The project focused on the development of nickel‑oxide (NiO) and zinc‑oxide (ZnO) tube targets for sputter deposition of the interfacial layer that connects a silicon bottom cell with a perovskite top cell in tandem solar modules. The interfacial layer must provide efficient hole transport while remaining compatible with the high‑temperature sputtering process required for the perovskite layer. To achieve this, the team investigated two distinct deposition routes for NiO: a reactive sputtering process from a metallic nickel tube and a direct sputtering from a ceramic NiO tube. The ceramic route proved advantageous because it eliminates the need for a strong magnetic field that would otherwise be weakened by the metallic nickel. Plasma spraying was employed to deposit the high‑melting‑point oxide onto a stainless‑steel substrate, and an iterative optimisation of process parameters was carried out to meet the key performance targets. The resulting NiO targets exhibited electrical conductivity sufficient for DC or AC sputtering, a high density that reduced porosity, and a residual magnetism below the threshold that would interfere with the sputtering magnetron. The sputter yield of the optimized targets exceeded 70 %, and the target thickness could be increased without compromising the deposition rate. Contamination control was a critical aspect; fine NiO particles were minimized to protect operators from the carcinogenic material, and the process was tuned to keep the temperature rise in the plasma plume to a minimum, thereby allowing the deposition of thicker targets without overheating the chamber.

Parallel to the NiO work, the project addressed the use of ZnO as a low‑cost alternative to indium‑tin‑oxide (ITO) for the transparent conductive oxide (TCO) layer. Industrially, aluminium‑doped ZnO (AZO) tube targets are already used for CIGS cells and for the silver seed layer in architectural glass. However, the higher sputtering temperatures (300–350 °C) required for perovskite deposition impose stricter thermal limits on the bond interface. The team performed bond tests using a Freiloc® bonding scheme and identified new material combinations that improved heat dissipation. As a result, the application limit of bonded ZnO targets was increased by roughly 50 %, enabling higher sputter powers without exceeding the thermal budget of the substrate. The ZnO targets also maintained the necessary electrical conductivity and optical transparency for the perovskite top cell.

The technical achievements of the project provide a foundation for industrial production of tube targets from carcinogenic materials, a capability that expands the product portfolio of the lead company, GfE Fremat GmbH. The project was carried out within a consortium that included several partners, though only GfE Fremat is listed as the executing institution. The consortium’s collaborative framework facilitated the exchange of expertise in sputtering technology, materials science, and bonding processes. The project was funded by the Federal Ministry of Economic Affairs and Climate Action (BMWK) under the grant code 03EE1118C and concluded on 31 May 2023. The results, while kept confidential for competitive reasons, are intended for use in sales and technical support activities and will contribute to the development of next‑generation perovskite tandem solar cells with reduced reliance on expensive ITO layers.