Transformation of Sintered CsPbBr3 Nanocrystals to Cubic CsPbI3 and Gradient CsPbBrxI3–x through Halide Exchange

Halide Ion Migration in Perovskite Nanocrystals and Nanostructures

ConspectusThe optical and electronic properties of metal halide perovskites provide insight into the operation of solar cells as well as their long-term operational stability. Halide mobility in perovskite films is an important factor influencing solar cell performance. One can visualize halide ion migration through halide exchange between two nanocrystal suspensions or between physically paired films of two different metal halide perovskites. The ability to tune band gap by varying halide ratios (Cl:Br or Br:I) allows the synthesis of mixed halide perovskites with tailored absorption and emission across the entire visible spectrum. Interestingly, mixed halide (e.g., MAPb(Br_0.5I_0.5)₃) films undergo phase segregation to form Br-rich and I-rich sites under steady state illumination. Upon halting illumination, segregated phases mix to restore original mixed halide compositions. Introducing multiple cations (Cs, formamidinium) at the A site or alloying with Cl greatly suppresses halide mobilities. Long-term irradiation of MAPb(Br_0.5I_0.5)₃ films also cause expulsion of iodide leaving behind Br-rich phases. Hole trapping at I-rich sites in MAPb(Br_0.5I_0.5)₃ is considered to be an important step in inducing halide mobility in photoirradiated films. This Account focuses on halide ion migration in nanocrystals and nanostructured films driven by entropy of mixing in dark and phase segregation under light irradiation.

Preserving a robust CsPbI3 perovskite phase via pressure-directed octahedral tilt

Functional CsPbI3 perovskite phases are not stable at ambient conditions and spontaneously convert to a non-perovskite δ phase, limiting their applications as solar cell materials. We demonstrate the preservation of a black CsPbI3 perovskite structure to room temperature by subjecting the δ phase to pressures of 0.1 - 0.6 GPa followed by heating and rapid cooling. Synchrotron X-ray diffraction and Raman spectroscopy indicate that this perovskite phase is consistent with orthorhombic γ-CsPbI3. Once formed, γ-CsPbI3 could be then retained after releasing pressure to ambient conditions and shows substantial stability at 35% relative humidity. First-principles density functional theory calculations indicate that compression directs the out-of-phase and in-phase tilt between the [PbI6]4- octahedra which in turn tune the energy difference between δ- and γ-CsPbI3, leading to the preservation of γ-CsPbI3. Here, we present a high-pressure strategy for manipulating the (meta)stability of halide perovskites for the synthesis of desirable phases with enhanced materials functionality.

Role of surface termination and quantum size in α-CsPbX3 (X = Cl, Br, I) 2D nanostructures for solar light harvesting

Quantum confinement of CsPbBr₃ nanoplatelets.

Photoinduced ion-redistribution in CH3NH3PbI3 perovskite solar cells

We use photoinduced absorption spectroscopy (PAS) to study the ionic motion in CH₃NH₃PbI₃ perovskite solar cells, consisting of indium tin oxide (ITO)/NiOx/perovskite/phenyl-C₆₁-butyric-acid-methyl ester (PCBM)/aluminum-doped zinc oxide (AZO)/ITO. We observed a slow (∼50 mHz) spectral blue shift (∼10^-4 eV) under modulated 520 nm illumination, which we interpreted in terms of the modulation in the bulk ion density. Numerical simulation shows that the mobile ion moves in and out from the double layers at the perovskite/charge transport layer interfaces in order to recover the bulk charge neutrality tipped off-balance by the photocarriers. The diffusion coefficient of the ion is 10^-10 to 10^-11 cm² s^-1, when we assume that the characteristic time constant of the ion motion is governed by the diffusion.

Half-filled intermediate bands in doped inorganic perovskites for solar cells

Efficiency of solar cells can be improved by introducing intermediate bands. However, width and occupation requirements on the intermediate bands bring challenges for materials design. In this study, we systematically investigate the electronic structure of doped CsPbX₃ (X = Cl, Br, or I). A screening in transition metals suggests that Cr and Mo doped perovskites have dispersive and half-filled intermediate bands, which is desirable for building solar cells. However, intermediate bands originated from degenerated d orbitals are easily split when the octahedral symmetry of the local chemical environment around the dopant is broken. To avoid this problem, we further perform a screening of non-transition metals aiming to generate sp orbital-based intermediate bands. It turns out that In and Ga can generate a half-filled intermediate band, which is not sensitive to the local symmetry. The estimated efficiency of In and Ga doped CsPbCl₃ at a doping concentration of 8.3% is 57.97% and 54.21%, notably higher than the Shockley-Queisser limit (40.7%). Results presented here demonstrate the potential of intermediate band engineering in photovoltaic applications.

Composition-dependent chemical and structural stabilities of mixed tin-lead inorganic halide perovskites

Alloying tin into lead-based halide perovskites is one of the strategies to reduce the chemical toxicities associated with lead-containing compounds, while retaining comparable physical properties. However, tin-based compounds possess their own shortcomings, with the most critical ones being their increased thermodynamic tendencies towards oxidative degradation, as well as vibrational anharmonicities due to the presence of shallow Sn-5s2 lone-pair electrons. Hereby, we performed density-functional-theory calculations to systematically examine the composition-dependent chemical and structural stabilities for Cs(PbxSn1-x)X3 (X = Cl, Br and I) alloys. We found that oxidative degradation to rhombohedral Cs2SnX6, SnO2 and cubic CsSnX3 tends to be the most favored pathway with no observable composition-dependent 'bowing behaviour', the latter is primarily governed by the bowing-effects in the demixing energies which are generated when the perovskite alloy phase-segregates into the two cubic end-members, which are two orders of magnitude smaller. Potential surface energy scans for the off-center B-site ion displacements further reveal the nonlinearity in the change of vibrational anharmonicity with respect to a linear change of Sn concentrations. Such nonlinearity is strongly modulated by the nature of the halide ions, in order to minimize the exchange repulsion between the charge densities of Sn-5s2 lone pairs and the octahedrally coordinating halogen anions.

Spectrum projection with a bandgap-gradient perovskite cell for colour perception

Optoelectronic devices for light or spectral signal detection are desired for use in a wide range of applications, including sensing, imaging, optical communications, and in situ characterization. However, existing photodetectors indicate only light intensities, whereas multiphotosensor spectrometers require at least a chip-level assembly and can generate redundant signals for applications that do not need detailed spectral information. Inspired by human visual and psychological light perceptions, the compression of spectral information into representative intensities and colours may simplify spectrum processing at the device level. Here, we propose a concept of spectrum projection using a bandgap-gradient semiconductor cell for intensity and colour perception. Bandgap-gradient perovskites, prepared by a halide-exchanging method via dipping in a solution, are developed as the photoactive layer of the cell. The fabricated cell produces two output signals: one shows linear responses to both photon energy and flux, while the other depends on only photon flux. Thus, by combining the two signals, the single device can project the monochromatic and broadband spectra into the total photon fluxes and average photon energies (i.e., intensities and hues), which are in good agreement with those obtained from a commercial photodetector and spectrometer. Under changing illumination in real time, the prepared device can instantaneously provide intensity and hue results. In addition, the flexibility and chemical/bio-sensing of the device via colour comparison are demonstrated. Therefore, this work shows a human visual-like method of spectrum projection and colour perception based on a single device, providing a paradigm for high-efficiency spectrum-processing applications.

Enhanced stability in CH3NH3PbI3 hybrid perovskite from mechano-chemical synthesis: structural, microstructural and optoelectronic characterization

Among the hybrid organic-inorganic perovskites MAPbX₃ (MA: methyl-ammonium CH₃-NH₃⁺, X = halogen), the triiodide specimen (MAPbI₃) is still the material of choice for solar energy applications. Although it is able to absorb light above its 1.6 eV bandgap, its poor stability in humid air atmosphere has been a major drawback for its use in solar cells. However, we discovered that this perovskite can be prepared by ball milling in a straightforward way, yielding specimens with a superior stability. This fact allowed us to take atomic-resolution STEM images for the first time, with sufficient quality to unveil microscopic aspects of this material. We demonstrated full Iodine content, which might be related to the enhanced stability, in a more compact PbI₆ framework with reduced unit-cell volume. A structural investigation from neutron powder diffraction (NPD) data of an undeuterated specimen was essential to determine the configuration of the organic MA unit in the 100-298 K temperature range. A phase transition is identified, from the tetragonal structure observed at RT (space group I4/mcm) to an orthorhombic (space group Pnma) phase where the methyl-ammonium organic units are fully localized. Our NPD data reveal that the MA changes are gradual and start before reaching the phase transition. Optoelectronic measurements yield a photocurrent peak at an illumination wavelength of 820 nm, which is redshifted by 30 nm with respect to previously reported measurements on MAPbI₃ perovskites synthesized by crystallization from organic solvents.

Improving the Quality and Luminescence Performance of All-Inorganic Perovskite Nanomaterials for Light-Emitting Devices by Surface Engineering

Lead halide perovskites and their applications in the optoelectronic field have garnered intensive interest over the years. Inorganic perovskites (IHP), though a novel class of material, are considered as one of the most promising optoelectronic materials. These materials are widely used in detectors, solar cells, and other devices, owing to their excellent charge-transport properties, high defect tolerance, composition- and size-dependent luminescence, narrow emission, and high photoluminescence quantum yield. In recent years, numerous encouraging achievements have been realized, especially in the research of CsPbX₃ (X = Cl, Br, I) nanocrystals (NCs) and surface engineering. Therefore, it is necessary to summarize the principles and effects of these surface engineering optimization methods. It is also important to scientifically guide the applications and promote the development of perovskites more efficiently. Herein, the principles of surface ligands are reviewed, and various surface treatment methods used in CsPbX₃ NCs as well as quantum-dot light-emitting diodes are presented. Finally, a brief outlook on CsPbX₃ NC surface engineering is offered, illustrating the present challenges and the direction in which future investigations are intended to obtain high-quality CsPbX₃ NCs that can be utilized in more applications.

Cation-doping matters in caesium lead halide perovskite nanocrystals: from physicochemical fundamentals to optoelectronic applications

All-inorganic caesium lead halide perovskite nanocrystals (PeNCs) with different dimensionalities have recently fascinated the research community due to their extraordinary optoelectronic properties including tunable bandgaps over the entire visible spectral region, high photoluminescence quantum yields (PLQYs) close to unity and narrow emission line widths down to 10-20 nm, making them particularly suitable as promising candidates for numerous applications ranging from light-emitting diodes (LEDs), solar cells to scintillators. Despite the considerable progress made in the past six years, the real-world applications of caesium lead halide PeNCs themselves especially in the category of CsPbX₃ (X = Cl, Br and I) are still restricted by their labile crystal lattices and downgraded luminescence when exposed to ambient air conditions. Recent experimental and theoretical studies on cation doping have proven to be an effective way to significantly improve the physicochemical properties of cesium lead halide PeNCs, which would have profound implications for a range of applications. In this review, we provide a brief overview of the most recent advances in cation-doped all-inorganic caesium lead halide PeNCs, aimed at developing high-performance and long-term stable optoelectronic and photovoltaic devices, which covers areas from their fundamental considerations of cation doping, controlled synthesis methodology and novel physicochemical properties to the optoelectronic applications with an emphasis on perovskite-based LEDs and solar cells. And finally, some possible directions of future efforts toward this active research field are also proposed.

Evidence of Large Polarons in Photoemission Band Mapping of the Perovskite Semiconductor CsPbBr_{3}

Lead-halide perovskite (LHP) semiconductors are emergent optoelectronic materials with outstanding transport properties which are not yet fully understood. We find signatures of large polaron formation in the electronic structure of the inorganic LHP CsPbBr_{3} by means of angle-resolved photoelectron spectroscopy. The experimental valence band dispersion shows a hole effective mass of 0.26±0.02 m_{e}, 50% heavier than the bare mass m_{0}=0.17 m_{e} predicted by density functional theory. Calculations of the electron-phonon coupling indicate that phonon dressing of the carriers mainly occurs via distortions of the Pb-Br bond with a Fröhlich coupling parameter α=1.81. A good agreement with our experimental data is obtained within the Feynman polaron model, validating a viable theoretical method to predict the carrier effective mass of LHPs ab initio.

Generation of low-symmetry perovskite structures forab initiocomputation

Ion displacements are the fundamental cause of ferroelectricity in perovskites. By properly shifting ions,ab initiocomputations have been extensively used to investigate the properties of perovskites in various structural phases. In addition to the relatively simple ion displacements, perovskites have another type of structural distortion known as antiferrodistortion or oxygen octahedron tilting. The interplay between these two types of distortions have generated abundant structural phases that can be tedious to prepare forab initiocomputation, especially for large supercells. Here, we design and implement a computer program to facilitate the generation of distorted perovskite structures, which can be readily used forab initiocomputation to gain further insight into the perovskite of a given structural phase.

Density functional theory study of mixed halide influence on structures and optoelectronic attributes of CsPb(I/Br)3

This paper reports on the influence of the bromine (Br) atoms substitution on the structures and optoelectronic traits of ${\text{CsPbI}_3}$CsPbI₃, wherein the density functional theory (DFT) simulation was performed, using all electrons full potential linearized augmented plane-wave method. Furthermore, the generalized gradient approximation, local density approximation, and modified Becke-Johnson exchange-correlation potential were used to improve the optimization and band structure calculations. The calculated lattice constants of ${\text{CsPbI}_3}$CsPbI₃ and ${\text{CsPbBr}_3}$CsPbBr₃ were consistent with the experimental values. All the studied compounds revealed wide and direct bandgap energies at the R-symmetry point, which varied from 1.74-2.23 eV. The obtained refractive indices of the ${\text{CsPbI}_3}$CsPbI₃, ${\text{CsPbBrI}_2}$CsPbBrI₂, ${\text{CsPbIBr}_2}$CsPbIBr₂, and ${\text{CsPbBr}_3}$CsPbBr₃ compounds were correspondingly 2.265, 2.245, 2.090, and 2.086. Present findings may contribute towards the development of experimental studies on the proposed compounds with controlled properties useful for the solar cells.

Facets Directed Connecting Perovskite Nanocrystals

Connecting nanocrystals with removal of interface ligand barriers is one of the key steps for efficient carrier transportation in optoelectronic device fabrication. Typically, ion migration for crystal deformation or connection with other nanocrystals needs a solvent as medium. However, on the contrary, this has been observed for CsPbBr₃ perovskite nanocrystals in film where nanocrystals were swollen to get wider and fused with adjacent nanocrystals in self-assembly on film during solvent evaporation. Depending on precursor composition and exposed facets, again these connections could be programmed for tuning their connecting directions leading to different shapes. Aging further on solid substrate, these were also turned to continuous film of nanostructures eliminating all interparticle gaps on the film. This transformation could be ceased at any point of time, simply by heating or adding sufficient ligands. Analysis suggested that these unique and controlled connections were only observed with polyhedron shaped nanostructures with certain compositions and not with traditionally cubes. Details of this solid-surface transformation during solvent evaporation were analyzed, and an interparticle material transfer type mechanism was proposed. As these observations were not seen in chalcogenide and oxide nanocrystals and exclusively observed in perovskite nanocrystals, this would add new fundamentals to the insights of crystal growths of nanocrystals and would also help in obtaining films of connecting nanocrystals.

Ferroelastic domains drive charge separation and suppress electron-hole recombination in all-inorganic halide perovskites: time-domain ab initio analysis

All-inorganic perovskites have great potential in photovoltaic applications and their performance is subject to phonon-assisted charge recombination dynamics. Local microstructures, such as ferroelastic domains, are considered to have a significant influence on the charge carrier lifetime in the CsPbBr₃ perovskite. Employing a combination of time-domain density functional theory and nonadiabatic (NA) molecular dynamics simulations, we demonstrate that the formation of ferroelastic domains weakens the NA coupling and suppresses the non-radiative electron-hole recombination. This effect originates from the ferroelastic domains separating electron and hole wave functions spatially and decreasing the NA coupling by a factor of 2.4 compared to pristine CsPbBr₃, delaying electron-hole recombination by a factor of 4.2. We also show that symmetry breaking enhances electronic-vibrational interactions, activating more phonon modes and accelerating quantum decoherence by 1 fs or so, which further slows recombination. Both factors compete successfully with the slightly reduced bandgap of about 0.2 eV and prolong the charge carrier lifetime to several nanoseconds. Our study advances the understanding of the atomistic mechanism for inhibited recombination in the CsPbBr₃ perovskite in the presence of ferroelastic domains, providing an effective route to design high-performance all-inorganic halide perovskites.

Recent Progress and Development in Inorganic Halide Perovskite Quantum Dots for Photoelectrochemical Applications

Inorganic halide perovskite quantum dots (IHPQDs) have recently emerged as a new class of optoelectronic nanomaterials that can outperform the existing hybrid organometallic halide perovskite (OHP), II-VI and III-V groups semiconductor nanocrystals, mainly due to their relatively high stability, excellent photophysical properties, and promising applications in wide-ranging and diverse fields. In particular, IHPQDs have attracted much recent attention in the field of photoelectrochemistry, with the potential to harness their superb optical and charge transport properties as well as spectacular characteristics of quantum confinement effect for opening up new opportunities in next-generation photoelectrochemical (PEC) systems. Over the past few years, numerous efforts have been made to design and prepare IHPQD-based materials for a wide range of applications in photoelectrochemistry, ranging from photocatalytic degradation, photocatalytic CO₂ reduction and PEC sensing, to photovoltaic devices. In this review, the recent advances in the development of IHPQD-based materials are summarized from the standpoint of photoelectrochemistry. The prospects and further developments of IHPQDs in this exciting field are also discussed.

Strain-induced structural phase transition, electric polarization and unusual electric properties in photovoltaic materials CsMI3 (M = Pb, Sn)

The structural phase transition, ferroelectric polarization, and electric properties have been investigated for photovoltaic films CsMI3 (M = Pb, Sn) epitaxially grown along (001) direction based on the density functional theory. The calculated results indicate that the phase diagrams of two epitaxial CsPbI3 and CsSnI3 films are almost identical, except critical transition strains varying slightly. The epitaxial tensile strains induce two ferroelectric phases Pmc21, and Pmn21, while the compressive strains drive two paraelectric phases P212121, P21212. The larger compressive strain enhances the ferroelectric instability in these two films, eventually rendering them another ferroelectric state Pc. Whether CsPbI3 or CsSnI3, the total polarization of Pmn21 phase comes from the main contribution of B-position cations (Pb or Sn), whereas, for Pmc21 phase, the main contributor is the I ion. Moreover, the epitaxial strain effects on antiferrodistortive vector, polarization and band gap of CsMI3 (M = Pb, Sn) are further discussed. Unusual electronic properties under epitaxial strains are also revealed and interpreted.

Photoluminescent Spectral Broadening of Lead Halide Perovskite Nanocrystals Investigated by Emission Wavelength Dependent Lifetime

Despite intensive efforts, the fluorescence of perovskite nanocrystals (NCs) still suffers from a poor color purity, which limits the applications in light emitting and multicolor display. A deep understanding on the fundamental of the photoluminescent (PL) spectral broadening is thus of great significance. Herein, the PL decay curves of the CsPbCl_xBr_3-x NCs are monitored at different wavelengths covering the entire PL band. Moreover, energy relaxation time τ and radiative recombination time β are obtained by numerical fittings. The dependences of τ and 1/β on the detection wavelength agree well with the steady-state PL spectrum, indicating the observed PL broadening is an intrinsic effect due to the resonance and off-resonance exciton radiative recombination processes. This work not only provides a new analysis method for time-resolved PL spectra of perovskites, but also gains a deep insight into the spectral broadening of the lead halide perovskite NCs.

Rapid Decoherence Induced by Light Expansion Suppresses Charge Recombination in Mixed Cation Perovskites: Time-Domain ab Initio Analysis

Using time-domain density functional theory combined with nonadiabatic molecular dynamics, we have investigated the effect of light-induced lattice expansion on the nonradiative electron-hole recombination in the mixed-cation perovskite FA_0.75MA_0.25PbI₃. We demonstrate that charge carrier lifetime extends by a factor of 1.5 within 1% lattice expansion; the bandgap grows only by 0.04 eV; the electron-phonon coupling increases by 13%; and the decoherence time shortens by 37%. The small bandgap change has negligible influence on recombination times. Lattice expansion enhances atomic fluctuations that lead to the enhancement of electron-phonon coupling and acceleration of decoherence. By creating several high-frequency phonon modes, the lattice expansion shortens the decoherence time further. As a result, rapid decoherence beats an enhanced electron-phonon coupling, playing the dominant role in suppressing the nonradiative electron-hole recombination. The light-induced lattice expansion or strain effects provide a rational route to improve the perovskite photovoltaic and photoelectronic device performance.

High-Brightness and Color-Tunable FAPbBr3 Perovskite Nanocrystals 2.0 Enable Ultrapure Green Luminescence for Achieving Recommendation 2020 Displays

To best catch human eyes in next-generation displays, the updated recommendation 2020 (Rec. 2020) standard has called for ultrapure green emitters to be qualified with a narrow emission of 525-535 nm with a full width at half-maximum (fwhm) below 25 nm. However, it is still challenging to find an emitter which can simultaneously cover these two criteria. Instead of traditional II-VI group semiconductor quantum dots, perovskite nanocrystals (NCs) can render versatile emitting tunability to allow them access to the Rec. 2020 standard. Herein, to realize the critical window of Rec. 2020, we have proposed a scalable, room temperature synthesis route of formamidinium lead bromide (FAPbBr₃) NCs using a sole ligand of sulfobetaine-18 (SBE-18). The as-synthesized FAPbBr₃ NCs exhibit an ideal emission at 534 nm with an ultranarrow fwhm of 20.5 nm and a high photoluminescence quantum yield of 90.6%, overwhelming the FAPbBr₃ nanoplates capped with oleic acid/oleylamine (OA/OAM). Introducing these high quality NCs into backlight displays, an ultrapure green backlight which covers ≈85.7% of the Rec. 2020 standard in the CIE 1931 color space is achieved, signifying the "greenest" backlight till now. Thus, we can foresee perovskite NCs as the most potential candidates for next-generation displays.

Insight into the Improved Phase Stability of CsPbI3 from First-Principles Calculations

The effect of organic cation doping with aziridinium (Az+) on the material properties of CsPbI3 was investigated by applying first-principles calculations. The results showed that the phase stability is greatly improved by incorporating the organic cation Az+ at the A site of CsPbI3. However, the band gap of CsPbI3 is further enlarged from 1.76 to 2.27 eV when 12.5% of Az doping is used. The optical absorption coefficient of Cs0.875Az0.125PbI3 is also decreased in the visible light region. The reasons of the improved phase stability and the enlargement of band gap arising from the organic cation doping are revealed. Our calculated results can provide theoretical guidance for improving the phase stability of halide perovskites.

Unravelling the Photocatalytic Behavior of All-Inorganic Mixed Halide Perovskites: The Role of Surface Chemical States

Within the most mesmerizing materials in the world of optoelectronics, mixed halide perovskites (MHPs) have been distinguished because of the tunability of their optoelectronic properties, balancing both the light-harvesting efficiency and the charge extraction into highly efficient solar devices. This feature has drawn the attention of analogous hot topics as photocatalysis for carrying out more efficiently the degradation of organic compounds. However, the photo-oxidation ability of perovskite depends not only on its excellent light-harvesting properties but also on the surface chemical environment provided during its synthesis. Accordingly, we studied the role of surface chemical states of MHP-based nanocrystals (NCs) synthesized by hot-injection (H-I) and anion-exchange (A-E) approaches on their photocatalytic (PC) activity for the oxidation of β-naphthol as a model system. We concluded that iodide vacancies are the main surface chemical states that facilitate the formation of superoxide ions, O₂^●-, which are responsible for the PC activity in A-E-MHP. Conversely, the PC performance of H-I-MHP is related to the appropriate balance between band gap and a highly oxidizing valence band. This work offers new insights on the surface properties of MHP related to their catalytic activity in photochemical applications.

Computational prediction of structural, electronic, and optical properties and phase stability of double perovskites K2SnX6 (X = I, Br, Cl)

The vacancy-ordered double perovskites K₂SnX₆ (X = I, Br, Cl) attract significant research interest due to their potential applications as light absorbing materials in perovskite solar cells. However, deeper insight into their material properties at the atomic scale is currently lacking. Here we present a systematic investigation of the structural, electronic, and optical properties and phase stabilities of the cubic, tetragonal, and monoclinic phases based on density functional theory calculations. Quantitatively reliable predictions of lattice constants, band gaps, effective masses of charge carriers, and exciton binding energies are provided and compared with the available experimental data, revealing the tendency of the band gap and exciton binding energy to increase on lowering the crystallographic symmetry from cubic to monoclinic and on moving from I to Cl. We highlight that cubic K₂SnBr₆ and monoclinic K₂SnI₆ are suitable for applications as light absorbers for solar cell devices due to their appropriate band gaps of 1.65 and 1.16 eV and low exciton binding energies of 59.4 and 15.3 meV, respectively. The constant-volume Helmholtz free energies are determined through phonon calculations, which predict phase transition temperatures of 449, 433 and 281 K for cubic–tetragonal and 345, 301 and 210 K for tetragonal–monoclinic transitions for X = I, Br and Cl, respectively. Our calculations provide an understanding of the material properties of the vacancy-ordered double perovskites K₂SnX₆, which could help in devising a low-cost and high performance perovskite solar cell.

HSE + SOC were used to calculate the band structures of the cubic, tetragonal, and monoclinic phases of the double perovskites K₂SnX₆ (X = I, Br, Cl).

Review on Recent Progress of All-Inorganic Metal Halide Perovskites and Solar Cells

All-inorganic perovskites are considered to be one of the most appealing research hotspots in the field of perovskite photovoltaics in the past 3 years due to their superior thermal stability compared to their organic-inorganic hybrid counterparts. The power-conversion efficiency has reached 17.06% and the number of important publications is ever increasing. Here, the progress of inorganic perovskites is systematically highlighted, covering materials design, preparation of high-quality perovskite films, and avoidance of phase instabilities. Inorganic perovskites, nanocrystals, quantum dots, and lead-free compounds are discussed and the corresponding device performances are reviewed, which have been realized on both rigid and flexible substrates. Methods for stabilization of the cubic phase of low-bandgap inorganic perovskites are emphasized, which is a prerequisite for highly efficient and stable solar cells. In addition, energy loss mechanisms both in the bulk of the perovskite and at the interfaces of perovskite and charge selective layers are unraveled. Reported approaches to reduce these charge-carrier recombination losses are summarized and complemented by methods proposed from our side. Finally, the potential of inorganic perovskites as stable absorbers is assessed, which opens up new perspectives toward the commercialization of inorganic perovskite solar cells.

Highly efficient nanocrystalline CsxMA1-xPbBrx perovskite layers for white light generation

Perovskite light converting layers optimization for cost-efficient white light emitting diodes (LED) was demonstrated. High excitation independent internal quantum efficiency (IQE) of 80% and weakly excitation dependent PL spectra suitable for white light generation were obtained in the mixed cation Cs_xMA_1-xPbBr₃ perovskite nanocrystal layers with optimal x = 0.3 being determined by effective surface passivation and phase mixing as revealed by x-ray diffraction. Enhancement of the PL homogeneity and the external quantum efficiency (EQE) were secured when using 2,2',2″-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole (TPBi) additive in the layer preparation process. Excitation dependent PL intensity, decay time, and IQE revealed that the high emission efficiency of the layers originates from a dominant radiative localized exciton recombination (130 ns) weakly influenced by the nonradiative free carrier recombination (750 ns). Warm and cool white LEDs with correlated color temperature 3000 K and 5600 K, and color rendering index 82 and 74, respectively, were realized by using the optimized perovskite layers, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) red emitter and a blue LED.

Unravelling the Effects of Pressure-Induced Suppressed Electron-Hole Recombination in CsPbBr3 Perovskite: Time-Domain ab Initio Analysis

Using nonadiabatic (NA) molecular dynamics simulations, we demonstrate pressure-dependent electron-hole recombination in all-inorganic CsPbBr₃ perovskite. In particular, electron-hole recombination under 1 atm takes place in several hundred picoseconds, agreeing well with experiments. An increase of pressure causes PbBr₆ octahedron distortion, including contraction of both Pb-Br-Pb angles and Pb-Br bond lengths, leading to a decrease in decoherence time and NA coupling and thus slowing electron-hole recombination. When the pressure reaches a critical pressure of 1.20 GPa, a phase transition occurs in which the charge carrier lifetime is longest and extends to several nanoseconds. When the pressure is increased over the threshold, the shrinkage of Pb-Br bond length is inhibited and the contraction of Pb-Br-Pb angles primarily induced the PbBr₆ octahedron distortion. Such a situation gives rise to a mild NA coupling and decoherence time, restoring the recombination time to over half of a nanosecond. Our study uncovers the mechanisms for the pressure-suppressed charge recombination and provides an advanced route toward further development of photovoltaic performance of perovskite materials.

Halide Perovskite Nanocrystals for Next-Generation Optoelectronics

Colloidal perovskite nanocrystals (PNCs) combine the outstanding optoelectronic properties of bulk perovskites with strong quantum confinement effects at the nanoscale. Their facile and low-cost synthesis, together with superior photoluminescence quantum yields and exceptional optical versatility, make PNCs promising candidates for next-generation optoelectronics. However, this field is still in its early infancy and not yet ready for commercialization due to several open challenges to be addressed, such as toxicity and stability. Here, the key synthesis strategies and the tunable optical properties of PNCs are discussed. The photophysical underpinnings of PNCs, in correlation with recent developments of PNC-based optoelectronic devices, are especially highlighted. The final goal is to outline a theoretical scaffold for the design of high-performance devices that can at the same time address the commercialization challenges of PNC-based technology.

Electrochemical Hole Injection Selectively Expels Iodide from Mixed Halide Perovskite Films

Halide ion mobility in metal halide perovskites remains an intriguing phenomenon, influencing their optical and photovoltaic properties. Selective injection of holes through electrochemical anodic bias has allowed us to probe the effect of hole trapping at iodide (0.9 V) and bromide (1.15 V) in mixed halide perovskite (CH₃NH₃PbBr_1.5I_1.5) films. Upon trapping holes at the iodide site, the iodide gradually gets expelled from the mixed halide film (as iodine and/or triiodide ion), leaving behind re-formed CH₃NH₃PbBr₃ domains. The weakening of the Pb-I bond following the hole trapping (oxidation of the iodide site) and its expulsion from the lattice in the form of iodine provided further insight into the photoinduced segregation of halide ions in mixed halide perovskite films. Transient absorption spectroscopy revealed that the iodide expulsion process leaves a defect-rich perovskite lattice behind as charge carrier recombination in the re-formed lattice is greatly accelerated. The selective mobility of iodide species provides insight into the photoinduced phase segregation and its implication in the stable operation of perovskite solar cells.

From Nonluminescent to Blue-Emitting Cs4 PbBr6 Nanocrystals: Tailoring the Insulator Bandgap of 0D Perovskite through Sn Cation Doping

All-inorganic cesium lead halide perovskite nanocrystals (NCs) with different dimensionalities have recently fascinated the research community due to their extraordinary optoelectronic performance such as tunable bandgaps over the entire visible spectral region. However, compared to well-developed 3D CsPbX₃ perovskites (X = Cl, Br, and I), the bandgap tuning in 0D Cs₄ PbX₆ perovskite NCs remains an arduous task. Herein, a simple but valid strategy is proposed to tailor the insulator bandgap (≈3.96 eV) of Cs₄ PbBr₆ NCs to the blue spectral region by changing the local coordination environment of isolated [PbBr₆ ]^4- octahedra in the Cs₄ PbBr₆ crystal through Sn cation doping. Benefitting from the unique Pb²⁺ -poor and Br^- -rich reaction environment, the Sn cation is successfully introduced into the Cs₄ PbBr₆ NCs, forming coexisting point defects comprising substitutional Sn_Pb and interstitial Br_i , thereby endowing these theoretically nonluminescent Cs₄ PbBr₆ NCs with an ultranarrow blue emission at ≈437 nm (full width at half maximum, ≈12 nm). By combining the experimental results with first-principles calculations, an unusual electronic dual-bandgap structure, comprising the newly emerged semiconducting bandgap of ≈2.87 eV and original insulator bandgap of ≈3.96 eV, is found to be the underlying fundamental reason for the ultranarrow blue emission.

CsPbBr3 Perovskite Powder, a Robust and Mass-Producible Single-Source Precursor: Synthesis, Characterization, and Optoelectronic Applications

A facile synthesis method is proposed for the mass production of high-quality CsPbBr₃ perovskite powder. It is shown that the proposed synthesis protocol is capable of producing polycrystalline CsPbBr₃ powder in quantities greater than 10 g. The derived thin films by thermal evaporation and spin-coating are of compact morphologies (root-mean-square roughness < 4 nm) without voids and pinholes. Moreover, the thin films show obvious photoluminescence (PL) with a narrow (bandwidth < 19 nm) peak centered at ∼520 nm, which is blue-shifted compared with the PL emission of the powder at 542 nm. The powder and the spin-coated film exhibit superior PL stability under long-term ambient conditions and in thermal cycling experiments performed at temperatures up to ∼120 °C. Accordingly, optoelectronic applications including the fabrication and characteristics of the electroluminescence device, the organic-inorganic powder doped with methylammonium and formamidinium ions, and fluorescent greenish-blue quantum dots are also demonstrated. On the basis of these demonstrations, the synthesized CsPbBr₃ perovskite powder can be expected to empower the advances in perovskite-related optoelectronics in the future.

Ultrafast carrier dynamics of metal halide perovskite nanocrystals and perovskite-composites

Perovskite nanocrystals (NCs), especially those based on cesium lead halides, have emerged in recent years as highly promising materials for efficient solar cells and photonic applications. The key to realization of full potential of these materials lies however in the molecular level understanding of the processes triggered by light. Herein we highlight the knowledge gained from photophysical investigations on these NCs of various sizes and compositions employing primarily the femtosecond pump-probe technique. We show how spectral and temporal characterization of the photo-induced transients provide insight into the mechanism and dynamics of relaxation of hot and thermalized charge carriers through their recombination and trapping. We discuss how the multiple excitons including the charged ones (trions), generated using high pump fluence or photon energy, recombine through the Auger-assisted process. We discussed the harvesting of hot carriers prior to their cooling and band-edge carriers from these perovskite NCs to wide band-gap metal oxides, metal chalcogenide NCs and molecular acceptors. How perovskites can influence the charge carrier dynamics in composites of organic and inorganic semiconductors is also discussed.

Improved phase stability of the CsPbI3 perovskite via organic cation doping

We have studied the effect of organic cation doping with dimethylammonium (DMA⁺), ethylammonium (EA⁺), and guanidinium (GA⁺) on the properties of the CsPbI₃ perovskite by performing first-principles calculations. It was found that these dopants, especially DMA⁺, can significantly improve the phase stability of the desired α phase of CsPbI₃ by compressing the transition temperature between the photoactive α phase (cubic perovskite structure) and the photoinactive δ phase (orthorhombic structure). However, the incorporation of organic cations decreases the absorption coefficient of the CsPbI₃ perovskite in the visible spectrum. The details of the improvement of the phase stability and the degradation of the optical properties arising from the organic dopants are revealed. Our results are valuable for developing stable and high performance hybrid perovskite photovoltaic materials.

Light Emission Enhancement by Tuning the Structural Phase of APbBr3 (A = CH3NH3, Cs) Perovskites

Lead halide perovskite (APbX₃) has recently emerged as a promising active layer in light-emitting diodes (LEDs) as well as an absorber for photovoltaic devices. For better LED properties, it is important to understand the fundamental mechanism of the optoelectronic behaviors, e.g., how the nanostructure of the APbX₃ thin film correlates with its emitting properties. We investigated the effect of APbBr₃ (A = CH₃NH₃, Cs) crystallite size on the photophysical properties regarding its crystallographic changes and spin-orbit coupling. Photoluminescence lifetime measurements, X-ray and electron diffraction analyses, and density functional theory calculations were performed. We demonstrate that the emitting properties of mesoscale APbBr₃ crystallites are improved due to the formation of a pure cubic phase that leads to the spin- and momentum-allowed carrier recombination. Our findings provide fundamental insights into the emitting behavior of APbBr₃, which suggests a control of its optoelectronic properties by means of modulating the crystal morphology and resultant electronic band structures.

Trivalent ion mediated abnormal growth of all-inorganic perovskite nanocrystals and their divergent emission properties

In this work, a new trivalent ion-mediated one-pot synthetic protocol is reported to create two well-defined optical absorbance and photoluminescence (PL) emissions in all-inorganic halide perovskite nanocrystals (NCs). The foreign M3+ cations (M = Bi, Al, In), typically from BiBr3, BiFeO3, BiCl3, AlBr3 or InBr3, function as capping ligands for generating a growth-constrained thinner nanoplatelet (NPL) population displaying the quantum confinement effect. The formation mechanism of the growth-constrained NPLs is proposed based on density functional theory (DFT) on the different slab energy of the representative NPLs achieved in the presence of Bi3+ ions and the density of states (DOS) of the supposed bulk perovskites. Notably, the formation of two groups of NCs with different sizes allows for the generation of dual optical absorbance and PL emissions. The influence of the M : Pb molar ratios on the precursors is systematically elucidated, from which the relative intensity and position of each PL emission can be fine-tuned. By virtue of the representative NPLs with well-defined green and blue emissions, the M3+-assisted synthetic protocol provides a facile and cost-effective route for producing unique NCs and nanostructures for optoelectronic device applications.

Postsynthesis Spontaneous Coalescence of Mixed-Halide Perovskite Nanocubes into Phase-Stable Single-Crystalline Uniform Luminescent Nanowires

All inorganic mixed-halide perovskite, CsPb(Br _xI_{1- x})₃ (0 ≤ x ≤ 1), nanocrystals possess tunable photoluminescence with high quantum yield in the visible window. However, the photoluminescence degrades rapidly with postsynthetic aging due to the spontaneous ion separation and phase instability. Here we show that the postsynthetic aging of CsPb(Br _xI_{1- x})₃ nanocubes spontaneously forms highly uniform single-crystalline nanowires with a diameter of 9 ± 0.5 nm and length of up to several micrometers. The nanowires show bright photoluminescence with an absolute photoluminescence quantum yield of 41%. Rietveld refinement identifies the stable orthorhombic phase of the nanowires, implying a phase transition from the cubic crystallographic phase of the nanocubes during the morphology evolution. Transient absorption spectroscopy reveals a faster excited-state decay dynamic with a large exciton delocalization length in 1D nanowires. Our findings elucidate the insights into the postsynthesis morphology evolution of mixed-halide perovskite nanocrystals leading to luminescent nanowires with excellent crystal phase stability for potential optoelectronic applications.

Water-stable and ion exchange-free inorganic perovskite quantum dots encapsulated in solid paraffin and their application in light emitting diodes

Perovskite quantum dots (PQDs) with high quantum efficiency and broad spectral tunability are promising luminescent materials and show potential applications in light-emitting diodes (LEDs). However, PQDs degrade rapidly in atmosphere moisture due to their easy hydrolysis and undergo undesirable spectral changes associated with ion exchanges. Herein, we encapsulated blue, green and red fluorescence-emitting all-inorganic PQDs respectively with hydrophobic solid paraffin to obtain multi-color luminescent paraffin-PQD composites (P-PQDs). The P-PQD composites not only preserve the bright luminescence, but also greatly improve the water resistance of PQDs. Moreover, the anion exchange phenomenon is effectively inhibited for the isolation of PQD nanoparticles by a solid paraffin layer. Finally, blue, green and red luminescent P-PQD microparticles have been synthesized and show promising applications in LEDs.

Metal Halide Perovskite Nanocrystals: Synthesis, Post-Synthesis Modifications, and Their Optical Properties

Metal halide perovskites represent a flourishing area of research, which is driven by both their potential application in photovoltaics and optoelectronics and by the fundamental science behind their unique optoelectronic properties. The emergence of new colloidal methods for the synthesis of halide perovskite nanocrystals, as well as the interesting characteristics of this new type of material, has attracted the attention of many researchers. This review aims to provide an up-to-date survey of this fast-moving field and will mainly focus on the different colloidal synthesis approaches that have been developed. We will examine the chemistry and the capability of different colloidal synthetic routes with regard to controlling the shape, size, and optical properties of the resulting nanocrystals. We will also provide an up-to-date overview of their postsynthesis transformations, and summarize the various solution processes that are aimed at fabricating halide perovskite-based nanocomposites. Furthermore, we will review the fundamental optical properties of halide perovskite nanocrystals by focusing on their linear optical properties, on the effects of quantum confinement, and on the current knowledge of their exciton binding energies. We will also discuss the emergence of nonlinear phenomena such as multiphoton absorption, biexcitons, and carrier multiplication. Finally, we will discuss open questions and possible future directions.

Tuning the Excited-State Dynamics of CuI Films with Electrochemical Bias

Owing to its high hole conductivity and ease of preparation, CuI was among the first inorganic hole-transporting materials that were introduced early on in metal halide perovskite solar cells, but its full potential as a semiconductor material is still to be realized. We have now performed ultrafast spectroelectrochemical experiments on ITO/CuI electrodes to show the effect of applied bias on the excited-state dynamics in CuI. Under operating conditions, the recombination of excitons is dependent on the applied bias, and it can be accelerated by decreasing the potential from +0.6 to -0.1 V vs Ag/AgCl. Prebiasing experiments show the persistent and reversible "memory" effect of electrochemical bias on charge carrier lifetimes. The excitation of CuI in a CuI/CsPbBr3 film provides synergy between both CuI and CsPbBr3 in dictating the charge separation and recombination.

Phase segregation due to ion migration in all-inorganic mixed-halide perovskite nanocrystals

Semiconductor mixed-halide perovskites featured with a tunable energy bandgap are ideal candidates for light absorbers in tandem solar cells as well as fluorescent materials in light-emitting diodes and nanoscale lasers. These device advancements are currently hindered by the light-induced phase segregation effect, whereby ion migration would yield smaller-bandgap domains with red-shifted photoluminescence. Here we show that upon laser excitation all-inorganic mixed-halide nanocrystals unexpectedly exhibit a blue shift in the photoluminescence peak that can revert back in the dark, thus depicting the processes of ion migration out of and back to the originally excited nanocrystals. Interestingly, this reversible photoluminescence shift can also be induced by electrical biasing of mixed-halide nanocrystals without the injection of charge carriers. The above findings suggest that it is the local electric field that breaks the ionic bonds in mixed-halide nanocrystals, which could be a universal origin for light-induced phase segregation observed in other mixed-halide perovskite materials.

Mixed-halide perovskites possess excellent semiconductor properties but suffer severely from notorious light-induced phase segregation effect. Here Zhang et al. employ simple photoluminescence measurements to link the effect to the local electric field induced ion migration process.

Mixed Cs and FA Cations Slow Electron-Hole Recombination in FAPbI3 Perovskites by Time-Domain Ab Initio Study: Lattice Contraction versus Octahedral Tilting

Using time domain density functional theory combined with nonadiabatic (NA) molecular dynamics, we show that electron-hole recombination takes subnanoseconds in FAPbI₃, showing excellent agreement with experiment. Cs doping retards charge recombination by factors of 1.1 and 3.1 due to lattice contraction and octahedral tilting, respectively. Lattice contraction decreases the NA coupling and increases the coherence time arising from the suppressed atomic fluctuations, slightly slowing recombination because the two factors have an opposite influence on quantum transition. In contrast, octahedral tilting simultaneously decreases the NA coupling, thanks to the reduced overlap between Pb and I orbitals, and the coherence time, extending the excited-state lifetime over 1 ns. Our simulations provide a mechanistic understanding for delayed charge losses in the mixed Cs and FA system, suggesting a rational strategy to improve perovskite solar cell performance.

Hydroxyl terminated mesoporous silica-assisted dispersion of ligand-free CsPbBr3/Cs4PbBr6 nanocrystals in polymer for stable white LED

Composite nanocrystals of CsPbBr3/Cs4PbBr6 have gained significant attention because of their high stability and unique photoelectronic property. However, their dispersion within polymers is rather difficult due to the absence of ligands, which limits further enhancement of stability and practical applications. Herein, a feasible, effective, and general method was developed to assist the dispersion of CsPbBr3/Cs4PbBr6 nanocrystals in polymer by using -OH terminated mesoporous silica as a micro-container. The composite film obtained is employed as the light emitter for the fabrication of white LEDs. It was found that silica loaded with composite nanocrystals disperse uniformly in the composite film which shows excellent stability with a half-life of 400 hours under the illumination with optical power density of 1.7 × 103 mW cm-2 and peak wavelength of 457 nm. The inner pore of the micro-container attracts precursors and confines the crystallization, leading to the incorporation of CsPbBr3/Cs4PbBr6 nanocrystals. Meanwhile, the hydrogen bonding of the outer surface with poly(methyl methacrylate) (PMMA) enables good dispersion of the loaded micro-containers in PMMA as evidenced by the optical microscopy characterization and water resistance test. Moreover, this strategy can also be applied to other kinds of polymers since the outside -OH group can react with siliane coupling agents. On the basis of stability tests associated with silica and polymer encapsulation, a possible mechanism is proposed for the enhancement of the stability of composite films under working condition.