Phonon-driven transient bandgap renormalization in perovskite single crystals

Tailoring the electronic structure of perovskite materials on ultrafast timescales is expected to shed light on optimizing optoelectronic applications. However, the transient bandgap renormalization observed upon photoexcitation is commonly explained by many-body interactions of optically created electrons and holes, which shrink the original bandgap by a few tens of millielectronvolts with a sub-picosecond time constant, while the accompanying phonon-induced effect remains hitherto unexplored. Here we unravel a significant contribution of hot phonons in the photo-induced transient bandgap renormalization in MAPbBr3 single crystals, as evidenced by asymmetric spectral evolutions and transient reflection spectral shifts in the picosecond timescale. Moreover, we performed a spatiotemporal study upon optical excitation with time-resolved scanning electron microscopy and identified that the surface charge carrier diffusion and transient bandgap renormalization are strongly correlated in time. These findings highlight the need to re-evaluate current theories on photo-induced bandgap renormalization and provide a new approach for precisely controlling the optical and electronic properties of perovskite materials, enabling the design and fabrication of high-performance optoelectronic devices with exceptional efficiency and unique properties.

Synthesis of Zwitterionic CsPbBr3 Nanocrystals with Controlled Anisotropy using Surface-Selective Ligand Pairs

Mechanistic studies of the morphology of lead halide perovskite nanocrystals (LHP-NCs) are hampered by a lack of generalizable suitable synthetic strategies and ligand systems. Here, the synthesis of zwitterionic CsPbBr3 NCs is presented with controlled anisotropy using a proposed "surface-selective ligand pairs" strategy. Such a strategy provides a platform to systematically study the binding affinity of capping ligand pairs and the resulting LHP morphologies. By using zwitterionic ligands (ZwL) with varying structures, majority ZwL-capped LHP NCs with controlled morphology are obtained, including anisotropic nanoplatelets and nanorods, for the first time. Combining experiments with density functional theory calculations, factors that govern the ligand binding on the different surface facets of LHP-NCs are revealed, including the steric bulkiness of the ligand, the number of binding sites, and the charge distance between binding moieties. This study provides guidance for the further exploration of anisotropic LHP-NCs.

Highly stable and pure single-photon emission with 250 ps optical coherence times in InP colloidal quantum dots

Quantum photonic technologies such as quantum communication, sensing or computation require efficient, stable and pure single-photon sources. Epitaxial quantum dots (QDs) have been made capable of on-demand photon generation with high purity, indistinguishability and brightness, although they require precise fabrication and face challenges in scalability. By contrast, colloidal QDs are batch synthesized in solution but typically have broader linewidths, low single-photon purities and unstable emission. Here we demonstrate spectrally stable, pure and narrow-linewidth single-photon emission from InP/ZnSe/ZnS colloidal QDs. Using photon correlation Fourier spectroscopy, we observe single-dot linewidths as narrow as ~5 µeV at 4 K, giving a lower-bounded optical coherence time, T2, of ~250 ps. These dots exhibit minimal spectral diffusion on timescales of microseconds to minutes, and narrow linewidths are maintained on timescales up to 50 ms, orders of magnitude longer than other colloidal systems. Moreover, these InP/ZnSe/ZnS dots have single-photon purities g(2)(τ = 0) of 0.077-0.086 in the absence of spectral filtering. This work demonstrates the potential of heavy-metal-free InP-based QDs as spectrally stable sources of single photons.

Long-Lived Exciton Coherence in Mixed-Halide Perovskite Crystals

Compositional engineering of the optical properties of hybrid organic-inorganic lead halide perovskites is crucial for the realization of efficient solar cells and light-emitting devices. We study the effect of band gap fluctuations on coherent exciton dynamics in a mixed FA0.9Cs0.1PbI2.8Br0.2 perovskite crystal by using photon echo spectroscopy. We reveal a narrow homogeneous exciton line width of 16 μeV at a temperature of 1.5 K. The corresponding exciton coherence time T2 = 83 ps is exceptionally long due to the localization of excitons at the scale of tens to hundreds of nanometers. From spectral and temperature dependences of the two- and three-pulse photon echo decay, we conclude that for low-energy excitons pure decoherence associated with elastic scattering on phonons is comparable with the exciton lifetime, while for excitons with higher energies, inelastic scattering to lower energy states via phonon emission dominates.

Optically detected magnetic resonance spectroscopic analyses on the role of magnetic ions in colloidal nanocrystals

Incorporating magnetic ions into semiconductor nanocrystals has emerged as a prominent research field for manipulating spin-related properties. The magnetic ions within the host semiconductor experience spin-exchange interactions with photogenerated carriers and are often involved in the recombination routes, stimulating special magneto-optical effects. The current account presents a comparative study, emphasizing the impact of engineering nanostructures and selecting magnetic ions in shaping carrier-magnetic ion interactions. Various host materials, including the II-VI group, halide perovskites, and I-III-VI2 in diverse structural configurations such as core/shell quantum dots, seeded nanorods, and nanoplatelets, incorporated with magnetic ions such as Mn2+, Ni2+, and Cu1+/2+ are highlighted. These materials have recently been investigated by us using state-of-the-art steady-state and transient optically detected magnetic resonance (ODMR) spectroscopy to explore individual spin-dynamics between the photogenerated carriers and magnetic ions and their dependence on morphology, location, crystal composition, and type of the magnetic ion. The information extracted from the analyses of the ODMR spectra in those studies exposes fundamental physical parameters, such as g-factors, exchange coupling constants, and hyperfine interactions, together providing insights into the nature of the carrier (electron, hole, dopant), its local surroundings (isotropic/anisotropic), and spin dynamics. The findings illuminate the importance of ODMR spectroscopy in advancing our understanding of the role of magnetic ions in semiconductor nanocrystals and offer valuable knowledge for designing magnetic materials intended for various spin-related technologies.

Low-Temperature Emission Dynamics of Methylammonium Lead Bromide Hybrid Perovskite Thin Films at the Sub-Micrometer Scale

We study the low-temperature (T = 4.7 K) emission dynamics of a thin film of methylammonium lead bromide (MAPbBr3), prepared via the anti-solvent method. Using intensity-dependent (over 5 decades) hyperspectral microscopy under quasi-resonant (532 nm) continuous wave excitation, we revealed spatial inhomogeneities in the thin film emission. This was drastically different at the band-edge (∼550 nm, sharp peaks) than in the emission tail (∼568 nm, continuum of emission). We are able to observe regions of the film at the micrometer scale where emission is dominated by excitons, in between regions of trap emission. Varying the density of absorbed photons by the MAPbBr3 thin films, two-color fluorescence lifetime imaging microscopy unraveled the emission dynamics: a fast, resolution-limited (∼200 ps) monoexponential tangled with a stretched exponential decay. We associate the first to the relaxation of excitons and the latter to trap emission dynamics. The obtained stretching exponents can be interpreted as the result of a two-dimensional electron diffusion process: Förster resonant transfer mechanism. Furthermore, the non-vanishing fast monoexponential component even in the tail of the MAPbBr3 emission indicates the subsistence of localized excitons. Finally, we estimate the density of traps in MAPbBr3 thin films prepared using the anti-solvent method at n∼1017 cm-3.

Time-Resolved Line Shapes of Single Quantum Emitters via Machine Learned Photon Correlations

Solid-state single-photon emitters (SPEs) are quantum light sources that combine atomlike optical properties with solid-state integration and fabrication capabilities. SPEs are hindered by spectral diffusion, where the emitter's surrounding environment induces random energy fluctuations. Timescales of spectral diffusion span nanoseconds to minutes and require probing single emitters to remove ensemble averaging. Photon correlation Fourier spectroscopy (PCFS) can be used to measure time-resolved single emitter line shapes, but is hindered by poor signal-to-noise ratio in the measured correlation functions at early times due to low photon counts. Here, we develop a framework to simulate PCFS correlation functions directly from diffusing spectra that match well with experimental data for single colloidal quantum dots. We use these simulated datasets to train a deep ensemble autoencoder machine learning model that outputs accurate, noiseless, and probabilistic reconstructions of the noisy correlations. Using this model, we obtain reconstructed time-resolved single dot emission line shapes at timescales as low as 10 ns, which are otherwise completely obscured by noise. This enables PCFS to extract optical coherence times on the same timescales as Hong-Ou-Mandel two-photon interference, but with the advantage of providing spectral information in addition to estimates of photon indistinguishability. Our machine learning approach is broadly applicable to different photon correlation spectroscopy techniques and SPE systems, offering an enhanced tool for probing single emitter line shapes on previously inaccessible timescales.

On-site growth of perovskite nanocrystal arrays for integrated nanodevices

Despite remarkable progress in the development of halide perovskite materials and devices, their integration into nanoscale optoelectronics has been hindered by a lack of control over nanoscale patterning. Owing to their tendency to degrade rapidly, perovskites suffer from chemical incompatibility with conventional lithographic processes. Here, we present an alternative, bottom-up approach for precise and scalable formation of perovskite nanocrystal arrays with deterministic control over size, number, and position. In our approach, localized growth and positioning is guided using topographical templates of controlled surface wettability through which nanoscale forces are engineered to achieve sub-lithographic resolutions. With this technique, we demonstrate deterministic arrays of CsPbBr₃ nanocrystals with tunable dimensions down to <50 nm and positional accuracy <50 nm. Versatile, scalable, and compatible with device integration processes, we then use our technique to demonstrate arrays of nanoscale light-emitting diodes, highlighting the new opportunities that this platform offers for perovskites' integration into on-chip nanodevices.

Exciton Dephasing by Phonon-Induced Scattering between Bright Exciton States in InP/ZnSe Colloidal Quantum Dots

Decoherence or dephasing of the exciton is a central characteristic of a quantum dot (QD) that determines the minimum width of the exciton emission line and the purity of indistinguishable photon emission during exciton recombination. Here, we analyze exciton dephasing in colloidal InP/ZnSe QDs using transient four-wave mixing spectroscopy. We obtain a dephasing time of 23 ps at a temperature of 5 K, which agrees with the smallest line width of 50 μeV we measure for the exciton emission of single InP/ZnSe QDs at 5 K. By determining the dephasing time as a function of temperature, we find that exciton decoherence can be described as a phonon-induced, thermally activated process. The deduced activation energy of 0.32 meV corresponds to the small splitting within the nearly isotropic bright exciton triplet of InP/ZnSe QDs, suggesting that the dephasing is dominated by phonon-induced scattering within the bright exciton triplet.

Design Rules for Obtaining Narrow Luminescence from Semiconductors Made in Solution

Solution-processed semiconductors are in demand for present and next-generation optoelectronic technologies ranging from displays to quantum light sources because of their scalability and ease of integration into devices with diverse form factors. One of the central requirements for semiconductors used in these applications is a narrow photoluminescence (PL) line width. Narrow emission line widths are needed to ensure both color and single-photon purity, raising the question of what design rules are needed to obtain narrow emission from semiconductors made in solution. In this review, we first examine the requirements for colloidal emitters for a variety of applications including light-emitting diodes, photodetectors, lasers, and quantum information science. Next, we will delve into the sources of spectral broadening, including "homogeneous" broadening from dynamical broadening mechanisms in single-particle spectra, heterogeneous broadening from static structural differences in ensemble spectra, and spectral diffusion. Then, we compare the current state of the art in terms of emission line width for a variety of colloidal materials including II-VI quantum dots (QDs) and nanoplatelets, III-V QDs, alloyed QDs, metal-halide perovskites including nanocrystals and 2D structures, doped nanocrystals, and, finally, as a point of comparison, organic molecules. We end with some conclusions and connections, including an outline of promising paths forward.

Size-Dependent Lattice Symmetry Breaking Determines the Exciton Fine Structure of Perovskite Nanocrystals

The order of bright and dark excitonic states in lead-halide perovskite nanocrystals is debated. It has been proposed that the Rashba effect, driven by lattice-induced symmetry breaking, causes a bright excitonic ground state. Direct measurements of excitonic spectra, however, show the signatures of a dark ground state, bringing the role of the Rashba effect into question. We use an atomistic theory to model the exciton fine structure of perovskite nanocrystals, accounting for realistic lattice distortions. We calculate optical gaps and excitonic features that compare favorably with experimental works. The exciton fine structure splittings show a nonmonotonic size dependence due to a structural transition between cubic and orthorhombic phases. Additionally, the excitonic ground state is found to be dark with spin triplet character, exhibiting a small Rashba coupling. We additionally explore the effects of nanocrystal shape on the fine structure, clarifying observations on polydisperse nanocrystals.

Promoting solution-phase superlattices of CsPbBr3 nanocrystals

We present a size-selective method for purifying and isolating perovskite CsPbBr₃ nanocrystals (NCs) that preserves their as-synthesized surface chemistry and extremely high photoluminescence quantum yields (PLQYs). The isolation procedure is based on the stepwise evaporation of nonpolar co-solvents with high vapor pressure to promote precipitation of a size-selected product. As the sample fractions become more uniform in size, we observe that the NCs self-assemble into colloidally stable, solution-phase superlattices (SLs). Small angle X-ray scattering (SAXS) and dynamic light scattering (DLS) studies show that the solution-phase SLs contain 1000s of NCs per supercrystal in a simple cubic, face-to-face packing arrangement. The SLs also display systematically faster radiative decay dynamics and improved PLQYs, as well as unique spectral absorption features likely resulting from inter-particle electronic coupling effects. This study is the first demonstration of solution-phase CsPbBr₃ SLs and highlights their potential for achieving collective optoelectronic phenomena previously observed from solid-state assemblies.

Zero-field quantum beats and spin decoherence mechanisms in CsPbBr3 perovskite nanocrystals

Coherent optical manipulation of exciton states provides a fascinating approach for quantum gating and ultrafast switching. However, their coherence time for incumbent semiconductors is highly susceptible to thermal decoherence and inhomogeneous broadening effects. Here, we uncover zero-field exciton quantum beating and anomalous temperature dependence of the exciton spin lifetimes in CsPbBr₃ perovskite nanocrystals (NCs) ensembles. The quantum beating between two exciton fine-structure splitting (FSS) levels enables coherent ultrafast optical control of the excitonic degree of freedom. From the anomalous temperature dependence, we identify and fully parametrize all the regimes of exciton spin depolarization, finding that approaching room temperature, it is dominated by a motional narrowing process governed by the exciton multilevel coherence. Importantly, our results present an unambiguous full physical picture of the complex interplay of the underlying spin decoherence mechanisms. These intrinsic exciton FSS states in perovskite NCs present fresh opportunities for spin-based photonic quantum technologies.

Elastic Phonon Scattering Dominates Dephasing in Weakly Confined Cesium Lead Bromide Nanocrystals at Cryogenic Temperatures

Cesium lead halide perovskite nanocrystals (PNCs) have emerged as a potential next-generation single quantum emitter (QE) material for quantum optics and quantum information science. Optical dephasing processes at cryogenic temperatures are critical to the quality of a QE, making a mechanistic understanding of coherence losses of fundamental interest. We use photon-correlation Fourier spectroscopy (PCFS) to obtain a lower bound to the optical coherence times of single PNCs as a function of temperature. We find that 20 nm CsPbBr₃ PNCs emit nearly exclusively into a narrow zero-phonon line from 4 to 13 K. Remarkably, no spectral diffusion is observed at time scales of 10 μs to 5 ms. Our results suggest that exciton dephasing in this temperature range is dominated by elastic scattering from phonon modes with characteristic frequencies of 1-3 meV, while inelastic scattering is minimal due to weak exciton-phonon coupling.

Colloidal CsPbX3 Nanocrystals with Thin Metal Oxide Gel Coatings

Lead halide perovskite (LHP) nanocrystals (NCs) have gathered much attention as light-emitting materials, particularly owing to their excellent color purity, band gap tunability, high photoluminescence quantum yield (PLQY), low cost, and scalable synthesis. To enhance the stability of LHP NCs, bulky strongly bound organic ligands are commonly employed, which counteract the extraction of charge carriers from the NCs and hinder their use as photoconductive materials and photocatalysts. Replacing these ligands with a thin coating is a complex challenge due to the highly dynamic ionic lattice, which is vulnerable to the commonly employed coating precursors and solvents. In this work, we demonstrate thin (<1 nm) metal oxide gel coatings through non-hydrolytic sol-gel reactions. The coated NCs are readily dispersible and highly stable in short-chain alcohols while remaining monodisperse and exhibiting high PLQY (70-90%). We show the successful coating of NCs in a wide range of sizes (5-14 nm) and halide compositions. Alumina-gel-coated NCs were chosen for an in-depth analysis, and the versatility of the approach is demonstrated by employing zirconia- and titania-based coatings. Compact films of the alumina-gel-coated NCs exhibit electronic and excitonic coupling between the NCs, leading to two orders of magnitude longer photoluminescence lifetimes (400-700 ns) compared to NCs in solution or their organically capped counterparts. This makes these NCs highly suited for applications where charge carrier delocalization or extraction is essential for performance.

Highly Emissive Zero-Dimensional Cesium Lead Iodide Perovskite Nanocrystals with Thermally Activated Delayed Photoluminescence

We utilized a modified reverse-microemulsion method to develop highly emissive and photostable zero-dimensional (0D) Cs₄Pb(Br_1-xI_x)₆ perovskite nanocrystals (PNCs). We employed single-particle photoluminescence (PL) spectroscopy to explore blinking statistics and demonstrate single-photon emission from individual PNCs. Low-temperature blinking and photon correlation studies revealed a transition from single- to multiphoton emission with progressively longer "delayed" PL components, reaching ∼70 ns at room temperature and representing a distinctive behavior to previously known iodide PNCs. Such thermally activated PL emission is explained by the existence of defect-related "reservoir" states, feeding back into the PNC's emissive state and providing multiple photons within a single excitation cycle. This work establishes a new member in the 0D class of perovskite materials, studies its photophysical properties, and reveals its potential for future optoelectronic applications.

In situgrowth strategy to construct perovskite quantum dot@covalent organic framework composites with enhanced water stability

Metal halide perovskite quantum dots (QDs) have excellent optoelectronic properties; however, their poor stability under water or thermal conditions remains an obstacle to commercialization. Here, we used a carboxyl functional group (-COOH) to enhance the ability of a covalent organic framework (COF) to adsorb lead ions and grow CH₃NH₃PbBr₃(MAPbBr₃) QDsin situinto a mesoporous carboxyl-functionalized COF to construct MAPbBr₃QDs@COF core-shell-like composites to improve the stability of perovskites. Owing to the protection of the COF, the as-prepared composites exhibited enhanced water stability, and the characteristic fluorescence was maintained for more than 15 d. These MAPbBr₃QDs@COF composites can be used to fabricate white light-emitting diodes with a color comparable to natural white emission. This work demonstrates the importance of functional groups for thein situgrowth of perovskite QDs, and coating with a porous structure is an effective way to improve the stability of metal halide perovskites.

Controlled Assembly and Anomalous Thermal Expansion of Ultrathin Cesium Lead Bromide Nanoplatelets

Quantum confined lead halide perovskite nanoplatelets are anisotropic materials displaying strongly bound excitons with spectrally pure photoluminescence. We report the controlled assembly of CsPbBr₃ nanoplatelets through varying the evaporation rate of the dispersion solvent. We confirm the assembly of superlattices in the face-down and edge-up configurations by electron microscopy, as well as X-ray scattering and diffraction. Polarization-resolved spectroscopy shows that superlattices in the edge-up configuration display significantly polarized emission compared to face-down counterparts. Variable-temperature X-ray diffraction of both face-down and edge-up superlattices uncovers a uniaxial negative thermal expansion in ultrathin nanoplatelets, which reconciles the anomalous temperature dependence of the emission energy. Additional structural aspects are investigated by multilayer diffraction fitting, revealing a significant decrease in superlattice order with decreasing temperature, with a concomitant expansion of the organic sublattice and increase of lead halide octahedral tilt.

How Pendant Groups Dictate Energy and Electron Transfer in Perovskite-Rhodamine Light Harvesting Assemblies

Energy and electron transfer processes allow for efficient manipulation of excited states within light harvesting assemblies for photocatalytic and optoelectronic applications. We have now successfully probed the influence of acceptor pendant group functionalization on the energy and electron transfer between CsPbBr₃ perovskite nanocrystals and three rhodamine-based acceptor molecules. The three acceptors─rhodamine B (RhB), rhodamine isothiocyanate (RhB-NCS), and rose Bengal (RoseB)─contain an increasing degree of pendant group functionalization that affects their native excited state properties. When interacting with CsPbBr₃ as an energy donor, photoluminescence excitation spectroscopy reveals that singlet energy transfer occurs with all three acceptors. However, the acceptor functionalization directly influences several key parameters that dictate the excited state interactions. For example, RoseB binds to the nanocrystal surface with an apparent association constant (K_app = 9.4 × 10⁶ M^-1) 200 times greater than RhB (K_app = 0.05 × 10⁶ M^-1), thus influencing the rate of energy transfer. Femtosecond transient absorption reveals the observed rate constant of singlet energy transfer (k_EnT) is an order-of-magnitude greater for RoseB (k_EnT = 1 × 10¹¹ s^-1) than for RhB and RhB-NCS. In addition to energy transfer, each acceptor had a subpopulation of molecules (∼30%) that underwent electron transfer as a competing pathway. Thus, the structural influence of acceptor moieties must be considered for both excited state energy and electron transfer in nanocrystal-molecular hybrids. The competition between electron and energy transfer further highlights the complexity of excited state interactions in nanocrystal-molecular complexes and the need for careful spectroscopic analysis to elucidate competitive pathways.

Many-Body Correlations and Exciton Complexes in CsPbBr3 Quantum Dots

All-inorganic lead-halide perovskite (LHP) (CsPbX₃ , X = Cl, Br, I) quantum dots (QDs) have emerged as a competitive platform for classical light-emitting devices (in the weak light-matter interaction regime, e.g., LEDs and laser), as well as for devices exploiting strong light-matter interaction at room temperature. Many-body interactions and quantum correlations among photogenerated exciton complexes play an essential role, for example, by determining the laser threshold, the overall brightness of LEDs, and the single-photon purity in quantum light sources. Here, by combining cryogenic single-QD photoluminescence spectroscopy with configuration-interaction (CI) calculations, the size-dependent trion and biexciton binding energies are addressed. Trion binding energies increase from 7 to 17 meV for QD sizes decreasing from 30 to 9 nm, while the biexciton binding energies increase from 15 to 30 meV, respectively. CI calculations quantitatively corroborate the experimental results and suggest that the effective dielectric constant for biexcitons slightly deviates from the one of the single excitons, potentially as a result of coupling to the lattice in the multiexciton regime. The findings here provide a deep insight into the multiexciton properties in all-inorganic LHP QDs, essential for classical and quantum optoelectronic devices.

Lead Halide Perovskite Nanocrystals with Low Inhomogeneous Broadening and High Coherent Fraction through Dicationic Ligand Engineering

Lead halide perovskite nanocrystals (LHP NCs) are an emerging materials system with broad potential applications, including as emitters of quantum light. We apply design principles aimed at the structural optimization of surface ligand species for CsPbBr₃ NCs, leading us to the study of LHP NCs with dicationic quaternary ammonium bromide ligands. Through the selection of linking groups and aliphatic backbones guided by experiments and computational support, we demonstrate consistently narrow photoluminescence line shapes with a full-width-at-half-maximum below 70 meV. We observe bulk-like Stokes shifts throughout our range of particle sizes, from 7 to 16 nm. At cryogenic temperatures, we find sub-200 ps lifetimes, significant photon coherence, and the fraction of photons emitted into the coherent channel increasing markedly to 86%. A 4-fold reduction in inhomogeneous broadening from previous work paves the way for the integration of LHP NC emitters into nanophotonic architectures to enable advanced quantum optical investigation.

Strongly Confined CsPbBr3 Quantum Dots as Quantum Emitters and Building Blocks for Rhombic Superlattices

The success of the colloidal semiconductor quantum dots (QDs) field is rooted in the precise synthetic control of QD size, shape, and composition, enabling electronically well-defined functional nanomaterials that foster fundamental science and motivate diverse fields of applications. While the exploitation of the strong confinement regime has been driving commercial and scientific interest in InP or CdSe QDs, such a regime has still not been thoroughly explored and exploited for lead-halide perovskite QDs, mainly due to a so far insufficient chemical stability and size monodispersity of perovskite QDs smaller than about 7 nm. Here, we demonstrate chemically stable strongly confined 5 nm CsPbBr3 colloidal QDs via a postsynthetic treatment employing didodecyldimethylammonium bromide ligands. The achieved high size monodispersity (7.5% ± 2.0%) and shape-uniformity enables the self-assembly of QD superlattices with exceptional long-range order, uniform thickness, an unusual rhombic packing with an obtuse angle of 104°, and narrow-band cyan emission. The enhanced chemical stability indicates the promise of strongly confined perovskite QDs for solution-processed single-photon sources, with single QDs showcasing a high single-photon purity of 73% and minimal blinking (78% "on" fraction), both at room temperature.

Enhancement of Single-Photon Purity and Coherence of III-Nitride Quantum Dot with Polarization-Controlled Quasi-Resonant Excitation

III-Nitride semiconductor-based quantum dots (QDs) play an essential role in solid-state quantum light sources because of their potential for room-temperature operation. However, undesired background emission from the surroundings deteriorates single-photon purity. Moreover, spectral diffusion causes inhomogeneous broadening and limits the applications of QDs in quantum photonic technologies. To overcome these obstacles, it is demonstrated that directly pumping carriers to the excited state of the QD reduces the number of carriers generated in the vicinities. The polarization-controlled quasi-resonant excitation is applied to InGaN QDs embedded in GaN nanowire. To analyze the different excitation mechanisms, polarization-resolved absorptions are investigated under the above-barrier bandgap, below-barrier bandgap, and quasi-resonant excitation conditions. By employing polarization-controlled quasi-resonant excitation, the linewidth is reduced from 353 to 272 µeV, and the second-order correlation value is improved from 0.470 to 0.231. Therefore, a greater single-photon purity can be obtained at higher temperatures due to decreased linewidth and background emission.

Interplay of Purcell effect and extraction efficiency in CsPbBr3 quantum dots coupled to Mie resonators

Inorganic halide perovskite quantum dots have risen in recent years as efficient active materials in numerous optoelectronic applications ranging from solar cells to light-emitting diodes and lasers, and have lately been tested as quantum emitters. Perovskite quantum dots are often coupled to photonic structures either to enhance their emission properties, by accelerating their emission rate thanks to the Purcell effect, or to increase light extraction. From a theoretical point of view, the first effect is often considered at the single-dipole level while the latter is often treated at the mesoscopic level, except possibly for quantum emitters. In this work we employ a layer of perovskite quantum dots coupled to dielectric Mie resonators to exploit both effects simultaneously and achieve an 18-fold increase in luminescence. Our numerical simulations, combined with spatially- and time-resolved photoluminescence measurements, reveal how the macroscopic response of the perovskite-on-Mie resonator structure results from the interplay of the two effects averaged over the whole spatial distribution of emitters. Our work provides thus guiding principles for maximizing the output intensity of quantum emitters embedded into photonic resonators as well as classical emitters integrated in perovskite-based optoelectronic devices.

Universal scaling laws for charge-carrier interactions with quantum confinement in lead-halide perovskites

Lead halide perovskites open great prospects for optoelectronics and a wealth of potential applications in quantum optical and spin-based technologies. Precise knowledge of the fundamental optical and spin properties of charge-carrier complexes at the origin of their luminescence is crucial in view of the development of these applications. On nearly bulk Cesium-Lead-Bromide single perovskite nanocrystals, which are the test bench materials for next-generation devices as well as theoretical modeling, we perform low temperature magneto-optical spectroscopy to reveal their entire band-edge exciton fine structure and charge-complex binding energies. We demonstrate that the ground exciton state is dark and lays several millielectronvolts below the lowest bright exciton sublevels, which settles the debate on the bright-dark exciton level ordering in these materials. More importantly, combining these results with spectroscopic measurements on various perovskite nanocrystal compounds, we show evidence for universal scaling laws relating the exciton fine structure splitting, the trion and biexciton binding energies to the band-edge exciton energy in lead-halide perovskite nanostructures, regardless of their chemical composition. These scaling laws solely based on quantum confinement effects and dimensionless energies offer a general predictive picture for the interaction energies within charge-carrier complexes photo-generated in these emerging semiconductor nanostructures.

A room-temperature polarization-sensitive CMOS terahertz camera based on quantum-dot-enhanced terahertz-to-visible photon upconversion

Detection of terahertz (THz) radiation has many potential applications, but presently available detectors are limited in many aspects of their performance, including sensitivity, speed, bandwidth and operating temperature. Most do not allow the characterization of THz polarization states. Recent observation of THz-driven luminescence in quantum dots offers a possible detection mechanism via field-driven interdot charge transfer. We demonstrate a room-temperature complementary metal-oxide-semiconductor THz camera and polarimeter based on quantum-dot-enhanced THz-to-visible upconversion mechanism with optimized luminophore geometries and fabrication designs. Besides broadband and fast responses, the nanoslit-based sensor can detect THz pulses with peak fields as low as 10 kV cm^-1. A related coaxial nanoaperture-type device shows a to-date-unexplored capability to simultaneously record the THz polarization state and field strength with similar sensitivity.

Lattice distortion inducing exciton splitting and coherent quantum beating in CsPbI3 perovskite quantum dots

Anisotropic exchange splitting in semiconductor quantum dots results in bright-exciton fine-structure splitting important for quantum information processing. Direct measurement of fine-structure splitting usually requires single/few quantum dots at liquid-helium temperature because of its sensitivity to quantum dot size and shape, whereas measuring and controlling fine-structure splitting at an ensemble level seem to be impossible unless all the dots are made to be nearly identical. Here we report strong bright-exciton fine-structure splitting up to 1.6 meV in solution-processed CsPbI₃ perovskite quantum dots, manifested as quantum beats in ensemble-level transient absorption at liquid-nitrogen to room temperature. The splitting is robust to quantum dot size and shape heterogeneity, and increases with decreasing temperature, pointing towards a mechanism associated with orthorhombic distortion of the perovskite lattice. Effective-mass-approximation calculations reveal an intrinsic 'fine-structure gap' that agrees well with the observed fine-structure splitting. This gap stems from an avoided crossing of bright excitons confined in orthorhombically distorted quantum dots that are bounded by the pseudocubic {100} family of planes.

Layer-by-layer assembly of CsPbX3 nanocrystals into large-scale homostructures

Advances in surface chemistry of CsPbX₃ (where X = Cl, Br or I) nanocrystals (NCs) enabled the replacement of native chain ligands in solution. However, there are few reports on ligand exchange carried out on CsPbX₃ NC thin films. Solid-state ligand exchange can improve the photoluminescence quantum yield (PLQY) of the film and promote a change in solubility of the solid surface, thus enabling multiple depositions of subsequent nanocrystal layers. Fine control of nanocrystal film thickness is of importance for light-emitting diodes (LEDs), solar cells and lasers alike. The thickness of the emissive material film is crucial to assure the copious recombination of charges injected into a LED, resulting in bright electroluminescence. Similarly, solar cell performance is determined by the amount of absorbed light, and hence the light absorber content in the device. In this study, we demonstrate a layer-by-layer (LbL) assembly method that results in high quality films, whose thicknesses can be finely controlled. In the solid state, we replaced oleic acid and oleylamine ligands with didodecyldimethylammonium bromide or ammonium thiocyanate that enhance the PLQY of the film. The exchange is carried out through a spin-coating technique, using solvents with strategic polarity to avoid NC dissolution or damage. Exploiting this technique, the deposition of various layers results in considerable thickening of films as proven by atomic force microscope measurements. The ease of handling of our combined process (i.e. ligand exchange and layer-by-layer deposition) enables thickness control over CsPbX₃ NC films with applicability to other perovskite nanomaterials paving the way for a large variety of layer permutations.

One-Dimensional Highly-Confined CsPbBr3 Nanorods with Enhanced Stability: Synthesis and Spectroscopy

One-dimensional (1D) colloidal lead halide perovskites (LHPs) have potential as quantum emitters. Their study, however, has been hampered by their previous instability, leaving a gap in our understanding of structure-property relationships in colloidal LHPs with anisotropic shapes. Here, we synthesize stable, highly-confined 1D CsPbBr₃ nanorods (NRs) and demonstrate their structural details and photoluminescence (PL) properties at both the ensemble and single particle levels. Using amino-terminated copolymers, we are able to stabilize and characterize 1D CsPbBr₃ NRs utilizing transmission electron microscopy (TEM) and small angle scattering (SAS). Scanning transmission electron microscopy reveals that these NRs possess structural defects, including twists and inhomogeneity. Solution-phase photon correlation spectroscopy shows low biexciton-to-exciton quantum yield ratios (QY_BX/QY_X) and broad spectral line widths dominated by homogeneous broadening.

Spheroidal Cesium Lead Chloride-Bromide Quantum Dots and a Fast Determination of Their Size and Halide Content

Lead halide perovskite (LHP) quantum dots (QDs), with their bright and narrow emission, are promising candidates for LEDs, lasers, and quantum light sources. However, current methods to synthesize monodisperse CsPb(Cl:Br)₃ and CsPbCl₃ QDs exhibiting multiple sharp absorption resonances are not as well developed compared to CsPbBr₃. Furthermore, both quantum confinement and the halide ratio in CsPb(Cl:Br)₃ QDs strongly influence the bandgap, making it impossible to optically determine their size. In this work, monodisperse spheroidal CsPb(Cl:Br)₃ QDs are synthesized in the 4-10 nm range, at any Cl:Br ratio, with up to five excitonic absorption transitions. Furthermore, in situ spectroscopy was used to cross-correlate the size and composition of these QDs directly to the energy of the first two excitonic absorption transitions. This work therefore provides not only a method for monodisperse CsPb(Cl:Br)₃ QDs but also a protocol to determine their size, concentration, and halide ratio, circumventing conventional expensive and time-consuming techniques.

A Fano Cavity-Photon Interface for Directional Suppression of Spectral Diffusion of a Single Perovskite Nanoplatelet

Colloidal nanocrystals that are capable of mass production with wet chemical synthesis have long been proposed as color-tunable, scalable quantum emitters for information processing and communication. However, they constantly suffer from spectral diffusion due to being exposed to a noisy electrostatic environment. Herein we demonstrate a cavity-photon interface (CPI) which effectively suppresses the temperature-activated spectral diffusion (SD) of a single perovskite nanoplatelet (NPL) up to 40 K. The spectrally stabilized single-photon emission is achieved at a specific emission direction corresponding to an inhibited dipole moment of the NPL as the result of the Fano coupling between the two photon dissipation channels of the NPL. Our results shed light on the nature of the SD of perovskite nanocrystals and offer a general cavity quantum electrodynamic scheme that controls the brightness and spectral dynamics of a single-photon emitter.

Superradiance and Exciton Delocalization in Perovskite Quantum Dot Superlattices

Achieving superradiance in solids is challenging due to fast dephasing processes from inherent disorder and thermal fluctuations. Perovskite quantum dots (QDs) are an exciting class of exciton emitters with large oscillator strength and high quantum efficiency, making them promising for solid-state superradiance. However, a thorough understanding of the competition between coherence and dephasing from phonon scattering and energetic disorder is currently unavailable. Here, we present an investigation of exciton coherence in perovskite QD solids using temperature-dependent photoluminescence line width and lifetime measurements. Our results demonstrate that excitons are coherently delocalized over 3 QDs at 11 K in superlattices leading to superradiant emission. Scattering from optical phonons leads to the loss of coherence and exciton localization to a single QD at temperatures above 100 K. At low temperatures, static disorder and defects limit exciton coherence. These results highlight the promise and challenge in achieving coherence in perovskite QD solids.

Biexciton dynamics in halide perovskite nanocrystals

Lead halide perovskite nanocrystals are attracting considerable interest as next-generation optoelectronic materials. Optical responses of nanocrystals are determined by excitons and exciton complexes such as trions and biexcitons. Understanding of their dynamics is indispensable for the optimal design of optoelectronic devices and the development of new functional properties. Here, we summarize the recent advances on the exciton and biexciton photophysics in lead halide perovskite nanocrystals revealed by femtosecond time-resolved spectroscopy and single-dot spectroscopy. We discuss the impact of the biexciton dynamics on controlling and improving the optical gain.

Exciton-Phonon and Trion-Phonon Couplings Revealed by Photoluminescence Spectroscopy of Single CsPbBr3 Perovskite Nanocrystals

Lead halide perovskite nanocrystals (NCs) have outstanding photoluminescence (PL) properties and excellent potential for light-emitting diodes and single-photon sources. Here, we report the multiple-peak structures originating from excitons, trions, and biexcitons in low-temperature PL spectra of single CsPbBr₃ NCs. We found fine-structure splitting in the PL peaks of bright excitons and biexcitons and also in the longitudinal-optical (LO)-phonon replicas of excitons. LO-phonon replicas of trions are clearly observed under strong photoexcitation, which do not show fine-structure splitting. From size-dependent analyses of these replicas, we clarified that both exciton-phonon and trion-phonon couplings become larger for smaller NCs and the coupling strengths of trions are larger than those of excitons in large NCs. These behaviors can be explained by the spatial distributions of the electron and hole wave functions in the NCs. Our findings provide essential information on electron-phonon couplings in perovskites and for the design of high-purity single-photon sources.

Thickness-Dependent Dark-Bright Exciton Splitting and Phonon Bottleneck in CsPbBr3-Based Nanoplatelets Revealed via Magneto-Optical Spectroscopy

The optimized exploitation of perovskite nanocrystals and nanoplatelets as highly efficient light sources requires a detailed understanding of the energy spacing within the exciton manifold. Dark exciton states are particularly relevant because they represent a channel that reduces radiative efficiency. Here, we apply large in-plane magnetic fields to brighten optically inactive states of CsPbBr3-based nanoplatelets for the first time. This approach allows us to access the dark states and directly determine the dark-bright splitting, which reaches 22 meV for the thinnest nanoplatelets. The splitting is significantly less for thicker nanoplatelets due to reduced exciton confinement. Additionally, the form of the magneto-PL spectrum suggests that dark and bright state populations are nonthermalized, which is indicative of a phonon bottleneck in the exciton relaxation process.

Controlling the nucleation and growth kinetics of lead halide perovskite quantum dots

Colloidal lead halide perovskite (LHP) nanocrystals are of interest as photoluminescent quantum dots (QDs) whose properties depend on the size and shape. They are normally synthesized on subsecond time scales through hard-to-control ionic metathesis reactions. We report a room-temperature synthesis of monodisperse, isolable spheroidal APbBr₃ QDs (A=Cs, formamidinium, methylammonium) that are size-tunable from 3 to over 13 nanometers. The kinetics of both nucleation and growth are temporally separated and drastically slowed down by the intricate equilibrium between the precursor (PbBr₂) and the A[PbBr₃] solute, with the latter serving as a monomer. QDs of all these compositions exhibit up to four excitonic transitions in their linear absorption spectra, and we demonstrate that the size-dependent confinement energy for all transitions is independent of the A-site cation.

Synthesis of Weakly Confined, Cube-Shaped, and Monodisperse Cadmium Chalcogenide Nanocrystals with Unexpected Photophysical Properties

Zinc-blende CdSe, CdS, and CdSe/CdS core/shell nanocrystals with a structure-matched shape (cube-shaped, edge length ≤30 nm) are synthesized via a universal scheme. With the edge length up to five times larger than exciton diameter of the bulk semiconductors, the nanocrystals exhibit novel properties in the weakly confined size regime, such as near-unity single exciton and biexciton photoluminescence (PL) quantum yields, single-nanocrystal PL nonblinking, mixed PL decay dynamics of exciton and free carriers with sub-microsecond monoexponential decay lifetime, and stable yet extremely narrow PL full width at half maximum (FWHM < 0.1 meV) at 1.8 K. Their monodisperse edge length, shape, and facet structure enable demonstration of unexpected yet size-dependent PL properties at room temperature, including unusually broad and abnormally size-dependent PL FWHM (∼100 meV), nonmonotonic size dependence of PL peak energy, and dual-peak single-exciton PL. Calculations suggest that these unusual properties should be originated from the band-edge electron/hole states of the dynamic-exciton, whose exciton binding energy is too small to hold the photogenerated electron-hole pair as a bonded Wannier exciton in a weakly confined nanocrystal.

Luminescence Enhancement Due to Symmetry Breaking in Doped Halide Perovskite Nanocrystals

Metal-halide perovskite nanocrystals have demonstrated excellent optoelectronic properties for light-emitting applications. Isovalent doping with various metals (M2+) can be used to tailor and enhance their light emission. Although crucial to maximize performance, an understanding of the universal working mechanism for such doping is still missing. Here, we directly compare the optical properties of nanocrystals containing the most commonly employed dopants, fabricated under identical synthesis conditions. We show for the first time unambiguously, and supported by first-principles calculations and molecular orbital theory, that element-unspecific symmetry-breaking rather than element-specific electronic effects dominate these properties under device-relevant conditions. The impact of most dopants on the perovskite electronic structure is predominantly based on local lattice periodicity breaking and resulting charge carrier localization, leading to enhanced radiative recombination, while dopant-specific hybridization effects play a secondary role. Our results suggest specific guidelines for selecting a dopant to maximize the performance of perovskite emitters in the desired optoelectronic devices.

First-Principles Nonequilibrium Green's Function Approach to Energy Conversion in Nanoscale Optoelectronics

Understanding photon-electron conversion on the nanoscale is essential for future innovations in nano-optoelectronics. In this article, based on nonequilibrium Green's function (NEGF) formalism, we develop a quantum-mechanical method for modeling energy conversion in nanoscale optoelectronic devices. The method allows us to study photoinduced charge transport and electroluminescence processes in realistic devices. First, we investigate the electroluminescence properties of a two-level model with two different treatments of inelastic scatterings. We show the regime where self-consistency between electron and photon is important for correct description of the inelastic scatterings. The method is then applied to model single-molecule junctions based on the density-functional tight-binding approach. The predicted emission spectra are found to be in very good agreement with experimental measurements. For nanostructured materials, the method is further applied to study the photoresponse of a two-dimensional graphene/graphite-C₃N₄ heterojunction photovoltaic device. The simulations demonstrate clearly the impact of atomistic details on the optoelectronic properties of nanodevices. This work provides a practical theoretical framework that can be applied to model and design realistic nanodevices.

Nanoscale 3D spatial addressing and valence control of quantum dots using wireframe DNA origami

Control over the copy number and nanoscale positioning of quantum dots (QDs) is critical to their application to functional nanomaterials design. However, the multiple non-specific binding sites intrinsic to the surface of QDs have prevented their fabrication into multi-QD assemblies with programmed spatial positions. To overcome this challenge, we developed a general synthetic framework to selectively attach spatially addressable QDs on 3D wireframe DNA origami scaffolds using interfacial control of the QD surface. Using optical spectroscopy and molecular dynamics simulation, we investigated the fabrication of monovalent QDs of different sizes using chimeric single-stranded DNA to control QD surface chemistry. By understanding the relationship between chimeric single-stranded DNA length and QD size, we integrated single QDs into wireframe DNA origami objects and visualized the resulting QD-DNA assemblies using electron microscopy. Using these advances, we demonstrated the ability to program arbitrary 3D spatial relationships between QDs and dyes on DNA origami objects by fabricating energy-transfer circuits and colloidal molecules. Our design and fabrication approach enables the geometric control and spatial addressing of QDs together with the integration of other materials including dyes to fabricate hybrid materials for functional nanoscale photonic devices.

Direct Patterning of Perovskite Nanocrystals on Nanophotonic Cavities with Electrohydrodynamic Inkjet Printing

Overcoming the challenges of patterning luminescent materials will unlock additive and more sustainable paths for the manufacturing of next-generation on-chip photonic devices. Electrohydrodynamic (EHD) inkjet printing is a promising method for deterministically placing emitters on these photonic devices. However, the use of this technique to pattern luminescent lead halide perovskite nanocrystals (NCs), notable for their defect tolerance and impressive optical and spin coherence properties, for integration with optoelectronic devices remains unexplored. In this work, we additively deposit nanoscale CsPbBr₃ NC features on photonic structures via EHD inkjet printing. We perform transmission electron microscopy of EHD inkjet printed NCs to demonstrate that the NCs' structural integrity is maintained throughout the printing process. Finally, NCs are deposited with sub-micrometer control on an array of parallel silicon nitride nanophotonic cavities and demonstrate cavity-emitter coupling via photoluminescence spectroscopy. These results demonstrate EHD inkjet printing as a scalable, precise method to pattern luminescent nanomaterials for photonic applications.

Lecithin Capping Ligands Enable Ultrastable Perovskite-Phase CsPbI3 Quantum Dots for Rec. 2020 Bright-Red Light-Emitting Diodes

Bright-red light-emitting diodes (LEDs) with a narrow emission line width that emit between 620 and 635 nm are needed to meet the latest industry color standard for wide color gamut displays, Rec. 2020. CsPbI₃ perovskite quantum dots (QDs) are one of the few known materials that are ideally suited to meet these criteria. Unfortunately, CsPbI₃ perovskite QDs are prone to transform into a non-red-emitting phase and are subject to further degradation mechanisms when their luminescence wavelength is tuned to match that of the Rec. 2020 standard. Here, we show that zwitterionic lecithin ligands can stabilize the perovskite phase of CsPbI₃ QDs for long periods in air for at least 6 months compared to a few days for control samples. LEDs fabricated with our ultrastable lecithin-capped CsPbI₃ QDs exhibit an external quantum efficiency (EQE) of 7.1% for electroluminescence centered at 634 nm─a record for all-inorganic perovskite nanocrystals in Rec. 2020 red. Our devices achieve a maximum luminance of 1391 cd/m² at 7.5 V, and their operational half-life is 33 min (T₅₀) at 200 cd/m²─a 10-fold enhancement compared to control samples. Density functional theory results suggest that the surface strain in CsPbI₃ QDs capped with the conventional ligands, oleic acid and oleylamine, contributes to the instability of the perovskite structural phase. On the other hand, lecithin binding induces virtually no surface strain and shows a stronger binding tendency for the CsPbI₃ surface. Our study highlights the tremendous potential of zwitterionic ligands in stabilizing the perovskite phase and particle size of CsPbI₃ QDs for various optoelectronic applications.

Simulations of nonradiative processes in semiconductor nanocrystals

The description of carrier dynamics in spatially confined semiconductor nanocrystals (NCs), which have enhanced electron-hole and exciton-phonon interactions, is a great challenge for modern computational science. These NCs typically contain thousands of atoms and tens of thousands of valence electrons with discrete spectra at low excitation energies, similar to atoms and molecules, that converge to the continuum bulk limit at higher energies. Computational methods developed for molecules are limited to very small nanoclusters, and methods for bulk systems with periodic boundary conditions are not suitable due to the lack of translational symmetry in NCs. This perspective focuses on our recent efforts in developing a unified atomistic model based on the semiempirical pseudopotential approach, which is parameterized by first-principle calculations and validated against experimental measurements, to describe two of the main nonradiative relaxation processes of quantum confined excitons: exciton cooling and Auger recombination. We focus on the description of both electron-hole and exciton-phonon interactions in our approach and discuss the role of size, shape, and interfacing on the electronic properties and dynamics for II-VI and III-V semiconductor NCs.

Coupling Perovskite Quantum Dot Pairs in Solution using a Nanoplasmonic Assembly

Perovskite quantum dots (PQDs) provide a robust solution-based approach to efficient solar cells, bright light emitting devices, and quantum sources of light. Quantifying heterogeneity and understanding coupling between dots is critical for these applications. We use double-nanohole optical trapping to size individual dots and correlate to emission energy shifts from quantum confinement. We were able to assemble a second dot in the trap, which allows us to observe the coupling between dots. We observe a systematic red-shift of 1.1 ± 0.6 meV in the emission wavelength. Theoretical analysis shows that the observed shift is consistent with resonant energy transfer and is unusually large due to moderate-to-large quantum confinement in PQDs. This demonstrates the promise of PQDs for entanglement in quantum information applications. This work enables future in situ control of PQD growth as well as studies of the coupling between small PQD assemblies with quantum information applications in mind.

Controlling Intrinsic Quantum Confinement in Formamidinium Lead Triiodide Perovskite through Cs Substitution

Lead halide perovskites are leading candidates for photovoltaic and light-emitting devices, owing to their excellent and widely tunable optoelectronic properties. Nanostructure control has been central to their development, allowing for improvements in efficiency and stability, and changes in electronic dimensionality. Recently, formamidinium lead triiodide (FAPbI₃) has been shown to exhibit intrinsic quantum confinement effects in nominally bulk thin films, apparent through above-bandgap absorption peaks. Here, we show that such nanoscale electronic effects can be controlled through partial replacement of the FA cation with Cs. We find that Cs-cation exchange causes a weakening of quantum confinement in the perovskite, arising from changes in the bandstructure, the length scale of confinement, or the presence of δ_H-phase electronic barriers. We further observe photon emission from quantum-confined regions, highlighting their potential usefulness to light-emitting devices and single-photon sources. Overall, controlling this intriguing quantum phenomenon will allow for its suppression or enhancement according to need.

Halide Perovskite Solar Cells for Building Integrated Photovoltaics: Transforming Building Façades into Power Generators

The rapid emergence of organic-inorganic lead halide perovskites for low-cost and high-efficiency photovoltaics promises to impact new photovoltaic concepts. Their high power conversion efficiencies, ability to coat perovskite layers on glass via various scalable deposition techniques, excellent optoelectronic properties, and synthetic versatility for modulating transparency and color allow perovskite solar cells (PSCs) to be an ideal solution for building-integrated photovoltaics (BIPVs), which transforms windows or façades into electric power generators. In this review, the unique features and properties of PSCs for BIPV application are accessed. Device engineering and optical management strategies of active layers, interlayers, and electrodes for semitransparent, bifacial, and colorful PSCs are also discussed. The performance of PSCs under conditions that are relevant for BIPV such as different operational temperature, light intensity, and light incident angle are also reviewed. Recent outdoor stability testing of PSCs in different countries and other demonstration of scalability and deployment of PSCs are also spotlighted. Finally, the current challenges and future opportunities for realizing perovskite-based BIPV are discussed.

High-Efficiency Fast-Radiative Blue-Emitting Perovskite Nanoplatelets and Their Formation Mechanisms

High-efficiency blue perovskite emitters with fast fluorescence radiation are not only crucial to achieving high-quality displays but also highly desired for optical wireless communications and quantum information technologies. Here, we demonstrate the preparation of blue-emitting Eu³⁺-, Sb³⁺-, and Ba²⁺-induced CsPbBr₃ nanoplatelets with narrow spectral widths. Among them, Sb³⁺-doped CsPbBr₃ NPLs can reach a photoluminescence quantum yield of 95%, with a very short fluorescence lifetime of 1.48 ns and greatly reduced ligand dosage. Through nuclear magnetic resonance analysis and density functional theory calculations, we find that the dopant-ligand interaction and dopant-induced growth energy barrier decide the growth kinetics of doped nanoplatelets. These mechanisms offer a fresh route to controlling the dimension of nanoscale perovskite emitters and benefit the development of fast-radiative perovskite emitters.

Structural Diversity in Multicomponent Nanocrystal Superlattices Comprising Lead Halide Perovskite Nanocubes

Nanocrystal (NC) self-assembly is a versatile platform for materials engineering at the mesoscale. The NC shape anisotropy leads to structures not observed with spherical NCs. This work presents a broad structural diversity in multicomponent, long-range ordered superlattices (SLs) comprising highly luminescent cubic CsPbBr₃ NCs (and FAPbBr₃ NCs) coassembled with the spherical, truncated cuboid, and disk-shaped NC building blocks. CsPbBr₃ nanocubes combined with Fe₃O₄ or NaGdF₄ spheres and truncated cuboid PbS NCs form binary SLs of six structure types with high packing density; namely, AB₂, quasi-ternary ABO₃, and ABO₆ types as well as previously known NaCl, AlB₂, and CuAu types. In these structures, nanocubes preserve orientational coherence. Combining nanocubes with large and thick NaGdF₄ nanodisks results in the orthorhombic SL resembling CaC₂ structure with pairs of CsPbBr₃ NCs on one lattice site. Also, we implement two substrate-free methods of SL formation. Oil-in-oil templated assembly results in the formation of binary supraparticles. Self-assembly at the liquid-air interface from the drying solution cast over the glyceryl triacetate as subphase yields extended thin films of SLs. Collective electronic states arise at low temperatures from the dense, periodic packing of NCs, observed as sharp red-shifted bands at 6 K in the photoluminescence and absorption spectra and persisting up to 200 K.