成果報告書詳細
管理番号20140000000596
タイトル*平成25年度中間年報 新エネルギー技術研究開発 革新的太陽光発電技術研究開発 (革新型太陽電池国際研究拠点整備事業) 低倍率集光型薄膜フルスペクトル太陽電池の研究開発(放熱基板カルコパイライト系集光型セル)
公開日2014/12/27
報告書年度2013 - 2013
委託先名学校法人東京理科大学
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 : NEDO Innovative PV Technology. Full Spectrum Thin Film Solar Cells with Low Concentration Ratios. Chalcopyrite-Based Thin Film Concentrator Solar Cells on Metal Substrates(FY2008-FY2014)FY2013 Annual Report

In our numerical simulations by using SCAPS3.2, we found that a back side grading of the absorber was necessary. However, the back side grading was not a critical parameter, since both a shallow grading slope and a steep grading slope led to high efficiencies. High Ga content films tend to be more defective, thus a relatively shallow grading at the back side may be preferable. At the front side, a flat profile led to the same efficiency as a limited bump of about 50 meV, but a too steep grading slope at the surface created an electron barrier which is detrimental for the PV performance of the device. Therefore, in our optimized design, no front Ga grading is necessary. We also investigated the valence band maximum (VBM) position, which can be tuned by implementing sulfur atoms as it is already reported in the literature. Our simulations were performed in order to optimize the depth of the sulfurized layer at the front side of the CIGS, as well as the VBM shift (ΔVBM) at the surface of the CIGS. As a result, we found that a sulfurized layer with a thickness comprised between 200 nm-300 nm, and a ΔVBM of about 0.15 eV-0.3 eV would lead the highest efficiency. Finally, the minimum bandgap of the absorber layer was optimized at 1.1 eV for the tandem application with a 620 nm optical splitting filter. If pure CIS (1.04 eV), the current reaches its maximum value but both the FF and the Voc are significantly degraded. For a minimum bandgap of 1.2 eV (like in state of the art CIGS), the low energy photons are not efficiently absorbed under filtered condition, the efficiency is lower. Such a solar cell could potentially lead to an AM 1.5 efficiency of 19.3% and a tandem efficiency (using a 620 nm filter) of 13.8%. Based on these results, we modified the classical three stage process to obtain a similar band profile in our experimental cells. The first and second stages were kept unchanged with a simultaneous deposition of In, Ga and Se followed by a simultaneous deposition of Cu and Se at higher temperature. Such process will create the desirable back side Ga grading. For the third stage, only In and Se were used, sometimes with a very limited amount of Ga in order to create a narrow bandgap region. By using CIGS thin films deposited by this process, the solar cells with a ZnO:B/ZnO/CdS/CIGS/Mo/SLG structure were fabricated and the cell performance were measured under AM1.5 and filtered (620nm) illuminations (100mW/cm2). The cell has a minimum bandgap of 1.11 eV, very close to the optimum from the simulations, and reaches 17.5% under AM 1.5 illumination. In the tandem conditions, the cell reaches 13.1% efficiency, very close to the maximum efficiency derived from simulation. This result suggests that narrow-bandgap CIGS-based solar cells is useful for spectral splitting tandem application as a bottom cell. Actually an efficiency of 22 % was achieved for the spectral splitting tandem device with a-Si top cell fabricated by Tokyo Tech.
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