Comparison of POCl3 diffusion with phosphorus ion implantation for Czochralski and Quasi-mono silicon solar cells

Both ion implanted and POCl₃ diffused emitters are used for industrial production of p-type Si solar cells. Formation of phosphorus doped emitter is known to perform gettering of impurities, however, gettering efficiency of these two diffusion techniques is not well quantified for single crystal Cz Si and Cast multi or Quasi-mono Si wafers. In addition, ion implantation can provide higher quality emitter with in situ oxide passivation[1]. This paper compares the performance of Quasi-mono and Cz Si cells fabricated with POCl₃ and ion-implanted emitters. Quasi-mono wafers have more defects and are expected to benefit from gettering. Large area screen printed p-type industrial cells with full Al-BSF cell structure were fabricated on commercial grade single crystal Cz Si and two different Cast Quasi-mono Si wafers with 50% and 80% mono-crystalline regions. Bulk lifetime was measured to evaluate the gettering efficiency of each technology before and after each emitter formation[2]. POCl₃ diffusion gave greater improvement in bulk lifetime of Quasi-mono wafers compared to ion implanted wafers, resulting in 0.7 ~ 1% higher absolute efficiency and over 5 ~ 15mV higher Voc compared to the implanted cells. However, in the case of Cz cells, bulk lifetime remained high and comparable for the two emitters. Therefore, Cz cells with implanted emitter gave 0.4% higher efficiency and 7mV higher Voc due to a higher quality emitter with in situ front oxide passivation.