Key takeaways:
LONGi’s Alex Li outlined why BC architectures may offer efficiency gains beyond the limits of advanced TOPCon structures
Presentation data showed BC modules maintaining a consistent power advantage and exhibiting reduced hotspot severity in shading scenarios
Modeled system results indicated potential CapEx and LCOE benefits when higher-power BC modules are deployed at utility scale
At the TaiyangNews webinar titled Unlocking the Utility Potential of BC Technology, Alex Li, Senior Director of Global Products at LONGi, presented the company’s rationale for focusing on back-contact (BC) technology as a pathway to higher module performance for utility-scale solar. Li began by explaining the technological evolution from standard TOPCon toward dual-poly structures and, ultimately, towards BC architectures. Improving TOPCon efficiency requires identifying the loss mechanisms; one of these is high-temperature front-side diffusion, which can introduce boron-related defects. Replacing this step with front-side polysilicon passivating contacts improves open-circuit voltage but also causes parasitic absorption losses. Laser patterning can localize polysilicon only beneath metal contact regions to reduce these optical losses, but once manufacturers reach this front-and-back TOPCon structure, further gains require eliminating front-side metallization entirely. Moving all contacts to the rear side allows optical and electrical pathways to be decoupled, which Li described as the fundamental reason LONGi considers BC the end-game architecture for single-junction silicon cells.
Li presented LONGi’s performance data for Hi-MO 9 modules, noting a maximum power output of 670 W and a mass-production level of around 655 W. He compared the power roadmaps of TOPCon and BC, assuming both use similar improvements, such as optimized metallization, multi-cut interconnection, and high-quality wafers. Even using those shared assumptions, Li argued that BC consistently maintains a 20-25 W advantage over TOPCon for modules of the same size. For example, he projected that by the end of the year, TOPCon modules may reach around 640 W, whereas BC modules could achieve approximately 660 W. This power margin, he said, has been stable across different forecasting periods and is expected to remain so as both technologies improve.
A substantial portion of the presentation addressed hotspot behavior. Li acknowledged that shading patterns in utility plants differ from those on residential rooftops, yet field inspections show frequent partial shading events caused by dust accumulation and bird droppings. Under 1,000 W/m², when a single cell is fully shaded, he explained, PERC and TOPCon modules can reach hotspot temperatures between 160 and 170°C. In contrast, LONGi’s BC modules have shown peak temperatures around 114°C under standard test conditions, and even lower – just above 40°C – in outdoor infrared comparisons under lower irradiance (550 W/m²). This reduced hotspot severity is attributed to the current-flow characteristics of BC cells. Li emphasized that, while BC cannot eliminate hotspots, it substantially mitigates their consequences over the 30-year life of a utility installation.
Li then discussed bifaciality. Traditional BC designs were considered unsuitable for large-scale ground-mounted systems due to very low rear-side response, but LONGi claims to have increased BC bifaciality to 65% in 2024, with 75% expected by year-end 2025, and 80% through further future developments. Although this remains lower than the typical 85% bifaciality of TOPCon, Li argued that the combined contribution of front and rear irradiance is more relevant in utility applications. With a higher front-side output (e.g., 670 W vs. 640 W for TOPCon), BC modules can deliver greater total power even with a bifaciality disadvantage. Under assumed operating conditions with 10% rear irradiance, Li calculated that BC maintains roughly a 25 W combined power lead.
Finally, Li connected the performance metrics to energy-system economics. A higher-power module reduces the number of trackers, cables, combiner boxes, and other balance-of-system components required for a given plant size, lowering CapEx. BC modules also feature a lower temperature coefficient and reduced linear degradation (e.g., 0.35% per year). When these factors are modeled over 30 years, Li stated that LCOE can be reduced by approximately 3.8% and IRR increased by about 5.3% in LONGi’s internal and field-test analyses. He concluded that BC offers what LONGi considers the most promising pathway for long-term performance improvements within single-junction crystalline silicon technology and is central to the company’s utility-scale product strategy.
