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Solar Knowledge
Introduction of IBC Solar Panel, Types, Progress and Challenge
What is IBC Solar Panel Cell ?
IBC solar panel cells, also known as Interdigitated Back Contact cells, are a type of solar cell where the positive and negative metal contacts are moved to the back of the cell. In 1975, Schwartz first proposed the back-contact solar cell, initially used in high-concentration systems. After years of development, the interdigitated back-contact (IBC) solar panel was developed. IBC solar panels / solar modules have no front-side grid lines, which significantly increases the light-receiving area of the cell and thus improves its efficiency. Unlike the passivation concepts of other crystalline silicon cells such as TOPCon and HJT, IBC solar modules mainly improve conversion efficiency through structural changes. IB cells (Interdigitated Back Contact), also known as interdigitated back-contact cells, feature positive and negative metal electrodes arranged in an interdigitated pattern on the rear surface of the cell. IBC cells have no metal grid lines on the front side, and the emitter, back field, and corresponding positive and negative metal electrodes are integrated in an interdigitated pattern on the back. This unshaded front structure completely eliminates the shading loss caused by grid electrodes, allowing for maximum utilization of incident light, effectively improving cell efficiency and power generation.
IBC Solar Panels Structure Features
On the back of a high-lifetime N-type silicon wafer substrate, alternating P+ and N+ diffusion regions are formed. The front surface is textured with a pyramid structure to enhance light absorption, while a front surface field (FSF) is created. The front surface typically uses a SiNx passivation and anti-reflection coating. The back surface utilizes passivation layers or stacks such as SiO2, AlOx, or SiNx. Finally, selective metal contacts for P and N are formed on the back surface.
Types of IBC Solar Panels
One of the core technologies of IBC cells is the design of the rear electrode, as it not only affects cell performance but also directly determines the manufacturing process of IBC modules. Based on different electrode designs, IBC cells can be categorized into three main types:
- Gridless IBC Cells
- Four-Busbar IBC Cells
- Point-Contact IBC Cells
These cells feature fine grid lines printed on the back without the need for insulation paste or busbars. Compared to busbar-type IBC cells, they have a simpler manufacturing process and lower costs. However, special equipment is required for assembling the modules, with high precision demands, leading to higher costs on the module side.
These cells allow for module assembly using conventional soldering methods, with lower precision requirements and no need for specialized equipment, making them highly adaptable. However, during cell manufacturing, insulation paste and busbars need to be printed, making the process more complex.
These cells do not require insulation paste, and both the main and fine grid lines are printed in a single step, simplifying the manufacturing process. For module assembly, metal foil is used to interconnect the cells, with lower precision requirements compared to gridless cells.
Recent Progress in IBC Solar Module
The U.S. company SunPower has developed three generations of IBC solar cells. Among them, the third generation IBC solar cell, produced in 2014 using N-type CZ silicon wafers, achieved a maximum efficiency of 25.2%. Reports show that SunPower's mass production efficiency reaches 25%, while LG's mass production efficiency reaches 24.5%.
In China, Trina Solar has been dedicated to the research and development of IBC monocrystalline silicon cells. In May 2017, they independently developed a large-area 6-inch (243.2 cm²) N-type monocrystalline silicon IBC cell with an efficiency of 24.13%. By February 2018, the efficiency of this cell was further improved to 25.04%, with an open-circuit voltage of 715.6 mV, and it was independently tested and certified by Japan's Electrical Safety & Environment Technology Laboratories (JET). This is the first time that a single-junction monocrystalline silicon solar cell made in China has surpassed 25% efficiency, as certified by a third-party authority. It also represents the highest conversion efficiency for a monocrystalline silicon solar cell produced on a large-area 6-inch crystalline silicon substrate in the world, marking a significant step forward for Trina in high-end photovoltaic cell technology research. 
Future Development of IBC Solar Module
The HBC (Heterojunction Back Contact) technology, which combines IBC and HJ technologies, can further improve cell efficiency. This method uses intrinsic amorphous silicon for surface passivation on the wafer and forms a heterojunction on the back using N-type and P-type amorphous silicon films. The advantage of this approach is that it leverages the excellent surface passivation properties of amorphous silicon, combined with the benefit of the IBC structure, which has no metal shading. Using the same device structure, a world-record efficiency of 26.6% was achieved in 2017. Its open-circuit voltage (Voc) reached 0.740V, short-circuit current density (Jsc) reached 42.5 mA/cm², and the fill factor (FF) was 84.6%. For crystalline silicon solar cells, the theoretical limit of Jsc is 43 mA/cm².
As shown in the figure, the HBC cell structure differs from traditional IBC cells in that the back emitter and BSF (back surface field) regions are made of p+ amorphous silicon and n+ amorphous silicon layers, with an intrinsic amorphous silicon passivation layer inserted in the heterojunction contact area. The combination of IBC and amorphous silicon passivation technologies is undoubtedly the direction for future efficiency improvements in IBC cells. 
The Challenge of IBC Solar Module
- The manufacturing process is complex, requiring the preparation of interdigitated P and N regions on the back of the cell, as well as the formation of metal contacts and grid lines on top of them. Key processes include diffusion doping, passivation coating, and metallization of the grid lines. Multiple masking and photolithography steps are needed, and the gap between the P and N regions must be highly precise to prevent leakage, which demands a high level of process development capability from cell manufacturers.
- The base material has high requirements, needing a long minority carrier lifetime. Since IBC cells are back-junction cells, in order to minimize or completely avoid the recombination of photo-generated carriers before they reach the back p-n junction, a longer minority carrier diffusion length is required.
- The complex manufacturing steps of IBC cells make their production costs significantly higher than traditional crystalline silicon cells. As mainstream PERC cells have already achieved an efficiency of 23%, and TOPCon and HJT cells can reach 24.5%, the efficiency premium of IBC cells is difficult to offset the increased costs, making their competitiveness less obvious.
