Heterojunction technology can fill the innovation gap in the residential and commercial rooftop photovoltaic market, and enhance the leading position of American solar energy through domestic manufacturing of cells and modules.

In September, the US Department of Energy released the "Future of Solar Energy Research", which outlines how solar energy can produce 45% of the country's electricity by 2050.

To achieve this goal, the United States needs to deploy 1,570 GW of solar energy by that date. The goal also depends on whether there are consistent policy efforts, aggressive cost reductions, the widespread deployment of other clean technologies, the expansion of transmission systems, and the large-scale electrification of end uses.

Research and development are essential to maintain or ideally accelerate the cost reduction trajectory of solar photovoltaic technology to achieve sustained growth in the industry. By supporting domestic manufacturing of next-generation solar cell technology, the United States has a special opportunity to establish itself as a global leader and achieve national decarbonization goals.

For many years, crystalline silicon solar cells have dominated the market and have become the industry standard for comparing all alternatives. In 2019, about 83% of the total US photovoltaic market was crystalline silicon-based products. Under the umbrella of crystalline silicon cells, single crystal p-type passivated emitters and backside cells (PERC) undisputedly dominate the market.

PERC batteries are expected to reach an efficiency platform of about 24% in mass production. With mass production efficiency approaching 23%, PERC seems to be approaching the limits of its possible efficiency and performance enhancements. As manufacturers try to close the gap between actual efficiency and maximum efficiency, PERC will continue to play an important role in photovoltaic deployments in the coming years. But the race to surpass PERC is already underway.

The successor of PERC has not yet been determined, but it can be said with certainty that it will be based on an efficient n-type battery that will help overcome the limitations of p-type batteries. The report indicates that the market share of n-type batteries may increase from only 5% in 2021 to 28% in 2028.

One such limitation of p-type batteries is bifaciality, that is, the ratio of the rear efficiency to the front efficiency under the same irradiance. The introduction of double-sided technology enables solar systems to generate higher energy. Single-sided PERC batteries can be easily converted to double-sided batteries, which helps PERC dominate the market.

However, the double-sidedness of PERC is limited to about 75%, while the double-sidedness of n-type batteries can reach more than 90%, depending on the battery structure.

Another limitation of p-type batteries is their high temperature coefficient, which reduces the performance of the module when operating at higher temperatures. The power temperature coefficient of PERC batteries ranges from 0.35%/oC to 0.40%/oC. The N-type wafer-based battery itself has a low temperature coefficient.

There are two main n-type cell technologies: tunnel oxide passivation contact (TOPCon) and heterojunction (HJT). Both of these technologies are likely to exceed 25% efficiency in large-scale manufacturing, and HJT may go further.

TOPCon cell manufacturing involves depositing a nano-scale silicon oxide layer between the silicon wafer and the metal contacts, followed by a thicker polysilicon layer. Because the battery architecture is very similar, TOPCon batteries can be manufactured with relatively low equipment costs by upgrading the existing PERC mass production line. This makes TOPCon superior to other n-type battery technologies in terms of ease of manufacturing expansion.

The manufacturing steps of heterojunction battery are much less than PERC or TOPCon.

Heterojunction cell manufacturing involves depositing an amorphous silicon layer on the top and bottom of the wafer, followed by transparent conductive oxide deposition and metal contacts. The manufacturing steps of heterojunction battery are much less than PERC or TOPCon.

However, the HJT battery production line is currently more expensive than TOPCon and must be built. HJT batteries also use more expensive metallization pastes. Heterojunction batteries are essentially double-sided, with a double-sided rate of over 90%, which is the highest among all battery technologies. The power temperature coefficient of the heterojunction battery is in the range of 0.25%/oC to 0.30%/oC. Compared with TOPCon battery, higher double-sidedness and lower temperature coefficient result in higher energy output. Mass production of heterojunction cells is expected to reach about 27%.

One of the most interesting developments in the solar industry is the development of tandem cells using HJT as the bottom cell. The use of these batteries has achieved a laboratory efficiency of 29%, making HJT batteries more advantageous than TOPCon batteries.

HJT technology has begun to be adopted more often by residential and light commercial installers.

As far as the current situation is concerned, the production cost of HJT batteries is higher than that of TOPCon batteries. This explains the acceptance of TOPCon technology by utility-scale solar developers, who are more sensitive to price increases. HJT technologies have begun to be adopted more frequently by residential and light commercial installers because they meet the owners' needs for enhanced aesthetics, higher durability, and increased output.

Although the current initial price of HJT modules may be higher, the owners are willing to pay for superior performance and longer service life.

Remember the solar future research goal of 1,570 gigawatts of solar deployment in 2050? Our cumulative deployment volume is approximately 110 GW, which is less than 1% of the required deployment volume in the next 30 years. More than a decade ago, the United States gave up its leadership in solar energy innovation and manufacturing; tariffs imposed on foreign imports during this period did not restore it to its original state. The good news is that the development of heterojunction cells (and modules) provides a huge opportunity for the United States to regain its solar leadership position.

HJT technology fills the existing innovation gap in the residential and commercial rooftop photovoltaic market, which currently consists entirely of standard crystalline silicon modules. With additional policy support, the HJT module can provide the required cost reduction to promote continuous growth beyond the roof and enter the field of utility scale.

For these reasons, the United States will benefit from expanding domestic manufacturing of heterojunction solar cells and modules. By doing so, the United States can regain its leadership in solar cell technology and achieve its ambitious decarbonization goals.

Policies such as the "US Solar Manufacturing Act", which was recently signed into law under the Infrastructure Act, will help reduce the capital intensity of new manufacturing facilities, expand next-generation photovoltaic cell technology in a cost-effective manner, and improve the competitiveness of American clean technologies. .

Nadeem Haque is Heliene's Chief Technology Officer.

The views and opinions expressed in this article are the author's own views and opinions, and do not necessarily reflect the views held by Photovoltaic Magazine.

This content is protected by copyright and cannot be reused. If you want to cooperate with us and want to reuse part of our content, please contact: editors@pv-magazine.com.

An important amendment to this review article. SEMA is not part of the Infrastructure Act, which contains photovoltaic specific clauses of less than US$100 million. Instead, most of the elements of SEMA have been incorporated into the "Better Rebuild Bill" passed by the House of Representatives and is currently under consideration in the Senate.

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