Light-emitting Transistors Research Articles

Near-Infrared to Visible Organic Upconversion Devices Based on Organic Light-Emitting Field Effect Transistors.

The near-infrared (NIR) to visible upconversion devices have attracted great attention because of their potential applications in the fields of night vision, medical imaging, and military security. Herein, a novel all-organic upconversion device architecture has been first proposed and developed by incorporating a NIR absorption layer between the carrier transport layer and the emission layer in heterostructured organic light-emitting field effect transistors (OLEFETs). The as-prepared devices show a typical photon-to-photon upconversion efficiency as high as 7% (maximum of 28.7% under low incident NIR power intensity) and millisecond-scale response time, which are the highest upconversion efficiency and one of the fastest response time among organic upconversion devices as referred to the previous reports up to now. The high upconversion performance mainly originates from the gain mechanism of field-effect transistor structures and the unique advantage of OLEFETs to balance betweenthe photodetection and light emission. Meanwhile, the strategy of OLEFETs also offers the advantage of high integration so that no extra OLED is needed in the organic upconversion devices. The results would pave way for low-cost, flexible and portable organic upconversion devices with high efficiency and simplified processing.

Low-voltage organic light-emitting field-effect transistors using n-Dodecylphosphonic acid-passivated HfOx dielectrics

Abstract In this paper, organic light-emitting field effect transistors (OLEFETs) operating at low voltages |Vop| below 10 V were first produced and characterized. n-Dodecylphosphonic acid (PA-C12)-self-assembled monolayer (SAM)-passivated HfOx dielectrics prepared using sol-gel chemistry decreased the device-operating voltages. Crystalline organic active layers of pentacene and N,N′-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (P13) acted as p-and n-type materials, respectively. 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran doped onto a tris(8-hydroxyquinoline) aluminum (Alq3:DCM) layer was used as a light-emitting medium to enhance the luminescence efficiency. Based on a tri-layered structure with balanced ambipolar transports and the combined high capacitance-low leakage current of the PA-C12-SAM-treated HfOx dielectrics, the OLEFETs demonstrated good device performance with light emission at |Vop|

Highly bendable luminescent semiconducting organic single crystal

Mechanical flexibility is one of the key benefits of organic electronics and optoelectronics. Although organic single crystals show the best performance in field-effect and light-emitting transistors, they are commonly considered to be brittle. In this work, we show that organic single crystals can be flexible, luminescent and semiconducting. We grew ∼1cm-size single crystals of a thiophene/phenylene co-oligomer, 1,4-bis(5′-hexyl-2,2′-bithiene-5-yl)benzene, from solution and resolved their structure by x-ray analysis. The crystals were bendable with a radius down to 0.2mm corresponding to a mechanical strain of 5%, had molecularly flat surface, showed the photoluminescence quantum yield up to 17% and charge mobility up to 0.07cm2/Vs. Multiple bending cycles of the crystals did not affect their photoluminescence but resulted in minor degradation of the charge mobility. These findings indicate that organic single crystals can be a platform for flexible optoelectronic devices.

Annulated 1,3,4-Azadiphospholides: Heterocycles with Widely Tunable Optical Properties.

Phosphorus heterocycles find applications in the synthesis of π-conjugated compounds and as precursors for optoelectronic materials such as organic light-emitting diodes (OLEDs), electronic switches, and transistors. A high-yield, one-pot synthesis of anionic annulated 1,3,4-azadiphospholides from Na(OCP) and 2-chloropyridines is presented. The synthesis proceeds without the use of transition metals and tolerates a wide range of substrates. Cyano-substituted compounds are especially deeply colored and have absorption maxima which range from λmax =525 to 596 nm. The optical properties are dominated by the spatial separation of an electron acceptor and donor unit within one molecule (push-pull chromophore). The anionic 1,3,4-azadiphospholides are silylated to neutral siloxy compounds with a strong blue-shifted absorption. This reaction can be reversed by addition of fluoride ions, which allows fluoride ions to be detected in optically low concentrations.

Highly Luminescent 2D-Type Slab Crystals Based on a Molecular Charge-Transfer Complex as Promising Organic Light-Emitting Transistor Materials.

A new 2:1 donor (D):acceptor (A) mixed-stacked charge-transfer (CT) cocrystal comprising isometrically structured dicyanodistyrylbenzene-based D and A molecules is designed and synthesized. Uniform 2D-type morphology is manifested by the exquisite interplay of intermolecular interactions. In addition to its appealing structural features, unique optoelectronic properties are unveiled. Exceptionally high photoluminescence quantum yield (ΦF ≈ 60%) is realized by non-negligible oscillator strength of the S1 transition, and rigidified 2D-type structure. Moreover, this luminescent 2D-type CT crystal exhibits balanced ambipolar transport (µh and µe of ≈10-4 cm2 V-1 s-1 ). As a consequence of such unique optoelectronic characteristics, the first CT electroluminescence is demonstrated in a single active-layered organic light-emitting transistor (OLET) device. The external quantum efficiency of this OLET is as high as 1.5% to suggest a promising potential of luminescent mixed-stacked CT cocrystals in OLET applications.

Multispectral electroluminescence enhancement of single-walled carbon nanotubes coupled to periodic nanodisk arrays.

The integration of periodic nanodisk arrays into the channel of a light-emitting field-effect transistor leads to enhanced and directional electroluminescence from thin films of purified semiconducting single-walled carbon nanotubes. The maximum enhancement wavelength is tunable across the near-infrared and is directly linked to the periodicity of the arrays. Numerical calculations confirm the role of increased local electric fields in the observed emission modification. Large current densities are easily achieved due to the high charge carrier mobilities of carbon nanotubes and will facilitate new electrically driven plasmonic devices.

Electrical pumping and tuning of exciton-polaritons in carbon nanotube microcavities

Exciton-polaritons are hybrid light-matter particles that form upon strong coupling of an excitonic transition to a cavity mode. As bosons, polaritons can form condensates with coherent laser-like emission. For organic materials, optically pumped condensation was achieved at room temperature but electrically pumped condensation remains elusive due to insufficient polariton densities. Here we combine the outstanding optical and electronic properties of purified, solution-processed semiconducting (6,5) single-walled carbon nanotubes (SWCNTs) in a microcavity-integrated light-emitting field-effect transistor to realize efficient electrical pumping of exciton-polaritons at room temperature with high current densities (>10 kA cm-2) and tunability in the near-infrared (1,060 nm to 1,530 nm). We demonstrate thermalization of SWCNT polaritons, exciton-polariton pumping rates ∼104 times higher than in current organic polariton devices, direct control over the coupling strength (Rabi splitting) via the applied gate voltage, and a tenfold enhancement of polaritonic over excitonic emission. This powerful material-device combination paves the way to carbon-based polariton emitters and possibly lasers.

Asymmetric Injection in Organic Transistors via Direct SAM Functionalization of Source and Drain Electrodes.

The fabrication of organic optoelectronic devices integrating asymmetric electrodes enables optimal charge injection/extraction at each individual metal/semiconductor interface. This is key for applications in devices such as solar cells, light-emitting transistors, photodetectors, inverters, and sensors. Here, we describe a new method for the asymmetric functionalization of gold electrodes with different thiolated molecules as a viable route to obtain two electrodes with drastically different work function values. The process involves an ad hoc design of electrode geometry and the use of a polymeric mask to protect one electrode during the first functionalization step. Photoelectron yield ambient spectroscopy and X-ray photoelectron spectrometry were used to characterize the energetic properties and the composition of the asymmetrically functionalized electrodes. Finally, we used poly(3-hexylthiophene)-based organic thin-film transistors to show that the asymmetric electronic response stems from the different electronic structures of the functionalized electrodes.

Luminescent Organic Semiconducting Langmuir Monolayers.

In recent years, monolayer organic field-effect devices such as transistors and sensors have demonstrated their high potential. In contrast, monolayer electroluminescent organic field-effect devices are still in their infancy. One of the key challenges here is to create an organic material that self-organizes in a monolayer and combines efficient charge transport with luminescence. Herein, we report a novel organosilicon derivative of oligothiophene-phenylene dimer D2-Und-PTTP-TMS (D2, tetramethyldisiloxane; Und, undecylenic spacer; P, 1,4-phenylene; T, 2,5-thiophene; TMS, trimethylsilyl) that meets these requirements. The self-assembled Langmuir monolayers of the dimer were investigated by steady-state and time-resolved photoluminescence spectroscopy, atomic force microscopy, X-ray reflectometry, and grazing-incidence X-ray diffraction, and their semiconducting properties were evaluated in organic field-effect transistors. We found that the best uniform, fully covered, highly ordered monolayers were semiconducting. Thus, the ordered two-dimensional (2D) packing of conjugated organic molecules in the semiconducting Langmuir monolayer is compatible with its high-yield luminescence, so that 2D molecular aggregation per se does not preclude highly luminescent properties. Our findings pave the way to the rational design of functional materials for monolayer organic light-emitting transistors and other optoelectronic devices.

(Invited) Optical and Electrical Generation of Exciton-Polaritons in Single-Walled Carbon Nanotubes

Exciton-polaritons are quasiparticles that form upon strong coupling between electronic excitations of a material and photonic states of a surrounding microcavity. In semiconducting single-walled carbon nanotubes the special nature of excited states leads to particularly strong coupling even at room temperature (Nature Comm. 2016, 7, 13078). While emission from exciton-polaritons is usually created by optical excitation, the high ambipolar charge carrier mobilities of single-walled carbon nanotubes make them suitable for electrically driven polariton light-emitting devices with extended functionalities. We show that strong coupling and near-infrared exciton-polaritons can be generated and tuned in light-emitting field-effect transistors with integrated microcavities.

Host-Free Blue Phosphorescent Dendrimer Organic Light-Emitting Field-Effect Transistors and Equivalent Light-Emitting Diodes: A Comparative Study

Light-emitting field-effect transistors (LEFETs) integrate functions of an organic light-emitting diode (OLED) and a field-effect transistor, and hence offer significant advantages in simplifying the device architecture for next-generation active matrix full-color displays as well as having potential in communications and electrically pumped lasers. There is little work on phosphorescent light-emitting field-effect transistors, particularly those based on host-free phosphorescent light-emitting materials. In this report, we demonstrate solution-processed host-free LEFETs and OLEDs based on a blue phosphorescent dendrimer as the active emitter and compare their performance. The LEFETs exhibit excellent electrical and optical characteristics with a luminance of 650 cd m–2 and an EQE of 2.1%, which is comparable to the equivalent OLEDs based on the same dendrimer. The LEFETs were fabricated in a heterostructure bilayer configuration using a p-type charge transport material, poly(2,5-bis(3-n-hexadecylthiophen...

Highly Efficient Red Organic Light-Emitting Transistors (OLETs) on High-k Dielectric

The investigation of the optoelectronic properties of a red organic light-emitting transistor using a high-k polymer (P(VDF-TrFE-CFE)) as gate dielectric is reported. Introducing the high-k polymer strongly reduces the threshold voltages (compared to poly(methyl methacrylate)) with improved efficiency, while maintaining comparable output light power. Use of a high-k polymer as gate dielectric enables the organic light-emitting transistor platform to be exploited in the display market, as it can be driven by commercially available electronics, thus enabling low-bias-driven organic electronics.

Morphology Engineering: A Route to Highly Reproducible and High Efficiency Perovskite Solar Cells.

Despite the rapid increase in the performance of perovskite solar cells (PSC), they still suffer from low lab-to-lab or people-to-people reproducibility. Aiming for a universal condition to high-performance devices, we investigated the morphology evolution of a composite perovskite by tuning annealing temperature and precursor concentration of the perovskite film. Here, we introduce thermal annealing as a powerful tool to generate a well-controlled excess of PbI2 in the perovskite formulation and show that this benefits the photovoltaic performance. We demonstrated the correlation between the film microstructure and electronic property and device performance. An optimized average grain size/thickness aspect ratio of the perovskite crystallite is identified, which brings about a highly reproducible power conversion efficiency (PCE) of 19.5 %, with a certified value of 19.08 %. Negligible hysteresis and outstanding morphology stability are observed with these devices. These findings lay the foundation for further boosting the PCE of PSC and can be very instructive for fabrication of high-quality perovskite films for a variety of applications, such as light-emitting diodes, field-effect transistors, and photodetectors.

Iminoborane units add inorganic flavor to polymers

Chemists in Germany have reported a novel inorganic–organic hybrid polymer that can be regarded as an amino borane analog of poly(p-phenylene vinylene), a semiconducting polymer commonly used in light-emitting diodes, field-effect transistors, solar cells, and other optoelectronic devices (Angew. Chem. Int. Ed. 2017, DOI: 10.1002/anie.201612476). Researchers are increasingly taking advantage of the technique of substituting selected C=C units in conjugated organic frameworks with structurally equivalent B=N units to create materials with new properties and functions. Because π-electron conjugation is often disrupted when substituting B=N for C=C, Holger Helten and coworkers of RWTH Aachen University became curious whether they could create a BN analog of poly(p-phenylene vinylene) and whether the polymer’s π-conjugation would persist. The researchers prepared the BN analog, poly(p-phenylene iminoborane), by a polycondensation reaction of boron- and silylamine-disubstituted benzenes. The team’s photophysical studies on the polymer and a series of oligomers provided evidence of π-conjugation across the B=N linkages,

Charge–Exciton Interaction Rate in Organic Field-Effect Transistors by Means of Transient Photoluminescence Electromodulated Spectroscopy

Organic light-emitting transistors (OLETs) offer a huge potential for the design of highly integrated multifunctional optoelectronic systems and of intense nanoscale light sources, such as the long-searched-for electrically pumped organic laser. In order to fulfill these promises, the efficiency and brightness of the current state-of-the-art devices have to be increased significantly. The dominating quenching process limiting the external quantum efficiency in OLETs is charge–exciton interaction. A comprehensive understanding of this quenching process is therefore of paramount importance. The present article reports a systematic investigation of charge–exciton interaction in organic transistors employing time-resolved photoluminescence electro-modulation (PLEM) spectroscopy on the picosecond time scale. The results show that the injected charges reduce the exciton radiative recombination in two ways: (i) charges may prevent the generation of excitons and (ii) charges activate a further nonradiative channe...

Vertical organic light-emitting transistor showing a high current on/off ratio through dielectric encapsulation for the effective charge pathway

We demonstrate a vertical organic light emitting transistor (VOLET) showing a high current on/off ratio through dielectric encapsulation of a finger-like source electrode, which is placed between the bottom gate electrode and the top drain electrode. In such a VOLET configuration, the charge transport occurs largely from the source electrode to an underlying organic semiconductor (OSC) layer through the interface between the OSC and the gate insulator. Accordingly, the current leakage is greatly reduced and the transistor-type switching behavior with a high on/off current ratio is achieved. The on/off current ratio of our VOLET is about 100 times higher than that of an existing VOLET with no dielectric encapsulation. Numerical simulations of the spatial distributions of the charge densities and the charge pathways performed in the two different VOLET configurations are in good agreement with the experimental results. Our dielectric encapsulation approach will provide a versatile method for developing a new class of high-performance OLET displays.

Harvesting Triplet Excitons with Exciplex Thermally Activated Delayed Fluorescence Emitters toward High Performance Heterostructured Organic Light-Emitting Field Effect Transistors.

The utilization of triplet excitons plays a key role in obtaining high emission efficiency for organic electroluminescent devices. However, to date, only phosphorescent materials have been implemented to harvest the triplet excitons in the organic light-emitting field effect transistors (OLEFETs). In this work, we report the first incorporation of exciplex thermally activated delayed fluorescence (TADF) emitters in heterostructured OLEFETs to harvest the triplet excitons. By developing a new kind of exciplex TADF emitter constituted by m-MTDATA (4,4',4″-tris(N-3-methylphenyl-N-phenylamino)triphenylamine) as the donor and OXD-7 (1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene) as the acceptor, an exciton utilization efficiency of 74.3% for the devices was achieved. It is found that the injection barrier between hole transport layer and emission layer as well as the ratio between donor and acceptor would influence the external quantum efficiency (EQE) significantly. Devices with a maximum EQE of 3.76% which is far exceeding the reported results for devices with conventional fluorescent emitters were successfully demonstrated. Moreover, the EQE at high brightness even outperformed the result for organic light-emitting diode based on the same emitter. Our results demonstrate that the exciplex TADF emitters can be promising candidates to develop OLEFETs with high performance.

Solvent Accommodation: Functionalities Can Be Tailored Through Co-Crystallization Based on 1:1 Coronene-F4TCNQ Charge-Transfer Complex.

Because organic donor/acceptor blending systems play critical roles in ambipolar transistors, photovoltaics, and light-emitting transistors, it is highly desirable to precisely tailor the stacking of cocrystals toward different intrinsic structures and physical properties. Here, we demonstrated that the structure-stacking modes and electron-transport behaviors of coronene-F4TCNQ cocrystals (1:1) can be tuned through the solvent accommodation. Our results clearly show that the solvent accommodation not only enlarges the inner mixed packing (...DAD···) distances, leading to the depressed short-contact interactions including the side-by-side and face-by-face intermolecular interactions, but also switches off electron-transport behavior of coronene-F4TCNQ cocrystals (1:1) in ambient atmosphere.

Optoelectronic parameters of TBADN organic molecule: New aspect to solution technique

2-tert-Butyl-9,10-di(naphth-2-yl)anthracene (TBADN) molecule have enormous applications in organic light-emitting diodes (OLEDs), transistors and organic lasers. These applications are based on its spectral, sensing and optical properties. The important spectral and sensing parameters of TBADN were calculated. The optical band gap values of TBADN were found to be 2.834, 2.775 and 2.674eV for molarities of 0.3, 0.9 and 2.7mM, while the optical band gap values of the TBADN for 0.25mM of chloroform, DCM and THF solvents are 2.892, 2.904 and 2.930eV, respectively. Impacts on the molarities and solvents on optical properties were investigated. In order to obtain lower optical energy gap of the TBADN, chloroform solvents can be preferred, while THF as a solvent can be preferred in applications of the TBADN molecule-based optoelectronic devices due to its higher transmittance spectrum.

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.