【作者】 肖志刚；

【导师】 周浪；

【作者基本信息】 南昌大学 ， 材料科学与工程， 2016， 博士

【摘要】 光伏电力是最具发展前景的清洁能源。光伏电力发展的主题之一依然是降低成本。国内外光伏电力主要基于硅片太阳电池。近年来发展的金刚石线锯切割硅片技术相对传统的砂浆线锯切割技术使成本有可观幅度的降低,2014年以来已全面应用于单晶硅太阳电池硅片生产。然而多晶硅太阳电池片却迄今不能得益于这项技术进步。其原因是金刚石线锯切割的多晶硅片与现行酸性湿法制绒技术不能兼容,国内外迄今仍没有理想的低成本替代技术。本研究组前期研究开发了一种基于氢氟酸-硝酸混合酸溶液热蒸气刻蚀作用的气相刻蚀制绒方法(简称VE制绒),对金刚石切割硅片具有良好的制绒效果,但其均匀稳定性尚不够理想,需要深入研究理解其绒面形貌的形成机理和控制方法,以发展成为一项有生产应用前景的技术。这是本文研究的主要意义和目的。近年来美国开发出一种低成本的直接生长法多晶硅片,同样面临与传统多晶硅酸性湿法制绒不兼容问题,本文对这种硅片也进行了VE制绒初步研究。对蒸气源混合酸溶液的配比、溶液温度、硅片预热保温方式、刻蚀时间等VE制绒工艺条件对刻蚀动力学和制绒效果的影响进行了系统的实验研究,采用失重分析考察刻蚀动力学,采用光反射率、表面粗糙度和表面微观形貌表征制绒效果。结果显示,VE制绒初期一般快速形成10微米以上直径的大而浅的刻蚀坑,且沿线痕方向连接成浅沟槽坑,绒面光亮,反射率较高;随后会形成大量1-5微米直径的小刻蚀坑,刻蚀坑深宽比增大,反射率降低;继续进行下去则又会出现复杂纳米尺度蚀坑。各工艺条件都能够影响从初期到后期不同阶段的时间和发展程度,从而影响到最终所得绒面蚀坑的形貌、大小、均匀性及稳定性。研究发现并提出VE制绒的“微液滴刻蚀机理”:晶体硅表面的VE刻蚀制绒,实际上并非是由酸溶液蒸气刻蚀而是由它在硅片表面凝结形成的微液滴的局域刻蚀作用而产生的;刻蚀过程中的反应放热及其对硅片温度的影响和演变对刻蚀效果有着关键作用。常温下开始制绒的硅片的VE过程大致分为以下三个阶段:初期液膜刻蚀阶段:初期硅片温度尚较低,酸蒸气在硅片上冷凝成片且迅速扩展润湿硅片表面形成液膜,以液膜为载体刻蚀硅片,去除表面损伤层,淡化切割痕带来的表面性质差别,此为第一阶段‘液膜刻蚀阶段’。此阶段类似于湿法酸刻蚀,与之有着一致的刻蚀反应机理和刻蚀形貌。中期微液滴刻蚀阶段:随刻蚀反应大量放热,硅片升温,液膜蒸发收缩破裂成微液滴,以及不断凝结新生的微液滴,以微液滴为载体刻蚀硅片,形成均匀分散的微米级蚀坑。这种局域刻蚀点的分布由外来凝结点决定,而不是循硅片表面切割纹分布,因此它能解决金刚石切割多晶硅片的制绒问题,消除表面切割纹。后期复杂循环刻蚀阶段:随着刻蚀继续进行,硅片局部或整体温度高于酸蒸气温度,微液滴细化至纳米尺度及至消失,过程中产生亚微米或纳米尺度蚀坑,反应放热减缓,使硅片温度下降,到一定程度后微液滴的凝结复而发生,而刻蚀亦复而进行…如此不断往复循环,形成亚微米或纳米尺度蚀坑形貌。这一过程中硅片温度处在酸蒸气的凝结-蒸发临界点附近,刻蚀形貌具有由临界现象带来的复杂特征。复杂纳米结构能够大幅提高表面减反射效果,但同时却使后续电池工艺困难,其利用需慎重对待。目前本VE制绒技术不包括这一阶段。基于上述“微液滴刻蚀机理”和VE三阶段特性,通过大量实验验证,本文对金刚石线锯切割多晶硅片表面VE制绒的形貌控制提出了一系列的工艺条件,并显著改进了其均匀稳定性,形成了一种刻蚀时间60秒的有量产前景的VE制绒工艺,所得绒面为均匀密布的微米级蚀坑,其平均光反射率低于18%。研究发现VE制绒对直接生长法多晶硅片也具有良好效果,平均光反射率可降低到15%。所提出的微液滴制绒机理和三阶段特性依然适用,与金刚石切割硅片的区别是其液膜刻蚀阶段较短,缩短了整体刻蚀制绒时间。VE可望为新兴的直接生长法硅片提供高效低成本制绒技术。更多还原

【Abstract】 Photovoltaic solar power is the most promising clean energy.Cost reduction is still one of the major topics of PV power development,which is mainly based on silicon wafer-based solar cells.Diamond wire saw wafering technology,which has been developed in recent years,can lower the cost significantly,as compared to the conventional slurry wire saw wafering technology.Since 2014,it has been completely applied for production of mono-crystalline silicon wafers for solar cells.However,production of multi-crystalline could not be benefited from this technical progress up to now.The reason is that the diamond wire sawn multi-crystalline wafers are incompatible to the acidic wet texturization technology,which is well adopted for texturing multi-crystalline silicon wafers in current PV industry.No ideal low cot solution to this problem comes up yet.The research group the present author works in has developed previously a vapor etching texturization method,with the vapor from thermal evaporation of HF-HNO3 solutions(VE texturization).A good texture has been achieved with the VE method.However its uniformity and stability are not satisfactory.A study for deeper understanding of the mechanisms for the morphological evolution,and for mastering their control,is required before this method can be developed into an industrially applicable technology.This is the major purpose of the present thesis.In recent years,a new kind of low cost direct grown multi-crystalline ribbon silicon wafers emerges up,which also suffers from incompatibility with the existing acidic wet texturization.The present thesis investigate the VE texturization behavior of this new concept wafers,too.Effects of the composition of the vapor source acid solution,its temperature,the pre-heating temperature of the wafer sample,the way for keeping its temperature,and the length of etching time,on the etching kinetics and the performances of the textures,have been experimentally investigated.The etching kinetics is examined by weight loss analysis,while the texture is characterized by light reflectivity,surface roughness and micro-morphology.The results show that,a VE process usually forms shallow,larger than 10 micron diameter etch-pits in initial stage,resulting in high light reflectivity;followed by a stage forming deep,1~5 micron diameter etch-pits,resulting in lower reflectivity;if keep continuing,complex nanometer-scale etch-pits may appear.Each of the listed processing conditions affects the lengths and development of the different stages,and thus affects the morphology,size,uniformity,and their stability of the obtained textures.We found and propose the “Micro-drop etching mechanism” for the VE texturization process.Briefly,the etching is actually not by the vapor directly,rather it is by the liquid droplets condensed from the vapor,on the wafer surface.The local attacks by the randomly condensed micro-drops created the etch-pits for the texture.The exothermic heat from the etching reaction and its effect on the wafer temperature play an important role in the morphological evolution of the textured surfaces.The VE process of a silicon wafer starting at ambient temperature may be divided into the following three stages:Initial stage of liquid film etching: the initial wafer temperature is still low,so the acid vapor is condensed onto the silicon wafer surface and forms a liquid film for its wetting with the silicon surface.The liquid film is the carrier film for etching silicon to remove the surface damage layer and reduce the difference of surface properties brought about by the cutting marks.This is the first stage of the ‘liquid film etching’.This stage is similar to wet acid etching with corresponding etching reaction mechanism and etching morphology.Middle stage of micro-drop etching: as the etching reaction emits a lot of heat,the silicon wafer is heated and its temperature rises,thus the liquid film is evaporated and breaks into micro-drops.These drops and new drops condensed constantly to as the etching carrier etch the silicon wafer and form lots of uniform micro-pits.The distribution of the localized etch points is determined by the external condensation points rather than the distribution of wafer surface cutting pattern,so the micro-drop etching can remove the saw marks on the surface and solve the problem of texturing on diamond wire sawn multi-crystalline silicon wafer.Later stage of complex cycling etching: as etching continues,the local or total temperature of the silicon wafer is higher than the temperature of the acid vapor,and the micro-droplets are refined to the nanometer scale and disappear.The sub-micron or nanometer scale pits are generated during the process.The exothermic reaction slows down to a certain extent then the condensation of micro-droplets occurs again,and etching reaction runs again and so on...the process continues back and forth,and forms the etched morphology on the surface with sub-micron or nano-scale pits.In this process,the temperature of the silicon wafer is near the critical point of condensation and evaporation of the acid vapor,and the etched morphology has the complex characteristics brought by the critical phenomenon.Complex nanostructures can significantly improve the surface antireflection effect,but at the same time make the follow-up battery technology difficult,its use must be treated with caution.At present the VE texturing technology does not include this stage.Based on the micro-drop etching mechanism and the three-stage features,with large amount of experimentations,a series processing conditions for controlling the VE textures of diamond wire sawn multi-crystalline silicon wafers are suggested.The uniformity and stability of the VE textures are significantly improved.A VE texturization process,which creates textures with micron-scale etch-pits densely distributed on diamond cut wafers and reduces their average light reflectivity to < 18%,with the etching time 60 sec and hence being promising for application in mass production,is developed.It was found that the VE texturization worked well with the direct grown ribbon silicon.The average light reflectivity can be reduced to 15%.The proposed microdroplet mechanism and three-stage characteristics are still valid here.The difference is that its liquid film etching stage is relatively shorter,and hence the overall texturization is faster.VE is expected to provide a high-performance and low-cost texturing technology for the emerging direct grown ribbon silicon wafers.更多还原