Role of the Recombination Zone in Organic Light-Emitting Devices

The impact of the recombination zone in organic light-emitting devices on the device efficiency and stability is discussed. We first summarize the basic idea of the dipole model to describe the optical outcoupling efficiency in planar organic light-emitting diodes (OLEDs) with a thin recombination zone. This model is further compared with the resonance interference effect in weak cavities, showing that maximum device outcoupling can be achieved when the device cavity is close to the condition of constructive interference. Recent developments in understanding the optical outcoupling efficiency for OLEDs with a broad recombination zone are further summarized, pointing out that carrier transport is crucial here. For emissive systems with balanced electron and hole transport, the broadening of the recombination zone has only a minor effect on the optical outcoupling efficiency. However, for emissive systems with inferior electron transport due to trapping effects, the recombination zone is located close to the metallic cathode side, leading to pronounced optical losses. Inverted device structures can reduce optical losses and therefore improve device performance. Based on emissive materials with balanced charge transport, high device efficiency and a long operational lifetime can be simultaneously achieved in OLEDs with a broad recombination zone. Potential research directions related to the recombination zone to achieve efficient and stable organic light-emitting devices are discussed as well.