Using hydrofluoric acid to reduce the contact resistance of screen-printed silicon solar cells – Its recombination impact and a method to eliminate it

Using hydrofluoric acid (HF) to improve the contact resistance of a screen-printed silicon solar cell with a high sheet resistance emitter was found to concurrently reduce its pseudo-fill factor. By treating the cell in phosphoric acid, this impact was found to be significantly reduced or eliminated. In the solar cell presented in this work, the pseudo-fill factor reduced from 82.1% to 79.9% after treating the cell in HF, but increased to 81.8% after a subsequent phosphoric acid treatment. Similar effects were found in three other screen-printed solar cells that were fired at different peak furnace temperatures and belt speeds. It was also shown that the phosphoric acid treatment alone does not affect the pseudo-fill factor of a cell. HF treatment can now be used to reduce the contact resistance of a screen-printed silicon solar cell with a high sheet resistance emitter without suffering from a lower pseudo-fill factor so that the full benefit of a HF treatment on a screen-printed silicon solar cell can be realized. This combination of HF and phosphoric acid treatments can potentially be a useful failure analysis method for screen-printed silicon solar cells in the production line and the laboratory.