Potassium Postdeposition Treatment-Induced Band Gap Widening at Cu(In,Ga)Se₂ Surfaces – Reason for Performance Leap?

Abstract

Direct and inverse photoemission were used to study the impact of alkali fluoride postdeposition treatments on the chemical and electronic surface structure of Cu(In,Ga)Se2 (CIGSe) thin films used for high-efficiency flexible solar cells. We find a large surface band gap (E(g)(Surf), up to 2.52 eV) for a NaF/KF-postdeposition treated (PDT) absorber significantly increases compared to the CIGSe bulk band gap and to the Eg(Surf) of 1.61 eV found for an absorber treated with NaF only. Both the valence band maximum (VBM) and the conduction band minimum shift away from the Fermi level. Depth-dependent photoemission measurements reveal that the VBM decreases with increasing surface sensitivity for both samples; this effect is more pronounced for the NaF/KF-PDT CIGSe sample. The observed electronic structure changes can be linked to the recent breakthroughs in CIGSe device efficiencies.

Injection Current Barrier Formation for RbF Postdeposition‐Treated Cu(In,Ga)Se2‐Based Solar Cells

“…[25] It was found that similar to a KF PDT, a surface layer forms. [5][6][7][8][9]26,30] Additionally, as presented here for a RbF PDT, with the introduction of a KF PDT, a blocking of the diode current was observed. [15] Recently, Malitckaya et al calculated the bandgap energies of AlkInSe 2 (Alk = Li, Na, K, Rb, Cs) secondary phases, which are expected to segregate on the surface of the CIGS absorber for K, Rb, and Cs.…”

“…A surface bandgap widening was observed after a KF PDT also experimentally. [8,9] These results hint to the fact that similar to a KF PDT, a RbF PDT introduces a surface layer, which might be responsible for the blocking of the diode current. That Rb-containing surface layer is subsequently called RIS layer.…”

“…Initially, it was found that the KF PDT causes the Cu depletion of the CIGS surface [4] and that a K-In-Se phase forms. [5][6][7][8] Additionally, shown in Figure 1f. A barrier at the front contact is introduced originating from a conduction band offset ΔE C at the iZnO/CdS interface.…”

NaF/KF post-deposition treatments and their influence on the structure of Cu(In, Ga)Se<inf>2</inf> absorber surfaces

“…[7], [10], [11] Furthermore, it was shown that this treatment has a distinct effect on the electronic surface structure of CIGSe absorbers. [8], [10] We have previously found a pronounced surface band gap (Eg surf ) wideningexplained by the formation of the Cu-and Ga-depleted surface and a K-In-Se type surface species -resulting in Eg surf =(2.52[+0.14/-0.51] eV).…”

Chalcopyrite compound semiconductors for thin film solar cells

“…The Cu-depleted layer was found to have a wider surface band gap than untreated absorbers [16,19] [24e26]). Whether the ion exchange reaction is limited to the grain boundaries or appears also in the bulk is not known currently.…”