磁控溅射Cl掺杂CdTe薄膜的孪晶结构与电学性质

CdTe用作薄膜太阳能电池吸收层需要经过氯处理才能得到高的光电转换效率,其中Cl原子的作用机理仍然没有完全被理解.实验发现Cl原子主要偏聚在CdTe晶界处,对晶界有钝化作用,而有第一性原理计算认为Cl原子掺入CdTe晶格能够引入浅能级提高光电转换效率.为了验证Cl原子掺杂是否对CdTe的光电转换效率有益,本文通过磁控溅射制备了100 ppm(ppm=1/1000000)Cl原子掺杂的CdTe(CdTe:Cl)薄膜并研究了薄膜的晶体结构与电学性质,同时对比了正常氯处理的无掺杂CdTe薄膜与CdTe:Cl薄膜之间的性质区别.实验发现Cl原子掺杂会在CdTe:Cl中形成大量仅由几个原子层构成的孪晶,电子和空穴在CdTe:Cl薄膜中没有分离的传导通道,而在氯处理后的CdTe薄膜中电子沿晶界传导,空穴沿晶粒内部传导.磁控溅射沉积的CdTe:Cl多晶薄膜属于高阻材料,退火前载流子迁移率很低,退火后载流子浓度降低到本征数量级,电阻率提高.CdTe:Cl薄膜电池效率远低于正常氯处理的无掺杂CdTe薄膜电池效率.磁控溅射制备的非平衡重掺杂CdTe:Cl多晶薄膜不适合用作薄膜太阳能电池的吸收层.

The twins structure and electric properties of Cl doped CdTe film by magnetron sputtering

CdTe is a promising material for fabricating high-efficient and low-cost thin film solar cell. To achieve high energy conversion efficiency, polycrystalline CdTe films must go through an annealing process in an atmosphere containing chlorine. Numerous researches of the mechanisms of chlorine treatment have been conducted. It is generally believed that chlorine treatment can increase the quantum efficiency of CdTe, cause CdTe grain to recrystallize, and reduce the defect density. In 2014 a research discovered that after chlorine treatment, Cl atoms are segregated at grain boundaries of CdTe and form p-n-p junction, which can separate electrons and holes, thus inhibiting the carrier recombination at grain boundaries. Another first-principle calculation research claimed that Cl atoms form V_Cd-Cl_Te complex, which is also named A-center, and provide extra shallow p-energy level to improve shallow p-doping of CdTe. It seems that both segregation and doping of Cl atoms can enhance cell performance.To test whether chlorine doping can contribute to the enhancement of cell performance, in this work we study chlorine doping in CdTe absorption layer by experiment. We deposit chlorine doped CdTe (CdTe:Cl) film by well controlling the chlorine concentration ((100±5) ppm) to investigate the effects of Cl doping on device performance. In this work, we also compare the lattice structure and electrical properties of CdTe:Cl films with those of conventional Cl treated CdTe films.The CdTe:Cl film deposited at low temperatures consists of both cubic and hexagonal phases. CdTe:Cl film deposited at high temperature consists of only cubic phase with (111) orientation. Phase structure remains stable after annealing. Serried twins can be observed in all CdTe:Cl rods and the twins each contain only several atom layers. The ultra-thin twins can be found in both as-deposited CdTe:Cl and post-annealing CdTe:Cl. There is neither separate conduction channel of electrons nor that of holes in CdTe:Cl. But for chlorine treated CdTe, grain boundaries are the conduction channels of electrons and holes traveling within grains. The resistivity of the CdTe:Cl film is found to increase drastically, and carrier density reduces to intrinsic state after annealing. The efficiency of CdTe:Cl cell is lower than that of chlorine treated CdTe cell. It seems that non-balanced heavy chlorine doping by magnetron sputtering is bad to CdTe absorption layer.