Passivated Emitter and Rear Cell (PERC) is not a new technology invented in recent years. As early as 1983, “Father of Solar Photovoltaics” Australian scientist Martin Green and his team put forward the idea of PERC and published related papers in 1989. Because this technology can greatly improve the efficiency of the battery, thereby reducing the manufacturing cost, it has been widely used in the new generation of solar cells in recent years. what is the difference between PERC cells and traditional P-type cells.
Figure 1 Standard Solar Cell P-Type
Most crystalline silicon solar cells are produced according to the architecture below.
From the front to the back:
– screen printed Ag-paste
– anti-reflection coating (ARC) (blue)
– Boron-doped P-type Silicon to N-type layer (n+ emitter) form the P-N junction
– aluminium Back Surface Field (Al-BSF)
– Screen-printed Al-paste
The silicon nitride on the front side is used both as a passivation layer and as an anti-reflective coating (ARC) for the cell. Since recombination speed in the Al-BSF layer is slow (generally above 200cm/s), only 60%-70% of the infrared light reaching the back layer can be reflected, resulting in more photoelectric loss and affecting the efficiency of the cell.
Figure 2 PERC Solar Cell
PERC cell designs are very similar except for the rear surface. In an Al-BSF cell, only the front surface is passivated using a thin layer of silicon nitride (SiN) which serves as both a passivation layer as well as the cell’s anti-reflective coating. Rear surface losses are dominant and drive the efficiency of the cell. In a PERC cell, both the front and the rear of the cell are passivated leading to an improvement in how efficiently the cell can convert light to electricity. There are three ways in which the passivation layer in a PERC solar cell increases overall efficiency:
- Reflection of light back through the cell: A back surface passivation layer reflects light, that passes through the silicon cell without being absorbed, back into the silicon, giving the solar cell a second absorption attempt. This reflection of light means that more incoming solar radiation will end up being absorbed by the silicon cell, and the cell becomes more efficient.
- Reduced electron recombination: The addition of a back-surface passivation layer reduces “electron recombination” in the solar cell. Electron recombination is the tendency of electrons to recombine, which causes a blockage in the free movement of electrons through the solar cell. This inhibition of free electron movement leads to less-than-optimal cell efficiencies. In a PERC solar cell, electron recombination is reduced in order to increase efficiency.
- Reduced heat absorption: A silicon wafer in a solar cell can only absorb light in wavelengths up to 1180 nm, and higher-wavelength light waves pass through the silicon and are absorbed by the solar panel’s metal back sheet, creating heat. When solar cells are heated, they operate at lower efficiencies. The back-surface passivation layer in PERC solar cells is specially designed to reflect light with a wavelength above 1180 nm, reducing the heat energy in the solar cell and consequently increasing efficiency.
Figure 3 Comparison between PERC module and Standard solar module
As can be seen from the above figure, in the case of low solar irradiance, such as cloudy or dusk, the efficiency of PERC cells is significantly higher than that of traditional cells. That is, under low light conditions, PERC batteries can still operate at an efficiency close to standard conditions and maintain good output.
Due to the relatively simple process of PERC technology, low cost, and high compatibility with existing battery production lines, large-scale expansion of PERC batteries has become an industry trend. Wang Yingge, general manager of LONGi, said: “P-type PERC battery efficiency has room for improvement in the next two to three years, and it is currently the most cost-effective technology for mass production.”
In this context, several leading photovoltaic companies in China have devoted themselves to the research and production of PERC cells. Taking Trina Solar for example, as early as December 2016, Trina set a world record for mono-crystalline PERC cells with a conversion efficiency of 22.61%. In March 2020, Trina made another success, using standard industrialized equipment to prepare PERC batteries with an efficiency of 23.39%.
Figure 4 Mono-Crystalline PERC solar cell efficiency records
According to the graph, LONGi and Jinko broke the records of PERC battery efficiency respectively. Currently, the highest efficiency record is 24.06% set by LONGi in 2019. Besides, photovoltaic companies such as JA Solar, Risen Energy, and Tongwei also have PERC cell modules for consumers to choose.
Although PERC technology is becoming more and more mature, grid connection with a reasonable price is still a challenge for now due to the solar cell efficiency and manufacture costs. On May 29, 2020, Ministry of Industry and Information Technology (China) published Standards for Photovoltaic Manufacturing Industry（2020 edition）, which required new monocrystalline silicon solar cells efficiency must be greater than 23%. But at present, the mass production of monocrystalline silicon solar cells can only achieve an efficiency between 22% to 22.4%. Therefore, how to cross the 23% threshold is a huge challenge for PERC solar cells, and many companies have therefore reconsidered their future product layouts, such as focusing on PERC+ cells with an efficiency of 23.5% or more or HJT technology, which is able to achieve above 24% efficiency.
Figure 5 Predicted Global PV cell Trend