Fabrication of perovskite solar cells

ITO glass was cleaned by successive sonication in detergent, deionized (DI) water, acetone and 2-propanol for 15 min respectively. The cleaned substrates were further treated with UV-ozone to remove the organic residual and enhance the wettability. A 30 mM SnCl₂ ∙ 2H₂O (Aldrich, >99.995%) solution was prepared in anhydrous ethanol (Decon Laboratories Inc.) and filtered by 0.2 μm syringe filter before use. The solution was spin-coated on the cleaned ITO substrate at 3000 rpm for 30 s, which was followed by heat-treatment at 150 °C for 30 min. After cooling down to room temperature, the spin-coating process was repeated one more time, and the resulting film was annealed at 150 °C for 5 min and 180 °C for 1 h. The SnO₂ coated ITO glass was further treated with UV-ozone before fabrication of the perovskite layer. Perovskite films were prepared according to the procedure described in an above section. The spiro-MeOTAD solution was prepared by dissolving 85.8 mg of spiro-MeOTAD (Lumtech) in 1 mL of anhydrous chlorobenzene (99.8%, Sigma-Aldrich) to which 33.8 μl of 4-tert-butylpyridine (96%, Aldrich) and 19.3 μl of Li-TFSI (99.95%, Aldrich, 520 mg/mL in acetonitrile) solution were added. The spiro-MeOTAD solution was spin-coated on top of the perovskite layer at 3000 rpm for 20 s by dropping 17 μl of the solution on the spinning substrate. The device was completed by thermal evaporation of ca. 100 nm-thick silver or gold layer at 0.5 Å/s to be used as an electrode. The size of active layer was 0.130 cm².