Volker Van Elsbergen

Investigation of FIrpic in PhOLEDs via LC/MS technique

The commercial breakthrough of phosphorescent organic white light sources is presently hampered due to the unavailability of a stable blue phosphorescent emitter material. Moreover, only few analytical investigations have been made regarding the chemical degradation of the phosphorescent emitter materials during the processing or the operation of the devices.Organic light emitting devices (OLEDs) containing phosphorescent metal complexes with iridium as central ion were investigated. Special attention was paid to the chemical degradation of the material. The devices were analyzed by means of high performance liquid chromatography coupled with mass spectrometry (HPLC/MS). Electron spray ionization (ESI) was employed as ionization source. Isomerization phenomena of the blue-green emitting heteroleptic iridium complex FIrpic could be observed after the device manufacture and after operation. These findings could give hints on the mechanisms that influence the lifetime of PhOLEDs based on FIrpic or similar blue emitters.

Influence of carrier conductivity and injection on efficiency and chromaticity in small-molecule white organic light-emitting diodes based on 4,4'-bis(2,2'-diphenylvinyl)-1,1'-spirobiphenyl and rubrene

Organic light-emitting devices (OLEDs) employing yellow-emitting 5,6,11,12-tetraphenylnaphthacene (rubrene) and blue-emitting 4,4′-bis(2,2′-diphenylvinyl)-1,1′-spirobiphenyl are optimized using a vacuum thermal evaporator. The influence of various hole injection/hole transport stacks and electron transport materials on the device performance and the electroluminescence spectra are discussed. Device characteristics are explained by the charge carrier distribution among the organic layers. OLEDs with warm-white emission with color coordinates of x=0.43 and y=0.42 were produced with power and current efficiencies of 5lm∕W and 10.9cd∕A, respectively, at a luminance of 1000cd∕m2. The maximum external quantum efficiency at a current density of 20mA∕cm2 was 4.6%.

White OLEDs for lighting applications

Efficient white OLEDs are becoming attractive as large area light sources for illumination and in future also for general lighting. We discuss device concepts for white OLEDs and their potential to achieve high efficacy and good lumen- and color-maintenance at the same time. We focus on OLEDs using a combination of fluorescent blue and phosphorescent red and green emitters (hybrid OLEDs). Hybrid OLEDs have high efficacy and lifetime in the white to warm white color region (color points B and A on the black-body-curve). Near illuminant A efficacy values of 28-29 lm/W without optical out-coupling can be achieved with a hybrid OLED. The external quantum efficiency (EQE) is 14%. A typical color rendering index (CRI) is 84. Recent results for monochrome OLEDs and for hybrid OLED stacks are presented.

66.3: Hybrid White OLEDs for General Lighting

OLEDs for general lighting require both high efficacy and good lumen maintenance. Hybrid stacks combining fluorescent blue and phosphorescent red and green emitters are a very good choice for both long lifetime and good efficacy values: 31 lm/W are demonstrated for a cold white color point. 44 lm /W are measured with improved out-coupling (ILO) using a scattering foil and 60 lm/W are demonstrated using a macro-extractor for light out-coupling. For warm white we realized 34 lm/W (with ILO 45 lm/W and 64 lm /W using a macro-extractor). LT50 lumen maintenance is for both stacks better than 30,000 hours.

Light extraction for a doubly resonant cavity organic LED: the RC2LED

The RC2LED is a substrate emitting OLED which has three additional interference layers between the ITO electrode and the glass substrate. This creates two resonant optical cavities. The RC2LED has 2 resonant optical cavities. The first cavity is also present in regular devices and is formed by metal/organic layers/ITO. The second cavity is formed by 3 additional layers: a high refractive index layer (Nb2O5), a low refractive index layer (SiO2) and a high refractive index layer (Nb2O5). The additional layers introduce a strong wavelength dependent improvement of the extraction efficiency compared to the OLED without the additional layers. Our simulations show an improvement of the extraction efficiency of over 70% over a wavelength range of 75 nm compared to an OLED without the 3 layers. Light extraction is worse compared to the reference OLED for wavelengths outside this wavelength range. the when compared to the OLED. This improvement has been experimentally verified for a green OLED with an emission between 500nm and 650 nm. A numerical study shows a relative improvement of 10% for the luminous power efficiency of a 3 color white OLED with the additional layers. The emitted white corresponds with the light emitted by illuminant A. The WOLED has been composed of a fluorescent blue emitter, green and red phosphorescent emitters.

High Efficiency OLEDs for Lighting Applications

Organic Light-Emitting Diode (OLED) technology is developing as a promising option for large area lighting applications, with basic properties such as efficiency, color stability and lifetime which approach or even exceed those of conventional lighting and inorganic LED technology and with various interesting additional complementing features. In this Chapter, an introduction is given on the development of OLED technology for lighting applications. We discuss the working principles of efficient white multilayer OLEDs, the factors which determine the efficiency, several key elements of the fabrication technology including encapsulation methods, and the state-of-the-art as realized in various institutes and companies.

52.3: OLEDs for Lighting Applications

OLEDs for lighting applications require the combination of several properties at the same time: Large emission area, high brightness, high efficiency, long lifetime, good color stability at different brightness levels, and low cost. In order to fulfill these demands, several OLED architecture concepts are under investigation: Hybrid layered OLEDs, stacked OLEDs, pixel-OLEDs. To achieve good color stability the diode-units used for stacking have to be optimized. The talk focuses on hybrid OLEDs and their properties. In this context we demonstrate a phosphorescent yellow diode (combination of red and green emitters) which has an efficacy between 50 and 60 lm/W without improved light out-coupling (ILO) and excellent color stability. Such highly optimized OLED architectures have to be combined with suitable optical out-coupling techniques to make OLEDs ready for lighting. Optical out-coupling techniques are briefly reviewed. We demonstrate a concept for extracting more light of an OLED using low refractive index hole transport layers.