Investigating the Role of Emissive Layer Architecture on the Exciton Recombination Zone in Organic Light-Emitting Devices

An experimental approach to determine the spatial extent and location of the exciton recombination zone in an organic light‐emitting device (OLED) is demonstrated. This technique is applicable to a wide variety of OLED structures and is used to examine OLEDs which have a double‐ (D‐EML), mixed‐ (M‐EML), or graded‐emissive layer (G‐EML) architecture. The location of exciton recombination in an OLED is an important design parameter, as the local optical field sensed by the exciton greatly determines the efficiency and angular distribution of far‐field light extraction. The spatial extent of exciton recombination is an important parameter that can strongly impact exciton quenching and OLED efficiency, particularly under high excitation. A direct measurement of the exciton density profile is achieved through the inclusion of a thin, exciton sensitizing strip in the OLED emissive layer which locally quenches guest excitons and whose position in the emissive layer can be translated across the device to probe exciton formation. In the case of the G‐EML device architecture, an electronic model is developed to predict the location and extent of the exciton density profile by considering the drift, diffusion, and recombination of charge carriers within the device. The impact of emissive layer architecture on the exciton recombination zone in organic light‐emitting devices is investigated. A technique to measure the location and spatial extent of exciton recombination in organic light‐emitting devices is demonstrated. The technique relies on the Förster energy transfer of an exciton from a luminescent guest to a dilute sensitizing molecule included in narrow strips within the emissive layer. It is found that the recombination zone depends strongly on emissive layer composition and architecture.