| Thin film solar cell as the new generation solar cell has been manufactured on large scale and has a powerfully tendency and profound prospect. Compared with the traditional solar cell, thin film cell can be applied in some certain significant occasion such as building integrated photovoltaics (BIPVs),large scale power station, space station and so on due to the advantages of lower weight, flexibility and lower cost of manufacture. Stainless steel foil is the best candidate used as the thin film solar flexible substrate. However, the drawbacks of stainless steel surface including high roughness and impurities hinder the cell’s performance contrasted with the nano-level surface roughness of glass substrate. Thus, this article intends to reduce the stainless steel substrate surface roughness to about 10 nm after polishing.An electrochemical mechanical polishing (ECMP) of stainless steel substrate is researched, which is aimed to overcome the problems of instability and high surface roughness from passive film during ECM. simultaneously keep a high polishing consequent. Based on the ECMP of flexible stainless steel substrate, the research content and work are as follows:(1) Analysis of composite polishing mechanism and establishment of material removal modelThe ECMP mechanism in theory is analyzed from the point of single ECM and mechanical polishing which separately comply with the Faraday’s Law, Preston equation and so on. on basis of which the key influential factors are determined and a model for ECMP material removal is formed.(2) Establishment of experimental apparatus, design and manufacture of monolithic cathodeThe main components in the apparatus include electrochemical power, electrolyte circulation cell, conductive and insulated structure, clamp structure, NC body and digital controller; besides, a compound cathode tool is designed and manufactured according to the stainless steel substrate characteristics. The cathode tool, whose material is copper, with precision abrasives uniformly distributed is directly linked to machine spindle and controlled by digital controller.(3) Determination of experimental parameters’ values and its experimentA disciplinary analysis of parameters is made under the orthogonal experiments of ECM and MP considering the theoretical analysis and experienced values from literatures. Finally, the experimental factors’ values are determined as a voltage of 5V～9V, an initial machining gap of 3 mm, a polishing time of 5 min～10 min, a pressure of 2.7 KPa～13.5 KPa, a rotational speed of 800 rpm～2000 rpm and a feed speed of 20mm/min～100 mm/min.(4) Establishment of regression model for Ra and MRR and the analysis of the experimental resultsThe ECMP experimental table is arranged through the quadratic orthogonal rotatable combination design. A series of measurements are taken including surface roughness and morphology with a Leica optic microscopy from Germany, light reflection rate with an UV visible-NIR spectrophotometer from America and surface phase composition with a XRD from Netherlands. Both regression models of Ra and MRR are achieved with a 97% fitting, besides the main and interactive effects of experimental factors on Ra and MRR are graphically analyzed with MATLAB software.The results show that under the condition of 1700 rpm,8 V,5.4 KPa and 80 mm/min the surface roughness (measured area 127.32×95.45 μm2) decreases from Ra 124 nm to Ra 13 nm and the corresponding light reflection increases from 56.8% to 62.4%, a material removal rate (MRR) of 0.026 g/min is obtained, besides a thin passive film of Fe2O3 and Cr2O3 has form. Thus, the stainless steel surface qualities are improved, further beneficial to enhance the performance of thin -film solar cell. |
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