Despite a decade of research, much remains unknown about the role that grain boundaries play in impacting the photophysics of lead halide perovskites and influencing their performance in solar cells. In this talk, I will describe how by combining experimental studies with theoretical device simulations, we find that the recombination at grain boundaries is diffusion limited and depends on the grain area with small grains acting as recombination hot spots. We show that the distribution of grain sizes not only influences the overall performance of perovskite solar cells, but also leads to significant current exchange between small and large grains at open-circuit conditions. 

On the wave of their success in strategic fields of application, as energy conversion and light-emission, great effort was devoted to better understand and clarify the native semiconducting properties of halide perovskite materials. As a result, the electronic properties of these systems are nowadays well understood, with clear identification of the influence of quantum confinement and chemical tuning on the corresponding band structure, as relevant in the case of dimensionally tailored materials,[1] and in mixed halide or double perovskites,[2] respectively. The situation however becomes more complex when one goes beyond the single particle picture. The Coulomb interaction between the photogenerated electron-hole pairs in fact may result in the formation of stable excitons, with distinct properties (energetics, polarization, degeneracy, etc.) with respect to the corresponding single particle transitions.

Here, we aim to provide a general frame for the discussion of the exciton properties of metal halide perovskite frames, which encompasses effects like quantum confinement and chemical composition. Symmetry-based, group theory analysis will provide a sounded ground for the discussion, with clear indication about the expected splitting of the excitonic features in the materials, when going from 3D, to 2D and to other systems of practical relevance.[3] These results will be then referred to recent spectroscopic experiments[4-6] and to cutting edge first-principle calculations,[7] so demonstrating the possibilities of modern first-principle simulations tools in supporting experimental investigations and predicting the excitonic properties of halide perovskite materials. Excitons dominate the optical response of two-dimensional layered perovskite materials, where quantum and dielectric confinement enhance the electron-hole interactions;[1] still, they are shown to play a fundamental role also for double perovskites,[8] with serious impact on the relative role of dimensional confinement.[9] In addition, also for 3D lead-based halide perovskites, featuring exciton binding as small as 14-25 meV at room temperature,[10] the clear understanding and assignment of the spectral signatures must rely on an excitonic picture.[4-5]

Two-dimensional (2D) hybrid organic-inorganic perovskites are among the most promising materials for optoelectronic applications thanks to the great synthetic versatility that allows tailoring their structural and photophysical properties. The alternation between organic ligands and inorganic layers creates a natural multiple quantum well (MQW) structure where the inorganic layers act as potential ‘wells’ while organics serve as energetic ‘barriers’. This structure provides strong electronic confinement and, as a consequence, creates stable excitons with high binding energy at room temperature. [1]

In this talk I will show that large single-crystal flakes of 2D perovskite are able to establish strong light-matter coupling with the generation of exciton−polaritons. These half-light half-matter quasi-particles have unique properties: they possess strong intrinsic nonlinearities, inherited from their excitonic component and extremely small effective mass and long coherence length, inherited from their photonic component.

By comparing different hybrid perovskites with the same inorganic layer but different organic interlayers, it is shown how the nature of the organic ligands controllably affects the out-of-plane exciton–photon coupling. [2]

Moreover, 2D single-crystals perovskite are able to sustain strong polariton nonlinearities at room temperature, with exciton-exciton interaction energies remarkably similar to the ones known for inorganic quantum wells at cryogenic temperatures and more than one order of magnitude larger than alternative room temperature polariton devices reported so far. [3]

Because of their easy fabrication, large dipolar oscillator strengths, and strong nonlinearities, these materials ideal candidate for integrated photonic circuits and electro-optical devices working at room temperature. [4 - 6]

Hybrid perovskites have generated a lot of interest in the field of photovoltaics during the past decade. In recent years, perovskite solar cells regularly output over 20 % of photoconversion efficiency, approaching conventional Silicon based devices.

Even though, remarkable optoelectronic properties of hybrid perovskites and easy fabrication routs have enabled fast development of devices, the common fabrication methods still result in formation of substantial density of defect states in hybrid perovskite thin films [1]. These defects when act as charge trapping centers, have been known to induce non-radiative losses and limit the maximum photoconversion efficiency of perovskite solar cells [2]. The progress in fundamental understanding of these defects, in particular about their formation sites and their exact impact on device efficiency, has been slower. Such knowledge, however, is critical to design successful passivation strategies and improve performance and viability of perovskite photovoltaic devices.

Here we elucidate the very origin of nanoscale defects that form in state-of-the-art solution processed triple cation mixed halide perovskite thin films, and evaluate their roles in charge trapping processes. We employed photoemission electron microscopy to directly image the distribution of defects on nanoscale [3] and revealed their spatial arrangement with respect to surface morphology. By adding the time resolution to our photoemission experiments, we accessed the exact roles of these defects in performance of perovskite thin films by monitoring charge trapping processes that occur at defect sites. We thus uncovered the presence of multiple types of nanoscale defect clusters, and found that depending on their origin, they play very different roles in photoexcited hole trapping – from very detrimental to relatively benign [4]. Our results highlight the need to develop targeted approaches to remove each undesired type of defect.

Spatial photoluminescence heterogeneity is a frequently recognized phenomenon in lead halide perovskite thin films meant for efficient photovoltaic devices. Deep trap states associated with local variation in structure, composition and strain are the main players dictating this photoluminescence heterogeneity and the device performance. These trap states can appear as nanoscale clusters and majorly reside in the grain boundaries.¹ Despite notable progress in understanding the fundamental aspects of these traps such as origin, location, and distribution, real time response of these trap states remains elusive. Temporal photoluminescence heterogeneity or intensity fluctuation introduced by the traps can be a major loss channel in the film and would require detailed investigation. In this context several questions that can arise are: (i) Are these photoluminescence quenching traps static or metastable? (ii) what are the quenching domains of these traps? (iii) Do they have any correlation with grain size and grain boundaries? (iv) Do they have any working timescale? (v) How does the surrounding environment influence formation and annihilation of these traps?

We observed large photoluminescence fluctuation in MAPbI₃ thin films when investigated in real time. The fluctuation varies as a function of grain size (500 nm-several µm) and surrounding environment. This fact indicates presence of metastable traps which can deplete significant amount of photogenerated carriers even in good quality films. Unlike spatially isolated crystals, proximity of the grains and crystallites in thin films provide a mixed photoluminescence signal making the trapping dynamics and carrier recombination more complex for individual grains. We have developed an advanced photoluminescence mapping technique based on correlation of the intensity fluctuation which divide the image into microscale clusters having high intra-cluster and low inter-cluster correlation, respectively. Size of the clusters changes as the grain size or the surrounding environment changes from air to inert. By correlating these clusters with electron microscopy image, we obtain important insight about the microscale quenching domain of these metastable traps. Power spectral density estimation of the intensity fluctuation of each cluster reveals existence of different types of metastable traps with timescales ranging from hundreds of milliseconds to tens of seconds.² Distinctly different response time of these quenching traps are noted when grains size is varied, and porosity is introduced into the sample. This approach demonstrates a methodology to understand several structural (quenching domain, grain size dependence) and dynamical aspects (metastability and working timescale) of the photoluminescence quenching traps in any thin films.

High environmental stability and surprisingly high efficiency of solar cells based on 2D perovskites have renewed interest in these materials. These natural quantum wells consist of planes of metal-halide octahedra, separated by organic spacers.  Remarkably the organic spacers play crucial role in optoelectronic properties of these compounds. The characteristic for ionic crystal coupling of excitonic species to lattice vibration became particularly important in case of soft perovskite lattice. The nontrivial mutual dependencies between lattice dynamics, organic spacers and electronic excitation manifest in a complex absorption and emission spectrum which detailed origin is subject of ongoing controversy. First, I will discuss electronic properties of 2D perovskites with different thicknesses of the octahedral layers and two types of organic spacer.  I will demonstrate that the energy spacing of excitonic features depends on organic spacer but very weakly depends on octahedral layer thickness. This indicates the vibrionic progression scenario which is confirmed by high magnetic fields studies up to 67T. Furtheremore, I will show that in 2D perovskites, the distortion imposed by the organic spacers governs the effective mass of the carriers.  As a result, and unlike in any other semiconductor, the effective mass in 2D perovskites can be easily tailored. Finally I will discuss the exciton fince structure. Optically inactive dark exciton states play an important role in light emission processes in semiconductors because they provide an efficient nonradiative recombination channel. Understanding the exciton fine structure in materials with potential applications in light-emitting devices is therefore critical. Our studies of the exciton fine structure in the family of two-dimensional (2D) perovskites show that in-plane magnetic field mixes the bright and dark exciton states, brightening the otherwise optically inactive dark exciton. The bright-dark splitting increases with increasing exciton binding energy. Hot photoluminescence is observed, indicative of a non-Boltzmann distribution of the bright-dark exciton populations. We attribute this to the phonon bottleneck, which results from the weak exciton–acoustic phonon coupling in soft 2D perovskites. Hot photoluminescence is responsible for the strong emission observed in these materials, despite the substantial bright-dark exciton splitting.

Quasi-two-dimensional (Q2D) organic-inorganic halide perovskites are an exciting family of heterogeneous layered semiconductors, which combine chemical and structural versatility of 3D organic-inorganic metal-halide perovskites with the ability to design light-matter interactions driven by structural and dielectric confinement [1]. They are a broad and versatile playground not only for materials discovery, but also for exploring fundamental excited state physics and development of novel optoelectronic applications. In particular, the structural and chemical diversity of this family of materials offers a unique opportunity to understand the role of chemistry and structure on photoexcited states of complex semiconductors in a systematic way; first principles computational modeling takes a key role in this context.
In this talk, I will present our recent work understanding optical excitations complex Q2D organic-inorganic halide perovskites using state-of-the-art first principles computational modeling techniques, such as the GW method [2] and the Bethe-Salpeter equation [3]. I will present our first principles calculations of optical excitations in single-layered Q2D halide perovskites, revealing the role of the organic cations in enhancing the dielectric screening of these complex systems, and show our recent work on understanding the optoelectronic properties of self-assembled perovskite-non-perovskite self-assembled interfaces [4].

References:
[1] Smith, Crace, Jaffee & Karunadasa, Annu. Rev. Mater. Res., 48, 111-136 (2018).
[2] Hybertsen & Louie, Phys. Rev. B 34, 5390 (1986).
[3] Rohlfing & Louie, Phys. Rev. Lett. 81, 2312 (1998).
[4] Aubrey, Valdes, Filip, Connor, Lindquist, Neaton & Karunadasa, Nature, 597, 355-359 (2021).

Two-dimensional Ruddlesden-Popper metal halides (2D-RPMHs) are materials composed of quasi-2D layers of metal-halide octahedra separated by long (~1nm) organic cationic layers. The latter facilitate electron and hole quantum confinement within the metal-halide layers resulting in a quantum-well like structure. Properties of excitons (i.e., the  electron-hole bound states) in such structures are characterized by strong binding energy (>200 meV) arising from the dynamically screened Coulomb interactions [1]. We have experimentally observed that polaronic effects arising from the lattice dressing of the carriers, are not only active but that they fundamentally define excitons in 2D-RPMHs [2]. We thus refer to such excitons as the exciton-polarons, with properties that are measurably distinct than those of free excitons in semiconductors [1]. In this talk, I will discuss the quantum dynamics of exciton-polarons and provide spectroscopic insights into the peculiar phonon-phonon [3], exciton-phonon and exciton-exciton [4] interactions. I will present our perspective on how the coherent optical response of 2D perovskites can be effectively rationalized within the “exciton-polaron” framework, in which lattice dressing of photo-carriers constitute an integral component of excitonic wavefunction [1], with consequences on exciton recombination dynamics and diffusion.

References

[1] A. R. Srimath Kandada and C. Silva, J. Phys. Chem. Lett., 11, 3173-3184 (2020).

[2] F. Thouin, D. Valverde-Chavez, C. Quarti, D. Cortecchia, I. Bargigia, D. Beljonne, A. Petrozza, C. Silva and A. R. Srimath Kandada, Nature Materials, 18, 349-356 (2019).

[3] E. Rojas-Gatjens, C. Silva-Acuna and A. R. Srimath Kandada, Peculiar anharmonicity of Ruddlesden Popper metal halides: Temperature dependent dephasing, Materials Horizons (2022).

[4] A. R. Srimath Kandada, H. Li, F. Thouin, E. R. Bittner and C. Silva, Stochastic scattering theory for excitation-induced dephasing: Time dependent nonlinear coherent exciton lineshapes, J. Chem. Phys., 153, 164706 (2020).

Metal halide double perovskites, especially the Cs₂AgBiBr₆, have attracted the attention of researchers in the search for less toxic candidates as active materials in optoelectronic devices.^[1,2] From this 3D structure, low-dimensional double perovskites are fabricated by introducing large organic cations, resulting in organic/inorganic architectures with one or more inorganic-octahedra layers separated by organic cations.^[3] Here, we synthesize a series of layered double perovskites from Cs₂AgBiBr₆ that consist of double (2L) or single (1L) inorganic octahedra layers, using ammonium cations of different size and chemical structure. By carrying out temperature-dependent Raman spectroscopy measurements, we highlight phase transition signatures in both inorganic lattice and organic moieties by detecting changes in the position and linewidth of the vibrational modes. Variations in the conformational arrangement of the organic cations from an ordered to disordered state match with a phase transition in the 1L systems with propyl- and butyl-ammonium moieties. Density functional theory calculations of the band structure reveal that the bandgap is direct only for the 1L crystal structures, and it becomes indirect in 2L systems. The direct bandgap character in 1L compounds stems from an extremely flat lowest conduction band, which enables small octahedral distortions to produce significant changes in the band structure across the bandgap. This translates to the optical properties, where we observe significant changes of photoluminescence intensity around the transition temperature as result of octahedral tilting rearrangement emerging at the phase transition.^[4] Our work provides novel insights into the structure-optical properties relationship in layered double perovskites, and demonstrates how the proper selection of the organic cations in the layered perovskites is relevant in terms of thermal stability or the use of the phase transition for active switching in future applications.

Solution processable semiconducting perovskites hold great promise for demanding applications involving light emission, such as printable lasers or quantum light sources. A case example is that of fully inorganic CsPbBr₃ quantum wells (CQWs) which display high quantum yield at room temperature. Less studied than their organic-inorganic counterparts, these CQWs sustain high single exciton binding energies and luminescence quantum yields, yet little is known on the nature of the exciton and multi-excitons states required for advanced applications. Here, we show that charge carriers in fully inorganic 2D perovskites exist as stable exciton - polarons, a complex between a charge neutral exciton and a lattice deformation. Next, we show that these unique species can fuse together to form a hereto unexplored bi-exciton polaron state, i.e. a two-particle complex bound by attractive Coulomb attraction whilst simultaneously being strongly coupled to the lattice. Finally, we show that net stimulated emission occurs through radiative recombination from this unique bi-exciton polaron state to a single free exciton polaron, showing the stability of the newly found species. Consequences of the polaronic character are identified as a low threshold for stimulated emission but with limited optical gain coefficients, both of which we can fully reproduce using a thermodynamic gain model. As such, our results provide a general framework to understand and predict the behavior of not only single, but also multi-exciton polaron states in perovskite materials.

Two-dimensional (2D) hybrid organic−inorganic perovskites are attracting interest thanks to their wide range of applications for solar energy conversion and beyond. Among their advantages, the tunability of their structural and optical properties is unique. For example, as the structural features of the organic spacers vary, the distancing between the inorganic layers can be affected – which affects the quantum confinement and hence the bandgap of the 2D perovskite. To experimentally elucidate the effect of the organic spacer on the optical and structural properties of perovskites, we applied hydrostatic pressures up to 375 MPa onto 2D materials with different spacers - e.g. butylammonium, benzylammonium, phenylehtylammonium, naphthalene-O-propylammonium, and perylene-O-ehtylammonium. In addition to a different degree of rigidity and extent of bandgap shift of the 2D materials for different spacers, we found that when butylammonium is used as the spacer, the 2D material undergoes a first-order phase transition at 350 MPa. By studying the specific effect of aromaticity and alkyl chain length of the organic spacers on the overall rigidity (hence, bandgap shift) of the 2D material and finding a pressure-dependent phase transition in one of the studied materials, our work is essential to designing 2D perovskites with finely tuned optical and structural properties.

Halide perovskites are the new wonder material of the optoelectronics community due totheir outstanding photoluminescence quantum yield, tunable emission wavelength and simplesolution or vapor-phase deposition. At the same time, their facile ion migration andtransformation under optical, electrical and chemical stress are seen as a major limitation fordevice implementation. Mixed halide perovskites are particularly problematic sinceoptical excitation can cause changes in the band gap that are detrimental for solar cell and light-emitting diode efficiency and stability. In this work, instead of preventing suchchanges, we exploit photo-induced halide segregation in perovskites to enable responsive,reconfigurable and self-optimizing materials. We show how a mixed halide perovskite film can betrained to give highly directional light emission using a nanophotonic microlens: through a self-optimized process of halide photosegregation, the system mimics the training stimulus. Longertraining leads to more highly directional emission, while the different halide migration kineticsin the light (fast training) and dark (slow forgetting) allow for material memory. This self-optimized material performs significantly better than lithographically aligned quantum dots[1], because it eliminates lens-emitter misalignment and automatically corrects for lens aberrations.Our system shows a combination of mimicking, improving over time, and memory, which make itcompatible with the basic requirements for learning,[2,3] and give the intriguing prospectof intelligent optoelectronic materials

Hybrid perovskite photovoltaic devices have rapidly emerged as promising contenders for next generation, low-cost solar cell technology. Yet, the presence of defect states critically impacts device operation, including device efficiency and potentially long-term stability. Understanding the nature of these defects and their role in photocarrier trapping, requires techniques that are capable of probing ultrafast photocarrier dynamics at the nanoscale.

            In this talk, I will discuss the development of time-resolved photoemission electron microscopy (TR PEEM) techniques in my lab [1], [2], applied to hybrid perovskite solar materials. Thereby, we directly visualize the presence of the performance limiting nanoscale defect clusters and elucidate the role of diffusion in the charge carrier trapping process [3]. By correlating PEEM measurements with other spatially resolved microscopies, we identify different types of defects that form, and study how passivation strategies may have a varied impact on them [4].

Lead-halide perovskite APbX₃ (A=Cs or organic cation; X=Cl, Br, I) quantum dots (QDs) are subject of intense research due to their exceptional properties as both classical¹ and quantum light sources.^2-4 Here^5,6 we present perovskite-type (ABO₃) binary nanocrystal superlattices, created via the shape-directed co-assembly of steric-stabilized, highly luminescent cubic CsPbBr₃ nanocrystals (which occupy the B and/or O lattice sites), assembled in combination with spherical Fe₃O₄ or NaGdF₄ nanocrystals (A sites). These ABO₃ superlattices, as well as the binary NaCl and AlB₂ superlattice structures that we demonstrated, exhibit a high degree of orientational ordering of the CsPbBr₃ nanocubes which preserve their high oscillator strength and long exciton coherence time in the assembly. Such superlattices exhibit superfluorescence—a collective emission that results in a burst of photons with ultrafast radiative decay (22 picoseconds) that could be tailored, by structural engineering of the nanoparticle assembly, for use in ultrabright (quantum) light sources. Our work paves the way for further exploration of complex, ordered and functionally useful perovskite mesostructures.

References

[1] Akkerman et al., Genesis, challenges and opportunities for colloidal lead halide perovskite nanocrystals. Nat. Mater. 17, 394–405 (2018).

[2] Becker et al., Bright triplet excitons in caesium lead halide perovskites. Nature 553, 189–193 (2018).

[3] Rainò et al., Superfluorescence from lead halide perovskite quantum dot superlattices. Nature 563, 671–675 (2018).

[4] Utzat et al., Coherent single-photon emission from colloidal lead halide perovskite quantum dots. Science 363, 1068–1072 (2019).

[5] Cherniukh et al., Perovskite-type superlattices from lead halide perovskite nanocubes. Nature 593, 535–542 (2021).

[6] Cherniukh et al., Shape-Directed Co-Assembly of Lead Halide Perovskite Nanocubes with Dielectric Nanodisks into Binary Nanocrystal Superlattices ACS Nano 15, 10, 16488–16500, (2021).

Lead halide perovskites have emerged as promising new semiconductor materials for high-efficiency photovoltaics, light-emitting applications and quantum optical technologies. Their luminescence properties are governed by the formation and radiative recombination of bound electron-hole pairs known as excitons, whose bright or dark character of the ground state remains debated.

Spectroscopically resolved emission from single lead halide perovskite nanocrystals at cryogenic temperatures provides unique insight into physical processes that occur within these materials. At low temperatures the emission spectra collapse to narrow lines revealing a rich spectroscopic landscape and unexpected properties, completely hidden at the ensemble level and in bulk materials.

In this talk, I will discuss how magneto-photoluminescence spectroscopy provides a direct spectroscopic signature of the dark exciton emission of single lead halide perovskite nanocrystals. The dark singlet is located several millielectronvolts below the bright triplet, in fair agreement with an estimation of the long-range electron hole exchange interaction. Nevertheless, these perovskites display an intense luminescence because of an extremely reduced bright-to-dark phonon-assisted relaxation. Spectral multiplets assigned to the biexciton recombination are also revealed at the single nanocrystal level, which directly provides the biexciton binding energy and evidence for extremely slow spin relaxation between the exciton fine structure sublevels.

Lead halide perovskites (LHP) nanomaterials have emerged as an exciting new class of semiconductor materials with outstanding electronic and optical properties, easy synthesis, and high photoluminescence quantum yield. Beyond their solar cell application, they have been extensively exploited in diverse fields such as LED technology, laser, and more recently in photocatalysis. [1]

They respond to the general formula APbX₃, where A is an organic cation such as methylammonium, formamidinium or inorganic cation such as Cs⁺, and X is the halide anion (Cl^-, Br^- or I^-). The key role of the organic ligands for surface passivation and colloidal dispersion has been demonstrated in the preparation of the first CH₃NH₃PbBr₃ nanocrystals in 2014, using octylammonium bromide and oleic acid as capping agents. [2] The nature of the organic ligands can determine the dimensionality of the inorganic framework and the dimension of the perovskite material, and therefore their optical properties.

The synthesis of colloidal low dimensional perovskites with deep blue-emissive will be presented. In particular, ultrathin [CsPbBr₃]PbBr₄ nanoplatelets and pure two-layered L₂[CH₃NH₃PbX₃]PbX₄ perovskite nanosheets were synthesized by thermal/ultrasound methodology and ligand assisted techniques, respectively. [3, 4] Their emissive properties and chemical/photochemical stability will be discussed. The long-term stability in the solid-state together with the high processability of the colloidal nanosheets enabled the preparation of blue-emissive solid films, which may be suitable for assembling on a large area on-demand and, consequently, be useful for preparing high-quality films for flexible and ultrathin optoelectronic devices, or they could be combined with other types of 2D materials.

Are all important non-radiative recombination centres in perovskites metastable?

Ivan G. Scheblykin

Chemical Physics and Nano Lund, Lund University, Sweden

Despite of very substantial efforts, it is still difficult to quantitatively account for charge dynamics in metal-halide perovskites over a broad range of excitation condition by the standard theoretical approaches of semiconductor physics.¹

One aspect which has never been taken into account in any modelling of this kind is defect metastability. A defect state is metastable if, for example, it undergoes transition from its active state, in this state it induces strong non-radiative (NR) recombination, to its passive state where it does not influence charge dynamics at all.² Obviously, defect metastability must influence charge carrier dynamics substantially, especially if switching of the defects is light or/and charge carrier concentration dependent. Evidence for the defect metastability is overwhelming:

1. Photoluminescence (PL) blinking of individual microcrystals and even grains of polycrystalline films.³

2. Reversible PL enhancement and PL bleaching^4,5

3. Self-healing property of metal-halide perovskites in general terms⁶

I will discuss how we can see the metastable NR centers in action by observation of PL blinking. I will argue, that most probably all important defect states causing NR recombination, which scientists from all fields from fundamental to applied care about, are, in fact, metastable and see then in PL blinking experiments. So far, I do not see any experimental observations which would disprove to this hypothesis.

Despite that PL blinking in perovskites is known from 2015,⁷ so far there had been no real connection drawn from the results obtained on individual crystals and properties of films and devices. If all important defects are metastable, understanding this metastability becomes very crucial with PL micro spectroscopy as an ideal tool.

Metal-halide perovskites (MHPs) hold great potential for next-generation optoelectronic technologies due to their remarkable characteristics. While the influence of strain on the intrinsic properties of MHPs is gaining interest, its combined effect with an external electric field has been largely overlooked. Here we perform an electric-field-dependent PL intensity study on heteroepitaxially strained surface guided CsPbBr₃ nanowires. We reveal an unexpected linear coupling between the alternating field and the PL intensity with a significant modulation depth (up to 40%), stemming from an induced internal dipole. Low-frequency polarized-Raman spectroscopy shows structural modifications under an external field, associated with the observed polarity. These results reflect the important interplay between an external field and strain in MHPs and offer new insight into the design of novel MHP-based nano-optoelectronics.

Perovskite solar cells, with solution-based, cheap synthesis methods and a rapid increase in power conversion efficiency, are a promising candidate for future solar cells. However, a major hurdle for commercialization remains, namely the intrinsic instability of these systems. Ion migration, the process by which the A, B and ions of the ABX₃ structure become mobile in the perovskite layer, represents a key challenge to tackle.

MAPbBr₃ was shown to be more stable under environmental conditions when compared to MAPbI₃. Using transient ion drift, we show that this stems from key changes in ion migration when going from MAPbI₃ to MAPbBr₃: methylammonium migration is suppressed, while bromide migration is reduced. Nowadays, state-of-the-art perovskite devices combine multiple ions: we therefore extend our study to ion migration in mixed-halide perovskites with varying ratios of iodide to bromide, and find interesting dynamics regarding the phase segregation phenomenon. In order to benefit both from the high efficiency of the 3D perovskites and from the stability of the 2D perovskites, new device architectures composed of a 2D layer on top of a 3D layer have emerged. We quantify the ion migration dynamics in these mixed-dimensionality perovskites, and find that ion migration is hindered in all systems incorporating a 2D layer. The specific hindering mechanism is however dependent on the 2D spacer molecule. Finally, we investigate the evolution of ion migration in different MAPbBr₃ solar cells as a function of the grain size of the active perovskite film. We show that beyond composition engineering, crystallinity can be another effective tool to control ion migration.

The broad implementation of halide perovskites into electronic applications beyond photovoltaics relies on effectively tuning free carrier concentrations in this class of semiconductors. However, doping in these materials remains difficult and is far from being fully understood. Specifically, detailed knowledge on the perovskite-dopant chemical interactions is key to enable effective doping strategies. This talk will cover our recent work on carrier compensation in inherently p-type MASn_xPb_1-xI₃ (where MA is methylammonium) by employing an n-type molecular dopant, i.e. n-DMBI. We observe a decrease in free hole concentration of nearly one order of magnitude, an increase in Seebeck coefficient and a shallower perovskite Fermi level, in consistence with electron donation from n-DMBI. This is shown to occur via dopant hydride loss and subsequent radical transfer to perovskite. We then find that only oxidised molecular dopant binds onto perovskite surface Sn sites via Lewis acid-base interactions, suggesting that charge transfer is facilitated by dopant/perovskite chemical coordination. We expect the detailed chemical insight on charge transfer doping provided herein to give design guidelines towards future molecular dopants for perovskite-based technologies.

In this work we show that thin polycrystalline films of spin-coated formamidinium tin triiodide (FASnI₃) perovskite demonstrate efficient random lasing (RL) possessing distinguished optical properties such as high mode stability, very high quality factor (~ 10⁴) and low threshold (≈ 2 and ≈ 25 microJ/cm² for 15 and 300 K, respectively). While usually RL is characterized by chaotic changes of the narrow lines spectral positions, the found in our case RL mode stability is a unique property, which is observed here for the first time for polycrystalline semiconductor systems. We demonstrate that the efficient random lasing and mode stability are due to the high efficiency of light scattering by FASnI₃ grains, which is a result of high refractive index of the material (higher than in the case of Pb-based halide perovskites) as well as of optimized grain size distribution (average size is close to the emission wavelength). We demonstrate also that different excitation conditions are required for generation of ultra-narrow RL lines and rather broad-band Amplified Spontaneous Emission (ASE): RL lines dominate when excitation spot size is a few tens of micrometers, while ASE dominates in case of large size excitation spot of a few hundreds of micrometers. From the above, it follows that the stability in time of narrow RL lines is ensured by the strong space localization of RL modes and the absence of interaction between them [1].

[1] Chirvony, V. S.; Suárez, I.; Sanchez-Diaz, J.; Sánchez, R. S.; Rodríguez-Romero, J.; Mora-Seró, I.; Martínez-Pastor, J. P. Stable non-chaotic high Q near-infrared random lasing in thin polycrystalline films of lead-free formamidinium tin triiodide perovskite. To be submitted.

Transient photoluminescence (trPL) is a powerful technique to study charge carrier dynamics and identify carrier recombination channels in luminescent thin films and devices. It has been widely used to quantify carrier radiative and non-radiative lifetimes,^[1] to compute the charge extraction velocity ^[2], and to study the influence of the interfaces on the recombination mechanisms.^[3]

Data fitting helps you extract this information from experimental trPL decays, but considering the number and nature of involved variables, the set of extracted values is most of the time not unique. Indeed, the trPL decay is ruled by the radiative and non-radiative recombination processes which follow, respectively, a quadratic and a linear dependence on the photogenerated carriers. The concentration of these carriers depends on several parameters, such as the injection level, the doping of the emitting material, the radiative recombination coefficient, and the trap density. Because of this interdependence, it is easy to obtain comparable trPL decay fits by using different combinations of values and, hence, interpreting incorrectly the recombination events. To obtain reliable information, it is better to use a robust simulation approach where the parameters are distinctly assessed and recursively optimized according to experimental data from the literature.

We demonstrate how to simulate experimental trPL curves of a halide perovskite on glass by fitting the data with a fully-coupled 1D optoelectronic simulation. The validity of the simulated trPL curves is also verified by fitting the decay with a bi-exponential function, which allows estimating the lifetimes of the different recombination mechanisms. We will present a robust routine to ascertain the reliability of the obtained results. This routine is based on experimentally measured material properties and on the analysis of the band diagram evolution during the PL transient. Our approach is not limited to the analysis of bare perovskites. We will show also a first analysis of the influence of the absorber interfaces on the trPL. The mentioned routine for trPL simulation can be further extended with the inclusion of photon recycling.^[4]

The renaissance of interest in halide perovskites, triggered by the discovery of their unprecedented performance in opto-electronic applications, elicited worldwide efforts to uncover a variety of intriguing physical properties, with a special interest about spin-orbit and Rashba/Dresselhaus effects. The current work presents for the first-time magneto-optical experimental evidence for non- uniaxial Rashba field with surplus contribution by a Dresselhaus effect arising from the bulk of MAPbBr3. Magneto-photoluminescence (MPL) spectra, monitored along several different crystallographic directions, were dominated by dual exciton emission peaks, while each exhibited a highly non-linear response to a magnetic field, with distinguished behavior at extreme points. Moreover, these plots depicted anisotropy from-B0 to +B0, with strong dependence on the axis of observations. Furthermore, ground state electron spin resonance spectroscopy, illustrated resonances bands coinciding with the extreme points in the MPL spectrum. Those resonance appeared only after illumination and disappeared a few hours later, designating an association with electron-nuclear coupling (the Overhauser effect. A theoretical model corroborating the experimental results, implementing anisotropic Rashba/Dresselhaus terms, anisotropy in the Landé g-factors, and a lesser contribution from an Overhauser effect, corroborated the experimental results. The origin of the Rashba/Dresselhaus effects in the bulk of halide perovskites is assumed to be attributed to the presence of grain boundaries. The research discoveries put a basic ground for expansion of possible applications of the halide perovskites toward spin-base devices.

During recent years, the picture of dynamic charge carrier screening in lead halide perovskites (LHPs) evolved to explain long carrier lifetimes and diffusion lengths, leading to the nowadays famous optoelectronic properties of this material class. However, the most important prerequisite for such anticipated structural screening, the direct observation of related lattice- or organic cation motion within the estimated polaron formation time (<1ps), remains elusive. By developing two-dimensional optical Kerr effect (2D-OKE) spectroscopy, we unveil that the ultrafast polarization response to near-bandgap excitation pulses is overwhelmed by the instantaneous electronic polarizability [1]. In this case, the nonlinear mixing of highly anisotropic and dispersive light propagations is responsible for oscillatory Kerr-type modulations at terahertz (THz) frequencies [1, 2]. Therefrom-arising 2D-OKE fingerprints can be harnessed to trace lattice anisotropy across non-trivial phase transitions or in multi-cation alloyed LHPs [3]. Despite this useful characterization of static lattice distortions, the dynamic lattice response seems elusive for the OKE with excitation pulses in the visible spectral range.

Ultimately, to study the ultrafast lattice response to a transient electric field, we move the excitation to the THz spectral region, resonant to the optical modes of the lead halide lattice, by employing intense phase-stable THz fields. Conceptually, this close to single-cycle electric field with a rise-time below 1 ps, can be seen as proxy for ultrafast electron-hole separation during optical excitation and consecutive thermalization. By probing the THz-induced Kerr effect (TKE), we observe a strong THz polarizability in both inorganic CsPbBr₃ and in the hybrid MAPbBr₃ materials. Also here, we show that it is crucial to account for dispersion and optical anisotropy in interpreting the transient responses via rigorous four-wave mixing simulations. Nevertheless, we finally observe a contribution from the lattice polarizability by witnessing the nonlinear excitation of coherent phonons, in full agreement with static Raman spectra. Moreover, we pinpoint a single phonon mode which dominates the lattice polarizability, providing another stepping stone towards a better understanding of how lattice dynamics enable charge carrier protection in this class of defect tolerant semiconductors.

References:

[1] S.F. Maehrlein et al., PNAS, 118,7, e2022268118 (2021)

[2] L. Huber et al., J. Chem. Phys., 154, 9, p. 094202 (2021)

[3] F. Wang et al., J. Phys. Chem. Lett., 12, 20, 5016–5022 (2021)