Influence of Na on grain boundary and properties of Cu(In,Ga)Se₂ solar cells

Abstract

Whether added from external sources or supplied via substrate, incorporation of Na has always improved the conversion efficiency of polycrystalline thin‐film Cu(In,Ga)Se2 (CIGS) solar cells. Segregation of Na along grain boundary (GB) is thought to be one of the main reasons for better efficiency, and changing its concentration levels may tune the device properties accordingly. Our previous works have shown that changes in GB chemistry over various Ga/In concentrations are the main reason for poor performance of Ga‐rich (18%) CIGS, besides its optimum band gap (1.4 eV) for energy conversion. It was suspected that changes in GB chemistry or device performance or both could be driven by variation in Na concentration at GB. In this research, atomic level fluctuations of Na atoms at GB for various CuIn1−xGaxSe2 thin films were measured accurately with atom probe tomography technique. Fluctuations observed in Na concentration at GB are restricted to the range 0.5–6.5 atoms per nm2 and is irrespective of x value (GGI) and GB surface density. Results also show that changes in GB chemistry for similar CIGS compositions are not necessary due to changes in Na concentration levels at GB. However, an increased density of GBs per unit volume with GGI may indicate an overall rise in Na in thin films and could explain the drop in CIGS performance. Copyright © 2017 John Wiley & Sons, Ltd.

Sodium enhances indium-gallium interdiffusion in copper indium gallium diselenide photovoltaic absorbers

“…A recent APT study of polycrystalline CIGS films suggests that higher GGI ratios may lead to higher Na concentration in the films, due to a higher density of grain boundaries per unit volume 50 . A correlation between Na and Ga concentrations was also shown for polycrystalline CIGS films obtained by selenization of Na-containing metallic precursors 51 .…”

“…The APT analyses show Na and In but no Ga enrichment at the planar defects observed in the film. Grain boundaries of polycrystalline films tend to show In 50 or Ga 57 enrichment, along with substantial amounts of Na.…”

“…refs. 50 and 57 , respectively), on the other hand, could suggest a different solubility of In and Ga supercomplexes in CIGS that may depend on CGI ratio. If either of the two supercomplexes segregates preferentially at the grain boundaries, it could hinder In/Ga interdiffusion across the boundary due to Zener pinning.…”

Alkali Metals Doping for High‐Performance Planar Heterojunction Sb2S3 Solar Cells

“…Therefore, doping of alkali ions into Sb 2 S 3 crystal lattice seems less possible. In this perspective, we speculate that alkali‐metal atoms may reside at the Sb 2 S 3 surface or grain boundaries and coordinate with sulfur, just as the doping of Na in CIGS solar cells . To affirm this assumption, we conducted X‐ray photoelectron spectroscopy (XPS) analysis.…”

“…In this perspective, we speculate that alkali-metal atoms may reside at the Sb 2 S 3 surface or grain boundaries and coordinate with sulfur, just as the doping of Na in CIGS solar cells. [33] To affirm this assumption, we conducted X-ray photoelectron spectroscopy (XPS) analysis. Figure 3a shows the S 2p spectra, there is a double peak at 161.0 and 162.2 eV which are ascribed to the S 2p 3/2 and S 2p 1/2 of Sb 2 S 3 in pristine Sb 2 S 3 sample.…”