Highly Simplified Tandem Organic Light-Emitting DevicesIncorporating a Green Phosphorescence Ultrathin Emitter within aNovel Interface Exciplex for High EfficiencyTing Xu,†,‡ Jun-Gui Zhou,† Chen-Chao Huang,† Lei Zhang,† Man-Keung Fung,† Imran Murtaza,§,∥

Hong Meng,*,‡,§ and Liang-Sheng Liao*,†

†Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials(FUNSOM), Soochow University, Suzhou, Jiangsu 215123, China‡School of Advanced Materials, Shenzhen Graduate School, Peking University, Shenzhen 518055, China§Key Laboratory of Flexible Electronics and Institute of Advanced Materials, Jiangsu National Synergistic Innovation Centre forAdvanced Materials, Nanjing Tech University, Nanjing 211816, China∥Department of Physics, International Islamic University, Islamabad 44000, Pakistan

ABSTRACT: Herein we report a novel design philosophy oftandem OLEDs incorporating a doping-free green phosphor-escent bis[2-(2-pyridinyl-N)phenyl-C](acetylacetonato)-iridium(III) (Ir(ppy)2(acac)) as an ultrathin emissive layer(UEML) into a novel interface-exciplex-forming structure of1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) and1,3,5-tri(p-pyrid-3-yl-phenyl)benzene (TmPyPB). Particularly,relatively low working voltage and remarkable efficiency areachieved and the designed tandem OLEDs exhibit a peakcurrent efficiency of 135.74 cd/A (EQE = 36.85%) which istwo times higher than 66.2 cd/A (EQE = 17.97%) of thedevice with a single emitter unit. This might be one of the highest efficiencies of OLEDs applying ultrathin emitters without lightextraction. Moreover, with the proposed structure, the color gamut of the displays can be effectively increased from 76% to 82%NTSC if the same red and blue emissions as those in the NTSC are applied. A novel form of harmonious fusion among interfaceexciplex, UEML, and tandem structure is successfully realized, which sheds light on further development of ideal OLED structurewith high efficiency, simplified fabrication, low power consumption, low cost, and improved color gamut, simultaneously.

KEYWORDS: organic light-emitting diodes, tandem structure, interface exciplex, ultrathin emissive layers, current efficiency

1. INTRODUCTIONAfter the invention of the first succinct organic light-emittingdevice (OLED) with two-layer structure in 1987 by C. W.Tang,1 flat-panel displays and illumination applications basedon OLED technology have developed sharply due to theirattractive features such as simple fabrication process, devisiblesmart structure, impressive color rendering, ultrathin structure,wide viewing angles, abundant organic materials, lightweight,and high compatibility with flexible substrates.2−9 OLEDstacking by two or more emitter units has drawn enormousattention due to impressive current efficiency (CE), brightness,external quantum efficiency (EQE), luminous efficiency, anddevice lifetime compared to conventional OLEDs.10−14

However, stacking multiple electroluminescence (EL) units intandem OLEDs increases driving voltage and complicates thefabrication process relative to their standard single-unitcounterparts. Moreover, commercial OLED products are stillexpensive for consumers.15 Therefore, it is urgently needed tosimplify the OLED structure and fabrication technology toreduce the cost of products.15 To be specific, in typical tandemOLEDs, the doping technique requires careful selection of

appropriate hosts for different dopants in the host−guestemitter system.16 Furthermore, the control of codeposition rateand dopant concentration in the fabrication procedure iscomplex and not absolutely accurate, which tends to lower theyield in the production line. Moreover, the physical vapordeposition equipment needs to be remolded with adequatesensors and evaporator sources to realize complex devicefabrication of tandem OLEDs, which increase their manufactur-ing costs.To loosen the bottlenecks, the doping-free technology

applying ultrathin emitting nanolayers (<1 nm) may bepractically helpful, as it can avoid the use of host, shorten thefabrication process by annihilating the doping process in atraditional fabrication of the host−guest luminous system,simplify the structure of OLEDs, and minimize the fabricationequipment requirement.17−19 Recently, some novel devicestructures applied doping-free technology, and tandem

Received: December 16, 2016Accepted: March 8, 2017Published: March 8, 2017

Research Article

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© 2017 American Chemical Society 10955 DOI: 10.1021/acsami.6b16094ACS Appl. Mater. Interfaces 2017, 9, 10955−10962

structure was demonstrated. High efficiency nondopedthermally activated delayed fluorescence (TADF)20 OLEDswith multiquantum well structure exhibited external quantumefficiency (EQE) of 22.6% and a current efficiency (CE) of 69cd/A as demonstrated by the Wang group.21 The Chen groupimproved the CE and EQE of OLEDs, achieving 96.28 cd/Aand 25.90%, respectively, with quantum dot and organic hybridtandem structure.22 Meanwhile, bilayer-type or cohost exciplex-based OLEDs demonstrated some classical cases for theapplication of a doping-free fabrication process en route torealize white OLEDs with EQE of 11.6% or highly simplifiedreddish OLEDs with high power efficiency (PE) of 31.80 lm/Wwith a low threshold voltage of 2.24 V.23−25 The He group alsodeveloped inverted phosphorescence OLEDs with novelemissive material Ir(tfmppy)2(tpip) as ultrathin emissive layers(UEMLs) showing a high EQE, above 31.1%, while conven-tional OLEDs reach an EQE of about 25%.26 The Fung groupand Ma group reported green phosphorescence OLEDs withUEMLs exhibiting a high EQE of 21.1% (with a CE of 79.5 cd/A) and 20.9% (with a CE of 79.1 cd/A), respectively.15,27

However, the performance of OLEDs independently applyingultrathin emitters, tandem structure, or interface exciplex isunsatisfactory.28−30

To combine the advantages and compensate for theweaknesses of ultrathin emitters, tandem structure, andinterface exciplex, we propose a novel tandem structureintegrating both ultrathin emitters and interface exciplex. Inthis paper, we report a highly simplified design of tandem

OLEDs incorporating a doping-free green phosphorescencebis[2-(2-pyridinyl-N)phenyl-C](acetylacetonato)iridium(III)(Ir(ppy)2(acac)) ultrathin emitter into a novel interface-exciplex-forming structure by 1,1-bis[(di-4-tolylamino)phenyl]-cyclohexane (TAPC) and 1,3,5-tri(p-pyrid-3-yl-phenyl)benzene(TmPyPB), interconnected by Bphen:LiNH2. Particularly,relatively low working voltage and remarkable efficiency areachieved, and the designed tandem OLEDs exhibit a peak CEof 135.74 cd/A (EQE = 36.85%) with exciplex-forming cohost,enhanced by nearly twice as compared with the 66.2 cd/A(EQE = 17.97%) of a single emitter device, which might be oneof the highest efficiencies of OLEDs applying ultrathin emitterswithout light extraction technology. We believe our work maygive a hand on further development of ideal OLEDs structurewith high efficiency, simplified fabrication, low powerconsumption and low cost simultaneously.

2. EXPERIMENTAL SECTION1,1-Bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC), 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN), 1,3,5-tris[(3-pyrid-yl)-phen-3-yl]benzene (TmPyPB), bis[2-(2-pyridinyl-N)phenyl-C]-(acetylacetonato)iridium(III) (Ir(ppy)2(acac)), 4,7-diphenyl-1,10-phe-nanthroline (Bphen), Ir(ppy)2(acac), and 8-hydroxyquinolinolato-lithium (Liq) were bought from Lumtec Company.

The structures of the reference and designed devices are ITO/HAT-CN(10 nm)/TAPC(55 nm)/Ir(ppy)2(acac) (0.1 nm, 0.3 nm,0.5 nm, 0.8 nm)/TmPyPB(40 nm)/Liq(2 nm)/Al(120 nm) and ITO/HAT-CN(10 nm)/TAPC(55 nm)/Ir(ppy)2(acac) (0.8 nm)/TmPyPB(40 nm)/BPhen:LiNH2 (10 nm, 50% by mole)/HAT-

Figure 1. Device structure of (a) single-unit OLED and (b) tandem OLED. Energy diagrams and carrier transfer route of proposed reference (c)single-unit OLED device and (d) tandem OLED. (e) Chemical structures of organic materials used to fabricate designed devices. Blue arrow meanselectron transfer route, red arrow means hole transfer route, and green imaginary line means recombination zone.

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CN(10 nm)/TAPC(55 nm)/Ir(ppy)2(acac)(0.8 nm)/TmPyPB(40nm)/Liq(2 nm)/Al(120 nm), respectively. The charge-generatinginterface or unit (CGU) is HAT-CN/TAPC in designed OLEDdevices. BPhen:LiNH2 (10 nm, 50% by mole) and TAPC serve aselectron-injecting layer (EIL) and hole-transporting layer (HTL),respectively. TmPyPB serves as electron-transporting layer (ETL).TAPC/TmPyPB is applied as interface exciplex and Ir(ppy)2(acac) asan emitter whereas in conventional single-unit OLEDs, differentthicknesses of the emitter were applied to optimize the referencedevice. The device structure, energy level, and carrier transport processof designed single-unit and tandem OLEDs are demonstrated inFigure 1. The chemical structures of applied materials are also depictedin Figure 1.All OLEDs were fabricated on patterned indium−tin-oxide (ITO)

glass as anode with a sheet resistance lower than 20 Ω/square. Light-emitting area of OLEDs made by the crossover of the ITO and thealuminum was 3 mm × 3 mm. The ITO glass substrates were cleanedfollowing a continuous ultrasound cleaning of acetone and 2-propanolfor 15 min each. After that, the substrates were dried in a drying ovenand treated in a UV−ozone instrument for 20 min. Then thesubstrates were taken into the vacuum chamber for OLED fabrication.The layers of OLEDs containing buffer layers, organic layers, andmetal material as cathode were sequentially deposited on the ITOglass substrates without breaking the vacuum (∼3.0 × 10−6 Torr). Thethermal evaporation rates of organic layers, Liq, and Al were about 1Å/s, 0.1 Å/s, and 5 Å/s, respectively.The electroluminescence (EL) and current density−voltage (J−V)

characteristics were tested with a Keithley 2400 sourcemeterconnected to a Photo Research SpectraScan PR 655. Photo-luminescence (PL) spectra were tested on a Hitachi F-4600fluorescence spectrophotometer. Delayed PL emission was tested onan ultrafast fluorescence spectrometer (HORIB-FM-2015).

3. RESULTS AND DISCUSSION

The charge-transporting layers should not only be doping freebut also effectively transport charges. Therefore, HAT-CN isselected as the HIL because of its strong hole injection ability,and TAPC has been used as HTL due to its high hole transportmobility (∼10−2 cm2 /V s),31,32 which also can confine theexcitons and electrons because TAPC has a high T1 (tripletstate) of 2.87 eV and LUMO (lowest unoccupied molecularorbital) of 1.8 eV to block the unwanted carriers leading toenergy loss. The common electron transport material mobilitiesof Bphen, BCP, TPBi, and TmPyPB are 3.9 × 10−4 cm2/(V s),6 × 10−7 cm2/(V s), 3.3 × 10−5 cm2/(V s), and 1 × 10−3 cm2/(V s), respectively.25 TmPyPB is selected as the ETL due to itshigh electron mobility (∼10−3 cm2 /V s),33 deep HOMO(highest occupied molecular orbital) of 6.7 eV, and high T1 of2.78 eV, which can effectively improve the electron injection,block the hole leakage, and confine the excitons. The thin activelayer Ir(ppy)2(acac) in between TAPC and TmPyPB has a PLpeak at about 528 nm, and its PLQY is as high as 95%.34,35

Because the active layer is very thin, the charge-transportingcharacteristics will be determined by both TAPC and TmPyPB.The hole-transporting property will be determined by TAPCwith its hole mobility of ∼10−2 cm2/(V s), and the electron-transporting property will be determined by TmPyPB with itselectron mobility of ∼1 × 10−3 cm2/(V s). TAPC and TmPyPBwith relatively high carrier mobility promote carrier injectionbalance in favor of designing highly efficient PhOLEDs.Fortunately, TAPC and TmPyPB can form an exciplex whichowns many distinguished properties widely used in designingnew types of exciplex−host OLEDs, such as balanced carriermobility, reduced device working votalge, enhanced efficiency,and simplified structure of the device.36 Moreover, an efficient

charge-generating interface HAT-CN/TAPC is applied in ourdesigned tandem OLEDs.37,38

PL spectra of TAPC, TmPyPB, mixed film, and layered filmwere tested as demonstrated in Figure 2 and Figure 3. The

electron-transporting layer also acts as a hole-blocking layer(TmPyPB −6.68 eV) compared with the HOMO level (−5.7eV) of TAPC as demonstrated in Figure 1. We chose TmPyPBas electron-acceptor in this interface exciplex system,39,40

because TmPyPB has electron transport capability better thanthat of TPBi, BCP, and Bphen. TAPC:TmPyPB shows anadditional exciplex peak, suggesting that the exciplex can beformed between TAPC and TmPyPB as demonstrated inFigure 2. Additionally, TAPC has a higher LUMO level of −2.0eV, leading to the possibility of acting as an electron-blockinglayer and forming an interfacial exciplex-forming donor,simultaneously.The photoluminescence (PL) of thin films of TAPC shows

major maxima at ∼370 nm and ∼450 nm, which is composedof molecular exciton (monomer) and excimer emission41 inFigure 2. Once new TAPC:TmPyPB film structure is formed totake the place of TAPC film structure, the molecular exciton(monomer) and excimer of TAPC both are observablyweakened by doping TmPyPB. Then the PL peaks at 370and 450 nm from molecular exciton (monomer) and excimeremission both disappeared while the PL of exciplex emission ofTAPC:TmPyPB is shown in 424 nm, which indicates that all ofthe excitons in the TAPC:TmPyPB film form exciplexeseffectively. The PL spectra of TAPC (10 nm)/TmPyPB(10

Figure 2. PL spectra of the TAPC, TmPyPB, and mixedTAPC:TmPyPB (1:1; w:w) films. (All are 30 nm thick).

Figure 3. PL spectra of TAPC (10 nm)/TmPyPB(10 nm) film.

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nm) film are presented in Figure 3. The PL spectra exhibitviolet blue emission with a peak at 424 nm (∼2.93 eV) andsecondary peak at 374 nm (∼3.3 eV), which correspond withthe PL spectra peak of TAPC:TmPyPB and TAPC. The mainpeak of TAPC:TmPyPB or TAPC/TmPyPB (∼2.93 eV) nearlycorresponds to the energy offset between the LUMO ofTmPyPB (∼2.7 eV) and the HOMO of TAPC (∼5.6 eV),verifying that the luminescence arises from exciplex emission.The secondary peak at 374 nm of the TAPC/TmPyPB filmcorresponds to the energy gap of TAPC as demonstrated inFigure 3 because TAPC has a narrower energy gap (∼3.7 eV)compared with TmPyPB (∼4 eV). This secondary peaksuggests that the energy of excitation can be transferred tothe T1 level of TAPC in the TAPC/TmPyPB interfacialexciplex system. The photoluminescence (PL) emission of theTAPC film is similar to that of the TAPC/TmPyPB film in the

range of 350−550 nm. In addition, the PL spectrum of theTAPC film or TAPC/TmPyPB film overlaps with theabsorption spectrum of Ir(ppy)2acac (350−550 nm)42 verywell, which ensures efficient energy transfer from the TAPC orTAPC/TmPyPB film to the Ir(ppy)2acac film. The goodoverlap between the emission of the TAPC or TAPC/TmPyPBfilm and the absorption of the Ir(ppy)2acac film shall lead toefficient Forster energy transfer from TAPC or TAPC/TmPyPB to Ir(ppy)2acac when the two molecular systemsare close enough to ∼10 nm.19

Compared with conventional host−guest energy transfersystems, as depicted in Figure 4(a),43 in interface exciplexOLEDs with UEMLs, most carriers meet at the interface ofTAPC and TmPyPB to form excitions whose energies are wellblocked by a higher T1 of TAPC and TmPyPB. Afterward, theenergy is transferred to the energy level of the interface exciplex

Figure 4. Triplet energy level Tl of TAPC, TmPyPB, CBP, and Ir(ppy)2(acac). Energy transfer or loss routing of (a) host−guest luminescencesystem and (b) exciplex:luminescence system.

Figure 5. (a) Current density−voltage−luminance characteristics of reference devices. (b) Current efficiency−current density characteristic ofreference devices. (c) Power efficiency−current density characteristic of reference devices. (d) Normalized EL spectra of reference devices.

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formed by TAPC and TmPyPB, which finally transfers toUEML, resulting in sparkling green light. However, part ofexcitons are formed at TAPC or part of the excitons diffuse ortunnel into TAPC near the interface of TAPC and TmPyPB,due to the exciton concentration gradient and very thin UEML,to completely block or consume the energy. This energy,located in the T1 of TAPC, transfers to the energy level ofinterface exciplex formed by TAPC and TmPyPB and finallytransfers to UEML, resulting in luminescence of green light.The entire energy transfer process of the exciplex:luminescencesystem is depicted in Figure 4(b).For the sake of optimizing the reference green OLEDs with

single-unit UEML, the thickness of (Ir(ppy)2(acac) wasincreased from 0.1 to 0.8 nm to obtain devices A−D, basedon some reported green OLEDs with UEML.17,26,27,44 Thecurrent density−voltage−luminance (J−V−L) characteristics,efficiency, and EL spectra of reference green OLEDs aredemonstrated in Figure 5. As demonstrated in Figure 5(a,d), J−V characteristics tend to increase with the thickness of(Ir(ppy)2(acac), while the peaks of normalized EL spectra ofreference OLEDs remain unchanged, and no emission of hostmaterial and interface exciplex implies sufficient transfer ofexcitons energy to UEML. But the width of PL spectra ofreference OLEDs tends to decrease with the increase in thethickness of UEML due to more effective utilization of energy.A thicker UEML prevents escape of the energy from the energylevels of the interface exciplex of TAPC and TmPyPB to the T1level of TAPC due to the significant inhibition of the tunnelingeffect, while luminance increases to the peak value with 0.8 nmthickness of Ir(ppy)2(acac) before declining.As depicted in Figure 5(b,c), 0.8 nm thickness of

Ir(ppy)2(acac) attains the maximum PE and CE in ouroptimized reference green OLEDs with UEML. The deviceperformance of the reference green OLEDs is summarized inTable 1, depicting peak current and power efficiencies (ηC, ηP)

of 66.2 cd/A and 55.2 lm/W, respectively, with the lightemission peaks at 528 nm. Threshold voltages of our optimizedreference green OLEDs are as low as 2.9 V, leading to theirdistinguished PE at 528 nm. Our designed interface exciplexgreen OLEDs with UEML show distinguished comprehensiveperformance (relatively low working voltage, high CE and PE)which are comparable with device E (with UEML), F, and G(traditional host−guest luminescent system), employing thesame functional material, as cited from the literature44,45 andsummarized in Table 1.

Novel interface exciplex green OLEDs with UEML as a highefficiency and highly simplified single unit without the dopingprocess paved the way for novel tandem OLEDs, realizing ourdesign philosophy of OLEDs as mentioned in Introduction.State-of-the-art OLEDs are proposed to combine theadvantages and compensate for the weaknesses of ultrathinemitters, tandem structure, and the interface exciplex. Figure1(d) exhibits the entire process of charge transport andgeneration in our designed single-unit and tandem OLEDsunder driving voltage. As to the charge generation process ofCGU, due to the good match of HOMO of TAPC (−5.7 eV)and LUMO of HAT-CN (−5.5 eV), electrons and holes can begenerated at the interface of HAT-CN/TAPC. The generatedcharge carriers then drift to UEML and recombine with thecarriers injected from the electrodes to luminesce by the drivingelectric field.Figure 6(a) exhibits a comparison of the J−V−L character-

istics of the designed tandem OLEDs with UEMLs (device H)with that of the reference single-unit OLEDs with UEML(device D). At all tested current densities, the designed tandemOLEDs show about twice the luminance and driving voltagethan that of the single-unit OLEDs, because the designedtandem OLEDs with UEMLs are formed by two single-unitOLEDs with UEML connected in series, so the driving voltageand luminance of the tandem OLEDs should be equal to thesum of that of individual single-unit OLEDs in theory under acertain voltage. For example, at a current density of 20 mA/cm2, the luminance (voltage) of 24699.9 cd/m2 (9.8 V) fordesigned tandem OLEDs corresponds to a luminance (voltage)of 11378.4 cd/m2 (4.9 V) for the single-unit OLEDs, which isnearly the twice that of individual single-unit OLEDs. Due tothe efficient electron injection and transportation in theBphen:LiNH2 and HAT-CN layer under bias voltage, theextra resistance induced can be negligible.37

Figure 6(b,c) exhibits the CE and PE as a function of currentdensity for reference OLEDs with UEML and tandem interfaceexciplex OLEDs with UEML, respectively. As depicted in Table2, the maximum current and power efficiencies (ηC, ηP) for theinterface exciplex tandem OLEDs with UEML are 135.74 cd/Aand 59.88 lm/W, respectively, whereas those for the referencesingle unit OLEDs with UEML are 66.19 cd/A and 55.16 lm/W, respectively. Apparently, the CE of interface exciplextandem OLEDs with UEML is more than two times as high asthat of reference single-unit OLEDs with UEML. Moreover, ahigher PE of the designed interface exciplex tandem OLEDswith UEML is realized, compared to that of the referencesingle-unit OLEDs. Three major reasons may contribute to theenhancement in PE and CE for the interface exciplex tandemOLEDs with UEML. First, plasmon quenching effect fromelectrodes is weakened by two single units, as plasmonquenching occurs only at the nearby electrode because theother electrode is too far away. Second, a high-quality CGUexists between each light-emitting unit offering equal amountsof holes and electrons under forward bias to improve balanceby preventing an excess of charges that would be lost duringOLEDs operation. Hence, the overall carrier recombinationbalance can be realized, leading to higher PE, CE, and EQE.Third, the electric-field-induced quenching effect, remarkable inphosphorescent OLEDs, can be restrained.10,46 The threshold-voltage of tandem OLEDs is as low as 5.74 V mainly due to Lidoping into the Bphen-reducing bulk resistor of tandemOLEDs and LUMO band bending of Bphen:LiNH2 asexhibited in Figure 1(d).10 Triplet−triplet annihilation

Table 1. Summary of Green Single-Unit OLED Performance

deviceavoltage(V)b

ηc(cd/A)c

ηp(lm/W)c

EQEc

(%)emission peak

(nm)

A 2.92 53.1 48.6 14.43 528B 2.94 53.2 45.7 14.44 528C 3.09 62.5 53.2 16.98 528D 3.15 66.2 55.2 17.97 528E 3.30 79.5 75.3 21.1 −F 3.80 72.2 54.1 18.1 −G 4.10 65.5 45.5 18.3 −

aThe reference PhOLEDs with different thicknesses of the ultrathinemission layer (0.1 nm, 0.3 nm, 0.5 nm, 0.8 nm). Devices E, F, and Gare cited from literature.44,45 bVoltages at 0.2 mA/cm2. cMaximumefficiencies between 100 and 5000 cd/m2.

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(TTA), triplet−polaron quenching (TPQ), and field-inducedquenching are possible quenching processes common inPhOLEDs.25 In our designed OLED with UEML, althoughthe emitting layer is very thin, the relatively high roll-off at thehigh current density could be TTA due to the short distancesamong the emitting molecules in the nondoped emitter.However, it should be noted that in most of the realapplications, to achieve an intensity up to 5000 cd/m2, thetandem device only needs to be operated at a current densitylower than 5 mA/cm2. In other words, there is no need tooperate the high efficiency tandem OLEDs at very high currentdensity. As a result, high roll-off at a high current density willnot occur in most real applications.Obviously, the EL characteristic spectra of tandem OLEDs

become narrower than that of single-unit OLEDs as shown inFigure 6(d)). Moreover, as Figure 7 and CIE (Table 2) exhibit,if the same red and blue emissions as those in the NTSC areapplied, improvement of the color gamut from 76% (single-unit

OLEDs with UEML) to 82% (designed interface exciplextandem OLEDs with UEML) can be realized at a currentdensity of 5 mA/cm2, which shows great superiority for thecolor gamut, compared with the liquid crystal display techniquewithout OLED or quantum dot LED color gamut of 45−72%in industrial products.47 This improvement is mainly due tomultifilm light interference in tandem OLEDs,48 whichdemonstrate our designed interface exciplex tandem OLEDs

Figure 6. (a) Current density−voltage−luminance characteristics of reference devices. (b) Current efficiency−current density characteristic ofreference devices. (c) Power efficiency−current density characteristic of reference devices. (d) Normalized EL spectra of reference devices.

Table 2. Summary of Tandem OLED Performance

deviceavoltage(V)b

ηc (cd/A)c

ηp(lm/W)c

EQEc

(%)emission peak

(nm)CTE (x,y)d

D 3.15 66.19 55.16 17.97 528 0.38,0.59

H 5.76 135.74 59.88 36.85 528 0.35,0.61

aThe best reference OLED (device D) and tandem OLED (device G)performance. bVoltages at 0.2 mA/cm2. cMaximum efficienciesbetween 100 and 5000 cd/m2. dCommission International deI′Eclairage coordinates measured at 5 mA/cm2. CIE coordinates ofNTSC standards are (0.21, 0.71).

Figure 7. Color gamut of NTSC standard and displays based ondesigned single-unit OLEDs and tandem OLEDs with UEL in a CIE1931 chromaticity diagram.

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with UEML and also help to improve color gamut, comparedwith conventional single-unit OLEDs with UEML, simulta-neously.

4. CONCLUSIONS

In summary, a novel design of tandem OLEDs incorporating adoping-free ultrathin emitter layer, with green phosphores-cence, into a novel interface-exciplex-forming structure issuccessfully demonstrated, showing high efficiency, high EQE,and improved color gamut. Particularly, the designed tandemOLEDs exhibit a record-high current efficiency of 135.74 cd/A(EQE = 36.85%) among OLEDs applying ultrathin emitterswithout light extraction technology. We believe our work mayshed light on the future development of ideal OLED structurewith high efficiency, simplified fabrication, low powerconsumption, and low cost, simultaneously.

■ AUTHOR INFORMATION

Corresponding Authors*E-mail: menghong@pkusz.edu.cn.*E-mail: lsliao@suda.edu.cn.

ORCIDHong Meng: 0000-0001-5877-359XLiang-Sheng Liao: 0000-0002-2352-9666NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work is supported by the National Natural ScienceFoundation of China (61575136, 51035008, 51373075),National Basic Research Program of China (973 Program,2015CB932200). This project was also funded by theCollaborative Innovation Center of Suzhou Nano Scienceand Technology (Nano-CIC), by the Priority AcademicProgram Development of Jiangsu Higher Education Institutions(PAPD), and by Shenzhen Key Laboratory of OrganicOptoelectromagnetic Functional Materials of Shenzhen Scienceand Technology Plan (ZDSYS20140509094114164). Thiswork was also financially supported by Shenzhen Science andTechnology Research Grant (JCYJ20160510144254604),Guangdong Key Research Project (2014B090914003), theShenzhen Peacock Plan (KQTD2014062714543296), andGuangdong Academician Workstation (2013B090400016).

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