Hefei Research Institute Makes Progress in Theoretical Research of Thin Film Solar Cell Materials

Recently, the research group of the Institute of Solid State Physics, Institute of Solid State Physics, Hefei Research Institute of Materials Science, Chinese Academy of Sciences, Zeng Ji, research group made new progress in the research of thin-film solar cell materials. Through theoretical calculations, theoretically selected Cu2ZnSnS4 (CZTS) Obstacles to the type of intrinsic defect that impedes the efficiency of the battery and proposes methods of regulation. The results are published online in the journal Solar Energy Materials and Solar Cells 180, 118-122 (2018).

All the elements that make up CZTS are abundant and non-toxic in the earth. They are recognized as environmentally friendly, low-cost, high-efficiency battery candidate materials. However, the maximum efficiency of current CZTS-based solar cells is about 12.7%, which is far below its equivalent. The highest efficiency of compound Cu(In,Ga)Se2 (20.3%), one of the important reasons is that there are many localized defects in the CZTS that impede the free transport of carriers. The current experimental techniques cannot judge the defects from the atomic scale. The type, and theoretically, can accurately determine the type of the main charge localized defect by studying the formation energy of the defect and the charge transfer energy level.

Based on the study of the hybrid functional theory, Zeng Hao's research group found that CuSn and CuZn are the main charge localized defects in CZTS, and their influence on carriers is not the same. CuSn forms a deep impurity level in the band gap. Electron-hole pairs are recombined through the deep level, so CuSn is a deep level recombination center. CuZn impurity levels are relatively low in position, easily ionized, contribute carriers, but ionized CuZn- tends to form a charge with ZnCu+ The mutually compensating acceptor-donor defect pairs, in which CuZn- and ZnCu+, which are electrically opposite to each other, attract large potential fluctuations in the material, which can trap carriers, thus reducing the carrier concentration in the material. After further research, the group proposed a method to suppress these two kinds of localized defects: (1) The Sn-rich growth environment suppresses CuSn because the chemical formula of Sn in CZTS has a very large range of changes, and can be suppressed by creating a Sn-rich growth environment. CuSn; (2) Cd doping suppresses CuZn, because doped Cd will occupy the position of Zn and reduce the possibility of CuZn formation. Both schemes have been experimentally supported.

The above work was supported by the national '973' project and the National Natural Science Foundation of China. The calculation work was completed at the Hefei Branch of the Supercomputing Center of the Chinese Academy of Sciences.

Figure 1. The formation energy of different charge localized defects at different points of the chemical potential stability interval varies with the Fermi level. The main formation energy is less than 1.5eV CuZn and CuSn.

Figure 2. CZTS chemical potential stability interval, the area enclosed by a polygon.

Figure 3. The formation energy of impurities introduced by Cd doping varies with the chemical potential of (a) Fermi level (b). The lowest formation energy of CdZn indicates that Cd impurities tend to occupy the position of Zn and suppress the formation of CuZn.