新穎矽量子點薄膜應用於太陽能電池之開發

Abstract

『溫室效應』與『能源危機』乃攸關未來人類生存的兩大重要議題，永續再生能源加速發展已刻不容緩，而太陽能電池(Solar Cell, SC)一直被視為極具潛力的再生能源之一。於考量電池效率、成本和壽命等關鍵因素下，矽基SC仍是最具有優勢可達到普及化的電池種類；為了達到高效率低成本(第三代SC)矽基SC，開發具有多重能隙的多接面元件結構以有效減低光損耗是不可或缺的要件。 有鑒於矽量子點的獨特光學特性，我們提出並開發『漸變矽過多氧化矽多層膜』與『氧化鋅矩陣材料整合』，期望能在保有矽量子點特性下，同時克服目前應用於SC所遭遇的載子傳輸效益明顯受限的瓶頸，以製作出更具應用潛力的新穎矽量子點薄膜。此篇論文首先即介紹SC的重要性和發展現況，與目前矽量子點應用於SC的優勢與挑戰，並提出我們的研究目的且簡介此研究過程中的相關製程與分析儀器。 為製作出超高密度矽量子點薄膜，我們捨棄傳統的[二氧化矽/矽過多氧化矽]多層膜結構，改以『漸變矽過多氧化矽多層膜』做為新沉積結構；實驗結果顯示，藉由週期性矽氧原子濃度差異分佈，可使矽過多原子於退火過程中自組織形成超高密度且均勻尺寸的奈米結晶矽量子點，此外，該新結構亦大幅提升矽量子點薄膜的光吸收係數與電傳輸效益，證明利用此新結構，可有效縮短矽量子點間距，以大幅提升矽量子點薄膜的光伏特性。 為形成更佳的載子傳輸機制，我們亦開發『氧化鋅矩陣材料整合』，因氧化鋅薄膜除具備直接寬能隙半導體特性之外，更罕見地同時擁有高透明度和高可調電性等優勢，故相當適合做為矽量子點矩陣材料並應用於SC；實驗結果指出，針對該矽量子點埋入式氧化鋅薄膜，除在長與短波長波段皆可分別保有氧化鋅的高光穿透與高光吸收特性之外，中波長波段亦具有來自矽量子點的光吸收與光激發光特性，證實來自矽量子點貢獻的光學次能隙形成；此外，相較於使用傳統矽相關介電矩陣材料，該矽量子點薄膜具有更高的導電度，且其載子主要經由氧化鋅矩陣傳輸而非傳統的量子點間穿隧效應傳輸，此經矩陣傳輸機制將可大幅提升矽量子點薄膜的載子傳輸效益，未來亦可更提升矽量子點應用於SC的優勢。 因此，此篇論文中，藉由分別開發新沉積結構與新矩陣材料，我們成功製作出更具潛力且更適合整合於矽基SC的新穎矽量子點薄膜，而根據此研究成果，我們更深信未來若能成功整合此新穎矽量子點薄膜，將可有利於矽基SC的發展，並加速邁向第三代SC的目標。

In order to resolve the critical issues of “Green House Effect” and “Energy Crisis” for humanity’s future, the accelerated developments of renewable energies are necessary. Among all of the renewable energies, solar cells (SCs) are highly considered as the most potential one. To ponder these key factors of efficiency, cost, and lifetime, undoubtedly, the Si-based SCs have the most advantages on popularized developments in the future. However, to successfully achieve high efficiency and low cost, also called the third generation SC, the tandem Si-based SCs with multi-bandgap is required to efficiently reduce the mismatched photon energy loss. Based on the unique properties of Si quantum dot (QD), we propose to develop the novel Si QD thin films by utilizing a gradient Si-rich oxide multilayer (GSRO-ML) structure and integrating with ZnO matrix material to overcome the bottlenecks of the largely limited carrier transport efficiency in the Si-based SCs integrating Si QDs. In the beginning of this dissertation, we talk about the importance and recent developments of SCs, and then, the advantages and challenges of SCs integrating Si QDs are discussed. After that, our motivations, fabrication process, and apparatus are also introduced in details. To achieve the formation of super-high density Si QD thin films, we forsake the traditional [SiO2/SRO]-ML structure and develop a new one, GSRO-ML. In our results, by utilizing the periodical variations in Si/O atomic concentration during deposition, the Si QDs with super-high density and good size control can be self-assembled from the uniform aggregations of Si-rich atoms during annealing. Besides, the considerable enhancements on photovoltaic properties are also obtained by using a GSRO-ML structure due to the improved carrier transport efficiency and larger optical absorption coefficient. To obtain the better carrier transport path for the Si QD thin films, we also develop a new matrix material, ZnO, because it has many desirable features, such as wide and direct bandgap, high transparency, and highly tunable electrical properties. In our results, though embedded with Si QDs, the optical properties of ZnO thin film can be preserved in the long- and short-wavelength ranges. In the middle-wavelength range, the significantly enhanced light absorption and the unusual PL emission peak, owing to embedding Si QDs, are observed. These results represent the sub-bandgap formation in ZnO thin film by utilizing Si QDs while maintaining the essential optical properties of ZnO matrix. In the electrical properties, the Si QD embedded ZnO thin film reveals the significantly higher conductivity than that using SiO2 matrix material. Besides, the carriers transport mainly via ZnO matrix, not through Si QDs, is clearly observed. This unique transport mechanism differing from those using the traditional Si-based dielectric matrix materials has great potential on leading to the much better carrier transport efficiency and electrical properties for SC applications. In this dissertation, we had demonstrated the proposed novel Si QD thin films, utilizing a GSRO-ML structure and integrating with ZnO matrix material, are more suitable and advantageous for the Si-based SCs integrating Si QDs. Therefore, the high-efficiency Si-based SCs integrating Si QDs can be most definitely expected using the novel Si QD thin films.