Back Contact Cells Demand Stricter Wafer Specs & Evolving Rear-Side Architectures

As with the other cell technologies, BC also has a special list of requisites in terms of wafer specifications, perhaps a more demanding one. One of the most critical differences is in wafer resistivity. While TOPCon cells typically require wafers with a resistivity range of around 0.4 to 1.6 Ω cm, BC cells employ wafers with much higher resistivity, often greater than 30 Ω cm. According to AIKO, higher resistivity enables longer minority carrier lifetimes, which is essential for optimizing the performance of BC cells. Similarly, LONGi emphasized that its HPBC 2.0 cells use TaiRay wafers – an independently developed, antimony-doped wafer type – designed specifically with higher purity, lower oxygen and metal impurity concentrations and, critically, higher resistivity. This allows BC cells to reach a higher theoretical efficiency limit vis-à-vis TOPCon cells, which cannot tolerate high-resistivity substrates (see Leading Manufacturers Back BC As The Future Of High-Efficiency Solar).

Beyond resistivity, carrier lifetime is another crucial factor. Both SPIC and AIKO highlighted that achieving a high minority carrier lifetime is even more important than resistivity alone. SPIC noted that BC wafers need to exhibit lower levels of oxygen, carbon, and metal impurities compared to TOPCon wafers, contributing to better electronic quality and, ultimately, improved cell performance. While SPIC acknowledged that exact specifications can vary, it points out that wafers meeting BC requirements typically command a higher price than those used for TOPCon, largely due to stricter quality standards (see AIKO Doubles Down On Innovation With Efficient & Smarter Modules).

In contrast to HJT, which can still tolerate relatively high oxygen content, BC demands wafers with very low contamination levels to minimize recombination losses and ensure superior device performance. This focus on material quality is in line with BC’s underlying strategy: achieving extremely high efficiencies by reducing all parasitic losses, including those stemming from the bulk material itself.

Cells
Although back contact (BC) technology follows the common principle of relocating all electrical contacts to the rear side of the solar cell, there are several variations.

BC structures
The base variant of the BC structure is diffused inter-digitated p+ and n- regions. There are also variations coming from the base wafer – p-type or n-type. Adapting the passivated contact strategy to the back contact architecture is another variant, and most BC structures fall into this category. However, the early BC designs passivated only one polarity of the contact, but later developments introduced passivation for both polarities. Additionally, the HJT structure can also be tweaked into BC, which liberates the architecture from one of its key limitations – parasitic absorption of light.

The cell structures adopted by leading manufacturers also have a few differences in how they balance light absorption, passivation, and carrier collection. At LONGi, the BC cell structure is centered on maximizing optical and passivation performance at the front, while dedicating the rear side solely to electrical collection. The ‘sunny’ side is covered with a uniform, full-area passivating anti-reflective coating that optimizes both surface passivation and light management without any interruptions from metal contacts. This avoids shading and parasitic absorption losses at the front surface – a clear advantage over conventional structures. The rear side consists of passivating as well as non-passivating contact zones. Within the passivating contact zones, LONGi uses a bipolar passivation strategy, meaning that both the p-type and n-type regions employ passivating contacts. LONGi refers to this careful design approach as a ‘division of labor,’ where each functional layer is optimized for either light management, passivation, or carrier collection.

AIKO calls its BC architecture ABC, abbreviated for All Back Contact technology. The front side of the ABC cell features an anti-reflection coating made from a combination of aluminum oxide and silicon nitride, providing both passivation and reflection control. Meanwhile, the rear side uses a high-temperature passivating contact structure, where both p and n regions integrate TOPCon-like layers for superior carrier selectivity and reduced recombination. AIKO emphasized that this rear-side design – combining dual TOPCon-based passivation – is key to achieving both very high efficiency and strong real-world performance.

SPIC, another early mover in BC technology, originally based its first generation of BC cells on the Zebra structure developed by ISC Konstanz, using a thermal diffusion process to form an n-type back surface field and a p-type emitter without the need for isolation zones. However, SPIC has since transitioned to its own in-house developed TBC (TOPCon Back Contact) structure. In the new design, TOPCon passivation is employed at the rear side, but with strict isolation between the p and n regions to minimize recombination losses. This evolution marks a shift from the simpler diffusion-based architecture toward more sophisticated selective passivation similar to other modern BC designs.

In an exclusive interview with TaiyangNews Managing Director Michael Schmela during the TaiyangNews SNEC Solar Leadership Conversations 2025 at SNEC 2025, LONGi Group Vice President Dennis She highlighted the company's efforts to make back contact technology a cost-effective solution across diverse application scenarios (see SNEC 2025 Exclusive: Interview With LONGi Group VP Dennis She).

This text is an excerpt from the TaiyangNews Cell & Module Technology Trends 2025 report, which can be downloaded for free here.