In this article, we demonstrated an exceptional palladium complex that exhibits both phosphorescence and delayed fluorescence for use as an efficient emitter in OLEDs. Devices employing PdN3N achieved external quantum efficiencies in excess of 22% and remarkable device operational lifetime to 90% initial luminance estimated at over 30,000 h at a practical luminance of 100 cd/m2. Further tuning of the phosphorescent and delayed fluorescent emission should have a great impact in the development of efficient and stable emitters for deep blue or white OLEDs.
Transparent conductive electrodes are one of the essential components for organic optoelectronic devices, including photovoltaic cells and light-emitting diodes. Indium-tin oxide (ITO) is the most common transparent electrode in these devices due to its excellent optical and electrical properties. However, the manufacturing of ITO film requires precious raw materials and expensive processes, which limits their compatibility with mass production of large-area, low-cost devices. The optical/electrical properties of ITO are strongly dependent on the deposition processes and treatment conditions, whereas its brittleness and the potential damage to underlying films during deposition also present challenges for its use in flexible devices. Recently, several other transparent conductive materials, which have various degrees of success relative to commercial applications have been developed to address these issues. Starting from the basic properties of ITO and the effect of various ITO surface modification methods, here we review four different groups of materials, doped metal oxides, thin metals, conducting polymers, and nanomaterials (including carbon nanotubes, graphene, and metal nanowires), that have been reported as transparent electrodes in organic optoelectronic materials. Particular emphasis is given to their optical/electrical and other material properties, deposition techniques, and applications in organic optoelectronic devices.
Recent progress in the development of materials and devices for single-doped white devices is presented with a particular focus on the development of platinum complexes exhibiting excimer emission. White organic light emitting diodes (WOLEDs) are strong candidates for the next generation of solid-state lighting, yet many of the best devices generate white light using multiple emitters embedded in a comparably complex device structure, raising the difficulty of consistently manufacturing these devices at a low cost and leading to challenges in color stability. These problems can be overcome by fabricating excimer-based WOLEDs, which construct a broad spectrum from a single emitter using blue monomer emission and red excimer emission. Through rational emitter design, the color quality and device efficiencies have steadily improved with recent achievements of external quantum efficiencies over 20% and a color rendering index greater than 80. Furthermore, recent applications of tetradentate platinum complexes for single-doped WOLEDs have yielded devices with a performance superior to many state-of-the-art multilayer WOLEDs as well as promising device operational stability.
Platinum-based blue and red emitters were used to make stacked organic light-emitting diodes devices for potential use in solid-state lighting. The electroluminescence (EL) spectra were strongly dependent on the red layer thickness and relative position of the red and blue emissive layers. Although all devices had very little EL in the green region, a color rendering index (CRI) as high as 65 was achieved. Addition of a suitable phosphorescent green emissive layer should produce higher CRI values. All devices tested exhibited external quantum efficiencies greater than 20%, indicating that the Pt-based emitters reported here are potentially useful for solid-state lighting applications.
We demonstrated enhanced efficiency in small molecule organic photovoltaic devices using dual organic interfacial layers of PEDOT:PSS followed by tetracene between the ITO anode and the organic donor material. The use of a small molecular templating layer, such as tetracene, proved to increase the molecular stacking of the subsequent phthalocyanine (Pc) based donor materials. Upon application in planar heterojunction devices of ZnPc and C60, an enhancement of over 80 percent in the donor contribution to the external quantum efficiency was observed attributed to the combination of exciton blocking by the higher band gap tetracene layer and enhanced exciton diffusion and charge transport resulting from the increased crystallinity.
Organic light emitting diodes using Pt-based red, green and blue dopants have been built for application to solid state lighting. All layers were deposited by thermal evaporation. Devices used a stacked design with separate red, green and blue layers. Blue devices achieved external quantum efficiencies of over 16% while devices with red / green / blue emissive layers reached quantum efficiencies of up to 15%, depending on the thickness of the red and green layers. We demonstrate an all-Pt based multiple emissive layer WOLED device.
White organic light-emitting devices (WOLEDs) can be fabricated using a simple, low-cost device structure with a single uniformly doped emissive layer. The Pt-17 emitter used in these devices obtains excellent color rendering (CRI = 80) as well as bright white electrophosphoresence (CIE x = 0.37, y = 0.40) by combining efficient monomer and efficient excimer emission as demonstrated by excellent external quantum efficiency (ηext = 15.9%). The Pt-17 based WOLED is also compatible with state-of-the-art charge injection and blocking materials as well as high out-coupling device structures. Application of these existing technologies is expected to extend luminance efficiencies of Pt-17 devices to world-class values (46 lm/?startWend? and 100 lm/?startWend? respectively). In addition to avoiding the difficulty and cost of fabricating more complex device structures, the color of a single-doped device also is uniquely independent of voltage, current density, and age. Molecules like fluorine-free Pt-17 are uniquely positioned to utilize excimer emissions in order to reduce manufacturing costs and provide solutions to satisfy many of the requirements for the next generation of organic solid-state lighting.