Contribution by: Christian Buchner – SCHMIDT
PERC technology is for sure one of the recent hottest topics for solar cell manufacturers. It has been extensively discussed on all recent conferences and shows and after all, it has found its way into mass manufacturing. Today high-efficiency mono and multicrystalline cells and modules are commercially available from different suppliers.
Mono based modules have even passed the magical barrier of 300Watt for a 60 cells module. The main topics in all presentations and discussions mainly focused on either the differences and potentials of the two main passivation technologies Al2O3 vs. Oxynitride or the different Al2O3 deposition methods PECVD vs ALD.
However, the manufacturing reality shows many more issues to solve in order to achieve real high efficiency. Besides using the right emitter design and related pastes to run the right firing temperatures that will not destroy the passivation again, proper cleaning and polishing of the rear side is essential. Taking a look at the landscape of PERC producers show that the leading ones in terms of efficiency are seeking new tools for junction isolation, rear polishing, and cleaning.
Due to the outstanding performance of the recent advancements in wet tools manufacturers are able to achieve the best passivation result their particular passivation process can deliver. Having achieved this, a cell performance loss analysis shows that now the rear side is not limiting anymore as it was with a standard Al-BSF.
Now the front side is back in focus since it is limiting efficiency and exactly for this reason, equipment providers are forecasting an extremely increasing demand for the selective emitter technology, which is the only proven manufacturing method using an ink-jetted mask followed by a wet chemical back etch. Supported by modern pastes for fine line printing which commonly also has an increased number of fingers on the front electrode etched backed emitters can go up as high as 160Ω/sq without sacrificing fill factor, but supporting efficiencies on mono PERC cell over 21%. Besides the efficiency advantage, another advantage of this process is that the emitter underneath the finger is still broad and low sheet resistance region, which extremely eases paste and firing parameter selection.
Having adjusted the front again with selective emitter leading manufacturers offers a second solution for boosting PERC to its full power. Taking a closer look to the rear side again brings attention to the bus bars. Although commonly quite small due to padded bus bar designs used today still in sum the rear bus bar is quite an area that shows increased recombination velocities due to insufficient passivation. An easy solution for this is a system, which replaces the rear bus bar screen printer that applies a thin tin layer on top of the aluminum to provide a solderable busbar. Such the rear can be fully covered with Aluminum and no passivation disturbance by the typical Silver/Aluminum bus bars, which typically contacts the Silicon rear directly, is introduced.
Summarizing the above, SCHMID offers three important solutions to achieve the maximum efficiency in a PERC manufacturing line, all proven in mass manufacturing and easy to upgrade.