4 tert butylpyridine

The compact TiO₂ layer was prepared by adding a weak acid (12 M hydrochloric acid) isopropanol titanium solution to anhydrous ethanol (10 mL). The solution was prepared by 1 : 6 mass ratio of mesoporous titanium dioxide and anhydrous ethanol. For the deposition of the mixed formamidinium–Cs lead iodide (Cs_xFA_1−xPbI_1.80Br_1.20) films, Cs_xFA_1−xPbI_1.80Br_1.20 solution was prepared by precursor solutions containing FAI (1.3 M), CsI (1.3 M), PbI₂ (1.3 M) and PbBr₂ (1.3 M) in a DMF : DMSO ratio of 4 : 1 (v/v). The solution was then stirred for 5 minutes and heated at 50 °C until it was completely dissolved, and formed a transparent and uniform yellow solution. The spiro-OMeTAD solution was prepared by dissolving 74 mg spiro-OMeTAD, 28.5 μL 4-tert-butylpyridine (Sigma-Aldrich), 17.5 μL of a stock solution of 520 mg mL⁻¹ lithium bis-(trifluoromethylsulphonyl) imide (Sigma-Aldrich) in acetonitrile (Sigma-Aldrich), and 29 μL of a stock solution of 200 mg mL⁻¹ tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)-cobalt(iii) tris(bis(trifluoromethylsulfonyl) imide) (Sigma-Aldrich) in acetonitrile in 1 mL anhydrous chlorobenzene.