Revisiting the Cu-Zn Disorder in Kesterite Type Cu2ZnSnSe4 Employing a Novel Approach to Hybrid Functional Calculations

Abstract

In recent years, the search for more efficient and environmentally friendly materials to be employed in the next generation of thin film solar cell devices has seen a shift towards hybrid halide perovskites and chalcogenide materials crystallising in the kesterite crystal structure. Prime examples for the latter are Cu2ZnSnS4, Cu2ZnSnSe4, and their solid solution Cu2ZnSn(SxSe1−x)4, where actual devices already demonstrated power conversion efficiencies of about 13 %. However, in their naturally occurring kesterite crystal structure, the so-called Cu-Zn disorder plays an important role and impacts the structural, electronic, and optical properties. To understand the influence of Cu-Zn disorder, we perform first-principles calculations based on density functional theory combined with special quasirandom structures to accurately model the cation disorder. Since the electronic band gaps and derived optical properties are severely underestimated by (semi)local exchange and correlation functionals, supplementary hybrid functional calculations have been performed. Concerning the latter, we additionally employ a recently devised technique to speed up structural relaxations for hybrid functional calculations. Our calculations show that the Cu-Zn disorder leads to a slight increase in the unit cell volume compared to the conventional kesterite structure showing full cation order, and that the band gap gets reduced by about 0.2 eV, which is in very good agreement with earlier experimental and theoretical findings. Our detailed results on structural, electronic, and optical properties will be discussed with respect to available experimental data, and will provide further insights into the atomistic origin of the disorder-induced band gap lowering in these promising kesterite type materials.

Disorder induced band gap lowering in kesterite type Cu2ZnSnSe4 and Ag2ZnSnSe4: a first-principles and special quasirandom structures investigation

“…Again, calculating the density of states (DOSs) for each of the ten different cation distributions for CZTSe and AZTSe and evaluating weighted averages, gave access to the DOSs for the disordered kesterite crystal structures as shown in the lower panels of figure 4 for CZTSe (a) and AZTSe (b), respectively. It should be mentioned that a recently devised approach to speed up hybrid functional calculations has been applied to investigate Cu-Zn disorder in CZTSe, and obtained very similar results to the ones presented here [30]. In comparison, the upper panels of figure 4 show the DOSs for the ordered kesterite crystal structures.…”

“…Due to the large number of special quasirandom structures that have to be taken into account, full structural relaxations utilising hybrid exchange and correlation functionals would be computationally too demanding and are not feasible yet. However, a recent paper [30] discussed a computational method with CZTSe as a benchmark case, that allows for a significant speed up of the underlying structural relaxations using a hybrid exchange and correlation functional, without having too much to compromise on the quality and validity of the results.…”

Current Status of the Open-Circuit Voltage of Kesterite CZTS Absorber Layers for Photovoltaic Applications—Part I, a Review

“…As those defects primarily involve the Cu and the Zn atoms, this type of disorder is also called “Cu/Zn disorder” within the space group [ 17 ]. In this disordered kesterite, the Cu atoms fully occupy the 2a Wyckoff position (the z = 0 plane), and the Cu an Zn atoms are mixed only in the 2c and 2d Wyckoff positions (the z = 1/4 and 3/4 planes) [ 15 , 18 , 19 , 20 ]. CZTS is intrinsically a p-type semiconductor due to the presence of the acceptor defects (i.e., copper vacancies (V Cu ) and copper-on-zinc (Cu Zn ) anti-site defects characterized by lower formation energies than the donor defects [ 21 ].…”