Distinct effects of Na and heavy alkali (K, Rb) incorporation in Cu(In, Ga)Se2 solar cells

Density functional theory calculations are performed to explore the nature of the alkali incorporation in Cu(In, Ga)Se2 (CIGS) thin-film solar cells. It is revealed that all the alkalis (Na, K, Rb) prefer to accumulate in the surface region of the absorber with occupying Cu sites. A hole barrier can be introduced in the absorber surface region, and the electron barrier at CdS/CuInSe2 interface is nearly changed after alkali incorporation. Distinct effects of Na and heavier alkalis (K and Rb) are explored. Post-deposition treatment (PDT) with Na leads to form NaxCu1−xInSe2 in the near-surface region. While monoclinic KInSe2 (or RbInSe2) secondary phase tends to form at the surface during K (or Rb) PDT. The hole barrier introduced by Na PDT can be much lower than that in the case of heavy alkali PDT due to the limited Na solubility in NaxCu1−xInSe2. The improved VOC in CIGS solar cells by heavy alkali PDT is mainly attributed to the absence of interfacial states and the elimination of isolated InCu and VCu defects in the surface region.