Impact of wet-chemical processing and Ozone rounding on the performance of Silicon heterojunction solar cells

With solar energy generation becoming one of the main energy sources of our world, the importance of methods that enable the production of high-efficiency solar cells, such as surface cleaning, conditioning and processing has grown at a similar pace. These processes not only prepare the wafers’ surface for the deposition of other layers, but also enhance solar cell
performance by improving their optical properties and device quality. In this work, we have studied the conditions and effects Saw Damage Etching (SDE), alkaline texturing and Ozone cleaning and rounding of the texture’s pyramid tips have on the
surface morphology and general performance of Silicon Heterojunction (SHJ) solar cells over a range of parameters. We have done so in two setups: a larger semi-industrial wet-chemistry batch operating tool and a smaller beaker and heating plate system. We have observed the effects different SDE times have on the wafer’s surface morphology by means of Scanning Electron Microscopy (SEM) and reflection response with UV-Vis measurements in the 400 to 1200 nm wavelength range. Results have shown that the surface gets smoother and reflection at the IR region decreases with processing time. Cell testing results indicate that having short processing times (1.5 minutes) or no SDE can deliver cells with efficiencies as good or better than cells with longer SDE times. We have also investigated the effects that processing time, KOH and texturing additive
concentrations, texturing additive type, texture size, temperature, stirring speed, reaction container material, reaction total volume and number of sequential runs have on the results of alkaline texturing of monocrystalline n-type Silicon. To do so, we have created and validated a system (the Beaker setup) to reproduce the reaction on a smaller scale using hand-cut wafer pieces. Further, we have studied the effects that performing O3 rounding of the pyramid tips have on the wafer and cell properties for 3 texture sizes and 5 process times. Simultaneously, we observed the effect variations in the surface’s texture has on the laydown of Ag-paste during metallization. To do so, we have studied the surface of the wafers and cells after different processing steps with confocal laser microscopy (LEXT) and SEM, their reflection response with UV-Vis measurements, paste consumption by mass measurements and the performance of cells and mini-modules by cell testing.
The results have shown that Ozone rounding smoothens the texture’s pyramid tips with time in a linear fashion, thereby increasing the surface’s reflection. With its effects being more pronounced for smaller than larger sized textures. Change of the wavelength reflection minimums after ITO deposition indicate that rounding can enable a reduction in the amount of TCO material used. Paste laydown results have shown that smaller textures lead to reduced material consumption, but we could observe no clear trend for the relation between this and rounding time. In terms of cell and mini-module performances, we have observed an improvement in VOC and concomitant loss in JSC which have led to a maximum in efficiency between 2 and 4 minutes of rounding. Considering that data variability was relatively high in these results, we suggest that new
experiments with a higher sampling have to be done to confirm the trends we observed.