Non-Aqueous One-Pot SnO2 Nanoparticle Inks and Their Use in Printable Perovskite Solar Cells

Metal halide perovskite materials are promising candidates for printable solar cells due to their feasibility for achieving high device efficiency at a low processing temperature. One of the key challenges in printed perovskite solar cell (PSC) research is to develop low-temperature-processable charge-transporting layers for both electron and hole-transporting materials, which can be used within large-scale roll-to-roll (R2R) printing techniques. Colloidal links allow for facile deposition, provided that the size of the nanoparticles (NPs) is controlled to less than a few tens of nanometres (ideally < 20 nm); they can be deposited as uniform films and can be processed at low temperatures (typically < 140 °C). Here, we report a facile and scalable route for the synthesis of SnO2 NP dispersions using a microwave-assisted "benzyl alcohol" approach that is compatible with all of these R2R printing requirements. The method enables crystalline SnO2 NPs to be synthesized with a controlled average particle size (∼6.5 nm) and be used directly as an ink without any post-synthesis purification (i.e., one-pot synthesis). The use of these SnO2 NPs has been explored as an electron transport layer (ETL) within planar PSCs using spin-coating and thermal processing at 140 °C for 2 min, yielding devices with over 18% photo-conversion efficiency. Comparable devices were also fabricated using slot die-coated SnO2 on glass substrates and R2R-coated SnO2 on plastic substrates, yielding efficiencies of 15.3 and 10.4%, respectively. Our results demonstrate the suitability of the developed SnO2 ink to be used for the deposition of ETLs in optoelectronic devices by industrial-scale R2R printing processes.