Enhancement of external quantum efficiency and reduction of roll-off in blue phosphorescent organic light emitt diodes using TCTA inter-layer

The improved external quantum efficiency (EQE) and reduced roll-off properties of blue phosphorescent organic light-emitting diodes (PHOLEDs), were fabricated with structure, ITO/NPB (400 Å)/TCTA (200 Å)/mCP:FIrpic (7%)(300 Å)/TPBi (300 Å)/Liq (20 Å)/Al (800 Å) by incorporating an 4,4′,4′′-tris(carbazol-9-yl)-triphenylamine (TCTA) interlayer. We compared the properties of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) and 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi) as the electron transport layer (ETL) with a typical structure of hole transport layer (HTL)/emissive layer (EML)/ETL in OLEDs and utilized inter-layer in the optimized structure to enhance EQE to 52% at 5.5 V, also stabilize the roll-off of 23%. The use of inter-layer in blue PHOLEDs exhibits a current efficiency of 10.04 cd/A, an EQE of 6.20% at 5.5 V and the highest luminance of 10310 cd/m² at 9.5 V. We have identified the properties of electroluminescence through the inter-layer in blue PHOLEDs which can be divided into singlet excitons and triplet excitons which emit fluorescence of N,N′-bis(1-naphthalenyl)-N,N′-bis-phenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) at 420 nm and phosphorescence of Iridium (III) bis[(4,6-difluorophenyl)-pyridinato-N,C²′] picolinate (FIrpic) at 470 nm, 494 nm, respectively.     

Organic solar cells with p-type amorphous chromium oxide thin film as hole-transporting layer

Efficient organic solar cells based on the blends of poly (3-hexylthiophene) (P3HT):fullerene derivative [6,6]-phenyl-C₆₁ butyric acid methyl ester (PCBM) composites have been fabricated by using the sputtered amorphous chromium oxide (ACO) film on fluorine-doped tin oxide (FTO) coated glass substrates as a hole-transporting layer (HTL). Through ACO layer sputtering temperature, film thickness and oxygen flow ratio optimization, the highest power conversion efficiency of 3.28% of FTO/ACO/P3HT:PCBM/Al solar cells on glass has been achieved under AM1.5G 100 mW/cm² illumination. It is found that the device with 10 nm thick ACO sputtered at 473 K and oxygen flow ratio f(O₂) (O₂/O₂ + Ar) = 40% shows the best photovoltaic properties. The photovoltaic properties in these devices are discussed in terms of the band diagrams and series resistance of the devices, and characteristics of ACO HTL. It is concluded that ACO is a suitable alternative to poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) as a HTL.     

Thermally and electrochemically stable amorphous hole-transporting materials based on carbazole dendrimers for electroluminescent devices

Amorphous hole-transporting carbazole dendrimers, 1,4-bis[3,6-di(carbazol-9-yl)carbazol-9-yl]-2,6-di(2-ethylhexyloxy)benzene (G2CB) and 1,4-bis[3,6-di(carbazol-9-yl)carbazol-9-yl]-9-(2-ethylhexyl)carbazole (G2CC), were synthesized by a divergent approach involving bromination and Ullmann coupling reactions. Compounds G2CB and G2CC showed high thermal stability (T_g = 206 to 245 °C) and excellent electrochemical reversibility. Double-layer organic light-emitting diodes were fabricated by using G2CB and G2CC as hole-transporting layers (HTLs) and tris(8-quinolinato)aluminum (Alq₃) as light-emissive layer with the device configuration of indium tin oxide/HTL/Alq₃/LiF:Al. Both devices exhibited bright green emission from Alq₃. The device using G2CC as HTL has the best performance with a maximum brightness of 8900 cd/m² at 14 V and a low turn-on voltage of 3.5 V.     

Highly Efficient p‐i‐n Perovskite Solar Cells Utilizing Novel Low‐Temperature Solution‐Processed Hole Transport Materials with Linear π‐Conjugated Structure

Alternative low‐temperature solution‐processed hole‐transporting materials (HTMs) without dopant are critical for highly efficient perovskite solar cells (PSCs). Here, two novel small molecule HTMs with linear π‐conjugated structure, 4,4′‐bis(4‐(di‐p‐toyl)aminostyryl)biphenyl (TPASBP) and 1,4′‐bis(4‐(di‐p‐toyl)aminostyryl)benzene (TPASB), are applied as hole‐transporting layer (HTL) by low‐temperature (sub‐100 °C) solution‐processed method in p‐i‐n PSCs. Compared with standard poly(3,4‐ethylenedioxythiophene): poly(styrenesulfonic acid) (PEDOT:PSS) HTL, both TPASBP and TPASB HTLs can promote the growth of perovskite (CH₃NH₃PbI₃) film consisting of large grains and less grain boundaries. Furthermore, the hole extraction at HTL/CH₃NH₃PbI₃ interface and the hole transport in HTL are also more efficient under the conditions of using TPASBP or TPASB as HTL. Hence, the photovoltaic performance of the PSCs is dramatically enhanced, leading to the high efficiencies of 17.4% and 17.6% for the PSCs using TPASBP and TPASB as HTL, respectively, which are ≈40% higher than that of the standard PSC using PEDOT:PSS HTL.     

Photo/electroluminescence properties of an europium (III) complex doped in 4,4′-N,N′-dicarbazole-biphenyl matrix

The photoluminescence properties of one europium complex Eu(TFNB)₃Phen (TFNB = 4,4,4-trifluoro-1-(naphthyl)-1,3-butanedione, Phen = 1,10-phenanthroline) doped in a hole-transporting material CBP (4,4′-N,N′-dicarbazole-biphenyl) films were studied. A series of organic light-emitting devices (OLEDs) using Eu(TFNB)₃Phen as the emitter were fabricated with a multilayer structure of indium tin oxide, 250 Ω/square)/TPD (N,N′-diphenyl-N,N′-bis(3-methyllphenyl)-(1,1′-biphenyl)-4,4′-diamine, 50 nm)/Eu(TFNB)₃phen (x): CBP (4,4′-N,N′-dicarbazole-biphenyl, 45 nm)/BCP (2,9-dimethyl-4,7-diphenyl-l,10 phenanthroline, 20 nm)/AlQ (tris(8-hydroxy-quinoline) aluminium, 30 nm)/LiF (1 nm)/Al (100 nm), where x is the weight percentage of Eu(TFNB)₃phen doped in the CBP matrix (1-6%). A red emission at 612 nm with a half bandwidth of 3 nm, characteristic of Eu(III) ion, was observed with all devices. The device with a 3% dopant concentration shows the maximum luminance up to 1169 cd/m² (18 V) and the device with a 5% dopant concentration exhibits a current efficiency of 4.46 cd/A and power efficiency of 2.03 lm/W. The mechanism of the electroluminescence was also discussed.     

Highly efficient dye sensitized solar cells based on a novel ruthenium sensitizer

A new sensitizer, Ru(2,2′-bipyridine-4,4′-dicarboxylic acid)-4,4′-bis(2-(4-N,N′-diphenylaminophenyl)ethenyl)-2,2′-bipyridine) (NCS)₂, denoted K77-7, was synthesized. The UV–vis spectrum of K77-7 was characterized. Dye sensitized solar cells (DSSC) based on K77-7 were fabricated and devices J/V curves were measured. The effects of co-adsorbent chenodeoxycholic acid, solvent in electrolyte, TiO₂ light scattering layer, and treatment of TiCl₄ aqueous solution on device efficiency were discussed. Under solar illumination of 100 mW cm^?2 (A.M. 1.5), the optimized DSSC device efficiency of 10.1 % was obtained.     

Organic light emitting devices with doped electron transport and hole blocking layers

This study reports on heterostructure OLEDs with n-type molecularly doped electron transport layer and hole blocking layer. The influence of doping on the operating voltage and on light emission performances was investigated. The n-type doping molecule is 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) dispersed into either an 8-(hydroquinoline) aluminum (Alq) electron transport layer (ETL) or a 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline (Bathocuproine BCP) hole blocking layer (HBL). The typical device structure is glass substrate/indium tin oxide/PEDOT/TPD–F4-TCNQ/Alq–DCM/BCP/Alq/Mg–Ag where Poly(3,4)ethylenedioxythiophene/Polystyrenesulphonate (PEDOT/PSS) is a hole injecting layer, TPD–F4-TCNQ is a hole transport layer (HTL) made of N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) doped with 2 wt.% of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F4-TCNQ) and Alq–DCM is the emitting layer (EML) made of Alq doped with 2 wt.% of 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) orange dye. The modified cathode consists in a combination of a BCP HBL and an Alq ETL where BCP or/and Alq were doped with PBD. Lowest operating voltage (3 V for a luminance of 10 Cd/m²) and brightest devices (6000 Cd/m²) were obtained with a hole blocking bilayer made of BCP doped with 28 wt.% deposited onto an undoped BCP (each one being 5 nm thick). Adding an undoped Alq layer improved the device current efficiency (4 Cd/A) but is detrimental to the operating voltage (6 V for a luminance of 10 Cd/m²). In the absence of real n-type doping with organic molecules, our results point out that the design of molecular doped injection layer at the cathode will need for a compromise between high luminance and efficiency on one hand and low operating voltage on the other hand.     

Layer-by-layer deposition films of copper phthalocyanine derivative; their photoelectrochemical properties and application to solution-processed thin-film organic solar cells

Ultrathin films serving as a light-harvesting and hole-transporting material were fabricated by layer-by-layer deposition of a water-soluble phthalocyanine derivative, copper(II) phthalocyanine-3,4′,4″,4″′-tetrasulfonic acid tetrasodium salt (CuPcTS), and poly(diallyldimethylammonium chloride). The blue-shift of absorption peak and the absorption dichroism of the Q band indicated that CuPcTS molecules in the layer-by-layer films form cofacial dimers or oligomers and that their molecular planes take a three- or two-dimensional orientation in a direction parallel to the substrate depending on a drying process of the film during the deposition. The diffusion constant of hole carriers among CuPcTS molecules in the film was evaluated to be 6.5 × 10^− 11 cm² s^− 1 in an acetonitrile solution from potential step chronoamperometry measurement. Solution-processed thin-film organic solar cells with a triple-layered structure were developed by combining a hole-transporting layer made of poly(3,4-ethylenedioxythiophene) oxidized with poly(4-styrenesulfonate), a light-harvesting layer of CuPcTS, and an electron-transporting layer of fullerene, in this sequence. Photovoltaic properties of the cells strongly depended on the thickness of CuPcTS films and can be maximized by controlling the thickness at ca. 10 nm.     

Enhancement of the efficiency and the color stabilization in green organic light-emitting devices with multiple heterostructures acting as a hole transport layer

The electrical and the optical properties of organic light-emitting devices (OLEDs), with and without multiple heterostructures consisting of N, N-bis-(1-naphthyl)-N, N-diphenyl-1,1-biphenyl-4,4-diamine (NPB)/5,6,11,12-tetraphenylnaphthacene (rubrene) acting as a hole transport layer (HTL), were investigated. The OLEDs with 3 periods of NPB/mixed rubrene:NPB multiple heterostructures acting as an HTL showed the highest luminances and efficiencies. While the electroluminescence (EL) peak corresponding to the rubrene layer did not appear for the OLEDs with 3 periods of NPB/mixed rubrene:NPB multiple heterostructures, only the EL peak related to the tris (8-hydroxyquinoline) aluminum layer was observed. The Commission Internationale de l’Eclairage chromaticity coordinates of the OLEDs with 3 periods of NPB/mixed rubrene:NPB multiple heterostructures at 14 V were (0.321, 0.531), indicative of a deep stabilized green color.     

Pyrazine-Functionalized Donor–Acceptor Covalent Organic Frameworks for Enhanced Photocatalytic H2 Evolution with High Proton Transport

The well-defined 2D or 3D structure of covalent organic frameworks (COFs) makes it have great potential in photoelectric conversion and ions conduction fields. Herein, a new donor–accepter (D–A) COF material, named PyPz-COF, constructed from electron donor 4,4′,4″,4′″-(pyrene-1,3,6,8-tetrayl)tetraaniline and electron accepter 4,4′-(pyrazine-2,5-diyl)dibenzaldehyde with an ordered and stable π-conjugated structure is reported. Interestingly, the introduction of pyrazine ring endows the PyPz-COF a distinct optical, electrochemical, charge-transfer properties, and also brings plentiful CN groups that enrich the proton by hydrogen bonds to enhance the photocatalysis performance. Thus, PyPz-COF exhibits a significantly improved photocatalytic hydrogen generation performance up to 7542 µmol g⁻¹ h⁻¹ with Pt as cocatalyst, also in clear contrast to that of PyTp-COF without pyrazine introduction (1714 µmol g⁻¹ h⁻¹). Moreover, the abundant nitrogen sites of the pyrazine ring and the well-defined 1D nanochannels enable the as-prepared COFs to immobilize H₃PO₄ proton carriers in COFs through hydrogen bond confinement. The resulting material has an impressive proton conduction up to 8.10 × 10⁻² S cm⁻¹ at 353 K, 98% RH. This work will inspire the design and synthesis of COF-based materials with both efficient photocatalysis and proton conduction performance in the future.     

Synthesis and blue electroluminescent properties of zinc (II) [2-(2-hydroxyphenyl)benzoxazole]

This study reports on the properties of organic light-emitting diodes (OLEDs) with zinc (II) [2-(2-hydroxyphenyl)benzoxazole] as a hole-blocking layer. OLEDs devices are prepared in a conventional OLEDs structure (i.e., anode/HTL/EL/HBL/cathode and anode/HTL/HBL/EL/cathode). The luminescence efficiencies and the turn-on voltage are significantly affected by the existence of the hole-blocking layer. This is attributed to an excellent hole-blocking property, which is in turn due to the high HOMO energy level (6.5 eV). The device showed luminous efficiency 2.46 lm/W at 5 V. The maximum luminance of about 10,000 cd/m² is obtained, and the turn-on voltage (2.5 V) is affected by the existence of the hole-blocking layer.     

Monitoring interface traps in operating organic light-emitting diodes using impedance spectroscopy

Electronic trap densities at the indium tin oxide (ITO)/hole transport layer (HTL) interface in operating organic light-emitting diodes (OLEDs) are characterized in situ using impedance spectroscopy. For OLEDs with a high density of active trap states, negative values of the frequency derivative of resistance are clearly observable for frequencies on the order of 10 kHz, whereas positive values are observed when the trap density is low With this technique, it is revealed that the trap density is minimized via the introduction of a TPD-Si₂ (4,4′-bis[(p-trichlorosilylpropylphenyl) phenylamino]-biphenyl) passivation layer at the ITO/HTL interface or by the application of large electric fields during device operation. Furthermore, impedance spectroscopy illustrates that the ITO/HTL interface is not a simple series resistance when traps are present since they are shown not to contribute to high frequency conduction. Overall, this paper demonstrates that the parasitic effects of interface traps can mask the underlying negative capacitive transport in OLEDs and presents a technique capable of monitoring the trap density of buried interfaces in organic electronic devices.     

Synergistic Ionic Liquid in Hole Transport Layers for Highly Stable and Efficient Perovskite Solar Cells

Perovskite solar cells (PSCs) with n-i-p structures often utilize an organic 2,2′,7,7′-tetrakis (N, N-di-p-methoxyphenyl-amine) 9,9′-spirobifluorene (spiro-OMeTAD) along with additives of lithium bis(trifluoromethanesulfonyl)imide salt (LiTFSI) and tert-butylpyridine as the hole transporting layer (HTL). However, the HTL lacks stability in ambient air, and numerous defects are often present on the perovskite surface, which is not conducive to a stable and efficient PSC. Therefore, constructive strategies that simultaneously stabilize spiro-OMeTAD and passivate the perovskite surface are required. In this work, it is demonstrated that a novel ionic liquid of dimethylammonium bis(trifluoromethanesulfonyl)imide (DMATFSI) could act as a bifunctional HTL modulator in n-i-p PSCs. The addition of DMATFSI into spiro-OMeTAD can effectively stabilize the oxidized spiro-OMeTAD⁺ cation radicals through the formation of spiro-OMeTAD⁺TFSI⁻ because of the excellent charge delocalization of the conjugated CF₃SO₂⁻ moiety within TFSI⁻. In addition, DMA⁺ cations could move toward the perovskite from the HTL, resulting in the passivation of defects at the perovskite surface. Accordingly, a power conversion efficiency of 23.22% is achieved for PSCs with DMATFSI and LiTFSI co-doped spiro-OMeTAD. Moreover, benefiting from the improved ion migration barrier and hydrophobicity of the HTL, still retained nearly 80% of their initial power conversion efficiency after 36 days of exposure to ambient air.     

Nanostructured Back Reflectors for Efficient Colloidal Quantum‐Dot Infrared Optoelectronics

Colloidal quantum dots (CQDs) can be used to extend the response of solar cells, enabling the utilization of solar power that lies to the red of the bandgap of c‐Si and perovskites. To achieve largely complete absorption of infrared (IR) photons in CQD solids requires thicknesses on the micrometer range; however, this exceeds the typical diffusion lengths (≈300 nm) of photoexcited charges in these materials. Nanostructured metal back electrodes that grant the cell efficient IR light trapping in thin active layers with no deterioration of the electrical properties are demonstrated. Specifically, a new hole‐transport layer (HTL) is developed and directly nanostructured. Firstly, a material set to replace conventional rigid HTLs in CQD devices is developed with a moldable HTL that combines the mechanical and chemical requisites for nanoimprint lithography with the optoelectronic properties necessary to retain efficient charge extraction through an optically thick layer. The new HTL is nanostructured in a 2D lattice and conformally coated with MoO₃/Ag. The photonic structure in the back electrode provides a record photoelectric conversion efficiency of 86%, beyond the Si bandgap, and a 22% higher IR power conversion efficiency compared to the best previous reports.     

Increasing open circuit voltage by adjusting work function of hole-transporting materials in perovskite solar cells

A series of conductive polymers, i.e., poly(3-methylthiophene) (PMT), poly(thiophene) (PT), poly(3-bromothiophene) (PBT) and poly(3-chlorothiophene) (PCT), were prepared via the electrochemical polymerization process. Subsequently, their application as hole-transporting materials (HTMs) in CH₃NH₃PbI₃ perovskite solar cells was explored. It was found that rationally increasing the work function of HTMs proves beneficial in improving the open circuit voltage (V _oc) of the devices with an ITO/conductive-polymer/CH₃NH₃PbI₃/C₆₀/BCP/Ag structure. In addition, the higher-V _oc devices with a higher-work-function HTM exhibited higher recombination resistances. The highest open circuit voltage of 1.04 V was obtained from devices with PCT, with a work function of–5.4 eV, as the hole-transporting layer. Its power conversion efficiency attained a value of approximately 16.5%, with a high fill factor of 0.764, an appreciable open voltage of 1.01 V and a short circuit current density of 21.4 mA·cm^–2. This simple, controllable and low-cost manner of preparing HTMs will be beneficial to the production of large-area perovskite solar cells with a hole-transporting layer.

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Microstructure and electrochemical properties of magnetron-sputtered LiCoO2/LiNiO2 multi-layer thin film electrode

LiCoO₂ single-layer and LiCoO₂/LiNiO₂ multi-layer thin film electrodes were successfully fabricated by magnetron sputtering. Their microstructure and electrochemical properties were investigated. Once annealed, both films had the (0 0 3) preferred orientation to minimize the surface energy. The initial discharge capacity of the multi-layer thin film was approximately 53.1 μAh/cm² μm, which was higher than that of the LiCoO₂ single-layer thin film having similar thickness. The capacity retention of the multi-layer thin film was superior to that of the single-layer thin film. These findings indicate that the multi-layer thin film is a promising cathode material for the fabrication of high-performance thin film batteries.     

Efficient triplet exciton confinement of white organic light-emitting diodes using a heavily doped phosphorescent blue emitter

We demonstrated efficient white electrophosphorescence with a heavily doped phosphorescent blue emitter and a triplet exciton blocking layer (TEBL) inserted between the hole transporting layer (HTL) and the emitting layer (EML). We fabricated white organic light-emitting diodes (WOLEDs) (devices A, B, C, and D) using a phosphorescent red emitter; bis(2-phenylquinolinato)-acetylacetonate iridium III (Ir(pq)₂acac) doped in the host material; N,N′-dicarbazolyl-3,5-benzene (mCP) as the red EML and the phosphorescent blue emitter; bis(3,5-Difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl) iridium III (FIrpic) doped in the host material; p-bis(triphenylsilyly)benzene (UGH2) as the blue EML. The properties of device B, which demonstrate a maximum luminous efficiency and external quantum efficiency of 26.83 cd/A and 14.0%, respectively, were found to be superior to the other WOLED devices. It also showed white emission with CIE_x,y coordinates of (x = 0.35, y = 0.35) at 8 V. Device D, which has a layer of P-type 4,4′,4″-tri(N-carbazolyl)triphenylamine (TCTA) material between the HTL and TEBL, was compared with device A to determine the 430 nm emission peak.     

Surface modified Al 2 O 3 in fluorinated polyimide/Al 2 O 3 nanocomposites: Synthesis and characterization

Organic–inorganic hybrid materials consisting of inorganic materials and organic polymers are a new class of materials, which have received much attention in recent years. In the present investigation, at first, the surface of nano-alumina (Al ₂ O ₃ ) was treated with a silane coupling agent of $\boldsymbol{\gamma} $ -aminopropyltriethoxysilane (KH550), which introduces organic functional groups on the surface of Al ₂ O ₃ nanoparticles. Then fluorinated polyimide (PI) was synthesized from 4,4 ^′ -(hexafluoroisopropylidene) diphthalic anhydride and 4,4 ^′ -diaminodiphenylsulfone. Finally, PI/modified Al ₂ O ₃ nanocomposite films having 3, 5, 7 and 10% of Al ₂ O ₃ were successfully prepared by an in situ polymerization reaction through thermal imidization. The obtained nanocomposites were characterized by fourier transform infrared spectroscopy, thermogravimetry analysis, X-ray powder diffraction, UV-Vis spectroscopy, field emission scanning electron microscopy and transmission electron microscopy. The results show that the Al ₂ O ₃ nanoparticles were dispersed homogeneously in PI matrix. According to thermogravimetry analysis results, the addition of these nanoparticles improved thermal stability of the obtained hybrid materials.     

Highly stable carbon-based perovskite solar cell with a record efficiency of over 18% via hole transport engineering

Carbon-based perovskite solar cells show great potential owing to their low-cost production and superior stability in air, compared to their counterparts using metal contacts. The photovoltaic performance of carbon-based PSCs, however, has been progressing slowly in spite of an impressive efficiency when they were first reported. One of the major obstacles is that the hole transport materials developed for state-of-the-art Au-based PSCs are not suitable for carbon-based PSCs. Here, we develop a low-temperature, solution-processed Poly(3-hexylthiophene-2,5-diyl) (P3HT)/graphene composite hole transport layer (HTL), that is compatible with paintable carbon-electrodes to produce state-of-the-art perovskite devices. Space-charge-limited-current measurements reveal that the as-prepared P3HT/graphene composite exhibits outstanding charge mobility and thermal tolerance, with hole mobility increasing from 8.3 × 10⁻³ cm² V^-1 s^-1 (as-deposited) to 1.2 × 10^-2 cm² V^-1 s^-1 (after annealing at 100 °C) - two orders of magnitude larger than pure P3HT. The improved charge transport and extraction provided by the composite HTL provides a significant efficiency improvement compared to cells with a pure P3HT HTL. As a result, we report carbon-based solar cells with a record efficiency of 17.8% (certified by Newport); and the first perovskite cells to be certified under the stabilized testing protocol. The outstanding device stability is demonstrated by only 3% drop after storage in ambient conditions (humidity: ca. 50%) for 1680 h (non-encapsulated), and retention of ca. 89% of their original output under continuous 1-Sun illumination at room-temperature for 600 h (encapsulated) in a nitrogen environment.     

Organic light-emitting diodes with 2-(4-biphenylyl)-5(4-tert-butyl-phenyl)-1,3,4-oxadiazole layer inserted between hole-injecting and hole-transporting layers

Organic light-emitting diodes (OLEDs) were fabricated based on copper phthalocyanine (CuPc) (hole-injecting layer), N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) (hole-transporting layer) and tris(8-hydroxyquinoline) aluminum (Alq₃) (emission and electron-transporting layer). A 2-(4-biphenylyl)-5(4-tert-butyl-phenyl)-1,3,4-oxadiazole (PBD) layer was inserted between CuPc and NPB. The effect of different thickness of PBD layer on the performance of the devices was investigated. The device structure was ITO/CuPc/PBD/NPB/Alq₃/LiF/Al. Optimized PBD thickness was about 1 nm and the electroluminescent (EL) efficiency of the device with 1 nm PBD layer was about 48 percent improvement compared to the device without PBD layer. The inserted PBD layer improved charge carriers balance in the active layer, which resulted in an improved EL efficiency. The performance of devices was also affected by varying the thickness of NPB due to microcavity effect and surface-plasmon loss.