Optical and structural properties of CsPbBr3 perovskite quantum dots/PFO polymer composite thin films

Electronic Structure and Optical Properties of Inorganic Pm3m and Pnma CsPbX3 (X = Cl, Br, I) Perovskite: A Theoretical Understanding from Density Functional Theory Calculations

In this study, we investigated the optoelectronic properties of cubic (Pm3m) and orthorhombic (Pnma) CsPbX3 (X = I, Br, and Cl). We utilized the full potential linear augmented plane wave method, which is implemented in the WIEN2k code, to facilitate the investigation. Different exchange potentials were used to analyze the optoelectronic behavior using the available density functional theory methods. Our findings revealed that CsPbX3 perovskites display direct band gaps at the R and Г points for cubic (Pm3m) and orthorhombic (Pnma) structures, respectively. Among the exchange potentials, the mBJ-GGA method provided the most accurate results. These outcomes concurred with the experimental results. In both Pm3m and Pnma structures, interesting changes were observed when iodide (I) was replaced with bromine (Br) and then chlorine (Cl). The direct band gap at the R and Г points shifted to higher energy levels. Similarly, when I was replaced with Br and Cl, there was a noticeable decrease in the absorption coefficient, dielectric constants, refractive index, and reflectivity, in addition to a band gap shift to higher energy levels.

Lead-free double perovskite Cs2MBiCl6 (M = Ag, Cu): insights into the optical, dielectric, and charge transfer properties

Recently, double perovskites have shown excellent potential considering the instability and toxicity problems of lead halide perovskites in optoelectronic devices. Here, the double perovskites Cs₂MBiCl₆ (M = Ag, Cu) were successfully synthesized via the slow evaporation solution growth technique. The cubic phase of these double perovskite materials was verified through the X-ray diffraction pattern. The investigation of Cs₂CuBiCl₆ and Cs₂AgBiCl₆ utilizing optical analysis showed that their respective indirect band-gap values were 1.31 and 2.92 eV, respectively. These materials, which are double perovskites, were examined using the impedance spectroscopy technique within the 10^-1 to 10⁶ Hz frequency and 300-400 K temperature ranges. Jonncher's power law was utilized to describe AC conductivity. The outcomes of the study on charge transportation in Cs₂MBiCl₆ (where M = Ag, Cu) suggest that the non-overlapping small polaron tunneling mechanism was present in Cs₂CuBiCl₆, whereas the overlapping large polaron tunneling mechanism was present in Cs₂AgBiCl₆.

Hybrid Ligand Polymerization for Weakly Confined Lead Halide Perovskite Quantum Dots

Rational ligand passivation is essential to achieve a higher performance of weakly confined lead halide perovskite quantum dots (PQDs) via a mechanism of surface chemistry and/or microstrain. In situ passivation with 3-mercaptopropyltrimethoxysilane (MPTMS) produces CsPbBr₃ PQDs with an enhanced photoluminescence quantum yield (PLQY, Φ_PL) of up to 99%; meanwhile, charge transport of the PQD film can be enhanced by one order of magnitude. Herein, we examine the effect of the molecular structure of MPTMS as the ligand exchange agent in comparison to octanethiol. Both thiol ligands promote crystal growth of PQDs, inhibit nonradiative recombination, and cause blue-shifted PL, while the silane moiety of MPTMS manipulates surface chemistry and outperforms owing to its unique cross-linking chemistry characterized by FTIR vibrations at 908 and 1641 cm^-1. Emergence of the diagnostic vibrations is ascribed to hybrid ligand polymerization arising from the silyl tail group that confers the advantages of narrower size dispersion, lower shell thickness, more static surface binding, and higher moisture resistance. In contrast, the superior electrical property of the thiol-passivated PQDs is mostly determined by the covalent S-Pb bonding on the interface.

Surface Passivation for Promotes Bi-Excitonic Amplified Spontaneous Emission in CsPb(Br/Cl)3 Perovskite at Room Temperature

Perovskite-type lead halides exhibit promising performances in optoelectronic applications, for which lasers are one of the most promising applications. Although the bulk structure has some advantages, perovskite has additional advantages at the nanoscale owing to its high crystallinity given by a lower trap density. Although the nanoscale can produce efficient light emission, its comparatively poor chemical and colloidal stability limits further development of devices based on this material. Nevertheless, bulk perovskites are promising as optical amplifiers. There has been some developmental progress in the study of optical response and amplified spontaneous emission (ASE) as a benchmark for perovskite bulk phase laser applications. Therefore, to achieve high photoluminescence quantum yields (PLQYs) and large optical gains, material development is essential. One of the aspects in which these goals can be achieved is the incorporation of a bulk structure of high-quality crystallization films based on inorganic perovskite, such as cesium lead halide (CsPb(Br/Cl)₃), in polymethyl methacrylate (PMMA) polymer and encapsulation with the optimal thickness of the polymer to achieve complete surface coverage, prevent degradation, surface states, and surface defects, and suppress emission at depth. Sequential evaporation of the perovskite precursors using a single-source thermal evaporation technique (TET) effectively deposited two layers. The PL and ASEs of the bare and modified films with a thickness of 400 nm PMMA were demonstrated. The encapsulation layer maintained the quantum yield of the perovskite layer in the air for more than two years while providing added optical gain compared to the bare film. Under a picosecond pulse laser, the PL wavelength of single excitons and ASE wavelength associated with the stimulated decay of bi-excitons were achieved. The two ASE bands were highly correlated and competed with each other; they were classified as exciton and bi-exciton recombination, respectively. According to the ASE results, bi-exciton emission could be observed in an ultrastable CsPb(Br/Cl)₃ film modified by PMMA with a very low excitation energy density of 110 µJ/cm². Compared with the bare film, the ASE threshold was lowered by approximately 5%. A bi-exciton has a binding energy (26.78 meV) smaller than the binding energy of the exciton (70.20 meV).

Successful Growth of TiO2 Nanocrystals with {001} Facets for Solar Cells

The growth of nanocrystals (NCs) from metal oxide-based substrates with exposed high-energy facets is of particular importance for many important applications, such as solar cells as photoanodes due to the high reactivity of these facets. The hydrothermal method remains a current trend for the synthesis of metal oxide nanostructures in general and titanium dioxide (TiO₂) in particular since the calcination of the resulting powder after the completion of the hydrothermal method no longer requires a high temperature. This work aims to use a rapid hydrothermal method to synthesize numerous TiO₂-NCs, namely, TiO₂ nanosheets (TiO₂-NSs), TiO₂ nanorods (TiO₂-NRs), and nanoparticles (TiO₂-NPs). In these ideas, a simple non-aqueous one-pot solvothermal method was employed to prepare TiO₂-NSs using tetrabutyl titanate Ti(OBu)₄ as a precursor and hydrofluoric acid (HF) as a morphology control agent. Ti(OBu)₄ alone was subjected to alcoholysis in ethanol, yielding only pure nanoparticles (TiO₂-NPs). Subsequently, in this work, the hazardous chemical HF was replaced by sodium fluoride (NaF) as a means of controlling morphology to produce TiO₂-NRs. The latter method was required for the growth of high purity brookite TiO₂ NRs structure, the most difficult TiO₂ polymorph to synthesize. The fabricated components are then morphologically evaluated using equipment, such as transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), electron diffraction (SAED), and X-ray diffraction (XRD). In the results, the TEM image of the developed NCs shows the presence of TiO₂-NSs with an average side length of about 20-30 nm and a thickness of 5-7 nm. In addition, the image TEM shows TiO₂-NRs with diameters between 10 and 20 nm and lengths between 80 and 100 nm, together with crystals of smaller size. The phase of the crystals is good, confirmed by XRD. The anatase structure, typical of TiO₂-NS and TiO₂-NPs, and the high-purity brookite-TiO₂-NRs structure, were evident in the produced nanocrystals, according to XRD. SAED patterns confirm that the synthesis of high quality single crystalline TiO₂-NSs and TiO₂-NRs with the exposed {001} facets are the exposed facets, which have the upper and lower dominant facets, high reactivity, high surface energy, and high surface area. TiO₂-NSs and TiO₂-NRs could be grown, corresponding to about 80% and 85% of the {001} outer surface area in the nanocrystal, respectively.

Organic–Inorganic Manganese (II) Halide Hybrid Combining the Two Isomers Cis/Trans of [MnCl4(H2O)2]: Crystal Structure, Physical Properties, Pharmacokinetics and Biological Evaluation

A manganese (II) complex templated by hexahydro-1,4-diazepinediium as a counter ion was grown by slow evaporation from an aqueous solution at room temperature. The X-ray diffraction analysis revealed that the compound (C5H14N2)[MnCl4(H2O)2] crystallizes in the centrosymmetric space group P2/c of the monoclinic system. The crystal structure of the Mn(II) complex is characterized by an alternation of 0-dimensional organic and inorganic stacks linked together by N/O-H…Cl and N-H…O hydrogen bonds, which lead to a three-dimensional supramolecular architecture. In this structure, the inorganic layer is built up by independent anionic moieties combining the two isomers cis/trans of [MnCl4(H2O)2]2−. The thermal decomposition was studied by TGA-DTA techniques. The optical band gap and Urbach energy were obtained by Tauc’s equation. The direct and indirect band gap values are found to be 4.58 and 4.44 eV, respectively. Weak antiferromagnetic interactions are present in the molecule under study, according to magnetic measurements. An agar well diffusion technique was used to assess the synthetic compound’s biological activity, and the results showed that it has potent antibacterial (Gram-positive and Gram-negative) properties. Interestingly, the synthesized compound also displayed antilipase activity. These biological activities have been confirmed by the bioavailability and pharmacokinetic analyses.

Conjugated Polymers-Based Ternary Hybrid toward Unique Photophysical Properties

The improvement of optical and optoelectronic properties of the individual poly [2-methoxy-5- (2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), poly[2-methoxy-5-(3,7-dimethyl-octyloxy)-1,4-phenylenevinylene]–End capped with Dimethyl phenyl (OC1C10–PPV–DMP), and poly (9,9′-di- n -octylfluorenyl-2,7-diyl) (F8) was revealed by blending them in ternary hybrid with optimal ratio (F8/2 wt.% MEH-PPV/2 wt.% OC1C10–PPV–DMP). All individual and optimal ternary solutions were prepared via the solution-blending method followed by depositing them onto glass and ITO substrates using spin-coating technique. The semi-crystalline phase of the ternary hybrid and the strong mixing between the conjugated polymers were evidenced by observing the X-ray diffraction patterns that related to F8 into the hybrid diffractogram. The optical and optoelectronic properties of all prepared thin films were investigated in terms of absorption and emission spectra, Commission International d′Eclairage (CIE) coordinates, and current–voltage (I-V) characterizations. Emission peaks at the entire range of visible spectrum can be revealed from the ternary hybrid of the three individual conjugated polymers, producing white emission as evidenced from the emission spectrum and CIE coordinates of the hybrid. Among all fabricated organic light-emitting diodes (OLEDs) devices, the ternary hybrid-based-OLED revealed the best performance in terms of current and turn-on voltage.

Multifunctional π-Conjugated Additives for Halide Perovskite

Additive is a conventional way to enhance halide perovskite active layer performance in multiaspects. Among them, π-conjugated molecules have significantly special influence on halide perovskite due to the superior electrical conductivity, rigidity property, and good planarity of π-electrons. In particular, π-conjugated additives usually have stronger interaction with halide perovskites. Therefore, they help with higher charge mobility and longer device lifetime compared with alkyl-based molecules. In this review, the detailed effect of conjugated molecules is discussed in the following parts: defect passivation, lattice orientation guidance, crystallization assistance, energy level rearrangement, and stability improvement. Meanwhile, the roles of conjugated ligands played in low-dimensional perovskite devices are summarized. This review gives an in-depth discussion about how conjugated molecules interact with halide perovskites, which may help understand the improved performance mechanism of perovskite device with π-conjugated additives. It is expected that π-conjugated organic additives for halide perovskites can provide unprecedented opportunities for the future improvement of perovskite devices.

A Multifunctional Ionic Liquid Additive Enabling Stable and Efficient Perovskite Light-Emitting Diodes

The electroluminescence performance and long-term stability of perovskite light-emitting diodes (PeLEDs) are greatly affected by the film quality of perovskite emitting layer. Herein, the authors employ an ionic liquid, 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIm]OTf), to manipulate the growth of quasi-2D perovskite films by providing heterogeneous nucleation sites. The [BMIm]OTf molecules simultaneously realize uniform perovskite films by reducing the contact angles of precursor solution on the hole transport layer (HTL), and eliminate defect states through bonding [BMIm]⁺ cations to negatively-charged uncoordinated Br and OTf^- anions to uncoordinated Pb²⁺ defects that effectively suppresses the defect states assisted nonradiative recombination in perovskite films. As a result, the efficiency and the operational lifetime of the resultant PeLED are enhanced by more than twofold and threefold, respectively, achieving a maximum external quantum efficiency of 17.6% and an operational lifetime of over 500 min at an initial brightness of 100 cd m^-2 .

Photophysical Characteristics of Multicolor Emitting MDMO-PPV-DMP/ZnO Hybrid Nanocomposites

The tuning of photophysical properties of the poly[2-methoxy-5-(3,7-dimethyl-octyloxy)-1,4-phenylenevinylene]—end capped with dimethylphenyl (DMP), MDMO-PPV–DMP, was achieved by incorporation of ZnO NPs with various contents. Hybrid nanocomposites of MDMO-PPV–DMP with ZnO NPs were prepared by solution blending method and then deposited onto glass substrates. The structural properties of the hybrid nanocomposites samples were characterized using X-ray diffraction, FTIR, and FE-SEM, while their optical properties were extracted from the absorption and photoluminescence spectra. The energy band gap, energy tail, steepness parameter, and CIE chromatic coordinates were tuned by increase the content of ZnO NPs into the polymer matrix. The ZnO NPs incorporation assists the emission wavelength shift and multicolor emitting from the hybrid nanocomposites.

Density Functional Theory Analysis of Structural, Electronic, and Optical Properties of Mixed-Halide Orthorhombic Inorganic Perovskites

Inorganic metal-halide perovskites hold a lot of promise for solar cells, light-emitting diodes, and lasers. A thorough investigation of their optoelectronic properties is ongoing. In this study, the accurate modified Becke Johnson generalized gradient approximation (mBJ-GGA) method without/with spin orbital coupling (SOC) implemented in the WIEN2k code was used to investigate the effect of mixed I/Br and Br/Cl on the electronic and optical properties of orthorhombic CsPb(I1-x Br x )3 and CsPb(Br1-x Cl x )3 perovskites, while the Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA) method was used to investigate their structural properties. The calculated band gap (E g) using the mBJ-GGA method was in good agreement with the experimental values reported, and it increased clearly from 1.983 eV for CsPbI3 to 2.420 and 3.325 eV for CsPbBr3 and CsPbCl3, respectively. The corrected mBJ + SOC E g value is 1.850 eV for CsPbI3, which increased to 2.480 and 3.130 eV for CsPbBr3 and CsPbCl3, respectively. The calculated photoabsorption coefficients show a blue shift in absorption, indicating that these perovskites are suitable for optical and optoelectronic devices.

Perovskite Light-Emitting Diodes with EQE Exceeding 28% through a Synergetic Dual-Additive Strategy for Defect Passivation and Nanostructure Regulation

Quasi-2D perovskites have long been considered to have favorable "energy funnel/cascade" structures and excellent optical properties compared with their 3D counterparts. However, most quasi-2D perovskite light-emitting diodes (PeLEDs) exhibit high external quantum efficiency (EQE) but unsatisfactory operating stability due to Auger recombination induced by high current density. Herein, a synergetic dual-additive strategy is adopted to prepare perovskite films with low defect density and high environmental stability by using 18-crown-6 and poly(ethylene glycol) methyl ether acrylate (MPEG-MAA) as the additives. The dual additives containing COC bonds can not only effectively reduce the perovskite defects but also destroy the self-aggregation of organic ligands, inducing the formation of perovskite nanocrystals with quasi-core/shell structure. After thermal annealing, the MPEG-MAA with its CC bond can be polymerized to obtain a comb-like polymer, further protecting the passivated perovskite nanocrystals against water and oxygen. Finally, state-of-the-art green PeLEDs with a normal EQE of 25.2% and a maximum EQE of 28.1% are achieved, and the operating lifetime (T₅₀ ) of the device in air environment is over ten times increased, providing a novel and effective strategy to make high efficiency and long operating lifetime PeLEDs.

Enhancement of Light Amplification of CsPbBr3 Perovskite Quantum Dot Films via Surface Encapsulation by PMMA Polymer

Photonic devices based on perovskite materials are considered promising alternatives for a wide range of these devices in the future because of their broad bandgaps and ability to contribute to light amplification. The current study investigates the possibility of improving the light amplification characteristics of CsPbBr3 perovskite quantum dot (PQD) films using the surface encapsulation technique. To further amplify emission within a perovskite layer, CsPbBr3 PQD films were sandwiched between two transparent layers of poly(methyl methacrylate) (PMMA) to create a highly flexible PMMA/PQD/PMMA waveguide film configuration. The prepared perovskite film, primed with a polymer layer coating, shows a marked improvement in both emission efficiency and amplified spontaneous emission (ASE)/laser threshold compared with bare perovskite films on glass substrates. Additionally, significantly improved photoluminescence (PL) and long decay lifetime were observed. Consequently, under pulse pumping in a picosecond duration, ASE with a reduction in ASE threshold of ~1.2 and 1.4 times the optical pumping threshold was observed for PQDs of films whose upper face was encapsulated and embedded within a cavity comprising two PMMA reflectors, respectively. Moreover, the exposure stability under laser pumping was greatly improved after adding the polymer coating to the top face of the perovskite film. Finally, this process improved the emission and PL in addition to enhancements in exposure stability. These results were ascribed in part to the passivation of defects in the perovskite top surface, accounting for the higher PL intensity, the slower PL relaxation, and for about 14 % of the ASE threshold decrease.

Study of charge transfer mechanism and dielectric relaxation of all-inorganic perovskite CsSnCl3

In the field of commercialization, lead-free metal halide perovskite materials are becoming more popular these days because of their prospective use in solar cells and also in other optoelectronic applications. In this paper, a non-toxic CsSnCl₃ metal halide is successfully synthesized via the slow evaporation solution growth technique. Such systematic characterizations as differential scanning calorimetry (DSC) measurements, dielectric measurements, and variable-temperature structural analyses indicate that CsSnCl₃ goes through a reversible phase transformation at T = 391/393 K from the monoclinic to the cubic system. Optical measurements of CsSnCl₃ reveal a direct band-gap value of about 3.04 eV. The study of the charge transfer mechanism of CsSnCl₃ is carried out based on Elliott's theory. The conduction mechanism in CsSnCl₃ is interpreted through the following two approaches: the non-overlapping small polaron tunneling (NSPT) model (monoclinic phase) and the overlapping large polaron tunneling (OLPT) model (cubic phase). Moreover, the high dielectric constant of CsSnCl₃ which is associated with a low dielectric loss makes it a possible candidate for energy harvesting devices.

The conduction mechanism in CsSnCl3 is interpreted through the following two approaches: the non-overlapping small polaron tunneling (NSPT) model (monoclinic phase) and the overlapping large polaron tunneling (OLPT) model (cubic phase).

Excitons competition regulation via organic cation-site and halogen-site co-halogenation of (X-p-PEA)2Pb(Cl/Br)4 perovskites

In this work, we report a family of co-halogenated two-dimensional hybrid perovskites (2DHPs) based on phenethylammonium lead halogen ((PEA)₂Pb(Cl/Br)₄) in which the organic cation-site (PEA) is substituted with halogen at the para-site, namely the formation of 4-halophenethylamine (X-p-PEA) (X = Cl, Br; p: para-site). The organic cations are regulated by introducing halogen ions at the para-site of the benzene ring to promote the structural distortion of the lead halide octahedral inorganic layer. Furthermore, (X-p-PEA) causes a shift in the energy band distribution of 2DHPs. In this case, the photoluminescence competition of free excitons (FEs) and self-trapped excitons (STEs) changes the microscopic relaxation process of excitons. In addition, we found that (Br-p-PEA) can increase the photoluminescence quantum yield (PLQY). At the same time, we regulate the halogen-site of perovskites from lead-chloride perovskites (LCPs) to lead bromine perovskites (LBPs), achieving emission from white light to blue light. Therefore, the co-halogenation regulation strategy of organic cation-site and halogen-site can effectively regulate the photoluminescence wavelength and improve the PLQY. This is of great significance for the development of perovskite materials with specific optoelectronic applications.

Achieving Optical Gain of the CsPbBr3 Perovskite Quantum Dots and Influence of the Variable Stripe Length Method

High-quality inorganic cesium lead halide perovskite quantum dot (CsPbBr₃ PQD) thin films were successfully deposited directly from a powdered source and used as an active laser medium following the examination of their distinctive surface and structural properties. To determine the suitability of the CsPbBr₃ PQDs as an active laser medium, amplified spontaneous emission (ASE) and optical gain properties were investigated under picosecond pulse excitation using the variable stripe length (VSL) method. The thin film of CsPbBr₃ PQDs has exhibited a sufficient value of the optical absorption coefficient of ∼0.86 × 10⁵ cm^-1 near the band edge and a direct band gap energy E _g ∼2.38 eV. The samples showed enhanced emission, and ASE was successfully recorded at a low threshold. The light emitted from the edge was observed near 2.40 and 2.33 eV for the stimulated emission (SE) and ASE regimes, respectively. The nonradiative decay contributes excitons dominant over biexcitons in the sample edge emission above the ASE threshold, making it practical for CsPbBr₃ PQDs to be used as optical gain media without undergoing repeated SE processes above the threshold over long periods. A high value of the optical gain coefficient was recorded at 346 cm^-1.

Achieving Optical Gain of the CsPbBr3 Perovskite Quantum Dots and Influence of the Variable Stripe Length Method

High-quality inorganic cesium lead halide perovskite quantum dot (CsPbBr₃ PQD) thin films were successfully deposited directly from a powdered source and used as an active laser medium following the examination of their distinctive surface and structural properties. To determine the suitability of the CsPbBr₃ PQDs as an active laser medium, amplified spontaneous emission (ASE) and optical gain properties were investigated under picosecond pulse excitation using the variable stripe length (VSL) method. The thin film of CsPbBr₃ PQDs has exhibited a sufficient value of the optical absorption coefficient of ∼0.86 × 10⁵ cm^-1 near the band edge and a direct band gap energy E _g ∼2.38 eV. The samples showed enhanced emission, and ASE was successfully recorded at a low threshold. The light emitted from the edge was observed near 2.40 and 2.33 eV for the stimulated emission (SE) and ASE regimes, respectively. The nonradiative decay contributes excitons dominant over biexcitons in the sample edge emission above the ASE threshold, making it practical for CsPbBr₃ PQDs to be used as optical gain media without undergoing repeated SE processes above the threshold over long periods. A high value of the optical gain coefficient was recorded at 346 cm^-1.

Tuning the Optical Properties of MEH–PPV/PFO Hybrid Thin Films via the Incorporation of CsPbBr3 Quantum Dots

The current work examines the effects of cesium lead bromide (CsPbBr3) perovskite quantum dots (PQDs) on the structural and optical properties of conjugated polymer blends of poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene] (MEH–PPV) and poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO). MEH–PPV/PFO composite thin-films containing PQDs with weight ratios between 0.5 wt.% and 10 wt.% were prepared via a solution-blending method prior to spin-coating on glass substrates. The MEH–PPV/PFO composites’ crystallinity was improved, and the roughness was dramatically increased with higher PQDs content, as confirmed by X-ray diffraction (XRD) and atomic force microscopy (AFM), respectively. Conversely, a higher PQDs content resulted in a gradual reduction of the Urbach tail and an increase in the steepness parameter, thereby reducing the localized density of the electronic states within the forbidden bandgap of the hybrids. Moreover, a slight reduction in the direct and indirect bandgaps was found in PQDs/(MEH–PPV/PFO) composite films containing a higher PQDs content and provided evidence of the low concentration of the localized states. The incorporation of the PQDs resulted in enhanced non-radiative energy transfer processes in the MEH–PPV/PFO hybrids, which are very important for the development of optimized optoelectronic devices.

Investigation of the exciton relaxation processes in poly(9,9-dioctylfluorene-co-benzothiadiazole):CsPbI1.5Br1.5 nanocrystal hybrid polymer–perovskite nanocrystal blend

The combination of lead halide perovskite nanocrystals and conjugated polymer in a blend film opens the way to the realization of hybrid active layers with widely tunable optical and electrical properties. However, the interaction between the polymeric and the perovskite component of the blends is mainly unexplored to date. In this work we perform temperature-dependent photoluminescence and time resolved photoluminescence measurements in order to deeply investigate the photophysics of a poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT):CsPbI_1.5Br_1.5 nanocrystal hybrid film. Our results suggest that the primary interaction channel is charge transfer, both from F8BT to the NCs and from the NCs to F8BT, while Förster resonant energy transfer has no visible effects. Moreover, we show that the charge transfer is assisted by energy migration within the F8BT excited state distribution and that it is dependent on the local micromorphology of the film. Our work improves the current understanding of the polymer:perovskite NC interactions in hybrid films, and it is expected to be relevant for the development of hybrid organic–perovskite optoelectronic devices.

The emission properties of a hybrid polymer:perovskite nanocrystals (NCs) blend film are investigated, evidencing that the main interaction process is not Förster transfer, but instead bidirectional polymer → NC and NC → polymer charge transfer.

Single-Source Thermal Evaporation Growth and the Tuning Surface Passivation Layer Thickness Effect in Enhanced Amplified Spontaneous Emission Properties of CsPb(Br0.5Cl0.5)3 Perovskite Films

High-quality inorganic cesium lead halide perovskite CsPb(Br_0.5Cl_0.5)₃ thin films were successfully achieved through evaporation of the precursors and deposition sequentially by a single-source thermal evaporation system. The different melting points of the precursors were enabled us to evaporate precursors one by one in one trip. The resulting films through its fabrication were smooth and pinhole-free. Furthermore, this technique enabled complete surface coverage by high-quality perovskite crystallization and more moisture stability oppositely of that produce by solution-processed. Then the perovskite films were encapsulated by evaporated a polymethyl methacrylate (PMMA) polymer as a specialized surface passivation approach with various thicknesses. The blue emission, high photoluminescence quantum yield (PLQY), stable, and low threshold of amplified spontaneous emission (ASE) properties of CsPb(Br_0.5Cl_0.5)₃ films in the bulk structure at room temperature were achieved. The effects of the surface-passivation layer and its thickness on the optical response were examined. Detailed analysis of the dependence of ASE properties on the surface passivation layer thickness was performed, and it was determined this achieves performance optimization. The ASE characteristics of bare perovskite thin film were influenced by the incorporation of the PMMA with various thicknesses. The improvement to the surface layer of perovskite thin films compared to that of the bare perovskite thin film was attributed to the combination of thermal evaporation deposition and surface encapsulation. The best results were achieved when using a low PMMA thickness up to 100 nm and reducing the ASE threshold by ~11 μJ/cm² when compared with free-encapsulation and by ~13 μJ/cm² when encapsulation occurs at 200 nm or thicker. Compared to the bare CsPb(Br_0.5Cl_0.5)₃, ASE reduced 1.1 times when the PMMA thickness was 100 nm.

Ultra-Stable Polycrystalline CsPbBr3 Perovskite-Polymer Composite Thin Disk for Light-Emitting Applications

Organic–inorganic halide organometal perovskites have demonstrated very promising performance in optoelectronic applications, but their relatively poor chemical and colloidal stability hampers the further improvement of devices based on these materials. Perovskite material engineering is crucial for achieving high photoluminescence quantum yields (PLQYs) and long stability. Herein, these goals are attained by incorporating bulk-structure CsPbBr₃, which prevents colloidal degradation, into polymethyl methacrylate (PMMA) polymer in thin-disk form. This technology can potentially realize future disk lasers with no optical and structural contributions from the polymer. The polycrystalline CsPbBr₃ perovskite particles were simply obtained by using a mechanical processing technique. The CsPbBr₃ was then incorporated into the PMMA polymer using a solution blending method. The polymer enhanced the PLQYs by removing the surface trap states and increasing the water resistance and stability under ambient conditions. In our experimental investigation, the CsPbBr₃/PMMA composites were extraordinarily stable and remained strongly luminescent after water immersion for three months and air exposure for over one year, maintaining 80% of their initial photoluminescence intensity. The CsPbBr₃/PMMA thin disk produced amplified spontaneous emission for a long time in air and for more than two weeks in water.

Structural, Electronic, and Optical Properties of CsPb(Br1-xClx)3 Perovskite: First-Principles Study with PBE-GGA and mBJ-GGA Methods

The effect of halide composition on the structural, electronic, and optical properties of CsPb(Br1-xClx)3 perovskite was investigated in this study. When the chloride (Cl) content of x was increased, the unit cell volume decreased with a linear function. Theoretical X-ray diffraction analyses showed that the peak (at 2θ = 30.4°) shifts to a larger angle (at 2θ = 31.9°) when the average fraction of the incorporated Cl increased. The energy bandgap (Eg) was observed to increase with the increase in Cl concentration. For x = 0.00, 0.25, 0.33, 0.50, 0.66, 0.75, and 1.00, the Eg values calculated using the Perdew-Burke-Ernzerhof potential were between 1.53 and 1.93 eV, while those calculated using the modified Becke-Johnson generalized gradient approximation (mBJ-GGA) potential were between 2.23 and 2.90 eV. The Eg calculated using the mBJ-GGA method best matched the experimental values reported. The effective masses decreased with a concentration increase of Cl to 0.33 and then increased with a further increase in the concentration of Cl. Calculated photoabsorption coefficients show a blue shift of absorption at higher Cl content. The calculations indicate that CsPb(Br1-xClx)3 perovskite could be used in optical and optoelectronic devices by partly replacing bromide with chloride.

Triplet Energy Transfer Mechanism of Ternary Organic Hybrid Thin Films of PFO/MEH-PPV/CsPbBr3 Perovskite Quantum Dots

The triplet energy transfer mechanism of novel poly(9,9-di-n-octylflourenyl-2,7-diyl) (PFO)/poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV)/CsPbBr₃ perovskite quantum dot (PQD) hybrid thin films was comprehensively investigated. The concentrations of PFO and MEH-PPV in all the specimens were fixed, while the PQD content was varied with various weight ratios and premixed by a solution blending method before it was spin-coated onto glass substrates. The triplet non-radiative Förster resonance energy transfers (FRETs) in the PFO/MEH-PPV/PQDs ternary blend, the dual FRET from PFO to both PQDs and MEH-PPV, and the secondary FRET from PQDs to MEH-PPV were observed. The values of the Förster radius (R_o) of FRET from PFO to MEH-PPV in the presence of various PQD contents (Case I) increased from 92.3 to 104.7 Å, and they decreased gradually from 68.0 to 39.5 Å for FRET from PFO to PQDs in the presence of MEH-PPV (Case II). These R_o values in both cases confirmed the dominance of FRET in ternary hybrid thin films. Upon increasing the PQD content, the distance between the donor and acceptor molecules (R_DA) and the conjugation length (A_π) in both cases gradually decreased. The small values of R_o, R_DA, and A_π with a decrease in the energy transfer lifetime (τ_ET) due to an increase in the PQD contents in both Cases I and II confirmed the efficient FRET in the hybrid. To prevent intermolecular transfer in PFO, the concentrations of MEH-PPV (Case I) and PQDs (Case II) should be decreased to a range of 0.57-0.39 mM and increased in the range of 1.42-7.25 mM.

Reducing Amplified Spontaneous Emission Threshold in CsPbBr3 Quantum Dot Films by Controlling TiO2 Compact Layer

Amplified spontaneous emission (ASE) threshold in CsPbBr3 quantum dot films is systematically reduced by introducing high quality TiO2 compact layer grown by atomic-layer deposition. Uniform and pinhole-free TiO2 films of thickness 10, 20 and 50 nm are used as a substrates for CsPbBr3 quantum dot films to enhance amplified spontaneous emission performance. The reduction is attributed indirectly to the improved morphology of TiO2 compact layer and subsequently CsPbBr3 active layer as grown on better quality substrates. This is quantified by the reduced roughness of the obtained films to less than 5 nm with 50 nm TiO2 substrate. Considering the used growth method for the quantum dot film, the improved substrate morphology maintains better the structure of the used quantum dots in the precursor solution. This results in better absorption and hence lower threshold of ASE. Besides that, the improved film quality results further in reducing light scattering and hence additional slight optical enhancement. The work demonstrates a potential venue to reduce the amplified spontaneous emission threshold of quantum dot films and therefore enhanced their optical performance.

Computational Investigation of the Folded and Unfolded Band Structure and Structural and Optical Properties of CsPb(I1−xBrx)3 Perovskites

The structural, electronic, and optical properties of inorganic CsPb(I1−xBrx)3 compounds were investigated using the full-potential linear augmented-plane wave (FP-LAPW) scheme with a generalized gradient approximation (GGA). Perdew–Burke–Ernzerhof generalized gradient approximation (PBE-GGA) and modified Becke–Johnson GGA (mBJ-GGA) potentials were used to study the electronic and optical properties. The band gaps calculated using the mBJ-GGA method gave the best agreement with experimentally reported values. CsPb(I1−xBrx)3 compounds were wide and direct band gap semiconductors, with a band gap located at the M point. The spectral weight (SW) approach was used to unfold the band structure. By substituting iodide with bromide, an increase in the band gap energy (Eg) values of 0.30 and 0.55 eV, using PBE-GGA and mBJ-GGA potentials, respectively, was observed, whereas the optical property parameters, which were also investigated, demonstrated the reverse effect. The high absorption spectra in the ultraviolet−visible energy range demonstrated that CsPb(I1−xBrx)3 perovskite could be used in optical and optoelectronic devices by partly replacing iodide with bromide.

Density Functional Study of Cubic, Tetragonal, and Orthorhombic CsPbBr3 Perovskite

Cesium lead bromide (CsPbBr3) perovskite has recently gained significance owing to its rapidly increasing performance when used for light-emitting devices. In this study, we used density functional theory to determine the structural, electronic, and optical properties of the cubic, tetragonal, and orthorhombic temperature-dependent phases of CsPbBr3 perovskite using the full-potential linear augmented plane wave method. The electronic properties of CsPbBr3 perovskite have been investigated by evaluating their changes upon exerting spin-orbit coupling (SOC). The following exchange potentials were used: the local density approximation (LDA), Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA), Engel-Vosko GGA (EV-GGA), Perdew-Burke-Ernzerhof GGA revised for solids (PBEsol-GGA), modified Becke-Johnson GGA (mBJ-GGA), new modified Becke-Johnson GGA (nmBJ-GGA), and unmodified Becke-Johnson GGA (umBJ-GGA). Our band structure results indicated that the cubic, tetragonal, and orthorhombic phases have direct energy bandgaps. By including the SOC effect in the calculations, the bandgaps computed with mBJ-GGA and nmBJ-GGA were found to be in good agreement with the experimental results. Additionally, despite the large variations in their lattice constants, the three CsPbBr3 phases possessed similar optical properties. These results demonstrate a wide temperature range of operation for CsPbBr3.