All-in-one-type organic light-emitting diodes for color tuning using phosphor in glasses with Pb-free silicate powders

We demonstrate all-in-one-type organic light-emitting diodes (OLEDs) that are fabricated using a color converting plate as a substrate. The color converting plate is Pb-free phosphor-in-glass (PiG), which is prepared by mixing Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺) and SiO₂–B₂O₃–RO (R = Ba, Zn) glass frit by sintering at 750 °C for 30 min. The maximum luminance, luminance efficiency, and power efficiency of blue OLEDs fabricated on commercial glass are measured as 10500 cd/m², 10.18 cd/A, and 2.95 lm/W, respectively. The Commission Internationale de l'Eclairge (CIE) coordinates of blue OLEDs is (0.167, 0.325). Our obtained results show that the luminance value decreased as the PiG thickness increased, and the glass to phosphor (GTP) ratio decreased. The OLED devices fabricated on the PiG substrate (GTP ratio = 9:1, thickness: 150 μm) showed a maximum luminance, luminance efficiency, and power efficiency of 7600 cd/m², 8.76 cd/A, and 2.85 lm/W, respectively. The CIE color coordinates changed to (0.286, 0.504) at 200 mA/cm². These results proved that color coordination can be easily adjusted by varying the GTP ratio and the thickness of the PiG.     

Upconversion Based Tunable White-Light Generation in Ln:Y<Subscript>2</Subscript>O<Subscript>3</Subscript> Nanocrystalline Phosphor (Ln = Tm/Er/Yb)

We report the generation of efficient white light based on upconversion (UC) in Tm³⁺/Er³⁺/Yb³⁺:Y₂O₃ nanocrystalline phosphor synthesized by simple and cost effective solution combustion technique on 976 nm laser excitation. The calculated color coordinates (using 1931 CIE standard) for samples annealed at different temperatures vary from (0.16, 0.30) to (0.32, 0.33) with dopant concentration, annealing temperature and the pump power; thus providing a wide color tunability including the white one. White emission is observed even at a very low laser power (60 mW). The maximum upconversion efficiency obtained for white emission is 2.79% with the color coordinates (0.30, 0.32) at laser power of 420 mW which is quite close to the standard white color coordinates.     

The characterization of electroplex generated from the interface between 2-(4-trifluoromethyl-2-hydroxyphenyl)benzothiazole] zinc and N,N′-diphenyl-N,N′- bis(1-naphthyl)-(1,1′-biphenyl)-4,4′-diamine

The electroplex between (2-(4-trifluoromethyl-2-hydroxyphenyl)benzothiazole) zinc [Zn(4-TfmBTZ)₂] as an electron-acceptor and N,N′-diphenyl-N,N′-bis(1-naphthyl)-(1,1′-biphenyl)-4,4′-diamine (NPB) as an electron-donor was characterized by bilayer, blend, and multilayer quantum-well (MQW) device, respectively. The blend composition and quantum-well number are effective parameters for tuning electroluminescence color. White light with high color purity and color rendering index (CRI) was observed from these devices based on Zn(4-TfmBTZ)₂/NPB. Moreover, the blend and MQW devices all exhibit high operation stability, hence excellent color stability. For the device with 5 mol% NPB in blend layer, its Commission International Del’Eclairage (CIE) coordinate region is x=0.28–0.31, y=0.33–0.35 and CRI is 83.3–91.2 at 5–9 V. For MQW structure device with NPB of 60 nm thickness, its CIE coordinate region is x=0.29–0.32, y=0.31–0.34 and CRI=87.9–92.5 at 10–15 V. Such high color stability and purity and CRI, being close to ideal white light, are of current important for white OLED.     

Simple-structure white organic light emitting diodes with high color temperature

We present high color temperature white organic light emitting diodes with a simple p-i-n structure. A sky blue phosphorescent dopant of iridium(III) bis[4,6-(difluorophenyl)-pyridinato-N,C^2’] picolinate and a red phosphorescent dopant of bis(2-phenylquinoline)(acetylacetonate)iridium(III) in the emissive layers is employed to make high color temperature devices. Very stable color variation under ?0.02 until a 5000 cd/m² brightness value is realized by efficient carrier control in a multi stacked emitting layer of blue/red/blue colors. Maximum current and power efficiencies of 23.8 cd/A and 22.9 lm/W in forward direction are obtained. With balanced emissions from the two emitters, the white light emission with very high correlated color temperature of 7308 K as well as CIE coordinates of (0.30, 0.33) is achieved.     

Tailoring red-green-blue emission from Er3+, Eu3+ and Tb3+ doped Y2O3 nanocrystals produced via PVA-assisted sol-gel route

Y₂O₃ luminescent nanoparticles were synthesized via PVA-assisted sol-gel method and their structural and optical properties were investigated. Effects of rare earth (Er³⁺, Eu³⁺ and Tb³⁺) doping on luminescence properties of the produced nanophosphors have been investigated under NIR (800 nm) and UV (240–300 nm) excitation. Intense infrared to red and green emissions were observed and a weak blue upconverted luminescence was also detected. Moreover, it was observed that changing the doping ions, the color emitted by the samples could be modified and different combinations of UV excitation and doping produced effective white light emissions. The obtained results demonstrate that PVA-assisted sol-gel is an effective methodology for the synthesis of rare-earth doped Y₂O₃ nanophosphors.     

Orange emissive Sr3Al2O5Cl2:Eu2+ phosphor for warm-white light-emitting diodes

Orange-emitting Sr₃Al₂O₅Cl₂:Eu²⁺ phosphors were synthesized by a high-temperature solid-state reaction. The excitation spectrum shows a broad band from the UV region to the blue region. The emission spectrum shows strong orange emission peaking at 610 nm, attributed to the d–f transition of the Eu²⁺ ion. By combining the Sr₃Al₂O₅Cl₂:Eu²⁺ phosphor with 420 nm and 460 nm chips, three white light-emitting diodes (LEDs) were fabricated. The warm-white LEDs show color rendering indexes of 76, 66 and 90 with color temperatures of 2447, 3546 and 4300 K, respectively. This new phosphor exhibits the potential to act as a single host doped with Eu²⁺ phosphor for UV or blue chip excited white LEDs.     

Improving the color purity and efficiency of blue organic light-emitting diodes (BOLED) by adding hole-blocking layer

This work demonstrates the fabrication of a bright blue organic light-emitting diode (BOLED) with good color purity using 4,4′-bis(2,2-diphenylvinyl)-1,1′-biphenyl (DPVBi) and bathocuproine (BCP) as the emitting layer (EML) and the hole-blocking layer (HBL), respectively. Devices were prepared by vacuum deposition on indium tin oxide (ITO)-glass substrates. The thickness of DPVBi used in the OLED has an important effect on color and efficiency. The blue luminescence is maximal at 7670 cd/m² when 13 V is applied and the BCP thickness is 2 nm. The CIE coordinate at a luminance of 7670 cd/m² is (0.165, 0.173). Furthermore, the current efficiency is maximum at 4.25 cd/A when 9 V is applied.     

Color variety of up-conversion emission of Er3+/Yb3+ co-doped phosphate glass ceramics

The sample of Er³⁺/Yb³⁺ co-doped phosphate glass ceramic was prepared. At 975 nm laser diode (LD) excitation, the strong up-conversion (UC) emissions were observed, which were the UC green emission at 510–570 nm and the UC red emission at 636–692 nm, respectively. At low pump power (126 mW), the red emission is primary, and the color purity R_cp is 0.81. With the increasing of pump power, the emission color gradually varies from red to green. The intensity of the green emission is stronger compared to that of the red emission at high power (868 mW), and the color purity R_cp is 0.76. Thus, this material can be applied to fluorescence anti-counterfeiting by the color variety of UC emission under different pump power.     

Blue organic light-emitting diodes with 2-methyl-9,10-bis(naphthalen-2-yl)anthracene as hole transport and emitting layer and the impedance spectroscopy analysis

2-methyl-9,10-bis(naphthalen-2-yl)anthracene (MADN) based fluorescent blue organic light-emitting diodes (OLEDs) are demonstrated. With MADN as emitting layer, experiments indicate that thick MADN (40–60 nm) is preferable for constructing efficient blue OLED. With MADN as hole-transport and emitting layer and tris(8-hydroxy-quinolinato)aluminium (Alq₃) as electron-transport layer, the OLED electroluminescent characteristics show a mixture emission of MADN and Alq₃ with Commission Internationale d'Eclairage (CIE) color coordinates of (0.25, 0.34), indicating feasible hole transporting in MADN. Using 4,7-diphenyl-1,10-phenanthroline (BPhen) replacing Alq₃ as electron-transport layer, the OLED shows deep blue emission with a maximum luminous efficiency of 4.8 cd/A and CIE color coordinates of (0.16, 0.09). The hole transport characteristics of MADN are further clarified by constructing hole-only device and performing impedance spectroscopy analysis. The results indicate that MADN shows superior hole-transport ability which is almost comparable to typical hole-transport material of N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine (NPB), suggesting a promising application for constructing efficient blue OLED with integrated hole-transport layer and emitting layer.     

White upconversion emission and color tunability of Y2O3:R(R=Yb3+, Er3+, Tm3+) nanophosphors

The Y₂O₃:R(R = Yb³⁺, Er³⁺, Tm³⁺) nanophosphors were synthesized by a solvothermal method and the temperature dependence of the white upconversion emission was studied using a 975 nm LD. The upconversion emission spectra in 1 mol% Er³⁺/5 mol% Yb³⁺/xTm³⁺ tri-doped Y₂O₃ nanophosphors were sintered at 1000 ^°C with x from 0 to 0.5 mol%. The blue emission intensity increases increasing Tm³⁺ concentration from 0 to 0.5 mol%, because the Tm³⁺ state can be easily reached due to the ²F_7/2 → ²F_5/2 transition of Yb³⁺ near 10,000 cm⁻¹. The Y₂O₃: Er³⁺/Yb³⁺/Tm³⁺ nanophosphors exhibit upconversion emission from white to green with increasing sintering temperature. The calculated CIE coordinates are located in the white region at a pump power of 700 mW at 1000 °C, and the color coordinates were very similar to the standard white light emission. Their upconversion process was described through energy level diagrams and results of upconversion emission spectra and pump power dependence.     

Crystal structure and photoluminescence correlations in white emitting nanocrystalline ZrO2:Eu3+ phosphor: Effect of doping and annealing

White emitting nanocrystalline ZrO₂:Eu³⁺ phosphors were synthesized by a simple precipitation route without using a capping agent. X-ray diffraction (XRD) study of ZrO₂ and ZrO₂:Eu³⁺samples revealed the presence of monoclinic and tetragonal phases. The monoclinic phase increases with increase in the annealing temperature while the tetragonal phase increases with increase in the concentration of Eu³⁺. This can be attributed to the presence of oxygen vacancy evolved when Zr⁴⁺ is replaced by Eu³⁺. Photoluminescence (PL) emission peaks of Eu³⁺ are observed at 591, 596, 606 and 613 nm on monitoring excitation wavelengths at 250, 286, 394 and 470 nm. The peaks at 591 and 606 nm were found to correlate with the tetragonal phase and those at 596 and 613 nm with the monoclinic phase. Intensities of these peaks are found to change as the crystal structure changes. The lifetime value corresponding to 591 nm peak increases with Eu³⁺ concentration at a particular heating temperature indicating increase of tetragonal phase with respect to monoclinic phase. The CIE co-ordinates of the doped samples were found to be close to that of white color (0.33, 0.33). The changes in the crystal structure of the doped samples due to doping and annealing did not affect the white color emission.     

Broadband yellow–white and near infrared luminescence from Bi-doped Ba10(PO4)6Cl2 prepared in reductive atmosphere

Bismuth ions doped Ba₁₀(PO₄)₆Cl₂ was synthesized by solid state reaction and its spectral properties were investigated. The results showed that the samples prepared in air only presented typical ultraviolet (UV) luminescence of Bi³⁺ at 370 nm, however broadband yellow–white and near infrared (NIR) emission could be observed in the samples prepared in reductive atmosphere, indicating those broadband emissions are related to the presence of low valent bismuth species. Correlating excitation and emission data with the charge of available lattice sites suggest that Bi³⁺ coexist with Bi⁺, and each locate on one of the two available Ba²⁺ lattice sites. We propose that broadband yellow–white and NIR emissions are ascribed to the related transitions of Bi⁺ ion.     

Transversely pumped laser using F 3 + and F2 mixed color centers in a LiF crystal at room temperature

A stable laser with F₃⁺ and F₂ mixed color centers in LiF crystal is constructed using a transversely pumped cavity at room temperature. The mixed color center laser is pumped with a nitrogen-laser-pumped dye laser. A pulse output of the laser is 0.23 mJ. The pulse widths of the F₃⁺ and F₂ color center lasers are about 12 and 8.5 ns, respectively. The optical–optical conversion efficiency is about 5.0%. The divergence of the F₃⁺ color center laser beam is about 2.2 mrad and that of the F₂ color center laser beam about 3.5 mrad. The polarization of the mixed color center laser is about 0.97. The output of the F₃⁺ color center laser extends from 515 to 575 nm and peaks at 540 nm, while that of the F₂ color center laser extends from 633 to 705 nm and peaks at 667 nm.     

Photoluminescence of SrGa2S4:Sn,Re(=Ce, Gd) phosphors

The photoluminescence (PL) spectra, PL excitation spectra, color coordinates, and X-ray diffraction spectra are reported for SrGa₂S₄:Sn,Re(=Ce and Gd, respectively) phosphors. By mixing SrGa₂S₄:Sn,Ce phosphors with different Ce³⁺ concentrations, white emissions can be obtained under the excitation of a 340-nm UV LED. Emissions in the green to yellow color range can be obtained from SrGa₂S₄:Sn,Gd phosphors. The rare earth ions enhance the green emission band, which peaks at 534 nm, instead of the yellow one. The origin of this enhancement is discussed. The resonant energy transfer rates are estimated in the cases from Ce³⁺ to the green and yellow centers of Sn²⁺ and between the yellow centers and the green centers.     

Highly efficient and stable organic light-emitting diodes employing MoO3-doped perylene-3, 4, 9, 10-tetracarboxylic dianhydride as hole injection layer

We report highly efficient and stable organic light-emitting diodes (OLEDs) with MoO₃-doped perylene-3, 4, 9, 10-tetracarboxylic dianhydride (PTCDA) as hole injection layer (HIL). A green OLED with structure of ITO/20 wt% MoO₃: PTCDA/NPB/Alq₃/LiF/Al shows a long lifetime of 1012 h at the initial luminance of 2000 cd/m², which is 1.3 times more stable than that of the device with MoO₃ as HIL. The current efficiency of 4.7 cd/A and power efficiency of 3.7 lm/W at about 100 cd/m² have been obtained. The charge transfer complex between PTCDA and MoO₃ plays a decisive role in improving the performance of OLEDs.     

Enhanced power efficiency for white OLED with MoO3 as hole injection layer and optimized charge balance

White OLEDs with a different hole injection layer (MoO₃ or m-MTDATA), and a different electron transport layer (Alq₃ or Bphen) have been investigated. With 9,10-bis (2-naphthyl)-2-t-butylanthracene (TBADN) doped with 3% P-bis (P-N, N-diphenyl-aminostyryl) benzene (DSA-ph) and 0.05% 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7,-tetramethyl-julolidy-9-enyl)-4H-pyran (DCJTB) as white emitting layer, the MoO₃/ /Bphen based device shows the lowest driving voltage and highest power efficiency among the referenced devices. At the current density of 20 mA/cm², its driving voltage and power efficiency are 5.43 V and 4.54 lm/W respectively, which is independently reduced 40% and improved 57% compared with those of the m-MTDATA/ / Alq₃ based one, respectively. The energy level diagram of the devices and single-carrier devices are studied to explain the reasons for the improvement. The results strongly indicate that carrier injection ability and balance shows a key significance in OLED performance.     

Improvement of performance of tetraphenylporphyrin-based red organic light emitting diodes using WO3 and C60 buffer layers

One of the porphyrin derivatives, meso-tetraphenylporphyrin (TPP), has been synthesized and examined as an emitter material (EM) for efficient fluorescent red organic light-emitting diodes (OLEDs). By inserting a tungsten oxide (WO₃) layer into the interface of anode (ITO) and hole transport layer N,N′-Di-[(1-napthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine (NPB) and by using fullerene (C₆₀) in contact with a LiF/Al cathode, the performance of devices was markedly improved. The current density–voltage–luminance (J–V–L) characterizations of the samples show that red OLEDs with both WO₃ and C₆₀ as buffer layers have a lower driving voltage and higher luminance compared with the devices without buffer layers. The red OLED with the configuration ITO/WO₃ (3 nm)/NPB (50 nm)/TPP (60 nm)/BPhen (30 nm)/C₆₀ (5 nm)/LiF (0.8 nm)/Al (100 nm) achieved the high luminance of 6359 cd/m² at the low driving voltage of 8 V. At a current density of 20 mA/cm², a pure red emission with CIE coordinates of (0.65; 0.35) is observed for this device. Moreover, a power efficiency of 2.07 lm/W and a current efficiency of 5.17 cd/A at 20 mA/cm² were obtained for the fabricated devices. The study of the energy level diagram of the devices revealed that the improvement in performance of the devices with buffer layers could be attributed to lowering of carrier-injecting barrier and more balanced charge injection and transport properties.     

Luminescence in Dy<Superscript>3+</Superscript> and Eu<Superscript>3+</Superscript> Activated K<Subscript>3</Subscript>Al<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>

The Dy³⁺ and Eu³⁺ activated K₃Al₂ (PO₄)₃ phosphors were prepared by a combustion synthesis. From a powder X-ray diffraction (XRD) analysis the formation of K₃Al₂ (PO₄)₃ was confirmed. In the photoluminescence emission spectra, the K₃Al₂(PO₄)₃:Dy³⁺ phosphor emits two distinctive colors: blue and yellow whereas K₃Al₂(PO₄)₃:Eu³⁺ emits red color. Thus the combination of colors gives BYR (blue–yellow–red) emissions can produce white light. These phosphors exhibit a strong absorption between 340 and 400 nm which suggest that present phosphor is a promising candidate for producing white light-emitting diodes (LED).     

Spectroscopic investigation and Judd–Ofelt analysis of silver nanoparticles embedded Er3+-doped tellurite glass

A series of silver nanoparticles (NPs) embedded zinc-tellurite glass is prepared by melt-quenching technique. The transmission electron microscopic images reveal spherical as well as anisotropic silver NPs having average diameter in the range of 14–48 nm. The Er³⁺-free glass sample containing AgCl exhibits surface plasmon resonance (SPR) band of Ag NPs centered at ∼ 501 nm. From Judd–Ofelt analysis, it is found that by increasing the concentration of NPs, the value of Ω₂ is enhanced suggesting increased covalency and decreased symmetry around the Er³⁺ ions. Integrated emission cross-section (IEC) is enhanced as the concentration of silver NPs is increased up to 0.5 mol% AgCl. Fourier infrared spectra show that the intensity of the vibrational band of the water molecule and fundamental stretching band of OH group are suppressed. Furthermore, under an excitation wavelength of 786 nm, three prominent upconversion emissions are observed at 520 nm, 550 nm and 650 nm which are attributed to ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2, and ⁴F_9/2 → ⁴I_15/2 transitions, respectively. The upconversion emissions are enhanced significantly by introduction of silver NPs. The enhancement is mainly attributed to the local field effect of silver NPs. Studied nanocomposites are potential candidates for the development of solid state lasers.     

Tandem organic light-emitting diodes with an effective charge-generation connection structure

The tandem organic light-emitting diodes (OLEDs) with an effective charge-generation connection structure of Mg-doped tris(8-hydroxyquinoline) aluminum (Alq₃)/Molybdenum oxide (MoO₃)-doped 3, 4, 9, 10-perylenetetracarboxylic dianhydride (PTCDA) were presented. At a current density of 50 mA/cm², the current efficiency of the tandem OLED with two standard NPB/Alq₃ emitting units is 4.2 cd/A, which is 1.7 times greater than that of the single EL device. The tandem OLED with the similar connection structure of Mg-doped PTCDA/ MoO₃-doped PTCDA was also fabricated and the influences of the different connection units on the current efficiency of the tandem OLED were discussed as well.