Cheap aluminum paste used to build TOPCon solar cells with 22.56% efficiency

Aiming to reduce expenses, Chinese researchers have developed a solar cell using TOPCon technology, replacing costly silver paste with screen-printed aluminum paste.

A team at Yangzhou University in China used a UV pulse laser to remove the rear passivation layer of TOPCon cells, and aluminum paste with varying silicon content was applied to reduce costs.

The laser reduced the open-circuit voltage (Voc) by about 6–7 mV at an energy density of 0.431 J/cm².

According to reseachers, the best performance was achieved with aluminum paste containing 25-29 percent silicon, resulting in a Voc of 663.60 mV and a conversion efficiency of 22.56 percent.

TOPCon cost reduction

The rapid advancement of science and technology drives the development of new energy sources. Cz-Si wafers, known for their purity and long carrier lifetime, enable high-efficiency photovoltaic (PV) devices.

Silicon solar cells, including TOPCon, heterojunction (HJT), and integrated-back contact (IBC) cells, are widely studied, with TOPCon being commercially popular due to its excellent passivation structure and compatibility with PERC cell production lines.

TOPCon cells use an ultrathin SiOx layer for chemical passivation and phosphorus-doped polycrystalline silicon (n+-poly-Si) for electric field passivation, consistently breaking efficiency records.

To reduce costs, improving efficiency and lowering material expenses are key, especially for the costly silver paste used in forming electrodes.

According to researchers, while the contact mechanism between silver paste and n+-poly-Si is well-researched, the use of aluminum paste in TOPCon cells is less understood, with limited studies exploring its contact mechanism.

“We are still adjusting further process and Al paste to improve efficiency. We are testing this innovation together with Chinese solar module manufacturer JA Solar and paste manufacturer Ruxing Technology Development Co. Ltd,” Jianning Ding, the research’ corresponding author told PV Magazine .

Al paste advantage

For the study, researchers built an 182 mm x 182 mm solar cell using a 140 μm thick n-type Czochralski wafer.

The rear passivation structure included a 1.6 nm silicon monoxide (SiOx) layer and a 250 nm phosphorus-doped polycrystalline silicon layer. Both were created through low-pressure chemical vapor deposition (LPCVD). An aluminum oxide (AlOx) layer was also added using atomic layer deposition (ALD).

They used a mixed silver and aluminum paste for the front side, while the rear side was coated with the full aluminum paste.

According to researchers, silicon was added to the aluminum paste to reduce the reaction with the n+-poly-Si, although it lowered the grid’s conductivity. Different silicon concentrations were tested for metallization.

“Adding silicon to aluminum paste can suppress the reaction between the aluminum paste and poly-Si, reducing metal recombination and improving open-circuit voltage,” Qinqin Wang, co-author of the research, told PV Magazine.

The team noted that an ohmic contact was formed through the creation of an Al-Si alloy. The thickness of the Al-Si alloy decreased as the silicon content in the aluminum paste increased.

The researchers reported that the solar cell with the highest silicon content showed the best results. Under standard testing conditions, the cell reached a power conversion efficiency of 22.56 percent, an open-circuit voltage of 663.6 mV, a short-circuit current density of 40.73 mA/cm², and a fill factor of 83.48 percent.

According to PV Magazine, the researchers noted that while the efficiency was 9.40 percent lower compared to TOPCon cells using silver paste, the aluminum paste cost only 10 percent of the silver paste.

They expect that in the future, TOPCon solar cells with aluminum paste on the rear side could achieve similar efficiency levels to commercial TOPCon cells using silver paste, which incorporate laser doping selective emitter technology and a bifacial poly-Si passivation structure.

The details of the team’s research were published in the journal Solar Energy .