Insights into Intrinsic Defects and the Incorporation of Na and K in the Cu₂ZnSnSe₄ Thin-Film Solar Cell Material from Hybrid-Functional Calculations

Abstract

We have performed density functional theory calculations using the HSE06 hybrid functional to investigate the energetics, atomic, and electronic structure of intrinsic defects as well as Na and K impurities in the kesterite structure of the Cu 2 ZnSnSe 4 (CZTSe) solar cell material. We found that both Na and K atoms prefer to be incorporated into this material as substitutional defects in the Cu sublattice. At this site highly stable (Na−Na), (K−K), and (Na−K) dumbbells can form. While Na interstitial defects are stable in CZTSe, the formation of K interstitial defects is unlikely. In general, the calculated formation energies for Na-related defects are always lower compared to their K-related counterparts. On the basis of thermodynamic charge transition level calculations, we can conclude that the external defects are harmless except Na Sn and K Sn . These defects induce gap states that might be detrimental for the device performance.

CZTSe solar cells developed on polymer substrates: Effects of low‐temperature processing

“…18 Finally, K has been calculated to preferably incorporate into CZTSe as K Cu antisite similarly to Na. 65 Thus, a similar increase in carrier concentration (through the avoidance of Zn Cu ) is expected and consistent with our results.…”

“…The creation of such an electronically inverted absorber surface (n‐type) is also plausible in CZTSe devices and would reduce recombination in the CZTSe/CdS interface improving Voc and FF . Finally, K has been calculated to preferably incorporate into CZTSe as K Cu antisite similarly to Na . Thus, a similar increase in carrier concentration (through the avoidance of Zn Cu ) is expected and consistent with our results.…”

Na‐Diffusion Enhanced p‐type Conductivity in Cu(In,Ga)Se2: A New Mechanism for Efficient Doping in Semiconductors

“…Second, unlike Na doping, K doping leads to the formation of a K-enriched layer on the CIGS surface, [14,46,52,53] which is water-insoluble and hinders the out-diffusion of K from the CIGS grain interiors. [55][56][57] Since Na is a fast diffusor in CZTSSe [58,59] and prefers replacing Cu when incorporated into the CZTSSe grains, [60,61] Na can also promote the formation of V Cu and thus increase the hole concentration in the CZTSSe grains, similar to the case in CIGS. We then conclude that K is not as efficient as Na in increasing the p-type conductivity of CIGS films, in good agreement with the experimental observations that K is not as efficient as Na in increasing the hole concentration in CIGS films.…”

Defect properties of Na and K in Cu2ZnSnS4 from hybrid functional calculation

“…Recently, there have been investigations on Na/K defects in CZTSe using GGA-PBE functional 16 and HSE hybrid functional. 17 Apart from the inadequacy of GGA functional as pointed out by Ref. 17, detailed investigations of the chemical potentials and phase diagrams have been lacking.…”

“…17 Apart from the inadequacy of GGA functional as pointed out by Ref. 17, detailed investigations of the chemical potentials and phase diagrams have been lacking. Selecting a chemical potential outside of the CZTSSe single phase region places the system into an unphysical thermodynamic situation.…”