Modeling the impacts of energetic disorder and charge transport on the exciton formation zone in organic light-emitting diodes

The width of exciton formation zone plays a vital role in determining the long-term stability of organic light-emitting diodes (OLEDs). Here, based on the general model of carrier device lifetimes, the dependences of the width and location of exciton formation zone on energetic disorder and charge transport have been simulated in OLEDs with emissive layer featuring hole and electron transport on host and guest molecules, respectively. The width of exciton formation zone increases with emissive layer's energetic disorder increasing. The emissive layer with properly imbalanced carrier mobilities enables larger width of exciton formation zone than that with balanced carrier mobilities, due to the different hole and electron trap densities. Moreover, increasing hole transport layer's hole mobility or decreasing electron transport layer's electron mobility helps increase the width of exciton formation zone. There is no width of exciton formation zone obtained, when hole (electron) transport layer's hole (electron) mobility is smaller than a certain value. The location of exciton formation relies on not only emissive layer's carrier mobilities but also hole (electron) transport layer's hole (electron) mobility. The general model provides the comprehensive picture to elucidate the roles of energetic disorder and charge transport in enhancing the stability of OLEDs, beneficial to improve the OLEDs designs towards the high stability at high luminance.