The samples were transferred to a nitrogen-filled glovebox immediately. As a second layer, the approximately 20 nm thin PVK film was deposited by spin-coating a 3 mg/mL concentrated PVKchlorobenzene solution at 3000 rpm for 30 s and heating at 175 °C for 30 min. The LHP NC solutions were spin-coated as described for the PL samples, after cooling the substrate to room temperature, resulting in a closed film of about 2-3 monolayers (effective thickness approximately 20 nm). To build a top contact, 55 nm B3PYMPM (rate: 100 pm/s) followed by 0.5 nm LiF (rate: 10 pm/s) were evaporated. The devices were finished with a 60 nm (rate: 100 pm/s) thick aluminum cathode, also deposited by thermal deposition. This fabrication is consistent with previously made perovskite LEDs published in reference 15.