LONGi framed BC as a structural response to 4 fundamental loss channels limiting conventional solar cell efficiency
Eliminating front metallization and doped-layer parasitic absorption was presented as a structural solution to expanding the efficiency ceiling
A ~1% absolute efficiency advantage over mainstream TOPCon was cited, equivalent to roughly 30 W higher module output
The roadmap targets approximately 28.5% cell efficiency in the coming years, highlighting compatibility with tandem architectures
As the solar industry continues its transition toward n-type technologies, the debate over the next structural efficiency plateau is intensifying. While TOPCon has emerged as the dominant high-volume architecture, back-contact (BC) technology is increasingly being positioned as an alternative pathway with a higher theoretical efficiency ceiling.
During the TaiyangNews flagship conference High-Efficiency Solar Technologies 2025, Dante Zeng, Product Marketing Manager at LONGi, presented the company’s interpretation of the BC White Paper, outlining why LONGi views back-contact architecture as a structural solution to fundamental power loss mechanisms. The BC White Paper was developed at the beginning of the year in collaboration with AIKO and 2 third-party organizations to evaluate the advantages of BC technology. The presentation, titled Why BC Leads – Interpreting the BC White Paper, analyzed optical losses, passivation constraints, thermal behavior, and shading response as major factors in long-term performance differentiation.
Rather than starting with efficiency records, Zeng explains why conventional solar cells cannot reach higher conversion efficiencies. LONGi categorized the constraints into 4 key areas: losses from incomplete light absorption, recombination linked to material defects, current losses due to imperfect passivation, and metallization-related losses that may also elevate reliability risks.
BC architecture was presented as addressing these loss channels at the structural level. By relocating all electrical contacts to the rear side of the cell, BC eliminates front-side gridline shading. This creates what LONGi described as a ‘zero-shading optical surface’, allowing greater incident photon utilization. Additionally, because the P-N junction is on the rear side, the front surface avoids parasitic absorption from the doped layer, further improving photon absorption and utilization.
LONGi described BC as an electrode and passivation engineering platform that can be combined with mainstream passivation approaches. The company cited its 27.8% laboratory efficiency achieved with a hybrid interdigitated back contact (IBC) structure that integrates advanced passivation concepts. The result aligns with global efficiency records reported by research institutions, such as UNSW, led by Martin Green.
The company confirmed that the hybrid BC architecture has entered mass production at roughly 1 GW scale and is commercially deployed in the European market. In its current module configuration of approximately 2 m², the platform delivers around 510 W, corresponding to an efficiency of about 25%.
One of the central arguments presented was a roughly 1% absolute efficiency advantage in favor of BC architecture. According to LONGi, its mass-produced BC modules exceed 24% efficiency and deliver around 655 W, compared with approximately 630 W for mainstream TOPCon modules. This difference corresponds to roughly 30 W additional power in standard module formats. In installations constrained by available area, such an increase in power density can directly influence system layout and overall energy yield.
Zeng further outlined a ‘comprehensive power’ framework combining front-side output, bifacial response, and location-dependent rear-side gain. In modeled water, grassland, and desert scenarios, BC modules showed a 3.9% to 4.5% higher comprehensive power value under the specified parameters.
Beyond nameplate efficiency, LONGi positioned BC as offering advantages in real-world operating conditions. The company cited a power temperature coefficient of approximately -0.26%/°C for BC modules, compared with around -0.29%/°C for TOPCon references. Under modeled high-temperature conditions, where ambient temperature approaches 40°C and module temperature reaches around 70°C, BC modules were shown to deliver more than 1.3% higher output per watt relative to the referenced comparison modules.
Long-term degradation was presented at approximately 0.35% annually for BC modules. The presentation further indicated that by year 30, cumulative degradation could be around 1.45% lower than for TOPCon modules under the modeled scenario. Lower degradation and improved uniformity were linked to reduced mismatch growth within strings over time.
Another advantage of BC architecture, according to LONGi, is shading tolerance. Zeng referenced IEC 60904 1 classification and indicated Class A plus performance for BC modules, compared with Class A for TOPCon. In a single spot shading example with about 50% shading on one cell, BC retained 91.8% output versus 84.69% for the referenced TOPCon module.
Hotspot behavior was also highlighted. Under IEC 61215 testing, slide data showed a maximum hotspot temperature of approximately 122.9°C for BC modules, compared with up to 185°C for the referenced TOPCon module. Normal hotspot temperatures for BC were shown below 100°C, whereas TOPCon values were cited in the 150-160°C range under comparable conditions.
In addition to performance gains, LONGi highlighted mechanical improvements. The company stated that its wafers are about 10 µm thicker than conventional designs, increasing breaking strength by around 16%. It also referenced its TaiRay wafer and 4-layer protection system, with slide data indicating a modeled crack-risk reduction of approximately 87.2%.
Looking ahead, LONGi positioned BC as a long-term efficiency platform. The company indicated that its current BC mass production cell efficiencies are around 27%, with a roadmap target of approximately 28.5% over the next 3 to 5 years. Module efficiencies above 26% were achievable under this development pathway. The company also cited its 33.7% perovskite-silicon tandem cell efficiency, built on the BC architecture.
The full presentation is available on the TaiyangNews YouTube channel here.