Communication: Optimization of the device structure of organic light-emitting-diodes (OLEDs) is required for improved reliability and performance. An apparatus is described that enables a matrix of OLEDs to be fabricated in a single experiment, i.e.,under comparable conditions, on a single substrate by combinatorial techniques. It is demonstrated that the influence of layer thickness variation of one or more layers on the device performance can be easily studied-as illustrated on the cover of this issue. The technique is also suitable for the fast screening and optimization off materials.