TOPCon Solar Cells

TOPCon, short for 'Tunnel Oxide Passivated Contact', is a more advanced N-type silicon cell technology. The concept was first proposed by the German solar research institution Fraunhofer ISE back in 2014. It was not until 2019 that the technology was scaled. As of January 2022, the world's largest solar panel manufacturers, Suntech, Trina Solar, JA Solar and LONGi Solar now use the technology to achieve solar panel with efficiencies above 22%.

PERC Cell Technology

In PERC (Passivated Emitter and Rear Cell or Passivated Emitter and Rear Contact) soalr cells, a dielectric passivation layer is added to the rear of the cell to allow more sunlight to be captured and turned into electricity with the goal of increasing the efficiency.

PERC technology comes in two different types, the n-type and the p-type PERC cells. The main difference between p-type and n-type solar cells is the number of electrons. In a p-type cell, the silicon wafer is doped with boron, an element with one less electron than silicon. This makes the cell positively charged. In an n-type, the cell is doped with phosphorus which has one more electron than silicon, making the cell negatively charged.

N-type cells return higher efficiency values than their p-type counterparts. They are also not affected by light-induced degradation unlike their p-type cells.

TOPCon: Beyond PERC Cell Technology

By 2019, three years after it was widely adopted by the solar industry and more than three decades after its invention by Australian scientist Martin Green in 1983, the PERC solar cell technology had became the dominant solar cell technology. In 2019, PERC solar cells held 65% of the market share. In just a year, it had sliced the share held by the more conventional back surface field (BSF) solar cell technology to slightly less than 32% globally from the 83% it held in 2017.

After years of continuous R&D efforts towards optimizing the technology, the efficiency improvement potential of PERC now seems to have been exhausted. At 23%, manufacturers have approached the theoretical upper limit imposed by scientific research on the efficiency of PERC cells, at 24.5%. Further optimization of the technology now seems elusive and since 2021, manufacturers have been looking beyond it to:

  • n-PERT (passivated emitter rear totally diffused),

  • Heterojunction (HJT)

  • Interdigitated back contact (IBC)

  • n-TOPCon

In a TOPCon cell, a tunnel oxide layer and an intrinsically polysilicon layer is added to the rear side. The front side is passivated by a dielectric stack of passivation and anti-reflection layer. Presently, the mainstream process grows a 1.5-2cm tunnel oxide layer through thermal oxidation, while a sediment 150-200nm of intrinsically polysilicon layer is added through a chemical depositing process known as LPCVD, and the cell is then doped through phosphorus diffusion.

A Jolywood TOPCon Solar Panel, Source: Jolywood
A Jolywood TOPCon Solar Panel, Source: Jolywood

TOPCon Great Attractions

Higher Upper Limit of Efficiency

In the move beyond PERC cells, the other options beside TOPCon and HJT, n-PERT and IBC due to their high manufacturing costs and complicated procedures and limited efficiency potential, are not as attractive.

TOPCon solar cells have the upper limit of theoretical efficiency set by scientific research in the field at 28.2%-28.7%. This is beyond that of HJT, which stands at 27.5% and well beyond that of PERC cells, at 24.5%. It approaches the theoretical limit of all crystalline-silicon based solar cells at 29.43%.

Adaptable Manufacturing Process

TOPCon merely adds an extra layer of processes to already existing and well mature PERC manufacturing processes and production lines. This compatibility means that TOPCon solar cells can be upgraded easily from existing PERC/PERT production lines with a small increase in cost. Though it does not compare very well with simpler HJT manufacturing processes which is totally incompatible with existing PERC manufacturing processes and requires an entirely new production line, it does compare better with the more complex IBC cell technology.

Though TOPCon is beset by its own challenges, all of which attempts to mitigate expands to the production costs, it represents solar cell technology's fastest growing frontier. Since 2019, manufacturers have begun reserving spaces in new production lines for upgrading into TOPCon. This has given TOPCon the prospects of fast industrialization. Bifacial solar panels giants, Jolywood now has a manufacturing capacity of up 3.6 GW of TOPCon solar panels with an extra 16GW planned. Major manufacturers and major market movers like Trina Solar, Canadian Solar, LONGi and Jinko are now embracing TOPCon and scaling their production lines. (In July 2021, LONGi launched its Hi-MO N module, a bifacial module manufactured with a next generation n-type TOPCon cells.)

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