Study Of Organic Light-emitting Diodes Research Articles

A facile and efficient preparation method for the doped light emitting layer in OLEDs: Blade-coated planar source evaporation

Although vacuum thermal evaporation (VTE) is a widely-used fabrication method in organic light-emitting diodes (OLED) industry, it has long been suffering from the sophisticated fabrication process, low material utilization, and high costs, which become much significant in the preparation of doped light emitting layer (EML). To overcome those intrinsic drawbacks of VTE, we propose a facile and efficient preparation method for the doped EML, i.e., blade-coated planar source evaporation (BCPSE), which combines the benefits of both solution-based blade-coating and combined planar source evaporation (BCPSE) techniques. BCPSE can simplify the fabrication process, increase the material utilization, and reduce the fabrication time, without degrading the device performance compared to that prepared by VTE. The film morphology and fabrication condition were carefully investigated and optimized, which show that BCPSE can increase the material utilization to 95 % and decrease the preparation time to several seconds, implying a great reduction in the fabrication cost. The validity of BCPSE was further demonstrated in a comparison study of a typical orange phosphorescent OLED, in which the mixed ternary EML comprising iridium-based dopant, n-type host, and p-type host was fabricated by using both VTE and BCPSE techniques. With the BCPSE technique, the device exhibited improved maximum external quantum efficiency (EQEmax) of 7.9 %, maximum current efficiency (CEmax) of 13.7 cd/A, and maximum power efficiency (PEmax) of 12.0 lm/W, which are comparable to the device employing VTE. In addition, both techniques gave the same electroluminescent spectra with the Commission International d'Eclairage (CIE) coordinates of (0.59, 0.38). The proposed method is not only helpful in reducing the fabrication costs while maintaining the device performance, but promising in advanced OLED applications, such as microOLEDs.

Lifetime Enhancement and Degradation Study of Blue OLEDs Using Deuterated Materials.

Significant lifetime enhancement, up to an eight-fold increase in T90, has been demonstrated in blue organic light-emitting diode (OLED) devices through the deuteration of host and hole transport materials. We observed a progressive increase in T90 using a series of anthracene-based hydrocarbon hosts with incremental deuteration in the emitting layer. In addition, we realized further lifetime improvement using a deuterated hole-transport layer along with the deuterated emitting layer. To elucidate the deuteration effects, we utilized laser desorption/ionization-time-of-flight (LDI-TOF) mass spectrometry for in situ UV irradiation to induce photodegradation and immediate chemical analysis of the resultant photodegradation species. Adducts between the host and moieties from transport materials were identified in UV-degraded films comprising a mixture of host and transport materials, indicating that similar species could be produced in OLED devices using these materials. Deuteration, in effect, mediated the formation of these adduct species, presumably electroluminescence quenchers, and thus improved the device lifetime. An approximate agreement was obtained between the kinetic isotope effect of the photodegradation reactions and the enhancement in device lifetime with deuteration.

Flexible organic light-emitting diodes for antimicrobial photodynamic therapy

Bacterial infection and the growth of antibiotic resistance is a serious problem that leads to patient suffering, death and increased costs of healthcare. To address this problem, we propose using flexible organic light-emitting diodes (OLEDs) as light sources for photodynamic therapy (PDT) to kill bacteria. PDT involves the use of light and a photosensitizer to generate reactive oxygen species that kill neighbouring cells. We have developed flexible top-emitting OLEDs with the ability to tune the emission peak from 669 to 737 nm to match the photosensitizer, together with high irradiance, low driving voltage, long operational lifetime and adequate shelf-life. These features enable OLEDs to be the ideal candidate for ambulatory PDT light sources. A detailed study of OLED–PDT for killing Staphylococcus aureus was performed. The results show that our OLEDs in combination with the photosensitizer methylene blue, can kill more than 99% of bacteria. This indicates a huge potential for using OLEDs to treat bacterial infections.

Recent Advances in Organic Light-Emitting Diodes Based on Pure Organic Room Temperature Phosphorescence Materials.

Pure organic room temperature phosphorescence (RTP) materials have attracted extensive attention in recent years due to their unique characteristics, such as flexible design method, low toxicity, low cost, as well as the ease of production at scale. The involvement of triplet state and direct radiative transition from the triplet state show that RTP materials have great potential as a new generation emitter in organic light-emitting diodes (OLEDs). Based on the mechanism of phosphorescence, various methods have been developed to achieve RTP emissions in the crystal state. However, the observation of RTP in the thin film state is much more difficult to achieve because of the lower degree of rigidity and suppression of the non-radiative transition. In this mini-review, molecular design strategies developed to achieve RTP emissions and their application in OLEDs are summarized and discussed. The conclusion and outlook point to great potential as well as the challenges for the continued study of pure organic RTP materials-based OLEDs.

The development of hybrid materials that combine polyamides with thienothiophene units and inorganic objects

The results of an experimental study of organic light-emitting diodes with poly-2,5-(3,4-diamino-thieno[2,3-b]thiophene)-4,4′-amidoarylene transport layers and CdSe/CdS/ZnS quantum dots with 4.1 nm CdSe core are presented.

Charge Recombination in Phosphorescent Organic Light-Emitting Diode Host–Guest Systems through QM/MM Simulations

Host–guest systems are crucial for achieving high efficiency in most organic light-emitting diode (OLED) devices. However, charge recombination in such systems is poorly understood due to complicated molecular environment, making the rational design of host–guest systems difficult. In this article, we present a computational study of a phosphorescent OLED with 2,8-bis(triphenylsilyl)dibenzofuran (BTDF) as the host and fac-tris(2-phenylpyridine) iridium (fac-Ir(ppy)3) as the guest, using a combined quantum mechanics/molecular mechanics (QM/MM) scheme. A new reaction coordinate is introduced to measure the electrostatic interactions between the host and guest molecules. Ionization potentials and electron affinities of the host show broader distributions as the host–guest interaction increases. On the basis of these distributions, we describe a molecular picture of charge recombination on the guest and find a direct charge trapping route for this system. Our results suggest several strategies for the design ...

Hybrid Organic/Inorganic Materials Depth Profiling Using Low Energy Cesium Ions.

The structures developed in organic electronics, such as organic light emitting diodes (OLEDs) or organic photovoltaics (OPVs) devices always involve hybrid interfaces, joining metal or oxide layers with organic layers. No satisfactory method to probe these hybrid interfaces physical chemistry currently exists. One promising way to analyze such interfaces is to use in situ ion beam etching, but this requires ion beams able to depth profile both inorganic and organic layers. Mono- or diatomic ion beams commonly used to depth profile inorganic materials usually perform badly on organics, while cluster ion beams perform excellently on organics but yield poor results when organics and inorganics are mixed. Conversely, low energy Cs(+) beams (<500 eV) allow organic and inorganic materials depth profiling with comparable erosion rates. This paper shows a successful depth profiling of a model hybrid system made of metallic (Au, Cr) and organic (tyrosine) layers, sputtered with 500 eV Cs(+) ions. Tyrosine layers capped with metallic overlayers are depth profiled easily, with high intensities for the characteristic molecular ions and other specific fragments. Metallic Au or Cr atoms are recoiled into the organic layer where they cause some damage near the hybrid interface as well as changes in the erosion rate. However, these recoil implanted metallic atoms do not appear to severely degrade the depth profile overall quality. This first successful hybrid depth profiling report opens new possibilities for the study of OLEDs, organic solar cells, or other hybrid devices.

Study of organic light-emitting diodes with exciplex and non-exciplex forming interfaces consisting of an ultrathin rubrene layer

Fluorescent organic light-emitting diodes (OLEDs) with exciplex and non-exciplex forming interfaces, where an ultrathin 5,6,11,12,-tetraphenylnaphtacene (rubrene) emissive layer was sandwiched, were fabricated. The performances of 4,4′,4″-tris(N-carbazolyl)-triphenylamine (TCTA)/1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi) exciplex-type device and 4,4′-di(9H-carbazol-9-yl)biphenyl (CBP)/TPBi no-exciplex-type device were optimized in terms of the thickness of rubrene ultrathin layer. The results showed that the exciplex-type device yielded high device efficiency, attributing to the utilization of triplet excitons as the triplets of the exciplex can be up-converted into singlets via reverse intersystem crossing. The light emission process in exciplex-type device was dominated by energy transfer, while the non-exciplex type device was dominated by charge trapping emission mechanism, which was verified by the single carrier devices.

Degradation study of organic light-emitting diodes with solution-processed small molecule phosphorescent emitting layers

Charge injection and solvent impurities effects on degradation of solution-processed small molecule phosphorescent organic light-emitting diodes.

Reliability Study of Organic Light-emitting Diodes by Continuous-wave and Pulsed Current Stressing

ABSTRACTThe stability of green phosphorescent OLEDs with different structures was evaluated through constant-current stressing. Through the modifications of the ITO anode by different plasma treatments and the hole transport layer (HTL) by incorporating inorganic dopants, we proved that energy level misalignment at the ITO/HTL interface leads to localized joule heating, accelerating defect generation and luminescence decay. Pulsed current stressing was then employed to suppress the joule-heating effect so as to differentiate the thermal and nonthermal factors governing the device degradation. For OLEDs with a large energy barrier at the ITO/HTL interface, the effective lifetime was markedly increased under pulsed operation, whereas in OLEDs with an appropriate interfacial energy level alignment, pulsed stressing with 10% duty cycle only improved the effective half life by ∼15% as compared to continuous-wave stressing, indicating a minor role played by joule heating.

Organic light-emitting devices with multi-shell quantum dots connected with polythiophene derivatives

The results of an experimental study of organic light-emitting diodes with poly-2,5-(3,4-diamino thieno[2,3-b]thiophene)-4,4’-amidoarylene transport layers and CdSe/CdS/ZnS quantum dots, the CdSe core of which has an average diameter of 4.1 nm, are presented. A suggestion is made regarding the possible conjugation of quantum dots and polythiophenes, which would substantially improve the characteristics of the light-emitting diodes in the case of optimization of their structure.

Mechanism of electronic-excitation transfer in organic light-emitting devices based on semiconductor quantum dots

The results of an experimental study of organic light-emitting diodes (LEDs) with luminescent layers based on two types of CdSe/CdS semiconductor quantum dots (QDs) with an average CdSe core diameter of 3 and 5 nm and a characteristic CdS shell thickness of 0.5 nm are presented. The dependences of the LED efficiency on the QD concentration are determined. The experimental data are used to determine the mechanism of electronic-excitation transfer from the organic matrix to the semiconductor QDs. Ways of optimizing the design of the LEDs in order to improve their efficiency are suggested on this basis.

Transient thermal characterization of organic light-emitting diodes

In this paper, a novel measurement approach of junction temperature and thermal resistance of organic light-emitting diodes (OLEDs) is reported. Transient thermal measurement is utilized to carry out the thermal study of OLEDs. A linear relationship between forward voltage and junction temperature is obtained at the sensor current of 0.56 µA for each pixel. The effects of input current and cooling conditions on the junction temperature and thermal resistance are discussed. It is found that the optical performance is greatly affected by the junction temperature. The average junction temperature of the OLED panel with the current density of 0.014 A cm−2 is 64.5 °C, while there exists a temperature difference of 14.5 °C and 43.5 °C with the center case and ambient temperature. In contrast to the natural cooling condition, a much smaller junction temperature rise and thermal resistance are obtained under the forced cooling condition. The thermal resistance from junction to ambient has an inverse relationship with input power under both natural and forced cooling conditions.

Organic semiconductor heterojunction charge generation layer and its application in tandem organic light-emitting diodes

Tandem organic light-emitting diodes (OLEDs) have attracted a great deal of attention from researchers because of their high luminance, efficiency and stability. This paper summarizes past advances in the study of tandem OLEDs. Then, we present a bilayer organic semiconductor heterojunction charge generation layer made of p- and n-type organic semiconductors. A low operational voltage, high power efficiency tandem OLED was fabricated using this charge generation layer as a connecting unit. We also analyze the physical mechanisms that guide the operation of the device, and discuss methods for reducing the voltage and enhancing the power efficiency. Our design concept provides a template for the fabrication of high performance tandem OLEDs. Finally, we discuss the future of tandem OLED technologies.

Ultrafast electric field-assisted pump-probe spectroscopy in poly(9,9-dioctylfluorene) light-emitting diodes

Here we report a detailed study of organic light-emitting diodes based on poly(9,9-dioctylfluorene) (PFO), to investigate polaron generation, due to field induced singlet dissociation, and their subsequent recombination. We find that initially free polarons coalesce into intermediate pairs of both singlets and triplets multiplicity which subsequently decay into the neutral state. There is indication that the efficiency of singlet formation ( β=0.7) is much higher than expected from simple state degeneracy arguments ( β=0.25).

Understanding fundamental processes in poly(9,9-dioctylfluorene) light-emitting diodes via ultrafast electric-field-assisted pump-probe spectroscopy.

Femtosecond electric-field-assisted pump-probe measurements are a new approach to the study of fundamental processes within organic optoelectronic devices. Here we report a detailed study of organic light-emitting diodes based on poly(9,9-dioctylfluorene), using both temporal and spectral information to investigate polaron generation, due to field-induced singlet dissociation, and their subsequent recombination. The fundamental event in electroluminescence is time resolved: we find that initially free polarons coalesce into intermediate pairs of both singlets and triplets multiplicity which subsequently decay into the neutral state. Our results indicate that the efficiency of singlet formation, beta approximately 0.7, is much higher than expected from simple state degeneracy arguments (beta approximately 0.25).