No Evidence for Passivation Effects of Na and K at Grain Boundaries in Polycrystalline Cu(In,Ga)Se₂ Thin Films for Solar Cells

Abstract

Thin‐film solar cells based on Cu(In,Ga)Se2 (CIGSe) absorber layers reach conversion efficiencies of above 20%. One key to this success is the incorporation of alkali metals, such as Na and K, into the surface and the volume of the CIGSe thin film. The present work discusses the impact of Na and K on the grain‐boundary (GB) properties in CIGSe thin films, i.e., on the barriers for charge carriers, Φb, and on the recombination velocities at the GBs, sGB. First, the physics connected with these two quantities as well as their impact on the device performance are revised, and then the values for the barrier heights and recombination velocities are provided from the literature. The sGB values are measured by means of a cathodoluminescence analysis of Na‐/K‐free CIGSe layers as well as on CIGSe layers on Mo/sapphire substrates, which are submitted to only NaF or only KF postdeposition treatments. Overall, passivating effects on GBs by neither Na nor K can be confirmed. The GB recombination velocities seem to remain on the same order of magnitude, in average about 103–104 cm s−1, irrespective of whether CIGSe thin films are Na‐/K‐free or Na‐/K‐containing.

Microscopic origins of performance losses in highly efficient Cu(In,Ga)Se2 thin-film solar cells

“…In contrast, band-gap fluctuations of 40–45 meV decrease the V oc by about 30 mV. Moreover, enhanced recombination at GBs decrease the V oc and FF of the device (compared with their theoretical limits) substantially, although rather low, average (effective) recombination velocities at GBs of several 100 cm s −1 are determined (as compared with values of several 1000 cm s −1 measured on Na/K-free or Na/K-containing CIGSe layers without any subject to RbF PDT 39 . It will be matter of future studies to reveal whether these low S GB values can indeed be traced to a passivation effect of Rb at CIGSe GBs.…”

“…In this respect, we would like to note that Siebentritt et al 38 reported evidence for the passivating effect of K, Rb, and Cs at GBs (via the reduction of barriers Φ b for charge carriers), while Abou-Ras et al 39 , using CL analyses, did not confirm that a KF PDT of CIGSe surfaces leads to passivation of the GBs in these absorber layers (i.e., the effective recombination velocities S GB,eff remain at about the same level with and without the KF PDT). This discrepancy can be traced back at least in part to the fact that Siebentritt et al applied surface-sensitive scanning Kelvin-probe force microscopy for the measurement of the barriers heights Φ b at the GBs and that GB properties at the very surface of a CIGSe layer need not be the same as in the CIGSe bulk.…”

Direct evidence for grain boundary passivation in Cu(In,Ga)Se2 solar cells through alkali-fluoride post-deposition treatments

“…More recent studies, where a KF-PDT was applied indicate also accumulation of K at GBs 29,30 . As a consequence of the relative concentrations of the different alkali elements at GBs and in the grain interior, several effects have been observed 31 , including Cu depletion, In and Se accumulation at GBs 32 , a diminished downward 33 and increased upward 16 band bending at GBs, increase 34–36 or decrease 10 of carrier concentration, and an impact on the GB recombination velocity 37 . A similar range of observations has been reported for RbF-PDT 38–41 .…”

Impact of Na Doping on the Carrier Transport Path in Polycrystalline Flexible Cu2ZnSn(S,Se)4 Solar Cells

“…[30][31][32] Although mainstream principles have been elucidated, there is still much debate on how alkali doping leads to performance improvement. [33] We fabricated a CZTSSe solar cell with an FF of over 60% on a flexible metal substrate and used it to analyze the effects of Na doping. From the results, we postulate that the carrier transport path depends on the amount of Na dopant, which affects the electrical resistance and FF of the polycrystalline CZTSSe solar cells.…”