Analyzing Silicon Solar Cells with X-Ray Diffraction

The key to the widespread adoption of photovoltaic (PV) cells is to reduce the cost and increase the efficiency of silicon devices. The commercial PV cells that perform the best have an efficiency around 21% and employ the same high-purity, single-crystal silicon use in the microelectronics industry.

“Mono-like” silicon is cheaper and quicker to grow than electronics-grade silicon, and the latest prototype devices achieve similar performance. The cost per Watt will be even further reduced by increasing the efficiency of mono-like silicon PV cells.

The Challenge

To improve crystal growth, it is important to understand how and why dislocations and other electrically active defects in silicon form as these defects can degrade device performance. It is vital to characterize the interaction between silicon and its metallic back plane, commonly fabricated by printing and annealing an aluminum paste, when optimizing the performance of silicon PV cells.

Commercially available aluminum pastes can give rise to different PV performances. Researchers at ESRF beamline BM05 collaborated with the CEA-INES to find out why. They exploited advanced synchrotron X-ray imaging techniques to quantify the role of lattice distortions in PV cells, with and without an aluminum backplane, and when fabricated with different aluminum plates.

The Results

X-ray diffraction topography showed significant distortion induced in the silicon when the aluminum backplane is present [Figure a]. However, in regions where the backplate was removed by etching, the samples showed much less distortion. These and other measurements showed that the distortion and strain of the silicon is directly caused by the contact with the eutectic layer and the aluminum back.

X-ray section topography rocking curve imaging was used to measure the degree of distortion [Figure b]. When compared with photovoltaic measurements, the results strongly suggest a correlation between photovoltaic efficiency and the lattice distortion of the silicon which is in contact with the aluminium and eutectic layers.

(a) Integrated diffracted intensity of a solar cell based on mono-like silicon, in which the left part of the sample has the full cell structure and the right part has the aluminium back plane removed. (b) Section topography rocking curve imaging of cells with back planes made from three different aluminium pastes: “paste 1”, in a cell associated with high photovoltaic efficiency, causes less distortion of the Si wafer, while “paste 3” (corresponding to a less efficient cell) induced misorientations up to 8 x 10–3 degrees. [T Thi et al. 2015 Solar Energy Materials and Solar Cells 135 17--21]

(a) Integrated diffracted intensity of a solar cell based on mono-like silicon, in which the left part of the sample has the full cell structure and the right part has the aluminium back plane removed. (b) Section topography rocking curve imaging of cells with back planes made from three different aluminium pastes: “paste 1”, in a cell associated with high photovoltaic efficiency, causes less distortion of the Si wafer, while “paste 3” (corresponding to a less efficient cell) induced misorientations up to 8 x 10–3 degrees. [T Thi et al. 2015 Solar Energy Materials and Solar Cells 135 17--21]

Conclusion

The results show that synchrotron methodologies are valuable techniques for studying features associated with the electrical efficiency in PV cells as well as the structure and crystalline perfection of silicon.

The ESRF/CEA-INES experiments demonstrated that the efficiency of mono-like silicon is strongly dependant on the grain size, precise composition and amount of aluminum paste used to fabricate the PV cell backplate. The distortion and strain at the Si back surface is a direct result of the eutectic layer, with a higher homogeneity producing the highest efficiencies due to less distortion. Therefore, for the production of high performance solar cells, choosing the appropriate paste is crucial.

The Technique

  • Defects in near-perfect crystals, such as voids, grain boundaries and dislocations, result in misorientation of the crystal lattice via long-range distortion and strain fields
  • X-rays pass through a sample and are diffracted, with a 2D detector recording the distribution of diffracted intensity in order to reveal any departures from crystal perfection
  • The technique is non-destructive and applicable from ingot growth through to functional solar cells
  • Users can run their own experiments of send their samples to be measured by ESRF scientists
  • BM05 is ideal for monochromatic beam topography, white beam topography and rocking curve imaging in both section and projection geometries

This information has been sourced, reviewed and adapted from materials provided by The Platform for Advanced Characterisation Grenoble (PAC-G).

For more information on this source, please visit The Platform for Advanced Characterisation Grenoble (PAC-G).

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