Role of Na and Ca as Isovalent Dopants in Cu₂ZnSnS₄ Solar Cells

Abstract

A pathway to improve the efficiency of Cu 2 ZnSnS 4 (CZTS)-based solar cells, which form an important class of beyond-Si, thin-film photovoltaic technology, is the employment of sustainable isovalent dopants (substituting for Cu and Zn) to suppress formation of disorder-inducing, performance-limiting antisite defects. Using calculations based on density functional theory, we examine the influence of Na and Ca as isovalent dopants for Cu and Zn, respectively, on defect formation, thermodynamic stability, and electronic properties of CZTS. On the basis of defect formation energies, we find that Na-doping should be feasible within CZTS while the incorporation of Ca will be difficult. Importantly, both Na and Ca effectively mitigate formation of antisites that cause disorder, if incorporated within the CZTS structure, across doping and Cu-chemical-potential conditions. Thermodynamically, doping high concentrations of Na into CZTS will result in phase separation between CZTS and Na 2 ZnSnS 4 domains, whereas large additions of Ca will lead to formation of other secondary phases, such as Cu 2 SnS 3 and CaS. With respect to electronic properties, Na (Ca) doping should cause a significant (marginal) increase in the band gap of kesterite CZTS. Overall, we suggest low Na-doping in CZTS as a promising pathway to improve performance of CZTS-based solar cells.

Sodium Effects on the Diffusion, Phase, and Defect Characteristics of Kesterite Solar Cells and Flexible Cu2ZnSn(S,Se)4 with Greater than 11% Efficiency

“…[25] Studies on the role of Na, the incorporation of Na, Na passivation, and Na-related defect characteristics have mainly been carried out by first-principles calculations based on density functional theory. [23,[26][27][28][29][30] However, these calculations do not provide sufficient information because the analysis of real devices that support the theoretical results is not linked to each other.…”

Optimizing kesterite solar cells from Cu2ZnSnS4 to Cu2CdGe(S,Se)4

“…Defect structures. All defects were generated in 2 Â 2 Â 2 supercells of their defect-free, bulk kesterite, stannite, or wurtzite structures (consistent with previous studies 57,72,77 ) using the protocol described in Table S6 in Section S5 of the ESI. † The objective of this study was to identify strategies, specically those involving ion substitution, for the suppression of Cu X + X Cu and 2Cu X + Y X and the promotion of V Cu .…”

“…59 Specically, Ag 2 ZnSnSe 4 affords a higher predicted maximum photovoltaic efficiency than Cu 2 ZnSnS 4 , 58 consistent with experiments. [62][63][64][65][66][67][68][69][70] The alkali metals are another promising group of isovalent replacements for Cu, [62][63][64][65][66][67][68][69][70][71][72][73] with theory indicating that <25% Na-doping in CZTS suppresses the formation of Cu Zn + Zn Cu . 72 In addition to the Cu + site, several studies establish isovalent doping on the Zn 2+ site as a promising strategy to improve the performance of CZTS-based solar cells as well.…”

“…[62][63][64][65][66][67][68][69][70] The alkali metals are another promising group of isovalent replacements for Cu, [62][63][64][65][66][67][68][69][70][71][72][73] with theory indicating that <25% Na-doping in CZTS suppresses the formation of Cu Zn + Zn Cu . 72 In addition to the Cu + site, several studies establish isovalent doping on the Zn 2+ site as a promising strategy to improve the performance of CZTS-based solar cells as well. Among the 2+ cations considered, which includes the alkaline earth 72,74,75 and transition metals, 57,[76][77][78] Cd has been identied, by both theory 57 and experiment, 77 as one of the most effective at reducing 1+/2+ and 2+/4+ cation disorder.…”

“…72 In addition to the Cu + site, several studies establish isovalent doping on the Zn 2+ site as a promising strategy to improve the performance of CZTS-based solar cells as well. Among the 2+ cations considered, which includes the alkaline earth 72,74,75 and transition metals, 57,[76][77][78] Cd has been identied, by both theory 57 and experiment, 77 as one of the most effective at reducing 1+/2+ and 2+/4+ cation disorder. Finally, on the 4+ site, Ge stands out as an exceptional candidate for 38,39 and the band gap (E g ), where E SQ g is the band gap that maximizes the Shockley-Queisser (SQ) limit.…”