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.
Figure 1: Difference between a p-type PERC and a n-type PERT solar cell
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.
What is a TOPCON solar cell?
TOPCON (also known as passivated contact) solar cell, is touted as the next generation of solar cell technology after PERC. This novel architecture is introduced by researchers at Fraunhofer Institute for Solar Energy Systems in Germany in 2013.
Compared to the other potential new technologies, such as HJT and IBC, TOPCON can be upgraded from the current PERC or PERT line. As a result, lower capital investment is needed for existing PERC or PERT manufacturers who are looking to upgrade their existing production lines. Moreover, a good gain in solar cell efficiency can also be achieved. This is ~1% in absolute value as reported in.
TOPCON is the acronym for “Tunnel Oxide Passivated Contact”. Figure 1 show this cell architecture as compared to a n-PERT solar cell.
Figure 1. n-PERT compared to n-TOPCON solar cell architectures
As shown in the figure, n-PERT and n-TOPCON are quite similar. Typically, to upgrade an n-PERT solar cell to a n-TOPCON solar cell, only an additional ultra thin SiO2 layer and a doped poly-Si layer are required.
The ultrathin SiO2 acts as surface passivation layer between the rear Si surface and the rear “contact” – the poly-Si layer. In addition, it also needs to be thin enough so that current can tunnel through it quantum mechanically.
The poly-Si layer is highly doped to produce a high conductivity layer. This high conductivity layer will then acts as a contact for current collection. Additionally, in a n-type TOPCON, the poly-Si layer is typically doped with phosphorus to provide field passivation (back surface field). This is similar to the phosphorus doped rear surface of n-PERT as shown in figure 1(a).
With the addition of the tunnel oxide layer, the original authors at Fraunhofer Institute for Solar Energy Systems reported an increase of ~1% in absolute solar cell efficiency.
What is a HJT solar cell?
HJT is the acronym for hetero-junction solar cells. Introduced by Japanese company Sanyo in the 1980s, then acquired by Panasonic in 2010s, HJT is considered as a potential successor to the popular PERC solar cell as of the time of writing, besides other technologies such as PERT and TOPCON.
Due to HJT’s fewer number of cell processing steps, and a much lower cell processing temperatures, this architecture has the potential to simplify the current solar cell manufacturing lines that are currently heavily based on PERC technology.
Figure 1: p-type PERC vs n-type HJT solar cell
As shown figure 1, HJT is very different to the popular PERC structure. As a result, manufacturing processes between these two architectures are very different. Compared to n-PERT or TOPCON, which can be upgraded from the current PERC lines, HJT requires significant capital investment in new equipment to start mass productions.
Additionally, as with a lot of new technologies, long-term operation/manufacturing stability of HJT is still under reviewed. This is due to processing challenges such as amorphous Si’s susceptibility to high temperature processes.
HJT demonstrates high solar cell efficiency thanks to the high quality hydrogenated intrinsic amorphous Si (a-Si:H in Figure 1) that can provide impressive defect passivation to both the front and rear surface of Si wafers (both n-type and p-type polarity).
The use of ITO as transparent contacts also improves current flows, while also acting the anti-reflection layer to provide optimal light capturing. Moreover, ITO can also be deposited via sputtering at low temperature, thus avoiding the re-crystallisation of the amorphous layer that will impact the passivation quality of the materials on the bulk Si surface.
In spite of its processing challenges and high capital investments, HJT is still an attractive technology. This technology demonstrates the ability to achieve >23% solar cell efficiency, compared to ~22% shown by TOPCON, PERT and PERC technologies.