The China Photovoltaic Industry Association (CPIA) recently published its latest guidance report, the “Photovoltaic Industry Development Roadmap 2020”, in which it provided professional suggestions for both the Chinese government and all industry players on key concerns, including market trends and future technologies. Thinner wafers and two possible next generation cell technologies, TOPCon and HJT, were identified, analyzed and discussed as a major area of focus due to their importance to the industry.
Firstly, thinner wafers can deliver higher cell conversion efficiencies due to a better match to diffusion length of minority carriers. Secondly, they also appear to be less fragile once thickness goes below 150μm. Thirdly, thinner wafers reduce the cost of crystalline silicon, especially during periods of supply shortages. Currently, slicing technology supports thinner wafers well, yet is limited by cell structure and respective panel encapsulation. Thinner wafers with conventional cells would cause a number of issues in downstream processes such as cracks, non-uniform resistance, hidden breakage and false welding, negatively impacting yield. According to the roadmap, going forward, the CPIA predicts a drop of 5μm in thickness every 2 years.
Meanwhile, the potential of PERC cells is being exhausted by the endless pursuit of higher efficiencies and this hitherto dominant cell technology is now approaching its conversion ceiling, with the entire PV industry expecting a new leader in next generation cell technology, very likely one from HJT, TOPCon or IBC, which all present much higher potential conversion rates than PERC.
This highlights the value of a special cell technology, Metal Wrap Through (MWT), which could be an effective route to add to those technologies identified in the CPIA roadmap.
What is MWT?
Different to conventional solar cell technologies, MWT technology transfers the current collected by fingers on the front side of a cell to the back side through silver paste, which is filled with tiny holes of around 120 μm in diameter made by a laser on a silicon wafer. With the elimination of bus-bars on the front side, the light receiving area is significantly increased, resulting in a direct improvement in panel efficiency. Silver consumption and minority carrier recombination loss at the metal electrode/emitter interface are also greatly reduced.
There are several advantages for MWT over other alternatives. The non-bus-bar structure eliminates welding stress which can cause tiny cracks and harm panel performance. The encapsulation of the MWT cell to panel reduces series resistance and working temperature, which improves panel reliability and is more appropriate for ultrathin wafer application.
Among conventional cell and panel technologies, MWT is quite unique, not only because of its technical characteristics, but also for its coupling of cell and panel. MWT can be combined with other technologies like PERC, HJT and TOPCon to achieve higher efficiency, the result being a hybrid solution, making it difficult to handle for those only focusing on cell or panel.
With high technical barriers, there are not many manufacturers utilizing MWT technology around the world, only some located in Europe, the US and China. Among these, Sunport Power, a China based leader in cell and panel manufacturing, is the only player with GW level MWT panel operations.
MWT fits thinner wafer
Benefitting from its unique encapsulation structure, MWT enables a solar cell to utilize thinner wafers to 130μm or even less while maintaining high production yield. According to Sunport Power, it has been proven that the encapsulation of an MWT panel can support super thin wafers with a thickness of only 100μm, 43% thinner than the current mainstream thickness of 175μm. The thickness Sunport uses in its existing MWT + PERC production lines for mass production output is currently 150μm, with plans to reduce this to 140μm in the near future.
Combination with other cell technologies
Considering the very close efficiency ceilings of current technologies, the winner might be that offering the best cost control. As a linking technology of cell and panel, MWT can easily combine with most other cell technologies, including HJT and TOPCon.
Researchers at Sunport have successfully combined MWT and PERC in mass production with almost the same yield as with PERC only, with lower costs if a thinner wafer is used. As with the combination with PERC, according to a study by the Netherlands Organization for applied scientific research (TNO), the combination of MWT and HJT can also increase total conversion efficiency by up to 4% compared to HJT only. MWT can additionally reduce consumption of silver paste, a major cost factor for the HJT process, pushing the advantage of HJT to a higher level.
The CPIA roadmap has indicated possible future technology routes. However, regardless of whether TOPCon or HJT wins, or both of them are applied, MWT will take on the role of an indispensable assistant and might become the real winner.