Features of KF and NaF Postdeposition Treatments of Cu(In,Ga)Se₂ Absorbers for High Efficiency Thin Film Solar Cells

Abstract

The introduction of a KF post-deposition treatment (KF PDT) of Cu(In,Ga)Se 2 (CIGS) thin films has led to the achievement of several consecutive new world record efficiencies up to 21.7% for the CIGS solar cell technology. The beneficial effect of the KF PDT on the photovoltaic parameters was observed by several groups in spite of differing growth methods of the CIGS layer. For CIGS evaporated at lower temperature on alkali-free, flexible plastic substrates, a postdeposition treatment to add Na was already successfully applied. However with the introduction of additional KF under comparable conditions, distinctly different influences on the final absorber alkali content as well as surface properties are observed. In this work we discuss in more details the intrinsically different role of both alkali-treatments by combining several microstructural and compositional analysis methods. The ion exchange of Na by K in the bulk of the absorber is carefully analyzed, and further evidences for the formation of a K-containing layer on the CIGS surface with increased surface reactivity are given. These results shall serve as a basis for the further understanding of the effects of alkali PDT on CIGS and help identifying research needs to achieve even higher efficiencies.

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

“…For the NaF/KF-PDT CIGSe samples, we detect no Na signal, which may be due to the suggested "ion-exchange mechanism" explaining that K replaces Na in the CIGSe during the PDT. [7], [18] Comparing the two NaF/KF-PDT CIGSe absorbers, it is apparent that for the alkali-poor NaF/KF-PDT CIGSe the intensity of the K-related photoemission signals (K 2s and K 2p) is significantly lower than the respective peak intensities for the alkali-rich NaF/KF-PDT CIGSe (see also Fig. 3c2)).…”

Passivation of Interfaces in Thin Film Solar Cells: Understanding the Effects of a Nanostructured Rear Point Contact Layer

“…Semiconductor surfaces, especially semiconductor-metal interfaces, can host several interface defects, leading to high surface recombination velocities, which in turn lower the electrical performance of solar cells. [36,37] These findings have motivated device simulations that have predicted gains up to 3% (in absolute power conversion efficiency) in fully passivated solar cells. The field effect passivation occurs by the presence of a built-in electric field that arises from the high density of fixed charge from the commonly used dielectric materials of the passivation layer.…”

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

“…Werner et al investigated a large set of samples with various PDTs and showed that the main capacitance step has the same origin independent of the alkaline PDT and is consistent with the capacitance step labeled "N1." [27] To extract the drop in capacitance and the activation energy of the capacitance step, the fitting method proposed by Weiss et al is applied to the capacitance spectra. [28] The activation energies of the main capacitance step show a good correlation to the incorporated Rb amount as depicted in Figure 7a.…”