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Molecular-scale simulation of electroluminescence in a multilayer white organic light-emitting diode

Abstract

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.

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Figure 1: OLED stack and its electrical characteristics.
Figure 2: Light-emission and exciton-generation profiles.
Figure 3: Energy-level diagram.
Figure 4: Inhomogeneity in exciton generation.
Figure 5: Spatial distribution of exciton generation.

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Acknowledgements

This research was supported by the European Community (Program No. FP7-213708 (AEVIOM), M.S., M.F., B.L., K.L., P.L., R.C. and P.A.B.), the Dutch nanotechnology programmes NanoNed (J.J.M.v.d.H.) and NanoNextNL (M.M., H.v.E.), and the Dutch Polymer Institute (DPI), projects No. 518 (M.C.) and 680 (R.J.d.V.). The authors thank J. Cottaar, H. Nicolai, R. Nitsche, B. Ruhstaller, P. Blom and N. Greenham for the many discussions we had at the AEVIOM meetings and other occasions. The value of the hole mobility of α-NPD in Table 1 was established with the help of R. Nitsche. M. de Vries performed the measurements from which the parameter values for NET5 and Spiro-DPVBi in Table 1 were obtained. We acknowledge the contributions of F. W. A. van Oost to the simulation codes.

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Contributions

M.M. and M.C. contributed equally to the work: M.M. performed the main Monte Carlo simulations and M.C. the main measurements on the OLED; R.J.d.V. determined the electron-transport parameters in Spiro-DPVBi and NET5; H.v.E. programmed the exciton diffusion software and prepared part of the figures; J.J.M.v.d.H. was involved in setting up the Monte Carlo simulations; M.S. and M.F. fabricated the OLED, provided its experimental optimization and characterization, and determined hole transport parameters, all supervised by B.L. and K.L.; P.L. was involved in the definition of the OLED stack, the UPS and optical measurements on the stack materials, and the fabrication of devices for the determination of electron transport parameters; R.C. supervised the whole project and contributed to the writing; P.A.B. supervised the simulation work and wrote the main part of the manuscript.

Corresponding author

Correspondence to Peter A. Bobbert.

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Mesta, M., Carvelli, M., de Vries, R. et al. Molecular-scale simulation of electroluminescence in a multilayer white organic light-emitting diode. Nature Mater 12, 652–658 (2013). https://doi.org/10.1038/nmat3622

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