Trap States in Lead Iodide Perovskites

Exciton-phonon coupling in quasi-two-dimensional Ruddlesden-Popper perovskites: impact of a mixed-phase structure

Two-dimensional (2D) metal halide perovskites (MHPs) hold great potential for optoelectronic and spintronic device applications due to their outstanding optical and electronic properties. In this study, we conducted a systematic temperature-dependent photoluminescence (PL) investigation to elucidate the influence of exciton-phonon scattering on the optoelectronic properties of 2D Ruddlesden-Popper (RP) perovskites, (C6H7SNH3)2 (CH3NH3)n-1PbnI3n+1 (n = 1-4). The results from our micro-PL study suggest that the optical band gap of each 2D phase (a perovskite with a specific number of inorganic layers, n) varies with temperature due to thermal lattice expansion and exciton-phonon interactions. The strength of exciton-phonon interactions differs in each 2D phase within a mixed-phase sample, with a notable increase as the layer number (n) rises. This enhancement is attributed to greater lattice mismatch, increased interface complexity, and a higher degree of disorder within the system. Additionally, we found that parameters such as exciton-phonon coupling strength and exciton binding energy exhibit significantly different behaviours across various mixed-phase perovskite samples, likely due to changes in the local environment around excitons in each specific phase. These findings offer valuable insights into the mechanisms underlying nonradiative processes and scattering phenomena and provide guidance for optimizing the efficiency of 2D-RP mixed-phase perovskite-based optoelectronic and spintronic devices.

Ordered Cationic Mixing in a 1D Organic-Inorganic Hybrid

Hybrid metal halides are a remarkably dynamic family of materials that offer a flexible platform for exploring the novel crystal chemistry that emerges at the intersection of organic and inorganic solids. Herein, we report the discovery of a hybrid that contains two molecules effectively adopting isostructural geometry, (1-NA)PbI3 and (1-MQ)PbI3, and our attempts to create solid solutions of the two beyond the 1:1 ratio. Single-crystal X-ray diffraction, combined with solid-state NMR measurements, clearly show that despite having nearly identical steric geometry, the only mixed phase attained was the composition (1-MQ)(1-NA)Pb2I6, which exhibits a high degree of order between the two molecules. We propose that this ordering is primarily driven by local molecular dipoles, which ultimately creates a band structure in the blended phase that is highly characteristic of the end members, with little sign of rehybridization between the organic or inorganic components.

Thermally Induced Neodymium-Doped Strategy toward Wide Band Gap Perovskite Solar Cells with a Fill Factor over 86

To approach the Shockley-Queisser (S-Q) limit in perovskite solar cells (PSCs), enhancing the fill factor (FF), a crucial parameter associated with carrier transport and nonradiative recombination, is of paramount importance. In this paper, the rare earths (RE), neodymium salt is used as dopant of 4-(3-,6-dimethoxy-9h-carbazol-9-butyl) phosphonic acid (MeO-4PACz) to obtain MeO-4PACz:Nd3+, and Nd3+ migration is induced during annealing. It is worth noting that the uniform diffusion of Nd3+ in the perovskite layer significantly increases the defect-formation energy of perovskite, thus reducing the density of the perovskite defect states, greatly improving the carrier transport rate and inhibited non-radiative recombination. The experimental results show that the wide-band gap (WBG) PSCs based on MeO-4PACz:Nd3+ has an excellent power conversion efficiency (PCE) of 22.82% with the FF of 86.35%. The proposed method provides a direct method for RE3+ to promote the FF of perovskites.

Hole Trapping in Lead Halide Perovskite Nanocrystal-Viologen Hybrids and Its Impact on Back Electron Transfer

Control of forward and back electron transfer processes in semiconductor nanocrystals is important to maximize charge separation for photocatalytic reduction/oxidation processes. By employing methyl viologen as the electron acceptor, we have succeeded in mapping the electron transfer from excited CsPbI3 nanocrystals to viologen as well as the hole trapping process. The electron transfer to viologen is an ultrafast process (ket = 2 × 1010 s-1) and results in the formation of extended charge separation as electrons are trapped at surface-bound viologen sites and holes at iodide sites. The I2─• formation, which is confirmed through the transient absorption at 750 nm, provides a convenient way to probe trapped holes and its participation in the back electron transfer process. By employing a series of mixed halide compositions, we were able to tune the bandgap and valence band energy of the perovskite donor. The back electron transfer rate constant (kbet = 1.3-2.6 × 107 s-1) is nearly three orders of magnitude smaller than that of forward electron transfer, thus extending the lifetime of the charge-separated state. The weak dependence of the back electron transfer rate constant on the valence band energy suggests that trapping of holes at halide (I or Br) sites is involved in the back electron transfer process. The ability to extend the lifetime of the charge-separated pair can offer new strategies to improve the redox properties of semiconductor-based photocatalytic systems.

Multiple defects renovation and phase reconstruction of reduced-dimensional perovskites via in situ chlorination for efficient deep-blue (454 nm) light-emitting diodes

Deep-blue perovskite light-emitting diodes (PeLEDs) based on reduced-dimensional perovskites (RDPs) still face a few challenges including severe trap-assisted nonradiative recombination, sluggish exciton transfer, and undesirable bathochromic shift of the electroluminescence spectra, impeding the realization of high-performance PeLEDs. Herein, an in situ chlorination (isCl) post-treatment strategy was employed to regulate phase reconstruction and renovate multiple defects of RDPs, leading to superior carrier cooling of 0.88 ps, extraordinary exciton binding energy of 122.53 meV, and higher photoluminescence quantum yield of 60.9% for RDP films with deep-blue emission at 450 nm. The phase regulation is accomplished via fluorine-derived hydrogen bonds that suppress the formation of small-n phases. Multiple defects, including halide vacancies (shallow-state defects) and lead-chloride antisite defects (deep-state defects), are renovated via C=O coordination and hydroxy-group-derived hydrogen bonds. Consequently, deep-blue PeLEDs with a record maximum external quantum efficiency of 6.17% and stable electroluminescence at 454 nm were demonstrated, representing the best-performing deep-blue PeLEDs.

High-pressure chemistry of functional materials

Functional materials, possessing specific properties and performing particular functions beyond their mechanical or structural roles, are the foundation of modern matter science including energy, environment, and quantum sciences. The atomic and electronic structures of these materials can be significantly altered by external stimuli such as pressure. High-pressure techniques have been extensively utilized to deepen our understanding of structure-property relationships of materials, while also enabling emergent or enhanced properties. In this feature article, we review the transformative impact of high pressure on the chemical and physical properties of functional materials, including perovskite materials, low-dimensional metal halides, metal chalcogenides, metal oxides, and inorganic molecular crystals. By analyzing recent advancements and methodological approaches in high-pressure research, we provide insights into the mechanisms driving structural and property changes in these materials. We also emphasize the significance of translating the knowledge gained from high pressure research to the design of new functional materials. Finally, we highlight the potential of high-pressure chemistry and nano-architectonics in advancing functional materials and discuss the future directions and challenges in this field.

Raman and Photoluminescence Studies of Quasiparticles in van der Waals Materials

Two-dimensional (2D) layered materials have received much attention due to the unique properties stemming from their van der Waals (vdW) interactions, quantum confinement, and many-body interactions of quasi-particles, which drive their exotic optical and electronic properties, making them critical in many applications. Here, we review our past years' findings, focusing on many-body interactions in 2D layered materials, including phonon anharmonicity, electron-phonon coupling (e-ph), exciton dynamics, and phonon anisotropy based on temperature (polarization)-dependent Raman spectroscopy and Photoluminescence (PL). Our review sheds light on the role of quasi-particles in tuning the material properties, which could help optimize 2D materials for future applications in electronic and optoelectronic devices.

Self-Trapped Excitons in 3R ZnIn2S4 with Broken Inversion Symmetry for High-Performance Photodetection

Exploring novel materials with intrinsic self-trapped excitons (STEs) is crucial for advancing optoelectronic technologies. In this study, 2D 3R-phase ZnIn2S4, featuring broken inversion symmetry, is introduced to investigate intrinsic STEs. This material exhibits a broadband photoluminescence (PL) emission with a full width at half maximum of 164 nm and a large Stokes shift of ≈0.6 eV, which arises from the distortion of [ZnS4]6- tetrahedral unit induced by the symmetry breaking and strong electron-phonon coupling. The photophysical properties of the STEs exhibit a high Huang-Rhys factor (15.0), rapid STEs formation time (166 fs), and extended STEs lifetime (1039 ps), as demonstrated by experimental evidence from temperature-dependent PL, Raman spectroscopy, and ultrafast absorption spectroscopy. Additionally, STE-induced photoconductive effect is elucidated, indicating that intrinsic STEs in 3R-ZnIn2S4 can provide a synergistic effect that enhances absorption capacity, localization, and lifetime by capturing the self-trapped hole state. Consequently, the 2D 3R-ZnIn2S4 photodetector exhibits remarkable broad-spectrum photosensitivity, including a photo-switching ratio of 11286, response times of less than 0.6 ms, responsivity of 15.2 A W-1, detectivity of 1.02 × 10¹¹ Jones, and external quantum efficiency of 5032% under 375 nm light. These findings provide new ideas for exploring materials with intrinsic STEs to achieve novel high-performance photodetector applications.

Interface Energy-Level Reorganization for Efficient Perovskite γ-Ray Detectors

Metal halide perovskites are promising candidates for gamma-ray (γ-ray) spectrum detectors. However, achieving high-resolution energy spectra in single-photon pulse-height analysis mode remains challenging, due to the inevitable leakage currents degrade the recognizable fingerprint energies which is critical for resolving γ-ray spectroscopy. We demonstrate under high bias voltage, a deficient contact barrier can lead to excessive surface charge injection, thereby increasing leakage current from electrodes to perovskites. Hence, we conceive to employ surface ligand engineering on perovskite single crystals to manipulate energy levels to suppress leakage current. In particular, anchoring a strong dipole ligand onto the perovskite induced surface charge-density displacement, leading to a downward band bending and heightened the corresponding contact barrier. Consequently, the strategy minimized the detectors' leakage current by an order of magnitude, to as low as 44 nA cm-2 at -100 V. The resulting detectors show a significant improvement in energy resolution, 3.9 % for 22Na 511 keV γ-rays has been achieved at room temperature. The resulting detector further resolves each fingerprint energy for 152Eu γ-spectrum, representing one of the best γ-rays perovskite detectors reported to date. Moreover, the detectors exhibited stabilized energy resolution without any degradation under a continuous electric field (1,000 V cm-1) for over 300 minutes, representing the longest longevity reported to date.

Halogen-Dependent Circular Dichroism and Magneto-Photoluminescence Effects in Chiral 2D Lead Halide Perovskites

Chiral lead halide perovskites (chiral LHPs) have emerged as one of the best candidates for opto-spintronics due to their large spin-orbit coupling (SOC) and unique chirality-induced spin selectivity (CISS) even in the absence of a magnetic field. Here, we report the impact of halide composition on circular dichroism (CD) and magneto-photoluminescence (PL) effects of chiral 2D LHPs (R/S-MBA)2PbBrxI4-x (MBA = C6H5CH2(CH3)NH3). By tuning the mixing ratio of Br/I halide anions, we find that (R/S-MBA)2PbBrxI4-x thin films exhibit tunable and wide wavelength range CD signals. Simultaneously, the main CD signals near the exciton absorption band gradually blue shift until they disappear. Moreover, the halogen-dependent negative magneto-PL effects of (R/S-MBA)2PbBrxI4-x thin films excited by left/right circularly polarized light can be detected at room temperature. We demonstrated that the halide composition can effectively modulate exciton splitting and chirality transfer in (R/S-MBA)2PbBrxI4-x owing to the chirality-induced SOC and crystalline structure transition, which lead to the adjustable CD signals. The interplay of Rashba-type band spin splitting and spin mixing among bright triplet exciton states is responsible for the halogen-dependent magneto-PL effect of chiral 2D LHPs. This study enables chiral 2D LHPs with CISS to be a new class of promising opto-spintronics materials for exploring high-performance spin-light-emitting diodes by halide engineering.

Enhancing Extraction and Suppressing Cooling of Hot Electrons in Lead Halide Perovskites by Dipolar Surface Passivation

Slowing hot carrier (HC) cooling and improving HC extraction are considered two pivotal factors for enhancing power conversion efficiency in emerging HC photovoltaic applications of perovskites and other materials. Employing ab initio quantum dynamics simulations, we demonstrate the simultaneous slow cooling and efficient extraction of hot electrons at the C60/CsPbI3 interface through dipolar surface passivation with phenethylammonium and 4-fluorophenethylammonium ligands. The passivation effectively suppresses I-Pb lattice vibrations, weakens the hot electron-phonon interaction in CsPbI3, and thus slows down the HC cooling. At the same time, the dipolar surface passivation elevates the LUMO + 1 state in C60 and reduces the energy gap for HC extraction. Concurrently, higher-frequency vibrations of the dipolar layer enhance the coupling between C60 and CsPbI3, promoting efficient HC extraction further. These phenomena are intensified with increased polarity of the dipolar layer. Furthermore, we find that dipolar passivation has the opposite influence on cold electron collection at the band edge, underscoring the fact that the observed improvement in photovoltaic performance stems preferentially from the effective utilization of HCs rather than cold electrons. The work provides a new strategy for achieving high-performance HC perovskite solar cells.

Disentangling the Optoelectronic Behavior of Lead Iodide Governed by Two-Dimensional Electron Confinement

We present a joint experimental and theoretical study for complete spectroscopic characterization and optoelectronic properties of lead iodide. Experimentally, we combine X-ray diffraction experiments to elucidate the structure with photoelectron spectroscopy to explore its electronic structure. Computationally, simulations are performed in the frame of density functional theory. We show that PbI2 presents a two-dimensional layered structure and exhibits a large transient photocurrent effect under visible light illumination, which are compatible with the surface photovoltage scenario. The transient photocurrent has an extremely long lifetime: when the sample is lightened with visible light, it shows very long relaxation times and, consequently, huge charge carrier diffusion lengths. We explain this anomalous behavior with the slow carrier mobility of holes and electrons caused by the 2D electron confinement in the layered material. Our results can be used as a simple model for understanding the optoelectronic properties of more complex 2D hybrid perovskites.

Ultrafast Interfacial Charge Transfer in Anisotropic One-Dimensional CsPbBr3/Pt Epitaxial Heterostructure

Colloidal one-dimensional (1D) perovskite nanorods (NRs) and metal epitaxial heterostructures (HSs) are the promising class of new materials for efficient photovoltaic and photocatalytic applications. Besides, fundamental photophysical properties and its device applications of 1D perovskite-metal HSs are limited due to their challenging synthetic protocols and difficulties in forming epitaxial growth between covalent and ionic bonds. Herein, we have synthesized the CsPbBr3 perovskite NRs-platinum (Pt) nanoparticles (NPs) (CsPbBr3/Pt) epitaxial HS using cation exchange followed by chemical reduction methods with the orthorhombic Cs2CuBr4 NRs. Here, the tertiary ammonium ions extensively helped to form the 1D Cs2CuBr4, CsPbBr3 NRs, and CsPbBr3/Pt HSs. For CsPbBr3/Pt HSs an epitaxial relationship has been established in the (020) plane of orthorhombic CsPbBr3 with the (020) plane of cubic Pt. Further, femtosecond transient absorption (TA) spectroscopy was employed to study the charge carrier dynamics of CsPbBr3/Pt HS. Upon 420 nm photoexcitation, excitons in the conduction band of CsPbBr3 NRs dissociate by electron transfer (with an ultrafast time of 1.1 ps) to the Pt domain. In addition, charge transfer (CT) was also demonstrated in the CsPbBr3/Pt HS, which is ascribed to strong electron coupling and epitaxial growth between CsPbBr3 and Pt states. This extensive understanding of the electron transfer dynamics of CsPbBr3/Pt epitaxial HS may pave the way to designing highly efficient photovoltaic and photocatalytic applications.

PEAI Surface Treatment for Low Ion Migration and High-Performance FAPbBr3 Single-Crystal X-ray Detectors

Organometal halide perovskite single crystals (SCs) are the most promising candidates for the next generation of radiation detection materials. However, surface defects severely affect their detection performance and limit further applications. Here, we identified the surface defect types of FAPbBr3 SCs and employed phenethylammonium iodide (PEAI) solution to treat the crystal surface and to investigate their effects on ion migration, photoelectric performance, and X-ray detection performance. Our experimental results demonstrated that the surface defects, such as the metallic Pb and Br vacancies, can be effectively passivated by both the PEAI and the two-dimensional (2D) PEA2PbI4 layers. The PEAI layer can elongate the carrier lifetime, lower the trap density, and suppress ion migration in FAPbBr3 SCs. The 2D PEA2PbI4 layer can form a dense and full surface coverage, suppress ion migration, and lower the dark current of the SCs. The X-ray sensitivity of the PEAI-passivated FAPbBr3 SC detectors is 227.93 μCGyair-1 cm-2, which is an order of magnitude higher than that of the pristine FAPbBr3 SC detectors. This work demonstrates that surface treatment plays a critical role in the crystal quality and the X-ray detection performance of SCs.

High-Contrast Thermochromism in Room-Temperature Transparent Layered Perovskite PEA2PbBr4 with a High Temperature-Induced Bandgap Change Rate of 0.8 meV/K

Two-dimensional organic-inorganic hybrid layered perovskites have emerged as a new generation of optoelectronic materials. However, the thermochromism in organic-inorganic hybrid layered perovskites has been rarely explored in depth. A further understanding of the mechanism is necessary and favorable for the application. Here, transparent centimeter-sized single crystals of the organic-inorganic hybrid layered perovskite (C6H5C2H4NH3)2PbBr4 (PEA2PbBr4) were synthesized using an improved evaporation method. As a typical organic-inorganic hybrid layered perovskite, the PEA2PbBr4 single crystal shows high-contrast and progressive thermochromism exhibiting a change from colorlessness and transparency to lemon yellow in a wide temperature range of 200-450 K. Based on the calculation through the Varshni equation, the temperature-induced bandgap change rate directly associated with the high-contrast thermochromism of PEA2PbBr4 reaching 0.8 meV/K. This value is higher than that of many three-dimensional perovskites and traditional IV-III semiconductors. Furthermore, the temperature-dependent 193 nm photoluminescence spectra suggest that this high temperature-induced bandgap change rate of PEA2PbBr4 is a result of the competitive interaction between lattice thermal expansion and electron-phonon coupling (Fröhlich coupling coefficient ΓLO = 2.215). Based on the characteristics introduced above, PEA2PbBr4 as an organic-inorganic hybrid layered perovskite has a better performance in achieving the balance between high-contrast and high room-temperature transmittance. Therefore, PEA2PbBr4 is a material with great potential in applications like temperature-indicating labels. This work provides valuable insights into the thermochromism of layered perovskites, offering a new material system and approach for developing thermochromic materials with higher sensitivity and efficiency.

Chiral 2D and quasi-2D hybrid organic inorganic perovskites: from fundamentals to applications

Chiral 2D and quasi-2D hybrid organic-inorganic perovskites (HOIPs) are emerging as promising materials for a variety of applications principally related to optoelectronics and spintronics, thanks to the combined benefits deriving from both the chiral cation and the perovskite structure. Since its recent birth, this research field is tremendously growing, focalizing on the chemical composition tuning to unveil its influence on the related functional properties as well as on developing devices for practical applications. In this review, we focused on the properties of 2D and quasi-2D chiral HOIPs, firstly providing an overview on their chiroptical behaviour followed by their potential exploitation in devices investigated so far for various applicative fields.

Rashba-Type Band Splitting Effect in 2D (PEA)2PbI4 Perovskites and Its Impact on Exciton-Phonon Coupling

Despite a few recent reports on Rashba effects in two-dimensional (2D) Ruddlesden-Popper (RP) hybrid perovskites, the precise role of organic spacer cations in influencing Rashba band splitting remains unclear. Here, using a combination of temperature-dependent two-photon photoluminescence (2PPL) and time-resolved photoluminescence spectroscopy, alongside density functional theory (DFT) calculations, we contribute to significant insights into the Rashba band splitting found for 2D RP hybrid perovskites. The results demonstrate that the polarity of the organic spacer cation is crucial in inducing structural distortions that lead to Rashba-type band splitting. Our investigations show that the intricate details of the Rashba band splitting occur for organic cations with low polarity but not for more polar ones. Furthermore, we have observed stronger exciton-phonon interactions due to the Rashba-type band splitting effect. These findings clarify the importance of selecting appropriate organic spacer cations to manipulate the electronic properties of 2D perovskites.

Unravelling Structural, Optical, and Band Alignment Properties of Mixed Pb-Sn Metal-Halide Quasi-2D Ruddlesden-Popper Perovskites

Lead-tin (Pb-Sn) mixed-halide perovskites show potential for single-junction and tandem solar cells due to their adjustable band gaps, flexible composition, and superior environmental stability compared to three-dimensional (3D) perovskites. However, they have lower power conversion efficiencies. Understanding band alignment and charge carrier dynamics is essential for enhancing photovoltaic performance. In this view, herein we have prepared thin films of mixed Pb-Sn-based two dimensional (2D) Ruddlesden-Popper (RP) perovskites BA2FA(Pb1-xSnx)2I7 using a solution-based method. XRD study revealed the formation of orthorhombic phases for pristine (BA2FAPb2I7) and mixed Pb-Sn perovskite thin films. UV-vis analysis showed that different n = 2 and n = 3 phases are present in the pristine sample. In contrast, Pb-Sn-doped samples showed no signature of other phases with a prominent red-shift in the visible spectral region. Cyclic voltammetry showed peaks for electron transfers at the band edges. Additionally, electrochemical and optical band gap matching was observed, along with decreased peak intensity due to less reactant and altered electrolyte-perovskite interface stability. Density functional theory (DFT) calculations revealed that the reduced band gap is due to the alteration of electrostatic interactions and charge distribution within the lattice upon Sn substitution. Low-temperature PL analysis provided insights into charge carrier dynamics with Sn substitution and suggested the suppression of higher n phases and self-trapped excitons/carriers in mixed Pb-Sn quasi-2D RP perovskite thin films. This study sheds light on the electron transfer phenomena between TiO2 and SnO2 layers by estimating band offsets from valence band maximum (VBM) and conduction band minimum (CBM), which is crucial for future applications in fabricating stable and efficient 2D-Pb-Sn mixed perovskites for optoelectronic applications.

A Soldering Flux Tackles Complex Defects Chemistry in Sn-Pb Perovskite Solar Cells

Developing tin-lead (Sn-Pb) narrow-bandgap perovskites is crucial for the deployment of all-perovskite tandem solar cells, which can help to exceed the limits of single-junction photovoltaics. However, the Sn-Pb perovskite suffers from a large number of bulk traps and interfacial nonradiative recombination centers, with unsatisfactory open-circuit voltage and the consequent device efficiency. Herein, for the first time, it is shown that abietic acid (AA), a commonly used flux for metal soldering, effectively tackles complex defects chemistry in Sn-Pb perovskites. The conjugated double bond within AA molecule plays a key role for self-elimination of Sn4+-Pb0 defects pair, via a redox process. In addition, C═O group is able to coordinate with Sn2+, leading to the improved antioxidative stability of Sn-Pb perovskites. Consequently, a ten-times longer carrier lifetime is observed, and the defects-associated dual-peak emission feature at low temperature is significantly inhibited. The resultant device achieves a power conversion efficiency improvement from 22.28% (Ref) to 23.42% with respectable stability under operational and illumination situations.

Phase Engineering Reinforced Energy Transfer for High-Performance Blue Perovskite Light-Emitting Diodes

Layered metal-halide perovskites, a category of self-assembled quantum wells, are of paramount importance in emerging photonic sources, such as lasers and light-emitting diodes (LEDs). Despite high trap density in two-dimensional (2D) perovskites, efficient non-radiative energy funneling from wide- to narrow-bandgap components, sustained by the Förster resonance energy transfer (FRET) mechanism, contributes to efficient luminescence by light or electrical injection. Herein, it is demonstrated that bandgap extension of layered perovskites to the blue-emitting regime will cause sluggish and inefficient FRET, stemming from the tiny spectral overlap between different phases. Motivated by the importance of blue LEDs and inefficient energy transfer in materials with phase polydispersity, wide-bandgap quasi-2D perovskites with narrow phase distribution, improved crystallinity, and the pure crystal orientation perpendicular to the charge transport layer are developed. Based on this emitter, high-performance blue perovskite LEDs with improved electroluminescence (EL) external quantum efficiency (EQE) of 7.9% at 478 nm, a narrow full width at half-maximum (FWHM) of 22 nm and a more stable EL spectra are achieved. These results provide an important insight into spectrally stable and efficient blue emitters and EL devices based on perovskites.

Electron Spectroscopy and Microscopy: A Window into the Surface Electronic Properties of Polycrystalline Metal Halide Perovskites

In the past years, an increasing number of experimental techniques have emerged to address the need to unveil the chemical, structural, and electronic properties of perovskite thin films with high vertical and lateral spatial resolutions. One of these is angle-resolved photoemission electron spectroscopy which can provide direct access to the electronic band structure of perovskites, with the aim of overcoming elusive and controversial information due to the complex data interpretation of purely optical spectroscopic techniques. This perspective looks at the information that can be gleaned from the direct measurement of the electronic band structure of single crystal perovskites and the challenges that remain to be overcame to extend this technique to heterogeneous polycrystalline metal halide perovskites.

Lead-free Cs2Ag1-xNaxIn1 - yBiyCl6 perovskite films with broad warm-yellow emission for lighting applications

Lead-free halide double perovskite Cs2AgInCl6 has been extensively studied in recent years due to the lead toxicity and poor stability of common lead halide perovskites. In this study, sodium (Na+) and bismuth (Bi3+) doped into Cs2AgInCl6 double perovskite, then Cs2Ag1-xNaxIn1 - yBiyCl6 films with broadband warm-yellow emissions were achieved by the blade coating method. Herein, Na and Bi content were changed as variables at a series of parameter optimization experiments, respectively. In the Cs2Ag1-xNaxIn1 - yBiyCl6 systems, Na+ broke the parity-forbidden transition of Cs2AgInCl6, and Bi3+ suppressed non-radiative recombination. The partial replacement of Ag+ with Na+ ions and doping with Bi3+ cations were crucial for increasing the intensity of the PL emission. The experimental results showed that the photoluminescence quantum yield of the Cs2Ag0.4Na0.6In0.8Bi0.2Cl6 film was 66.38%, which was the highest data among all samples. It demonstrated remarkable stability under heat and ultraviolet conditions. After five thermal cycles, the PL intensity of the Cs2Ag0.4Na0.6In0.8Bi0.2Cl6 film is only reduced to approximately 5.7% of the initial value. After 720 h continuous ultraviolet irradiation, there occurred 31.9% emission decay of the film.

Effective Small Organic Molecule as a Defect Passivator for Highly Efficient Quasi-2D Perovskite Light-Emitting Diodes

The use of a small organic molecular passivator is proven to be a successful strategy for producing higher-performing quasi-2D perovskite light-emitting diodes (PeLEDs). The small organic molecule can passivate defects on the grain surround and surface of perovskite crystal structures, preventing nonradiative recombination and charge trapping. In this study, a new small organic additive called 2, 8-dibromodibenzofuran (diBDF) is reported and examines its effectiveness as a passivating agent in high-performance green quasi-2D PeLEDs. The oxygen atom in diBDF, acting as a Lewis base, forms coordination bonds with uncoordinated Pb2+, so enhancing the performance of the device. In addition, the inclusion of diBDF in the quasi-2D perovskite results in a decrease in the abundance of low-n phases, hence facilitating efficient carrier mobility. Consequently, PeLED devices with high efficiency are successfully produced, exhibiting an external quantum efficiency of 19.9% at the emission wavelength of 517 nm and a peak current efficiency of 65.0 cd A-1.

Lead-free Perovskite with Distorted [InX6]3- Octahedron Induced by Organic Cation and Enhanced PLQY by Sb Doping

In-based halide perovskites have attracted a lot of attention because of their unique broadband emission properties. Herein, a series of In-based hybrid perovskites of (H2MP)2InCl7·H2O (1), (H2EP)2InCl7·H2O (2), (H2MP)2InBr7·H2O (3), and (H2EP)2InBr7·H2O (4) were synthesized under the control of halogen ions and organic cations. 1, 2, and 4 exhibit obvious photoluminescence properties with peaks at 392, 442, and 652 nm, respectively. The effects of the different components on the crystal structure and photoluminescence properties are discussed by calculating the structural distortion of the [InX6]3- octahedron. The photoluminescence properties of 1 and 4 were significantly improved after Sb3+ doping with PLQY values of 57.12 and 41.53%. Finally, a white LED was successfully fabricated with the two doped compounds coated onto the 365 nm blue LED chip.

Structurally Flexible 2D Spacer for Suppressing the Electron-Phonon Coupling Induced Non-Radiative Decay in Perovskite Solar Cells

This study presents experimental evidence of the dependence of non-radiative recombination processes on the electron-phonon coupling of perovskite in perovskite solar cells (PSCs). Via A-site cation engineering, a weaker electron-phonon coupling in perovskite has been achieved by introducing the structurally soft cyclohexane methylamine (CMA+) cation, which could serve as a damper to alleviate the mechanical stress caused by lattice oscillations, compared to the rigid phenethyl methylamine (PEA+) analog. It demonstrates a significantly lower non-radiative recombination rate, even though the two types of bulky cations have similar chemical passivation effects on perovskite, which might be explained by the suppressed carrier capture process and improved lattice geometry relaxation. The resulting PSCs achieve an exceptional power conversion efficiency (PCE) of 25.5% with a record-high open-circuit voltage (VOC) of 1.20 V for narrow bandgap perovskite (FAPbI3). The established correlations between electron-phonon coupling and non-radiative decay provide design and screening criteria for more effective passivators for highly efficient PSCs approaching the Shockley-Queisser limit.

Intrinsic Self-Trapped Excitons in Graphitic Carbon Nitride

Graphitic carbon nitrides (g-C3N4) as low-cost, chemically stable, and ecofriendly layered semiconductors have attracted rapidly growing interest in optoelectronics and photocatalysis. However, the nature of photoexcited carriers in g-C3N4 is still controversial, and an independent charge-carrier picture based on the band theory is commonly adopted. Here, by performing transient spectroscopy studies, we show characteristics of self-trapped excitons (STEs) in g-C3N4 nanosheets including broad trapped exciton-induced absorption, picosecond exciton trapping without saturation at high photoexcitation density, and transient STE-induced stimulated emissions. These features, together with the ultrafast exciton trapping polarization memory, strongly suggest that STEs intrinsically define the nature of the photoexcited states in g-C3N4. These observations provide new insights into the fundamental photophysics of carbon nitrides, which may enlighten novel designs to boost energy conversion efficiency.

Modulating Inorganic Dimensionality of Ultrastable Lead Halide Coordination Polymers for Photocatalytic CO2 Reduction to Ethanol

Lead halide hybrids have shown great potentials in CO2 photoreduction, but challenging to afford C2+ reduced products, especially using H2O as the reductant. This is largely due to the trade-off problem between instability of the benchmark 3D structures and low carrier mobility of quasi-2D analogues. Herein, the lead halide dimensionality of robust coordination polymers (CP) was modulated by organic ligands differing in a single-atom change (NH vs. CH2), in which the NH groups coordinate with interlamellar [PbI2] clusters to achieve the important 2D→3D transition. This first CP based on 3D cationic lead iodide sublattice possesses both high aqueous stability and a low exciton binding energy of 25 meV that is on the level of ambient thermal energy, achieving artificial photosynthesis of C2H5OH. Photophysical studies combined with theoretical calculations suggest the bridging [PbI2] clusters in the 3D structure not only results in enhanced carrier transport, but also promotes the intrinsic charge polarization to facilitate the C-C coupling. With trace loading of Rh cocatalyst, the apparent quantum efficiency of the 3D CP reaches 1.4 % at 400 nm with a high C2H5OH selectivity of 89.4 % (product basis), which presents one of the best photocatalysts for C2 products to date.

Exciton engineering of 2D Ruddlesden-Popper perovskites by synergistically tuning the intra and interlayer structures

Designing two-dimensional halide perovskites for high-performance optoelectronic applications requires deep understanding of the structure-property relationship that governs their excitonic behaviors. However, a design framework that considers both intra and interlayer structures modified by the A-site and spacer cations, respectively, has not been developed. Here, we use pressure to synergistically tune the intra and interlayer structures and uncover the structural modulations that result in improved optoelectronic performance. Under applied pressure, (BA)2(GA)Pb2I7 exhibits a 72-fold boost of photoluminescence and 10-fold increase of photoconductivity. Based on the observed structural change, we introduce a structural descriptor χ that describes both the intra and interlayer characteristics and establish a general quantitative relationship between χ and photoluminescence quantum yield: smaller χ correlates with minimized trapped excitons and more efficient emission from free excitons. Building on this principle, we design a perovskite (CMA)2(FA)Pb2I7 that exhibits a small χ and an impressive photoluminescence quantum yield of 59.3%.

Chirality-Induced Crystallization and Defect Passivation of Perovskites: Toward High-Performance Solar Cells

As an additive for perovskites, in addition to functional groups, the steric configuration of molecules is worthy of consideration because it influences perovskite crystallization, thus determining whether defect passivation is effective without any side effects. In this work, the chiral molecules l- and d-pyroglutamic acid (l-PA and d-PA) were chosen as additives for perovskite passivators to reveal the reasons for the differences in passivation between amino acids with different steric configurations. Functional groups, such as the C═O groups and N-H groups of l-PA and d-PA, can passivate the perovskite defects. However, l-PA exhibited a more distorted steric configuration, while d-PA was more planar, leading to differences in the distances between the two C═O groups. Taking the Pb-Pb bond length as a reference, the shorter distance between the two C═O groups of l-PA distorts the perovskite lattice structure, which results in poor device stability. Conversely, the similar distance between the two C═O groups of d-PA promoted the preferred orientational growth of the perovskite. Finally, the d-PA-doped device accomplished an excellent efficiency of 24.11% with an improved open-circuit voltage of 1.17 V. Furthermore, the efficiency of the unencapsulated d-PA-doped device was maintained at 93% in N2 for more than 3000 h and 74% after 500 h of operation at maximum power point tracking under continuous illumination.

Structural Properties of Some Vacancy-Ordered Platinum Halide Perovskites

The structural changes that accompany the dehydration of Na2PtX6·6H2O (X = Cl, Br) were studied using in situ variable temperature synchrotron X-ray diffraction. The two hexahydrates are isostructural, containing isolated PtX6 octahedra separated by Na cations. Removal of the water results in the formation of the anhydrous vacancy ordered double perovskites Na2PtX6. The Na cation is too small for the cuboctahedron site of the parent cubic structure, resulting in cooperative tilting of the PtX6 octahedra and lowering of the symmetry. Replacing Na with a larger alkali metal (K, Rb, or Cs) invariably enabled the isolation of the anhydrous hexahalide, and we found no evidence that these readily hydrated. For all cations, other than Na, it was possible to observe the archetypical cubic structure, although for the two potassium salts K2PtBr6 and K2PtI6, this was only observed above a critical temperature of 175 and 460 K, respectively. As these two samples were cooled, symmetry lowering was observed, yielding a tetragonal structure initially and ultimately a monoclinic structure: Fm3̅m → P4/mnc → P21/n. These phase transitions are associated with the onset of long-range cooperative tilting of the PtX6 octahedra described using the Glazer tilt notation as a0a0a0 → a0a0c+ → a-a-c+.

Synthesis, Structure, and Optical Properties of a 0D Hybrid Organic-Inorganic Metal Halide (C5N2H14Cl)GeCl3

Due to the flexible structural tunability and excellent photoelectric performance, hybrid organic-inorganic metal halides (OIMHs) have attracted intensive attention and become a hot topic in the field of materials. It is important and necessary to explore new OIMHs and study their structure-property relationship. In this work, a new lead-free OIMH, (C5N2H14Cl)GeCl3, is synthesized by the combination of hydrothermal and solution methods. This compound features a zero-dimensional structure composed of inorganic [GeCl3]- trigonal pyramids surrounded by isolated Cl- anions and organic (C5N2H14)2+ cations. Preliminary characterization and first-principles calculations are performed to study its basic optical properties. Interestingly, (C5N2H14Cl)GeCl3 shows weak blue emission under ultraviolet excitation, and the intrinsic mechanism is discussed.

Sulfonate-Containing Polyelectrolytes for Perovskite Modification: Chemical Configuration, Property, and Performance

Three sulfonate-containing polyelectrolytes are elaborately designed and used to passivate perovskite film with the anti-solvent method. Under the influence of the secondary monomer, three copolymers present various chemical configurations and deliver different modification effects. Fluorene-thiophene copolymer STF has linear and highly-conjugated chain. STF-perovskite film presents large crystal grains. Fluorene-carbazole copolymer SCF has flexible chain and easily enters into grain boundary areas. SCF-perovskite film is homogenous and continuous. Fluorene-fluorene copolymer SPF agglomerates on the surface and is not applicable to the anti-solvent method. The full investigation demonstrates that STF and SCF not only conduct surface defect passivation, but also improve the film quality by being involved in the perovskite's crystallization process. Compared with the control device, the devices with STF and SCF deliver high efficiency and excellent stability. The unencapsulated devices with STF and SCT maintain ≈80% of the initial power conversion efficiency (PCE) after 40 days of storage under 30-40% relative humidity. SCF performs better and the device maintains 60% of the initial PCE after 20 days of storage under 60-80% relative humidity.

Synergistic nucleation regulation using 4,4',4''-tris(carbazol-9-yl)-triphenylamine and moisture for stably air-processed high-performance perovskite photodetectors

Perovskite photodetectors (PPDs) offer a promising solution with low cost and high responsivity, addressing the limitations of traditional inorganic photodetectors. However, there is still room for improvement in terms of the dark current and stability of air-processed PPDs. In this study, 4,4',4''-tris(carbazol-9-yl)-triphenylamine (TCTA) was utilized as a nucleation agent to enhance the quality of perovskite films. The synergistic effect of TCTA and moisture promotes rapid nucleation of PbI2-PbCl2, resulting in an increased nucleation rate and the elimination of pinholes in the film. By employing additive engineering, we obtained a PbI2-PbCl2 layer with high coverage, leading to a low density of traps in the corresponding perovskite film. Consequently, the modified PPD exhibits a remarkable reduction in dark current density by over one order of magnitude, reaching 2.4 × 10-10 A cm-2 at -10 mV, along with a large linear dynamic range (LDR) of 183 dB. Furthermore, the resulting PPD demonstrates remarkable stability, retaining 90% of the initial external quantum efficiency (EQE) value even after continuous operation for over 3200 hours. Owing to a fast response time in the nanosecond range, the PPD could convert modulated light signals into electrical signals at a speed of 588 Kbit s-1, highlighting the great potential in the field of optical communication.

3-Thiophenemalonic Acid Additive Enhanced Performance in Perovskite Solar Cells

The development of ambient-air-processable organic-inorganic halide perovskite solar cells (OIHPSCs) is a challenge necessary for the transfer of laboratory-scale technology to large-scale and low-cost manufacturing of such devices. Different approaches like additives, antisolvents, composition engineering, and different deposition techniques have been employed to improve the morphology of the perovskite films. Additives that can form Lewis acid-base adducts are known to minimize extrinsic impacts that trigger defects in ambient air. In this work, we used the 3-thiophenemalonic acid (3-TMA) additive, which possesses thiol and carboxyl functional groups, to convert PbI2, PbCl2, and CH3NH3I to CH3NH3PbI3 completely. This strategy is effective in regulating the kinetics of crystallization and improving the crystallinity of the light-absorbing layer under high relative humidity (RH) conditions (30-50%). As a result, the 3-TMA additive increases the yield of the power conversion efficiency (PCE) from 14.9 to 16.5% and its stability under the maximum power point. Finally, we found that the results of this work are highly relevant and provide additional inputs to the ongoing research progress related to additive engineering as one of the efficient strategies to reduce parasitic recombination and enhance the stability of inverted OIHPSCs in ambient environment processing.

Insights into the Growth Orientation and Phase Stability of Chemical-Vapor-Deposited Two-Dimensional Hybrid Halide Perovskite Films

Chemical vapor deposition (CVD) offers a large-area, scalable, and conformal growth of perovskite thin films without the use of solvents. Low-dimensional organic-inorganic halide perovskites, with alternating layers of organic spacer groups and inorganic perovskite layers, are promising for enhancing the stability of optoelectronic devices. Moreover, their multiple quantum-well structures provide a powerful platform for tuning excitonic physics. In this work, we show that the CVD process is conducive to the growth of 2D hybrid halide perovskite films. Using butylammonium (BA) and phenylethylammonium (PEA) cations, the growth parameters of BA2PbI4 and PEA2PbI4 and mixed halide perovskite films were first optimized. These films are characterized by well-defined grain boundaries and display characteristic absorption and emission features of the 2D quantum wells. X-ray diffraction (XRD) and a noninteger dimensionality model of the absorption spectrum provide insights into the orientation of the crystalline planes. Unlike BA2PbI4, temperature-dependent photoluminescence measurements from PEA2PbI4 show a single excitonic peak throughout the temperature range from 20 to 350 K, highlighting the lack of defect states. These results further corroborate the temperature-dependent synchrotron-based XRD results. Furthermore, the nonlinear optical properties of the CVD-grown perovskite films are investigated, and a high third harmonic generation efficiency is observed.

Broadband Emission Origin in Metal Halide Perovskites: Are Self-Trapped Excitons or Ions?

It has always been a goal to realize high efficiency and broadband emission in single-component materials. The appearance of metal halide perovskites makes it possible. Their soft lattice characteristics and significant electron-phonon coupling synergistically generate self-trapped excitons (STEs), contributing to a broadband emission with a large Stokes shift. Meanwhile, their structural/compositional diversity provides suitable active sites and coordination environments for doping of ns2 ions, allowing 3 Pn ( n =0,1,2) →1 S0 transitions toward broadband emission. The ns2 ions emission is phenomenologically similar to that of STE emission, hindering in-depth understanding of their emission origin, and leading to failure to meet the design requirements for practical applications. In this scenario, herein, the fundamentals and development of such two emission mechanisms are summarized to establish a clear and comprehensive understanding of the broadband emission phenomenon, which may pave the way to an ideal customization of broadband-emission metal halide perovskites.

Diammonium spacer-induced stable zigzag type 2D Dion-Jacobson lead/tin-based perovskite solar cells

Hybrid halide two dimensional (2D) perovskites have attracted considerable attention because they exhibit an improvement in perovskite solar cells compared with their 3D analogs. However, their bulky organic space group, leading to higher bandgaps and exciton binding energy, limits the charge transport in solar cells. Herein, the 3-(aminomethyl)pyridinium (3API2, C6H10N2I2) dication is incorporated into FA(Pb0.5Sn0.5)I3 to develop zigzag type 2D Dion-Jacobson-phase perovskites, which have low band gaps in the range of 1.44 to 1.53 eV for concentrations from 5 mol% to 20 mol% due to structural distortion. The introduction of the 3API2 cation increases the carrier conductivity and produces a high-quality perovskite film with no pinhole and connected grains, which is favorable for efficient carrier transport. Consequently, solar cells employing FA(Pb0.5Sn0.5)I3 with 10 mol% 3API2 added as a light absorber achieve a power conversion efficiency of 5.46% with an open-circuit voltage of 0.47 V, a fill factor of 58.07% and a short-circuit current density of 20.18 mA cm-2. With this class of new 2D Dion-Jacobson perovskite compositions, this work suggests potential future directions for improving the performance and device stability of perovskite solar cells.

Charge carrier dynamics and transient spectral evolutions in lead halide perovskites

Lead halide perovskites (LHPs) have emerged as promising materials for solar cell applications due to their unique photophysical properties. Most of the crucial properties related to solar cell performance such as carrier mobility, diffusion length, recombination rates, etc. have been estimated using ultrafast spectroscopic methods. While various methods have been developed to prepare and fabricate high-quality perovskite films for photovoltaic applications, understanding the charge carrier dynamics is also crucial at each stage of the charge generation, cooling, and recombination processes. Using femtosecond (fs) transient absorption (TA) spectroscopy, various stages of charge carrier dynamics in perovskite materials could be monitored. In this article, we focus on some of the recent experimental developments related to charge carrier dynamics in perovskites and discuss the current understanding of (1) exciton dissociation, (2) charge carrier thermalization, (3) hot carrier cooling, and (4) electron-phonon coupling along with some of the crucial spectral emergence in the pump-probe experiments of LHP materials.

The Scale Effects of Organometal Halide Perovskites

Organometal halide perovskites have achieved great success in solution-processed photovoltaics. The explorations quickly expanded into other optoelectronic applications, including light-emitting diodes, lasers, and photodetectors. An in-depth analysis of the special scale effects is essential to understand the working mechanisms of devices and optimize the materials towards an enhanced performance. Generally speaking, organometal halide perovskites can be classified in two ways. By controlling the morphological dimensionality, 2D perovskite nanoplatelets, 1D perovskite nanowires, and 0D perovskite quantum dots have been studied. Using appropriate organic and inorganic components, low-dimensional organic-inorganic metal halide hybrids with 2D, quasi-2D, 1D, and 0D structures at the molecular level have been developed and studied. This provides opportunities to investigate the scale-dependent properties. Here, we present the progress on the characteristics of scale effects in organometal halide perovskites in these two classifications, with a focus on carrier diffusion, excitonic features, and defect properties.

Photovoltaic properties of new solar cell based on ideal cubic NaNbO3 thin films: a combined experimental and density functional theory study

We explore the photovoltaic properties of a novel homojunction solar cell based on NNO(p)/NNO(n) perovskite by employing a combination of material synthesis, characterization and density functional theory calculations that are novel ideas compared to those previously reported in the literature. The band structure reveals that NaNbO3 introduces a n-type semiconductor. Moreover, using DFT calculation, we created n-NNO by a simple substitution in the O site by 4.16% fluorine atoms. Experimental and DFT calculation reveals that NNO perovskite exhibits a direct bandgap of ∼1.6 eV, with a slightly larger two other direct bandgaps of ∼2.13 and 3.24 eV. After extracting the necessary parameters, an electrical modelization of an n-NNO/p-NNO solar cell is performed by Maple software revealed that the cell conversion efficiency can reach 17% which presents a first path to identify a new solar cell based only on perovskite material.

Trap or Triplet? Excited-State Interactions in 2D Perovskite Colloids with Chromophoric Cations

Movement of energy within light-harvesting assemblies is typically carried out with separately synthesized donor and acceptor species, which are then brought together to induce an interaction. Recently, two-dimensional (2D) lead halide perovskites have gained interest for their ability to accommodate and assemble chromophoric molecules within their lattice, creating hybrid organic-inorganic compositions. Using a combination of steady-state and time-resolved absorption and emission spectroscopy, we have now succeeded in establishing the competition between energy transfer and charge trapping in 2D halide perovskite colloids containing naphthalene-derived cations (i.e., NEA2PbX4, where NEA = naphthylethylamine). The presence of room-temperature triplet emission from the naphthalene moiety depends on the ratio of bromide to iodide in the lead halide sublattice (i.e., x in NEA2Pb(Br1-xIx)4), with only bromide-rich compositions showing sensitized emission. Photoluminescence lifetime measurements of the sensitized naphthalene reveal the formation of the naphthalene triplet excimer at room temperature. From transient absorption measurements, we find the rate constant of triplet energy transfer (kEnT) to be on the order of ∼109 s-1. At low temperatures (77 K) a new broad emission feature arising from trap states is observed in all samples ranging from pure bromide to pure iodide composition. These results reveal the interplay between sensitized triplet energy transfer and charge trapping in 2D lead halide perovskites, highlighting the need to carefully parse contributions from competing de-excitation pathways for optoelectronic applications.

Manipulation of Shallow-Trap States in Halide Double Perovskite Enables Real-Time Radiation Dosimetry

Storage phosphors displaying defect emissions are indispensable in technologically advanced radiation dosimeters. The current dosimeter is limited to the passive detection mode, where ionizing radiation-induced deep-trap defects must be activated by external stimulation such as light or heat. Herein, we designed a new type of shallow-trap storage phosphor by controlling the dopant amounts of Ag+ and Bi3+ in the host lattice of Cs2NaInCl6. A distinct phenomenon of X-ray-induced emission (XIE) is observed for the first time in an intrinsically nonemissive perovskite. The intensity of XIE exhibits a quantitative relationship with the accumulated dose, enabling a real-time radiation dosimeter. Thermoluminescence and in situ X-ray photoelectron spectroscopy verify that the emission originates from the radiative recombination of electrons and holes associated with X-ray-induced traps. Theoretical calculations reveal the evolution process of Cl-Cl dimers serving as hole trap states. Analysis of temperature-dependent radioluminescence spectra provides evidence that the intrinsic electron-phonon interaction in 0.005 Ag+@ Cs2NaInCl6 is significantly reduced under X-ray irradiation. Moreover, 0.025 Bi3+@ Cs2NaInCl6 shows an elevated sensitivity to the accumulated dose with a broad response range from 0.08 to 45.05 Gy. This work discloses defect manipulation in halide double perovskites, giving rise to distinct shallow-trap storage phosphors that bridge traditional deep-trap storage phosphors and scintillators and enabling a brand-new type of material for real-time radiation dosimetry.

Two-Second-Annealed 2D/3D Perovskite Films with Graded Energy Funnels and Toughened Heterointerfaces for Efficient and Durable Solar Cells

The 2D/3D perovskite heterostructures have been widely investigated to enhance the efficiency and stability of perovskite solar cells (PSCs). However, rational manipulation of phase distribution and energy level alignment in such 2D/3D perovskite hybrids are still of great challenge. Herein, we successfully achieved spontaneous phase alignment of 2D/3D perovskite heterostructures by concurrently introducing both 2D perovskite component and organic halide additive. The graded phase distribution of 2D perovskites with different n values and 3D perovskites induced favorable energy band alignment across the perovskite film and boosted the charge transfer at the relevant heterointerfaces. Moreover, the 2D perovskite component also acted as a "band-aid" to simultaneously passivate the defects and release the residual tensile stress of perovskite films. Encouragingly, the blade-coated PSCs based on only ≈2 s in-situ fast annealed 2D/3D perovskite films with favorable energy funnels and toughened heterointerfaces achieved promising efficiencies of 22.5 %, accompanied by extended lifespan. To our knowledge, this is the highest reported efficiency for the PSCs fabricated with energy-saved thermal treatment just within a few seconds, which also outperformed those state-of-the-art annealing-free analogues. Such a two-second-in-situ-annealing technique could save the energy cost by up to 99.6 % during device fabrication, which will grant its low-coast implementation.

Electric Field-Induced Quenching of MAPbI3 Photoluminescence in PeLED Architecture

Photoluminescence (PL) measurements are a widely used technique for the investigation of perovskite-based materials and devices. Although electric field-induced PL quenching provides additional useful information, this phenomenon is quite complex and not yet clearly understood. Here, we address the PL quenching of methylammonium lead iodide (MAPbI3) perovskite in a light-emitting diode (PeLED) architecture. We distinguish two quenching mechanisms: (a) indirect quenching by slow irreversible or partially reversible material changes that occur gradually under the applied light and electric field and (b) direct quenching by the influence of the electric field on the charge carrier densities, their spatial distributions, and radiative recombination rates. Direct quenching, observed under the abrupt application of negative voltage, causes a decrease of the PL intensity. However, the PL intensity then partially recovers within tens of milliseconds as mobile ions screen the internal electric field. The screening time increases to hundreds of seconds at low temperatures, indicating activation energies for ion motion of about 80 meV. On the other hand, ultrafast time-resolved PL measurements revealed two main phases of direct quenching: an instantaneous reduction in the radiative carrier recombination rate, which we attribute to the electron and hole displacement within individual perovskite grains, followed by a second phase lasting hundreds of picoseconds, which is due to the charge carrier extraction and spatial separation of electron and hole "clouds" within the entire perovskite layer thickness.

Tailoring passivators for highly efficient and stable perovskite solar cells

There is an ongoing global effort to advance emerging perovskite solar cells (PSCs), and many of these endeavours are focused on developing new compositions, processing methods and passivation strategies. In particular, the use of passivators to reduce the defects in perovskite materials has been demonstrated to be an effective approach for enhancing the photovoltaic performance and long-term stability of PSCs. Organic passivators have received increasing attention since the late 2010s as their structures and properties can readily be modified. First, this Review discusses the main types of defect in perovskite materials and reviews their properties. We examine the deleterious impact of defects on device efficiency and stability and highlight how defects facilitate extrinsic degradation pathways. Second, the proven use of different passivator designs to mitigate these negative effects is discussed, and possible defect passivation mechanisms are presented. Finally, we propose four specific directions for future research, which, in our opinion, will be crucial for unlocking the full potential of PSCs using the concept of defect passivation.

Higher stability and better photoluminescence quantum yield of cesium lead iodide perovskites nanoparticles in the presence of CTAB ligand

Inorganic halide perovskites, such as CsPbI3, have unique optoelectronic properties which made them promising candidates for several applications. Unfortunately, these perovskites undergo rapid chemical decomposition and transformation into yellow δ-phase. Thus, the synthesis of stable cesium lead iodide perovskites remains an actual challenging field and it is imperative to develop a stabilized black phase for photovoltaic applications. For this purpose, a surfactant ligand was used to control the synthesis of inorganic perovskite CsPbI3 nanoparticles. Herein we demonstrate a new avenue for lead halide perovskites with the addition of either hexadecyltrimethylammonium bromide (CTAB) or silica nanoparticles to maintain in the first place; the stability of the α-CsPbI3 phase, and later on to boost their photoluminescence quantum yield (PLQY). The prepared perovskites were characterized using UV-visible absorption spectroscopy, fluorescence spectroscopy, scanning electron microscopy, thermogravimetric analysis and X-Ray diffraction technique. Results show higher stability of α-CsPbI3 phase and improvement in PLQY % to reach 99% enhancement in presence of CTAB. Moreover, the photoluminescence intensity of CsPbI3 nanoparticles was higher and was maintained for a longer duration in the presence of CTAB.

Thermalization and relaxation mediated by phonon management in tin-lead perovskites

Understanding and control of ultrafast non-equilibrium processes in semiconductors is key to making use of the full photon energy before relaxation, leading to new ways to break efficiency limits for solar energy conversion. In this work, we demonstrate the observation and modulation of slow relaxation in uniformly mixed tin-lead perovskites (MASnxPb1-xI3 and CsSnxPb1-xI3 nanocrystals). Transient absorption measurements reveal that slow cooling mediated by a hot phonon bottleneck effect appears at carrier densities above ~1018 cm-3 for tin-lead alloy nanocrystals, and tin addition is found to give rise to suppressed cooling. Within the alloy nanoparticles, the combination of a newly introduced high-energy band, screened Fröhlich interaction, suppressed Klemens decay and reduced thermal conductivity (acoustic phonon transport) with increased tin content contributes to the slowed relaxation. For inorganic nanocrystals where defect states couple strongly with carriers, sodium doping has been confirmed to benefit in maintaining hot carriers by decoupling them from deep defects, leading to a decreased energy-loss rate during thermalization and an enhanced hot phonon bottleneck effect. The slow cooling we observe uncovers the intrinsic photophysics of perovskite nanocrystals, with implications for photovoltaic applications where suppressed cooling could lead to hot-carrier solar cells.

Facet-Dependent Photoelectrochemistry on Single Crystal Organic-Inorganic Halide Perovskite Electrodes

Organometallic halide perovskites have garnered significant attention in various fields of material science, particularly solar energy conversion, due to their desirable optoelectronic properties and compatibility with scalable fabrication techniques. It is often unclear, however, how carrier generation and transport within complex polycrystalline films are influenced by variations in local structure. Elucidating how distinct structural motifs within these heterogeneous systems affect behavior could help guide the continued improvement of perovskite-based solar cells. Here, we present studies applying scanning electron microscopy (SECCM) to map solar energy harvesting within well-defined model systems of organometallic halide perovskites. Methylammonium lead bromide (MAPbBr3) single crystals were prepared via a low-temperature solution-based route, and their photoelectrochemical properties were mapped via SECCM using p-benzoquinone (BQ) in dichloromethane as a redox mediator. Correlated SECCM mapping and electron microscopy studies enabled facet-to-facet variations in photoelectrochemical performance to be revealed and carrier transport lengths to be evaluated. The photoelectrochemical behavior observed within individual single crystals was quite heterogeneous, attributable to local variations in crystal structure/orientations, intrafacet junctions, and the presence of other structural defects. These observations underscore the significance of controlling the microstructure of single perovskite crystals, presenting a promising avenue for further enhancement of perovskite-based solar cells.

Chiral Metal Halide Perovskites: Focus on Lead-Free Materials and Structure-Property Correlations

Hybrid organic-inorganic perovskites (HOIPs) are promising materials in several fields related to electronics, offering long carrier-diffusion lengths, high absorption coefficients, tunable band gaps, and long spin lifetimes. Recently, chiral perovskites have attracted huge interest thanks to the possibility of further widening the applications of HOIPs. Chiral materials, being intrinsically non-centrosymmetric, display several attractive physicochemical properties, including circular dichroism, circularly polarized photoluminescence, nonlinear optics, ferroelectricity, and spin-related effects. Recent studies have shown that chirality can be transferred from the chiral organic ligands into the inorganic perovskite framework, resulting in materials combining the advantages of both chirality and perovskite superior optoelectronic characteristics. As for HOIPs for photovoltaics, strong interest is currently devoted towards the development of lead-free chiral perovskites to overcome any toxicity issue. While considering the basic and general features of chiral HOIPs, this review mainly focuses on lead-free materials. It highlights the first attempts to understand the correlation between the crystal structure characteristics and the chirality-induced functional properties in lead and lead-free chiral perovskites.

Accurate Dimension Prediction for Low-Dimensional Organic-Inorganic Halide Perovskites via a Self-Established Machine Learning Strategy

Low-dimensional perovskites (LDPs) have enormous potential for the development of advanced optoelectronic devices and tackling the stability issue for the commercial application of perovskites. However, quantified structural dimensionality prediction for LDPs is still an intractable issue. Herein, we develop a self-established machine learning (ML)-assisted approach to predict the dimensionality of LDPs based on 195 reported amines that are classified as two-dimensional, one-dimensional, and zero-dimensional. The optimal K-nearest neighbor model allows us to realize an accuracy rate of 92.3% for the test data set containing 39 reported amines. Two features, i.e., ATSC1pe and SlogP_VSA2, associated with polarity and electrostatic potential on the van der Waals surface of an organic spacer, are identified from >1800 descriptors as key controlling factors determining the structure dimensionality. This work develops a typical paradigm for the application of a multiple-classification strategy of ML with an extremely high accuracy rate, which would thereby motivate the development of new types of LDPs.