In Heterojunction (HJT) solar cells manufacturing, deposition, which is the heart of the processing, has two subsections. While previous articles The Core of HJT and Reducing HJT Costs With CAT-CVD have discussed the various important aspects of the core layer deposition, an overview on the second half of deposition that deals with the application of the transparent conductive oxide (TCO) is provided here. TCO acts as both an antireflection coating and a conductive electrode to extract and laterally conduct the electrical current. These thin films are usually applied on both sides. While the majority of HJT aspirants aim at gaining from the bifacial aspect, one can apply silver on the rear side to make it monofacial.
Applying TCO is a key step in HJT processing. Here, enough care must be taken to retain the passivation properties of the underlying amorphous silicon layers. The quality of the TCO influences lateral charge collection. In addition, the transparency and resistivity of the TCO are also very important. The TCO application is typically accomplished in PVD tools by means of sputtering, and indium tin oxide (ITO) is the most commonly used material for it.
There are a larger number of suppliers of PVD systems including Von Ardenne, Singulus, GS-Solar, Ulvac and Maxwell. The most important development associated with these tools is high throughput and flexibility to process different wafer formats.  Today’s PVD tools can support a throughput level of up to 10,000 wafers per hour referring to M6 wafer size and most of the equipment suppliers are offering machine platforms that are flexible to process three important wafers sizes – M6, M10 and G12.
Reactive Plasma Deposition (RPD) is an alternative method. RPD utilizes an evaporating mechanism for coating the target material on the wafer surface without damaging the existing films. Typically, indium tungsten oxide (IWO) targets are used in this process. Japan’s Sumitomo holds the patents for RPD along with the target. Archers had licensed this technology in the past. S.C. New Energy has also acquired a license to use and develop the technology further. However, RPD is typically a single side process, which necessitates the need for two of such systems. For this, S.C New Energy offering an extensive configuration, one single side RPD system, a machine platform integrated with two RPD systems and a combination RPD and PVD in one system.
The most important specs of the tools from both streams – PVD and RPD – are summarized in the table (see above).
For detailed analysis on TCO deposition tools used for HJT production, please check TaiyangNews report on Heterojunction Solar Technology 2020, which can be downloaded for free here.