铸造晶体硅的杂质与缺陷

Photovoltaic electricity based on solar cells is one important form to utilize the renewable energy, which is clean and environment-friendly. The basic material to fabricate the solar cells is cast crystalline silicon (c-Si). The impurities and defects in cast c-Si is the key scientific and technology issue, which determine the efficiency of solar cell as well as its cost. This project will fabricate the cast c-Si using the nucleation control technology based on our own independent-intellectual-property rights, including the cast mono-like c-Si and high performance multi-crystalline silicon (mc-Si). The properties of impurities and defects in cast c-Si will be systematically investigated, especially in n-type c-Si, the c-Si grown under nitrogen ambient protection and the germanium co-doped c-Si. Firstly, we will demonstrate the causes of the dislocation and grain boundary generation, and the behaviors of impurity diffusion and clustering in cast c-Si. Then, the interaction mechanism between impurity and defect as well as the evolution in their electrical properties will be illustrated. Finally, the mechanism of various impurities and defects influencing the properties of solar cells will be revealed. On this basis, we will further study the influence of impurities and defects on the performances of cast c-Si solar cell, and meanwhile illustrate the mechanisms. This project will not only achieve some creative results and breakthrough in the basic theory of impurity and defects in cast c-Si, but also explore the novel growth technology of cast c-Si, which is appealing to the improvement of both crystal quality and solar cell performances. The achievements in this project will be of significance for the sustainable development of photovoltaic industry and the improvement of product international competitiveness in our country from the viewpoint of basic knowledge and technology.

利用太阳电池进行光伏发电是一种重要的清洁环保可再生能源利用形式，而铸造晶体硅是制备太阳电池的主要基础材料。铸造晶体硅的关键科学技术问题是杂质和缺陷，它决定着太阳电池效率和成本。本项目将在利用自主知识产权的铸造晶体硅生长技术基础上，通过晶核诱导等技术制备铸造晶体硅（包括铸造单晶硅和高效铸造多晶硅），并系统研究铸造晶体硅中杂质和缺陷的性质，重点研究n型、氮保护气和掺锗铸造晶体硅。项目将揭示晶体生长过程中位错、晶界等缺陷的产生原因以及杂质扩散和聚集的规律；阐明杂质和缺陷相互作用的机理以及缺陷的电学性质的变化规律；系统阐述杂质和缺陷对太阳电池性能的影响机理。研究结果不仅会在基础研究方面取得创新和突破，而且对开发具有我国特色的铸造晶体硅生长新技术，改善晶体质量，进而提高太阳电池的光电转换效率具有重要的推动作用，为我国光伏产业的可持续发展和提高其国际竞争力提供知识和技术基础。

铸造晶体硅是制备太阳电池的主要基础材料，其关键科学技术问题是杂质和缺陷。本项目首先研究了铸造晶体硅的制备技术，主要包括高效铸造多晶硅和铸造单晶硅。采用多孔坩埚、铺设碎硅片等方法来控制硅熔体在坩埚底部的形核，实现了高质量小晶粒高效铸造多晶硅的生长。采用坩埚底部铺设单晶硅块作为籽晶，有效控制了晶体的单一晶向，可利用碱制绒工艺获得较高的陷光效应。研究发现铸造晶体硅中杂质主要来源坩埚及其氮化硅涂层的沾污，这些杂质通过扩散方式进入晶体，以复合体或沉淀的形式影响晶体质量。其中，最有害杂质的金属元素—Fe，与p型掺杂剂B和Ga易结合形成Fe-B对和Fe-Ga对，严重降低材料的少子寿命，通过坩埚侧壁的高纯涂层技术可以有效降低金属杂质Fe的污染。而轻元素杂质—氧会与硼原子结合形成硼氧复合体，引起光衰减效应，采用热辅助电注入法能有效降低硼氧复合体浓度。另外，在铸造晶体硅共掺氮或锗等元素杂质可以有效提高硅材料的机械强度，有利于晶体硅太阳电池的薄片化，降低电池制造成本。研究还发现，位错是铸造晶体硅中最主要的缺陷，可利用低指数西格玛型晶界来抑制铸造单晶硅制备过程中由于籽晶拼接过程引入的位错源，将位错密度降低一个数量级。位错和晶界的电学复合活性严重依赖于其杂质沾污水平，随着杂质沾污程度的增加而增大，采用氢钝化或者磷（硼）吸杂技术可以有效降低杂质沾污的影响，提高太阳电池的效率。目前，铸造类单晶硅的晶界工程技术已转让给国际最大的铸造晶体硅制造企业—江苏协鑫，铸造类晶硅片年产量3亿片/年；晶体硅太阳电池抗衰减的热辅助电流注入技术已转让给常州时创能源，该公司的抗衰减设备在光伏产业得到了广泛的应用。项目的研究结果不仅在基础研究方面取得创新和突破，而且开发出了具有我国特色的铸造单晶硅生长新技术以及太阳电池的抗衰减新技术，为我国光伏产业的可持续发展和提高其国际竞争力提供知识和技术基础。