| 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. |
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