In order to spread solar cells widely, it is desirable that solar cell materials are made of low-cost and earth-abundant elements. In this light, we focus on the II-IV-V2 chalcopyrite semiconductor compound, ZnSnP2, which is a p-type semiconductor with a bandgap of 1.66 eV. ZnSnP2 is reported to have high absorption coefficient. Therefore, ZnSnP2 is a promising material for solar absorber. For the application to solar cells, the formation of thin film is required.
In this work, the method to obtain thin film of ZnSnP2 is thus proposed and the properties are investigated. As a technique to obtain ZnSnP2 thin film, we adopted the phosphidation of Zn-Sn thin film prepared by sputtering technique. In this method, an appropriate source of phosphorus should be selected. We thus considered to use samples with Sn / Sn4P3 dual phase as a phosphorus source, by which the phosphorus vapor pressure could be accurately controlled by temperature.
The phosphidation of Zn-Sn thin film was carried out at 500 C, under the phosphorus vapor pressure of 10 to -2 atm. In phosphidation for 30 min, the formation of ZnSnP2 was observed, while metal Zn and Sn were not identified. This indicates that phosphidation of Zn-Sn thin film was completed. The obtained ZnSnP2 thin film shows a p-type semiconductor, and the density and the mobility of holes were 10 to 16 - 10 to 18 cm-3 and about 1 cm2V-1s-1, respectively. These properties are attractive for a solar absorber. However, many protrusions, which might be obstacles for making photovoltaics, were observed on the surface of the thin films. To improve the surface morphology, we discussed the mechanism of the phosphidation. When the phophidation was carried out for various period, some particles with high Sn concentration were formed in the initial stage, and they may grow up to the protrusions of ZnSnP2.
It was concluded that the formation of the particles might be caused by the compressive stress from phosphides, Zn3P2 and ZnSnP2, formed at the initial stage. Therefore, we consider to add elements, which make intermetallic compounds with Sn, to supress the phase separation. In the conference, the results of its improvement and the performance of photovoltaics will be presented.