Succession to the Throne: Looking Beyond PERC

PVTIME – In the past ten years, the price of photovoltaic modules has dropped from RMB 4.05/W at the beginning of 2015 to RMB 1.8/W in May 2021 (a decrease of up to 55%). The decline in module price is inseparable from the advancement of solar cell technology.

The last three years clearly belonged to p-mono PERC (passivated emitter and rear contact) cell technology as it dominated the global market with its efficiency and mass production maturity thanks to continuous research and development efforts. These continuous improvements in the price-performance ratio up to now enable PERC to be the preferred choice for utility-scale installations. According to the China Photovoltaic Industry Association’s 2020 Edition roadmap, PERC market share reached 86.40% in 2020.

The current average mass-produced conversion efficiency of PERC cells is 23.3%, and it is expected to reach 23.5% by the end of this year. However, the industry is certain of the fact that the conversion efficiency of PERC technology is close to reaching its theoretical ceiling of 24.5%. The industry’s hunger for higher solar cell conversion efficiencies will not be satiated by exhaustible technology. Looking beyond PERC, n-type solar cells, namely heterojunction (HJT) and TOPCon are generally believed to be potential heirs to the PERC throne.

TOPCon (Tunnel oxide passivated contacts) solar cells are widely seen as the forthcoming technology to PERC. Its cell architecture is based on the concept of ‘passivating and carrier selective contacts’, where the recombination of minority charge carriers is suppressed by a stack of thin tunnel oxide and heavily-doped polysilicon (poly-Si) layers. The current mass production efficiency TOPCon is 23.5%, and there is room for further improvement to 26%.

TOPCon technology has good compatibility with existing PERC production lines where more than 200GW of upgrade opportunities exist. The technological maturity and the proven industrial readiness of process technologies used in PERC are widely believed to provide an easy transition of the PV industry towards the higher efficiency cell concepts such as TOPCon. Another potential advantage of carrying this evolutionary approach is the possibility of upgrading the existing PERC production lines for TOPCon processing with the addition of a minimum number of process steps, provided that the required area is available in the PERC facility.

However, for the mass production of TOPCon cells, the economic competitiveness against the current mainstream PERC cell is still an open question. The additional capital and operating costs required to establish and operate a TOPCon manufacturing facility need to be offset by an increment in conversion efficiency on the cell/module/system level, in order to further lower the levelized cost of electricity (LCOE) of large scale PV installations.

On the other hand, Heterojunction technology (HJT) technology is another option to increase cell efficiency and power output to higher levels.

HJT a special PN junction, which combines two different technologies into one cell: a crystalline silicon cell sandwiched between two layers of amorphous “thin-film” silicon. The HJT solar cell structure was first developed in 1983 and commercialized by Sanyo/Panasonic of Japan. By 2010, these solar cells had attained full technical maturity. At that time, their application was restricted to small-sized niche applications.

One of the advantages of HJT is its low-temperature coefficient. By having a lower temperature coefficient than other modules, HJT can produce more energy over the life of the system than a module of similar nameplate wattage. At the same time, HJT exhibits better power performance in low light conditions, is less susceptible to degradation mechanisms compared to traditional p-type PERC cells. Another possibility this technology brings is that you can transition to tandem cells which go even above 25-30% in cell efficiency. Presently, the mass production efficiency of many companies can reach 24%,

In terms of manufacturing processes, HJT requires less than steps than both PERC and TOPCon but high upfront equipment cost is restrictive to the large-scale industrialization as it is not compatible with existing production lines. For it to be considered as a potention route, the equipment investment cost must be <300 million/GWp. For reference, the equipment cost for 1GWp of PERC is between150-200 million while for TOPCon it’s between 200-250 million). In the future, in order for heterojunction to compete with PERC, scaling is the key.

There is no doubt that n-type cell technology will replace PERC battery technology when the main driving force of cost to effectiveness begins to favor one over the other. At this time, there is no clear answer as to which technological path will produce the heir to PERC as many cell producers are already cautiously investing in both. However, some experts find it unlikely that a scenario where manufacturers forgo technical upgrades to TOPCon from existing PERC capacity and go directly to HJT would occur.