Hybrid-Functional Calculations on the Incorporation of Na and K Impurities into the CuInSe₂ and CuIn₅Se₈ Solar-Cell Materials

Abstract

We have studied the energetics, atomic, and electronic structure of Na and K point defects, as well as the (Na−Na), (K−K), and (Na−K) dumbbells in CuInSe 2 and CuIn 5 Se 8 solar cell materials by hybrid functional calculations. We found that although Na and K behaves somewhat similar; there is a qualitative difference between the inclusion of Na and K impurities. Namely, Na will be mostly incorporated into CuInSe 2 and CuIn 5 Se 8 either as an interstitial defect coordinated by cations, or two Na impurities will form (Na−Na) dumbbells in the Cu sublattice. In contrast to Na, K impurities are less likely to form interstitial defects. Instead, it is more preferable to accommodate K either as K Cu substitutional defect, or to form (K−K) dumbbells on Cu substitution positions. Our data show that all (Na−Na), (Na−K), and (K−K) dumbbells can form in both CuInSe 2 and CuIn 5 Se 8 . In the Cu-poor CuIn 5 Se 8 material the pristine Cu vacancies act as the most stable sites where Na and K can be inserted. The formation energy of Na-related defects is generally lower than the corresponding K-related defects, which would mean that if a defect site is already occupied by Na, then it is less likely that K is able to substitute Na during the postdeposition treatment. Regarding the electronic structure of the materials, Na and K point defects located in the Cu sublattice do not create deep defect levels in the gap, so they are not detrimental for the solar cell. In contrast, Se-related substitutional defects introduce defect levels in the gap, which act as charge traps, leading to severe degradation of the device efficiency. However, the formation energy of these Se-related defects are high so that they should have a low concentration in the material.

Effect of KF absorber treatment on the functionality of different transparent conductive oxide layers in CIGSe solar cells

“…Recent work on hybrid‐functional computation of Na and K extrinsic defects in CuInSe 2 and CuIn 5 Se 8 has proven that both elements behave similar in this respect. It is stated that K does not form interstitial defects but rather occupies the Cu sublattice, which leads to a preferred incorporation in copper‐poor material . Indeed, a significantly larger amount of K was found in Cu‐poor absorber regions, which emphasizes the formation of K Cu point defects as suggested in this study.…”

“…It is stated that K does not form interstitial defects but rather occupies the Cu sublattice, which leads to a preferred incorporation in copper-poor material. 24 Indeed, a significantly larger amount of K was found in Cu-poor absorber regions, 25 which emphasizes the formation of K Cu point defects as suggested in this study. It is proposed here, that for a higher anneal- The observed changes in doping density give a possible explanation for the trends in V oc .…”

Alkali Atoms Diffusion Mechanism in CuInSe2 Explained by Kinetic Monte Carlo Simulations

“…Experimentally, point defects in CuInSe 2 (CISe), CuGaSe 2 (CGSe), and CIGSe have been studied using different techniques such as photoluminescence, [15][16][17][18][19][20][21] cathodoluminescence, [22][23][24] and optical absorption measurement. [31][32][33][34] Diffusion of constituent and external elements in the absorber layer were studied extensively as well. [28,29] In addition, deep-level transient spectroscopy probed at least two transition levels in the CIGSe absorber.…”

A short review on the advancements in electroplating of CuInGaSe2 thin films

“…The crystal structure of CIGSe chalcopyrite material is shown in Fig. 1a [13]. The crystal structure can be realized as a doubled unit cell of zinc blend structure with alternating Cu and In atoms [15].…”