K Improves Cu(In,Ga)Se2 Surface Band Alignment through Reconstruction

K, Rb, and Cs improve Cu(In,Ga)Se2 (CIGS) solar cell performance, but the mechanism remains unclear. Here we use air-free transfer of multiple samples to study KF post-deposition treatments (PDTs) by X-ray photoelectron spectroscopy. The KF PDTs do not change the majority carrier concentration or Cd in-diffusion, but they boost efficiency by 6.1% absolute, improve minority carrier lifetime, and shift the surface valence band further from the Fermi energy. Unlike former reports, the valence-band shift is not a result of lower Cu/(Ga+In) or higher K/(K+Cu) composition ratios. We propose that instead, KF PDTs alter the surface valence-band alignment through a reconstructive phase transformation from chalcopyrite CIGS to K2CuIn3Se6 or KInSe2, which have layered structures. These compounds can leave Cu-free cation planes after K is rinsed away, unlike the structure of chalcopyrite CIGS or CuIn3Se5.