TOPCon Suppliers Turn To Rear Poly-Fingers For Additional Efficiency Gains

The current state-of-the-art in TOPCon technology is rapidly evolving into the next-generation technology, which the industry characterizes with different suffixes after TOPCon, such as ‘3.0 / 4.0 / 5.0 / 6.0 / + /….’. The enhancements focus on reducing optical and electrical losses through more sophisticated passivation and contact strategies. Major components of this evolution are rear poly-fingers, edge passivation, and stencil printing at the cell level and multi-cut layout at the module level. The major configurational difference is the transition from full-area passivated contacts to selective polysilicon fingers, or ‘poly-fingers’, on the rear side. By removing the polysilicon layer from the non-contacted areas of the cell, manufacturers can significantly reduce parasitic light absorption, thereby boosting the short-circuit current and overall efficiency (see TOPCon Progress Moves Toward Passivation And Contact Optimization). 

Another important aspect of this latest generation of TOPCon is edge passivation. It offers performance benefits not at the cell level, but at the module level, enabling the multi-cut module layout. On the metallization front, adapting to stencil printing is also part of the development package. In fact, most of the latest product series from leading TOPCon manufacturers are inherently based on these 3 technology upgrades. These latest product platforms from different companies, featuring a range of technological enhancements, have been the focus of discussions and presentations at recent TaiyangNews conferences. Some companies have also made incremental developments to complement these gains.  

Rear Poly-Fingers  

Another important trend catching up quickly among TOPCon suppliers is rear poly-fingers. Localized rear fingers is a concept inspired by BC 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 BC cells. Indeed, several mainstream equipment vendors in China, most recently LAPLACE, have also started developing such laser tools. This approach reduces parasitic absorption of sunlight in 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. 

The crux of the process lies in the laser tools, with a high-power laser source being the key to thinning the polysilicon. While stability was an issue with such laser sources in the past, recent developments in the segment have enabled very stable high-power lasers. The laser spot is also an important parameter here; the roundness, shape uniformity, and its transition along the radius also influence the final poly-finger formations. Leadmicro is offering a laser tool with about 110 W and a throughput range of 6,300 to 7,000 pieces per hour. 

With lasers being the key, which are not side-sensitive, why not use them on the front side? First, while the effort to realize localized poly-fingers on the front is nearly the same, this approach poses new alignment challenges during contact application. Given the sensitivity of the front-side emitter, any misalignment could lead to shunting and defects, in which case the benefits are less pronounced. LECO already decouples metal contact recombination and contact resistivity, narrowing the performance benefits with the bifacial poly. On the other hand, applying poly-fingers to the rear side provides greater patterning tolerance, making implementation easier. 

The text is an edited excerpt from TaiyangNews’ latest report on Cell & Module Technologies Trends 2026, which can be downloaded for free here.