According to many market studies, the global solar photovoltaic technology market is expected to grow by more than 150GW by 2021. Compared with the cumulative installed capacity of solar photovoltaic as high as 100GW in 2012, the solar market is now growing into the king of the new energy market. Titus Koech, JinkoSolar's SSA technical service manager, wrote that this development was caused by various technologies provided by the upper-tier suppliers in the market.

Technologies such as passivated emitter rear contact (PERC), half cut (Cut), multiple busbar (MBB), flat tape and double-sided batteries have been proven to be mature and reliable. They have become mainstream technologies in the world and the Middle East and North Africa.

PERC technology is a milestone in the solar photovoltaic market. PERC is today's mainstream technology (p-type gallium doping), which occupies 90% of the photovoltaic market. This technology changes the market mode from low-efficiency low-power modules to high-efficiency high-power modules, paving the way for double-sided modules to become mainstream. It even led the market to enter the transition from polycrystalline to single crystal. But what is the next technology that will revolutionize the solar industry like PREC?

Most people are familiar with traditional p-type battery technology. With the large-scale manufacturing of monocrystalline silicon wafers, p-type monocrystalline PERC cell technology has been deployed in most projects since 2017. Due to the simple manufacturing process, low cost, and high battery conversion efficiency, mainstream module manufacturers have used mono-crystalline PERC to improve.

However, the efficiency of PERC cells has approached the limit of 24.5%, and the cost reduction has also slowed down. At the same time, new battery technologies are developing rapidly, and n-type batteries are most likely to replace p-type batteries as the next-generation mainstream technology.

Traditional p-type batteries use boron-doped silicon wafer substrates, which can easily form boron-oxygen pairs after initial irradiation. The formation of recombination centers by trapping electrons in the silicon wafer substrate will cause photo-induced degradation.

In contrast, the silicon wafer substrate of the n-type battery is doped with phosphorus, so there is almost no loss of the recombination center formed by boron oxygen, which greatly reduces the photodegradation. For example, in TOPCon technology, the structure of the tunnel oxide layer further reduces the subsurface recombination rate, greatly optimizing the battery conversion efficiency, and the upper limit can reach 28.2%~28.7%

The PV market is now dominated by p-type modules, which constitute most of the production capacity of Tier 1 suppliers, as well as limited n-type module capacity for specific markets and applications. However, since the industry is currently reaching the theoretical limit of PERC technology (approximately 24.5%), it is clear that the conversion from p-type to n-type modules will be the next step in the market.

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N-type modules have been on the market for some time, but they have always been considered high-quality products. However, current mature technologies such as tunnel oxide passivation contact (TOPCon), n-type phosphorus-doped solar cell technology with thin oxide doped between the emitter and the silicon wafer (base), and heterojunction technology (HIT), etc. The development of n-type phosphorus doped amorphous layers on c-Si wafers, and improvements in n-type production machines have made these technologies more competitive in p-type modules. Therefore, many Tier1 suppliers are considering increasing the production capacity of n-type modules starting in 2022.

On the one hand, heterojunction oxide tunnel (HOT), or commercially known as tunnel oxide passivation contact (TOPCon), is considered to be the second generation of solar photovoltaic technology after PERC. In order to manufacture it, some additional steps should be included in the production line to convert PERC to TOPCon. Therefore, minimum capital investment is required. Ultimately, this will affect the price difference between p-type PERC and n-type TOPCon technology, which may be less than 10% compared with other n-type technologies.

The TOPCon battery adopts cutting-edge high-efficiency passivation contact technology, and the back side adopts a micro-nano tunnel oxide layer and a carrier-selective microcrystalline silicon film laminated functional structure.

This innovative structure demonstrates two-way improvements in passivation performance and conductivity, thereby significantly improving battery conversion efficiency and power generation performance. The highest efficiency of n-type HOT2.0 battery is close to 25% in mass production, showing broad application prospects.

In short, TOPCon and HIT reduce the number of subsurface recombination rates through passivation, thereby achieving efficiency improvements. The former uses a tunnel oxide layer, while HIT uses an intrinsic amorphous silicon film. Differences in methods lead to their respective manufacturing processes, leading to differences in commercial costs.

The simple concept of TOPCon technology is to add a tunnel oxide layer. This layer will allow passage of majority carriers while restricting passage of minority carriers. Therefore, it will reduce the recombination process that occurs in the band gap and result in lower resistance loss and carrier recombination, opening the circuit voltage (Voc) increase. 

Compared with the standard PERC technology, TOPCon has significant advantages, some of the advantages are shown in the following table:

Due to the advantages listed in Table 1, it is estimated that n-type TOPCon will increase power generation by at least 3% and reduce the levelized cost of electricity (LCOE) by at least 5%.

On the contrary, the concept of heterojunction technology (HJT or HIT) is to add two layers of amorphous silicon around the crystalline silicon cell. These layers have a different energy band gap than crystalline silicon. Ultimately, this structure will provide a selective barrier for electrons and holes to pass through the other side, which will also lead to a reduction in their recombination.

Although the current structure of TOPCon is considered a new technology, in the past few years, it has witnessed the rapid development of battery conversion efficiency. The battery conversion efficiency of TOPCon has been greatly improved. The theoretical limit of TOPCon is 28.7%.

Judging from the rapid development of TOPCon technology, JinkoSolar broke the four records of TOPCon technology in 2021 alone, including three battery efficiency (24.90%, 25.25% and 25.40%) and one module efficiency (23.53%).

Other suppliers and research centers have also broken many records in the past 2 to 3 years. Therefore, TOPCon is expected to be widely adopted worldwide. Major suppliers have announced to increase TOPCon production capacity from 2022. JinkoSolar will begin mass production of Tiger Neo TOPCon modules based on M10 wafers (182mm) in the first quarter of 2022. The production capacity is expected to reach 10GW by the end of 2022.

In addition to high efficiency and high power, Tiger Neo has the following advantages compared with monocrystalline PERC photovoltaic modules:

Compared with the traditional p-type module, the power supply warranty can reach 30 years. The degradation rate in the first year is less than 1%, which means that the power output in the 30th year is guaranteed to be at least 87.4%.

The double-sided factor or traditional PERC component is 70%, while the TOPCon component is modified to 85%. The corresponding power gain is about 2.03%. According to the formula, the power gain of the traditional PERC module due to the back power gain is 9.45%, while the TOPCon module due to the double-sided factor increased by 15% (STC conditions and average ground reflectivity), the power gain increased by 2%.

Comparison of power gain caused by double-sided factor increase

PERC: BSI*Bifi(70%)≈9.45% TOPCon: BSI*Bifi(85%)≈11.48% P*=Pfront*(1+BSI * Bifi) *Bifi: module double-sided coefficient *BSI: double-sided stress radiation Illumination coefficient (depending on actual irradiance and ground reflectivity)

Titus Koech, Technical Service Manager-SSA, JinkoSolar

Koech is an electrical and electronics engineer with more than eight years of experience in the renewable energy industry in Sub-Saharan Africa (SSA). He holds a master's degree in Renewable Energy Enterprise and Management (REEM). Although his industry experience is accompanied by knowledge of project management and deployment, where he has supervised the development of many renewable energy projects (photovoltaic + diesel + energy storage) in Africa, his scope of involvement has been extended to technical support and business for many projects Development of the Social Security Administration.

JinkoSolar (NYSE: JKS) is one of the world's largest and most innovative solar module manufacturers. The company distributes its solar products and sells its solutions and services to a diversified international utility, commercial and residential customer base.

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