Engineering better solar cells

Adding selenium (Se) to cadmium telluride (CdTe)-based thin-film solar cells is known to improve performance, but the way this occurs throughout a solar cell has not been understood in detail. In a recently published paper, careful comparisons of Se composition to material properties, including carrier lifetime, cathodoluminescence (CL), grain size, photocurrent and voltage, reveal insights into how performance is improved.

Polycrystalline CdSeTe thin films were first deposited on glass substrates with a technique known as close-spaced sublimation. A CdSeTe alloy source was used for this first step, and a CdTe layer was then deposited on top. Relative to solar cells without Se, the selenium alloying effectively lowers the bandgap and increases photocurrent without commensurate voltage loss, resulting in greater efficiency.

Auger electron spectroscopy and electron backscatter diffraction (EBSD) were used to determine the correlation between Se content and grain size and orientation. EBSD measurements clearly revealed a bilayer structure, with smaller grains in the front (CdSeTe) region and much larger grains in the back (CdTe) region. Auger profiles showed an approximate 8% incorporation of Se at the front interface where small grains exist. CL further revealed reduced bandgap and enhanced CL intensity in the CdSeTe regions compared to the CdTe regions.

Both time-resolved photoluminescence and CL revealed the carrier lifetime is longer in the CdSeTe alloy regions than in the CdTe regions. The authors conclude that Se both passivates grain boundaries and improves grain-interior lifetime, an effect which occurs only with significant alloying. These methods and results give valuable insights for engineering CdTe solar cells by alloying and can be applied to other technologies.

Source: “Recombination and bandgap engineering in CdSeTe/CdTe solar cells,” by X. Zheng, D. Kuciauskas, J. Moseley, E. Colegrove, D. S. Albin, H. Moutinho, J. N. Duenow, T. Ablekim, S. P. Harvey, A. Ferguson, and W. K. Metzger, APL Materials (2019). The article can be accessed at https://doi.org/10.1063/1.5098459 .