Solar modules: PERC vs. TOPCon solar cell technology

Although the first solar cell, developed in the USA in 1954, was an N-type cell, the P-type cell became dominant in subsequent years. This was because, in the early days, the modules were primarily used in the aerospace industry, where they proved to be more robust. Only in recent years have solar cell manufacturers begun to rethink their approach, due to the greater power output of N-type cells. This is mainly due to the longer lifespan of these cells, as, unlike the P-type, they are not susceptible to the "boron-oxygen defect." This defect leads to a decrease in efficiency over time. Furthermore, N-type solar cells are less prone to metallic impurities in the silicon.

In photovoltaic (PV) technology, even the slightest discrepancies in chemical composition can lead to significant differences in efficiency and economic viability. A prime example is the comparison of P-type and N-type solar cells. These differ in their cell structure: P-type solar cells are based on a positively charged silicon substrate, while N-type solar cells are designed in the opposite way, with the negatively doped side serving as the cell's base.

However, N-type solar cells are currently more expensive to produce, due to decades of focus on P-type cells. Their production led to economies of scale in the value chain, which still need to be established for N-type manufacturing. Furthermore, the production of N-type solar modules involves additional steps, which further increases costs. However, due to their higher efficiency, the market share of N-type cells is steadily increasing, and it is likely only a matter of time before they replace P-type cells as the dominant solar cell technology.

A similar rivalry between two cell types can be seen when comparing the solar cell technologies PERC (Passivated Emitter and Rear Cell) and TOPCon (Tunnel Oxide Passivated Contact), which have recently caused a stir.

PERC modules increase the efficiency of PV modules because they convert incoming light into energy more effectively. This is achieved by reflecting the portion of the light that reaches the back of the cell back into the cell. This is made possible by a layer applied to the back of the module, known as backside passivation. The increased efficiency of PERC cells compared to conventional modules is 1%, which in turn helps to reduce production costs per kWp generated.

To achieve maximum efficiency, PERC cells primarily use mono-wafers. This type of solar cell has undergone significant development in recent years. The increased efficiency has led to their almost complete replacement of conventional Al BSF cells in professional PV systems. However, PERC cells are anything but new; their principle was first described in 1983 by the University of New South Wales in Australia. It was only through continuously improved production processes that the technology's potential could be fully realized for mass production.

However, PERC technology is not without its problems. These cells are more susceptible to light-induced degradation (LID) than conventional cells. This LID effect causes a drop in the solar cells' performance after their first exposure to light. The risk of potential-induced degradation (PID) is also higher. PID defects can have far-reaching consequences, as they can significantly reduce the output of entire PV systems. Therefore, investors should pay particular attention to ensuring that PERC cells are certified for PID resistance according to IEC TS 62804.

A rising star in solar cell technology is TOPCon, developed at the Fraunhofer ISE in Freiburg. These cells can be both mono- and bifacial and, unlike PERC, primarily utilize N-type wafers. They exhibit a higher efficiency potential compared to PERC. The Chinese manufacturer Jinko Solar demonstrated just how high this potential can be last year, presenting a monocrystalline, bifacial N-type TOPCon solar module with an efficiency of up to 23.53%. Longi Solar's TOPCon module, presented in 2021, achieved even higher performance. Using a P-type TOPCon cell, it set a new world efficiency record for this technology, achieving 25.19%. Given the rapid progress, it is likely only a matter of time before this record is surpassed. Intensive research on these modules will ensure that development continues for a long time to come.

Although the performance of TOPCon solar cells is hard to beat, PERC technology still dominates the market. This is likely to remain the case for some time due to the continued increase in cell efficiency and simultaneously decreasing production costs.

However, TOPCon cells are gaining ground due to their exceptionally high efficiency. Currently, high production costs are a drawback for this type of PV system. However, significant economies of scale are expected as mass production ramps up, which would allow TOPCon modules to become serious competitors to PERC technology in Germany. Researchers at the Fraunhofer Institute for Solar Energy Systems (ISE) confirm this. According to them, however, in addition to reducing manufacturing costs, increasing production output to the level of PERC cells is necessary to establish the economic viability of TOPCon modules.

According to the scientists' findings, TOPCon technology results in 13.5 to 18.6% higher total costs for solar cells and 3.6 to 5.5% higher total module costs compared to PERC in ground-mounted PV systems with a capacity of 5 megawatts. Nevertheless, the researchers at Fraunhofer ISE noted that the 0.4 to 0.55% higher cell efficiency of TOPCon solar cells compared to bifacial p-PERC enables cost-effective mass production.

Regardless of whether N-type or P-type solar cells are used, there is a way to significantly increase the efficiency of solar modules: bifacial technology. Unlike monofacial solar cells, which only generate PV power when their top surface is illuminated, bifacial solar cells are designed to generate power from both their top and bottom surfaces. This increased light absorption considerably improves the module's efficiency.

Naturally, the efficiency on the underside is not as high as on the top side, which faces the sun. Nevertheless, depending on the location, distance from the ground, and external conditions, the efficiency can increase by more than 19% due to the increased irradiance on the underside. This can lead to an increase in the overall system capacity of between 10 and 30 Wp. For example, the output of a module that previously delivered 290 Wp increases to 320 to 360 Wp.

When installing bifacial systems, it is important to ensure that they are mounted at a sufficient distance from the surface below to allow for the additional light transmission. For surfaces with low to medium reflectivity, such as a tiled roof or grass, the minimum distance should be at least 40 centimeters. For highly reflective surfaces (e.g., snow), the distance to the ground should be greater than 1.5 meters.

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