Diffuse electroreflectance of thin-film solar cells: Suppression of interference-related lineshape distortions

Electroreflectance (ER) is a standard method to determine the band gap of semiconductor materials that has also been applied to thin-film solar cells (TFSCs). However, the lineshapes in typical ER spectra of TFSCs are significantly distorted compared to the model lineshapes, which are used for spectrum evaluation. These distortions are mainly due to thin-film interferences in the stratified system. In this letter, we demonstrate that these distortions are significantly suppressed in diffuse ER (D-ER) where the diffuse instead of the specular reflection of TFSCs is evaluated. The existence of an ER signal in the diffuse reflectance is shown by two-dimensional finite-difference time-domain simulations. Experimentally, the suppression of interference-related lineshape distortions is demonstrated on a series of Cu₂ZnSnSe₄ solar cells with different layer thicknesses and therefore different optical path lengths for interference. The same working principle is demonstrated for a Cu(In,Ga)(S,Se)₂ solar cell as well. The resulting lineshapes in D-ER can then be interpreted using standard analysis methods such as Aspnes' Third-Derivative Functional Form.