Quantifying the Damage Induced by X-ray Photoelectron Spectroscopy Depth Profiling of Organic Conjugated Polymers

Abstract

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 bulkheterojunction 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 2 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.

Energy Level Alignment in Ternary Organic Solar Cells

“…We would like to point out that, while being a destructive method, UPS depth profiling utilizes an argon gas cluster ion etching beam which has been shown to minimize etching damage in organic materials and to preserve the electronic and compositional structure of organic materials. [44,45] To compare the values found by UPS depth profiling to the measured VOC, we average the smaller photovoltaic gap over the entire active layer thickness. We also estimate the error in the photovoltaic gap, which is influenced by the experimental error of the UPS measurement, errors introduced by fitting as well as the result of averaging over the entire active layer.…”

“…UPS and XPS depth profiling was carried out with the MAGCIS dual mode ion source (ESCALAB 250Xi, Thermo Scientific) operated in the argon gas cluster ion beam mode, which has been shown to minimize etching induced material damage. [44,45] The cluster mode was used at an energy of 4 keV with "large" clusters (in the order of Ar2000) and an etch crater size of (2.5 x 2.5) mm 2 . To create a depth profile, XPS or UPS measurements were altered with cluster etch steps and a 10s break after etching to prevent charging.…”

Sequentially Deposited versus Conventional Nonfullerene Organic Solar Cells: Interfacial Trap States, Vertical Stratification, and Exciton Dissociation

“…Depth profiling uses Argon (Ar) ion cluster sputtering which does not induce damage to the electronic and chemical structures of the organic materials. [63] The combination of Ar cluster etching with the highly surfacesensitive UPS offers a superior vertical resolution of 1-2 nm, surpassing the capabilities of traditional XPS depth profiling (normally 5-10 nm). We first probe the spectra of pure PBDB-T and NCBDT films ( Figure S3), which show different distribution of filled states and will be later be used for fitting.…”

Surface and interface characterization of Ru/C/Ru trilayer structure using grazing incidence X‐ray reflectivity and X‐ray fluorescence

“…[4][5][6] However, due to destructive nature of these techniques, significant relaxations and modifications may occur in the film structure during measurements, and therefore, reliable interpretations from the measurements become quite difficult. 7,8 In grazing incidence X-ray fluorescence (GIXRF) technique, antinode positions of X-ray standing waves (XSWs) can be varied across the depth by changing grazing incidence angle. One can selectively excite provides reliable information about the structural parameters and film composition.…”