Fig. 1 Fabrication schemes: (a) Laser-interference lithography setup. The laser beam is split into two separate beams, which are widened up and are brought to interference under a certain angle at the substrate surface. (b) After illumination the resist is developed, which leaves a resist grating on the sample. (c) TiO₂ is evaporated onto the sample. (d) After lift-off a TiO₂ grating results. (e) SEM image of a TiO₂ grating with a 330 nm period. (f) The WOLED stack is fabricated on top. (g) SEM image of a cross section of a finished WOLED with grating. (h) A PDMS microlens stamp is pressed into a layer of PMMA/anisol, which results into a microlens array at the substrate/air interface. (i) SEM image of a microlens array.

Fig. 2 (a) Measured I-V characteristics of the reference device and the devices with 15 nm and 35 nm grating height. (b) Relative current efficiencies of the devices. (c) Relative power efficiencies of the devices.

Fig. 3 (a) Scheme of light propagation in the reference device. Generated light is partly trapped in waveguide modes and in substrate modes. Light in the escape cone has the chance to exit the device (radiation modes). (b) Possible scattering of photons due to the grating. Yellow arrows: the emitter couples to waveguide modes, substrate modes and radiation modes. Green arrows: the grating can scatter waveguided photons to substrate modes and radiation modes. Photons that were initially in substrate modes can also scatter into radiation modes. Red arrows: back scattering of photons from radiation modes or substrate modes into waveguide modes. Photons from radiation modes can also scatter into substrate modes. (c) Overview off the different devices and their respective enhancement factors.

Fig. 4 (a) Comparison of the relative efficiencies of the 15 nm grating device with and without the microlens array. (b) Relative efficiencies of the 35 nm grating device with and without the microlens array.

Fig. 5 (a)-(d) Measured angle resolved normalized spectral emission of the same luminous areas. The 35 nm grating device without (a) and with (b) the microlens array and the 15 nm grating device without (c) and with (d) the microlens array.