Low-Temperature-Processed Transparent Electrodes Based on Compact and Mesoporous Titanium Oxide Layers for Flexible Perovskite Solar Cells

The demand for lightweight, low-cost, flexible optoelectronic devices poses unique challenges in terms of material selection, mechanical stability, and fabrication processes. The use of flexible substrates requires low-temperature processing and a replacement of brittle transparent conductive oxides such as indium–tin-oxide (ITO), commonly used as transparent electrodes. To this end, dielectric–metal–dielectric (DMD) stacks have emerged as a compelling alternative to ITO. In this study, sputter-deposited, niobium-doped TiO2 (TNO) is investigated for the design of such flexible transparent DMD electrodes. Using DC magnetron sputtering from a conductive oxide target allows for high deposition rates, resulting in transparent TNO films even in inert Ar atmosphere and without substrate heating. The combination with ultrathin Ag and Au layers yields flexible DMD electrodes on poly(ethylene terephthalate) (PET) substrate with visible transmittance above 0.70 and sheet resistance below 10 Ω/sq. Bending tests show superior mechanical stability under tensile stress throughout 3000 bending cycles and down to 3 mm bending radius, proving their compatibility with roll-to-roll production requirements and flexible devices. Further, this study shows that the developed DMD electrodes can be ideally combined with a low-temperature and solution-processed mesoporous TiO2 layer to act as electron transport layer for subsequent implementation in perovskite solar cells. As a proof-of-concept, this approach yielded indium-free, flexible perovskite solar cells on PET, with a power conversion efficiency of ∼8%.