Tandem white organic light-emitting diodes adopting a C60：rubrene charge generation layer

Organic bulk heterojunction fullerence(C60) doped 5, 6, 11, 12-tetraphenylnaphthacene(rubrene) as the high quality charge generation layer(CGL) with high transparency and superior charge generating capability for tandem organic light emitting diodes(OLEDs) is developed. This CGL shows excellent optical transparency about 90%, which can reduce the optical interference effect formed in tandem OLEDs. There is a stable white light emission including 468 nm and 500 nm peaks from the blue emitting layer and 620 nm peak from the red emitting layer in tandem white OLEDs. A high efficiency of about 17.4 cd/A and CIE coordinates of(0.40, 0.35) at 100 cd/m2 and(0.36, 0.34) at 1000 cd/m2 have been demonstrated by employing the developed CGL, respectively.     

Effect of NaCl doped into Bphen layer on the performance of tandem organic light-emitting diodes

To improve the performance of tandem organic light-emitting diodes （OLEDs）, we study the novel NaCl as n-type dopant in Bphen：NaCl layer. By analyzing their relevant energy levels and cartier transporting characteristics, we discuss the mechanisms of the effective charge generation layer （CGL） of Bphen：NaCl （6 wt%）/MoO3. In addition, we use the Bphen：NaC1 （20 wt%） layer as the electron injection layer （ELL） combining the CGL to further improve the performance of tandem device. For this tandem device, the maximal current efficiency of 9.32 cd/A and the maximal power efficiency of 1.93 lm/W are obtained, which are enhanced approximately by 2.1 and 1.1 times compared with those of the single- emissive-unit device respectively. We attribute this improvement to the increase of electron injection ability by introducing of Bphen：NaCl layer. Moreover, the CGL is almost completely transparent in the visible light region, which is also important to achieve an efficient tandem OLEDs.     

Exciplex elimination in an organic light-emitting diode based on a fluorene derivative by inserting 4,4’-N,N’-dicarbazole-biphenyl into donor/acceptor interface

Organic light-emitting diodes (OLEDs) composed of a novel fluorene derivative of 2,3-bis(9,9-dihexyl-9H-fluoren-2-yl)-6,7-difluoroquinoxaline (F2Py) were fabricated, and exciplex emission was observed in the device. To depress the exciplex in an OLED for pure colour light emission, 4, 4'-N,N'-dicarbazole-biphenyl (CBP) was inserted as a separator at the donor/acceptor interface. It was found that the device without the CBP layer emitted a green light peaking at 542~nm from the exciplex and a shoulder peak about 430~nm from F2Py. In contrast, the OLED with CBP layer emitted only a blue light peak at about 432~nm from F2Py. Device efficiencies were calculated by a simulative mode in an injection controlled type mechanism, and the results showed that exciplexes yield much lower quantum efficiency than excitons. The device with CBP has a higher power efficiency as no exciplex was present.     

Pure RGB Emissions Based on a White OLED Combined with Optical Colour Filters

We report on a white organic light emitting device （OLED） with a single light emitting layer consisting of a greenish-white emitting host bis-（2-（2-hydroxyphenyl） benzothiazole）zinc （Zn（BTZ）2 ） and an orange-red dopant 5,6,11,12-tetraphenylnaphthacene （rubrene）. The Commission Internationale De L＇Eclairage （CIE） coordinates, external quantum efficiency, and brightness of the white OLED are （0.341, 0.334）, 0.63% and 4000 Cd/m^2 at the bias of 20 V, respectively. Pure red-green-blue （RGB） emissions have been successfully achieved from the white OLED combined well with several built-in optical colour filters （CFs）. The CIE coordinates of the white mixture calculated in theory are very close to the coordinates of the white mixture which recorded with spectrophotometer in practice.     

Efficiency of a blue organic light-emitting diode enhanced by inserting charge control layers into the emission region

We demonstrate high current efficiency of a blue fluorescent organic light-emitting diode (OLED) by using the charge control layers (CCLs) based on Alq3 . The CCLs that are inserted into the emitting layers (EMLs) could impede the hole injection and facilitate the electron transport, which can improve the carrier balance and further expand the exciton generation region. The maximal current efficiency of the optimal device is 5.89 cd/A at 1.81 mA/cm2 , which is about 2.19 times higher than that of the control device (CD) without the CCL, and the maximal luminance is 19.660 cd/m2 at 12V. The device shows a good color stability though the green light emitting material Alq3 is introduced as the CCL in the EML, but it has a poor lifetime due to the formation of cationic Alq3 species.     

A red oxide phosphor,SreScAlO5：Eu^2＋ with perovskite-type structure,for white light-emitting diodes

Sr2ScAlO5：Eu^2＋, a red oxide phosphor with a perovskite-type structure, has been synthesized through a solid-state reaction and its luminescence properties have been investigated. An absorption band centering at 450 nm is observed from the diffuse reflection spectra and the excitation spectra, indicating that the phosphor can match perfectly with the blue light of InGaN light-emitting diodes. A broad red emission band at 620 nm is found from the emission spectra, originating from the 4f^65d-4f^7 transition of the Eu^2＋ ions. The best doping content of Eu in this material is about 5%. S Sr2ScAlO5：Eu^2＋ is a highly promising red phosphor for use in white light-emitting diodes.     

Optical Interference Effects by Metal Cathode in Organic Light-Emitting Diodes

The dependence of light intensities of organic light-emitting diodes (OLEDs) on the distance of emission zone to metal cathode is investigated numerically. The investigation is based on the half-space optical model that accounts for optical interference effects of metal cathode. We find that light intensities of OLEDs are functions of the distance of emission zone from the metal cathode because of the effect of interference of the metal cathode.This interference leads to an optimal location of emission zone in OLEDs for the maximum of light intensities.Optimal locations of emission zone are numerically shown in various emitting colour OLEDs with different metalcathodes and these results are expected to give insight into the preparation of high efficiency full colour or white light OLEDs.     

Novel Field Emission Organic Light Emitting Diodes with Dynode

This work presents novel field emission organic light emitting diodes(FEOLEDs) with dynode,in which an organic EL light-emitting layer is used instead of an inorganic phosphor thin film in the field emission display(FED).The proposed FEOLEDs introduce field emission electrons into organic light emitting diodes(OLEDs),which exhibit a higher luminous efficiency than conventional OLED.The field emission electrons emitted from the carbon nanotubes(CNTs) cathode and to be amplified by impact the dynode in vacuum.These field emission electrons are injected into the multi-layer organic materials of OLED to increase the electron density.Additionally,the proposed FEOLED increase the luminance of OLED from 10 820 cd/m2 to 24 782 cd/m2 by raising the current density of OLED from an external electron source.The role of FEOLED is to add the quantity of electrons-holes pairs in OLED,which increase the exciton and further increase the luminous efficiency of OLED.Under the same operating current density,the FEOLED exhibits a higher luminous efficiency than that of OLED.     

Enhancing The Efficiency of White Organic Light-emitting Diode Using Energy Recyclable Photovoltaic Cells

We demonstrate that power recycling is feasible by using a semi-transparent stripped Al electrode as interconnecting layer to merge a white organic light-emitting devices (WOLED) and an organic photovoltaic(OPV) cell. The device is called a PVOLED. It has a glass/ITO/CuPc/m-MTDATA∶V2O5/NPB/CBP∶FIrpic∶DCJTB/BPhen/LiF/Al/P3HT∶PCBM/V2O5/Al structure. The power recycling efficiency of 10.133% is achieved under the WOLED of PVOLED operated at 9 V and at a brightness of 2 110 cd/m2, when the conversion efficiency of OPV is 2.3%. We have found that the power recycling efficiency is decreased under high brightness and high applied voltage due to an increase input power of WOLED. High efficiency (18.3 cd/A) and high contrast ratio (9.3) were obtained at the device operated at 2 500 cd/m2 under an ambient illumination of 24 000 lx. Reasonable white light emission with Commission Internationale De L'Eclairage (CIE) color coordinates of (0.32,0.44) at 20 mA/cm2 and slight color shift occurred in spite of a high current density of 50 mA/cm2. The proposed PVOLED is highly promising for use in outdoors display applications.     

Fine-tuning the thicknesses of organic layers to realize high-efficiency and long-lifetime blue organic light-emitting diodes

By using p-bis(p-N,N-diphenyl-aminostyryl)benzene doped 2-tert-butyl-9,10-bis-β-naphthyl)-anthracene as an emitting layer,we fabricate a high-efficiency and long-lifetime blue organic light emitting diode with a maximum external quantum efficiency of 6.19% and a stable lifetime at a high initial current density of 0.0375 A/cm2.We demonstrate that the change in the thicknesses of organic layers affects the operating voltage and luminous efficiency greater than the lifetime.The lifetime being independent of thickness is beneficial in achieving high-quality full-colour display devices and white lighting sources with multi-emitters.     

Pure blue and white light electroluminescence in a multilayer organic light-emitting diode using a new blue emitter

We characterized the 6,12-bis{[N-（3,4-dimethylphenyl）-N-（2,4,5-trimethylphenyl）]amino} chrysene （BmPAC）, which has been proven to be a blue fluorescent emission with high EL efficiency. The blue fluorescent device exhibits good performance with an external quantum efficiency of 5.8% and current efficiency of 8.9 cd/A, respectively. Using BmPAC, we also demonstrate a hybrid phosphorescence/fluorescence white organic light-emitting device （WOLED） with high efficiency of 36.3 cd/A. In order to improve the relative intensity of blue light, we plus a blue light-emitting layer （BEML） in front of the orange light emitting layer （YEML） to take advantage of the excess singlet excitons. With the new emitting layer of BEML/YEML/BEML, we demonstrate the fluorescence/phosphorescence/fluorescence WOLED exhibits good performance with a current efficiency of 47 cd/A and an enhanced relative intensity of blue light.     

Efficient Phosphorescent Organic Light Emitting Diodes Using FITCNQ as the Indium-Tin-Oxide Modification Layer

A phosphorescent organic light emitting diode by using tetrafluorotetracyanoquinodimethane （F4 TCNQ） as the indium-tin-oxide modification layer and 4,4＇-bis（earbazol-9-yl）biphenyl （CBP） as the hole transporting layer is reported. CBP doped with a green phosphorescent dopant, tris（2-（p-tolyl）pyridine） iridium（III） （Ir（mppy）3） is used as the emission layer in this device, and the maximum current efficiency of 31.3 cd/A is achieved. Further- more, low efficiency roll-off of 10.4% is observed with device luminance increasing from 100 cd/m2 （29. 7 cd/A） to lO000 cd/m2 （26.5 cd/A）. It is demonstrated that a charge-generation area is formed at F4 TCNQ/CBP interface, which will benefit hole injection into the hole transporting layer. Moreover, use of the CBP hole transporting layer will benefit the low efficiency roll-off by broadening triplet exciton formation, as well as by avoiding accumulation of unbalanced carrier at the hole transporting layer/emission layer interface.     

GaInN light-emitting diodes with omni-directional reflector using nanoporous SnO_2 film

Enhancement of light extraction in a GaInN light-emitting diode （LED） employing an omni-directional reflector （ODR） consisting of GaN, SnO2 nanorod and an Ag layer was presented. The ODR comprises a transparent, quarterwave layer of SnO2 nanorod claded by silver and serves as an ohmic contact to p-type GaN. Transparent SnO2 sols were obtained by sol-gel method from SnCl2·2H2O, and SnO2 thin films were prepared by dip-coating technique. The average size of the spherical SnO2 particles obtained is 200 nm. The refractive index of the nanorod SnO2 film layer is 2.01. The GaInN LEDs with GaN/SnO2/Ag ODR show a lower forward voltage. This was attributed to the enhanced reflectivity of the ODR that employs the nanorod SnO2 film layer. Experimental results show that ODR-LEDs have lower optical losses and higher extraction efficiency as compared to conventional LEDs with Ni/Au contacts and conventional LEDs employing a distributed Bragg reflector （DBR）.     

White electroluminescence of n-ZnO：Al/p-diamond heterostructure devices

An n-ZnO：A1/p-boron-doped diamond heterostructure electroluminescent device is produced, and a rectifying be- havior can be observed. The electroluminescence spectrum at room temperature exhibits two visible bands centred at 450 nm-485 nm （blue emission） and 570 nm-640 nm （yellow emission）. Light emission with a luminance of 15 cd/m2 is observed from the electroluminescent device at a forward applied voltage of 85 V, which is distinguished from white light by the naked eye.     

AlGaInP thin-film LED with omni-directionally reflector and ITO transparent conducting n-type contact

In this paper a novel A1GalnP thin-film light-emitting diode （LED） with omni-directionally reflector （ODR） and transparent conducting indium tin oxide （ITO） n-type contact structure is proposed, and fabrication process is developed. This reflector is realized with the combination of a low-refractive-index dielectric layer and a high reflectivity metal layer. This allows the light emitted or internally reflected downwardly towards the GaAs substrate at any angle of incidence to be reflected towards the top surface of the chip. ITO n-type contact is used for anti-reflection and current spreading layers on the ODR-LED with ITO. The sheet resistance of the ITO films （95 nm） deposited on n- ohmic contact of ODR-LED is of the order 23.5Ω/△ with up to 90% transmittance （above 92% for 590-770 nm） in the visible region of the spectrum. The optical and electrical characteristics of the ODR-LED with ITO are presented and compared to conventional AS-LED and ODR-LED without ITO. It is shown that the light output from the ODR-LED with ITO at forward current 20mA exceeds that of AS-LED and ODR-LED without ITO by about a factor of 1.63 and 0.16, respectively. A favourable luminous intensity of 218.3 mcd from the ODR-LED with ITO （peak wavelength 620 nm） could be obtained under 20 mA injection, which is 2.63 times and 1.21 times higher than that of AS-LED and ODR-LED without ITO, respectively.[第一段]     

Optimization of a GaN-based irregular multiple quantum well structure for a dichromatic white LED

GaN-based irregular multiple quantum well(IMQW) structures assembled two different types of QWs emitting complementary wavelengths for dichromatic white light-emitting diodes(LEDs) are optimized in order to obtain near white light emissions.The hole distributions and spontaneous emission spectra of the IMQW structures are analysed in detail by fully considering the effects of strain,well-coupling,valence band-mixing and polarization effect through employing a newly developed theoretical model from the k · p theory.Several structure parameters such as well material component,well width,layout of the wells and the thickness of barrier between different types of QWs are employed to analyse how these parameters together with the polarization effect influence the electronic and the optical properties of IMQW structure.Numerical results show that uniform hole distributions in different types of QWs are obtained when the number of the QWs emitting blue light is two,the number of the QWs emitting yellow light is one and the barrier between different types of QWs is 8nm in thickness.The near white light emission is realized using GaN-based IMQW structure with appropriate design parameters and injection level.     

Active Loss of Light Yield of PbWO4：Y Scintillation Crystals After Irradiation

PWO crystals doped with yttrium were grown with the Bridgman method in platinum crucible and by using an indigenously developed resistive heating furnace. After an exposure of γ-ray from a ^60Co source, with the dose rate of lS rad/h for 20h, the light output increases for about 15%, accompanied with vanishing of an optical absorption band at 420 nm. The excitation and emission spectra of PWO crystals were measured before and after irradiation with different dose rates. The optical absorption band at 420nm was also found in the PWO sample annealed in oxygen-rlch atmosphere. It is suggested that the absorption band at 42Onm is related to Pb^3 point defects existing in the PWO crystal. The unusual change of light output after irradiation probably results from the transformation of lead ions from Pb^3 to Pb^2 .     

Tandem organic light-emitting diode with a molybdenum tri-oxide thin film interconnector layer

A 10-nm-thick molybdenum tri-oxide(MoO3) thin film was used as the interconnector layer in tandem organic lightemitting devices(OLEDs).The tandem OLEDs with two identical emissive units consisting of N,N-bis(naphthalen-1-yl)N,N-bis(phenyl)-benzidine(NPB) /tris(8-hydroxyquinoline) aluminum(Alq3) exhibited current efficiency-current density characteristics superior to the conventional single-unit devices.At 20 mA/cm2,the current efficiency of the tandem OLEDs using the interconnector layers of MoO3 thin film was about 4.0 cd/A,which is about twice that of the corresponding conventional single-unit device(1.8cd/A).The tandem OLED showed a higher power efficiency than the conventional single-unit device for luminance over 1200cd/m2.The experimental results demonstrated that a MoO3 thin film with a proper thickness can be used as an effective interconnector layer in tandem OLEDs.Such an interconnector layer can be easily fabricated by simple thermal evaporation,greatly simplifying the device processing and fabrication processes required by previously reported interconnector layers.A possible explanation was proposed for the carrier generation of the MoO3 interconnector layer.     

White Emitting ZnS Nanocrystals： Synthesis and Spectrum Characterization

Spherical organic-bonded ZnS nanocrystals with 4.0 4-0.2 nm in diameter are synthesized by a liquid-solid-solution method. The photoluminescence spectrum of sample （[S^2-]/[Zn^2＋] = 1.0） shows a strong white emission with a peak at 490nm and - 170 nm full widths at half maximum. By Gauss fitting, the white emission is attributed to the overlap of a blue emission and a green-yellow emission, originating from electronic transitions from internal S^2- vacancies level to valence band and to the internal Zn^2＋ vacancy level, respectively. After sealingZnS nanocrystals onto InGaN chips, the device shows CIE coordinates of （0.29,0.30）, which indicates their potential applications for white light emitting diodes.     

Nondoped Electrophosphorescent Organic Light-Emitting Diodes Based on Platinum Complexes

An undoped electrophosphorescent organic light-emitting diode is fabricated using a pure platinum（Ⅱ） （2-phenylpyridinato-N, Ca） （3-benzoyl-camphor） [（ppy）pt（bcam）] phosphorescent layer acting as the emitting layer. A maximum power efficiency Tlp of 6.621m/W and current efficiency of 14.78 cd/A at 745 cd/m2 are obtained from the device. The roll-off percentage of ηp of the pure phosphorescent phosphor layer device is reduced to 5% at a current density of 20mA/cm2, which is about 11% for conventional phosphorescent devices. The low roll-off efficiency is attributed to the phosphorescent material, which has the molecular structure of a strong steric hindrance effect.