銅銦鎵硒太陽能電池之緩衝層材料製備與特性分析

Abstract

本研究第一部分利用化學水浴沉積法製備硫化銦薄膜，並探討醋酸濃度對薄膜特性與太陽能電池電性表現之影響。研究結果顯示，添加醋酸於反應溶液中可降低成核速率，使得顆粒形貌自纖維狀逐漸變為顆粒狀，並成功於基板製備緻密之硫化銦薄膜。隨著醋酸濃度增加，氫氧化銦生成減緩使得薄膜內之In2S3比例增加，造成薄膜之能隙值降低並導致短波長範圍之穿透率產生red-shift現象。將實驗數據結合文獻模擬之結果進行元件電性表現之探討，顯示於醋酸濃度為0.3M下所製備之硫化銦緩衝層薄膜與CIGS吸收層具有合適的能帶結構，可有效改善Cu(In,Ga)Se2太陽能電池之光電轉換效率。當醋酸濃度減少時，造成硫化銦薄膜之能隙值過高使得接面產生能障，導致元件之載子傳遞受到抑制而使元件之電性表現下降。 本研究第二部分藉由微波輔助化學水浴沉積法，有效縮短沉積硫化鎘薄膜之反應時間，成功製備均勻覆蓋之硫化鎘薄膜做為緩衝層材料。研究結果顯示，於低氨水濃度條件時，反應以同質成核為主並於溶液中產生嚴重的顆粒團聚，導致薄膜表面呈現大顆粒之形貌。氨水濃度增加可有效抑制同質成核反應，使薄膜沉積顆粒粒徑逐漸降低且薄膜厚度逐漸下降。當氨水濃度控制在2.5M時，可製備緻密且均勻覆蓋CIGS吸收層之硫化鎘薄膜，並有效降低薄膜厚度至30 nm以下。透過降低緩衝層薄膜厚度可增加元件於短波長範圍之載子收集，進而提升Cu(In,Ga)Se2太陽能電池之短路電流密度與光電轉換效率。當氨水濃度過高時，所生成之硫化鎘薄膜易生成孔洞出現造成元件之漏電流，並導致電池之電性表現下降。

Indium sulfide films were successfully synthesized via chemical bath deposition in first part. When acetic acid was added into the solution, the morphology of obtained films changed from fiber-shaped to a granular-shaped because the reaction of nucleation was restrained. Increasing the concentration of acetic acid, the bandgaps of the prepared films gradually reduced owing to the raise of In2S3 content, leading to the red-shift phenomenon in the short-wavelength region of transmittance spectrum. In this study, photovoltaic properties of Cu(In,Ga)Se2 solar cells were discussed in detail with literatures for simulation. As the concentration of acetic acid was increased to 0.3M, the prepared indium sulfide films had a suitable band alignment with CIGS absorber, and thus improved efficiency of Cu(In,Ga)Se2 solar cell. When the concentration of acetic acid was decreased, the bandgaps of indium sulfide films were increased and the transferring of photo-generated carriers were hindered by energy barrier, resulting in the deterioration of photovoltaic properties. In second part, CdS films with completely coverage on CIGS absorbers were successfully prepared via microwave-assisted chemical bath deposition in a short reaction period. As the ammonia concentrations were low, the homogeneous nucleation was dominated, resulting in the serious agglomeration of particles in the solution. Large particles on the prepared films were attributed to the aggregates in the solution. Upon elevating ammonia concentrations, the particle sizes and the thickness of CdS films decreased owing to the suppression of homogeneous nucleation. When the ammonia concentration was 2.5M, a compact and uniform CdS film was observed on the Cu(In,Ga)Se2 absorber, and the thickness of CdS film was below 30 nm. Therefore, reducing the thickness of buffer layer increased the carrier collection in the short wavelength and enhanced the Jsc and efficiency of the Cu(In,Ga)Se2 solar cell. However, further increasing ammonia concentration formed holes on CdS film, resulting in the increase of shunting paths. Hence, the undesirable leakage deteriorated the cell performance of Cu(In,Ga)Se2 solar cell.