Lead 2 bromide

1-Octadecene
(ODE, tech, 90%), oleic acid
(OA, tech, 90%), benzoic acid (BA, 99%), oleylamine (OLA, tech, 70%),
lead(II) bromide (PbBr₂, 98%), and cesium carbonate (Cs₂CO₃, 99%) were purchased from Sigma-Aldrich. The
reagents were used as received without any further experimental purification.
ODE and OLA were degassed before use.

Lead(II) bromide (PbBr₂, ≥98%) and N,N-dimethylformamide (DMF, anhydrous, 99.8%) were purchased from Sigma-Aldrich; methylammonium bromide (MABr) was purchased from Dyesol. All chemicals were used without any further purification. Glass substrates were cleaned in acetone and isopropyl alcohol for 10 min by sonication. The cleaned glass substrates were treated with Oxygen plasma for 10 min before the perovskite deposition. An equimolar solution of PbBr₂ and MABr was prepared in DMF (20 wt%) and spin coated on the substrate at 3000 rpm for 60 s, and immediately annealed at 100 °C for 15 min, under inert atmosphere. In order to avoid photo-degradation, the sample was encapsulated by a thick polymethyl methacrylate (PMMA) layer and measured in air, which hampers the formation of intra-gap defect states. The details of the experimental setup are reported in Supplementary Note 1.

Cesium carbonate (Cs₂CO₃, 99.995% trace-metal basis), lead oxide (PbO, 99.999% trace-metal
basis), lead(II) bromide (PbBr₂, 98%), oleic acid (OA,
technical grade 90%), oleylamine (OAm, technical grade 70%), 1-octadecene
(ODE, technical grade 90%), toluene (anhydrous, 99.8%), and hexane
(95%) were received from Sigma-Aldrich. Hydrobromic acid (HBr, 48%)
was purchased from VWR Chemicals BDH. OA and OAm were dried with molecular
sieves under an argon atmosphere before use. Other chemicals were
used as received.

Lead (II) bromide (99.999%), N, N′-dimethylethylenediamine (99%), hydrobromic acid (48 wt.% in H₂O), octanoic acid (98%) and hexane (98.5%, mixture of isomers) were purchased from Sigma-Aldrich. All reagents and solvents were used without further purification unless otherwise stated. Spectroscopic grade solvents were used in the UV-Vis and photoluminescence spectroscopic measurements.

Indium-doped tin oxide (ITO) substrates (15 Ω/sq) were purchased from Nippon Sheet Glass (Osaka, Japan). N,N-dimethylformamide (DMF; 99.8%), dimethyl sulfoxide (DMSO; ≥99.9%), lead(II) bromide (PbBr₂; 99.9%), and 2-propanol (IPA;99.5%) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Lead(II) iodide (PbI₂; >98.0%) and cesium iodide (CsI; >99.0%) were purchased from TCI (Tokyo, Japan). Formamidinium iodide (FAI), methylammonium bromide (MABr) were purchased from GreatCell Solar (New South Wales, Australia). Bathocuproine (BCP; 99.5+%) and fullerene (C₆₀) were purchased from Nano-C (Westwood, MA, USA), and nanoparticles (NiCT-7, 2.5 wt% NiO in Ethanol) were purchased from Nano Clean Tech (Seoul, Republic of Korea). SAM materials (HC-A1 and HC-A4) were synthesized following previous work [27 (link),28 (link)], and ethanol (>99.9%) and tetrahydrofuran (THF; 99%) were purchased from EMSURE® (Merck, Darmstadt, Germany) and JUNSEI (Kyoto, Japan), respectively.