Current knowledge of the intrinsic defect properties of Cu 2 ZnSnS 4 ͑CZTS͒ is limited, which is hindering further improvement of the performance of CZTS-based solar cells. Here, we have performed first-principles calculations for a series of intrinsic defects and defect complexes in CZTS, from which we have the following observations. ͑i͒ It is important to control the elemental chemical potentials during crystal growth to avoid the formation of secondary phases such as ZnS, CuS, and Cu 2 SnS 3. ͑ii͒ The intrinsic p-type conductivity is attributed to the Cu Zn antisite which has a lower formation energy and relatively deeper acceptor level compared to the Cu vacancy. ͑iii͒ The low formation energy of many of the acceptor defects will lead to the intrinsic p-type character, i.e., n-type doping is very difficult in this system. ͑iv͒ The role of electrically neutral defect complexes is predicted to be important, because they have remarkably low formation energies and electronically passivate deep levels in the band gap. For example, ͓Cu Zn − +Zn Cu + ͔, ͓V Cu − +Zn Cu + ͔, and ͓Zn Sn 2− +2Zn Cu + ͔ may form easily in nonstoichiometric samples. The band alignment between Cu 2 ZnSnS 4 , CuInSe 2 and the solar-cell window layer CdS has also been calculated, revealing that a type-II band alignment exists for the CdS/ Cu 2 ZnSnS 4 heterojunction. The fundamental differences between CZTS and CuInSe 2 for use in thin-film photovoltaics are discussed. The results are expected to be relevant to other I 2-II-IV-VI 4 semiconductors.

Tuning Ir d-orbital occupation via doping Cu into the IrO2 lattice to prepare a highly efficient oxygen evolution reaction catalyst, Cu0.3Ir0.7Oδ.

A thin-film solar cell based on Cu2ZnSn(S,Se)4 (CZTSSe) alloy was recently found to exhibit a light to electricity conversion efficiency of 10%, making it competitive with the more mature Cu(In,Ga)Se2 based technologies. We study the compositional dependence of the physical properties of CZTSSe alloys through first-principles calculations and find that, these mixed-anion alloys are highly miscible with low enthalpies of formation, and the cations maintain the same ordering preferences as the parent compounds Cu2ZnSnS4 and Cu2ZnSnSe4. The band gap of the CZTSSe alloy decreases with the Se content almost linearly, and the band alignment between Cu2ZnSnS4 and Cu2ZnSnSe4 is of type-I, which allows for more facile n-type and p-type doping for alloys with high Se content. Based on these results we analyze the influence of composition on the efficiency of CZTSSe solar cells and explain the high efficiency of the cells with high Se content.

The I 2-II-IV-VI 4 quaternary chalcogenide semiconductors ͑e.g., Cu 2 ZnGeS 4 , Cu 2 ZnSnS 4 , Cu 2 ZnGeSe 4 Cu 2 CdSnSe 4 , and Ag 2 CdGeSe 4 ͒ have been studied for more than 40 years but the nature of their crystal structures has proved contentious. Literature reports exist for the stannite and kesterite mineral structures, which are zinc-blende-derived structures, and wurtzite-stannite, which is a wurtzite-derived structure. In this paper, through a global search based on the valence octet rule ͑local charge neutrality͒, we report a wurtzitederived structure corresponding to the kesterite structure, namely, wurtzite-kesterite ͑space group Pc͒, which is the ground state for some I 2-II-IV-VI 4 compounds, but is easily confused with the wurtzite-stannite ͑spacegroup Pmn2 1 ͒ structure. We show that there is a clear relationship between the properties of the wurtzitekesterite and zinc-blende-derived kesterite structures, as well as between wurtzite-stannite and stannite. Contributions from the strain and Coulomb energies are found to play an important role in determining the structural stability. The underlying trends can be explained according to the size and ionicity of the group-I,-II,-IV, and-VI atoms. Electronic-structure calculations show that the wurtzite-derived structures have properties similar to the zinc-blende-derived structures, but their band gaps are relatively larger, which has also been observed for binary II-VI semiconductors.

Using an all-electron band structure approach, we have systematically calculated the natural band offsets between all group IV, III-V, and II-VI semiconductor compounds, taking into account the deformation potential of the core states. This revised approach removes assumptions regarding the reference level volume deformation and offers a more reliable prediction of the “natural” unstrained offsets. Comparison is made to experimental work, where a noticeable improvement is found compared to previous methodologies.

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The ternary semiconductors Cu2SnX3 (X=S, Se) are found frequently as secondary phases in synthesized Cu2ZnSnS4 and Cu2ZnSnSe4 samples, but previous reports on their crystal structures and electronic band gaps are conflicting. Here we report their structural and electronic properties as calculated using a first-principles approach. We find that: (i) the diverse range of crystal structures such as the monoclinic, cubic and tetragonal phases can all be derived from the zinc-blende structure with tetrahedral coordination. (ii) The energy stability of different structures is determined primarily by the local cation coordination around anions, which can be explained by a generalized valence octet rule. Structures with only Cu3Sn and Cu2Sn2 clusters around the anions have low and nearly degenerate energies, which makes Cu and Sn partially disordered in the cation sublattice. (iii) The direct band gaps of the low energy compounds Cu2SnS3 and Cu2SnSe3 should be in the range of 0.8-0.9 eV and 0.4 eV respectively, and are weakly dependent on the long-range structural order. A direct analogy is drawn with the ordered vacancy compounds found in the Cu(In, Ga)Se2 (CIGS) solar cell absorbers.

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