BC Technology Places Higher Demands On Wafers

Back contact (BC) cell architecture is widely regarded as the ultimate structure for single-junction PV devices. It is also emerging as one of the most important technology trends within China’s PV manufacturing landscape. And BC is currently the only cell architecture attracting new production capacity additions amid prevailing overcapacity in China. Nearly every leading PV manufacturer is now establishing at least a pilot line and actively working on BC. However, the technology is being driven primarily by a few key players. These include the pioneer of BC technology, Maxeon (now part of TZE), as well as AIKO, LONGi, and SPIC. That’s also the reason much of the innovation and development in BC is proprietary and confidential. While TaiyangNews will present a separate, dedicated, detailed report on BC technology, the following section summarizes the most important recent developments that were presented at several conferences.

For BC technology, wafer requirements are significantly more stringent, reflecting the complexity of the cell architecture. One of the most critical differences is in wafer resistivity. BC cells employ wafers with much higher resistivity. Since the areas of both polarities are located on the rear side of the cell, the carrier path is long. Higher resistivity enables a longer minority-carrier lifetime, known as the bulk lifetime, which is crucial for carriers generated at the front to reach the back electrodes. Achieving a higher minority-carrier lifetime is even more crucial.

Gokin Solar presented the wafer perspective of BC technology at the Global BC Technology Innovation Summit, as part of bifiPV 2025 held in China. For BC, 3 parameters are critical: resistivity uniformity, material purity, and mechanical strength. High resistivity uniformity is essential to enable consistent high conversion efficiency across the wafer. At the same time, extremely low levels of metallic impurities are required to achieve high minority-carrier lifetimes, placing a strong emphasis on advanced gettering and purification processes. Mechanical robustness is equally important because BC cells are asymmetrical, and thinning influences mechanical yield. The complex processing steps also require greater strength to prevent breakage.

To meet these requirements, Gokin highlights several process innovations. These include advanced doping strategies that reduce resistivity variation along the ingot from ~30% to below 5%, supported by automated real-time doping control systems. In parallel, dedicated surface-purification techniques are used to remove impurities at the raw-material stage. Oxygen content is another critical parameter, managed through closed-loop thermal field control and advanced crystal growth processes, enabling levels below 10 ppm.

LONGi has also developed TaiRay wafers, which are antimony-doped wafers designed specifically with higher purity, lower oxygen and metal impurity concentrations and, critically, higher resistivity.

Regarding the wafer dimensions, BC also closely follows TOPCon. The companies leading the BC segment are also using different wafer formats, at least for their top product range. AIKO, for example, is relying on a 183 × 197.34 mm wafer format for its ABC series, while LONGi uses 182.2 × 182 mm. SPIC’s usage is more representative of the old format than a product of differentiation. As to the wafer thickness, BC typically uses thicker wafers compared to TOPCon and HJT at 135 μm, according to CPIA, which is not expected to change much; only drop to 130 μm between 2026 and 2028, and remain so until 2035.

The text is an edited excerpt from TaiyangNews’ report on Cell & Module Technology Trends 2026, which can be downloaded for free here.