The optical outcoupling of top-emitting organic light-emitting diodes (OLEDs) can be improved by a thin dielectric capping layer on top of the transparent cathode. We investigate the emission properties of a set of top-emitting OLEDs with the same device structure, but different organic capping layer thicknesses to understand the capping layer effect. We demonstrate that the distribution pattern of the emitted light from top-emitting OLEDs depends strongly on the capping layer thickness, showing not only a maximum current efficiency enhancement by a factor of 1.38 (up to 78cd/A in forward direction at 1000cd/m2), but also a quantum efficiency improvement by 35%. This leads to a device efficiency of up to 17.8% and 69lm/W at a brightness of 1000cd/m2. We show that this efficiency enhancement is not due to the redistribution of emitted light, but mainly due to the improvement of outcoupling efficiency by changing the optical structure of the devices with the organic capping layer. The maximum outcoupling efficiency is achieved at a capping layer thickness where the top contact stack (cathode+capping layer) shows low absorption and high reflection.