BC technology is expected to reach an average mass-scale efficiency of 25.5% in the next 7 years
Bifacial BC technology is expected to dominate the market from 2027 or 2028
ZEBRA TBC cell architecture is expected to outpace the double poly TOPCon as the mainstream cell technology
High efficiency and lower breakdown voltage are the major technological advantages of BC technology
The emergence of TOPCon as the dominant n-type cell technology, surpassing p-PERC by the end of 2024, has raised the question: What comes next? The recent commercialization of the LECO metallization process in TOPCon cell structures has further reduced the efficiency gap compared to HJT cell technology. However, a few PV module manufacturers have adopted back-contact (BC) cell technology, which features both positive and negative contacts on the rear side, offering higher efficiency, greater power output, and improved aesthetics compared to mainstream TOPCon.
At the recently concluded TaiyangNews Annual Flagship Conference – High-Efficiency Solar Technologies 2024 – Co-founder and Director of ISC Konstanz, Radovan Kopecek, delved into the PV market outlook, the status of n-type and BC cell technologies, and the future of BC technology (see ISC Konstanz presentation here). ISC Konstanz, a renowned 19-year-old German research institute, having an interest in the development of n-type cell technology, is lobbying for BC as the future mainstream cell technology. An early adopter of bifacial n-type cell technology, the institute is staffed by 100 skilled scientists and 65 employees. It has achieved up to 24.6% cell efficiency at an industrial scale and has transferred advanced cell technologies totaling 3.8 GW to date.
Starting with the PV installation outlook report from BloombergNEF, Kopecek predicted that global annual PV capacity additions will increase to 1,000 GW (1 TW) by the end of 2024, up from the current 600 GW. In line with the expected exponential growth over the next decade, the deployment of TW-scale PV capacity, coupled with a reduced levelized cost of energy (LCOE) – with or without storage – will be driven by rapid technological innovation, solutions to PV supply chain constraints, and an evolving technology mix, according to the PV installation graph presented at the TW workshop.
Further, new LCOE calculations by Nijsse reveal that c-Si technology is still on a steep learning curve. Starting with Al-BSF or PERC-based cell technology in 2020 and advancing to TOPCon technology by 2023 with an average module efficiency of up to 22.5%, it is expected that the industry will enter an era of TOPCon and BC technology integration after 2027. However, with advancements in BC technology over the next 7 years, average module efficiency is expected to reach up to 25.5%. Nijsse further predicted that bifacial BC technology will emerge as the dominant solution post-2030.
ISC Konstanz offers a wide range of n-type cell technologies for PV production, including the standard TOPCon 1.0 cell structure, which achieves over 24.5% efficiency. The ongoing development of the double poly TOPCon structure provides advantages in flexibility, enabling the inclusion of screen-printed copper technology in collaboration with Copprint. Additionally, ISC Konstanz offers BC technology ZEBRA 1.0, which features no passivating contacts, while also developing the next-generation ZEBRA 2.0 with both polarities passivated (TBC). This bifacial TBC technology, which is adaptable to copper screen-printing technology, will be transferred to FuturaSun and is expected to be ready for production in 2025, according to ISC Konstanz.
Using the TaiyangNews November TOP SOLAR MODULES report as an example, the institute noted that the top 3 modules featuring BC technology offer an efficiency range between 23% and 24.2%. In comparison, HJT module technology boasts an efficiency range of 22.5% to 23.18%, while TOPCon modules deliver an efficiency range of 22.5% to 23%. Furthermore, according to the nPV Book 2023 on crystalline silicon solar cell technology, the successful implementation of the LECO metallization process on the front-side metal contact of TOPCon cells achieved a Voc of more than 735 mV. To realize further technological improvement, development of both-sided passivating contacts, known as the double poly TOPCon structure, started. While some experts were initially pessimistic about the back-contact structure, or ZEBRA 2.0, which has both polarities passivated, the poly TOPCon structure presents challenges, such as the difficulty of achieving selective polysilicon. In contrast, the BC structure is relatively simpler if the necessary technology, such as fast lasers, is available, explained Kopecek.
Presenting the historical trend in the evolution of revolutionary cell technologies, ISC Konstanz highlighted the incorporation of low-cost Cz-Si cells and PECVD-based AlOx film deposition in p-PERC technology in 2018. This progressed into TOPCon technology following the successful implementation of LECO metallization and ALD-based AlOx deposition by 2024. This trend is expected to shift towards the TBC back-contact structure by 2027, according to ISC Konstanz.
Regarding BC technology, the research institute has successfully incorporated cost-effective BC interconnect technology, although work remains on copper screen printing and laser structuring technology. However, the institute remains skeptical about when perovskite tandem technology will become mainstream. In line with the technology evolution, the research institute has a vision of 2027+ for the BC module which will feature a temperature coefficient (Tc) below 0.3%/K and bifaciality of up to 90% as well as more than 25% efficiency.
Regarding the commercialization of BC technology, Maxeon has been the pioneer of the technology. Maxeon produces IBC solar cells and modules and has been developing IBC intellectual property (IP) since at least 2005. Maxeon spun off from SunPower Corporation in 2020 and produces IBC solar cells with passivated polysilicon contacts (“poly-IBC”)*. Of late, the BC technology diversified into 3 mainstream concepts with different cell architectures – PERC, nPERT, and TOPCon – LONGi Solar has adopted the POLO IBC structure, the BC version of the PERC cell. In 2019, ISC Konstanz transferred the ZEBRA 1.0 version of BC technology to SPIC, which was equipped with laser structuring technology from DR Laser and the classical stringing method for BC half-cell technology. The BC version of the TOPCon cell architecture, has been adopted by several other companies, including LONGi, SPIC, FuturaSun, Valeo Cell, and CARBON*.
The German research institute also shared a summary of the recent bifiPV Workshop in Zhuhai, China, and the BCworkshop in Delft. The bifiPV Workshop, organized in collaboration with AIKO and ISC Konstanz, concluded that BC manufacturers will adopt bifacial BC technology, material quality will advance to semiconductor grade, and laser technology, which may also be used in module production, becomes increasingly important. Meanwhile, by 2025, BC module efficiency is expected to reach 25%, with a bifaciality factor of 78%. In addition, LONGi predicted that BC technology will be the next big thing by 2027 or 2028.
The key takeaway from the BCworkshop in Delft was that the low breakdown voltage of the BC cell will be a crucial technological feature.
* This paragraph of the article has been updated, as it was erroneously stated that Maxeon is an adopter of Zebra technology, while Maxeon has been the pioneer of the IBC technology.