  X-ray photoemission spectroscopy (XPS) depth profiling using monoatomic Ar+
ion etching sources is a common technique that allows for the probing of the
vertical compositional profiles of a wide range of materials. In polymer-based
organic photovoltaic devices, it is commonly used to study compositional
variations across the interfaces of the organic active layer with charge
extraction layers or electrodes, as well as the vertical phase separation
within the bulk-heterojunction active layer. It is generally considered that
the damage induced by the etching of organic layers is limited to the very top
surface, such that the XPS signal (acquired from the top ~10 nm of the layer)
remains largely unaffected, allowing for a reliable measurement of the sample
composition throughout the depth profile. Herein, we investigate a range of
conjugated polymers and quantify the depth of the damage induced by monoatomic
etching for Ar+ ion energies ranging from 0.5 keV to 4 keV using argon gas
cluster ion beam depth profiling. The results demonstrate that even when
etching with the lowest available monoatomic ion energy for as little as 3 s,
the damaged polymer material extends deeper into the bulk than the XPS probing
depth. We show that the damaged material distorts the compositional information
obtained by XPS, resulting in erroneous depth profiles. Furthermore, we propose
that only gas cluster ion beam etching sources should be used for depth
profiling of organic conjugated polymers, as those induce significantly less
damage and maintain the compositional information throughout the entire
profile.
