First-principles studies on the properties of Cu2ZnSnS4 grain-boundaries due to photovoltaic effect

Microstructures and electronic structures of Cu2ZnSnS4 (CZTS) grain-boundaries (GB) are studied by the first-principles electronic structure method. Some special twist grain-boundaries have low grain-boundary energies and exhibit similar electronic structure as that in a perfect crystal. The twist grain-boundaries such as 3[221] and 6[221] have grain-boundary planes parallel to (112) plane, the easiest cleavage plane, so that they have small damages to the crystal structure and small influence on the properties of the materials. Grain-boundary plays two roles in CZTS thin-films: (1) capturing and trapping holes from p-n junctions, and (2) providing fast channels for transportation of majority carriers. As the majority of carriers, the positively charged holes need override a barrier before being trapped by a potential-well in the grain-boundary region. For the minority of carriers, the grain boundary is a high barrier to prevent electrons from transporting across it. The intrinsic nature of the potential barrier is not very clear. By calculating the distributions of static potentials across different grain boundaries of CZTS and also by comparing them with those across different surfaces, we find that the potential barriers at grain boundaries are the remnants of the potential barriers of surfaces, which trap the electrons in the bulk and prevent the electrons from escaping from the bulk to vacuum. When two surfaces get contact to form a grain boundary the corresponding surface barriers will be merged together as one potential barrier of the grain boundary. It is obvious that if a grain boundary intersects with the surface, the escaping work function near the grain boundary is lower than that near the prefect crystal surface. Experiment shows the coexistence of Sn4+ and Sn2+ions. The Sn4+ ions are located in the bulk by bonding 4 S atoms as neighbors. Our results show that Sn2+ ions can appear in the grain-boundary regions, on the surfaces or in the bulk with lattice defects so that Sn2+ ions have the lower coordination number by bonding 3 S atoms. The Sn atom is favored to be at the center of S octahedron with six neighboring S (or O) atoms in most sulfides (oxides) of tin. In CZTS, Sn atom is at the center of tetrahedron with 4 neighboring S atoms so that Sn atom is very active to move by structural relaxations. Most importantly the conduction-bands in CZTS are formed by the hybridizations between the s electrons of Sn and p electrons of S so that the conduction-bands of CZTS are sensitively dependent on the distributions and properties of Sn atoms. The appearing of Sn2+ ions and the strong structural relaxations of Sn atoms in grain-boundary regions and on surfaces induce extra in-gap states as a new source for the recombination of electron-hole pairs that are un-favored to the photo-voltage effects. Generally, the grain boundary plays a negative role in brittle photo-voltage materials such as Si and GaAs, and the positive role in ductile photo-voltage materials such as CdTe and CIGS (Cu(InGa)Se2). It means that the growth of the hard and brittle films is very difficult, the micro-cracks and micro-pores are easily created. Our calculations show that CdTe, CIGS and CZTS are all ductile with Poisson-ratio greater than 0.33. This means that CZTS can be used as the absorber of flexible solar cell. By comparing the optical absorption-coefficients of crystals, grain-boundaries, surfaces and nano-particles, we find that the internal surfaces in thin-films with high pore-ratio can create new energy-levels in band-gap, which enhances the recombination between electrons and holes and decreases the optical absorption-coefficients (1.3 eV). As a result, the high dense CZTS thin-film is required for high-efficient CZTS solar-cell. The positive role of grain boundary is more important if the CZTS film has the large, unique oriented grains and the uniform distribution of grain sizes. The simple and regular grain-boundary network is more beneficial to the coherent transport of majority carriers.