Microlenses with diameters of 10, 20, and (almost equal to the periodicity of the array) and with an aspect ratio of 1 (hemispheres) were fabricated according to this process. The resulting MLAs have a typical transmission above 80% between 400 and 800 nm. Furthermore, their haze factor in transmission, defined as the ratio of the nonspecularly transmitted light to the overall transmitted light, reaches more than 70% regardless of the diameter considered. Thus, MLAs can help diffusing angular and, to a certain extent, spatial inhomogeneities of the emission. However, in case of a large variation in the local brightness, as the one introduced by shadowing metal grids used for charge carrier transport, more advanced concepts are required to obtain a spatially homogeneous emission.30 In order to test their potential for external outcoupling, those MLAs were integrated in white OLEDs (WOLEDs) by using a single layer of the white-emitting co-polymer SPW087 by Merck KGaA, and the measured efficiencies (defined in terms of luminous flux normalized to the electrical power consumption) were compared with the ones obtained without the MLA on the same devices.29 The results shown in Fig. 5 demonstrate that an efficiency enhancement of around 50% is achieved compared to a reference device which exhibits a luminous efficacy of . In addition, the diameter of the microlenses has only a weak influence on the enhancement factor, an observation which is corroborated by other studies.26 It can be added here that most of the light outcoupling was found to occur after the first interaction with the MLA. Lastly, the MLA did not affect the spectral and angular emission characteristics of the WOLEDs investigated. To summarize, the fabrication process presented above enables production of high-quality MLAs on large areas to efficiently extract substrate modes. It can be directly adapted and extended to flexible substrates such as the ones based on polyethylene terephthalate.31 In addition to the periodical array of identical microlenses, disordered configurations (in both the position and size distribution of the lenses) have been recently proposed. For example, such ensembles were obtained spontaneously using a phase separation technique,32 or almost autonomously via the so-called “microbial approach.”33 Their integrated enhancement could not overcome that obtained with regular MLA, but adding disorder enabled improving the uniformity of the luminance angular distribution while suppressing spectral distortion. Lastly, in their recent work, Kim et al.34 combined all the previously mentioned attributes (patterning on a flexible polyethylene naphthalate substrate, cost-effective fabrication process based on the self-assembly of microparticles, patterning of irregular MLA) and enhanced the MLA properties by incorporating nanoparticles. Due to the improved light extraction thanks to the nanoparticles scattering, the maximum power efficiency increased from (irregular MLA) to (irregular MLA including nanoparticles). In addition, the authors demonstrated that the nanoparticles block the penetration of oxygen and water vapor molecules into the organic layers, so that this approach also helps in improving the stability of the devices.