Multifunctional Ti(Al)Ox layers for silver and indium-free perovskite/silicon tandem solar cells

Perovskite solar cells are one candidate to enable higher photovoltaic conversion efficiencies in tandem solar cells at reasonable costs for terrestrial applications in the future. Currently, these cells require indium in the transparent conductive oxide (TCO) layer and are typically metallized using silver. Alternatives for these scarce materials are needed in a fast-growing PV-industry. A novel process concept, which was previously demonstrated on silicon heterojunction (SHJ) solar cells has been realized for the first time on perovskite/silicon tandem solar cells (PSTSCs). A thin layer of a so-called valve-metal, in this case titanium or titanium-aluminum, is sputtered onto the TCO layer. The contact grid is applied onto this metal layer and the metal layer between the grid positions is then transformed into its transparent oxide in an electrochemical anodization process. By choice of suitable process conditions and valve-metal layer composition, the refractive index of the resulting layer can be tuned to match the optical requirements of the resulting solar cell. PSTSCs with various valve-metal, TCO and metallization combinations have been investigated, showing that this approach has the potential to yield optically competitive surfaces without using any scarce materials. In the presented experiment, the created solar cells exhibit increased series resistance and low shunt resistance, limiting the fill factors to ∼50 % as compared to around ∼75 % for standard reference groups. The reason behind this limitation needs to be understood in the future.