OLED Architecture Research Articles

OLEDs on Down-Converting Fabric by Using a High Scalable Planarization Process and a Transparent Polymeric Electrode

Textile-based electronics represents a key technology for the development of wearable devices. Light-emitting textiles based on OLED architecture are particularly promising due to their intrinsic flexibility and possibility to be fabricated on large areas using scalable processes. Fabric planarization is one of the most critical issues in their fabrication. Here we report a fast, simple, and industrially scalable planarization method based on the transfer of surface morphological properties from silicon to fabric. A liquid resin is used as a planarization layer, and by exploiting the low roughness of a ‘guide substrate’ it is possible to replicate the smooth and uniform surface from the silicon to the planarization layer. The result is a fabric with a flat and homogeneous polymer layer on its surface, suitable for OLED fabrication. In particular, the effect of resin viscosity on the surface morphology was evaluated to obtain the best planarization layer. The best device shows high luminance and current efficiency values, even after 1000 bending cycles. We also explored the possibility of tuning the color emitted by the device by using a fluorescent fabric as a down-converting layer. Thanks to this approach, it is in principle possible to achieve white emission from a very simple device architecture.

White Organic Light emitting diodes based On exciplex states by using a new carbazole derivative as single emitter Layer

In this work we report the fabrication of organic light-emitting diodes based on the carbazole derivative 1,4-bis(9H-carbazol-9-yl)phenyl)ethynyl)benzene, showing excellent CIE coordinates (0.31, 0.33). To achieve this, we developed an initial architecture where the emissive layer was sandwiched between ITO/PEDOT:PSS, that acted as anode/hole transport layer, and Ca/Ag as cathode. The electroluminescence for this initial architecture was similar to the photoluminescence of the carbazole derivative, showing blue emission with CIE coordinates (0.20, 0.22). Subsequently, when the thickness of the PEDOT:PSS layer was reduced from 60 nm to 30 nm, the electroluminescence changed from blue to greenish, with new CIE coordinates (0.26, 0.45). We attributed this effect to intermolecular interactions producing the observed emission from excitons within the emissive layer and exciplexes at the interface of the hole transport and the emissive layer.Based on these preliminary results, a second architecture using 15 nm of 4,7-diphenyl-1,10-phenanthroline as the electron transport layer (ETL) was developed. In this manner, the observed electroluminescence spectrum became wider and the devices showed white emission, with outstanding CIE coordinates (0.31, 0.33). For this architecture, new exciplexes generated at the interface between emissive and electron transport layer emitted at longer wavelengths giving the complementary emission for white light.The generation of exciplex states at the two interfaces of the emitting layer made possible the fabrication of white light organic diodes with the simplest and more commonly used OLED architecture. Interestingly, by using an ETL in the second architecture, and by changing the thickness of the hole transport layer from 60 to 30 nm, it was possible to further fine-tune the white light from cold to warm emission.

Performance evaluation and analysis of two-stack warm white organic light emitting diodes with three spectral peaks

Abstract In this paper, we report a highly efficient two-stack warm white organic light emitting diodes (WOLEDs) with three spectral peaks using all phosphorescent organic emitters. A multi-stack OLED architecture enhances the color stability along electric field variation by separating emissive layer despite its complex structure. Performances of two- and three-stack WOLEDs having three spectral peaks are theoretically evaluated using our optical structure design method. The theoretically designed two- and three-stack structures show power efficiency of 51.5 and 33.7 lm/W at a constant current density of 1 mA/cm2 with color rendering index (CRI) of 84 and 88, respectively. Two-stack WOLED is expected to have higher power efficiency due to its lower driving voltage and smaller blue portion than that of three-stack WOLED. An optically optimized and fabricated two-stack WOLED with red and co-doped green-blue emissive layers exhibits a power efficiency as high as 58.1 lm/W and CRI of 85. The driving voltage of fabricated WOLED is (5.9 V) well correlated with the expected value of connected unit devices. In addition, our two-stack WOLED with diffusive film and hemispherical structure shows an enhanced power efficiency of 71.0 and 101.3 lm/W, respectively.

White and Saturated Blue Phosphorescent OLED based on the Non-Emissive Homoleptic Complex Ir(ppz)3 as single active material

AbstractThe well-known homoleptic iridium (III) complex Ir(ppz)

High‐Transconductance Organic Thin‐Film Electrochemical Transistors for Driving Low‐Voltage Red‐Green‐Blue Active Matrix Organic Light‐Emitting Devices

AbstractSwitching and control of efficient red, green, and blue active matrix organic light‐emitting devices (AMOLEDs) by printed organic thin‐film electrochemical transistors (OETs) are demonstrated. These all‐organic pixels are characterized by high luminance at low operating voltages and by extremely small transistor dimensions with respect to the OLED active area. A maximum brightness of ≈900 cd m−2 is achieved at diode supply voltages near 4 V and pixel selector (gate) voltages below 1 V. The ratio of OLED to OET area is greater than 100:1 and the pixels may be switched at rates up to 100 Hz. Essential to this demonstration are the use of a high capacitance electrolyte as the gate dielectric layer in the OETs, which affords extremely large transistor transconductances, and novel graded emissive layer (G‐EML) OLED architectures that exhibit low turn‐on voltages and high luminescence efficiency. Collectively, these results suggest that printed OETs, combined with efficient, low voltage OLEDs, could be employed in the fabrication of flexible full‐color AMOLED displays.

66.2: Efficiency Enhancement in White PIN OLEDs by Simple Internal Outcoupling Methods

AbstractHighly efficient white PIN OLED structures based on internal outcoupling approaches using vacuum processable materials have been developed. In combination with external MLA outcoupling films an efficiency enhancement of 1.8 can be achieved resulting in a power efficiency of 50 lm/W at color coordinates x,y = 0.46/0.41 for a tandem OLED using fluorescent blue and phosphorescent green and red emitter systems. In a single unit 2‐color fluorescent white system with the same combination of internal and external outcoupling methods an efficiency of 37 lm/W at a color x,y = 0.48/0.42 is reached. This is to our knowledge the highest power efficiency reported for a fluorescent white OLED with a flat light extraction method. A similar approach using a vacuum processable scattering material in top emission OLED architectures on metal substrates allows to reach efficiencies of 36.5 lm/W with a CRI of 75 at a color x,y = 0.45/0.41.

41.3: Charge Carriers And Triplets In OLED Devices Studied by Electrically Detected Magnetic Resonance

AbstractOrganic Light‐Emitting Diodes (OLEDs) were investigated with an Electron Paramagnetic Resonance (EPR) technique that uses the effective device conductance as the detection channel. This technique enables us to identify and study charge carriers and triplet excitons with high sensitivity. Using a series of model devices, we demonstrate that this type of spectroscopy provides information about triplet energy transfer and the location of the recombination zone. The fundamental understanding about the extent of the recombination zone in various OLED architectures helps us to design devices with improved performance.