Alkali-induced Grain Boundary Reconstruction on Cu(In, Ga)Se2 Thin Film Solar Cells using Cesium Fluoride Post Deposition Treatment

Heavy alkali metal treatment is a key factor to approach high efficiency in Cu(In, Ga)Se₂ (CIGS) solar cells. Here, we show that the Cs-induced surface modification on CIGS thin-film solar cells, especially on grain boundary reconstruction generated from cesium fluoride post-deposition treatment (CsF-PDT). The CsInSe₂ phase can be synthesized by CsF-PDT process without extra assistant species, and specifically favors grain boundaries (GBs), which was further characterized by FE-EPMA, TEM, and SIMS analysis. The Cs-contained GBs may both create the valence-band downshift and conduction-band upward. By controlling the Cs-contained layer thickness and modifying absorber, the spike at conduction-band can be overcome by tunneling while remaining high valence-band downshift to suppress the GBs recombination. The efficiency was improved from 15.1 to 18.3% after CsF-PDT; the voltage loss of E_g/q–V_oc was down below 0.4 V. Those improvements are mainly contributed from the grain boundary reconstruction at the junction area using CsF-PDT to reduce the recombination.