Dynamic Impurity Redistributions in Kesterite Absorbers

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.

Alkali Dispersion in (Ag,Cu)(In,Ga)Se2 Thin Film Solar Cells—Insight from Theory and Experiment

“…Traces of O were detected at some GBs for each of the samples investigated, which is a common finding in CIGSe and CZTS absorbers. 66,67 Oxygen contamination is argued to arise primarily through diffusion from the substrate and/or surface oxidation before deposition of a buffer layer. Preferential O segregation is reported for GBs with a specific misorientation angle, 40 but no such correlation has been established for alkali elements.…”

Manipulating the Distributions of Na and Cd by Moisture‐Assisted Postdeposition Annealing for Efficient Kesterite Cu2ZnSnS4Solar Cells

“…HJA can also facilitate the supplementary diffusion of alkali metals from the deep bulk to the front surface and within CdS, [ 3 ] which has been reported even at 50–200 °C. [ 38,48 ] In Figure 4e, SIMS result from the device with the combined treatments shows a distinctive Na peak within the CdS layer, in contrast to the removal of the near‐surface Na peak caused by MA‐350C, shown in Figure 1b. This rediffusion of alkali metals during HJA tends to compensate the substantial loss of Na within the junction region that occurred during MAPDA, but the concentration of Na within the junction still remains lower than that of the HJA‐Only devices, shown in Figure 4d.…”

Small Atom Doping: A Synergistic Strategy to Reduce SnZn Recombination Center Concentration in Cu2ZnSnSe4

“…In addition, it has been shown that Na is able to migrate in CZTSSe via Cu vacancies at room temperature. [ 50,51 ] Thus, unless strong electrostatic interactions arise (i.e., bonding), atoms smaller than Na could show a similar behavior, allowing them to outgas or accumulate at the grain boundaries during the cooling down step, while their solubility decreases with temperature.…”