Finding the X factor - X-ray photoelectron spectroscopy
Manufacturing materials consistently to identical standards is becoming more feasible. Tim Nunney and Richard White, from global research company Thermo Fisher Scientific Inc, outline the uses of X-ray photoelectron spectroscopy.
Increasing economic and environmental concerns in relation to energy production have resulted in the advancement of a new era of complex materials and devices, such as thin film solar cells, fuel cells and batteries. Throughout each phase of development, material classification and analysis is essential. Specifically, materials must be examined to confirm compositional homogeneity across the surface, alongside consistent thickness and interfacial chemistry.
As a result of recent advances in spectrometer hardware and software design, the benefits of X-ray photoelectron spectroscopy (XPS) are now increasingly available to those involved in the research of thin film devices. Here, key assessment criteria for two well established thin film devices are explored – the copper indium gallium selenide (CIGS) solar cell, and the proton exchange membrane fuel cell (PEMFC) will be discussed, with particular focus on how XPS as a technique facilitates this assessment.
Down to basics
Accuracy and regulation of surface, thin film and interface structure is vital to the effectiveness of many materials and devices. In applications, such as solar cells, products depend on conductive, semi-conductive and insulating layers. The structure of precise layers within a thin film device is calculated to produce specific physical, chemical and electrical properties. However, absolute control over the intended chemistry is one of the biggest challenges within chemical engineering. Deviations can frequently arise, in particular within interfacial regions. Indeed, it is rare for analytical techniques to successfully examine these variant materials in one isolated analysis.
X-ray photoelectron spectroscopy uses the photoelectric effect. Taken from an atom situated in the surface region, an electron is removed by an X-ray photon. The electron can only be released if that photon contains energy beyond that of the electron’s binding energy to the nucleus. The resulting electron’s kinetic energy is measured by the spectrometer. This enables the binding energy to be calculated using the relation – Kinetic energy (KE) = Photon energy (hv) – Binding energy (BE).
An X-ray source, electron energy analyser and detection system are the three principle elements of the XPS spectrometer (see image left). To reduce energy losses frequently suffered by emitted photoelectrons when encountering gas molecules, XPS spectrometers must function under ultra-high vacuum conditions, with 1x10-7mbar or higher as the standard. All samples compatible with this environment can be analysed, with minimal preparation.
X-ray photoelectron spectroscopy has unique surface sensitivity, enabling a vertical depth resolution on the nanometre scale (typically <10nm). This surface sensitivity is a result of the limited path length of the photoelectrons in a solid. Only those photoelectrons that originate from within close proximity to the surface possess a high probability of escaping without the loss of energy. Sharp peaks on a wide background form the XPS spectrum (see images, below, and charts D and E on this page) with the peaks resulting from electrons that can escape without energy loss and the stepped background from those inelastically scattered in the surface.
Fuel cell characterisation
This study examines how XPS is used to analyse a PEMFC. Reduced operating temperatures and increased higher conversion efficiencies are two benefits of this device over solid oxide fuel cells.
The membrane electrode assembly is at the core of these devices and contains layers of platinum in carbon black, which mobilise the response of hydrogen and oxygen. Its key purpose is to increase the surface area of platinum, while constraining it to within the electrode layers. Any reduction of the surface area reduces device competency. If high currents erode the carbon black structure, platinum loss can arise, therefore releasing the active metal and enabling it to transfer to the nearby polymer electrolyte.
Hydrogen ion mobility in the electrolyte is also prevented by platinum migration into the electrolyte layer. The purpose of this analysis is to ascertain whether the platinum has migrated from the electrode layers into the electrolyte.
Despite the low concentration of platinum (<0.5%), XPS easily identifies its distribution within the device. The platinum was found only within the carbon black electrode layer and had not migrated into the Nafion electrolyte.
Inside the cell
Solar cells founded on CIGS thin films are under development as a potentially more efficient and low-cost alternative to silicon-based solar cells. Typically, CIGS cells consist of a thin-film stack on a glass substrate. The molybdenum and the zinc oxide layers form the electrical contacts at the front and back of the device. Between these layers the p-type CIGS film acts as the sunlight absorber layer, with a thin n-type CdS layer forming a p-n junction at the interface.
The XPS depth at the interface profile reveals the chemical structure of the solar cell. The profile indicates the crucial composition gradient of gallium and indium inside the CIGS layer. Precise control of the Ga in gradient is key to the device’s efficiency as this ratio determines the band gap of the material. Band gap tuning of the device controls the range of wavelengths of light that are converted to electrical energy. The ability of XPS spectrometers to provide composition assessments out of complicated multi-component films alongside continuing good depth resolution is indicated by the calibre of this profile.
The need for new thin film devices for cleaner power sources continues to intensify and is equalled by a need for instrumentation that can clarify the chemical characteristics of ultra-thin films and interfaces. As demonstrated, XPS is an important tool in the surface analysis process, offering unique materials information to complement other analytical techniques.
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