A-site Cation Research Articles

Effect of Cs+ Doping on the Carrier Dynamics of MAPbI3 Perovskite.

Organic inorganic perovskite materials have received increasing attention in the optoelectronic field because of their unique properties. The ultrafast dynamics of photogenerated carriers determine photoelectric conversion efficiency, thus, it is feasible to influence the dynamics behavior of photogenerated carriers by regulating A-site cations. This paper mainly used transient absorption spectra (TAS) technology to study the photogenerated carriers relaxation processes of organic-inorganic perovskite CsxMA1-xPbI3 materials at different x values. Three sets of time constants were obtained by global fitting at different values of x. The experimental results showed that the crystal structure of perovskite could be affected by adjusting the Cs+ doping amount, thereby regulating the carrier dynamics. The appropriate amount of A-cation doping not only maintained the organic-inorganic perovskite crystal phase, but also prolonged the photogenerated carrier's lifetime. The 10% Cs+ doping CsxMA1-xPbI3 perovskite has potential for solar cell applications. We hope that our research can provide dynamics support for the development of organic-inorganic perovskite in solar cells.

Restoring La1.2Sr0.8Ni0.5Mn0.5O4+δ electrocatalysis for proton-conducting solid oxide fuel cells with the elimination of bi-shell surface

Meliorating the particle surface could alter the surface properties, thus potentially enhancing the electrode reactions. In this assignment, a recovery of the intrinsic nanostructure at La1.2Sr0.8Ni0.5Mn0.5O4+δ (LSNM) particle surface with the elimination of the bi-shell nano-surface layers caused by dopant segregation was attempted based on the lower powder calcination temperature and introduction of A-site cation deficiency. The A-site deficient (La1.2Sr0.8)0.9Ni0.5Mn0.5O4+δ [D-LSNM], accompanied with no phase separation on surface, is successfully obtained, in which the smaller particles and more oxygen vacancies would promote the electrocatalysis, oxygen reduction reaction and oxygen migration, thus boosting the electrode reactions. The surface nanostructure-recovered D-LSNM cathode achieves a preeminent performance with power output of 1547 and 1030 mW cm−2 at 700 and 600 °C, being better than other K2NiF4-type R-P cathodes in the literature. Anyhow, the preferable cell performance along with the fine stability suggests that the original surface-nanostructured D-LSNM is a promising cathode alternative for low-temperature proton-conducting solid oxide fuel cells. This work provides a new kind of reverse thinking to design highly catalytically active cathode materials via the restoration of the intrinsic nanostructure at surface, which would be beneficial to the related electrocatalytic fields.

A-site cations tailoring the activity of LnMnO3 perovskites for CO and propane oxidation

To understand the role of A-site cations of perovskites on the reactivity, the LnMnO3 perovskites (Ln = La, Nd, Sm) with varied A-site cations were prepared by a facile glycine combustion method and evaluated for CO and propane oxidation. The activity for both reactions followed the trend: NdMnO3 > SmMnO3 > LaMnO3, demonstrating that lanthanide cations at A sites significantly affect the catalytic performance. Combined with Fourier transmission infrared spectra, temperature programmed reduction and desorption techniques, it was revealed that both the strength of Mn–O bonds and the adsorption of reactants played crucial roles on the activity for CO oxidation and C3H8 combustion on the LnMnO3. The adsorption of the reactants was responsible for the higher activity on the NdMnO3 than on the SmMnO3, while the strength of Mn–O bonds for the higher activity on the SmMnO3 than on the LaMnO3. The NdMnO3 possessed both weak Mn–O bonds and favorable adsorption of the reactants achieved the highest activity for CO oxidation and C3H8 combustion. The best NdMnO3 catalyst achieved CO and propane complete conversion at 160 and 360 °C, 90 and 60 °C lower than the LaMnO3, respectively.

Cathode Surface Segregating Modification for Boosting Oxygen Reduction Reactions: Coupling Theory and Experiment in Proton Conducting Solid Oxide Fuel Cells (H-SOFCs)

The surface electrochemical catalysis reaction is complex due to the unpredictable surface morphology and severe reaction environment, especially for cathodes in proton conducting solid oxide fuel cells (H-SOFCs) where water vapor is generated and evaporated at a high operating temperature. The first-generation cathode materials (LaxSr1-xMO3-δ, M=Fe, Co, Mn) tend to have A-site cation segregation, and this segregation layer reduces the performance. Surface segregation phenomena tends to be ignored by computer simulations because surface termination is described as a highly crystallized layer, misleading the development of material computing science. Therefore, a combination of theory and surface investigation is essential for future work in this community. In this work, we use density functional theory (DFT) for designing a higher-performing cathode material, La0.35Bi0.15Sr0.5FeO3-δ (LBSF), for improvement of both bulk and surface electrochemical character in an intrinsic manner, predicting a candidate material with good performance that can be used in solid oxide proton conducting fuel cells (H-SOFCs). By combining a sensitive surface analysis technique, low energy ion scattering (LEIS), surface segregation is finely considered and determined that the particles are fully covered with a Sr segregation layer on the outer most surface. Aberration-Corrected transmission electron microscopy (ACTEM) is also applied and shows an amorphous structure at the particle edge. With the determination of this segregation layer, a reality-closed model is given, and related surface reactions are modelled based on this layer instead of a conventional cleaved, well-crystallized surface. Also, as bismuth diffuses towards the surface due to the lone pair effect, the segregation becomes an oxygen reduction reaction preferred layer instead of a blocking layer. After computational prediction and surface analysis, LBSF is successfully applied into H-SOFCs and shows excellent performance with a peak power density of 1630 mW cm−2 at 700 oC. In addition, long term stability tests were conducted to ensure stability in future commercial use, and the experimental results show that LBSF works stable under extremely high temperature operating environments. In addition, a full cell was stabilized for about 300 hours, indicating a potential commercial application in the future.

Effect of a-Site Cation Defect on Electrochemical Performance and Stability of La0.8Sr0.2Co0.2Fe0.8O3- δ Cathode

The application of Solid oxide fuel cells (SOFCs) is restricted by the insufficient catalytic activity in oxygen reduction reaction and the impedance increase of cathode in long-term operation at medium and low temperature[1]. In this work, we prepare La0.8Sr0.2Co0.2Fe0.8O3- δ (LSCF) nanofiber cathode material with different A-site defects by electrospinning technique, and systematically explore the influence of A-site cation defect on the electrochemical performance and long-term stability of LSCF cathode. The experimental results show that the cathode with La defects at A-site (La0.77Sr0.2Co0.2Fe0.8O3- δ) obtains better electrochemical performance than the cathode with Sr defects (La0.8Sr0.17Co0.2Fe0.8O3- δ). With the increase of the proportion of La deficiency at A-site, the long-term operation stability of LSCF cathode is improved, and Sr surface segregation is suppressed. However, excessive A-site deficiency cause lattice instability of LSCF, resulting in the separation of Co3O4, which reduces the electrochemical catalytic activity of LSCF cathode. Proper A-site defects can not only increase the oxygen vacancy of LSCF material, but also inhibit the Sr surface segregation, which makes La0.77Sr0.2Co0.2Fe0.8O3- δ a promising optimized SOFCs cathode material. Figure 1

Protonic Ceramic Fuel Cells with Stoichiometric Electrolytes

Among various types of solid oxide electrochemical cells, protonic ceramic fuel cells (PCFCs) have been expected to reduce the operating temperature below 600 °C with their significantly high ionic conductivity and activation energy. For example, recently developed proton conductor (BaZr0.4Ce0.4Y0.1Yb0.1O3-δ, BZCYYb) ideally has ~12-fold higher ionic conductivity (2.810-2 S/cm at 600 °C) and low activation energy (Ea < 0.4 eV) than those of conventional oxide ion conductor (Y0.08Zr0.92O2-δ; 2.810-3 S/cm at 600 °C and Ea ~0.87 eV). However, the low chemical and physical stabilities of proton conducting oxides during fabrication processes, primarily due to the high mobility and volatility of Barium (Ba), induce the substantially lower electrochemical performance than their predictions, limiting their utilization and application. Here, we present the chemically and physically stable BZCYYb electrolyte with the desired stoichiometry and the average grain size of ~10 µm by controlling the chemical potential of the A-site cation, Ba, near the BZCYYb electrolyte surface during the sintering process. A stoichiometric BZCYYb-based PCFC in an anode-supported configuration exhibits 1.90/cm2 and 1.01 W/cm2 with an extremely low ohmic resistance of ~0.060 ohm·cm2 at 650 °C and ~0.082 ohm·cm2 at 550 °C, respectively. Our results may open the possibility of finding a technical breakthrough for various next-generation electrochemical systems having desirable material properties without the requirement for complicated and expensive fabrication processes.

Electrospun Nanofiber Electrodes with in situ Exsolved Nanocatalysts for Symmetrical SOCs

Solid oxide cells (SOCs) offer great promise for efficient energy generation and storage in a variety of applications. Symmetrical SOCs with identical anode and cathode materials configuration favoring an efficient reversible operation of the cells, allow both electricity production from traditional and renewable energy sources in fuel cell mode, and clean, efficient fuel production (in reverse/electrolysis mode). In addition, the symmetrical cell design is particularly advantageous, including: decreased number of cell components, simplification of fabrication process and alleviating the chemical stability issues related with materials, as well as the enhanced resistance to coking and sulphur poisoning, when cells are fuelled by alternative sources (non-H2) [1]. However, electrode material candidates with excellent redox stability and electrocatalytic activities in both oxidizing and reducing conditions for symmetrical SOCs are limited due to the strict criteria. The design of electrode materials with in situ exsolution of nanoparticles significantly benefits the performance of SOCs [2]. In the anode operation condition, metallic nanoparticles are in situ exsolved from the parent electrode materials decorating on the electrode surface, substantially improving the catalytic activity and electronic conductivity of anode, thus boosting the cell performance. The in situ exsolved nanoparticles can be socketed to the parent oxide, enhancing the stability and hydrocarbon coking tolerance, making the symmetrical SOCs fuelled by cheaper and readily available non-hydrogen alternatives. Moreover, the possible reversible exsolution/dissolution of nanoparticles can potentially resolve the particle agglomeration and coke formation during usage of non-hydrogen fuels [2, 3]. The electrospinning technique has been reported to obtain nanofiber-structured electrodes for SOCs, which efficiently contributes to the enhancement of the electrochemical performance of SOCs [4].In this work, the A-site deficient (Ln, Ba/Sr)2-α(Fe,M)2-x(Ni,Co)xO6-δ (Ln = selected lanthanides; M = Mn, Ti, Cr, Mo, W) perovskites were successfully obtained and evaluated as novel electrode materials for SOCs. It has been found that the A-site nonstoichiometry favours the in situ exsolution of metallic nanocatalysts (NixCo) from the parent materials in reducing condition. The selection of A-site cations (lanthanides) significantly affects the crystal structure (simple or double perovskite structure), while the B-site cations determine the stability of materials in air and reducing atmospheres. The reversibility of exsolved nanoparticles is systematically investigated under reducing and oxidizing conditions with different annealing temperatures. The possibility of in situ exsolution of nanooxides on the cathode (NixCoOy-like nanooxides) is also investigated. Nanofiber-structured electrodes with the ability of in situ exsolution of nanocatalysts are fabricated using the electrospinning technique, which significantly boost the electrochemical properties of constructed SOCs.

Ground state structural, electronic and magnetic properties of Mg-doped LaMnO[formula omitted] perovskite: GGA+U study

The effect of Mg doping on the ground state structural, electronic and magnetic properties has been investigated by means of GGA+U calculations. The inspection of the structure, magnetic moment data and density plots, show that by substituting La by Mg at the La site at a concentration of 0.25, the compound exhibits a Pbnm orthorhombic structure and a half metallic ferromagnetic behaviour with degenerated Mn-eg states. The corresponding average A-site cation radius 〈rA〉 value does not allow metallic-insulator phase transition in the compound. However, by doping Mg on Mn site with the same proportion, the resulting compound shows a Pbnm orthorhombic structure and a half metallic behaviour with degeneracy lifting in Mn-eg states. Our results obtained for LaMn0.75Mg0.25MnO3 agree more with experimental data than those obtained for La0.75Mg0.25MnO3, suggesting that Mg will prefer to substitute at the Mn site due to their close ionic radii.

Tin perovskite transistors and complementary circuits based on A-site cation engineering

Tin halide perovskites have the general chemical formula ASnX3, where A is a monovalent cation and X is a monovalent halide anion. These semiconducting materials can be used to fabricate p-type transistors at low cost and temperature and could be potentially integrated with n-type oxide-based transistors to create complementary circuits. However, the materials suffer from low crystallization controllability and high film defect density, resulting in uncompetitive device performance. Here we show that pure-tin perovskite thin-film transistors can be created using triple A cations of caesium–formamidinium–phenethylammonium. The approach leads to high-quality cascaded tin perovskite channel films with low-defect, phase-pure perovskite/dielectric interfaces. The optimized thin-film transistors exhibit hole mobilities of over 70 cm2 V−1 s−1 and on/off current ratios of over 108, which are comparable with commercial low-temperature polysilicon transistors. The transistors are fabricated using solution-processing methods at temperatures no higher than 100 °C. We also integrate the devices with n-type metal oxide transistors to create complementary inverters with voltage gains of 370, and NOR and NAND logic gates with rail-to-rail switching performance.

Effect of alkali-metal cations on synthesis of stoichiometric layered perovskite oxynitrides

Layered perovskite oxynitrides, which contain a stoichiometric lattice N content, are promising visible-light-driven photocatalysts. However, their synthesis remains a challenge for inorganic chemists. Herein, we report the successful synthesis of layered perovskite oxynitrides A2A'2Ta3O9N with different cations at A/A′-sites (A = Na, K, Rb, Cs; A' = Ca, Sr, Ba) via thermal ammonolysis from layered Dion–Jacobson phase oxide precursors (AA'2Ta3O10). We present the first experimental study of layered oxynitride formation across the A and A′ cation size series, revealing that the A-site alkali-metal cation rather than the A′-site alkaline-earth metal cation is critical to the successful synthesis of the stoichiometric layered oxynitrides. Nitrided products with A = Na, K, or Rb exhibited structural transitions, while no structural transition was observed for Cs. Interestingly, nitrided products with A = Rb and K exhibited the same structural transition behavior, i.e., alkali-metal cations entered the interlayer to form an oxynitride with a composition A2A'2Ta3O9N·nH2O. By contrast, with A = Na, it formed an oxynitride that differs from the Ruddlesden–Popper structure but has the same composition of A2A'2Ta3O9N·nH2O. Our finding suggests that, as the cation size decreases from Cs to Na, stoichiometric A2A'2Ta3O9N compositions become more favorable than compounds with A-sites. This work provides a new perspective for the future design and synthesis of novel layered oxynitrides.

A-site effects of titanate-perovskite (ATiO3)-based catalysts on dehydrogenation of N-heterocyclic molecules

Dehydrogenation reactions in liquid organic hydrogen carrier (LOHC) systems present significant challenges, particularly when aiming for low-temperature operations while ensuring that no hydrogen remains in the substrate molecules. Enhancing catalytic performance requires modifying the adsorption behavior of the reactants and products during dehydrogenation. Perovskites have emerged as promising catalyst supports because of their ability to modify the surface chemical properties by manipulating the cations present at the A- and B- sites. This study investigated the effects of A-site cations (Ca, Sr, and Ba) in titanate-type perovskite (ATiO3)—a prototypical perovskite—on the dehydrogenation activity in LOHC systems. Remarkably, Pd/SrTiO3 exhibited outstanding performance by completely converting octahydro-N-methylindole to N-methylindole and releasing 5.76 wt% hydrogen over 8 h. Additionally, it dehydrogenated dodecahydro-N-ethylcarbazole to N-ethylcarbazole with a hydrogen release of 5.70 wt%. Furthermore, the catalyst demonstrated a stable performance after recycling tests for three times without degradation or loss of activity. The chemical state of the catalyst surface was characterized through X-ray photoelectron spectroscopy, H2-temperature programmed reduction, and chemisorption using NH3, CO2, and H2. The results revealed that the exceptional dehydrogenation activity of Pd/SrTiO3 is due to the presence of suitable surface oxygen vacancies and abundant acid–base sites.

Cation-Radius-Controlled Sn-O Bond Length Boosting CO2 Electroreduction over Sn-Based Perovskite Oxides.

Despite the intriguing potential shown by Sn-based perovskite oxides in CO2 electroreduction (CO2 RR), the rational optimization of their CO2 RR properties is still lacking. Here we report an effective strategy to promote CO2 -to-HCOOH conversion of Sn-based perovskite oxides by A-site-radius-controlled Sn-O bond lengths. For the proof-of-concept examples of Ba1-x Srx SnO3 , as the A-site cation average radii decrease from 1.61 to 1.44 Å, their Sn-O bonds are precisely shortened from 2.06 to 2.02 Å. Our CO2 RR measurements show that the activity and selectivity of these samples for HCOOH production exhibit volcano-type trends with the Sn-O bond lengths. Among these samples, the Ba0.5 Sr0.5 SnO3 features the optimal activity (753.6 mA ⋅ cm-2 ) and selectivity (90.9 %) for HCOOH, better than those of the reported Sn-based oxides. Such optimized CO2 RR properties could be attributed to favorable merits conferred by the precisely controlled Sn-O bond lengths, e.g., the regulated band center, modulated adsorption/activation of intermediates, and reduced energy barrier for *OCHO formation. This work brings a new avenue for rational design of advanced Sn-based perovskite oxides toward CO2 RR.

Nucleation-mediated growth of chiral 3D organic-inorganic perovskite single crystals.

Although their zero- to two-dimensional counterparts are well known, three-dimensional chiral hybrid organic-inorganic perovskite single crystals have remained difficult because they contain no chiral components and their crystal phases belong to centrosymmetric achiral point groups. Here we report a general approach to grow single-crystalline 3D lead halide perovskites with chiroptical activity. Taking MAPbBr3 (MA, methylammonium) perovskite as a representative example, whereas achiral MAPbBr3 crystallized from precursors in solution by inverse temperature crystallization method, the addition of micro- or nanoparticles as nucleating agents promoted the formation of chiral crystals under a near equilibrium state. Experimental characterization supported by calculations showed that the chirality of the 3D APbX3 (where A is an ammonium ion and X is Cl, Br or mixed Cl-Br or Br-I) perovskites arises from chiral patterns of the A-site cations and their interaction with the [PbX6]4- octahedra in the perovskite structure. The chiral structure obeys the lowest-energy principle and thereby thermodynamically stable. The chiral 3D hybrid organic-inorganic perovskites served in a circularly polarized light photodetector prototype successfully.

Boosting photodetection performance of Cs2AgBiBr6 through A-site Rb substitution and interfacial engineering

Bismuth-based double perovskite Cs2AgBiBr6 shows promise as a photodetection material. However, its detection performance and application are limited by high-exciton binding energy and poor carrier mobility. In this study, we address these limitations by delicately designing a solution-based method for incorporating A-site Rubidium (Rb) substitution into Cs2AgBiBr6 double perovskite films. The introduction of Rb resulted in a significant decrease in trap defect density and an improvement in film quality. The trap-filled limit voltage (VTFL) of pure and Rb-doped CABB film is determined to be 1.71 V and 0.48 V, respectively. Subsequently, by introducing an ultrathin atomic-layer-deposited (ALD) TiO2 films, the fabricated CABB photodetectors exhibit significantly improved photoresponse performance. The response speed and −3dB bandwidth are boosted from ∼93 ms to ∼350 μs and broadened from 1.4 kHz to 17 kHz, respectively. Density Functional Theory (DFT) calculations indicate Rb-substitution shortens the bond length and weaken exciton binding energy. Furthermore, we demonstrate a wireless near ultraviolet (UV) light communication system using CABB photodetectors as light receivers. Our findings provide an efficient approach to utilize A-site cation substitution as a tuning parameter for photodetection in high-exciton binding energy perovskite materials, thereby extending the potential applications of other functional perovskites.

Piezoelectric and Dielectric Properties in Bi0.5(Na,K)0.5TiO3-x Ag2O Lead-Free Piezoceramics.

Lead-free piezoceramics of Bi0.5(Na0.825K0.175)0.5TiO3 with varying concentrations of x mol% Ag2O (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, denoted as BNKT-xA) were fabricated using the solid-state technique. An extensive investigation was undertaken to analyze the structural, piezoelectric, and dielectric properties of these piezoceramics in the presence of Ag ions. There is no evidence of any secondary solid solution in the BNKT-xA piezoceramics. The ceramics with x mol% Ag2O in BNKT still demonstrate the presence of both rhombohedral (R) and tetragonal (T) phases. The addition of Ag+ is helpful to increase the relative density of the BNKT-xA piezoceramics. It is noteworthy that the BNKT-0.3 mol% A piezoceramics show remarkable improvements in their properties (d33 = 147 pC/N, kp = 29.6%, ε = 1199, tanδ = 0.063). These improvements may be ascribed to the denser microstructure and the preservation of morphotropic phase boundaries between R and T phases caused by the appropriate addition of Ag cations. The addition of Ag+ results in the relaxor behavior of the BNKT-xA ceramics, characterized by disorder among A-site cations. With the increase in temperature, the d33 value of BNKT-xA ceramics does not vary significantly in the range of 25 to 125 °C, ranging from 127 to 147 pC/N (with a change of d33 ≤ 13.6%). This finding shows that piezoelectric ceramics have a reliable performance over a certain operating temperature range.

Enhanced Hot-Phonon Bottleneck Effect on Slowing Hot Carrier Cooling in Metal Halide Perovskite Quantum Dots With Alloyed A-Site.

A deep understanding of the effect of the A-site cation cross-exchange on the hot-carrier relaxation dynamics in perovskite quantum dots (PQDs) has profound implications on the further development of disruptive photovoltaic technologies. In this study, we investigated the hot carrier cooling kinetics of pure FAPbI3 , MAPbI3 , CsPbI3 and alloyed FA0.5 MA0.5 PbI3 , FA0.5 Cs0.5 PbI3 , and MA0.5 Cs0.5 PbI3 QDs using ultrafast transient absorption (TA) spectroscopy. The lifetimes of the initial fast cooling stage (<1ps) of all the organic cation-containing PQDs were shorter than those of the CsPbI3 QDs, as verified by the electron-phonon coupling strength extracted from the temperature-dependent photoluminescence spectra. The lifetimes of the slow cooling stage of the alloyed PQDs were longer under illumination greater than 1 sun, which was ascribed to the introduction of co-vibrational optical phonon modes in the alloyed PQDs. This facilitated efficient acoustic phonon upconversion and enhanced the hot-phonon bottleneck effect, as demonstrated by first-principles calculations. This article is protected by copyright. All rights reserved.

Thermoelectric enhancement in A-site deficient high-entropy perovskite (Sr0.25Ca0.25La0.25Ba0.25)1-xTiO3±δ ceramics by fine manipulating cation vacancies

SrTiO3-based perovskite structured oxides have attracted much attention as an n-type thermoelectric material over a wide temperature range. An enhancement in performance requires strategies that simultaneously optimize electron and phonon transports. In this work, a series of A-site cation vacancies were introduced into a high-entropy system to fabricate A-site deficient (Sr0.25Ca0.25La0.25Ba0.25)1-xTiO3±δ(0＜x ≤ 0.125) samples which optimize configuration entropy ranges 10.08–11.24 J mol−1K−1 and simultaneously manipulate electronic and phonon transports properties. Most importantly, the direct evidence of positron annihilation and iDPC-STEM results unambiguously confirms these A-site cation vacancies. Consequently, carrier mobility was improved by 16.5 times which significantly contributed to an increase in the electrical conductivity. The low lattice thermal conductivity of 2.54 W m−1K−1 was obtained at 1073 K which was significantly reduced by 21.4% compared to the majority values of SrTiO3-based ceramics. The maximum ZT = 0.24 was attained for the x = 0.100 sample with an enhancement of 27.8% than that of stoichiometric ceramic due to the synergistic effect of higher PF and relatively low κ. This study reveals that in a distorted, high-entropy perovskite structure, multiple band degeneracy is advantageous for enhancing the electrical characteristics due to the delocalized electrons that result from enhanced crystal symmetry. In the meanwhile, anharmonicity and localized phonons inhibited the propagation of transverse phonons resulting in a markedly reduced lattice thermal conductivity. This feasible strategy of defect engineering combined with entropy manipulation demonstrates that it is not necessary to simply pursue a much higher configuration entropy in TE ceramics to optimal ZT values.

Phase structure and electrical properties of BiFeO3–BaTiO3 ceramics near the morphotropic phase boundary

As a promising lead-free high-temperature piezoelectric material, BiFeO3–BaTiO3 (BF-BT) is still under debate on its crystal structure, especially for the compositions in the morphotropic phase boundary (MPB). Herein, the crystal structure of (1-x)BF-xBT ceramics near the MPB with x = 0.20–0.33 was analyzed by X-ray diffraction and Raman spectroscopy. With the addition of BT, the phase transition from distorted rhombohedral to pseudo-cubic structure was determined by Rietveld refinement. Raman analysis showed the weakening interactions between the A-site cations and BO6 octahedron, as well as the degenerated distortions of BO6 octahedron. Intriguingly, the results of X-ray photoelectron spectroscopy suggested the inhibition of Fe3+ reduction and decrease in oxygen vacancy concentration by the incorporation of BT. In terms of the electrical properties, the significant decline in Curie temperature, improvement in permittivity, and reduction in high-temperature dielectric loss can be observed with the increase of BT content. The increased remnant polarization, enhanced dielectric response and decreased coercive field were helpful for the piezoelectric performance. The maximum room-temperature piezoelectric coefficient of ∼160 pC/N and high-temperature piezoelectric coefficient of ∼324 pC/N were acquired in the x = 0.30 ceramics. This work provides a clear scope for further research on the structural transformation and high-temperature piezoelectric applications of BF-BT system.

Bulky cation hinders undesired secondary phases in FAPbI3 perovskite solar cells

Interfacial treatments with bulky cations over 3D lead halide perovskites (LHPs) have enabled power conversion efficiencies beyond 25 %. However, the mechanisms behind these improvements still need to be better understood. In this work, we focus on a specific type of perovskite, formamidinium lead iodide (FAPI), without additives, and demonstrate that phenethylammonium iodide (PEAI) can prevent the transformation of the FAPI perovskite into undesirable hexagonal non-perovskite phases when exposed to air. We use various analytical techniques to understand how the organic salt interacts with the 3D perovskite absorber. Through grazing incidence wide-angle X-ray scattering analysis, we study the structure at the surface and bulk, observing that hexagonal FAPI phases dominate the phase composition in FAPI films without surface treatments. Time-resolved photoluminescence (TRPL) and terahertz (THz) spectroscopy show an enhancement in the carrier lifetime and photoconductivity, respectively, when the FAPI films are coated with PEAI. In solar cells, introducing PEAI enables power conversion efficiencies of up to 20.2 % and open circuit voltages (VOC) of up to 1.14 V, which are among the highest reported values for a FAPI solar cells without additional A-site cations.

Realizing the Lowest Bandgap and Exciton Binding Energy in a Two-Dimensional Lead Halide System.

Finding stable analogues of three-dimensional (3D) lead halide perovskites has motivated the exploration of an ever-expanding repertoire of two-dimensional (2D) counterparts. However, the bandgap and exciton binding energy in these 2D systems are generally considerably higher than those in 3D analogues due to size and dielectric confinement. Such quantum confinements are most prominently manifested in the extreme 2D realization in (A)mPbI4 (m = 1 or 2) series of compounds with a single inorganic layer repeat unit. Here, we explore a new A-site cation, 4,4'-azopyridine (APD), whose size and hydrogen bonding properties endow the corresponding (APD)PbI4 2D compound with the lowest bandgap and exciton binding energy of all such compounds, 2.19 eV and 48 meV, respectively. (APD)PbI4 presents the first example of the ideal Pb-I-Pb bond angle of 180°, maximizing the valence and conduction bandwidths and minimizing the electron and hole effective masses. These effects coupled with a significant increase in the dielectric constant provide an explanation for the unique bandgap and exciton binding energies in this system. Our theoretical results further reveal that the requirement of optimizing the hydrogen bonding interactions between the organic and the inorganic units provides the driving force for achieving the structural uniqueness and the associated optoelectronic properties in this system. Our preliminary investigations in characterizing photovoltaic solar cells in the presence of APD show encouraging improvements in performances and stability.