Origin of Luminescent Centers and Edge States in Low-Dimensional Lead Halide Perovskites: Controversies, Challenges and Instructive Approaches

Quasi-2D Ruddlesden-Popper Lead Halide Perovskites: How Edge Matters

A band-mapping technique is introduced to investigate the formation of low-energy edge states in quasi-2D Ruddlesden-Popper (RP) perovskites, (BA)₂(MA)_n-1Pb_nI_3n+1, through a localized mode of measurement, namely, scanning tunneling spectroscopy. The local band structures measured at different points reveal the formation of 3D CH₃NH₃PbI₃ (MAPbI₃) at the edges of the perovskite nanosheets; for thin films, the 3D phase (n = ∞) could be seen to form at grain boundaries. The presence of MAPbI₃ at the edges or grain boundaries of the perovskites has led to self-forming type-II band alignment in BA₂MA₂Pb₃I₁₀ (n = 3). The rationale behind achieving a high-efficiency solar cell based on the material, which has a large exciton binding energy, has been inferred. Kelvin probe force microscopy studies under illumination have yielded a higher surface photovoltage at the edges compared to the interior and supported the inference of exciton dissociation due to internal type-II band alignment in the quasi-2D RP perovskites.

Photoluminescence and Raman Spectra of One-Dimensional Lead-free Perovskite CsCu2I3 Single-Crystal Wires

Lead-free highly luminescent CsCu₂I₃ perovskite has attracted much attention recently, but agreements on basic optical properties have remained unsettled. By correlating X-ray diffraction with the photoluminescence (PL) of CsCu₂I₃ single-crystal wires, we first show that blue PL at 420 nm originates from CuI. We then exclude defect states as a source for the broadband emission centered at 570 nm from the lack of defect absorption, PL under sub-bandgap photoexcitation, observations of a linear dependence of PL intensity on excitation laser power, and a strong spectral blueshift under mild hydrostatic pressure. Finally, using a model of the self-trapped exciton and the associated coordinate configuration diagram, we explain pressure evolutions of PL energy, intensity, and lifetime. Single-crystal wires also enable us to obtain polarization-dependent Raman spectra down to 10 cm^-1 and confirm their respective ambient crystal structure of orthorhombic Cmcm and phase transition to Pbnm at ∼5 GPa.

Insights Into to the KX (X = Cl, Br, I) Adsorption-Assisted Stabilization of CsPbI2 Br Surface

Despite the excellent optoelectronic properties, organic-inorganic hybrid perovskite solar cells (PSCs) still present significant challenges in terms of ambient stability. CsPbI₂ Br, a member of all-inorganic perovskites, may respond to this challenge because of its inherent high stability against light, moisture, and heat, and therefore has gained tremendous attraction recently. However, the practical application of CsPbI₂ Br is still impeded by the notorious phenomenon of photoinduced halide segregation. Herein, by applying first-principles calculations, the stability, electronic structure, defect properties, and ion-diffusion properties of the stoichiometric CsPbI₂ Br (110) surface and that with the adsorption of KX (X = Cl, Br, I) are systematically investigated. It is found that the adsorbed KX can serve as an external substitute of the halogen vacancies on the surface, therefore inhibiting halogen segregation and improving the stability of the CsPbI₂ Br surface. The KX can also eliminate deep-level defect states caused by antisites, thereby contributing to the promoted optoelectronic properties of CsPbI₂ Br. The mechanistic understanding of surface passivation in this work can lay the foundation for the future design of CsPbI₂ Br PSCs with optimized optoelectronic performance.

Study of Construction and Performance on Photoelectric Devices of Cs-Pb-Br Perovskite Quantum Dot

White LEDs were encapsulated using Cs₄PbBr₆ quantum dots and Gd₂O₃:Eu red phosphor as lamp powder. Under the excitation of a GaN chip, the color coordinates of the W-LED were (0.33, 0.34), and the color temperature was 5752 K, which is close to the color coordinate and color temperature range of standard sunlight. The electric current stability was excellent with an increase in the electric current, voltage, and luminescence intensity of the quantum dots and phosphors by more than 10 times. However, the stability of the quantum dots was slightly insufficient over long working periods. The photocatalytic devices were constructed using TiO₂, CsPbBr₃, and NiO as an electron transport layer, light absorption layer, and catalyst, respectively. The Cs–Pb–Br-based perovskite quantum dot photocatalytic devices were constructed using a two-step spin coating method, one-step spin coating method, and full PLD technology. In order to improve the water stability of the device, a hydrophobic carbon paste and carbon film were selected as the hole transport layer. The TiO₂ layer and perovskite layer with different thicknesses and film forming qualities were obtained by changing the spin coating speed. The influence of the spin coating speed on the device’s performance was explored through SEM and a J–V curve to find the best spin coating process. The device constructed by the two-step spin coating method had a higher current density but no obvious increase in the current density under light, while the other two methods could obtain a more obvious light response, but the current density was very low.

Revealing photoluminescence mechanisms of single CsPbBr3/Cs4PbBr6 core/shell perovskite nanocrystals

CsPbBr3 nanocrystals (NCs) encapsulated by Cs4PbBr6 has attracted extensive attention due to good stability and high photoluminescence (PL) emission efficiency. However, the origin of photoluminescence (PL) emission from CsPbBr3/Cs4PbBr6 composite materials has been controversial. In this work, we prepare CsPbBr3/Cs4PbBr6 core/shell nanoparticles and firstly study the mechanism of its photoluminescence (PL) at the single-particle level. Based on photoluminescence (PL) intensity trajectories and photon antibunching measurements, we have found that photoluminescence (PL) intensity trajectories of individual CsPbBr3/Cs4PbBr6 core/shell NCs vary from the uniform longer periods to multiple-step intensity behaviors with increasing excitation level. Meanwhile, second-order photon correlation functions exhibit single photon emission behaviors especially at lower excitation levels. However, the PL intensity trajectories of individual Cs4PbBr6 NCs demonstrate apparent "burst-like" behaviors with very high values of g 2(0) at any excitation power. Therefore, the distinguishable emission statistics help us to elucidate whether the photoluminescence (PL) emission of CsPbBr3/Cs4PbBr6 core/shell NCs stems from band-edge exciton recombination of CsPbBr3 NCs or intrinsic Br vacancy states of Cs4PbBr6 NCs. These findings provide key information about the origin of emission in CsPbBr3/Cs4PbBr6 core/shell nanoparticles, which improves their utilization in the further optoelectronic applications.

Enhanced Stability of All-Inorganic Perovskite Light-Emitting Diodes by a Facile Liquid Annealing Strategy

Ensuring the stability of all-inorganic halide perovskite light-emitting diodes (LEDs) has become an obstacle that needs to be broken for commercial applications. Currently, lead halide perovskite CsPbX₃ (X = Br, I, Cl) nanocrystals (NCs) are considered as alternative materials for future fluorescent lighting devices due to their combination of superior optical and electronic properties. However, the temperature of the surface of the LEDs will increase after long-term power-on work, which greatly affects the optical stability of CsPbX₃ NCs. In order to overcome this bottleneck issue, a strategy of annealing perovskite materials in liquid is proposed, and the changes in photoluminescence and electroluminescence (EL) behaviors before and after annealing are studied. The results show that the luminescence stability of the annealed perovskite materials is significantly improved. Moreover, the EL stability of different perovskite LED devices under long-term operation is monitored, and the performance of the annealed materials is particularly outstanding. The results have proved that this convenient and low-cost liquid annealing strategy is suitable for large-scale postprocessing of perovskite materials, granting them stable fluorescence emission, which will bring a new dawn to the commercialization of next-generation optoelectronic devices.

Photophysics of 2D Organic-Inorganic Hybrid Lead Halide Perovskites: Progress, Debates, and Challenges

2D organic-inorganic hybrid Ruddlesden-Popper perovskites (RPPs) have recently attracted increasing attention due to their excellent environmental stability, high degree of electronic tunability, and natural multiquantum-well structures. Although there is a rapid development of photoelectronic applications in solar cells, photodetectors, light emitting diodes (LEDs), and lasers based on 2D RPPs, the state-of-the-art performance is far inferior to that of the existing devices because of the limited understanding on fundamental physics, especially special photophysics in carrier dynamics, excitonic fine structures, excitonic quasiparticles, and spin-related effect. Thus, there is still plenty of room to improve the performances of photoelectronic devices based on 2D RPPs by enhancing knowledge on fundamental photophysics. This review highlights the special photophysics of 2D RPPs that is fundamentally different from the conventional 3D congeners. It also provides the most recent progress, debates, challenges, prospects, and in-depth understanding of photophysics in 2D perovskites, which is significant for not only boosting performance of solar cells, LEDs, photodetectors, but also future development of applications in lasers, spintronics, quantum information, and integrated photonic chips.

Highly efficient radiative recombination in intrinsically zero-dimensional perovskite micro-crystals prepared by thermally-assisted solution-phase synthesis

Zero-dimensional (0D) quantum confinement can be achieved in perovskite materials by the confinement of electron and hole states to single PbX₆⁴⁻ perovskite octahedra. In this work, 0D perovskite (Cs₄PbBr₆) micro-crystals were prepared by a simple thermally-assisted solution method and thoroughly characterized. The micro-crystals show a high level of crystallinity and a high photoluminescence quantum yield of 45%. The radiative recombination coefficient of the 0D perovskite micro-crystals, 1.5 × 10⁻⁸ s⁻¹ cm³, is two orders of magnitude higher than that of typical three-dimensional perovskite and is likely a strong contributing factor to the high emission efficiency of 0D perovskite materials. Temperature dependent luminescence measurements provide insight into the role of thermally-activated trap states. Spatially resolved measurements on single 0D perovskite micro-crystals reveal uniform photoluminescence intensity and emission decay behaviour suggesting the solution-based fabrication method yields a high-quality and homogenous single-crystal material. Such uniform emission reflects the intrinsic 0D nature of the material, which may be beneficial to device applications.

0D Cs₄PbBr₆ perovskite microcrystals exhibit a radiative recombination coefficient two orders of magnitude higher than typical 3D perovskite.

Emergence of Impurity-Doped Nanocrystal Light-Emitting Diodes

In recent years, impurity-doped nanocrystal light-emitting diodes (LEDs) have aroused both academic and industrial interest since they are highly promising to satisfy the increasing demand of display, lighting, and signaling technologies. Compared with undoped counterparts, impurity-doped nanocrystal LEDs have been demonstrated to possess many extraordinary characteristics including enhanced efficiency, increased luminance, reduced voltage, and prolonged stability. In this review, recent state-of-the-art concepts to achieve high-performance impurity-doped nanocrystal LEDs are summarized. Firstly, the fundamental concepts of impurity-doped nanocrystal LEDs are presented. Then, the strategies to enhance the performance of impurity-doped nanocrystal LEDs via both material design and device engineering are introduced. In particular, the emergence of three types of impurity-doped nanocrystal LEDs is comprehensively highlighted, namely impurity-doped colloidal quantum dot LEDs, impurity-doped perovskite LEDs, and impurity-doped colloidal quantum well LEDs. At last, the challenges and the opportunities to further improve the performance of impurity-doped nanocrystal LEDs are described.

Determining In-Plane Carrier Diffusion in Two-Dimensional Perovskite Using Local Time-Resolved Photoluminescence

The diffusion length of photogenerated carriers is a crucial parameter in semiconductors for optoelectronic applications. However, it is a challenging task to determine the diffusion length in layered nanoplatelets due to their anisotropic diffusion of photogenerated carriers and nanometer-thin thickness. Here, we demonstrate a novel method to determine the in-plane diffusion length of photogenerated carriers in layered nanoplatelets using local time-resolved photoluminescence. Also, the in-plane carrier diffusion length of 1.82 μm is obtained for an exfoliated (BA)₂PbI₄ (BA = CH₃(CH₂)₃NH₃) perovskite nanoplatelet. This method is particularly useful for weak luminescent materials and the materials that are easily damaged by long-term laser beam because of the high detection sensitivity. This technique is extendable to other layered materials and therefore plays a valuable role in the development and optimization of two-dimensional (2D) and three-dimensional (3D) semiconductor materials and devices for photovoltaic and photonic applications.

First-principles calculations of electronic structure and optical and elastic properties of the novel ABX3-type LaWN3 perovskite structure

The development of ABX₃-type advanced perovskite materials has become a focus for both scientific researchers and the material genome initiative (MGI). In addition to the traditional perovskite ABO₃ and halide perovskite ABX₃, LaWN₃ is discovered as a new ABX₃-type advanced perovskite structure. The elastic and optical properties of this novel LaWN₃ structure are systematically studied via DFT. Based on the calculated elastic constants, the bulk modulus, shear modulus, Young's modulus and Pugh modulus ratio are precisely obtained. Results show that (1) LaWN₃ is an indirect bandgap semiconductor with a hybrid occuring near the Fermi level and the main contributions are La-d, W-d and N-p. (2) LaWN₃ has a certain ductility. The optical constants, such as absorption spectrum, energy-loss spectrum, conductivity, dielectric function, reflectivity and refractive index, are analyzed and the static dielectric constant is 10.98 and the refractivity index is 3.31. (3) The optical constants of LaWN₃ are higher than those of other existing ABX₃-type materials, showing very promising application as a functional perovskite in the future. The existence of this stable LaWN₃ structure might widen the perovskite material's application, such as in photodetectors, light-emitting diodes, perovskite solar cells, fuel cells and so on.

Using first-principles calculation, the stable R3c LaWN₃ as a new ABX₃-type advanced perovskite structure is designed in the plan of the material genome initiative (MGI), which helps to widen the nowadays nitride perovskite material's application.

Down-Shifting and Anti-Reflection Effect of CsPbBr3 Quantum Dots/Multicrystalline Silicon Hybrid Structures for Enhanced Photovoltaic Properties

Over the past couple of decades, extensive research has been conducted on silicon (Si) based solar cells, whose power conversion efficiency (PCE) still has limitations because of a mismatched solar spectrum. Recently, a down-shifting effect has provided a new way to improve cell performances by converting ultraviolet (UV) photons to visible light. In this work, caesium lead bromide perovskite quantum dots (CsPbBr3 QDs) are synthesized with a uniform size of 10 nm. Exhibiting strong absorption of near UV light and intense photoluminescence (PL) peak at 515 nm, CsPbBr3 QDs show a potential application of the down-shifting effect. CsPbBr3 QDs/multicrystalline silicon (mc-Si) hybrid structured solar cells are fabricated and systematically studied. Compared with mc-Si solar cells, CsPbBr3 QDs/mc-Si solar cells have obvious improvement in external quantum efficiency (EQE) within the wavelength ranges of both 300 to 500 nm and 700 to 1100 nm, which can be attributed to the down-shifting effect and the anti-reflection property of CsPbBr3 QDs through the formation of CsPbBr3 QDs/mc-Si structures. Furthermore, a detailed discussion of contact resistance and interface defects is provided. As a result, the coated CsPbBr3 QDs are optimized to be two layers and the solar cell exhibits a highest PCE of 14.52%.

Negative photoconductivity in Cs4PbBr6 single crystal

Negative photoconductivity is firstly observed in large size Cs₄PbBr₆ single crystal that grown from Cs-rich solution. The Br vacancy and free excitons are responsible for this novel phenomena.

Extrinsic Green Photoluminescence from the Edges of 2D Cesium Lead Halides

Since the first report of the green emission of 2D all-inorganic CsPb₂ Br₅ , its bandgap and photoluminescence (PL) origin have generated intense debate and remained controversial. After the discovery that PL centers occupy only specific morphological structures in CsPb₂ Br₅ , a two-step highly sensitive and noninvasive optical technique is employed to resolve the controversy. Same-spot Raman-PL as a static property-structure probe reveals that CsPbBr₃ nanocrystals are contributing to the green emission of CsPb₂ Br₅ ; pressure-dependent Raman-PL with a diamond anvil cell as a dynamic probe further rules out point defects such as Br vacancies as an alternative mechanism. Optical absorption under hydrostatic pressure shows that the bandgap of CsPb₂ Br₅ is 0.3-0.4 eV higher than previously reported values and remains nearly constant with pressure up to 2 GPa in good agreement with full-fledged density functional theory (DFT) calculations. Using ion exchange of Br with Cl and I, it is further proved that CsPbBr_{3- x} X_x (X = Cl or I) is responsible for the strong visible PL in CsPb₂ Br_{5- x} X_x . This experimental approach is applicable to all PL-active materials to distinguish intrinsic defects from extrinsic nanocrystals, and the findings pave the way for new design and development of highly efficient optoelectronic devices based on all-inorganic lead halides.