Stefan Peter Grabowski

Analysis of lead oxide (PbO) layers for direct conversion X-ray detection

Lead oxide (PbO) is a candidate direct conversion material for medical X-ray applications. We produced various samples and detectors with thick PbO layers. X-ray performance data such as dark current, charge generation yield and temporal behavior were evaluated on small samples. The influence of the metal contacts was studied in detail. We also covered large a-Si thin-film transistor (TFT)-plates with PbO. Imaging results from a large detector with an active area of 18 cm /spl times/ 20 cm are presented. The detector has 960 /spl times/ 1080 pixels with a pixel pitch of 184 /spl mu/m. The modulation transfer function at the Nyquist frequency of 2.72 linepairs/mm is 50%. Finally, a full size X-ray image is presented.

PbO as direct conversion X-ray detector material

A flat X-ray detector with lead oxide (PbO) as direct conversion material has been developed. The material lead oxide, which has a very high X-ray absorption, was analysed in detail including Raman spectroscopy and electron microscopy. X-ray performance data such as dark current, charge yield and temporal behaviour were evaluated on small functional samples. A process to cover a-Si TFT-plates with PbO has been developed. We present imaging results from a large detector with an active area of 18 × 20 cm2. The detector has 1080 × 960 pixels with a pixel pitch of 184 μm. The linearity of detector response was verified. The NPS was determined with a total dark noise as low as 1800 electrons/pixel. The MTF was measured with two different methods: first with the analysis of a square wave phantom and second with a narrow slit. The MTF at the Nyquist frequency of 2.72 lp/mm was 50 %. We calculated first DQE values of our prototype detector plates. Full size images of anatomic and technical phantoms are shown.

OLEDs for lighting applications

Organic light emitting diodes (OLEDs) provide potential for power-efficient large area light sources that combine revolutionary properties. They are thin and flat and in addition they can be transparent, colour-tuneable, or flexible. We review the state of the art in white OLEDs and present performance data for three-colour hybrid white OLEDs on indexmatched substrates. With improved optical outcoupling 45 lm/W are achieved. Using a half-sphere to collect all the light that is in the substrate results in 80 lm/W. Optical modelling supports the experimental work. For decorative applications features like transparency and colour tuning are very appealing. We show results on transparent white OLEDs and two ways to come to a colour-variable OLED. These are lateral separation of different colours in a striped design and direct vertical stacking of the different emitting layers. For a striped colour tuneable OLED 36 lm/W are achieved in white with improved optical outcoupling.

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.

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.