New group of chalcopyrite-type semiconductor for solar cells

“New group of chalcopyrite-type semiconductor for solar cells”Yoshitaro, NOSEAssistant ProfessorDepartment of Materials Science and Engineering, Kyoto University AbstractIn late years, solar cells are mainly fabricated by silicon, however, higher-performance and lower-cost solar cells are demanded for prevalence. So, various materials such as compound semiconductors and organic materials are studied for solar cell applications. Among all, CuInSe2(CIS) or Cu(Ga, In)Se2(CIGS) with chalcopyrite structure and II-VI semiconductor CdTe are attractive materials and mass-produced by some companies. However, the conversion efficiency of solar cells using the above materials are still lower than that of silicon solar cells. Additionally, rare metals, In and Ga, and toxic elements, Se or Cd, are used in them. Recently, CuZnSnSe4 (CZTS), which is obtained by substituting In with Zn and Sn in CIS, was thus proposed as a candidate material for solar cells [1]. However, the conversion efficiency of its solar cell is about 7 %, that is much lower compared with CIGS. Therefore, it is necessary to investigate another group of solar cell materials for achieving high performance and low cost.CIGS belongs to a so-called I-III-V2 type chalcopyrite semiconductor. In this type, various materials are available as shown in Fig. 1 [2] and studied from fundamental properties to practical applications. On the other hand, the chalcopyrite semiconductors with II-IV-V2 type are also considered to be available but there are few reports on their solar cells.Considering from consisting elements and bandgap for solar cells, we focus on zinc-tin-diphosphide, ZnSnP2 among II-IV-V2 type semiconductors. As an interesting feature, ZnSnP2 ware reported to show an order-disorder transition at 720 ºC [3]. By this transition, the crystal structure of ZnSnP2 changes from the chalcopyrite to the sphalerite as shown in Fig. 2. In the chalcopyrite structure, so-called ordered structure, Zn and Sn occupy on determined sites of fcc positions, while they randomly occupy on fcc sites in the sphalerite structure, so-called disordered structure. The bandgap of ZnSnP2 varies from 1.66 eV to 1.25 eV with changing the structure [4]. This suggests that the bandgap of ZnSnP2 can be controlled by controlling atomic configuration. The theoretical conversion efficiency of solar cell shows the highest value around the bandgap at 1.4 eV, which is within the bandgap range of ZnSnP2. Therefore, ZnSnP2 is promising to be a candidate material for high-performance solar cells.Precise phase diagrams are helpful for crystal growth of ZnSnP2 with controlled bandgap, but no diagrams were experimentally determined. In this study, we thus establish the phase diagram of the Zn-Sn-P ternary system in order to investigate phase relationship of the ternary system and the Sn-ZnSnP2 pseudo-binary system. Additionally, we try to make bulk crystals by flux method based on the phase diagram.The phase diagram was established by equilibrium experiments and differential thermal analysis. From the isothermal section of the ternary diagram, the single-phase region of ZnSnP2 ranges to Zn-rich composition. This non-stoichiometry is considered to appear by the occupation of excess Zn on Sn sites and ZnSnP2shows a p-type semiconductor in this situation. In the diagram of the Sn-ZnP2 pseudo-binary system shown in Fig. 3, ZnSnP2 shows an peritectic reaction and another phase are observed at higher temperatures. Therefore, it is difficult to obtain ZnSnP2 from the melt directly. So, we tried to fabricate ZnSnP2 crystals by flux method using Sn-rich solution based on the diagram and obtained two-phase crystals with Sn and ZnSnP2 as shown in Fig. 4. Only Sn could be dissolved using hydrochloric acid andZnSnP 2 powder was obtained. The bandgap of the powder crystals was estimated to be 1.65 eV by spectrophotometer. In this way, ZnSnP 2 crystals were obtained based on the phase diagram. The production process for thin film is also considered based on the phase diagram.Figures (can also be inserted in the text):References[1] K. Ito, T. Nakazawa, "Electrical and Optical-properties of Stannite-type Quaternary Semiconductor Thin-Films", Jpn J. Appl. Phys. 27, (1988), 2094.[2] J. L. Shay and J. H. Wernick, "Ternary Chalcopyrite Semiconductors: Growth,Electric Properties and Applications", Pergamon Press, New York, (1975).[3] S. A. Mughal, A. d. Payne, B. Ray, J. Mater. Sci . 4 (1969) 897-801.[4] M.A. Ryan, M.W. Peterson, D.L. Williamson, B.A. Parkinson, J. Mater. Res. 2(1987) 528-537.Keywordscompound semiconductor, chalcopyrite structure, order-disorder transition,phase diagram, crystal growth Fig. 2 Order-disorder transition of ZSP.Fig. 1 Chalcopyrite compounds. Fig. 4 Crystals obtained by flux method. Fig. 3 The Sn-ZnP 2 pseudo-binary system.。