The electroluminescence mechanism of non-doping PhOLEDs based on CBP/Ir(ppy)3 investigated by delayed EL measurements

In this paper, the non-doped PhOLED based on 4,4′-bis(9-carbazolyl)-1,1′-biphenyl(CBP) and fac-tris(2-phenylpyridine) iridium(Ir(ppy)₃) with the emitting layer of [CBP/Ir(ppy)₃]_n/CBP in which n is equal to 2,3,4 and 5, respectively, are prepared. The electroluminescence and the luminance-voltage-current density characteristics of devices are detected. The wavelength range of these devices is varied from 517 nm to 540 nm. And the delay EL measurements are used to elucidate the carrier recombination and light emission mechanism in non-doped PhOLEDs. In this technique, the devices are driven using a square pulse driving scheme, with a forward bias pulse width of 1 ms, which is sufficiently long for prompt EL to reach its steady-state intensity. The delay EL results show that changing the number of the non-doped EML leads to marked changes in the charge-trapping and host–host TTA patterns, which suggests that the carrier transport and recombination processes depends on the number of non-doped EML. When the number of non-doped EML is less than 3, only the trapped carrier recombination signal are detected in the delay EL measurement. For the devices with more non-doped EML than 3, both the trapped carrier recombination and host–host TTA signal are detected. All these results are discussed and give the evidence for the electroluminescence mechanism of prepared devices.     

Harvesting Excitons Via Two Parallel Channels for Efficient White Organic LEDs with Nearly 100% Internal Quantum Efficiency: Fabrication and Emission‐Mechanism Analysis

By incorporating two phosphorescent dyes, namely, iridium(III)[bis(4,6‐difluorophenyl)‐pyridinato‐N,C²′]picolinate (FIrpic) for blue emission and bis(2‐(9,9‐diethyl‐9H‐fluoren‐2‐yl)‐1‐phenyl‐1H‐benzoimidazol‐N,C³)iridium(acetylacetonate) ((fbi)₂Ir(acac)) for orange emission, into a single‐energy well‐like emissive layer, an extremely high‐efficiency white organic light‐emitting diode (WOLED) with excellent color stability is demonstrated. This device can achieve a peak forward‐viewing power efficiency of 42.5 lm W^?1, corresponding to an external quantum efficiency (EQE) of 19.3% and a current efficiency of 52.8 cd A^?1. Systematic studies of the dopants, host and dopant‐doped host films in terms of photophysical properties (including absorption, photoluminescence, and excitation spectra), transient photoluminescence, current density–voltage characteristics, and temperature‐dependent electroluminescence spectra are subsequently performed, from which it is concluded that the emission natures of FIrpic and (fbi)₂Ir(acac) are, respectively, host–guest energy transfer and a direct exciton formation process. These two parallel pathways serve to channel the overall excitons to both dopants, greatly reducing unfavorable energy losses. It is noteworthy that the introduction of the multifunctional orange dopant (fbi)₂Ir(acac) (serving as either hole‐trapping site or electron‐transporting channel) is essential to this concept as it can make an improved charge balance and broaden the recombination zone. Based on this unique working model, detailed studies of the slight color‐shift in this WOLED are performed. It is quantitatively proven that the competition between hole trapping on orange‐dopant sites and undisturbed hole transport across the emissive layer is the actual reason. Furthermore, a calculation of the fraction of trapped holes on (fbi)₂Ir(acac) sites with voltage shows that the hole‐trapping effect of the orange dopant is decreased with increasing drive voltage, leading to a reduction of orange emission.     

White organic light-emitting diodes based on emission from DPVBi-doped 4,48-bis (2,28-diphenylvinyl)-1, 18-biphenyl

Organiclight-emitting diodes (OLEDs) have beenin-vestigated for many years on account of their highlumi-nance,low driven voltages ,wide visual range,flexiblesubstratesinflat-panel ,full color displays and backlightapplications .For high brightness and eff…     

Exciton formation and light emission near the organic–organic interface in small-molecule based double-layer OLEDs

We investigate the efficiency and emission color of small-molecule based double-layer organic light-emitting diodes (OLEDs) based on 4,4′-bis[1-naphthyl (phenyl) amino]-1,1′-biphenyl (α-NPD) and aluminum (III) bis (2-methyl-8-quinolinato)4-phenylphenolato (BAlq) by studying the charge transport and photophysics near the organic–organic interface between the emitting layers. For that purpose, the light-emission profile is reconstructed from full angle, wavelength and polarization dependent electroluminescence spectra. By increasing the thickness of the BAlq layer from 100 to 300 nm, at a fixed 160 nm α-NPD layer thickness, the emission color is found to vary from deep blue to green, yellow-green, white and back to blue. We demonstrate that this is due to a gradual emission profile shift, in combination with a wavelength and layer thickness dependent light outcoupling efficiency. The emission profile shift, from an approximately 20 nm-wide zone on the α-NPD-side of the interface to a very narrow zone on the BAlq-side of the interface, gives rise to a changing balance between the contributions from BAlq excitons, α-NPD excitons and charge-transfer excitons. It also contributes to a pronounced layer thickness dependence of the external quantum efficiency. The shift of the emission profile is explained by a charge transport and recombination model.     

Efficiency enhancement of organic light emitting diode via surface energy transfer between exciton and surface plasmon

Organic light emitting diodes (OLEDs) with surface plasmon (SP) enhanced emission have been fabricated. Gold nanoclusters (GNCs) deposited using thermal evaporation technique has been used for localization of surface plasmons. Size of GNCs and distance of GNCs from the emissive layer have been optimized using steady state and time resolved photoluminescence (PL) results. 3.2 Times enhancement in PL intensity and 2.8 times enhancement in electroluminescence intensity of OLED have been obtained when GNCs of size 9.3 nm has been introduced at a distance of 5 nm from emissive layer. Distance dependence of energy transfer efficiency between exciton and SPs was found to be of 1/R⁴ type, which is typically the dependence for dipole-surface energy transfer.     

High-efficiency host free deep-blue organic light-emitting diode with double carrier regulating layers

A bright high-efficiency host-free deep-blue organic light-emitting diode (OLED) is demonstrated. Without the aid of any carrier regulating layer, the deep-blue OLED shows a power efficiency of 1.7 lm W⁻¹ with CIE coordinates of (0.143, 0.098) at 1000 cd m⁻². The respective power efficiency is increased from 1.7 to 2.1 and 2.2 lm W⁻¹ as a single- and double-carrier regulating layers were incorporated. The respective peak luminance also increases from 5250 to 7620 and 9130 cd m⁻², an increment of 45% and 74%. The marked brightness improvement may be attributed to the incorporated carrier regulating layers that effectively lead carriers to recombine in a wider zone. Moreover, the blue emission can be hypsochromic shifted by varying the incorporation position of the carrier regulating layer and the emissive layer thickness.     

Thermal measurements and analysis of AlGaInP/GaInP MQW red LEDs with different chip sizes and substrate thicknesses

Thermal properties of AlGaInP/GaInP MQW red LEDs are investigated by thermal measurements and analysis for different chip sizes and substrate thicknesses. To extract the thermal resistance (R_th), junction temperature (T_j) is experimentally determined by both forward voltage and electroluminescence (EL) emission peak shift methods. For theoretical thermal analysis, thermal parameters are calculated in simulation using measured heat source densities. The T_j value increases with increasing the injection current, and it decreases as the chip size becomes larger. The use of a thin substrate improves the heat removal capability. At 450 mA, the T_j values of 315 K and 342 K are measured for 500 × 500 μm² LEDs with 110 μm and 350 μm thick substrates, respectively. For 500 × 500 μm² LEDs with 110 μm thick substrate, the R_th values of 13.99 K/W and 14.89 K/W are obtained experimentally by the forward voltage and EL emission peak shift methods, respectively. The theoretically calculated value is 13.44 K/W, indicating a good agreement with the experimental results.     

White emission polymer light-emitting devices with efficient electron injection from alcohol/water-soluble polymer/Al bilayer cathode

We demonstrate highly efficient white emission polymer light-emitting diodes (WPLEDs) from multilayer structure formed by solution processed technique, in which alcohol/water-soluble polymer, poly [(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) was incorporated as electron-injection layer and Al as cathode. It was found that the device performance was very sensitive to the solvents from solution of which the PFN electron-injection layer was cast. Devices with electron-injection layer cast from methanol solution show degraded performance while the best device performance was obtained when mixed solvent of water and methanol with ratio of 1:3 was used. We attribute the variation in device performance to washing out the electron transport material in the emissive layer due to rinse effect. As a result of alleviative loss of electron transport material in the emissive layer, the optimized device with a peak luminous efficiency of 18.5 cd A^?1 for forward-viewing was achieved, which is comparable to that of the device with same emissive layer but with low work-function metal Ba cathode (16.6 cd A^?1). White emission color with Commission International de I’Eclairage coordinates of (0.321, 0.345) at current 10 mA cm^?2 was observed.     

Efficient polymer light-emitting diodes with ZnO nanoparticles and interpretation of observed sub-bandgap turn-on phenomenon

Low-temperature liquid-phase synthesized ZnO nanoparticles (NPs) are used to realize highly-efficient polymer light-emitting diodes (PLEDs) containing a poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) emissive layer. It is shown that a 77-nm-thick ZnO electron transport layer (ETL) increases the maximum luminance and current efficiency of the PLED from ∼450 to ∼13,100 cd/m² and 0.55–4.7 cd/A, respectively. In addition, the ZnO layer shifts the peak in the electroluminescence (EL) spectrum from ∼535 nm for a PLED with a CsN₃ layer or no NP layer to ∼585 nm. After ruling out the possibility of ZnO-defect-related emissions and micro-cavity or interference effects, it is argued that the sub-bandgap turn-on behavior of the PLED with a ZnO ETL is the result of emission zone transition and enhanced hole injection rate near the anode, based on the trap-filled space-charge limited conduction (SCLC) phenomenon in the current density-voltage curves and the current-dependent EL spectra.     

Novel Heterolayer Organic Light‐Emitting Diodes Based on a Conjugated Dendrimer

We demonstrate a novel organic light‐emitting diode (LED) heterolayer structure that contains a conjugated dendrimer as the light‐emitting molecule. The LED was prepared by spin‐coating two dendrimer layers from the same solvent. The device consists of a graded bilayer structure formed from a neat dendrimer film covered with a film consisting of the same dendrimer but doped with the electron‐transporting material 2‐(4‐biphenylyl)‐5‐phenyl‐1,3,4‐oxadiazole (PBD). In this device, the heterojunction interface present in conventional bilayer organic light‐emitting diodes is eliminated, and is replaced by a graded interlayer. By optimizing the concentration of PBD in the dendrimer, a peak electroluminescence (EL) external quantum efficiency of 0.16 % at 600 cd m^–2 was obtained. The EL quantum efficiency is significantly enhanced in comparison with devices based on a single layer, a conventional bilayer, and a single‐layer doped with PBD. The EL quantum efficiency is a factor of eight larger than that of a conventional bilayer LED made with the conjugated dendrimer as the emissive layer and poly(methylmethacrylate) (PMMA) doped with PBD as the electron‐transporting layer. The best blended device exhibited only one third of the efficiency of the graded device. The improvement in the operating characteristics of the graded device is attributed to the efficient device structure, in which exciton formation is improved by a graded doping profile of electron‐ and hole‐transporting components.     

Solution-processed high-efficiency cadmium-free Cu-Zn-In-S-based quantum-dot light-emitting diodes with low turn-on voltage

High-efficiency green, yellow and red quantum-dot light emitting devices (QLEDs) have been fabricated via a simple solution-processed technique, in which different-colored Cu-In-Zn-S/ZnS core/shell colloidal semiconductor nanocrystals (NCs) are used as emissive layers sandwiched between an organic holes transport layer and an electron transport layer of ZnO nanoparticles. The yellow QLEDs exhibit a low turn-on voltage of 1.8 V and high luminance of 3061 cd/m² as well as the peak external quantum efficiency (EQE) of 2.42%. In contrast, the green and red QLEDs show a low turn-on voltage of 2 V, and their electroluminescence (EL) peaks are located at 540 and 642 nm with the peak EQE of 0.25% and 0.91%, respectively. The solution-processed high-efficiency cadmium-free QLEDs with a low turn-on voltage and good reproducibility may become one of the most promising next generation of display.     

Improve on/off ratio of organic heterojunction transistors by adopting single-sandwich configuration

Organic heterojunction transistors (OHJTs) based on 5,5′″-bis(naphtha-2-yl)-2,2′:5″,2′″-quaterthiophene (NaT₄)/copper-hexadecafluoro-phthalocyanine (F₁₆CuPc) heterojunction were fabricated in single-sandwich and sandwich configurations, respectively. All the devices operated in depletion-accumulation (normally-on) mode. High field-effect mobility of 0.35 cm²/Vs was obtained for all devices, which was higher than that, 0.20 cm²/Vs of the devices with NaT₄ as active layer. The on/off ratio of 1 × 10⁵ was obtained for OHJTs with single-sandwich configuration, which is three orders of magnitude higher than that of OHJTs with sandwich configuration. Compared with OHJTs with sandwich configuration, the higher on/off ratio was mainly determined by the lower off state current in OHJTs with single-sandwich configuration. In OHJTs with single-sandwich configuration, the well-type shield effect of the source and drain electrodes caused a very narrow empty region in F₁₆CuPc film, which is responsible for the lower off state current.     

Over 1000 V/30 mA operation GaN-on-Si MOSFETs fabricated on Si substrates

We demonstrated the operation of GaN-on-Si metal-oxide-semiconductor field effect transistors (MOSFETs) for power electronics components. The interface states at SiO₂/GaN were successfully improved by annealing at 800 °C for 30 min in N₂ ambient. The interface state density was less than 1 × 10¹¹ cm^-2 eV⁻¹ at E_c − 0.4 eV. The n⁺ contact layers as the source and drain regions as well as the reduced surface field (RESURF) zone were formed using a Si ion implantation technique with the activation annealing at 1200 °C for 10 s in rapid thermal annealing (RTA). As a result, we achieved an over 1000 V and 30 mA operation on GaN-on-Si MOSFETs. The threshold voltage was +2.6 V. It was found that the breakdown voltage depended upon the RESURF length and nitride based epi-layer thickness. In addition, we discussed the comparison of each performance of GaN-on-Si with -sapphire devices.     

Increase of current density and luminance in organic light-emitting diode with reverse bias driving

When applying the voltage pulses (6 V) to the organic light-emitting diode based on tris(8-hydroxyquinolinato) aluminium (Alq₃) as the electron transporting layer, current density and luminance increased by 16% and 20%, respectively, by providing the reverse bias (−16 V) during the off-period. By using displacement current measurement, we can deduce that such an enhancement resulted from the interfacial positive charges trapped at the Alq₃/cathode interface, with the relaxation time ∼0.4 ms. By doping the organic material as the carrier trapping sites at Alq₃/cathode interface, such current density and luminance increase can be further enhanced. 25% and 36% increase in current density and luminance was demonstrated with such driving technique, respectively.     

High morphology stability and ambipolar transporting host for use in blue phosphorescent single-layer organic light-emitting diodes

The authors report a small molecule host of 2,7-bis(diphenylphosphoryl)-9-[4-(N,N-diphenylamino)phenyl]-9-phenylfluorene (POAPF) doped with 8 wt% iridium(III)-bis[(4,6-difluorophenyl)pyridinato-N,C^2′]picolinate (FIrpic) for use in efficient and single-layer blue phosphorescent organic light-emitting diodes (PHOLEDs) exhibiting a maximum external quantum efficiency of ∼20.3% at brightness of 100 cd/m². The high performance of such single layer PHOLEDs is attributed to the POAPF host’s high morphological stability, suitable triplet energy level, and equal charge carrier mobilities of hole and electron to form the broad carrier recombination zone in the emitting layer, thus reducing the triplet-triplet annihilation and resulting in a slight efficiency roll off of 0.5% from the brightness of 1 and 1000 cd/m². This work also systematically investigated the arrangement of the POAPF:FIrpic recombination zone for optimizing the performance of the single layer PHOLED.     

The effects of interface states on the capacitance and electroluminescence efficiency of InGaN/GaN light-emitting diodes

The capacitance-voltage characteristics and external quantum efficiency of electroluminescence in blue GaN light-emitting diodes (LEDs) with an InGaN quantum well have been investigated in the temperature range 77–300 K. The results obtained are interpreted taking into account the effect of the InGaN/GaN interface states of structural defects and impurities on the capacitance of the GaN LEDs. The nonlinearity of the C^?2(U) characteristics observed at low forward bias is attributed to an increase in the interface charge resulting from tunneling of free electrons and their trapping at the interface states. According to estimates, states with a density of about 3 × 10¹² cm^?2 are present at the interface. A recombination current in the interface region suppresses the injection of charge carriers into the quantum well and decreases the electroluminescence efficiency at high forward bias. Degradation of the optical power of the LEDs, accompanied by an increase in the measured capacitance, is attributed to an increase in the density of charged interface states and changes in their distribution in the band gap.     

Morphology-dependent electroluminescence in poly(N-vinyl carbazole)-based multi-component single emissive layer polymer light-emitting diodes

We investigate the electroluminescent performance of single-layer polymer light-emitting diodes with the emissive layer comprised of poly(N-vinyl carbazole) (PVK): electron-transporting molecules (ETMs):iridium(III) [bis(4,6-difluorophenyl)-pyridinato-N,C²]-picolinate (FIrpic) trinary components. A series of ETMs with comparable energy levels is tentatively utilized to modulate the morphology. The morphology of the emissive layer is studied using the transmission electron microscopy and atomic force microscopy in combination with the interfacial energy calculation between these components, with purpose to understand the multi-component miscibility and morphological properties of these emissive layers. It is found that the multi-component miscibility has dramatic influence on the morphology of the multi-component emissive layer and the final electroluminescent performance of the devices. The results reveal that the aggregation of ETMs mainly results in an increase of driving voltages, while the aggregation of FIrpic emitters would lead to the reduction in the luminous efficiencies as a result of their self-quenching effects. The PVK:1,3-bis[(p-tert-butyl)phenyl-1,3,4-oxadiazoyl]benzene (OXD-7): FIrpic blend demonstrates a more uniform morphology resulting in an optimal luminous efficiency of 15 cd A^?1.     

Highly efficient inverted blue light-emitting diodes by thermal annealing and interfacial modification

Inverted polymer light-emitting diodes (IPLED) were fabricated by using polyfluorene (PF-FSO10) as the emissive layer, polyethyleneimine ethoxylate (PEIE) as the interlayer, and 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) as the electron-transport layer with the configuration of ITO/ZnO/PEIE/TPBI/PF-FSO10/MoO₃/Al. Thermal annealing PF-FSO10 made the hole transporting ability decreased, the electron transporting ability enhanced, and thus, jointly promoted the carrier balances in the emissive layer. Inserting a TPBI layer between the PEIE and PF-FSO10 much enhanced the electron transportation and reduced the exciton quench on the ZnO interface. Therefore, the inverted blue light-emitting diode was obtained with a luminous efficiency of 6.8 cd A⁻¹ (an external quantum efficiency of 7.1%), and the CIE color coordinates of (0.15, 0.15), which was among the highest efficiency values in solution-processed inverted blue fluorescent light-emitting diodes.     

Investigation of blue phosphorescent organic light-emitting diode host and dopant stability

By incorporating prospective materials for blue PHOLED emissive layers into model OLEDs, we have investigated how hole transport in a prototypical blue phosphorescent emitter, FIrpic (bis(4,6-difluorophenyl-pyridinato-N,C2) picolinate Iridium) doped mCBP (4′-bis(3-methylcarbazol-9-yl)-2,2′-biphenyl), can impact PHOLED device operational stability. We found the host mCBP to be stable in supporting hole transport, but unstable with respect to electron–hole recombination. As a dopant, FIrpic was found to be unstable with respect to both hole transport and charge recombination processes. Our results indicate that FIrpic doped mCBP is unsuitable for use as an emissive layer in OLED devices and we provide a general strategy to screen materials and better understand their stability.     

Low-voltage p-channel, n-channel and ambipolar organic thin-film transistors based on an ultrathin inorganic/polymer hybrid gate dielectric layer

A key issue in research into organic thin-film transistors (OTFTs) is low-voltage operation. In this study, we fabricated low-voltage operating (below 3V) p-channel, n-channel and ambipolar OTFTs based on pentacene or/and C₆₀ as the active layers, respectively, with an ultrathin AlO_X/poly(methyl methacrylate co glycidyl methacrylate) (P(MMA–GMA)) hybrid layer as the gate dielectric. Benefited from the enhanced crystallinity of C₆₀ layer and greatly reduced density of electron trapping states at the interface of channel/dielectric due to the insertion of ultrathin pentacene layer between C₆₀ and P(MMA–GMA), high electron mobility can be achieved in present pentacene/C₆₀ heterostructure based ambipolar OTFTs. The effect of the thickness of pentacene layer and the deposition sequence of pentacene and C₆₀ on the device performance of OTFTs was studied. The highest electron mobility of 3.50 cm²/V s and hole mobility of 0.25 cm²/V s were achieved in the ambipolar OTFT with a pentacene (3.0 nm)/C₆₀ (30 nm) heterostructure.