Layered 2D halide perovskites with their alternating organic and inorganic atomic layers that form a self‐assembled quantum well system are analogues of the purely inorganic 2D transition metal dichalcogenides. Within their periodic structures lie a hotbed of photophysical phenomena such as dielectric confinement effect, optical Stark effect, strong exciton–photon coupling, etc. Detailed understanding into the strong light–matter interactions in these hybrid organic–inorganic semiconductor systems remains modest. Herein, the intricate coherent interplay of exciton, spin, and phonon dynamics in (C6H5C2H4NH3)2PbI4 thin films using transient optical spectroscopy is explicated. New insights into the hotly debated origins of transient spectral features, relaxation pathways, ultrafast spin relaxation via exchange interaction, and strong coherent exciton–phonon coupling are revealed from the detailed phenomenological modeling. Importantly, this work unravels the complex interplay of spin–quasiparticle interactions in these layered 2D halide perovskites with large spin–orbit coupling.

We utilized two organic dications containing, respectively, a pyridinium and an imidazolium core to construct new n = 1 (where n refers to the number of contiguous 2D inorganic layers; i.e., not separated by organic cations) twodimensional (2D) lead-iodide perovskites 1 and 2. The former material exhibits a (100)-and the latter a very rare 3  3 (110)structural type. Compared with primary ammonium functionality, their constituent ring-centred positive charges have lower charge density. As a result, [PbI6]4-inter-octahedral distortions of the inorganic lattice are reduced (Pb-I-Pb bond angles are as high as 166o and 174o, respectively). This results in bathochromically shifted optical features. In addition, the compact nature of the dications produce super short lead-iodide sheet separations, with respective iodide-iodide (I•••I) distances as small as 4.149 Å and 4.278 Å. These are amongst the shortest separations of adjacent lead-iodide layers, in such materials, ever reported. When crystallized as thin films on top of substrates, the resulting 2D perovskite layers do not adopt a regular growth direction parallel to the surface. Instead, the crystallites grow with no fixed orientation. As a consequence of their proximate inorganic distances and unusual crystallization tendencies, the resulting 2D perovskites exhibit low excitonic activation energies (93.59 meV and 96.53 meV, respectively), enhanced photoconductivity in solar cells, and unprecendented incident photon-to-current conversion rates of up to 60%. More importantly, mesoporous 2D layered perovskite solar cells with power conversion efficiencies (PCEs) of 1.43% and 1.83% were achieved for 1 and 2, respectively. These are the highest values obtained, thus far, for pure n = 1 lead-iodide perovskites and more than 20 times higher than those obtained for materials templated by more conventional cations, such as phenylethylammonium (0.08%).

Excitonic effects underpin the fascinating optoelectronic properties of 2D perovskites that are highly favorable for photovoltaics and light‐emitting devices. Analogous to switching in transistors, manipulating these excitonic properties in 2D perovskites using coherent phonons could unlock new applications. Presently, a detailed understanding of this underlying mechanism remains modest. Herein, the origins of the carrier‐phonon coupling in 2D perovskites using transient absorption (TA) spectroscopy are explicated. The exciton fine structure is modulated by coherent optical phonons dominated by the vibrational motion of the PbI6 octahedra via deformation potential. Originating from impulsive stimulated Raman scattering, these coherent vibrations manifest as oscillations in the TA spectrum comprising of the generation and detection processes of coherent phonons. This two‐step process leads to a unique pump‐ and probe‐energy dependence of the phonon modulation determined by the imaginary part of the refractive index and its derivative, respectively. The phonon frequency and lattice displacement of the inorganic octahedra are highly dependent on the organic cation. This study injects fresh insights into the exciton–phonon coupling of 2D perovskites relevant for emergent optoelectronics development.

most successful to date were complex lead halides comprising simultaneously several univalent cations (Cs + , CH 3 NH 3 + or MA + , [H 2 NCHNH 2 ] + or FA +) and halide anions (typically Br − , I −) in their crystal lattice. [2] However, these materials suffer from low photostability. In particular, Hoke et al. first demonstrated that the mixed-halide MAPb(I 1−x Br x) 3 absorbers undergo rapid light-induced halide segregation with the formation of I-rich and Br-rich phases leading to both structural and energetic disorder resulting in a significant decrease in solar cell performance. [3,4] While the effect of short light exposure was found to be essentially reversible in the dark, long-term irradiation of the mixed halide perovskite films results in their complete degradation. [5] Therefore, light-induced halide phase segregation is considered as a severe limitation for achieving long-term operational stability of perovskite solar cells based on the absorbers incorporating more than a single halide anion. [6] Overcoming this problem is crucially important for the development of tandem devices with the upper cell based on the perovskite absorber with the tailored optical properties realized through halide mixing. Since the discovery of the light-induced halide phase segregation in complex lead halides, many research groups have investigated this phenomenon in detail in an attempt to reveal its mechanism. Multiple models varying in the origin

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Zero-dimensional (0D) hybrid organic-inorganic lead halides have been shown to display efficient broadband photoluminescence and are, therefore, of significant interest for artificial lighting applications. However, work that investigates the formability of the materials as a function of templating organic cation structure are rare. This severely limits our ability to rationally design new materials displaying specific structural and photophysical properties. With the goal of accessing rare 0D trimeric bromoplumbates, we have systematically varied templating N-alkylpyridinium cations and examined their impact upon inorganic lattice structure. Whereas comparatively short and flexible N-alkyl substituents (ethyl, 2-hydroxyethyl, and pentyl) yield one-dimensional (1D) inorganic chains, more rigid substituents (benzyl, acetamidyl, and cyanomethyl) afford hybrids composed of lead-bromide face-sharing trimers ([Pb3Br12]6-). Of the rigid substituents studied, benzyl groups were found to enforce the highest level of distortion of the [PbBr6]4-octahedra that comprise their trimeric structures. Upon exposure to ultra-violet (UV) light, N-benzylpyridinium lead-bromide (1)6[Pb3Br12] exhibits a broadband emission, centered at 571 nm, which spans from 400 to 800 nm. More specifically, it displays a large Stokes shift of ca. 1.39 eV and a full width at half maximum (FWHM) of ca. 146 nm. This broadband emission decays with a comparatively long lifetime of 426 ns at room temperature, which increases to 5.8 µs at 77 K. The reduced size and dimensionality of its inorganic lattice also results in a photoluminescence quantum yield (at least 10 %) that is approximately one order magnitude higher than that of its 1D congeners. Mechanistically, broadband emission in (1)6[Pb3Br12] is believed to originate from triplet excited state(s) obtained from excited-state structural reorganization of the [Pb3Br12]6-moiety.

Extending halide perovskites' optoelectronic properties to stimuli-responsive chromism enables switchable optoelectronics, information display, and smart window applications. Here, we demonstrate a band gap tunability (chromism) via crystal structure transformation from three-dimensional FAPbBr 3 to a ⟨110⟩ oriented FA n+2 Pb n Br 3n+2 structure using a mono-halide/cation composition (FA/Pb) tuning. Furthermore, we illustrate reversible photochromism in halide perovskite by modulating the intermediate n phase in the FA n+2 Pb n Br 3n+2 structure, enabling greater control of the optical band gap and luminescence of a ⟨110⟩ oriented monohalide/cation perovskite. Proton transfer reaction-mass spectroscopy carried out to precisely quantify the decomposition product reveals that the organic solvent in the film is a key contributor to the structural transformation and, therefore, the chromism in the ⟨110⟩ structure. These intermediate n phases (2 ≤ n ≤ ∞) stabilize in metastable states in the FA n+2 Pb n Br 3n+2 system, which is accessible via strain or optical or thermal input. The structure reversibility in the ⟨110⟩ perovskite allowed us to demonstrate a class of photochromic sensors capable of self-adaptation to lighting.

Multilayers consisting of alternating soft and hard layers offer enhanced toughness compared to all-hard structures. However, shear instability usually exists in physically sputtered multilayers because of deformation incompatibility among hard and soft layers. Here, we demonstrate that 2D hybrid organic-inorganic perovskites (HOIP) provide an interesting platform to study the stress–strain behavior of hard and soft layers undulating with molecular scale periodicity. We investigate the phonon vibrations and photoluminescence properties of Ruddlesden–Popper perovskites (RPPs) under compression using a diamond anvil cell. The organic spacer due to C4 alkyl chain in RPP buffers compressive stress by tilting (n = 1 RPP) or step-wise rotational isomerism (n = 2 RPP) during compression, where n is the number of inorganic layers. By examining the pressure threshold of the elastic recovery regime across n = 1–4 RPPs, we obtained molecular insights into the relationship between structure and deformation resistance in hybrid organic-inorganic perovskites.