Organic Electronics 2011 Volume 12 №03

Elsevier. — 151 p. — ISSN: 1566-1199.«Organic Electronics» is a journal whose primary interdisciplinary focus is on materials and phenomena related to organic devices such as light emitting diodes, thin film transistors, photovoltaic cells, sensors, memories, etc.Papers suitable for publication in this journal cover such topics as photoconductive and electronic properties of organic materials, thin film structures and characterization in the context of organic devices, charge and exciton transport, organic electronic and optoelectronic devices.«Organic Electronics» provides the forum for applied, fundamental and interdisciplinary contributions spanning the wide range of electronic properties and applications of organic materials. A Letters section is included for rapid publication of short articles announcing significant and highly original results.405-410
Yu Yang, Soichiro Omi, Ryo Goto, Masayuki Yahiro, Masanao Era, Hirofumi Watanabe, Yuji Oki. Wavelength sensitive photodiodes in the visible based on J-type aggregated films patterned by inkjet method
Abstract:
In this study, a kind of disposable and miniaturized organic photodiodes in the visible has been developed based on the J-type aggregated cyanine dyes films by the drop-on-demand inkjet method. Wavelength sensitivity has been found by measuring the open-circuit voltage (V_oc) of the organic photodiodes under the excitation of laser pulses at various wavelengths and high spectral resolution around the absorption peak of each cyanine dye J-aggregates can be obtained, where a sharp decrease on the Voc and a Voc in reverse direction are observed. The mechanism responsible for the sharp decrease and reverse Voc are discussed and the feasibility of using the laser output from a solid-state dye laser waveguide as the excitation source for the organic photodiodes which may be possibly integrated together onto the flow-cytometry chips in the future are also demonstrated.411-418
Chanwoo Yang, Youngjin Kwack, Se Hyun Kim, Tae Kyu An, Kipyo Hong, Sooji Nama, Mijeong Park, Woon-Seop Choi, Chan Eon Park. Ambipolar thin-film transistors and an inverter based on pentacene/self-assembled monolayer modified ZnO hybrid structures for balanced hole and electron mobilities
Abstract:
We have fabricated high performance ambipolar thin-film transistors (TFTs) and an inverter based on organic–inorganic bilayer structures composed of an upper pentacene layer and a lower atomic-layer-deposited zinc oxide (ZnO) layer. The insertion of a dodecanoic acid (DA) self-assembled monolayer (SAM) into the interface between pentacene and ZnO results in an improvement in the morphology of the pentacene layer and in well-balanced ambipolarity with hole and electron mobilities of 0,34 and 0,38 cm²V^-1s^-1, respectively. The ambipolar TFTs with DA-treated ZnO exhibit a hole to electron mobility ratio of approximately 0.90, which is higher by a factor of ~2,8 than that of ambipolar TFTs with untreated ZnO.We also tested the introduction of a perfluorooctyltriethoxysilane (PFOTES) SAM; the effects of the permanent dipole fields of the SAMs on the electrical and ambipolar characteristics of the hybrid TFTs were investigated.419-423
A.C. Hübler, G.C. Schmidt, H. Kempa, K. Reuter, M. Hambsch, M. Bellmann. Three-dimensional integrated circuit using printed electronics
Abstract:
A fully printed ring oscillator circuit is reported, which has not only horizontally arranged parts, but inverter stages stacked vertically on top of each other in four through-connected substrate layers. For the preparation of the inverters exclusively gravure and flexographic printing were used. Characteristics of the stacked ring oscillator are similar to planar circuits. A very interesting path of development for printed electronics is opened with this approach, because three-dimensional circuits become possible.424-428
Insun Park, Younhee Lim, Sangtaik Noh, Donggu Lee, Michael Meister, Jason J. Amsden, Frédéric Laquai, Changhee Lee, Do Y. Yoon. Enhanced photovoltaic performance of ZnO nanoparticle/poly(phenylene vinylene) hybrid photovoltaic cells by semiconducting surfactant
Abstract:
Hybrid films of ZnO nanoparticles and poly[2-methoxy-5-(3',7'-dimethyloctyloxyl)-1,4-phenylene vinylene] (MDMO-PPV) were investigated as a model hybrid bulk heterojunction (HBHJ) photovoltaic cell which combines the simple processability and excellent electrical characteristics of inorganic nanoparticle acceptors and conjugated polymer donors. Improved photovoltaic performance was observed when the ZnO nanoparticles were stabilized with a new semiconducting surfactant, 2-(2-ethylhexyl)-1,3-dioxo-2,3-dihydro-1H-benzo[de]isoquinoline-6,7-dicarboxylic acid (BQ). We found that using the BQ surfactant increased the power conversion efficiency (1,23%) while a typical insulating surfactant, oleic acid (OA), decreased the efficiency (0,65%) despite the improvement of the ZnO dispersion, as compared to the HBHJ films prepared without a surfactant (0,75%). The underlying mechanism was delineated by hybrid-morphology study, photoluminescence spectroscopy, and photo-induced absorption spectroscopy.429-434
Guozheng Nie, Junbiao Peng, Linfeng Lan, Ruixia Xu, Jianhua Zou, Yong Cao. Tuning on threshold voltage of organic field-effect transistor with a copper oxide layer
Abstract:
Organic field-effect transistors (OFETs) based on the pentacene semiconductor with an embedded thin layer of copper oxide (CuO) were investigated. The drain current of OFETs with a thin CuO layer embedded in pentacene increases more than three times compared to that of traditional OFETs without the CuO layer, and the threshold voltage shifts from -17,5 V to -7,9 V. A possible mechanism for OFETs with the CuO layer was discussed via analysis of electron transfer near the contact between CuO and pentacene. OFETs with the CuO layer exhibit an interesting result of a systematic tuning of threshold voltage by controlling the initial voltage of gate-to-source upon scanning transfer curves.435-441
Steffen Pfuetzner, Christine Mickel, Jens Jankowski, Moritz Hein, Jan Meiss, Christoph Schuenemann, Chris Elschner, Alexandr A. Levin, Bernd Rellinghaus, Karl Leo, Moritz Riede. The influence of substrate heating on morphology and layer growth in C₆₀:ZnPc bulk heterojunction solar cells
Abstract:
The change of morphology in mixed layers due to different substrate temperature Tsub of organic solar cells containing C₆₀ and zinc phthalocyanine (ZnPc) is studied. Heating the substrate during deposition of the bulk heterojunction C₆₀:ZnPc leads to a significant improvement of solar cell performance, mainly due to an increase in photocurrent and fill factor (FF). This is attributed to improved charge carrier percolation pathways within the C₆₀:ZnPc blend. Using atomic force microscopy, scanning electron microscopy, transmission electron microscopy, organic field effect transistor, X-ray diffraction, and absorption measurements, we observe aggregation, cluster-like, and polycrystalline growth of the heated bulk layer. This provides better transport percolation paths by inducing a phase separation of the molecules. Heated blend layer with thickness of 60 nm shows high performance without loss in FF. When heating the substrate to the optimum temperature of 110 °C, a power conversion efficiency of 3,0% is achieved, compared to 1,4% for an identical device prepared on a substrate held at room temperature.442-446
Thorsten Wagner, Daniel Roman Fritz, Peter Zeppenfeld Standing and flat lying α-6T molecules probed by imaging photoelectron spectroscopy
Abstract:
Photoelectron-emission microscopy (PEEM) is a very versatile tool for studying dynamic processes like growth and phase transitions on surfaces. But besides the real time characterization of the morphology it can provide local spectroscopic information on a sub-lm scale. To this end we have combined a conventional PEEM setup with a He-discharge lamp delivering high energy photons for electron excitation. The setup thus allows probing the electronic structure of individual crystallites of α-sexithiophene on Ag(110). Upon growth of this organic semiconductor on the surface held at 370 K, needles consisting of flat lying molecules and platelets containing near upright standing molecules are formed. Both types of crystallites can not only be distinguished by their shape but also by their photoelectron emission spectra. We also identify a wetting layer consisting of flat lying molecules which have essentially the same spectroscopic signature as the molecules in the needles.447-452
D. Simeone, M. Rapisarda, G. Fortunato, A. Valletta, L. Mariucci. Influence of structural properties on environmental stability of pentacene thin film transistors
Abstract:
We studied environmental stability of top contact pentacene TFTs with active layer evaporated at different growth rates. We measured the transfer characteristics in vacuum and in air and after storing the devices in oxygen for several days. Different pentacene growth rates result in different grain size of active layer. This morphology difference influences the hysteresis of transfer characteristics induced by water absorption. On the contrary, aging effects on the transfer characteristics of pentacene O-TFTs, induced by oxygen diffusion into the active layer, are not related to structural characteristics of pentacene film.453-460
Ting Lei, Hai-Bo Chen, Jie Yin, Shan Huang, Xing Zhu, Jian Pei. Microwires and microtwists from X-shaped conjugated molecules as low-loss, long distance photo waveguide materials
Abstract:
Single crystalline microwires and polycrystalline microtwists were self-assembled from X-shaped conjugated molecules through solution process in high yield. The excellent longrange optical waveguide ability and efficient light transfer between wires made the as-prepared microwires potential building blocks for complex optical devices at micrometer scale. By slight structural change, well-dispersed polycrystalline microtwists were also successfully obtained. We further demonstrated that such chiral structures could also be used as optical waveguiding materials. The investigation of photo waveguide property in the well-dispersed single microtwist is the first demonstration of chiral microstructures in the application of optical waveguide, which opens the possibility of coupling the chirality of material to that of light.461-471
Zhuoying Chen, Junfeng Fang, Feng Gao, Thomas J.K. Brenner, Kulbinder K. Banger, Xingzhu Wang, Wilhelm T.S. Huck, Henning Sirringhaus. Enhanced charge transport by incorporating additional thiophene units in the poly(fluorene-thienyl-benzothiadiazole) polymer
Abstract:
We report a comparative study of optical properties, structure and morphology, field-effect transistor (FET) and solar cell performance between poly(4-(3,4'-dihexyl-2,2'-bithiophen-5-yl)-7-(5'-(9,9-dioctyl-9H-fluoren-2-yl)-3,4'-dihexyl-2,2'-bithiophen-5-yl)benzo[c][1,2,5]-thiadiazole) (F8TTBTT), and its predecessor poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2',2''-diyl) (F8TBT). Compared to F8TBT, F8TTBTT has two more thiophene units incorporated in its monomer structure. Such a modification leads to a reduced optical band gap, improved charge injection and significantly enhanced ambipolar field-effect mobilities reaching 5 x 10^-2 cm² V^-1 s^-1 for holes and 4 x 10^-3 cm² V^-1 s^-1 for electrons. The enhanced carrier mobilities are most likely a result of an increased backbone planarization and interchain interaction. As a consequence of ambipolar transport, light-emission was observed from the transistor channel during operation. The reduced band gap and improved charge transport make F8TTBTT an interesting candidate also for solar cell applications. Unoptimized solar cells based on F8TTBTT:PCBM blends were found to exhibit power conversion efficiency under AM 1,5 illumination of ~1,54%.472-476
Sang-Hyun Eom, Edward Wrzesniewski, Jiangeng Xue. Close-packed hemispherical microlens arrays for light extraction enhancement in organic light-emitting devices
Abstract:
Close-packed hemispherical microlens arrays have been fabricated using a soft lithography method and used to enhance the light extraction efficiency in organic light-emitting devices (OLEDs). A close-packed monolayer of polystyrene microspheres was used to produce a soft template of polydimethylsiloxane for fabricating microlens arrays from a photopolymerizable optical adhesive. The microlens contact angle and array fill-factor increase with the polystyrene microsphere size, reaching (85° ± 5°) and (85 ± 3)% for 100 μm size microspheres, respectively. Attaching such a large-area (2 cm in diameter) microlens array on the glass substrate of an OLED leads to up to 70% efficiency enhancement, which also shows dependencies on the area and detailed structure of the OLED.477-485
Emanuele Orgiu, Simone Locci, Beatrice Fraboni, Erika Scavetta, Paolo Lugli, Annalisa Bonfiglio. Analysis of the hysteresis in organic thin-film transistors with polymeric gate dielectric
Abstract:
Controlling threshold voltage (VTH) and field-effect mobility (μ_FET) in organic thin-film transistors (OTFTs) is of primary importance to attain reliable devices that can be harnessed in more complicated circuits and eventually commercialized. In particular hysteresis in OTFT transfer curves is an issue that has to be better understood and analyzed. In this regard, even if the interface between organic dielectric and organic semiconductor seems to play an important role, our study shows that a further and relevant factor is played by the transport of charges across the bulk of the dielectric layer. Here, an analytical approach is applied to identify and understand the different components that give rise to the hysteresis in the transfer curves of pentacene-based OTFTs using poly(vinyl alcohol) (PVA) and poly(-vinyl alcohol) cross-linked with ammonium dichromate (PVAad) as the (polymeric) gate dielectric. Transfer curves simulations which include charge transport in the PVA show a hysteretic behavior in good agreement with the experimental data. Moreover, the hysteresis measured in OTFTs can be reduced by the insertion of an underlying dielectric layer of SiO₂ that blocks the above-mentioned effect. The residual contribution to the hysteresis is then analyzed by investigating the chemistry nature of both PVA and PVAad, which show different electron trapping efficiency at the interface with the pentacene layer. Therefore, a consistent explanation of how the polymeric dielectric is able to impact the transfer curves requires considering all mentioned factors.486-491
Marcus Lehnhardt, Thomas Riedl, Torsten Rabe, Wolfgang Kowalsky. Room temperature lifetime of triplet excitons in fluorescent host/guest systems
Abstract:
We studied the triplet decay dynamics in the fluorescent polymer–polymer host/guest system F8BT/MEH-PPV and in the small molecule host/guest system Alq3/DCM. The triplet lifetime was determined by a highly sensitive time-resolved measurement of the triplet state absorption via a pump and probe experiment in a waveguide structure. We were able to identify triplet–triplet annihilation as a major decay mechanism of triplet states in the pristine amorphous organic materials at room temperature (rate constant: k_TT ≈ 10^-12 cm³ s^-1). On the contrary, the host/guest systems showed a significantly increased triplet lifetime caused by a reduced triplet–triplet annihilation rate which we attribute to a substantially lowered mobility of the localized triplet excitons. Our results are expected to be of paramount importance for organic light emitting devices based on fluorescent emitters (e.g. organic lasers and organic light emitting diodes).492-496
Sung-Jin Kim, Yadong Zhang, Carlos Zuniga, Stephen Barlow, Seth R. Marder, Bernard Kippelen. Efficient green OLED devices with an emissive layer comprised of phosphor-doped carbazole/bis-oxadiazole side-chain polymer blends
Abstract:
High-efficiency organic light-emitting devices (OLEDs) were fabricated in which solutionprocessed ambipolar blends of hole- and electron-transport polymer hosts doped with a green-emitting iridium complex are sandwiched between a photocrosslinked hole-transporting layer and a vacuum-deposited electron-transporting layer. The ambipolar host blends consist of blends of bis-oxadiazole-functionalized poly(norbornene) electron-transport materials and poly(N-vinylcarbazole). For the best device examined, an external quantum efficiency of 13,6% and a maximum luminous efficiency of 44,6 cd/A at 1000 cd/m² with a turn-on voltage of 5,9 V were obtained.497-503
Mamatimin Abbas, Gulbeden Cakmak, Nalan Tekin, Ali Kara, Hasan Yuksel Guney, Elif Arici, Niyazi Serdar Sariciftci. Water soluble poly(1-vinyl-1,2,4-triazole) as novel dielectric layer for organic field effect transistors
Abstract:
Water soluble poly(1-vinyl-1,2,4-triazole) (PVT) as a novel dielectric layer for organic field effect transistor is studied. Dielectric spectroscopy characterization reveals it has low leakage current and rather high breakdown voltage. Both n-channel and p-channel organic field effect transistors are fabricated using pentacene and fullerene as active layers. Both devices show device performances with lack of hysteresis, very low threshold voltages and high on/off ratios. Excellent film formation property is utilized to make AlOx and thin PVT bilayer in order to decrease the operating voltage of the devices. All solution processed ambipolar device is fabricated with simple spin coating steps using poly(2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylenevinylene) (MEH–PPV) end capped with polyhedral oligomeric silsesquioxanes (POSS) as active layer. Our investigations show that PVT can be a very promising dielectric for organic field effect transistors.504-508
Sha Wu, Shaohu Han, Yina Zheng, Hua Zheng, Nanliu Liu, Lei Wang, Yong Cao, Jian Wang. pH-neutral PEDOT:PSS as hole injection layer in polymer light emitting diodes
Abstract:
In the application of polymer light emitting diodes (PLEDs), to prevent etching of ITO by the acidic poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), the pHneutral PEDOT:PSS was introduced as the hole injection layer (HIL). For the double layer PEDOT:PSS HIL, the pH-neutral PEDOT:PSS on top of ITO stops the indium migration into the polymer, while the regular acidic PEDOT:PSS beneath the light emitting layer keeps the efficient hole injection. The performance of the double layer HIL PLEDs is comparable to or even better than that of the single layer HIL light emitting device. To use pH-neutral PEDOT:PSS as a single layer HIL, the neutral PEDOT:PSS layer was treated by UV and oxygen plasma. Such treatments reduced the hole injection barrier between the HIL and the light emission layer to improve the hole injection, leading to a device performance better than that of the regular acidic PEDOT:PSS HIL device.509-515
Jer-Wei Chang, Po-Wei Liang, Min-Wei Lin, Tzung-Fang Guo, Ten-Chin Wen, Yao-Jane Hsu. An ambipolar to n-type transformation in pentacene-based organic field-effect transistors
Abstract:
We report an ambipolar to n-type transformation in pentacene-based organic field-effect transistors (OFETs) of aluminum (Al) source–drain electrodes. The hole currents of the ambipolar OFETs were decreased by over two orders of magnitude within 24 h after the device fabrication, but the electron currents remained unchanged, which makes the devices present n-type only characteristics. The transformation is correlated with the interfacial interactions between Al electrodes and pentacene, as characterized by analyzing near-edge X-ray absorption fine structure spectra and Micro-Raman spectra. This study highlights the importance of metal/organic interfaces in the performance of OFETs, and demonstrates decent n-type pentacene-based OFETs with electron mobility of 0,1 cm²/V/s with proper annealing treatment.516-519
Kipyo Hong, Se Hyun Kim, Chanwoo Yang, Tae Kyu An, Hyojung Cha, Chanjun Park, Chan Eon Park. Photopatternable, highly conductive and low work function polymer electrodes for high-performance n-type bottom contact organic transistors
Abstract:
We demonstrate the use of n-type N,N0-ditridecyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C13) bottom contact organic field-effect transistors that employ photopatternable highly conductive poly(3,4-ethylenedioxythiophene):tosylate (PEDOT:Tos) source/drain electrodes characterized by a very low work function (4,3 eV). Due to the low work function of this material, the electron injection barrier between PTCDI-C13 and PEDOT:Tos was 0,25 eV lower than that between PTCDI-C13 and gold. The low injection barrier reduced the contact resistance, yielding a high field effect mobility in transistors based on PEDOT:Tos (0,145 cm²/Vs); the field effect mobility was 16 times higher than that in transistors based on gold (0,009 cm²/Vs).520-528
Toshinori Matsushima, Guang-He Jin, Yoshihiro Kanai, Tomoyuki Yokota, Seiki Kitada, Toshiyuki Kishi, Hideyuki Murata. Interfacial charge transfer and charge generation in organic electronic devices
Abstract:
We have recently proposed that improvement of device performance using a buffer layer of molybdenum trioxide (MoO₃) originates from interfacial charge generation at an interface of MoO₃ and an organic hole-transport layer [17]. However, there is no clear experimental evidence enough to support the charge generation in our recent report. In this study, from comparison of current density–voltage characteristics of organic hole-only devices and ultraviolet/visible/near-infrared absorption spectra of composite films, we can conclude that the interfacial charge generation surly occurs to realize space-charge-limited currents of a wide variety of organic hole-transport layers. Moreover, a drastic increase in current density of a bilayer device of n-type C₆₀ and p-type N,N'-diphenyl-N,N'-bis(1-naphthyl)-1,1'-biphenyl-4,4'-diamine (α-NPD) by using a MoO₃ layer can provide the evidence of the charge generation.529-533
Jin-Young Kim, Min Jong Bae, Shang Hyeun Park, Taewon Jeong, Sunjin Song, Jeonghee Lee, Intaek Han, Ji Beom Yoo, Donggeun Jung, SeGi Yu. Electroluminescence enhancement of the phosphor dispersed in a polymer matrix using the tandem structure
Abstract:
An alternating current driven electroluminescence (EL) device consisting of a phosphor dispersed in a polymer matrix device was fabricated as a tandem structure. This structure can be formed by duplicating the emitting and conducting layers. The EL device with the tandem structure, particularly the device with the triple emitting layer, was six times brighter than (1460 cd/m² at an alternating voltage application of 50 V) the structure with a single emitting layer. This tandem structure plays important roles by increasing the chance of electron–hole tunneling into the phosphor with leading to an increase in brightness by combining the light emitted from several emitting layers in a device. In addition, the adoption of a spin-coating method for the deposition of an emitting layer can allow the production of a dielectric part on the lower-lying emitting phosphor powder particles. The implementation of a dielectric part and emitting part by one spin-coating procedure simplifies the overall fabrication process, particularly for the tandem structure. The significant increase in brightness with each additional emitting layer as well as a facile fabrication process would help to make the EL device a competitive candidate in future displays and lightings.534-540
Hong Ying Mao, Fabio Bussolotti, Dong-Chen Qi, Rui Wang, Satoshi Kera, Nobuo Ueno, Andrew Thye Shen Wee, Wei Chen. Mechanism of the Fermi level pinning at organic donor–acceptor heterojunction interfaces
Abstract:
We investigate the energy level alignment and the Fermi level pinning mechanism at organic donor–acceptor heterojunctions interfaces by using the model organic–organic heterojunctions (OOHs) with well-defined molecular orientation of the standing copper (II) phthalocyanine (CuPc) and zinc phthalocyanine (ZnPc) films on the standing copperhexadecafluoro-phthalocyanine (F₁₆CuPc) thin films on SiO₂. We identify two distinct regions for the energy level alignment by in situ ultraviolet photoelectron spectroscopy investigation. In region (I) where the work function (WF) of the underlying substrate is larger than the ionization potential (IP) of the top organic layers, the substrate Fermi level is pinned at the leading edge of the HOMO peak accompanied by a decreasing of the WF; in region (II) where the WF is smaller than the IP of the top organic layers, a downward shift of both the HOMO and vacuum level is observed. In connection with the defect induced gap states, we provide a detailed explanation for this thickness dependent energy level alignment and Fermi level pinning mechanism at the organic donor–acceptor OOH interface.541-546
S.L. Lai, Q.X. Tong, M.Y. Chan, T.W. Ng, M.F. Lo, C.C. Ko, S.T. Lee, C.S. Lee. Carbazole–pyrene derivatives for undoped organic light-emitting devices
Abstract:
Two carbazole–pyrene derivatives, namely 3,6-dipyrenyl-9-(4'-tert-butylphenyl) carbazole (BPyC) and 3,6-dipyrenyl-9-(4'-pyrenylphenyl) carbazole (TPyC), have been designed and synthesized for application in organic light-emitting devices (OLEDs). While the two compounds have similar chemical structures and photoluminescent properties, OLEDs based on them show distinct electroluminescence (EL) spectra. The BPyC-based devices show a single peak saturated blue emission with CIE coordinates of (0,15, 0,18); whereas the TPyC-based devices exhibit two emission peaks at blue and yellow hues with CIE coordinates of (0,22, 0,29). The difference in their EL spectra is attributed to the substitution of the t-butyl unit of BPyC with a pyrenyl group to form TPyC, which effectively increases the electron-donating property and results in exciplex formation at its interface with the electron-accepting TPBI. A high external quantum efficiency of 3,11% is achieved in the TPyC-based devices. Influences of chemical structure and fluorescent quantum yield on the efficiency of exciplex emission are discussed.547-555
Chih-Hung Hsiao, Yi-Hsin Lan, Pei-Yu Lee, Tien-Lung Chiu, Jiun-Haw Lee. White organic light-emitting devices with ultra-high color stability over wide luminance range
Abstract:
A white organic light-emitting device (WOLED) with ultra-high color stability was achieved by introducing an appropriate emitting layer (EML) structure with a spacer, and engineering a blue EML (B-EML) with a selectively doped profile. The advantage of the selectively doped profile over the conventional, uniformly doped profile was to minimize direct exciton formation on dopants with lower exciton energies to suppress electroluminescence (EL) spectrum variations. The recombination zone was found to be located at the spacer/B-EML interface, with a width of 4,5 nm. With the selectively doped profile, the WOLED exhibited ultra-high color stability, with the CIE coordinates shifting from (0,399, 0,483) to (0,395, 0,479) as the luminance increased from 145 to 12,100 cd/m² and from (0,401, 0,481) to (0,400, 0,479) as the luminance increased from 1240 to 4850 cd/m², the practical luminance range for display and lighting applications. In addition to the small CIE coordinates variation of (-0,004, -0,004) over the broad luminance range of about two orders of magnitude, we also achieved a high device efficiency of 34,1 cd/A, which stayed larger than 30 cd/A below 2000 cd/m².