The kesterite-structured semiconductors Cu2ZnSnS4 and Cu2ZnSnSe4 are drawing considerable attention recently as the active layers in earth-abundant low-cost thin-film solar cells. The additional number of elements in these quaternary compounds, relative to binary and ternary semiconductors, results in increased flexibility in the material properties. Conversely, a large variety of intrinsic lattice defects can also be formed, which have important influence on their optical and electrical properties, and hence their photovoltaic performance. Experimental identification of these defects is currently limited due to poor sample quality. Here recent theoretical research on defect formation and ionization in kesterite materials is reviewed based on new systematic calculations, and compared with the better studied chalcopyrite materials CuGaSe2 and CuInSe2 . Four features are revealed and highlighted: (i) the strong phase-competition between the kesterites and the coexisting secondary compounds; (ii) the intrinsic p-type conductivity determined by the high population of acceptor CuZn antisites and Cu vacancies, and their dependence on the Cu/(Zn+Sn) and Zn/Sn ratio; (iii) the role of charge-compensated defect clusters such as [2CuZn +SnZn ], [VCu +ZnCu ] and [ZnSn +2ZnCu ] and their contribution to non-stoichiometry; (iv) the electron-trapping effect of the abundant [2CuZn +SnZn ] clusters, especially in Cu2ZnSnS4. The calculated properties explain the experimental observation that Cu poor and Zn rich conditions (Cu/(Zn+Sn) ≈ 0.8 and Zn/Sn ≈ 1.2) result in the highest solar cell efficiency, as well as suggesting an efficiency limitation in Cu2ZnSn(S,Se)4 cells when the S composition is high.

No abstract

The structural and electronic properties of Cu 2 ZnSnS 4 and Cu 2 ZnSnSe 4 are studied using first-principles calculations. We find that the low energy crystal structure obeys the octet rule and is the kesterite ͑KS͒ structure. However, the stannite or partially disordered KS structures can also exist in synthesized samples due to the small energy cost. We find that the dependence of the band structure on the ͑Cu,Zn͒ cation ordering is weak and predict that the band gap of Cu 2 ZnSnSe 4 should be on the order of 1.0 eV and not 1.5 eV as was reported in previous absorption measurements.An ideal thin-film solar cell absorber material should have a direct band gap around 1.3-1.5 eV with abundant, inexpensive, and nontoxic elements. Cu͑In, Ga͒Se 2 ͑CIGS͒ is one of the most promising thin-film solar cell materials, demonstrating an efficiency of about 20%. 1 However, In and Ga are expensive components, and the band gap is usually not optimal for high efficiency CIGS solar cells. Currently, designing and synthesizing novel, high-efficiency, and low cost solar cell absorbers to replace CIGS has attracted much attention. Among the materials that have been investigated, quaternary Cu 2 ZnSnS 4 ͑CZTS͒ and Cu 2 ZnSnSe 4 ͑CZTSe͒ compounds have drawn significant interest because they contain only abundant and nontoxic elements Cu, Zn, Sn, S, and Se, and the reported band gaps for both materials are about 1.5 eV ͑Table I͒, 2-15 which is ideal for solar cell application.Unfortunately, the fundamental physical properties of these materials are not well understood. For example, what are the ground state structures, and how does the crystal structure affect their band structure and optical properties. From a theoretical point of view, it is also difficult to understand why the recently reported band gap of CZTSe at about 1.5 eV is basically the same as that of CZTS. Selenides ͑ZnSe, CuGaSe 2 with band gaps of 2.82 and 1.68 eV, respec-tively͒, with larger lattice constants and higher p orbital energies, usually have much smaller band gaps than sulfides ͑ZnS, CuGaS 2 with band gaps of 3.78 and 2.43 eV, respec-tively͒.In this paper, we systematically investigate the structural and electronic properties of these two quaternary compounds using first-principles total energy and band structure calculations within the density functional formalism as implemented in the VASP code. 16 For the exchange-correlation potential, we used the generalized gradient approximation ͑GGA͒ of Perdew and Wang, known as PW91. 17 The projector augmented-wave pseudopotentials 18 with an energy cutoff of 300 eV for plane waves in the 4 ϫ 4 ϫ 4 Monkhorst-Pack k-point meshes were employed to give converged results.It has been shown that the ternary CuGaX 2 , CuInX 2 , and the quaternary Cu 2 ZnSnX 4 compounds can be obtained through cation mutation of their II-VI analogs. 19 For example, by mutating two Zn in ZnS to Cu+ Ga, we obtain CuGaS 2 . There are two fundamental I-III-VI 2 structures that obey the octet rule: chalcopyrite ͑CH͒ and CuAu-like ͑CA͒ structure...

An approach is introduced to calculate the thermodynamic oxidation and reduction potentials of semiconductors in aqueous solution. By combining a newly developed ab initio calculation method for compound formation energy and band alignment with electrochemistry experimental data, this approach can be used to predict the stability of almost any compound semiconductor in aqueous solution. Thirty photocatalytic semiconductors have been studied, and a graph (a simplified Pourbaix diagram) showing their valence/conduction band edges and oxidation/reduction potentials relative to the water redox potentials is produced. On the basis of this graph, the thermodynamic stabilities and trends against the oxidative and reductive photocorrosion for compound semiconductors are analyzed, which shows the following: (i) some metal oxides can be resistant against the oxidation by the photogenerated holes when used as the n-type photoanodes; (ii) all the nonoxide semiconductors are susceptible to oxidation, but they are resistant to the reduction by the photogenerated electrons and thus can be used as the p-type photocathodes if protected from the oxidation; (iii) doping or alloying the metal oxide with less electronegative anions can decrease the band gap but also degrade the stability against oxidation.

No abstract

2 based technologies. Zinc-blende related structures are formed by quaternary compounds, but the complexity associated with the multi-component system introduces diffi culties in material growth, characterization, and application. First-principles electronic structure simulations, performed over the past fi ve years, that address the structural, electronic, and defect properties of this family of compounds are reviewed. Initial predictions of the bandgaps and crystal structures have recently been verifi ed experimentally. The calculations highlight the role of atomic disorder on the cation sub-lattice, as well as phase separation of Cu 2 ZnSnS 4 into ZnS and CuSnS 3 , on the material performance for light-to-electricity conversion in photovoltaic devices. Finally, the current grand challenges for materials modeling of thin-fi lm solar cells are highlighted. 401 www.MaterialsViews.com www.advenergymat.de

Hybrid halide perovskites such as methylammonium lead iodide (CH3NH3PbI3) exhibit unusually low free-carrier concentrations despite being processed at low-temperatures from solution. We demonstrate, through quantum mechanical calculations, that an origin of this phenomenon is a prevalence of ionic over electronic disorder in stoichiometric materials. Schottky defect formation provides a mechanism to self-regulate the concentration of charge carriers through ionic compensation of charged point defects. The equilibrium charged vacancy concentration is predicted to exceed 0.4 % at room temperature. This behavior, which goes against established defect conventions for inorganic semiconductors, has implications for photovoltaic performance.

No abstract