Bistability and improved hole injection in organic bistable light-emitting diodes using a quantum dot embedded hole transport layer

Organic bistable light-emitting diodes (OBLED) were developed by using a quantum dot embedded hole transport layer in the organic light-emitting diodes. The driving voltage of the OBLED was decreased due to the good hole transport properties of quantum dot embedded hole transport layer and the OBLED also showed bistability at negative bias due to the switching behavior of the quantum dot based hole transport layer. The origin for the switching behavior of the OBLED was confirmed by fabricating organic bistable device with the quantum dot embedded hole transport layer.     

Efficiency improvement of blue phosphorescent organic light emitting diodes by using a stacked emitting structure

Quantum efficiency of blue phosphorescent organic light emitting diodes (PHOLEDs) was improved by using a stacked emitting structure which can balance holes and electrons in light emitting layer. N,N′-dicarbazolyl-3,5-benzene (mCP) doped with iridium(III) bis(4,6-(di-fluorophenyl)-pyridinato-N,C2′) picolinate (FIrpic) was used as an emitting layer with good hole injection properties near hole transport layer and 1,3-bis(triphenylsilyl)-benzene (TSB) doped with FIrpic was used as an emitting layer with good electron injection properties near electron transport layer. High quantum efficiency of 10.2% was obtained at 500 cd/m² compared with 7.5% and 7.6% of mCP and TSB reference devices.     

High-performance organic light emitting diodes fabricated with a ruthenium oxide hole injection layer

We report enhanced hole injection using an RuO_x layer between indium tin oxide anodes and 4,4′-bis[N-(1-naphtyl)-N-phenyl-amino]biphenyl in organic light emitting diodes (OLEDs). The operation voltage of OLEDs at a current density of 100 mA/cm² decreased from 17 V to 14 V and the maximum luminance value increased from 120 cd/m² to 2500 cd/m² upon transformation of the Ru layer to RuO_x by surface treatment using O₂ plasma. Synchrotron radiation photoelectron spectroscopy results showed that the work function increased by 0.4 eV as the Ru layer was transformed to RuOx. Thus, the hole injection energy barrier was lowered, reducing the turn-on voltage and increasing the quantum efficiency of the OLEDs.     

Organic interlayer for high power efficiency in organic light-emitting diodes

The power efficiency of organic light-emitting diodes (OLEDs) was improved using hexaazatriphenylene-hexacarbonitrile (HAT) as the interlayer between hole injection layer and hole transport layer for efficient hole injection. The hole injection was enhanced and the driving voltage was lowered by the HAT interlayer. A high power efficiency of 73.3 lm/W was obtained from the green phosphorescent organic light-emitting diode with the HAT interlayer.     

Multilayer blue polymer light-emitting devices with spin-coated interlayers

Employment of multilayer heterostructures is a common approach to achieve efficiency and stable organic light emitting diodes (OLEDs). In this work, we report multilayer blue polymer light-emitting devices (PLEDs) by using spin-coated fluorene-triarylamine copolymers as interlayers between the conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT) and the emitting layer. A blue PLED with stepped hole injection profile yields an external quantum efficiency of 6.0% at a luminance of 9500 cd/m² at 5.5 V and an extrapolated lifetime of more than 18,000 h from 100 cd/m².     

Photoreactive main chain conjugated polymer containing oxetane moieties in the side chain and its application to green electrophosphorescence devices

The utilization of a photoreactive hole injection/transport layer in multilayer electrophosphorescence polymer light-emitting diodes (PLEDs) is demonstrated in this study. A new photoreactive polymer was synthesized using 3,6-dibromo-9-(6-((3-methyloxetan-3-yl)methoxy)hexyl)-9H-carbazole and 2,4-dimethyl-N,N-bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)aniline via the Suzuki coupling reaction. When oxetane groups were photopolymerized in the presence of a cationic photoinitiator, the cured film showed good compatibility with PEDOT:PSS and the indium tin oxide (ITO) layer due to the hydrophilic nature of the cross-linked section. The resulting green light-emitting device bearing PVK:PBD:Ir(Cz-ppy)₃ exhibits a maximum external quantum efficiency of 8.73%, corresponding to a luminous efficiency of 28.2 cd/A when using the device configuration of ITO/POx-TPACz/PEDOT:PSS/PVK:PBD:Ir(Cz-ppy)₃/TAZ/Alq₃/LiF/Al. These values are higher than those of PLEDs using conventional PEDOT:PSS as a single hole injection layer (HIL). The slight degree of improvement in device efficiency is due to the reduced hole injection barrier.     

The use of cross-linkable interlayers to improve device performances in blue polymer light-emitting diodes

High efficiency blue PLEDs were fabricated by adding a thin interlayer between PEDT:PSS and emitting polymer layers. Two different cross-linkable alkoxysilane-based interlayer materials, X-NPB and X-PDA, were synthesized based on N,N′-bis(4-methylphenyl)-N,N′-diphenyl-1,4-phenylenediamine (PDA) and N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1-biphenyl-4,4-diamine (NPB) which are well-known OLED HTLs. The devices, with configuration of indium tin oxide (ITO)/PEDT:PSS (65 nm)/interlayer (10–20 nm)/emitting polymer layer (70 nm)/BaF₂ (2 nm)/Ca (50 nm)/Al (300 nm), were fabricated by spin coating and thermal evaporation. In this device structure, the cross-linked X-NPB or X-PDA interlayers are more adherent and mechanically robust as well as impervious to spin coating of next emitting polymer layer. In addition, the devices with these interlayers exhibit a higher luminescence and current efficiency than those without interlayers because interlayers have two functions which are blocking electrons and preventing from severe quenching by PEDT:PSS.     

Oxadiazole-containing phenylene vinylene ether linkage copolymer as blue-green luminescent and electron transport material in polymer light-emitting diodes

We report studies on a new ether-type poly(phenylene vinylene) (PPV) copolymer containing oxadiazole groups in the conjugated main chain. It can be used as a blue-green electroluminescent material and as an electron transport/hole blocking material in polymer light-emitting diodes using PPV as the emitting material. The bilayer devices with aluminum cathode show a maximum brightness of about 300 cd/m² at about 21 V and a maximum external quantum efficiency of 0.1%. The quantum efficiency of the bilayer device is enhanced by a factor of 195 in comparison with that of the single layer device of PPV.     

Role of mixed hole transport layer with exciton blocking properties in phosphorescent organic light-emitting diodes

Improved efficiency in green phosphorescent organic light-emitting diodes using a composite hole transport layer (HTL) with hole transport and exciton blocking function was investigated. Mixed layer of (4,4′-N,N′-dicarbazole)biphenyl (CBP) and N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine (NPB) was used as a HTL and quantum efficiency could be enhanced from 4.3% to 8.1% at 100 cd/m². Best performances could be obtained in the device with 50% CBP and 50% NPB in the HTL.     

Low driving voltage in organic light-emitting diodes using MoO3 as an interlayer in hole transport layer

Driving voltage of organic light-emitting diodes (OLEDs) was lowered by applying MoO₃ as an interlayer between hole injection layer (HIL) and hole transport layer (HTL). MoO₃ was effective as an interlayer between HIL and HTL due to its valence band of around 5.3 eV which is suitable for hole injection. Hole injection from HIL to HTL was enhanced by MoO₃ interlayer and driving voltage of green fluorescent device could be lowered by 1.3 V at 1000 cd/m² by using thin MoO₃ interlayer.     

Photoluminescence and electroluminescence of a series of terbium complexes

Photoluminescence and electroluminescence of a series of terbium complexes based on 1-phenyl-3-methyl-4-R-5-pyrazolone were analyzed. The first absorption band of the pyrazolone derivative ligands gradually shifts toward the shorter wavelength region as the R changes from an electron acceptor to an electron donor. Correspondingly, the photoluminescence quantum efficiency of the terbium complexes increases. The neutral ligands also affect the photoluminescence and electroluminescence of the terbium complexes. A photochemical explanation for the influence of the R group and neutral ligand on the photoluminescence was proposed in relation to ligand-to-metal energy transfer. The electroluminescence of the terbium complexes having a neutral ligand comes from both the light emitting layer and the hole transport layer while the electroluminescence of the terbium complex without a neutral ligand is pure green coming solely from the light-emitting layer. It therefore demonstrates that the former have higher electron transport ability than the latter.     

Efficient multilayer organic light emitting diode

Some organic light emitting diodes with Alq₃ as emission layer, multilayer composed of m-MTDATA, NPB, TPD or SA as hole transport layer and lithium doped Alq₃ as electron injection assistant layer have been investigated. The current voltage characteristics and the electroluminescent (EL) output voltage characteristics have been investigated. It is found that the m-MTDATA is suitable as a hole injection layer close to the hole injection electrode ITO, NPB is suitable in contact with the emission layer, SA and TPD can be inserted between them to form an energy ladder structure, with which the efficiency of device has been increased. When thin lithium doped Alq₃ layer is inserted between Alq₃ and the aluminium electrode, the driving voltage of the devices has been clearly decreased while the current density and the EL output increased.     

Enhanced performance of polythiophene derivative based light emitting diodes by addition of europium and ruthenium complexes

Fabrication of polymer light emitting diodes (PLEDs) from a urethane containing processable polythiophene derivative, poly[2-(3-thienyl) ethanol n-butoxy carbonylmethyl urethane], (PURET) and its composites with luminescent Ruthenium (II) and Europium (III) complexes is discussed. Enhanced electroluminescence (EL) performance was observed, when 1% by weight of europium (III) or ruthenium (II) complexes were added to the PURET polymer. Multi-layered devices with polyaniline as a hole injecting layer and tris-8-hydroxyquinoline-aluminum as an electron injecting layer have also been fabricated. PLEDs from PURET polymer have a threshold voltage of 3.6 V and emit an orange-red light with a brightness of about 500 cd/m² under forward bias of 9 V, upon the addition of 1% of europium (III) thenoyl trifluoroacetonate complex to the polymer.     

Comparison of the carrier mobility,unipolar conduction,and light emitting characteristics of phosphorescent host–dopant system

We have investigated the charge carrier transport mobility and hole-only current behavior of phosphorescent host–dopant mixture, and evaluated the efficiency and lifetime behavior of organic light emitting device (OLED) containing the corresponding light emitting host–dopant system. The carrier drift mobilities of the phosphorescent host, 4,4′-N,N′-dicarbazole-biphenyl (CBP), doped with green-emitting fac-tris(2-phenylpyridine) iridium (Ir(ppy)₃) or bis(2-(2′-benzo[4,5-a]thienyl)pyridinato-N,C3′) iridium(acetyl-acetonate) (Btp₂Ir(acac)) red-emitting dopants, were directly investigated with time of flight (TOF) photoconductivity method. The resolved electron mobility of phosphorescent host–dopant layer by TOF-PC method showed the significant reduction at CBP:(Btp₂Ir(acac)) layer, which can be explained by the electron trapping. Measured hole-only current data also shows the reduction, which is more significant at CBP:(Btp₂Ir(acac)) as expected from larger energy level offset. The efficiency, spectral emitting properties, and device stability of phosphorescent OLEDs with identical host–dopant layer were evaluated. Compared with CBP:(Btp₂Ir(acac)), device with CBP:Ir(ppy)₃ emitter shows the spectral response and half-lifetime less dependent upon the hole/exciton blocking layer and its thickness. Such device data was well-correlated with probed TOF and hole current behavior.     

Color stable and interlayer free hybrid white organic light-emitting diodes using an area divided pixel structure

Color stable and interlayer free hybrid white organic light-emitting diodes were fabricated by using an area divided pixel structure. The area divided pixel structure was realized by stacking red and blue emitting layers using a fine metal mask. A phosphorescent red emitting layer was patterned by a metal mask and a blue fluorescent emitting layer was commonly deposited on the patterned red emitting layer. The blue fluorescent emitting layer could play a role of a hole-blocking layer and a white emission could be obtained due to separate emission of red and blue emitting layers. The interlayer free hybrid WOLEDs showed color stability from 100 cd/m² to 10,000 cd/m².     

Research progress in electron transport layer in perovskite solar cells

Since perovskite solar cells appeared in 2009, its simple preparation process, high photoelectric conversion efficiency and the characteristic of low cost in preparation process let it become the hot spot of both at-home and abroad. Owing to the constant efforts of scientists, the conversion efficiency of perovskite solar cells is more than 20% now. Perovskite solar cells are mainly composed of conductive glass, electron transport layer and hole transport layer, perovskite layer and electrode parts. This paper will briefly introduce the working principle and working process about the electron transport layer of perovskite solar cells. The paper focuses on aspects such as material types(e.g., inorganic electron transport materials, organic small molecule electron transport materials, surface modified electron transport materials and doped electron transport materials), preparation technology of electron transport layer, the effects of electron transport layer on the photovoltaic performance of the devices, and the electron transport layer in the future research.     

Electroluminescence of phenylene–vinylene random copolymers with different conjugation lengths

PPV random derivates were synthesized and characterized. Polymer light emitting diodes (PLEDs) were assembled using the random copolymers as emissive layer and showed EL in the blue-green region in function of the method of preparation. The increase in the average conjugation degree in the polymer chain led to the reduction of the turn-on voltage of the device. The addition of Alq3 as ETL increased tenfold the luminescence efficiency.     

Low voltage organic light emitting diode using p–i–n structure

Efficient n-type doping has been achieved by doping Liq in electron transport material Alq₃. Detailed investigation of current density–voltage characteristics of electron only devices with different doping concentrations of Liq in Alq₃ has been performed. An increase in current density by two orders of magnitude has been achieved with 33 wt% of Liq doped in Alq₃. Organic light emitting diode with p–i–n structure was fabricated using F₄-TCNQ doped α-NPD as hole transport layer, Ir(ppy)₃ doped CBP as emitting layer and 33 wt% Liq doped Alq₃ as electron transport layer. Comparison of OLEDs fabricated using undoped Alq₃ and 33 wt% Liq doped Alq₃ as electron transport layer shows reduction in turn on voltage from 5 to 2.5 V and enhancement of power efficiency from 5.8 to 10.6 lm/W at 5 V.     

Efficient light emitting diodes with Teflon buffer layer

The polytetrafluoroethylene (Teflon) was utilized as buffer layer to improve the performance of organic light emitting diodes (OLEDs). In the ITO/Teflon/N,N′-diphenyl-N,N′-(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD)/tris(8-hydroxy-quinoline) aluminum (Alq₃)/Ca/Ag device, the Teflon film helped to enhance the hole tunneling injection and effectively impede indium diffusion from the ITO electrode. Compared with the devices without Teflon, the turn-on voltage was lowered by 1.5 V due to the introduction of Teflon buffer, and the optimized devices exhibited a luminous efficiency double that of the devices without Teflon layer, and the device lifetime proved to be dramatically increased.     

Investigation of double emissive layer structures on phosphorescent blue organic light-emitting diodes

The performances of blue phosphorescent organic light-emitting diodes (PHOLEDs) at high current densities have been investigated with double emissive layer structures (D-EMLs). The D-EMLs are comprised of two emissive layers with a hole-transport-type host of N,N′-dicarbazolyl-3,5-benzene (mCP) and an electro transport-type ultrawide band-gap host of m-bis-(triphenylsilyl)benzene (UGH3) both doped with a blue electro-phosphorescent dopant of iridium(III)bis(4,6-difluorophenyl-pyridinato-N,C^2′) picolinate (FIrpic). The expansion of hole/electron recombination zone in D-EMLs has been successfully achieved by controlling of each EML properties, therefore external quantum efficiency, especially at high current density region was significantly enhanced. Moreover, the blue PHOLED with D-EMLs showed substantially reduced roll-off with the external quantum efficiency of 10.0% at 5000 cd/m².