Quantum Dot Light‐Emitting Transistors—Powerful Research Tools and Their Future Applications

In this progress report, the recent work in the field of light‐emitting field‐effect transistors (LEFETs) based on colloidal quantum dots (CQDs) as emitters is highlighted. These devices combine the possibility of electrical switching, as known from field‐effect transistors, with the possibility of light emission in a single device. The properties of field‐effect transistors and the prerequisites of LEFETs are reviewed, before motivating the use of colloidal quantum dots for light emission. Recent reports on these quantum dot light‐emitting field‐effect transistors (QDLEFETs) include both materials emitting in the near infrared and the visible spectral range—underlining the great potential and breadth of applications for QDLEFETs. The way in which LEFETs can further the understanding of the CQD material properties—their photophysics as well as the carrier transport through films—is discussed. In addition, an overview of technology areas offering the potential for large impact is provided.     

High Performance p‐ and n‐Type Light‐Emitting Field‐Effect Transistors Employing Thermally Activated Delayed Fluorescence

Light‐emitting field‐effect transistors (LEFETs) are an emerging type of devices that combine light‐emitting properties with logical switching function. One of the factors limiting their efficiency stems from the spin statistics of electrically generated excitons. Only 25% of them, short lived singlet states, are capable of light emission, with the other 75% being long lived triplet states that are wasted as heat due to spin‐forbidden processes. Traditionally, the way to overcome this limitation is to use phosphorescent materials as additional emission channel harnessing the triplet excitons. Here, an alternative strategy for triplet usage in LEFETs in the form of thermally activated delayed fluorescence (TADF) is presented. Devices employing a TADF capable material, 4CzIPN (2,4,5,6‐tetra[9H‐carbazol‐9‐yl]isophthalonitrile), in both n‐type and p‐type configurations are shown. They manifest excellent electrical characteristics, consistent brightness in the range of 100–1,000 cd m^‐2 and external quantum efficiency (EQE) of up to 0.1%, which is comparable to the equivalent organic light‐emitting diode (OLED) based on the same materials. Simulation identifies the poor light out‐coupling as the main reason for lower than expected EQEs. Transmission measurements show it can be partially alleviated using a more transparent top contact, however more structural optimization is needed to tap the full potential of the device.     

Solution‐Processed Zinc Oxide as High‐Performance Air‐Stable Electron Injector in Organic Ambipolar Light‐Emitting Field‐Effect Transistors

Electron injection from the source–drain electrodes limits the performance of many n‐type organic field‐effect transistors (OFETs), particularly those based on organic semiconductors with electron affinities less than 3.5 eV. Here, it is shown that modification of gold source–drain electrodes with an overlying solution‐deposited, patterned layer of an n‐type metal oxide such as zinc oxide (ZnO) provides an efficient electron‐injecting contact, which avoids the use of unstable low‐work‐function metals and is compatible with high‐resolution patterning techniques such as photolithography. Ambipolar light‐emitting field‐effect transistors (LEFETs) based on green‐light‐emitting poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole) (F8BT) and blue‐light‐emitting poly(9,9‐dioctylfluorene) (F8) with electron‐injecting gold/ZnO and hole‐injecting gold electrodes show significantly lower electron threshold voltages and several orders of magnitude higher ambipolar currents, and hence light emission intensities, than devices with bare gold electrodes. Moreover, different solution‐deposited metal oxide injection layers are compared. By spin‐coating ZnO from a low‐temperature precursor, processing temperatures could be reduced to 150 °C. Ultraviolet photoemission spectroscopy (UPS) shows that the improvement in transistor performance is due to reduction of the electron injection barrier at the interface between the organic semiconductor and ZnO/Au compared to bare gold electrodes.     

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.     

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.     

Integration of a Rib Waveguide Distributed Feedback Structure into a Light‐Emitting Polymer Field‐Effect Transistor

Ambipolar light‐emitting organic field‐effect transistors (LEFETs) possess the ability to efficiently emit light due to charge recombination in the channel. Since the emission can be made to occur far from the metal electrodes, the LEFET structure has been proposed as a potential architecture for electrically pumped organic lasers. Here, a rib waveguide distributed feedback structure consisting of tantalum pentoxide (Ta₂O₅) integrated within the channel of a top gate/bottom contact LEFET based on poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole) (F8BT) is demonstrated. The emitted light is coupled efficiently into the resonant mode of the DFB waveguide when the recombination zone of the LEFET is placed directly above the waveguide ridge. This architecture provides strong mode confinement in two dimensions. Mode simulations are used to optimize the dielectric thickness and gate electrode material. It is shown that electrode absorption losses within the device can be eliminated and that the lasing threshold for optical pumping of the LEFET structure with all electrodes (4.5 µJ cm^?2) is as low as that of reference devices without electrodes. These results enable quantitative judgement of the prospects for realizing an electrically pumped organic laser based on ambipolar LEFETs. The proposed device provides a powerful, low‐loss architecture for integrating high‐performance ambipolar organic semiconductor materials into electrically pumped lasing structures.     

Conjugated‐Polymer‐Based Lateral Heterostructures Defined by High‐Resolution Photolithography

Solution processing of polymer semiconductors provides a new paradigm for large‐area electronics manufacturing on flexible substrates, but it also severely restricts the realization of interesting advanced device architectures, such as lateral heterostructures with defined interfaces, which are easily accessible with inorganic materials using photolithography. This is because polymer semiconductors degrade, swell, or dissolve during conventional photoresist processing. Here a versatile, high‐resolution photolithographic method is demonstrated for patterning of polymer semiconductors and exemplify this with high‐performance p‐type and n‐type field‐effect transistors (FETs) in both bottom‐ and top‐gate architectures, as well as ambipolar light‐emitting field‐effect transistors (LEFETs), in which the recombination zone can be pinned at a photolithographically defined lateral heterojunction between two semiconducting polymers. The technique therefore enables the realization of a broad range of novel device architectures while retaining optimum materials performance.     

Charge transport and recombination in heterostructure organic light emitting transistors

Light-emitting field effect transistors (LEFETs) are a class of organic optoelectronic device capable of simultaneously delivering the electrical switching characteristics of a transistor and the light emission of a diode. We report on the temperature dependence of the charge transport and emissive properties in a model organic heterostructure LEFET system from 300 K to 135 K. We study parameters such as carrier mobility, brightness, and external quantum efficiency (EQE), and observe clear thermally activated behaviour for transport and injection. Overall, the EQE increases with decreasing temperature and conversely the brightness decreases. These contrary effects can be explained by a higher recombination efficiency occurring at lower temperatures, and this insight delivers new knowledge concerning the optimisation of both the transport and emissive properties in LEFETs.     

Organic Light Emitting Field Effect Transistors: Advances and Perspectives

Light emitting field effect transistors based on molecular and polymeric organic semiconductors are multifunctional devices that integrate light emission with the current modulating function of a transistor. The planar geometry of organic light emitting field effect transistors (OLEFETs) offers direct access to the light emission region, providing a unique experimental configuration to investigate fundamental optical and electronic properties in organic semiconductors. OLEFETs show great potential for technological applications such as active matrix full color electroluminescent displays. In this Feature Article we review advances in OLEFETs since their first demonstration in 2003 and we highlight exciting challenges associated with their future development.     

Programmable polymer light emitting transistors with ferroelectric polarization-enhanced channel current and light emission

We present a voltage programmable polymer light emitting field-effect transistor (LEFET), consisting of a green emitting polymer (F8BT), and a ferroelectric polymer, P(VDF-TrFE), as the gate dielectric. We show by both experimental observations and numerical modeling that, when the ferroelectric gate dielectric is polarized in opposite directions at the drain and source sides of the channel, respectively, both electron and hole currents are enhanced, resulting in more charge recombination and ～10 times higher light emission in a ferroelectric LEFET, compared to the device with non-ferroelectric gate. As a result of the ferroelectric poling, our ferroelectric LEFETs exhibit repeated programmability in light emission, and an external quantum efficiency (EQE) of up to 1.06%. Numerical modeling reveals that the remnant polarization charge of the ferroelectric layer tends to ‘pin’ the position of the recombination zone, paving the way to integrate specific optical out-coupling structures in the channel of these devices to further increase the brightness.     

Ambipolar Light‐Emitting Transistors of a Tetracene Single Crystal

The first ambipolar light‐emitting transistor of an organic molecular semiconductor single crystal, tetracene, is demonstrated. In the device configuration, electrons and holes injected from separate magnesium and gold electrodes recombined radiatively within the channel. By varying the applied voltages, the position of the recombination/emission zone could be moved to any position along the channel. Because of the changes made to the device structure, including the use of single crystals and polymer dielectric layers and the adoption of an inert‐atmosphere fabrication process, the set of materials that can be used for light‐emitting transistors has been expanded to include monomeric molecular semiconductors.     

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.     

Integrating Efficient Optical Gain in High‐Mobility Organic Semiconductors for Multifunctional Optoelectronic Applications

Simultaneously integrating efficient optical gain and high charge carrier mobility in organic semiconductors for multifunctional optoelectronic applications is challenging. Here, a new thiophene/phenylene derivative, 5,5′‐bis(2,2‐diphenylvinyl)‐bithiophene (BDPV2T), containing an appropriate butterfly molecular configuration in a π‐conjugated structure, is designed to achieve both solid‐state emission and charge transport properties. The prepared BDPV2T crystals exhibit excellent light‐emitting characteristics with a photoluminescence quantum yield of 30%, low light‐amplification threshold of 8 kW cm^?2, high optical net gain up to 70 cm^?1, and high charge carrier mobility up to 1 cm² V^?1 s^?1 in their J‐aggregate single crystals. These BDPV2T single crystal characteristics ensure their application potential for photodetectors, field‐effect transistors, and light‐emitting transistors. High optoelectronic performances are achieved with photoresponsivity of 2.0 × 10³ A W^?1 and light on/off ratio of 5.4 × 10⁵ in photodetectors, and efficient ambipolar charge transport (µ_h: 0.14 cm² V^?1 s^?1, µ_e: 0.02 cm² V^?1 s^?1) and electroluminescence characteristics in light‐emitting transistors. The remarkably integrated optoelectronic properties of BDPV2T suggest it is a promising candidate for organic multifunctional and electrically pumped laser applications.     

Light‐Emitting Field‐Effect Transistors Consisting of Bilayer‐Crystal Organic Semiconductors

A novel device structure for organic light‐emitting field‐effect transistors has been developed. The devices comprise bilayer‐crystal organic semiconductors of a p‐type and an n‐type. The pn‐junction can readily be formed by successively laminating two crystals on top of a gate insulator. This structure enables the efficient injection and transport of electrons and holes, leading to their effective recombination. As a result, bright emissions are attained. The devices are operated by AC gate voltages. Gate‐voltage phase‐resolved drain‐current and emission‐intensity measurements enable us to study the relationship between the emissions and carrier transport. The maximum external quantum efficiency reaches 0.045%.     

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.     

Enhancement and Concurrence of Emissions from Multiple Fluorophores in a Single Emitting Layer of Micellar Nanostructures

For concurrent emission of multiple fluorophores from a single emitting layer with a highly efficient energy transfer from antenna molecules to emitting molecules by fluorescence resonance energy transfer (FRET), a paradoxical requirement in an emitting layer is necessary, that is, close placement of an emitting fluorophore and a harvesting molecule, and isolation of emitting fluorophores. Here we demonstrate how to overcome this paradox by full utilization of a micellar nanostructure consisting of a core and a corona, that is, the core is used as a place for FRET between light‐collecting donors and light‐emitting fluorophores, and the corona is used as a barrier for FRET between light‐emitting fluorophores. Enhancement of light emission from fluorophores was achieved by locating emitting fluorophores and light‐harvesting molecules at the same core of micelles. Moreover, with the same micellar nanostructure, concurrent emission of multiple fluorophores with enhanced intensity was induced by isolating them in independent micelles, the corona structure of which worked as an effective blockade for FRET.     

Organic Light‐Emitting Diodes with Field‐Effect‐Assisted Electron Transport Based on α‐bi;,ω‐bi;‐Diperfluorohexyl‐quaterthiophene

Materials commonly used in the carrier transport layers of organic light‐emitting diodes, where transport occurs through the bulk, are in general very different from materials used in organic field‐effect transistors, where transport takes place in a very thin accumulation channel. In this paper, the use of a high‐performance electron‐conducting field‐effect transistor material, diperfluorohexyl‐substituted quaterthiophene (DFH‐4T), as the electron‐transporting material in an organic light‐emitting diode structure is investigated. The organic light‐emitting diode has an electron accumulation layer in DFH‐4T at the organic hetero‐interface with the host of the light‐emitting layer, tris(8‐hydroxyquinoline) aluminum (Alq₃). This electron accumulation layer is used to transport electrons and inject them into the active emissive host‐guest layer. By optimizing the growth conditions of DFH‐4T for electron transport at the organic hetero‐interface, high electron current densities of 750 A cm^?2 are achieved in this innovative light‐emitting structure.     

Exciplex Emission in Light‐Emitting Electrochemical Cells and Light Outcoupling Methods for More Efficient LEC Devices

Much effort has gone into research on light‐emitting electrochemical cells (LECs) in recent years. LECs have a simple structure and can be fabricated using low‐cost methods and materials and are seen as the next big thing in organic devices after organic light‐emitting diodes (OLEDs). In particular, expectations are high, in that LECs could be used to create a new generation of low‐cost lighting systems, making use of their surface‐emitting property. Getting such systems to the market will require the development of highly efficient white light‐emitting LECs. A variety of methods for obtaining white emission based on the light‐mixing principle have been explored. Among these, the use of exciplexes formed between donor‐type and acceptor‐type molecules is one of the more promising. Exciplex emission is broad in spectrum and can be used to produce LECs with a high color rendering index. In this progress report, the recent developments in research into LECs designed to utilize exciplex emission and present technologies used to obtain white emission are discussed. The potential for using thermally activated delayed fluorescence to improve efficiency is described. Finally, the latest developments in optical engineering techniques for LECs are also discussed.     

The Fabrication and Characterization of Single‐Component Polymeric White‐Light‐Emitting Diodes

By using Ni⁰‐mediated polymerization, we have systematically synthesized a series of fluorene‐based copolymers composed of blue‐, green‐, and red‐light‐emitting comonomers with a view to producing polymers with white‐light emission. 2,7‐Dibromo‐9,9‐dihexylfluorene, {4‐(2‐[2,5‐dibromo‐4‐{2‐(4‐diphenylamino‐phenyl)‐vinyl}‐phenyl]‐vinyl)‐phenyl}‐diphenylamine (DTPA), and 2‐{2‐(2‐[4‐{bis(4‐bromo‐phenyl)amino}‐phenyl]‐vinyl)‐6‐tert‐butyl‐pyran‐4‐ylidene}‐malononitrile (TPDCM) were used as the blue‐, green‐, and red‐light‐emitting comonomers, respectively. It was found that the emission spectra of the resulting copolymers could easily be tuned by varying their DTPA and TPDCM content. Thus with the appropriate red/green/blue (RGB) unit ratio, we were able to obtain white‐light emission from these copolymers. A white‐light‐emitting diode using the polyfluorene copolymer containing 3 % green‐emitting DTPA and 2 % red‐emitting TPDCM (PG3R2) with a structure of indium tin oxide/poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonic acid)/PG3R2/Ca/Al was found to exhibit a maximum brightness of 820 cd m^–2 at 11 V with Commission Internationale de L'Eclairage (CIE) coordinates of (0.33,0.35), which are close to the standard CIE coordinates for white‐light emission (0.33,0.33).     

High‐Performance,Phosphorescent, Top‐Emitting Organic Light‐Emitting Diodes with p–i–n Homojunctions

High‐performance, green, orange, and red top‐emitting organic light‐emitting diodes (TOLEDs) with p–i–n homojunction are demonstrated. An excellent ambipolar host, 2,5‐bis(2‐(9H‐carbazol‐9‐yl)phenyl)‐1,3,4‐oxadiazole (o‐CzOXD), which has good thermal and morphological stabilities, a high triplet energy level, and equally high electron and hole mobilities, is chosen as the organic host material for the homojunction devices. By electrical doping, the carrier injection and transporting characteristics are greatly improved. The optical structure is optimized in view of light emission of different colors to enhance the color purity and improve the view characte­ristics. As a result, high efficiency p–i–n homojunction TOLEDs with saturated intrinsic emission of the emitting materials and angular independence of the emission are realized. The performances of these p–i–n homojunction TOLEDs are even higher than the multi‐layer heterojunction bottom‐emitting devices using the same emitting layers.