Cu_2ZnSnS_4晶界性质与光伏效应的第一性原理研究

First-principles studies on the properties of Cu_2ZnSnS_4 grain-boundaries due to photovoltaic effect

本文应用第一性原理电子结构计算方法研究了锌黄锡矿Cu_2ZnSnS_4(CZTS)晶界的性质:包括微结构和电子结构及其对光伏效应的影响.计算结果表明:从p-n结区扩散过来空穴可以翻越一定势垒后被晶界俘获,晶界进一步提供载流子扩散的快速通道,使得这些空穴可以快速运动到阳极.少数载流子电子在晶界中心区附近感受到很高的静电势垒,但其高势垒两侧存在的势阱可以束缚少量电子.对多数载流子空穴,晶界中心则是势阱,势阱两边有阻止空穴扩散到晶界中心的势垒.由于CZTS晶体的易解理面是(112),晶界面与(112)面平行的扭转晶界Σ3*[221]和Σ6*[221]等不破坏原有晶体的基本结构,它们的晶界能很小,而且其电子结构与晶体内部基本相同,因此尽管它们大量存在于CZTS材料中,但是对材料性质仅有很小的影响.通过比较晶体、晶界、空腔的表面和纳米棒的电子结构和光吸收系数,我们可以看出:这些微结构会在带隙内引入新的能级(复合中心),同时高的孔隙率会降低(大于1.3 eV)光的吸收系数,因此提高CZTS薄膜的致密度是提高CZTS太阳能电池效率关键.

Microstructures and electronic structures of Cu2ZnSnS4（CZTS） grain-boundaries（GB） are studied by the firstprinciples 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 Σ3[221] and Σ6[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 Ga As,and the positive role in ductile photo-voltage materials such as Cd Te 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 Cd Te,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 cancreate 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.