Solar Cell Materials Characterisation

Defect analysis of multicrystalline Si solar cell material

An example for this is the characterisation of inclusions in multicrystalline Si solar grade material. Feedstock impurities and impurities introduced from other sources during crystal growth and wafer sawing can have detrimental effect on solar cell performance. The severity of the impact of a particular type of inclusion is determined by the chemistry and the predominant location of the impurity in the material. Oxford Instruments provides the tools to identify the type of impurity (Fe, SiC, Si₃N₄, etc.) and characterise its environment in terms of crystallography and chemistry. EBSD pattern quality maps (fig.1) show the grain structure of the Si sample and location of inclusions and other defects. Combined EDS and EBSD maps provide chemical analysis of individual defects as well as their location with respect to grain boundaries. X-Ray maps acquired using the X-Max 80 detector attached to an SEM show a sub-micron Fe inclusion. Typically this level of detail required expensive and time consuming synchrotron studies. With the NordlysNano EBSD detector the orientation of grains around the inclusion are mapped, and the misorientation between the grains can be determined. The nature of the grain boundary will control whether the boundary is likely to ‘gather’ impurities which will impact on the cell efficiency.

The crystallographic (EBSD) and chemical (EDS) data can be collected and viewed simultaneously using AZtec, the new integrated nanoanalysis platform from Oxford Instruments. 

Thin film solar cells

Whether you are a manufacturer of thin film solar cells engaged in the constant drive to bring down costs and increase efficiency, or an R&D facility working at the cutting edge of thin film solar cell research, Oxford Instruments offers the right tools to answer key questions about the film structures in your solar cell materials.

For example, to optimise CdTe based solar cells as well as CIS and CIGS solar cells understanding the relationship between layer chemistry, thickness and grain structure is vital as performance is closely linked to these materials properties.

Oxford Instruments gives you the tools to investigate the structure of your layers in two and three dimensions. EDS analysis can give rapid information about the layer composition  and thickness. In many cases, this is possible without having to cross-section the sample, by collecting X-ray spectra from the sample surface (Fig. 2b) and using ThinFilmID, a solution developed by Oxford Instruments which can determine the composition and thickness of surface and subsurface layers. This enables a non-destructive evaluation of your cell either for R&D or quality control purposes.

In cross-section (Fig. 2c), EBSD analysis, can characterise the grain morphology, orientation and prevalent types of grain boundaries (Fig 3). The example shows EBSD maps of the microstructure of CIS thin films. Grain size and grain boundaries are linked to cell efficiency.  Fig. 2 (a) A flexible CIGS thin film solar cell, (b) electron image of a top view of a CIGS solar cell in an SEM, (c) cross-sectional image showing the different layers in the cell.  Fig. 3 EBSD map of a CIS thin film. The colours correspond to various crystallographic directions given by the legend, indicating the local orientations of the CIS thin film.

Compositional and crystallographic Information in 3D

Oxford Instruments 3D solutions in combination with FIB-SEM tools (focused ion beam in combination with electron beam) now make compositional and crystallographic analysis in 3D a reality. An X-Max 80 and/or a Nordlys EBSD detector installed on a FIB-SEM can be combined with Oxford instruments 3D analytical solutions. Samples can be sectioned by FIB milling and compositional or crystallographic data can be acquired from each slice without user intervention. This data can generate virtual 3D models of your sample which can be used to visualise and analyse critical sample properties such as compositional variations, precipitates or grain boundaries in three dimensions (Fig. 4).