BC and Perovskite: Technological Advancement and Critical Challenges

 

Recently, BC manufacturers released a technical white paper, proposing that “BC three-terminal stacking is expected to become the ultimate solution for stacking.” The word “ultimate” was mentioned again, and it seems to have become the signature word of BC manufacturers. As the photoelectric conversion efficiency of single-crystal silicon solar cells gradually approaches the theoretical limit, major manufacturers have invested in the race for stacking technology. So, is this conclusion of BC manufacturers exaggerated?

 

The choice of the “bottom battery” architecture of stacking technology is tantamount to laying the foundation before building a high-rise building: the stability of the foundation determines whether the upper space can truly realize its potential. At present, TOPCon is generally considered to be the most suitable bottom battery for efficient “stacking” with perovskite due to its mature process and excellent photoelectric matching; although the BC architecture has certain theoretical efficiency advantages, it is like building a three-story building on soft soil in terms of three-terminal interconnection, process complexity and cost control. Not only is the engineering difficulty and risk high, but it is also a typical “giving up the near and seeking the far”.

 

Optical matching is limited, and BC advantages are difficult to continue

 

First, the core advantage of BC cells is that the positive and negative electrodes are all moved to the back through laser grooving or masking processes, thereby eliminating the occlusion of the front grid line and improving the light absorption efficiency of single-junction cells. However, this advantage is no longer valid in the stacked structure-the top perovskite cell still needs to set the grid line electrode on the front, and the optical occlusion problem still exists.

 

In contrast, TOPCon cells optimize the design of the front and back electrodes to maximize the light absorption efficiency while reducing optical losses, and show higher process compatibility and better overall performance during the stacking process.

 

There are many challenges on the front surface, and the BC process is difficult

 

The BC cell itself has a complex structure, and in order to achieve effective coupling with the perovskite stacked cell, the passivation structure on its front surface needs to be further optimized. While ensuring a good passivation effect, the charge transfer capability of the top cell must also be taken into account. This puts higher requirements on the design of the front surface structure and the process technology.

 

Although BC manufacturers use “leave complexity to manufacturers” as a marketing strategy, the complexity of the structure not only increases the difficulty of manufacturing, but also hides a higher risk of failure – such as the difficulty in controlling the front interface, the narrow process window, and the difficulty in verifying long-term reliability. It is especially easy to expose hidden dangers in complex environments such as humidity and high temperature.

 

On the other hand, the TOPCon battery has excellent charge transfer and efficient passivation performance on its front surface. It can meet the stacking requirements without additional structural adjustments, providing a more stable “support layer” for perovskite batteries.

 

The interconnection method is complex, and the difficulty of the component end is increased.

 

Whether the TOPCon stacked perovskite adopts a two-terminal or four-terminal stacking structure, the internal output and standard packaging can be achieved. Due to the special distribution of the back electrode, the BC battery is usually combined with the perovskite using a three-terminal stacking (3T) design, which is what we often call a three-terminal. Although the three-terminal stacking battery has the potential to integrate the advantages of the two-terminal and four-terminal structures, in actual design, it is still necessary to make a trade-off between the two and tend to approach one of the performance paths. Therefore, it is difficult for the three-terminal structure to take into account all the advantages of both the two-terminal and four-terminal in the same device.

 

On the component side, part of the back electrode of the BC three-terminal stacked battery needs to be used as the electrode of the bottom battery and also needs to be connected to the top battery, which not only increases the complexity of packaging and process, but also makes it difficult for the components to achieve standard parallel output, bringing significant challenges to system integration. In addition, in the 3T structure, after the top battery and the bottom battery are integrated, the voltage cannot be adjusted independently, and it is difficult to achieve component-level parallel connection like the 4T component.

 

If parallel connection is required, complex jumpers and serial-parallel design are required, and the overall solution is complex and costly. From a cost perspective, the cost of BC three-terminal batteries is higher than that of two-terminal batteries, and significantly higher than that of four-terminal stacking, while the performance improvement is limited. Of course, BC batteries can also be used to make four-terminal stacked components, but the efficiency advantage of the front side can only be maintained at 30%, and the back side power generation is still an unavoidable shortcoming. Its comprehensive power still has disadvantages, and coupled with the relatively high component cost, it is difficult to gain an advantage in the cost per kilowatt-hour.

 

The core battlefield of stacked batteries: cost & landing speed

 

If the breakthrough in stacking efficiency is the “technical solution”, then cost, scale, and landing speed are the “industrial solution”. In fact, in the stacked structure, the bottom battery has limited improvement on the overall efficiency, accounting for only about one-third. With a target efficiency of 30%, the bottom battery contributes only 10%. This means that in the competition, whoever can complete the bottom battery integration at a lower cost, larger scale, and faster speed will have a more realistic advantage.

 

TOPCon technology is not only efficient and stable in process, but also has established a mature production capacity of nearly 1,000 GW, which can directly “seamlessly connect” the stacked industry without the need to rebuild the production line, significantly reducing the initial investment. At the same time, TOPCon technology is still evolving and has entered the TOPCon 2.0 stage. It has made continuous breakthroughs in the directions of higher current carrying capacity, better interface passivation, and lower process costs, laying a more solid foundation for the future integration with new technologies such as perovskite. However, the BC architecture is difficult to compete with TOPCon in terms of process complexity, equipment matching, and industrial collaborative efficiency, and it is extremely difficult to achieve large-scale mass production in the short term.

 

High efficiency is only the starting point, and industry is the end point. The goal of perovskite stacking silicon stacking is not only high-efficiency laboratory data, but also stable and reliable large-scale application. From the completeness of the industry chain to process compatibility, and then to component design and system integration, TOPCon undoubtedly meets the standards of the “next generation bottom cell”. Although the BC architecture is not lacking in innovation, facing the comprehensive test of the stacking era, it seems more like a love affair that is destined to be complicated and difficult to achieve a happy ending.