Top-emitting Organic Light-emitting Devices Research Articles

Bright flexible organic light-emitting devices on copy paper substrates

We have developed flexible top emission organic light-emitting devices (OLEDs) using copy paper substrates. Poly-2-chloro-p-xylylene and SiO2 buffer layers were coated on the copy paper for protecting the water absorption and improving the barrier and morphological characteristics. Sputtered Ni and thermally evaporated Ca/Ag films were used as anode and semitransparent cathode, respectively. The electrical conduction, luminance, and efficiency characteristics of the copy paper-based flexible OLEDs were investigated. The SiO2 buffer layer was critically important in obtaining the high performance devices. The device exhibited a high luminance of 2200 cd/m2 at a driving voltage of 13 V.

Surface potential behaviors of UV treated of Ag anode for high-performance T-OLED by nanotribology

A high reflective bottom anode is essential for high-performance top-emitting organic light emitting devices (OLEDs). Ag has high reflectivity for visible light but its electronic properties are not ideal for anodes of OLEDs. Thus, we investigated the UV treatment effect of Ag film with a thin silver oxide layer at the surface for application of top-emitting OLEDs. In this study, we measured the change of potential difference between Ag film and ITO (indium tin oxide) films, according to UV treatment time and the mechanical properties by KPFM (Kelvin Probe Force Microscope) method and nano-indenter system, respectively. The KPFM and nano-indenter results show that the differences of surface potential and surface volume change according to UV treatment time.

Nanograting transfer for light extraction in organic light-emitting devices

Damage-free transfer of nanogratings as waveguide mode extraction elements onto the top of top-emission organic light-emitting devices (OLEDs) was realized with the assistance of cyclic olefin copolymer molds. Photoluminescence and electroluminescence measurements together with transfer matrix calculations were used to identify the extracted waveguide modes. The presented one-step, wet-free, and etchless nanotransfer approach can avoid damage on the top-emission OLED structure and does not adversely affect the OLED performance. It offers the possibility for the independent optimization of OLED fabrication and nanofabrication, and has potential applications in organic optoelectronic devices, especially top-emission OLEDs.

Organic light emitting diodes with p-Si anodes and semitransparent Ce/Au cathodes

The Ce (x nm)/Au (15 nm) stacked layers were used as semitransparent cathodes in the top-emission organic light emitting devices (TOLEDs) fabricated on a p-type silicon anodes and substrate, where x varies from 4 to 16. The consequence of the Ce layer thickness on transmittance and the device performance were studied when the organic layers NPB (60 nm)/ALQ (60 nm) were kept unchanged, where NPB was N, N′-bis-(1-naphthl)-diphenyl-1, 1′-biphenyl-4, 4′-diamine, and AlQ is tris-(8-hydroxyquinoline) aluminum. The cathode of Ce (11 nm)/Au (15 nm) has a transparency of 46%, and the TOLED with it achieves the highest luminescence efficiencies: a current efficiency of 0.91 cd/A at 13.7 V and a peak power efficiency of 0.28 lm/W at 9 V. The turn-on voltage is 3.0 V. The Ce/Au cathode is both chemically and electrically stable.

Optical simulation and optimization of ITO-free top-emitting white organic light-emitting devices for lighting or display

We present a simulation of the optical properties of ITO-free top-emitting white organic light-emitting devices (TEWOLEDs). Metals (Al, Ag, Mo, Cu, Au, Sm) are used as anode or cathode depending on their work functions. Besides, devices with 1D metallic–dielectric photonic crystal anode are also demonstrated. The influence of the capping layer or the metallic–dielectric multilayer structure on the optical properties (such as efficiency, angular emission characteristics, color gamut and color rendering index) of the devices has been investigated. The results indicated that TEWOLEDs can be used as lighting with high color rendering index or as a display with wide color gamut by carefully designing the structure of the devices.

Blue top-emitting organic light-emitting devices using Alq3 as phase shift adjustment layer

Blue top-emitting organic light-emitting devices (TEOLEDs) are demonstrated by employing Alq3 as phase shift adjustment layer (PSAL) to increase the phase shift on reflection of the top electrode within a range, which also improves the light out-coupling. By adjusting the thickness of PSAL, the CIEx,y of devices, which utilize 2, 7-Di-pyrenyl-9, 9-spiro-bifluorene (DPSF) as emitting layer, changes from (0.16, 0.50) to (0.18, 0.37). The maximum current efficiency of 7.1 cd/A is acquired under 4.5 V with an increasing luminance of 139 cd/m2. Compared with adjusting the total thickness of organic layer, it is more beneficial for achieving blue TEOLEDs with high efficiency.

Simulation of optical performance of red top emitting organic light emitting devices based on phase-shift on reflection theory

In this paper, we study the phase shift of the reflection from a semitransparent electrode of a top-emitting organic light-emitting device (TOLED) which is able to adjust spectrum of TOLED. Based on the micro-cavity theory and the transfer matrix theory, a model is set up to simulate the red TOLED. The simulation results show that the modulation of the device is not only limited to organic layer thickness, but also related to the reflection phase shift. The phase shift of the reflection of top electrode is adjusted through changing the thickness of organic layer capped on the top electrode, thus the TOLED optical performance can be changed. This result provides a useful method of improving the performance of the device.

Transparent oxide/metal/oxide trilayer electrode for use in top-emitting organic light-emitting diodes

The most commonly used transparent electrode, indium-tin oxide (ITO), is costly and requires methods of deposition that are highly destructive to organic materials when it is deposited on top of the organic layers in top-emitting organic light-emitting devices (OLEDs). Here we have employed a trilayer electrode structure consisting of a thin layer of metal sandwiched between two MoO3 layers, which can be deposited through vacuum thermal evaporation without much damage to the organic active layers. Such MoO3/Au/MoO3 trilayer electrodes have a maximum transmittance of nearly 90% at 600 nm and a sheet resistance of <10 ohms per square (Ω/sq) with a 10-nm thick Au intermediate layer. Using these trilayers as the top transparent anode, we have fabricated top-emitting OLEDs based on either a fluorescent or phosphorescent emitter, and observed nearly identical emission spectra and similar external quantum efficiencies as compared to the more conventional bottom-emitting OLEDs based on the commercial ITO anode. The power efficiency of the top-emitting devices is 20% to 30% lower than the bottom-emitting devices due to the somewhat inferior charge injection in the top-emitting devices. The performance and emission characteristics of these devices indicate that this trilayer structure is a promising candidate as a transparent anode in top-emitting OLEDs.

Blue top-emitting organic light-emitting devices based on wide-angle interference enhancement and suppression of multiple-beam interference

Blue top-emitting organic light-emitting devices (TEOLEDs) based on blue phosphor iridium (III) bis[(4,6-difluorophenyl)-pyridinato-N,C 2′] picolinate (FIrpic) are demonstrated. Instead of using microcavity effects to obtain the blue top emission in conventional reports, the blue emission with a good chromaticity is achieved in this paper through the suppression of the multiple-beam interference by introducing a 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline light outcoupling layer onto the semitransparent Sm/Ag cathode. The electroluminescent performances of the blue emission is further improved based on the effective carrier transport and the consideration of the wide-angle interference at the emission peak of FIrpic with a maximum luminance of 8029 cd/m 2 and a luminous efficiency of 4.02 cd/A achieved at 14 V and 10 V, respectively. With a polarizer on the top cathode, a high pixel contrast ratio of 113:1 is realized under an ambient illumination of 140 lx and a pixel brightness of 1000 cd/m 2.

Improved performance of mixed single layer top-emission organic light emitting devices using capping layer

The performance of top-emission organic light emitting devices (TEOLEDs) can be improved by using a thin capping layer on top of the semitransparent metal electrode. We investigated the emission properties of inverted mixed single layer TEOLEDs with the same device structure but different capping materials. The thickness of capping layer was optimized by calculation. The power efficiency of device was 2.5times enhanced when 45nm TPD capping layer was added. The enhancement is not simply dependent on the transmittance and reflectance of the top contact, but also on other complex phenomena such as the interference effects in the device. The results of properties and dependence of EL spectra on viewing angle for all devices indicated that the large enhancement factor may be related to the complex interference phenomenon in our mixed single layer devices due to the emitter center and recombination region is different from conventional heterojunction devices.

Efficient top-emitting organic light-emitting devices using Fe 3O 4 modified Ag anode

Highly efficient top-emitting organic light-emitting devices (TOLEDs) using a Fe 3O 4 modified Ag anode have been demonstrated. The tris-(8-hydroxyquinoline) aluminum-based TOLEDs exhibit a very low turn-on voltage of 2.5 V and a high current efficiency of 8.1 cd/A. The improved properties for the TOLEDs is mainly due to the enhanced hole injection by introducing the anodic buffer. The mechanism of this enhanced hole injection is studied by the X-ray and ultra-violet photoemission spectroscopy, which demonstrated that the dipole layer is formed at the anode/organic interface and the hole-injection barrier is therefore reduced after introducing the thin Fe 3O 4 film between the Ag anode and the hole-transport layer.

Flexible top emission organic light-emitting devices using sputter-deposited Ni films on copy paper substrates

We have developed flexible top emission organic light-emitting devices by using sputtered Ni films on the parylene coated copy paper substrates. The Ni film grew through the coalescence of the Ni islands nucleated on the cellulose fiber patterns. The parylene, acting as a buffer layer for preventing water absorption, affected the morphological and electrical characteristics of the deposited Ni films. The flexible top emission diode device composed of Ni anode and Ca/Ag semitransparent cathode exhibited a luminance of ∼1 cd/m 2 at a driving voltage of 14.5 V.

Top-emitting organic light-emitting devices based on silicon substrate using Delta -doping technique

We have fabricated a Top-emitting organic light-emitting device on silicon substrate Delta -doping technique. Using ultrathin quinacridone as emitting layer, the performance of Top-emitting organic light-emitting device is improved obviously. However, when increasing the thickness of the anode, the performance of device is enhanced dramatically. The max power efficiency of device is 5.9 Lm/W at 5 V corresponding to the current efficiency of 9.3 cd/A. The max current efficiency of device is also increases to 11 cd/A at 7 V.

Investigation of emission location in top-emitting green organic light-emitting devices by optical analysis

A series of top-emitting organic light-emitting devices with different thicknesses of carrier transporting layers (N,N′-di(1-naphtyl)-N,N′-diphenylbenzidine, tris(8-hyroxyquinloine) aluminum (Alq 3)) and emitting layer (EML, 10-(2-Benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,11H-(1)-benzopyropyrano(6,7-8-i,j)quinolizin-11-one (C545T)-doped Alq 3) were fabricated. C545T-doped Alq 3 was found to bring about double recombination peaks in EML. As the distance between EML and reflective anode was increased, the outcoupling efficiency greatly deviated from optically-simulated values due to charge imbalance in EML and optical loss at the EML/Alq 3 interface. The device with 30 nm of EML exhibited maximized outcoupling efficiency and further increase of EML thickness brought about decrease in efficiency due to decrease in hole–electron recombination at the EML/Alq 3 interface.

Improved Performance of Top-Emission Organic Light Emitting Device With a Mixed Single Layer

Top-emission organic light emitting devices (TEOLED) with a mixed single layer by mixing of electron transport materials, hole transport material and dope material were investigated. With an optimized mixing ratio of PyPySPyPy:Alq3:α-NPD:Rubrene [25:50:25:1], maximum power efficiency, 3.45 lm/W, and maximum luminance, 7,000 cd/m2 was obtained. Compared to the conventional heterostructure device, the performance was improved with a factor of more than two. The dependence of organic layer thickness on device performance was also investigated and the result showed that 100 nm was an appropriate thickness for good performance of mixed single layer TEOLED. In addition, the maximum luminance and power efficiency were obtained at 20,000 cd/m2 and 7.5 lm/W, respectively, when TPD layer of 45 nm was capped onto top metal electrode.

Red top-emitting organic light-emitting device using 6, 13-di-(3, 5-diphenyl)phenylpentacene doped emitting system

Abstract Fluorescent red organic light-emitting devices (OLEDs) with narrow emission and negligible current-induced quenching by using a pentacene derivative of 6,13-di-(3,5-diphenyl)phenylpentacene (PDT) doped emitting system were demonstrated. With the incorporation of C 60 outcoupling layer, the PDT-based top-emitting OLED showed superior electroluminescence performances with Commission Internationale d’Eclairage color coordinates of (0.64, 0.36) and luminous efficiency of ∼3 cd/A. To clarify the actual energy transfer process in PDT-doped emitting system, the Forster’s radius was calculated. The results indicated that the energy transfer process is predominantly from the host of tris(8-hydroquinoline) aluminum to guest of PDT via the assistant dopant of 5,6,11,12-tetraphenylnaphthacene.

High efficiency rubrene based inverted top-emission organic light emitting devices with a mixed single layer

Inverted top-emission organic light emitting devices (TEOLEDs) with a mixed single layer by mixing of electron transport materials (PyPySPyPy and Alq 3), hole transport material (α-NPD) and dope material (rubrene) were investigated. Maximum power efficiency of 3.5 lm/W and maximum luminance of 7000 cd/m 2 were obtained by optimizing the mixing ratio of PyPySPyPy:Alq 3:α-NPD:rubrene=25:50:25:1. Luminance and power efficiency of mixed single layer device were two times improved compared to bi-layer heterojunction device and tri-layer heterojunction device. Lifetime test also shows that the mixed single layer device exhibits longer operational lifetimes of 343 h, which is three times longer than the 109 h for tri-layer device, and two times longer than the 158 h for bi-layer device. In addition, the maximum luminance and power efficiency were obtained at 20,000 cd/m 2 and 7.5 lm/W, respectively, when a TPD layer of 45 nm was capped onto the top metal electrode.

Improved performance of Si-based top-emitting organic light-emitting device using MoO x buffer layer

Highly efficient Si-based top-emitting organic light-emitting device (TOLED) using MoO x buffer layer is demonstrated. With tris(8-hydroquinoline) aluminum as emitting and electron-transport layer, the p-Si/MoO x based TOLED shows a maximum luminous efficiency of 1.1 cd/A and a power efficiency of 0.68 lm/W, which are almost double those (0.64 cd/A and 0.34 lm/W) of p-Si/SiO 2 based TOLED. Moreover, in comparison with the widely used thermally grown SiO 2 buffer layer, MoO x can be deposited by conventional evaporation technology and thereby simplifying fabrication process.

Realization of blue, green, and white inverted microcavity top-emitting organic light-emitting devices based on the same emitting layer

High efficiency blue, green, and white inverted microcavity top-emitting organic light-emitting devices based on blue emitter p-bis(p-N,N-diphenyl-amino-styryl) benzene-doped 2-methyl-9,10-di(2-naphthyl) anthracene as the emitting layer are demonstrated. The different colors can be well realized by simply changing the thickness of the hole-transporting layer, thus modifying the cavity length to obtain various resonance wavelengths. The comprehensive analysis on the emission mechanism is presented.

Top-emitting organic light-emitting device with high efficiency and low voltage using a silver–silver microcavity

Top-emitting organic light-emitting devices (TOLEDs) with high efficiency and low driving voltage using silver–silver microcavity structure were demonstrated. With tris(8-hydroquinoline) aluminum as emission layer, the Ag–Ag based TOLED showed a maximum luminous efficiency of 9.21 cd/A which was much higher than conventional Ag–Al/Ag based TOLED (4.21 cd/A) and corresponding bottom-emitting OLED (3.77 cd/A). The Ag–Ag based TOLED also exhibited low driving voltage and thereby an enhanced power efficiency of 4.25 lm/W (at 40 mA/cm 2). The excellent performance of Ag–Ag based TOLED was attributed to the significant microcavity effect and efficient carrier injection from Ag electrode.