應用於銅銦鎵硒薄膜太陽能電池之硒化製程於吸收層鎵元素暨能隙分佈優化

Abstract

銅銦鎵硒是一種具有發展潛力的薄膜太陽能電池材料。在硒化製程中，由於鎵堆積在吸收層的底部，導致開路電壓大幅下降，進而使轉換效率無法提高。因此，本論文研究在三階段退火(硒化)製程中，每個階段對鎵分佈的影響，以及優化三階段退火製程。在此研究結果中，在退火的第一階段，隨著退火時間的增加，愈多的鎵元素會往吸收層的底部擴散；在第三階段中，鎵反而會往吸收層的表面擴散，使得鎵分佈變得更平坦。在優化三階段退火製程後，以30min/350 oC、15min/450 oC、30min/550 oC的退火參數，可在吸收層中得到最佳的鎵分佈。但是，此吸收層表面之鎵含量過低，因此針對優化後的吸收層，提出新的預鍍層結構，用以改善吸收層表面鎵濃度過低的問題。在原始的預鍍層上濺鍍具有高濃度鎵的銅銦鎵，形成雙層結構的預鍍層，此結構不僅可提升吸收層表面的鎵含量，還可增加元件的開路電壓，使得銅銦鎵硒薄膜太陽能電池的轉換效率提升至7.25%。

Cu(In,Ga)Se2 (CIGSe) is a potential material for developing high efficiency thin film solar cells. In the selenization processes, gallium (Ga) accumulates near the back-side region of CIGSe absorber, resulting in low open-circuit voltage (VOC) and conversion efficiency. This thesis investigates the effect of Ga distribution in each annealing step and optimizes the three step annealing process. Ga diffused to the CIGSe/Mo interface in the first-step annealing, and diffused to surface of CIGSe absorber in the third-step. The three-step annealing parameters, therefore, were optimized as the annealing temperatures of 350, 450, and 550oC for 30, 15, and 30 minutes, respectively. The Ga/(In+Ga) ratio is 0.15 by using the optimized three-step annealing parameters. However, the Ga concentration remains low near the surface region, and consequently results in high Ga concentration at the CIGSe/Mo interface. To improve this issue, a new double-stacked precursor structure was proposed in the thesis. This precursor structure not only enhances the VOC due to the increase of Ga content near the surface region of CIGSe absorber, but also increases the cell efficiency from 4.88 to 7.25%. The use of double-stacked precursor effectively enhances conversion efficiency in the selenization processes.