Abstract
Cu2ZnSn(S,Se)4 is a promising nontoxic earth‐abundant solar cell absorber. To optimize the thin films for solar cell device performance, postdeposition treatments at temperatures below the crystallization temperature are normally performed, which alter the surface and bulk properties. The polycrystalline thin films contain relatively high concentrations of impurities, such as sodium, oxygen and hydrogen. During the treatments, these impurities migrate and likely agglomerate at lattice defects or interfaces. Herein, impurity redistribution after air annealing for temperatures up to 200 °C and short heavy water treatments are studied. In addition, nonuniformities of the sodium distribution on a nanometer and micrometer scale are characterized by atom probe tomography and secondary ion mass spectrometry, respectively. Sodium and oxygen correlate to a greater extent after heat treatments, supporting strong binding between the two impurities. Redistributions of these impurities occur even at room temperature over longer time periods. Heavy water treatments confirm out‐diffusion of sodium with more incorporation of oxygen and hydrogen. It is observed that the increased hydrogen content does not originate from the heavy water. The existence of an “ice‐like” layer on top of the Cu2ZnSnS4 layer is proposed.
Highlights
To optimize the thin films for solar cell device performance, postdeposition treatments at temperatures below the crystallization temperature are normally performed, which alter the surface and bulk properties
Sodium and oxygen correlate to a greater extent after heat treatments, supporting strong binding introduced into CZTSSe and may originate from the Mo layer and through contaminations during the processing.[12]
Na is intentionally introduced via the soda-lime We have previously identified the correlation between Na glass (SLG) substrate, or added during the fabrication process, and O in Cu2ZnSnS4 (CZTS) and shown that O can be used for instance, through NaF treatment.[4]
Summary
Spatial variations of sodium are prevalent in the samples of this study, in agreement with previous studies on CZTSSe.[9,10,29,30,31] Figure 1 shows the spatial sodium distribution of two CZTS absorber specimens on the nanometer and micrometer scale using atom probe tomography (APT) and secondary ion mass spectrometry (SIMS) image depth profiling. Other areas in the reconstruction show similar behavior (i.e., bottom left region in Figure 1b) of Na and Cu inhomogeneities that are correlated at the same regions, which is expected to be caused by the same mechanism stemming from migrating lattice defects in the absorber. SIMS image depth profile measurements of Na reveal a nonuniform spatial distribution of the SIMS intensity on the micrometer scale (Figure 1e). Inhomogeneities of Na on a millimeter scale has been observed by performing SIMS at several locations (not shown) This has already been identified by Gershon et al and has been attributed to inhomogeneities in the SLG.[31] To limit the effects of these inhomogeneities, SIMS depth profiles are measured as close as possible to each other after heat treatments and heavy water treatments
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