Layer-by-layer assembly of CsPbX3 nanocrystals into large-scale homostructures

Advances in surface chemistry of CsPbX3 (where X = Cl, Br or I) nanocrystals (NCs) enabled the replacement of native chain ligands in solution. However, there are few reports on ligand exchange carried out on CsPbX3 NC thin films. Solid-state ligand exchange can improve the photoluminescence quantum yield (PLQY) of the film and promote a change in solubility of the solid surface, thus enabling multiple depositions of subsequent nanocrystal layers. Fine control of nanocrystal film thickness is of importance for light-emitting diodes (LEDs), solar cells and lasers alike. The thickness of the emissive material film is crucial to assure the copious recombination of charges injected into a LED, resulting in bright electroluminescence. Similarly, solar cell performance is determined by the amount of absorbed light, and hence the light absorber content in the device. In this study, we demonstrate a layer-by-layer (LbL) assembly method that results in high quality films, whose thicknesses can be finely controlled. In the solid state, we replaced oleic acid and oleylamine ligands with didodecyldimethylammonium bromide or ammonium thiocyanate that enhance the PLQY of the film. The exchange is carried out through a spin-coating technique, using solvents with strategic polarity to avoid NC dissolution or damage. Exploiting this technique, the deposition of various layers results in considerable thickening of films as proven by atomic force microscope measurements. The ease of handling of our combined process (i.e. ligand exchange and layer-by-layer deposition) enables thickness control over CsPbX3 NC films with applicability to other perovskite nanomaterials paving the way for a large variety of layer permutations.

Synthesis of CsPbBr3 Nanocubes. NCs were synthesized via hot injection (165°C) of a cesium oleate precursor (500 uL) into a lead bromide (72 ± 2 mg) solution in 1-octadecene (ODE) (5mL) in presence of surfactants: oleic acid (50 uL) and oleylamine (500 uL). The injection was carried out during cooling towards room temperature under stirring (no ice bath). When the solution reached 30°C, it was transferred in 4 vials and centrifuged at 4000 rpm for 3 minutes. Supernatant was discarded and the walls of the vials dried with a swab to remove excess of solvent. Then, precipitate was centrifuged once more at 4000 rpm for 1 minute and dried again. Finally, precipitate was redispersed in 1 mL of toluene for each vial.
Preparation of Stock Solution of RNH3I (OLA-HI). 10 ml of OLA and 1 ml of HI were loaded in a 25 ml three neck round bottomed flask. Then the solution was heated at 120 °C for 2 hour with nitrogen gas purging to remove water present in the acid. The solution was then collected under hot condition in an airtight syringe and stored in a deaerated 30 ml vial fitting with a screw type septa. This stock solution was solidified at room temperature and for every reaction this stock solution was heated to 80°C and the desired amount of the melted stock solution was introduced in the reaction system. Synthesis of CsPbI3 Nanocubes. 92,2 mg of PbI2 and 5 ml ODE were loaded in a 25 ml round bottomed flask and deaerated at 120 °C for 1 hour by purging nitrogen gas. Then 0.5 ml each of OLA and OA were injected into the reaction flask. 0,5 mL of pre-heated stock solution of OLA-HI was injected to the reaction system, and it was kept under heating condition for 10 minutes.
Then the temperature was increased to 260 °C. Once the solution became clear, 0.5 ml Cs-oleate was injected swiftly. The reaction was stopped by removing heating mantel after 1 min of annealing and then it was allowed to cool down to room temperature naturally. The crude solution was taken in a centrifuge tube and centrifuged at 6000 rpm for 15 min at temperature of 17 °C.
After centrifugation, the supernatant solution was discarded carefully and redispersed in hexane.
To re-precipitate further, methyl acetate (hexane:methylacetate = 1:4) was added to the dispersed solution and again centrifuged at 6000 rpm for 2 min. The precipitate was redispersed in hexane. Photoluminescence quantum yield. PLQY measurements were carried out on pristine, ligandtreated NCs and multiple layer films with an Edinburgh Instruments fluorescence spectrometer (FLS920), which included a Xenon lamp with a monochromator for steady-state PL. The PL spectra recorded from films were obtained with an excitation wavelength of 400 nm. A calibrated integrating sphere was used to record the PLQY values from films on ITO/glass substrates.

SOLUTION PHASE LIGAND EXCHANGE OF CsPbBr3 NANOCRYSTALS.
Solution phase ligand exchange was carried out according to the procedure reported in the Experimental Section. X-Ray diffraction pattern show a cubic phase for the treated sample as well as the pristine one ( Figure S1 a,b,c). In Figure S1 d,e,f we report the size distributions for pristine, DDAB and NH4SCN treated samples respectively. Abs/PL spectrum of DDAB and NH4SCN treated samples present an excitonic peak at 504 nm ( Figure S2a). PL emission of the NH4SCN treated sample blue shifts by 3 nm to 508 nm (FWHM = 15 nm, Figure S2a)

SOLID PHASE LIGAND EXCHANGE AND LAYER-BY-LAYER PROCEDURE.
A concentrated solution of CsPbBr3 is obtained by mixing 2/3 batches of reported synthesis and leaving the obtained solution in the fridge (4°C) for one night. For an optimal result, once taken the supernatant, this procedure is repeated twice. A variable amount of counter-solvent (preferably ethyl acetate or acetonitrile) is added until the solution becomes turbid. After that, it is centrifuged at 5000 rpm for 6 minutes and redispersed in 400 μL of toluene.
A variable amount of obtained solution (depending on substrate dimension) is deposited on ITO/glass and spin-coated at 1500 rpm for 2 minutes. For DDAB treatment, 6 mM DDAB solution in methyl acetate is drop-cast on the nanocrystal film and left spinning for 2 minutes. Same procedure for 0.01 mM of NH4SCN solution in methyl acetate. From now on, it is possible to deposit again the concentrated solution on CsPbBr3 and repeat the ligand treatment 3/4 times until the desired film thickness is obtained. Optical density measured at every step of deposition are reported in Figure S3 a and b respectively for the DDAB and NH4SCN treatment.

S3:
Optical density spectra recorded with increasing number of layers deposited following Layer-by-Layer assembly for DDAB (a) and NH4SCN (b).