Since their invention by Tang and Van Slyke in 1987 [1] , organic light-emitting diodes (OLEDs) have matured into highly efficient and versatile light sources. By today they have secured a substantial share of the market for mobile phone displays and are prime candidates for a range of applications including large area displays, luminescent signage and large lighting panels for glare-free solid-state illumination. [2][3][4][5] The organic conjugated molecules on which OLEDs are based offer nearly unlimited possibilities for chemical tuning of their characteristics, such as color of emission [6] , and enable light-weight devices with inherent mechanical flexibility [7][8][9] . Compared to conventional inorganic LEDs, OLEDs are based on less toxic materials and their production has significantly lower environmental impact. OLEDs achieve sub-µs switching and their excellent efficiency allows high brightness levels without excessive heat production. Integration with suitable backplane driver electronics enables spatially controlled generation of light as required for high-resolution displays. These features also render the technology attractive for applications in biotechnology and biomedicine where controlled illumination is crucial, e.g. in optogenetics -a technique that enables precise control of neuronal behavior with light [10,11] . However, device encapsulation represents a major challenge in this context, because contact with biological material typically

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Organic light emitting diodes (OLED) are promising candidates offering in optical sensor applications to detect different gas compositions and excitable optical marker groups in chemical and biological processes. They enable attractive solutions for monitoring the gas phase composition of e.g., dissolved molecular oxygen (O2) species in bio reactors or excitation of fluorescent markers. In this work, we investigate different OLED devices for biomedical applications to excite the fluorescent dye rhodamine 6G (R6G). The OLED devices are built in top emission geometry comprising a distributed Bragg reflector (DBR) acting as optical mirror. The OLED is optimized to provide a very narrow emission characteristic to excite the R6G at 530 nm wavelength and enabling the possibility to minimize the optical crosstalk between the OLED electroluminescence and the fluorescence of R6G. The DBR includes a thin film encapsulation and enables the narrowing of the spectral emission band depending on the number of DBR pairs. The comparison between optical simulation data and experimental results exhibits good agreement and proves process stability. OPEN ACCESSElectronics 2015, 4 983

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