  Organic light-emitting diodes (OLEDs) have successfully entered the display
market and continue to be attractive for many other applications. As
state-of-the-art OLEDs can reach an internal quantum efficiency (IQE) of almost
100 %, light outcoupling remains one of the major screws left to be turned. The
fact that no superior outcoupling structure has been found underlines that
further investigations are needed to understand their prospect. In this paper,
we use two-dimensional titanium dioxide (2D TiO$_2$) block arrays as a model of
an internal light outcoupling structure and investigate the influence of its
geometrical parameters on achieving the highest external quantum efficiency
(EQE) for OLEDs. The multivariable problem is evaluated with the visual
assistance of scatter plots, which enables us to propose an optimal period
range and block width-to-distance ratio. The highest EQE achieved is 45.2 %
with internal and external structures. This work contributes to the highly
desired prediction of ideal light outcoupling structures in the future.
