Effect of Ge inclusion on surface morphologies and the growth mechanism of Cu 2 (Sn 1-x Ge x )S 3 films grown by the sulfurization of Ge/Cu/ SnS precursors

The effects of the [Ge]/([Ge]+[Sn]): x ratio on the increase in the surface roughness and growth mechanism of Cu2Sn1-xGexS3 (CTGS) films were examined as a first step toward realizing high-efficiency CTGS solar cells. The surface roughness of the CTGS films did not change significantly up to 450 degrees C during the sulfurization of the Ge/ Cu/SnS precursors, whereas that of the CTGS films increased beyond 500 degrees C. Further, Cu2GeS3 (CGS) and germanium sulfide (GeS) were observed via X-ray diffraction; these CGS and GeS compounds remained in the upper layer up to 540 degrees C, and CTGS was formed at 560 degrees C by the reaction with Cu2SnS3. The increased surface roughness in the CTGS films is attributed to the larger CTGS grain size that increases the x ratios of CTGS and the re-evaporation of vapor atoms (e.g., GeS) that generates a high vapor pressure on the surface of films by forming CGS and GeS compounds. These results confirm that the formation of CGS and GeS compounds, and the formation of dense grains in the CTGS films caused by the deposition of lower x ratios in the CTGS films are effective measures for improving the overall surface roughness.