Kanta Sugimoto

Microstructural characterization of Cu2ZnSn(S,Se)4 solar cells fabricated from nanoparticles

We characterized the microstructure of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells fabricated from Cu-poor, Zn-rich Cu2ZnSnSe4 (CZTSe) nanoparticles. Various sintering atmospheres of sulfur and selenium led to different microstructural properties. For the samples sintered under sulfur atmosphere, a large number of ZnS secondary phases were observed and the Zn/Sn ratio of the grains was significantly lower than 1. Regions of the ZnS secondary phases showed a dark contrast in electron-beam-induced current (EBIC) images, indicating that the existence of the ZnS phases reduced the minority carrier collection efficiency. In contrast, for the samples sintered under selenium atmosphere, Cu was found to accumulate near the grain boundaries to form the Cu-rich CZTSe phases and only a small number of ZnSe secondary phases were detected.

Crystal growth mechanism of Cu2ZnSn(S,Se)4 thin films fabricated from nanoparticles

In this paper, we study the reaction mechanisms involved in the transformation from the precursor prepared with nanoparticles to the Cu2ZnSn(S,Se)4 phase during solid-phase sulfurization and selenization. The top-view of the film observed by scanning electron microscopy showed that crystal grains suddenly grow at temperatures above 550 °C. For all samples, impurity phases were not detected by X-ray diffraction and Raman spectroscopy, which indicates that intermediate phases such as binary and ternary compounds are not necessary to produce the Cu2ZnSn(S,Se)4 phase. During sintering, the film was first sulfurized because of the difference between the vapor pressures of S and Se. Subsequently, the S of Cu2ZnSn(S,Se)4 was replaced by Se, causing the generation of a defective layer. For the sample sintered at 600 °C, Cu-rich, Zn-rich, and Sn-rich phases were detected by transmission electron microscopy analysis. Additionally, a diagrammatic Cu2ZnSn(S,Se)4 film growth model was proposed to illustrate the detailed reaction mechanism during precursor sintering.

Characterization of CuInS2 thin films prepared from materials grown by using the mechanochemical method and their photovoltaic applications

Cu–In–S thin films were deposited on glass substrates using single-source thermal evaporation with ternary compounds as source materials. Polycrystalline CuInS2 powder grown using the mechanochemical method was employed as the source material. After deposition, the films were annealed in H2S gas at different temperatures from 250 to 500 °C for 60 min. X-ray diffraction patterns indicated that single-phase CuInS2 was formed when annealed above 400 °C. The grain size of the crystals in thin films was approximately 0.2 to 2.0 µm. The best Al/ZnO:Al/ZnO/CdS/CuInS2/Mo solar cell had an open-circuit voltage of 360 mV, a short-circuit current density of 18.6 mA/cm2, and a fill factor of 35.5%, resulting in 2.38% efficiency.