Efficiency Boost of (Ag_(0.5),Cu_(0.5))(In_(1-x),Ga_x)Se_2 Thin Film Solar Cells by Using a Sequential Process: Effects of Ag-Front Grading and Surface Phase Engineering

摘要

Post-selenization-fabricated (using elemental Se vapor) Cu(In,Ga)Se_2 solar cellefficiency is limited by a low open-circuit voltage, which is attributable to theGa-deficient surface and insufficient grain growth. In this study, aband-grading structure is demonstrated by combining Ag-front and Ga-backgrading in selenized (Ag,Cu)(In,Ga)Se_2 (ACIGSe) absorbers with a properlydesigned precursor structure (Mo/CuGa/In/AgGa) and high Ag content([Ag]/([Ag]+[Cu]) = 0.5). The phase evolution during post-selenization revealsthat the precursor structure suppresses Ag2Se formation and promotes theACIGSe phase formed at a low temperature with enhanced grain growth. Awidened surface bandgap by Ag-front grading substantially increases theopen-circuit voltage. Furthermore, Ag addition promotes ordered vacancycompound (OVC) formation on the front surface to enlarge the valence bandoffset, which in turn reduces interface recombination. Furthermore, the OVCphase also assists interface passivation. Promoting surface OVC phase by Agaddition is also validated by first-principles calculations. Furthermore, theK-doped CuGa precursor is used for a ACIGSe absorber to address thesignificantly reduced carrier density by the Ag addition. With a band-gradingstructure and surface OVC phase, the superior device achieves an efficiency of> 19%, the highest efficiency by post-selenization with an elemental Sesource.