Analysis of alloying and built-in voltage in thin film chalcogenide solar cells using modulation spectroscopy

Modulation spectroscopy has been performed on superstrate CdS/CdTe solar cells prepared by close-spaced sublimation. Photoreflectance (PR) measurements were conducted near the CdTe E₀ band edge using a variable-wavelength probe beam in conjunction with a chopped pump beam. A fit to the observed PR spectrum was made using two third-derivative, low-field lineshapes. The band gap of the active material near the junction is 1.46 eV (CdTe_1-xS_x). The envelope function varied as 1 - V/Vbi· The PR spectrum shifted with a temperature coefficient of -0.31 meV/K. Photoreflectance spectroscopy offers a powerful method of determining absorber band gaps, alloying, and built-in voltage in thin-film solar cells.