Paper AS+BI-TuM11
XPS Profiling and Work Function Mapping of a Damaged Solar Cell

Tuesday, October 30, 2012, 11:20 am, Room 20

In many areas of materials technology, it is important to control both the chemical composition and the electrical properties of the material. One example of this need is in the manufacturing of solar cells. In this case, the solar cell is based on a thin film of CIGS (Cu (In, Ga) Se2). The full structure of the device includes an upper electrode containing indium tin oxide (ITO), zinc oxide, and cadmium sulfide. The whole structure is separated from a steel substrate using layers of molybdenum and chromium.

It has been demonstrated previously that X-ray Photoelectron Spectroscopy (XPS) is the ideal technique for characterizing the compositional depth profiles of CIGS solar cells, similar to the one described above. Using XPS it is possible to measure elemental composition gradients in the CIGS layers (allowing engineers to tune the band gap of the device) and also to investigate chemistry at interfacial layers. XPS can also be used to measure another very important parameter of solar cells, i.e. the work function. This measurement relies upon the spectrometer being accurately calibrated and the photon energy being accurately known. On a modern XPS instrument, internal standard samples (copper, silver, and gold) may be used to automatically calibrate the XPS binding energy scale. The photon energy can be checked by measuring the position of an X-ray induced Auger peak on the binding energy scale and adding it to the known kinetic energy for that peak in the Auger spectrum.

This work demonstrates the use of XPS to characterize a damaged solar cell, using depth profiling to identify the delamination zone in the solar cell stack. The surface of the delaminated cell has also been mapped for elemental and work function information.