【作者】 周潘兵；

【导师】 周浪；

【作者基本信息】 南昌大学 ， 材料物理与化学， 2012， 博士

【摘要】 太阳能光伏发电是解决世界能源危机和环境污染的重要途径。目前硅晶太阳电池占据了光伏市场约88%的份额。硅晶材料的热改性指通过热处理或优化硅晶太阳电池制造中的热过程参数来提高晶体硅材料的光伏应用性能。它是一条低成本地提高硅晶太阳电池转换效率的重要的潜在途径。本文系统地研究了热过程参数对晶体硅中杂质状态与分布、缺陷密度的影响,以其由此导致的晶体硅电学性能的变化规律。所涉及的硅晶材料包括直拉单晶硅与定向凝固铸造多晶硅。研究主要取得以下结果：1、发现铸造多晶硅在连续冷却过程中,间隙固溶态的杂质氧有很强的沉淀趋势,而替位固溶态的碳则不然。前者在高达10℃/s的冷却速率下仍会发生可观沉淀析出；而后者在低至0.017℃/s冷却速率下也基本不发生沉淀析出。计算分析显示,硅晶中杂质碳不易发生沉淀析出的原因在于碳在硅中的扩散激活能大,扩散速率极低。2、发现无论是直拉单晶硅还是铸造多晶硅,其铁杂质的脱溶沉淀与分解重溶对热过程十分敏感,需密切关注。两种原始态的硅晶材料在300～1050℃范围加热后快速冷却至室温,其间隙铁含量均显著增加；加热温度越高,快冷后硅晶中的间隙铁含量越高；而经一定温度加热后,间隙铁含量均随冷却减缓而降低。在900～1050℃范围加热,继以50℃/s的速率快冷至室温后,两种硅晶材料中90%以上的铁仍以沉淀形式存在,其余的铁以过饱和固溶间隙态存在。对铸造多晶硅,这些过饱和的间隙铁经加热随即缓慢冷却处理后,又发生沉淀而大幅度回落下降,并随加热温度的升高而逐渐降低,900℃加热时即接近原始铸造多晶硅的间隙铁含量水平。3、0.2mm厚的多晶硅片经过1320℃以上高温退火并缓慢冷却后,其内部位错数量会显著下降。例如在1340℃退火2小时并以0.12℃/s速率冷却后,其内部位错密度降低约一半。实验显示,高温下硅晶体中位错的消长对热应力十分敏感,从而受冷却条件和材料厚度影响很大—在1320℃以上高温退火后冷却速率稍高(大于0.13℃/s)即会使多晶硅片内部位错密度增加；而厚度为13mm的铸造多晶硅块经此高温退火后即使以0.03℃/s的速度缓慢冷却,其内部的位错密度也有一定程度的升高。实验还发现,经1100℃以下温度退火并缓慢冷却后,0.2mm多晶硅片中位错密度并不降低,反而增加。4、提出原始态硅晶材料少子寿命的热衰减概念并进行系统研究。实验发现,原始态的直拉单晶硅与铸造多晶硅在300-1050℃范围加热后快冷至室温,其少子寿命均显著下降,加热温度越高,下降程度越大；而经相同的温度加热后,其少子寿命随冷却速率增加而降低；热衰减后的多晶硅片经不同加热并缓慢冷却处理后,其少子寿命均能得到不同程度的恢复,在900℃达到最大值,约为原始多晶硅片的92%。实验还发现,经过加热淬冷热衰减的铸造多晶硅比原始铸造多晶硅具有更强的抗高温热衰减能力。5、综合硅晶材料少子寿命热衰减／恢复现象与热过程中硅晶内部氧、碳和铁杂质杂质状态与位错密度变化的关联情况,提出热衰减的主要机理是硅晶中铁等金属沉淀相的分解造成的铁等金属杂质的固溶释放,它们在冷却过程中未能充分聚集沉淀,而造成较高含量的过饱和固溶铁等金属杂质或高度分散细密的二次沉淀,成为少子复合中心,使少子寿命大幅下降；而已经热衰减硅晶的少子寿命恢复的主要机理则是过饱和固溶铁等金属杂质的聚集沉淀,或细密沉淀的溶解与再次聚集沉淀为较大尺度沉淀相。6、采用650℃保温40min的退火可以有效地消除直拉单晶硅中形成的热施主,并使虚高的少子寿命和电阻率恢复到实际值。在后续高温加热后的缓慢冷却过程中,硅片的热施主会再次生成,其含量随冷却速率降低而增加,快速冷却可避免热施主的再次生成。实验还发现,慢冷至550℃后再快冷同样可避免热施主的再次生成,而且还避免了少子寿命热衰减。这一发现可用以解决消除热施主处理与少子寿命热衰减的矛盾。以上结果对晶硅太阳电池的生产中热过程的优化利用以及对硅片附加热处理的设计将具有重要的参考价值,对于低成本地提高现有晶硅太阳电池效率意义重大。部分结果公开发表后已为多晶硅太阳电池制造企业采纳应用。更多还原

【Abstract】 Solar photovoltaic power is an important solution to the energy and environment crisis of the world. Silicon wafer-based solar cells occupy about88%of the cells installed in solar power systems. Thermal engineering of crystalline silicon refers to enhancing photovoltaic performance of the material through thermal treatments or optimizing thermal processes in fabrication of silicon wafer-based solar cells. It is a potential route to higher energy conversion efficiency at low cost.In the present thesis, effects of thermal parameters on the state and distribution of impurities and defect densities in crystalline silicon, and hence on electrical properties of crystalline silicon, are investigated systematically. Both Cz type mono-crystalline silicon and directionally solidified cast multi-crystalline silicon are involved. The following major results are obtained.1) It has been found that, in continuous cooling of cast multi-crystalline silicon, the dissolved interstitial oxygen has very strong tendency to precipitate, while carbon does not. The former can precipitate even in cooling as fast as10℃/s, while the latter basically does not precipitate even in cooling as slow as0.017℃/s. Calculation shows that the reason for difficulty of carbon precipitation is the high activation energy for diffusion of carbon in crystalline silicon, and its extremely low diffusion rate.2) It has been found that, in both the mono-crystalline silicon and the multi-crystalline silicon, precipitation or dissolving of iron impurity is very sensitive to thermal processes, which worth close attentions. Concentration of dissolved interstitial iron in the two crystalline silicon materials of as-solidified state remarkably increases after heating to300～1050℃followed by a rapid cooling to ambient temperature. The higher the heating temperature, the higher the iron concentration in the rapidly cooled material. For heating to a certain temperature, slower cooling leads to lower concentration of interstitial iron. In both crystalline silicon materials,900～1050℃heated followed by50℃/s quenching to ambient temperature, over90%of iron exists in state of precipitates, with rest in dissolved interstitial state. For multi-crystalline silicon, the supersaturated interstitial silicon will precipitate again after a heating followed by slow cooling procedure, leading to remarkable decrease of its concentration. Higher heating temperature leads to lower level of interstitial iron concentration after slow cooling, down to the as-cast level when the heating temperature reaches900℃.3) Dislocation density in0.2mm thick multi-crystalline wafers significantly decreases after annealing at1320℃or above followed by slow cooling. For instance, in the sample annealed at1340℃for2hours followed by cooling at0.12℃/s, about50%decrease of dislocation density was found. At high temperatures, it is found that generation/disappearing of dislocations in crystalline silicon is very sensitive to thermal stress, and hence to cooling condition and thickness of materials--in the wafers cooled at slightly higher rate (greater than0.13℃/s) after annealing at1320℃/s or above, the dislocation density increased, and in13mm thick sample of multi-crystalline silicon block, increase of dislocation density appeared even when the cooling is as slow as0.03℃/s after the same kind of annealing. It is further shown that annealing at1100℃/s or lower followed by slow cooling results in increase of dislocation density, rather than decrease.4) The concept of thermal degradation of minor carrier lifetime is proposed and supported by experimental studies. Both mono-crystalline silicon and multi-crystalline silicon in their as-solidified state are found to have decreased minor carrier lifetime after300～1050℃heating followed by rapid quench to ambient temperature. Higher the heating temperature, shorter the lifetime after quench. With the same heating temperature, the lifetime decreases as the cooling rate increases. The thermally degraded multi-crystalline silicon can be recovered to various extent by heating followed by slow cooling, with the maximum recovery extent achieved at900℃heating,～92%of the as-solidified multi-crystalline silicon. It was also found that, the thermally degraded multi-crystalline silicon by fast heating-quenching showed better resistance to high temperature thermal degradation than the as-solidified multi-crystalline silicon.5) Based on the phenomena of thermal degradation/recovery, and their correlation with the evolution of the state and distribution of iron, oxygen, carbon, and dislocation density in thermal processes, it is proposed that the major mechanism for thermal degradation is dissolving of the precipitates of iron and other similar metal impurities, which releases iron and other metals, resulting in supersaturated iron and other metal impurities, or fine scattered secondary precipitates in fast cooling. They act as recombination centers of the minor carriers and cause remarkable decrease of the lifetimes. The major mechanism for the recovery of the thermally degraded silicon is then the precipitation of supersaturated iron and other similar metal impurities, or dissolving of the fine precipitates followed by secondary precipitation forming larger size precipitates.6) Thermal donors formed in Cz mono-crystalline silicon can be effectively eliminated by a40min annealing at650℃, which make the measured minor carrier lifetime and resistivity recovered to their real value. In the subsequent heating to high temperature followed by slow cooling, the thermal donors form again, with its concentration increasing with decrease of cooling rate. The re-formation of thermal donors can be avoided with fast cooling. It is found that if the material is slowly cooled to550℃, and then cool quickly, formation of thermal donors can also be avoided. This discovery can be utilized to solve the problem of contradiction between the thermal donor elimination and thermal degradation of minor carrier lifetimes.The results presented above are helpful for optimization of thermal processes in fabrication of solar cells from crystalline silicon wafers, and for designing of added heat treatment of silicon wafers, which in turn are of great value to increase of energy conversion efficiency of crystalline silicon wafer-based solar cells. Part of the results, after publication, has already been used in solar cell manufacturing industry.更多还原