【作者】 储清梅；

【导师】 戴永年； 张鹏翔； 刘翔；

【作者基本信息】 昆明理工大学 ， 有色金属冶金， 2014， 博士

【摘要】 太阳电池的发展至今有几十年了,晶体硅电池一直都是其市场主流,而高效率、低成本是其发展方向。腐蚀技术(即表面织构)是提高晶体硅电池转换效率的重要手段,但诸如金字塔的初始形成机制和腐蚀各向异性等问题仍未得到很好的解决。为了进一步了解腐蚀机理,本论文提出了NaOH/NH4F和NaOH/NH4F/Na2CO3两种腐蚀体系,与纯NaOH溶液进行对比分析研究。通过对表形貌、腐蚀速率和拉曼光谱等实验结果的分析研究,提出了腐蚀-聚合机理及其模型,解释了腐蚀过程中表形貌、腐蚀速率的变化以及其它一些腐蚀体系中出现的实验现象。纯NaOH溶液腐蚀所得的硅表面为金字塔结构,反射率最低。加入NH4F后,硅表面主要为较短小的山丘状小丘覆盖,反射率较高,一般为15-16%,腐蚀速率则明显降低且完全由它的浓度控制,OH-离子的影响非常微弱。再加入Na2CO3,表面小丘为长长的山链状,反射率比较接近纯NaOH溶液腐蚀(本文实验所得的最低值为12.85%),腐蚀速率比NaOH/NH4F体系的略低。为了进一步的深入研究,实验测定了三种腐蚀体系的拉曼光谱,结果发现这三种腐蚀体系和硅片表面硅酸盐的聚合都有明显变化,其聚合程度都是NaOH/NH4F/Na2CO3体系强于NaOH/NH4F体系,NaOH/NH4F体系又强于纯NaOH溶液,并且都没有Si-F键存在。基于这些实验结果,本文提出了腐蚀-聚合理论。这一理论认为硅腐蚀后的表形貌受产物硅酸盐聚合的影响,聚合所产生的氧化硅是金字塔、小山丘和山链形成的微掩膜。在碱性含氟溶液中,硅腐蚀的中间产物为SiHxFy(OH)z,它并不稳定,最终会变为硅酸盐。硅酸盐的聚合方式有三种：path1(亲核脱质子硅醇三Si-O-与中性硅酸盐物种的反应)、path2(Si-OH群的桥接反应)和path3(Si-F直接与Si-OH反应)。path1和path2聚合所形成的产物聚合度低,path3不仅能在硅酸盐浓度低时发生而且产物的聚合度还比较高。纯NaOH溶液聚合按path1和path2进行,所形成的微掩膜尺寸最小,表面小丘为金字塔。NaOH/NH4F体系三种聚合方式都发生,所形成的微掩膜尺寸较大,硅表面为条状小山丘覆盖。C032-离子虽也能促进聚合,但加入纯NaOH溶液只能增加金字塔的密度,而加入NaOH/NH4F体系与F-离子共同作用却能提高聚合度,使条状小山丘变成长长的山链。根据腐蚀过程中硅表面反应物H2O和OH-离子的浓度变化以及氧化硅掩膜的形成,腐蚀-聚合理论解释了本文三种腐蚀体系形成的表形貌差异。再结合腐蚀过程中表形貌引起的腐蚀速率变化,它还能解释一些其它实验现象,如金字塔崩顶和各种添加剂的影响等。基于腐蚀-聚合理论,本文建立了腐蚀-聚合原子模型。这一模型不仅解释了多种形状的小丘形成,还解释了它们形成的难易程度。金字塔是以一个氧化硅顶点为掩膜形成的小丘,其氧化物聚合度最低,最易于形成。(111)条状小丘以氧化硅密度最高、最不易形成的多个氧化硅顶点为掩膜,所以(111)条状小丘很难形成。六面小丘以氧化硅密度相对(111)条状小丘较低的多个顶点为掩膜,相对容易形成,本文NaOH/NH4F体系腐蚀出的小丘就近似这种形状。此外,氧化硅掩膜若中途形成在金字塔的(111)面上则会形成(110)面。若(111)面上氧化硅形成得很少,(110)面小得可忽略或是氧化硅形成在金字塔的棱上,那么金字塔(111)面的倾角变小,金字塔发生延展,这解释了金字塔的倾角小于理论值54.735°；若(111)面上氧化硅形成得很多,(110)面不可忽略,那么金字塔变成八面锥,这解释了八面锥的出现。腐蚀-聚合理论及其模型成功解释了金字塔初始形成以及腐蚀过程中表形貌变化、腐蚀速率变化等实验现象,成为Si表面腐蚀技术的理论组成部分,为高效率、低成本硅太阳电池的研究开发奠定了较好的理论基础。更多还原

【Abstract】 Solar cells industry began to evolve for decades and crystalline silicon cells are the market mainstream, whose trend is high-efficiency and low-cost. Etching technology plays an important role in the increase of cell conversion efficiency whereas several questions such as initial growth of pyramid and etching anisotropy aren’t answered clearly. In order to understand etching mechanism thoroughly, we proposed NaOH/NH4F and NaOH/NH4F/Na2CO3etching solutions and made a theoretical and experimental investigation in detail by comparison with pure NaOH. Owing to experimental result such as morphology, etch rate, Raman spectra and so on, etching-condensation mechanism and its atomistic model are put forward. And alteration of morphology and etch rate as well as some phenomena concerning other etching solutions are clearly explained.Si(100) surface etched in pure NaOH solution is covered with pyramids that leads to a lowest reflectance. After adding NH4F to pure NaOH solution, the shape of hillocks is similar to pyramid or ridge and reflectance is in a higher range of15-16%, but etch rate decreases rapidly with NH4F concentration and increases a little with NaOH concentration. After another reagent Na2CO3is added into the solution containing NaOH and NH4F, the ridges become so long as to look like mountain chain whereas reflectance is close to the value obtained from pure NaOH solution(the lowest experimental value is12.85%in this paper) and the etch rate is a little lower than that in NaOH/NH4F system.For a thorough investigation, Raman spectra of different etching solutions and silicon wafer surfaces were measured. It was found that condensation of Si-OH groups at the wafer surfaces and in etching solutions varied obviously as the same tendency. The degree of condensation is highest in NaOH/NH4F/Na2CO3system and it is lowest in pure NaOH solution. Moreover, no Si-F bond is formed.Based on these experimental results, etching-condensation mechanism is proposed. It is believed that the silicate products affect the Si(100) surface morphologies and silicon oxides are the masks of all hillocks including pyramids, ridges and mountain chains. In alkaline fluoride solutions, SiHxFy(OH)z is the intermediate product of silicon etching, but it isn’t stable and should be eventually transformed into silicates. There are three paths of condensation:path1(attack of a nucleophilic deprotonated silanol on neutral silicate species), path2(condensation of Si-OH groups) and path3(polymerization of Si-F and Si-OH groups). The degree of condensation are lower by path1and path2, on the contrary, it is higher by path3. And path3should occur even if silicate concentration is very low. In pure NaOH solutions, silicon oxides are generated by path1and path2, consequently the degree of condensation is so low that the masks are smallest and pyramids are formed. In NaOH/NH4F system, three paths should take place and they result in the formation of bigger masks so that infinite ridges arise. CO32-ions only increase the pyramid population when they are added into pure NaOH solution, but in alkaline fluoride solution, they accelerate condensation in company with F" ions and an increase of condensation degree leads to the appearance of mountain chain.According to concentrations of H2O and OH-ions at the silicon surface and formation of oxide masks, three different surface morphologies have been explained by etching-condensation mechanism in this paper. If variation of etch rate resulting from surface morphologies is taken into account during the etching process, we can understand more phenomena such as softened pyramids loosing their tetragonal shape with defined triangular faces and effects of various additives.An atomistic model founded on etching-condensation mechanism is put forward systematically. It describes the formation of multifarious hillocks and reflects their difficulties in the growth process. A pyramid derives from an oxidized silicon atom and is easiest to come into being because of the minimum size of oxides. A long (111) hillock needs a mask composed of many tightest oxidized silicon atoms, so it is most difficult to emerge. For a long hillock with6planes, the density of those oxidized silicon atoms is lower than that of a long (111) hillock and it is possible to appear accordingly. The hillocks obtained in NaOH/NH4F system are similar to this shape. Furthermore,(110) planes should be formed if condensation occurs at (111) planes of a pyramid during etching process. On condition that oxides are located on the edge of the pyramid or formed (110) planes are too small to be ignored owing to a very small quantity of oxides produced at (111) planes, the angles between (111) and (100) planes should be slightly lower than the expected value54.735°. When oxides at (111) planes are so plenty that (110) planes are macroscopically visible, a pyramid with8planes should appear. By etching-condensation mechanism and its atomistic model, not only the question about initiator of pyramid has been answered but also lots of phenomena during etching process, such as alteration of morphology and etch rate, could be explained. It is an important supplement to theory of silicon etching and lay the foundation for developing low-cost and high-efficiency crystalline silicon solar cells.更多还原