Polyhedral oligomeric silsesquioxanes (POSS) of general the chemical composition (RSiO_3/2)_n are an important and extensively studied class of hybrid nanoscale materials. While the inorganic cage of the best-known POSS is usually octahedral, the chemical structure of organic functional groups R on the silicon atoms may vary (Scheme 1) [1]. The unique structure of POSS makes them a versatile component of various organic-inorganic hybrid materials. Functionalized POSS were thus applied for the synthesis of inorganic complexes and catalysts [2,3,4,5,6], including extended MOF architectures [7,8] as well as photoactive [9], self-healing [10] and self-assembling systems [11], hybrid polymer composites [12,13,14,15,16,17], and materials for biomedical applications [18,19,20,21].

Interesting results obtained in the field of materials engineering have inspired the use of polyhedral silsesquioxanes to improve the structure and properties of hybrid organo-halide perovskites. Materials based on this type of perovskite material are of great interest in the field of organic electronics, both in basic research and in a wide range of applications [22,23,24,25]. Techniques for their preparation are facile and relatively inexpensive, and thus they offer significant potential for thin-film solar cell technologies. Their characteristic structural features are sheets of organic and inorganic components alternately stacked on the molecular scale [26]. Alkylammonium cations are often used as layer separators in metal halide perovskites, acting as a barrier against magnetic and electronic coupling. Three-dimensional perovskites with the formula ABX₃ (A: amine, B: metal, and X: halide) are typically obtained when small organic amine molecules are used as the precursors of cationic species, whereas A₂BX₄ two-dimensional perovskite structures are produced when larger amine molecules are employed.

Inorganic cesium lead halide CsPbX₃ (X = Cl, Br, and I or mixed halide) perovskite nanocrystals (NCs) are dominant in this field, owing to their high photoluminescence quantum yield (PLQY, up to 90% in solution) and narrow emission peaks, as well as their easy solution processing [27]. The exceptional optoelectronic characteristics of CsPbX₃ NCs, and their photovoltaic performance, make them valuable emissive materials in electroluminescent (EL) devices, including light emitting diodes (LED), solar cells, lasers, and photodetectors [28]. Small intrinsic defects do not act as electronic trap states [29], and their characteristic emission colors can be adjusted across the visible range by tuning the size of NCs and their chemical features [30,31,32,33].

The interfacial layers play an important role in determining the efficiency and stability of perovskite structures, and various strategies have been developed to cope with the problem of surface engineering [35]. The successful application of functionalized trialkoxysilanes for the formation of siloxane networks between perovskite layers and interface passivation was reported [36,37,38,39,40]. Cage-like POSS molecules, that exhibit a characteristic tendency to self-organize, can also be a suitable interlayer component of perovskite structures. The advantage of POSS over the random silsesquioxane networks obtained in the sol-gel process with trialkoxysilanes is the uniform morphology of the inorganic layer due to the defined size of silsesquioxane nanocages.

A new class of low-dimensional layered POSS-metal halide hybrid perovskites was prepared under ambient conditions using octa(ammoniumpropyl)octasilsesquioxane chloride (A-POSS) and several metal halide salts (CuCl₂, PdCl₂, PbCl₂, and MnCl₂) [41]. The components were first dissolved in aqueous hydrochloric acid or water, and then the complexes were isolated as microcrystalline precipitates. The structural features of the hybrid complexes indicate the dependence of the stability of octahedral POSS on the synthesis conditions. The cage silsesquioxane structure mostly remained intact in the complexes, with metal halides formed in the process. Solid-state ²⁹Si MAS NMR recorded for the hybrid species showed a slight upfield shift of the peak attributed to the T³-type silicon atoms in the cubic POSS structure. The extent of the shift depended on the type of perovskite cation (δSi = −63.9 ppm; −67.1 ppm; −65.1 ppm; −64.4 ppm; and −64.1 ppm for, respectively, A-POSS, Cu-A-POSS; Pd-A-POSS; Pb-A-POSS; and Mn-A-POSS). A slight peak broadening was attributed to the magnetic effect of the metal halide layers, especially in the case of the Cu and Mn complexes. For Pb-A-POSS, the broadening of the main peak was accompanied by the appearance of a weak resonance peak around −55 ppm. The chemical shift range indicates that the band may have arisen from the partial degradation of the silsesquioxane cage. Pd-A-POSS was an exception, as their ²⁹Si NMR spectra featured a sharp T³ peak.

Interestingly, the use of octa(propylammonium)octasilsesquioxane bromide (Br-POSS) or iodide (I-POSS) for the co-crystallization with PbCl₂ did not involve the degradation of the silsesquioxane cube [43]. The plate-like products precipitated quite rapidly, and their PXRD diffractograms indicated layered structures with interlayer spacings (1.61 nm and 1.62 nm, respectively) similar to those calculated for Pb-A-POSS. The ionic silsesquioxane interlayers stabilized the excited electron-hole pairs in the semiconducting Pb-X-POSS lead halide perovskites (X = Cl⁻NH₃⁺; Br⁻H⁺; I⁻H⁺) with well isolated layers, even if they lacked porosity. This effect is important for potential applications of hybrid POSS-perovskite materials as light-emitting diodes and light absorbers in solar cells. The absorption and the exciton emission peak positions shifted towards longer wavelengths by changing the ionic group from Cl⁻NH₃⁺ to Br⁻H⁺ to I⁻H⁺. For Pb-Br-POSS and Pb-I-POSS, strong absorption bands were observed at ~390 and 510 nm, while sharp PL peaks appeared at 424 and 526 nm, respectively, when excited with λ_ex = 355 nm. These results showed that by selecting the composition of hybrid complexes, it is possible to adjust both the structure and the properties of NCs. This may have a significant impact on the design process of, e.g., solar cell technology.

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 N₂ 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 N₂ 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