Potassium versus Sodium in Cu(In,Ga)Se₂—Similarities and Differences in the Electrical Characteristics of Solar Cells and Thin Films after NaF or KF Postdeposition Treatment

Abstract

The improvement of CIGSe solar cells efficiency due to potassium fluoride postdeposition treatment (KF‐PDT) is often interpreted as being caused by changes in the interface region. The impact of KF‐PDT on bulk properties of the CIGSe absorbers is investigated by measuring hole densities in the solar cells and conductivities of corresponding thin films and is then analyzed together with similar data for NaF‐treated absorbers and thin films. It is found that both alkali metals change the electrical properties of the whole CIGSe layer and not only the absorber surface. By measuring electrical conductivity and doping level in thermal equilibrium (relaxed) and metastable state (after light soaking) as well as their evolution with the amount of Na and K after postdeposition treatment (PDT) at various temperatures, one can observe effects that can only be explained by the influence of alkali metals on grain boundary properties. In contrast to Na, low concentration of potassium leads to a decrease in conductivity. Persistent photoconductivity exhibits a linear relationship with the relaxed conductivity in both NaF‐ and KF‐treated samples. These effects directly show that electrical transport in CIGSe is limited by potential barriers at the grain boundaries, which are impacted by the alkali treatment.

Anisotropic Charge Transport in Cu(In,Ga)Se2 by Heavy Alkali Postdeposition Treatment for Reducing Cell‐to‐Module Efficiency Loss in Monolithically Integrated Photovoltaic Modules

“…Czudek et al also reported 18 times lower in-plane conductivity for CIGS sample with KF PDT compared to the NaF PDT case in the dark at room temperature. [16] Although their absolute conductivities are different from ours due to the use of ZrO 2 substrate as well as the smaller channel length (1 mm apart between Au electrodes on CIGS), the relative difference is quite similar and meaningful. Particularly, we discover that the lower conductivity with NaF þ KF PDT than NaF PDT is mostly because of the much lower mobility, not because of the carrier concentration.…”

“…Given that other heavy alkali PDTs are also energetically favorable to form a secondary phase at the GB where the stability range increases in the order of Na, K, Rb, and Cs, [5b] it is worth trying RbF or CsF PDTs in our future works. Furthermore, it is reported that the horizontal conductivity with KF PDT becomes much lower at lower PDT temperatures, [ 16 ] so manipulating the PDT temperature may offer further rooms for an advancement in the module performance.…”

Chalcopyrite solar cells —state-of-the-art and options for improvement

“…This defect shows metastable transitions between donor type and acceptor type [50], which can explain the metastable increase of the doping level. It was shown recently that Na and K play a role in the metastable increase of the doping level [51,52]. The In-on-Cu antisite has also been described as a metastable defect [53] which transitions from a shallow donor (the antisite) to a deep donor (a DX defect, where the In atom becomes interstitial) thereby trapping electrons.…”

“…In particular the metastable increase of the doping level has been found to anneal out at temperature above about 200 K [52,55]. Thus, it could be expected that this metastable effects have little influence on solar cell efficiency.…”