Polarization-Dependent Photoluminescence of a Highly (100)-Oriented Perovskite Film

Lead halide perovskite is one of the attractive functional materials owing to its outstanding opto-electronic properties, which have been addressed in numerous studies. This study aims to clarify the link between the growth pattern and the charge carrier related properties for the highly oriented perovskite film along the [100] direction. For this purpose, a CH₃NH₃PbI₃ thin film mainly grown along the [100] direction was fabricated and subjected to spectroscopic analysis to understand the basic optoelectronic features of the oriented film. A perovskite film with random growth was also fabricated for comparison. In particular, results from excitation polarization photoluminescence spectroscopy (ExPPS) revealed that the orientation of transition dipole moment, which is relevant to the anisotropic property of the film, is attributed to the growth direction of the perovskite film. This study suggests that the absorption anisotropy can affect the anisotropy in properties of the perovskite device. Furthermore, photodetectors with the perovskite films were fabricated to investigate the effect of growth direction on the photodetector performances, revealing that a photodetector with the oriented perovskite film showed larger photoresponses. In order to provide an explanation for such result, we performed a PL lifetime imaging study of the oriented and randomly grown perovskite films.     

Thin‐Film Transformation of NH4PbI3 to CH3NH3PbI3 Perovskite: A Methylamine‐Induced Conversion–Healing Process

Methylamine‐induced thin‐film transformation at room‐temperature is discovered, where a porous, rough, polycrystalline NH₄PbI₃ non‐perovskite thin film converts stepwise into a dense, ultrasmooth, textured CH₃NH₃PbI₃ perovskite thin film. Owing to the beneficial phase/structural development of the thin film, its photovoltaic properties undergo dramatic enhancement during this NH₄PbI₃‐to‐CH₃NH₃PbI₃ transformation process. The chemical origins of this transformation are studied at various length scales.     

Molecular Intercalation and Electronic Two Dimensionality in Layered Hybrid Perovskites

In layered hybrid perovskites, such as (BA)₂PbI₄ (BA=C₄H₉NH₃), electrons and holes are considered to be confined in atomically thin two dimensional (2D) Pb–I inorganic layers. These inorganic layers are electronically isolated from each other in the third dimension by the insulating organic layers. Herein we report our experimental findings that suggest the presence of electronic interaction between the inorganic layers in some parts of the single crystals. The extent of this interaction is reversibly tuned by intercalation of organic and inorganic molecules in the layered perovskite single crystals. Consequently, optical absorption and emission properties switch reversibly with intercalation. Furthermore, increasing the distance between inorganic layers by increasing the length of the organic spacer cations systematically decreases these electronic interactions. This finding that the parts of the layered hybrid perovskites are not strictly electronically 2D is critical for understanding the electronic, optical, and optoelectronic properties of these technologically important materials.     

Lead‐ and Iodide‐Deficient (CH3NH3)PbI3 (d‐MAPI): The Bridge between 2D and 3D Hybrid Perovskites

3D and 2D hybrid perovskites, which have been known for more than 20 years, have emerged recently as promising materials for optoelectronic applications, particularly the 3D compound (CH₃NH₃)PbI₃ (MAPI). The discovery of a new family of hybrid perovskites called d ‐MAPI is reported: the association of PbI₂ with both methyl ammonium (MA⁺) and hydroxyethyl ammonium (HEA⁺) cations leads to a series of five compounds with general formulation (MA)_1−2.48x(HEA)_3.48x[Pb_1−xI_3−x]. These materials, which are lead‐ and iodide‐deficient compared to MAPI while retaining 3D architecture, can be considered as a bridge between the 2D and 3D materials. Moreover, they can be prepared as crystallized thin films by spin‐coating. These new 3D materials appear very promising for optoelectronic applications, not only because of their reduced lead content, but also in account of the large flexibility of their chemical composition through potential substitutions of MA⁺, HEA⁺, Pb²⁺ and I⁻ ions.     

Phase Regulation Strategy of Perovskite Nanocrystals from 1D Orthomorphic NH4PbI3 to 3D Cubic (NH4)0.5Cs0.5Pb(I0.5Br0.5)3 Phase Enhances Photoluminescence

This work reports this first synthesis of 1D orthomorphic NH₄PbI₃ perovskite nanocrystals (NCs) considering the role of inorganic ammonium ions at the nanoscale. The addition of bromide ions at the halogen site did not improve the photoluminescence properties. Furthermore, the 3D cubic phase of (NH₄)_0.5Cs_0.5Pb(I_0.5Br_0.5)₃ NCs with bright photoluminescence was synthesized by adding Cs ions into the crystal lattice of (NH₄)Pb(I_0.5Br_0.5)₃. Moreover, the photophysical properties of different phase structures were studied using femtosecond transient absorption (FTA) spectroscopy. The ultrafast trap state capture process is a key factor in the change of photoluminescence properties and the cubic phase may be the best structure for photoluminescence. These results suggest that the ammonium ion perovskite (AIP) nanocrystals could be potential materials for optoelectronic applications through A‐site cation substitution.     

Ion‐Exchange‐Induced 2D–3D Conversion of HMA1−xFAxPbI3Cl Perovskite into a High‐Quality MA1−xFAxPbI3 Perovskite

High‐quality phase‐pure MA_1?xFA_xPbI₃ planar films (MA=methylammonium, FA=formamidinium) with extended absorption and enhanced thermal stability are difficult to deposit by regular simple solution chemistry approaches owing to crystallization competition between the easy‐to‐crystallize but unwanted δ‐FAPbI₃/MAPbI₃ and FA_xMA_1?xPbI₃ requiring rigid crystallization conditions. Here A 2D–3D conversion to transform compact 2D mixed composition HMA_1?xFA_xPbI₃Cl perovskite precursor films into 3D MA_1?xFA_xPbI₃ (x=0.1–0.9) perovskites is presented. The designed Cl/I and H/FA(MA) ion exchange reaction induced fast transformation of compact 2D perovskite film, helping to form the phase‐pure and high quality MA_1?xFA_xPbI₃ without δ‐FAPbI₃ and MAPbI₃ impurity. In all, we successfully developed a facile one‐step method to fabricate high quality phase‐pure MA_1?xFA_xPbI₃ (x=0.1–0.9) perovskite films by 2D–3D conversion of HMA_1?xFA_xPbI₃Cl perovskite. This 2D–3D conversion is a promising strategy for lead halide perovskite fabrication.     

Exploring Lead‐Free Hybrid Double Perovskite Crystals of (BA)2CsAgBiBr7 with Large Mobility‐Lifetime Product toward X‐Ray Detection

Halide double perovskites have recently bloomed as the green candidates for optoelectronic applications, such as X‐ray detection. Despite great efforts, the exploration of promising organic–inorganic hybrid double perovskites toward X‐ray detection remains unsuccessful. Now, single crystals of the lead‐free hybrid double perovskite, (BA)₂CsAgBiBr₇ (BA⁺ is n‐butylammonium), featuring the unique 2D multilayered quantum‐confined motif, enable quite large μτ (mobility‐lifetime) product up to 1.21×10^?3 cm² V^?1. This figure‐of‐merit realized in 2D hybrid double perovskites is unprecedented and comparable with that of CH₃NH₃PbI₃ wafers. (BA)₂CsAgBiBr₇ crystals also exhibit other intriguing attributes for X‐ray detection, including high bulk resistivity, low density of defects and traps, and large X‐ray attenuation coefficient. Consequently, a vertical‐structure crystal device under X‐ray source yields a superior sensitivity of 4.2 μC Gy_air^?1 cm^?2.     

Control of Charge Transport in the Perovskite CH3NH3PbI3 Thin Film

Carrier density and transport properties in the CH₃NH₃PbI₃ thin film have been investigated. It is found that the carrier density, the depletion field, and the charge collection and transport properties in the CH₃NH₃PbI₃ absorber film can be controlled effectively by different concentrations of reactants. That is, the carrier properties and the self‐doping characteristics in CH₃NH₃PbI₃ films are strongly influenced by the reaction thermodynamic and kinetic processes. Furthermore, by employing mixed solvents with ethanol and isopropanol to deposit the CH₃NH₃PbI₃ film, the charge collection and transport efficiencies are improved significantly, thereby yielding an overall enhanced cell performance.     

Optoelectronic and morphology properties of perovskite/silicon interface layer for tandem solar cell application

Silicon (Si) solar cell has low optical absorption because of the low and indirect bandgap of Si, and the efficiency was trapped at 25% for 15 years. Si solar cell is able to achieve efficiency up to 30% by adding perovskite as multiple bandgap material through tandem formation. In this paper, the Si/perovskite interface layer was characterized to study the compatibility of perovskite on fluorine-doped tin oxide (FTO) glass and p-type Si wafer (p-Si). The single solution deposition step of methyl ammonium lead iodide, CH₃NH₃PbI₃ (MAPbI₃) perovskite film, was spin-coated at different concentration. The physical properties of the MAPbI₃/FTO and MAPbI₃/p-Si were obtained by profilometer, atomic force microscope, X-ray diffraction, and Raman spectroscopy. The optical properties were analyzed by ultraviolet-visible spectroscopy, photoluminescence, and infrared transmission. Then the electrical properties were measured by Hall effect. From the measurement, it is observed that 1.2M concentration of MAPbI₃ thin film has the highest thickness, smoothest film surface, and largest crystallite size compared with 0.8M and 1.0M. It is found that there is an interaction in perovskite/Si interface and caused in a low-wavelength shift, and the increase in concentration of MAPbI₃ helped in intensifying the Raman signal produced. 1.2M MAPbI₃ thin film had the highest enhancement in light trapping property rather than 0.8M and 1.0M. The bulk concentration and conductivity of 1.2M perovskite were higher, but the resistivity was lower than 0.8M MAPbI₃ because of more CH₃NH₃I and PbI₂ concentration within MAPbI₃ perovskite compound.     

Coordination Chemistry Dictates the Structural Defects in Lead Halide Perovskites

We show the influence of species present in precursor solution during formation of lead halide perovskite materials on the structural defects of the films. The coordination of lead by competing solvent molecules and iodide ions dictate the type of complexes present in the films. Depending on the processing conditions all PbIS₅⁺, PbI₂S_4, PbI₃S₃^?, PbI₄S₂^2?, PbI₅S₂^3?, PbI₆^4?and 1D (Pb₂I₄)_n chains are observed by absorption measurements. Different parameters are studied such as polarity of the solvent, concentration of iodide ions, concentration of solvent molecules and temperature. It is concluded that strongly coordinating solvents will preferentially form species with a low number of iodide ions and less coordinative solvents generate high concentration of PbI₆^?. We furthermore propose that all these plumbate ions may act as structural defects determining electronic properties of the photovoltaic films.     

Templated‐Assembly of CsPbBr3 Perovskite Nanocrystals into 2D Photonic Supercrystals with Amplified Spontaneous Emission

Perovskite nanocrystals (NCs) have revolutionized optoelectronic devices because of their versatile optical properties. However, controlling and extending these functionalities often requires a light‐management strategy involving additional processing steps. Herein, we introduce a simple approach to shape perovskite nanocrystals (NC) into photonic architectures that provide light management by directly shaping the active material. Pre‐patterned polydimethylsiloxane (PDMS) templates are used for the template‐induced self‐assembly of 10 nm CsPbBr₃ perovskite NC colloids into large area (1 cm²) 2D photonic crystals with tunable lattice spacing, ranging from 400 nm up to several microns. The photonic crystal arrangement facilitates efficient light coupling to the nanocrystal layer, thereby increasing the electric field intensity within the perovskite film. As a result, CsPbBr₃ 2D photonic crystals show amplified spontaneous emission (ASE) under lower optical excitation fluences in the near‐IR, in contrast to equivalent flat NC films prepared using the same colloidal ink. This improvement is attributed to the enhanced multi‐photon absorption caused by light trapping in the photonic crystal.     

Structure-property relationship of acceptor-substituted oligothiophenes

A series of oligothiophenes that are end-capped with dicyanovinyl (DCV) and 1,3,2-(2H)-dioxaborine (DOB) moieties has been prepared using standard procedures. Their optoelectronic properties have been investigated by cyclic voltammetry and optical absorption. The optical absorption has been measured both in solution and thin film state.     

Influence of Synthesis Conditions on the Morphology and Spectral-Luminescent Properties of Films of Organic-Inorganic Perovskite CH<Subscript>3</Subscript>NH<Subscript>3</Subscript>PbI<Subscript>2.98</Subscript>Cl<Subscript>0.02</Subscript>

The influence of the synthesis conditions (rotation speed used for spin-coating deposition of the film, film drying temperature, and the ratio of the PbI₂ and CH₃NH₃I reactants in solutions) on the microstructure, phase composition, and spectral-luminescent properties of films of organic-inorganic perovskite CH₃NH₃PbI_3–xCl _x (x = 0, 0.02) was elucidated.     

Comparison of optical and structural properties of nanostructure TiO2 thin film doped by Sn and Nb

The thin films of TiO₂ doped by Sn or Nb were prepared by sol–gel method under process control. The effects of Sn and Nb doping on the structural, optical and photo-catalytic properties of applied thin films have been studied by X-ray diffraction (XRD) high resolution transmission electron microscopy and UV–Vis absorption spectroscopy. Surface chemical state of thin films was examined by atomic X-ray photoelectron spectroscopy. XRD results suggest that adding impurities has a great effect on the crystallinity and particle size of TiO₂. Titania rutile phase formation in thin film was promoted by Sn⁴⁺ addition but was inhibited by Nb⁵⁺ doping. The activity of the photocatalyst was evaluated by photocatalytic degradation kinetics of aqueous methylene blue under UV and Visible radiation. The results show that the photocatalytic activity of the Sn-doped TiO₂ thin film have a larger degradation efficiency than Nb-doped TiO₂ under visible light, but under UV light photocatalytic activity of the Nb-doped TiO₂ thin film is better.     

The effects of the lattice modulation on the intermolecular motions of the MA cations of the tetragonal MAPbI3 phase

Quite recently, it was shown both experimentally and theoretically that the electronic properties of MAPbI₃ in the tetragonal phase can be affected by both MA⁺ cations and distortions of PbI₃⁺ and phonons play an important role in the dynamical electronic properties. In particular, the fluctuation of MAPbI₃ due to phase transition is also important. In this work, we shed light on the nature of the phonons in the THz region by constructing a model system, performing the first-principles simulation method, and analyzing the normal mode via the energy decomposition method. Our simulations show that, near the phase transition condition with a particular MA1 and MA2 orientational configuration, the hydrogen atoms of MA2 will participate in the phonon in the THz region with framework distortion of PbI₃⁺.     

Transition metal dichalcogenide thin films for solar hydrogen production

In the last three decades, transition metal dichalcogenides (TMDs) have been extensively studied for electronic, photonic, and energy applications. Different efforts are directed to find a holy grail of efficient and economically feasible materials that could be simple in production and available on a large scale. The interest in TMDs (MoS₂, WS₂, MoSe₂, WSe₂) stems from their suitable electronic structure for efficient solar light absorption and simple exfoliation technique of 2D crystallites due to the van der Waals bonding of these materials. This led to various designs and combinations of 2D single layers that could form heterojunctions and multijunctions for efficient light absorption, charge carrier generation/separation, and its transfer in optoelectronic and energy harvesting devices. Herein, TMD thin films are reviewed as photoelectrodes for solar hydrogen evolution and compared to that of other more developed materials.     

Effect of the heat treatment of CH3NH3PbI3 perovskite on its electrical and photoelectric properties

The effect of the annealing of CH₃NH₃PbI₃ perovskite on its electrical, photoelectric and optical properties has been estimated. The annealing leads to a two-phase structure consisting of perovskite and lead iodide, whose relative concentrations depend on the annealing temperature. The formation of a PbI₂ phase in a perovskite film upon heating leads to a decrease in the conductivity and photoconductivity of two-phase material, which contradicts the assumption of a decrease in recombination associated with PbI₂, obtained by measuring the parameters of a solar cell.     

The Second Spacer Cation Assisted Growth of a 2D Perovskite Film with Oriented Large Grain for Highly Efficient and Stable Solar Cells

The fabrication of high‐quality film with large grains oriented along the direction of film thickness is important for 2D Ruddlesden–Popper perovskite‐based solar cells (PVSCs). High‐quality 2D BA₂MA_n?1Pb_nI_3n+1 (BA⁺=butylammonium, MA⁺=methylammonium, n=5) perovskite films were fabricated with a grain size of over 1 μm and preferential orientation growth by introducing a second spacer cation (SSC⁺) into the precursor solution. Dynamic light scattering showed that SSC⁺ addition can induce aggregation in the precursor solution. The precursor aggregates are favorable for the formation of large crystal grains by inducing nucleation and decreasing the nucleation sites. Applying phenylethylammonium as SSC⁺, the optimized inverted planar PVSCs presented a maximum PCE of 14.09 %, which is the highest value of the 2D BA₂MA_n?1Pb_nI_3n+1 (n=5) PVSCs. The unsealed device shows good moisture stability by maintaining around 90 % of its initially efficiency after 1000 h exposure to air (Hr=25±5 %).     

Benign‐by‐Design Solventless Mechanochemical Synthesis of Three‐, Two‐, and One‐Dimensional Hybrid Perovskites

Organic–inorganic hybrid perovskites have attracted significant attention owing to their extraordinary optoelectronic properties with applications in the fields of solar energy, lighting, photodetectors, and lasers. The rational design of these hybrid materials is a key factor in the optimization of their performance in perovskite‐based devices. Herein, a mechanochemical approach is proposed as a highly efficient, simple, and reproducible method for the preparation of four types of hybrid perovskites, which were obtained in large amounts as polycrystalline powders with high purity and excellent optoelectronics properties. Two archetypal three‐dimensional (3D) perovskites (MAPbI₃ and FAPbI₃) were synthesized, together with a bidimensional (2D) perovskite (Gua₂PbI₄) and a “double‐chain” one‐dimensional (1D) perovskite (GuaPbI₃), whose structure was elucidated by X‐ray diffraction.     

Star‐Shaped Molecules as Dopant‐Free Hole Transporting Materials for Efficient Perovskite Solar Cells: Multiscale Simulation

On the reported TCP‐OH (See Scheme 1), other two star‐shaped molecules are theoretically designed by replacement of side group of TCP‐OH by N,N‐di(4‐methoxyphenyl)aniline for TPAP‐OH and oxygen‐bridged triarylamine for TBOPP‐OH . The core group, phenol, is kept in three molecules. Their potential to be hole transport material in perovskite solar cells without dopants is evaluated by multiscale simulations. The properties of isolated molecules are estimated by the frontier molecular orbital, absorption spectrum, and hole mobility. After that, the HTM@CH₃NH₃PbI₃ adsorbed system is studied to consider the influence of adsorption on HTM performance. Besides the primary judgment, the glass transition temperature is also simulated to determine the stability of amorphous film. Not only the chemical stability is evaluated but also the amorphous film stability is considered. The latter is almost neglected in previous theoretical studies to evaluate the properties of HTMs. The performance of a designed molecule is evaluated from both the isolated molecules and HTM@CH₃NH₃PbI₃ adsorbed system including aforementioned items, which is favorable to build reliable structure‐property relationship.