Achilles Heels of Phosphine Oxide Materials for OLEDs: Chemical Stability and Degradation Mechanism of a Bipolar Phosphine Oxide/Carbazole Hybrid Host Material

For long-living organic light-emitting diodes (OLEDs), the chemical stability of all employed materials is essential. In this work, we take a typical bipolar material, 9-(3,5-bis(diphenylphosphoryl)phenyl)-9H-carbazole (CzPO2), as an example for exploring the intrinsic chemical stability of the hot-spot phosphine oxide (PO) based materials for OLEDs. Compared to the carbazole-only counterparts, PO-based carbazole materials typified by CzPO2 have prominent advantages in terms of electrochemical stability and bipolar character, which are generally required for improving the device stability. However, we discovered that CzPO2 suffers a fatal chemical instability just originating from the PO moieties. Under UV irradiation or electrical stress, the identified degradation products of CzPO2 point to the dissociation of relatively weak C-P bonds as the initiating step. Quantum chemical calculations were carried out to gain further insight into the role of the C-P single bond in the intrinsic degradation mechanism associated with the aging process of OLEDs. The cleavage of vulnerable C-P single bonds may occur not only in excited states, but also more easily in charged states. These findings strongly suggested that the chemically unstable C-P bond of PO derivatives could undermine the stability of the corresponding OLEDs, regardless of the function that the PO materials played in devices. For improving the lifetimes of OLEDs, it is highly suggested to consider the relative bond strengths in charged states or excited states of OLED materials, in addition to the generally required thermal stability.