Caesium iodide

Lead (II) iodide (PbI₂, 99.99%) was purchased from TCI America, while N,N dimethylformamide (DMF), dimethylsulfoxide (DMSO), anhydrous ethanol, toluene, chlorobenzene, methylammonium bromide (MABr), lead (II) bromide (PbBr₂), caesium iodide (CsI), methylammonium iodide (MAI, 99.9%), and formamidinium iodide (FAI, > 98%), tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)cobalt(III) tris-(bis(trifluoromethyl sulfonyl)imide) (FK209). were purchased from Sigma-Aldrich. The 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) was obtained from Lumtec. Finally, fluorine-doped tin oxide (FTO) coated glass substrates (dimensions: 25 mm × 25 mm × 2.2 mm; sheet resistance: 7–8 Ω sq⁻¹), were purchased from Zhuai Kaivo, China. All chemicals were used as received.

All solvents were purchased from Sigma Aldrich. PbI₂ and PbBr₂ were purchased from TCI Chemicals, MAI and FABr were purchased from Ossila, and Caesium Iodide was purchased from Sigma Aldrich. The stoichiometric inks prepared contained 467.5 mg mL⁻¹ of PbI₂, 167.2 mg mL⁻¹ of MAI, 68.3 mg mL⁻¹ of PbBr₂ and 18.8 mg mL⁻¹ of MABr. These were weighed out and dissolved in each of the relevant solvents (DMF/DMSO or DMSO). A stock solution of CsI was prepared in DMSO (389 mg mL⁻¹), with 40 μL of this solution then added to the solution. The final stoichiometry of the resultant solutions was Cs_0.05FA_0.81MA_0.14PbI_2.55Br_0.45.

Lead iodide (PbI₂, 99.999%), lead bromide (PbBr₂, 99.999%), caesium iodide (CsI, 99.999%), bis(trifluoromethane)sulfonimide lithium salt (Li-TFSI, 99.95%) and 4-tert-butylpyridine were purchased from Sigma Aldrich. Formamidinium iodide, methylammonium (MA) bromide and methylammonium chloride (MACl) were synthesized in house by reacting equal molar amounts of formamidine (FA) acetate and methylamine alcohol solution with the corresponding halogen acid³³ . PSP was synthesized in a laboratory according to the method mentioned in the Supplementary Information. PTAA, 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (spiro-OMeTAD) and BCP were purchased from Lumtec. Tin (IV) oxide (SnO₂, 15% in H₂O colloidal dispersion) was purchased from Alfa Aesar. Fullerene (C₆₀) was purchased from Nano-C. All solvents used in the experiments, including N,N-dimethylformamide (99.8%), dimethyl sulfoxide (anhydrous, 99.9%), chlorobenzene (anhydrous, 99.8%), acetonitrile (99.9%) and isopropyl alcohol (anhydrous, 99.8%), were purchased from Sigma Aldrich. All chemicals were used as received without any further purification.