport.     

In-plane light emission of organic light-emitting transistors with bilayer structure using ambipolar semiconducting polymers

The concept of using an ambipolar bilayer semiconducting heterostructure in organic light-emitting transistors (OLETs) is introduced to provide a new approach to achieve surface emission. The properties of top-gate-type bilayer OLETs with ambipolar materials based on two types of fluorene-type polymers used as an emissive layer and an electron blocking layer are investigated. Line-shaped yellow–green emission occurs near a hole-injection electrode. When hole transport is dominant in the upper layer which acts as an electron blocking layer, and electrons are injected into the lower layer, an in-plane light-emitting pattern is observed. The measured in-plane emission zone confirms that both hole and electron transport are determined to occur mainly along the different organic layers between the source and drain electrodes, and an in-plane recombination zone of electrons and holes exists near the bilayer organic interface. This work is anticipated to be useful for the development of in-plane light-emitting transistors.     

Selectively Fluorinated Furan-Phenylene Co-Oligomers Pave the Way to Bright Ambipolar Light-Emitting Electronic Devices

Linearly conjugated oligomers attract ever-growing attention as promising systems for organic optoelectronics because of their inherent lucky combination of high charge mobility and bright luminescence. Among them, furan-phenylene co-oligomers (FPCOs) are distinguished by outstanding solubility, very bright luminescence, and good hole-transport properties; however, furan-containing organic semiconductors generally lack electron transport, which makes it impossible to utilize them in efficient light-emitting electronic devices, specifically, ambipolar light-emitting transistors. In this work, 1,4-bis(5-phenylfuran-2-yl)benzene (FP5) derivatives are synthesized with the fully/partially fluorinated central and edge phenyl rings. It is shown that the selective fluorination of FPCOs lowers the energies of frontier molecular orbitals, maintaining the bandgap, solubility, and bright luminescence, dramatically improves the photostability, tunes the π-π stacked packing, and allows the first realization of electron transport in FPCOs. It is found that selectively fluorinated 2,2′-(2,3,5,6-tetrafluoro-1,4-phenylene)bis[5-(3,5-difluorophenyl)furan] demonstrates well-balanced ambipolar charge transport and efficient electroluminescence in an organic light-emitting transistor (OLET) with external quantum and luminous efficiencies as high as 0.63% and 5 cdA⁻¹, respectively, which are among the best reported for OLETs. The findings show that “smart” fluorination is a powerful tool to fine-tune the stability and performance of linearly conjugated small molecules for organic optoelectronics.     

Organic Light‐Emitting Transistors Containing a Laterally Arranged Heterojunction

An approach to produce organic light‐emitting transistors (OLETs) containing a laterally arranged heterojunction structure, which minimizes exciton quenching at the metal electrodes, is described. This device configuration provides an organic light‐emitting diode (OLED) structure where the anode (source) electrode, hole‐transport material (field‐effect material), light‐emitting material, and cathode (drain) electrode are laterally arranged, thus offering a chance to control the electroluminescent intensity by changing the gate bias. Pentacene and tris(8‐quinolinolato)aluminum (Alq₃) are employed as the field‐effect and light‐emitting materials, respectively. The laterally arranged heterojunction structures are achieved by successively inclined deposition of the field‐effect and light‐emitting materials. After deposition of pentacene, a narrow gap of about 10–20 nm between the drain electrode and pentacene was obtained, thereby creating an opportunity to fabricate a laterally arranged heterojunction. In the OLETs, unsymmetrical source and drain electrodes, that is, Au and LiF/Al ones, are used to ensure efficient injection of holes and electrons. Visible‐light emission from OLETs is observed under ambient atmosphere. This result is ascribed to efficient carrier injection and transport, formation of a heterojunction, as well as good luminescence from the organic emissive layer. The device structure serves as an excellent model system for OLETs and demonstrates a general concept of adjusting the charge‐carrier injection and transport, as well as the electroluminescent properties, by forming laterally arranged heterojunctions.     

Intrinsically Stretchable Light-Emitting Polymer Semiconductors with High Charge Mobility Through Micro-Crystalline Aggregation-Limited Morphology

Intrinsically stretchable light-emitting polymer semiconductors are essential building blocks for bioelectronics and display textiles. Stretchability is challenging for rigid conjugated polymers unless sacrificing charge mobility by introducing amorphous domains. High-performance light-emitting properties designed with twisted angle are undesirable for conductive materials. Hence, the concurrent strategies hardly satisfy the balance of stretchability, light-emitting and mobility. Herein, a morphology engineering is proposed by controlling micro-crystalline and limiting aggregation, that four intrinsically stretchable emissive polymers with good charge mobility based on indacenodithiophene (IDT) are obtained. Polymers reveal good emission properties with high photoluminescence quantum yields (PLQY) of about 20%, while stretchable modulus and charge mobility are tunable by backbone and weight. Specifically emphasizing, IDT-2T-H retains high performance of charge mobility and PLQY even at 100% strain. Therefore, organic light emitting diodes are fabricated based on it and showing the luminance of 176.2 cd cm⁻², which verifies the potential of technique to reconcile integration of stretchability, light-emitting, and mobility. This is the first attempt to integrate balanced mechanical, optical, and electrical properties through micro-crystalline aggregation-limited morphology in one polymer, offering a feasible approach to advanced integrated circuit and multi-functional electronics in the future.     

基于低温多晶硅薄膜晶体管的AMOLED交流像素电路

This work presents a new voltage programmed pixel circuit for an active-matrix organic light-emitting diode(AMOLED) display.The proposed pixel circuit consists of six low temperature polycrystalline silicon thinfilm transistors(LTPS TFTs),one storage capacitor,and one OLED,and is verified by simulation work using HSPICE software.Besides effectively compensating for the threshold voltage variation of the driving TFT and OLED,the proposed pixel circuit offers an AC driving mode for the OLED,which can suppress the degradation of the OLED.Moreover,a high contrast ratio can be achieved by the proposed pixel circuit since the OLED does not emit any light except for the emission period.     

Three-terminal light-emitting device with adjustable emission color

A three-terminal organic light-emitting device with a periodic interrupted middle electrode is developed to allow for an adjustable emission color. The emission results from three independent light-emitting diodes with one diode utilizing exciplex emission. An equivalent electrical circuit is suggested taking the current–voltage characteristics and the direction of current flow through the organic structure into account. Two diodes are formed between the embedded middle electrode and the LiF/Al top and ITO bottom electrode, respectively, and the third diode utilizes that part of the device without the middle-electrode exhibiting exciplex emission. It will be shown that the spectrum of the emitted light can be tuned from blue to orange by controlling the applied potentials to the device terminals.     

In-plane electroluminescence from microcavity organic light-emitting transistors

We report the observation of in-plane emission beneath the drain electrode in multilayer heterostructure organic light-emitting transistors (OLETs). A novel modification method for the interface between the hole transport layer and the emission layer has been proposed, which brought a great enhancement for the light power and external quantum efficiency. Further, distributed Bragg reflector was incorporated to the in-plane-emitted OLETs, which combined with the top thin layer of Au, forming a vertical microcavity. The electroluminescence spectra were significantly altered by the microcavity and much narrower linewidth was obtained. The results will help to develop high color purity and white OLETs with high performance, which would be useful for multifunctional displays.     

Flexible Low‐Power Operative Organic Source‐Gated Transistors

Low‐voltage operation and fast switching ability are necessary for wearable electronic devices. Recently, electrolyte dielectric materials have been widely used to decrease driving voltages; however, they often exhibit unwanted doping effects and power dissipation problems. Here, a method for dramatically lowering driving voltages is reported in organic electronics via source‐gated transistor (SGT) structures. SGTs are fabricated by evaporating asymmetric metals with different work functions for the source and drain electrodes. Versatile organic semiconductor‐based SGTs demonstrate a significantly lower drain voltage (<10 V) for the saturation regime compared to that of typical field‐effect transistors with the same dielectric layer (>80 V). Furthermore, coating reduced Pyronin B (rPyB) onto n‐type SGTs decreases the threshold voltage from 51.2 to 0.1 eV and improves air‐stability, exhibiting a maintained electron mobility (>90%) for 40 d. The air‐stability is due to both the energetic and kinetic factors, including a decreased lowest unoccupied molecular orbital level of the n‐type semiconductor after doping and covering the active layer with rPyB. Finally, flexible SGTs are fabricated on a Parylene‐C substrate that shows highly stable operation in a bending test. The results demonstrate a promising technology for low‐power, flexible electronic devices via electrode engineering.     

High-Gain Chemically Gated Organic Electrochemical Transistor

Organic electrochemical transistors (OECTs) have exhibited promising performance as transducers and amplifiers of low potentials due to their exceptional transconductance, enabled by the volumetric charging of organic mixed ionic/electronic conductors (OMIECs) employed as the channel material. OECT performance in aqueous electrolytes as well as the OMIECs’ redox activity has spurred a myriad of studies employing OECTs as chemical transducers. However, the OECT's large (potentiometrically derived) transconductance is not fully leveraged in common approaches that directly conduct chemical reactions amperometrically within the OECT electrolyte with direct charge transfer between the analyte and the OMIEC, which results in sub-unity transduction of gate to drain current. Hence, amperometric OECTs do not truly display current gains in the traditional sense, falling short of the expected transistor performance. This study demonstrates an alternative device architecture that separates chemical transduction and amplification processes on two different electrochemical cells. This approach fully utilizes the OECT's large transconductance to achieve current gains of 10³ and current modulations of four orders of magnitude. This transduction mechanism represents a general approach enabling high-gain chemical OECT transducers.     

Solution‐Processed Organic Light‐Emitting Transistors Incorporating Conjugated Polyelectrolytes

Improved performance of p‐type organic light‐emitting transistors (OLETs) is demonstrated by introducing a conjugated polyelectrolyte (CPE) layer and symmetric high work function (WF) source and drain metal electrodes. The OLET comprises a tri‐layer film consisting of a hole transporting layer, an emissive layer, and a CPE layer as an electron injection layer. The thickness of the CPE layer is critical for achieving good performance and provides an important structural handle for consideration in future optimization studies. We also demonstrate for the first time, good performance solution‐processed blue‐emitting OLETs. These results further demonstrate the simplification of device fabrication and improved performance afforded by integrating CPE interlayers into organic optoelectronic devices.     

Enhanced Charge Injection Through Nanostructured Electrodes for Organic Field Effect Transistors

Nanosphere lithography is used to process nanopore‐structured electrodes, which are applied into the fabrication of bottom‐gate, bottom‐contact configuration organic field effect transistors (OFETs) to serve as source/drain elecrodes. The introduction of this nanopore‐structure electrode facilitates the forming of nanopore‐structure pentacene layers with small grain boundaries at the electrode interface, and then reduces the contact resistance, contact‐induces the growth of pentacene and accordingly improves the mobility of charge carriers in the OFETs about 20 times as compared with results in literature through enhancing the charge carrier injection. It is believed that this structure of electrode is a valuable approach for improving organic filed effect transistors.