Black Multicrystalline Silicon Solar Cells Prepared By All-liquid Two-step Metal-catalyzed Chemical Etching

In recent years, black silicon of extremely low reflectivity in visible and near-infraredattracted great attention for effectively improving the conversion efficiency of solar cell.Among many methods to fabricate the black silicon, metal-catalyzed chemical etching(MCCE) showed remarkable advantages such as simple equipment, low cost, and easilyintegrating into the current solar cell processing. There are many groups engaged in theresearch of MCCE and also obtained some achievements, which mostly focused onone-step MCCE method. However, one-step MCCE will consume a large amounts of noblemetal such as Ag, thus bring a difficult to removal them from the wafer surface completelyin the follow-up process.In this paper, the research and development of all-liquid two-step MCCE method notonly can be compatible with existing production line perfectly, but also solve thedrawbacks of one-step method effectively. It reduces the production cost largely andsimplifies the process steps of the production line. Therefore, it well be extreme importantto explore this technique for the PV industry.Firstly, the Ag nanoparticles will be deposited on the surface of silicon wafer byimmerging the silicon wafer in the solution of AgNO3, and in the second step nanostructurewill be formed by etching Ag-loaded wafer in a mixed solution of HF and H2O2. This papermainly studied the influence of the solution concentration and reaction time to thenanostructure, which was investigated by scanning electron microscopy (SEM) and opticalmicroscope (OM) and etc. The results showed that: different solution concentration andreaction time will result in various nanostructures.Secondly, the obtained Ag nanoparticles through two-step method and one-stepmethod were characterized by SEM/EDX and particle size analysis software. It is foundthat two-step method consumes much less silver, and the distribution and size of Agnanoparticles is very regularity. Therefore, the residual metal will be easily removed toavoid the serious surface recombination introduced by metal.Finally, the black silicon has double-scale texture of nanostructure and microstructure, which makes the reflectivity down from25%to very low values, i.e5%. The opticalproperties of black silicon were mainly investigated by standard8degrees Angle ofintegral type reflectometer (D8reflectometer). However, the black silicon with very lowreflectivity has the deep structure and large specific surface area, thus it will increase thecarrier recombination centers, which will depress the conversion efficiency of the cell. Inpractice, we add a step of post-etching to black silicon to adjust the depth of nanostructureto an ideal range. Several batches of black multicrystalline silicon by two-step methodhave been fabricated on an industrial production line at Canadian Solar Inc.@Suzhou.The results show that the average18.05%efficiency is higher than the average17.52%efficiency in the production line.In summary, we confirmed the feasibility of fabricating black silicon by usingtwo-step MCCE methods in both laboratory and production line. The presented results andmethods have great impacts to the application of photovoltaic community, and we believethat it will have a very bright application in the future.