Solar module technology has been evolving so rapidly that Indian manufacturers, relying on importing the production lines mainly from China, are caught between the fear of falling behind the technology curve or being saddled with a bad investment.

The manufacturer’s dilemma is whether to upgrade from widely used passivated emitter and rear contact (PERC) technology to a more advanced and efficient tunnel oxide passivated contact (TOPCon) solar cell.

TOPCon technology offers higher efficiency and performance than MonoPERC technology for solar cells, but many manufacturers hesitate to adopt TOPCon because of the high costs and risks involved.

The decision becomes even more complex because another advanced Heterojunction (HJT) technology has the potential to upstage TOPCon.

TOPCon cell uses a thin silicon oxide layer to form a tunnel junction between the cell’s n-type and p-type silicon layers. This tunnel junction reduces the recombination losses of charge carriers, which are electrons and holes that generate electricity when sunlight hits the cell. By reducing these losses, TOPCon cells can increase their open-circuit voltage and fill factor, two critical parameters that determine the efficiency of a solar cell.

TOPCon cells have several advantages over MonoPERC cells, which are another type of solar cell technology that uses aluminum oxide as a passivation layer on the rear side of the cell. Passivation layers prevent unwanted chemical reactions that can degrade the cell’s performance. However, aluminum oxide is less effective than silicon oxide in reducing recombination losses, making TOPCon cells achieve higher efficiencies than MonoPERC cells.

Another benefit of TOPCon cells is that they have lower temperature coefficients, which means they perform better under high temperatures. This is important for solar cells because they are exposed to varying weather conditions and temperatures. High temperatures can reduce the output power and lifespan of solar cells. TOPCon cells are more resistant to these effects than MonoPERC cells, which makes them more suitable for hot and humid climates.

TOPCon cells are also compatible with bifacial modules that can generate electricity from both sides by capturing the reflected light from the ground or other surfaces. This can increase the energy yield significantly, depending on the albedo and installation conditions. MonoPERC cells are unsuitable for bifacial modules because they have a metal layer on the rear side that blocks the light from entering.

Heterojunction technology combines crystalline silicon and amorphous silicon layers to create a high-efficiency solar cell that does not need any passivation layer or metal contact on the rear side.

Catch-22 for Manufacturers

A TOPCon line can be built on top of the MonoPERC line at approximately 40% additional cost.

“Manufacturers with the latest lines can upgrade to TOPCon, but it is still expensive. TOPCon technology requires more advanced equipment and processes, such as plasma-enhanced chemical vapor deposition, atomic layer deposition, and laser doping,” a prominent module manufacturer said.

Manufacturers who have already invested in MonoPERC production lines have two options —continue with their existing facilities and set up an entirely new TOPCon line or upgrade the current line to TOPCon.

The first option requires substantial capital infusion while ignoring the existing base. The second option, while more cost-effective, doesn’t allow the manufacturer to recover the return on investment on the MonoPERC line promised at installation time.

The increasing attractiveness of HJT muddies the water further for manufacturers. “HJT technology can achieve efficiencies higher than both MonoPERC and TOPCon. How can we justify changing our production lines to TOPCon and risk losing our market share to HJT in a few years? We cannot afford to chase every new technology that comes along,” an industry source said.

Other manufacturers were concerned that TOPCon technology requires a high level of skill and experience to implement successfully. They suggested that Indian manufacturers concentrate on perfecting the current technology — MonoPERC — and wait until TOPCon technology becomes more stable and reliable before switching.

“A drop in the yield of TOPCon cells from 95% to 90%, for example, would increase the production cost by 10%. This is because more cells would have to be produced to achieve the same output, resulting in higher material and labor costs. Therefore, having an effective and knowledgeable clean room management team is crucial to maintaining a high yield and keeping costs low,” the manufacturer quoted earlier said.

The current market demand for a product is another element that manufacturers must consider before making investment decisions. MonoPERC cells are currently the dominant technology in the solar market and are widely available at much lower prices than TOPCon modules.

“This may reduce the appeal of investing in TOPCon technology for manufacturers who target these markets. We must carefully evaluate the market potential and policy environment before adopting new technologies,” another manufacturer said.

Leading industry experts from the renewable energy sector recently converged at the Mercom India Renewables Summit 2023 to discuss the current technological advancements in the solar industry and their relevance to the local weather conditions in India.