Device preparation for single-particle EL imaging and spectroscopy

The EL devices were prepared on microscope cover slides (22 × 22 mm, 0.15 mm thickness) on which an ITO electrode (56.2 nm thickness, 100 Ω sq⁻¹; Geomatec) has been custom-deposited by sputtering. The ITO layer was etched to form stripes 5 mm in width. The substrates were pre-cleaned by sonicating in acetone and 1-propanol. A 58 nm hole-injection layer of poly(3,4ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS, Clevios P AI4083, H. C. Starck) was spin coated (4000 r.p.m., 40 s) on the ITO substrate and baked at 200 °C for 90 min. The active layer is a mixture of PVK and 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD, B8378-5G; Merck) at a ratio of 7:3, doped with the appropriate concentration of the suspensions of CsPbBr₃. The layer was spin-coated from chlorobenzene to form a film of 52 nm thickness. The sample was placed in a vacuum chamber where a 15 nm electron-transport layer of 2,2′,2″-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi), 1 nm layer of LiF and an Al electrode (200 nm) were subsequently evaporated. The Al electrode was deposited in form of stripes 5 mm in widths, orthogonal to the ITO stripes. The device element area was 0.25 cm². The device was used for experiments immediately.