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).     

High-efficiency orange and white phosphorescent organic light-emitting diodes based on homojunction structure

We demonstrate high-efficiency orange and white phosphorescent organic light-emitting diodes based on homojunction structure. Excellent performance is realized by using step-graded p- and n-type doping structure in orange homojunction device. The resulting orange homojunction device exhibits a maximum current efficiency of 30.0 cd/A and low efficiency roll-off. The improvements are mainly attributed to the utilization of step-graded doped profile, which facilitates balanced charge carrier injection and transport. Moreover, one optimized white homojunction device based on two complementary colors shows a maximum efficiency of 15.4 cd/A, and superior color-stability in a wide range of luminance.     

Efficient single layer RGB phosphorescent organic light-emitting diodes

We report efficient single layer red, green, and blue (RGB) phosphorescent organic light-emitting diodes (OLEDs) using a “direct hole injection into and transport on triplet dopant” strategy. In particular, red dopant tris(1-phenylisoquinoline)iridium [Ir(piq)₃], green dopant tris(2-phenylpyridine)iridium [Ir(ppy)₃], and blue dopant bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium [FIrpic] were doped into an electron transporting 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi) host, respectively, to fabricate RGB single layer devices with indium tin oxide (ITO) anode and LiF/Al cathode. It is found that the maximum current efficiencies of the devices are 3.7, 34.5, and 6.8 cd/A, respectively. Moreover, by inserting a pure dopant buffer layer between the ITO anode and the emission layer, the efficiencies are improved to 4.9, 43.3, and 9.8 cd/A, respectively. It is worth noting that the current efficiency of the green simplified device was as high as 34.6 cd/A, even when the luminance was increased to 1000 cd/m² at an extremely low applied voltage of only 4.3 V. A simple accelerated aging test on the green device also shows the lifetime decay of the simplified device is better than that of a traditional multilayered one.     

High-efficiency and superior color-stability white phosphorescent organic light-emitting diodes based on double mixed-host emission layers

We demonstrate high-efficiency and superior color-stability white phosphorescent organic light-emitting diodes based on double blue mixed-host emission layers (EMLs) with different mixed ratios. The key feature of the concept is to introduce double blue mixed-host EMLs with an orange ultrathin layer sandwiched between them. The improved white device without spacer or interlayer achieves superior color-stability and reduced efficiency roll-off, which are consistent with the good ambipolar conductivity of the mixed-host layer. Moreover, peak efficiency of 40.8 lm/W and low turn-on voltage of 2.71 V are realized. The double mixed-host EMLs concept proves to be quite useful in achieving excellent device performance.     

Deep blue phosphorescent organic light-emitting diodes with excellent external quantum efficiency

Highly efficient deep blue phosphorescent organic light-emitting diodes (PHOLEDs) using two heteroleptic iridium compounds, (dfpypy)₂Ir(acac) and (dfpypy)₂Ir(dpm), as a dopant and 9-(3-(9H-carbazol-9-yl)phenyl)-9H-carbazol-3-yl)diphenylphosphine oxide as a host material have been developed. The electroluminescent device of (dfpypy)₂Ir(dpm) at the doping level of 3 wt% shows the best performance with external quantum efficiency of 18.5–20.4% at the brightness of 100–1000 cd/m² and the color coordinate of (0.14, 0.18) at 1000 cd/m².     

Doping-free hybrid white organic light-emitting diodes with fluorescent blue,phosphorescent green and red emission layers

Industrialized white organic light-emitting diodes (OLEDs) currently require host-guest doping, a complicated process necessitating precise control of the guest concentration to get high efficiency and stability. Two doping-free, hybrid white OLEDs with fluorescent blue, and phosphorescent green and red emissive layers (EMLs) are reported in this work. An ultra-thin red phosphorescent EML was situated in a blue-emitting electron transport layer (ETL), while the ultra-thin green phosphorescent EML was placed either in the ETL (Device 1), or the hole transport layer (HTL) (Device 2). Device 2 exhibits higher efficiency and more stable spectrum due to the enhanced utilization of excitons by ultra-thin green EML at the exciton generation zone within the HTL. Values of current efficiency (CE), power efficiency (PE), and CRI obtained for the optimized hybrid white OLEDs fabricated through a doping-free process were of 23.2 cd/A, 20.5 lm/W and 82 at 1000 cd/m², respectively.     

Efficient non-doped monochrome and white phosphorescent organic light-emitting diodes based on ultrathin emissive layers

Efficient red, orange, green and blue monochrome phosphorescent organic light-emitting diodes (OLEDs) with simplified structure were fabricated based on ultrathin emissive layers. The maximum efficiencies of red, orange, green and blue OLEDs are 19.3 cd/A (17.3 lm/W), 45.7 cd/A (43.2 lm/W), 46.3 cd/A (41.6 lm/W) and 11.9 cd/A (9.2 lm/W). Moreover, efficient and color stable white OLEDs based on two complementary colors of orange/blue, three colors of red/orange/blue, and four colors of red/orange/green/blue were demonstrated. The two colors, three colors and four colors white OLEDs have maximum efficiencies of 30.9 cd/A (27.7 lm/W), 30.3 cd/A (27.2 lm/W) and 28.9 cd/A (26.0 lm/W), respectively. And we also discussed the emission mechanism of the designed monochrome and white devices.     

Management of charge carriers and excitons for efficient and color-stable white phosphorescent organic light-emitting diodes with simplified structure

We have demonstrated color-stable and highly efficient simplified white phosphorescent organic light-emitting diodes. The key feature is the use of a novel approach to confine the distribution of charge carriers and excitons across the whole blue emission layer. The resulting two-color white device has the maximum power efficiency and current efficiency of 45.5 lm/W and 43.5 cd/A with a very low color shift over a wide range of luminance. By systematically investigating the working mechanisms, we found that the ambipolar charge carrier transport ability of co-host layer which ensures the distribution of excitons to form in the whole blue emission layer was the critical factors for constructing color-stable white devices. Our results show that simplified white devices based on two organic materials achieving excellent color stability are possible.     

Tailoring electronic structure of organic host for high-performance phosphorescent organic light-emitting diodes

We investigated highly efficient phosphorescent organic light-emitting diodes (PHOLEDs) based on three novel 1,3,5-triazine derivatives as the host materials and two kinds of iridium complexes as the guests, respectively. For comparison, the devices using a common phosphorescent host 4,4′-N,N′-dicarbazolebiphenyl (CBP) have also been fabricated. Results show that the devices using 9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9H-carbazole (PTC) and 4-(4,6-diphenoxy-1,3,5-triazin-2-yl)-N,N-diphenylaniline (POTA) as host have better performance than that of CBP. In comparison with the PHOLEDs based on CBP host, PTC- and POTA-based PHOLEDs show significantly lower driving voltages and higher power efficiencies. The high bipolar carrier mobility of the host is found to be critical to this kind of doping system, which would balance the injection of both carriers and improve efficiency.     

Novel spiro-based host materials for application in blue and white phosphorescent organic light-emitting diodes

Two novel spiro-based host materials, namely 3-(9,9′-spirobi[fluoren]-6-yl)-9-phenyl-9H-carbazole (SF3Cz1) and 9-(3-(9,9′-spirobi[fluoren]-6-yl)phenyl)-9H-carbazole (SF3Cz2) were designed and synthesized. Due to the meta-linkage of spirobifluorene backbone, both SF3Cz1 and SF3Cz2 possess triplet energies over 2.70 eV, indicating they could serve as suitable hosts for blue and even white phosphorescent organic light-emitting diodes (PHOLEDs). The fabricated bis(4,6-(difluorophenyl)-pyridinato -N,C′)picolinate (FIrpic) based PHOLEDs hosted by SF3Cz1 and SF3Cz2 exhibited excellent performance with maximum external quantum efficiencies (EQEs) of 18.1% and 19.7%, respectively. Two-color warm white PHOLEDs fabricated by utilizing SF3Cz1 and SF3Cz2 as hosts also achieved high EQEs and low efficiency roll-offs. The results demonstrate that SF3Cz1 and SF3Cz2 are promising hosts for blue and white PHOLEDs.     

Novel scattering and color converting substrates for simple-structured white organic light-emitting diodes

Organic light-emitting diodes (OLEDs) have shown great success in the display applications recently. However, the applications of OLEDs in lighting are still limited due to their complex device structures. Here, we developed a novel phosphor doped glass substrate with both high scattering and excellent color conversion capability to greatly simplify the device structures of white organic light-emitting diodes (WOLEDs). A simple-structured WOLED comprising a blue OLED and the scattering fluorescent substrate was demonstrated to realize high quality white light for lighting applications. The WOLED exhibits a turn-on voltage of 2.7 V, a maximum power efficiency of 29.8 lm/W, an external quantum efficiency (EQE) of 14.2%, a color rendering index (CRI) of 86, and a correlated color-temperature (CCT) of 3900 K. The low turn-on voltage can be attributed to the single emissive layer structure used in the WOLED. The high power efficiency as well as the high EQE are due to both the high color conversion efficiency and the high scattering capability of the fluorescent substrate. In addition, the WOLED is favorable for high-quality solid-state lighting in our daily life due to its high color rendering ability along with an adequate CCT CC.     

Phosphorescent dye-doped hole transporting layer for organic light-emitting diodes

Hole transport materials are critical to the performance of organic light-emitting diodes (OLEDs). While 1,1-bis(di-4-tolylaminophenyl)cyclohexane (TAPC) with a high triplet energy is widely used for high efficiency phosphorescent OLEDs, devices using TAPC as a hole transport layer (HTL) have a short operating lifetime due to the build-up of trapped charges at the TAPC/emitting layer (EML) interface during device operation. In this work, to solve the operating stability problem, instead of using conventional HTLs, we use a(fac-tris(2-phenylpyridine)iridium (III))(Ir(ppy)₃) doped layer as an HTL to replace the conventional HTLs. Because of the hole injecting and transporting abilities of the phosphorescent dye, holes can be directly injected into the emitting layer without an injection barrier. OLEDs based on a phosphorescent dye-doped HTL show significant improvement in operational stability without loss of efficiency.     

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.     

Transient electroluminescence in heavy metal complex-based phosphorescent organic light-emitting diodes

In phosphorescent organic light-emitting diodes (PHOLEDs), both the rise time and decay time decrease with increasing amplitude of the applied voltage pulse. The rise time τ_r of the transient electroluminescence (TEL) increases linearly with increasing value of the ratio of voltage V to the current j, that is, with V/j. Using the equations for the dynamics of charge carriers an expression is derived for the rise time τ_r of the TEL in OLEDs. It is shown that τ_r should increase with increasing values of the ratio (V/j), dielectric constant ε, and area of cross-section of the emission layer, however, it should decrease with the thickness of emission layer. For higher values of the applied voltage nonlinearity occurs in the τ_r versus V/j plot because the increase in mobility of carriers at high electric field causes increase in the current flowing through the OLEDs. In fact, the rise time of TEL is related to the product of capacitance and effective resistance of the OLED. Considering the rate of generation and decay of radiative triplet excitons in the emission layer, an expression is derived for the decay time of TEL in PHOLEDs and it is shown that, for higher values of the time-constant of OLED, the decay time should be equal to the time-constant, however, for lower values of the time-constant, the decay time should be equal to the lifetime of radiative triplet excitons in the emission layer. A good agreement is found between the theoretical and experimental results.     

High triplet,bipolar polymeric hosts for highly efficient solution-processed blue phosphorescent polymer light-emitting diodes

Two polymeric hosts PCzTPP and PCzTPPO with twisted geometrical configurations for blue phosphorescent polymer light-emitting diodes (PhPLEDs) were designed and synthesized by incorporating electron-accepting carbazole units with electron-donating TPP/TPPO groups. This molecular design endows PCzTPP and PCzTPPO with high glass transition temperatures of 204 °C and 215 °C, high triplet energies of 2.72 eV and bipolar features. In addition, the HOMO and LUMO of these polymers matched well with the HOMO of the hole-transport layer and the Fermi level of cathode compared with PVK, which facilitated the injection of holes and electrons. PCzTPP- and PCzTPPO-based single-emissive-layer blue PhPLEDs were fabricated with simplified device configuration by solution process using FIrpic as a dopant. These devices exhibited lower turn on voltages (<8 V) than PVK-based devices (12 V). The maximum luminances of PCzTPP- and PCzTPPO-based devices were twofold and threefold that of PVK-based devices, and the maximum current efficiencies were nearly threefold and ninefold, respectively. Moreover, PCzTPPO-based solution processed blue PhPLEDs with improved configuration showed maximum current efficiency and external quantum efficiency of 14.5 cd/A and 6.6%, respectively.     

Synthesis,characterization of the phenylquinoline-based on iridium(III) complexes for solution processable phosphorescent organic light-emitting diodes

A new series of highly efficient Ir(III) complexes, (DPQ)₂Ir(pic-N-O), (F₄PPQ)₂Ir(pic-N-O), (FPQ)₂Ir(pic-N-O), and (CPQ)₂Ir(pic-N-O) were synthesized for phosphorescent organic light-emitting diodes (PhOLEDs), and their photophysical, electrochemical, and electroluminescent (EL) properties were investigated. The Ir(III) complexes, including picolinic acid N-oxide (pic-N-O) ancillary ligand, are comprised with the various main ligands such as 2,4-diphenylquinoline (DPQ), 4-phenyl-2-(2,3,4,5-tetrafluorophenyl)quinoline (F₄PPQ), 2-(9,9-diethyl-9H-fluoren-2-yl)-4-phenylquinoline (FPQ) and 9-ethyl-3-(4-phenylquinolin-2-yl)-9H-carbazole. Remarkably, high performance PhOLEDs using a solution-processable (DPQ)₂Ir(pic-N-O) doped CBP host emission layer were fabricated to give a high luminance efficiency (LE) of 26.9 cd/A, equivalent to an external quantum efficiency (EQE) of 14.2%.The calculated HOMO–LUMO energy gaps for (DPQ)₂Ir(pic-N-O), (F₄PPQ)₂Ir(pic-N-O), (FPQ)₂Ir(pic-N-O) and (CPQ)₂Ir(pic-N-O) were in good agreement with the experimental results.     

Effect of coupling between excitons and gold nanoparticle surface plasmons on emission behavior of phosphorescent organic light-emitting diodes

Enhanced efficiency and reduced efficiency roll-off in phosphorescent organic light-emitting diodes (PhOLEDs) are realized by interposing a solution-processed gold nanoparticle (GNP)-based interlayer between the anode and the hole-injection layer. Transient photoluminescence measurements elucidate that a reduced lifetime of the triplet excitons was observed for samples having a GNP-interlayer as compared to a control sample without the GNP-interlayer. The decrease in the triplet exciton lifetime, caused by the coupling between the triplet excitons and the localized surface plasmons (LSPs) excited by the GNPs, enables reducing the triplet–triplet and triplet–polaron annihilation processes, thereby a reduced efficiency roll-off in PhOLEDs. The presence of a GNP-interlayer also acts as an optical out-coupling layer contributing to the efficiency enhancement and was demonstrated by the theoretical simulation.     

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

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

Highly efficient phosphorescent organic light-emitting diodes using a beryllium metal–chelate complex as electron-transporting host material

A classical fluorescent metal–chelate complex bis(2-(2-hydroxyphenyl)-pyridine)beryllium (Bepp₂) has been used as an efficient electron-transporting host material to construct highly efficient phosphorescent organic light-emitting diodes (PHOLEDs) with an orange-emitting phosphorescent guest bis(7,8-benzoquinolinato) iridium (III) (N,N′-diisopropyl-benzamidine) ((bzq)₂Ir(dipba)). Due to the well-matched energy levels of Bepp₂ with the corresponding hole-/electron- transporting (HT/ET) materials and the high-efficiency and complete energy transfer of this host–guest system, the Bepp₂-based PHOLEDs exhibit rather low driving voltage (2.8 V) and high peak EL efficiencies of over 70 cd A⁻¹ for luminous efficiency, 55 lm W⁻¹ for power efficiency, and 23% for external quantum efficiency, a performance significantly better than that using CBP as the host.     

Simplified and high-efficiency warm/cold phosphorescent white organic light-emitting diodes based on interfacial exciplex co-host

Possessing the reverse intersystem crossing (RISC) process, exciplex system has vast potential to enhance the efficiency of the white organic light-emitting diodes (WOLEDs). Nevertheless, general structures of the emitting layer always employ triple-doping in a long range (20–30 nm) which is complicated on fabrication progress. In this paper, based on the interfacial exciplex co-host, a flexible and simplified structure design is proposed to realize both warm and cold phosphorescent WOLEDs. In the two devices, with strategically locating the ultrathin orange phosphorescent emitting layers at two sides of the blue phosphorescent emitting layer (2 nm), respectively, multiple energy transfer channels are created to carry out highly efficient exciton utilization. Owing to the different energy transfer mechanisms, different organic emission ratios are obtained in two WOLEDs. The cold WOLEDs exhibited superior maximum external quantum efficiency (EQE), current efficiency (CE) and power efficiency (PE) of 28.37%, 72.17 cd A⁻¹ and 87.17 lm W⁻¹, respectively. Also, the warm WOLEDs showed high values as EQE of 23.80%, CE of 67.70 cd A⁻¹ and PE of 81.10 lm W⁻¹. Furthermore, both the devices presented rather stable color output in the luminance range from 2000 cd m⁻² to 10000 cd m.⁻²