Charge transport layers manage mobility and carrier density balance in light-emitting layers influencing the operational stability of organic light emitting diodes

Organic light emitting diodes (OLEDs) consist of several organic layers, including the charge injection layer, charge transport layer, and light emitting layer (EML). Of these layers, the charge transport layer is crucial for ensuring device longevity, but its overall effects on charge transport and corresponding device stability are poorly understood. Herein we report the factors influencing differences in lifetime between two OLEDs with different hole transporting layers (HTLs). Comprehensive electrical analysis of the materials and the devices reveals that the mobility, accumulation, trapping, and the transport path of holes in the EML are totally changed by the HTLs. The charge transport layers affect mobility and carrier density balance in the EML through the modification of the charge transport path and the energetic barrier. This results in a reduction of overbalanced polaron density, which is critical for bond dissociation in excitonic interactions. Consequently, device lifetime is increased sevenfold through modification of the HTL structure without any alteration of the EML. These results imply that the analysis of polaronic transport through impedance spectroscopy is a crucial step in determining the requisite electrical properties for charge transport layers, with a view to maximizing the operational stability of OLEDs.