Efficient simplified orange and white phosphorescent organic light-emitting devices with reduced efficiency roll-off

Efficient orange phosphorescent organic light-emitting devices based on simplified structure with maximum efficiencies of 46.5 lm/W and 51.5 cd/A were reported. One device had extremely low efficiency roll-off with efficiencies of 50.6 cd/A, 45.0 cd/A and 39.2 cd/A at 1000 cd/m², 5000 cd/m² and 10,000 cd/m² respectively. The reduced efficiency roll-off was attributed to more balanced carrier injection and broader recombination zone. The designed simplified white device showed much lower efficiency roll-off than the control one based on multiple emitting layers. The efficiency of simplified white device was 40.8 cd/A at 1000 cd/m² with Commission Internationale de I’Eclairage coordinates of (0.39, 0.46).     

Simplified single-emitting-layer hybrid white organic light-emitting diodes with high efficiency,low efficiency roll-off,high color rendering index and superior color stability

Single-emitting-layer hybrid white organic light-emitting diodes (SEL-hybrid-WOLEDs) are promising candidates for large-area lightings, however, ideal hybrid WOLEDs with a simple structure and high-efficiency, low roll-off, high color rendering index (CRI) and superior CIE coordinates have been rarely reported. In this paper, high-performance SEL-hybrid-WOLEDs are demonstrated by utilizing a thermally activated delayed fluorescence (TADF) host emitter combined with green and red phosphors. The optimized WOLED exhibits an external quantum efficiency (EQE) of 20.2%, CIE coordinates of (0.360, 0.390) and a CRI of 85. Remarkably, an extremely low efficiency roll-off is also realized, with an EQE of 19.4% remained even at the practical luminance of 1000 cd/m², resulting from the wide recombination zone as well as the well-tuned energy transfer in the emitting layer. Moreover, benefited from the stable recombination zone, superior color stability was also achieved. The intriguing results, we believe, greatly manifest the great potential of such a strategy and may pave the way towards real applications.     

High-efficiency and reduced efficiency roll-off of top-emitting organic light-emitting diodes

Top-emitting organic light-emitting diodes (TEOLEDs) have attracted extensive attention for their high brightness and flat-panel display. However, the efficiency roll-off at high brightness is the issue that needs to be resolved for further practical applications using TEOLED devices. Herein, a serial of high-efficiency tandem TEOLED introducing a fullerene/zinc-phthalocyanine organic semiconductor heterojunction as a charge generation layer is demonstrated. With unique photovoltaic properties, the charge generation layer can absorb part of the photons emitted by the emission layer (Ir(ppy)3) and generate electrons and holes. By optimizing the thickness of the charge generation layer, the pure green electroluminescent TEOLED device manufactured has a high brightness of 156 000 cd/cm2 and a maximum current efficiency of 86 cd/A. Importantly, relying on the energy between the photovoltaic and the microcavity effects, only 1.5% of the efficiency roll-off is obtained at 1 000—10 000 cd/cm2. Introducing fullerene/zinc-phthalocyanine as the charge-generating layer provides a promising alternative for developing high-efficiency tandem TEOLED devices.     

Low driving voltage white organic light-emitting diodes with high efficiency and low efficiency roll-off

Single emission layer white organic light-emitting diodes (WOLEDs) showing high color stability, low turn-on voltage, high efficiency and low efficiency roll-off by incorporating iridium(III) bis[(4,6-difluo-rophenyl)-pyridinato-N,C2] (FIrpic) and bis(2-phenylbenzothiazolato) (acetylacetonate)iridium(III) (Ir(BT)₂(acac)) phosphors dyes have been demonstrated. Our WOLEDs without any out-coupling schemes as well as n-doping strategies show low operating voltages, low turn-on voltage (defined for voltage to obtain a luminance of 1 cd/m²) of 2.35 V, 79.2 cd/m² at 2.6 V, 940.5 cd/m² at 3.0 V and 10 300 cd/m² at 4.0 V, respectively, and achieve a current efficiency of 40.5 cd/A, a power efficiency of 42.6 lm/W at a practical brightness of 1000 cd/m², and a low efficiency roll-off 14.7% calculated from the maximum efficiency value to that of 5000 cd/m². Such improved properties are attributed to phosphors assisted carriers transport for achieving charge carrier balance in the single light-emitting layer (EML). Meanwhile the host–guest energy transfer and direct exciton formation process are two parallel pathways serve to channel the overall excitons to dopants, greatly reduced the unfavorable energy losses.     

Improved efficiency,stable spectra and low efficiency roll-off achieved simultaneously in white phosphorescent organic light-emitting diodes by strategic exciton management

Despite their merits of high efficiency and environmental friendliness, phosphor based white organic light-emitting diodes (WOLEDs) for commercial applications still face tough challenges of efficiency roll-off and color stability. Herein, we fabricated high-efficiency phosphor WOLEDs with extremely low roll-off and stable white emission by employing mixed spacer layer between the two complementary emissions as well as mixed host in the orange layer. The strategic exciton management in our proposed device structure greatly balanced the transport of charge carriers due to the excellent exciton manipulation of the mixed spacer, and significantly suppressed the exciton quenching owing to the extended exciton recombination region, which significantly minimized the efficiency roll-off of the fabricated WOLEDs. The resulting phosphor WOLED exhibited the maximum current efficiency (CE) and power efficiency (PE) of 47.5 cd A⁻¹ and 44.7 lm W⁻¹, respectively, and the CE still had 43.1 cd A⁻¹ at 5000 cd m⁻², showing a suppressed efficiency roll-off of only 9.2%. Additionally, the device achieved fairly stable spectra over a wide range of luminance with suitable CIE coordinates for indoor lighting and outdoor displays.     

A spectrum-adjusted white organic light-emitting diode for the optimization of luminous efficiency and color rendering index

High luminous efficiency and high color rendering index (CRI) are both the foremost factors for white organic light-emitting diodes (WOLEDs) to serve as next generation solid-state lighting sources. In this paper, we show that both luminous efficiency and CRI can be improved by adjusting the green/red spectra of WOLEDs. With green emission spectra matching with the human photopic curve, the WOLEDs exhibit higher luminous efficiency and higher CRI. Theoretical calculation shows that by tuning the white emission spectra to maximally match with the human photopic curve, the luminous efficiency can be improved by 41.8% without altering the color coordinates, the color correlated temperature (CCT) and the external quantum efficiency (EQE) of the WOLEDs.     

Color stable and low driving voltage white organic light-emitting diodes with low efficiency roll-off achieved by selective hole transport buffer layers

White organic light-emitting diodes (WOLEDs) showing high color stability, low operating voltage, high efficiency and low efficiency roll-off by adopting different hole transport buffer layers which also behaves as electron/exciton blocking layers (EBL) have been developed. The characteristics of WOLEDs based on blue–green and orange phosphors could be easily manipulated by hole transport buffer layer, which tailors charge carrier transportation and energy transfer. Our WOLEDs show low operating voltages, 100 cd/m² at 3.2 V, 1000 cd/m² at 3.7 V and 10000 cd/m² at 4.8 V, respectively, and achieve a current efficiency of 35.0 cd/A, a power efficiency of 29.0 lm/W at a brightness of 1000 cd/m², and a low efficiency roll-off 8.7% calculated from the maximum efficiency value to that of 5000 cd/m².     

Fabrication of high efficiency and color stable white organic light-emitting diodes by an alignment free mask patterning

Small molecule based white organic light-emitting diodes were fabricated by using an alignment free mask patterning method. A phosphorescent red/green emitting layer was patterned by a metal mask without any alignment and a blue phosphorescent emitting layer was commonly deposited on the patterned red/green emitting layer. A white emission could be obtained due to separate emission of red/green and blue emitting layers. A maximum current efficiency of 30.7 cd/A and a current efficiency of 26.0 cd/A at 1000 cd/m² were obtained with a color coordinate of (0.39, 0.45). In addition, there was little change of emission spectrum according to luminance because of balanced red/green and blue emissions.     

Color stable white organic light-emitting diode based on a novel triazine derivative

A novel red–orange emitting material with a branched molecular structure, 2,4,6-tris[2-(N-ethyl-3-carbazole)carboxethenyl]-1,3,5-s-triazine (TC3), has been synthesized and characterized using UV–visible, photoluminescence (PL) and electroluminescence (EL) spectroscopy. White EL devices were fabricated using TC3 as a red–orange emitter and 8-hydroxyquinolinolato lithium (Liq) as a blue–green emitter. N,N-bis(3-methylphenyl)-N,N-diphenylbenzidine (TPD) as the adjustor for charge carrier mobility was introduced between the two emitting layers to improve the stability of the white emission color on bias voltage. The EL devices of ITO/poly(N-vinylcarbazole) (PVK):TC3 (56 nm)/TPD (5 nm)/Liq (30 nm)/Mg:Ag exhibited good quality white emission. The Commission Internationale De L’Eclairage chromaticity coordinates are (0.34, 0.39) and are stable on the bias voltage.     

Phosphorescent white organic light-emitting devices with color stability and low efficiency decay by using wide band-gap interlayer

High-performance phosphorescent white organic light-emitting devices (PhWOLEDs) with color stability and low efficiency decay are demonstrated by inserting wide band-gap materials between emitting layers. The two devices with N,N’- dicarbazolyl-3,5-benzene (mCP) and p-bis(triphenylsilyl)benzene (UGH2) as the interlayer exhibit both slight Commission Internationale del’Eclairage (CIE) coordinates variations of (±0.010, ±0.005) and (±0.013, ±0.006) in a wide voltage range, and low decay in current efficiency which shifts from the peak value 35.4 cd·A-1 and 27.4 cd·A-1 to 28.8 cd·A-1 and 23.5 cd·A-1 at 40000 cd·m-2 , respectively. The improvements are attributed to the charge carriers balance and the elimination of energy transfer loss by confining the carrier accumulation at the exciton formation interface through the interlayer.     

A novel intramolecular charge transfer blue fluorophor for high color stability hybrid di-chromatic white organic light-emitting diodes

A novel intramolecular charge transfer (ICT) based blue fluorophor, 5,11-di(40-dimesitylboronphenyl)indolo[3,2-b]carbazole(DDBICZ), possessing a high fluorescent quantum yield of 0.52, a high triplet energy level (2.59 eV), and an intriguing bipolar charge transporting ability, was used as a highly efficient blue fluorophor and a host for a yellow phosphor. Doping a yellow phosphor(bis(2-(3-trifluoromethyl-4-fluorophenyl)-4-methylquinolyl)(acetyl-acetonate)iridium(III) (Ir(ffpmq)₂(acac)) in the host DDBICZ, a simplified fluorescence/phosphorescence (F/P)-based hybrid white organic light-emitting diode (WOLED) with a symmetrical BYB-EML of DDBICZ (10 nm)/DDBICZ: 6 wt% Ir(ffpmq)₂(acac) (10 nm)/DDBICZ (10 nm) is demonstrated. The hybrid WOLED exhibits excellent high color stability and good color quality. Under wide operating voltage range from 5 V to 10 V, the Commission Internationale de L'Eclairage (CIE) coordinates of the hybrid WOLED only change from (0.35, 0.36) to (0.34, 0.35), with a high color rending index (CRI) of 79–81. The high color stability is due to the fact that the BYB-EML effectively offsets the change of emission intensity from different emitters caused by the shift of carrier recombination zone with the increase of voltage. In addition, the hybrid WOLED also reveals a considerable current efficiency of 24.4 cd/A. These results demonstrate that efficient F/P hybrid WOLEDs with high color stability could be achieved by such simple BYB-EML structure using single-dopant strategy.     

Color stable multilayer all-phosphor white organic light-emitting diodes with excellent color quality

The color stability of all-phosphor white organic light-emitting diodes (WOLEDs) is crucial and remains a challenge that must be overcome before the wide application of phosphor WOLEDs technology. Besides, color stable all-phosphor WOLEDs should also offer high color rendering index (CRI) and ideal correlated color temperature (CCT) simultaneously to make the technology competitive against other alternative technologies such as inorganic LEDs. In this work, we demonstrate a series of color stable all-phosphor WOLEDs with two emitters (blue and yellow), three emitters (blue, green/red, and yellow) and four emitters (blue, green, yellow and red) by introducing tris (phenylpyrazole) Iridium [Ir(ppz)₃] as interlayer. The results show that appropriate thickness of Ir(ppz)₃ interlayer not only can control exciton distribution in the emission zone, but also can improve the spectra stability. In particular, one efficient four-color device with double-interlayer yields fairly high CRI of 92 and 90, ideal CCT of 3703 K and 3962 K at illumination-relevant luminance of 100 cd m^–2 and 1000 cd m^–2, respectively, which is very appropriate to indoor lighting application. By further employing appropriate hosts to regulate the carrier injection, ultrahigh stable four-color devices with applicable CRI are finally achieved.     

High efficiency green phosphorescent organic light-emitting diodes with a low roll-off at high brightness

Highly efficient green phosphorescent organic light-emitting diodes (PHOLEDs) with low efficiency roll-off at high brightness have been demonstrated with a novel iridium complex. The host material 1,3-bis(carbazol-9-yl)benzene (mCP) with high triplet energy is also used as the hole transporting layer to avoid carrier accumulation near the exciton formation interface and reduce exciton quenching. It provides a new approach for easily fabricating PHOLED with high triplet energy emitter. Moreover, the hole blocking layer is extended into the light emitting layer to form a co-host, realizing better control of the carrier balance and broader recombination zone. As a consequence, a maximum external quantum efficiency of 20.8% and current efficiency of 72.9 cd/A have been achieved, and maintain to 17.4% and 60.7 cd/A even at 10,000 cd/m², respectively.     

High-efficiency phosphorescent organic light-emitting devices with low efficiency roll-off using a thermally activated delayed fluorescence material as host

Phosphorescent organic light-emitting devices (PHOLEDs) with high efficiency and low efficiency roll-off were fabricated. The emissive layer was composed of a thermally activated delayed fluorescence (TADF) material 4,5-bis(carbazol-9-yl)-1,2-dicyanobenzene (2CzPN) as host and an orange iridium complex bis(4-tert-butyl-2-phenylbenzothiozolato-N,C^2′)iridium(III)(acetylacetonate) [(tbt)₂Ir(acac)] as dopant. At a low dopant concentration of 1 wt%, a PHOLED without light extraction optimization achieved a maximum power efficiency of 42.1 lm/W, a luminance efficiency of 77.9 cd/A and an external quantum efficiency (EQE) of 26.8%, respectively. Meanwhile, the EQE maintained 26.6% at 1000 cd/m² and 25.8% at 5000 cd/m², respectively. Moreover, a critical current density of 300 mA/cm² was realized, indicating significantly improved efficiency roll-off. The efficient utilization of triplet excitons on 2CzPN for phosphorescence via reverse inter-system crossing of 2CzPN followed by Fӧrster resonance energy transfer from 2CzPN to (tbt)₂Ir(acac) is responsible for the superior performance.     

High general and special color rendering index white organic light-emitting device with bipolar homojunction emitting layers

High general and special color rendering index (CRI) together with gamut area index (GAI) is demanded for lighting source. In a tolerable Duv, an efficient warm white organic light-emitting device (WOLED) with a high CRI and GAI is developed by using a bipolar homojunction emitting layers, i.e., junction between different emitting layers with the same bipolar host material. At a low drive voltage of 4 V, the WOLED shows a high luminance of 1594 cd/m² and a power efficiency of 12.95 lm/W, and, it shows a high general CRI of 82, a special CRI R₉ of 75, an average of R₉ to R₁₂ of 69 and a GAI of 95. Besides, the WOLED shows relative stable color coordinates due to the elimination of the charges accumulation at interface between different emitting layers. And the current efficiency roll-off of the WOLED is also reduced due to the balance of carrier injection and transport in emitting layers.     

用于有机发光二极管的具有开关漏电抑制能力的像素电路

本文介绍了一种用于有机发光二极管微显示系统的像素电路,该电路具有开关漏电抑制能力。在信号保持期间,电路采用无外部输入的自参考环路跟踪内部节点电压,达到漏电抑制的目的。采用该漏电抑制技术可以用更小的存储电容获得更长的保持时间。采用0.35-μm CMOS 工艺实现了一个60×80像素阵列的试验系统,像素面积是15×15 μm^2。测试结果表明,本文提出的像素电路获得了超过500ms的保持时间,在100pA~3nA输出电流范围内获得了良好的精度和线性度。     

ITO-free down-conversion white organic light-emitting diodes with structured color conversion layers for enhanced optical efficiency and color rendering

We report our study on white organic light-emitting diodes (WOLEDs) implemented in a down-conversion scheme based on an ITO-free, cavity-enhanced blue phosphorescent OLED and a micro-structured color conversion layer (CCL) containing red and green phosphors. Cavity resonance induced by a ZnS/Ag/MoO₃ anode structure enables both efficiency enhancement/spectral refinement of blue phosphorescent OLED. In accordance with the resonance-induced effect, outcoupling assistance provided by micro-structuring of CCLs works to yield WOLEDs with both high efficiency and illumination-quality color rendering. Highly flexible WOLEDs are also demonstrated in the proposed scheme and tested at a radius of curvature of 10.8 mm to illustrate its advantages in realizing versatile next-generation light sources.     

High quantum efficiency and color stability in white phosphorescent organic light-emitting diodes using carboline derivative as a host material

Highly efficient and color stable phosphorescent white organic light-emitting diodes were developed using a high triplet energy host material, 3,3′-bis(9H-pyrido[2,3-b]indol-9-yl)-1,1′-biphenyl (CbBPCb), derived from carboline. Two color phosphorescent white organic light-emitting diodes were fabricated by co-doping of blue and orange triplet emitters or double emitting layer structure of blue and orange emitting layers. High quantum efficiency above 20% and color stability were achieved in the white device by optimizing the doping concentration and emitting layer thickness.     

Concentration-insensitive and low-driving-voltage OLEDs with high efficiency and little efficiency roll-off using a bipolar phosphorescent emitter

A series of simplified trilayer phosphorescent organic light-emitting diodes (PHOLEDs) with high efficiency and little efficiency roll-off based on a bipolar iridium emitter Iridium(III) bis(2-phenylpyridinato)-N,N′-diisopropyl-diisopropyl-guanidinate (ppy)₂Ir(dipig) has been demonstrated. They are dominated by the efficient direct-exciton-formation mechanism and show gratifying concentration-insensitive and low-driving-voltage features. In particular, very high and stable electroluminescence (EL) efficiencies (maximum power efficiency and external quantum efficiency >98 lm W^?1 and 25% respectively, and external quantum efficiency >20% over a wide luminance range of 1–15,000 cd m^?2) are achieved in the PHOLEDs based on emitting layers (EMLs) consisting of (ppy)₂Ir(dipig) codeposited with common host CBP in an easily controlled doping concentration range (15–30 wt%). The EL performance of the PHOLEDs is comparable to the highest PHOLEDs reported in scientific literature.     

High efficiency pure white organic light-emitting diodes using a diphenylaminofluorene-based blue fluorescent material

High efficiency pure white organic light-emitting diodes (WOLEDs) were developed using a highly efficient diphenylaminofluorene-based deep blue fluorescent material (DAF). A high quantum efficiency of 7.1% with color coordinates of (0.15, 0.18) were obtained from the DAF-doped blue device, which was then combined with phosphorescent red/green devices. A mixed interlayer was used to control the color coordinates and charge balance in the emitting layer of the WOLEDs. The pure white hybrid WOLEDs showed a high quantum efficiency of 12.3%.