Role of Guest Materials in the Lower Stability of Solution-Coated versus Vacuum-Deposited Phosphorescent OLEDs

Utilizing different phosphorescent materials as emitter guests, this work investigates the root causes of the lower electroluminescence (EL) stability of solution-coated (SOL) organic light-emitting devices (OLEDs) relative to their vacuum-deposited (VAC) counterparts. The results show that emitter guest molecules aggregate under electrical stress, leading to the emergence of new longer-wavelength bands in the EL spectra of the devices over time. However, the intensity of these aggregation emission bands is much stronger in the case of SOL host:guest systems than that of their VAC counterparts, indicating that guest aggregation occurs much faster in the former. The results reveal that the phenomenon arises from differences in the initial morphologies and are likely associated with the use of solvents in the solution-coating process. Moreover, although excitons can drive this aggregation in the case of SOL emissive layer (EML) devices, the coexistence of excitons and polarons accelerates this phenomenon significantly. The results uncover one of the main causes of the lower stability of OLEDs made by solution coating and reveal the importance of adopting new molecular designs that make them less susceptible to aggregation for the development of SOL OLEDs with high performance.