Synthesis of CsPbI3 perovskite nanocrystals (PNCs)
For cesium oleate solution, Cs2CO3 (407 mg), 1.25 mL of pre-degassed oleic acid (OA) and 20 mL octadecene (ODE) were put into a 100 mL 3-neck flask and degassed under vacuum at 120 °C for 1 hour. After degassing, N2 gas was filled into the flask and then the solution was elevated to 150 °C, which was ready for injection.
PbI2 (498 mg), 2.5 mL pre-degassed OA, 2.5 mL pre-degassed oleylamine (OLA) and 25 mL ODE were put into a 100 mL 3-neck flask and degassed under vacuum at 120 °C for 1 hour. After degassing, N2 gas was filled into the flask and then the temperature was increased to 180 °C.
When the temperature reached 180 °C, cesium oleate solution (2 mL) was swiftly injected into flask. After 10 s, the flask was cooled down in an ice water bath.
For purification, 15 mL of the crude solution and 35 mL of MeOAc (antisolvent) were mixed and centrifuged at 8,000 rpm for 5 minutes. After that, the supernatant was discarded and the remaining PNCs were re-dispersed in 5 mL of hexane. Then, 8 mL of MeOAc was added and centrifuged at same condition. Finally, the precipitant was re-dispersed in 5 mL of hexane.
Ligand exchange
CsPbI3 PNCs were dispersed in toluene (30 mg mL-1). 120 μL of DPAI solution (83.3 mg mL-1 in toluene:BuOH 1:1 v/v) were added into NCs dispersion.
For purification, 5 mL MeOAc was added to 1 mL of the exchanged PNC dispersion 1 mL, and centrifuged at 10,000 rpm for 5 min. The final PNCs were dispersed in toluene for LED fabrication.
Figure 1. Schematic illustrations on the surface chemistry of PNCs before (a) and after (b) SPLE process. TEM images (scale bar: 10 nm) and photographs of PNC dispersion for neat (c,e) and SPLE-PNC (d,f) with DPAI, respectively. (g) 13C solid-state NMR spectra of DPAI powder, neat and SPLE-PNCs. (h) Absorption and PL spectra of the PNC films. (i) J–V curves of hole-only devices (ITO/NiOx/PNC/MoO3/Ag) employing the PNC active
layers. The estimated NT of both PNC films are labeled.
Characterization
Solid-state Nuclear Magnetic Resonance (NMR) measurements were performed at 9.4 T (KBSI Western Seoul Center) using a 4 mm Bruker HXY probe. All samples were spun at 13 kHz MAS in zirconia rotors. Perovskite samples were packed under a nitrogen atmosphere. 13C spectra were acquired using 1H–13C Hartmann–Hahn cross polarization sequence with 100 kHz SPINAL64 1H decoupling. 13C shifts were referenced to tetramethylsilane. Transmission electron microscopy (TEM) images were taken using a JEM-2100F TEM instrument (JEOL). UV–vis absorption spectra were measured with a Shimadzu UV-2600 UV–Vis spectrometer. Steady-state photoluminescence (PL) measurements were carried out using a pulsed xenon lamp. For SCLC analysis, the NT of the PNC films was calculated from the following relationship: VTFL = qNTL2/2ε0εr, where q was the elemental charge, L was the thickness of the device, and ε0 and εr were the vacuum permittivity and dielectric constant of the film (≈6.32).