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EnPV GmbH has developed a TOPCon solar cell concept with a novel rear contact technology. The cell can be produced at a significantly lower cost than conventional TOPCon cells and its manufacturing process is easy to introduce into existing TOPCon production equipment.
EnPV GmbH, a subsidiary of German energy provider EnBW Energie Baden-Württemberg AG, has introduced a solar cell based on n-type tunnel oxide passivated contact (TOPcon) technology and a new type of back contact architecture, described as self-aligned back contact (SABC) technology.
“Common interdigitated back contact (IBC) solar cells require an interdigitated pattern that is not homogeneous by nature and need to protect parts of the back surface of the wafer during some of the manufacturing steps,” EnPV CEO Massimo Centazzo told pv magazine . “Our SABC cell manufacturing avoids masking to structure the back surface and, by using laser structuring instead, manages phase isolation without any additional dedicated process steps.”
Centazzo explained that phase isolation involves “reliably” isolating the p phase and n phase from each other through a byproduct of other process steps, which explains why it is “self-aligning.”
“The use of both techniques – laser processes for structuring and self-aligned phase isolation – leads to a dramatic simplification of the manufacturing process compared to any other downstream cell contact process,” the CEO said. “While still allowing for higher cell efficiency, SABC cells are produced at a lower cost than TOPCon and heterojunction (HJT) cells.”
The manufacturer claims the new back-contact technology could easily be used on existing TOPCon cell production lines, with the upgrade requiring the addition of a wet chemical process, a laser structuring process step, deposition of an etch barrier, and physical deposition based on doped amorphous silicon vapor deposition for the n-contact.
“Our method does not require a specific trench to isolate the polysilicon layers of opposite polarity,” Centazzo adds. “Instead, the recess in the trench wall separates the two polarities, so that the first p-type poly-Si layer is located on the surface of the trench, while the second n-type poly-Si layer is located at the bottom of the trench.”
Below the undercut, which is made by wet chemical etching of the trench, no poly-Si is deposited during the physical vapor deposition (PVD) process. The n-type poly-Si also covers the p-type poly-Si emitter forming a low resistivity tunnel contact, since both poly-Si layers are highly doped with dopant concentrations greater than 1,020 cm3. The n-type poly-Si layer covers the entire surface and allows the same metallization scheme to be used for both polarities, according to the manufacturer.
Centazzo also explained that the SABC target process starts with texturing and boron diffusion on both the front and back sides. “A single-sided borosilicate glass (BSG) etch followed by an alkaline etch of the boron emitter on the back side prepares this surface for poly-Si deposition,” he stressed. “Instead of boron diffusion forming the front floating emitter (FFE), a phosphor diffusion forming a front surface field (FSF) or a diffusion-free surface could also be used for the SABC process flow.”
Wet alkaline chemical etching etches the poly-Si on the front side down to the BSG layer and on the back side etches the p-type poly-Si in the exposed areas. The back side etching not only removes the poly-Si layer, but also etches the silicon base vertically and laterally, creating an undercut on the surface. After the removal of the etch barrier and the BSG and the growth of a second interfacial oxide, the n-type poly-Si subsequently deposited by PVD will not cover the entire surface on the back side.
“With the highly directional deposition technique, doped silicon is mainly deposited on vertically exposed surfaces, which are not shadowed by the undercut,” Centazzo emphasizes. “Therefore, at the bottom of the trench, the n-type poly-Si forms a passivating contact, but there will also be an n-type poly-Si above the p-type poly-Si that will form a tunnel junction with the emitter. The shadowed part of the trench wall isolates both passivating contacts, without the need for additional processing steps such as structuring, local etching or masking.”
The manufacturing process then ends with high-temperature annealing, which crystallizes the poly-Si layers and activates the dopants, standard passivation with aluminum oxide (AlOx) and silicon nitride (SiNx), and metallization by screen printing. “As n-type poly-Si covers the surfaces of both passivation contacts, the same screen printing paste can be applied in a single printing step for both polarities,” Centazzo said.
In his opinion, SABC TOPCon cells could be cheaper than conventional TOPCon cells.
“We expect an efficiency increase when switching from TOPCon to SABC of more than 0.5% absolute at the cell level, and an additional 0.7% at the module level due to the lower cell-to-module losses of the back-contact modules compared to regular modules,” he added. “The cost of conversion from cell to system amounts to €0.75 ($0.83)/W, while in a SABC-based system, due to the higher efficiency of the modules, it drops to €0.71/W, bringing the cost of cell production down from €0.100/W to €0.85/W.” In total, a system based on SABC modules would cost €0.80/W, which is 6% less than one with TOPCon cells.”
The novel cell concept has a Technology Readiness Level (TRL) of 6. TRL measures the maturity of the technological components of a system and is based on a scale of one to nine, with nine representing technologies that are mature enough for full commercial application. According to the company, it would only be possible to move up to TRL 8 when implemented on a pilot production line in a series production environment.
EnBW has currently committed to funding the development of SABC cells, but the industrialisation strategy is still being discussed. “As EnBW’s business territory of activity is traditionally Germany and Europe, where PV manufacturing is in decline, the search for suitable partners for the industrialisation of SABC had to be extended beyond Europe,” said Centazzo, noting that EnPV is currently assessing opportunities for the launch of SABCs in cell factories located in the world’s hotspots for PV manufacturing, namely the US, India and China.
"We plan to launch a pilot line by the end of 2025," he concluded. "The partner has not yet been decided." |
