Organic Light-emitting Transistors Research Articles

Highly Efficient Flexible Organic Light Emitting Transistor Based on High‐k Polymer Gate Dielectric

AbstractIn this work, the multilayer organic light emitting transistors (OLETs) based on high‐k polymer crosslinked poly(vinyl alcohol) (C‐PVA) as the dielectric layer are studied. The devices show an excellent brightness of 14 500 cd m−2 and a record breaking external quantum efficiency (EQE) of about 9.0% in inert atmosphere. The use of perfluoro(1‐butenyl vinyl ether) polymer (CYTOP) modified C‐PVA as a hydrophobic dielectric affords OLETs with good stability, displaying a maximum brightness of 13 400 cd m−2 and EQE of 7.31% in ambient conditions with high humidity (relative humidity RH = 70%). Furthermore, the flexible OLETs based on CYTOP/C‐PVA dielectric layer show a brightness of 8300 cd m−2 and a peak EQE at 9.01%, which is the first reported flexible OLET with >500 cd m−2 brightness. The excellent OLET device performance is ascribed to the excellent hole transport structure, high efficiency of guest–host system, and the better carrier balance in the emitting layer, which could be a practical strategy to develop high‐performance flexible OLETs. It is proven for the first time that OLETs could achieve such high EQE and brightness, even for flexible OLETs, paving the way for their future application in industry.

Enhanced ambipolar charge transport for efficient organic single crystal light-emitting transistors with a narrowed ambipolar regime

An efficient ambipolar OLET was constructed based on a high-mobility emissive 2,6-diphenylanthracene semiconductor via energy-level engineering, giving a narrowed ambipolar regime with strong light emission.

High-performance amorphous organic semiconductor-based vertical field-effect transistors and light-emitting transistors.

Herein, two kinds of vertical organic optoelectronic devices, vertical organic field-effect transistors (VOFETs) and light-emitting transistors (VOLETs), were constructed based on amorphous organic semiconductors of N,N'-di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine (NPB) as hole injecting and transport layers and tris(8-hydroxy-quinolinato) aluminum (Alq3) as the emitting layer. High device performances with a large on/off ratio of ∼6 × 103, current density of ∼40 mA cm-2, and fast response of ∼5 ms at a frequency of 20 Hz and a brightness of 126 cd m-2 were demonstrated for these two vertical devices with good device stability and repeatability. These results suggest the potential applications of amorphous organic semiconductors with good film-forming characteristics and easy device fabrication ability in vertical optoelectronic circuits.

Foldable semi-ladder polymers: novel aggregation behavior and high-performance solution-processed organic light-emitting transistors.

A critical issue in developing high-performance organic light-emitting transistors (OLETs) is to balance the trade-off between charge transport and light emission in a semiconducting material. Although traditional materials for organic light-emitting diodes (OLEDs) or organic field-effect transistors (OFETs) have shown modest performance in OLET devices, design strategies towards high-performance OLET materials and the crucial structure–performance relationship remain unclear. Our research effort in developing cross-conjugated weak acceptor-weak donor copolymers for luminescent properties lead us to an unintentional discovery that these copolymers form coiled foldamers with intramolecular H-aggregation, leading to their exceptional OLET properties. An impressive external quantum efficiency (EQE) of 6.9% in solution-processed multi-layer OLET devices was achieved.

Design of High-Performance Organic Light-Emitting Transistors.

Organic light-emitting transistors (OLETs) integrate the light-emitting and gate-modulated electrical switching functions in a single device. Over the past decades, progress has been made in developing new fluorescent semiconductors and device engineering that pushed efficiencies of OLET devices to 8%. However, this efficiency of transistors is still too low to be competitive with organic light-emitting diodes (OLEDs). Currently, there are relatively few suitable organic fluorescent semiconductors suitable for OLETs, and the mechanism of electroluminescence in the devices is still not fully understood. In this mini-review, we discuss the state of highly efficient OLETs and plausible approaches to those unsettled problems. Since this is a mini-review, we will not be able to cover all the excellent work in the literature. Readers are encouraged to read other excellent reviews published earlier.

Indication of cooperative light amplification in 5,5″-bis(4-biphenylyl)−2,2′:5,2″-terthiophene single crystals at room temperature

Among the large number of π-conjugated compounds, thiophene/phenylene co-oligomers have been one of the promising candidates for both optically pumped light amplification media and light emission media in electroluminescence devices owing to their high thermal stability, high photoluminescence (PL) quantum yield and high carrier mobility. Of these, the single-crystal state of 5,5″-bis(4-biphenylyl)−2,2′:5′,2″-terthiophene (BP3T), which has been frequently used as an active medium in organic light-emitting transistor devices, indicates an unusual light amplification process in its PL spectra. Herein, we explain two insights about the indication of a cooperative light amplification process in BP3T single crystals under optical pumping. The first one is excitation beam area dependence for the nonlinear amplification threshold in PL spectra at room temperature, which has previously been carried out at 12 K. The second one is time-delayed amplified emission in various excitation stripe lengths.

Prospect of silver nanowire (AgNW) in development of simple and cost-effective vertical organic light-emitting transistors

Despite a great potential for low-voltage display applications, vertical organic light-emitting transistors (VOLETs) suffer serious issues of high-cost and complex fabrication techniques, notably for the intermediate electrode. To address this problem, this study demonstrates a cost-effective and simple approach to fabricate a VOLET device by utilising spin-coated silver nanowires (AgNWs) as an intermediate electrode. AgNWs exhibit high electrical conductivity, high porosity and high optical transparency, which qualify them as a perfect candidate for the intermediate electrode in VOLETs. To show the potential of AgNWs in VOLET devices using a facile, cost-effective spin-coated method, two types of VOLETs, namely, the Schottky barrier (SB) VOLET and static induction transistor (SIT) VOLET, were fabricated and analysed. Interestingly, both the devices show transistor behaviour when the Vg is varied, implying a fully functional VOLET device. We believe that this is one of the simplest methods to fabricate VOLETs without compromising the device characteristics demonstrated to date.

Annealing effects on output characteristics of solution processable vertical organic light-emitting transistor (VOLET)

The influence of annealing temperature on the output characteristics of solution processable vertical organic light-emitting transistor (VOLET) was investigated. Poly[(vinylidenefluoride-co-trifluoroethylene] (PVDF-TrFE) and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) have been used as a dielectric and emissive layer, respectively, in a silver nanowire (AgNW) sourced VOLET. It was found that the performance of the VOLET is the best after annealed at 100 oC. A high polarization of the PVDF-TrFE promotes more charged carrier to be accumulated at AgNW/MEH-PPV interface. Surface morphology and photoluminescence results show that a high surface roughness of MEH-PPV exhibit bright emission due to reduction of internal reflection of light emission.

Solution-Processable Vertical Organic Light-Emitting Transistors (VOLETs) with Directly Deposited Silver Nanowires Intermediate Source Electrode.

A simple spin-coating process for fabricating vertical organic light-emitting transistors (VOLETs) is realized by utilizing silver nanowire (AgNW) as a source electrode. The optical, electrical and morphological properties of the AgNW formation was initially optimized, prior VOFET fabrication. A high molecular weight of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] MEH-PPV was used as an organic semiconductor layer in the VOFET in forming a multilayer structure by solution process. It was found that current density and luminance intensity of the VOLET can be modulated by a small magnitude of gate voltage. The modulation process was induced by changing an injection barrier via gate voltage bias. A space-charge-limited current (SCLC) approach in determining transistor mobility has been introduced. This preliminary and fundamental work is beneficial towards all-solution processing display devices.

Strong Near-Infrared Solid Emission and Enhanced N-Type Mobility for Poly(naphthalene Diimide) Vinylene by a Random Polymerization Strategy

Organic semiconductors possessing both strong solid emission and charge transport ability are crucial for certain applications such as organic light-emitting transistors and injection lasers, yet t...

Fabrication of Pixelated Organic Light‐Emitting Transistor (OLET) with a Pure Red‐Emitting Organic Semiconductor

AbstractA pure red‐emitting organic light‐emitting transistor (OLET) is successfully fabricated using a π‐extended dicyanodistyrylbenzene‐type organic semiconductor material (Hex‐4‐TFPTA), which shows outstanding charge transport and solid‐state luminescence at the same time. Based on the structural (X‐ray) and photophysical analyses, it is found that the appropriate molecular stacking and highly allowed S1→S0 transition of Hex‐4‐TFPTA created the ambidextrous balance between electrical mobility (µe, FET ≈ 1 cm2 V−1 s−1) and solid‐state luminescent quantum yield, even in the deep‐red (DR) region (ΦF = 28%). Remarkably, the color purity of the red Hex‐4‐TFPTA thin film (0.70, 0.30) is close to the National Television System Committee (NTSC) standard primary red (0.67, 0.33) in Commission Internationale de I'Eclairage (CIE) color space. Moreover, in this work, micropixelated areal OLET emission is demonstrated for the first time by patterning the organic layer via a novel soft‐lithographic technique called “patterned taping.”

High-Efficiency Single-Component Organic Light-Emitting Transistors.

Construction of high-performance organic light-emitting transistors (OLETs) remains challenging due to the limited desired organic semiconductor materials. Here, two superior high mobility emissive organic semiconductors, 2,6-diphenylanthracene (DPA) and 2,6-di(2-naphthyl) anthracene (dNaAnt), are introduced into the construction of OLETs. By optimizing the device geometry for balanced ambipolar efficient charge transport and using high-quality DPA and dNaAnt single crystals as active layers, high-efficiency single-component OLETs are successfully fabricated, with the demonstration of strong and spatially controlled light emission within both p- and n- conducting channels and output of high external quantum efficiency (EQE). The obtained EQE values in current devices are approaching 1.61% for DPA-OLETs and 1.75% for dNaAnt-based OLETs, respectively, which are the highest EQE values for single-component OLETs in the common device configuration reported so far. Moreover, high brightnesses of 1210 and 3180 cd m-2 with current densities up to 1.3 and 8.4 kA cm-2 are also achieved for DPA- and dNaAnt-based OLETs, respectively. These results demonstrate the great potential applications of high mobility emissive organic semiconductors for next-generation rapid development of high-performance single-component OLETs and their related organic integrated electro-optical devices.

Vertical Organic Tunnel Field-Effect Transistors

Doping organic semiconductors has become a key technology to increase the performance of organic light-emitting diodes, solar cells, or field-effect transistors (OFETs). However, doping can be used...

A strategy to boost external quantum efficiency of organic light-emitting transistors

Organic light-emitting transistors (OLETs) have emerged as promising light-emitting electronics, combining the functions of a transistor and a diode in a single device. OLETs typically possess outstanding electrical properties but have inefficient optical properties. Here, based on optical simulations, a strategy of how to improve the light output as well as reduce optical energy losses in OLETs is performed. The results reveal that the main optical energy losses occur via the waveguide mode by the organic layers and through absorption by the gate material. These energy losses in OLETs can be significantly reduced by appropriately adjusting the thicknesses and refractive indices of the transporting layers as well as those of the gate materials. This device design opens perspectives in the development of the efficiency of OLETs as high as 40% of external quantum efficiency.

Separation in the Roles of Carrier Transport and Light Emission in Light-Emitting Organic Transistors with a Bilayer Configuration.

To develop high-performance organic light-emitting organic field-effect transistors (LE-OFETs), a fundamental problem in organic semiconductors is to compromise light luminescent efficiency for high carrier mobility and vice versa. Therefore, LE-OFETs can avoid this problem by separating the light-emission and carrier-transport functions. Here, a bilayer LE-OFET composed of a tetracene crystal as a carrier transporter (bottom crystal) and a 4-(dicyanomethylene)-2-methyl-6-( p-dimethylaminostyryl)-4 H-pyran (DCM1)-doped tetracene crystal as a light emitter (top crystal) was fabricated. Red light-emission color, which is distinct from the green emission color of tetracene, was detected in the top crystal. Light emission from the top layer was prohibited when an insulating thin film was inserted between the two crystals. These observations indicate that excitons are formed in the bottom crystal and transferred to the top crystal, emitting reddish light. Bilayer LE-OFETs have the advantage of providing both high current density and a bright emission for high-performance light-emitting FETs.

Full-surface emission of graphene-based vertical-type organic light-emitting transistors with high on/off contrast ratios and enhanced efficiencies

Surface-emitting organic light-emitting transistors (OLETs) could well be a core element in the next generation of active-matrix (AM) displays. We report some of the key characteristics of graphene-based vertical-type organic light-emitting transistors (Gr-VOLETs) composed of a single-layer graphene source and an emissive channel layer. It is shown that FeCl3 doping of the graphene source results in a significant improvement in the device performance of Gr-VOLETs. Using the FeCl3-doped graphene source, it is demonstrated that the full-surface electroluminescent emission of the Gr-VOLET can be effectively modulated by gate voltages with high luminance on/off ratios (~104). Current efficiencies are also observed to be much higher than those of control organic light-emitting diodes (OLEDs), even at high luminance levels exceeding 500 cd/m2. Moreover, we propose an operating mechanism to explain the improvements in the device performance i.e., the effective gate-bias-induced modulation of the hole tunnelling injection at the doped graphene source electrode. Despite its inherently simple structure, our study highlights the significant improvement in the device performance of OLETs offered by the FeCl3-doped graphene source electrode.

Optically switchable organic light-emitting transistors.

Organic light-emitting transistors are pivotal components for emerging opto- and nanoelectronics applications, such as logic circuitries and smart displays. Within this technology sector, the integration of multiple functionalities in a single electronic device remains the key challenge. Here we show optically switchable organic light-emitting transistors fabricated through a judicious combination of light-emitting semiconductors and photochromic molecules. Irradiation of the solution-processed films at selected wavelengths enables the efficient and reversible tuning of charge transport and electroluminescence simultaneously, with a high degree of modulation (on/off ratios up to 500) in the three primary colours. Different emitting patterns can be written and erased through a non-invasive and mask-free process, on a length scale of a few micrometres in a single device, thereby rendering this technology potentially promising for optically gated highly integrated full-colour displays and active optical memory.

Organic Light-Emitting Field-Effect Transistors: Device Geometries and Fabrication Techniques.

Organic light-emitting transistors (OLETs), as novel and attractive kinds of organic electronic devices, have gained extensive attention from both academia and industry. The unique device architectures can simultaneously combine the electrical switching functionality of organic field-effect transistors and the light generation capability of organic light-emitting diodes in a single device, thereby holding great promise for reducing the complicated processes of next-generation pixel circuitry. This review involves the design, fabrication, and applications of OLETs with a comprehensive coverage of this field with the aim to give a deep insight into the intrinsic mechanisms of devices. Challenges and future prospects of OLETs are also discussed.

Organic Semiconductor Single Crystals for Electronics and Photonics.

Organic semiconducting single crystals (OSSCs) are ideal candidates for the construction of high-performance optoelectronic devices/circuits and a great platform for fundamental research due to their long-range order, absence of grain boundaries, and extremely low defect density. Impressive improvements have recently been made in organic optoelectronics: the charge-carrier mobility is now over 10 cm2 V-1 s-1 and the fluorescence efficiency reaches 90% for many OSSCs. Moreover, high mobility and strong emission can be integrated into a single OSSC, for example, showing a mobility of up to 34 cm2 V-1 s-1 and a photoluminescence yield of 41.2%. These achievements are attributed to the rational design and synthesis of organic semiconductors as well as improvements in preparation technology for crystals, which accelerate the application of OSSCs in devices and circuits, such as organic field-effect transistors, organic photodetectors, organic photovoltaics, organic light-emitting diodes, organic light-emitting transistors, and even electrically pumped organic lasers. In this context, an overview of these fantastic advancements in terms of the fundamental insights into developing high-performance organic semiconductors, efficient strategies for yielding desirable high-quality OSSCs, and their applications in optoelectronic devices and circuits is presented. Finally, an overview of the development of OSSCs along with current challenges and future research directions is provided.

Organic Light-Emitting Transistors with Simultaneous Enhancement of Optical Power and External Quantum Efficiency via Conjugated Polar Polymer Interlayers.

Organic light-emitting transistors (OLETs) show the fascinating combination of electrical switching characteristics and light generation capability. However, to ensure an effective device operation, an efficient injection of charges into the emissive layer is required. The introduction of solution-processed conjugated polyelectrolyte (CPE) films at the emissive layer/electrode interface represents a promising strategy to improve the electron injection process by dipole formation. However, their use in optoelectronic devices also involves some limitations because of the ionic nature of CPEs. In this context, neutral conjugated polar polymers (CPPs) represent a valid alternative to CPEs because the conjugated backbones of CPPs are functionalized with polar nonionic side groups, thus avoiding ion-dependent drawbacks. By introducing a layer of polyfluorene-containing phosphonate groups underneath the metal electrodes, we here demonstrate a substantial improvement of the electron injection properties into the OLET-emissive layer and, accordingly, a more than 2-fold increased light power and a 5 times higher external quantum efficiency of p-type OLETs in comparison with reference devices without any interlayer. The great benefit of using a transparent glass substrate allowed to selectively investigate the morphological and photoluminescent characteristics of both CPE- and CPP-buried interlayers within complete OLETs by means of an optical scanning probe technique. This, together with a thorough optoelectronic characterization of the figures of merit of working light-emitting devices, allowed to disclose the origin of the improved optical performance of CPP-based devices as well as the operation mechanisms of the investigated interlayer in the corresponding OLETs.