Efficiency Boost of (Ag_0.5,Cu_0.5)(In_1-x,Ga_x)Se₂ Thin Film Solar Cells by Using a Sequential Process: Effects of Ag-Front Grading and Surface Phase Engineering

Post-selenization-fabricated (using elemental Se vapor) Cu(In,Ga)Se₂ solar cell efficiency is limited by a low open-circuit voltage, which is attributable to the Ga-deficient surface and insufficient grain growth. In this study, a band-grading structure is demonstrated by combining Ag-front and Ga-back grading in selenized (Ag,Cu)(In,Ga)Se₂ (ACIGSe) absorbers with a properly designed precursor structure (Mo/CuGa/In/AgGa) and high Ag content ([Ag]/([Ag]+[Cu]) = 0.5). The phase evolution during post-selenization reveals that the precursor structure suppresses Ag₂Se formation and promotes the ACIGSe phase formed at a low temperature with enhanced grain growth. A widened surface bandgap by Ag-front grading substantially increases the open-circuit voltage. Furthermore, Ag addition promotes ordered vacancy compound (OVC) formation on the front surface to enlarge the valence band offset, which in turn reduces interface recombination. Furthermore, the OVC phase also assists interface passivation. Promoting surface OVC phase by Ag addition is also validated by first-principles calculations. Furthermore, the K-doped CuGa precursor is used for a ACIGSe absorber to address the significantly reduced carrier density by the Ag addition. With a band-grading structure and surface OVC phase, the superior device achieves an efficiency of > 19%, the highest efficiency by post-selenization with an elemental Se source.