Reduced Efficiency Roll-off Research Articles

Reduced Efficiency Roll-Off in White Phosphorescent Organic Light-Emitting Diodes Based on Double Emission Layers.

We demonstrate high-efficiency white phosphorescent organic light-emitting diodes with low efficiency roll-off. The feature of the device concept is employing two phosphorescent emission layers (EMLs) separated by a mixed interlayer. Both the EMLs are doped by two phosphorescent dyes. The resulting white device with the optimized doping concentration shows a maximum efficiency of 31.0 cd/A with extremely low efficiency roll-off of 30.7 cd/A at 1000 cd/m2, 27.2 cd/A at 5000 cd/m2, and 25.5 cd/A at 10,000 cd/m2, respectively, without any outcoupling structures. This is enabled by the balanced charge carrier transport in EMLs, leading to broader exciton recombination zone.

Ultra-Bright Near-Infrared Perovskite Light-Emitting Diodes with Reduced Efficiency Roll-off

Herein, an insulating biopolymer is exploited to guide the controlled formation of micro/nano-structure and physical confinement of α-δ mixed phase crystalline grains of formamidinium lead iodide (FAPbI3) perovskite, functioning as charge carrier concentrators and ensuring improved radiative recombination and photoluminescence quantum yield (PLQY). This composite material is used to build highly efficient near-infrared (NIR) FAPbI3 Perovskite light-emitting diodes (PeLEDs) that exhibit a high radiance of 206.7 W/sr*m2, among the highest reported for NIR-PeLEDs, obtained at a very high current density of 1000 mA/cm2, while importantly avoiding the efficiency roll-off effect. In depth photophysical characterization allows to identify the possible role of the biopolymer in i) enhancing the radiative recombination coefficient, improving light extraction by reducing the refractive index, or ii) enhancing the effective optical absorption because of dielectric scattering at the polymer-perovskite interfaces. Our study reveals how the use of insulating matrixes for the growth of perovskites represents a step towards high power applications of PeLEDs.

Exciplex Cohosts Employing Nonconjugated Linked Dicarbazole Donors for Highly Efficient Thermally Activated Delayed Fluorescence-Based Organic Light-Emitting Diodes.

Two new nonconjugated linked dicarbazole materials, dCzPSi and dCzPSO2, with high triplet energy were synthesized and characterized. dCzPSi and dCzPSO2 were adopted as unipolar host materials for the green thermally activated delayed fluorescence (TADF) emitter (4CzIPN) to achieve high-efficiency organic light-emitting diodes (OLEDs). The electron-transporting acceptor, PO-T2T, was introduced to mix with dCzPSi and dCzPSO2 to give two new exciplex-forming systems that can improve the exciton formation propensity in the emitting layer. The relevant properties of these new exciplexes were characterized, and they were suggested as promising cohosts for the green TADF emitter, 4CzIPN. The characteristics of the devices employing single hosts (dCzPSi and dCzPSO2) and exciplex-forming cohosts (dCzPSi:PO-T2T and dCzPSO2:PO-T2T) were explored. The obtained results indicate that the Si-bridged dicarbazole compound dCzPSi outperforms its counterpart dCzPSO2 in which two carbazole groups are linked by an SO2 group. The device employed with the dCzPSi:PO-T2T cohost with 10 wt % 4CzIPN achieved a low Von of 2.2 V and maximum efficiencies of 21.1% (external quantum efficiency), 56.4 cd A-1 (current efficiency), 59.1 lm W-1 (power efficiency), as compared to those (18.7%, 56.6 cd A-1, and 68.5 lm W-1) of the dCzPSO2:PO-T2T-hosted device. This work verifies the advantages of using a cohost that can form an exciplex for boosting the device efficiency with reduced efficiency roll-off of TADF-based OLEDs.

Reduced Efficiency Roll-Off and Enhanced Stability in Perovskite Light-Emitting Diodes with Multiple Quantum Wells.

Mixed-formamidinium (FA) and -cesium (Cs) cations were used to fabricate multiple quantum well (MQW) perovskite light-emitting diodes (PeLEDs). The partial substitution of FA with Cs facilitates the formation of wider quantum wells, which can effectively reduce efficiency roll-off by suppressing Auger recombination. The device has a peak external quantum efficiency (EQE) of 7.8% at a current density of 125 mA cm-2, maintaining an EQE of ∼6.6% at a high current density of 500 mA cm-2. Due to the improved stability of mixed-cation structure, we achieve a PeLED device with a lifetime of ∼31 h under a constant current density of 10 mA cm-2.

4-Diphenylaminocarbazole: Switching Substituent Position for Voltage Reduction and Efficiency Enhancement of OLEDs

Simple but exceptionally efficient 4-diphenylaminocarbazole host material, 4-DPACz, is presented and compared with its positional isomer, 1-DPACz. The shift of diphenylamino substituent from the 1-position to 4-position of carbazole resulted in an increase in the HOMO energy level as well as an increase in triplet energy level. Having a high triplet energy level (2.76 eV) and well-matched HOMO energy level (-5.61 eV), 4-DPACz showed reduced driving voltage and higher efficiencies for solution-processed green PhOLEDs compated to PVK as well as 1-DPACz. Maximum luminous, power, and external quantum efficiencies reaching to 47.9 cd A-1, 25.2 lm W-1, and 14.3%, respectively, were achieved with a device configuration of [ITO/PEDOT:PSS/4-DPACz:Ir(mppy)3/TPBi/CsF/Al]. Additional enhancement of efficiencies of 4-DPACz was verified when incorporating another dopant, Ir(Si-bppy)2(acac), resulting in 59.1 cd A-1, 29.5 lm W-1, and 15.8%. Furthermore, reduced efficiency roll-off was clearly observed for 4-DPACz compared with PVK. Such improved device characteristics of 4-DPACz were attributed to its high hole mobility and charge balance inside the emitting layer therof. The excellent results using such a simple-structured 4-DPACz could promote various applications of this 4-DPACz unit as a building block structure for further possible oligomeric, dendritic, and polymeric materials.

Highly Efficient TADF Polymer Electroluminescence with Reduced Efficiency Roll-off via Interfacial Exciplex Host Strategy.

Solution-processed organic light-emitting diodes (s-OLED) consisting of TAPC/TmPyPB interfacial exciplex host and polymer PAPTC TADF emitter are prepared, simultaneously displaying ultralow voltages (2.50/2.91/3.51/4.91 V at luminance of 1/100/1000/1000 cd m-2), high efficiencies (14.9%, 50.1 lm W-1), and extremely low roll-off rates (J50 of 63.16 mA cm-2, L50 of ca. 15000 cd m-2). Such performance is distinctly higher than that of pure-PAPTC s-OLED. Compared to pure-PAPTC, the advanced emissive layer structure of TAPC:PAPTC/TmPyPB is unique in much higher PL quantum yield (79.5 vs 36.3%) and nearly 4-fold enhancement in kRISC of the PAPTC emitter to 1.48 × 107 s-1.

Versatile Donor-π-Acceptor-Type Aggregation-Enhanced Emission Active Fluorophores as Both Highly Efficient Nondoped Emitter and Excellent Host.

By employing triphenylamine (TPA) and/or carbazole as donor and cyano group as acceptor, three donor-π-acceptor (D-π-A)-type fluorophores, 4,6-di(4-(diphenylamino)phenyl)isophthalonitrile (DTPAIPN), 4-(4-(diphenylamino)phenyl)-6-(3,5-(9,9'-dicarbazolyl)phenyl)isophthalonitrile (TPAmCPIPN), and 4,6-di(3,5-(9,9'-dicarbazolyl)phenyl)isophthalonitrile (DmCPIPN), were designed, synthesized, and characterized for their thermal, photophysical, electrochemical, and electroluminescent properties. All three compounds exhibited excellent thermal and morphological stabilities with thermal decomposition (Td) and glass transition temperature (Tg) values ranging from 396 to 516 °C and from 114 to 211 °C, respectively. The investigation into the photophysical properties revealed the aggregation-enhanced emission (AEE) feature of these chromophores. The performances of the nondoped devices were significantly improved as compared to those of the doped devices, further confirming the AEE nature. In particular, DmCPIPN-based nondoped device rendered the maximum current efficiency (CE), power efficiency (PE), and external quantum efficiency (EQE) values of 26.6 cd A-1, 22.4 lm W-1, and 8.3%, respectively, in comparison with 9.1 cd A-1, 4.9 lm W-1, and 4.2% for the doped device. The versatility of these new luminogens was demonstrated through the utilization of them as host for highly efficient red phosphorescent organic light-emitting diodes (PHOLEDs). The EQE of the DmCPIPN-hosted device reached as high as 19.3% with a reduced efficiency roll-off of 10.3% at 1000 cd m-2. These results demonstrate that the new luminogens of this work can be used as both nondoped emitter and host for high-performance OLEDs, which facilitates the development of multifunctional materials for OLEDs applications.

Novel thioxanthone host material with thermally activated delayed fluorescence for reduced efficiency roll-off of phosphorescent OLEDs

2,7-Di(9,9-dimethyl-9H-fluoren-1-yl)-9H-thioxanthen-9-one (DMBFTX) with thermally activated delayed fluorescence (TADF) was well designed and synthesized. The phosphorescent organic light-emitting device (PHOLED) based on this novel TADF host material displays a stable red phosphorescence region, a peak external quantum efficiency (EQE) value of 12.9% and a low EQE roll-off of 38.8% at a luminance of 10000cd/m2, which is benefited from the reverse intersystem crossing (RISC) of TADF host and less populated triplet exitons. Notably, the red device based on the TADF host DMBFTX exhibits superior electroluminescence performance and reduced efficiency roll-off compared with the one hosted by commercially available host 1,3-bis(9-carbazolyl)benzene (mCP), illustrating the high potential of employing the TADF host material with small energy gap to reduce efficiency roll-off in PHOLED.

Triphenyl phosphine oxide and carbazole-based polymer host materials for green phosphorescent organic light-emitting diodes

Four novel polymers, poly(3,6-9-decyl-carbazole-alt-1,3-benzene) (PB13CZ), poly(3,6-9-decyl-carbazole-alt-bis(4-phenyl) (phenyl) phosphine oxide) (PTPPO38CZ), poly(3,6-9-decyl-carbazole-alt-2,4-phenyl(diphenyl) phosphine oxide) (PTPPO13CZ) and poly(3,6-9-decyl-carbazole-alt-bis(3-phenyl) (phenyl) phosphine oxide) (PTTPO27CZ) were synthesized, and their thermal, photophysical properties and device applications were further investigated to correlate the chemical structures with the photoelectric performance of bipolar host materials for phosphorescent organic light emitting diodes. All of them show high thermal stability as revealed by their high glass transition temperatures and thermal decomposition temperatures at 5% weight loss. These polymers have wide band gaps and relatively high triplet energy levels. As a result, the spin coating method was used to prepare the green phosphorescent organic light emitting diodes with polymers PTPPO38CZ, PTPPO13CZ and PTTPO27CZ as the typical host materials. The green device of polymer PTPPO38CZ as host material shows electroluminescent performance with maximum current efficiency of 2.16 cd·A−1, maximum external quantum efficiency of 0.7%, maximum brightness of 1475 cd·m−2 and reduced efficiency roll-off of 7.14% at 600 cd·m−2, which are much better than those of the same devices hosted by polymers PTTPO27CZ and PTPPO13CZ.

Highly Simplified Reddish Orange Phosphorescent Organic Light-Emitting Diodes Incorporating a Novel Carrier- and Exciton-Confining Spiro-Exciplex-Forming Host for Reduced Efficiency Roll-off.

A novel exciplex-forming host is applied so as to design highly simplified reddish orange light-emitting diodes (OLEDs) with low driving voltage, high efficiency, and an extraordinarily low efficiency roll-off, by combining N,N-10-triphenyl-10H-spiro [acridine-9,9'-fluoren]-3'-amine (SAFDPA) with 4,7-diphenyl-1,10-phenanthroline (Bphen) doped with trivalent iridium complex bis(2-methyldibenzo[f,h]quinoxaline) (acetylacetonate)iridium(III) (Ir(MDQ)2(acac)). The reddish orange OLEDs achieve a strikingly high power efficiency (PE) of 31.80 lm/W with an ultralow threshold voltage of 2.24 V which is almost equal to the triplet energy level of the phosphorescent reddish orange emitting dopant. The power efficiency of the device with the exciplex-forming host is enhanced, achieving 36.2% mainly owing to the lower operating voltage by the novel exciplex forming cohost, compared with the reference device (23.54 lm/W). Moreover, the OLEDs show extraordinarily low current efficiency (CE) roll-off to 1.41% at the brightness from 500 to 5000 cd/m2 with a maximal CE of 32.87 cd/A (EQEmax = 11.01%). The devices display a good reddish orange color (CIE of (0.628, 0.372) at 500 cd/m2) nearly without color shift with increasing brightness. Co-host architecture phosphorescent OLEDs show a simpler device structure, lower working voltage, and a better efficiency and stability than those of the reference devices without the cohost architecture, which helps to simplify the OLED structure, lower the cost, and popularize OLED technology.

Highly efficient thermally activated delayed fluorescence materials with reduced efficiency roll-off and low on-set voltages

Two carbazole and diphenylamine terminated D–A type molecules were developed as TADF materials, achieving highly efficient blue and green emitting devices with low on-set voltages and reduced efficiency roll-off.

Simple-Structured Phosphorescent Warm White Organic Light-Emitting Diodes with High Power Efficiency and Low Efficiency Roll-off.

We present phosphorescent WOLEDs fabricated simply by inserting an ultrathin nondoped orange layer within the blue emissive zone, where an efficient exciplex system is applied as the host. The resulting WOLED shows maximum power efficiency of 75.3 and 63.1 lm/W at the luminance of 1000 cd/m(2). The exciton density profile in the emitting layer, the operational mechanism and the quenching process at high luminance are systematically investigated by experimental and theoretical methods, from which it is concluded that the efficient utilization of excitons via exciplex host and the wide recombination zone are the key factors for the prominent achievement of high efficiency and greatly reduced efficiency roll-off.

Achieving Extreme Utilization of Excitons by an Efficient Sandwich-Type Emissive Layer Architecture for Reduced Efficiency Roll-Off and Improved Operational Stability in Organic Light-Emitting Diodes.

It has been demonstrated that the efficiency roll-off is generally caused by the accumulation of excitons or charge carriers, which is intimately related to the emissive layer (EML) architecture in organic light-emitting diodes (OLEDs). In this article, an efficient sandwich-type EML structure with a mixed-host EML sandwiched between two single-host EMLs was designed to eliminate this accumulation, thus simultaneously achieving high efficiency, low efficiency roll-off and good operational stability in the resulting OLEDs. The devices show excellent electroluminescence performances, realizing a maximum external quantum efficiency (EQE) of 24.6% with a maximum power efficiency of 105.6 lm W(-1) and a maximum current efficiency of 93.5 cd A(-1). At the high brightness of 5,000 cd m(-2), they still remain as high as 23.3%, 71.1 lm W(-1), and 88.3 cd A(-1), respectively. And, the device lifetime is up to 2000 h at initial luminance of 1000 cd m(-2), which is significantly higher than that of compared devices with conventional EML structures. The improvement mechanism is systematically studied by the dependence of the exciton distribution in EML and the exciton quenching processes. It can be seen that the utilization of the efficient sandwich-type EML broadens the recombination zone width, thus greatly reducing the exciton quenching and increasing the probability of the exciton recombination. It is believed that the design concept provides a new avenue for us to achieve high-performance OLEDs.

Extremely high external quantum efficiency of inverted organic light-emitting diodes with low operation voltage and reduced efficiency roll-off by using sulfide-based double electron injection layers

We developed efficiency roll-up blue fluorescent and green phosphorescent inverted light-emitting diodes with very-low energy consumption by using advanced double electron injection layers composing of metal sulfide.

Evaporation- and Solution-Process-Feasible Highly Efficient Thianthrene-9,9',10,10'-Tetraoxide-Based Thermally Activated Delayed Fluorescence Emitters with Reduced Efficiency Roll-Off.

Two novel evaporation- and solution-process-feasible thermally activated delayed fluorescence emitters, green-light-emission ACRDSO2 and yellow-light-emission PXZDSO2, based on a brand-new electron-acceptor moiety thianthrene-9,9',10,10'-tetraoxide, are developed for organic light-emitting diodes. The solution-processed devices, without any hole-transport layer, exhibit competitive performance and reduced efficiency roll-off compared with corresponding vacuum-deposited devices.

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 46.5lm/W and 51.5cd/A were reported. One device had extremely low efficiency roll-off with efficiencies of 50.6cd/A, 45.0cd/A and 39.2cd/A at 1000cd/m2, 5000cd/m2 and 10,000cd/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 40.8cd/A at 1000cd/m2 with Commission Internationale de I’Eclairage coordinates of (0.39, 0.46).

Highly Efficient Blue Fluorescent Organic Light-Emitting Diodes by Engineering Hole-Transporting/Exciton-Blocking Layer

This paper reports low-voltage and high power efficiency bluefluorescent organic light-emitting diodes (FLOLEDs) using 4,4 � -bis[4(di-4-tolylamino)styryl]biphenyl (DPAVBi) as a fluorescent dye and 9,10-di(naphth-2-yl)anthracene (ADN) as a host. We studied the effect of using 1,1-bis{4-[N,N-di(p-tolyl)amino]-phenyl}cyclohexane (TAPC) as an exciton blocking layer in the blue FLOLEDs, and found the formation of electromers in TAPC at high current density. Based on the efforts toward high power efficiency and stability, we fabricated the blue FLOLEDs exhibiting low driving voltages of 2.9 V for 100 cd m−2 and 3.6 V for 1000 cd m−2 ,a maximum luminance of 43256 cd m−2 at 6.6 V, and a high power efficiency of 9.7 lm W−1 with reduced efficiency roll-off.

Using an ultra-thin non-doped orange emission layer to realize high efficiency white organic light-emitting diodes with low efficiency roll-off

By adopting an ultra-thin non-doped orange emission layer sandwiched between two blue emission layers, high efficiency white organic light-emitting diodes (WOLEDs) with reduced efficiency roll-off were fabricated. The optimized devices show a balanced white emission with Internationale de L'Eclairage of (0.41, 0.44) at the luminance of 1000 cd/m2, and the maximum power efficiency, current efficiency (CE), and external quantum efficiency reach 63.2 lm/W, 59.3 cd/A, and 23.1%, which slightly shift to 53.4 lm/W, 57.1 cd/A, and 22.2% at 1000 cd/m2, respectively, showing low efficiency roll-off. Detailed investigations on the recombination zone and the transient electroluminescence (EL) clearly reveal the EL processes of the ultra-thin non-doped orange emission layer in WOLEDs.

Tailoring the hole-injection layer in organic light-emitting devices by introducing Au@SiO_2 nanoparticles

A simple and efficient method to improve the internal and external quantum efficiency of phosphorescent organic light-emitting devices (PhOLEDs) is demonstrated, i.e., combining the localized surface plasmon (LSP) and optical scattering effects by introducing gold nanoparticles (NPs) coated by SiO₂ (named as Au@SiO₂). The effects of LSP and optical scattering on PhOLEDs are studied. The lifetime of the triplet exciton in the device is modified by surface plasmon coupling induced by Au@SiO₂ NPs, which results in increased internal quantum efficiency of the NP-containing devices. The optical scattering effect induced by Au@SiO₂ NPs enhances the light outcoupling efficiency of the PhOLEDs and is demonstrated by measuring the angularly resolved photoluminescence. As a result, a PhOLED with 101% improved efficiency and 21% reduced efficiency roll-off is achieved using the composite HTL.

Dicarbazolyldicyanobenzenes as Thermally Activated Delayed Fluorescence Emitters: Effect of Substitution Position on Photoluminescent and Electroluminescent Properties

Abstract We demonstrate two efficient blue-green thermally activated delayed fluorescence (TADF) compounds comprising a dimeric phenylcarbazole and four cyano substituents on the phenyl rings. A comparison of the 2,6-dicyano-substituted derivative (26IPNDCz) with the 3,5-dicyano-substituted derivative (35IPNDCz) shows that 26IPNDCz provides a larger dihedral angle and a lower decrease in the energy difference between the first singlet and triplet excited states (ΔEST) and the TADF lifetime. An organic light-emitting diode based on 26IPNDCz afforded an external quantum efficiency of 10% with reduced efficiency roll-off.