Supramolecular Engineering of Hybrid Materials in Photovoltaics and Beyond

: Solar-to-electric energy conversion has provided one of the most powerful renewable energy technologies. In particular, hybrid organic–inorganic halide perovskites have recently emerged as leading thin-film semiconductors for new generation photovoltaics. However, their instability under operating conditions remains an obstacle to their application. To address this, we relied on supramolecular engineering in the development of organic systems that can interact with the surface of hybrid perovskites through different noncovalent interactions and enhance their operational stabilities. Moreover, we have utilized the uniquely soft yet crystalline structure of hybrid perovskites and their mixed ionic–electronic conductivity to provide a platform to advance their functionality beyond photovoltaics. This account reviews our recent progress in supramolecular engineering of hybrid perovskites in photovoltaics and discusses their perspectives in the development of smart technologies.


Introduction
Smart technologies that can autonomously adapt to their operating conditions are increasingly required in modern optoelectronics. [1][2][3] They can be realized by controlling material functions in response to external stimuli. [4][5][6][7] Some of the most versatile strategies for accessing adaptive functional materials rely on supramolecular chemistry, which plays a critical role in natural systems and biological processes. [5][6][7][8] However, the potential of stimuli-responsive (supra)molecular materials has not been fully realized in optoelectronics due to the challenges of transferring their function to the solid state in functional devices. [8][9][10] This creates the need for 'amphidynamic' materials that are crystalline yet capable of hosting dynamic and stimuli-responsive functionalities in the solid state (Fig. 1). [8][9][10][11][12] Hybrid organic-inorganic metal halide perovskites present a unique class of materials that display an untapped capacity to act as amphidynamic scaffolds (Fig. 2). [11][12][13] These versatile and soft, yet crystalline, semiconductors that feature mixed ionic-electronic conductivities have recently attracted considerable attention due to their exceptional optoelectronic characteristics. [14][15][16][17][18][19] In particular, they have demonstrated remarkable solar-to-electric power conversion in photovoltaics, despite routinely being prepared by solution-processing. [18,19] However, they feature limited stabilities under atmospheric conditions of oxygen and moisture, as well as in response to voltage and light under operating conditions. [20][21][22][23][24][25] This has stimulated extensive research on establishing the strategies to stabilize hybrid perovskite materials and optoelectronic devices.
In our work, we have relied on (supra)molecular engineering for the stabilization of hybrid halide perovskites by designing organic molecules that can interact with the perovskite interface through tailored noncovalent interactions. This has resulted in the enhancements of operational stabilities in perovskite solar cells without compromising their performances, while opening a path for enhancing their functionality. This account details some of the latest developments and future perspectives of supramolecular engineering in hybrid photovoltaics and beyond towards smart technologies.

Hybrid Halide Perovskites and their Supramolecular Control in Hybrid Photovoltaics
Hybrid organic-inorganic halide perovskites can be described by the AMX 3 formula (Fig. 2a-b), which defines the inorganic {MX 6 }-based corner-sharing octahedral framework (Fig. 2b) comprised of divalent metal ions (M = Pb 2+ , Sn 2+ ), halide anions (X = Cl -, Br -, I -), and central 'A-cations' (e.g. Cs + , methylammonium (MA), formamidinium (FA)). [13] These solution-processed materials show excellent light absorption and long-living charge carriers with long diffusion lengths and high defect tolerance, which has stimulated their use in thin-film photovoltaics since 2009 . [13,[26][27][28][29][30][31] nium (ADAM) organic spacers (Fig. 3d). [41,42] These materials were analyzed by a combination of techniques complemented by solid-state NMR spectroscopy to reveal atomic-level interactions in Ruddlesden-Popper phases. Adamantane is known to form ordered assemblies based on vdW interactions that were applied in functional materials, such as plastic crystals and molecular machines. [41] The adamantane core was thereby functionalized with a methylammonium group and introduced into FA-based layered hybrid perovskites to provide a hydrophobic backbone through stronger vdW interactions as compared to conventional BA spacers. [40] As a result, we obtained layered hybrid perovskites with power conversion efficiencies that exceed 7% in conventional perovskite solar cells. [41] This was accompanied by long-term operational stability in one of the record-performing FA-based layered hybrid perovskite solar cells at the time.

Enhanced Functionality of Hybrid Perovskites
While supramolecular control offers an effective strategy to control the properties of hybrid perovskite materials and their photovoltaic devices, it also provides a platform for enhancing their functionality. [36] In particular, the assembly of organic spacer layers have so far been mostly used for templating layered hybrid perovskites and stabilizing them due to the increased hydrophobicity and suppression of ion migration. However, their structural versatility enables enhancing the functionality of hybrid perovskite assemblies towards multifunctional materials. This can be achieved through the incorporation of electro-and/or photoactive organic species, as well as chiral moieties (Fig. 5a) within the perovskite frameworks, tuning their optoelectronics and rendering them more responsive to various external stimuli. [36] The electroactive spacers in layered perovskites have been particularly relevant since the insulating character of organic spacer layer limits charge extraction, thereby leading to inferior performances of layered hybrid perovskites as compared to photovoltaic performances and operational stabilities are greater than for either of the spacers applied individually (Fig. 4a,b). [54] We used NMR crystallography to assess the atomic-level structure in a model comprising PEA and/or FEA spacers (S) in S 2 PbI 4 (n = 1) layered perovskite compositions. 19 F→ 13 C cross polarization (CP) NMR spectra for mixtures of PEA and FEA spacer precursors and their layered perovskites showed atomiclevel mixing between the two components (Fig. 4c), since CP relies on through-space dipole-dipole interactions at the subnanometer distance. [54] However, layered perovskites based on a single type of spacer were found to exhibit similar spectral features to those with mixed spacers, suggesting the possibility of a nanoscale segregation. To scrutinize this, the chemical shifts were calculated using density functional theory (DFT) for different trial structures (Fig. 4d) that were generated by identifying low energy structures from molecular dynamics simulations of various assemblies, some of which were based on reported crystal structures optimized by DFT. The comparison of experimental and calculated 13 C and 19 F chemical shifts revealed the best agreement for a segregated model, indicating that the nonsegregated system matches the experimental data with 99.9% probability based on the Bayesian analysis (Fig. 4e-f). [54] The generality of this observation was further confirmed in Dion-Jacobson layered perovskite analogues based on 1,4-phenylenedimethylammonium (PDMA) spacers and their perfluoroarene analogues (F-PDMA), which were also found to feature nanosegregation in thin films and mechanosynthetic powders. [55] In contrast to resented despite their attractive optoelectronic properties and higher thermal stability. [40] This is partly since the photoactive α-FAPbI 3 perovskite phase is not thermodynamically stable at room temperature and stabilizing it remains an ongoing challenge. We have thereby studied NDI/FA-based low-dimensional hybrid perovskites by means of MD simulations and DFT, revealing the capacity to form layered hybrid perovskites featuring Type II QW structure in which spacer orbitals contribute to the band edge (Fig. 5b). [56] This was evidenced experimentally in thin films and mechanochemically prepared powders by a combination of techniques, including solid-state NMR spectroscopy, X-ray diffraction (XRD), and transient absorption (TA) spectroscopy, complemented by time-resolved microwave conductivity (TRCM) measurements. XRD confirmed the formation of low-their three-dimensional analogues. [36] The use of electroactive spacers could thereby overcome this limitation by changing the electronic structure in hybrid materials (Fig. 5b). We recently applied a functionalized naphthalenediimide (NDI) based spacer, namely 2,2'-(1,3,6,8-tetraoxo-1,3,6,8-tetrahydrobenzo[lmn] [3,8] phenanthroline-2,7-diyl)bis(ethylammonium) (NDIEA), in the formation of Dion-Jacobson perovskite phases. [56] NDI materials are common electron acceptors used in organic electronics due to high electron affinity and charge carrier mobility, as well as thermal stability. [56][57][58] An NDI-containing low-dimensional perovskite system has been previously described based on MA A-cation compositions. [57] In general, the research on layered hybrid perovskites has been predominantly focused on MAbased systems while FA-based compositions remain underrep- dimensional perovskite phases, whereas solid-state NMR spectroscopy revealed that they contribute to stabilizing the α-FAPbI 3 phase (Fig. 5c-g). Moreover, TA corroborated electron transfer between the NDI and the perovskite layers through the appearance of a positive feature in the spectra between 450-550 nm upon excitation at 510 nm of thin films (Fig. 5h-i), which was associated with the NDIEA-based radical anion. [56] This was also apparent in the neat NDIEA spacer films excited at 400 nm ( Fig. 5j) but not upon excitation at 510 nm (Fig. 5k), confirming that the spectral signatures originated from the electron transfer. This exchange did not take place in reference layered perovskite systems incorporating electronically inactive spacers, such as the PDMA. [45,46] Moreover, the characteristic spectral shape did not change over time, suggesting the formation of long-lived radical anions in Type IIA QW structure predicted by DFT. However, TRCM revealed lower photoconductivity that could be attributed to the structural disorder in thin films and a highly localized nature of the bottom of the CB. [56] This highlights the complexity and challenges in the development of multifunctional layered perovskites, which stimulates further investigations to enable their use in optoelectronics.
In the efforts to develop multifunctional materials, it would be of interest to control the properties of hybrid perovskites by relying on physical stimuli without changing their composition. This would open another path for their utility beyond photovoltaics, such as in flexible electronics and sensing. [12] The characteristics of hybrid perovskites permit using mechanical stimuli for this purpose towards realizing mechanoresponsive materials. [11,12] While there has been a number of studies on hybrid perovskites under pressure, this has so far not involved pressure levels below 1 GPa that are compatible with practical applications. This pressure range is further of interest since the corresponding strain levels are comparable to those of intrinsic strain associated with polaron effects, conformational changes or lattice mismatches. [12] The comparatively low bulk modulus of halide perovskites (of the order of 10s of GPa) with respect to their oxide analogues (>100 GPa) renders them more easily compressible across pressure ranges. This is even more pronounced in layered hybrid perovskites, although they remain underexplored in pressure-induced transformations (Fig. 6a). [12] The optoelectronic properties of hybrid perovskites can be altered by affecting M-X interactions and octahedral connectivity under pressure, as well as the octahedral tilting and quantum confinement determined by the width of the potential barrier defined by the spacer in layered hybrid perovskites (Fig. 6a). The resulting changes in the optoelectronic properties in response to pressure would render these materials mechanochromic. [12] Towards this goal, we have relied on external pressure in the 0-0.35 GPa range in representative RP and DJ phases as model systems based on benzylammonium (BzA) and PDMA spacers, respectively. [12] For a better understanding of their structure-property relationships, we have also investigated the role of halide anions in both iodide and bromide compositions. Pressure-dependent X-ray scattering evidenced that lattices of these perovskite materials monotonically shrink through compression, with a largest compression along the stacking direction (a-axis; Fig. 6b-c). Although it was expected that the additional degrees of freedom in RP phases would render them more compressible than DJ systems, with further differences for I-and Brbased compositions, the bulk moduli of RP and DJ phases were comparable under this pressure. [12] Moreover, DJ-based phases (a) Schematic representation of emerging multifunctionalities associated with layered hybrid perovskites involving different functions of organic spacers, from their structure-directing roles, through electro-and photoactivity, to chirality and mechanoresponsiveness. Adapted with permission from ref. [36]. Copyright 2021 Royal Society of Chemistry. (b) Representative electroactive spacers applied in layered hybrid perovskites based on NDIEA moieties featuring Ty pe II electronic structure. (c-g) 15 N solid-state NMR spectra at 21.1 T, 100 K, with 12.5 kHz magic angle spinning of (c) α-FAPbI 3 , (d) δ-FAPbI 3 , (e) neat NDIEAI 2 and (f-g) (NDIEA)FA n-1 Pb n I 3n+1 (n = 1, 3) powders prepared mechanosynthetically. (h-k) Transient absorption spectra of (h-i) (NDIEA) FA n-1 Pb n I 3n+1 composition and (j-k) NDIEAI 2 thin films upon excitation at 510 nm (h,i,k) and 400 nm (j). The spectral shape does not change over time and the generated species are long lived. Adapted with permission from ref. [56]. Copyright 2021 American Chemical Society.
were not susceptible to major structural changes associated with the halide ion, which was presumably due to their higher geometric constraints. On the contrary, BzA-based RP phases featured larger structural rearrangements in Br-based compositions in contrast with their higher level of rigidity compared to I-based systems. This was more pronounced for (BzA) 2 PbBr 4 , with a compression in the stacking direction up to -6% relative change, accompanied by an isostructural phase transition, which was associated with the non-centrosymmetric crystal structure featuring two distinct Pb-X-Pb angles. [12] DFT calculations ascribed these structural changes to the interplay of interactions between the spacer layer and the inorganic slabs, suggesting that the isostructural transition can be related to a decrease in the distance between spacers, leading to an increased penetration depth into the inorganic lattice. We further assessed this effect by means of pressure-dependent UV-vis absorption and photoluminescence spectroscopy, revealing a mechanochromic response and a gradual red-shift with increasing pressure (Fig.  6d). (BzA) 2 PbBr 4 showed the most significant shift of the optical absorption (-55 meV) as compared to the other compositions, featuring a rather comparable change (~-30 meV; Fig. 6e). [12] Unlike previous reports on pressure-induced changes in hybrid perovskites, these optical responses were fully reversible, which is critical for their practical application. The reversible mechanochromism of these materials has been associated with an interplay of interactions between the organic and inorganic layers in layered hybrid perovskite materials, which creates potential for perovskites as emerging mechanophores and amphidynamic platforms for smart materials. [10][11][12]

Summary and Outlook
The use of supramolecular strategies in stabilizing hybrid organic-inorganic perovskites has shown potential for advancing hybrid materials and their application in renewable energy conversion in photovoltaics. In particular, the unique characteristics of hybrid perovskites as crystalline yet soft and solutionprocessable mixed (semi)conductors permitted tailoring their properties through molecular design of the organic components. These developments were further enabled by the use of NMR crystallography in assessing atomic-level interactions and determining their supramolecular structure. This has led to a new generation of layered hybrid perovskites as versatile platforms for enhancing the functionality of hybrid materials through the use of (photo)electroactive components. While this unlocks an attractive direction for opto(electro)ionics, the complexity of these systems requires an interdisciplinary approach for a better understanding of their structure-property relationships to guide advanced material design, which is the subject of our ongoing research (Fig.  7). This could set the stage for realizing the potential of hybrid multifunctional materials in smart and sustainable technologies in the future.   7. Future perspectives for supramolecularly engineered hybrid perovskites. Schematic of the challenges and opportunities for the further development of hybrid perovskites through supramolecular design by exploiting their performance in renewable power generation, addressing their stability by controlling mixed conduction, and relying on their versatility and compatibility with flexible substrates for enhanced functionality towards smart and autonomous technologies. Device image adapted from @iaremenco/123rf.com