Chinese scientists achieve the world’s highest efficiency (26.81%) crystalline silicon solar cells

Recently, LONGi Green Energy Technology Co., Ltd. (LONGi) and the team of Professor Gao Pingqi of Sun Yat-sen University published an article in the journal Nature Energy, reporting crystalline silicon heterojunction solar cells with a conversion efficiency of up to 26.81%, which is currently the world’s highest efficiency of silicon-based photovoltaics.

On May 4, 2023, the results were published in the journal Nature Energy under the title “Silicon heterojunction solar cells with up to 26.81% efficiency achieved by electrically optimized nanocrystalline-silicon hole contact layers”.

Lin Hao of Sun Yat-sen University, Yang Miao and Ru Xiaoning of LONGi are the co-first authors of the paper, Dr. Xu Xixiang and Dr. Yin Shi of LONGi and Professor Gao Pingqi of Sun Yat-sen University are the co-corresponding authors of the paper, and the relevant team of Delft University of Technology in the Netherlands provided support in charge transport modeling.

Silicon-based semiconductor technology supports the two pillar industries of information and energy, and silicon-based photovoltaics pursue the continuous improvement of photoelectric conversion efficiency, just as silicon-based silicon-based chips continue to chase size miniaturization. The rapidly emerging silicon-based photovoltaic industry based on semiconductor technology and new energy demand has become an important way to achieve energy open source, which is conducive to accelerating the realization of “carbon peak and carbon neutrality”, which is conducive to promoting the energy revolution and ensuring national energy security.

Solar energy is the cheapest and most accessible form of energy. Now, it will be more efficient than ever. LONGi Green Energy Technology Co., Ltd. (LONGi) and the team of Professor Gao Pingqi of Sun Yat-sen University reported that crystalline silicon heterojunction solar cells with conversion efficiency as high as 26.81%, which is currently the world’s highest efficiency of silicon-based photovoltaics, breaking the highest efficiency in a small area maintained by Kaneka in Japan since 2017, marking that China has truly entered the center of the world photovoltaic stage as a technology leader. This new solar cell is also made of mainstream crystalline silicon materials, but with much higher conversion efficiency, so this innovation will further consolidate the key role of solar cells in the energy transition and potentially change the rules of the game for the world’s transition to renewable energy.

Key breakthrough: The research team used a p-type nanocrystalline silicon thin film contact layer to replace the traditional p-type amorphous silicon contact layer. Although the theoretical feasibility of this improvement is relatively easy to understand, the successful introduction of high-efficiency heterojunction cells and the creation of a world record for conversion efficiency is unique. The core technology includes breakthroughs in the structural factors and doping efficiency of this layer.

A huge improvement: Compared to any other type of solar cell made of crystalline silicon, the new hole contact layer enables the transfer of electrical energy with less resistance and leads to higher power conversion efficiency. LONGi researchers developed the new technology on standard industrial-grade silicon wafers, making it almost immediately applicable to the production of solar panels. Compared to previous technologies, the improvement in battery performance is very significant, with an absolute leap of 1.5% in conversion efficiency. Dr. Xixiang Xu of LONGi Academia Si said: “This is the best performance of all crystalline silicon solar cell structures to date, and corresponds to a product that accounts for more than 95% of the global photovoltaic market. ”。

Physical mechanism: After careful research and analysis of the current channels in the new contact layer, it is found that the new p-type nanocrystalline silicon contact layer has lower activation energy, which is conducive to reducing the interface barrier of hole transport and helping to stimulate more efficient interband tunneling, so it presents a lower contact resistance. As the surface passivation level of crystalline silicon heterojunction solar cells becomes higher and higher, surface recombination will gradually retreat to a secondary position, and the Auger recombination process of crystalline silicon itself will gradually play a leading role, at this time the ideal factor of the diode can be less than 1, and the optimization of the contact resistance will drive the rapid improvement of the filling factor index, and then promote the breakthrough of power conversion efficiency. Professor Gao Pingqi from the School of Materials Science of Sun Yat-sen University said: “The study of silicon-based hole contact layers with low activation energy is very timely and extremely important, and our work represents great progress in exploring the electrical properties of hole contact, which is conducive to improving the performance of heterojunction, hybrid cells and other types of silicon-based solar cells.” ”

Figure 1: Comparison of characteristic parameters of various high-efficiency crystalline silicon solar cells.

In this paper, from the comparison of photoelectric performance (Figure 1a), it is pointed out that LONGi’s silicon heterojunction (n-SHJ, red box) battery has outstanding electrical performance compared with other battery technologies, and the optical optimization of superposition can achieve a breakthrough in cell efficiency on the traditional front and back battery. A more detailed electrical analysis shows (Figure 1b) that a rapid increase in FF, especially beyond the limit curve (blue dashed line) with an ideal factor of 1, is the key to further improvement in battery electrical performance. This improvement is attributed to the rapid decrease in contact resistance after the doped film layer of the battery back junction is replaced by a p-type doped amorphous film (p-a-Si:H) with a p-type doped nanocrystalline film (p-nc-Si:H) (as shown in Figure 2).

Figure 2: Comparison of the electrical properties of two SHJ crystalline silicon solar cells (p-a-Si:H and p-nc-Si:H coatings).

Figure 3: Microstructure, electrical properties and band structure of p-a-Si:H and p-nc-Si:H hole selection contact layers.

Further analysis shows that p-nc-Si:H has obvious advantages over p-a-Si:H in terms of crystallization (Figure 3a), conductivity and activation energy (Figure 3d). This makes holes more easily transmitted on a heterojunction composed of p-nc-Si:H than a heterojunction composed of p-a-Si:H (Figures 3e and f) with better passivation. The test shows that the contact resistance and surface recombination of the heterojunction composed of p-nc-Si:H are reduced to 5 m·cm2 and 0.5 fA·cm-2, respectively. This raises the open circuit voltage and FF of the battery to 751.4 mV and 86.07%, respectively. According to the certification of the Hamelin Solar Energy Institute (ISFH) in Germany, the energy conversion efficiency of the cell is as high as 26.81%, which is the current world record for the energy conversion efficiency of single-crystal silicon solar cells (regardless of technical routes)!

The above work is supported by the Key R&D Program of the Ministry of Science and Technology (2022YFB4200203, 2022YFB4200200) and the key and general projects of the National Foundation of China (62034009, 62104268). (Source: Science Network)

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