合金直拉法制备太阳级硅过程中B、P杂质相重构及迁移机制研究

Preparation of solar grade silicon (SoG-Si) with metallurgical method has many advantages such as much less investment and pollution, low energy consumption, low cost etc., which has attracted wide attention. How to remove the impurities from metallurgical grade silicon(MG-Si) such as boron and phosphorus with low energy consumption, high efficiency to obtain solar grade silicon is urgently needed. Based on the comment problems of removal and migration mechanisms of elements such as boron and phosphorus, a new method that was combined reconstructing impurity phases with Czochralski method, was proposed to remove boron and phosphorus during solvent refining process. By adding metallic elements, the reconstructed impurities phase composition, morphologies, distribution rule will be conducted to master the characteristics of B and P reconstructed impurity phases. The migration mechanism and distribution rule of reconstructed impurity phases containing B and P will be studied by alloy Czochralski process, which will provide the theoretical support for the removal of boron and phosphorus with stable and high efficiency in MG-Si, and promote the technology progress in the preparation of solar grade silicon. Moreover, electrochemical method on MG-Si will be applied to reveal the occurrence state and physical and chemical properties of boron and phosphorus in silicon.

冶金法制备太阳级硅以其投资少、环境污染少、工艺简单等优点，引起了国内外的广泛关注。如何由冶金级硅通过稳定高效的工艺去除B、P等杂质元素获得高纯度的太阳级多晶硅是亟需解决的问题。针对合金法提纯硅中B、P杂质元素的去除机理和迁移机制两个共性问题，本项目提出利用杂质相重构与直拉法相结合的新方法，来实现B、P杂质元素的深度去除。通过不同金属添加剂对B、P杂质相重构机制影响的研究，明确杂质（相）在合金化过程中的形成机理；通过合金直拉过程中重构杂质相定向迁移机制的研究，以期实现B、P杂质的稳定高效去除，促进合金法制备太阳级硅的技术进步。同时，本项目也提出了将电化学法应用于硅中杂质赋存状态及物化特性的表征。

太阳能以其分布广泛、储量丰富、清洁无污染等特点而成为未来解决能源短缺的一条重要途径。晶硅太阳电池占整个太阳电池份额的90%以上，硅中微量的杂质元素，特别是B/P会显著影响电池的转化效率。本课题通过原位观察法对不同热处理工艺条件下杂质相赋存状态的演化规律及物化特性进行了表征和分析；采用Ga-Si合金法研究了合金化过程对精炼硅形貌及杂质相的影响规律，从动力学的角度考察了合金配比、冷却速率等对B/P去除率的影响规律，同时提出了表观偏析系数的概念来表征B/P在合金精炼过程中的偏析系数。Ga-Si合金化提纯可以显著降低B/P的偏析系数，从热力学角度证明Ga-Si合金法除B/P是可行的。采用Al-Si合金法，研究了含P元素在合金化-熔析分离-酸洗处理三个阶段的杂质相的赋存状态及迁移规律，结果表明P杂质主要以Al-Si-Ca-P四元杂质相的形式赋存在初晶硅的晶界处。研究了Al-Si合金吹气精炼对B杂质相的重构和去除机制，与未通气相比，Al-Si合金熔体经过2.5h吹气精炼B的去除率提高了28.9%。揭示了B在合金吹气精炼过程中的去除机制，证明B在铝硅合金中主要的杂质相是以TiB2，AlB2和VB化合物的形式析出。研究了Al-Si合金掺Ga对B/P去除机制，考察了掺Ga对初晶硅晶粒生长尺寸及对B/P去除率的影响规律，合金内部产生明显的三相组成，包括初晶硅相、三元共晶相(α Al + Si + β Ga)和GaxAl1-x相。随着Ga掺入量的增加，初晶硅的回收率显著提高。提出了合金直拉法提纯硅的新方法，研究了合金直拉过程中B/P杂质的迁移和分配机制，并对直拉过程中B/P的除杂机制进行了解析，通过提拉工艺的优化，可以实现提拉样品中初晶硅的富集和分离，提高B/P杂质的分离和除杂效率。将直拉法工艺与合金法相结合，充分利用合金法对B、P杂质相重构和拉晶法对杂质相定向迁移方面的优势，实现B、P的稳定高效的去除，促进了冶金法提纯硅技术的进步。