White OLEDs White organic light emitting diodes (white OLEDs) show promise for a major role in ambient lighting in the future. Low material costs, a wide choice of materials with customized properties, and easy production methods are features of the OLED technology which have favoured its fast development and industrial application in recent time. The energetically broad emission spectra and almost Lambertian emission of OLEDs are especially favourable for lighting applications, since they lead to homogeneous illumination and high quality color rendering. The possibility to produce large area OLED panels will also open new ways for lighting design apart from common incandescent bulbs or fluorescent tubes.
To take the chance for Europe to hold its leadership in the lighting market by developing the technology of white OLEDs, the European Commission has started the OLLA - project ( www.olla-project.org) in which our group is also essentially involved. Currently, the simultaneous maximization of device lifetime and efficiency of white OLEDs is most challenging and requires the development of new device strategies. Our main focus is the combination of fluorescent blue dyes with phosphorescent green and red emitter systems in one OLED to get white light. We have developed a device concept which is most flexible and ready for the incorporation of various emitter systems with nearly arbitrary properties. Key feature of this concept is an interlayer between phosphorescent and fluorescent regions with: - high triplet energy
- high band gap
- ambipolar transport properties
The reason why this is necessary is shown here: 
The interlayer suppresses Dexter energy transfer (2) from the phosphorescent emitters to the non-radiative triplet state of the fluorescent blue emitter. To still make exciton generation possible on both sides of the interlayer, it should be as ambipolar as possible.
A successful realisation of this device concept is shown here (for more details see reference 1):  The device comprises electrically doped charge transport layers which leads to very low driving voltage. OLED lighting will be realized by large lighting panels with a typical brightness of 1000 cd/m. The color of the emitted light is very similar to the color of a common light bulb, i.e. warm white "standard illuminant A".
The color rendering index (CRI) is a measure for the quality of color reproducability of objects under a light source. We reach 91 (of 100), which is excellent.
The power efficiency is a measure for the conversion efficiency of electric energy to visible light. 
references:
G. Schwartz, M. Pfeiffer, K. Walzer, and K. Leo, Proc. SPIE 6192, 61920Q/1 (2006).
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