Introduction

Phosphorus diffusion is one of the most wide-spread method for emitter formation in silicon solar cells due to its several beneficial effects among which stands out its ability to improve bulk lifetime by reducing metal impurities, its reduction in surface recombination parameters by means of acting as a selective contact for minority carriers and its capability to create an effective ohmic contac.
Focusing on its gettering effect, some extended processes such as low temperature annealing after phosphorus diffusion has been proposed for some materials of lower quality (such as mc-Si), mainly because of the reduction in the concentration of interstitial iron. Moreover, our research group has recently found that there is a strong correlation between the amount of electrically inactive phosphorus in the dead layer created in conventional emitters and the optimum phosphorus diffusion conditions to get the largest gettering potential of phosphorus diffusion.
Taking into consideration the vast growth of PV industry in the last decades, and as a way to continue reducing manufacturing costs, it is believed that other alternative source of silicon such as the promising upgraded metallurgical-grade (UMG) silicon will have an increasingly penetration in the market in a medium term scenario. Although UMG (mc and mono) has already shown comparable efficiencies to solar grade silicon, more efforts should be done to identify the main technical challenges associated with the use of this material to find adjusted solar cells processing methods with the aim of reducing cost without compromising solar cells performance.
Thus, the aim of this study is to optimize the standard phosphorus diffusion method to be applied in lower quality materials. In the present work, the effect of low and high temperature annealing after the phosphorus diffusion step is investigated in order to figure out if this step could have a beneficial impact on the solar cell behavior by reducing the amount of impurities as it happens in the case of solar grade silicon. For this goal, some p-type UMG wafers were used to compare the effect of the different phosphorus diffusion gettering step. The most promising result, obtained with the low temperature annealing was also implemented in a simple manufacturing Al-BSF solar cells process. Characterization of the samples was mainly based on the Electrochemical Capacitance-Voltage profile and the Transient Photoconductance Decay (PCD) method.