Dielectric function of Cu(In, Ga)Se2-based polycrystalline materials

Abstract

The dielectric functions of Cu(In, Ga)Se2(CIGS)-based polycrystalline layers with different Ga and Cu compositions have been determined by applying spectroscopic ellipsometry (SE) in a wide energy range of 0.7–6.5 eV. To suppress SE analysis errors induced by rough surface and compositional fluctuation, quite thin CIGS layers (<60 nm) with high uniformity toward the growth direction have been characterized using a self-consistent SE analysis method. We find that the optical model used in many previous studies is oversimplified particularly for the roughness/overlayer contribution, and all the artifacts arising from the simplified analysis have been removed almost completely in our approach. The CIGS dielectric functions with the variation of the Ga composition [x = Ga/(In + Ga)] revealed that (i) the whole CIGS dielectric function shifts toward higher energies with x, (ii) the band gap increases linearly with x without the band-gap bowing effect, and (iii) the overall absorption coefficients are significantly smaller than those reported earlier. Furthermore, the reduction of the Cu composition [y = Cu/(In + Ga)] leads to (i) the linear increase in the band-edge transition energy and (ii) the decrease in the absorption coefficient, due to the smaller interaction of the Cu 3d orbitals near the valence band maximum in the Cu-deficient layers. When y > 1, on the other hand, the free-carrier absorption increases drastically due to the formation of a semi-metallic CuxSe phase with a constant band gap in the CIGS component. In this study, by using a standard critical-point line-shape analysis, the critical point energies of the CIGS-based layers with different Ga and Cu compositions have been determined. Based on these results, we will discuss the optical transitions in CIGS-based polycrystalline materials.

Band gap widening at random CIGS grain boundary detected by valence electron energy loss spectroscopy

“…According to the literature, it is expected that Cu depletion causes a downward bending of the valence band that could explain the observed band gap widening at the grain boundary. 8 However, in the present specimens, the actual energy changes of the valence band due to compositional variations still need to be determined. In general, the downwards bending of the valence band at the grain boundary and the related valence barrier are supposed to repel the holes from the grain boundary.…”

“…The valence band maximum (E V ) is determined by the Cu 3d and Se 4p antibonding states, and therefore Cu depletion lowers the valence band maximum. 8,11,12 The conduction band minimum (E C ) strongly depends on the GGI present: increasing Ga concentration shifts the conduction band minimum towards higher energies. 8,[12][13][14] Regarding the three models, the measured band gap widening at the grain boundary can be explained by the valence band barrier model (Figure 3(b)), which is also in agreement with the coincident Cu depletion.…”

A comparative study on the electronic and optical properties of Sb-=SUB=-2-=/SUB=-Se-=SUB=-3-=/SUB=- thin film

“…4) and Table 2 it is apparent that the experimeantal results are much closer to the theoretical calculations. Importantly, the obtained dielectric constant is larger than the copper indium gallium diselenide (CIGS) [54]. The large value of dielectric constant of Sb 2 Se 3 suggests that smaller exciton binding energy and possibly transient separation of electrons and holes pair upon photons irradiation and the large value of ε ′′ related to the dielectric loss is favorable for light harvesting in Sb 2 Se 3 film.…”

On-line monitoring of solar cell module production by ellipsometry technique

“…This approach reduces the number of fitting parameters for (ε 1 , ε 2 ) from several to just one: the Ga content x. [15][16][17][18] Because (ε 1 , ε 2 ) is now represented by this single parameter, the chances of parameter correlations during fitting are reduced, enabling production-scale compositional mapping of chalcopyrite films by SE.…”