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.

中文翻译：

---

用于OLED的氧化膦材料的致命弱点：双极性氧化膦/咔唑混合主体材料的化学稳定性和降解机理

对于长寿命的有机发光二极管（OLED），所有采用的材料的化学稳定性至关重要。在这项工作中，我们采用一种典型的双极性材料9-（3,5-双（二苯基磷酰基）苯基）-9 H-咔唑（CzPO2），以探索用于OLED的基于热点氧化膦（PO）的材料的固有化学稳定性为例。与仅咔唑的对应物相比，以CzPO2为代表的PO基咔唑材料在电化学稳定性和双极性特性方面具有突出的优势，这通常是提高设备稳定性所必需的。但是，我们发现CzPO2遭受致命的化学不稳定性，而该化学不稳定性仅源自PO部分。在紫外线照射或电应力作用下，已鉴定出的CzPO2降解产物表明，相对较弱的C-P键解离为起始步骤。进行了量子化学计算，以进一步了解C–P单键在与OLED老化过程相关的固有降解机理中的作用。易损的C–P单键的断裂不仅可能在激发态下发生，而且在带电状态下更容易发生。这些发现强烈表明，无论PO材料在设备中发挥何种作用，PO衍生物的化学不稳定的C-P键都可能破坏相应OLED的稳定性。为了提高OLED的寿命，强烈建议除了通常需要的热稳定性之外，还要考虑OLED材料在充电状态或激发状态下的相对结合强度。无论PO材料在设备中发挥的功能如何。为了提高OLED的寿命，强烈建议除了通常需要的热稳定性之外，还要考虑OLED材料在充电状态或激发状态下的相对结合强度。无论PO材料在设备中发挥的功能如何。为了提高OLED的寿命，除通常需要的热稳定性外，强烈建议考虑OLED材料在充电状态或激发状态下的相对结合强度。