Depth profiling organic light-emitting devices by gas-cluster ion beam sputtering and X-ray photoelectron spectroscopy

The composition depth profile in organic light-emitting devices (OLEDs) is investigated using X-ray photoelectron spectroscopy (XPS) coupled with gas-cluster Ar-ion beam milling (Ar-GCIB). The XPS technique gives precise information about the surface chemistry of the organic thin films and is capable of differentiating the various charge transport, host and guest materials used in an OLED. The use of large Ar ion clusters (∼1500–2500 atoms/cluster) in the milling process allows for small amounts of the organic thin film to be sputtered away without contaminating the surface or damaging the underlying material chemistry. By probing OLEDs as a function of depth, key parameters including emissive layer composition and interface quality can be assessed directly. It is found that the depth profile for graded-composition emissive layer OLEDs closely matches the intended deposition profiles, maintaining both the composition gradient and the intended endpoint compositions. The ability to resolve and correlate subtle changes in film composition to variations in device performance will help inform efforts in device design, while also serving as a diagnostic tool to better understand the mechanisms for device degradation and failure.