Small-molecule Organic Light-emitting Diodes Research Articles

Recent research on the status and advances of OLED

Organic Light-Emitting Diodes (OLEDs) have already been a prominent display technology in various applications for many decades, ranging from smartphones and televisions to wearable devices and lighting. It is important to understand the current state of OLED research in order to assess its progress, identify challenges, and explore more potential ways for further development. In this paper, the significance and widespread adoption of OLED technology are briefly introduced, and its advantages over traditional display technologies are highlighted. The paper then provides a concise overview of the major components and working principles of OLEDs without delving into basic notions. The paper discusses both small molecule OLEDs (SMOLEDs) and polymer OLEDs (POLEDs), emphasizing their unique structure, characteristics and properties. Different strategies employed to enhance OLED performance is investigated, including the regulation of band gaps that influence the efficiency and brightness. This paper also focuses on the evaluation of device reliability and stability aspects. The techniques and materials employed in studies related to OLED development are also discussed with many searched experimental data and ongoing research activities. Furthermore, the research content covered the aspects of recent commercial advancements in OLED technology, such as Active matrix OLEDs (AMOLED) and Passive matrix (PMOLED). The review provides a guideline for researchers, industry professionals, and enthusiasts to better understand the OLED technology into its details, potential, and the areas that require further exploration.

The Introduction of Different Types of OLEDs

As display technology advances, organic light emitting diodes (OLEDs) have attracted a lot of attention worldwide due to their light weight, flexibility, wide viewing angle, etc. This paper briefly introduces the history and the different historical OLEDs, including phosphorescence, thermally activated delayed fluorescence (TADF), and triplet-triplet annihilation (TTA). Then, this article mainly presents two kinds of OLEDs. The first is small-molecule OLED with typical three organic layers. The purity of the material is quite high, and it can produce high-quality films with high fluorescence quantum efficiency, which can produce a variety of colors of light. Its disadvantages are poor thermal stability and limited carrier transport capacity. The second is polymer based OLED. Compared with small molecule organic luminescence materials, polymer materials have higher physical strength, better processibility, mechanical properties, and thermal stability. In this regard, recent research progress and the comparison between these two types of OLEDs are proposed. They both are considered the most promising display technologies.

Efficient small molecule organic light-emitting diodes fabricated by brush-coating

White and RGB phosphorescent OLEDs were constructed by continuously brush-coating the hole-injection layer and small molecule light-emitting layer.

Multiscale molecular simulations of the morphological evolution of small-molecule organic solar cells during the vacuum codeposition process

The mesoscale morphologies of organic small molecular films fabricated via vacuum deposition processes are critical to the performance of small-molecule solar cells and organic light-emitting diodes. In the present study, the morphological evolution of the active layer of DPDCPB:${\mathrm{C}}_{70}$ small-molecule solar cells during vacuum codeposition processes was revealed by a series of GPU-accelerated coarse-grained molecular-dynamics simulations. The ${\mathrm{C}}_{70}$ and DPDCPB molecules were coarsened into ellipsoids and bonded ellipsoids, respectively. The interactions between ellipsoids were described by the Gay-Berne formulation and were parametrized to reproduce potential energy surfaces from all-atom atomistic simulations using a genetic algorithm. Due to the significantly reduced overall degrees of freedom, this coarse-grained scheme allowed us to simulate the vacuum codeposition processes and monitor the morphological evolution of systems with system length scales compatible with those of the experiments (\ensuremath{\sim}30 nm). Our simulations indicate that the film morphologies are closely correlated with the DPDCPB:${\mathrm{C}}_{70}$ blending ratio. High ${\mathrm{C}}_{70}$ concentration leads to a rough film surface, which is in accordance with experimental observations and can be attributed to the strong self-aggregation behavior of ${\mathrm{C}}_{70}$ molecules. The morphological property analysis indicates that the rough film surface has an almost negligible impact on the DPDCPB/${\mathrm{C}}_{70}$ domain percolations, and the device with the optimal deposition ratio should give the most balanced hole/electron transfer in respective DPDCPB/${\mathrm{C}}_{70}$ domains. The present study demonstrates that by using the ellipsoid-based coarse-grained model, it is possible to study the morphological evolution of small-molecule organic thin film during vacuum deposition processes with molecular scale details, which can provide valuable insights for experimental teams to further optimize device fabrication protocols for the next generation of organic optoelectronic devices.

The Synthesis of Polyethersulfone (PES) and its Derivatives as Polymer Light Emitting Diode (PLED) Material

Nowadays some types of polymer are being developed as Polymer Light Emitting Diode (PLED) materials because they have some advantages compared to small molecule organic light emitting diode (SM-OLED). Polymers which have numerous conjugated double bonds can be used as PLED materials, such as the polyethersulfone (PES) and its derivatives. Therefore, further research on the synthesis of PES and its derivatives is needed to explore their potential as PLED materials. In this research, the synthesis of polyethersulfone has been performed utilizing Microwave Assisted Organic Synthesis (MAOS) method and subsequently the synthesized PES was being transformed into the nitrated PES (PES-NO2) and the aminated PES (PES-NH2) utilizing the conventional method (reflux). Polymer structure is elucidated through FT-IR and 1H-NMR spectrum. Polymer application as PLED material is characterized by fluorescence emission spectrum. The maximum wavelengths in the fluorecence emission spectra of polymer in NMP were 444 nm for PES, 356 nm and 444 nm for PES-NO2, also 440 nm for PES-NH2.Based on the various analyses of data, the synthesis of PES, PES-NO2, and PES-NH2 has been successfully performed and all of polymers have the potent to be used as PLED materials because of its ability to emit light (blue) in the visible area.

Transparent bacterial cellulose nanocomposites used as substrate for organic light-emitting diodes

In this work, high transparent bacterial cellulose (HTBC) biocompatible membranes were produced to be used as substrates in organic light-emitting diodes (OLEDs). These multifunctional membranes are based on bacterial cellulose (BC) and an organic–inorganic sol, composed of boehmite (Boe) nanoparticles and epoxi modified siloxane (GTPS). In order to be used as substrates, BC/Boe-GPTS membranes were covered with silicon dioxide (SiO2) and indium tin oxide (ITO) thin films deposited at room temperature using radio frequency (RF) magnetron sputtering. Visible light transmission improves to 88%, instead of 40% previously achieved. The electrical properties for HTBC/SiO2/ITO substrate shows that the ITO deposited films are n-type doped semiconductors with resistivity of 2.7 × 10−4 Ω cm, carrier concentration of − 1.48 × 1021 cm−3, and mobility of 15.2 cm2 V−1 s−1. These values are comparable to those of commercial ITO deposited onto glass substrates. After the characterization of the HTBC film, we used it as a substrate for the fabrication of a small molecule organic light-emitting diode OLED. The maximum efficiencies obtained were 1.95 cd/A and 1.68 cd/A for the reference OLED and the HTBC OLED, respectively. The HTBC OLED efficiency is then around 86% of the standard ITO-based OLED. This is clearly a good improvement, since previous BC-based simple architecture devices without Boe-GPTS have an efficiency 50% smaller than that of the standard OLED.

Enhanced near-infrared electroluminescence from a neodymium complex in organic light-emitting diodes with a solution-processed exciplex host

We report enhanced near-infrared (NIR) electroluminescence from a Nd3+-complex with thenoyltrifluoroacetone and 1,10-phenanthroline ligands. The NIR-emitting complex was blended into an exciplex-forming co-host system comprising 2,7-bis(diphenylphosphoryl)-9,9′-spirobifluorene as the electron transport material and 4,4′,4″-tris(carbazol-9-yl)triphenylamine as the hole transport material in solution-processed small molecule organic light-emitting diodes (OLEDs). This binary ambipolar host system favors direct charge trapping and exciton formation on the Nd3+-complex molecules. Efficient energy transfer from the singlet and triplet exciplexes formed between the host molecules to the Nd3+ ions contributes to the enhanced luminescence efficiency. The photoluminescence quantum yield of this blend is 1.2%, and the optimized OLED shows a maximum electroluminescence external quantum efficiency of 0.034%. The device also exhibits a low efficiency roll-off of only 12% over a current density range of 100 mA/cm2, due to the reduced triplet-polaron annihilation.

Improving the performance of solution-processed small molecule OLEDs via micro-aggregation formed by an alcohol additive incorporation

An alcohol additive 1, 8-Octanediol (ODO) with a simple carbon chain structure was first introduced into the light-emitting layer (LEL) of solution-processed small molecule organic light-emitting diodes (OLEDs) in this work. We found a typical host material of CBP presented a micro-aggregation film state with low percent of ODO adding in the LEL. The corresponded solution-processed devices with the LEL of CBP and 4CzIPN were investigated. The reduced device turn-on voltage and improved efficiency was achieved with low percent addition of 2 wt% ODO in LEL. Moreover, in the atmosphere, the device with ODO additive showed alleviated device degradation compared to no additive device. These results demonstrate the micro-aggregated state of the host is helpful, which can be attributed to the better charge transfer ability. Significantly, this additive process was also applied to other devices with reduced device turn-on voltage and improved efficiency.

Anchoring a Sulfonate Group to an Electron‐Transporting Molecule by an Alkyl Chain and Its Use as the Counter Anion in a Phosphorescent Cationic Iridium Complex

An electron-transporting anion prepared by anchoring a sulfonate group onto an electron-transporting molecule through a flexible alkyl chain and its use as the counter anion in a cationic iridium complex is reported. The flexible alkyl chain improves the solubility of the bulky anion in water or in polar organic solvents. The anion exhibits similar photophysical and electrochemical properties to the parent electron-transporting molecule. Within the complex, the anion does not disturb the phosphorescence of the cation in the solid film. Solution-processed small-molecule organic light-emitting diodes (OLEDs) using the complex as the dopant show superior performances over the reference device using the conventional complex with a PF6 - counter anion. It is revealed that anchoring anionic groups to optoelectronically active molecules with flexible alkyl chains is a feasible approach to develop optoelectronically active anions for assembling ion pairs with advanced optoelectronic properties.

Inkjet-Printed Small-Molecule Organic Light-Emitting Diodes: Halogen-Free Inks, Printing Optimization, and Large-Area Patterning.

Manufacturing small-molecule organic light-emitting diodes (OLEDs) via inkjet printing is rather attractive for realizing high-efficiency and long-life-span devices, yet it is challenging. In this paper, we present our efforts on systematical investigation and optimization of the ink properties and the printing process to enable facile inkjet printing of conjugated light-emitting small molecules. Various factors on influencing the inkjet-printed film quality during the droplet generation, the ink spreading on the substrates, and its solidification processes have been systematically investigated and optimized. Consequently, halogen-free inks have been developed and large-area patterning inkjet printing on flexible substrates with efficient blue emission has been successfully demonstrated. Moreover, OLEDs manufactured by inkjet printing the light-emitting small molecules manifested superior performance as compared with their corresponding spin-cast counterparts.

Stable and efficient blue fluorescent organic light-emitting diode by blade coating with or without electron-transport layer

Abstract Multi-layer small-molecule blue fluorescent organic light-emitting diode (OLED) is fabricated by blade coating. The emission layer is based on a mixed host of 1-(7-(9,9′-bianthracen-10-yl)-9,9-dioctyl-9 H -fluoren-2-yl)pyrene (PT-404) and electron-transport material 2,7-Bis(diphenylphosphoryl)-9,9′ -spirobifluorene (SPPO13), and the blue guest emitter is 4-4′-(1E,1′E)-2,2′-(naphthalene-2,6-diyl)bis(ethane-2,1-diyl)bis(N,N-bis(4-hexyl- Phenyl) aniline) (Blue D). A hole-transport layer of Poly-(9, 9-dioctylfluorenyl-2, 7-diyl)-co-(4, 4-( N -(4-sec-butylphenyl)) diphenylamine) (TFB) is added on top of PEDOT: PSS anode. The electrons are blocked away from TFB by a layer of pure host emission layer of PT-404 between TFB and the mixed –host emission layer. For the device with the electron transport layer of Tris(8-hydroxyquinolinato)aluminum (Alq 3 ) blade-coated over the emission layer the efficiency and lifetime at initial brightness of 500 cd m −2 are 7.5 cd A −1 and 150 h for Alq 3 /CsF/Al cathode. When the Alq 3 /CsF/Al is replaced by simply CsF/Al over the mixed-host emission layer the efficiency and lifetime are 6.4 cd A −1 and 300 h (2 times longer than that of the Alq 3 /CsF/Al cathode). The lifetime depends on the electron-hole balance tuned by the mixed-host blending ratio as well as the electron injection from the cathode. This work shows good stability is possible for all-solution-processed blue OLED.

Electrolyte-Gated Red, Green, and Blue Organic Light-Emitting Diodes.

We report vertical electrolyte-gated red, green, and blue phosphorescent small-molecule organic light-emitting diodes (OLED), in which light emission was modified by tuning the electron injection via electrochemical doping of the electron injection layer 4,4-bis(N-carbazolyl)-1,1-biphenyl (CBP) under the assistance of a polymer electrolyte. These devices comprise an electrolyte capacitor on the top of a conventional OLED, with the interfacial contact between the electrolyte and electron injection layer CBP of OLEDs achieved through a porous cathode. These phosphorescent OLEDs exhibit the tunable luminance between 0.1 and 10 000 cd m-2, controlled by an applied bias at the gate electrode. This simple device architecture with gate-modulated luminance provides an innovative way for full-color OLED displays.

Near infrared electroluminescence from Nd(TTA)3phen in solution-processed small molecule organic light-emitting diodes

We report on the near infrared electroluminescence properties of a Nd3+ complex with thenoyltrifluoroacetone and 1,10-phenantroline ligands in solution-processed organic light-emitting diodes. Spin-coated blends containing a 1,3-bis(9-carbazolyl)benzene host doped with the Nd3+ complex were found to exhibit a photoluminescence quantum yield of about 0.5%, regardless of the doping concentration level. Electroluminescent devices based on these small molecule blends showed the characteristic emission of Nd3+ at 890, 1060 and 1330 nm with an external quantum efficiency as high as 0.022%. These improved performances were mainly attributed to direct charge trapping and exciton formation on the near infrared emitter. Importantly, the efficiency roll-off at high current densities due to triplet-triplet exciton annihilation in the device containing 20 wt% of the complex was lower than what is typically observed in lanthanide complex-based electroluminescent devices. This is presumably due to the high triplet energy of the host material, which prevents guest-to-host energy-back transfer and thus host-guest triplet-triplet exciton annihilation.

Ultrahigh-efficiency solution-processed simplified small-molecule organic light-emitting diodes using universal host materials.

Although solution processing of small-molecule organic light-emitting diodes (OLEDs) has been considered as a promising alternative to standard vacuum deposition requiring high material and processing cost, the devices have suffered from low luminous efficiency and difficulty of multilayer solution processing. Therefore, high efficiency should be achieved in simple-structured small-molecule OLEDs fabricated using a solution process. We report very efficient solution-processed simple-structured small-molecule OLEDs that use novel universal electron-transporting host materials based on tetraphenylsilane with pyridine moieties. These materials have wide band gaps, high triplet energy levels, and good solution processabilities; they provide balanced charge transport in a mixed-host emitting layer. Orange-red (~97.5 cd/A, ~35.5% photons per electron), green (~101.5 cd/A, ~29.0% photons per electron), and white (~74.2 cd/A, ~28.5% photons per electron) phosphorescent OLEDs exhibited the highest recorded electroluminescent efficiencies of solution-processed OLEDs reported to date. We also demonstrate a solution-processed flexible solid-state lighting device as a potential application of our devices.

Highly efficient solution-processed small-molecule white organic light-emitting diodes

Solution-processed small-molecule white organic light-emitting diodes (WOLEDs) were fabricated with a co-host of hole-transporter 4,4′,4″-Tris(carbazol-9-yl)triphenylamine (TCTA) and electron-transporter 2,7-Bis(diphenylphosphoryl)-9,9'-spirobifluorene (SPPO13). By doping 15 wt% FIrpic or F3Irpic and 0.5 wt% Ir(MDQ)2(acac) in to the TCTA/SPPO13 host, highly efficient white OLEDs have been achieved which exhibit nearly identical emission spectra at different luminance. The F3Irpic and Ir(MDQ)2(acac)-based WOLED shows maximum efficiencies of 40.9 cd/A, 36.7 lm/W and 16.9%, and even high efficiencies of 30.1 cd/A and 12.3% at the practical luminance of 1000 cd/m2, which are among the highest efficiencies of the solution-processed small-molecule WOLEDs. These results demonstrate a convenient way to realize solution-processed WOLEDs with high efficiency and high spectral stability through full small-molecule materials system.

Multi-solution processes of small molecule for flexible white organic light-emitting diodes

Most small molecule organic light emitting diode (SM-OLED) device structures are made in one layer using solution-based processing because the solution is usually a high dissolvent material that easily attacks the layer below it. We demonstrate a simple and reliable stamping technique for fabricating multi-solution process flexible white SM-OLEDs. The structure is anode/spin-hole injection layer/spin-emitting layer/stamping-electron transport layer/cathode. Poly(di-methyl silane) (PDMS) stamp is used for transferring electron transport layer. An intermediate ultraviolet-ozone surface treatment is introduced to temporarily modify the PDMS stamp surface. Then, the solution-based electron transport layer film can therefore be uniformly formed on top of the PDMS surface. After that the electron transport layer film on the PDMS stamp is transfer-printed onto the emitting layer with suitable heating and pressing. A solution-based processing is successfully established to efficiently fabricate flexible white SM-OLEDs. The SM-OLEDs were obtained at the current density of 20mA/cm2, luminance of 1062cd/m2, current efficiency of 5.57cd/A, and Commission internationale de l'éclairage coordinate of (0.32, 0.35).

Effects of Gold-Nanoparticle Surface and Vertical Coverage by Conducting Polymer between Indium Tin Oxide and the Hole Transport Layer on Organic Light-Emitting Diodes.

PSS), which was used as a capping layer between indium tin oxide (ITO) and a hole transport layer (HTL) on small-molecule fluorescent organic light-emitting diodes (OLEDs), was systemically investigated. With respect to the Au-NP loading amount and size, the resultant current densities influenced the charge balance and, therefore, the OLED device performance. When the capping layer consisted of ITO/Au-NPs/ PSS+Au-NPs, superior device performance was obtained with 10-nm Au-NPs through increased surface coverage in comparison to other Au-NP PEDOT:PSS coverage conditions. Furthermore, the Au-NP size determined the vertical coverage of the capping layer. The current densities of OLEDs containing small Au-NPs (less than 30 nm, small vertical coverage) covered by PSS decreased because of the suppression of the hole carriers by the Au-NP trapping sites. However, the current densities of the devices with large Au-NPs (over 30 nm, large vertical coverage) increased. The increased electromagnetic fields observed around relatively large Au-NPs under electrical bias were attributed to increased current densities in the OLEDs, as confirmed by the finite-difference time-domain simulation. These results show that the coverage conditions of the Au-NPs by the PSS clearly influenced the OLED current density and efficiency.

P‐125: Maskless RGB Color Patterning via Dye Diffusion for Vacuum‐Deposited Small Molecule OLED Displays

A maskless RGB color patterning technique based on dye diffusion is proposed here for vacuum‐deposited small molecule OLED displays. This approach utilizes selective diffusion of dyes through thermal diffusion via physical contact for color patterning. The proposed maskless color patterning technique enables us to overcome challenging issues in the conventional color patterning technique using fine metal shadow masks. The maskless color patterning technique based on dye diffusion has been suggested as a color patterning technique for polymer OLEDs. However, it has not yet been applied to vacuum‐deposited small molecule OLEDs likely due to several expected concerns such as limited diffusion and contact‐induced damage in small molecule films. The purpose of the present study is therefore to test whether the color pattering technique based on dye diffusion can be applied to vacuum‐deposited small molecule OLEDs. In order to investigate that, red, green, and blue OLEDs are fabricated side by side on one substrate by doping dyes into host through thermal diffusion via physical contact. Device performance of the fabricated devices, including electroluminescence spectra and IVL characteristics, is tested to investigate if molecular diffusion is sufficient for obtaining desired color spectrum and investigate effects of the physical contact.

High-resolution electrohydrodynamic jet printing of small-molecule organic light-emitting diodes

The development of alternative organic light-emitting diode (OLED) fabrication technologies for high-definition and low-cost displays is an important research topic as conventional fine metal mask-assisted vacuum evaporation has reached its limit to reduce pixel sizes and manufacturing costs. Here, we report an electrohydrodynamic jet (e-jet) printing method to fabricate small-molecule OLED pixels with high resolution (pixel width of 5 μm), which significantly exceeds the resolutions of conventional inkjet or commercial OLED display pixels. In addition, we print small-molecule emitting materials which provide a significant advantage in terms of device efficiency and lifetime compared to those with polymers.

Interface and thickness tuning for blade coated small-molecule organic light-emitting diodes with high power efficiency

We developed a general method based on fluorescence microscopy to characterize the interface dissolution in multi-layer organic light-emitting diodes (OLEDs) by blade coating. A sharp bi-layer edge was created before blade coating, with the bottom layer being insoluble and top layer soluble. After blade coating, fluorescence images showed that the edge of the top layer shifted when the layer dissolved completely, whereas the bottom layer's edge remained in place as a positioning mark. The dissolution depth was determined to be 15–20 nm when the emissive-layer host of 2,6-bis (3-(9H-carbazol-9-yl)phenyl) pyridine (26DCzPPy) was coated on the hole-transport layer of N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine(NPB), which was consistent with a sudden drop in efficiency of orange OLEDs with layer thickness below 20 nm. Thus, the layer thickness of OLEDs was optimized to stay more than 20 nm for blade coating. For a two-color white OLED with the structure TCTA/26DCzPPy:PO-01-TB:FIrpic/TPBI, efficiency was 24 cd/A and 8.5 lm/W at 1000 cd/m2. For a three-color white OLED with Os(fptz)2(dhpm) added as the emitter, the efficiency was 12.3 cd/A and 3.7 lm/W at 1000 cd/m2. For a green device with the structure TCTA/26DCzPPy:Ir(mppy)3/TPBI, the efficiency was 41.9 cd/A and 23.4 lm/W at 1000 cd/m2.