Naoki Suyama

Grain Boundary Evaluation of Cu(In1-xGax)Se2 Solar Cells

The grain boundary (GB) properties of polycrystalline Cu(In1-x,Gax)Se2 (CIGS) have been characterized using electron beam-induced current (EBIC) measurements, electron backscatter diffraction (EBSD) patterns, and scanning spreading resistance microscopy (SSRM) measurements. The polished cross section of CIGS solar cells was evaluated by these three methods, and the surface EBIC image was obtained by scanning electron microscopy (SEM). A combination of the EBIC and EBSD techniques makes it possible to investigate the effect of the GBs on the minority carrier collection. Furthermore, the SSRM mapping enables the analysis of grain-by-grain carrier profiling by measuring the spreading resistance of CIGS solar cells. It was found from these results that the twin boundaries of CIGS grains do not contribute to carrier recombination. Furthermore, the brighter EBIC signals were observed at the GBs of CIGS, which showed that the produced electron–hole pairs easily separate from each other and that the minority carriers are repelled from the GBs. This remarkable property of the GBs is suitable for application of CIGS to solar cells.

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.

Effects of N2/H2 Sintering Atmosphere on the Properties of Cu2ZnSn(S,Se)4 Solar Cells Fabricated by a Nanoparticle-Based Approach

A N2/H2 mixture gas was used for the first time to sinter Cu2ZnSn(S,Se)4 (CZTSSe) films fabricated by a nanoparticle-based approach. It was found that N2/H2 sintering treatment is effective to achieve a dense CZTSSe film. We compared the effects of N2/H2 and N2 atmospheres on the device properties of CZTSSe solar cells using electron beam-introduced current (EBIC) measurement, and the results revealed that the minority carrier collection length of the CZTSSe solar cells was improved by sintering under N2/H2. We obtained a CZTSSe solar cell of 5.4% what by this technique of sintering CZTSSe films under N2/H2 atmosphere.

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.