Symposium EN08—Low-Dimensional Halide Perovskites—From Fundamentals to Applications

Halide perovskite nanocrystals have emerged as excellent light sources for light-emitting devices and lasers due to quantum yields approaching unity and an emission wavelength tunable throughout the visible range. As recently shown, it is the only material currently known that could possess a lowest-lying bright exciton state, greatly enhancing radiative efficiency. This last feature, known as bright-dark exciton inversion, is induced through the Rashba effect, resulting from breaking the nanocrystal inversion symmetry. Nanoplatelets, colloidal semiconductor quantum wells, have additional advantages for light emission, as their radiative rates and outcoupling efficiencies are strongly enhanced through quantum confinement and dielectric confinement effects.
Here, we investigate the excitonic fine structure of thickness tunable halide perovskite nanoplatelets by merging time- and temperature-resolved photoluminescence spectroscopy with an effective mass model. We find a thickness-dependent bright-dark exciton splitting reaching up to 33 meV, the largest ever reported value. Additionally, we find that the bright exciton is further split by up to 16 meV for the in-plane and out-of-plane polarized states. Due to these immense splitting energies, the radiative properties of the nanoplatelets will also be affected at room temperature. The experimental results obtained in this paper are in excellent agreement with the novel theory, which takes quantum confinement and dielectric confinement anisotropy into account. Notably, the model we develop can be directly applied to any semiconductor nanoplatelet and generalized for any nanostructure.

Multicomponent perovskite solar cells have reached the recent efficiency breakthrough of 25.5%, higher than silicon polycrystalline photovoltaics. Such fantastic result was only possible due to a precise control and engineering of the morphology, interfaces and the use of multiple cations in perovskite A-site. For tandem perovskite solar cells, a mixture of different anions, as Br and I is also desired to adjust the band gap. Such cocktail of different cations and anions influences the formation of intermediates, new phases, favours halide homogenization, etc; so that at the end, the efficiency of the device is closely related to not only the optical quality of the film (e.g. crystallinity), but morphology and composition.
In this presentation, we will summarize important results using in situ experiments to probe perovskite formation (2D and 3D), stability and composition. We employed time-resolved grazing incidence wide angle x-ray scattering (GIWAXS), small angle x-ray scattering (SAXS) and high-resolution XRD taken at the Brazilian Synchrotron National Laboratory.
In situ GIWAXS experiments allowed us to understand the influence of the relative humidity and time to drop the antisolvent during the preparation of perovskite films and get important information about final composition and morphology [1]. It is well known that a 2D layer on the top of a 3D bulk perovskite improves stability and performance. In situ GIWAXS revealed us that during thermal annealing the 2D layer transforms itself into a disorder layer, improving hole transfer and stability [2]. This technique was also employed to identify the first intermediates formed during the degradation of different Cs and Br perovskite compositions under ambient conditions [3].
In situ SAXS is another powerful technique to follow the first stages of the 2D perosvkite’s formation. Our results suggest that the formation of the individual slabs in BA₂[FAPbI₃]PbI₄ is quite fast (within the first 10 s) and, then, these slabs self-assemble into bulk crystallites during the next 40 minutes [4].
Another topic of interest is the photoinduced phase segregation in FACs-based perovskite films. We investigated this phenomenon by photoluminescence (PL) studies and in situ high-resolution XRD (dark and under illumination). We found out that cubic-tetragonal transition is able to stabilize the samples with higher Br and Cs content, however this is not true for the films with 17% Br. Thus, other mechanisms must be operating and ruling phase segregation [5].
[1] R. Szostak, S. Sanchez, P. E. Marchezi, A. Marques, J. C. Silva, M. S. Holanda, H. C. N. Tolentino, A. Hagfeldt, A. F. Nogueira, “Revealing the perovskite film formation using the gas quenching method by in situ GIWAXS: morphology, properties and device performance”, Advanced Functional Materials, 31, 2007473 (2021)
[2] A. Sutanto, R. Szostak, N. Drigo, V. Queiroz, P. E. Marchezi, J. C. Germino, H. N. Tolentino, M. Nazeeruddin, A. F. Nogueira, G. Grancini “In Situ Analysis Reveals the Role of 2D Perovskite in Preventing Thermal-Induced Degradation in 2D/3D Perovskite Interfaces", Nano Letters, 20(5) 3992-3998 (2020)
[3] P. E. Marchezi, E. M. Therézio, R.Szostak, H. C. Loureiro, K. Bruening, A. Gold-Parker, M. A. Melo Jr., C. J. Tassone, H. C. N. Tolentino, M. F. Toney, A. F. Nogueira, “Degradation mechanisms in mixed-cation and mixed-halide Cs_xFA_1-xPb(Br_yI_1-y)₃ perovskite films under ambient conditions” J. Mater. Chem. A, 9, 9302-9312 (2020)
[4] R. F. Moral, L. G. Bonato, J. C. Germino, W. X. Oliveira, R. Kamat, J. Xu, C. Tassone, D. D. Stranks, M. F. Toney, A. F. Nogueira, “Synthesis of Polycrystalline Ruddlesden-Popper Organic Lead Halides and Their Growth Dynamics”, Chemistry of Materials, 31 (22) (2019), 9472-9479

Zero-dimensional (0D) inorganic perovskites have emerged as promising candidates over their higher dimensional (3D, 2D) analogs by showing superior photoluminescence quantum yield (PLQY) and significant physiochemical stability under ambient conditions. In previous efforts, the inclusion of the 0D phase into a 3D perovskite matrix improved photoluminescence (PL) characteristics by passivating surface defects. Nonetheless, the 0D phase itself is generally non-luminescent, limiting its applicability. Given this, we designed a novel solvent engineering method to incorporate highly emissive 0D perovskite nanocrystals (NCs) into a 3D perovskite composite film (CsPbBr₃, a polyelectrolyte and Li salt) to form the emissive layer in single-layer perovskite light-emitting electrochemical cells. An optimized 3D-0D composite achieved a maximum luminance of 11200 cd m⁻², an external quantum efficiency of 8.2%, a current efficiency of 33 cd/A, and a power efficiency of 31 lm/W. Constant current driving of the 3D-0D composite exhibited 150 h operation at over 400 cd m⁻² brightness. Extrapolating this to a 100 cd m⁻² initial luminance, a luminance half-life of over 37000 h is obtained, among the highest for perovskite devices to date. The inclusion of 0D NCs increases photoluminescence lifetime by surface passivation, suppresses non-radiative losses, induces thin-film smoothness, produces smaller crystallites for efficient charge confinement.

2D thick-layer lead bromide perovskites remain an unfathomed phase space with the lack of systematic studies to establish the structure, photophysical properties and stability behavior of this family of 2D halide perovskites. Herein, we present new members of n=2 lead bromide perovskites (C_mH_2m+1NH₃)₂(CH₃NH₃)Pb₂Br₇ (m = 6−8) that belong to the Ruddlesden-Popper structure type, incorporating long chain alkyl-monoammonium cations (C_mH_2m+1NH₃) of hexylammonium (m=6), heptylammonium (m=7) and octylammonium (m=8). A universal solution synthetic methodology for bulk, thick-layer lead bromide perovskites is presented with all structures solved single crystal X-ray diffraction. The studied bilayer lead bromide perovskites demonstrate a decrease in the lattice rigidity and lattice match of the inorganic perovskite layer - organic layer, as the monoammonium chain length increases. In comparison to their iodide analogues, the bilayer lead bromide compounds exhibit elongation of their stacking axis despite the smaller dimensions of the [PbBr₆]^4- lattice with a reduced calculated internal lattice strain, inferring a greater lattice match between the inorganic [PbBr₆]^4- perovskite layer and organic layer. The titled compounds exhibit narrow-band green emission near 2.5 eV. Time-resolved photoluminescence (PL) displays longer carrier lifetimes compared to their iodide analogues, where electronic structure calculations indicate that the increase of the monoammonium chain length and thus, lattice softness enhances non-radiative recombinations. A complete set of air, light and heat stability tests on unencapsulated thin films of ((C_mH_2m+1NH₃)₂(CH₃NH₃)Pb₂Br₇ (m = 4, 6−8) and MAPbBr₃ show they are stable in ambient air for at least 5 months. Extraordinarily, 3D MAPbBr₃ films prove to be more stable than films of 2D lead bromide perovskites, in contrast to MAPbI₃ which is less stable than the 2D lead iodide perovskites.

The partial incorporation of large organic cations into the octahedral sites of 3D perovskites has been widely investigated but few works have reported their addition in 2D Ruddlesden–Popper (RP) perovskites. Here, the gradual substitutiont of the methylammonium (MA) cation in thin films of 2D RP perovskites (BA)₂(MA)₂Pb₃I₁₀ (BA = n-butylammonium) by guanidinium (Gua), dimethylammonium (DA), ethylammonium (EA), rubidium (Rb), propylammonium (PA), cesium (Cs) or formamidinium (FA) to synthesize mixed A-cation (BA)₂(MA_1-xA_x)₂Pb₃I₁₀ perovskites has been studied. The limit in the percentage of the A-site cation at which it is homogeneously incorporated in all n phases without destabilizing the overall structure is determined. It is found that the structural changes in the 2D RP perovskites may lead to modifications of the optoelectronic properties as larger bandgap (EA), blue-shift of the PL peak position (above the threshold), strong quenching of the PL (Cs) or reduced concentration of trap states (Gua and EA). Solar devices fabricated with the different mixed A-cation 2D RP display an enhanced performance for those perovskites containing low percentages of Gua or EA cations. This characterization is very valuable to correlate the size and shape of the A-cation with the changes of the properties of the material.

Metal-halide perovskites offer numerous opportunities for use in large-area optoelectronics due to their strong optical absorption, long charge carrier diffusion lengths, low trap densities and processing versatility. However, full exploitation of these important attributes for the development of large-area opto/electronics remains challenging. In this presentation I will present and discuss results from our recent efforts to utilize perovskite semiconductors for a variety of applications. Particular emphasis will be placed on our recent work that aims to combine organic with perovskite semiconducting materials to produce prototype devices with unusual microstructural characteristics and advanced functionalities. Devices to be discussed include nanogap photodiodes, phototransistors, transistor memories, memtransistors, and light-emitting devices.

The applications of two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) have broadened as functional molecules are explored and incorporated into this emerging material.¹ However, obtaining quality films of HOIPs with functional molecules remains a challenge.¹ One group successfully engineered and incorporated new conjugated functional molecules that changed the band alignment, but only created small crystals, not contiguous films.² Mitzi et al. created layered perovskites with functional molecules via single source thermal ablation, however film thickness was difficult to control and the risk of organic cation damage remained.³ Solution processing is challenged by finding a single solvent for both the inorganic and organic precursor materials. Two-step solution-processing partially avoids the single solvent criterion, but solvent choice is still a critical challenge and a limiting factor for synthesis of 2D HOIPs with functional molecules.⁴

Herein, a deposition technique is presented that better accommodates organic cation deposition to address the challenge of consistently producing controlled, quality thin films of HOIPs with functional molecules. Resonant-infrared, matrix-assisted pulsed laser evaporation (RIR-MAPLE) is a modified pulsed laser deposition technique that uses a frozen target solution with precursor materials. The laser energy is resonantly absorbed by the matrix solvent (monoethylene glycol), the sublimation of which ejects the inorganic and organic precursor materials onto the substrate, making this method gentle on the organic precursor unlike physical vapor deposition. RIR-MAPLE has been used previously to successfully deposit high quality thin films of oligothiophene-based HOIPs.⁵ This demonstration was enabled by an extremely low solution concentration (22 mM) of the organic and inorganic precursors in a single target solution (known as simultaneous deposition scheme). However, this simultaneous deposition scheme may not suffice for other large, functional molecules for which a low concentration target solution is not adequate to address solvent incompatibility between the organic and inorganic precursors.

Therefore, this work investigates an alternative deposition scheme to demonstrate a potential benefit of RIR-MAPLE: the ability to completely separate the inorganic and organic precursor target solutions. In this sequential deposition scheme, the frozen inorganic and organic precursor solutions are spatially separated within the target. Each precursor is deposited alternately according to the laser raster pattern on a time-scale less than that needed for full substrate coverage. The RIR-MAPLE sequential deposition scheme has been demonstrated for polymer-nanomaterial composites.^6,7 In this work, film characterization (including X-ray diffraction, energy dispersive spectroscopy, photoluminescence, UV-vis absorbance spectroscopy, scanning electron microscopy, and atomic force microscopy) will be used to compare film quality resulting from simultaneous and sequential deposition schemes. Specifically, thin films of (C₈H₁₂N)₂PbI₄ are investigated, instead of oligothiophenes, to confirm film quality because phenethylammonium-based HOIPs have been well characterized by other deposition techniques. The conclusive demonstration of high-quality 2D HOIPs by the RIR-MAPLE sequential deposition scheme will expand eligible functional molecules by circumventing the well-known solubility and vapor deposition challenges.

REFERENCES
1. Saparov, B. & Mitzi, D. B. Chemical Reviews 116, 4558–4596 (2016).
2. Gao, Y. et al. Nat. Chem. 11, 1151–1157 (2019).
3. Mitzi, D. B., Chondroudis, K. & Kagan, C. R. Inorg. Chem. 38, 6246–6256 (1999).
4. Liang, K., Mitzi, D. B. & Prikas, M. T. Chem. Mater. 10, 403–411 (1998).
5. Dunlap-Shohl, W. A. et al. Mater. Horizons 6, 1707–1716 (2019).
6. Ajnsztajn, A. et al. Crystals 10, 152 (2020).
7. Ge, W., et al. Appl. Phys. Lett. 104, 223901 (2014).

Increases in the global energy demand necessitate the development of new approaches to energy conversion and storage. In particular, the utilisation of solar energy could provide a basis of evolving technologies capable of meeting modern and future demands.

By combining the photovoltaic and newfound electrochemical properties of organo-metal hybrid perovskite materials in a single device, a novel photobattery technology is proposed [1]. Utilising the photovoltaic performance of perovskite materials in combination with the intercalation and conversion mechanisms available to Lithium ion species [2], a device with the ability both to convert light to electrochemical energy and store it is demonstrated. The motivation for such a device will be discussed, with its inherent impact in areas such as off-grid energy solutions and the internet of things. The fabrication techniques are described and characterisation techniques common to both photovoltaic and electrochemical disciplines, with their recent results are discussed. Modifications are made to conventional electrochemical coin cells and pouch cells, in order to facilitate optical access to the electrode material. Using in-operando x-ray diffraction and optical probing, the fundamental mechanisms of charge storage and ion conversion are investigated both in the presence of light, to replicate a light-charging cycle and in the absence of light.

[1] Nano Lett. 2018, 18, 3, 1856-1862
[2] Mater. Adv., 2021,2, 3370-3377

Inorganic cesium-lead-halide perovskites are of interest for optoelectronic applications such as solar cells, LEDs and lasers, since they not only exhibit excellent optical properties, but are also more thermally stable [1]. However, inorganic perovskites frequently come with serious processing challenges and high-quality thin films (no pinholes, high photoluminescence quantum yield) are difficult to achieve. Amplified spontaneous emission (ASE) in thin films of these materials, which is an important prerequisite for lasing, can often only be reached at cryogenic temperatures. The blue to ultraviolet spectral range, which would be reached by CsPbCl₃, is a particular challenge in this regard. Solution-based deposition of CsPbCl₃ is extremely hampered by the poor concomitant solubility of the precursor salts PbCl₂ and CsCl.

Within this study, the preparation of wide-gap CsPbCl₃ perovskite thin films is presented by the use of thermal evaporation of the individual salts. Specifically, a stack of alternating layers of PbCl₂ and CsCl is designed, that leads to some kind of self-limiting formation of the desired 3D - CsPbCl₃ perovskite at the interfaces of the superlattice. We analysed the formation of CsPbCl3 by photoelectron spectroscopy and X-ray diffraction as well as grazing incidence wide angle x-ray scattering (GIWAXS), to unravel the appearance of the various phases (0D-Cs₄PbCl₆, 2D-CsPb₂Cl₅, 3D-CsPbCl₃). The insights gained from these studies allowed us to optimize the superlattice architecture.

The resulting CsPbCl₃ shows an intense photoluminescence at 413 nm with a narrow line width of 10 nm (FWHM), which agrees with reports of CsPbCl₃ single crystals [2]. Most notably, we were able to optimize the CsPbCl₃ thin film to support amplified spontaneous emission in the deep blue spectral region at 427 nm (line width 2.2nm) under pulsed optical excitation (355nm, 300 ps) above a threshold energy density of 190 µJ/cm² at room-temperature (RT). This is the first report of CsPbCl₃ thin films showing ASE at RT. The integration of this active material in a resonator to achieve the first thin film CsPbCl₃ lasers at RT will be presented.

1. Pourdavoud N., et al., Adv. Mater. 2019, 31, 1903717, DOI: 10.1002/adma.201903717
2. Gui P., et al., Small 2019, 15, 1902618, DOI: 10.1002/smll.201902618

Perovskite-based visible light-emitting diodes (LEDs) with blue, green, and red emissions have successfully been demonstrated with high external quantum efficiencies (EQEs) and bright luminance for next-generation display and solid-state lighting applications. In spite of the high performances of perovskite-based visible LEDs, however, their applications will be limited due to the toxicity of lead-based halide perovskites. While there have currently been many reports about lead-free metal halide emitters to solve this toxicity issue, most lead-free halide LEDs show still significantly low performance, resulting in below 1 %.
In this work, lead-free halide LEDs are fabricated using non-toxic and earth-abundant CsCu₂I₃ with a strong yellow emission at a peak wavelength of 568 nm. CsCu₂I₃–based host-dopant emitters are formed by vacuum thermal evaporation (VTE) film co-deposition process instead of the commonly-used solution-based film deposition process. Using the VTE process, extremely thin (30 nm) host-dopant emitters have successfully been formed with the CsCu₂I₃ dopant and various organic host molecules. A bright yellow emission with a photoluminescence quantum yield value of 84.8% is achieved in the 0.5% CsCu₂I₃-doped halide emitter film due to the successful spatial localization of charge carriers and excitons using an organic host with appropriate energy levels to CsCu₂I₃. With the further enhancement in charge balance using the co-host system, a record-breaking lead-free halide LED has been fabricated with an EQE of 7.4%. The lead-free halide LEDs are also highly stable in the device operation with LT70 of 20 hours at 100 cd/m².

Recent discoveries have revealed a breakthrough in the field using inorganic-organic hybrid layers called perovskites as the light harvester in the solar cell. The inorganic-organic arrangement is self-assembled as alternate layers, being a simple, low cost procedure. These organic-inorganic hybrids promise several benefits not delivered by the separate constituents. In this lecture I will discusses new directions in low dimensional perovskite and their applications in solar cells.
In low dimensional systems, stability of excitons in quantum wells is greatly enhanced due to the confined effect and the coulomb interaction. The exciton binding energy of the typical 2D organic-inorganic perovskites is up to 300 meV and their self-assembled films exhibit bright photoluminescence at room temperature.
In this work we will show the dimensionality in the perovskite structure. The 2D perovskite structure should provide stable perovskite structure compare to the 3D structure. The additional long organic cation, which is added to the perovskite structure (in the 2D structure), is expected to provide hydrophobicity, which will enhance the resistivity of the perovskite to humidity. We will demonstrate the use of 2D perovskite using unique barrier molecultes in high efficiency solar cells.
Moreover, we will show a highly efficient semitransparent perovskite solar cell. The semitransparency was achieved through an inkjet printing of “holes” in the perovskite film. The “holes” in the perovskite allows to control the semitransparency of the solar cell.
Organic-inorganic halide perovskite is used mainly in its “bulk” form in the solar cell. Confined perovskite nanostructures could be a promising candidate for efficient optoelectronic devices, taking advantage of the superior bulk properties of organo-metal halide perovskite, as well as the nanoscale properties. In this talk, I will present our recent progress related to the synthesis and characterization of perovskite NPs- i.e. Inorganic and hybrid organic-inorganic NPs. New nanostructures such us: NRs and NWs will be presented and the introduction of other cations such us Rb will be shown.

The impressive flexibility exhibited by halide perovskites [1] offers many opportunities for chemists to design application driven materials [2,3]. Among the challenges still to be overcome is the replacement of toxic lead in materials for optoelectronic devices. If bismuth seems to be an ideal candidate owing to its low toxicity and natural abundancy, the performances of 3-dimensional (3D) Bi-based materials in solar cells remain far from their Pb-based competition [4]. However, the low-dimension side of the family (2D and 1D) still holds promises.

Here, we focus on 2D and 1D Bi-based halide perovskites thanks to joint experimental characterization and computational investigations. By highlighting, among others, the key aspects of connectivity between octahedra, we will describe the challenges of designing efficient lead-free materials, as well as the opportunities offered by this not yet fully explored territory [5,6].

[1] B. Saparov, D. B. Mitzi, Chem. Rev. 2016, 116, 4558.
[2] L. Pedesseau, M.K. et al., ACS Nano 2016, 10, 9776.
[3] C. Katan, N. Mercier, J. Even, Chem. Rev. 2019, 119, 3140.
[4] Z. Jin et al., J. Mater. Chem. A 2020, 8, 1616.
[5] N. Louvain, N. Mercier, F. Boucher, Inorg. Chem. 2009, 48, 879.
[5] E. S. Vasileiadou, M.K. et al., Chem. Mater., in press.
[6] N. Mercier, M.K. et al., manuscript in preparation.

2D materials have captured the imagination of the materials science community, yet the chemical processes leading up to the formation of these materials are often difficult to probe. Working with colloidal nanomaterials allows us to use wet-chemical control and fast in-situ techniques to understand the reactions occuring during synthesis. In this study, we demonstrate that 2 D perovskites can form through two processes at the nanoscale: Either from a layer-by-layer chemical exfoliation process from lead bromide nanocrystals or through direct nucleation from precursors. We study these processes in detail through in-situ spectroscopy in a stopped flow chamber. Chemical exfoliation appears to leave entire layers of the lead bromide nanocrystal intact – stopped flow experiments show little lateral confinement of the 2D exciton peak at short time scales, and the level of confinement is correlated to the size of the initially synthesized lead bromide nanocrystals. Direct nucleation leads to initial exciton peaks that are about 200 meV higher in energy compared to bulk values. The ratio of organic amine to lead controls which formation pathway dominates, as well as the size at which lead bromide nanocrystals exfoliate into 2D perovskite nanoflakes. We demonstrate qualitative agreement with simulations of a kinetic nanocrystal growth model with a single layer exfoliation step. Isotropic absorption, measurable lateral confinement and observations of growth kinetics make in-situ experiments of colloidal nanomaterials an ideal platform for understanding the complicated processes of nucleation, growth and phase transformation of 2D perovskites and 2D materials more generally. Deeper understanding of the formation processes, especially in their initial stages, will be important for ongoing development of synthetic procedures in this field.

Quasi-2D perovskites (q2D PVKs) are promising candidates for stable and highly efficient solar cells. On top of the superior environmental stability compared to their 3D counterparts, q2D PVKs possess tunable optoelectronic properties, as it is possible to change the number of inorganic layers (n) sandwiched between the organic spacers to tune bandgap and exciton binding energy. However, centering the phase distribution on a specific n-value is challenging. Together with a variety of processing techniques, solvent engineering has previously been used in the literature to tune both phase purity and device efficiency. Nevertheless, no reports have investigated the effect of co-solvents on the crystallization mechanism and the reasons behind the changes in phase distribution.

To fill this gap, we first investigated the kinetics of film formation of BA₂MA₃Pb₄I₁₃ processed from DMF/co-solvent mixtures (DMSO and NMP) via in-situ absorption measurements conducted during room-temperature spin coating or thermal annealing. By combining these results with photoluminescence (PL) spectroscopy and angle-dependent grazing-incidence wide-angle X-ray scattering (GIWAXS), we concluded that the crystallization starts as a 3D-like perovskite from the liquid-air interface, both when the q2D PVK forms during spin coating (low amounts of DMSO or NMP in the solvent mixture) and during annealing (high amounts). The structure of the 3D-like perovskite on the top of the film, however, is impacted by the crystallization at the bottom, as suggested by the redshift of the absorption onset.

In agreement with previous reports, we observed a 2D-3D gradient in the film, with q2D PVKs formed mostly at the bottom of the film and 3D-like PVKs at the top. Even though the crystallization mechanism is the same when using DMSO and NMP as co-solvents, the phase distribution at the bottom of the film is different. More specifically, for the 4:1 DMF/NMP ratio, the distribution shifts toward small n-values, mainly n = 2. On the contrary, when using DMSO as a co-solvent with the same volumetric ratio, a broader phase distribution appears. During the crystallization, as described before, a 3D-like layer is initially formed on the top of the film, while underneath a wet film consisting of perovskite precursors has yet to crystallize. The interactions of the co-solvents with the precursors in the wet film are important in shifting the resulting phase distribution, as they influence the availability of MAI or BAI molecules to participate in the crystallization process. We investigated the ternary interactions between BAI or MAI, PbI₂, and the solvents with FT-IR and DFT. By analyzing the S=O and C=O stretching peaks, we observed that NMP interacts more strongly with MAI-PbI₂ than with BAI-PbI₂, indicating that BAI is less bound to the ternary complex, compared to MAI, and thus more available to participate in the formation of quasi-2D perovskite phases. This explains the phase distribution shifted to smaller n-values when using NMP. The same analysis showed that DMSO, instead, interacts similarly in ternary complexes with both MAI and BAI, agreeing with a broader phase distribution.

Overall, we propose a crystallization mechanism for BA₂MA₃Pb₄I₁₃ in a DMF/co-solvent mixture where 3D-like perovskites crystallize first at the liquid-air interface. While the crystallization proceeds, the phase distribution in the underlying perovskite layer is influenced by the availability of MAI and BAI molecules, which is in turn influenced by the co-solvent’s choice. This conclusion confirms the importance of the co-solvents (or additives) in the challenging formation of phase-pure quasi-2D perovskites. By carefully tuning the DMF/co-solvent ratio, we finally fabricated a more phase-pure q2D PVK with device efficiencies up to 11%, a low degree of 2D-3D vertical gradient, and a narrow phase distribution.

In this presentation, we discuss how the electronic landscape of metal halide perovskites may be modified under two scenarios, first, through intrinsic quantum confinement effects found to occur in a nominally bulk semiconducting perovskite [1], and second, in thin films of nanocrystals within the weak electronic confinement regime [2].

We have recently reported the surprising presence of above-bandgap oscillatory features in the absorption spectra of formamidinium lead triiodide thin films [1]. We attribute these discrete features to intrinsically occurring quantum confinement effects, for which the related energies change with temperature according to the inverse square of the intrinsic lattice parameter, and with peak index in a quadratic manner. By determining the threshold film thickness at which the amplitude of the peaks is appreciably decreased, and through ab initio simulations of the absorption features, we estimate the length scale of confinement to be 10-20 nm. Such absorption peaks present a new and intriguing quantum electronic phenomenon in a nominally bulk semiconductor, offering intrinsic nanoscale optoelectronic properties without necessitating cumbersome additional processing steps. We discuss two potential explanations for the formation of such quantum well or superlattice structures, the inclusion of thin electronic barrier layers of hexagonal delta phase, or the presence of ferroelectric domains.

We further discuss the vibrational and electronic transport properties of thin films comprising metal-halide perovskites nanocrystals (NCs) which offer an effective approach for processing films from inks for low-cost, high-performance device fabrication. We disentangle the effects of surface ligands, morphology, and boundaries on charge-carrier transport in thin films fabricated with these high-quality NCs [2]. We employ terahertz (THz) spectroscopy to optically probe the photoconductivity of CsPbBr₃ NC films. The vibrational and optoelectronic properties of the NCs are compared with those of the corresponding bulk polycrystalline perovskite and significant deviations are found. Charge-carrier mobilities and recombination rates are demonstrated to vary significantly with the NC size. Such dependences derive from the localized nature of charge carriers within NCs, with local mobilities dominating over interparticle transport. It is further shown that the colloidally synthesized NCs have distinct vibrational properties with respect to the bulk perovskite, exhibiting blue-shifted optical phonon modes with enhanced THz absorption strength that also manifest as strong modulations in the THz photoconductivity spectra. Such fundamental insights into NC versus bulk properties will guide the optimization of nanocrystalline perovskite thin films for optoelectronic applications.

[1] Intrinsic quantum confinement in formamidinium lead triiodide perovskite,
A. D. Wright, G. Volonakis, J. Borchert, C. L. Davies, F. Giustino, M. B. Johnston, and L. M. Herz,
Nature Materials 19, 1201–1206 (2020).

[2] CsPbBr₃ nanocrystal films: Deviations from bulk vibrational and optoelectronic properties,
S. G. Motti, F. Krieg, A. J. Ramadan, J. B. Patel, H. J. Snaith, M. V. Kovalenko, M. B. Johnston, and L. M. Herz,
Advanced Functional Matererials 30, 1909904 (2020).

2D hybrid organic-inorganic perovskites (HOIPs) are low-cost, high-performance semiconductor materials with great potential demonstrated in various fields, including photovoltaics, light-emitting diodes, lasers, and field-effect transistors. In these applications, it is common for 2D HOIPs to encounter charged species, including defects, molecular dopants, interfacial dipoles, and excitons, which could modulate the carrier transport properties inside the functional materials. How the 2D HOIPs will re-arrange the internal charges to screen the Coulombic interaction and how this charge screening behavior is related to the structure and composition of the 2D HOIPs are vital for the device design and optimization and yet largely remain unknown. Here we report a systematic study of the charge screening length of prototypical 2D HOIPs with a general formula of (C₄H₉-NH₃)₂(CH₃NH₃)_n-1Pb_nI_3n+1 (n= 1 to 5) by measuring the work function of 2D HOIP flakes on a charged dielectric surface SiO₂ using kelvin probe force microscopy. The work functions of the 2D HOIP nanoflakes monotonically change as the flake thickness increases, showing a progressive screening of the influence from the SiO₂ surface charges. The screening length is short, less than 20 nm, which decreases monotonically with respect to the n number, i.e., the thickness of the inorganic slab within each repeating unit. 2D HOIP with n = 5 exhibits the shortest screening length, which could completely shield the electrostatic interaction with only 2 repeating units. The screening lengths of 2D HOIPs are much larger than, comparable to, and shorter than those of graphene, MoS₂, and h-BN, respectively. While the inorganic framework dominates the charge screening behavior, the organic spacer molecules can affect the charge screening by modulating the interlayer charge coupling through the van der Waals interfaces. Our results can provide valuable insights from the charge screening perspectives into the carrier dynamics and device design involving 2D HOIPs.

Lead-halide perovskite colloidal nanocrystals (LHP NCs) have emerged over the past decade as leading candidates for next-generation, efficient light-emitting diodes (LEDs). Owing to their high photoluminescence quantum yields (PLQYs), LHPs efficiently convert injected charge-carriers to light and vice versa. In the synthesis of high-quality LHP NCs, the purification process is a critical step that influences the final PLQY, transport properties and polydispersity of the NCs. Despite the important role of the antisolvent in the purification process, a detailed understanding of how the antisolvent influences the surface chemistry of the NCs is missing in the field. Here, we address this knowledge gap, using CsPbBr_xI_3-x NCs as the model system. We find that as the polarity of the antisolvent increased (from methyl acetate through to butanol), there was an increased blue-shift in the photoluminescence (PL) peak of the NCs, along with a decrease in PLQY. Through transmission electron microscopy and X-ray photoemission spectroscopy measurements, we identify the change of PL spectra to be due to solvent-induced iodide removal, which leads to a change in halide composition and band-gap. By using detailed nuclear magnetic resonance (NMR) measurements, along with density functional theory calculations, the mechanism we propose is that the oleic acid and oleylamine ligands undergo an amide condensation reaction at the surface of NCs in the presence of the polar antisolvent during purification. The conversion of amine to amide lead to ligand and surface halide detachment. Solvent polarity plays an important role in this reaction, in which solvents with higher polarity tend to remove more surface ligands and halides. This leads to higher surface halide vacancy densities, resulting in decreases in the PLQY. Furthermore, washing with high polarity solvent would cause an inhomogeneous distribution of surface halides, and reduces electroluminescence spectral stability of LED devices. Our work shows that maintaining high-PLQY NCs with low defect density requires the use of low-polarity antisolvents during

The effective mass of charge carriers is a crucial parameter for the design of any optoelectronic device, as it is directly linked to key material characteristics such as the carrier mobility, exciton binding energy, and diffusion length. Consequently, a precise determination of its value is of paramount importance for the in-depth understanding of any semiconductor. The estimated values of the effective mass of 2D halide perovskites currently span a broad range, providing a source of uncertainty in this promising material system.

Using high magnetic fields up to 65 T we were able to observe multiple interband Landau level transitions in the absorption spectra of (PEA)₂SnI₄ and (PEA)₂PbI₄, providing us with a direct measure for the reduced effective mass of charge carriers in these systems. The determined reduced effective masses are found to be very close to or even lower than that of their 3D analogues, which is atypical in the presence of quantum confinement effects. We highlight how the distortion imposed by the organic spacers, and orbital hybridization effects by the metal cation, govern the effective mass in 2D halide perovskites. Combining the experimental results with electronic band-structure calculations, we propose a scaling diagram for the effective mass value versus the distortion of the octahedra imposed by the organic cations. As a result, we postulate that the effective mass in these systems can be easily tailored over a wide range by appropriate selection of organic spacers and metal ions.

Perovskite nanocrystals have attracted great interest due to tunable optoelectronic characteristics and potential applications in photonics, energy storage, lighting and display. To date, most synthesis methods have involved organic solvent, such as N,N-dimethylformamide and dimethyl sulfoxide, which poses a potential threat to the environment, health and security. In this work, we develop an environmental-friendly and cost-efficient method to synthesize CsPbBr₃ powders at room temperature with water. Using ultrasonication and centrifugation, we obtain CsPbBr₃ nanocrystals with green (~522 nm) and blue (~493 nm) emissions from the powders. The photoluminescence quantum yield of the blue-emitting nanocrystals is 80%, much larger than 61.4% of the CsPbBr₃ nanocrystals made by an anti-solvent method. We prepare polymer composite films with PMMA and the green-emitting CsPbBr₃ nanocrystalz on a polyethylene terephthalate plate. The composite films exhibit deflection-endurable photoluminescence, i.e. there is no observable change in the PL characteristics for local radius of curvature up to 10.07 mm. The method developed in this work provides a unique approach to produce halide perovskite nanocrystals without harmful organic solvents in precursor solutions.
This work is supported by the NSF through the grant CMMI-1854554 and CBET- 2018411.

Microscale patterning of solution-processed nanomaterials is important for integration in functional devices. Colloidal lead halide perovskite (LHP) nanocrystals (NCs) can be particularly challenging to pattern due to their incompatibility with polar solvents and lability of surface ligands. Here, we introduce a direct photo-patterning approach for LHP NCs through the binding and subsequent cleavage of a photosensitive oxime sulfonate ester. The photosensitizer binds to the NCs through its sulfonate group and is cleaved at the N-O bond during photo-irradiation with 405 nm light. This bond cleavage decreases the solubility of the NCs which allows patterns to emerge upon development with toluene. Post-patterning ligand exchange results in photoluminescence quantum yields up to 76%, while anion exchange provides tunability in the emission color. The patterned NC films also show photoconductive behavior, proving that good electrical contact between the NCs can be established.

Recently, inorganic lead-free metal halide perovskites have gained attention as promising optoelectronic materials for various applications due to their nontoxicity, environmental moisture, and thermal stability. In particular, the lead-free cesium bismuth halide perovskites with the general formula A_xB_yX_z; z = x +3y, where A=Cesium (Cs), B=Bismuth (Bi), X=Iodide (I), Bromide (Br) have generated enormous interest among the perovskite research community as emerging materials for optoelectronic applications. Among the Bi-based perovskites, the CsBi₃I₁₀ perovskite (where x=1, y=3, and z=10) has the lowest bandgap of about 1.77 eV with a broad light absorption spectrum that is beneficial for solar cell, broad photodetection, and mechanoluminescence pressure sensing application. However, studies focused on understanding the microstructural and electronics properties of solution-processed CsBi₃I₁₀ perovskite is still lacking. Moreover, it is still challenging to prepare uniform, compact, and void-free CsBi₃I₁₀ perovskites. Herein, we systematically study the microstructural, optical, charge transport, defect density, and electronic properties of CsBi₃I₁₀ perovskite with and without the incorporation of the chlorinated interface modifier. The experimental results show that the interface modifier led to enhanced light absorption, crystallinity, and microstructural features. Furthermore, this work explores the underlying mechanism for such enhancement and assesses the influence of chlorinated interface modifiers for improving the performance of a self-powered photodetector for mechanoluminescence pressure sensing applications.

Two-dimensional (2D) halide perovskites have emerged as outstanding semiconducting materials thanks to their superior stability and structural diversity. However, the ever-growing field of optoelectronic device research using 2D perovskites requires systematic understanding of the effects of the spacer on the structure, properties, and device performance. So far, many studies are based on trial-and-error tests of random spacers with limited ability to predict the resulting structure of these synthetic experiments, hindering the discovery of novel 2D materials to be incorporated into high-performance devices. In this presentation, we provide guidelines on successfully choosing spacers and incorporating them into crystalline materials and optoelectronic devices. Useful insights are emerging on what kind of spacer cations can stabilize 2D perovskites thanks to an extensive collection of spacer cations, which have been shown to stabilize 2D perovskites. There will be emphasis on the effects of the spacer on the structure and optical properties.

The optoelectronic properties of two-dimensional metal halide perovskites (2D MHPs), such as large exciton binding energies, narrow excitonic emission and small Stokes shifts, make them excellent candidates for light emitting applications. However, the purely 2D MHPs, characterized by large organic cations (e.g. phenetylammoinum PEA, butylammonium BA, naphtylammonium NMA), are outperformed in LED and lasing devices by other low-dimensionality systems such as fully inorganic nanoparticles or multidimensional perovskites based on a mixture of smaller and bulkier organic cations. Importantly, an unexpected limitation comes from 2D MHPs poor luminescence efficiency: to rationalize this, different mechanisms have been proposed involving recombination on defects and grain boundaries, competing multi-exciton processes, poorer charge transport qualities and effects on luminescence of exciton-phonon coupling. [1-4]. However, the lower radiative recombination efficiencies of 2D MHPs compared to 3D counterparts is not yet fully understood.
In this work, we investigate exciton transport properties in the prototypical 2D MHP phenethylammonium lead iodide ((PEA)₂PbI₄) to identify the effect of surface states on the excitonic luminescence, and assess their role in photoluminescence (PL) quenching compared to bulk defects. More specifically, we investigate exciton transport in polycrystalline thin films, establishing an exciton diffusion length of 150 nm in good agreement with values reported in other 2D MHPs [5, 6]. By comparing the radiative lifetime and luminescence efficiencies in films of varying thicknesses [7], we identify the role of the surface as a quencher of excitonic radiative recombination. In films thicker than the diffusion length, where most excitons recombine before interacting with the surface, the PL lifetime is comparable to that of single crystals, indicating a limited role of grain boundaries as non-radiative recombination centers. In thinner films however, a significant quenching of radiative lifetime is observed. The quenching role of surface defects is further confirmed by comparison with (PEA)₂PbI₄ nanoparticles, where no quenching is observed despite their characteristically smaller spatial scale. Finally, we relate surface defectivity to the presence of undercoordinated lead atoms, and explore how ad-hoc passivating strategies may be exploited to reduce exciton quenching at the film surface.

[1] Adv. Funct. Mater. 2020, 30, 1907979
[2] Nat. Mater. 17, 550–556 (2018)
[3] Science 355, 1288–1292 (2017)
[4] ACS Nano 2016, 10, 11, 9776–9786
[5] J. Phys. Chem. Lett. 2021, 12, 4003−4011
[6] Nat Commun 11, 2035 (2020)
[7] J. Phys. Chem. B 2009, 113, 9104–9109

Many electronic and structural properties of halide perovskite evade conventional categorization. In the talk, we will present two synthetic platforms consisting of atomically-defined one-dimensional halide perovskite nanowires, where we will address the lingering scientific questions of the intriguing optical and structural behaviors of halide perovskite nanostructures.
In the first synthetic platform, we synthesized uniform colloidally stable nanowires with fixed diameter but varied aspect ratios. Using this system, we revealed the scaling laws of photoluminescence quantum yield and radiative lifetime with respect to the aspect ratio of nanocrystals. The scaling laws derived herein are not only a phenomenological observation but proved a powerful tool disentangling the carrier dynamics of microscopic systems in a quantitative and interpretable manner. The investigation of our model system and theoretical formulation bring to light the dimensionality, as a hidden constraint on carrier dynamics, and identify the diffusion length as an important parameter that distinguishes nanoscale and macroscale carrier behaviors.
In the second synthetic platform, we successfully encapsulated single-unit-cell thick halide perovskite nanowires inside single-walled carbon nanotubes. State-of-art transmission electron microscope coupled with a fast electron detection camera was employed to observe the transient structure dynamics of the atomically-thin halide perovskite nanowires. We found that due to the protection of carbon nanotubes, the notoriously beam-sensitive perovskite nanostructure behaved “resiliently” to the electron beam so that their lattice is dynamic but maintain a degree of integrity throughout the acquisition.
Overall, with the atomic-level synthetic control we created two types of 1D halide perovskite nanowires, and due to their structural and morphological uniqueness, new photophysics and structure dynamics were explored. We will demonstrate in the talk how to use these systems and their interesting properties to articulate the explanatory gaps in the field of low dimensional halide perovskite.

Two-dimensional hybrid halide perovskites (2D HHPs) intrigue material scientists from both the scientific and technological points of views. They exhibit pronounced and easily tunable excitonic properties, which are strongly coupled to thermal fluctuations. The structure of 2D HHPs is closely related to that of their extensively studied 3D counterparts, in which the electronic properties are strongly influenced by anharmonic lattice dynamics.
In this presentation, I will show a study[1] of the structural dynamics of the prototypical 2D HHP (CH₃(CH₂)₃NH₃)₂PbI₄ (BAPI) by comparing it to its 3D counterpart CH₃NH₃PbI₃ (MAPI) and aromatic counterpart (C₆H₅-(CH₂)₂NH₃)₂PbI₄. We use single-crystal x-ray diffraction and temperature dependent Raman scattering to show that stronger intermolecular interactions between the organic molecules force the structure to a lower symmetry with more harmonic thermal fluctuations. Moreover, we use polarization-orientation Raman scattering and density-functional-perturbation-theory to relate the low-temperature thermal fluctuations of MAPI and BAPI, showing strong similarities between the structural dynamics of the materials. Finally, we uncover the mechanism of the order-disorder phase transition in BAPI which preserves the crystal symmetry due to a skewed double-well potential, biased by the hydrogen bonding between the organic amine and the inorganic scaffold. The phase transition involves unlocking of an anharmonic octahedral tilting motion which increases the anharmonicity of the lattice dynamics and decreases other phonons’ lifetime.
These anharmonic fluctuations are important for technological applications and novel designs of materials and devices since they induce charge carrier localization and affect the optoelectronic performance.

[1] M. Menahem et al., ACS Nano, In Press. DOI: 10.1021/acsnano.1c02022

Ruddlesden-Popper (RP) perovskite phases of nominal composition PEA₂(Cs_0.75MA_0.25)Pb₂Br₇ with and without the addition of the isopropylammonium additive, are demonstrated to emit in the wavelength range around 480 nm and 500 nm, respectively. The sample with isopropylammonium when deposited on a PEDOT-PSS coated substrate displays an exceptional PL QY 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 funneling of the photoexcitations towards the low dimensional phases is blocked in samples using the isopropylammonium additive, mostly due to the random orientation of very small crystalline domains. All the features of this new RP phase allowed us to fabricate LEDs emitting at an average wavelength of 483 nm, with FWHM of the electroluminescence of 25 nm and an external quantum efficiency up to 6%.
These results demonstrate that there is an opportunity to achieve high efficiency LEDs also in the blue spectra range by engineering RP phases.

2D/3D perovskites are attractive for a variety of applications due to their enhanced stability over their 3D perovskite-based counterparts. However, improvements to 2D/3D perovskite-based devices are hampered by missing knowledge about the 2D perovskite itself. In particular, greater understanding is needed about how processing and composition engineering affect film structure and optoelectronic properties. Because charge generation in 2D perovskites is primarily excitonic, understanding excitonic emission and its link(s) to film processing is crucial for improving 2D/3D perovskite-based devices.

Here, we show that film processing can turn on or off exciton emission that is phonon-coupled. We made thin films of (EA)4Pb3Br10, where EA is ethylammonium. The films are found to be phase-pure; this phase-purity is attributed to using a single A/A’-site cation in the Ruddlesden-Popper n=3 structure, which prohibits crystallization of off-target phases. Composition engineering can thus be used to select for specific Ruddlesden-Popper phases in films. Thin films and bulk crystals (EA)4Pb3Br10 have three emission features: a narrow feature at higher energy, and two features at lower energy. Temperature dependent luminescence measurements reveal that the lower-energy features are phonon-coupled, and are perhaps either phonon replicas or self-trapped excitons. Notably, the intensity of the broad, lowest-energy emission feature depends on film casting kinetics. This intensity dependence is attributed to the presence/absence of film strain imparted by the casting. These results provide an additional knob with which to change the proportions of different types of exciton emission in films. We anticipate that the results here will lead to new 2D/3D perovskite heterostructures with better control over composition and excitonic transitions. [1]

[1] Kennard, R.M.; Dahlman, C.J.; Chung, J.; Cotts, B.; Mikhailovsky, A.A.; Mao, L.; DeCrescent, R.A.; Stone, K.H.; Venkatesan, N.R.; Mohtashami, Y.; Assadi, S.; Salleo, A.; Schuller, J.A.; Seshadri, S.; Chabinyc, M.L. Under Review, 2021.

Metal halide perovskite, as an emerging semiconductor, provides a unique opportunity for high-definition display and solid-state lighting. Rapid and breakthrough improvements have been achieved in green, red, and near-infrared perovskite light-emitting diodes (PeLEDs). However, blue PeLEDs are still far behind with much lower performance. Herein, we introduce a new dual ligand strategy of 2D/3D perovskite thin film preparation by using the appropriate ratio of Ethane-1,2-diammonium bromide (EDBr) and Phenethyl ammonium bromide (PEABr). The steady-state photoluminescence spectra demonstrate that double side amido terminals EDBr ligand can suppress efficiently the small n domains and non-radiative recombination, leads to strong energy transfer to larger n domains. Based on such films, a sky blue PeLED at 485 nm with about 730 cd/m2 brightness and low turn-on voltage (2.8v) was achieved.

Two-dimensional hybrid perovskites have attracted renewed interest of the condensed matter community due to their intriguing electronic and vibrational properties. They offer excellent platforms to study fundamental many-body physics with promising opportunities for optoelectronic applications. Composed by self-assembly from nanometer-thin sheets of inorganic molecules separated by organic spacers, layered perovskites represent natural, two-dimensional quantum wells. An important consequence of the reduced dimensionality together with weak dielectric screening is the formation of tightly bound electron-hole complexes, commonly known as excitons. In these systems, the excitons are exceptionally robust and, most importantly, couple strongly to light. Following photon absorption, they not only serve as the main energy carriers in the crystal, but also determine the conversion of that energy back to light. Motivated by these properties, a wealth of recent research activities emerged, aimed at the study of exciton transport in two-dimensional hybrid perovskites.
A central question in this context is how the excitons move as a function of temperature. In close analogy to electrons, temperature-dependent transport is considered key to understand the physics of the excitonic mobility. Here, we present the results of our study of exciton propagation in two-dimensional hybrid perovskites, performed from liquid helium up to room temperature conditions [1]. The exciton diffusion is directly monitored by optical means using spatially- and time-resolved photoluminescence microscopy. To avoid material degradation under illumination, the samples are fully encapsulated between thin layers of hexagonal boron nitride. At all studied temperatures our measurements demonstrate that the excitons are highly mobile during their nanosecond lifetimes. The extracted diffusion coefficients range from 1 up to 30 cm²/s and the corresponding diffusion lengths are as high as 1 micrometer.
Two distinct regimes of exciton transport are identified. For all temperatures above 50 K, the excitons propagation follows the conventional diffusion law. Moreover, the extracted diffusion coefficient is either constant or decreases with increasing temperature. This behavior is a characteristic hallmark of free exciton propagation, that is one of our key results. In the temperature regime below 50 K, however, the diffusion is anomalous. First, the exciton cloud expands rapidly after the excitation and then the expansion slows down over time. At liquid helium temperature, the exciton distribution even shrinks again after 100 ps. This observation corresponds to an effectively negative diffusion coefficient that subsequently converges towards zero at longer timescales. Our findings are discussed in the context of an effective mass model from first-principles calculations and temperature-dependent collision broadening based on spectral linewidth analysis. Further considered is the potential impact of non-equilibrium dynamics and exciton localization. Finally, we demonstrate the apparent applicability of a semi-classical diffusion model and provide an outlook towards understanding exciton transport in two-dimensional halide perovskites.

[1] J. D. Ziegler et al., Nano Lett. 20, 6674 (2020), DOI:10.1021/acs.nanolett.0c02472

Two-dimensional halide perovskites are exciting new semiconductors that show great promising in low cost and high-performance optoelectronics devices including solar cells, LEDs, photodetectors, transistors, etc. In the first part of this talk, I will present a molecular approach to the synthesis of high-quality organic-inorganic hybrid perovskite quantum wells through incorporating widely tunable organic semiconducting building blocks as the surface capping ligands. By introducing sterically tailored groups into the molecular motif, the strong self-aggregation of the conjugated organic molecules can be suppressed, and single crystalline organic-perovskite hybrid quantum wells and superlattices can be easily obtained via one-step solution-processing. Energy transfer and charge transfer between adjacent organic and inorganic layers are extremely fast and efficient, owing to the atomically-flat interface and ultra-small interlayer distance. Furthermore, this conjugated ligand design greatly enhances materials chemical stability and suppresses halide anion migration. Based on this, we demonstrate for the first time an epitaxial halide perovskite heterostructure with near atomically-sharp interface. Finally, we demonstrate stable and efficient LEDs and FETs using the novel 2D hybrid materials.

Metal-halide perovskites have emerged over the past years as a versatile class of semiconductors for high-performance optoelectronic devices.[1] However, despite their unique properties, this family of perovskites has been observed to display strong instability, especially when electrostimulated, limiting their definitive integration in real-world light-emitting devices.[2-4]

In this talk, we will present a detailed photophysical characterisation of quasi 2D green-emitting halide perovskite films in which different passivation strategies are employed to improve both their optical and electrical properties. We will introduce a new powerful technique with which we can evaluate the performance of perovskite-based LEDs at the nanoscale by hyperspectral wide-field imaging. We show how to obtain maps at the diffraction limit scale for their External Quantum Efficiency (EQE), luminance, and luminous efficacy. Our observations reveal the degradation pathways of this emerging class of LEDs, induced by microscale heterogeneities that hamper the ultimate device performance. The lessons learned from our multimodal microscopy approach allow us to identify a novel dual molecular treatment that selectively 1) optimises the thin film quality (e.g. suppression of pinholes while maximising light outcoupling) and 2) enhances the intrinsic optoelectronic quality of the quasi 2D perovskite (e.g. PLQE, IQE). This methodology results in bright (>14,000 cd/m2) and efficient (>20% EQE) green LEDs with operational lifetimes surpassing 100 h. Our work represents a step forward in diagnosing the in-operando properties of low dimensional perovskite-based devices at the nanoscale and opens avenues to identify strategies for stable and efficient LEDs with spatially maximised performance.[5]

[1] Stranks, S. D. and Snaith, H. J. Metal-halide perovskites for photovoltaic and light-emitting devices. Nat. Nanotechnol. 2015, 10, 391.
[2] Anaya, M.; et al. Best practices for measuring emerging light-emitting diode technologies. Nature Photonics, 2019, 12, 818.
[3] Hoye, R. L. Z.; Lai, M.-L.; Anaya, M. et al. Identifying and Reducing Interfacial Losses to Enhance Color-Pure Electroluminescence in Blue-Emitting Perovskite Nanoplatelet Light-Emitting Diodes. ACS Energy Lett. 2019, 5, 1181-1188.
[4] Ji, K.; Anaya, M.; Abfalterer, A.; Stranks, S. Halide Perovskite Light Emitting Diode Technologies. Adv. Opt. Mater. 2021, 2002128.
[5] Anaya, M.; Cui, L.; Frohna, K.; Stranks, S. D. et al. to be submitted.

While 3-dimensional inorganic and hybrid halide perovskites have rightly seen great success in photovoltaics and LEDs, lower dimensional perovskites and perovskite-like materials, whether layered or ordered defect ‘0D’, have also shown similarly promising optoelectronic properties. The onset of self-trapped excitonic behaviour that has also been observed and associated with reduction in structural or electronic dimensionality make this class of materials highly promising as light emitters.¹ In the stable, wholly inorganic low-D perovskites, such strong excitonic behaviour has been seen in the ‘0D’ perovskite-like Cs₂M^(IV)X₆ compounds,² as well as at room temperature in the 2D ordered-defect pnictide-based perovskites of the Cs₃M^(III)₂X₉ family,³ and also resulting in high quantum efficiency in the highly promising Cs₂M^(I)M^(III)X₆ double perovskites,⁴ which while 3-dimensional in structure, can demonstrate a ‘0D’-like electronic structure due to the substitution of the perovskite ‘B’ site;⁵ all of these materials maintain the added benefit of moving away from toxic Pb.
Nevertheless, a reliable theoretical description of the physical origin of these phenomena across the range of different structures available to lower-dimensional perovskites is lacking due to the inability of standard ‘independent particle’ methods such as Density Functional Theory (DFT) to describe excitons. In this study, we select example compounds from each of the aforementioned materials families and use ab-initio calculations to establish their electronic, optical and excitonic properties: Cs₂M^(IV)X₆ (M^(IV) = Zr, Sn; X = Cl, Br), Cs₃M^(III)₂X₉ (M^(III) = Bi, X = Br) and Cs₂M^(I)M^(III)X₆ (M^(I) = Ag, Na; M^(III) = Bi, In; X = Cl, Br). We initially establish a comparison in the electronic structures predicted through accurate standard ab-initio methods, such as hybrid DFT, and higher level many-body perturbation methods (QSGW) available within the open-source code Questaal.⁶ Then, where standard DFT fails, we calculate the excitonic spectrum, within the Bethe-Salpeter Equation, of each material to establish a consistent understanding of the excitonic properties and establish the influence of both structure and composition. We also computationally examine the dynamic and thermodynamic stability of these materials to confirm whether the advantage of stability attributed in both lower-dimensional and inorganic perovskites is maintained. In performing this survey across a range of inorganic perovskite-like structures, we hope to establish the structure-property relationships that can potentially provide insight into the behaviour of current inorganic or hybrid inorganic-organic low-dimensional perovskites and also guide future syntheses of lead-free, efficient light emitters.
(1) Smith, M. D.; Karunadasa, H. I. Acc. Chem. Res. 2018, 51, 619.
(2) Abfalterer, A.; Shamsi, J.; Kubicki, D.J.; Savory, C.N.; Xiao, J., et al. ACS Materials Letters, 2020, 2, 1633-1652
(3) McCall, K.M.; Stoumpos, C.C.; Kostina, S.S.; Kanatzidis, M.G.; Wessels, B.W. Chem. Mater. 2017, 29, 4129-4145
(4) Luo, J.; Wang, X.; Li, S.; Liu, J.; Guo, Y. et al. Nature, 2018, 563, 541
(5) Xiao, Z.; Meng, W.; Wang, J.; Mitzi, D.B.; Yan, Y. Mater. Horiz., 2017, 4, 206-216
(6) Pashov, D.; Acharya, S.; Lambrecht, W.R.L.; Jackson, J.; Belashchenko, K.D.; Chantis, A.; Jamet, F.; van Schilfgaarde, M. Comp. Phys. Commun., 2020, 249, 107065

Over the past decade, metal halide perovskites, with a particular focus on traditional structures such as CsPbX₃ (X = Br^-, Cl^-, I^-) have garnered much interest as potential photocatalysts for many reactions such as solar fuel production (CO₂ reduction, HER), and in the degradation of organic pollutants. The fully ionic nature of these materials, however, has resulted in major challenges for their practical use. More stable perovskite alternative structures such as Cs₃B₂X₉ (B = Sb³⁺, Bi³⁺ X = Br^-, Cl^-, I^-) have shown great promise as novel photocatalysts and have not been heavily studied yet, especially at the 0D structural level in the form of NCs. Thus, this study was aimed at investigating the photocatalytic capabilities of Cs₃Bi₂Br₉ NCs synthesized by hot injection method. As synthesized Cs₃Bi₂Br₉ NCs with an observed optical bandgap of 2.85 eV were applied to the photocatalytic degradation of Rhodamine G (RhG) dye in an aqueous environment. For comparison, CsPbBr₃ NCs synthesized by hot injection method and P25 TiO₂ (Sigma) were also tested. The degradation of RhG follows pseudo-zeroth order kinetics with Cs₃Bi₂Br₉ NCs reaching catalytic efficiencies near 90%. Furthermore, the observed stability of the Cs₃Bi₂Br₉ NCs is much greater than the CsPbBr₃ NCs in that near zero catalytic efficiency was observed for CsPbBr₃. This is due to the degradation of CsPbBr₃ NCs in water occurring at a much faster rate than RhG degradation. Despite this, it was observed that as synthesized Cs₃Bi₂Br₉ NCs do degrade over time and thus the overall stability of the system must be improved for practical use. Current efforts to enhance the stability of the system include overcoating via ALD in order to passivate the surface and protect the perovskite layer, with an overall goal of producing a suitable water stable photocatalyst with high efficiency for wastewater purification.

Lead halide perovskites have been well explored in various optoelectronic applications due to their unique properties. While issues like toxicity and instabilities hinder their further use, which aroused great interest for the development of alternative lead-free perovskites. Recently, a new family of layered double perovskites (LDPs) with a chemical formula of Cs₄M(II)M(III)₂X₁₂ provide a new route to solve these problems by combining two main strategies including composition substitution and dimensionality reduction. On the one hand, Pb²⁺ cations are substituted by a combination of M(II) and M(III) cations with the enlarged composition tuning space. On the other hand, the layered structure consists of one layer of [M(II)X₆]^4- octahedra sandwiched by two layers of [M(III)X₆]^3- octahedra in between two adjacent vacancy layers. However, accessibilities to this class of perovskite-type nanomaterials remain inadequate, hindering their practical implementations in various applications. Here, we report the colloidal synthesis of several types of LDP NCs using a modified hot-injection method, such as Cs₄CuSb₂Cl₁₂, Cs₄MnBi₂Cl₁₂, Cs₄CdBi₂Cl₁₂ and Cs₄CdSb₂Cl₁₂ LDP NCs. The resulting LDP NCs show unique optical, electronic and magnetic properties with different crystal structures, inspiring to build the composition-structure-property relations between various LDP NCs. Therefore, a series of Cs₄Cu_xAg_2-2xSb₂Cl₁₂, Cs₄Cd_1-xMn_xBi₂Cl₁₂, and Cs₄Cd_1-xCu_xSb₂Cl₁₂ perovskite-type NCs (0≤x≤1) are synthesized to investigate the composition-induced crystal structural transformation, and thus, the evolution of electronic bandgap, magnetic ordering and charge carrier dynamics. These transformation processes are explored by experimental observations and further confirmed by theoretical DFT calculations on the band structures, charge density differences and electron localization function. Photo-generated charge carrier dynamics of the LDP NCs are investigated using ultrafast spectroscopic techniques. Taking advantage of both the unique electronic structures and solution processability, we demonstrate that the LDP NCs can be solution-processed as high-speed photodetectors with ultrafast photo-response and narrow bandwidth. We anticipate that our study will prompt future research to design and fabricate novel and high-performance lead-free perovskite-type NCs for a range of photovoltaic applications in the future.

Cs2AgBiBr6 (CABB) has been proposed as a less toxic and more stable alternative to lead halide perovskites. However, low charge carrier collection efficiencies remain an obstacle for the incorporation of this material in optoelectronic applications. In this talk, I will elaborate on the dynamics of charge transport, localization and recombination in CABB thin films, studied using time-resolved photoconductivity and transient absorption/reflectance measurements. Temperature-dependent photoconductivity measurements indicate band-like charge transport of free charges, similar to lead-based perovskites.¹ We find that replacement of Bi with Sb shifts the absorption onset, while preserving the charge transport mechanism. However, as charges lose mobility within tens of nanoseconds, diffusion lengths remain relatively short in these lead-free materials. While mobile charges have a nanosecond-lifetime, we observe microsecond-long lifetimes in transient absorption measurements.³ This suggests that the conductivity loss in CABB films is due to a combined effect of carrier recombination and localization. Moreover, we find that the charge carrier diffusion length is similar to the and sample thickness, suggesting that charge localization occurs at the materials surface. Therefore, improving surface quality could be a strategy to optimize performance of CABB-based optoelectronic devices. In parallel to investigating CABB, we performed a toxicity study of methylammonium lead iodide perovskites on A. Thaliana plants, and found that the iodide was actually much more toxic than the lead.² Altogether, these results stress the importance of further understanding which perovskites are most harmful to the environment, while optimizing the optoelectronic quality of materials with the lowest toxicity.
(1) Hutter, E. M.; Gélvez-Rueda, M. C.; Bartesaghi, D.; Grozema, F. C.; Savenije, T. J. Band-Like Charge Transport in Cs2AgBiBr6 and Mixed Antimony-Bismuth Cs2AgBi1- XSbxBr6 Halide Double Perovskites. ACS Omega 2018, 3 (9), 11655–11662. https://doi.org/10.1021/acsomega.8b01705.
(2) Hutter, E. M.; Sangster, R.; Testerink, C.; Ehrler, B.; Gommers, C. M. M. Metal Halide Perovskite Toxicity Effects on Plants Are Caused by Iodide Ions. arXiv:2012.06219 2020.
(3) Jöbsis, H. J.; Caselli, V. M.; Askes, S. H. C.; Garnett, E. C.; Savenije, T. J.; Rabouw, F. T.; Hutter, E. M. Recombination and Localization : Unfolding the Pathways behind Conductivity Losses in Cs 2 AgBiBr 6 Thin Films. arXiv:2107.01852 2021.

Lead halide compounds, including lead halide perovskite nanocrystals (NCs), have attracted the interest of researchers in optoelectronics and photonics due to their high photoluminescence quantum yields (PLQYs) coupled with relatively short PL lifetimes (on the order of a few nanoseconds). However, lead-free metal halides of high PLQY, including double perovskites and their doped NCs, typically possess long PL lifetimes (up to microseconds), due to the nature of their excitons, which limits their application space. Here, I introduce CsMnBr₃ NCs, which are lead-free and combine a high PLQY with an exceptionally short radiative lifetime (on the order of picoseconds). We find that the octahedral coordination of Mn²⁺ in CsMnBr₃ induces a red emission centered at ~643 nm with a PLQY of ~54% and a fast radiative decay rate. Femtosecond transient absorption and transient PL spectroscopies reveal the existence of a low-lying excited state of Mn²⁺ that relaxes to the ground state within ~600 ps by emitting light at ~643 nm. At greater excitation energies, higher excited states of Mn²⁺ relax within a sub-nanosecond timescale to this low-lying excited state. A similarly positioned PL peak with a short picosecond-scale PL lifetime and a PLQY of ~ 6.7% was also detected in bulk CsMnBr₃ single crystals – a relatively high quantum yield for a bulk material. Our experimental results and density functional theory modelling show that the crystal structure and the strong coupling among Mn²⁺ ions govern those luminescence properties of CsMnBr₃ NCs and single crystals. These findings pave the way for new lead-free materials that combine high PLQY and ultrafast luminescence.

Ferroelasticity represents material domains possessing spontaneous strain that can be switched by external stress. Three-dimensional perovskites like methylammonium lead iodide are determined to be ferroelastic. Layered perovskites have been applied in optoelectronic devices with outstanding performance. However, the understanding of lattice strain and ferroelasticity in layered perovskites is still lacking. Here, using the in-situ observation of switching domains in layered perovskite single crystals under external strain, we discover the evidence of ferroelasticity in layered perovskites with layer number more than one, while the perovskites with single octahedra layer do not show ferroelasticity. Density functional theory calculation shows that ferroelasticity in layered perovskites originates from the distortion of inorganic octahedra resulting from the rotation of aspherical methylammonium cations. The absence of methylammonium cations in single layer perovskite accounts for the lack of ferroelasticity. These ferroelastic domains do not induce non-radiative recombination or reduce the photoluminescence quantum yield.

Low-dimensional metal halides have been the focus of intense investigations in recent years following the success of the three-dimensional (3D) hybrid lead halide perovskites as optoelectronic materials for solar cells, semiconductor radiation detectors, and narrow emitters. In particular, the light emission of zero-dimensional (0D) metal halides have found utility in a variety of applications (including X-ray scintillation and remote thermometry) that are complementary to those of the 3D halide perovskites, thanks to their unusual properties including intrinsic broadband emission and highly temperature-dependent lifetimes. Inorganic, lead-free compositions are especially sought after thanks to their enhanced stability and reduced toxicity.
Here, we flesh out the family of fully inorganic, 0D pnictogen halides based on the Rb₇M₃X₁₆ (M=Sb, Bi; X=Br, I) stoichiometry by solvothermal and solid-state synthesis of the heavier halides, namely Rb₇Sb₃Br₁₆, Rb₇Bi₃Br₁₆, and Rb₇Bi₃I₁₆.The title compound exhibits a 0D structure comprised of two isolated units: [SbBr₆]^3- octahedra and edge-sharing bioctahedra [Sb₂Br₁₀]^4- dimers which order into layers along the c-axis. The compound is air-stable and exhibits weak orange-red photoluminescence (PL) at room temperature, but intriguingly at low temperatures has two distinct emission bands with different excitation characteristics. Temperature-dependent PL and PL excitation spectra reveal that these bands correspond to these two motifs, and that there is a crossover near 150 K where the high-energy emission quenches as the units begin to interact.

We are also able to obtain Rb₇Bi₃Br₁₆ and Rb₇Bi₃I₁₆ which both crystallize in orthorhombic symmetry, with Rb₇Bi₃Br₁₆ presenting weak low-temperature luminescence while Rb₇Bi₃I₁₆ is non-luminescent. These trends in PL intensity with changing composition can be directly linked to the lone pair activity, which is maximized in the Sb analogues with lighter halides, yielding insight into how to design optically emissive materials. This work expands the library of emissive inorganic metal halides and provides further evidence for the efficacy of low-dimensional Sb-X luminescent centers based on octahedral and edge-sharing [Sb₂X₁₀]^4- dimers.

I will review advances in perovskite LEDs, focusing on the use of quantum wells and quantum dots.

Lead halide perovskite nanocrystals (LHP NCs_, NCs, A=Cs⁺, FA⁺, FA=formamidinium; X=Cl, Br, I) exhibit spectrally narrow (<100 meV) fluorescence, tunable over the entire visible spectral region of 400-800 nm [1,2]. Owing to their high oscillator strength, slow dephasing (long coherence times of up to 80 ps), minimal inhomogeneous broadening of emission lines, and a bright triplet exciton character with orthogonal dipole orientation [2], these NCs make for a highly versatile platform for creating controlled, aggregated states exhibiting collective phenomena. Long-range ordered superlattices (SLs) with the simple cubic packing of cubic perovskite NCs exhibit sharp red-shifted lines in their emission spectra and superfluorescence (a fast collective emission resulting from coherent multi-NCs excited states) [4]. We now present perovskite-type ABO₃ binary and ternary NC SLs by a shape-directed co-assembly of CsPbBr₃ nanocubes (occupying B- and/or O-sites) with spherical dielectric Fe₃O₄ or NaGdF₄ NCs (A-sites) and truncated-cuboid PbS NCs (B-site) [5]. Such ABO₃ SLs, as well as other newly obtained SL structures (binary NaCl- and AlB₂-types, columnar assemblies with disks, etc.), exhibit a high degree of orientational ordering of CsPbBr₃ nanocubes. These mesostructures exhibit superfluorescence as well, characterized, at high excitation density, by emission pulses with ultrafast (22 ps) radiative decay and Burnham-Chiao ringing behavior with a strongly accelerated build-up time.

[1] M. V. Kovalenko et al. Science 2017, 358, 745-750
[2] Q.A. Akkerman et al. Nature Materials 2018, 17, 394–405
[3] M. A. Becker et al, Nature 2018, 553, 189-193
[4] G. Rainò et al. Nature 2018, 563, 671–675
[5] I. Cherniukh et al. Nature 2021, 593, 535–542

Lighting applications require cheap and sustainable materials for broadband white-light emission generation. Recently, layered hybrid perovskites have been proposed as a potential material system, in which broad emission from self-trapped excitons was reported, among varying optical properties and unfortunately with low stability [1], [2].

Here, we report the generation of broad spectra from free, bright excitons by producing thin films of layered Ruddlesden-Popper lead iodide perovskites with tailored composition of multiple phases with varying layer number.

We investigate structure and photophysics of our hybrid perovskite films with selected linear and aromatic monoammonium cations [3], and a range of small cations (Cs, FA, MA). The chosen materials were produced by optimizing fabrication parameters, which were reported to influence the optical and structural properties [3]. We find best performance from the Cs containing compositions.

The resulting optimized materials are optically investigated with time resolved photoluminescence spectroscopy and transient absorption spectroscopy both at room temperature and at cryogenic temperatures. Furthermore, we characterize photoluminescence quantum efficiency. A detailed structural analysis with X-ray diffraction spectroscopy, atomic force microscopy and scanning electron microscope is performed to correlate structural and optical properties.

For the materials which show the broadest emission spectrum and best optoelectronic performance, we fabricate LEDs and use them as the emitter layer in combination with a blue LED for broadband white light emission.

Our findings have promise as a new approach for using Ruddlesden-Popper lead iodide perovskites in the broader application of white LEDs, which could result in lower cost and more efficient lighting.

[1] M. Bidikoudi, E. Fresta, and R. D. Costa, “White perovskite based lighting devices,” Chem. Commun., vol. 54, no. 59, pp. 8150–8169, 2018, doi: 10.1039/c8cc03166e.
[2] S. Kahmann, E. K. Tekelenburg, H. Duim, M. E. Kamminga, and M. A. Loi, “Extrinsic nature of the broad photoluminescence in lead iodide-based Ruddlesden–Popper perovskites,” Nat. Commun., vol. 11, no. 1, pp. 1–8, 2020, doi: 10.1038/s41467-020-15970-x.
[3] X. Li, J. M. Hoffman, and M. G. Kanatzidis, “The 2D halide perovskite rulebook: How the spacer influences everything from the structure to optoelectronic device efficiency,” Chem. Rev., vol. 121, no. 4, pp. 2230–2291, 2021, doi: 10.1021/acs.chemrev.0c01006.

Recently, 2-dimension (2D) organic-inorganic halide perovskites have become one of the key materials in emission diode. The electronic structure and structural stability of these materials show a significant dependence on chemical composition. Therefore the emission properties and durability of the halide perovskite can be tuned by changing the combination of composition. In this study, using density functional theory (DFT) and molecular dynamics, we theoretically investigated a crystal structure and electronic structure of the partially substituted cubic (A)2PbX3 (A = butylammonium (BA), octylammonium (OA), X = halide). Our calculation results indicate that a bandgap is associated with Pb-X distance in the lattice, although the contribution of A-site ions is little. The relation of bandgap with Pb-X length is further confirmed and refined by expanding the investigated composition based on the 2D perovskite database reported by Marchenko et al. [Chem. Mater. (2020) 32, 7383]. It is found that the structural parameter of Pb-X simply explains the bandgap for different compositions. The relationship of bandgap and emission peaks in photoluminescence (PL) is further investigated with machine learning. Finally, PL emission peaks are predicted for the database and some of them are compared with the experiments for validation. This model explained the trend of the Stokes shift in 2D halide perovskite. The broadness of emission peaks, which may be related to the A site species, is also discussed.

Hybrid lead halide perovskites and their lower dimensional related-structures are rising materials with extensive applications in photovoltaics and light-emitting diodes (LEDs), in part due to their luminescent properties. Herein, we present a family of low-dimensional lead halides with strong photoluminescence, incorporating into their structure the symmetric, triamine chromophore: 1,3,4-tris(1,3,5-tris(4-aminophenyl)benzene, symbolized as T. A zero-dimensional (0D) lead bromide structure is synthesized with the formula: (T)7Pb3Br27.DMF. The novel 0D material contains the triamine molecules with isolated [PbBr6]4- octahedra and (DMF) molecules dispersed throughout.
The distance of the terminal amino-tail of the triamine to the nearest bromine atom is 3.652 Å. As a result of the extended conjugation of the triamine chromophore, (T)7Pb3Br27.DMF displays cation- π interactions, with π- π interactions of 3.498 Å distance between the closest aromatic rings of the adjacent triamine molecules. The DMF molecules are orientated adjacent to the lead bromide octahedra with an average distance of 3.05 Å of the oxygen atom with the nearest bromine atoms.
The (T)7Pb3Br27.DMF compound exhibits high distortion levels of the [PbBr6]4- octahedra, with an average bond length of 3.01Å, an average bond angle variance of 46.696 deg.2 and an average bond length distortion index of 0.02676. The measured UV absorption spectra of the triamine itself shows two features, one with an absorption edge at 2.85 eV and another absorption edge at 2.58 eV. The (T)7Pb3Br27.DMF compound also displays two features in its absorption spectra with absorption edges at 2.95 eV and 2.00 eV. The studied compounds demonstrate vibrant, strong photoluminescence emission. The triamine chromophore exhibits blueish-white emission with a broad emission peak centered at 428 nm along with a shoulder at 370 nm. The (T)7Pb3Br27.DMF compound demonstrates strong and narrow blue emission at 368 nm with a lifetime of 5 ms due to self-trapped excitons. Additionally, the lead chloride and lead iodide analogues incorporating the triamine molecule have been synthesized. Our work proposes a synthetic strategy in designing hybrid lead halide compounds with symmetric chromophore molecules that exhibit strong photoluminescent emission.

Next-generation terahertz (THz) sources demand light-weight, low-cost, defect-tolerant, and robust components with synergistic, tunable capabilities. However, a paucity of materials systems simultaneously possessing these desirable attributes and functionalities has made device realization difficult. Here we report the observation of asymmetric spintronic-THz radiation in Two-Dimensional Hybrid Metal Halides (2D-HMH) interfaced with a ferromagnetic metal, produced by ultrafast spin current under femtosecond laser excitation. The generated THz radiation exhibits an asymmetric intensity toward forward and backward emission direction whose directionality can be mutually controlled by the direction of applied magnetic field and linear polarization of the laser pulse. Our work demonstrates the capability for the coherent control of THz emission from 2D-HMHs, enabling their promising applications on the ultrafast timescale as solution-processed material candidates for future THz emitters.

Nanophotonics and meta-optics based on optically resonant all-dielectric structures is a rapidly developing research area driven by its potential applications for low-loss efficient metadevices. Recently, the study of halide perovskites has attracted enormous attention due to their exceptional optical and electrical properties. As a result, this family of materials can provide a prospective platform for modern nanophotonics [1] and meta-optics [2], allowing us to overcome many obstacles associated with the use of conventional semiconductor materials. Here, we overview the recent progress in the field of halide perovskite nanophotonics starting from single-particle light-emitting nanoantennas [3,4], nanolasers [5], microlasers [6,7], and to the large-scale designs working for surface coloration, anti-reflection, and optical information encoding [8-9].


References:
[1] Makarov, S., Furasova, A., Tiguntseva, E., Hemmetter, A., Berestennikov, A., Pushkarev, A., Zakhidov, A. and Kivshar, Y., 2019. Halide-Perovskite Resonant Nanophotonics. Advanced optical materials, 7(1), p.1800784.
[2] Berestennikov, A.S., Voroshilov, P.M., Makarov, S.V. and Kivshar, Y.S., 2019. Active meta-optics and nanophotonics with halide perovskites. Applied Physics Reviews, 6(3), p.031307.
[3] Tiguntseva, E.Y., Zograf, G.P., Komissarenko, F.E., Zuev, D.A., Zakhidov, A.A., Makarov, S.V. and Kivshar, Y.S., 2018. Light-emitting halide perovskite nanoantennas. Nano letters, 18(2), pp.1185-1190.
[4] Tiguntseva, E.Y., Baranov, D.G., Pushkarev, A.P., Munkhbat, B., Komissarenko, F., Franckevicius, M., Zakhidov, A.A., Shegai, T., Kivshar, Y.S. and Makarov, S.V., 2018. Tunable hybrid Fano resonances in halide perovskite nanoparticles. Nano letters, 18(9), pp.5522-5529.
[5] Tiguntseva, E., Koshelev, K., Furasova, A., Tonkaev, P., Mikhailovskii, V., Ushakova, E.V., Baranov, D.G., Shegai, T., Zakhidov, A.A., Kivshar, Y. and Makarov, S.V., 2020. Room-Temperature Lasing from Mie-Resonant Non-Plasmonic Nanoparticles. ACS Nano 13 (4), 4140-4147.
[6] Zhizhchenko, A., Syubaev, S., Berestennikov, A., Yulin, A.V., Porfirev, A., Pushkarev, A., Shishkin, I., Golokhvast, K., Bogdanov, A.A., Zakhidov, A.A. and Kuchmizhak, A.A., 2019. Single-mode lasing from imprinted halide-perovskite microdisks. ACS nano, 13(4), pp.4140-4147.
[7] Trofimov, P., Pushkarev, A.P., Sinev, I.S., Fedorov, V.V., Bruyère, S., Bolshakov, A., Mukhin, I.S. and Makarov, S.V., 2020. Perovskite–Gallium Phosphide Platform for Reconfigurable Visible-Light Nanophotonic Chip. ACS nano, 14(7), pp.8126-8134.
[8] Zhizhchenko, A.Y., Tonkaev, P., Gets, D., Larin, A., Zuev, D., Starikov, S., Pustovalov, E.V., Zakharenko, A.M., Kulinich, S.A., Juodkazis, S. and Kuchmizhak, A.A., 2020. Light-Emitting Nanophotonic Designs Enabled by Ultrafast Laser Processing of Halide Perovskites. Small, 16(19), p.2000410.
[9] Zhizhchenko, A.Y., Cherepakhin, A.B., Masharin, M.A., Pushkarev, A.P., Kulinich, S.A., Porfirev, A.P., Kuchmizhak, A.A. and Makarov, S.V., 2021. Direct Imprinting of Laser Field on Halide Perovskite Single Crystal for Advanced Photonic Applications. Laser & Photonics Reviews, p.2100094.

Halide perovskite semiconductors can merge the highly efficient operational principles of conventional inorganic semiconductors with the low temperature solution processability of emerging organic and hybrid materials, offering a promising route towards cheaply generating electricity as well as light. Following a surge of interest in this class of materials, research on the corresponding halide perovskite nanocrystals (NCs) as well has gathered momentum in the last years.¹ A hot area of research in this field is the fabrication of nano-heterostructures in which the perovskite is one of the domains, which will briefly reviewed, including work from our group. Another class of materials that has been recently investigate by us is that of metal chalcohalides. These materials offer a broad solid-state chemistry and they been investigated for applications in solar energy conversion, thermoelectrics, hard radiation detection, and superconductivity. Among them, lead chalcohalides have been rarely studied in the past. We have recently reported the synthesis of a series of lead chalcohalides, by means of colloidal approaches, delivering phases and compositions that had not been previously identified in the bulk.² These materials present indirect band gaps, they emit in the NIR region of the spectrum at cryogenic temperatures and have unique crystal structures. We will also show our recent results on the synthesis and advanced characterization of heterostructured nanocrystals in which one domain is a lead chalcohalide and the other domain is a cesium lead halide perovskite.³ The two domains are separated by a flat, atomically defined, epitaxial interface. In these materials, the photogenerated carries are separated at their interface, and as such they might be promising in applications ranging from catalysis to photovoltaics. The talk will also discuss on how the presence of the halide perovskite domain dictates the phase and chemical composition of the metal halide domain.

References
1. Q. Akkerman et al. “Genesis, challenges and opportunities for colloidal lead halide perovskite nanocrystals”, Nat. Mater. 2018, 17, 394.
2. S. Toso et al. “Nanocrystals of Lead Chalcohalides: A Series of Kinetically Trapped Metastable Nanostructures”, J. Am. Chem. Soc. 2020, 142, 22, 10198.
3. S. Toso et al. “Nanocrystals of Lead Chalcohalides: A Series of Kinetically Trapped Metastable Nanostructures”, J. Am. Chem. Soc. 2021, 143, 3, 1435.

Low-dimensional metal halides have attracted interest from researchers in the field of optoelectronics due to their high photoluminescence quantum yield (PLQY) [1-2]. The fact that these metal halides exhibit a high bulk PLQY, without preparing them in nanocrystal form, makes them an interesting class of optoelectronic materials. One characteristic feature is a large Stokes shift - the difference between absorbed and emitted light wavelength - which is usually attributed to the formation of self-trapped excitons (STEs). STE indicates the state where a photoexcited electron-hole pair is stabilized and trapped at a lattice site. This phenomenon is the result of strong electron-phonon coupling and can be enhanced by a soft crystal host [3]. Although low-dimensional metal halides shows the high PLQY of > 90% via STE emission, the emission processes happening within low-dimensional metal halides is poorly understood at the moment. Indeed, (i) why self-trapping occurs so efficiently; (ii) how self-trapped exciton emission competes against intrinsic band-to-band transitions are not known.

Here, we performed first-principles density-functional theory (DFT) calculations to explain why this class of materials exhibit such a high PLQY. We selected Cs₃Cu₂I₅ as the model system for this work as the material is a well-known example of STE emission [4]. Optical properties originate from its intrinsic bulk electronic structure (e.g. high-frequency dielectric function and absorption coefficient) will be firstly assess. Then explicit modelling of STE formation will be carried out to calculate the STE emission energy and self-trapping barrier in Cs₃Cu₂I₅. Based on these quantities, we will evaluate the rates of band-to-band recombination and STE emission, and consequently, we predict the quantum yield of blue emission as a function of the excited carrier concentration.

[1] X. Li, X. Gao, X. Zhang, X. Shen, M. Lu, J. Wu, Z. Shi, V. L. Colvin, J. Hu, X. Bai, W. W. Yu, and Y. Zhang, Adv. Sci. 8, 2003334 (2021)
[2] M. Li and Z. Xia, Chem. Soc. Rev. 50, 2626 (2021)
[3] S. Li, J. Luo, J. Liu, and J. Tang, J. Phys. Chem. Lett. 10, 1999 (2019).
[4] T. Jun, K. Sim, S. Iimura, M. Sasase, H. Kamioka, J. Kim, and H. Hosono, Adv. Mater. 30, 1804547 (2018).

Organic-inorganic hybrid perovskites emerged as a very promising material for many applications such as photovoltaics, LEDs, and transistors. These materials have the composition of ABX₃ where A is a small organic cation such as MA⁺, FA⁺, B is Pb^- or Sn^- and X is a halide anion such as I^-, Br^- and Cl^-.
Partial substitution of organic cations in the 3D structure with hydrophobic organic cations can lead to the formation of lower-dimensional perovskites. This led to methods of forming 2D/3D bilayered perovskite, whereby a single-step fabrication process a phase separation is driven by self-assembly is obtained forming an upper 2D layer that serves as a capping layer and a bottom 3D layer.
In our research, we present the influence of the deposition method, the stoichiometry of the precursors’ solution, and the surface energy of the substrate on the quality of the perovskite layers in terms of grain size and crystallinity as well as on the composition of the 2D/3D structure in terms of n=1,2,...
This film optimization allowed us to fabricate field-effect transistors that are based on layered perovskite and use the transistor as a tool to evaluate the material’s properties. We’ll describe the performance of the transistors in terms of device J-V characteristics as well as extraction of residual doping and mobility, as a function of the processing methods. These parameters would assist in designing efficient and stable photovoltaic devices.

The rapid relaxation of above-gap ‘hot’ carriers in absorber layer materials currently places a limit on the open-circuit voltage achievable in perovskite photovoltaic devices. The dimentions and dimentionality of perovsikte materials is expected to affect carrier cooling on account of changin coupling between electronic states as well as coupling electronic system to lattice phonon energy levels. Here, ultrafast pump-push-probe spectroscopy is used to compare hot carrier cooling dynamics in 3D perovskites, 8 and 5 nm cuboidal nanocrystals,
3.5 nm nanoplatelets and bulk 2D perovskites prepared using different techniques. Carrier cooling rates are shown to be similar to bulk materials in all but 2D-confined perovskite nanosystems. Surprisingly, cooling rates are observed to increase in such systems, while hot phonon bottleneck phenomena become much less pronounced. This work offers further insight into fundamental physical processes occurring in nanoscale materials and aid in the design of future solar cell absorber systems.

It is common in the context of hybrid metal halide perovskites and its derivatives to use temperature dependence of photoluminescence linewidths to estimate electron-phonon coupling parameters. However, due to comparable contributions from homogeneous and inhomogeneous mechanisms to the spectral broadening, linear spectroscopies like photoluminescence are incapable of isolating the pure dephasing rates and thus are not representative of the system-bath interactions. In these circumstances, nonlinear coherent optical spectroscopy offers insights into the nature of many-body interactions that are entirely inaccessible to temperature-dependent linear spectroscopies. When applied to Ruddlesden-Popper metal halides, these techniques have indeed enabled us to quantify the elastic scattering effects on the exciton dephasing rates. Here, we will discuss the mechanistic nature of exciton-phonon, exciton-exciton and phonon-phonon interactions in prototypical Ruddlesden Popper metal halides based on such experimental observations. We will discuss our perspective on how the coherent lineshapes of Ruddlesden-Popper metal halides can be effectively rationalized within an exciton polaron framework. We will also discuss the critical role of the metal cation in the observed coherent exciton dynamics.

Two dimensional (2D) layered lead halide perovskites with large exciton binding energies, efficient radiative recombination, and outstanding environmental stability are regarded as supreme candidates for realizing highly compact and ultralow threshold lasers. However, continuous-wave (CW) pumped lasing of 2D Lead halide perovskites, as the precondition for the electrically pumped lasing, is still challenging. Here, the phenylethylammonium lead iodide [(PEA)₂PbI₄] thin flakes, exfoliated from the solution-processed millimeter-sized single crystal, were utilized as gain media to construct a vertical-cavity surface-emitting laser (VCSEL), which presents the robust single-mode CW lasing operation with an ultra-low threshold of 5.7 W cm⁻² at room temperature, attributed to strong optical confinement in the high-Q cavity. Moreover, the multi-mode emission has been synchronously discussed through experiements and simulations. This work opens up a new window for developing novel 2D VCSELs based on the layered lead halide perovskites.

Understanding the transport properties of layered perovskite optoelectronics requires the characterization of carrier dynamics both in the plane of and transverse to the layer structure. We explore carrier transport using polarization-dependent terahertz (THz) conductivity measurements in layered (2D) hybrid perovskites. We have synthesized ~300 nm thick films of n = 1 butylammonium and phenethylammonium lead iodide layered perovskite semiconductors, with layers oriented parallel to sapphire substrates and with crystalline domain sizes of approximately 1 micron diameter. In the measurements, around 110 femtosecond pump pulses from a Ti:sapphire laser source were converted to THz radiation with picosecond pulse duration via an electro-optic crystal. Transmission measurements at normal incidence yielded values of complex transmission coefficients which enable determination of the complex dielectric function in the 0.3 - 2 THz range, corresponding to the in-plane complex conductivities of these materials. We will also discuss results of non-normally incident transmission measurements varying s and p THz polarization to probe the differences in ligand-mediated out-of-plane transport between these layered perovskites, as well as results of density functional theory calculations. This work expands on the utility of oblique incidence terahertz experiments which are rarely implemented for complex parameter extraction in uniaxial thin films. Furthermore, these results underscore the potential for THz spectroscopy to probe fundamental charge transport phenomena in complex photovoltaic materials with layered structures.

Halide perovskite has been investigated intensively as an attractive candidate for some optoelectronic applications such as solar cells, light-emitting diodes, lasers and photodetectors. Especially, the sharp and easily changeable emission with halide anion exchange make this material group even more attractive for the light-emitting applications. Among this material group, inorganic 3D CsPbX₃ (X: Cl^-, Br^-, I^-) semiconductors produce 0D Cs₄PbX₆ insulators in specific conditions. This can be easily done with vacuum co-evaporation with excess CsX. In this regard, we investigate the anion diffusion and emission property of the halide perovskite. These heterojunctions are electrically excited between two electrodes as the light-emitting electrochemical cells (LECs), as a proof-of-concept application. With this simplified structure, the optoelectronic property can be explored without the effects from any charge transport layer. Herein we demonstrate different behavior of anion diffusion depending on the dimension of vacuum co-evaporated cesium lead chloride.

In recent years, lead halide perovskite solar cells(PSCs) have been attracting attention as the next generation of solar cells because they can be fabricated by the solution process and their high energy conversion efficiency. On the other hand, the lead halide perovskite compounds that constitutes PSCs have problems of instability and toxicity under various conditions. In recent years, researches on solar cells using a copper halide perovskite compounds as a photoactive layer in which Pb is replaced with Cu, which has low toxicity, have been reported. It has been reported that copper halide perovskite compounds have a high absorption coefficient and high charge mobility depending on the compound. However, a solar cell using them has a conversion efficiency of about 2%, and it has been pointed out that further improvement including selection of the compound is necessary.
We have systematically investigated the effects of cation and halide ion types of copper halide perovskite compound films on the stability, morphology and optical properties under various conditions. In this presentation, we will report the detailed experimental results on the electronic characteristics of these thin films and the characteristics of solar cells.
In order to solve these problems, research has been conducted on perovskite compounds in which the Pb at the B site is replaced by Cu, which is less toxic. The Cu-based perovskites have been reported to have high absorption coefficient and charge mobility depending on the compound. However, the conversion efficiency of solar cells using Cu-based perovskite compounds as the photoactive layer is only about 2 %, indicating the need for further improvement, including the selection of compounds.
In this study, we prepared thin films of (C6H5CH2NH3)2CuX4 by changing the X site of the perovskite layer and investigated the application of the compound. The optical band gap energies obtained from the optical absorption spectra are 2.03 eV for (C6H5CH2NH3)2Cu Br4 and 2.05 eV for (C6H5CH2NH3)2Cu Cl4. Both compounds were found to be highly stable in air. The detailed physical properties of the films will be reported in the day.

Halide perovskites have opened up a new world of semiconductors that can be processed crudely yet exhibit exceptional optoelectronic performance. Their performance has continued to improve not just in photovoltaic configurations, but also in light-emitting diodes (LEDs) and quantum technologies. While the bulk majority of work over the last decade has focused on 3D halide perovskite materials, there is a vast family of perovskites of different dimensions, for example 2D and quasi-2D layered structures in both bulk and nanostructured forms, which can show modular, tunable and high-quality performance.

Here I will present recent progress on synthesis, understanding and device development of several low-dimensional halide perovskite systems. First, I will show results using local time-resolved luminescence measurements to track exciton diffusion in low-dimensional 2D halide perovskite flakes, revealing how the local energy landscape can drive long-range exciton diffusion on the order of hundreds of nanometres. I will show how we can controllably monitor the growth of 2D/3D lead-tin bulk films, and control energy transfer between the different species in the subsequently tailored films. Finally, I will demonstrate the use of low-dimensional perovskites in efficient light emission systems towards white light applications, including in LED device configurations.

2D layered hybrid perovskites are receiving increased attention as active materials for use in various optoelectronic applications, including solar cells, light-emitting diodes, photodetectors, and transistors. Understanding the phase formation behavior of these materials is crucial for their use in optoelectronic devices since different crystal phases possess different optical and electronic properties. The design rules to obtain the desired 2D layered perovskite crystal phase have not been fully determined yet.

For this study, we explored the formation of 2D layered perovskites and low-dimensional hybrids containing oligothiophene derivatives in the organic layer. On the one hand, we varied the number of thiophene rings in the oligothiophene. On the other hand, we varied the halide that is used. We studied the formation of the hybrids in thin films as well as through crystal growth methods.

For the series of lead iodide hybrids, we show that the use of two different processing methods (spin-coating or drop-casting) leads to the formation of two different crystal phases for bithiophene, terthiophene, and quaterthiophene propylammonium cations. One of these phases corresponds to the typical 2D layered perovskite, while the other phase has optical properties corresponding to a dimensionality intermediate between typical 2D and 1D hybrids.

For the bithiophene propylammonium cation, we varied the halide that is used. The crystal structure and phase behavior of the 2D layered perovskites were studied in detail. It is suggested that via halide substitution from iodide to bromide to chloride, the molecular degrees of freedom of the bithiophene cations are reduced by spatial confinement due to a smaller inorganic framework. In this way, the formation of lower-dimensional hybrids besides the targeted 2D layered perovskite is limited.

2D Ruddlesden Popper perovskites are considered potential candidates for next generation optoelectronic devices due to their favourable properties like large excitonic binding energy, superior stability and structural tunability as compared to 3D perovskites^1,2. The strong quantum and dielectric effects arising due to the layered structure of these materials offer easy modulation of band-gap in the entire visible spectra, establishing great prospects in laser and light emitting applications. While the quantum confinement effects are stronger in lower dimensional Ruddlesden Popper perovskites resulting in narrow emission linewidth, these perovskites suffer from low intrinsic photoluminescence quantum yield (PLQY), which is found to be less than 1% for completely 2D perovskite thin films³, thus hampering their optoelectronic performance. Since PL intensity has direct correlation with their emission characteristics, it is important to engineer the device to compensate for the low PLQY of perovskite emitter layer. In this work, we demonstrate a significant fluorescence enhancement can be obtained using a plasmonic nanocavity. Optical simulations based on Lumerical FDTD show a strong near field enhancement coupled with radiative decay enhancement due to Purcell factor results in around 5-fold enhancement in PL intensity for a 2D BA2PbI4 perovskite thin film sandwiched between a metal layer and nanoparticle cavity.

References:
1. X. Gao, X. Zhang, W. Yin, H. Wang, Y. Hu, Q. Zhang, Z. Shi, V. L. Colvin, W. W. Yu, and Y. Zhang, “Ruddlesden–Popper perovskites: synthesis and optical properties for optoelectronic applications”, Adv. Sci. 6, (2019) 1900941.
2. T. C, Sum, M. Righetto, and S. S. Lim, “Quo vadis, perovskite emitters?”, J. Chem. Phys. 152, (2020) 130901.
3. D. N. Congreve, M. C. Weidman, M. Seitz, W. Paritmongkol, N. S Dahod, and W. A Tisdale, “Tunable light emitting diodes utilizing quantum-confined layered perovskite emitters'', ACS Photonics 4, (2017) 476.

The ability to generate and manipulate spin information underpins the field of spintronics, which utilizes the electron’s spin degree of freedom to transfer and process information. Identifying new 2D magnetic materials empowers the direct control of spin information through the manipulation of magnetization at a low-dimensional limit. However, the Mermin–Wagner restriction induced by thermal fluctuations presents a significant barrier to this pursuit. Here, we report the observation of chirality-induced pseudo-magnetization native to a non-magnetic 2D chiral hybrid organic-inorganic perovskite (chiral-HOIP) identified by spin Hall magnetoresistance measurements. We find pronounced magnetoresistance signals persisting up to room temperature despite the lack of any magnetic elements in the hybrid structures, as well as tunable magnetic hysteretic behaviors controlled by injected spin currents. Our work paves the way for utilizing solution-processed 2D chiral hybrid perovskites in room temperature spintronic applications.

Photovoltaics (PV) play a very key role in achieving the target of world’s economic powers to achieve low or zero carbon emission based economy. The present PV market is dominated by silicon based solar cells, however, the high cost of materials have led the scientific community to look for the alternative ways to harness the sunlight. In the recent years, hybrid perovskites have emerged as a possible solar cell technology to achieve the power conversion efficiency in excess of 20%. Despite the very high efficiency due to the strong absorption coefficient, defect tolerance, and high ambipolar carrier mobility, the hybrid perovskite materials suffer from the problems of device stability, and high content of lead (Pb) which makes the commercialization of these materials a gigantic task.

In the present work we have focused on to develop highly tunable lead-free halometalates for use in hybrid solar cells.¹ The materials developed in this work can be best described as the materials between OPV and perovskites. In the present work we have used the conjugated organic molecules rather than the ammonium ions as the organic component to mimic the perovskite packing.² This design principle enables us to avoid lead (Pb) as the metal component in the molecule while the 3-D packing of the molecular system is maintained. The 3-D packing of the organic components lead to high charge transport properties and strong absorption in the visible region. Moreover, the inorganic post-transition metal iodides (BiI₃, SbI₃ and SnI₂)³ have been chosen to coordinate with the organic part due to the presence of filled metal s states and large spin-orbit coupling which leads to small band gaps and high carrier mobility. Such inorganic-organic coordinated system lead to high tunability of optical, electronic and structural properties which can lead to efficient PV absorber materials.
References:
1. Liu, J.; Guan, Y.; Jiao, C.; Lin, M.; Huang, C.; Dai, W. Dalton Trans. 2015, 44, 5957-5960.
2. Travis, W.; Knapp, C. E.; Savory, C. N.; Ganose, A. M.; Kafourou, P.; Song, X.; Sharif, Z.; Cockcroft, J. K.; Scanlon, D. O.; Bronstein, H.; Palgrave, R. G. Inorg. Chem. 2016, 55, 3393-3400.
3. Cheetham, A. K.; Rao, C. N. R.; Feller, R. K. Chem. Commun. 2006, 46, 4780-4795.

For solution-processing perovskite nanocrystal photonics and optoelectronics, it remains a challenge to uncover the carrier drift dynamics with an intrinsic phonon-scattering mechanism in in-situ devices, and within an ultrafast timescale, since the classic time-resolved photoluminescence and transient absorption spectroscopies mainly capture the carrier diffusion dynamics prior to the defect scattering mechanism. Here, we use ultrafast photocurrent spectroscopy to observe in-situ the band-like transport prior to carrier trapping in a CsPbI₃ nanocrystal array. We attribute transport to the longitudinal optical phonon scattering, while the coupling between phonon and carriers is predominant among classic polar semiconductors and similar perovskites. At room temperature, the carriers preserve band-like transport for more than 70 nm long range drift length and a mobility of more than 14 cm²/Vs.

Transitions from three-dimensional (3D) hybrid organic-inorganic perovskite (HOIP) structures to lower dimensions including 2D and quasi 2D structures opened new range of functional properties in these materials systems. Yet, the performance of solar cells made from 2D HOIP is below 3D compositions due to the higher band gap energy, low carrier concentration and anisotropy in charge transport. Controlling ferroelectricity in 2D HOIPs is considered as a powerful strategy to drive the spatial separation of photogenerated charge carriers to improve the photovoltaic action in this class of HOIPs. In this study, we reveal the strain-dependent ferroelectricity and charge carrier properties in strain-engineered 2D 4,4-difluoropiperidinium (DFPD) lead iodide or (4,4-DFPD, C₅H₁₀F₂N)₂PbI₄ perovskite utilizing advanced scanning probe microscopy (SPM) techniques¹. We demonstrate crystallization- and strain-dependent thin films with different thickness prepared by drop-casting, spin-coating, and vacuum-poling treatment¹. Band-excitation piezo-response force microscopy (BE-PFM) and contact mode Kelvin probe force microscopy (cKPFM) reveal ferroelectric domains and switchable dynamics with significantly low switching voltages in the low strain film. Photoluminescence (PL) spectroscopy shows strain-dependent optical properties and non-centrosymmetric structure with strong second harmonic generation (SHG) peaks in in the low strain film. In addition, KPFM results demonstrate a higher surface potential in the low strain films while the photovoltage and local current are the highest on the high strain film. Our study demonstrates the critical role of strain engineering on electromechanical and charge carrier dynamics in 2D HOIPs, being important for development of 2D HOIP devices. Additionally, we will present the underlying physical mechanisms of light-ferroelectricity interaction in 2D MHPs². X-ray diffraction (XRD) patterns exhibit significant change in preferential orientation of individual lattice planes under external stimuli. Particularly, the reduced intensity of the (1 1 1) lattice plane under illumination leads to transitioning from a ferroelectric to paraelectric phase. The instability of organic cations is also observed under illumination by time-of-flight secondary ion mass spectrometry (ToF-SIMs). The combination of lattice variation and chemical changes under illumination clearly contribute to the origin of light-ferroelectricity interaction in 2D (4,4-DFPD, C₅H₁₀F₂N)₂PbI₄.

References:
1 Kim, D. et al. Ferroelectric and Charge Transport Properties in Strain-Engineered Two-Dimensional Lead Iodide Perovskites. Chemistry of Materials 33, 4077-4088, doi:10.1021/acs.chemmater.1c00679 (2021).
2 Kim, D., Ievlev, A. V., Ovchinnikova, O. O., Kalinin, S. V. & Ahmadi, M. Light-Ferroelectricity Interaction in Two-Dimensional Lead Iodide Perovskites. Submitted (under review) (2021).

Flexible semiconductor materials, where structural fluctuations and transformation are tolerable and have low impact on electronic properties, focus interest for future applications. Two dimensional and thin layer lead halide perovskites are hailed for their unconventional optoelectronic features. We report a surprising structural deformation that appears in thin layer colloidal nanobelts upon their adsorption on hydrophobic carbon substrates. We show that the variations in microstructure we detect, are mechanical buckling of the nanobelts. These are determined using transmission electron microscopy (TEM), including selected orientation dark field imaging (SODFI) and tilting experiments. Atomic force near-field microscopy (SNOM-AFM) was used to correlate structure properties with 10nm resolution. By employing plate buckling theory to the data, we approximate adhesion forces between the buckled nanobelt and the substrate. The effect of the buckled perovskite crystal lattice on optoelectronic and other physical properties is still being unveiled. However, the forces and the capillary action leading to the buckling of metal-halide perovskites at the nanoscale, are critical for the design of future devices and heterostructures based on Nano-perovskites. If time permits we will show also preliminary results of spontaneous self-healing/annealing of structural defect in perovskite nanocrystals.

Chirality in semiconductor materials enables a number of powerful applications, reaching from consumer electronics and 3D displays to drug screening and encryption [1-3]. Recently, chiral hybrid perovskites have emerged as a promising class of materials for optoelectronic applications based on their ability to preferably absorb and emit one type of circularly polarized light. However, so far, the chiral organics used to introduce chirality into lead halide perovskites almost exclusively consist of a small group of commercially available, primary amines [1].

A textbook example for chiral molecules are the naturally occurring amino acids. These represent a diverse group of organic compounds whose application for industrial production is highly desirable as they are non-toxic, biodegradable and are available in large quantities at relatively low cost. In this work, we now introduce the chiral small organic 3-aminobutyric acid into the 1D perovskite dimethylammonium lead iodide (DMAPbI₃) using a simple spin-coating based approach. We find that the resulting material displays strong circular dichroism up to 400 mdeg (g_abs = 17.5 × 10^-3) in the blue spectral region that is tunable in strength by adjusting the amount of amino acid introduced. We resolve the detailed structural origin of the chirality from XRD experiments of the crystal structure. We employ transient optical spectroscopy to investigate the dynamics of excited state polarization and recombination.

Our findings expand the pool of chiral hybrid materials and provide fresh impulses for future maximization of chiral effects in hybrid perovskite materials.


[1] G. Long, R. Sabatini, M. I. Saidaminov, G. Lakhwani, A. Rasmita, X. Liu, E. H. Sargent, W. Gao Chiral-perovskite optoelectronics. Nat. Rev. Mater. 5, 423–439 (2020).
[2] B. Thaom, X. Gao, K. Pan, J. Deng Chiral Helical Polymer/Perovskite Hybrid Nanofibers with Intense Circularly Polarized Luminescence. ACS Nano 15, 7463-7471 (2021).
[3] C. Chen, L. Gao, W. Gao, C. Ge, X. Du, Z. Li, Y. Yang, G. Niu, J. Tang Circularly polarized light detection using chiral hybrid perovskite Nat. Comm. 10, 1927 (2019).

One of the main reasons for the success of perovskite photovoltaics is the presence of free carriers as majority optical excitations in 3D materials at room temperature. To get efficient solar cells, stability and duration improving can be achieved thanks to variations in materials composition, including the use of layered 2D perovskites. However, the current understanding is that in 2D perovskites or at cryogenic temperatures insulating bound excitons form, which need to be split are not beneficial to photoconversion. Here we present a tandem spectroscopy technique, applied to several 3D and 2D hybrid perovskite thin films, that combines ultrafast time-resolved photoluminescence and differential transmission (pump and probe). Our results demonstrate a chemical equilibrium between a majority of non-emitting large polarons and a minority phase of bright excitons, even in 2D perovskites and at low temperatures. The presence of a conductive polaron plasma foresees novel mechanisms for LEDs and lasers, as well as new perspectives for 2D perovskites in photovoltaics.

Experiments show low to ultra-low densities of optoelectronically active, static, defects in Lead Halide Perovskites, HaPs. We first present a simple logic argument that this must be because of an active reaction to damage, rather than to tolerance to damage. For 3D HaPs we and others presented evidence for self-healing,* a process that, in inanimate matter, is explained by equilibrium thermodynamics. Indeed, we argued that the main cause for such healing and the resulting low (static) defect densities is the small free energy of reaction that just stabilizes the materials against decomposition.*
Here we will show the effect of kinetics, by reporting and discussing results of studying single crystals of 2D and 2D-3D HaPs, and compare these to those obtained on 3D single crystals, in terms of their recovery from photodamage.

* cf. Rakita et al., Mater. Hor. (2019); Ceratti et al., Mater. Hor. (2021); Adv. Mater. (2018); Kumar et al., MRS Bull (2020)

Metal halide perovskites have attracted enormous attention in the scientific community in recent years. This attention has been drawn by breakthroughs in perovskite optoelectronics, mainly in photovoltaics and LEDs. Although there has been great progress in their use in optoelectronics, fundamentally understanding this class of materials is still challenging, in particular the interplay of several physical and chemical processes. Many fundamental questions are left unanswered, such as what makes them unique/excellent semiconductors; how to tune their optoelectronic properties; why some compositions are more stable than others; the consequence of their chemical instability on their optoelectronic properties; and how to stabilize them.

My research group focuses on addressing such questions using computer simulations, combining DFT calculations and MD simulations to go from the atomistic scale to molecular scale. In this talk, I will highlight our recent progress on improving the stability of this class of materials by composition engineering, the use of additives, ect. I will also give an outlook of our materials engineering strategies for going from 3D systems to systems with lower dimensions.

To minimize surface reactivity, defects and the possibility of water and O₂ absorption of/by 3D metal Halide Perovskites (HaPs), covering them with an ultra-thin layer of hydrophobic 2D HaP has been reported to be beneficial. At the same time the stability of a 2D-on-3D HaP composite has been questioned, especially based on results from GI-XRD and PL studies. The question of what is happening on an atomic scale is a vexing one, if higher stability is the goal. It is also a fascinating one because of the dynamic nature of the lattices, which, for 3D HaPs at times seems to approach, from the literature, “panta rhei” i.e., everything flows. We studied the 2D-on-3D HaP composite by TEM-based methods, using FIB-prepared cross-sections of films; the films were prepared by a new controlled gas-phase cation exchange process of the top layer of a spin-coated 3D MAPbI₃ film and characterized by GI-XRD and PL. The 2D HaP was the n=1 Ruddlesden-Popper (F)PEA₂PbI₄, where PEA = phenyl-ethylamine and F-PEA, its mono-fluorinated derivative. All through the study we strove to minimize electron and ion fluences, often below what could be deduced from fluences reported in other works. The results allow us to follow the effects of beam damage and thus to determine conditions of measurement before that stage. We used 4D-STEM and scanning nanobeam electron diffraction (NBED), on relatively fresh and aged (all in inert atmosphere) samples. We find direct evidence for the 2D n=1 phase appearing inside the 3D matrix and also for the formation of quasi-2D phases near the interface with the carbon coat. The results imply 2D/3D perovskite heterointerface lattice rearrangements involving migration of not only (the smaller) MA⁺, but also of (the larger) FPEA⁺ cation. As PEA was found too labile for controlled measurements F-PEA was used to be able to get clear “before” and “after” results and measure actual structural evolution under the beam, which primarily shows the loss of long-range periodicity as well structural rearrangements, leading to quasi-2D phase formation. Our results complement extensive empirical and semi-empirical data to engineer stable 2D-on-3D composites and define conditions for atomic resolution studies of real 2D/3D samples in device-relevant configurations.

The synthesis of perovskite nanocrystals (NCs) and the control of their composition and morphology is critical to tuning their optical and electronic properties. Beyond direct synthesis methods, in recent years the indirect synthesis of perovskite NCs based on the post-synthetic transformation reactions of zero-dimensional perovskite NCs has been developed to fine-tune the nucleation and growth of NCs. Herein, we report two examples of the transformation reaction of 0D perovskite NCs to obtain perovskite NCs with uniform morphology, and well-defined crystal structure. Firstly we present the synthesis of ultra-thin CsPbBr₃ NWs through the reaction between PbBr₂ and Cs₄PbBr₆ 0D perovskite NCs. The synthesized CsPbBr₃ NWs averaged 2.5 nm in width and tens of micrometers in length, and show blue photoluminescence (PL) with a quantum yield of 15%. We tentative attribute the formation mechanism of CsPbBr₃ NWs to the formation of the PbBr₂-ligand intermediates, whose structure and be controlled by the length of organic ligands. We further expand this reaction scheme to the synthesis of CsPbI₃ NCs with a low-dimensional morphology. Secondly, we present a versatile synthesis technique of bismuth-based perovskites based on the Cs₃BiX₆ (X = Cl, Br) NCs. Both Cs₂AgBiX₆ double perovskite nanocubes and Cs₄MBi₂Cl₁₂ (M = Cl, Br) layered double perovskite NCs were obtained in this manner. Furthermore, we found the intermediates during the transformation of Cs₃BiCl₆ to Cs₂AgBiCl₆ double perovskite NCs showed a red-shifted PL at 770 nm with enhanced PL intensity, which was tentatively attributed to the passivation of Cs cation in the Cs₂(Cs_1-xAgx)BiCl₆ NC and the large lattice deformation leading to the formation of self-trapped exciton with large trapping energy. Our works provide methods to tune the morphology and composition of perovskite NCs and give insights into the chemical activity of 0D perovskite NCs.

Low-dimensional Ruddlesden-Popper (RP) perovskites have steadily risen to prominence as stable and efficient photovoltaic materials for solar cell applications due to enhanced structure stability and reduced moisture intrusion in comparison to standard 3D perovskites. However, thin-films processed from stoichiometric 2D precursor solutions tend to possess polydisperse distribution of different number of inorganic layers with the organic dielectric layers behaving as charge transport inhibitors with large binding energies, crippling device performance. Precise processing and composition engineering is necessary to accurately control nucleation and crystallization events to fabricate 2D RP perovskite films of high phase purity devoid of 3D mixtures. Perpendicular crystallographic growth is equally desirable to achieve fluent charge transport along charge collection direction.
In this work, we fabricated 2D RP perovskites (PEA)₂(MA)₄Pb₅I₁₆ (Nominal n-value = 5 ) using anti-solvent and hot-casting methods with different combinations of processing additives to improve the vertical orientation and reduce polydispersity of low n 2D RP phases and 3D phase. The critical influence of drip-time on phase-distribution and crystal growth was revealed during the application of toluene as anti-solvent with methylammonium chloride (MACl, MACl/MAI = 0.5 mol ratio) as additive. Earlier drip times during spin-coating resulted in greater density of 2D and 3D-like phases, while later times yielded a narrow distribution of vertically oriented polydisperse high n-value 2D phases (n = 3, 4, and 5) and no 3D phase, confirmed by UV-Vis absorption spectral and XRD study of thin films and EQE measurements of devices. Fine-tuning the anti-solvent discharge time and phase control afforded inverted solar cells of ITO/PEDOT:PSS/(PEA)₂(MA)₄Pb₅I₁₆/PC₆₁BM/BCP/Ag with PCEs up to 8.51%.
Hot-casting was found to yield RP perovskite films (MACl/MAI = 0.5 mol ratio) with dramatically increased crystallinity and “perfect” perpendicular orientation compared to anti-solvent method. Substrate temperature was found to have a strong impact on the phase distribution and orientation, with sub-optimal heating resulting in high degree of polydispersity and poor crystallinity. Hot-cast films exhibited higher PCE values up to 10.7% owing to improved FF and V_oc whilst maintaining an identical EQE profile to anti-solvent treated devices having pure 2D phases.
Thin-films of (PEA)₂(MA)₄Pb₅I₁₆ with different concentrations of MACl and NH₄Cl additives were fabricated to understand whether the absence of 3D-character was due to the crystallisation rate, presence of excess [MA⁺] or Cl^-. It was found that NH₄Cl additive induced drastically enhanced crystal growth along [111] and [202] planes and formed smooth films with low pinhole density upon hot-casting compared to the MACl additive. However, thin films prepared with NH₄Cl additive displayed a broader distribution of 2D phases with n ranging from 1 to 5 and 3D character. Furthermore, devices with the thin films prepared by adding NH₄Cl additive exhibited lower PCEs of 6.1% upon hot-casting, despite favourable orientation, thus demonstrating the benefit of phase purity with large n values. These results will be augmented with Transient Absorption Spectroscopy (TAS) to probe the energy transfer mechanism of graded structure of the polydisperse perovskite and Grazing-Incidence Wide Angle Scattering (GIWAXS) to underpin the time-resolved structural evolution of 2D RP perovskites and quantify individual phases.
This mechanisms on orientational control and fine-tuning the phase distribution of 2D RP phases via adding additives and controlling nucleation and crystallization kinetics as well as how these effects are reflected by photophysical properties of solar cells will be discussed.

Perovskite quantum dots (QDs) have the advantages of quantum confinement effect, defect-tolerant nature, and the capability of developing lightweight and flexible films, thus attracting much recent research for a variety of functional device developments including QD solar cells and LEDs. Here we report our recent progress on a novel surface ligand engineering strategy in designing new hybrid perovskite QDs, which leads to not only fundamental understanding on the optoelectronic working mechanism of the QDs, but also remarkably improve the optoelectronic quality of the perovskite QDs. The new classes of perovskite quantum dots have been used as building blocks in Quantum Dot Solar Cells with a certified record efficiency of 16.6% with excellent long-term operation stability. By using QDs as light absorbing materials, the QD based photocatalysts also exhibited good performance in photocatalytic gaseous hydrogen production.

Organic-inorganic two-dimensional halide perovskites (2DPKs) are soft semiconductors formed by combining organic and inorganic moieties, which self-assemble in solution to form highly ordered periodic stacks. They exhibit a large compositional and structural phase space, which has led to novel and emergent physical properties offering potential for applications in optoelectronics, energy conversion, photonics, spintronics, and quantum devices. To achieve this paradigm, there is a need to understand both the fundamental physics and the growth of both 2DPK crystals and thin films used for integration in devices. In this presentation, I will describe three exciting results.
First we overcome a long-standing challenge in 2D perovskites of realizing phase pure thin-films. Briefly, we demonstrate a scalable approach involving dissolution of single-phase crystalline powders with homogeneous perovskite layer thickness in desired solvents, to fabricate 2D perovskite thin-films with high phase purity. In-situ characterizations reveal the presence of sub-micron-sized seeds in solution that preserve the memory of the dissolved single-crystals, and dictates nucleation and growth of grains with identical layer thickness of the perovskite layers in thin-films. Photovoltaic devices fabricated with such films yield an efficiency of 17.1% and 1.20V open-circuit voltage, while preserving 97.5% of their peak-performance after 800 hours under illumination without any external thermal management.
Second, we report sunlight-activated anisotropic lattice contraction in 2DPKs, which is reversible and strongly dependent on both the nature of the interlayer organics and the details of the structure. Our theory suggests that light-generated charge accumulation results in the build-up of a bulk compressive strain, which induces a continuous lattice contraction over minutes. In-situ structural measurements on photovoltaic devices directly correlate the light-induced lattice contraction to an increase in the photovoltaic efficiency of Dion-Jacobson 2DPK solar cells from 15.6% to 18.3%. We found that compressive strain enhances inter-layer electronic interactions in 2DPKs leading to three-fold increase in carrier mobility and conductivity in 2DPK thin films.
Third, we employed ultrafast electron diffraction (UED) to capture at the picosecond time scale how light generated hot carriers dissipate their energy to the lattice and affect the 2DPK structure. The carrier cooling dynamic in 2DPKs yields a fast lattice re-ordering that is distinct from 3DPKs, followed by classic slow thermal dissipation of carriers excess energy to the lattice. The former process suggests the light-induced activation of an incipient phase transition, toward a hypothetical undistorted phase due charge-lattice interactions. The effect of anisotropy and the nature of the bulky organic cations were also confirmed to play a role in tailoring carrier-phonon interactions.

Perovskite solar cells (PSCs) are one of the most promising candidates for the next generation of solar cells because they have high power conversion efficiency (PCE) exceeding 20% and they can be manufactured by a high-productivity fabrication process involving low-temperature processing and a roll-to-roll coating method. To meet the growing demand for renewable energy generation, it is necessary to mass-produce solar cells with high PCE at low cost. The PCE limit of MAPbI₃ (MA: methylammonium), which is a typical perovskite material, is 31% based on the Shockley-Queisser (S-Q) limit due to its band gap of 1.55 eV. It is thought that the maximum theoretical PCE of a material can be achieved at a band gap of around 1.1-1.4 eV and that a SnPb iodide perovskite compound is a suitable candidate for realizing the maximum PCE value of 33% based on its S-Q limit. In addition, SnPb PSCs are relatively environmentally friendly because they require far less Pb compared with Pb-based PSCs. Studies of SnPb PSCs have been increasing and PCEs exceeding 20% have been reported recently [1]. However, many of these SnPb PSCs are millimeter-scale cells fabricated using spin-coating method. Thus, it is necessary to find a way to manufacture SnPb PSCs with larger areas in a more highly productive manner.
To realize large-area coating, we have been developing the “meniscus coating method” [2,3], which is capable of producing uniform layers with low variations in thickness over a large area. By using this coating method in MAPbI₃-based PSCs, we have achieved a PCE of 14.1% in a polymer film module with an active area of 703 cm², and 16.1% with an active area of 781 cm² in a glass-based module [4]. Accordingly, we expect to realize high-productivity fabrication of SnPb PSCs by utilizing a roll-to-roll process and applying the meniscus coating method.
In this study, we developed the meniscus coating method for a SnPb perovskite layer. This coating realizes high productivity via the 1-step process at a speed of over 5 m/min. In addition, by employing the equimolar constitution of Sn and Pb (Sn:Pb = 1:1), a band gap of around 1.2 eV was realized in the SnPb perovskite layer. We also examined the surface passivation of the SnPb perovskite layer and found a PCE of 18.3% in a 1-cm² cell of the SnPb PSC having an inverted structure (ITO/hole transport layer/SnPb perovskite layer/PCBM/BCP/Ag).

Acknowledgments
This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) project “Development of Technologies to Promote Photovoltaic Power Generation as a Main Power Source.”

References
[1] Jiupeng Cao and Feng Yan, Recent progress in tin-based perovskite solar cells, Energy Environ. Sci. 14. 3 (2021), 1286-1325. https://doi.org/10.1039/D0EE04007J

[2] H. Oooka, Y. Shinjo, T. Sawabe, T. Sugizaki, A. Amano, T. Ono, K. Sugi, I. Takasu, Y. Mizuno, J. Yoshida, S. Enomoto, A. Hirao, and I. Amemiya, High-Brightness Large-Area White OLED Fabricated by Meniscus Printing Process. Society for information display, International symposium (2012). https://doi.org/10.1889/1.3500459

[3] S. Mori, H. Oooka, H. Nakao, T. Gotanda, Y. Nakano, H. Jung, A. Iida, R. Hayase, N. Shida, M. Saito, K. Todori, T. Asakura, A. Matsui, and M, Hosoya, Organic photovoltaic module development with inverted device structure. Proceedings of MRS Meetings mrsf14-1737-u17-02 (2015). https://doi.org/10.1557/opl.2015.540

[4] Kenji Todori, Hiroyuki Miyauchi, Toshiba Review, vol.76, No.3, 17-20 (2021).

While power conversion efficiencies of perovskite solar cells using the standard (n-i-p) mesoporous and planar architecture have surpassed 24% some time ago, inverted (p-i-n) planar PSCs lag behind considerably. Severe non-radiative interfacial recombination at the perovskite/electron transport layer and at the grain boundaries still limits their open-circuit voltage (V_OC) and fill factor (FF) as compared to their n-i-p counterparts. Particularly for next generation monolithic perovskite/Si tandem photovoltaics, which commonly apply a p-i-n perovskite top solar cell, these recombination processes introduce a conceptual hurdle.
To address this challenge, we introduce a novel dual 2D/3D perovskite heterostructure passivation approach by adding phenethylammonium chloride (PEACl) to our methylammonium-free double cation p-i-n perovskite solar cell. Using PEACl:PbCl₂ as the additive and PEACl for surface treatment, our approach simultaneously passivates recombination centers at the grain boundaries in the bulk of the perovskite film and recombination centers at the perovskite/C₆₀ interface, respectively. The dual 2D/3D heterostructure passivation results in one of the highest reported PCEs for p-i-n PSCs of 22.7% with a remarkable V_OC and FF of 1.162 V (E_g = 1.57 eV) and 83.2%, respectively. Furthermore, a remarkable stabilized efficiency of 22.3% is demonstrated (5 min at maximum power point tracking), one of the highest reported for MA-free p-i-n PSCs.
Our study researches the advanced use of dual passivation as a key mechanism to manage the detrimental defects both in the grain boundaries and at the perovskite/ETL interface to achieve highly efficient p-i-n PSCs. By means of time resolved photoluminescence and photoluminescence quantum yield, we show that the charge carrier lifetime and quasi-Fermi level splitting are significantly enhanced compared to reference devices. We attribute the improvements to the formation of a 2D Ruddlesden Popper perovskite phase at the film surface and GBs, which leads to efficient chemical passivation of GB and surface defects as well as additional electronic passivation at the perovskite/C₆₀ interface. At the same time, the activation energy for ion migration is significantly increased, resulting in enhanced stability of the PSCs under light, humidity, and thermal stress.

Two-dimensional (2D) Ruddlesden–Popper (RP) phase hybrid perovskites have since received a lot of attention as one of the most promising materials for making long-term, stable, high-performance perovskite solar cells (PSCs). The presence of big ammonium cations, such as butylammonium (BA) and phenylethylammonium (PEA), in a 2D or mixed 2D and three-dimensional (3D) structure has recently been shown to limit moisture infiltration at the boundaries of Sn-based perovskites, resulting in exceptional high film and device stability.¹ However, even when synthesized with stoichiometric concentrations of precursors, solution processed 2D tin perovskites still exhibit a mixed 2D phase with varying number of inorganic frameworks that is a signal for spreading of the quantum well (QW) width, inhibiting charge transport, increasing interfacial charge accumulation and affecting final device performance.^2,3 Therefore, it is highly desired to control the QW width into an extremely small distribution and optimize the charge-transport characteristic. In this study, we fabricated 2D RP phase tin perovskite PEA₂FA_n-1Sn_nI_3n+1 thin films with targeted n = 5, because of their suitable bandgaps for solar cells, using phenylethylammonium acetate (PEAAc) instead of commonly used PEAI. It is highly expected that compounds with a uniformly distributed intermediate phase in precursors will result in narrower distributed QW width and higher film and device stability due to the strong coordination effect between Ac^- group and Sn²⁺. The precursor solution prepared from PEAAc has a narrower size distribution and better stability than the solution prepared from PEAI, ensuring the uniform and narrower distribution of QW width and better stability of the thin films prepared via one-step plus anti-solvent solution process. Fourier transform infrared (FTIR) spectra reveal the coordination interaction between Ac^- group and Sn²⁺ via the C=O bond vibration shift in the thin film. In addition, X-ray diffraction (XRD) patterns and UV-vis absorption spectra demonstrate that the films have reduced the amount of small n number of 2D perovskites, which is preferred for device charge transport. The orientation of 2D perovskites and QW distribution are studied by grazing incidence wide-angle X-ray scattering (GIWAXS) and femtosecond transient absorption (TA) measurements. In comparison to standard organic spacers PEAI, the photovoltaic performance for the devices made under optimal conditions using PEAAc has a best power conversion efficiency (PCE) of over 9% with no hysteresis using a device structure of ITO/PEDOT:PSS/2D tin perovskite/ICBA/Ag. This is among the highest efficiency for 2D tin perovskite solar cells. This work demonstrates that anion in large organic spacers could play a critical role in forming a narrower distribution of QW, enhancing phase and device stability, and improving device performance. Therefore, anion engineering has the potential to aid in the further development of solar cells, photodetectors, and LEDs utilizing 2D perovskites.

References
1. Tsai, H. et al. High-efficiency two-dimensional Ruddlesden-Popper perovskite solar cells. Nature 2016, 536 (7616), 312-6.
2. Gabriella A T. et al. Tuning cesium–guanidinium in formamidinium tin triiodide perovskites with an ethylene diammonium additive for efficient and stable lead-free perovskite solar cells. Mater. Adv., 2020, 1, 3507-3517.
3. Gabriella A T. et al. Impact of cesium on the phase and device stability of triple cation Pb–Sn double halide perovskite films and solar cells. J. Mater. Chem. A, 2018,6, 17426-17436.

Colloidal lead halide perovskite quantum dot (QD) has emerged as a promising material for next generation optoelectronic devices due to its unique properties, such as quantum confinement effect, high photoluminescence quantum yield, defect tolerance, and multiexciton generation. For photovoltaic applications, native insulating ligands on QD surfaces need to be removed to achieve strong electronic coupling of adjacent QDs. Unlike metal chalcogenide QDs, the ionic crystalline structure of perovskite is susceptible to decomposition under polar solvents and harsh ligand exchange conditions. Therefore, fundamental insights are needed to establish a mild ligand exchange method to gently modify surface ligand density without causing decomposition. Among the choices of solvents, methyl acetate (MeOAc) strikes to have the moderate polarity to remove surface ligands without decomposing the QDs. However, how inter-QD coupling can be improved during ligand exchange has not been well understood and developed. Here, we report an ion-assisted ligand exchange strategy that enhances the electronic coupling and prevents defect formation in FAPbI₃ QD films for perovskite QD solar cells. We synthesized FAPbI₃ QDs with hot injection method, followed by a series of purification steps. An average QD size of 12 nm with narrow size distribution was measured from TEM images. UV-Vis absorption spectra of QD solutions show an absorption edge of 775 nm and photoluminescence emission spectra show a peak at 785 nm. Iterative spin coating steps were applied to stack QD films to reach ~300 nm thickness. To conduct the solid-state ligand exchange, spin coated QD films after each spin coating step were dipped into MeOAc solutions, in which acetate salts with different cations (FA⁺, GA⁺, PEA⁺, Pb²⁺) were added to manipulate ligand exchange capability and suppress defect formation. Acetate, with a higher affinity to Pb²⁺ than oleate, can accelerate the removal of oleate. In addition, it can fill the X-site vacancy and therefore prevent the migration of iodide ions. The cations can fill the vacancies cause by removal of oleylammonium (A-site) and Pb-oleate complex (B-site). The ligand exchange reaction rates with different acetate salts were estimated from the intensity change of the vibrational mode in FTIR spectra and compared. SEM images show that the packing of QDs become more compact after ligand exchange while smooth film surfaces are retained. A slight redshift was observed from photoluminescence emission spectra, verifying successful inter-QD coupling. XRD confirms that no phase transitions happened during ligand exchange, indicating the preservation of nanocrystalline structure. The coupled film exhibits a long PL lifetime because of suppressed trap formation. We fabricated QD solar cells with the structure of ITO/PEDOT:PSS/FAPbI₃ QD/PCBM/BCP/Ag. Consequently, all the acetate salts contributed to a significant improvement of J_SC, and fill factors were maintained above 75%. The improved J_SC was attributed to a net reduction of insulating ligands due to stronger ligand exchange effect of acetate. Except FAOAc-MeOAc, all other acetate salts MeOAc treatments led to increased V_OC because of suppressed trap formation. Charge carrier mobilities were measured with electron-/hole-only devices. Light intensity dependent J-V measurements were conducted to investigate the charge recombination mechanisms. This work demonstrates a more reliable solid-state ligand exchange strategy, which enhances the electronic coupling and prevents defect formation in FAPbI₃ QD films, and sheds light on the roles of ions in solid-state ligand exchange.

Mixing halides in metal halide perovskites (MHPs) is an effective approach to adjust MHPs bandgap for applications in tandem solar cells. However, mixed-halide (MH-) MHPs undergo light-induced-phase-segregation (LIPS) under continuous illumination. Therefore, understanding the mechanism of LIPS is necessary for developing stable MH-MHPs. In this work, we investigated LIPS in layered (L)-MHPs, unveiling that spacer cation plays a crucial effect on LIPS. Our chemical characterization of LIPS in L-MHPs revealed an iodide-repulsion effect and consequently the formation of pure-Br-phase or extremely Br-rich phase in the illuminated region, where iodide repulsion is dependent on light intensity and chemical environment (e.g., oxygen penetration into MHPs affect iodide repulsion process). By further probing chemical changes in a MAPb(Br0.5I0.5)3 film under illumination, we observed, in addition to the formation of I-rich and Br-rich domains (that is a generally believed mechanism of LIPS), LIPS also leads to a significant vertical chemical redistribution, which is critical to charge carrier transport and extraction in MHPs devices. Moreover, we discovered that LIPS is more significant in the bulk of the MHP film, which is probably due to a large population of photogenerated charge carriers in the bulk. Overall, by directly characterizing chemical changes due to LIPS, we revealed the chemical changes associated with LIPS in MHPs, which is the key for understanding LIPS mechanism and developing stable mixed-halides MHPs. Our studies about the role of spacer cation in LIPS and extrinsic ion penetration also offers insight into improving the stability of MHPs.

Acknowledgments: This work is conducted at the Center for Nanophase Materials Sciences, a US Department of Energy Office of Science User Facility.

The thermodynamics of interfaces, encapsulated in the parameter of the work of interface formation, dictate the stability and evolution of suspensions, gels, emulsions, and many other dispersed systems. In particular, it is one of the main axes along which workers engineer the properties, especially size and shape, of colloidal nanoparticles. A widespread assumption is that such colloidal solutions are never thermodynamically stable: they will tend to aggregate, ripen, or coarsen, limiting their performance and long-term usefulness. This need not be the case, however, and truly stable solid colloidal systems have been contemplated or even proposed for many decades. We build on this work using the framework of classical nucleation theory for colloidal solids, from which a simple expression for the size distribution depending on the concentration of the particle components and the interface energy can be obtained. The need for a statistical-mechanical model describing the particle degrees of freedom hints that colloidal solutions could serve as tests of this contentious part of nucleation theory. Conversely, an experimentally measurable size distribution corresponds directly to a size-dependent interface energy, and thus thermodynamically stable colloids are probes of solid-solution interactions at nanoscopic dimensions. We invert this relation to examine previous data sets from colloidal nanoparticles, and suggest that, from the apparent dependence of interface energy on size, the mechanism of colloid stabilization may be more complex than one of those already proposed, e.g. interface bending energy or dielectric charging. We conclude with a note of caution, however, that so far it remains to be demonstrated conclusively that the systems considered in our work are really thermodynamically stable with a definite place in a multicomponent phase diagram.

Inverted perovskite solar cells passivated via organic cations exhibit superior power conversion efficiency compared to unpassivated ones. These record efficiencies have been reached thanks to the use of large organic cations to passivate the interface between the perovskite absorber and the transport layers. However, the passivation mechanism of 2D perovskites on top of the 3D bulk absorber layer is still under intense debate in the community. In particular, issues regarding the complete coverage of the passivating layer, the full formation of a 2D layer on top of the 3D bulk perovskite and the possible effects on carrier extractions still need to be clarified. Here, we study the optoelectronic properties and chemical structure of interface doped perovskite solar cells where large cations, namely Cl-PEAI and F-PEAI, were incorporated at both front and rear interfaces of the absorber. The effect of the cation addition led to an increase of all the main PV characteristic, reaching PCE values up to 23.7%. We combined steady-state and time-resolved multidimensional photoluminescence imaging techniques to probe the main optoelectronic and transport properties of such devices. We thus obtained quantitative maps for physical parameters such as Quasi Fermi Level Splitting (QFLS), Urbach Energy and surface recombination rate, which proved a homogenous passivation by Cl-PEAI and F-PEAI cations over the 3D surface. Moreover, we identified interfacial passivation as the main mechanism leading to a clear improvement of the Voc which increases from 1.10 to 1.16 eV on full devices, whereas the addition of the cations on bare perovskite thin films resulted into an increase of the QLFS in the range of 0.01 -0.02 eV. Complementary Raman analyses proved that passivation effect and deeper interaction of delocalised ∏ - electron from the F-PEAI and Cl-PEAI with the Pb-I perovskite octahedral are cause for the enhanced thermal fluctuation and formation of large polaron in the perovskite lattice. These passivation effect induces structural deformation which are the main responsible phenomena for the observed delayed carrier recombination and increased photovoltaic performance. By linking optical and electrical measurements we highlight the benefits of this passivation method in maximising all the main photovoltaic characteristics and in approaching inverted perovskite solar cell theoretical limit.

In recent years, lead halide perovskite solar cells, which have been attracting attention as next-generation solar cells, have an energy conversion efficiency of over 25% and are expected to be practical use in the near future. However, the lead halide perovskite compounds contains lead, which is harmful to the human body and the environment, and is an issue for practical use.
Many studies have been reported on solar cells using perovskite compounds based on tin, germanium, bismuth, etc., which are less toxic than lead.
Among them, research on solar cells using bismuth halide perovskite(Ca₃Bi₂I₉) has been actively conducted and it has been reported that the energy conversion efficiency(PCE) is 3.20 %.
In this study, for the purpose of developing high-quality films applicable to optoelectric applications with adjusted band gap energy in A₃Bi₂X₉ compound semiconductors, we fabricated mixed bismuth halide perovskite films with various chemical compositions. Crystal morphology, crystal structure and physical properties were evaluated using SEM, powder XRD measurement, and light absorption spectrum measurements. In this presentation, we will report these detailed results and the characteristics of solar cells using mixed bismuth halids perovskites.

Recently, owing to their excellent optoelectronic properties combined with an enhanced material stability, two-dimensional (2D) organic halide perovskites have emerged as an attractive alternative to 3D perovskites for photovoltaics^1,2. More, their ability to emit white-light at room temperature with a good color rendering projected this family of materials into the spotlight of potential future low-cost white LED components³. However, the underlying physical mechanisms behind these emissive properties are not fully understood. Herein we propose to investigate the origin of the broad-band emission observed at different temperatures in a series of (C₆H₁₁NH₃)₂PbX₄ 2D Ruddlesden Popper perovskites (X= Br or I) ^4,5 by coupling advanced structural measurements with an atomistic approach based on the density functional theory (DFT). Indeed, as the temperature is decreased, phase transitions between undistorted and distorted structures were observed for the two compounds of the series by means of temperature dependent X-ray diffraction. While this crystallographic phase transition goes along with a measured polarization-electric field hysteresis for the bromide compound, apparition of satellite incommensurate reflections was instead obtained for the Iodine one. We believe that these satellite reflections might also be connected to a low temperature ferroelectric ordering. In order to set a clear interplay between the structural distortions and the observed optoelectronic properties, theoretical studies intend to provide insights of the electronic band structures and polarization characteristics for these compounds.

(1) Tsai, H.; Nie, W.; Blancon, J.-C.; Stoumpos, C. C.; Asadpour, R.; Harutyunyan, B.; Neukirch, A. J.; Verduzco, R.; Crochet, J. J.; Tretiak, S.; Pedesseau, L.; Even, J.; Alam, M. A.; Gupta, G.; Lou, J.; Ajayan, P. M.; Bedzyk, M. J.; Kanatzidis, M. G.; Mohite, A. D. High-Efficiency Two-Dimensional Ruddlesden–Popper Perovskite Solar Cells. Nature 2016, 536 (7616), 312–316. https://doi.org/10.1038/nature18306.
(2) Katan, C.; Mercier, N.; Even, J. Quantum and Dielectric Confinement Effects in Lower-Dimensional Hybrid Perovskite Semiconductors. Chem. Rev. 2019, 119 (5), 3140–3192. https://doi.org/10.1021/acs.chemrev.8b00417.
(3) Dohner, E. R.; Jaffe, A.; Bradshaw, L. R.; Karunadasa, H. I. Intrinsic White-Light Emission from Layered Hybrid Perovskites. J. Am. Chem. Soc. 2014, 136 (38), 13154–13157. https://doi.org/10.1021/ja507086b.
(4) Yangui, A.; Garrot, D.; Lauret, J. S.; Lusson, A.; Bouchez, G.; Deleporte, E.; Pillet, S.; Bendeif, E. E.; Castro, M.; Triki, S.; Abid, Y.; Boukheddaden, K. Optical Investigation of Broadband White-Light Emission in Self-Assembled Organic–Inorganic Perovskite (C ₆ H ₁₁ NH ₃ ) ₂ PbBr ₄. J. Phys. Chem. C 2015, 119 (41), 23638–23647. https://doi.org/10.1021/acs.jpcc.5b06211.
(5) Yangui, A.; Pillet, S.; Mlayah, A.; Lusson, A.; Bouchez, G.; Triki, S.; Abid, Y.; Boukheddaden, K. Structural Phase Transition Causing Anomalous Photoluminescence Behavior in Perovskite (C ₆ H ₁₁ NH ₃ ) ₂ [PbI ₄ ]. J. Chem. Phys. 2015, 143 (22), 224201. https://doi.org/10.1063/1.4936776.

Targeted functionalisation of 3D perovskite with a 2D passivation layer via R-NH₃I treatment has emerged as an effective strategy for enhancing both the efficiency [1,2] and chemical stability [3] of ABX₃ perovskite solar cells, but the underlying mechanisms behind these improvements remain unclear. Here, we correlate the chemical evolution of the 3D/2D|charge transport layer (CTL) interface with the injection yield to the CTL through a mixture of surface sensitive structural and time-resolved optical techniques.
We assign a passivation mechanism where R-NH₃I reacts with excess PbI₂ in the MAPbI₃ film and unsaturated PbI₆ octahedra to form (R-NH₃)₂(MA)_1-nPb_nI_3n+1. Crucially, we show that precise control of the 2D (R-NH₃)₂(MA)_1-nPb_nI_3n+1 layer underpins performance improvements: n = 1 yields over a two-fold improvement in hole injection to the HTL; n > 1 deteriorates hole injection. Ultrafast transient absorption spectroscopy suggests that this n-dependence is rooted in the fact that fast (< 6ns) hole injection does not occur between the 3D perovskite and 2D layer. Finally, we correlate these improvements in hole transfer with a 30% enhancement in device efficiency. [4] These results help explain contemporary empirical findings in the field and set out an important design rule for the further optimisation of multidimensional perovskite optoelectronics.

[1] Cho et al, Energy Environ. Sci., 2018, 11, 952–959.
[2] Jung et al, Nature, 2019, 567, 511–515.
[3] Grancini et al, Nat. Commun., 2017, 8, 15684.
[4] Westbrook et al, J. Phys. Chem. Lett. 2021, 12, 13, 3312.