2, University of Tsukuba, Tsukuba, Ibaraki, Japan
The tunable bandgap, high efficiencies, and inexpensive fabrication techniques of perovskites coupled with the high performance and reliability of silicon create an intriguing partnership for a tandem solar cell. In such two-terminal, series-connected, monolithic cells, indium tin oxide (ITO) is commonly used as a transparent electrode due to its high conductivity and transparency to broadband wavelengths. However, sputtering ITO onto the soft, organic perovskite and electron transport layers induces degradation, leading to poor device performance. This work describes the development of an ITO nanoparticle buffer layer deposited via spray deposition to mitigate such damage. ITO has not been previously utilized in this capacity but is a promising candidate material due to its matching work function with the adjacent electrode. Similarly to perovskites, crystalline silicon wafers passivated with hydrogenated amorphous silicon experiences sputter damage, which results in severe carrier lifetime reduction. Consequently, silicon represents a convenient model system for development of these buffer layers intended for subsequent use on perovskites. Films of ITO nanoparticles deposited onto high-lifetime silicon wafers exhibited 40% absolute higher lifetime compared to bare wafers without the buffer layer following ITO sputtering. Additionally, these films demonstrated nearly identical transmittance to glass in the visible range and >80% transmittance at 1200nm. The continuity of these films has been confirmed by scanning electron microscopy and they exhibit a high degree of uniformity across areas typical of perovskite cell. During deposition, the solvent containing the ITO nanoparticles is completely evaporated and has no interaction with the substrate, a crucial step in limiting degradation. Before deposition, the nanoparticles are synthesized using solution chemistry, allowing flexibility of ITO particle diameter and ligand properties. The film porosity and thickness can also be tuned thereby providing further opportunities for process optimization. Future work includes additional optical and electrical characterization as well as full integration into perovskite-silicon tandems with subsequent characterization.