Symposium EL02—Fundamentals of Halide Semiconductors for Optoelectronics

High environmental stability and surprisingly high efficiency of solar cells based on 2D perovskites have renewed interest in these materials. These natural quantum wells consist of planes of metal-halide octahedra, separated by organic spacers. Remarkably the organic spacers play crucial role in optoelectronic properties of these compounds. The characteristic for ionic crystal coupling of excitonic species to lattice vibration became particularly important in case of soft perovskite lattice. The nontrivial mutual dependencies between lattice dynamics, organic spacers and electronic excitation manifest in a complex absorption and emission spectrum which detailed origin is subject of ongoing controversy. First, I will discuss electronic properties of 2D perovskites with different thicknesses of the octahedral layers and two types of organic spacer. I will demonstrate that the energy spacing of excitonic features depends on organic spacer but very weakly depends on octahedral layer thickness. This indicates the vibrionic progression scenario which is confirmed by high magnetic fields studies up to 67T. Finally, I will show that in 2D perovskites, the distortion imposed by the organic spacers governs the effective mass of the carriers. As a result, and unlike in any other semiconductor, the effective mass in 2D perovskites can be easily tailored.

All inorganic Metal halide perovskite (MHP) CsPbBr₃ nanowires (NWs) arrays have shown excellent performance in various optoelectronic applications such as laser, LED, and photodetection, etc. owing to its special properties have, such as one-dimensional photon/electron transport, enhanced light absorption, larger active surface area and higher charge injection efficiencies, etc.
The growth of CsPbBr₃ NWs and their various optoelectronic applications have been explored, e.g. for lasing, multicolor displays, control of emission anisotropy, and light guiding, as well as photodetection. However, most of these studies used unprotected in-plane NWs with horizontal alignment. A few studies have investigated vertically aligned MHP NW arrays, which have been proven to have high light extraction capability for LED devices and high resolution for pixel image sensors. So far, studies on vertically aligned CsPbBr₃ NWs arrays are limited.
Here, we report a low temperature solution growth of vertically aligned CsPbBr₃ NWs arrays with excellent stability, using anodized aluminum oxide (AAO) templates. AAO template is commonly used for nanocrystals synthesis, and it has also been used for MHP nanocrystals synthesis. However, the growth behavior and mechanism of micrometer length CsPbBr₃ NWs in AAO from a solution precursor have not been clearly investigated.
In this work, the growth behavior of CsPbBr₃ NWs in 5 μm thick AAO template from solution is clearly elucidated. Owing to the low concentration of precursor, the growth of micrometer length pure phase CsPbBr₃ NWs in AAO is different from the common way used for growing other MHP NWs such as MAPbI₃. The low solubility of its precursors makes it more challenging to fill the nanopores with CsPbBr₃, but it can be overcome by supplying sufficient solution as shown by our results. The NW diameter (10-250 nm) and the length (tens of nm to few μm) can be independently controlled. With decreasing diameter, the CsPbBr₃ NWs show a gradual photoluminescence blue-shift from 250 nm to 10 nm and crystal structure change from orthorhombic to cubic phase below 20 nm. This is the first observation of physical confinement induced phase transition for CsPbBr₃. The physical confinement of the CsPbBr₃ NWs in the AAO gives a remarkable stability to long term air storage. Any significant degradation of these structures is not observed within 4 months, despite storing the sample in ambient air. Additionally, the CsPbBr₃-NWs/AAO composite shows good resistance to X-ray exposure, which makes it promising for applications in X-ray scintillation. The growth method proposed in this work should be viable for a wide range of MHP materials. The demonstrated stability makes the vertically aligned CsPbBr₃-NWs a promising foundation for a wide range of applications in many fields of optoelectronics.

There is an increasing interest in two-dimensional (2D) Ruddlesden-Popper perovskites for solar harvesting and light-emitting applications due to their superior chemical stability as compared to bulk perovskites.[1,2] Both, purely 2D and blends of 2D/3D phases have been successfully employed in solar cells with efficiencies of >18% and >21%, respectively.[3,4] As with earlier advances in the field of perovskites, these technological improvements are advancing at a pace that far exceeds our understanding of the physical mechanisms underlying their performance. Particularly, the reduced dimensionality in 2D perovskites results in excitonic excited states which dramatically modify the dynamics of charge collection. While the carrier dynamics in bulk systems is increasingly well understood, a detailed understanding of the spatial dynamics of the excitons in 2D perovskites is lacking.[5]

Here, we present direct measurements of exciton transport in single-crystalline layered perovskites. Using transient photoluminescence microscopy, we can follow the temporal evolution of a near-diffraction-limited exciton population with sub-nanosecond resolution revealing the spatial and temporal exciton dynamics. We observe two distinct temporal regimes: For early times excitons undergo unobstructed normal diffusion, while at later times exciton transport becomes subdiffusive as excitons get trapped. Interestingly, the diffusivity at early times depends sensitively on the choice of the organic spacer which can yield diffusion lengths that differ by up to an order of magnitude. We find a clear correlation between lattice stiffness and diffusivity, suggesting exciton–phonon interactions to be dominant in the spatial dynamics of the excitons in perovskites, consistent with the formation of exciton–polarons. Our findings provide a clear design strategy to optimize exciton transport in these systems.[6] In addition, we show that the complex exciton dynamics observed at later times can be leveraged to provide a detailed map of the trap-state landscape in 2D perovskites, in particular when used in combination with transient photoluminescence spectroscopy and a rigorous diffusion model that accounts for a distribution of radiative shallow trapping sites.[7]
1. Krishna, A., Gottis, S., Nazeeruddin, M. K. & Sauvage, F. Mixed
Dimensional 2D/3D Hybrid Perovskite Absorbers: The Future of Perovskite Solar Cells? Adv. Funct. Mater. 29, 1806482 (2019).
2. Ortiz-Cervantes, C., Carmona-Monroy, P. & Solis-Ibarra, D.
Two-Dimensional Halide Perovskites in Solar Cells: 2D or not 2D?
ChemSusChem 12, 1560–1575 (2019).
3. Li, P. et al. Phase Pure 2D Perovskite for High-Performance 2D-3D
Heterostructured Perovskite Solar Cells. Adv. Mater. 30, 1805323 (2018).
4. Yang, R. et al. Oriented Quasi-2D Perovskites for High Performance
Optoelectronic Devices. Adv. Mater. 30, 1804771 (2018).
5. Straus, D. B. & Kagan, C. R. Electrons, Excitons, and Phonons in
Two-Dimensional Hybrid Perovskites: Connecting Structural, Optical, and Electronic Properties. J. Phys. Chem. Lett. 9, 1434–1447 (2018).
6. Seitz, M. et al. Exciton diffusion in two-dimensional metal-halide
perovskites. Nat. Commun. 11, 2035 (2020).
7. Seitz, M. et al. Mapping the Trap-State Landscape in 2D Metal-Halide Perovskites using
Transient Photoluminescence Microscopy. Adv. Opt. Mat. (accepted 2021).

High environmental stability and surprisingly high efficiency of solar cells based on 2D perovskites have renewed interest in these materials. This natural quantum wells consists of planes of metal-halide octahedra, separated by organic spacers. Remarkable the organic spacers plays crucial role in optoelectronic properties of these compounds. Despite the bandedge states are composed from metal and halide orbitals the organic spacer imposes octahedral distortion and in this way controls band structure and carriers-phonon coupling. The characteristic for ionic crystal coupling of excitonic species to lattice vibration became particularly important in case of soft perovskite lattice. The nontrivial mutual dependencies between lattice dynamics, organic spacers and electronic excitation manifest in a complex absorption and emission spectrum which detailed origin is subject of ongoing controversy, because observed multiple sidebands are attributed to phonon replicas or distinct excitonic states.
Here we address this issue by systematic studies of 2D perovskites of different thickness of octahedral layer, different organic spacer, (namely phenethylammonium C₆H₅C₂H₄NH₃ (PEA) and long aliphatic chains (AC) C_nH_2n+1NH₃ (with n=4,6,8,10,12)) and halide cage composition. We found that energy spacing of excitonic feature depends on organic spacer but very weakly depends on octahedral layer thickness and metal ion. This support the vibrionic progression scenario originating from coupling to organic spacer or halide cage motion. Our results show that in case of PEA spacer exciton couple dominantly to PEA vibration, while in case of AC excitons are coupled to octahedral layer vibration. We found that exciton-phonon coupling can be tuned by the quantum well thickness. The phonon related origin of complex spectral feature is additionally supported by high magnetic fields studies up to 67T. In all cases the equally spaced features exhibit identical shifts in magnetic field as can be expected for phonon replicas. The observed difference in exciton-phonon coupling can be consistently explained taking in to account mutual dependence between organic spacer, octahedral distortion and dielectric screening.

As easy-to-grow quantum wells with direct bandgaps, two dimensional Ruddleson-Popper perovskites carry great potential as excitonic components in strongly coupled polaritonic applications. Their combination of narrow excitonic features at room temperature and enhanced collective effects (i.e., coupling with multiple excitons) enable the observation of various regimes of light-matter interaction, including evidence of strong coupling with dielectric cavities or plasmonic nano-arrays.

Here we demonstrate that in (C₄H₉NH₃)₂PbI₄, butylammonium lead iodide (BAPI) thin films prepared via spin coating, the geometric morphology of the perovskite emitters needs to be considered to understand the excitonic spectrum and the light-matter interactions with localized cavities, in our case plasmonic silver nanoparticles. Specifically, we provide proof that a distinct exciton-photon Fano resonance in dark field scattering emerges from the coupling between the band edge exciton and the geometry-driven Rayleigh-like scattering background of the BAPI material itself, supported by finite-difference time-domain (FDTD) simulations and a classical coupled-oscillator model.
Through sequential spin coating, we combine the BAPI thin films with individual plasmonic silver nanocavities. Combining single particle dark field scattering spectroscopy and FDTD simulations, we demonstrate that the BAPI Fano resonance exhibits strong coupling to the plasmon resonance of colloidal silver nanoparticle plasmonic cavities, although the total dark field scattering of the combined system is still dominated by the perovskite Fano resonance. We estimate a Rabi splitting of ~ 300 meV -- amongst the highest values ever reported for comparable material systems.

By adding a gold layer beneath the perovskite, we formed particle-on-mirror cavities, allowing us to access the out-of-plane excitonic component of the 2D perovskite. Due to the higher energy dissipation rate of the combined cavity, the combination of BAPI and particle-on-mirror cavity is found to exhibit intermediate coupling.
Our work not only highlights the potential of Ruddleson-Popper perovskites for polariton applications, but in addition provides new and unexpected insights into the principles governing the light matter-interaction of 2D halide perovskites.

In spite of rapid improvements in efficiency and brightness of metal halide perovskite light-emitting diodes (PeLEDs), the poor operational stability remains a critical challenge hindering their practical applications. Herein, we demonstrate greatly improved operational stability of high-efficiency PeLEDs, enabled by incorporating dicarboxylic acids into precursor for the deposition of perovskite emissive layers. We reveal the critical role of carboxyl groups in enhancing the device stability through converting the active organic ingredients in perovskite emissive layers to stable amides, a process catalyzed by the alkaline zinc oxide substrate. The formed inert amides prohibit detrimental interfacial reactions between the perovskites and the charge injection layer underneath, greatly improving the thermal stability of the perovskite emissive layers and ensuring the long operational stability of the resulting PeLEDs. Through rationally optimizing the amidation reaction in the perovskite emissive layers, we achieve efficient PeLEDs with a peak external quantum efficiency of 18.6% and an outstandingly long half-life time of 682 h at 20 mA cm^-2, presenting an important breakthrough in PeLEDs.

Organic-inorganic metal halide hybrids have attracted great research attention for their remarkable and useful optical and electronic properties with applications in a variety of areas, ranging from photovoltaic cells to light emitting devices, sensors, and detectors. In addition to the well-known 3D ABX₃ perovskite structure, organic-inorganic metal halide hybrids can have many other crystallographic structures, in which inorganic metal halide units, for instance, metal halide octahedrons, form 2D, 1D, and 1D structures surrounded by organic cations. These low dimensional materials exhibit unique and remarkable properties that are completely different from those of 3D ABX₃ perovskites. For instance, corrugated-2D and 1D organic metal halide hybrids exhibit broadband emissions from both direct and self-trapped excited states, and 0D organic metal halide hybrids exhibit broadband emissions from the reorganized excited states with high photoluminescence quantum efficiencies of up to 100 %. In this talk, I will present our recent efforts on the development and study of organic-inorganic metal halide hybrids beyond perovskites. From 3D to 2D, 1D, and 0D structures at the molecular level, from mononuclear molecular metal halide species to multinuclear metal halide clusters, and from single-component homogeneous systems to multi-component heterogenous systems, I will discuss how we can achieve synthetic control of this class of hybrid materials to realize desired properties with applications in various types of optoelectronics.

Recently, astounding optoelectronic properties have been discovered in a group of halide perovskite materials. In this talk, our recent work of developing perovskite-polymer composites towards the realization of fully printed and stretchable light-emitting diodes (LEDs) will be presented. The perovskite-polymer composites possess all the remarkable optoelectronic characteristics of pristine perovskites. For instance, we have demonstrated their use for blue, green, and red LEDs. In addition, the device efficiencies are compatible to those of pristine perovskite LEDs. The perovskite-polymer composites have shown advantages in improving the processability and quality of the perovskite thin films; and enhancing the structural stability of the perovskites especially at humid fabrication and service environments. By embedding the perovskite crystals inside a polymer matrix, the perovskites can be less toxic and more environmentally benign compared to pristine perovskites; moreover, to enable new mechanical properties of perovskite LEDs.

Organometal halide perovskite has already attracted extensive research attention owing to its great potential for ubiquitous optoelectronic applications. However, most hybrid perovskite-based optoelectronic devices are fabricated on rigid indium-tin-oxide (ITO) glass substrate inside the glove box with a vacuum-evaporated metal top electrode, which is a tedious and time-consuming process. In this work, we demonstrate an all-solution-processed perovskite light-emitting diodes (PeLEDs) fabricated entirely by inkjet printing in ambient conditions for high-resolution flexible display applications. The PeLEDs were constructed with an extremely simple 4-layer sandwich structure, consisting of a composite transparent bottom electrode of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and poly(ethylene oxide) (PEO), a composite light-emissive layer of methylammonium lead tribromide (CH₃NH₃PbBr₃) and poly(ethylene oxide) (PEO), and a silver nanowire (AgNW) top electrode, all directly printed onto an ultrathin polydimethylsiloxane (PDMS) substrate. In addition, a printed polyethyleneimine (PEI) buffer layer inserted between the emissive layer and the top electrode was found to be effective in protecting the perovskite crystals from being attacked by the solvent in AgNW ink during the top electrode printing step. The device exhibits bright green emission with a turn-on voltage of 3.18 V, a maximum luminance intensity of 10 227 cd m^-2, and a peak current efficiency of 2.01 cd A^-1. By optimizing the ink formulation and printing recipe and taking full advantage of direct printing, we have also achieved scalable patterning of PeLEDs with resolution down to 250 μm. The printed PeLEDs also exhibit good flexibility and can be conformably bent to a 3.5 mm radius convex curvature for over 100 bending cycles without degradation in performance. The combination of superior optical performance, simplified device structure, all-solution processibility, and low-cost printing strategy is promising to pave way for emerging flexible display and wearable electronics applications.

Hybrid perovskites offer an unprecedented degree of tunability through mixing and matching of diverse functionalities for inorganic and organic components [1]. The interfaces among organic cations and between the organic and inorganic components (mediated by hydrogen bonding and steric interactions) play an essential role in determining emergent properties within this semiconductor family. In this talk, we will examine how organic cation choice can profoundly impact spin, optical and thermodynamic properties, with a goal towards understanding how these characteristics can be rationally controlled. As an example, selection of appropriate chiral organic cations can lead to a transfer of chirality between the organic and inorganic components and an associated breaking of symmetry within the inorganic layer, which in turn provides a large Rashba-Dresselhaus splitting of the conduction band and associated control over spin texture [2]. Alternatively, organic cations with controlled HOMO and LUMO position can offer unique opportunities to fabricate self-assembling quantum well structures with distinct alignment of the energy levels [3]. Appropriate choice of the organic cation also leads to hybrids with melting temperature well below the decomposition point [4]. These examples of structure-property versatility yield new opportunities for fundamental science and prospective device applications.

References:
[1] B. Saparov, D. B. Mitzi, Chem. Rev. 116, 4558 (2016).
[2] M. K. Jana, R. Song, H. Liu, D. R. Khanal, S. M. Janke, R. Zhao, C. Liu, Z. V. Vardeny, V. Blum, D. B. Mitzi, Nature Commun. 11, 4699 (2020).
[3] W. A. Dunlap-Shohl, E. T. Barraza, A. Barrette, S. Dovletgeldi, G. Findik, D. J. Dirkes, C. Liu, M. K. Jana, V. Blum, W. You, K. Gundogdu, A. D. Stiff-Roberts, D. B. Mitzi, Mater. Horiz. 6, 1707 (2019).
[4] T. Li, W. A. Dunlap-Shohl, E. W. Reinheimer, P. Le Magueresc D. B. Mitzi, Chem. Sci. 10, 1168 (2019).

Metal halide perovskites are emerging class of semiconducting materials that make high performance photovoltaics. Recently, low dimensional perovskites, such as 2D perovskites and nanocrystals, show unique carrier transport and recombination processes. Low dimensional perovskites are thus demonstrated as ideal candidate for high efficiency light emitting diodes.

In my talk, I will discuss both 2D perovskites and perovskite nanocrystals and their applications in light emitting devices and radiation detectors. Firstly, the charge transport properties and surface properties will be discussed by scanning photocurrent microscopy measurement, where we found a long carrier diffusion length in 2D perovskite single crystals along in-plane direction. Secondly, we observed large degree of charge localization in the bromide 2D perovskite which yielded strong emission properties. Finally, I will introduce new perovskite nanocrystal structure that show stable and bright emission properties. Using low dimensional perovskite materials, we demonstrate bright light emitting diodes with external quantum efficiency of 10%~15% with stable operational lifetime.

Reduced dimensionality forms of perovskites with alternating layers of organic ligands are a promising class of materials for achieving stable perovskite solar cells. Most work until now has focused on phases utilizing two ammonium terminated ligands per formula unit. However, phases utilizing a single diammonium ligand per formula unit are advantageous in that they can potentially have a thinner insulating organic layer between Pb-halide layers, yet the structural effects on their optoelectronic properties remain to be well understood. In this talk, we first rapidly summarize our recent work on the energetics of 2D Ruddlesden-Popper phase metal halide perovskites (MHP). We then turn to two Dion-Jacobson n=1 MHPs formed with butane 1,4- diammonium (BDA) and N,N–dimethylpropane diammonium (DMPD) spacers. Using ultraviolet and inverse photoelectron spectroscopies, BDAPbI₄ is shown to have a larger transport gap by 350 meV and a larger exciton binding energy by 140 meV than DMPDPbI₄. Through density functional theory calculations, the cause of this difference is traced to the out-of-plane tilting of the Pb-halide octahedra provoked by the asymmetric ligand in DMPDPbI4. Parallel channels of nearly straight Pb-I-Pb bonds are formed in one direction, leading to enhanced electronic coupling and higher band dispersion in that direction. In BDAPbI₄, no such channels exist, resulting in greater electronic confinement and a larger band gap and exciton binding energy.

Two-dimensional (2D) halide perovskites have great promise in optoelectronic devices because of their stability and optical tunability, but the subtle effects on the inorganic layer when modifying the organic spacer remain unclear. Here, we introduce two homologous series of Ruddlesden-Popper (RP) structures using the branched isobutylammonium (IBA) and isoamylammonium (IAA) cations with the general formula (RA)₂(MA)_n-1Pb_nI_3n+1 (RA = IBA, IAA; MA = methylammonium n = 1-4).[1] Surprisingly, the IAA n = 2 member results in the first modulated 2D perovskite structure with a ripple with a periodicity of 50.6 Å occurring in the inorganic slab diagonally to the [101] direction of the basic unit cell. This leads to an increase of Pb-I-Pb angles along the direction of the wave. Generally, both series show larger in-plane bond angles resulting from the additional bulkiness of the spacers compensating for the MA’s small size. Larger bond angles have been shown to decrease the bandgap which is seen here with the bulkier IBA leading to both larger in-plane angles and lower bandgaps except for n = 2, in which the modulated structure has a lower bandgap because of its larger Pb-I-Pb angles. Photo-response was tested for the n = 4 compounds and confirmed, signaling their potential use in solar cell devices. We made films using an MACl additive which showed good crystallinity and preferred orientation according to grazing-incidence wide-angle scattering (GIWAXS). As exemplar, the two n = 4 samples were employed in devices with champion efficiencies of 8.22% and 7.32% for IBA and IAA, respectively.
References

[1] Hoffman, J. M.; Malliakas, C. D.; Sidhik, S.; Hadar, I.; McClain, R.; Mohite, A. D.; Kanatzidis, M. G. Chem. Sci., 2020, Advance Article.

Two-dimensional (2D) hybrid organic-inorganic halide perovskites are a promising class of environmentally stable semiconductors. Their inherent structural tunability and technological features provide a vast compositional space to engineer new materials for optoelectronic applications. Within this compositional space, several different homologous series and structure types of 2D perovksites have been developed for their successful fabrication in solar cells, light-emitting diodes and radiation detectors with attractive efficiencies. Although 2D lead iodide perovskites exhibit superior stability over their 3D parent structures, a systematic understanding of their observed stability is missing. Herein, we actuate a comprehensive study on the relative stability of several distinct families of 2D halide perovskites in bulk crystal and film form. Thermochemical evaluation of the representative 2D structure types of Ruddlesden-Popper (RP) and Dion-Jacobson (DJ) perovskites was undertaken based on calorimetric measurements that reveal that the enthalpy of formation for the RP perovskites is negative while for the DJ perovskites is positive. Film stability tests demonstrate consistent observations with the thermochemical findings, where RP lead iodide perovskites are both thermodynamically and environmentally stable candidates for optoelectronic applications. Additionally, the methodical tailoring of the 2D perovskite structure’s composition (organic and inorganic component), unveils trends in the comparison of 2D lead iodide and bromide perovskites, for the acquisition of halide perovskites with enhanced stability. Structural – crystallographic analysis of bulk, layered perovskites provides atomistic insight to control these materials’ stability. Our work highlights the importance of the rational assessment of stability in 2D hybrid halide perovskites for the optimal synthetic design and engineering of environmentally robust perovskite materials for next-generation optoelectronic devices.

Metal halide perovskites are promising for applications in light-emitting diodes (LEDs), but still suffer from defects-mediated nonradiative losses, which represent a major efficiency-limiting factor in perovskite-based LEDs (PeLEDs). Here, we synthesized a series of new phenyl and thienyl-based molecular passivators with ammonium, formamidinium and imidazolium terminal groups, to examine the effects of different anchoring groups on defect passivation. After introducing the passivators, the as-prepared perovskite thin films display improved optoelectronic properties with low nonradiative recombination rate and high PLQY as well as reduced grain size and surface roughness. On this basis, we demonstrate highly efficient PeLEDs with an EQE of 15.6% using a novel imidazolium terminated passivator. Employing grazing incidence wide-angle X-ray scattering technique and single-crystal analysis, we further confirm that the in-situ formation of low-dimensional perovskite phase on the surface of 3D perovskite nanograins is responsible for the surface defects passivation, which lead to significantly enhanced device performances. This work not only provides us a deep insight into the interaction of organic molecules with perovskite lattice for defects passivation, but also guides the beginning of a new realm of molecular engineering for the development of high-performance perovskite-based optoelectronic devices.

Metal halide perovskite nanocrystals (PNCs) are regarded as promising light-emitters due to their high photoluminescence quantum efficiency and color purity. However, the electroluminescence efficiencies of perovskite light-emitting diodes (PeLEDs) based on PNCs are limited by lack of comprehensive materials design strategy both to suppress formation of defects causing non-radiative recombination and to enhance charge carrier confinement for more radiative recombination inside PNCs. In this work, we report a reproducible method for producing PeLEDs based on PNCs that leads to unprecedentedly high current efficiency (CE) of 108 cd×A^-1 (external quantum efficiency (EQE) of 23.4 %) in the PeLEDs and even higher CE of 205 cd×A^-1 (EQE of 45.5 %) with a hemispherical lens. To accomplish this, we design a simple materials alloying strategy that generates smaller, monodisperse colloidal particles (improving charge carrier confinement) with fewer surface defects (suppressing non-radiative recombination). The governing concept is elegant only with one type of large organic cation dopant in colloidal PNCs. Doping of larger guanidinium (GA) into formamidinium (FA) lead bromide PNCs yields limited bulk solubility, above which extra GA segregates to the surface and stabilizes the under-coordinated sites. In bulk of PNCs, a small concentration of GA creates an entropy-stabilized phase. Increasing GA concentration requires higher surface/volume, leading to smaller PNCs for more carrier confinement inside PNCs. Additionally, the one dopant alloying strategy in PNCs was combined with a surface stabilizing a 1,3,5-tris(bromomethyl)-2,4,6-triethylbenzene overcoat which acts as a Br vacancy healing agent. Our world-leading efficiencies demonstrate that this design strategy provides a clear pathway to translate PNCs into PeLEDs for a new generation of high-efficiency display applications.

In recent years metal halide perovskite based LEDs (PeLEDs) have attracted increasing research attention due to their solution processability and extraordinary electroluminescence performances. However, many PeLEDs exhibit significantly degraded radiance or shifted emission wavelength within hours of operation, thus limiting the practical applications of this technology. In this talk I will introduce our recent efforts in addressing the stability issues in high-efficiency PeLEDs. Firstly, the importance of eliminating iodide residues in the perovskite emissive layer will be discussed. Applying extensively excess ammonium halides in forming perovskites is a widely used approach to achieve high-performance PeLEDs. However, most of these PeLEDs suffer from severe external quantum efficiency (EQE) roll-off at high current densities, thereby restricting the realization of high-brightness PeLEDs and laser diodes. By combining voltage-dependent electrical stress measurements and ex-situ ion distribution analysis of the PeLEDs, we found that the electric-field driven diffusion of excess iodide ions, originated from the non-stoichiometric precursors, plays a dominant role in the EQE roll-off. Based on this discovery, we introduced a simple wash-off treatment with chloroform to remove the excess iodides from the perovskite surface and demonstrated that the treatment is highly effective in suppressing the roll-off behavior. Secondly, we further explored surface treatment of the FAPbI₃ perovskite films with phenylalkylammonium iodide molecules of varying alkyl chain lengths. Combining experimental characterization and theoretical modelling, we show that these molecules stabilize the perovskite through suppression of iodide ion migration. The stabilization effect is enhanced with increasing chain length due to the stronger binding of the molecules with the perovskite surface, as well as the increased steric hindrance to reconfiguration for accommodating ion migration. The passivation also reduces the surface defects, resulting in a high radiance and delayed EQE roll-off. Using the optimized passivation molecule, phenylpropylammonium iodide, we achieved a high radiance of 1282.8 W/(sr × m²), a high critical current density (the current density corresponding to half of the EQE_max) of 2400 mAcm^-2 and a record T₅₀ half-lifetime of 130 hrs under a high bias current density of 100 mA/cm².

Metal halide perovskite light-emitting diodes (PeLEDs) show great potentials to be the next-generation lighting technology, with external quantum efficiencies (EQEs) exceeding 20% for infrared, red and green LEDs. However, the efficiencies of blue and white devices severely lag behind. To improve the performance of blue PeLEDs, we employed an integrated strategy combining dimensional engineering of perovskite film and recombination zone modulation in the LED device to obtain an EQE up to 5 %.^[1] While further incorporating the strategy of interfacial engineering, highly efficient blue PeLEDs with EQEs over 10% have been successfully realized in our group, establishing an excellent platform for white-light emission. In our latest work, we demonstrated efficient white PeLEDs by optically coupling a blue PeLED with a red emitting perovskite nanocrystal layer in an advanced device structure, which allows to extract the trapped optical modes (waveguide and SPP modes) of blue photons in the device to the red perovskite layer via near-field effects. As a result, a white PeLEDs with EQE over 10% is achieved, which represents the state-of-the-art performance for white PeLEDs.

References:
[1] Z. Li^†, Z. Chen^†, H.-L. Yip* et al. Nat. Commun. 2019, 10, 1027; Z. Chen, H.-L. Yip* et al. Adv. Mater. 2017, 29, 1603157.

Quasi-2D layered halide perovskites have been demonstrated as a promising class of materials for light-emitting applications. The interests in this class of materials derives from its attractive features including strong light absorption, narrow emission and unprecedented defect tolerance. Furthermore, halogen alloying offers an effective way for the finetuning of bandgap energy, hence a wide range of emission colors can be achieved by using mixed halide perovskites (MHPs). However, light-emitting diodes based on MHPs exhibit significant spectral shifts during the operation, which suggests that the fundamental understanding of the effects of mixed halide compositions should be preceded. Here, we report a novel mechanism of the spectral instability of layered mixed halide perovskites containing bromide and iodide. X-ray diffraction and absorption measurement indicate the uniform distribution of anions in the ambient environment. However, strong electric field which is applied during the device operation is found to drive the systematic migration of halides. The quantum mechanical density functional theory calculations reveal that slow equatorial-to-apical diffusion of bromide is responsible for the halide redistribution. In-depth analysis on the electroluminescence spectra also confirm that the spectral instability is responsible for the anion migration. Finally, we suggest that the dominant red electroluminescence from layered MHPs is attributed to the thermodynamically favored hole injection process. It is envisioned that our comprehensive study will provide new insights into the mechanism of spectral instability of multilayer electroluminescence devices based on MHPs.

Tin halide perovskite solar cells have attracted attention due to the less toxic nature of tin compared to lead compound. Currently, the efficiency of tin-based perovskite solar cells now reaching more than 13 % slowly catching up with that of lead-based perovskite solar cells showing the promise of tin-based perovskites. However, there are several issues with tin-based perovskites that need to be addressed in order for these materials to compete with lead-based perovskites. The susceptibility of Sn²⁺ to oxidize into Sn⁴⁺ upon exposure to air reduces the long-term device stability and this phenomenon also caused major surface recombination which will result in low open-circuit voltage. In addition, the energy levels in tin halide perovskites are shallow compared to that of lead-based perovskites which result in large energy mismatch when common charge transport carriers are used. In this work, we performed simultaneous A-site substitution and surface treatment on tin-halide perovskites to achieve efficiency of more than 10 % and device stability of more than 300 h. Upon partial A-site substitution, the energy barrier has been effectively reduced especially at the hole transport layer/perovskite interface leading to efficient hole carrier injection. While the surface passivation suppressed the formation of unreacted Sn species and hence surface recombination issue has been addressed successfully. Our hypotheses have been supported using Mott-Schottky and Electrochemical Impedance Spectroscopy measurements showing reduced intrinsic carrier density and enhanced carrier recombination resistance. Using X-ray Photoelectron Spectroscopy measurements, we confirmed the suppression of under-coordinated Sn species and oxidized Sn⁴⁺.

Lead iodide (PbI₂) with van der Waals (vdW) layered crystal structure is widely used as a precursor to prepare lead halide perovskites (LHPs) for diverse applications such as photovoltaics (PVs) or optoelectronics. Although the recent advances in the LHPs-based device applications are mainly based on the thin film-based technologies, fabrication of low-dimensional structures in precise manner would be beneficial for the enhancement of their physical properties. 1D nanowire is of particular interest since its structure intrinsically allows the increase in the light absorption, carrier lifetime, and mobilities, compared to its bulk counterpart. Furthermore, the ability to engineer the structure of 1D nanowires (e.g., phase, orientation, and heterojunction) promises advanced exploitation of the properties, as previously demonstrated for group VI or III-V semiconductor material system.
Here, we demonstrate that PbI₂ nanowires grown via vapor – liquid – solid (VLS) method, originally composed of vdW layers stacked along [0001] axis, can switch their orientation (i.e., kinking) by introducing PbBr₂ during the growth. Specifically, we can control two different types of kinking modes: (1) orientation with twin boundary (TB) formation, (2) orientation without TB, by differing the Br exposure time. Our systematic study controlling VLS growth parameters indicates that the incorporation of Br to the catalytic Pb seeds disturbs the triple-phase-line (TPL) and change interface energy, which determines the mode of kinking according to the Br concentration. Our kinked PbI₂ nanowires having different vdW stacking in a single domain exhibit interesting optical property such as localized photoluminescence and waveguide effect, suggesting their potential use in photonics or optoelectronics.

Standard enthalpies of formation of APbX₃ (A = methylammonium, Cs; X = Cl, Br, I) perovskites and their solid solutions APb(X_1-zX^/_z)₃ from halides and from elements at 298 K were measured using solution calorimetry. Standard entropies of CsPbX₃ (X = Cl, Br, I) at 298 K were determined using the results of EMF measurements of galvanic cells. Intrinsic and extrinsic stabilities of studied halides were analyzed and compared with each other. The main difference between the stabilities of CsPbX₃ and CH₃NH₃PbX₃ halides was found to stem from the different chemical nature of cesium and methylammonium cations. Indeed, the enthalpies of formation of CsPbX₃ from binary constituent halides, Δ_fH_hal^○, are only slightly more negative than those of CH₃NH₃PbX₃. Small values of Δ_fH_hal^○ imply that the entropic contribution to the Gibbs free energy of formation of CsPbX₃ and CH₃NH₃PbX₃ is significant and, hence, of utmost importance for understanding the intrinsic stability of these compounds and their analogs. Regarding the extrinsic stability, the presence of gaseous O₂, H₂O, and CO₂ was shown to be crucial for the stability of the iodide, CsPbI₃, for which several decomposition reactions, exergonic at 298 K, were identified. At the same time, chloride, CsPbCl₃, and bromide, CsPbBr₃, are much less sensitive to these chemical agents. However, liquid water should degrade all the CsPbX₃ halides.

Here, I will summarize our understanding of the nature of defects and their photo-chemistry in tin-halide perovskites thin films. We show that, in inert conditions, tin, p-doped, and lead (intrinsic) based perovskite thin film show comparable photoluminescence quantum yield, at comparable morphology. The thin film is also extremely stable under light soaking. On the other hand, photovoltaic devices, showing comparable device architecture show a dramatic reduction in power conversion efficiency for tin based devices. While so far, most of the community effort has been focused on the optimization of the perovskite thin film, very little work has been done on the design of the solar cell architecture. Here we will show new architectures for making tin based solar cells a competitive solution.

Inorganic Cs-based lead halide perovskites have recently experienced a surge in interest, in part due to their enhanced stability compared with hybrid perovskites. The mixed CsPb(Br_xI_1−x)₃ is especially interesting due to the tunable bandgap in the optimal range suitable for tandem solar cells as well as light emitting devices. However, in contrast to the pure CsPbI₃ and CsPbBr₃, there has been very few investigations on the crystal phase dynamics of the halide mixture. In this work, we present the phase diagram of CsPb(Br_xI_1−x)₃ (0 ≤ x ≤ 1, 300–585 K) obtained by high-throughput in situ GIWAXS measurements of a combinatorial thin film library¹. The thin film library is produced by combinatorial inkjet printing and measured at a high-flux liquid metal jet source. After synthesis, the films with lower Br/I ratio exhibit the non-photoactive delta-phase at room temperature, whereas the films with higher I/Br ratios are present in the metastable perovskite gamma phase. We find that during heating all compositions convert to the cubic perovskite phase at high temperature and that the presence of bromide in the films stabilizes the metastable perovskite phases upon cool down. In accordance with recent predictions from DFT-calculations, the phase transition temperatures are found to monotonically decrease with increasing bromide content.

¹Näsström et al., J. Mater. Chem. A, 2020, DOI: 10.1039/d0ta08067e

Crystalline metal halide perovskites (MHPs) have shown tremendous significance in the past decade, leading to momentous advancement in the interdisciplinary fields of materials, electronics and photonics. So far, these studies heavily focused on crystalline MHPs. Though crystallinity offers numerous advantages, the ability to induce glass formation in such semiconductors could provide unique opportunities to extend the associated structure-property relationships and broaden their application space. Despite significant efforts to amorphize MHPs under high pressure, an immediate reversal to a crystalline state upon pressure removal has so far impeded the study of the glassy MHP state and associated practical application, thus necessitating alternative routes. Further, the ability to melt-quench hybrid MHPs has largely been limited due to (i) degradation/loss of the organic component upon heating above 200^○C and (ii) super-fast ordering kinetics of these melts. Herein, we demonstrate the first example of glass formation at ambient pressure and reversible switching between glassy and crystalline states in hybrid MHPs. Drawing inspiration from structure-property studies of chiral vs. racemic organic systems, we exploit bulky aromatic chiral organic cations to modify the bonding and packing characteristics and thereby enable an exceptionally low melting temperature (T_m = 175^○C) below the degradation point (T_d ≈ 205^○C), as well as slow ordering kinetics in the exemplary 2D S-(-)-1-(1- naphthyl)ethylammonium lead bromide chiral MHP. These factors provide facile access to a stable glassy state of the said chiral MHP by melt-quenching both in thin-film and monolith configurations. Furthermore, the components of the melt can recrystallize (at T_x ≈ 100^○C) upon heating above the glass-transition temperature (T_g ≈ 67^○C), allowing reversible switching between the glassy and crystalline states with characteristic temperatures obeying the atypical sequence for MHPs, T_g < T_x < T_m < T_d. Formation of a semiconducting MHP glass, along with the demonstration of thermally-induced reversible switching between the glassy and crystalline states, each offering distinct semiconducting properties, unlocks a new research domain for MHPs with numerous prospective applications spanning the research fields of memory, computing, photonics, catalysis, batteries and meta-surfaces.

Reference: A. Singh et. al., Adv. Mater. 2021, 33(3), 2005868.

Successful application of metal halide perovskites in different technologies stems from their demonstrated exceptional optoelectronic properties such as a long charge carrier diffusion length, tunable bandgap, low mid-bandgap trap density, high absorption coefficient, and efficient photoluminescence. However, one question still to be answered is whether these properties, particularly for those polycrystalline films used in most optoelectronic devices, are of intrinsic or extrinsic nature. For example, among all the intriguing optoelectronic properties of the polycrystalline thin perovskite films, the long carrier recombination lifetime is most attractive as it enables efficient perovskite solar cells. I will present some intriguing observation of how extrinsic metal ions affect the materials Morphology, optoelectronic properties and stability.

Despite showing great promise for optoelectronics, the commercialization of halide perovskite nanostructure-based devices is hampered by inefficient electrical excitation and strong exciton binding energies. To counteract these problems, an understanding of energy- and charge transfer processes in this material is crucial. Here, we study Förster resonance energy transfer (FRET) and charge carrier diffusion processes in quantum-confined two-dimensional CsPbBr₃-based nanoplatelets (NPls). In thin films of NPls with two predetermined thicknesses, we observe an enhanced acceptor photoluminescence (PL) emission and a decreased donor PL lifetime. This indicates a FRET-mediated process, benefitted by the structural parameters of the NPls and with efficiencies of nearly η_FRET = 70%.[1] Additionally, films comprising NPls of one thickness show signs of FRET-enabled long-range exciton diffusion with diffusion lengths of several hundred nanometers. Ultimately, a tailored energy funnel, in combination with a high diffusion length for perovskite NPls could substantially improve the efficiency of blue-emitting nanostructure-based optoelectronic devices.

[1] A. Singldinger, M. Gramlich, C. Gruber, C. Lampe, A. S. Urban, ACS Energy Letters 5, 1380-1385 (2020)

Lead halide perovskites are one of the leading candidates as the light absorbing layer for new photovoltaic devices¹. As the fabrication of high quality perovskite layers has matured, the optimisation of the charge selective contacts has become increasingly important².

In a solar cell, the device asymmetry necessary to generate a photocurrent and photovoltage is provided by electron or hole selective transport layers (TLs) on either side of the light absorbing layer. At each interface, carrier selectivity may be provided by the relative band alignment, and can also be affected by any built-in fields at the interface. Built-in fields can also affect the interfacial recombination that competes with current extraction, so can have an important role in device performance. Together the two TLs may provide a built-in field throughout the perovskite layer which aids charge separation in the bulk.

Here we report time-resolved measurements of charge extraction into commonly used charge transport layers, by using the terahertz photo-conductivity, transient optical absorption and photoluminescence as non-contact spectroscopic probes. In order to isolate the carrier dynamics at a single interface, experiments were performed on bilayers consisting of the perovskite layer and a single transport layer. By investigating each transport layer in isolation, bulk fields were avoided and the behaviour of each interface was isolated independently.
In this work we studied bilayers consisting of commonly used transport layers (Spiro-OMeTAD, PCBM and C60) deposited on top of a high-performance triple cation mixed halide perovskite³ layer (FA_0.79MA_0.16Cs_0.05)Pb(I_0.83Br_0.17)₃. The ground-state bleach of the interband transitions (observed via transient absorption spectroscopy) probed the carrier density change, while the THz photoconductivity probed the product of the carrier density and mobility. By comparing the two techniques, the relative mobility of electrons and holes can also be observed, with holes found to have higher mobility. The optical techniques used here avoided the need for additional contacts, which may influence the data or confuse the interpretation of the results.

By tuning the optical excitation wavelength we created different initial carrier distributions in the perovskite. When using a short excitation wavelength to generate carriers close to the interface with the transport layer, rapid carrier extraction (within a picosecond) was observed, whereas when photogeneration was concentrated on the far side, negligible carrier extraction was observed over the first 3ns. Conversely, when using longer wavelengths to generate carriers more uniformly through the depth of the perovskite layer, very little extraction was observed over the first few nanoseconds. Therefore, concentrating carriers near the transport layer interface gave extraction rates that were order of magnitudes faster than expected from diffusion alone.

We identify band bending near the interface as a potential route by which rapid charge extraction can occur. We explored the possible scenarios of drift-limited and diffusion-limited transport through a numerical solution to the drift-diffusion and Poisson equations. The importance of band bending (built-in fields) at the interface in perovskite heterojunctions was assessed.

1. Jena, A. K., Kulkarni, A. & Miyasaka, T. Halide Perovskite Photovoltaics: Background, Status, and Future Prospects. Chem. Rev. 119, 3036–3103 (2019).
2. Schulz, P., Cahen, D. & Kahn, A. Halide Perovskites: Is It All about the Interfaces? Chemical Reviews 119, 3349–3417 (2019).
3. Saliba, M. et al. Cesium-containing triple cation perovskite solar cells: Improved stability, reproducibility and high efficiency. Energy Environ. Sci. 9, 1989–1997 (2016).

The Shockley-Queisser limit for solar cell efficiency of ~ 33% can be overcome if hot carriers can be harvested before they thermalize. Recently, carrier cooling time up to 100 picoseconds was observed in hybrid organic-inorganic lead halide perovskites, but it is unclear whether these long-lived hot carriers can migrate long distance for efficient collection. We report direct visualization of hot carrier migration in CH₃NH₃PbI₃ thin films by ultrafast transient absorption microscopy, demonstrating three distinct transport regimes. Quasi-ballistic transport was observed to correlate with excess kinetic energy; resulting in up to 230 nanometers transport distance in 300 fs that could overcome grain boundaries. The nonequilibrium transport persisted over tens of picoseconds and ~ 600 nanometers before reaching the diffusive transport limit. These results suggest potential applications of hot carrier devices based on hybrid perovskites.

We recenlty used electric force microscopy to discover persistent light-induced conductivity in a range of 3D lead-halide perovskite semiconductors. Here electric force microscopy was used to record the light-dependent impedance spectrum and time-resolved photoconductivity of a film of butylammonium lead iodide, BA₂PbI₄, a 2D Ruddlesden--Popper perovskite semiconductor. The impedance spectrum of BA₂PbI₄ showed modest changes as the illumination intensity was varied up to 1400 mW/cm², in contrast with the comparatively dramatic changes seen for 3D lead-halide perovskites under similar conditions. BA₂PbI₄ 's light-induced conductivity had a rise time and decay time of 100 microseconds, 10⁴ slower than expected from direct electron-hole recombination and yet 10⁵ faster than the conductivity-recovery times recently observed in 3D lead-halide perovskites and attributed to the relaxation of photogenerated vacancies. What sample properties are probed by electric force microscope measurements remains an open question. A Lagrangian-mechanics treatment of the electric force microscope experiment was recently introduced by Dwyer, Harrell, and Marohn which enabled the calculation of steady-state electric force microscope signals in terms of a complex sample impedance. Here this impedance treatment of the tip-sample interaction is extended, through the introduction of a time-dependent transfer function, to include time-resolved electrical scanned probe measurements. It is shown that the signal in a phase-kick electric force microscope experiment, and therefore also the signal in a time-resolved electrostatic force microscope experiment, can be written explicitly in terms of the sample's time-dependent resistance (i.e., conductivity).

I will discuss the synthesis and surface-management of perovskite quantum dots. I will address progress in managing ligand exchanges towards well-passivated inks that can then be processed directly into films. I will discuss the resultant photophysics and how the materials can be deployed in devices such as narrow-linewidth LEDs.

Understanding the relationship between structure and functionality is particularly important for the future development of energy devices such as photovoltaics, batteries, catalysts, supercapacitors, optoelectronics, among a myriad of other important applications. In virtually all cases, it is the local inhomogeneities and defects that ultimately control macroscopic behavior and device performance by acting as nucleation centers for new phases, interactions, or failure sites. It is only by understanding the local mechanisms that we can hope to address these problems, and in doing so understand how to improve and enable the next generation of high conversion efficiency and long lifetime for optoelectronic materials. Hybrid organic-inorganic perovskites (HOIPs) such as methylammonium lead iodide (CH3NH3PbI3) have attracted broad research interest due to their outstanding optoelectronic performance. However, fundamental understandings of the origin of high PCE and the anomalous current-voltage (I-V) hysteresis of HOIPs optoelectronic devices has proven elusive, even controversial. Although ferroelectricity has been suggested as the reason for this hysteric behavior, convincing evidence supporting this hypothesis is still incomplete. This is due in part to the strong ion motion in HOIPs that complicates the ferroic characterization. Here, using multi-modal functional and chemical imaging methods, we unveil the interaction between ferroic behavior and ion distribution in methylammonium lead iodide (CH3NH3PbI3 or MAPbI3). We demonstrate in-situ interaction of ions under the effect of local fields by introducing a new in-situ imaging mobility for time-of-flight secondary ion mass spectrometry (ToF-SIMS). By combining x-ray diffraction and ToF-SIMS we show that the ion redistribution is accompanied by a change in lattice strain, which is reversible. Additionally, Kelvin Probe Force Microscopy (KPFM) demonstrated a screening effect of the electric field and polarization in CH3NH3PbI3 caused by ion migration. Overall, we demonstrate that local ion distribution can manipulate the formation of ferroelastic twin domain, providing a pathway to manipulate ferroelastic twin domains and hence optimize optoelectronic properties of HOIPs. This work offers an understanding of ferroic-ionic interplay in HOIPs, providing a pathway to develop novel devices based on HOIPs.

Hybrid halide perovskites have proven to be prominent candidates for many commercial applications, and despite their solution processable synthetic protocols, the resulting films and single crystals demonstrate a combination of optoelectronic properties which are not found to their fully inorganic semiconductor competitors. These features are attributed partly to their inherent defect tolerance.
In this work we take advantage of their defect tolerance to engineer a new family of 3D highly defective “hollow” bromide perovskites with general formula (FA)_1-x(en)_x(Pb)_1-0.7x(Br)_3-0.4x (FA = formamidinium, en = ethylenediammonium, x = 0-0.44). The corresponding materials were characterized by a battery of techniques, including single crystal X-ray diffraction (XRD), high resolution powder X-ray diffraction (PXRD), and solid state nuclear magnetic resonance (NMR) measurements. Pair distribution function (PDF) analysis shed light on the local structural coherence, revealing a wide distribution of Pb-Pb distances in the crystal structure, validating further that the corresponding materials are lead deficient and that en inclusion has the same structural effect as in the case of the “hollow” iodide analogs.
By manipulating the number of defects, we finely tuned the optical properties of the pristine FAPbBr₃, by blue shifting the band gap from 2.20 eV to 2.60 eV for the 42% en sample. A most unexpected outcome was the fact that above 40% en incorporation the material exhibits strong broad light emission with 1% photoluminescence quantum yield (PLQY), that is maintained after exposure in air for more than a year. This is the first example of strong broad light emission from a 3D hybrid halide perovskite and is a clear demonstration that defect engineering can be utilized in our favour to add unusual, exotic optoelectronic properties to this versatile class of materials.


References
[1] Spanopoulos, I.; Hadar, I.; Ke, W.; Guo, P.; Mozur E. M.; Morgan E.; Wang S.; Reddy G. N. M.; Seshadri R.; Schaller, R. D.; Kanatzidis, M. G., under submision. 2020.

Metal halide perovskites (MHPs) solar cells have shown great potential for optoelectronic applications. Although it has been well established how grain, grain boundary, and grain facet affect MHPs optoelectronic properties, less is known about subgrain structures. Recently, a sub-grain feature, twin domain, in MHPs has stimulated extensive discussion. Numerous calculation works have indicated that both domains and domain walls can significantly influence the optoelectronic properties of MHPs, while related experimental results are still missing. In this work, using electron backscatter diffraction (EBSD) and multiple advanced piezoresponse force microscopy (PFM) we studied the crystallographic and ferroic properties of twin domains in MHPs, respectively. Using EBSD, we identified the orientation relationship across the twin boundaries in CH₃NH₃PbI₃ and the directions parallel to the surface normal. Then, we performed multiple advanced PFM measurements to investigate whether the origin of PFM signals of these twin domains is ferroelectricity. We also investigated the behavior of twin domains and domain walls in polarization switching measurement and discovered nonferroelectric contributions to polarization switching measurement. Combining EBSD and PFM study, we found that the PFM signal in CH₃NH₃PbI₃ in the directions parallel to the surface normal (identified by EBSD) is not originated from piezoelectricity/ferroelectricity. These measurements were also carried out to study low-dimensional (2D) metal halide materials. A number of interesting phenomena have also been observed in 2D metal halide materials, including ferroic twinning, ion segregation, photoinduced strain, etc. This work offers a picture describing the crystallographic orientation, twin domains, domain walls in MHPs, which is crucial for understanding the influence of subgrain structure on the optoelectronic properties of metal halide semiconductors.

The inorganic lead halide perovskite CsPbBr₃ exhibits many outstanding properties, in addition to possibly offering environmental stabilities better than their hybrid perovskite counterparts. The full utilization of these materials in optoelectronic applications would be aided by gaining the ability to control the electrical conductivity via impurity doping. One possible donor, bismuth, has been found to enhance solar cell efficiency, and an increase in the position of the Fermi level upon doping. Here I examine how bismuth incorporates into CsPbBr₃ using first-principles hybrid density functional theory. The stability of different configurations is considered as a function of chemical potential and Fermi level. Although bismuth prefers to substitute for lead under most conditions, it introduces a deep donor level ~500 meV from the conduction-band minimum, indicating it will not efficiently generate free carrier concentrations. Based on these results, doping strategies for CsPbBr₃ are reconsidered.

This work was supported by the Office of Naval Research through the Naval Research Laboratory’s Basic Research Program

Hybrid organic−inorganic lead halide perovskites have attracted much attention in the field of optoelectronic devices because of their desirable properties such as high crystallinity, smooth morphology, and well-oriented grains. In addition, they can be easily implemented in many platforms using deposition techniques such as spin coating, methylamine gas vapor annealing, and vapor-assisted/gas-assisted solution processing.¹ However, direct patterning of perovskites is known to be challenging because they degrade when exposed to polar solvents and to UV and high electron energies that are used in photo- and e-beam lithography.² Recently, it was shown that thermal nanoimprint lithography (NIL) is an effective method not only to directly pattern but also to improve the morphology, crystallinity, and crystallographic orientations of annealed perovskite films.^3,4 However, the underlining mechanisms behind the positive effects of NIL on perovskite material properties have not been understood. In this work, we study the kinetics of perovskite grain growth with surface energy calculations by first-principles density functional theory (DFT) and reveal that the surface energy-driven preferential grain growth during NIL, which involves multiplex processes of restricted grain growth in the surface-normal direction, abnormal grain growth, crystallographic reorientation, and grain boundary migration, is the enabler of the material quality enhancement. Moreover, we develop an optimized NIL process and prove its effectiveness by employing it in a perovskite light-emitting electrochemical cell (PeLEC) architecture, in which we observe a fourfold enhancement of maximum current efficiency and twofold enhancement of luminance compared to a PeLEC without NIL, reaching a maximum current efficiency of 0.07598 cd/A at 3.5 V and luminance of 1084 cd/m² at 4 V.

Reference
(1) Jung, M.; Ji, S. G.; Kim, G.; Seok, S. il. Perovskite Precursor Solution Chemistry: From Fundamentals to Photovoltaic Applications. Chemical Society Reviews 2019, 48 (7), 2011–2038. https://doi.org/10.1039/c8cs00656c.
(2) Melvin, A. A.; Stoichkov, V. D.; Kettle, J.; Mogilyansky, D.; Katz, E. A.; Visoly-Fisher, I. Lead Iodide as a Buffer Layer in UV-Induced Degradation of CH3NH3PbI3films. Solar Energy 2018, 159 (November 2017), 794–799. https://doi.org/10.1016/j.solener.2017.11.054.
(3) Li, Z.; Moon, J.; Gharajeh, A.; Haroldson, R.; Hawkins, R.; Hu, W.; Zakhidov, A.; Gu, Q. Roomerature Continuous-Wave Operation of Organometal Halide Perovskite Lasers. ACS Nano 2018, 12 (11), 10968–10976. https://doi.org/10.1021/acsnano.8b04854.
(4) Pourdavoud, N.; Haeger, T.; Mayer, A.; Cegielski, P. J.; Giesecke, A. L.; Heiderhoff, R.; Olthof, S.; Zaefferer, S.; Shutsko, I.; Henkel, A.; Becker-Koch, D.; Stein, M.; Cehovski, M.; Charfi, O.; Johannes, H. H.; Rogalla, D.; Lemme, M. C.; Koch, M.; Vaynzof, Y.; Meerholz, K.; Kowalsky, W.; Scheer, H. C.; Görrn, P.; Riedl, T. Room-Temperature Stimulated Emission and Lasing in Recrystallized Cesium Lead Bromide Perovskite Thin Films. Advanced Materials 2019, 31 (39). https://doi.org/10.1002/adma.201903717.

Lead halide perovskites such as CsPbBr3 have attracted widespread attention as optoelectronic materials, due in large part to their good performance despite high defect densities. This “defect tolerance” has often been explained by hypothesizing that there is negligible trap-assisted nonradiative recombination in these materials because none of the dominant defects give rise to deep levels in the gap. We refer to this as the “shallow defect hypothesis” (SDH). In this work, we reject the SDH for CsPbBr3. Via a thorough first-principles inventory of native defects and hydrogen impurities, we show that a number of relevant defects do in fact have deep levels, most notably the bromine interstitial and hydrogen interstitial. This adds to a growing body of evidence against the SDH, suggesting that the observed defect tolerance may be due instead to relatively low recombination rates at deep levels. Guided by the theoretical identification of these defects, experiments can take steps to mitigate trap-assisted non-radiative recombination, further boosting the efficiency of lead halide perovskite optoelectronics.
* M.W.S. was supported by the Naval Research Laboratory Postdoctoral Fellowship through the American Society for Engineering Education. J.L.L. was supported by the ONR/NRL 6.1 Base
Research Program.

The world around us consists of a myriad of objects with curved geometries and complex surfaces, yet the majority of sensors and detectors that we use to study them are rigid and planar. While state-of-the-art X- and gamma-ray detectors based on Si, a-Se, HgI₂ and CdZnTe have tremendously advanced the field of digital X-ray radiography, their mechanical stiffness, high thickness, and mass impose limits on their application and ultimate performance. Here we present first ultraflexible, lightweight, and highly conformable perovskite X-ray detectors, showing excellent performance that was achieved by means of thorough interfacial engineering study [1]. Our devices with an active area of 0.05 cm² show sensitivity of 9.3 ± 0.5 μC Gy^-1 cm^-2 operated at 0 V (passive mode operation) with limit of detection down to 0.58 ± 0.05 μGy s^-1 setting a current record for passive thin film perovskite detectors. Moreover, these ultraflexible detectors allow for isotropic operation, reliably detecting X-rays impinging either on the back or the front side of the device. Ultraflexible, low-cost, and highly sensitive high energy radiation detectors are of great interest to the fields of medical diagnostics, dosimetry, industrial inspection,security, and defense. Low weight and high conformability of X-ray wearable dosimeters are appealing features for astronauts, nuclear power plants, and laboratory workers, as well as for imagers used in structural inspection and cultural heritage preservation.

[1] Demchyshyn S, Verdi M, Basiricò L, Ciavatti A, Hailegnaw B, Cavalcoli D, Scharber MC,Sariciftci NS, Kaltenbrunner M, Fraboni B. Designing Ultraflexible Perovskite X-Ray Detectorsthrough Interface Engineering. Advanced Science. 2020 Dec;7(24):2002586.

Metal halide perovskite solar cells are now nearly as efficient as the best silicon solar cells but could cost less and be made with a less energy-intensive fabrication process. One of the most significant, but frequently overlooked challenges that must be addressed to allow commercialization of perovskite devices is their degradation under partial shading. When a solar cell is shaded in a module, the photogenerated current drops and a negative voltage (reverse bias) builds up as the other illuminated cells in series try to drive current through, leading to undesired efficiency losses.(1, 2) Moreover, metal-halide perovskite-based photodetectors are showing real potential as x-ray detectors for medical imaging purposes. X-ray detectors are mainly operated in reverse bias for fast extraction of charge carriers.(3)However there have been very few studies, investigating the behavior of devices under prolonged reverse bias. (2, 4, 5)
It is imperative to understand how these devices behave under reverse bias. Applying -1 V to -5 V (reverse bias) for a few seconds on perovskite devices with metal contacts induces irreversible device shunting and significant device degradation.(2) Additionally, reports have shown some semi-transparent device architectures display substantial “s-kink” behavior in their J-V scans after placed under extended reverse bias.(4) However, the “s-kink” behavior is only one particular case of the reverse bias behavior. Many devices generally undergo short circuit, open circuit voltage and/or fill factor losses. Additionally, there have been observations of reverse bias induced halide phase segregation.(2, 4) Consequently, while several processes may be occurring at reverse bias, the mechanisms provided do not entirely explain the severe efficiency losses (>50%) measured after reverse bias.
Here, we use an advanced drift-diffusion approach incorporating an electrochemical term to elucidate the short-circuit, open circuit and fill factor losses we experimentally measure after prolonged reverse bias. We show that holes can tunnel into the perovskite due to sharp band bending near the contacts, accumulate within the bulk of the perovskite absorber, and trigger the oxidation of halides. The density of the oxidized halide species is much higher in reverse bias as there are scarcely any electrons available to reduce the halogens. The resultant halogens act as bulk recombination centers. While the interstitial halogen density does decay when the cell is operated in forward bias, permanent degradation can occur if the iodine diffuses out of the perovskite layer. Our theory shows the significance of the mobile ions in controlling device performance and stability and suggests that reverse bias could be used to perform accelerated tests to quickly assess the robustness of perovskite devices to halide reactivity.

1. T. J. Silverman, M. G. Deceglie, X. Sun, R. L. Garris, M. A. Alam, C. Deline, S. Kurtz, Thermal and Electrical Effects of Partial Shade in Monolithic Thin-Film Photovoltaic Modules. IEEE J. Photovoltaics. 5, 1742–1747 (2015).
2. A. R. Bowring, L. Bertoluzzi, B. C. O’Regan, M. D. McGehee, Reverse Bias Behavior of Halide Perovskite Solar Cells. Adv. Energy Mater. 8, 1702365 (2018).
3. H. Wei, Y. Fang, P. Mulligan, W. Chuirazzi, H.-H. Fang, C. Wang, B. R. Ecker, Y. Gao, M. A. Loi, L. Cao, J. Huang, Sensitive X-ray detectors made of methylammonium lead tribromide perovskite single crystals. Nat. Photonics. 10, 333–339 (2016).
4. R. A. Z. Razera, D. A. Jacobs, F. Fu, P. Fiala, M. Dussouillez, F. Sahli, T. C. J. Yang, L. Ding, A. Walter, A. F. Feil, H. I. Boudinov, S. Nicolay, C. Ballif, Q. Jeangros, Instability of p–i–n perovskite solar cells under reverse bias. J. Mater. Chem. A. 8, 242–250 (2020).
5. A. Rajagopal, S. T. Williams, C.-C. Chueh, A. K. Y. Jen, Abnormal Current–Voltage Hysteresis Induced by Reverse Bias in Organic–Inorganic Hybrid Perovskite Photovoltaics. J. Phys. Chem. Lett. 7, 995–1003 (2016).

There is a continued and growing need for miniaturization of sensors to enable their integration into technologies, such as wearable electronics in medical testing and (bio)chemical analyses. Perovskite-based photodetectors (PPDs) are of special interest for integration with optical sensor components due to their thin structure and exceptional attributes of high responsivity and fast response. In this presentation we demonstrate the suitability and advantage of PPDs for optical analyte sensing by monitoring not only dose-dependent changes in the photoluminescence (PL) intensity of analyte-sensitive dyes, but also by measuring the PL decay times following an excitation pulse, which is a preferred monitoring approach. Two types of highly responsive PPD structures are described: (1) PPDs with wide band responsivity that are suitable for light detection over a broad spectral range, and (2) PPDs sensitive only over a tunable narrow spectral band, which makes them suitable for multi-analyte compact sensors arrays, with analytes that are detected in different spectral ranges. We show the functionality of such PPDs for monitoring gas-phase and dissolved oxygen (DO), as well as glucose. The effect of the PPDs attributes, e.g., size, response time, and responsivity spectrum on the sensors’ components, mode of operation, and characteristics, including dynamic range, sensitivity, and limit of detection will be discussed. Advantages of such PPDs in comparison to Si photodiodes, thin film amorphous and nano-crystalline Si, as well as all-organic photodetectors will also be presented.

Despite recent rapid advances in metal halide perovskites for use in optoelectronics, the fundamental understanding of the electrical poling induced ion migration, accounting for many unusual attributes and thus performance in perovskite based devices, remain comparatively elusive. Herein, the electrical poling promoted polarization potential is reported for rendering hybrid organic inorganic perovskite photodetectors with high photocurrent and fast response time, displaying a tenfold enhancement in the photocurrent and a twofold decrease in the response time after an external electric field poling. First, a robust meniscus assisted solution printing strategy is employed to facilitate the oriented perovskite crystals over a large area. Subsequently, the electrical poling invokes the ion migration within perovskite crystals, thus inducing a polarization potential, as substantiated by the surface potential change assessed by Kelvin probe force microscopy. Such electrical poling induced polarization potential is responsible for the markedly enhanced photocurrent and largely shortened response time. This work presents new insights into the electrical poling triggered ion migration and, in turn, polarization potential as well as into the implication of the latter for optoelectronic devices with greater performance. As such, the utilization of ion migration produced polarization potential may represent an important endeavor toward a wide range of high performance perovskite based photodetectors, solar cells, transistors, scintillators, etc.

Volatile Organic Compound (VOC) gases can contribute to the environmental pollution, leading to difficulty in breathing, nausea, eye irritation, and even damage of the central nervous system, specially in indoor environment. Sensing these gases plays a crucial role in environmental monitoring. This study aims to test the sensitivity and selectivity of methylammonium lead iodide (MAPbI₃) perovskites to various different VOC gases. Here, at first, microchannels were created on indium tin oxide (ITO) coated plastic substrates. The microchannels were filled with the perovskite precursor solution using the capillary motion force. After crystallization, the current-voltage (I-V) characteristic responses of the perovskite before and after the exposure to the gasses were studied, both under dark and light conditions. The variation of the photocurrent in response to ethanol, methanol, and acetone have been studied. While the sensor showed a negligible I-V changes in response to ethanol and acetone, the photocurrent was decreased by 25% when the device was exposed to methanol at 2.0 V voltage bias. The experimental results of detecting VOCs at room temperature are promising for the development of an array of low-cost and low-power perovskite-based gas sensors that can be integrated in lab-on-a-chip devices.

In this study, optical multispectral sensors based on perovskite semiconductors have been proposed, simulated, and characterized. The perovskite material system combined with the 3D vertical integration of the sensor channels allows for the realization of sensors with high sensitivities and a high spectral resolution. The sensors can be applied in several emerging areas, including biomedical imaging, surveillance, complex motion planning of autonomous robots or vehicles, artificial intelligence, and agricultural applications. The sensor elements can be vertically integrated on a readout electronic to realize sensor arrays and multispectral digital cameras. In this study, three- and six-channel vertically stacked perovskite sensors are optically designed, electromagnetically simulated, and colorimetric characterizations to evaluate the color science. The proposed sensors allow for the implementation of snapshot cameras with high sensitivity. The proposed sensor is compared to other sensor technologies in terms of sensitivity and selectivity.

To further improve the powder conversion efficiency of halide perovskite solar cells (PSCs), it is highly preferred to understand the electrical properties of the perovskite absorbers, since the PCE is determined by the electrical properties of PSCs, such as defect activation energy and density, carrier concentration, and dielectric constant. Capacitance–based techniques, such as thermal admittance spectroscopy (TAS) and capacitance–voltage (C–V), have been the choice of method for measuring electrical properties of semiconductor devices and have played important roles in the development of thin-film solar cell technologies. These techniques have been used to measure the electrical properties of PSCs such as defect activation energy and density, carrier concentration, and dielectric constant, which provide key information for evaluating the device performance. We show that charge-transport layers can have significant influence on the measured capacitance, affecting the interpretation of the results. For example, the hole-transport layers (HTLs) can introduce high-frequency capacitance signature due to the response of charge carriers in HTLs. In literature, the so-called D1 signature was attributed to defects in perovskite defects. However, we found that the D1 signature is due to HTLs. For HTL-free PSCs, the D1 signature disappears and the high-frequency capacitance can be considered as the geometric capacitance for analyzing the dielectric constant of the perovskite layer.

Understanding hot-carrier cooling processes in hybrid (organic-inorganic) halide perovskites is crucial in photovoltaic applications. The dynamics of carrier-lattice interactions in perovskites strongly affect carrier lifetimes and diffusion lengths, which are key factors in device efficiency. Studies of 2D perovskite systems have revealed unique physics in terms of electron-lattice interactions, such as light-induced phonon coherences, phonon dynamical lattice disorder, and electron-phonon scattering through deformation potentials. Besides spectroscopic studies, we have performed direct structural probing of 2D perovskites using ultrafast electron diffraction (UED), and have shown multiple electron-phonon interactions during non-thermal carrier relaxation. Those electron-phonon scattering dynamics are dominated by structural deformation related to low-frequency phonon modes, induced by in-plane rotation of perovskite octahedra. Compared with the structural responses of 3D perovskites (MAPbI₃), our results on 2D crystals show distinct behavior in terms of both anisotropy and anharmonicity. These understandings could provide deeper insight in the dynamics of carrier-lattice interactions in 2D perovskites, and will bridge the knowledge gap of hot-carrier dynamics between 3D and 2D perovskite systems.

Lead halide perovskites uniquely offer long carrier diffusion lengths and high resilience against electronic defects, which are highly surprising for a solution-processed semiconductor. The microscopic mechanisms behind such favorable properties have not been fully understood to date. Main hypotheses revolved around dynamic structural responses and formation of large polarons in order to explain the slower carrier recombination and defect tolerances. Nevertheless, these dynamic structural aspects have not been monitored before mainly due to lack of available techniques that can track atomic-scale motions after photoexcitation. In this work, we perform optical pump / femtosecond diffuse x-ray scattering probe measurements using an x-ray electron free laser, the so-called LCLS at SLAC/Stanford [1]. The experiments clearly show that localized expansive strains build up following photoexcitation. It takes ~20 ps for the local distortions, on average, to reach about 5 nm in diameter. These lattice distortions then relax over the time window of the carrier recombination. Our finding undoubtedly demonstrates that the charge carriers in the perovskites strongly distort the atomic structure transiently while the distortion size extending over many unit cells, hence forming large polarons. In this experiment, we resolve the spatio-temporal evolution of the polarons for the first time enabled by phonon-momentum-resolved measurements in our experiment based on time-resolved diffuse x-ray scattering.

[1] B. Guzelturk et al. Nature Materials (2020) DOI: 10.1038/s41563-020-00865-5

Energy carrier transport and recombination in emerging semiconductors can be directly monitored with optical microscopy, leading to the measurement of the diffusion coefficient (D), a critical property for design of efficient optoelectronic devices. D is often determined by fitting a time-resolved expanding carrier profile after optical excitation using a Mean Squared Displacement (MSD) Model, where D = [σ²(t) - σ²(0)]/2t, with σ²(t) being the Gaussian variance as a function of time. Although this approach has gained widespread adoption, its utilization can significantly overestimate D due to the non-linear recombination processes that artificially broaden the carrier distribution profile. Here, we simulate diffusive processes in both excitonic and free carrier semiconductors and present revised MSD Models that take into account second-order (bimolecular) and third-order (Auger) processes to accurately recover D for various materials. For perovskite thin films, utilization of these models can reduce fitting error by orders of magnitude, especially for commonly deployed excitation conditions where carrier densities are > 5x10¹⁶ cm^-3. Contrary to what is generally accepted, we find that photon recycling does not have a significant impact on carrier profiles and that differences in grain size and boundary behavior, present in most polycrystalline films, can lead to distinct profiles that are not captured by MSD Models. Finally, we present clear strategies to investigate energy transport in disordered materials for more effective design and optimization of electronic and optoelectronic devices.

Controlled functionalization of colloidal quantum dot surfaces is key to the manipulation of their optoelectronic properties. Applying this principle to the study of organometallic halide perovskite quantum dots (PQD’s), we investigate temperature-dependent optical and energy transfer properties of four PQD species, all functionalized with variations of benzoic ligands. These include benzoic acid (BA), phenylacetic acid (PAA), benzylamine (BZA), and isopropyl benzylamine (IPBZA). While the chemical structure of BZA/BA and IPBZA/PAA differ by just a few carbon atoms, this difference manifests as a significant variation in PQD properties. Charge transfer efficiency in PQD films comprising BA-ligated samples varies between 12- 95% as dot density is tuned from 10² – 10⁵ dots/µm² but is consistently ~ 92% over that entire range for PAA-ligated PQDs. As temperature T decreases, initially, recombination is dominated by bound or trapped excitons, but for T < 80 K, spectral broadening accompanied by free excitonic behavior is observed. Our results indicate enhanced charge delocalization at lower T, which reduces exciton confinement and recombination decay rates, underlining the importance of investigating PQD-ligand interactions at the fundamental level given the significant effect minute changes in ligand structures have on optoelectronic properties of quantum dots.

Ion dissociation in perovskite lattices has been recently identified to determine the intrinsic stability of perovskite solar cells (PVSCs), the underlying degradation mechanism is still elusive. In this work, we demonstrate that by combining highly sensitive sub-bandgap external quantum efficiency (s-EQE) spectroscopy, impedance analysis as well as theoretical calculations, the evolution of defect states in PVSCs during the degradation can be monitored. It is found that the degradation of PVSCs can be divided into three steps: (1) dissociation of iodine ions (I^-) from perovskite lattices, (2) migration of I^- to interfaces, and (3) consumption of I^- by reacting with the metal electrode. Importantly, step (3) is found to be crucial as it will accelerate the first two steps and lead to continuous degradation. By replacing the metal with more chemically robust indium tin oxide (ITO) as the electrode, we find that the dissociated I^- under light soaking will only saturate at the perovskite/ITO interface without being consumed. Importantly, the dissociated I^- will subsequently restore to the iodine vacancies under dark condition to heal the perovskite and photovoltaic performance. Such shuttling of I^- without consumption in the ITO-contact PVSCs results in Harvest-Rest-Recovery (HRR) cycles in the natural day/night operation. We envision that the mechanism of the intrinsic perovskite material degradation reported in this work will lead to clearer research directions towards highly stable PVSCs.

Lead-free perovskite-inspired materials (PIMs) are receiving ever-growing attention in photovoltaics, optoelectronics, and beyond, due to their similarity to mainstream lead-based perovskites while being free of the toxicity concerns associated with the latter.^[1–3] Specifically, antimony- and bismuth-based PIMs have been identified as particularly promising. Nonetheless, the efficiencies of such PIMs in single-junction outdoor solar photovoltaics are yet to approach the levels of the lead-based counterparts. An important limiting factor lies in the bandgaps of these materials, which are in the region of 1.9 eV or greater, thereby preventing the optimal absorption of solar light for single-junction operation.
Going beyond the mainstream view of solely considering lead-free PIMs for outdoor solar photovoltaics, herein we show that these materials have considerable potential for indoor photovoltaics (IPV),^[4] a rapidly growing sector in energy harvesting for smart devices of the Internet of Things (IoT) ecosystem.^[5] With a focus on two representative lead-free PIMs with high photoconversion efficiencies, Cs₃Sb₂Cl_xI_9-x^[6] and BiOI,^[7] we show that their IPV efficiencies are up to ~5%, i.e., four times higher than under outdoor solar illumination and already within the performance range of mainstream commercial IPV based on hydrogenated amorphous silicon (a-Si:H).^[4] Further, based on power-dependent measurements and optical loss analyses, we provide insight into the current performance bottlenecks and identify strategies for future improvements toward the ultimate IPV efficiencies of these materials. Finally, by combining millimeter-scale Cs₃Sb₂Cl_xI_9-x and BiOI IPV devices with ultralow-power printed electronics,^[8] we present the first-ever demonstration of printed thin-film-transistor electronics powered by IPV.^[4] By revealing the capability and potential of lead-free PIMs for indoor photovoltaics, our findings point to the opportunity provided by such environmentally-friendly semiconductors to sustainably power the growing IoT ecosystem.

References
[1] R. Nie, R. R. Sumukam, S. H. Reddy, M. Banavoth, S. Il Seok, Energy Environ. Sci. 2020, 13, 2363.
[2] V. Pecunia, L. G. Occhipinti, A. Chakraborty, Y. Pan, Y. Peng, APL Mater. 2020, 8, 100901.
[3] Y.-T. Huang, S. R. Kavanagh, D. O. Scanlon, A. Walsh, R. L. Z. Hoye, Nanotechnology 2021, 32, 132004.
[4] Y. Peng, T. N. Huq, J. Mei, L. Portilla, R. A. Jagt, L. G. Occhipinti, J. L. MacManus-Driscoll, R. L. Z. Hoye, V. Pecunia, Adv. Energy Mater. 2021, 11, 2002761.
[5] Q. Hassan, Ed., Internet of Things A to Z, John Wiley & Sons, Inc., Hoboken, NJ, USA, 2018.
[6] Y. Peng, F. Li, Y. Wang, Y. Li, R. L. Z. Hoye, L. Feng, K. Xia, V. Pecunia, Appl. Mater. Today 2020, 19, 100637.
[7] R. L. Z. Hoye, L. C. Lee, R. C. Kurchin, T. N. Huq, K. H. L. Zhang, M. Sponseller, L. Nienhaus, R. E. Brandt, J. Jean, J. A. Polizzotti, A. Kursumović, M. G. Bawendi, V. Bulović, V. Stevanović, T. Buonassisi, J. L. MacManus-Driscoll, Adv. Mater. 2017, 29, 1702176.
[8] L. Portilla, J. Zhao, Y. Wang, L. Sun, F. Li, M. Robin, M. Wei, Z. Cui, L. G. Occhipinti, T. D. Anthopoulos, V. Pecunia, ACS Nano 2020, 14, 14036.

The exceptional optoelectronic performance of lead-halide perovskites (LHPs) has motivated enormous research efforts toward the discovery of ‘perovskite-inspired materials’ – compounds which aim to replicate the astonishing performance of LHPs while avoiding the infamous stability and toxicity pitfalls of these materials.^1–3 Recently, quaternary chalcogen halides of group IV / V elements have begun to attract attention, due to the presence of valence ns² lone pairs, a performance-defining feature of LHPs.⁴
Following the first experimental report of stable, solution-grown tin-antimony sulfoiodide (Sn₂SbS₂I₃) solar cells,⁵ with power conversion efficiencies above 4% (exceeding the first reported solar efficiency of methylammonium lead-iodide (MAPI)),⁶ we comprehensively characterize the structural and electronic properties of this emerging material. We find that the experimentally reported (non-ferroelectric) Cmcm crystal structure is in fact an equipment artefact, representing an average over multiple (ferroelectric) Cmc2₁ configurations. The resemblance of this dynamic crystal structure and ferroelectric behavior to that of MAPI begs the question of its importance in high-performance defect-tolerant solar materials.
Moreover, using state-of-the-art ab-initio methods (hybrid Density Functional Theory including spin-orbit coupling effects), we rigorously assess the efficiency limits of this material on the basis of its electronic structure and predicted defect behavior.
Our work provides valuable insight regarding both the potential success of this emerging class of optoelectronic materials and structure-property relationships in perovskite-inspired materials, guiding design strategies and expanding the compositional space of candidate materials.

1 Y.-T. Huang, S. R. Kavanagh, D. O. Scanlon, A. Walsh and R. L. Z. Hoye, arXiv:2008.08959.
2 S. R. Kavanagh, Z. Li et al, J. Mater. Chem. A, 2020, 8, 21780–21788.
3 M. Buchanan, Nature Physics, 2020, 16, 996–996.
4 R. Nie, R. R. Sumukam, S. H. Reddy, M. Banavoth and S. I. Seok, Energy Environ. Sci., 2020, 13, 2363–2385.
5 R. Nie, K. S. Lee, M. Hu, M. J. Paik and S. I. Seok, Matter, 2020, S2590238520304471.
6 A. Kojima, K. Teshima, Y. Shirai and T. Miyasaka, Journal of the American Chemical Society, 2009, 131, 6050–6051.

The emergence of lead halide perovskites for optical and electronic applications has been one of the most important discoveries of materials science and chemistry of the past two decades. To accelerate the industrial use of metal halide perovskites, there is a global search for alternative non-toxic, lead-free halides that demonstrate higher environmental stability and similarly outstanding optoelectronic properties while preserving the advantageous properties of lead halides. In this talk, our recent discoveries of brand-new families of lead-free high efficiency light-emitting materials will be summarized. These include all-inorganic copper(I) halides demonstrate low-dimensional non-perovskite structures, and consequently, very flat bands around the band gap, leading to very localized charges. Such charge localization and low-dimensional structures typically result in the presence of high stability self-trapped excitons at room temperature producing record high photoluminescence efficiencies approaching unity. On the other hand, families of low-dimensional hybrid organic-inorganic halides prepared in our show very diverse crystal structures and optical emission properties, including controllable emission form organic or inorganic components, or simultaneous emission from both. The talk will be concluded with a few examples of the potential practical applications of the luminescent metal halides mentioned in this talk.

Lead-based halide perovskite crystals have unique structural dynamics in that they exhibit, an-harmonic, liquid-like thermal fluctuations. This type of structural dynamics, that can not be treated within the classic framework of semiconductor physics, is intimately to the beneficial electronic properties of the lead based perovskites. In recent years, the double perovskite Cs2AgBiBr6, was studied as part of an effort to find a lead-free perovskite for optoelectronic applications. While the electronic properties of Cs2AgBiBr6 were extensively studied, little is known about its structural dynamics. We investigate the structural dynamics of Cs2AgBiBr6 and compare it to CsPbBr3 which is its lead-based analog. By using temperature-dependent Raman measurements we found that both materials are strongly anharmonic. However, the expression of their anharmonic behaviour is markedly different. Cs2AgBiBr6 shows a soft-mode and a displacive phase transition similar to archetypical oxides perovskites. In contrary, CsPbBr3 show an increase of acentral Raman component, related to a large amplitude, relaxational motion. These differences area result of different evolution with temperature of the tilting instability that exists in both crystals at low temperatures. This difference is important because it may explain why Cs2AgBiBr6 is not an effective material for optoelectronic applications compared to the lead-perovskites. In the course of our investigations, we also discovered a new phase of Cs2AgBiBr6 below35 K.

The bismuth-based (oxy)iodides BiI₃, BiOI and Ag_xBiI_x+3 share similar layered crystal structures, optimal band gaps for top absorbers in tandem solar cells, and moderate growth temperatures. Similarly to halide perovskite absorbers, they contain a heavy cation with a lone pair of electrons (Bi³⁺) which has been proposed as one of the features enabling defect tolerance in perovskites.

We have grown and characterized BiI_3, BiOI, and Ag₃BiI₆ absorbers and solar cells in a systematic manner by employing a consistent synthesis method ((oxy)iodization of metallic precursor) and a consistent analysis routine [1]. In this way, the individual strengths and weaknesses of the three absorbers, as well as their common challenges, can be outlined.

The radiative efficiency of the three materials is within the same order of magnitude, indicating a similar degree of defect tolerance. Apart from that, each material has its own advantages and drawbacks, which will be outlined in this presentation.

Control of growth orientation should be a priority for all three materials in view of their anisotropic properties. P-type bulk doping and selection of alternative hole transport layers with deep valence bands would also be desirable to improve performance. At the device level, we report an improved open circuit voltage of BiI₃ solar cells with respect to the state of the art, and we report a proof-of-concept Ag₃BiI₆/silicon tandem cell.

Finally, we have observed at least one unusual, exciting property in each material: BiI₃ diodes have a very low dark ideality factor, on par with the highest-quality silicon solar cells. BiOI has a very convenient mix of moderate growth temperature and air stability. Ag₃BiI₆ is a mixed electronic-ionic conductor with fast ionic diffusion, which may find applications in neuromorphic, memory, or battery devices.

[1] Crovetto et al., Parallel Evaluation of the BiI₃, BiOI, and Ag₃BiI ₆ Layered Photoabsorbers. Chem. Mater. 2020, 32, 3385–3395.

All-inorganic group 11 halides have recently gained interest in the optical materials community as viable, relatively non-toxic alternatives to luminescent lead halides. Bulk powder samples and single crystals of A₂MX₃ (A=Rb, K, NH₄; M= Cu, Ag; X= Cl, Br, I) were prepared using high temperature solid-state and solution techniques. The all-inorganic group 11 halides have been investigated through an in-depth optical characterization, including diffuse reflectance, photoluminescence, and radioluminescence. A₂CuX₃ are found to exhibit narrow blue photoluminescence peaks with record-high efficiencies as evidenced by the measured photoluminescent quantum yields up to unity (100 %). Conversely, A₂AgX₃ displays broad, white emission with tunability in color temperature. Through variation in the A⁺ cation site, the stability of the material can be dramatically increased, while retaining similar optical performance. For A₂CuX₃ the bright emission in this family has been attributed to self-trapped excitons localized on [CuX₃]^2- anionic substructure based on exhaustive photoluminescence studies supported by the density functional theory (DFT) calculations. Our combined experimental and computational study suggests strong potential of A₂MX₃ as phosphors for radiation detection and solid-state lighting applications.

Room-temperature continuous-wave (CW) lasing in quasi-two-dimensional perovskites was just recently demonstrated in thin films on a distributed feedback grating. However, CW lasing and amplified spontaneous emission (ASE) in three-dimensional (3D) perovskites was so far, only achieved at cryogenic temperatures. Factors limiting the 3D materials to sustain CW gain at higher temperatures include the temperature dependence of the charge carrier distribution in energy; the build-up of photo-induced non-radiative recombination channels; and the rates of bimolecular and Auger recombination. We investigate the latter in a phase-stable triple cation perovskite, by performing transient reflection measurements to determine the charge carrier dynamics from 80 K up to 290 K.

To accurately determine the Auger rate coefficient, we vary the initial carrier density reaching up to very high carrier concentrations where the Auger recombination plays a significant role. These carrier concentrations are above the ASE threshold for thin films prepared on conventional substrates, such as glass or sapphire. Therefore, we use a silicon substrate with a higher refractive index to completely suppress ASE in the film and its associated carrier depletion. Based on the power and temperature-dependent carrier dynamics measurements, we implemented a global fitting of the rate equation across the considered temperature range to extract the bimolecular and Auger recombination coefficients.

The results show that the bimolecular recombination rate decreases with increasing temperature (from 6.4×10^-10 cm^-3 s^-1 at 80 K to 1.1×10^-10 cm^-3 s^-1 at 290 K), as expected due to increased phonon scattering, whereas the Auger rate coefficient stays approximately constant across the entire temperature range (at approximately 3×10^-29 cm^-6 s^-1). Based on these results, we consider the carrier density at which the Auger recombination will dominate over the bimolecular recombination and discover that above 250 K, the ASE threshold density is lower than the carrier density at which the non-radiative Auger recombination dominates over the radiative bimolecular recombination. This means, that above 250 K, the ASE performance is limited by the Auger recombination and the threshold carrier density is in the roll-off region of the radiative emission efficiency due to Auger losses. At lower temperatures, the ASE threshold density is below the point at which Auger recombination starts dominating and the increase in ASE threshold is mainly determined by two factors: the rapid decrease in the bimolecular rate coefficient with temperature, and the energy dilution of the charge carriers. This energy dilution results in a reduced fraction of radiative emission in the ASE band, compared to the total photoluminescence spectrum. Both effects lead to a necessary increase in carrier density (and therefore pump rate) to maintain the same total emission rate in the ASE band. To increase the temperature at which CW ASE (and lasing) can be achieved in 3D perovskite films, strategies to increase the bimolecular recombination coefficient and keep a narrow emission bandwidth at higher temperatures should be investigated.

Rapid advances in perovskite photovoltaics have produced efficient solar cells, with stability and duration improving thanks to variations in materials composition, including the use of layered 2D perovskites. A major reason for the success of perovskite photovoltaics is the presence of free carriers as majority optical excitations in 3D materials at room temperature. On the other hand, the current understanding is that in 2D perovskites or at cryogenic temperatures insulating bound excitons form, which need to be split in solar cells and are not beneficial to photoconversion. Here we apply a tandem spectroscopy technique that combines ultrafast photoluminescence and differential transmission to demonstrate a plasma of unbound charge carriers in chemical equilibrium with a minority phase of light-emitting excitons, even in 2D perovskites and at cryogenic temperatures. We validate the technique with 3D perovskites and investigate 2D compounds basded on both Pb and Sn as metal cation. The underlying photophysics is interpreted as formation of large polarons, charge carriers coupled to lattice deformations, in place of excitons. A conductive polaron plasma foresees novel mechanisms for LEDs and lasers, as well as a prominent role for 2D perovskites in photovoltaics.

Recent advancements in perovskite solar cell performance were achieved by stabilizing the α-phase of FAPbI₃ in nip-type architectures. However, these advancements could not be directly translated to pin-type devices. Here, we fabricated a high-quality double-cation perovskite (MA_0.07FA_0.93PbI₃) with low bandgap energy (1.54 eV) using a two-step approach on a standard polymer (PTAA).¹ The perovskite films exhibit large grains (1 µm), high external photoluminescence quantum yields of 20% and outstanding Shockley-Read-Hall carrier lifetimes of 18.2 µs without further passivation. The exceptional opto-electronic quality of the neat material was translated into efficient (up to 22.5%) pin-type cells with improved stability under illumination. The low-gap cells stand out by their high fill factor (83%) due to reduced charge transport losses and short-circuit currents 24 mAcm^-2. Using intensity dependent QFLS measurements, we quantify an implied efficiency of 28.4% in the neat material which can be realized by minimizing interfacial recombination and optical losses.
¹Gutierrez-Partida, et al. Large-grain double cation perovskites with 18 µs lifetime and high luminescence yield for efficient inverted perovskite solar cells. ASC Energy Letters.2021, https://dx.doi.org/10.1021/acsenergylett.0c02642

Perovskite nanostructures have been engineered for LEDs, lasers and photodetectors^[1], their reduced dimensionality resulting in quantum confinement of charge carriers which yields dramatically different optoelectronic properties, including enhanced photoluminescence quantum yield^[2] and lower thresholds for amplified spontaneous emission^[3]. Although the creation of such perovskite nanostructures has clear advantages, it often relies on challenging top-down fabrication methods. It would therefore be highly advantageous if instead nanoscale domains were found to form intrinsically through self assembly in the perovskite.

In this study^[4], I report the discovery of intrinsically-occurring nanostructures in FAPbI₃, which exhibit quantum confinement effects manifested as an oscillatory absorption feature above the band gap. These features are present at room temperature but sharpen and become more apparent as the temperature is lowered towards 4 K. I demonstrate that the energetic spacings and temperature-dependence of the peaks vary in a manner consistent with quantum confinement intrinsically associated with the lattice of the material. I suggest the origin of this confinement to be nanodomains with an extent of approximately 10-20 nm. This interpretation is supported by correlating absorption spectra against ab initio calculations based on the bandstructure of FAPbI₃ in the presence of infinite barriers, and simulations for superlattices with moderate barrier heights. I further explore ferroelectricity/ferroelasticity and delta-phase twin boundaries as two possible causes of these domains. Altogether, such absorption peaks present a novel and intriguing quantum electronic phenomenon in a nominally bulk semiconductor, offering intrinsic nanoscale optoelectronic properties without necessitating cumbersome additional processing steps.

[1] Y. Fu, H. Zhu, J. Chen, M. P. Hautzinger, X.-Y. Zhu, S. Jin, Nat. Rev. Mater. 2019, 4, 169.
[2] L. Polavarapu, B. Nickel, J. Feldmann, A. S. Urban, Adv. Energy Mater. 2017, 7, 1.
[3] M. Li, Q. Gao, P. Liu, Q. Liao, H. Zhang, J. Yao, Adv. Funct. Mater. 2018, 28, 1707006.
[4] A. D. Wright, G. Volonakis, J. Borchert, C. L. Davies, F. Giustino, M. B. Johnston, L. M. Herz, Nat. Mater. 2020. https://doi.org/10.1038/s41563-020-0774-9

Organolead halide perovskite thin films are an emerging class of triplet sensitizer for photon upconversion applications, with favourable semiconductor properties such as strong and broad optical absorption, large carrier diffusion lengths and pronounced spin-mixing. Our study aimed to identify connections between key properties of the perovskite film and the upconversion efficiency achieved in a standard bilayer upconverter structure. The perovskite film properties were deliberately altered by changing film processing conditions such as spin-coating speed, antisolvent type and antisolvent dripping time, while triplet-triplet annihilation photon upconverters were prepared in a bilayer structure consisting of thin film methaylammonium lead iodide perovskite (MAPI) combined with a DBP-doped rubrene annihilator layer.

Photoexcitation of the perovskite film at 715 nm lead to delayed photoluminescence at 605 nm from the annihilator layer, with triplet formation in the rubrene layer sensitized by photogenerated charge carriers in the MAPI film. A stronger upconversion effect was observed for films which displayed brighter and more uniform perovskite photoluminescence. These attributes were highly sensitive to antisolvent dripping time, and were optimized at 20 seconds, measured relative to the start of the spinning process. Further, the choice of antisolvent also had a significant contribution in upconversion performance. Two different antisolvents, anisole and chlorobenzene, were utilized in the MAPI formation process. Films treated with anisole had on average a 10-fold increase in upconversion efficiency compared to the chlorobenzene-treated films.

Time-resolved photoluminescence measurements of the two MAPI variants revealed a stark difference in carrier lifetimes, which were 52 ns and 306 ns in the chlorobenzene and anisole-treated films, respectively. Owing to the near-identical appearance of the two films in a range of bulk-sensitive measurements, we proposed that the difference in carrier lifetime is due to a changing defect density at the MAPI/rubrene interface induced by the two antisolvent treatments. This may itself explain the pronounced difference in upconversion performance, since triplet exciton formation is driven by charge carrier interactions across the same interface, and presumably competes with defect-mediated electron-hole recombination.

We have recently performed several fundamental magneto-transport (Hall effect), photoconductivity (PC) and photoluminescence (PL) studies of lead-halide perovskite single crystals, leading to a better understanding of the important charge transport and recombination properties of these materials [1,2,3]. Among these are the first reliable measurements of the dark- and photo-Hall effects in perovskites [1,2], revealing the intrinsic charge carrier mobilities and recombination parameters, as well as the first proposal of large polarons being an important type of carrier in these materials [4]. In this talk, I will focus more on the discovery of unconventional power exponents in photoexcitation flux dependence of PC and PL (including the “strange” power 3/2 in PL) in perovskite single crystals [3], and the demonstration of a novel type of an optoelectronic field-effect transistor device, where PL (rather than electric current) is modulated by a gate voltage [5].
References:
1. Y. Chen, H. T. Yi, X. Wu, R. Haroldson, Y. N. Gartstein, Y. I. Rodionov, K. S. Tikhonov, A. Zakhidov, X.-Y. Zhu, V. Podzorov, "Extended carrier lifetimes and diffusion lengths in hybrid perovskites revealed by steady-state Hall effect and photoconductivity measurements", Nature Comm. 7, DOI: 10.1038/ncomms12253 (2016).
2. H. T. Yi, X. Wu, X.-Y. Zhu, V. Podzorov, "Intrinsic charge transport across phase transitions in hybrid organo-inorganic perovskites", Adv. Mater. 28, 6509-6514, DOI: 10.1002/adma.201600011 (2016).
3. H. T. Yi, P. Irkhin, P. P. Joshi, Y. N. Gartstein, X. Zhu and V. Podzorov, "Experimental Demonstration of Correlated Flux Scaling in Photoconductivity and Photoluminescence of Lead-Halide Perovskites", Phys. Rev. Applied 10, 054016, DOI: 10.1103/PhysRevApplied.10.054016 (2018).
4. X.-Y. Zhu and V. Podzorov, “Charge carriers in hybrid Organic–Inorganic Lead Halide perovskites might be protected as large polarons”, J. Phys. Chem. Lett. 6 (23), 4758-4761 (2015).
5. H. T. Yi, S. Rangan, B. Tang, C. D. Frisbie, R. A. Bartynski, Y. N. Gartstein and V. Podzorov, "Electric-field effect on photoluminescence of lead-halide perovskites", Materials Today, DOI: 10.1016/j.mattod.2019.01.003 (2019).

Optical excitation creates excitons in semiconductors. These quasiparticles most commonly exist in incoherent states. In extremely rare situations under critical conditions, these excitons might gain a collective electronic coherence through a process called spontaneous synchronization. This process is a symmetry breaking, i.e., second-order quantum phase transition, similar to Bose-Einstein condensation and superconductivity. When this happens, all oscillators form a giant dipole and radiate collectively, and act like a giant atom. This process was first proposed by Dicke and hence called Dicke superradiance. Since the electronic quantum phase is extremely fragile due to thermal phonon interactions, Dicke superradiance is only observable in a handful of solid state systems at cryogenic temperatures. In this presentation, we will present the results of our recent discovery of high-temperature Dicke superradience in hybrid perovskites, and discuss the fundamental mechanism leading to such a macroscopic quantum phenonomena in hybrid perovskites. Observation of superradient phase transition in hybrid perovskites is important for developing emerging quantum applications using this versatile material family. Specifically, quantum sensing, communication, and computation applications require robust quantum oscillators. Observation of superfluorescence in hybrid perovskite can pave the way for such applications.

Two-dimensional (2D) materials are outstanding candidates for a variety of optoelectronic applications but are inhibited by low carrier mobilities and high exciton binding energies compared to their bulk counterparts. To that end, rapid Förster resonance energy transfer (FRET) presents an opportunity to quickly and purposefully direct energy through devices, which could improve the efficiency of existing applications and establish new types of energy-funneling devices. The exciting observation of fast energy transfer that outcompetes other multiexcitonic effects, on the order of picoseconds, has been experimentally observed in 2D materials with requisite spectral overlap and electronic coupling between donor and acceptor, but high quality, clear data in such systems is sparse. Predictive models for FRET in 2D systems, particularly with respect to the distance between states, are underdeveloped.
In this work, we approach FRET in 2D perovskite quantum wells; these materials, in which layers of perovskite are electronically decoupled by organic spacer cations into 2D quantum wells of n octahedra in thickness, provide ideal systems for studying FRET given the precision with which various electronic and optical parameters may be synthetically adjusted. Though research into devices fabricated from such materials has skyrocketed, as they present ambiently stable and highly tunable alternatives to well-known hybrid organic inorganic perovskites, synthetic control for desired mixtures of n phases in thin-films that would enable a reliable energy transfer study are non-existent. Few studies exist that aim to discern energy transfer processes in these materials, and each suffers from a complete lack of control over sample composition, resulting in reported lifetimes of energy transfer that are orders of magnitude apart. In order to provide insights regarding FRET in this material class, we prepare thin-films with controlled mixtures of specific n phases of 2D perovskite quantum wells for the first time and use them to investigate binary arrangements of quantum well thicknesses to reveal fundamental behavior.
We examine rates of Förster resonance energy transfer in binary mixtures of 2D perovskites as a function of the distance between layers by incorporating alkylammonium cations of increasing chain length. Energy transfer is observed using transient absorption spectroscopy in each case. Evaluated rates in some instances outpace biexcitonic Auger recombination, and lifetimes become slower with increasing separation. We model these systems computationally, obtaining results in agreement with our empirically obtained lifetimes, and expand upon the model to survey effects of other influencing factors on energy transfer in 2D materials that may be, in principle, synthetically controlled. Our work begins building the necessary foundation for predictive models of FRET in 2D PQWs and further guides design principles for employing FRET in devices fabricated from 2D materials.

Organic-inorganic halide perovskite (OIHP) semiconductors are currently the focus of significant research interest due to their potential optoelectronic device applications. Quantum-confined nanocrystals (NCs) of these materials emit with high photoluminescence quantum yields and a narrow, tunable spectrum. These properties, coupled with the potential for low-cost, facile solution-based syntheses, make these NCs ideal for LEDs, lasers, and optical sensors. Due to their high surface area-to-volume ratio, the surface plays a critical role in determining the excited state dynamics, and thus optoelectronic properties, in these materials. In particular, the evolution of NC surface quality and the mechanisms by which surface states become bound by ligands during growth is still poorly understood. Under-coordinated surface atoms act as traps for charge carriers and can promote unwanted non-radiative recombination. The goal of this work is to understand how the NC surface becomes passivated during NC synthesis.

The fate of photogenerated species in NCs is typically studied using non-linear optical spectroscopies, such as transient absorption (TA), but these techniques have inherently long measurement timescales of up to several hours. OIHP NCs are traditionally formed through either a hot-injection or ligand-assisted reprecipitation synthesis. These reactions are typically complete withing a few minutes, and nascent NCs are highly unstable and difficult to kinetically trap, precluding in situ investigation of dynamics with non-linear optical spectroscopies. This has limited our understanding of nucleation and growth processes in these systems. To overcome these limitations, we have developed a TA spectrometer that reduces the timescale required for measurement by spatially encoding the time delay in a tilted-pulse geometry. This allows collection of an entire broadband transient spectrum with a 60 ps time delay in just a few laser shots, and spectra with a good signal-to-noise ratio can be acquired within seconds. In this work we apply this novel instrumentation to a NC synthesis whose reaction kinetics are limited by the solvation of precursors in a highly nonpolar solvent. Coupling this solvation-mediated synthesis to a rapid sampling technique has enabled the first-ever characterization of exciton dynamics in growing methylammonium lead triiodide NCs.

We observe the appearance of unique TA spectral features in newly formed NCs that slowly disappear over the remainder of the synthesis. Comparison to first and second order derivatives of the linear absorbance spectrum shows that photogenerated charge carriers become localized at surface trap states in nascent NCs, inducing a Stark effect that manifests in the TA measurements. The amplitude of the lineshape arising from this Stark effect declines as NC growth continues, suggesting that intermediate NCs possess unpassivated surface sites that can trap charge carriers and hamper optoelectronic performance. These sites are slowly passivated by capping ligands over the course of the synthesis. Our results contribute to a growing body of evidence suggesting that surface passivation occurs toward the end of NC growth in a final surface ligation stage of the reaction. This experimental technique provides researchers with a new tool to report on surface quality in growing NCs and has the potential to yield fundamental insights into the mechanism of nanocrystal nucleation and growth. A full understanding of these processes would allow for further synthetic control of NC properties by providing researchers the ability to tune reaction conditions in situ to target desired properties.

Inorganic lead halide perovskite (CsPbX₃, X=Cl, Br, and/or I) have fascinating optical properties. By tuning the halide composition, the luminescence of CsPbX₃ can be tuned across the entire visible spectrum. The optical property of CsPbX₃ can also be modified upon the introduction of a new cation species, either a divalent or a trivalent metal ion. Depending on the type of cation, the synthesis strategy, and the nature of the perovskite host, how the introduced cations influence the optical property of the pristine perovskite could be entirely different. In this presentation, I am going to discuss three systems: (1) Mn-doped CsPbX₃, in which the introduction of Mn results in dual-band emission, (2) Pr³⁺-doped CsPbX₃, in which heterovalent doping drastically enhance the perovskite bandgap emission, and (3) Eu²⁺-doped CsPbBr₃, in which Eu²⁺ induces a phase transformation of the perovskite host and producing a blue-emission band. The electronic structure of the doped perovskite and the luminescence mechanism is investigated using synchrotron-based X-ray absorption fine structure (XAFS) and X-ray excited optical luminescence (XEOL). These characterization techniques provide detailed structural information and energy transfer processes in these doped perovskites, and the information obtained is valuable for designing CsPbX₃ light-emitters with desired properties.

Understanding and tailoring the structure-induced physical behavior of materials under practical environments is critical for designing efficient and durable optoelectronic devices. Here, we report a new phenomenon - a sunlight-activated anisotropic lattice contraction in organic-inorganic 2D perovskites, which is reversible and strongly dependent of the specific structural phase and the organic interlayer cation bridging the perovskite octahedra. Modeling 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 2D perovskite solar cells from 13.8% to 16.4%. The increase in efficiency results from a combined increase in the open circuit voltage and fill factor, arising from the reduction of the potential barrier heights upon light-induced contraction

Halide perovskites have exceptional optoelectronic properties, making them attractive for photovoltaics, light-emitting technologies, and radiation detection. However, a poor understanding of the relationship between crystal dimensions, composition, and properties limits their use in integrated devices. In this presentation, we will discuss a multiplexed cantilever-free scanning probe method for synthesizing compositionally diverse halide perovskite nanocrystals spanning cm² areas. Single-particle photoluminescence studies reveal multiple independent emission modes due to defect-defined band edges with relative intensities that depend on crystal size for a fixed composition. Smaller particles, but ones with dimensions that exceed the quantum confinement regime, exhibit blue-shifted emission due to the reabsorption of higher-energy modes. The method reported herein is generalizable and has been used to synthesize six different halide perovskites, including a layered Ruddlesden-Popper phase. Importantly, it also enables the preparation of functional solar cells based upon a single nanocrystal. Additionally, the ability to pattern arrays of multi-color light-emitting nanocrystals opens avenues towards the development of sophisticated optoelectronic devices, including a wide variety of optical displays.