TCO Deposition In HJT

Reduction Of Indium Consumption Is A Hot Topic In HJT Processing As Aluminum Doped Zinc Oxide Emerges As Alternative To ITO For TCO Application

TCO Deposition In HJT

Elephant in the room: The industry has been evaluating various options to eliminate or reduce indium. While eliminating is not an option yet, it can be reduced by 85%. (Source: Forschungszentrum Jülich)

  • The TCO layer is important in HJT processing for extracting and conducting electrical current and its quality affects lateral charge collection
  • Indium tin oxide is the most commonly used material for TCO, but its high cost and limited availability led to consumption reduction
  • PVD tools are used for TCO application and utilization of these tools for higher throughput, ease of operation and maintenance are key factors.

After we discussed microcrystalline silicon as a promising alternative to doped amorphous silicon for HJT emitter layers in an earlier article, we now look into the second half of deposition in HJT processing (see HJT Processing: Shifting To Micro Silicon Layers).

The second part of the deposition process deals with the application of the TCO layer, which acts as both an antireflective coating and a conductive electrode to extract and laterally conduct the electrical current. Applying TCO is also a very important 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 application is typically accomplished in PVD tools by means of sputtering, and indium tin oxide (ITO) is the most commonly used material. Reactive Plasma Deposition (RPD) is an alternative method, but it is yet to enter mainstream commercial production

The hot topic in the TCO segment is how to reduce indium consumption – the key ingredient of ITO – if not eliminate it altogether. The fact that indium is only second to silver in terms of non-silicon costs of HJT cell production is what puts it in focus. Its limited availability is another issue. Aluminum-doped zinc oxide is becoming an effective alternative. It is low cost and non-toxic, but it is a relatively less inert material chemically. However, eliminating indium does not seem possible, at least as of now, according to Forschungszentrum Jülich’s Head of R&D Department of Research Center Dr. Kaining Ding, who provided a keynote on the latest updates in HJT technology development during the last TaiyangNews High Efficiency Solar Conference  (see Presentation here). Cells based on indium free TCO suffer from high level of efficiency losses, whereas Ding emphasizes that indium consumption can be reduced by 85% through the use of a stack of AZO/ITO. Approaches such as using thinner ITO with ARC are also under evaluation. This helps move the spotlight away from indium consumption issues for some time at least.

Touching a bit on the equipment side while on the topic, Maxwell’s Vice President Overseas Sales Peter Wolf said that his company’s tool platform is ready with a solution, underlining “we don’t see a shortage at the moment.” However, to be future ready in case such a need arises, Maxwell’s PVD can already use AZO and apply coatings as stacks starting with ITO; 2 layers of AZO and then ITO on top, for example.

Optimization for better target utilization has remained an important topic on the tool vendors’ end. However, the major focus in optimization at an equipment level after ITO consumption is higher throughput. There are more PVD vendors compared to PECVD, including Von Ardenne, Singulus, Maxwell, S.C New Energy, GS-Solar and H2GEMINI. Among these, Germany’s Von Ardenne has the highest throughput of 7,500 wafers per hour, followed by Maxwell at 7,200. In order to support this high throughput, Maxwell, for example, is integrating more processing units – 4 on top and 4 on the bottom. Ease of operation and maintenance without the need for industrial line cranes is also a common attribute of the PVD tools from leading suppliers.

Day-2 of TaiyangNews virtual conference om High Efficiency Solar Technology focussed on HJT, for more presentations click here.

For more details on HJT, download the TaiyangNews report on Heterojunction Solar Technology 2023 for free here.

About The Author

Shravan Chunduri

Shravan Chunduri is Head of Technology at TaiyangNews. Shravan caught the solar bug vey early in this career, starting 20 years ago in research, followed by solar manufacturing, then writing and consulting. His responsibility spans from writing technology articles and reports.

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