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

Abstract

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.

Electroreflectance of Cu2(Cd1−x,Znx)SnS4Levcenko1, Hadke2, Wong3 et al. 2021Phys. Rev. Materials0000View full textAdd to dashboardCite

“…However, the interpretation of the high-energy feature is more complex and it can be associated with Franz-Keldysh oscillations (FKO) induced by the internal p-n junction field [12,13] or with distortion of the ER signal due to interference effect of the window overlayer [14,18] or even to contribution from the high-energy unknown transition. In the case of the interference effect, it has been experimentally demonstrated that different thicknesses of the ZnO:Al window of epitaxial Cu 2 ZnSnSe 4 thin film solar samples resulted in different shape ER signal in the band-edge region of the absorber layer [19]. To obtain relevant information on the possible origin of the spectral ER signal of CCTS sample, we perform a quantitative analysis based on the ER theory [12,13].…”

CZTSe solar cells prepared by co-evaporation of multilayer Cu–Sn/Cu,Zn,Sn,Se/ZnSe/Cu,Zn,Sn,Se stacks

“…Using a function generator, a square wave modulation in reverse bias ( f∼223 Hz, U∼−1.5 V) was applied and the relative change of the reflected intensity (ΔR/R) under an angle of incidence of 30°and an angle of detection of 0°was measured utilizing a lock-in technique. Further details may be found in our previous publication [18].…”

The Bandgap as a Moving Target: Reversible Bandgap Instabilities in Multiple-Cation Mixed-Halide Perovskite Solar Cells

“…Using this technique, we are able to investigate complete thin-film solar cells in a nondestructive way under operation-relevant conditions. The principle of the analysis is based on the fact that segregation of halide ions directly affects the bandgap energy of the absorber layer. − In particular, electroreflectance (ER) spectroscopy is well suited for the precise determination of small energy shifts of critical points in the band structure, such as the bandgap. − By applying an ac bias to the solar cell and, thereby, periodically modulating the electric field in the absorber layer, the dielectric function ε is modulated. The dielectric function, in turn, is directly related to measurable optical properties, e.g., the reflectance R .…”