|タイトル||*平成25年度中間年報 太陽光発電システム次世代高性能技術の開発 ＣＺＴＳ薄膜太陽電池の高効率化技術の研究開発 （－機能層・セル内界面の電子構造評価・最適化に関する研究開発－）|
|報告書年度||2013 - 2013|
|英文要約||Title: Development of high-efficiency CZTS solar cells and submodules (FY2012-FY2014) FY2013 Annual Report
1. Objectives: Electronic structures of buffer and absorber layers and interfaces and electronic activity of grain boundaries are to be investigated for developing key technologies for higher conversion efficiency in CZTS based cells. Correlations between these characteristics and fabrication parameters and guiding principle for process-optimization are to be clarified.
2. For FY 2013 (1) Development of the determination method of the S/(S+Se) compositional ratio of the surface of the CZTSSe films; The linear correlation between the S/(S+Se) ratio and the Auger parameter of Zn in the CZTSSe layers has been found. By utilizing this correlation, the anion mixing-ratio can be determined even for the CZTSSe with compositions close to S/(S+Se)= 0.0 or 1.0, where determination from the deconvolution of the overlapped S and Se XPS signals.
(2) Experimental determination of the changes of band gap energy, band-edge positions of the CZTSSe layers with the S/(S+Se) ratio; The dependence of the electronic structure of the cleaned surfaces of the CZTS absorber layers on the anion mixing ratio has been examined by means of UPS and IPES. With an increase of S/(S+Se) ratio, the band gap energy was expanded up to 1.4 ~ 1.5 eV of CZTS. The rise of conduction band minimum (CBM) is the major origin of this expansion. These reveal the similarity between the S/(S+Se)-dependence of electronic structure of the CZTSSe films and the Ga substitution-dependence of that of the CIGS.
(3) Characterization of the band alignment at the interface between MBE-CdS buffer and CZTSSe; S/(S+Se) ~ 0.3
The experiments in the previous financial year revealed that ion etching, which is an ordinary method for the depth profiling of electronic structure, induced obvious degradations in the surface of the CZTSSe layer. In order to clarify the intrinsic nature, the interfaces were fabricated by the stepwise evaporation of the CdS on the cleaned CZTSSe. Then, the band alignment have been determined from the dependence of in-situ XPS, UPS/IPES spectra on thickness of the CdS buffer. The deposition of the 12 nm thick CdS layer results in the bulk-like electronic structure of the buffer. This also induces the lowering of CBM and VBM by 0.2 and 1.6 eV, respectively. The downward band bending by 0.5 eV was also induced by the 12 nm CdS deposition. From these values, conduction band offset (CBO) and valence band offset were determined as +0.3 and +1.1 eV, respectively. This CBO value is within the proper range for suppressing the interface-recombination, which is consistent with the high conversion efficiency above 10% achieved in the cells using the identical junction. These results strongly indicate that the band alignment at the buffer/absorber interface is one of the dominant factors for the performance of the CZTS cells.