Review Paper: Progress on efficient cathodes for organic light‐emitting diodes

Abstract— Balanced charge injection is essential for highly efficient and stable OLEDs. Various cathode materials, such as elemental metals, metal alloys, and metal compounds, have been adopted to facilitate electron injection. Currently, composite cathodes utilizing an electron‐injection layer and an air‐stable metal, such as LiF/Al, is the most common choice. This article will review the progress of efficient vacuum‐evaporated cathodes for OLEDs and their mechanisms.     

Simulated light extraction from a light‐emitting diode integrated with a microlens array

Abstract— Light extraction from InGaN‐based light‐emitting diodes (LEDs) on which microlens arrays were integrated using ray‐tracing methods was simulated. Enhancement of the total light extraction and beam shaping in the forward direction of the output of microlens‐integrated LEDs compared to conventional LEDs were observed. The diameter, curvature radius, and density of the microlens arrays on the LEDs were varied and the optimal conditions for external efficiency was determined.     

Displacement‐current analysis of organic light‐emitting diodes

Abstract— Organic light‐emitting diodes (OLEDs) having multiple organic layers were fabricated to analyze the physical phenomena occurring in an OLED according to the amplitude of the applied voltage. The staircase voltage with both an increasing period and a constant period was designed and applied to an OLED. The displacement current began to change at a voltage where the conduction current began to change, and partly originated from the formation of space charge due to the low mobility of the majority carrier. The displacement current was shown to be constant at low voltage and decreased after showing a maximum value as the applied voltage increased. The exact voltage for the injection of two types of carriers and light emission could be obtained from the variation in the displacement current.     

Composite silver electrode for double‐sided‐emitting stacked organic light‐emitting diode

Abstract— A reflective composite silver electrode is proposed and characterized as the middle electrode of a stacked organic light‐emitting diode (OLED) with double‐sided light emission. The proposed electrode is composed of a thermally evaporated stack of LiF (1 nm)/Al (3 nm)/Ag (70 nm) layers. The LiF/Al and the plasma‐treated Ag of the electrode function well as the respective cathode and anode of the bottom‐ and top‐emitting stacked OLEDs, with both being of the non‐inverted type. Power efficiencies of 10.3 and 12.1 lm/W at 100 cd/m² have been measured for bottom‐ and top‐emitting OLEDs, respectively, using dye doping. The stacked OLED having this bipolar middle electrode can be constructed as a two‐terminal‐only device, allowing for simpler driving schemes in double‐side‐emitting passive‐/active‐matrix OLED displays.     

Charge‐recombination processes in oligomer‐ and polymer‐based light‐emitting diodes: A molecular picture

Abstract— An overview of our recent work on the mechanisms of singlet and triplet exciton formation in electroluminescent π‐conjugated materials will be presented. According to simple spin statistics, only one‐fourth of the excitons are formed as singlets. However, deviations from that statistics can occur if the initially formed triplet charge‐transfer (CT) excited states are amenable to intersystem crossing or dissociation. Although the electronic couplings between the CT states and the neutral exciton states are expected to be largest for the lowest singlet and triplet excitons (S¹ and T¹, respectively), the possibility for direct recombination into T¹ is always very small due to the large exchange energy. In small molecules, spin statistics is expected to be observed because both singlet and triplet exciton formations proceed via higher‐lying S_n/T_n states with similar electronic couplings and fast formation rates. In extended conjugated chains, however, that the ¹CT → S¹ pathway is faster while the ³CT → ^Tn channels become much slower, opening the route to intersystem crossing or dissociation among the ³CT states.     

Full color quantum dot light‐emitting diodes patterned by photolithography technology

Quantum dot light‐emitting diodes are promising candidates for next generation displays. For display application, a pixel consists of red (R), green (G), and blue (B) side‐by‐side sub‐pixels, which thereby requires a high resolution patterning of the light‐emission layers. In this work, the quantum dot (QD) light‐emitting layers are fine patterned by using the photolithography and the lift‐off techniques. To facilitate the lift‐off process, reverse photoresist AZ5214E is used because of its special inverted trapezoidal structure after developing. To prevent the QDs being washed off during the lift‐off process, the ZnMgO layer is treated by the hydrophobic material hexamethyldisilazane. With hexamethyldisilazane treatment, the adhesion between the QDs and the ZnMgO is effectively improved. As a result, side‐by‐side R/G/B QD with pixel size of 30 μm × 120 μm is successfully achieved. After patterning, the R, G, and B‐quantum dot light‐emitting diodes exhibit a maximum current efficiency of 11.6 cd/A, 29.7 cd/A, and 1.5 cd/A, respectively. This work confirms the feasibility of patterning QDs by using the photolithography and the lift‐off techniques.     

Passive‐matrix polymer light‐emitting displays

Abstract— Organic light‐emitting device research focuses on the use of small‐molecule and polymer materials to make organic electroluminescent displays, with both passive‐ and active‐matrix technologies. This paper will focus on the characteristics of red, green, and blue electroluminescent polymers suitable for fabricating monochrome and full‐color passive‐matrix displays. The stability of polymer OLEDs, and the use of ink‐jet printing for direct high‐resolution patterning of the light‐emitting polymers will also be discussed. It will be shown that the performance of light‐emitting polymers is at the brink of being acceptable for practical applications.     

Nanorod‐form ZnO‐homojunction ultraviolet light‐emitting diodes

Abstract— Ultraviolet (UV) light‐emitting diodes based on ZnO‐homojunction nanorods is reported. p‐type doping can be obtained from intrinsic (or close to) ZnO by introducing acceptors, such as P or As, using ion implantation followed by appropriate thermal annealing and dopant activation. Our approach provides a possible solution to p‐type doping of ZnO and ZnO‐homojunction light‐emitting diodes. It is interesting to note that this solution is offered in the form of nanorods.     

Organic light‐emitting diodes with enhanced out‐coupling efficiency using high‐refractive‐index glass frit

We have fabricated a novel type of substrate for organic light‐emitting diodes (OLEDs) to improve the light out‐coupling efficiency. It was fabricated by forming an excellent flat layer using a high‐refractive‐index B₂O₃‐SiO₂‐Bi₂O₃ frit glass on the light diffusive glass substrate. By using this substrate, we have sufficiently reduced the total internal reflection of OLEDs, and we successfully obtained more than 1.9 times higher light out‐coupling efficiency without spectral changes and viewing angle dependency. Furthermore, we have also successfully demonstrated 50 × 50 mm large‐area white OLEDs with this novel substrate.     

Indium tin oxide anode modification for enhanced polymer light‐emitting devices

Abstract— Effort has been focused on exploring indium tin oxide (ITO) anode modification for enhanced performance of polymer light‐emitting diodes (PLEDs). It was found that oxidative treatment, e.g., a commonly used oxygen plasma, modifies ITO surface effectively to produce a low‐conductivity oxygen‐rich region. As a consequence, oxygen plasma‐treated ITO behaves somewhat similarly to specimens where there is an ultra‐thin insulating layer on its surface. It shows that the presence of such an ultra‐thin insulating interlayer between the ITO and the polymer layer favors the efficient operation of the PLEDs. The result of this effort provides an insight to better understand optimal anode contact for enhanced PLED performance.     

Very‐high‐brightness diodes for passive‐matrix display applications

Abstract— The development of a high‐brightness low‐voltage yellow‐light‐emitting polymer system suitable for use in low‐cost passive‐matrix displays will be reported. Average device efficiencies of 16 lm/W at 100 and 1000 cd/m² are achieved at 2.1 and 2.4 V, respectively. A luminance level of 100,000 cd/m² is achieved at 5.5 V.     

Linearly polarized light‐emitting backlight

A new polarized backlight system for liquid‐crystal displays (LCDs) is presented in which one linear polarization is preferentially coupled out by anisotropic scattering. The lightguide consists of a polymeric polarization‐dependent scattering film adhered to a transparent polymeric substrate. By changing the scattering power of the film, the polarized light outcoupling angles can be influenced and optimized to achieve a maximum outcoupling centered along the normal direction. The other linear polarization is mainly trapped in the lightguide and is shown to be recycled to enhance the overall light and/or energy efficiency. With a proper substrate choice, the achieved local contrast exceeds 14 over a 50‐mm range. A collimated light input further enhances the polarized contrast to well over 17.     

Active‐matrix organic light‐emitting diode displays with indium gallium zinc oxide thin‐film transistors and normal,inverted, and transparent organic light‐emitting diodes

Abstract— Active‐matrix organic light‐emitting diode (AMOLED) displays have gained wide attention and are expected to dominate the flat‐panel‐display industry in the near future. However, organic light‐emitting devices have stringent demands on the driving transistors due to their current‐driving characteristics. In recent years, the oxide‐semiconductor‐based thin‐film transistors (oxide TFTs) have also been widely investigated due to their various benefits. In this paper, the development and performance of oxide TFTs will be discussed. Specifically, effects of back‐channel interface conditions on these devices will be investigated. The performance and bias stress stability of the oxide TFTs were improved by inserting a SiO^x protection layer and an N²O plasma treatment on the back‐channel interface. On the other hand, considering the n‐type nature of oxide TFTs, 2.4‐in. AMOLED displays with oxide TFTs and both normal and inverted OLEDs were developed and their reliability was studied. Results of the checkerboard stimuli tests show that the inverted OLEDs indeed have some advantages due to their suitable driving schemes. In addition, a novel 2.4‐in. transparent AMOLED display with a high transparency of 45% and high resolution of 166 ppi was also demonstrated using all the transparent or semi‐transparent materials, based on oxide‐TFT technologies.     

All solution‐processed white quantum‐dot light‐emitting diodes with three‐unit tandem structure

Quantum‐dot light‐emitting diodes (QLEDs) are promising candidates for next generation displays. White QLEDs which can emit red, green and blue colors are particularly important; this is because the combination of white QLEDs and color filters offers a practical solution for high‐resolution full‐color displays. In this work, we demonstrate all‐solution processed three‐unit (red/green/blue) white tandem QLEDs for the first time. The white tandem devices are achieved by serially connecting the red bottom sub‐QLED, the green middle sub‐QLED and the blue top sub‐QLED using the inter‐connecting layer (ICL) based on ZnMgO/PEDOT:PSS heterojunction. With the proposed ICL, the two‐unit tandem QLEDs exhibit a high current efficiency of 22.22 cd/A, while the three‐unit white QLEDs exhibit evenly separated red, green and blue emission with a CIE coordinate of (0.30, 0.44), a peak current efficiency of 4.75 cd/A and a high luminance of 4206 cd/m². Displays based on the developed white QLEDs exhibit a wide color gamut of 114% NTSC. This work confirms the effectiveness of the proposed ZnMgO/PEDOT:PSS ICL and the feasibility of making all‐solution processed tandem white QLEDs by using the proposed ICL.     

Electrically commanded surfaces: A new liquid‐crystal‐display concept

Abstract— Fast in‐plane switching of the optic axis was realized in liquid‐crystal displays (LCDs) based on the concept of Electrically Commanded Surfaces (ECS). According to this concept, the liquid‐crystal layer in such a display is aligned by means of thin ferroelectric liquid‐crystal‐polymer (FLCP) film deposited onto the inner side of the display substrates. An electric field, applied normal to the substrates, switches the molecules of the ferroelectric film, representing the commanded surface that, via elastic forces, further transfers to the liquid‐crystal layer. The concept of electrically commanded surfaces opens the door to a new generation of advanced LCDs exhibiting extraordinary performance such as fast in‐plane switching.     

The influence of the hole transport layers on the performance of blue and color tunable quantum dot light‐emitting diodes

The performance of the blue quantum dot light‐emitting diodes (QLEDs) is largely affected by the hole transport layers (HTLs). As a consequence of the deep valance band level of blue quantum dots (QDs), hole injection is relatively difficult in blue QLEDs. To favor the hole injection, HTLs with high hole mobility and deep‐lying highest occupied molecular orbital level are desired. In this work, various HTLs and their influence on the performance of blue QLEDs are demonstrated. Devices with poly(N‐vinylcarbazole) (PVK) HTL exhibit the highest external quantum efficiency while devices with poly[9,9‐dioctylfluorene‐co‐N‐(4‐(3‐methylpropyl))‐diphenylamine] (TFB) exhibit the lowest driving voltage. By combining the advantages of PVK and TFB, the blue QLEDs with TFB/PVK bilayered HTL simultaneously exhibit a low driving voltage of 2.6 V and a high external quantum efficiency of 5.9%. Moreover, the exciplex emission at the interface of HTL/QDs is also observed, and the emission intensity can be tuned by modulating the hole injection. By utilizing PVK doped with 25% poly(3‐hexylthiophene) (P3HT) as HTL, exciplex emission is significantly enhanced at low driving voltage while QD emission is dominant at high driving voltage. By combining the exciplex emission and the QD emission, the emission color can be effectively tuned from red to blue as the driving voltage changing from 2 to 10 V.     

Towards a polarized light‐emitting backlight: micro‐structured anisotropic layers

A backlight for liquid‐crystal‐display illumination is presented, in which s‐polarized light is preferentially coupled out by micro‐optical structures in a birefringent layer. In the experiments, contrasts higher than 15 have been obtained. A polarization dependent ray‐tracing model has been developed. Important guidelines for finding an optimal backlight configuration have been derived from the calculations.     

Improved performance of organic light‐emitting devices with ultra‐thin hole‐blocking layers

Abstract— Tris‐(8‐hydroxyqunoline) aluminum (Alq₃)‐based organic light‐emitting devices (OLEDs) using different thickness of 2,9‐Dimethyl‐4,7‐diphenyl‐1,110‐phenanthorline (BCP) as a hole‐blocking layer inserted both in the electron‐ and hole‐transport layers have been fabricated. The devices have a configuration of indium tin oxide (ITO)/m‐MTDATA (80 nm)/BCP (X nm)/NPB (20 nm)/Alq₃ (40 nm)/BCP (X nm)/Alq₃ (60 nm)/Mg: Ag (200 nm), where m‐MTDATA is 4, 4′, 4″‐Tris(N‐3‐methylphenyl‐N‐phenyl‐amino) triphenylamine, which is used to improve hole injection and NPB is N,N′‐Di(naphth‐2‐yl)‐N,N′‐diphenyl‐benzidine. X varies between 0 and 2 nm. For a device with an optimal thickness of 1‐nm BCP, the current and power efficiencies were significantly improved by 47% and 43%, respectively, compared to that of a standard device without a BCP layer. The improved efficiencies are due to a good balance between the electron and hole injection, exciton formation, and confinement within the luminescent region. Based on the optimal device mentioned above, the NPB layer thickness influences the properties of the OLEDs.     

Light‐emitting polymers for full‐color display applications

The latest developments in light‐emitting‐polymer (LEP) technology at CDT continue to show steady progress. Device performance for blue, green, and red systems as well as a high‐performance yellow system in terms of device efficiency and stability will be described. Some of the issues associated with the commercialization of LEP technology including the development of direct‐patterning techniques enabling full‐color passive‐ and active‐matrix display will be discussed.     

Flexible quantum dot light‐emitting devices for targeted photomedical applications

Quantum dot light‐emitting devices (QLEDs), originally developed for displays, were recently demonstrated to be promising light sources for various photomedical applications, including photodynamic therapy cancer cell treatment and photobimodulation cell metabolism enhancement. With exceptional emission wavelength tunability and potential flexibility, QLEDs could enable wearable, targeted photomedicine with maximized absorption of different medical photosensitizers. In this paper, we report, for the first time, the in vitro study to demonstrate that QLEDs‐based photodynamic therapy can effectively kill Methicillin‐resistant Staphylococcus aureus, an antibiotic‐resistant bacterium. We then present successful synthesis of highly efficient quantum dots with narrow spectra and specific peak wavelengths to match the absorption peaks of different photosensitizers for targeted photomedicine. Flexible QLEDs with a peak external quantum efficiency of 8.2% and a luminance of over 20,000 cd/m² at a low driving voltage of 6 V were achieved. The tunable, flexible QLEDs could be employed for oral cancer treatment or diabetic wound repairs in the near future. These results represent one fresh stride toward realizing QLEDs' long‐term goal to enable the wide clinical adoption of photomedicine.