Equipment makers are reducing polysilicon layer thickness to around 80 nm, improving tool throughput and lowering absorption losses
Edge passivation is emerging as a key step to minimize recombination losses from laser-cut half/multi-cut cells, delivering up to 0.3% efficiency gains
Rear poly-finger structures reduce parasitic absorption and improve bifaciality, further pushing cell efficiency by 0.1% to 0.2% absolute
Having discussed the deposition technologies enabling passivation in different cell architectures, this section highlights recent process innovations such as thinner poly layers, edge passivation, and localized rear poly designs that further enhance cell performance (see Passivation – A Key Differentiator Across Solar Cell Technologies).
The 2 important topics related to passivation are edge passivation and rear poly. But before we get into the details, here is a brief update on polysilicon thickness development.
Film Configuration: The thickness of the polysilicon layer has been a topic of interest. Here, equipment makers follow the developments in pastes, which currently support 80 nm. Equipment vendors, paste suppliers, and PV manufacturers are collectively working to reduce this thickness further. Reducing the film thickness has a double impact; in addition to increasing deposition tool throughput, the thin layers also reduce absorption losses.
Edge Passivation: An important trend increasingly becoming mainstream in the context of passivation, especially within TOPCon technology, is edge passivation. This approach aims to eliminate the undesired losses that occur when cutting the cells in half or, in fact, into several pieces as required. For background, cutting a fully processed cell in half, which is inevitable with wafer sizes of M6 and beyond, 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. It is a coin-stack batch process, which enables precise application of the passivation layers to the cut edges without undesirable deposition on the cell surface. The process is only relevant to TOPCon, as HJT has long adopted 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, various tool vendors and manufacturers have announced efficiency gains ranging from 0.1% to 0.3%.
Another benefit of edge passivation is that it enables PV manufacturers to revisit the concept of cutting cells into more than 2 pieces. Several such products were highlighted at the recently held leading solar shows – Intersolar Europe and SNEC 2025. On the other hand, a particularly interesting development is that Leadmicro has introduced thermal and deposition tool platforms that can support half-cell processing for TOPCon. This not only avoids the need for an additional edge passivation tool in the line, but the approach also improves ingot utilization in the upstream and supports efforts to reduce wafer thickness. The downside of this approach is that the process must stick to half-cell formats and is not supportive of slicing cells into 3 pieces, thereby constraining flexibility (see Deposition Technologies For Passivation In Advanced Solar Cells).
Rear Poly: Another trend that is catching up quickly among TOPCon suppliers is the use of rear poly-fingers. ‘Localized rear fingers’ is a concept inspired by back-contact cell designs, where the passivated contact structure is realized 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 will be applied. 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 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.
The text is an edited excerpt from TaiyangNews’ latest Market Survey on Solar Cell Production Equipment 2025, which can be downloaded for free here.