Organic Light-emitting Transistors Research Articles

Electroluminescence emission patterns of organic light-emitting transistors based on crystallized fluorene-type polymers

The electroluminescence (EL) emission patterns of organic light-emitting transistors (OLETs) based on crystallized poly(9,9-dioctylfluorene) (F8), poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) and poly(9,9-dioctylfluorene-co-dithienyl-benzothiadiazole) (F8TBT) films are investigated. For the single-layer devices and the mixed-layer device without an F8/F8BT interface, only line-shaped EL emission patterns are observed between source/drain (S/D) electrodes. For an F8BT (F8TBT)/F8 heterostructure device, a localized electric field is generated by the positive (negative) charges of the accumulated holes (electrons) in the F8 upper layer, which allow the injection of electrons (holes) in the F8BT (F8TBT) lower layer at a lower (higher) gate voltage. The F8/F8BT device exhibits unique light emission properties with a surface like EL emission pattern between S/D electrodes at a lower gate voltage. The interfacial structure is important for forming field-effect transistor channels along different organic layers to obtain a surface like emission between S/D electrodes. For the F8TBT/F8 OLET, the hole carrier transport mainly occurs at the F8TBT lower layer, and line-shaped EL emission patterns are observed in the vicinity of the source electrode upon varying the gate voltages owing to the worse carrier balance between the F8TBT lower layer and the F8 upper layer.

Doped polymer semiconductors with ultrahigh and ultralow work functions for ohmic contacts.

To make high-performance semiconductor devices, a good ohmic contact between the electrode and the semiconductor layer is required to inject the maximum current density across the contact. Achieving ohmic contacts requires electrodes with high and low work functions to inject holes and electrons respectively, where the work function is the minimum energy required to remove an electron from the Fermi level of the electrode to the vacuum level. However, it is challenging to produce electrically conducting films with sufficiently high or low work functions, especially for solution-processed semiconductor devices. Hole-doped polymer organic semiconductors are available in a limited work-function range, but hole-doped materials with ultrahigh work functions and, especially, electron-doped materials with low to ultralow work functions are not yet available. The key challenges are stabilizing the thin films against de-doping and suppressing dopant migration. Here we report a general strategy to overcome these limitations and achieve solution-processed doped films over a wide range of work functions (3.0-5.8 electronvolts), by charge-doping of conjugated polyelectrolytes and then internal ion-exchange to give self-compensated heavily doped polymers. Mobile carriers on the polymer backbone in these materials are compensated by covalently bonded counter-ions. Although our self-compensated doped polymers superficially resemble self-doped polymers, they are generated by separate charge-carrier doping and compensation steps, which enables the use of strong dopants to access extreme work functions. We demonstrate solution-processed ohmic contacts for high-performance organic light-emitting diodes, solar cells, photodiodes and transistors, including ohmic injection of both carrier types into polyfluorene-the benchmark wide-bandgap blue-light-emitting polymer organic semiconductor. We also show that metal electrodes can be transformed into highly efficient hole- and electron-injection contacts via the self-assembly of these doped polyelectrolytes. This consequently allows ambipolar field-effect transistors to be transformed into high-performance p- and n-channel transistors. Our strategy provides a method for producing ohmic contacts not only for organic semiconductors, but potentially for other advanced semiconductors as well, including perovskites, quantum dots, nanotubes and two-dimensional materials.

Novel Light-Emitting Electrochemical Transistor Using Ruthenium Complex

Recently, organic light-emitting transistors (OLETs) which combined organic light-emitting diodes (OLEDs) with organic thin film transistors (OTFTs) have being intensively investigated for application to display. They can control light emission by the gate voltage. So far, we have investigated light-emitting electrochemical cells (LECs) and electrochemical transistors (ECTs) independently. LECs have some advantages about a simple device structure and usage of air-stable metal cathode compared to OLEDs. ECTs also have unique features for lower driving gate voltage than that of OTFTs because the electrochemical doping becomes easier for the formation of the electric double layer in the ECTs. Therefore, a novel concept for the light-emitting electrochemical transistor (LECT) which consists of the light-emitting electrochemical cell (LEC) and the electrochemical transistor (ECT) was introduced. To aim at constructing LECT, four-electrode electrochemical system using interdigitated array (IDA) electrodes was employed to demonstrate the concept of LECT in solution. Cyclic voltammetric (CV) measurement was carried out in acetonitrile solution containing Ru(bpy)3(PF6)2 as redox species and a chemiluminescent material. We confirmed that the light emission was controlled at the electrode potential difference between both fingers of IDA electrodes as source and drain electrodes of the transistor and reference/counter electrodes as gate electrode. The light emission was observed at the electrogenerated chemiluminescent condition where the excited state Ru(II) was produced on the gap distance by the electron exchange reaction of Ru(I) and Ru(III) produced at both fingers of IDA electrodes, respectively. If the electrode potential would be shifted to anodic or cathodic directions, whether Ru(I) or Ru(III) would be not produced, the light emission disappear even at the same bias voltage between the source and drain electrodes (VDS) where the light emission is possible. If VDS is not enough to produce Ru(I) and Ru(III), the light emission does not occur. If VDS is enough to produce Ru(I) and Ru(III) in the diffusion limited condition, the light emission becomes steady state intensity. Therefore, we demonstrated that the light emission can be controlled by VDS and the gate electrode potential in the LECT. We also confirmed that current behavior flowed between source and drain electrodes shows bipolar transistor characteristics. We are now trying to solidify the LECT system in solution to realize LECT device.

Effects of Doping and Electrode Contacts on Performance of Organic Light-Emitting Transistors Based on Pentacene and Tris(8-hydroxyquinoline)aluminum

Heterojunction-based organic light-emitting field-effect transistors (OLEFETs) with a top-contact, long-channel geometry were fabricated and comparatively characterized. The neutral cluster beam deposition (NCBD) method was used to successively deposit two layers of p-type pentacene and n-type tris(8-hydroxyquinoline)aluminum (Alq3). For doped OLEFETs, 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) was used as a highly fluorescent dye dopant to enhance the light-emission efficiency and change the emission color. OLEFETs revealed fine device characteristics based on deposition of highly crystalline active layers. The combination of the highly fluorescent DCM doping and asymmetric electrode configuration (Au and Li:Al or LiF/Al) exhibited efficient energy transfer and enhanced electroluminescence (EL) emission. The operating light-emission mechanisms were discussed based on EL photos acquired using a charge-coupled device (CCD) camera.

Improved Performance of Organic Light-Emitting Field-Effect Transistors by Interfacial Modification of Hole-Transport Layer/Emission Layer: Incorporating Organic Heterojunctions.

Organic heterojunctions (OHJs) consisting of a strong electron acceptor 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) and an electron donor N,N'-di(naphthalene-1-yl)-N,N'-diphenyl-benzidine (NPB) were demonstrated for the first time that they can be implemented as effective modification layers between hole transport layer (HTL) and emission layer in the heterostructured organic light-emitting field effect transistors (OLEFETs). The influence of both HAT-CN/NPB junction (npJ) and NPB/HAT-CN junction (pnJ) on the optoelectronic performance of OLEFETs were conscientiously investigated. It is found that both the transport ability of holes and the injection ability of holes into emissive layer can be dramatically improved via the charge transfer of the OHJs and that between HAT-CN and the HTL. Consequently, OLEFETs with pnJ present optimal performance of an external quantum efficiency (EQE) of 3.3% at brightness of 2630 cdm(-2) and the ones with npJs show an EQE of 4.7% at brightness of 4620 cdm(-2). By further utilizing npn OHJs of HAT-CN/NPB/HAT-CN, superior optoelectronic performance with an EQE of 4.7% at brightness of 8350 cdm(-2) and on/off ratio of 1 × 10(5) is obtained. The results demonstrate the great practicality of implementing OHJs as effective modification layers in heterostructured OLEFETs.

54-3:Invited Paper: Flexible Active-Matrix OLET Display on a Plastic Substrate

The organic light-emitting transistors (OLETs) with a novel tri-layer organic semiconducting heterostructure architecture and good performance, including record high EQE, is presented. In addition, an active-matrix display based on OLETs, without driving TFTs, will be demonstrated on a plastic substrate. The potential advantages of such technologies will be discussed.

P-176: Innovative Trilayer Organic Light Emitting Transistor (OLET) Structure for Blue Emission

We report on a three-layer organic light-emitting transistor (3L-OLET) comprising a novel set of semiconductors as transport layers, 2,7-Dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) and 2,5-bis[4-(heptadecafluorooctyl)phenyl]thieno[3,2-b]thiophene (N-F2-6). These materials, combined with an emissive layer of a 4,4'-Bis(N-carbazolyl)-1,1'-biphenyl (CBP) derivative, 4,4'-bis(3,6-dineopentyl-9H-carbazole-9-yl)-1,'-biphenyl (NP4-CBP) and a new dye (BLUE1), have been used to obtain blue electroluminescent device.

Vertical Organic Field-Effect Transistors for Integrated Optoelectronic Applications.

Direct integration of a vertical organic field-effect transistor (VOFET) and an optoelectronic device offers a single stacked, low power optoelectronic VOFET with high aperture ratios. However, a functional optoelectronic VOFET could not be realized because of the difficulty in fabricating transparent source and gate electrodes. Here, we report a VOFET with an on/off ratio up to 10(5) as well as output current saturation by fabricating a transparent gate capacitor consisting of a perforated indium tin oxide (ITO) source electrode, HfO2 gate dielectric, and ITO gate electrode. Effects of the pore size and the pore depth within the porous ITO electrodes on the on/off characteristic of a VOFET are systematically explained in this work. By combining a phosphorescent organic light-emitting diode with an optimized VOFET structure, a vertical organic light-emitting transistor with a luminance on/off ratio of 10(4) can be fabricated.

Organic light-emitting transistors with a thin metal layer covering a diffraction grating

ABSTRACTWe fabricated organic light-emitting transistors (OLETs) characterized by an Ag layer deposited on a one-dimensional (1D) or two-dimensional (2D) diffraction grating that acts as a combined gate insulator with SiO2. The Ag layer was entirely covered with an organic crystal. Upon photoexcitation that crystal showed narrow linewidth emissions (NLEs) parallel to the substrate plane. The narrowed lines were either redshifted or blueshifted with rotation of the crystal around a normal to its surface with respect to the grating wave vector. Strong emissions (∼104–106 cd m−2) accompanied by current-injected NLEs were observed from the 1D and 2D grating OLETs.

Physical Property Evaluation of ZnO Thin Film Fabricated by Low-Temperature Process for Flexible Transparent TFT.

The usual silicon-based display back planes require fairly high process temperature and thus the development of a low temperature process is needed on flexible plastic substrates. A new type of flexible organic light emitting transistor (OLET) had been proposed and investigated in the previous work. By using ultraviolet/ozone (UV/O3) assisted thermal treatments on wet processed zinc oxide field effect transistor (ZnO-FET), through low-process temperature, ZnO-FETs were fabricated which succeeded to achieve target drain current value and mobility. In this study, physical property evaluation of ZnO was conducted in term of their crystallinity, the increase composition of ZnO formed inside the thin film and the decrease of the carbon impurities originated from aqueous solution of the ZnO itself. The X-ray diffraction (XRD) evaluation showed UV/03 assisted thermal treatment has no obvious effect towards crystallinity of ZnO in the range of low process temperature. Moreover, through X-ray photoelectron spectroscopy (XPS) evaluation and Fourier transform infrared (FT-IR) spectroscopy evaluation, more carbon impurities disappeared from the ZnO thin film and the increase of composition amount of ZnO, when the thin film was subjected to UV/O3 assisted thermal treatment. Therefore, UV/O3 assisted thermal treatment contributed in carbon impurities elimination and accelerate ZnO formation in ZnO thin film, which led to the improvement in the electrical property of ZnO-FET in the low-process temperature.

Improved carrier balance and polarized in-plane light emission at full-channel area in ambipolar heterostructure polymer light-emitting transistors

We report polarized in-plane emission at full-channel area in ambipolar heterostructure organic light-emitting transistors (OLETs) utilizing bilayer oriented fluorene-type polymers. The present work demonstrates that carrier balance between bilayers is a key factor in achieving in-plane emission at almost full-channel area when hole transport is dominant in the upper layer, which acts as an electron blocking layer, and electrons are injected into the lower layer. Hole accumulation in the upper layer affects electron injection from the electrode to the lower layer and the probability of recombination of holes in the upper layer with electrons in the lower layer near the bilayer interface. For OLETs having an oriented bilayer with the channel direction parallel to orientation of the polymer chains, a uniform in-plane emission pattern, an increased electroluminescence intensity, and a high external quantum efficiency can be achieved by improving the hole mobility in the upper layer and optimizing the carrier balance. These results are expected to be very useful for the development of surface micro-light sources.

The emergence of charge coherence in soft molecular organic semiconductors via the suppression of thermal fluctuations

Organic semiconductors are already widely used in electronics. Nevertheless, their fundamental properties are still being debated. In particular, charge transport, which determines the performance of organic light-emitting diodes, solar cells and organic field-effect transistors, has been described by a wide range of complementary but incompatible theories. These theories involve either localized charge carriers hopping from molecule to molecule, leading to incoherent charge transport, or delocalized charge carriers moving around freely in the semiconductor, leading to coherent transport. In this communication, we reveal the first experimental evidence that charge coherence can be tuned from partially to fully coherent in one and the same material—pentacene—showing a continuous transition from one transport mechanism to the other. Microscopically, the transport mechanism depends on the overlap of adjacent molecular orbitals, which in turn is sensitive to molecular thermal fluctuations. We control these fluctuations through moderate variations of pressure and temperature, leading to a mobility increase of 75%. We quantify the influence of these thermal fluctuations by estimating the critical value below which fully coherent charge transport emerges. The ability to control thermal fluctuations and therefore to effectively tune the charge coherence is an important key to improving charge transport in soft molecular materials. A team in Japan experimentally shows that charge transport in an organic semiconductor can be tuned by varying the temperature and pressure. Charge transport in organic semiconductors has been much debated, with incompatible theories invoking either localized charge carriers hopping from molecule to molecule (incoherent transport) or delocalized charge carriers moving freely among the molecules (coherent transport) being put forward. Jun Takeya and colleagues at the University of Tokyo and the University of Tsukuba have realized a continuous change from one transport mechanism to the other in the aromatic hydrocarbon pentacene. They found that the transport mechanism depends on the degree to which the orbitals of neighbouring molecules overlap, which is sensitive to thermal fluctuations. Pushing molecules closer together through raising the pressure and lowering the temperature enhanced electronic coupling, boosting pentacene's mobility by 75 per cent. Our study reveals the first experimental evidence that charge transport can be tuned from partially to fully coherent. We used pentacene, a soft organic material, to measure transport properties depending on temperature and pressure. Under ambient conditions, charge transport was only partially coherent. At 1 GPa and below 220 K fully coherent charge transport emerged. Microscopically, we find that the thermal fluctuations are reduced so that the charge carriers start moving around freely leading to a coherent charge transport.

Organic crystal light-emitting transistors combined with a metal oxide layer

We improved organic light-emitting transistors (OLETs) characterized by aluminum-doped zinc oxide (AZO) layer insertion between organic and gate insulator layers using organic oligomer semiconductor crystals. (i) To ensure firm contact between the crystal and the AZO layer, we shaped the AZO layer into a rectangle (250 × 500 µm2) and covered it with a vapor-phase-grown crystal. (ii) To enhance contact between the crystal and the AZO layer, we placed the crystal used as a mask on the patternless AZO layer and etched parts of AZO not covered with the crystal with hydrochloric acid vapor. We completed OLETs by forming electron- and hole-injection contacts on the crystal. We modified these contacts with an oxide and/or a carbonate. The devices showed bright light emission from the part of the crystal sandwiched between the electron- and hole-injection contacts located on the AZO layer.

Organic Light-Emitting Transistors: Materials, Device Configurations, and Operations.

Organic light-emitting transistors (OLETs) represent an emerging class of organic optoelectronic devices, wherein the electrical switching capability of organic field-effect transistors (OFETs) and the light-generation capability of organic light-emitting diodes (OLEDs) are inherently incorporated in a single device. In contrast to conventional OFETs and OLEDs, the planar device geometry and the versatile multifunctional nature of OLETs not only endow them with numerous technological opportunities in the frontier fields of highly integrated organic electronics, but also render them ideal scientific scaffolds to address the fundamental physical events of organic semiconductors and devices. This review article summarizes the recent advancements on OLETs in light of materials, device configurations, operation conditions, etc. Diverse state-of-the-art protocols, including bulk heterojunction, layered heterojunction and laterally arranged heterojunction structures, as well as asymmetric source-drain electrodes, and innovative dielectric layers, which have been developed for the construction of qualified OLETs and for shedding new and deep light on the working principles of OLETs, are highlighted by addressing representative paradigms. This review intends to provide readers with a deeper understanding of the design of future OLETs.

ChemInform Abstract: Molecular Materials That Can Both Emit Light and Conduct Charges: Strategies and Perspectives

AbstractReview: [51 refs.

Red phosphorescent trilayer organic light-emitting field-effect transistors with a wide recombination zone

We report on red phosphorescent trilayer organic light-emitting field-effect transistors (LEFETs), which exhibit ambipolar transport characteristics but strong emission in the unipolar hole regime, with an exceptionally wide recombination zone of 60–70 µm and a maximum external quantum efficiency of 0.21%. From the results of detailed electroluminescence characterization, we clarify how the energy-level matching condition and transport geometry of the heterostructure govern the charge distribution and recombination, and affect the overall device performance.

Anthracene-based molecular emitters for non-doped deep-blue organic light emitting transistors

Twisted anthracene derivatives, including the newly synthesized DIPAXA compound, are here investigated as blue emitters in non-doped organic light emitting transistors (OLETs).

High mobility emissive organic semiconductor.

The integration of high charge carrier mobility and high luminescence in an organic semiconductor is challenging. However, there is need of such materials for organic light-emitting transistors and organic electrically pumped lasers. Here we show a novel organic semiconductor, 2,6-diphenylanthracene (DPA), which exhibits not only high emission with single crystal absolute florescence quantum yield of 41.2% but also high charge carrier mobility with single crystal mobility of 34 cm2 V−1 s−1. Organic light-emitting diodes (OLEDs) based on DPA give pure blue emission with brightness up to 6,627 cd m−2 and turn-on voltage of 2.8 V. 2,6-Diphenylanthracene OLED arrays are successfully driven by DPA field-effect transistor arrays, demonstrating that DPA is a high mobility emissive organic semiconductor with potential in organic optoelectronics.

Molecular Materials That Can Both Emit Light and Conduct Charges: Strategies and Perspectives.

Molecular materials with concomitant light-emissive and semiconducting properties have received increasing attention in recent years. Such dual functional materials ensure the development of multifunctional devices (e.g., organic light-emitting transistors) and the emergence of new technologies. However, owing to the fact that intermolecular interactions and dense packing have opposite effects on photoluminescence and charge-carrier mobility, it is still rather challenging to rationally design high-performance molecular materials that exhibit both semiconducting and light-emissive properties. In fact, only a limited number of such dual functional materials are available, and most of their performances need to be further improved. In this concept article we discuss the design strategies and perspectives of this challenging area with the introduction of representative examples of such dual functional materials reported in recent years.

Organic light-emitting transistors: a key enabling photonic device

Organic light-emitting transistors: a key enabling photonic device