Organic Electroluminescent Devices Research Articles

Realization of an organic semiconductor electroluminescent device with high directionality and color purity

AbstractWe are introducing an innovative organic semiconductor electroluminescent device capable of emitting light with unprecedented characteristics, that is, high directionality with divergence angle of 1.1° and high color purity with the full‐width at half‐maximum of 2.5 nm. This achievement was realized through design techniques aimed at enhancing gain via meticulous recombination zone control and complemented by the reduction of the optical losses. These results demonstrate the simultaneous achievement of a narrow spectrum and high directionality in an organic electroluminescent device.

Recent Achievements for Flexible Encapsulation Films Based on Atomic/Molecular Layer Deposition.

The purpose of this paper is to review the research progress in the realization of the organic-inorganic hybrid thin-film packaging of flexible organic electroluminescent devices using the PEALD (plasma-enhanced atomic layer deposition) and MLD (molecular layer deposition) techniques. Firstly, the importance and application prospect of organic electroluminescent devices in the field of flexible electronics are introduced. Subsequently, the principles, characteristics and applications of PEALD and MLD technologies in device packaging are described in detail. Then, the methods and process optimization strategies for the preparation of organic-inorganic hybrid thin-film encapsulation layers using PEALD and MLD technologies are reviewed. Further, the research results on the encapsulation effect, stability and reliability of organic-inorganic hybrid thin-film encapsulation layers in flexible organic electroluminescent devices are discussed. Finally, the current research progress is summarized, and the future research directions and development trends are prospected.

Highly efficient co-doped blue phosphorescent organic electroluminescent devices due to improved carriers balance and broadening recombination zone

High performance blue phosphorescent organic electroluminescent devices were realized by utilizing trivalent terbium complex and iridium complex as co-dopant and emitter, respectively. In this case, the co-doped trivalent terbium complex molecules within light-emitting layer function as deeper electron trappers due to its low-lying energy levels. Compared with reference device, co-doped devices displayed relatively higher electroluminescent performances due to improved carriers’ balance as well as broadening recombination zone. Finally, the optimal device with main emission peak at 462 nm obtained the maximum brightness, external quantum efficiency, current efficiency and power efficiency up to 24396 cd/m2, 26.3%, 46.93 cd/A and 50.82 lm/W, respectively. Meanwhile, even at the practical brightness of 1000 cd/m2 (4.3 V), prominent external quantum efficiency and current efficiency as high as 24.0% and 40.73 cd/A, respectively, can still be retained because the broadening recombination zone helps to decrease exciton density thus suppress exciton quenching.

Hole-transporting materials boost optoelectronic performance: Their first application in electrochromic device based on 1,1′-Bis(3-sulfopropyl) viologen

In this paper, we present the first application of organic hole-transporting semiconductors 4,4′,4′'-tris(carbazol-9-yl)triphenylamine (TCTA) and 4,4′-cyclohexylbis[N,N-bis(4-methylphenyl) aniline] (TAPC) in electrochromism area, demonstrating that TCTA or TAPC is suitable for use as hole-transporting materials on the anode of 1,1′-bis(3-sulfopropyl) viologen (SPV)-based electrochromic devices. For such devices, we obtain a decreased driving voltage of SPV-based electrochromic devices down to −2.0 V in the presence of TCTA or TAPC with comparison to that of ca. −2.5 V in the absence of the hole-transporting material. The optical contrast of the device increased from 57.19 % (without hole-transport layer) to 66.41 % (TCTA) or 62.97 % (TAPC) at 533 nm. The response time of 66.2 s in the absence of hole-transporting layer can decrease to 30.6 s with TCTA and 62.7 s with TAPC, respectively. The coloration efficiency of the as-fabricated devices at 533 nm can reach from 67.3 cm2/C (SPV-only) up to 153.2 cm2/C (TCTA) and 80.5 cm2/C (TAPC). This work paved a new way for boosting electrochromic performances of viologen-based devices via electrode modification of anode by hole-transporting material TCTA and TAPC which have been used effectively and efficiently in organic electroluminescent devices and solar cells.

(Invited) SAE-MEMS Technology for Electrostatic Vibrational Energy Harvesters

Energy harvesting [1] is expected to be a self-powering technology for small wireless sensor nodes in the next generation Internet-of-Things (IoT). In particular, small vibrational energy harvesters (VEHs) based on micro-electro-mechanical systems (MEMS) technology, which are battery-free and can generate electricity even at night or in the dark, are being investigated worldwide for next-generation power sources for small wireless IoT nodes [2,3]. Electrostatic MEMS VEHs, which use semi-permanent charge-holding dielectrics called "electrets," are useful for power generation using environmental vibrations because they have a higher output power density at lower frequencies than other methods [4]. Further miniaturization, higher performance, and higher productivity will be required to use electret-type MEMS VEHs as power sources for compact wireless IoT nodes and other devices. Although the integration of MEMS VEHs with power management circuits enables miniaturization, higher performance, and improved productivity, the monolithic integration of electret-type MEMS VEHs and electronic circuits is an unexplored area in conventional technology. Conventional electrostatic MEMS VEHs involve corona discharge, electron beam irradiation, X-ray irradiation, or high-temperature treatment as electret charge processing, which imposes restrictions on the design and manufacturing of MEMS structures and the integration of MEMS and electronic circuits. Recently, self-assembled electrets (SAEs), which are electrets that do not require any charging process, have been developed using materials for organic electroluminescent devices [5]. The surface charge density of SAEs is equivalent to those of previously reported electrets for VEHs, and thus SAE is expected to improve the performance of VEH and simplify the manufacturing process. However, there have been no reports of MEMS devices using SAEs.Our proposed method is the first technology to form SAEs inside MEMS structures using only a vacuum evaporation process at room temperature, and we have succeeded in actual vibration power generation [6]. This technology can be integrated into existing semiconductor processes because the electret formation requires only room-temperature deposition, making it possible to monolithically integrate electret-type MEMS VEHs and electronic circuits on the same substrate. In addition, since the SAE surface potential is proportional to the film thickness, the amount of electricity generated can be further increased by increasing the thickness of the SAE film. Therefore, this technology is expected to accelerate the miniaturization, performance, and productivity improvement of MEMS environmental vibration power generation devices. Akinaga, “Recent advances and future prospects in energy harvesting technologies,” Jpn. J. Appl. Phys. 59 (2020) 110201.D. Mitcheson, E. M. Yeatman, G. K. Rao, A. S. Holmes, and T. C. Green, “Energy Harvesting from Human and Machine Motion for Wireless Electronic Devices” Proc. IEEE 96 (2008) 1457–1486.Suzuki, “Recent progress in MEMS electret generator for energy harvesting,” IEEJ Trans. Elec. Electron. Eng. 6 (2011) 101-111.Toshiyoshi, S. Ju, H. Honma, C.-H. Ji, and H. Fujita, “MEMS vibrational energy harvesters,” Sci. Technol. Adv. Mater. 20 (2019) 124-143.Tanaka, N. Matsuura, and H. Ishii, “Self-Assembled Electret for Vibration-Based Power Generator,” Sci. Rep. 10 (2020) 6648.Yamane, H. Kayaguchi, K. Kawashima, H. Ishii, and Y. Tanaka, “MEMS post-processed self-assembled electret for vibratory energy harvesters,” Appl. Phys. Lett. 119 (2021) 254102.

Boosting the generality of catalytic systems by the synergetic ligand effect in Pd-catalyzed C-N cross-coupling

In the areas of catalysis and organic chemistry, the development of versatile and efficient catalytic systems has long been a challenge, primarily due to the intricate relationship between ligands and transition metal centers. This study addresses this challenge by exploring the concept of ligand synergy to enhance the generality of catalytic systems, a crucial metric for their practical utility. By combining N-heterocyclic carbene (NHC) and phosphine ligands, we unveil a novel catalytic system that exhibits high level of generality in the Buchwald-Hartwig cross-coupling reaction. Our findings not only demonstrate the enhanced efficiency of this system, leading to the synthesis of valuable compounds with applications in organic electroluminescent devices and the pharmaceutical industry, but also shed light on the broader potential of ligand synergy in catalysis. Through machine learning analysis, we uncover the critical role of specific ligand properties, further paving the way for rational catalyst design.

High efficiency double light-emitting layer sky blue phosphor organic light-emitting diode with slow emission efficiency attenuation

In this study, by doping the iridium (III) double ((4,6-difluorobenzene) -pyridine-N, C2’Pyridinformate (FIrpic) into 4,4′, 4″ three (carbazole) trioxilamine (TcTa) and 9 (4 tert-butyl-benzene) −3,6 double (triphenylsilyl) -9H carbazole (CzSi) layers as light emitting layers (EMLs) to achieve sky blue phosphor organic electroluminescence (EL) devices with very low efficiency roll-off. The dependence of the EL performance on the doping concentration of the light-emitting molecules and the different EML thicknesses are studied in detail. Experimental data show that the thin-film-doped TcTa layer is very important for the efficiency of the device. Finally, the optimal device achieves extremely high power efficiency, current efficiency, and external quantum efficiency (EQE), reaching 74.82 lm/W, 66.68 cd/A, and 31.9 %, respectively. At a certain luminance of 2000 cd/m2 (3.4 V), the same device retained power efficiency with current efficiency and external quantum efficiency of 57.71 lm/W, 62.46 cd/A and 29.3 %, respectively, based on theoretical discussion, the improved EL efficiency was attributed to the energy levels of the host and luminescent molecules, which helps to balance the charge carrier in the emitter molecule distribution and expand the recombination region.

Pure Polycyclic Aromatic Hydrocarbon Isomerides with Delayed Fluorescence and Anti-Kasha Emission: High-Efficiency Non-Doped Fluorescence OLEDs.

Pure polycyclic aromatic hydrocarbons (PAHs) consisting solely of carbon-hydrogen or carbon-carbon bonds offer great potential for constructing durable and cost-effective emitters in organic electroluminescence devices. However, achieving versatile fluorescence characteristics in pure PAHs remains a considerable challenge, particularly without the inclusion of heteroatoms. Herein, an efficient approach is presented that involves incorporating non-six-membered rings into classical pyrene isomerides, enabling simultaneous achievement of full-color emission, delayed fluorescence, and anti-Kasha emission. Theoretical calculations reveal that the intensity and distribution of aromaticity/anti-aromaticity in both ground and excited states play a crucial role in determining the excited levels and fluorescence yields. Transient fluorescence measurements confirm the existence of thermally activated delayed fluorescence in pure PAHs. By utilizing these PAHs as emitting layers, electroluminescent spectra covering the entire visible region along with a maximum external quantum efficiency of 9.1% can be achieved, leading to the most exceptional results among non-doped pure hydrocarbon-based devices.

High‐Performance Multi‐Resonance Thermally Activated Delayed Fluorescence Emitters for Narrowband Organic Light‐Emitting Diodes

AbstractMulti‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters have drawn considerable attention because of their remarkable optoelectronic properties of high emission efficiency and narrow emission profile, and represent an active subject of cutting‐edge research in the organic electroluminescence (EL). However, the slow reverse intersystem crossing (RISC) rate of MR‐TADF emitter caused by the large energy gap (ΔEST) and small spin‐orbit coupling (SOC) matrix elements between the singlet and triplet excited states limits their further development in organic EL devices. Currently, innovative molecular design strategies have been developed including heavy atom integration, π‐extended MR framework and metal perturbation, and so on to improve the RISC process of MR‐TADF emitters for high‐performance EL devices. Here, an overview is presented on the recent progress of MR‐TADF emitters with fast RISC rate ( > 10−5 s−1), with particular attention to the molecular design, optoelectronic properties, and device performance of organic light‐emitting diodes (OLEDs), which intends to systematize the knowledge in this subject for the thriving development of highly efficient MR‐TADF emitters. Finally, the challenges and future prospects of MR‐TADF materials are discussed comprehensively.

Synthetic Strategies for Accessing Dibenzophosphole Scaffolds

AbstractDibenzophospholes, phosphorus-containing π-conjugated cyclic compounds, have attracted considerable attention because of their potential applications in various functional materials such as those required for organic electroluminescent devices. Moreover, their synthetic methods have been widely developed. This review summarizes the construction strategies of dibenzophosphole scaffolds, including those developed recently.1 Introduction2 Construction of a Phosphole Skeleton Using Aryl Compounds2.1 [4+1] Cyclization between Biaryl Derivatives and P1 Units2.2 Intramolecular P–C Bond Formation of Biarylphosphines2.3 Intramolecular C–C Bond Formation of Diarylphosphines2.4 [3+2] Cyclization between Arylphosphines and Arenes3 Fused-Benzene Ring Formation4 Successive Phosphole Skeleton and Fused-Benzene Ring Formation5 Conclusions

MgF2 as an interlayer to enhance the stability of thermally activated delayed fluorescence based organic electroluminescence devices

Stability is one of the major challenges in organic semiconductor based optoelectronic devices. A comparative study of thermally activated delayed fluorescence (TADF) based organic light emitting diodes (OLEDs) with alkali-halide lithium fluoride (LiF) vs alkaline halide magnesium fluoride (MgF2) inorganic electron injection interlayers is presented. A TADF emitter 4CzIPN doped in CBP is used as an active layer (thickness = 15 nm @6wt. % doping) in an OLED structure: Glass/ITO/PEDOT:PSS/NPD/CBP/CBP:4CzIPN/TPBi/interlayer/Al. Prior to this comparative study, a separate exercise is carried out to obtain an optimal thickness of an MgF2 interlayer on the basis of leakage current and efficiency in the TADF-OLEDs. OLEDs with an LiF interlayer showed an external quantum efficiency (EQE) of 19.7% in comparison with an MgF2 interlayer-based OLED showed slightly lower average EQE ∼19.1% at a luminance level of 100 cd/m2; these efficiency numbers are averaged over ∼60 OLEDs. These slight changes in EQE are supported by the relative photoluminescence quantum yield measurements with a whole device stack. However, alkaline halide MgF2 based TADF-OLEDs showed approximately seven-fold enhancement in the stability (LT60) under identical operating conditions. In situ photoluminescence monitoring of operational TADF-OLEDs confirmed that the probable cause of reduced lifetime is degradation of an LiF/TPBi interface.

Efficient Red Light Emitting Diodes Based on a Zero‐Dimensional Organic Antimony Halide Hybrid (Adv. Mater. 9/2023)

Organic Metal Halide Hybrids Efficient electroluminescent devices can now be fabricated using solution-processed thin films based on 0D organic metal halide hybrids (OMHHs). In article number 2209417, Biwu Ma and co-workers develop a highly luminescent 0D OMHH containing organic semiconducting cations, which successfully addresses the issues of low conductivity and poor energy alignment of existing 0D OMHHs for electrically driven light-emitting diodes (LEDs).

Cyanopyridone-cored fluorophores with triphenylamine peripheries: From molecular design to OLED fabrication studies

• Seven novel cyanopyridone-based D-A-D type molecules with a fixed triphenylamine donor, were synthesized. • Theoretical, optical, solvatochromic, thermal, electrochemical, electrical, and EL studies were performed to evaluate structure-property relationships. • Doped OLEDs using novel fluorophores as dopants outperform non-doped devices. • OLEDs emit yellow light with maximum efficiencies of 10.72 cd/A, 7.87 lm/W, and 5.32%. Amongst many types of fluorescent organic materials, compounds with π-conjugation between an electron donor (D) and electron acceptor (A) have been intensively investigated as effective organic electroluminescence (EL) device components. Herein, we report the synthesis and structural characterization of a new series of D-A-D configured small molecule-based fluorophores ( TPA-CyP1-7 ) having triphenylamine ( TPA ) as a donor unit and 3-cyanopyridine-2-one ( CyP ) as an acceptor core with different architects for use as efficient yellow light emitters in fluorescent organic light-emitting diodes (OLED). The detailed photophysical, solvatochromic, thermal, electrochemical, and EL properties, including quantum chemical calculations, were systematically investigated to study their relation between structure and properties. All the fluorophores show high fluorescent quantum yields in the solid film state and display high thermal stability with decomposition temperatures above 350 o C. The study reveals that they possess appropriate HOMO and LUMO energies level for effective charge injection. Finally, these yellow luminogens were employed to fabricate new OLED devices as sole emitters and dopants with CBP host materials. Interestingly, the host-guest devices doped with CBP host emitters show a remarkable improvement in the overall device performance. Among them, the TPA-CyP3 -based doped device has achieved a maximum current efficiency ( η c ), high power efficiency ( η p ), and good external quantum efficiency ( η EQE ) of 10.72 cd/A, 7.87 lm/W, and 5.32%, respectively.

Triphenylamine‐Functionalized Multiple‐Resonance TADF Emitters with Accelerated Reverse Intersystem Crossing and Aggregation‐Induced Emission Enhancement for Narrowband OLEDs

AbstractMultiple‐resonance (MR) thermal activated delayed fluorescence (TADF) emitters have attracted increasing attention in organic electroluminescence devices, owing to their superior quantum efficiency and narrowband emission for high color purity. However, MR‐TADF materials often suffer from severe aggregation‐caused quenching (ACQ) and efficiency roll‐off problems due to their rigid planar structures and the lack of sufficient charge‐transfer (CT) characters with inefficient reverse intersystem crossing (RISC). Herein, by attaching electron‐rich triphenylamine (TPA) with twisted spatial conformation to the MR framework, two efficient narrowband MR‐TADF emitters, namely BNCz‐pTPA and BNCz‐mTPA, are developed. The TPA substituent endows the new emitters with aggregation‐induced emission enhancement (AIEE) for ACQ suppression. The unprecedented AIEE‐MR‐TADF emitters exhibit CT character in high‐lying triplet excited states for faster RISC, while the locally‐excited (LE) character of the first singlet excited state is retained for narrowband emission with high emission efficiency. An organic light‐emitting diode (OLED) based on BNCz‐pTPA exhibits a maximum external quantum efficiency of 27.3% with slow efficiency roll‐off, demonstrating much higher performances than those of the BNCz‐based OLED. This study may provide a simple but effective approach to constructing high‐performance emitters for wide‐color‐gamut OLED displays.

Exciton regulation mechanism of Alq<sub>3</sub>/HAT-CN tandem electroluminescent devices

Tandem organic electroluminescent devices (OLEDs) have attracted widespread attention due to their long lifetime and high current efficiency. In this study, a double-emitting unit tandem OLED is fabricated by using Alq<sub>3</sub>/HAT-CN as an interconnect layer. Its photovoltaic properties and exciton regulation mechanism are investigated. The results show that the luminance (11189.86 cd/m<sup>2</sup>) and efficiency (13.85 cd/A) of the tandem OLED reaches 2.7 times that of the single electroluminescent (EL) unit OLED (luminance and efficiency of 4007.14 cd/m<sup>2</sup> and 5.00 cd/A, respectively) at a current density of 80 mA/cm<sup>2</sup>. This proves that the Alq<sub>3</sub>/HAT-CN is an efficient interconnect layer. At room temperature, the polaron pair undergoes intersystem crossing (ISC) due to hyperfine interaction (HFI) when a magnetic field is applied to the device. This increases the concentration of the triplet excitons (T<sub>1</sub>), thus promoting the charge scattering. The result is a rapid increase in the low magnetic field and a slow increase in the high magnetic field of the MEL. When the injection current strength is constant, there is less uncompounded charge in the Alq<sub>3</sub>/HAT-CN device than in other connected layer devices. Triplet-charge annihilation (TQA) is weak, resulting in a relative increase in the value of T<sub>1</sub>, which is not involved in the TQA. This suppresses the ISC and leads to a minimal increase in the MEL. As the current strength increases, the T<sub>1</sub> value increases, causing TQA to increase and ISC to decrease. Since the TQA is related to charge and T<sub>1</sub> value, lowering the temperature reduces the carrier mobility in the device, resulting in the relative decreasing of charge concentration and the weakening of TQA. Lowering the temperature reduces the quenching of thermal phonons and increases the T<sub>1</sub> value while extending its lifetime, resulting in the enhancement of triplet-triplet annihilation (TTA). At low temperatures, the high magnetic field shape of the MEL changes from slowly increasing to rapidly decreasing. Therefore, the T<sub>1</sub> value can be regulated by varying the current strength and temperature, which further affects the strength of ISC, TQA and TTA, and the luminescence and efficiency of the device can be effectively improved by reducing TQA and ISC. This work is of great significance in understanding the luminescence mechanism of small molecule tandem devices and studying the mechanism for improving their photovoltaic properties.

Investigation on potential applications of thermal activated delayed fluorescence materials in organic electroluminescent devices

In the past few years, organic battery light emitting devices have attracted a lot of attention, showing great application potential. Fluorescence material, phosphorescence material and thermal activated delayed fluorescence (TADF) material, have attracted more and more attention. However, phosphorescent material is limited by the useage of a heavy metal, Ir. So people want to find a more efficient and environmental-friendly material:TADF. TADF polymer can be used as a new type of organic photoelectric functional materials due to its high exciton utilization. This work focuses on the working principle, advantages, applications of TADF materials and the imagination for future applications. OLED emission of different light colors is also worthy of attention, the differences and efficiency of the three colors have sparked discussion as well. On this basis, starting from the luminescence principle and device structure of TADF, the advantages and disadvantages of TADF materials are summarized. Finally, the solution to “aggregation-caused quenching” problem (ACQ) is found, so as to focus on the application of technology and envisage the future application direction.

High‐Performance Blue Electroluminescence Devices Based on Linear Gold(I) Complexes as Ultrafast Triplet Exciton Harvesters

AbstractAchieving large external quantum efficiencies, narrow bandwidths, and a long operational lifetime at high brightness remains the largest hurdle to developing organic blue‐emitting devices. Here, a material strategy is demonstrated that can meet these conditions. The strategy is based on linear heteroleptic Au(I) complex exciton harvesters and multiresonance thermally activated delayed fluorescence (MR‐TADF) emitters. The organic electroluminescence devices produce blue emission with Commission Internationale de l'Eclairage chromaticity coordinates of (0.108, 0.160), a narrow full‐width at half‐maximum value of 20 nm, and a maximum external quantum efficiency (EQE) as high as 30.2%. Notably, the EQE value remains 22.2% at 2000 cd m−2, whereas conventional control devices with an organic exciton harvester suffer from huge roll‐offs in quantum efficiency. An additional benefit of the device is a one‐order‐of‐magnitude improvement in its operational lifetime compared with that of the control device. Finally, the investigations reveal that the improvements are attributable to the unique ability of the Au(I) complexes for ultrafast triplet exciton harvest. In addition, the Au(I) complexes can facilitate Förster energy transfer to the MR‐TADF emitter, with effective suppression of hazardous triplet–triplet Dexter energy transfer. It is believed that the research is helpful in commercializing high‐efficiency and stable blue electroluminescence devices.

Fluorinated β‐diketone‐based Sm(III) complexes: spectroscopic and optoelectronic characteristics

The synthesis and characterization of a series of octa-coordinated Sm(III) complexes with 4,4,4-trifluoro-1-(2-naphthyl)-1,3-butanedione (TFNB) and 2,2'-bipyridine (Bpy) derivatives as ancillary ligand are described here. The complexes were analyzed by elemental, spectroscopic such as infrared spectroscopy, 1 H NMR, and thermogravimetric analyses. The fluorinated TFNB ligand absorbs in the range from 200 to 400 nm. The complexes show the sharp and structured Sm-based emissions in visible region upon irradiation in UV range. Excitation spectra of complexes show similarity to the absorption spectra of ligands suggesting that excitation energy is transferred from ligands to Sm(III) centre by the antenna effect. Photoluminescence emission spectra and colour parameters affirmed that the complexes show luminescence in orange-red region. These luminous Sm(III) complexes might be applied as emissive layer in organic electroluminescent devices.

Carrier mobility in field effect transistors based on copper-phthalocyanine thin films with different phase structure

Using annealing procedure at different temperatures after deposition at room temperature we obtained copper- phthalocyanine (CuPc) thin films with a — and p —phase structure. A phase structure of thin films was controlled by X-ray diffraction method, morphology was controlled by SEM. The field effect transistors was fabricated by high vacuum deposition of CuPc thin film (thickhess of 100 nm) on Si02 substrate which acting as gate contact. Gold drain and source contacts deposited on the top of active layer of FET. From measured current voltage measurements calculated mobility and concentration of charge carriers in FET. These parameters depend on phase structure of CuPc thin film, and characteristics of our FET are comparable with values of other authors.
 CuPc is commercially available macro cyclic metal complex that can be easily obtained in large quantity and high purity. Together with other phthalocyanine derivatives, the chemically and thermally stable CuPc has wide applications in dye processing, spectral sensitization, chemical sensors, and optical data storage [3-4]. The semiconducting behavior of metal phthalocyanines was first observed in 1948 and they have since attracted great interest in advancement of protptype organic semiconductors. Among the metal substituted phthalocyanines CuPc has been found superior properties [5-7], such as phenomena of field dependent and wavelength-dependent efficiency in an organic static induction transistors, and stabilizing role of CuPc layer on a highly stable organic electroluminescent device based on thin film Alq, and indicated a very weak interaction between CuPc and Au at the interface.
 In general, phthalocyanine materials can exist in several crystalline polymorphs, including a-, p -, x-and y - structure, and the most well known are the thermally metastable a- and p - [8-10], Phthalocyanine films deposited at room temperature usually consist of a-phase crystallites (at sublimation pressure of less than 10 Pa). At higher deposition pressures or at substrate temperature above 210°C, the p -phase directly obtained [11], Although it is well known that a-phase crystallites undergo a phase transformation into P -phase by treatment in various organic suspension media [12,13] or during annealing at higher temperatures [14,15], only some authors describe the nature of the a—>p phase transformation of copper phthalocyanine thin films with a thickness of less than 100 nm [8,14-15].

Investigation of 4,4′-bis[(N- carbazole) styryl] biphenyl (BSB4) for a pure blue fluorescent OLED with enhanced efficiency nearing the theoretical limit

4,4′-bis[(N-carbazole) styryl] biphenyl (BSB4 or BSBCz) is one of the widely studied organic fluorescent materials for blue organic electroluminescent devices in the recent times. In this work, BSB4 is used as a guest material to construct the host-guest matrix for the emissive layer (EML) of a pure blue fluorescent organic light-emitting diode (OLED). A pure blue emission suitable for display application with a Commission Internationale de l’Eclairage coordinate of (0.147,0.070) is achieved by the blue-shift of the emission spectrum of the host-guest matrix from that of the pristine guest (BSB4) molecules. The optimization of OLED structures is carried out by considering (a) charge balance in the EML for high exciton density, and (b) optical interference of generated light in the organic layers for increased light outcoupling. A thorough comparative study on the use of different combinations of widely used hole and electron transport layers to obtain charge balance in the EML of the OLED, thereby enhancing the external quantum efficiency (EQE) is shown. Optical interference effects in the fabricated OLEDs are analyzed by optical simulation of each device structure by transfer matrix method. With the optimized device structures, we are able to overcome the 2% EQE limit that has been reported so far for blue fluorescent OLEDs with BSB4 as light emitting material and achieve a maximum EQE of 4.08%, which is near to the theoretical limit of EQE for fluorescent OLEDs.