成果報告書詳細
管理番号20130000000120
タイトル*平成24年度中間年報 太陽エネルギー技術研究開発 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 大面積サブモジュールの高効率化技術の研究開発
公開日2013/6/22
報告書年度2012 - 2012
委託先名昭和シェル石油株式会社
プロジェクト番号P07015
部署名新エネルギー部
和文要約
英文要約Title: Development of High Efficiency CZTS Solar Cells and Submodules (FY2012-FY2013) FY2012 Annual Report

We, Showa Shell Sekiyu K.K., have been focusing on following items to achieve our intermediate target for FY2013, a conversion efficiency of 11% on a 10 × 10 cm2-sized Cu2ZnSnS4 (CZTS) submodule. In order to improve conversion efficiency for large area submodule, we set our sights on developing a technology which enables to improve crystalline quality of CZTS light-absorbing layer. The CZTS layer has been formed by sulfurization process, i.e., annealing metal precursors deposited by sputtering or evaporation methods under sulfur atmosphere, just like the sulfurization after selenization (SAS) process used to form CuInSe2 film. In this year, we have first considered suppressing phase separation and improving flatness of CZTS light-absorbing layer. Previously, our CZTS layer contained a lot of voids due to the volume expansion during the crystal growth. Hence, the voids and poor flatness were the origin of the formation of leak points and inhomogeneous carrier collection. Therefore we have optimized the precursor composition, its source material, and the condition of the sulfurization process. By these optimizations, we have suceeded in substantially reducing voids and, especially, improving flatness of the CZTS layer. Second, we have considered the thickness of the absorber layer. If the CZTS layer was too thick compared with the thickness requierd to fully absorb the incident light, the layer introduces redundant resistance to the device, resulting in the reduction of fill-factor (FF). Thus, we have optimized the thickness of CZTS layer and found that approximately 600 nm is enough for full absorption. Finally, we have tried to optimize the thickness of CdS buffer layer to minimize the loss of light absorption at short wavelength region. As a result, we have achieved a conversion efficiency of 9.2% (8.8% without anti-refrection coating (ARC)) on a 7 × 7 cm2-sized CZTS submodule with CdS buffer layer, which is the world record efficiency for the CZTS submodule and also for the Se-free CZTS including small area cell. In order to attain much higher efficiency, we have started to controll the band gap of the light-absorbing layer by adding Se to CZTS. We have found that Voc × Jsc for CZTSSe submodule reached 16.6 mW/cm2, while it for our best performing CZTS submodule (8.8% without ARC) was 14.6 mW/cm2. This significant enhancement of Voc × Jsc primarily comes from the reduction of Voc deficit compared with the band gap. If FF reached 0.7 after tuning band offset between the buffer layer and the absorber layer, the conversion efficiency will reach 12%. It is clarified that the reduction of Voc loss is one of the highest priority issues for achieving higher efficiency in the further study.
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