Non-Destructive Evaluation of Toxic-Less Approach on Emitter Formation by Water-based Phosphoric Acid for n-Type Silicon

Silicon solar cells are well known as a major technology used in solar photovoltaics due to their high lifetime and highly efficient performance. The conventional process of silicon solar cell wells involves a highly toxic process that is harmful to the environment and human health because of the application of liquid-based phosphorous oxychloride (POCl3) and boron tribromide (BBr3) for emitter formation. This paper discusses the potential of water-based phosphoric acid (H3PO4) as emitter formation on n-type Si wafer. An alternative method for fabrication of Si solar cells by replacing POCl3 or BBr3 by phosphoric acid (H3PO4) is proposed. Phosphoric acid is less harmful toward the environment and health because of in-house synthesis. The emitter formation uses heavily doped/ highly concentrated phosphoric acid by dip coating at various temperatures and times. Temperature was varied within the high range of 875 degrees C to 975 degrees C and low range of 700 degrees C to 850 degrees C. Deeper junction depth was observed at high temperatures with sheet resistance value of 7 omega/sq, whereas a heavily doped junction was determined at 700 degrees C for 5 min with sheet resistance (R-sheet) value of 81 omega/sq. The variation in sheet resistance (R-sheet) ranging from 5 omega/sq to 100 omega/sq depended on time and temperature, where deeper junction depth occurred at high temperatures while shallow junction occurred at low temperatures during diffusion. Resistance measured was consistent from the front and back surface of n-Si wafer. Junction depth (x(j)) was determined through mathematical calculation depending on time, temperature, peak doping and base doping of n-Si wafers. The junction depth before and after PSG removal on n-Si wafers during low-temperature diffusion of H3PO4 ranged from 0.08801 mu m to 4 mu m. The value of junction depth, (x(j)) at high temperature was approximately 4 mu m to 28.844 mu m. These data were supported by the activation of phosphorous by 20% H3PO4 on n-Si wafer by using dark I-V, with series and shunt resistance of 7.763 ohm and 18.315 ohm, respectively. PC1D simulation was used to estimate the junction depth, which was found to be correlated with the measured sheet resistance from the experiment. The overall efficiency of the n-type emitter on n-Si solar cell was determined as 8.6% by using the average peak doping concentration. The FESEM and EDX profiles indicated the presence of 20% H3PO4 on n-Si wafer. Thus, fully coated 20% H3PO4 can be applied to a toxic-less approach for commercial fabrication of silicon solar cells.

Automated summary

This paper explores the potential of using phosphoric acid as the emitter formation for silicon solar cells, which is less harmful to the environment and human health compared to the conventional toxic process. Different depths of junction were achieved through dip coating experiments at various temperatures and times. The experimental results of phosphorus activation using 20% H3PO4 supported the calculated junction depth. The overall efficiency of the n-type emitter on n-Si solar cell was determined using the measured sheet resistance and was found to be 8.6%.