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

作　　者：范巍 曾雉 

机构地区：[1]中国科学院固体物理研究所中国科学院材料物理重点实验室中国科学院合肥物质科学研究院,合肥230031

出　　处：《物理学报》2015年第23期403-415,共13页Acta Physica Sinica

基　　金：国家重点基础研究发展计划(973计划)(批准号:2012CB933702)资助的课题~~

摘　　要：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 lo

关 键 词：锌黄锡矿Cu2ZnSnS4 太阳能电池 晶界微结构 密度泛函理论 

分 类 号：O469[理学—凝聚态物理]