Optimization of phosphoric acid based limited-source-diffusion to obtain high quality emitter for screen printed contacts

Limited source diffusion using commercial dopant sources have been used for obtaining high quality emitters for high efficiency cells. In this work, the effect of gas flow on the quality of limited source diffusion is investigated. An emitter is diffused using an in-house prepared limited phosphoric acid dopant source. The effect of gas flow in this process is studied by measuring the uniformity of sheet resistance and emitter saturation current density. In addition, to improve these two parameters, an alternate diffusion process was developed which involves the use of a short in-situ oxidation at a lower temperature (700°C) before the actual diffusion that resulted in excellent J_oe <; 100 fA/cm². The modified diffusion process also made this process much less sensitive to ambient humidity. 4 cm² full Al BSF cells fabricated on 0.6 Ω·cm p-type FZ silicon wafers using this emitter with screen printed contacts resulted in a peak efficiency of 19.6% with a V_OC of 648 mV and a J_SC of 37.5 mA/cm².