Edge passivation is an emerging trend in the TOPCon area. The aim with this approach is to eliminate the undesired losses that originate from cutting cells in half or, as a matter of fact, several pieces as and when required. For some background, cutting a fully processed cell in half creates defects along the cut corners. These defects act as recombination centers, possibly leading to slight performance losses. The edge passivation neutralizes these defects and eliminates the losses. In the process, the passivation layers are precisely applied to the cut edges without undesirable effects on the cell surface. The process is only relevant to TOPCon, as HJT has long adapted to processing half cells all along the line. Being a batch process, TOPCon is not exactly compatible with half-wafer processing. During the process, an aluminum oxide film is applied on the laser-cut edges using ALD. Most leading equipment makers, including Laplace, Leadmicro, and Ideal Energy, among others, are offering edge passivation tools. Depending on the baseline process, different tool vendors and manufacturers have announced an efficiency gain ranging from 0.1% to 0.3%.
On the other hand, a particularly interesting development is that Leadmicro has introduced a thermal and deposition tool platform capable of supporting half-cell processing for TOPCon. The most immediate benefit is the elimination of edge losses. Additionally, this approach improves ingot utilization upstream and supports efforts to reduce wafer thickness (see UVID In TOPCon: Leadmicro’s Perspective).
Rear Poly-Fingers
Another important trend that is catching up quickly among the TOPCon suppliers is rear poly-fingers. Localized rear fingers is a concept inspired by back-contact cell designs, where polysilicon is applied in a fingered pattern on the rear side. The process involves laser ablation of the rear passivation stack in non-contact regions, meaning the stack of silicon oxide and doped polysilicon is etched off in all open areas on the cell’s rear side except where the contacts are applied. The technology can benefit from the laser tool ecosystem developed for back-contact cells. Indeed, several mainstream equipment vendors in China, most recently Laplace, have also started developing such laser tools. This approach reduces the parasitic absorption of poly-layers on the rear side, improving the bifaciality from 80% to 85%. These localized rear fingers have the potential to improve the overall cell efficiency from 0.1% to 0.2% absolute.
Although front-side localized poly could theoretically provide further benefits, its implementation is far more complex due to alignment challenges and the sensitivity of front-side emitters, which increases the risk of shunting and defects. The introduction of laser-induced metallization (LIM) has already significantly reduced front-side recombination, making localized poly less attractive. Additionally, applying poly fingers on the rear side provides greater tolerance in patterning, making it easier to implement. However, this is not unanimous, and the jury is still out.
Despite these challenges, a few companies have bifacial poly, essentially implementing poly-fingers also on the front side. This approach also includes optimization of contact formation, passivation, and bulk properties. Paste manufacturers are actively developing new pastes specifically designed for bifacial poly applications.
