Color-saturated and angle-stable blue top-emitting organic light-emitting diodes based on semitransparent bilayer cathode: Theory and experiment

Based on a modified electromagnetic theory, a bilayer metal cathode consisting of an electron injection layer and a silver (Ag) layer is designed to improve the color chromaticity in blue top-emitting organic light-emitting diodes (TEOLEDs). The effects of the complex refractive index of the electron injection material on the reflectivity and transmittivity of the bilayer cathode are investigated in detail, and then samarium (Sm) is selected as the electron injection material due to its proper refractive index of ～1.22 + 1.12i and work function of ～2.7 eV. Then, the emission peak wavelength, the full width at half maximum, and the Commission International de L’Eclairage coordinates of the blue TEOLEDs with different Sm/Ag bilayer cathodes are calculated and discussed. According to the theoretical results, a blue TEOLED with the optimized bilayer cathode of Sm (15 nm)/Ag (5 nm) is fabricated. The measurement results indicate that the blue TEOLED possesses an excellent chromaticity which is even better than that of a bottom-emitting organic light-emitting diode. Besides, the excellent angle stability is observed in the blue TEOLED even with a large viewing angle change of 0–75°.     

Dual enhancing properties of LiF with varying positions inside organic light-emitting devices

A multilayer organic light-emitting device (OLED) has been fabricated with a thin (0.3 nm) lithium fluoride (LiF) layer inserted inside an electron transport layer (ETL), aluminum tris(8-hydroxyquinoline) (Alq₃). The LiF electron injection layer (EIL) has not been used at an Al/Alq₃ interface in the device on purpose to observe properties of LiF. The electron injection-limited OLED with the LiF layer inside 50 nm Alq₃ at a one forth, a half or a three forth position assures two different enhancing properties of LiF. When the LiF layer is positioned closer to the Al cathode, the injection-limited OLED shows enhanced injection by Al interdiffusion. The Al interdiffusion at least up to 12.5 nm inside Alq₃ rules out the possible insulating buffer model in a small molecule bottom-emission (BE) OLED with a thin, less than one nanometer, electron injection layer (EIL). If the position is further away from the Al cathode, the Al diffusion reaches the LiF layer no longer and the device shows the electroluminescence (EL) enhancement without an enhanced injection. The suggested mechanism of LiF EL efficiency enhancer is that the thin LiF layer induces carrier trap sites and the trapped charges alters the distribution of the field inside the OLED and, consequently, gives a better recombination of the device. By substituting the Alq₃ ETL region with copper phthalocyanine (CuPc), all of the electron injection from the cathode of Al/CuPc interface, the induced recombination at the Alq₃ emitting layer (EML) by the LiF EL efficiency enhancer, and the operating voltage reduction from high conductive CuPc can be achieved. The enhanced property reaches 100 mA/cm² of current density and 1000 cd/m² of luminance at 5 V with its turn-on slightly larger than 2 V. The enhanced device is as good as our previously reported non-injection limited LiF EIL device [Yeonjin Yi, Seong Jun Kang, Kwanghee Cho, Jong Mo Koo, Kyul Han, Kyongjin Park, Myungkeun Noh, Chung Nam Whang, Kwangho Jeong, Appl. Phys. Lett. 86 (2005) 213502].     

Alcohol-soluble polyfluorenes containing dibenzothiophene-S,S-dioxide segments for cathode interfacial layer in PLEDs and PSCs

A series of alcohol-soluble amino-functionalized polyfluorene derivatives (PF-N-S, PF-N-SC8 and PF-N-SOC8) comprising various ratios of dibenzothiophene-S,S-dioxide segments (S/SC8/SOC8) in the main chains, respectively, were synthesized and utilized as cathode interfacial layer (CIL) in polymer light-emitting diodes (PLEDs) and polymer solar cells (PSCs) with high-work-function Al (or Au) electrode. The polymers possess LUMO/HOMO levels at −2.78 to −3.53 eV/−5.69 to −6.32 eV. Multilayer PLEDs and PSCs with device configurations of ITO/PEDOT:PSS (40 nm)/P-PPV or PFO-DBT35:PCBM = 1:2 (80 nm)/CIL (3–15 nm)/Al (or Au) (100 nm) were fabricated. The PF-N-S-10/Al (or Au) cathode PLEDs displayed maximum luminous efficiency of 24.4 cd A⁻¹ (or 11.9 cd A⁻¹), significantly higher than bare Al (or Au) cathode device, exceeding well-known Ba/Al and poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN)/Al (or PFN/Au) cathode devices. The enhanced open-circuit voltages (V_ocs), electron reflux and reduced work functions clarify that the electron injection barrier from the Al (or Au) electrode can be lowered by inserting the polymers as CIL. The resulted PSCs also show device performances exceeding Al and PFN/Al cathode devices. The results indicate that PF-N-S, PF-N-SC8 and PF-N-SOC8 are excellent CIL materials for PLEDs and PSCs with high-work-function Al or Au electrode.     

Colored reflective organic light-emitting device without bias

Under white ambient illumination and without bias, a reflective organic light-emitting device (ROLED) comprising a microcavity cathode exhibited various colors for static information display applications by means of internal interference and absorption effects. The configuration of this microcavity cathode was a metal/organometallic/metal structure of Al (10 nm)/Ag (15 nm)/Ag nanoparticles doped inside tris(8-hydroxyquinolinato) aluminum (Alq₃) (x nm)/Al (100 nm) with excellent conductivity. The thickness of the Ag:Alq₃ played a crucial role in determining the reflection color; for example, varying it from 20, 40, 60, 80 and 100 nm yielded the colors light yellow, light orange, reddish purple, greenish blue, and light green, respectively. In the dark, this ROLED can be used to display information with an ultra-high contrast ratio by applying on a small bias, like conventional OLED displays. Hence, this ROLED is a highly promising candidate for applications in energy-saving electronic fixed-pattern signs, logos, indicators, and manual information displays.     

Enhancing the electron injection in polymer light-emitting diodes using a sodium stearate/aluminum bilayer cathode

In this contribution the molecule sodium stearate (NaSt) is used for the first time as electron injection layer in combination with the fluorescent polymer phenylene substituted poly (para-phenylenevinylene) (Ph-PPV) in organic light-emitting diodes (OLEDs). The fabricated devices show current efficiencies up to 8.4 cd/A, indicating that the employed NaSt/aluminum (Al) bilayer cathode has adequate electron injection capabilities in conjunction with Ph-PPV and, therefore, NaSt has the potential to become a non-toxic alternative to the widely-used alkali halide lithium fluoride (LiF).Numerical simulations of the device structure are performed which are in good agreement with the experiments. Additionally, it is shown that the NaSt/Al cathode of the presented device cannot be simply modeled by using a low work function contact, as it is commonly done for the LiF/Al cathode in simulations of multilayer devices. Instead, an alternative approach is introduced in which an insulator in combination with the Fowler–Nordheim tunneling and the direct tunneling model is chosen to describe the charge carrier injection through the NaSt layer.     

Synthesis and characterization of fluorene-based copolymers as electron-transporting materials for PLEDs

Owing to their low cost, easy processing, and the possibility of flexible fabrication, polymer light-emitting diodes (PLEDs) are emerging as an important class of materials. Despite promising characteristics, the relatively easy ionization of the well-known low-work-function cathodes such as Ca and Ba prevents the full usage of these materials. Herein, we report the syntheses of three alcohol-soluble conjugated polymers with different conjugation lengths and electron affinities as electron injection and transport materials for PLEDs: poly[9,9-bis(2-dihexylaminoethoxy)fluorene-co-tetrafluorobenzene] (PFOH-1), poly[9,9-bis(2-dihexylaminoethoxy)fluorene-co-thiophene] (PFOH-2), and poly[9,9-bis(2-dihexylaminoethoxy)fluorene-co-benzo-thiadiazole] (PFOH-3). For comparison, devices using Al, Ca, and Al cathodes were also fabricated. The device based on the Al cathode showed lower performance with a luminescence efficiency of 0.93 cd/A and a luminance of 248 cd/m²; that based on the low-work-function metal Ca as the cathode showed a near-threefold increase in luminescence efficiency at 2.51 cd/A and brightness at 856 cd/m² owing to greatly enhanced electron injection from the cathode; and the device employing the PFOH-3/Al cathode exhibited a luminescence efficiency of 2.35 cd/A and a brightness of 667 cd/m² at a current density of 35 mA/cm², which is comparable with the performance of the device with the Ca cathode.     

Electrical and thermal stability of Ag ohmic contacts for GaN-based flip-chip light-emitting diodes by using an AgAl alloy capping layer

We have investigated Ag(200 nm)/AgAl(100 nm) ohmic contacts to p-type GaN for near-UV (405 nm) flip-chip light-emitting diodes (LEDs). It is shown that the use of an AgAl alloy capping layer (with 8 at% Al) results in better electrical and optical properties as compared to single Ag contacts when annealed at 430 °C. For example, Ag/AgAl (8 at% Al) contacts give specific contact resistance of 4.6×10^–4 Ω cm² and reflectance of 90% at a wavelength of 405 nm. However, use of an AgAl (with 50 at% Al) layer is not effective. LEDs fabricated with the Ag/AgAl (8 at% Al) reflectors produce higher light output as compared with the ones with single Ag reflectors. Ohmic mechanisms of the Ag/AgAl (8 at% Al) contacts are described and discussed.     

Fluorene trimers with various 9,9′-substituents: The synthesis,characteristics, condensed state structures,and electroluminescence properties

The four fluorene-based trimers with various aromatic and alkyl substituents (T1–T4) are synthesized and characterized. These oligomers show the similar electronic absorption and emission characteristics (e.g., absorption peak at 351 nm, and highly efficient deep blue emission at 394 nm in solution), indicating that the major electronic properties of the core chromophore are essentially independent of the substituents. However, the condensed state structures and thermal properties of four trimers are found to be different from each other, from crystalline (full alkyl (T1) or full aromatic (T2) substituted trimers) to amorphous (mixed aromatic and alkyl (T4) substituted trimers). The effect of different condensed state structures on electroluminescence device properties is presented: The blue light-emitting devices with accordant structure of ITO/PEDOT:PSS/TCTA (40 nm)/trimers (40 nm)/BCP (10 nm)/Alq₃ (20 nm)/LiF/Al exhibit different EL efficiency (2.9% of T2, 1.8% of T3 and 2.7% of T4). Using amorphous T4, the white light-emitting device of ITO/TCTA (40 nm)/rubrene (0.1 nm)/T4 (8 nm)/Alq₃(52 nm)/LiF/Al is fabricated with high efficiency (6.15 cd A⁻¹), high brightness (9500 cd m⁻²) and good white light CIE coordinates (0.32, 0.37).     

Bright and efficient white stacked organic light-emitting diodes

High brightness and efficient white stacked organic light-emitting diodes have been fabricated by connecting individual blue and red emissive units with the anode–cathode layer (ACL) consisting of LiF (1 nm)/Ca (25 nm)/Ag (15 nm). Use 1,3-bis(carbazol-9-yl)benzene (mCP):bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl) iridium III (FirPic) as the blue emitter and tris(8-hydroxy-quinolinato)aluminium (Alq₃):4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vinyl)-4H-pyran (DCJTB) as the red emitter, white light emission with CIE coordinates of (0.32, 0.38) was obtained at a driving voltage of 26 V with a luminance of 40,000 cd/m². By replacing the red fluorescent emitter with a phosphorescent one, the color coordinates were improved to (0.33, 0.31). The peak external quantum efficiency was enhanced from 5.3% (at 28.2 mA/cm²) to 10.5% (at 1.4 mA/cm²) as well.     

Top illuminated organic photodetectors with dielectric/metal/dielectric transparent anode

The top illuminated organic photodetectors (OPDs) with a Dielectric/Metal/Dielectric (DMD) transparent anode are fabricated. The transparent electrode is composed of molybdenum trioxide (MoO₃)/silver (Ag)/MoO₃ layers and zinc oxide (ZnO)/aluminum (Al) is used for bottom cathode. The optimized DMD electrode has an optical transmittance of 85.7% at the wavelength of 546 nm and sheet resistance of ∼6 Ω/sq. The fabricated OPDs exhibit a high detectivity and wide range linearity.     

New wide-bandgap organic donor and its application in UVB sensors with high responsivity

A triarylamine-containing fluorene derivative (FP) with wide bandgap and excellent thermal stability was synthesized and used as electron donor to construct planar heterojunction organic ultraviolet sensor (UVS), while bis(4-(4,6-diphenyl-1,3,5-triazine-2-yl)phenyl)diphenylsilane (NSN) was used as electron acceptor. The UVS ITO/PEDOT:PSS/FP/NSN/LiF/Al showed sensitive visible-blind response to UV illumination from both ITO and cathode sides. When no bias applied, the peak responsivity to UV light through ITO and cathode side was 47 and 33 mA/W, respectively. To the irradiation from the semitransparent Al side, the most sensitive response range covers the UVB region. Under a bias of ?4 V, the peak responsivity at 300 nm reaches 473 mA/W.     

Synthesis,characterization and high-efficiency blue electroluminescence based on coumarin derivatives of 7-diethylamino-coumarin-3-carboxamide

A novel tripodal compound, tris[2-(7-diethylamino-coumarin-3-carboxamide)ethyl]amine (tren-C), and a model compound, N-butyl-7-(diethylamino)-coumarin-3-carboxamide, were synthesized and characterized by elemental analysis, infrared and ¹H NMR spectra. The structure of the model compound was characterized by single crystal X-ray crystallography. The electroluminescence devices of ITO/2-TNATA (5 nm)/NPB (40 nm)/CBP: tren-C or model compound (wt%, 30 nm)/Bu-PBD (30 nm)/LiF (1 nm)/Al (100 nm) were fabricated and characterized. The EL spectra of the devices comprising vacuum vapour-deposited films using tren-C as a dopant are similar to the PL spectrum of tren-C in chloroform solutions. At the concentration of 0.5 wt% tren-C, a blue-emitting OLED with an emission peak at 464 nm, a maximum external quantum efficiency (EQE) of 1.39% and a maximum luminous efficiency of 2 cd/A at the current density of 20 mA/cm², and a maximum luminance of 1450 cd/m² at 12 V are achieved.     

Highly efficient two component phosphorescent organic light-emitting diodes based on direct hole injection into dopant and gradient doping

A series of two component phosphorescent organic light-emitting diodes (PHOLEDs) combing the direct hole injection into dopant strategy with a gradient doping profile were demonstrated. The dopant, host, as well as molybdenum oxide (MoO₃)-modified indium tin oxide (ITO) anode were investigated. It is found that the devices ITO/MoO₃ (0 or 1 nm)/fac-tris(2-phenylpyridine)iridium [Ir(ppy)₃]:1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi) (30 → 0 wt%, 105 nm)/LiF (1 nm)/Al (100 nm) show maximum external quantum efficiency (EQE) over 20%, which are comparable to multi-layered PHOLEDs. Moreover, the systematic variation of the host from TPBi to 4,7-diphenyl-1,10-phenanthroline (Bphen), dopant from Ir(ppy)₃ to bis(2-phenylpyridine)(acetylacetonate)iridium [Ir(ppy)₂(acac)], and anodes between ITO and ITO/MoO₃ indicates that balancing the charge as well as controlling the charge recombination zone play critical roles in the design of highly efficient two component PHOLEDs.     

Fluorene-based cathode interlayer polymers for high performance solution processed organic optoelectronic devices

A hydrophilic polyfluorene-based conjugated polyelectrolyte (CPE) Poly[9,9-bis(4′-(6″-(diethanolamino)hexyloxy) phenyl)fluorene], PPFN-OH (Scheme 1) has been synthesized and utilized as cathode interlayer for both polymer light emitting diodes (PLEDs) and solar cells (PSCs). For comparison, another CPE namely Poly[9,9-bis(6′-(diethanolamino)hexyl)fluorene] (PFN-OH) has also been investigated. They comprise the same polyfluorene backbone structures with, respectively, diethanolaminohexyl (PFN-OH) and diethanolaminohexoxyphenyl (PPFN-OH) substituents attached to the C9 carbon of the fluorene repeat unit. In comparison to reference devices with more reactive Ca/Al cathodes, utilizing these CPEs as interlayers allowed an Al cathode to be used for blue light emission PLEDs, yielding 51% and 92% enhancement of maximum luminous efficiency (LE) for PFN-OH and PPFN-OH, respectively. The PLEDs with PPFN-OH showed both higher maximum LE and maximum luminance (L) (LE = 2.53 cd/A at 6.2 V, L = 9917 cd/m² at 8.3 V) than devices with PFN-OH (2.00 cd/A at 4.1 V, 3237 cd/m² at 7.2 V). The PPFN-OH PLEDs also showed no significant roll-off in efficiency with increasing current density up to 400 mA/cm², indicating excellent electron injection ability and stability for this interlayer. The insertion of alkoxy-phenyl groups at the C9-position in PPFN-OH is clearly advantageous. This simple modification significantly improves the CPE cathode interlayer performance. Parallel investigations of the electron extraction properties of PPFN-OH in inverted architecture PSCs with PCDTBT:PC₇₀BM bulk heterojunction active layers demonstrated a power conversion efficiency enhancement of ∼19% (from 4.99% to 5.95%) for indium tin oxide cathode devices compared with reference devices using Ca/Al cathodes. These results confirm PPFN-OH to be a promising interlayer material for high performance solution processed organic optoelectronic devices.     

Bulk-like Al/Ag bilayer film due to suppression of surface plasmon resonance for high transparent organic light emitting diodes

We demonstrate the enhanced optical and electrical properties of an ultrathin silver (Ag) film by applying an aluminum (Al) seed layer between LiF and Ag as a transparent cathode for higher-transparency organic light-emitting diodes (OLEDs). Although the thickness ranges from 4 to 8 nm, the ultrathin Ag film is a continuous and uniform bulk-like film with an Al seed layer, which suppresses the surface plasmon absorption. Compared to an Ag-only cathode, the measured transmittance spectra were considerably increased, comparable with the theoretical calculations of a bulk Al/Ag bilayer film. The Al/Ag bilayer cathode has a transmittance of 87% at a 550 nm wavelength and a sheet resistance of 19.5 Ω/sq with a 4-nm-thick Ag layer. The transparent OLED devices that employed the Al/Ag cathode showed a transmittance of 72% at a 550 nm wavelength for an Ag thickness of 6 nm.     

Effects of thin film processing on pentacene/C60 bilayer solar cell performance

Solar cells based on pentacene/C₆₀ bilayer heterojunctions have been fabricated with a structure of ITO/poly(styrenesulfonate) (PEDOT:PSS)/pentacene (40 nm)/fullerene (C₆₀)(40 nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) (10 nm)/Al. The effect of pentacene crystalline domain size on performance was investigated by controlling the pentacene deposition rate. The devices show improved light-to-electricity conversion efficiencies from 0.49% to 1.12% under an AM 1.5 solar simulator (100 mW/cm²), when the pentacene evaporation rate is in a range of 5 Å/s–0.5 Å/s. Atomic force microscopy (AFM) measurements show that the pentacene films deposited by a slow evaporation rate have larger crystalline domains and a fewer amorphous domains, compared to films obtained by faster evaporation rates. Upon thermal annealing at 200 °C for 1 min, there is merging of pentacene crystalline domains. These changes in film morphology impact the charge separation at the donor/acceptor interface and the hole and electron mobilities, and hence, directly affect the device performance.     

Effect of Al:Ag alloy cathode on the performance of transparent organic light-emitting devices

Transparent organic light-emitting devices (TOLEDs) based on a stacked alloy cathode of LiF/Al:Ag are investigated. The devices have a structure of indium-tin-oxide (ITO)/4,4′,4′′-Tris[2-naphthyl(phenyl)amino]triphenylamine (2T-NATA) (25 nm)/N,N''-Di-[(1-naphthyl)-N,N''-diphenyl]-1,1''-biphenyl-4,4''-diamine (NPB) (40 nm)/tris-(8-hydroxyquinoline) aluminum (Alq3) (50 nm)/LiF (1 nm)/Al:Ag (1:3) (x), where the thicknesses of cathode metal layers (Al:Ag) are adjusted, respectively, from 70 nm to 100 nm. In the experiment, it is found that the LiF (1 nm)/Al:Ag (1:3) (75 nm) has good electron injection efficiency. Compared with an Al-only cathode, the turn-on voltage is lowered. At the voltage of 10 V, the luminances for bottom emission from ITO anode side and top emission from metal cathode side are 2 459 cd/m2 and 1 729 cd/m2, respectively. Thanks to electron injection enhancement by using Al:Ag cathode, we can obtain a better energy level matching between the cathode and the organic layer, thus the devices have lower turn-on voltage and higher luminance. The total transmittance of the devices can achieve about 40% at the wavelength of 550 nm.     

High performance top-emitting and transparent white organic light-emitting diodes based on Al/Cu/TcTa transparent electrodes for active matrix displays and lighting applications

It is challenging to obtain broadband emission covering as much of the visible light spectrum as possible in top-emitting white organic light-emitting diodes (TEWOLEDs) due to the well known microcavity effects. In this work, we achieved TEWOLED with three separate peak and negligible angular dependence by employing a high transmittance stack cathode Al (2 nm)/Cu (18)/TcTa (60 nm). The TEWOLED shows an efficiency of 25.6 cd/A, 20.1 Lm/W at 1000 cd/m², and low voltage of 4.2 V for 1222 cd/m². Synchronously, we achieved transparent white organic light-emitting diode (TWOLED) using this high transmittance stack cathode, the TWOLED exhibits similar spectrum and comparable luminance from both sides, and the maximum total efficiencies of the TWOLED are 28.6 cd/A, 24.9 Lm/W.     

Formation of low-resistance and transparent indium tin oxide ohmic contact for high-brightness GaN-based light-emitting diodes using a Sn–Ag interlayer

We report on the formation of low-resistance and highly transparent indium tin oxide (ITO) ohmic contacts to p-GaN using a Sn–Ag alloy interlayer. Although the as-deposited Sn–Ag(6 nm)/ITO(200 nm) contacts show non-ohmic behaviors, the scheme becomes ohmic with specific contact resistance of 4.72×10⁻⁴ Ω cm² and produce transmittance of ∼91% at wavelength of 460 nm when annealed at 530 °C. Blue light-emitting diodes (LEDs) fabricated with the Sn–Ag/ITO contacts give forward-bias voltage of 3.31 V at injection current of 20 mA. LEDs with the Sn–Ag/ITO contacts show the improvement of the output power by 62% (at 20 mA) compared with LEDs with Ni/Au contacts.     

Fluorene-based Tröger’s base analogues: Potential electroluminescent materials

Two new Λ-shaped fluorene-based Tröger’s base (TB) analogues with aryl substitutions are successfully synthesized and their photophysical and electroluminescent properties are examined in detail. Both compounds exhibit strong fluorescence emission in dilute solutions and aggregated states. Some abnormal photophysical behaviors have been observed; that is, the amorphous films of the two TB analogues show multiple blue–green emissions similar to the emissions of some polyfluorenes and oligofluorenes, while both the dilute solutions and the polycrystalline powders of two compounds show single blue–violet emission. Furthermore, the emissions of the amorphous film are obviously red-shifted in comparison with the polycrystalline powders. Organic light emitting diodes (OLEDs) using the two compounds as non-doped emitters with device structure of ITO/NPB (30 nm)/TBFB-BP or TBFB-FB (40 nm)/TPBI (40 nm)/LiF (1 nm)/Al (80 nm) were fabricated and high brightness (22047 cd/m² for TBFB-BP and 13434 cd/m² for TBFB-FB), high efficiency (2.78 cd/A, 1.82 lm/W for TBFB-BP and 2.76 cd/A, 1.93 lm/W for TBFB-FB) and low turn-on voltage (4.6 V for TBFB-BP and 4.5 V for TBFB-FB) were obtained. Our studies suggest that TB analogues could be excellent light emitting materials for OLED applications.