Thin-film Electroluminescent Research Articles

Red and Blue Thin Film Electroluminescence in Eu‐doped orthophosphates

AbstractSrLiPO4, SrZn2(PO4)2 and SrMg2(PO4)2 powders doped with Eu have been synthesized and bright blue Eu2+ photoluminescence (PL) has been observed. Blue electroluminescence (EL), which peaks at 445 nm (CIE coordinates x=0.26, y=0.19) has also been observed in SrLiPO4:Eu2+, which is believed to be a promising new blue EL phosphor.

Light-Emitting Diodes Based on Poly 3-(2-(5-chlorobenzotriazolo)ethyl) Thiophene Langmuir-Blodgett Films

A multilayer polymer thin film electroluminescence (EL) device has been prepared by fully using the LB film method. We synthesized a novel polythiophene derivative poly-3-(2-(5-chlorobenzotriazole)ethyl)thiophene (PCBET) and studied the EL effect of LB LEDs PCBET as the emitting materials. The PL spectra of polymer shows a peak at around 605 nm, which is about 65 nm blue shifted compared with poly 3-decylthiophene results from the benzotrizole group.

Characterization of thin films deposited by RF magnetron sputtering for use in a.c. TFEL devices

Thin films of have been deposited by RF magnetron sputtering onto 100 mm diameter n-type single-crystal Si wafers. Full deposition and post-deposition variables have been investigated with respect to their effect on the dielectric constant and refractive index of the thin films. Specifically for use as insulators for thin film electroluminescent (TFEL) devices, the films need to exhibit a high dielectric constant and a low refractive index. The optimum fabrication route was determined to be deposition at C in a 30% in Ar atmosphere at 7 mTorr with a post-deposition anneal at C for 1 h. Demonstrated here is that films exhibiting suitable characteristics, namely, and n = 2.1, for use in TFEL devices can be fabricated using RF magnetron sputter deposition.

The relation between luminous properties and oxygen content in ZnS:TbOF thin-film electroluminescent devices fabricated by radio-frequency magnetron sputtering method

The purpose of this paper is to study the relationship between the oxygen concentration and brightness degradation in ZnS:TbOF green thin-film electroluminescent (EL) devices. The characteristics including crystallinity, optical, and electrical properties were discussed. The brightness-voltage (B-V) measurement results shelved that with higher oxygen-content in ZnS:TbOF phosphor layer, lower brightness was measured. It was consistent with the poor crystallinity, worse photoluminescent intensity, and easier to get moisture in the oxygen-rich (O/Tb>1) phosphor film. Furthermore, deep level transient spectroscopy (DLTS) measurements identified that when the O/Tb ratio was greater than 1, the oxygen-related deep hole traps E/sub H1/ and/or E/sub H2/ could be detected in the ZnS:TbOF phosphor layer. These E/sub H1/ and/or E/sub H2/ traps were believed to be the main killers for the brightness of the device since they capture most of the holes from the generated electron-hole pairs. This evidence strongly supports that the modified energy transfer model is more dominant than direct impact excitation during the luminescent process.

Study on optical efficiency of alternating-current thin-film electroluminescent devices

An expression of the optical efficiency for alternating current thin film electroluminescent (ACTFEL) devices is given by a model of an optical microscopic cavity, and the optimization of the structure of the ACTFEL device is discussed. The model is reasonable since the radiation patterns developed by it coincide with the experimental results. The model indicates that in the ACTFEL device, the wide-angle interference is obvious, but the multiple-beam interference is slight.

Effect of Zn codoping on Ce3+ emission in SrS electroluminescent thin films

Abstract— Ce emission in SrS thin films has been investigated in the temperature range of 6–300 K. Films codoped with Cl show a strong thermal quenching around 50 K with a thermal activation energy of 0.3–0.4 eV. The low activation energy indicates the presence of shallow quenching centers. These centers are probably created out of various possible complex centers formed due to pairing of strontium and sulfur vacancies with co‐activators and are associated with Ce emission centers. Ce in the presence of Cl has a greater tendency of forming Sr vacancies as compared to that in the presence of excess S. Zn codoping along with K has been found to reduce these vacancies and significantly improve the thermal stability of the luminescent characteristics of SrS:Ce thin films.

Optimization of alternating-current thin-film electroluminescent displays

Different factors affecting the performance of alternating-current thin-film electroluminescent (ACTFEL) devices are discussed. The Chen-Krupka model of electron transport was applied to examine the case of non-uniform distribution of the dopant concentration in the active layer. Based on the results, a ZnS doping profile is proposed which incorporates higher doping at the interfaces than in the bulk. Also, the asymmetry in the excitation efficiency of electrons near the two insulator-semiconductor interfaces (ISIs) of ACTFEL devices was studied and was ascribed to the difference in the interface quality arising from the choice of insulating material and to the selection of the annealing process. The role of the space charge stored near the ISI and the influence of drive conditions on the operational characteristics of the device are discussed.

Electroluminescence in thin solid films of closely-packed CdS nanocrystals

Electroluminescence in thin solid films of closely-packed CdS nanocrystals

Electroluminescence in thin solid films of closely-packed CdS nanocrystals

An all-inorganic structure has been prepared with thin 200–300 nm solid films of closely packed 1.6–1.8 nm CdS nanocrystals sandwiched between ITO and Ag contact layers and electroluminescence behavior has been examined at room temperature. The spectral position of the electroluminescence band is voltage-dependent and exhibits blue shift at increased bias demonstrating the color tunability of electroluminescence output. Voltage-current and bias-intensity curves indicate the hopping conductivity of carriers through the nanogranular semiconductor film. The mechanism of electroluminescence in nanogranular CdS film is suggested to be an impact ionization of deep radiative centers in CdS nanocrystals.

Static space charge in evaporated ZnS:Mn alternating-current thin-film electroluminescent devices

The operation of alternating-current thin-film electroluminescent (ACTFEL) devices may be strongly affected by the presence of dynamic or static positive space charge within the phosphor layer during device operation. Dynamic space charge is a positive charge in the phosphor layer which is periodically created and annihilated during each period of the applied voltage waveform. In contrast, static space charge is a positive space charge in the phosphor layer whose charge state does not change appreciably during steady-state operation of the ACTFEL device. The static space charge density of evaporated ZnS:Mn ACTFEL devices is estimated to be ∼7×1016 cm−3 from measured trends in the phosphor clamping field as a function of phosphor thickness. This static space charge density estimate implies a cathode clamping field of ∼ 2.2 MV/cm and a clamping interface trap depth of ∼1.5 eV. Furthermore, from transferred charge trends as a function of the phosphor thickness it is concluded that the static space charge in evaporated ZnS:Mn ACTFEL devices arises from metastable hole trapping in the phosphor.

The effect of rubrene as a dopant on the efficiency and stability of organic thin film electroluminescent devices

Rubrene was doped into the hole transport layer of an organic thin film electroluminescent (OTFEL) device with a double-layered structure. It was found that the dopant has a profound influence on the EL characteristics - it changed the region of light emission, increased the luminescence efficiency by more than 50% and improved the device stability tenfold. The reasons for these effects are discussed based on injection theory and the energy level diagram of the device.

Electroluminescent Displays

AbstractThis paper will review the electrical and optical properties of both monochrome and color electroluminescent (EL) displays. A simple electrical model for thin film electroluminescent (TFEL) device operation will be presented and used to describe the luminance and power consumption of TFEL devices. The basic material characteristics desirable for EL phosphors will be described. Progress in the development of monochrome and color EL displays will be presented including TFEL and Active Matrix EL devices and phosphors.

The structure, device physics, and material properties of thin film electroluminescent displays

Abstract Thin film electroluminescent (TFEL) displays are complex optoelectronic devices with challenging material requirements. The multilayer device structure includes two electrodes (one metallic and one transparent), two insulators, and a semiconducting ‘phosphor’ layer. Each layer has unique electronic and optical properties that must be satisfied for device operation. In this article, we review the device structure, the electrical and optical device physics, and the material properties of TFEL displays. Particular attention is given to the phosphor layer properties and the radiative recombination phenomenon that is responsible for luminescence. The current status of the red, green, blue and white TFEL phosphors is also reviewed, and the current and future applications are discussed.

Alternating-current thin-film electro-luminescent devices: Effect of fabrication conditions on aging and failure defect formation

Various types of failure defects can be encountered or unintentionally generated in the alternating-current thin-film electroluminescent (ACTFEL) devices by various growth, environmental, and driving conditions. The review summarizes the existing information on the failure formation and aging phenomena as well as methods of defect analysis and classification. The material reported in this paper is intended to serve as a useful tool for the new failure analyst as well as a convenient reference source for the expert in field of thin-film electroluminescent devices. In this study, different kinds of failures, such as localized destructive breakdown defects (LDBD), propagating defects, blistering, layer de-lamination, stress-induced defects, defects of packaging and metallization, and defects due to moisture and ion migration are covered. The optical observation is found to be one of the most valuable tools for large-area defect investigation. The layer-by-layer etch technique is applied to analyze the position of each type of defect in the stack. Most of these defects were found to start at the edges of the pixel. Both the material quality and uniformity of the semiconductor and insulator layers as well as quality of the front transparent electrode shoulders are found to be prerequisites of the failure defect-free structure. Some peculiarities of the active layer growth are found to be responsible for the stoichiometry perturbation and inclusion formation.

Complementary analysis of ALE-grown SrS based thin film electroluminescent structures with ion beam methods

Ion beam analysis techniques available at low energy accelerators (<10 MV) were utilised in the complementary analysis of thin film electroluminescent (TFEL) structures based on SrS phosphor material. The methods used include backscattering (BS), nuclear resonance broadening (NRB), deuteron induced reactions, elastic recoil detection, particle induced X-ray emission, etc. These methods are characterised by their non-destructive nature, good reproducibility and quantitativity. They were used to determine the depth profiles of the main components in a multilayered structure and also the amounts of impurities or dopants such as hydrogen, carbon, oxygen, fluorine, sodium, manganese and cerium. The capability and accuracy of these methods are discussed in the frame of examples and the analysis of measured spectra.

Study on EL emission region of polymer thin film in PPV LED

In this paper, the intensity ratios I(λ 1 = 515 nm)/ I(λ 2 = 550 nm) of both electroluminescence (EL) and photoluminescence (PL) spectra of poly(phenylene vinylene) (PPV) thin film in the same PPV light-emitting diode (LED) are measured in order to discuss the EL emission dominant region. In the EL spectrum, the ratio I 1(λ 1)/ I 1(λ 2) decreased from 1.35 to 1.25, when the bias voltage increased from 4 to 12 V. In the PL spectrum, the ratios I 1(λ 1)/ I 1(λ 2) and I 2(λ 1)/ I 2(λ 2) (here I 1 and I 2 denote the intensities emitting from the indium-tin oxide (ITO) glass side and the opposite side, respectively) approach 1.5 and 1.3, separately. Based on these results, it is deduced that the emission dominant region of EL of PPV thin film in a single-layer PPV LED is close to the top electrode and apart from that of PL.

Sputter deposition of phosphors for electroluminescent flat panel displays

Thin film electroluminescent (TFEL) phosphors for flat panel displays have been produced by several different growth methods including atomic layer epitaxy, chemical vapor deposition, and sputter deposition. There is a great deal of interest in sputter deposition due to the extensive knowledge base and equipment from other existing thin film manufacturing. However, deposition of sulfide-based TFEL phosphors by conventional radio frequency magnetron sputtering has proven to be difficult due to the formation of negative sulfur ions near the target which are accelerated to the growing film by the self-bias developed on the target. This negative-ion resputtering can result in amorphization or re-sputtering of the phosphor films. In severe cases, a net sputtering of the substrate can result. In order to remedy this negative ion resputtering problem, modifications of the magnetron geometry and ion-beam sputtering have been evaluated for production of CaxSr1−xGa2S4:Ce and SrS:Ce TFEL phosphors. Sputter deposited TFEL films also typically require a postdeposition anneal which adds to expense and can cause other problems for the flat panel display. Ion-beam assist during deposition of undoped ZnS was studied as a method to induce surface-atom mobility and a more crystalline as-deposited film for use in monochrome TFEL displays.

Sputter deposition of ZnS:Mn/SrS:Ce multilayered thin film white phosphor

A full color thin film electroluminescent (TFEL) display can be fabricated by using color filters in combination with a high efficiency ‘white’ phosphor, such as a thin film multilayered stack of ZnS:Mn and SrS:Ce (denoted ZnS:Mn/SrS:Ce). To date, deposition of these multilayers has been limited to vacuum evaporation techniques and atomic layer epitaxy, both of which require different substrate temperatures for growth of high quality ZnS:Mn and SrS:Ce. This repeated thermal cycling during multilayer deposition can adversely affect electroluminescent (EL) performance and manufacturability. Sputter deposition of ZnS:Mn and SrS:Ce produces high quality phosphors for a wider range of substrate temperatures. We have determined a common set of radio frequency (rf) sputter deposition parameters for ZnS:Mn and SrS:Ce that result in high performance, multilayered white phosphors for use in TFEL devices. The EL performance of our samples is comparable to the best performance reported for evaporated multilayered samples. The major improvement is that the rf sputtered ZnS:Mn and SrS:Ce layers were deposited at the same substrate temperature. We report on the effects of sputter deposition parameters on the resultant composition and morphology of ZnS:Mn and SrS:Ce thin films and multilayers. Their EL performance was evaluated and correlated to composition and morphology.

Preparation of CaGa 2S 4:Ce thin films for blue emitting thin-film EL device

Blue emitting thin-film electroluminescent (TFEL) devices with Ce 3+-doped calcium thiogallate (CaGa 2S 4:Ce) phosphor films have been prepared by multi-source deposition (MSD). Calcium (Ca), gallium(III) sulfide (Ga 2S 3) and cerium chloride (CeCl 3) were employed as the source materials. The thin film deposited at a substrate temperature of 550°C with a Ga 2S 3 Ca flux ratio of 70 has a CaGa 2S 4 phase, but this film contained a gallium(II) sulfide (GaS) phase as well. The CaGa 2S 4:Ce thin films were successfully prepared by carrying out the annealing in H 2S atmosphere. The film annealed at 800°C showed a bright blue photoluminescence (PL) with peaks at around 460 and 510 nm.

Structural and electroluminescent characteristics of sputtered SrS:Ce thin films by rapid thermal process

Thin film electroluminescent (EL) structure of Y 2O 3/SrS:Ce/BaTiO 3 has been deposited by rf-magnetron reactive sputtering and subsequently submitted to various thermal treatments. The effects of rapid thermal process (RTP) on the structural and EL characteristics of SrS:Ce thin films have been investigated, and compared with the results obtained by the conventional furnace. After the post-deposition annealing, the SrS film has the tendency to recrystallize preferentially in the (200) plane. The higher RTP temperature also results in the larger grain size of SrS. Auger measurements show that the RTP method can overcome the nonstoichiometry and interdiffusion problems encountered in the furnace-treated sample. It was found that the EL peaks gradually recovered to the band transitions of Ce 3+ ions (475 and 530 nm) in the SrS crystal field as the RTP temperature increased from 650 to 850°C. The threshold voltage can be further reduced to 150 V and the brightness increases eight times as much as the optimum furnace-treated device. The improvements in EL performance are due to the facts that the RTP can enhance the crystallinity of SrS, alleviate oxygen-induced defect centers from interdiffusion, activate Ce 3+ ions in phosphor, and yield a blue shift in emission colors.