Investigating Free Charge‐Carrier Recombination in Organic LEDs Using Open‐Circuit Conditions

Abstract Although organic light‐emitting diodes (LEDs) are widely used in commercial applications, a comprehensive knowledge about the occurring recombination pathways is still absent. Especially access to the bandgap is needed to figure out which path the charge carriers take, key to further understand and optimize the latest generations of thin film LED technology. Here, state‐of‐the‐art organic LEDs are treated like solar cells by measuring the open‐circuit voltage under UV illumination. A variation of the illumination intensity and temperature gives access to the effective bandgap, the ideality factor and the recombination strength. A differential evaluation approach is implemented, yielding the full temperature and intensity dependence of these parameters. A radiative and a non‐radiative recombination process are identified and used to reconstruct the current density dependence of the external quantum efficiency. All recombination events are related to an effective bandgap that suggests free charge recombination between the guest and the host material of the emission layer. This approach will thus help to identify and eliminate non‐radiative recombination losses in LEDs based on other type of molecules emitters and materials, such as perovskites or quantum dots, as well. Knowing the effective bandgap will also be essential to realize LEDs with the lowest possible driving voltages.