Perovskite Samples Research Articles

Detailed chemical mechanism of the phase transition in nano-SrTiO3 perovskite with visible luminescence

In present study, we have explained the detailed synthesis chemistry of perovskite material SrTiO3 and the role of annealing temperature on its phase transition, which has not been reported till date. Rare-earth ion Eu was explored as a dopant and its concentration was optimized for achieving maximum luminescence. At annealing temperature of 600 °C, mixed phase consisting of TiO2, SrCO3 and Eu2O3 was observed in the XRD pattern, which eventually vanished at 1000 °C and a pure phase of SrTiO3 was obtained. Besides this, the SrTiO3 undoped and Eu doped samples were synthesized in bulk as well as nano forms, to study their structural, morphological and optical behavior. The SrTiO3:Eu optimized sample annealed at 1000 °C exhibited maximum luminescence due to improved crystallinity and phase purity of the host lattice. The synthesized perovskite sample can have multidisciplinary applications in advanced areas of photovoltaics.

Controlling and Optimizing Amplified Spontaneous Emission in Perovskites.

Perovskites are currently attracting extensive research interest as a wavelength-tunable lasing material. As a first step toward electrically pumped lasers, numerous investigations have recently reported amplified spontaneous emission (ASE) of optically pumped perovskites with remarkably low thresholds. Here, we investigate the optical aspects of perovskite ASE, to establish the design principle of materials and devices. We show that compared to solution-processed CsPbBr3, vacuum deposition yields superior ASE characteristics with a threshold of 35 μJ/cm2. The optical loss (Rloss) during lateral photon propagation in the waveguide mode is identified as a key parameter to determine the ASE quality. With spatially resolved photoluminescence, we determine Rloss as 40 and >1000 cm-1 for vacuum and solution-processed perovskites, respectively. We present a comprehensive model that relates ASE, gain length, optical loss, temperature, and density of states. Finally, we succeed in demonstrating ASE in perovskite samples with metal electrodes, mimicking a diode architecture for electrical pumping. Optical spacer layers are shown to play a crucial role in preventing metal absorption loss in waveguide modes.

Perovskite solar cells stability enhancement via analytical fabrication conditions

The rapid interest of low-cost hybrid organic-inorganic lead halide perovskite solar cells (PSCs) paved the way to develop its efficiency through different fabrication techniques. The inclusion of different monovalent organic or inorganic cation and halide anion in the A-site and X-site of the perovskite structure made the material to be stable at ambient conditions. In this study, a triple cation was carefully evaluated by stressing the importance of minimal processing steps, such as spin-coating, thermal annealing, and the addition of antisolvent. For enhancing the stability of perovskite film, different concentration of cesium cation was incorporated into the perovskite film. 10% cesium cation incorporated perovskite film showed a high reflectance with flat and full surface coverage. Considering the importance of other parameters and conditions, its stability was enhanced that made the perovskite film retain its black perovskite phase for four months even after exposure to ambient air. Power conversion efficiency (PCE) of 10.90% was measured under reverse scanning for 10% cesium incorporated perovskite solar cells. This structural, thermal and environmental stability, and reproducible characteristic behavior of the 10% cesium incorporated perovskite samples were achieved because of the modified fabrication techniques which are useful for future and large-scale applications.

Origin of Ionic Inhomogeneity in MAPb(IxBr1-x)3 Perovskite Thin Films Revealed by In-Situ Spectroscopy during Spin Coating and Annealing.

Irradiation-induced phase segregation in mixed methylammonium halide perovskite samples such as methylammonium lead bromide-iodide, MAPb(IxBr1-x)3, is being studied intensively because it limits the efficiency of wide band gap perovskite solar cells. It has been postulated that this phenomenon depends on the intrinsic ionic (in)homogeneity in samples already induced during film formation. A deeper understanding of the MAPb(IxBr1-x)3 formation processes and the influence of the halide ratio, solvents, and the perovskite precursor composition as well as the influence of processing parameters during deposition, e.g., spin coating and annealing parameters, is still lacking. Here, we use a fiber optic-based optical in-situ setup to study the formation processes of the MAPb(IxBr1-x)3 series on a subsecond time scale during spin coating and thermal annealing. In-situ UV-vis measurements during spin coating reveal the influence of different halide ratios, x, in the precursor solution on the preferential crystallization of the phase. Pure bromide samples directly form a perovskite phase, samples with high iodide content form a solvate intermediate phase, and samples with a mixed stoichiometry between 0.1 ≤ x ≤ 0.6 form both. This leads to a heterogeneous formation process via two competing reaction pathways, that leads to a heterogeneous mixture of phases, during spin coating and rationalizes the compositional heterogeneity of mixed bromide-iodide samples even after annealing.

Effect of Amount of Nb/Ti Co-Doping on the Microstructure and Electrochemical Performance of SrCoO<sub>3-δ</sub>-Based Perovskite

An increasing number of attention is being paid to perovskite oxides due to their relatively low prices and comparable ability in being an alternative choice to replace high-priced metal catalyst in many fields. In this work, a group of Ti/Nb co-doped SrCoO3-δ-based perovskite oxides SrTixNb0.2-xCo0.8O3-δ (x=0, 0.05, 0.1, 0.15, 0.175, 0.2) were synthesized and measured as candidates for oxygen evolution reaction (OER). Among all the perovskite samples, SrTixNb0.2-xCo0.8O3-δ (x=0.175, STNC-4) displayed the lowest OER overpotential of 0.4 V and the lowest Tafel plots as well as fastest charge transfer ability. This catalyst also revealed a great durability in alkaline solution.

Effect of PbI2 surface treatment with DMSO vapor on the properties and photodetector characteristics of CH3NH3PbI3–Cl perovskite films synthesized by a PbS-to-PbI2-to-perovskite sequence

Abstract In this work, we present the effect of the treatment of PbI2 thin films with dimethyl sulfoxide (DMSO) vapor on the final characteristics and properties of CH3NH3PbI3–xClx perovskite synthesized from a three-step approach. This is an additional step, thus, it changes the previous approach to a four-step sequence, which, for the present research, consists in the following: (1) chemical solution deposition of PbS thin films over glass substrates; (2) chemical transformation of the deposited PbS thin films into PbI2 by means of a gas–solid reaction with iodine vapor; (3) surface treatment of the synthesized PbI2 thin films with DMSO vapor, being this step the driving force of the present research; and (4) chemical transformation of the DMSO-treated PbI2 thin films into CH3NH3PbI3–xClx perovskite films by means of a liquid–solid reaction with a spin-coated solution mix of methylammonium iodide/chloride. We particularly present here the effect of four exposure times to DMSO vapor (5, 10, 15, and 20 min), which were compared against an untreated reference. The thin films of all the synthesized materials were uniform and strongly adhered to the glass substrate. The PbI2 and perovskite samples were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and ultraviolet–visible (UV–Vis) and fluorescence spectroscopies. It was observed that all the DMSO-treated PbI2 films present pores and, thus, show a decrease of transmittance. They also lead to subsequent perovskite samples that present blurred grain boundaries, which are less pronounced in comparison to the reference perovskite, which was synthesized from PbI2 with no DMSO treatment. This resultant morphology, translated in a better continuity of the perovskite material, was attributed as the cause of the increased photocurrent and responsivity of the photodetector fabricated from perovskite synthesized from DMSO-treated PbI2 film, as compared with the photodetector fabricated with the reference perovskite. Therefore, the treatment of PbI2 thin films with DMSO vapor is beneficial for improving the photodetector characteristics of CH3NH3PbI3–xClx perovskites.

Comparative studies of optoelectrical properties of prominent PV materials: Halide perovskite, CdTe, and GaAs

We compare three representative high performance PV materials: halide perovskite MAPbI3, CdTe, and GaAs, in terms of photoluminescence (PL) efficiency, PL lineshape, carrier diffusion, and surface recombination and passivation, over multiple orders of photo-excitation density or carrier density appropriate for different applications. An analytic model is used to describe the excitation density dependence of PL intensity and extract the internal PL efficiency and multiple pertinent recombination parameters. A PL imaging technique is used to obtain carrier diffusion length without using a PL quencher, thus, free of unintended influence beyond pure diffusion. Our results show that perovskite samples tend to exhibit lower Shockley–Read–Hall (SRH) recombination rate in both bulk and surface, thus higher PL efficiency than the inorganic counterparts, particularly under low excitation density, even with no or preliminary surface passivation. PL lineshape and diffusion analysis indicate that there is considerable structural disordering in the perovskite materials, and thus photo-generated carriers are not in global thermal equilibrium, which in turn suppresses the nonradiative recombination. This study suggests that relatively low point-defect density, less detrimental surface recombination, and moderate structural disordering contribute to the high PV efficiency in the perovskite. This comparative photovoltaics study provides more insights into the fundamental material science and the search for optimal device designs by learning from different technologies.

Influence of NaBH4 Liquid Reduction Over LaCuZn Perovskite for CO2 Hydrogenation to Methanol

The Cu-based perovskite shows good methanol selectivity but low CO2 conversion in CO2 hydrogenation to methanol. After treated by NaBH4 liquid reduction, perovskite LaCuZn sample (l-LCZ) is reduced partially, particle size becomes smaller and BET surface area increases. This sample (l-LCZ) was tested for CO2 hydrogenation to methanol direct without H2 reduction, and the catalytic activity of it is higher obviously than that over the catalyst LaCuZn which is reduced by H2 at 623 K for 6 h. NaBH4 liquid reduction has good influence on the perovskite properties.

Electronic structure of MAPbI3 and MAPbCl3: importance of band alignment

Since their first appearance, organic-inorganic perovskite absorbers have been capturing the attention of the scientific community. While high efficiency devices highlight the importance of band level alignment, very little is known on the origin of the strong n-doping character observed in the perovskite. Here, by means of a highly accurate photoemission study, we shed light on the energy alignment in perovskite-based devices. Our results suggest that the interaction with the substrate may be the driver for the observed doping in the perovskite samples.

Compositional and Morphological Changes in Water-Induced Early-Stage Degradation in Lead Halide Perovskites

With tremendous improvements in lead halide perovskite-based optoelectronic devices ranging from photovoltaics to light-emitting diodes, the instability problem stands as the primary challenge in their development. Among all factors, water is considered as one of the major culprits to the degradation of halide perovskite materials. For example, CH3NH3PbI3 (MAPbI3) and CH(NH2)2PbI3 (FAPbI3) decompose into PbI2 in days under ambient conditions. However, the intermediate changes of this degradation process are still not fully understood, especially the changes in early stage. Here we perform an in-situ investigation of the early-stage MAPbI3 and FAPbI3 degradation under high water vapor pressure. By probing the surface and bulk of perovskite samples using near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and XRD, our findings clearly show that PbI2 formation surprisingly initiates below the top surface or at grain boundaries, thus offering no protection as a water-blocking layer on surface or grain boundaries to slow down the degradation process. Meanwhile, significant morphological changes are observed in both samples after water vapor exposure. In comparison, the integrity of MAPbI3 film degrades much faster than the FAPbI3 film against water vapor. Pinholes and large voids are found in MAPbI3 film while only small number of pinholes can be found in FAPbI3 film. However, the FAPbI3 film suffers from its phase instability, showing a fast α-to-δ phase transition. Our results highlight the importance of the compositional and morphological changes in the early stage degradation in perovskite materials.

Energetics of Nonradiative Surface Trap States in Nanoparticles Monitored by Time-of-Flight Photoconduction Measurements on Nanoparticle-Polymer Blends.

Nanoparticles (NPs) have had increasingly successful applications including in emissive or photovoltaic devices; however, trap states associated with the surface of NPs often drastically reduce the efficiency of devices and are difficult to detect spectroscopically. We show the applicability of photoconduction as the means of detecting and quantifying trap states in NPs. We performed time-of-flight (ToF) photoconduction measurements, using semiconducting poly[bis(4-phenyl)(4-butyphenyl)amine] (P-TPD) doped with either core/shell CdSeS/CdS quantum dots (QDs) or perovskite CsPbBr3 NPs, both of which are carefully designed to be energetically matched. In the case of the QDs, a drop in the hole mobility from ∼10-3 to ∼10-4 cm2 V-1 s-1 was observed when compared to a control sample, suggesting the presence of a hole trapping. These trap states were found to be around -5.0 to -4.9 eV from the vacuum level. The presence of the trap states was further supported by a coincident reduction in the photoluminescence (PL), quantum yield (QY), and lifetime of the core/shell QDs after purification. Using the measured reductions in the PL, QY, and lifetime, the surface trap state density was estimated to increase by between 20 and 40%, most likely due to a ligand detachment. In the case of the perovskite NP-doped samples, a mobility of ∼10-3 cm2 V-1 s-1 was measured. Thus, doping with perovskite NPs did not generate any obvious hole trapping from the P-TPD matrix. The absence of a trapping may be related to the reduced surface-to-volume ratio and NP number density of the perovskite NPs compared to the core/shell QDs, since the perovskite NPs are approximately 10 times larger in radius than that of the core/shell QDs. Our results suggest that to minimize the presence of trap states with a view to improving device performance, large-size perovskite NPs appear to be better than small-size QDs.

Adsorption of Formic Acid on CH3NH3PbI3 Lead–Halide Organic–Inorganic Perovskites

The adsorption kinetics of formic acid (HCOOH) was studied under ultrahigh vacuum (UHV) on lead–halide organic–inorganic perovskites (CH3NH3PbI3) supported on indium-doped tin oxide (ITO). The thin film samples were made at ambient pressure using the common liquid-phase synthesis, but samples were degassed under UHV. We studied real-world samples which do have some carbon impurities. Adsorption and decomposition kinetics were characterized with thermal desorption spectroscopy. Furthermore, samples’ composition (including impurities) were inspected under UHV with Auger electron spectroscopy and X-ray photoelectron spectroscopy. Accordingly, formic acid adsorbs molecularly at 100 K but decomposed above 600 K. On ITO or thermally decomposed perovskite samples, formic acid adsorbs only molecularly. The gas-phase species detected are consistent with a decomposition mechanism via the formation of formate (HCOO) and formyl (HCO) intermediates. The perovskite itself also decomposed thermally, but it is stabilized...

Pulsed electric current boosts electrochemical performance and electro-conductivity of LaxSr1−xCryNi1−yO3 perovskite via exsolution of nanoparticles

In situ exsolved nanoparticles (NPs) on surfaces perform well in many catalytic applications. Herein, an emerging technique of pulsed electric current (PEC) for facilitating NPs exsolution was firstly utilized in functional materials towards tailoring structure and morphology of materials. PEC facilitated Ni NPs exsolution in situ and the exposed active sites were highly dispersed on the LaCrO3 substrate surface. Such result was confirmed by combining x-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy. As a consequence, PEC-treated and untreated LaCrO3 perovskite samples were employed as electrocatalysts for oxygen evolution reaction (OER). The PEC-treated samples exhibit obviously improved OER activity (lower overpotential, smaller Tafel slope) and lower reaction resistance than the untreated samples. This illustrates that the exsolved Ni NPs driven by PEC are beneficial for the OER performance and are probably the components of the OER active sites.

Mitigating Measurement Artifacts in TOF-SIMS Analysis of Perovskite Solar Cells.

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is one of the few techniques that can specifically distinguish between organic cations such as methylammonium and formamidinium. Distinguishing between these two species can lead to specific insight into the origins and evolution of compositional inhomogeneity and chemical gradients in halide perovskite solar cells, which appears to be a key to advancing the technology. TOF-SIMS can obtain chemical information from hybrid organic-inorganic perovskite solar cells (PSCs) in up to three dimensions, while not simply splitting the organic components into their molecular constituents (C, H, and N for both methylammonium and formamidinium), unlike other characterization methods. Here, we report on the apparently ubiquitous A-site organic cation gradient measured when doing TOF-SIMS depth-profiling of PSC films. Using thermomechanical methods to cleave perovskite samples at the buried glass/transparent conducting oxide interface enables depth profiling in a reverse direction from normal depth profiling (backside depth profiling). When comparing the backside depth profiles to the traditional front side profiled devices, an identical slight gradient in the A-site organic cation signal is observed in each case. This indicates that the apparent A-site cation gradient is a measurement artifact due to beam damage from the primary ion beam causing a continually decreasing ion yield for secondary ions of methylammonium and formamidinium. This is due to subsurface implantation and bond breaking from the 30 keV bismuth primary ion beam impact when profiling with too high of a data density. Here, we show that the beam-generated artifact associated with this damage can mostly be mitigated by altering the measurement conditions. We also report on a new method of depth profiling applied to PSC films that enables enhanced sensitivity to halide ions in positive measurement polarity, which can eliminate the need for a second measurement in negative polarity in most cases.

Optical Properties of Thin-Film Hybrid Organic-Inorganic Perovskites Under the Influence of Pyridine Vapor

Perovskite solar cells (PSС) attracted much attention due to their excellent optoelectronic characteristics and efficiency reached 24.2% only in the last few years. Despite their high efficiency, there is still a limited stability of the PSC, which must be overcome. It was recognized that a humid environment and oxygen is the main problem in the decomposition of perovskite materials. This article is devoted to the study of the perovskite thin films stability by treating the surface with pyridine. Perovskite samples were studied by UV-Vis spectroscopy as well as FTIR spectroscopy. FTIR indicated a slight change in the structure of perovskite films. While no damage under the action of pyridine was detected, the material retained its semiconductor properties. It is concluded that increasing the long-term stability of organo-inorganic perovskites requires further research.

Defect structure transformation during substitution in quadruple perovskite CaCu3-Ti4-Fe2O12 studied by positron annihilation spectroscopy

Positron annihilation spectroscopic studies have been performed to investigate the defect structural evolution in polycrystalline CaCu3-xTi4-xFe2xO12 (x = 0.0, 0.1, 0.3, 0.5 and 0.7) cubic perovskite samples. The positron lifetimes, relative intensities and Doppler-broadened annihilation lineshape parameters all indicate about the annulment of crystalline vacancies from the structure and attending porous ceramic structure at higher concentration of substitution. The compositional dependence of the intensity of the defect specific positron lifetime component clearly suggests that, in the initial stages of substitution, positron annihilates in vacancy-type defects. But, for higher concentrations of Fe3+, the materials are devoid of such defects and positron trapping takes place within the pores. The results of coincidence Doppler broadening spectroscopy measurements support such transformation in which the positron trapping sites are relocated to the ceramic pores leading to the modification of the material properties.

Bifunctional electrocatalytic activity of La0.8Sr0.2MnO3-based perovskite with the A-site deficiency for oxygen reduction and evolution reactions in alkaline media

The development of efficient noble-metal free electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is of great importance for energy storage devices, such as fuel cell and zinc-air battery. In this work, we report a facile approach to enhance the electrocatalytic activity of La0.8Sr0.2MnO3-based perovskite by introducing the deficiency in the A-site and transition-metal Fe in the B-site. Bifunctional electrocatalysts La0.8Sr0.2MnO3, (La0.8Sr0.2)0.98MnO3, (La0.8Sr0.2)0.95MnO3 and (La0.8Sr0.2)0.95Mn0.5Fe0.5O3 were prepared by a facile sol–gel process. The material characterization results showed that compared with La0.8Sr0.2MnO3, (La0.8Sr0.2)0.98MnO3 and (La0.8Sr0.2)0.95MnO3 have smaller particle size, more oxygen vacancies and proper Mn valence, which will benefit both ORR and OER. The results were verified by testing the electrocatalytic activities using rotating-disk electrode (RDE) in alkaline media. For the perovskite oxides with only A-site cation deficiency, the bifunctional electrocatalytic activities increase in the following order: La0.8Sr0.2MnO3 < (La0.8Sr0.2)0.98MnO3 < (La0.8Sr0.2)0.95MnO3. With partial substitution of Mn by Fe in the B-site, the (La0.8Sr0.2)0.95Mn0.5Fe0.5O3 perovskite oxide exhibits even better electrocatalytic activity. Further experiments reveal that (La0.8Sr0.2)0.95Mn0.5Fe0.5O3 has the highest current density (4.5 mA cm−2) in ORR which is comparable to commercial Pt/C (5 mA cm−2) and the enhancement of the OER is more obvious than that of the ORR. Subsequently, the perovskite samples were used as the cathode catalysts in zinc-air batteries. The results further prove that proper use of A-site deficiency and B-site Fe doping in perovskite oxides can boost up the electrocatalytic activities.

Phase Transition of La0.62Sr0.38MnO3 Perovskite Manganites

Bulk and nano La0.62Sr0.38MnO3 perovskite manganite samples were prepared using solid state and sonochemical reaction respectively. The ultrasonic velocities measurement was made on prepared samples using ultrasonic through transmission method, at a fundamental frequency of 5 MHz over wide range of temperatures. The temperature dependence of the ultrasonic parameters shows an interesting anomaly in bulk and nano perovskite samples. The observed dramatic softening and hardening in sound velocities are related to phase transitions. Further, a decrease in grain size in the nanostructured sample leads to a shift in the ferromagnetic transition temperature (TC) from 375 to 370 K.

EVOLUTION AND CHARACTERIZATIONS OF AURIVILLIUS BISMUTH TITANATE BY MODIFIED SOLID STATE PROCESSING

Mild agate mortar activation followed by sintering of precursors, in resemblance to modified solid state process, was studied for synthesis of the aurivillius bismuth titanate. Hand on mill activation was carried out for eight hours followed by annealing treatment at 1000°C for 12 hours in presence of air atmosphere to obtain the proper phase. Scherrer’s formula was utilized to estimate crystallite size along with planes of orientation. Crystallite size was about 65nm while prominent peaks of orientations were (-117), (006), (111), (200) and others. FESEM and TEM studies were carried to obtain the morphology and estimated grain size of the synthesized aurivillius compound. Morphological features execute the material system to be nanocrystalline in nature as estimated from grain size measurement in correspondence with XRD crystallite size estimation. FTIR analysis confirms M-O coordinations of synthesized modified perovskite (aurivillius) sample. Optical property was evaluated from UV-VIS analysis with prominent absorption spectra in the visible region. Tauc plot was used to estimate the band gap to be about 2.86eV &amp; 2eV for both direct and indirect transitions.

Development of LaFeO3 modified with potassium as catalyst for coal char CO2 gasification

Coal catalytic gasification is an effective way to realize clean coal utilization. The core of this technology is the preparation of highly efficient and stable catalysts. At present, perovskite-type oxides have attracted increasing attention as a promising catalyst because of its good thermal stability and redox activity. In this study, perovskite-type oxides were proposed to act as catalyst for coal char CO2 gasification, and K was further used for LaFeO3 catalyst modification to improve its catalytic activity. The catalytic performances of La1-xKxFeO3 (x=0-0.9) were tested by thermogravimetric analysis and fixed-bed tube reactor. The crystal structure was determined by X-ray diffraction (XRD), the percentage content and valence state of surface elements were examined by X-ray photoelectron spectroscopy (XPS), the surface morphology of perovskite samples was observed by scanning electron microscopy (SEM). Besides, the specific surface area and average particle diameter were analyzed by N2 absorption/desorption technology (BET) and laser particle size analyzer, respectively. Results suggested that the perovskite-type catalyst can effectively lowered the initial reaction temperature and exhibited improved catalytic activity after a part of La3+ at A-site was replaced by K+. The time required to achieve 50% carbon conversion in the initial reaction was reduced by 87.3% when the molar ratio of K+ increased from 0 to 0.7. The La0.3K0.7FeO3 perovskite also exhibited relatively stable catalytic activity during cyclic process, and its optimal temperature in coal char CO2 gasification was approximately 850 °C. The XRD patterns indicated that the perovskite structure of catalyst can remain unchanged when K+ doping ratio was lower than 0.8. Therefore, 0.7 is a suitable doping ratio. The perovskite La0.3K0.7FeO3 still exhibited perovskite crystal structure after 10 cycles of experiment. The XPS results indicated that additional oxygen vacancies and Fe4+ formed after K+ doping, which contributed to gas adsorption and electron transfer and improved oxidation performance. SEM and BET showed that the pore structure of catalyst was destroyed during cyclic gasification, resulted in the enlargement of particle size detected by laser particle size analyzer. Leading to the decrease in effective contact area and catalytic performance. Thus, K+-modified perovskite La1-xKxFeO3 is a kind of equipment-friendly, efficient and recyclable catalyst and can be used as a promising catalyst for coal char CO2 gasification.