In the previous post, I have discussed the difference between a PERC and a conventional crystalline Si solar cell. In this post, I will talk about the PERT solar cell, which is also based on crystalline Si wafers.
PERT is an acronym for passivated emitter rear totally diffused. It has recently attracted wide attentions from the solar PV industry and research institutions. In particular, with the popular PERC structure seems to reach the bottleneck of its practical power conversion efficiency limit, PV researchers are looking for other cell architectures to continue to boost the efficiency of industrially viable Si solar cells.
Figure 1 shows the difference between a p-type PERC and a n-type PERT structure.
PERC (passivated emitter rear contact) structure has a localised back surface field(BSF). The BSF is created from the doping of Al into Si during metal co-firing processes. BSF helps to improve the solar cell efficiency by forming a high-low junction with the p-type Si base wafer. This junction repels minority carriers and prevent them from recombining at the rear surface of the Si wafer.
On the other hand, for PERT structure, the rear surface is “totally diffused” with either boron (p-type) or phosphorus (n-type). Usually PERT technology is implemented on n-type Si solar cells. This is to take full advantage of n-type Si wafers’ higher tolerance to metallic impurities, lower temperature coefficient and lower light induced degradation than p-type Si wafers. The light induced degradation is lower in n-type Si, possibly due to lower boron-oxygen pairs, as the bulk in n-type wafer is doped with phosphorus.
Nevertheless, the “totally diffused” BSF requires additional novel processes, such as high temperature POCL and BBr3 diffusion. As a result, PERT is more expensive to manufacture than PERC.
Nonetheless, the full area BSF in PERT solar cells may provide more effective high-low junction passivation effect, than the localised, coarser Al based BSF in PERC. In addition, n-type PERT also allows the integration of the so-called tunnel oxide passivated contact (TOPCON) structure. This TOPCON structure has potential to further improve the efficiency of the device. TOPCON structure will be discussed in the next post.