The observed phenomena were explained by detailed structural analysis. The PXRD patterns showed differences between the interlayer thicknesses of the perovskites obtained from EA (1.06 nm) or PEA (1.86 nm) alone and those containing a mixture of one of the amines and A-POSS (1.71 nm regardless of the type of organic amine). Increasing the amount of EA resulted in a product of bimodal structure with the layer distances of 1.06 nm (EA) and 1.71 nm (A-POSS or a mixture of A-POSS and EA), as indicated by the characteristic diffraction patterns. The results of the N
2 sorption studies corroborated the PXRD data. All the hybrid materials containing A-POSS (also those admixed with EA) contained micropores smaller than 2 nm and sorption/desorption isotherms of type I were obtained. The amount of adsorbed N
2 was greater for Cu-A-POSS/EA than Cu-A-POSS, and the micropore volume and BET surface increased linearly with the replacement ratio (x) of POSS with EA(44% increment if x = 0.202). When x was >20%, the micropore volume and BET surface area decreased and an increase in the fraction of nonporous layered perovskites was observed with PXRD (EA-derived layered perovskites are nonporous). On the contrary, the shrinkage of the interlayer pores was observed in the case of the layered perovskites separated by mixtures of the cubic silsesquioxanes and PEA (comparing to Cu-PEA). It implies the influence of π–π stacking of the aromatic amine molecules on the porosity of the layered perovskites. The reduction in the interlayer distance suggests that the ordering of PEA was altered in the presence of A-POSS. The effect allows for the precise structural engineering of the interlayer distance and, correspondingly, the properties of such complex systems.
2.2. Encapsulation with POSS to Increase Brightness and Stability of Perovskites
Perovskite nanocrystals of the CsPbX
3 structure are promising and valuable emissive materials in electroluminescent devices. However, the quality of an as-prepared film of NCs may be relatively poor due to the presence of the long-chain surface ligands used during the coating process, which hampers the efficiency of the charge injection. The processing problems may lead to uneven, patchy film coverage over the device sublayers. Perovskite nanocrystals can be protected from moisture by encapsulation in a hydrophobic polymer matrix, which provides a physical barrier
[45,46,47,48][42][43][44][45]. However, the formation of stable perovskite-polymer composites is not trivial due to the phenomena of phase separation and nanocrystal agglomeration.
Functionalized POSS can be a solution in the engineering of perovskite materials to enhance the low stability of the ionic nanocrystal lattice. Hydrophobic polyhedral silsesquioxanes of the (RSiO
3/2)
7(R′SiO
3/2)
1 (R = i-Bu and R′ a functional group) structure can play a beneficial role as materials that can encapsulate the perovskite NCs or form an intermediate hydrophobic passivation layer for their thin films. Monofunctional POSS of this type can improve the surface coverage and the morphological features of the perovskite films and possibly improve their miscibility with polymer matrices. For example, molecules of (iBuSiO
3/2)
7[HS(CH
2)
3SiO
3/2] were used as a surface protecting additive to the CsPbX
3 (X = Br or I) nanocrystals
[49,50][46][47]. However, low amounts of POSS were required since the silsesquioxane cages may act as insulators.
The treatment provided the moisture resistant hybrid perovskite nanopowders-quantum dots (PQD) that can be used as solid state luminophores in all-perovskite white light-emitting devices. The hybrid silsesquioxane molecules acted as a hole-blocking layer between the thin coating made of the perovskite NCs and the 1,3,5-tris(N-phenylbenzimidazol-2-yl) benzene (TPBi) film that operated as the electron-transporting layer. The POSS-PQD exhibited (HRTEM and PXRD) the lattice plane distance of 0.58 nm, characteristic for cubic phase CsPbBr
3 perovskite, and high output performance. Moreover, the silsesquioxane coating prevented anion exchange between the perovskite nanocrystals in the solid state, thereby increasing the stability of the mixtures of the perovskite NC powders with the different halide compositions. The distinct emission spectra of the different POSS-passivated CsPbX
3 were preserved, while the uncoated NCs underwent ion exchange, resulting in a broadening of their characteristic PL signals in the solid state. It should be noted that their PLQY in toluene solutions slightly decreased to 62% (X = Br) and 45% (X = Br/I) upon passivation with POSS, but the absolute PLQY in the solid state did not change and remained very high (respectively, 61% and 45%).
This strategy yielded single layer, all-perovskite devices that emitted white light by mixing the nanopowders of the green-emitting POSS-CsPbBr
3 and the red-emitting POSS-CsPb(Br/I)
3. The POSS-passivated perovskites were thus used as solid state luminophores for the fabrication of all-perovskite (a single down-conversion layer) white LEDs with a CIE chromaticity coordinate of (0.349, 0.383), CRI = 81, and luminous efficiency of 14.1 lm W
−1. The characteristic electroluminescence spectrum is a combination of the three emission peaks (the blue one originated from the blue-emitting InGaN LED chip).
The improvement in the morphology and coverage of the CsPbBr
3 NCs in the presence of POSS did not always lead to luminance enhancement. If the POSS molecules were present in the active layer, they acted as insulators. As a result, the external quantum efficiency (EQE) value was not high and the average PL lifetimes were reduced (from 434 to 134 ns, and from 115 to 63 ns for the suspension and supernatant solutions, respectively). Nevertheless, with the optimized POSS concentrations, higher loading of the separated NCs resulted in an increase in the overall LED brightness.
It was more beneficial to use POSS as a separate layer on top of the active perovskite NC layer. In this case, the average PL decay time only decreased from 434 to 342 ns, and the average recombination rate increased by 21%. The effect of the upper POSS layer was attributed to the more efficient electron and hole recombination in the NCs zone and the blocking of hole transport between the perovskite NCs and the TPBi layers. In this case, the peak LED luminance was almost eight times higher (2983 cd/m
2 at 11.5 V; LE = 1.20 cd/A; EQE = 0.35%) than that obtained without the hole-blocking layer. In addition, POSS enhanced the stability of the LED devices and their operation lifetime was five times longer.
MA-NCs can further participate in radical polymerization reactions. The copolymerization with methyl methacrylate and/or (iBuSiO
3/2)
7[(H
3C)H
2C=CC(O)O(CH
2)
3SiO
3/2] yielded composite materials (PMPNC and PMPOPNC, respectively). Their diffraction patterns corresponded to that of the orthorhombic perovskite NCs crystals. The thin films cast from the dispersions of PMPOPNC in the organic solvents contained well-dispersed crystalline nanoparticles with a size of ∼12 nm, embedded within the amorphous PMMA-
co-P(ME-POSS) (PMPO) matrix. No phase separation was observed. The nanoparticles of PMPOPNC were larger (average size 68 nm) than the free ME-NCs and their size distribution was wider. The PL spectra of PMPNC and PMPOPNC corroborated those of the free ME-NCs and the ME-NC/PMMA composites (λ
e at 516 nm). Their PLQY (respectively, 68% and 72%) was similar to that of the free ME-NCs (PLQY above 80%) and larger than the PLQY of the MENCs/PMMA blend (ca. 54%). The emission wavelength and PLQY of the PMPOPNC can be tuned by adjusting the ratio of halides in the perovskite structure CsPbX
3 by using different lead halide precursors.
The PMPOPNC film was hydrophobic (static water contact angle of 120°, comparing to 105° of PMPNC and 104° of ME-NCs/PMMA blend). The effect was assigned to the micro-nanorough features revealed in the SEM micrographs, caused by the migration of the POSS-containing fraction to the air-solid interface (confirmed by XPS gradient concentration data).
2.3. POSS As a Moisture Barrier in Perovskite Films
As was mentioned in the previous section, apart from the employment of the POSS coated perovskite NCs as solid state luminophores, the large size of the POSS macromolecules and the presence of hydrophobic organic groups grafted to Si atoms can be an advantage with regard to their barrier action against moisture. The significant potential in this area is offered by monofunctional polyhedral silsesquioxanes of type (RSiO
3/2)
7(R′SiO
3/2), where R = iBu and R′ is a reactive organic residue that enables the adsorption of POSS on the surface of perovskite crystals or thin films. The passivated perovskites can be applied as water resistant light-emitting materials.
It was also shown that [3-(2-aminoethyl)amino]propyl-heptaisobutyl substituted POSS (iBuSiO
3/2)
7[H
2N(CH
2)
3SiO
3/2] (POSS-NH
2) can be applied as a capping ligand for (CH
3NH
3)PbBr
3 (MAPbBr
3) (MA—methylammonium)
[52][48]. POSS-NH
2 passivated the surface of the MAPbBr
3 NCs, controlling the crystal size and increasing the perovskite material stability in the LED devices. The importance of the presence of sterically demanding POSS ligands was demonstrated by the comparison of the structure and properties of hybrid NCs with their analogs modified with (3-aminopropyl)triethoxysilane (APTES) as the caping ligands.
POSS-NH
2 made the surface of a thin layer of the perovskite NCs waterproof, influenced the structure of the film, and helped to tune its optoelectronic behaviour
[53][49]. The photovoltaic performance with a power conversion efficiency (PCE) over 20% was observed. It was also shown that POSS-NH
2 can effectively passivate the surface of thin coatings of NCs composed of the mixed halide perovskites, MA
xFA
1−xPbI
3−yBr
y (FA—formamidinium), as well as influence the crystal grain boundary and the number of ionic defects
[54][50]. The temperature-dependent admittance measurements proved that the presence of POSS-NH
2 reduced the charge trap density and, as a result, the trap-state energy level of 0.045 eV was achieved. Those features were reflected in an enhancement of the open-circuit voltage (V
OC) and power conversion efficiency from 18.1% to 20.5%.
As was already mentioned, the presence of POSS is beneficial, but its quantity must be optimized because the inorganic cube acts also as an insulator. It was shown that the surface composition of the FA
0.85MA
0.15Pb(I
0.85Br
0.15)
3 films and their morphology changed in a regular manner in the presence of POSS-NH
2 as a result of its interaction with the perovskite film
[55][51]. When the amount of silsesquioxane exceeded the optimum concentration (10 mg/mL), the quality of the perovskite film surface morphology was compromised by the formation of cracks. The AFM height profiles displayed by the variations in the root mean square (RMS) values show that the size of NCs changed on modification with POSS-NH
2 to a degree that depended on the amount of POSS used for the modification.
2.4. Application of Water-Resistant Hybrid POSS-Perovskite Systems
The stability of the MAPbI
3-based solar cells, based on the passivated films under ambient environment, was shown by the stable V
OC and J
SC values during 90 days. After that time, the solar cells made of the perovskites passivated by POSS-NH
2 and POSS-SH retained, respectively, 100% and 94% of their initial J
SC (fill factors around 89% and 88%, respectively, of their original performance and 72% of the initial FF for the non-passivated devices). The better protection offered by POSS-NH
2 corroborated the stronger interactions of this compound with the perovskite film. The protective effect provided by POSS was less pronounced in the case of the solar cells based on (FA)
0.85(MA)
0.15Pb(I
3)
0.85(Br
3)
0.15 with formamidinium ions embedded in the perovskite structure. Passivation with POSS played a minor role in such stable systems, which already had good moisture resistance.
[3-(2-Aminoethyl)amino]propyl-heptaisobutyl-POSS was used for the synthesis of an amphiphilic copolymer (
ap-POSS-PMMA-
b-PDMAEMA, methylmethacrylate, MMA, and 2-(dimethylamino)ethylmethacrylate, DMAEMA) further applied to prepare the stable core-shell colloidal perovskite nanocrystal-polymer micelle composites (
ap-POSS-PMMA-
b-PDMAEMA@CsPbBr
3)
[58][52]. The presence of the hydrophobic POSS-PMMA segment of
ap-POSS-PMMA-
b-PDMAEMA was crucial in the process of self-assembling into the “reverse” micelles in DMF/toluene. The reverse micelles acted as confined nanoreactor templates during the perovskite crystallization, passivating the perovskite surface with a multidentate capping shell.
3. Conclusions
Polyhedral oligomeric silsesquioxanes can be effectively used for the modification of inorganic cesium-halide perovskites CsPbX
3 (X = Cl, Br, I), both nanocrystals and thin films. Depending on the chemical structure of POSS, they can act as an interlayer component of perovskite structures (ionic octafunctional POSS) or as a passivating and structure-controlling agent of perovskite nanocrystals (bifunctional polyhedral silsesquioxanes of (iBuSiO
3/2)
7(R′SiO
3/2)
1 type). The literature reports show that the crystallization kinetics of perovskites can be modified by the presence of structure-directing POSS. This approach results in a reduction in the number of crystallographic defects in the perovskite-based materials and, consequently, an improvement/modification in their optoelectronic properties. In addition, POSS can enhance the hydrolytic and thermal stability of perovskite structures.