Back contact technology: How LONGi’s HPBC 2.0 technology improves performance in partial shade

Anyone who has lived with solar knows the frustration: a small patch of shade from a tree or chimney can cause a disproportionate drop in output. Traditional solar panels are highly sensitive to shade. Even when just a small part of the surface is obstructed, output losses of 15–25% are common. In more severe cases, studies and field tests have reported losses of 30–40%, and in extreme scenarios - when a single cell in a string is shaded - entire modules can lose more than half their power. This wide range highlights why shading is considered one of the most critical performance risks in conventional solar design.

LONGi’s Hybrid Passivated Back Contact (HPBC) technology represents a significant step forward in solar cell design. First launched in November 2022 with the Hi-MO 6 series and enhanced in 2024 with the Hi-MO X10, HPBC moves all electrical contacts to the rear of the cell. This architecture maximizes light absorption on the front surface and enables current to flow through multiple pathways, reducing the performance penalty under partial shade. With HPBC 2.0, LONGi has gone further by introducing a built-in shading optimizer. This feature intelligently redirects current around shaded areas, for example from a leaf or branch, ensuring that the module maintains higher output even under non-ideal conditions.

For homeowners and businesses, this means higher yields where partial shading has always been a limiting factor.

What is HPBC 2.0 technology?

HPBC stands for Hybrid Passivated Back Contact, a solar cell architecture pioneered by LONGi. The design merges advanced passivation techniques with a full back-contact layout, leaving the front surface free of metal grid lines. The absence of front-side metallization improves aesthetics, but more importantly, it enables the cell to capture more photons. The result is higher conversion efficiency and a fundamentally different way of managing current flow, one that reduces the performance penalty under shade.

LONGi’s HPBC 2.0 technology has achieved a world-record module efficiency of 25.4 %, independently certified by Fraunhofer ISE under laboratory conditions. This figure demonstrates the technology’s ultimate potential. The Hi-MO X10 series, built on the same HPBC 2.0 platform, delivers commercial module efficiencies up to 24.8 % and power outputs of up to 670 W.

Thanks to its optimized current management and built-in shading control, HPBC 2.0 can reduce power loss from partial shading by up to 70 % compared with conventional module technologies, ensuring stable energy generation even in complex rooftop environments.

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Why conventional panels struggle in shade

Conventional PERC and TOPCon modules connect cells in series, much like a string of lights. When part of one cell is shaded, it increases resistance for the entire string, dragging down output. Even light shading of 5% of a module’s surface can cause a 15–25% or higher loss in production.

This structural weakness has always limited solar performance in built environments where chimneys, trees, or nearby buildings cast intermittent shadows.

Back-contact design: How HPBC 2.0 handles shade differently

In HPBC 2.0 cells, both positive and negative contacts are positioned on the rear in an interdigitated pattern. This frees the entire front for light absorption and reduces optical shading.

Instead of forcing current through a single route, HPBC’s rear design allows electricity to bypass shaded regions by flowing through alternative paths. This is comparable to traffic rerouting around a blocked road.

Together, these features deliver more stable power output. According to testing by China’s National Photovoltaic Quality Inspection Center (CPVT), LONGi’s HPBC 2.0 modules have shown daily gains exceeding 10% compared to conventional modules in dynamic shading scenarios.

In real projects, customers report notable improvements. One installation exposed to tree shading saw production rise by nearly 18% after switching to HPBC modules, with the greatest benefit during late afternoon when shadows used to cause severe losses.

Comparing HPBC 2.0, TOPCon, and PERC

The solar market today essentially offers three key technologies. This is how LONGi’s HPBC 2.0 performs compared to the two conventional technologies.

Technological characteristics explained

LONGi’ back contact technology HPBC 2.0

HPBC 2.0 marks the next evolution of LONGi’s back-contact technology. Building on the first-generation Hi-MO 6 launched in 2022, the Hi-MO X10 introduced in 2024 combines a clean, grid-free front with an integrated shading optimization layout. This innovation not only boosts efficiency to record levels of 26.6% at cell and 24.8% at module, but also ensures more stable performance when parts of a panel are shaded. With its higher baseline efficiency and enhanced real-world resilience, HPBC 2.0 is particularly compelling for rooftops, façades, commercial and industrial (C&I) projects, as well as utility-scale plants where energy density and reliability directly impact system returns.

Here is a summary on its key features:

Shadow-free front surface: By moving both electrical contacts to the back of the cell, the front surface is entirely clear of obstructions, allowing maximum sunlight to reach the light-absorbing layer.

Enhanced light absorption: A multi-layer anti-reflection film and improved micro-texture uniformity reduce short-wave light reflection, capturing more sunlight and increasing short-circuit current.

Increased open-circuit voltage: The use of self-developed bipolar hybrid passivation technology boosts the open-circuit voltage, contributing to higher overall cell efficiency.

Reduced shading losses and hotspots: Bipolar low-resistance passivation contacts and shading optimizer technology significantly lower power losses from shading by over 70% and reduce hotspot temperatures by 28%.

Improved reliability and longevity:

• Uniform passivation minimizes UV-induced degradation, enhancing long-term efficiency and resilience.
• The use of thicker, more robust proprietary TaiRay wafers improves mechanical stability and reduces the risk of microcracks, contributing to greater module reliability.
• The special 0BB (Zero Busbar) structure further increases module power output and reliability.

Superior temperature performance: The technology features a power temperature coefficient of -0.26%/°C, an improvement of 0.03%/°C over TOPCon modules, ensuring superior performance in hot climates.

Enhanced resistance: HPBC 2.0 modules demonstrate superior resistance to UV-induced degradation, damp heat, and thermal cycling compared to previous technologies.

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PERC (Passivated Emitter and Rear Cell)

This technology has long been the workhorse of the solar industry. Compared to conventional Al-BSF cells, PERC adds a dielectric passivation layer and locally opened rear contacts, which reduce recombination and reflect unused photons back into the cell. This improvement often yields a gain of up to ~1–2 percentage points in efficiency in practical high-quality implementations. Today, commercial PERC modules typically deliver ~20–21.5 % efficiency. However, as the industry advances, many believe PERC’s practical efficiency ceiling is being approached, and the architecture faces inherent vulnerabilities such as light-induced or LeTID degradation - especially in p-doped formulations. As a result, many manufacturers are shifting toward n-type and back-contact alternatives like TOPCon and Back Contact technology, which promise higher efficiency potential and greater degradation resilience.

TOPCon (Tunnel Oxide Passivated Contact)

It builds on PERC by adding a thin tunnel-oxide layer and a doped polysilicon film on the rear surface. This configuration reduces recombination losses and supports higher open-circuit voltage. In commercial modules, TOPCon routinely achieves ~22–23% efficiency, and advanced cell designs are pushing toward 24–25%. Because its manufacturing steps are largely compatible with PERC production lines, TOPCon is an attractive upgrade path for existing factories.

The future of back contact technology

The future of back-contact technology is widely regarded as one of the most promising directions in solar. Experts such as Radovan Kopecek of ISC Konstanz predict that BC cell and module production could expand to 1 TW of global capacity by 2030, calling it 'the final evolution step for crystalline silicon.' According to Kopecek, the expiry of key BC patents around 2028 will accelerate market adoption, lowering barriers for more manufacturers to enter the field. TÜV Rheinland and other certification institutions have highlighted BC’s architectural strengths - a clean, unobstructed front surface and higher conversion potential - as reasons why it is well positioned to become the next mainstream PV technology.

The outlook is reinforced by industry roadmaps: the International Technology Roadmap for Photovoltaics (ITRPV) sees BC designs capturing a steadily growing share of new installations through the late 2020s, with especially strong prospects in rooftops, façades, and distributed generation. With real-world advantages under shade, long-term reliability improvements, and record-setting efficiency benchmarks, BC is increasingly viewed not as a niche but as a cornerstone technology that will define the next phase of solar growth.

LONGi’s leadership sees back-contact technology as central to the industry’s future. “In the next five to six years, back-contact cells will be the mainstream of crystalline silicon,” said LONGi Chairman Zhong Baoshen in 2023.

Is LONGi’s HPBC 2.0 the right choice for you?

When deciding whether HPBC 2.0 and its related Hi-MO X10 Series suit your project, consider these factors:

• Shade exposure: If your site experiences partial shading, HPBC 2.0 may significantly increase real-world output.
• Visual requirements: The sleek, uniform front face appeals to homeowners and architects.
• Space constraints: Where roof space is limited, higher efficiency per square meter can be decisive.
• Budget: HPBC 2.0 delivers additional energy yield that compensates for higher initial costs.
• Climate: HPBC’s stable temperature coefficient ensures reliable yields even in hot regions.
• Low-load bearing rooftops: LONGi also introduced a light design module with HPBC 2.0 which is 30% lighter than conventional single-glass modules called Hi-MO X10 Guardian Light Design
• High Yield, fast ROI: For C&I projects, HPBC 2.0 delivers faster ROI and higher lifetime yields. Its superior efficiency and shading resilience improve near-term cash flow while maximizing long-term value.

HPBC 2.0 is an option for challenging conditions – in the premium segment and beyond

By rethinking how cells collect light and conduct electricity, HPBC 2.0 tackles one of solar’s longest-standing weaknesses: shade sensitivity. This innovation has also made back-contact technology accessible beyond premium rooftops, extending its benefits to a wider range of applications. With its higher conversion efficiency, superior shading tolerance, proven resistance to snow, hail, and high winds, and stable long-term output, HPBC 2.0 is an attractive choice for all – residential, commercial, industrial, and utility-scale projects.

For C&I customers, BC delivers faster ROI through higher yields per square meter, while in utility-scale plants, it enhances land-use efficiency, reduces BOS costs, and maximizes energy generation across the project lifetime. As manufacturing expands and costs decline, BC is positioned to become a leading technology across all major market segments.

For homeowners and businesses with shading challenges, performance or design priorities, HPBC 2.0 offers a way to secure higher yields, greater reliability, and a sleek aesthetic - making it one of the most future-ready solar technologies available today.

Learn more about our Hi-MO X10 Series as part of the EcoLife Series here.