Hybrid perovskites have recently emerged as a promising material for optoelectronic applications due to their unique combination of properties. In particular, their 2D crystalline form is of great interest since the alternating inorganic and organic layers naturally form a multiple quantum-well structure, leading to the formation of stable excitonic resonances at room temperature. However, a controlled modulation of the quantum well width, which is defined by the number of inorganic layers (n) between two organic ones, is not straightforward and represents the main synthetic challenge in the field. This study presents a novel approach to easily tune the number of inorganic layers (n) in lead iodide perovskite single-crystalline flakes. This approach exploits an iodide salt as an additive that acts as an additional source of I−, inducing the generation of iodoplumbate species and resulting in an increase of the monomer density, resulting in the fine-tuning of the n value. Unlike other methods, our protocol allows us to fix the molar ratio of precursors and to control the "n" value by simply modulating the amount of potassium iodide (KI) added to the solution. The excellent optical quality of the synthesized flakes enables an in-depth analysis by Fourier-space microscopy, which reveals that the excitons orientation can be manipulated by modifying the number of inorganic layers. It is found that the excitonic out-of-plane component is enhanced when n is increased. This combined advance in synthesis and optical characterization provides a better understanding of the exciton behavior in low-dimensional perovskites, opening up possibilities for the design of materials with improved optoelectronic characteristics.

Tin halide perovskites display similar or even superior electronic and optical properties when compared to well-established Pb-based perovskites. Despite their favorable optoelectronic properties, the tin perovskite solar cells still exhibit power conversion efficiency (PCE) much lower than that of their lead analogs. This is mainly attributed to the fact that metastable Sn(II) in perovskite lattice is prone to the oxidation. The presence of Sn(IV) in the material results in self-doping and formation of non-radiative recombination centers. One of the strategies to overcome this obstacle is addition of bulky A-site cations which stabilize the resulting 3D films in a quasi-2D fashion or preparation of 2D tin halide perovskites. Especially in recent years, single crystal 2D-tin halide perovskites have attracted significant attention. However, the synthesis and growth of such materials high-quality remain very challenging. In this work, we report a comprehensive study of the synthesis and the growth of single-crystal 2D tin halide perovskites (Ruddlesden-Popper), which is still poorly explored (excluding extensively studied materials containing PEA derivatives acting as an A-site cations). By careful control the growth conditions (temperature, and composition), we obtained mm-sized (up to 10 mm), high-quality single crystals of 2D tin halide perovskites. The single crystals were characterized structurally and optically by means of inter alia X-ray diffraction, steady-state photoluminescence, and optical absorption.

The introduction of chiral organic spacers in 2D and 1D perovskites breaks the inversion symmetry operation in the crystal, due to the asymmetric hydrogen bonding between the H atom of the ammonium group in the organic cation and the halide atom (Cl, Br, or I) in the lead octahedra. [1,2] The absence of inversion symmetry operation results in the emergence of Rashba/Dresselhaus spin-splitting, i.e. the splitting of the double degenerated electronic band into two bands with the same energy, but shifted in k-space,[3,4] triggering spin-dependent properties even in the absence of magnetic field. Such a feature has been utilized for applications in polarized light detectors,[5] spin-dependent charge transport,[6] and polarized light-emitting diodes.[7] In this work, we investigate how the processing of chiral 2D perovskites impacts their optical properties, microstructure, and phase purity. Specifically, we synthesized the R-, S-, and rac-MBA2PbI4 (MBA: methylbenzylammonium) and varied the solvent used for film deposition (dimethylformamide, DMF, or acetonitrile, ACN), and the subsequent thermal annealing procedure. We found that the processing conditions strongly impact the optical properties and microstructure of the chiral 2D perovskites. Moreover, we identified the anisotropic emergence of a 1D phase, which is dependent on the molecule's chirality and seems more prone to occur with pure enantiomers than in the racemic mixture. We demonstrate that its formation is suppressed when the 2D perovskite is processed from DMF, due to its higher boiling point and a stronger interaction between the solvent and Pb2+ ions, thus enabling a higher phase purity. These observations give fundamental insights into the film formation processes of chiral 2D perovskites and offer processing strategies to control anisotropic growth in 2D perovskite and to tune their chiroptical response.

Hybrid organic-inorganic perovskites have received tremendous research attention over the past decade for use in optoelectronic applications such as solar cells and photodetectors. Despite their excellent optoelectronic properties, their commercialization is hindered by their limited intrinsic stability. A prime route to obtain more stable perovskite materials is the addition of a large organic ammonium cation to form quasi-2D perovskites.^[1-2]

We employ a functionalized benzothienobenzothiophene (BTBT) organic cation either as an additive in the precursor solution to obtain a quasi-2D perovskite for use in photodetectors^[1], or as an interlayer in perovskite solar cells.

The BTBT cation was added to a CsPbI₃ precursor solution to obtain a n = 2 quasi-2D perovskite film. We compared the stability and performance of this film to that of a state-of-the-art (BA)₂CsPb₂I₇ (n = 2) film. While the (BA)₂CsPb₂I₇ film starts to degrade at 130 °C, the thermal stability of the film containing the BTBT cation is significantly enhanced to 230 °C. Furthermore, while the (BA)₂CsPb₂I₇ film degrades into (BA)₂PbI₄ and other compounds after 1 day of storage in air at 77% relative humidity, the film containing the BTBT cation shows excellent resistance against humidity, with no apparent degradation after 1 year of storage at high relative humidity. We fabricated planar photodetectors using both films. The specific detectivity of the photodetectors with films containing our tailored cation is similar in magnitude compared to those containing the state-of-the-art (BA)₂CsPb₂I₇ films. In summary, we prepared a quasi-2D perovskite with a similar optoelectronic performance as the literature reference but with significantly enhanced stability.

The same BTBT cation was also used as an interlayer in a solar cell between a NiO_x hole-transporting layer and a triple-cation 3D perovskite leading to an enhanced lifetime of photo-generated charge carriers based on TRPL measurements. Furthermore, a significant V_OC improvement is achieved in solar cells leading to a higher power conversion efficiency. The moisture stability of the solar cells was enhanced with the presence of the interlayer.

Starting from the well-studied ABX₃ (A = CH₃NH₃⁺, Cs⁺; B = bivalent metal cation, X = Cl^-, Br^-, I^-) perovskite, new derivates can be obtained with the partial or full substitution of the A and B cations with different metal ions [1]. Besides a change in the elemental composition, the dimension of the ion in the A site enlarges the formation of polymorphs of a lower dimensionality (2D or 0D). Indeed, the use of an organic cation, usually bigger than the inorganic metal ion, is one of the most promising strategies to tune and confer new optoelectronic properties in metal halide compounds [2]. For example, the 2D layered lead halides have excellent emission comparable with the 3D counterpart but with better electronic conductivity and quantum confinement effect observable even in the bulk materials [3].

Here, we present our recent results on the preparation of emissive 2D (Pb,Mn)-based hybrid metal halide as single crystals. Starting from the reported synthesis of Bz₂PbBr₄ (Bz⁺ = (C₆H₅CH₂NH₃)⁺), the Pb²⁺ was progressively replaced by Mn²⁺, even if the percentage of Pb substitution strongly depends on the metal ratio in the starting materials. The presence of Bz cation in the structure is confirmed by FTIR analyses. The emission varies from the light blue of Bz₂PbBr₄ to the typical orange one of the Mn. The PL spectrum exhibits a weak emission for the excitonic peak at 410 nm and a strong Mn emission, centered at 610 nm, ascribable to the spin-forbidden Mn²⁺ d−d transition (⁴T₁ → ⁶A₁). The ABS spectra demonstrate that even at low Pb concentrations it is observable an energy-transfer process from the Pb²⁺ to Mn²⁺ emitter. Interestingly, the emission intensity reaches the maximum for the experimental Mn composition of 8.0% (PLQY » 47.9%), even if a notable value is measurable also for the Mn-rich composition (PLQY » 37.2% for Mn = 66.5%).

Incorporating organic semiconductor (OS) spacer cations into layered lead halide perovskites provides a powerful approach to tune the optoelectronic properties of the resulting organic-inorganic hybrid materials. Forming type-II nano heterostructures using OS spacers, for example, allows the partitioning of charge carriers either in the organic or inorganic layer and provides a potential strategy to overcome intrinsically high exciton binding energies in layered perovskites by facilitating charge separation.[1] However, large OS cations with suitable energy levels remain challenging to incorporate into a layered perovskite structure resulting in relatively few systems reported and characterized.

In this regard, our lab developed novel OS cations and optimized processing conditions to afford incorporation in layered perovskites.[2,3] In this presentation, we elucidate novel OS cations based on rylene dyes chromophores incorporation into a layered perovskite and elaborate the size limitations of the chromophore when it comes to the formation of (2D) layered perovskite structure. Using transient absorption (TA) and time resolved photoluminescent spectroscopy (TRPL) we indicate the formation of charge-transfer excitons at the perovskite-OS interface and further investigate the effect of the distance of the chromophore to the perovskite layer on the electron-transfer process from the perovskite layer to the OS cation. Time resolved microwave conductivity (TRMC) measurements and terahertz spectroscopy was used to elucidate the effect of the type-II nano heterostructure on the photogenerated free charge-carrier showing enhanced carrier lifetimes compared to layered perovskites incorporating aliphatic spacers cations. The presented work implies the possibility to mitigate the inefficient charge-separation process due to the high exciton-binding energy in lead-halide layered perovskite by forming OS-perovskite nano heterostructures opening the door for a new class of material.

Double perovskites are promising candidates for less toxic and highly stable metal halide perovskites, but their optoelectronic performances still lag behind those of the lead halide counterpart, due to the indirect nature of the bandgap and the strong electron-phonon coupling. Reducing the dimensionality of Cs₂AgBiBr₆ down to a 2D layered form is strategic in order to tune the band gap from indirect to direct and provides new insights into the structure-property relationships of double perovskites. Herein, we report on a series of monolayer 2D hybrid double perovskites of formula (RA)₄AgBiBr₈, where RA represents different primary ammonium large cations with alkyl- and aryl-based functionalities.[1] An in-depth experimental characterization of structure, film morphology and optical properties of these perovskites is carried out. Interestingly, the variation of the ammonium cation and the inter-planar distance between adjacent inorganic monolayers has peculiar effects on the film-forming ability and light emission properties of the perovskites. Experiments have been combined with DFT calculations in order to understand the possible origin of the different emissive features. Our study provides a toolbox for future rational developments of 2D double perovskites, with the aim of narrowing the gap with lead halide perovskite optoelectronic properties. We further discuss on the achievement of a bathochromic shift in the absorption features of a butylammonium-based silver-bismuth bromide monolayer double perovskite through doping with iodide and study the optical properties and stability of the resulting thin films in environmental conditions.[2] Finally, we report on recent, on-going work on the 2D/3D interface engineering to tune the efficiency of solar cells based on the double perovskite and carbon electrode charge extracting layers.[3] 

Epitaxial heterostructures based on oxide perovskites and III–V, II–VI and transition metal dichalcogenide semiconductors form the foundation of modern electronics and optoelectronics. Halide perovskites—an emerging family of tunable semiconductors with desirable properties—are attractive for applications such as solution-processed solar cells, light-emitting diodes, detectors and lasers. Their inherently soft crystal lattice allows greater tolerance to lattice mismatch, making them promising for heterostructure formation and semiconductor integration. Atomically sharp epitaxial interfaces are necessary to improve performance and for device miniaturization. However, epitaxial growth of atomically sharp heterostructures of halide perovskites has not yet been achieved, owing to their high intrinsic ion mobility and their poor chemical stability. Therefore, understanding the origins of this instability and identifying effective approaches to suppress ion diffusion are of great importance. In this talk I will present an effective strategy to substantially inhibit in-plane ion diffusion in two-dimensional halide perovskites by incorporating rigid π-conjugated organic ligands. Highly stable and tunable lateral and vertical epitaxial heterostructures, multiheterostructures and superlattices will be demonstrated. Furthermore, using these 2D heterostructures as a new platform, I will present our recent efforts in 1) quantitatively understanding the anion inter-diffusions and migrations, and 2) controlling and manipulating exciton transport and light-emission in halide perovskites.

Metal halide perovskites are emerging materials for optoelectronics due to their excellent optoelectronic properties, such as direct and tunable band gaps, large absorption cross section, long lifetimes and diffusion paths of the charge carriers, coupled to an elevated defects tolerance.[1] The band gaps and the exciton binding energies of these materials can be tuned by varying the chemical composition of the inorganics or by reducing dimensionality by introducing large cations. Interestingly, reduction in dimensionality leads to the emergence of novel optical features, such as the sub-gap broad emission (BE) in 2D perovskites, whose origin is hotly debated.[2] Contrasting hypotheses assign BEs to the recombination of intrinsic self-trapped excitons (STEs) or to emission from native defects.[3-4] 

In this presentation the defect chemistry and photophysics of PEA2MX4 (PEA=phenethylammonium; M=Pb, Sn; X=I, Br, Cl) <100> 2D perovskites is discussed by a computational perspective, in order to provide a microscopic picture of the defects processes taking place in these quantum confined materials. The trends in the defects chemistry moving from 3D to 2D and the effects of the metal by replacing lead with tin will be discussed. Hence, by comparing predictions with experiments, the diverse hypothesis about the origin of the BE in this class of materials will be analyzed. DFT results show that sensibly lower defect densities are expected in 2D perovskites compared to 3D analogues, but a deepening of defect charge transitions in the band gap is reported due to QC. Despite the low calculated defect densities, emission from halide vacancies is compatible with the experimentally observed sub-gap features in the relatively large set of compounds considered in the study, suggesting that the density of these optically active centers is modulated by the crystallization kinetics. On the other hand, the simulation of STEs indicates that the self-trapping of holes and electrons is a feasible process only in the wide band gap Br- and Cl-based 2D perovskites, even though the process is thermodynamically hindered by the shallow nature of the transition.[5] 

Our work provides useful insights into the intrinsic and extrinsic mechanisms generating BEs in 2D perovskites and it demonstrates that a control of the crystallization process, e.g. by tuning PEA-halide stoichiometry in the precursors, is key to selectively control the optical features of these materials.

2D perovskites have emerged as more stable analogues for photovoltaic applications as their 3D counterparts. However, in contrast to the free charge carriers in 3D perovskites, the confined space of the layered 2D perovskites leads to the formation of excitonic excited states. To harvest the energy of the excitonic excited state in a solar cell, transport of the excitons to charge-separating interfaces is required. Understanding the transport of excitons in 2D perovskites is therefore a crucial step for the development of perovskite photovoltaics containing 2D phases which benefit from both improved stability and maximized efficiency. 

In this talk I will present our recent efforts to visualize exciton diffusion dynamics in a variety of 2D perovskite materials using Transient Photoluminescence Microscopy (TPLM). [1-4] TPLM combines diffraction limited excitation with time and spatially resolved detection of excitonic emission to reconstruct a movie-like representation of exciton transport with sub-nanosecond and few-nanometer resolution. Using this technique, a number of important structure-property relationships for exciton transport in 2D perovskites have been revealed, including the importance of lattice rigidity [1], the influence of trap states [3] and halide mixing [4], as well as the influence of doping strategies. I will highlight the importance of our use of Brownian dynamics simulations to explain the various anomalous transport regimes that can be encountered in these materials. 

In recent years, the interest in 2D materials and low-dimensional perovskites (LDP) as building blocks for photovoltaic devices has flourished. This is due to their chemical and electronic properties, which enable them to minimise performance losses and improve the stability of the devices. In this talk, it is presented an innovative strategy to improve the performances of inverted perovskite solar cells (PSC) by incorporating high-quality graphene flakes (GF) in the fullerene-based electron transporting layer (ETL) [1]. It has been proved that the so-formed ETL can be processed as a composite (Graphene:Fullerene) ultra-thin film atop the perovskite, reducing the defect-mediated recombination while creating preferential paths for the extraction of the electrons towards the current collector. Moreover, the properties and the synthesis of LDP based on thiophene cations will be discussed. At first, it will be shown how to control the properties of LDP: the optical and structural ones, and the segregation of different phases upon the modification of the halide composition. Indeed, the rational substitution of halides led to a spatial distribution of phases in the thin film, where the minor phase was responsible for the light emission while the major one gave a human-eye transparent appearance to the material [2]. Lastly, it will be discussed the implementation of this LDP in highly efficient PSC, giving rise to multidimensional 2D/3D perovskite heterostructure by means of two different passivation approaches.

[1] A.Zanetta, I.Bulfaro, F.Faini, M.Manzi, G.Pica, M.De Bastiani, S.Bellani, M.I.Zappia, G.Bianca, L.Gabatel, J.K.Panda, A.E.Del Rio Castillo, M.Prato, S.Lauciello, F.Bonaccorso, G.Grancini; Journal of Materials Chemistry A, 2023.

[2] A.Zanetta, Z.Andaji‐Garmaroudi, V.Pirota, G.Pica, F.U.Kosasih, L.Gouda, K.Frohna, C.Ducati, F.Doria, S.D.Stranks, G.Grancini; Advanced Materials, 34 (1), 2105942.

Metal halide perovskites are hot contenders as next generation of light emitters. Bright and colour-pure light-emitting diodes (LEDs) were demonstrated based on bulk 3D, nanocrystals, and quasi-2D structures (Ruddlesden-Popper phases) with targeted compositions that allow for accessing different spectral regions. Green and red LEDs have reached external quantum efficiencies exceeding 20%, but success in the blue spectral region has so far been limited.

Here we demonstrate blue LEDs with a peak wavelength of 481 nm, a colour purity of up to 88 % (CIE coordinates (0.1092, 0.1738)), an external quantum yield of 5.2 % and a luminance of 8260 cd m^-2. These devices are based on quasi-2D PEA₂(Cs_0.75MA_0.25)Pb₂Br₇, which is cast from solutions containing isopropylammonium (iPAm). The iPAm-modified sample when deposited on a PEDOT:PSS coated substrate, displays an exceptional PLQY as high as 64%. Cross-correlation of the optical and structural investigations indicate that the RP phase is composed of domains of n=3 phases surrounded by higher dimensionality phases, which allow the efficient transport of charge carriers towards the low dimensional domains. Interestingly, the energy transfer of the photoexcitations towards the low dimensional phases is blocked in samples using the iPAm additive, mostly due to the random orientation of very small crystalline domains. These interesting features present in our iPAm-modified system allowed us to fabricate bright blue-emitting PeLEDs with an average wavelength of 483 nm and FWHM of the electroluminescent of 25 nm for our champion device.

Our work demonstrates the great potential to tailor the composition and the structure of thin films based on Ruddlesden-Popper phases to boost performance of optoelectronic devices – specifically for blue-LEDs.

Low-dimensional perovskite structures are at the forefront of light emitting applications, mainly owing to high carrier and exciton confinement. The ability to tune the emission wavelength through halide alloying and addition of organic cations promoting formation of 2D structure enables fabrication of light emitting diodes (LEDs) with external quantum efficiency comparable with solutions based on organic semiconductors, with the benefit of high colour purity. One of the challenges in the development of perovskite LEDs is their fundamental absorption-related optical characterisation, including assessing the influence of possible processes at the interfaces between electrodes and the active layer compared to the pristine material. Photocurrent spectroscopy remains the standard high-sensitivity technique for probing defect-related absorption and material disorder, applicable but limited to fully contacted devices, using LEDs in reverse as photodetectors. Here we demonstrate a complementary approach of incorporating modulated photocurrent and photothermal deflection spectroscopy (PDS) experiments for characterisation of 2D perovskite LEDs on different fabrication stages: bare perovskite thin films, the same deposited on top of transparent transport layers from the substrate-side (half-devices), and full LED structures. The results confirm that in optimised device preparation arrangements the sandwiched perovskite layer can fully preserve its optical quality, confirmed by quantifying absorption related to sub-gap trap states and sharpness of the absorption edge (the Urbach energy), revealing a crucial information for future development of LEDs.

Among hybrid perovskites (HP), 2D are playing a major role in many optoelectronics applications including solar cells, thanks also to their stabilizing effect. Despite their huge success, the dynamics of photogenerated carriers in these materials still needs to be fully assessed. In this work we report the result of ultrafast optical spectroscopy measurement in  “tandem” configuration, combining transient absorption and time resolved photoluminescence in the same excitation conditions, as already applied to 3D and 2D HP thin films [1]. To get clearer information about intrinsic property, for the first time we apply our technique on single crystals of 2D HP (phenethylammonium (PEA)2PbI4), comparing nonresonant excitation, generating hot carriers, with resonant excitation, directly injecting cold excitons in the material. The results of our measurements show that both in the tihn film and in the crystal the photophysics consists in an initial massive formation of excitons, followed by a spontaneous ultrafast exciton splitting, occuring on a sub ps timescale. The evidences of this phenomenon are shown both in thin film and crystals wiht different excitation condition, eventually at low temperature, demonstrating that is an intrinsic effect. We believe that this results provide a better understanding in the photophysics laying under the exceptional optoelectronic properties of 2D HPs. 

 

Layered metal-halide perovskites have shown great promise for applications in optoelectronic devices, where a large number of suitable organic cations give the opportunity to tune their structural and optical properties. However, especially for Sn-based perovskites, a detailed understanding of the impact of the cation on the crystalline structure is still missing. By employing two cations, 2,2’-oxybis(ethylammonium) (OBE) and 2,2’-(ethylenedioxy)bis(ethylammonium) (EDBE), we obtain a planar <100> and a corrugated <110>-oriented perovskite, respectively, where the hydrogen bonding between the EDBE cations stabilises the corrugated structure. We will show that OBESnI₄ exhibits a relatively narrow band gap and photoluminescence bands compared to EDBESnI₄. An in-depth analysis shows that the markedly different optical properties of the two compounds have an extrinsic origin. Interestingly, thin films of OBESnI₄ can be obtained both in black and red colours. This effect is attributed to a second crystalline phase that can be obtained by processing the thin films at 100 °C. Our work highlights that the design of the crystal structure as obtained by ligand chemistry can be used to obtain the desired optical properties, whereas thin film engineering can result in multiple crystalline phases unique to Sn-based perovskites.