Simple-structured efficient white organic light emitting diode via solution process

High-efficiency white emission is crucial to the design of energy-saving display and lighting panels, whereas solution-process feasibility is highly desirable for large area-size and cost-effective roll-to-roll manufacturing. In this study, we demonstrate highly-efficient, bright and chromaticity stable white organic light emitting diodes (OLEDs) with solution-processed single emissive layer. The resultant best white OLED shows excellent electroluminescence performance with forward-viewing external quantum efficiency, current efficiency and power efficiency of 22.7%, 48.8 cd A^− 1 and 27.8 lm W^− 1 at 100 cd m^− 2, respectively, with a maximum luminance of 19,590 cd m^− 2. Furthermore, we also observed an increment of 112% in the power efficiency, 86.9% in the current efficiency and a decrement of 39.2% in the external quantum efficiency at 100 cd m^− 2 as the doping concentration of blue dye was increased from 10 wt% to 25 wt% in the devices. The better efficiency performance may be attributed to the effective exciton-confining device architecture and low-energy barrier for electrons to inject from the hole-blocking electron-transport layer to the host layer.     

High-efficiency hybrid organic–inorganic light-emitting devices

We report efficient red, orange, green and blue organic–inorganic light emitting devices using light emitting polymers and polyethylenimine ethoxylated (PEIE) interlayer with the respective luminance efficiency of 1.3, 2.7, 10 and 4.1 cd A⁻¹, which is comparable to that of the analogous conventional devices using a low work-function metal cathode. This is enabled by the enhanced electron injection due to the effective reduction of the ZnO work-function by PEIE, as well as hole/exciton-blocking function of PEIE layer. Due to the benign compatibility between PEIE and the neighboring organic layer, the novel phosphorescent organic–inorganic devices using solution-processed small molecule emissive layer show the maximum luminance efficiency of 87.6 cd A⁻¹ and external quantum efficiency of 20.9% at 1000 cd m⁻².     

Synthesis and photophysical characterization of an ionic fluorene derivative for blue light-emitting electrochemical cells

A highly fluorescent an ionic fluorene derivative 1 was synthesized and its photophysical, electrochemical and electroluminescence characteristics were investigated. Deep blue emissions were observed for compound 1 in solid as well as in dilute solutions. The synthesized compound shows high fluorescence quantum yield around 77% indicates that compound 1 can perform its role as efficient ionic emitter in LEC devices. Light-emitting electrochemical cell (LEC) devices were fabricated incorporating compound 1 without (device I) and with (device II) ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM·PF₆). Devices I and II exhibited blue electroluminescence maximum centered at 455 and 454 nm with CIE coordinates of (0.15, 0.21) and (0.16, 0.22), respectively. Maximum luminance and current efficiency of 1105 cd m⁻² and 0.14 cd A⁻¹ respectively, has achieved for device I while that of device II resulted in 1247 cd m⁻² and 0.14 cd A⁻¹ respectively.     

Highly efficient blue and all-phosphorescent white polymer light-emitting devices based on polyfluorene host

We report efficient blue electrophosphorescent polymer light emitting devices with polyfluorene (PFO) as the host and iridium bis[2-(4,6-difluorophenyl)-pyridinato-N,C²] picolinate (FIrpic) as the dopant. Despite the low-lying triplet energy level of the polyfluorene polymer host, phosphorescent quenching can be suppressed by using poly(N-vinylcarbazole) (PVK) as anode buffer layer, resulting in a high luminous efficiency of 26.4 cd A^?1, which is one of the best results in the literature based on conjugated polymer reported to date. The reduced phosphorescent quenching is found to be associated with the exciton formation and charge carrier recombination within the PVK layer and the PVK/PFO interface due to the accumulation of holes. As compared with the devices based on non-conjugated host polymer PVK, the devices based on PFO showed a lower turn-on voltage (3.6 V vs. 4.4 V) and higher power efficiency (17 lm W^?1 vs. 8.3 lm W^?1) due to the higher mobility of PFO. When doubly doped with a newly synthesized yellow-emitting metallophosphor, white polymer light-emitting devices with superior device performance (a peak device efficiency of 40.9 cd A^?1, a CIE coordinates of (0.32, 0.48), and a power efficiency of 31.4 lm W^?1) was achieved. These findings can broaden our selection in polymer hosts for highly efficient phosphorescent blue emitting devices and can find potential applications in full color displays and solid-state lighting applications in the future.     

Multifunctional homoleptic iridium(III) dendrimers towards solution-processed nondoped electrophosphorescence with low efficiency roll-off

A series of new iridium dendrimers comprised of bifunctional charge transport peripheral groups have been designed and facilely synthesized. The relationship between the structures and their photophysical, electrochemical and electrophosphorescent performances is investigated. Through the incorporation of rigid electron-transporting phosphine oxide groups and/or hole-transporting arylamine units, the new complexes all have good thermal stabilities with high glass-transition temperature up to 284 °C. Besides, the peripheries sufficiently shield the emissive core from the intermolecular interactions and prevent luminance quenching in neat films. Solution-processed phosphorescent organic light-emitting device (PhOLED) based on bipolar phosphor 2 as neat emitter achieves a maximum current efficiency of 12.4 cd A⁻¹ with Commission Internationale de l’Eclairage coordinates of (0.57, 0.42), and the value remains at 11.5 cd A⁻¹ at a practical luminance of 1000 cd m⁻². This low roll-off can be attributed to the bipolar nature of the emitter. This indicates that rational incorporation of charge-transporting moieties into the sphere of iridium(III) core is a simple and effective approach to develop efficient host-free phosphors for solution-processable nondoped PhOLEDs.     

Tuning the energy gap and charge balance property of bipolar host by molecular modification: Efficient blue electrophosphorescence devices based on solution-process

Three bipolar hosts composed of electron-accepting diphenylphosphine oxide and electron-donating carbazole/triphenylamine have been synthesized and characterized. With structural topology modification, the particular physical properties of the materials can be subtly optimized, such as the thermal stability, singlet–triplet energy gap and charge balance ability. Both DFT calculation and experiment results demonstrate that the introduced triphenylamine can effective minimize the HOMO–LUMO energy gap, while the carbazole units can prevent the excited energy loss and keep high triplet energy (E_T = 3.0 eV) due to the enhanced molecular rigidity. As a result, solution-processed blue PHOLEDs exhibited a high current efficiency of 25.2 cd A⁻¹ and a power efficiency of 11.5 lm W⁻¹, which implies that the unique molecular modulation is very cost-effective and competitive for the device performance improving.     

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.     

Highly efficient and color stable single-emitting-layer fluorescent WOLEDs with delayed fluorescent host

We demonstrated highly efficient and color stable single-emitting-layer fluorescent WOLEDs using blue thermally activated delayed fluorescent material of bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone (DMAC-DPS) as host and traditional orange fluorescent material of (5,6,11,12)-tetraphenyl-naphthacene (rubrene) as dopant. At a low dopant concentration of 0.6 wt%, we achieved the efficient white emission that comprised of blue host and orange dopant. The maximum current efficiency, power efficiency and external quantum efficiency were 20.2 cd A⁻¹, 15.9 lm W⁻¹ and 7.48%, respectively. Besides, the Commission Internationale de I’Eclairage coordinates were almost the same with the increased voltage, which shifted from (0.359, 0.439) to (0.358, 0.430) as the voltage rose from 5 V to 8 V. The achievement of so high efficiency was attributed to the efficient up-conversion of DMAC-DPS triplet excitons and efficient energy transfer from host to dopant by Förster transfer mechanism. The more detailed working mechanism was also argued.     

Indolo[3,2-b]carbazole: Promising building block for highly efficient electroluminescent materials

Two luminescent materials based on indolo[3,2-b]carbazole have been designed and synthesized. They were highly fluorescent both in solution and in the solid state. High-performance electroluminescent devices with indolo[3,2-b]carbazole luminescent derivatives as the emissive materials were fabricated for the first time with low turn-on voltage of 2.65 V, high luminescence efficiency of 7.92 lm W^?1, and high brightness of 68729 cd m^?2. The results demonstrated that indolo[3,2-b]carbazole has great potentials as promising building block for highly efficient electroluminescent materials.     

Cadmium-free quantum dots based violet light-emitting diodes: High-efficiency and brightness via optimization of organic hole transport layers

Bright and efficient violet quantum dot (QD) based light-emitting diodes (QD-LEDs) with heavy-metal-free ZnSe/ZnS have been demonstrated by choosing different hole transport layers, including poly(4-butyl-phenyl-diphenyl-amine) (poly-TPD), poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine] (TFB), and poly-N-vinylcarbazole (PVK). Violet QD-LEDs with maximum luminance of about 930 cd/m², the maximum current efficiency of 0.18 cd/A, and the peak EQE of 1.02% when poly-TPD was used as HTL. Higher brightness and low turn-on voltage (3.8 V) violet QD-LEDs could be fabricated when TFB was used as hole transport material. Although the maximum luminance could reach up to 2691 cd/m², the devices exhibited only low current efficiency (∼0.51 cd/A) and EQE (∼2.88%). If PVK is used as hole transport material, highly efficient violet QD-LEDs can be fabricated with lower maximum luminance and higher turn-on voltages compared with counterpart using TFB. Therefore, TFB and PVK mixture in a certain proportion has been used as HTL, turn-on voltage, brightness, and efficiency all have been improved greatly. The QD-LEDs is fabricated with 7.39% of EQE and 2856 cd/m² of maximum brightness with narrow FWHM less than 21 nm. These results represent significant improvements in the performance of heavy-metal-free violet QD-LEDs in terms of efficiency, brightness, and color purity.     

A highly efficient OLED based on terbium complexes

A neutral ligand 9-(4-tert-butylphenyl)-3,6-bis(diphenylphosphineoxide)-carbazole (DPPOC) and its complex Tb(PMIP)₃DPPOC (A, where PMIP stands for 1-phenyl-3-methyl-4-isobutyryl-5-pyrazolone) were synthesized. DPPOC has a suitable lowest triplet energy level (24,691 cm^?1) for the sensitization of Tb(III) (⁵D₄: 20,400 cm^?1) and a significantly higher thermal stability (glass transition temperature 137 °C) compared with the familiar ligand triphenylphosphine oxide (TPPO). Experiments revealed that the emission layer of the Tb(PMIP)₃DPPOC film could be prepared by vacuum co-deposition of the complex Tb(PMIP)₃(H₂O)₂ (B) and DPPOC (molar ratio = 1:1). The electroluminescent (EL) device ITO/N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-diphenyl-4,4′-diamine (NPB; 10 nm)/Tb(PMIP)₃ (20 nm)/co-deposited Tb(PMIP)₃DPPOC (30 nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP; 10 nm)/tris(8-hydroxyquinoline) (AlQ; 20 nm)/Mg_0.9Ag_0.1 (200 nm)/Ag (80 nm) exhibited pure emission from terbium ions, even at the highest current density. The highest efficiency obtained was 16.1 lm W^?1, 36.0 cd A^?1 at 6 V. At a practical brightness of 119 cd m^?2 (11 V) the efficiency remained above 4.5 lm W^?1, 15.7 cd A^?1. These values are a significant improvement over the previously reported Tb(PMIP)₃(TPPO)₂ (C).     

Green and blue–green light-emitting electrochemical cells based on cationic iridium complexes with 2-(4-ethyl-2-pyridyl)-1H-imidazole ancillary ligand

The ionic iridium complexes, [Ir(ppy)₂(EP-Imid)]PF₆ (Complex 1) and [Ir(dfppy)₂(EP-Imid)]PF₆ (Complex 2) are used as the light-emitting material for the fabrication of light-emitting electrochemical cells (LECs). These complexes have been synthesized, employing 2-(4-ethyl-2-pyridyl)-1H-imidazole (EP-Imid) as the ancillary ligand, 2-phenylpyridine (ppy) and 2-(2,4-difluorophenyl)pyridine (dfppy) as the cyclometalated ligands, which were characterized by various spectroscopic, photophysical and electrochemical methods. The photoluminescence (PL) emission spectra in acetonitrile solution show blue–green and blue light emission for Complexes 1 and 2 respectively. However, LECs incorporating these complexes resulted in green (522 nm) light emission for Complex 1 with the Commission Internationale de L’Eclairage (CIE) coordinates of (0.33, 0.56) and blue–green (500 nm) light emission for Complex 2 with the CIE coordinates of (0.24, 0.44). Using Complex 1, a maximum luminance of 1191 cd m⁻² and current efficiency of 1.0 cd A⁻¹ are obtained while that of Complex 2 are 741 cd m⁻² and 0.88 cd A⁻¹ respectively.     

Efficient non-doped pure blue organic light-emitting diodes based on an asymmetric phenylenevinylene with cis double bond

We reported an asymmetric phenylenevinylene with a cis double bond 2-(4-(p-tolyl)styryl)-1,4-dip-tolylbenzene (cis-TSDTB) and its use as efficient deep-blue emitter for organic light-emitting diodes (OLEDs) applications. The crystal structure of cis-TSDTB showed torsion configuration and asymmetric geometry, which make it packing in a reduced intermolecular interaction arrangement. And its single crystals showed excellent fluorescence owing to this unique molecular configuration. Typical OLEDs using cis-TSDTB as non-doped emitters exhibited saturated blue light with the CIE 1931 coordinates of (0.15, 0.10), which is quite close to the National Television Standards Committee (NTSC) blue standard. High luminescence efficiency (3.4 cd A⁻¹) and high brightness (9855 cd m⁻²) have been realized in the device. All of these outstanding results indicated that cis-phenylenevinylene will be a promising candidate as blue light-emitting materials.     

Ultra-bright alternating current organic electroluminescence

We report on an alternating current (AC) field induced organic electroluminescence (EL) device with internal charge carrier generation and recombination luminance of over 5000 cd m^?2 under AC drive without charge carrier injection from external electrodes. The ultra-bright AC-EL is attributed to an optical optimization performed on the devices via numerical optical simulations based on an optical thin film model as well as an increase in the number of charge carriers achieved via the concept of molecular doping within the device. The luminance levels achieved are highest reported so far in literature for AC organic light emitting devices.     

Tetrasubstituted adamantane derivatives with arylamine groups: Solution-processable hole-transporting and host materials with high triplet energy and good thermal stability for organic light-emitting devices

Four new host/hole-transporting materials, namely 4,4′,4″,4‴-(adamantane-1,3,5,7-tetrayl)tetrakis(N,N-diphenylaniline) (4TPA-Ad, 1),4,4′,4″,4‴-(adamantane-1,3,5,7-tetrayl)tetrakis(N,N-di-p-tolylaniline) (4MTPA-Ad, 2), 1,3,5,7-tetrakis(4-(9H-carbazol-9-yl)phenyl)adamantane (4Cz-Ad, 3) and 1,3,5,7-tetrakis(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)adamantane (4tBuCz-Ad, 4), were designed and synthesized by incorporating four electron-donating arylamine units into the rigid adamantane skeleton via a simple C–N coupling reaction. Their thermal, photophysical and electrochemical properties were investigated. The molecular design endows the materials with high triplet energies of ∼3.0 eV, good solution processability, high thermal stability and appropriate HOMO levels. Two types of electroluminescent devices using 1–4 as hole-transporting or host materials were fabricated. The device based on 2 as solution-processed hole-transporting material and tris(quinolin-8-yloxy)aluminum as an emitter revealed a maximum current efficiency of 4.2 cd A⁻¹, which was comparable with the TAPC-based control device. The sky-blue device employing 2 as solution-processed host material and 4,6-(difluorophenyl)pyridine-N,C^2′)picolinate (FIrpic) as an emitter showed a maximum current efficiency of 16.6 cd A⁻¹ with Commission Internationale de I’Eclairage (CIE) coordinates of (0.16, 0.32).     

Solution-processable and thermal-stable triphenylamine-based dendrimers with truxene cores as hole-transporting materials for organic light-emitting devices

Two solution processable π-conjugated triphenylamine-based dendrimers, Tr-TPA3 and Tr-TPA9 were served as hole-transporting materials (HTMs) for organic light-emitting devices (OLEDs). The two dendrimers exhibit similar absorption and emission behaviors in solutions and thin films, which demonstrate that these dendrimers can form amorphous states in their films. The dendrimers showed excellent solubility, which are soluble in common organic solvents such as chloroform, tetrahydrofuran, and 1,1,2,2-tetrachloroethane, high thermal stability with high glass-transition temperature (T_g) of 115 °C for Tr-TPA3 and 140 °C for Tr-TPA9, high the highest unoccupied molecular orbital (HOMO) energy level (?5.12 eV for Tr-TPA3 and ?4.95 eV for Tr-TPA9, respectively) and good film forming property. When we employed these dendrimers as hole transport layer (HTL) in tris-(8-hydroxyquinoline) aluminum (Alq₃)-emitting electroluminescence (EL) devices, the Tr-TPA9-based double-layer device exhibited the turn-on voltage of 2.5 V, the maximum luminance of about 11,058 cd m^?2 and the maximum current efficiency of 4.01 cd A^?1. The comparison of the properties between the EL devices with dendrimers as HTL and the EL device with 1,4-bis(1-naphthylphenylamino)biphenyl (NPB) as HTL indicated that this series of dendrimers can be good candidates for HTM in OLEDs.     

High-performance green phosphorescent top-emitting organic light-emitting diodes based on FDTD optical simulation

We have successfully applied finite-difference time-domain (FDTD) method in top-emitting organic light-emitting diodes (TOLEDs) for structure optimization, demonstrating good agreement with experimental data. A mixed host with both hole transport and electron transport materials is employed for the green phosphorescent emitter to avoid charge accumulation and broaden the recombination zone. The resulting TOLEDs exhibit ultra-high efficiencies, low current efficiency roll-off, and a highly saturated color, as well as hardly detectable spectrum shift with viewing angles. In particular, a current efficiency of 127.0 cd/A at a luminance of 1000 cd/m² is obtained, and maintains to 116.3 cd/A at 10,000 cd/m².     

A facile chemical reduction approach for effectively tuning thermoelectric properties of PEDOT films

Poly(3,4-ethylenedioxythiophene)–tosylate–polyethylene glycol–polypropylene glycol–polyethylene glycol (PEDOT–Tos–PPP) films were prepared via a vapor phase polymerization (VPP) method. The films possess good electrical conductivity (1550 S cm⁻¹), low Seebeck coefficient (14.9 μV K⁻¹) and thermal conductivity (0.501 W m⁻¹ K⁻¹), and ZT ∼ 0.02 at room temperature (RT, 295 K). Then, the films were treated with NaBH₄/DMSO solutions of different NaBH₄ concentrations to adjust the redox level. After the NaBH₄/DMSO treatment (dedoping), the electrical conductivity of the films continuously decreased from 1550 to 5.7 S cm⁻¹, whereas the Seebeck coefficient steeply increased from 14.9 to 143.5 μV K⁻¹. A maximum power factor of 98.1 μW m⁻¹ K⁻² has been achieved at an optimum redox level. In addition, the thermal conductivity of the PEDOT–Tos–PPP films decrease from 0.501 to 0.451 W m⁻¹ K⁻¹ after treated with 0.04% NaBH₄/DMSO solution. A maximum ZT value of 0.064 has been achieved at RT. The electrical conductivity and thermal conductivity (Seebeck coefficient) of the untreated and 0.04% NaBH₄/DMSO treated PEDOT–Tos–PPP films decrease (increases) with increasing temperature from 295 to 385 K. And the power factor of the films monotonically increases with temperature. The ZT at 385 K of the 0.04% NaBH₄/DMSO treated film is 0.155.     

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 and good color quality white light-emitting devices based on polymer blend

Here we report efficient and color-stable white polymer light-emitting devices (WPLEDs) based on a newly synthesized efficient blue emitting polymer poly[(9,9-bis(4-(2-ethylhexyloxy)phenyl)fluorene)-co-(3,7-dibenziothiene-S,S-dioxide10)] (PPF-3,7SO10) which dually function as host material and blue emitter, with appropriate blending ratio with two typical electroluminescent polymers, green emitting poly[2-(4-(3′,7′-dimethyloctyloxy)-phenyl)-p-phenylenevinylene] (P-PPV) and orange–red emitter poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH–PPV) with appropriate blending ratio. In a single active layer WPLEDs with a blending ratio of 100:0.8:0.5 (B:G:R) by weight, white light emission with CIE coordinate of (0.34, 0.35) was realized. The resulted device shows a high luminous efficiency (LE) of 8.7 cd A^?1, which could be further enhanced to 14.0 cd A^?1 with incorporation of a thin hole transporting layer poly (vinylcarbazole) (PVK) at the anode side. The obtained luminous efficiency is listed as one of the highest reported value for WPLEDs based on all fluorescent polymer emitters. The devices had appropriate color temperature of 2500–6500 K and high color rendering index (CRI) of 72–79, and are characterized with stable electroluminescent spectra upon change of current density, stress and annealing at high temperature, thus can find application in solid-state lighting.