Impact of Sn(S,Se) Secondary Phases in Cu₂ZnSn(S,Se)₄ Solar Cells: a Chemical Route for Their Selective Removal and Absorber Surface Passivation

Abstract

The control and removal of secondary phases is one of the major challenges for the development of Cu 2 ZnSn(S,Se) 4 (CZTSSe)-based solar cells. Although etching processes have been developed for Cu(S,Se), Zn(S,Se), and CuSn(S,Se) secondary phases, so far very little attention has been given to the role of Sn(S,Se). In this paper, we report a chemical route using a yellow (NH 4 ) 2 S solution to effectively remove Sn(S,Se). We found that Sn(S,Se) can form on the surface either because of stoichiometric deviation or by condensation. After etching, the efficiency of devices typically increases between 20 and 65% relative to the before etch efficiencies. We achieved a maximum 5.9% efficiency in Se-rich CZTSSe-based devices. It is confirmed that this feature is related not only to the removal of Sn(S,Se) but also to the unexpected passivation of the surface. We propose a phenomenological model for this passivation, which may open new perspectives for the development of CZTSSe-based solar cells.

Complex Interplay between Absorber Composition and Alkali Doping in High‐Efficiency Kesterite Solar Cells

“…The degradation of FF at high Sn contents is associated with the appearance of Sn(S,Se) 2 secondary phase that segregates on top of the samples ( Figure S2, Supporting Information) and appears at Cu/Sn ratio < 1.65 in the absorber as measured by XRF. [18] At low Sn concentrations the devices show a degradation of V OC , J SC , and FF simultaneously, which is caused by shunting due to the formation of Cu x Se phases which occur at Cu/Sn ratios > 2.0 measured by XRF. [19] The bandgap trends of the kesterite layers are presented in Figure 3d as derived from the inflection point in the long wavelength region of the EQE.…”

Optical properties of Cu_2ZnSnSe_4 thin films and identification of secondary phases by spectroscopic ellipsometry

“…Despite these suitable optical properties and rising interest, there are difficulties in applying this material commercially as an absorber in solar cells. One of the major obstacles is the formation of secondary phases during the growth process, which can be detrimental to the solar cell performance [6,[10][11][12]. Therefore, it is important to investigate CZTSSe absorbers with a quick and non-destructive method to identify structural properties and the existence of secondary phases.…”

Investigation of the SnS/Cu2ZnSnS4 Interfaces in Kesterite Thin-Film Solar Cells

“…3 It has been shown that both etching with (NH 4 ) 2 S solution and using thermal evaporation during the annealing process are effective approaches for removing SnS on CZTS surface. 8 Regarding the CZTS rear, SnS could form due to the initial CZTS composition 23 as well as the back contact decomposition. 24 Therefore, to control the SnS formation on the CZTS rear, efforts to develop suitable CZTS composition and to passivate the back contact are required.…”