In multilayer white organic light-emitting diodes the electronic processes in the various layers--injection and motion of charges as well as generation, diffusion and radiative decay of excitons--should be concerted such that efficient, stable and colour-balanced electroluminescence can occur. Here we show that it is feasible to carry out Monte Carlo simulations including all of these molecular-scale processes for a hybrid multilayer organic light-emitting diode combining red and green phosphorescent layers with a blue fluorescent layer. The simulated current density and emission profile are shown to agree well with experiment. The experimental emission profile was obtained with nanometre resolution from the measured angle- and polarization-dependent emission spectra. The simulations elucidate the crucial role of exciton transfer from green to red and the efficiency loss due to excitons generated in the interlayer between the green and blue layers. The perpendicular and lateral confinement of the exciton generation to regions of molecular-scale dimensions revealed by this study demonstrate the necessity of molecular-scale instead of conventional continuum simulation.

We use photoelectron spectroscopy to study the electronic structure and energy level alignment throughout an organic light-emitting diode. The structure under investigation is a state-of-the-art long-living red phosphorescent device composed of doped charge-injection layers, charge-blocking layers, and an emission layer. By consecutively building up the whole device, the key parameters of every interface are measured. Our results show that the doped layers have a significant influence on the device energetics, especially in controlling the built-in potential, and that there are mostly only small dipoles present at the interfaces of the intrinsic organic layers

We present investigations of top emitting organic light emitting devices (OLED) comprising n- and p-doped organic charge transport layers. It has been found previously that in comparison to noninverted p-i-n OLEDs, inverted n-i-p OLEDs show reduced device performances after fabrication. These differences can be eliminated by subsequent thermal annealing of the whole n-i-p OLED. After this process, the n-i-p OLED exhibits a superior low driving voltage of 2.9 V at 1000 cd/m2 and shows an increase in external quantum efficiency from 11% to almost 15% which we ascribe to a modified charge balance within the intrinsic organic emission layer.

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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