Two-dimensional hybrid perovskites are a highly intriguing class of materials, composed of alternating inorganic and organic molecular layers. Their reduced dimensionality combined with weak dielectric screening leads to the formation of tightly bound excitons that efficiently absorb and emit radiation. A central questions for excitons in perovskites from the perspectives of both fundamental physics and applications is their mobility. In addition, the external control of their optical response has proven difficult due to challenges to introduce electrical doping into this class of materials.

In this talk I will focus on studies of optically detected exciton transport in 2D perovskites via ultrafast microscopy, discussing different regimes of propagation including free and localized states. I will show how the excitons behave at the structural phase transition and how their transport is determined by strong interactions with the lattice vibrations. In the second part, I will illustrate how the formation of electrically tunable trions, bound states of excitons in free carriers can be achieved in these hybrid semiconductors to optically detect doping. The trions in 2D perovskites are stable for both n- and p- doped regimes, feature unusually large binding energies and a substantial mobility at elevated temperatures.

Two-dimensional (2D) metal halide perovskite semiconductors have promising prospects for enhancing the stability of perovskite-based photovoltaic devices. In addition, these low-dimensional materials with electronic confinement offer further opportunities in light emission and quantum technologies. However, their technological applications still require a comprehensive understanding of the nature of charge carriers and their transport mechanisms.

In this work, we employ ultrafast optical and terahertz spectroscopy to investigate the exciton formation dynamics, the charge-carrier mobilities, and the charge-phonon couplings of 2D Ruddlesden-Popper perovskites (PEA)₂PbI₄ and (BA)₂PbI₄ (where PEA is phenylethylammonium and BA is butylammonium). We reveal band transport with high in-plane mobilities that give rise to efficient long-range conductivity. We show how the organic cation moderates the coupling of charge carriers to optical phonon modes, impacting the charge-carrier mobilities. Furthermore, we demonstrate a new experiment for simultaneously recording the terahertz and optical transmission transients, thus allowing us to monitor the exciton formation dynamics over the picosecond timescale. The observed dynamics reveal a long-living population of free charge-carriers that greatly surpasses the theoretical predictions of the Saha equation even at temperatures as low as 4K.

Our findings provide new insights into the temperature-dependent interplay of exciton and free charge carriers in 2D Ruddlesden-Popper perovskites. Furthermore, the sustained free charge-carrier population and high mobilities revealed by this work demonstrate the potential of these semiconductors for applications that require efficient charge transport, such as solar cells, transistors, and electrically driven light sources.

Metal-halide perovskite materials demonstrated extraordinary performance in solar cells and light emission in recent years, and their layered low-dimensional counterparts promise even greater tunability due to the huge variety of molecules available for the organic phase. [1, 2] Single octahedra-layer structures act as two-dimensional quantum wells, showing strong confinement and large exciton binding energies. Band gap and light emission can be designed by the choice of the organic cations.[3-5]

We investigate the emission from single plates of two-dimensional Ruddlesden-Popper metal-halide perovskites with different organic cations by angle-resolved polarized photoluminescence (PL) spectroscopy. The choice of the organic cation strongly influences the number of PL peaks that we observe in the band edge emission, and the different PL peaks manifest a strong angle dependence in their linear polarization, manifesting an angle-dependent intensity maximum along one of the principal axes of the octahedra lattice. Such a marked intensity maximum along one in-plane direction is surprising, since theory predicts isotropic in-plane exciton wave function distributions [6]. Therefore, electron-phonon coupling could be at the origin of the distinct anisotropy of the emission polarization, since a large number of phonon bands in such materials show strong directionality. [7, 8] We investigate the angle resolved phonon response together with the emission polarisation of the single plates with resonant and non-resonant Raman spectroscopy, which allows for a detailed correlation of the directionality of the emission with the phonons. The combined angle-resolved emission and Raman spectroscopy allows for a detailed investigation of the exciton fine structure and assignment of confined exciton states and phonon side bands.

Layered perovskites are quasi-twodimensional crystals in which n layers of metal-halide octahedra are spatially separated by organic molecular layers. A wide variety of organic molecules can be used in the fabrication of these materials, allowing for variation of the interlayer distance and orientation, as well as functionalization, for example by introduction of chiral or electroactive organic molecules. Furthermore, chemical substitution at the metal and halide sites can drastically change the band gap size and symmetry in these materials. In this talk, I will discuss how chemical substitution affects the exciton fine structure of layered perovskites, focusing on metal site substitution and different organic interlayers. Our first-principles calculations based on the GW and Bethe-Salpeter Equation approaches demonstrate the importance of chemical, structural, and dielectric heterogeneity for controlling the nature and binding energy of excitons and the need for first-principles calculations to understand the electronic and excited state structure of this complex material class.

Perovskite-derived two-dimensional (2D) materials are emerging as an excellent combination to the three-dimensional (3D) metal halide perovskites for enhancing its stability.[1] In this 2D/3D approach, bulky ammonium organic cations are deposited on top of the 3D perovskite active layer forming a very thin coating of a low dimensional 2D perovskite.[2] 2D/3D inverted perovskite solar cells (PSCs) have achieved to retain >95% of their initial value after >1000 hours at damp-heat test conditions (85 ºC and 85% RH), while power conversion efficiencies (PCE) above 25% have been recently reported in 2D/3D PSCs.[3]

In this work, we investigate the deposition of two diammonium spacers with similar chemical composition (4,4′-Dithiodianiline, 2S, and 4,4’-ethylenedianiline, ET) but with a totally different molecular geometry on top of a 3D perovskite with Cs_0.09FA_0.91PbI₃ formula to manipulate the efficiency and stability of the PSCs. Our results demonstrate an improved PCE of 21% when using the 2S spacer in opposition to an inferior PCE of 16% in the cells covered with the ET spacer. The stability tests display no loss in the PCE upon constant illumination at RT at the MPPT when using the 2S spacer vs. a significant drop in the PCE for the cells with the ET spacer. This divergent behavior is ascribed to the formation of a parallel oriented layer of a 2D perovskite with the 2S spacer that facilitates the charge extraction in the PSC in opposition to the isotropically layer of a 1D perovskite detected upon addition the ET spacer.

Perovskite materials have gained attention in the field of photovoltaics due to their high conversion efficiency, tunable bandgaps, high charge carrier mobility, low exciton binding energy, and ease of synthesis [1]. Perovskite Solar Cells (PSCs) have demonstrated promising Power Conversion Efficiencies (PCE) of up to 25\%, comparable to the widely commercialized crystalline silicon counterparts. However, achieving highly efficient PSCs requires careful optimization of the perovskite absorbing layer and interfaces with charge-selective contacts.

This study focuses on interface passivation strategies, with a particular emphasis on the use of organic molecules for surface treatment. Organic molecules, especially those containing ammonium (NH3$^+$), show promise in passivating surface trap states [2-3]. The introduction of ammonium-based organic molecules in post-surface treatments leads to the formation of low-dimensional 2D perovskite structures, impacting the quantum confinement regime.

The investigation approaches the correlation between molecular flexibility and the formation of 2D/3D perovskite heterointerfaces, exploring their subsequent effects on solar cell performance. The study synthesizes two variations of a well-studied phenylethylammonium iodide (PEAI): a more rigid trans-2-phenylcyclopropylammonium iodide (PCPEAI) and a more flexible cyclohexylethylammonium iodide (CHEAI).

Detailed analyses, including SEM-Cathodoluminescence [4], reveal that the 2D phases present a heterogeneous distribution over the 3D surface. The study also highlights that the absence of $\pi$-$\pi$ stacking interactions and a higher degree of freedom in the more flexible CHEAI molecule prevent undesirable aggregation, resulting in improved device efficiency. The CHEAI-based devices exhibit a commendable efficiency of close to 21\%. The results indicate that molecular flexibility plays a crucial role in facilitating the formation of 2D phases.

This research contributes valuable insights into the intricate relationship between molecular flexibility, 2D/3D perovskite heterointerfaces, and PSC performance. The findings offer guidance for designing more efficient and stable perovskite solar cells, making progress in next-generation photovoltaic technologies.

Halide perovskites have garnered significant attention due to their unique properties and potential applications in optoelectronics and energy-related fields. However, the reduction in crystal dimension of perovskite from 3D to low dimensional structures introduces challenges, particularly regarding quantum and dielectric confinements, resulting in larger band gaps and exciton-binding energies. The low dimensional structures imply intrinsically bigger unit cells which makes challenging the application of standard density functional theory (DFT) calculation in terms of computational cost. Furthermore, DFT tends to significantly underestimate band gaps, which is extremely problematic for optoelectronic materials. Density Functional Tight-Binding (DFTB) is a flexible, semi-empirical method based on DFT which capable of simulating large system sizes and offers the possibility of accurate band gap prediction with low computational cost.^1,2

In this work, we highlight the application of DFTB methodology for studying the electronic structure, effective masses, and charge density localization in low-dimensional perovskite materials. By employing empirical fitting and parameterization, the DFTB method captures the electronic band structures of model 2D halide perovskites (e.g., Cs₂PbI₄, BA₂PbI₄, PEA₂PbI₄, and BA₂PbBr₄). We show good agreement between DFTB results and experimental electronic band gaps, as well as reduced effective masses. This first attempt is promising for further applications to other low-dimensional (1D, 0D, hollow) perovskite nanostructures or 2D/3D perovskite heterostructures with large sizes and complexity, which demonstrated excellent operational stability in solar cell architectures.

: A wide range of physical properties associated with layered (two-dimensional) perovskites has been realized due to the vast composition space of the inorganic layer and molecular diversity in organoammonium cations. A new class of layered halide perovskites recently discovered by our group, termed “mosaic” perovskites, form through alloying of single and double layered perovskites and feature three distinct metal ions. The first example of these disordered alloys comprised Cu(I), Cu(II), and In(III) in the perovskite layer, leading to emergent optoelectronic properties owing to intervalence charge transfer between Cu ions.

Here, we discuss facile synthetic methods to produce mosaic layered perovskites, the role of metal-cation ordering in optoelectronic and magnetic properties, and the interplay between stability and disorder in these complex mixtures. Moreover, we expand the diversity of mosaic layered perovskite alloys through exploration of transition-metal ion pairs that engender strong intra-layer coupling. We demonstrate that features of the known Cu(I)-Cu(II)-In(III) mosaic alloy can be used to systematically evaluate candidates and experimentally realize new examples of mosaic alloys.

Overall, these mosaic halide-perovskite alloys represent a platform for rational design of intra-layer coupling interactions within layered halides beyond the conventional compositional limitations of single or double perovskites.

The development of efficient solar cells based on perovskite materials has emerged as a promising path for advancing photovoltaic technology. Specifically, 2D perovskites (2D-PVKs) have been found to exhibit higher stability properties against their 3D counterparts [1]. However, many factors are essential for an effective construction of these devices. Solvent engineering plays a critical role in controlling the crystalline structure and optoelectronic properties of 2D perovskites [2]. However, the existing and research-dominant perovskite solar cells (PSCs) base their formulations on highly toxic solvents such as DMF, increasing potential health risks for researchers and the environment [3].

Herein, we mainly focused on the development of novel solvent formulations, tailored to the specific requirements of 2D perovskite fabrication. Following solvent’s solubility parameters, we predict novel and green solvents that significantly enable the formation and growth of perovskite crystals. These new solvents along with the state-of-the-art fabrication processes aim to provide an enhanced control over crystal growth, minimizing defects, while meeting existing efficiencies of solar cells. All while prioritizing sustainable and environmentally green practices.

Our research extends beyond solvent considerations, seeking to use these new formulations to not only enhance efficiencies but also to contribute to the long-term stability that 2D PVKs offer. The strategic selection of solvents aims to mitigate degradation mechanisms, such as ion migration and moisture-induced instabilities, which are well-known challenges in perovskite solar cell technology.

In conclusion, this project underscores the role of solvent engineering and the selection of non-hazardous solvents in the fabrication of 2D perovskite films, and its further use in solar devices. The ongoing efforts to develop novel solvent formulations represent a stride towards a deeper understanding of perovskite crystallization and heighten the potential of these materials for its widespread adoption in energy conversion technologies.

Two-dimensional (2D) organic-inorganic metal halide perovskites are persistently emerging as a promising class of semiconductors, not only due to their superior intrinsic and extrinsic stability but also due to their virtually endless compositional possibilities. Especially the versatile organic cations offer many options to study the underlying physics of these materials as well as to modify key properties on a device and application oriented research path. Looking for more stable and less toxic alternatives to 2D Pb-based perovskites, double perovskites based on Ag, Cu, Bi and Sb are at the focus of the most recent research efforts.

We synthesized 2D n=1 Ruddlesden-Popper double perovskite based on large, conjugated cations, creating different types of band gap alignments to break the quantum confinement of the 2D layered crystal structure. We discuss the structural and optical properties, the photoluminescence, the electronic structure, the charge-carrier mobilities and the mixed- and out-of-plane conductivities for 8 new materials. We isolate the most promising organic cation and successfully incorporate it into the first pure n=1, parallel oriented, 2D RP double perovskite solar cell.

Two-dimensional layered metal-halide perovskites (2DLPs) represent an emerging class of materials where a semiconducting metal-halide octahedral layer is sandwiched between two layers of bulky organic cations. The distinctive structural characteristic of these materials results in high in-plane mobility of excitons and charge carriers, but at the same time, hinders the out-of-plane mobility due to the presence of mostly isolating organic cations [1], [2]. This limits the possibility of studying charge and energy transfer processes in vertical heterostructures of this material class. Instead, lateral heterostructures, wherein the composition changes along the in-plane direction, offer an intriguing approach for investigating potential transfer processes at the junction region.

We studied the formation of lateral heterostructures in the system PEA₂PbBr₄-PEA₂PbI₄ by developing a facile room-temperature anion exchange method in solution. Ion exchange methods are a common tool used to tune the optical properties in 3D metal-halide perovskites but are underrepresented in the case of 2DLPs [3]. The approach takes advantage of the highly anisotropic structure of 2DLPs, where the bulky organic cations suppress the vertical diffusion of the ions while lateral diffusion is preferred [4]. We observed that 2DLPs can be stabilized in polar solvents such as octanol by exposing them to the corresponding halide salt of the organic cation. Introducing a different halide salt of the cation compared to the parent 2DLP leads to the formation of lateral heterostructures, which initiate at the edges of the microcrystals and propagate toward the center. This process ultimately results in a core-crown-like microstructure containing 2DLPs of two different halides, each contributing to a distinct emission profile. We demonstrate the influence of different processing parameters like the type of ion source and the solvent on the microstructure and optical properties of the resulting heterostructure. Furthermore, we observe a strong dependency on the direction of the exchange process. Treating PEA₂PbBr₄ with an iodine source results in the predominant formation of nearly phase-pure PEA₂PbI₄ along the microcrystal peripheries. Conversely, subjecting PEA₂PbI₄ to a bromide source consistently yields an alloyed phase, with no discernible presence of a pure bromide phase. This behavior stands in stark contrast to that of 3D perovskites and is attributed to the preferential occupancy of halides in specific positions within the [PbX₆]^4- octahedral [5].

The formation of such a heterojunction in the in-plane direction of the semiconducting layer provides an opportunity to control the directionality of the charge carrier or energy flow toward the edges or the center of these microstructures. This, in turn, contributes significantly to a better understanding of the optoelectronic properties in heterostructured 2DLPs.

Halogen perovskites represent a family of semiconductors with optical and transport properties that can be widely modified through element substitutions or crystal structure distortions. They are processable in solution and find application in solar cells, LEDs, lasers and detectors. Despite the high efficiency of 3D perovskites, 2D halide perovskites are more stable, durable, and chemically versatile. These materials, characterized by a two-dimensional crystal structure, are composed of thin layers of perovskite separated by organic cations and specifically the single crystal atomic configuration confers several advantageous properties and characteristics. Indeed, in 2D single crystals the absence of grain boundaries enhances electronic properties, such as charge transport.
Achieving the right crystal thickness is crucial for device performance too, preventing carrier loss, and ensuring effective light absorption to unlock the full potential of carrier transport. This can be attained by a successful space-confined method, which allows to control the thickness during perovskite preparation. The last one is crucial for 2D and quasi-2D perovskites, since it is essential for perovskites in solar cells to align their crystallography orientation with the carrier collection direction into the device, to permit the conduction. Indeed, 2D perovskites have to deal with their preferential orientation growth, which is typically parallel to the substrate with which they are in touch, insulating the carriers into the layers and causing transport carrier problems into the device. To avoid this problem, the task is to modify the orientation growth from parallel to vertical to the substrate.

In this contribution my work related to space-confined and orientation-growth techniques will be explained, in which the details of the processes and results obtained will be explored. Specifically, by confining the growth within specific spaces introducing modifications to confinement surfaces, it was possible to dictate the size and shape of the resulting single crystal. Moreover, to change the crystallography orientation, we worked with the addition of additives and using functionalized cations as spacers, in order to favor the vertical layer disposition. The samples obtained have been analyzed by different instruments, such as AFM, confocal microscope, XRD and others, investigating their thickness, morphology and orientation.

Two dimensional perovskites 2DPs – intended to be single/few inorganic layers (typically <3) spaced by large organic cations – have attracted a wide interest for their modular wide band gap and proved stability, but their application in solar cells has been so far unsuccessful. Lack of knowledge on how to manipulate the orientation of the inorganic sheets during solution-based film deposition only enabling a parallel orientation to the substrate is the actual bottleneck. This limits vertical charge percolation and efficiency, ultimately making them uncompetitive in the solar cells arena.

Here I will discuss a novel effective strategy to break such efficiency limit by controlling the orientation of the inorganic backbone of thiophene-based 2DPs (with n<3) inducing a vertical growth of the inorganic framework with respect to the substrate. Integrated in a solar cell, we enable vertical charge percolation, reaching a “record” efficiency of 9.4 %. Such proof of concept intimately demonstrates the potential of controlling the crystalline orientation of 2DPs providing an essential strategy for boosting their performances. Beyond such demonstration and its validation monitoring the crystal orientation at the nanometric scale, we reveal the fundamental mechanism behind. Two key ingredients play a concomitant role: 1) inducing the formation of nanometer-size crystallites already in the precursor solution and 2) creating a Cl-rich 2DP phase at the bottom interface where Chlorine – differently from what commonly happens in 3D perovskite- substitutes Iodine entering in the structure and ultimately inducing the vertical growth.

This talk summarizes recent advances in tuning layered perovskite materials from a first-principles perspective, covering energy level alignment, spin character of energy levels, manipulating carrier concentrations by heteroatom doping as well as by coherent stimulation of charge oscillations between the organic and inorganic components. Energy level alignments between the organic and inorganic components are satisfactorily covered by large-scale, spin-orbit coupled hybrid density functional theory (HSE06) calculations, with simulation sizes up to several thousand atoms enabled by the FHI-aims code in an accurate all-electron approach. Using chiral organic cations, chirality is imparted to the inorganic part of the system via structural asymmetries and hydrogen bonds, leading to conduction band spin splitting that is characterized by a simple structural descriptor. In order to utilize this and similar effects in devices, manipulating carrier concentrations in the conduction bands and valence bands is an important prerequisite. We study Bi as a potential n-dopant in Pb based layered perovskites, as well as Sn as an effective p-dopant. Direct, large supercell calculations of these substitutions show that n-doping by Bi should be weak but attainable (with 0.55 eV offset to the conduction band edges in phenethylammonium lead iodide, Bi-induced doping levels are not shallow). However, a population of compensating deeper defects appears to limit the experimentally attainable doping efficiency. For Sn, we show that an enhanced tendency of forming Pb vacancies nearby is likely responsible for experimental observations of slight p doping. As a final point, we rationalize experimental observations of laser-controlled coherent charge oscillations in a layered perovskite in terms of electron-phonon coupling, offering a potential model for direct, picosecond control of carrier populations in perovskite materials.

The family of hybrid organic-inorganic lead-halide perovskites are the subject of intense interest for optoelectronic applications, including solar cells, light-emitting diodes and detectors. Interest in the sub-class of 2D layered hybrid perovskites has been increasing due to their structural and compositional versatility. This versatility allows for the incorporation of spacer cations with an extended conjugated system that are electronically active, as opposed to the electronically inactive alkylammonium- and phenylethylammonium-derived spacer cations that are most often used.[1]

I will present our work on 2D layered perovskites containing optically and electronically active carbazole-based Cz-Ci molecules [2-3], where Ci indicates an alkylammonium chain and i indicates the number of CH2 units in the chain, varying from 3−5. These 2D perovskites, with a formula of (Cz-Ci)2PbI4, show a tunable electronic coupling between the inorganic lead-halide and organic layers. The strongest interlayer electronic coupling was found for (Cz-C3)2PbI4, containing the carbazole spacer with the shortest alkyl chain length. Using ultrafast spectroscopy, we measure ultrafast hole transfer from the photoexcited lead-halide layer to the Cz-Ci molecules, the efficiency of which increases by varying the chain length from i=5 to i=3. The charge transfer results in long-lived carriers (10 – 100 ns) and quenched emission. Electrical charge transport measurements using single-carrier devices show markedly increased out-of-plane carrier mobilities compared to the reference 2D perovskite (PEA)2PbI4, with carrier mobility increasing from i=5 to i=3.[4]

Quasi-2D perovskites with Ruddlesden-Popper structure, with the formula C₂A_n-1Pb_nBr_3n+1 , where C is a bulky spacer cation, A is a small organic cation (methylammonium (MA), or formamidinium (FA)) or Cs⁺, and n is the number of octahedral layers between the spacer cation bilayers, have been attracting increasing attention for applications in light emission in blue and green spectral ranges. Quasi-2D perovskites with different spacer cations exhibit vastly different crystallization, photoluminescence, and stability. One of the methods to adjust crystallization and achieve favourable energy landscape for efficient funnelling to result in bright sky-blue or green emissions is to use a mixture of spacer cations. In this work, we investigated the use of different carboxylic group containing spacers, including 5-ammonium valeric acid (5AVA), 4-ammonium butyric acid (4ABA), 3-ammonium propionic acid (3APA) and their mixtures, for preparation of n=2 and n=3 quasi-2D perovskites with MA and FA small cations. The films generally exhibited bright emission, which could be further increased with different additives, and the cations used affected the ratios of different n phases present in the film, and ultimately emission colour. However, the films exhibited strong tendency toward disproportionation upon exposure to elevated temperature or ambient. Thus, we explored mixing the carboxylic group containing spacers (5AVA was selected as it produces very bright films) with a spacer cation forming films with exceptional stability, 2-thiopheneethylammonium, namely TEA. For the optimal 5AVA-TEA ratio, we can obtain films with predominantly n=2 phase even after ambient exposure for n=2 solution stoichiometry, very different from pure 5AVA-based films (entirely dominated by n=1 phase after 3 min ambient exposure), as well as pure TEA-based films (showing prominent presence of n=1 and n=2 phases with no significant change with ambient exposure). For n=3, pure TEA-based films again show negligible change with atmosphere exposure, while for pure 5AVA there is a significant presence of 3D perovskite.  In addition, for the same solution stoichiometry, changing the small cation affects the crystallization and consequently light emission, with sky blue emission achievable only for MA small cation and n=2 solution stoichiometry. The effect of solution composition (spacer cations used and their ratios to small cations) on the phase composition of resulting film and consequently light emission is discussed.

Hybrid organic-inorganic perovskites have become one of the leading materials in
photovoltaics, although their instability under operating conditions interferes with practical
applications. This can be overcome by using tailored organic systems that interact with hybrid
perovskite frameworks through various noncovalent interactions, which predominantly involve
aromatic π-based systems as interfacial modulators or spacers forming layered hybrid
perovskites. We demonstrate the unique capacity to broaden this material space by relying on
unorthodox σ-hole-based interactions that can enhance the functionality of hybrid perovskites,
improving their operational stability without compromising photovoltaic performances,
providing a versatile supramolecular strategy for advancing hybrid photovoltaics.

1. J. V. Milić*, Chimia 2022, 76, 784.
2. W. Luo, G. AlSabeh, J. V. Milić*, Photochemistry 2022, 50, 342.
3. J. V. Milić, J. Phys. Chem. Lett. 2022, 13, 9869.
4. M. A. Ruiz Preciado, D. J. Kubicki, A. Hofstetter, L. McGovern, M. H. Futscher, A.
Ummadisingu, R. Gershoni-Poranne, S. M. Zakeeruddin, B. Ehrler, L. Emsley, J. V. Milić, M.
Grätzel, J. Am. Chem. Soc. 2020, 142, 1645.
5. W. Luo, J. V. Milić*, et al. 2023, manuscript submitted