Perovskite solar cells (PSCs) are attracting increasing commercial interest due to their potential as cost-effective, lightweight sources of solar energy. Low-cost, large-scale printing and coating processes can accelerate the development of PSCs from the laboratory to the industry. The present work demonstrates the use of microwave-assisted solvothermal processing as a new and efficient route for synthesizing crystalline SnO2 nanoparticle-based aqueous dispersions having a narrow particle size distribution. The SnO2 nanoparticles are analyzed in terms of their optical, structural, size, phase, and chemical properties. To validate the suitability of these dispersions for use in roll-to-roll (R2R) coating, they were applied as the electron-transport layer in PSCs, and their performance was compared with equivalent devices using a commercially available aqueous SnO2 colloidal ink. The devices were fabricated under ambient laboratory conditions, and all layers were deposited at less than 150 °C. The power conversion efficiency (PCE) of glass-based PSCs comprising a synthesized SnO2 nanoparticle dispersion displayed champion levels of 20.2% compared with 18.5% for the devices using commercial SnO2 inks. Flexible PSCs comprising an R2R-coated layer of synthesized SnO2 nanoparticle dispersion displayed a champion PCE of 17.0%.