Phase Transitions In Perovskites Research Articles

Revealing the Role of Polyacrylonitrile for Highly Efficient and Stable Perovskite Solar Cells at Extremely Low Temperatures

AbstractPerovskite solar cells (PSCs) are considered to be a promising candidate for near‐space applications due to their high specific power, excellent radiation resistance, and compatibility with flexible substrates. Nevertheless, the low‐temperature cycles impose considerable challenges to their long‐term stability. Herein, the extremely low‐temperature photovoltaic process of PSCs is investigated and the effect mechanisms of PAN (Polyacrylonitrile) as an additive are revealed. It is found that the additive PAN regulates perovskite lattice stress and stabilizes perovskite lattice structure and thus retards perovskite phase transition at low temperatures. Moreover, the PAN improves perovskite dielectric properties and ion migration activation energy and effectively passivates perovskite defects at low temperatures. Therefore, the carrier transport/extraction/collection abilities at low temperatures are enhanced effectively by alleviating exciton effect and increasing dielectric screening effect. Benefiting from these positive effects of PAN, the PAN‐modified device achieves a maximum efficiency of 24.34% at 150K and maintains 72% of its initial value after 120 thermal cycles (290‐130K). Also, the PAN‐modified flexible devices exhibit excellent bending stability and thermal cycle stability. This study provides a deep insight into understanding the extremely low‐temperature photovoltaic behavior of PSCs and demonstrates the potentiality of PAN additive strategy for achieving efficient and stable PSCs at low temperatures.

Ab-initio investigation of the structural, electronic and optical properties of CsPbBr3 perovskite under pressure using the DFT-1/2 method

Pressure-induced phase transitions in metal halide perovskites lead to significantly different material properties and offer great potential for diverse applications. The electronic, structural, and optical properties of Cesium lead bromide (CsPbBr3) crystals are investigated using density functional theory (DFT) to draw an integrated picture of orbital morphology, absorption spectra, and band gap values. Spin-orbit coupling and DFT-1/2 correction are used to gain a better insight into electronic properties and band splitting in the cubic phase. Calculations of pressure-induced phase transitions, electronic band gap engineering, and manipulation of optical absorption edges of CsPbBr3 are conducted. In the cubic and tetragonal crystal lattices, a band gap closure and opening occurs demonstrating a Dirac cone upon application and increase of pressure. Indeed, in the tetragonal lattice of CsPbBr3, a topological band occurs. Orthorhombic perovskite crystal lattice undergoes successive direct-indirect and indirect-direct band gap transitions, followed by a space group change at extreme pressures. However, under pressure, orthorhombic non-perovskite lattice experiences a sudden band gap breakdown. Phase and band gap transitions are explained by orbital hybridizations and structural symmetry evolution (e.g., bond length contraction/dilation and octahedral cage rotation/distortion). These results provide comprehensive guidelines for further experimental studies on pressure engineering, advanced photovoltaic and optoelectronic applications, discovering and understanding new topological band structures of perovskite materials.

(Benzimidazolium)2GeI4: A layered two-dimensional perovskite with dielectric switching and broadband near-infrared photoluminescence

Two-dimensional (2D) lead halide perovskites have attracted tremendous attention due to their outstanding physical properties. However, the presence of toxic lead is a major problem for large-scale applications. Here, we report the synthesis, phase transition, dielectric and photoluminescence (PL) properties of a lead-free 2D Ge-based perovskite (BIM)2GeI4 (BIM = benzimidazolium) which experiences a phase transition at 405 K accompanied by an order-disorder change of organic spacer. Moreover, the step-like dielectric transition near the phase transition temperature makes it suitable for dielectric switching. Meanwhile, (BIM)2GeI4 has a direct bandgap of 2.23 eV and broadband emission from 550 to 1000 nm, implying its unique optical property for potential near-infrared lighting and displaying. This work provides a fresh example for the development of lead-free 2D Ge-based perovskite with potential applications in the fields of optoelectronics and high temperature dielectric switching.

Mixing I and Br in Inorganic Perovskites: Atomistic Insights from Reactive Molecular Dynamics Simulations.

All-inorganic halide perovskites have received a great deal of attention as attractive alternatives to overcome the stability issues of hybrid halide perovskites that are commonly associated with organic cations. To find a compromise between the optoelectronic properties of CsPbI3 and CsPbBr3, perovskites with CsPb(BrxI1-x)3 mixed compositions are commonly used. An additional benefit is that without sacrificing the optoelectronic properties for applications such as solar cells or light-emitting diodes, small amounts of Br in CsPbI3 can prevent the inorganic perovskite from degrading to a photo-inactive non-perovskite yellow phase. Despite indications that strain in the perovskite lattice plays a role in the stabilization of the material, a full understanding of such strain is lacking. Here, we develop a reactive force field (ReaxFF) for perovskites starting from our previous work for CsPbI3, and we extend this force field to CsPbBr3 and mixed CsPb(BrxI1-x)3 compounds. This force field is used in large-scale molecular dynamics simulations to study perovskite phase transitions and the internal ion dynamics associated with the phase transitions. We find that an increase of the Br content lowers the temperature at which the perovskite reaches a cubic structure. Specifically, by substituting Br for I, the smaller ionic radius of Br induces a strain in the lattice that changes the internal dynamics of the octahedra. Importantly, this effect propagates through the perovskite lattice ranging up to distances of 2 nm, explaining why small concentrations of Br in CsPb(BrxI1-x)3 (x ≤ 1/4) have a significant impact on the phase stability of mixed halide perovskites.

Enhanced stability of FA-based perovskite: Rare-earth metal compound EuBr2 doping

It is highly desirable to enhance the long-term stability of perovskite solar cells (PSCs) so that this class of photovoltaic cells can be effectively used for the commercialization purposes. In this contribution, attempts have been made to use the two-step sequential method to dope EuBr2 into FAMAPbI3 perovskite to promote the stability. It is shown that the device durability at 85 °C in air with RH of 20%–40% is improved substantially, and simultaneously the champion device efficiency of 23.04% is achieved. The enhancement in stability is attributed to two points: (i) EuBr2 doping effectively inhibits the decomposition and α–δ phase transition of perovskite under ambient environment, and (ii) EuBr2 aggregates in the oxidized format of Eu(BrO3)3 at perovskite grain boundaries and surface, hampering humidity erosion and mitigates degradation through coordination with H2O.

Investigating the role of dispersion corrections and anharmonic effects on the phase transition in SrZrS3: A systematic analysis from AIMD free energy calculations.

A thermally driven needle-like (NL) to distorted perovskite (DP) phase transition in SrZrS3 was investigated by means of abinitio free energy calculations accelerated by machine learning. As a first step, a systematic screening of the methods to include long-range interactions in semilocal density functional theory Perdew-Burke-Ernzerhof calculations was performed. Out of the ten correction schemes tested, the Tkatchenko-Scheffler method with iterative Hirshfeld partitioning method was found to yield the best match between calculated and experimental lattice geometries, while predicting the correct order of stability of NL and DP phases at zero temperature. This method was then used in free energy calculations, performed using several approaches, so as to determine the effect of various anharmonicity contributions, such as the anisotropic thermal lattice expansion or the thermally induced internal structure changes, on the phase transition temperature (TNP→DP). Accounting for the full anharmonicity by combining the NPT molecular dynamics data with thermodynamic integration with harmonic reference provided our best estimate of TNL→DP = 867K. Although this result is ∼150K lower than the experimental value, it still provides an improvement by nearly 300K compared to the previous theoretical report by Koocher et al. [Inorg. Chem. 62, 11134-11141 (2023)].

Temperature-dependent excited states for detecting reversible phase transitions in 2D lead(II) iodide perovskites.

Significant interest exists in water-tolerant 2D lead iodide perovskites owing to their stability and proven potential in photovoltaic and photonic applications. These materials have solid-state phase transitions that are accessible below 100 °C. Here, the study witnesses the multiple phase transitions of the last members of a series of organic-inorganic hybrid materials, [(CnH2n+1NH3)2PbI4], with even n as n = 14, 16, and 18, once again. By employing temperature-dependent steady-state photoluminescence (PL) and temperature-dependent time-resolved photoluminescence (TRPL) spectroscopy in the temperature range of -18 to +90 °C and at -196 °C, we explore the thermal responses of these materials. The investigation reveals reversible phase transitions occurring between room temperature (RT) and elevated temperatures, impacting the optical properties and emitting colors of the perovskite compounds. The longer the alkyl chain, the higher the phase transition temperature, attributed to increased conformational disorder and enhanced perovskite symmetry. The decay constants for all compounds are very close in value, which confirms the underlying excited-state dynamics, pointing to contributions primarily from inorganic components across different phases. We anticipate that our results on the detection of phase transitions in 2D perovskites will not only motivate the use of these techniques for detecting phase transitions but also would help to understand their excited states in more details to selectively use them for solar cell and next-generation display technologies.

CsZnPbBr3/ZnS core/shell perovskite nanocrystals for stable and efficient white light-emitting diodes.

The widespread applicability of perovskite nanocrystals (PeNCs) is impeded by their intrinsic instability. A promising solution is utilizing robust chalcogenides as a protective shell to shield the sensitive luminescent cores from the external environment. However, the inferior structural stability and surface lability of PeNCs usually lead to perovskite phase transition during shell growth. Herein, we introduced smaller Zn ions to partially replace Pb ions in perovskites, which reduces the Pb-X bond length and enhances the Pb-X bond energy for inner lattice stabilization. Simultaneously, extra oleylammonium bromide (OAmBr) was added to protect the labile surface of PeNCs by compensating for the detachment of ligands and the loss of surface Br ions. As a result, the dual strategies enable the epitaxial growth of a ZnS shell and significantly enhance the chemical stability of CsZnPbBr3/ZnS core/shell PeNCs. After three thermal cycles ranging from 300 to 450 K, the core/shell PeNCs retained 70% of their initial photoluminescence (PL) intensity. In stark contrast, the pristine CsPbBr3 PeNCs exhibit complete PL quenching after just the first temperature cycle. For practical applications, the green core/shell PeNCs were integrated with commercially available red-emitting phosphors on a blue-emitting InGaN chip to fabricate a white light-emitting diode (WLED), which demonstrates a high luminous efficacy (LE) of 61.3 lm W-1 and nearly constant Commission Internationale de l'Eclairage (CIE) coordinates under varying operating currents.

Phase Transitions, Dielectric Response, and Nonlinear Optical Properties of Aziridinium Lead Halide Perovskites.

Hybrid organic-inorganic lead halide perovskites are promising candidates for next-generation solar cells, light-emitting diodes, photodetectors, and lasers. The structural, dynamic, and phase-transition properties play a key role in the performance of these materials. In this work, we use a multitechnique experimental (thermal, X-ray diffraction, Raman scattering, dielectric, nonlinear optical) and theoretical (machine-learning force field) approach to map the phase diagrams and obtain information on molecular dynamics and mechanism of the structural phase transitions in novel 3D AZRPbX3 perovskites (AZR = aziridinium; X = Cl, Br, I). Our work reveals that all perovskites undergo order-disorder phase transitions at low temperatures, which significantly affect the structural, dielectric, phonon, and nonlinear optical properties of these compounds. The desirable cubic phases of AZRPbX3 remain stable at lower temperatures (132, 145, and 162 K for I, Br, and Cl) compared to the methylammonium and formamidinium analogues. Similar to other 3D-connected hybrid perovskites, the dielectric response reveals a rather high dielectric permittivity, an important feature for defect tolerance. We further show that AZRPbBr3 and AZRPbI3 exhibit strong nonlinear optical absorption. The high two-photon brightness of AZRPbI3 emission stands out among lead perovskites emitting in the near-infrared region.

Quantum-Accurate Modeling of Ferroelectric Phase Transition in Perovskites from Message-Passing Neural Networks

Quantum-Accurate Modeling of Ferroelectric Phase Transition in Perovskites from Message-Passing Neural Networks

Effect of isosymmetric phase transition in MIEC perovskite on the kinetic parameters of its interaction with oxygen

Oxygen partial pressure relaxation technique was used to study transient kinetics of re-equilibration in the reaction of nonstoichiometric oxide Ba0.5Sr0.5Co0.75Fe0.2Mo0.05O3-δ with oxygen within a region of 3-δ stoichiometry close to the nominal oxidation state 3+ for cobalt where an isosymmetric phase transition cubic perovskite P1 – cubic perovskite P2 is observed. The relaxation data were analyzed using a macrokinetic model that explicitly takes into account the power-law dependence of the surface reaction rate on the oxygen partial pressure. A twofold decrease in the power-law exponent (n = 0.7 to n = 0.3) was found at the point of the P1-P2 phase transition, which indicates a change in the mechanism of the rate-determining step of the reaction caused by the transition. At the same time, the functional dependence of the diffusion mobility of oxygen vacancies on the equilibrium oxygen pressure does not change at the transition point, which means that the transition does not affect the mechanism of bulk vacancy mobility.

Effect of Heat and Moisture on the Phase Transition in Dimethylammonium-Facilitated CsPbI3 Perovskite

Effect of Heat and Moisture on the Phase Transition in Dimethylammonium-Facilitated CsPbI3 Perovskite

Light‐Induced Dielectric Enhancement in a 2D Dion–Jacobson Type Perovskite Phase Transition Material

AbstractStimuli‐responsive materials, showing bistable behaviors between two distinct states under external stimuli, can be potentially applied as basic components of modern electrical and electronic devices. 2D hybrid perovskites hold a promise to switch and tune dielectric bistability based on their solid‐state phase transitions. Here, strong dielectric enhancements are successfully actualized by virtue of light illumination in a Dion–Jacobson (DJ) type 2D hybrid perovskite, (HDA)(MA)2Pb3Br10 (1, where HDA is 1,6‐hexanediammonium and MA is methylammonium). Upon heating, 1 exhibits an ordered–disordered phase transition ≈ 324 K (Tc), accompanying a transition from its low to high dielectric state. Strikingly, its light‐induced dielectric constant changes are greatly enhanced with a magnitude up to 250% near the Tc, superior to most traditional inorganic phase transition materials. This process closely relates to the photoexcited dielectric effect that is caused by the interband photo‐excitation. Benefitting from the stability of DJ‐type perovskite structure, the switching cycles of dielectric enhancement display operational stability with superior anti‐fatigue characteristics. As per the knowledge, the findings of this strong dielectric enhancement are unprecedented in the family of 2D DJ‐type hybrid perovskites, which will facilitate broad interest in exploiting new stimuli‐responsive materials to assemble high‐performance smart electronic devices.

First-principles Study of the Structural Properties of RbSnI2Br Perovskites Under Pressure

As a member of lead-free perovskite, tetragonal phase perovskite RbSnI2Br has a long-term development prospect. Due to the effect of structural changes on physical properties, we have explored the pressure-induced phase transition of RbSnI2Br perovskite. For this project, the structural properties of the tetragonal phase perovskite RbSnI2Br are studied by using the Perdew-Burke-Ernzerhof (PBE) functional for the generalized gradient approximation (GGA) under pressure conditions of 0 GPa–1 GPa. The calculation results of structural factors were in excellent agreement with those obtained from previous studies at the ground state, which ensures the accuracy of the study. The results show phase transitions occur with increasing pressure. The structural space group of β-RbSnI2Br changes from P4/MMM (tetragonal system) to P1 (triclinic system) under pressure in the range of 0 GPa–1 GPa. All these provide a relative theoretical basis for our further understanding of α-RbSnI2Br perovskite.

Limits of the phonon quasi-particle picture at the cubic-to-tetragonal phase transition in halide perovskites

The soft modes associated with continuous-order phase transitions are associated with strong anharmonicity. This leads to the overdamped limit where the phonon quasi-particle picture can break down. However, this limit is commonly restricted to a narrow temperature range, making it difficult to observe its signature feature, namely the breakdown of the inverse relationship between the relaxation time and damping. Here we present a physically intuitive picture based on the relaxation times of the mode coordinate and its conjugate momentum, which at the instability approach infinity and the inverse damping factor, respectively. We demonstrate this behavior for the cubic-to-tetragonal phase transition of the inorganic halide perovskite CsPbBr3 via molecular dynamics simulations, and show that the overdamped region extends almost 200 K above the transition temperature. Further, we investigate how the dynamics of these soft phonon modes change when crossing the phase transition.

Phase Transitions in Inorganic Halide Perovskites from Machine-Learned Potentials

Phase Transitions in Inorganic Halide Perovskites from Machine-Learned Potentials

Pressure-Induced Phase Transition versus Amorphization in Hybrid Methylammonium Lead Bromide Perovskite.

The crystal structure of the CH3NH3PbBr3 perovskite has been investigated under high-pressure conditions by synchrotron-based powder X-ray diffraction. We found that after the previously reported phase transitions in CH3NH3PbBr3 (Pm3̅m→Im3̅→Pmn21), which occur below 2 GPa, there is a third transition to a crystalline phase at 4.6 GPa. This transition is reported here for the first time contradicting previous studies, which reported amorphization of CH3NH3PbBr3 between 2.3 and 4.6 GPa. Our X-ray diffraction measurements show that CH3NH3PbBr3 remains crystalline up to at least 7.6 GPa, the highest pressure covered by experiments. The new high-pressure phase is also described by the space group Pmn21; however, the transition involves abrupt changes in the unit-cell parameters and a 3% decrease of the unit-cell volume. Our conclusions are confirmed by optical-absorption experiments, by visual observations, and by the fact that pressure-induced changes up to 10 GPa are reversible. The optical studies also allow for the determination of the pressure dependence of the band-gap energy, which is discussed using the structural information obtained from X-ray diffraction.

Origin of Phase Transitions in Inorganic Lead Halide Perovskites: Interplay between Harmonic and Anharmonic Vibrations

Inorganic lead halide perovskites (ILHPs) exhibit a series of phase transitions, and stabilization of the phases with desirable optoelectronic properties remains a major challenge. However, the intrinsic origins of structural instabilities in CsPbX3 (X = Br, I) are still elusive. Herein, the important role of harmonic and anharmonic vibrations in influencing thermodynamic fluctuations of ILHPs was revealed, through combined lattice dynamics and multiphonon theory calculations, and verified by diffraction experiments. Our results demonstrate that the transition between δ- and γ-CsPbI3 is driven by harmonic vibrations, unveiling the mysterious mechanism for stabilizing γ-CsPbI3 via applying strain. Moreover, the successive transitions from the α- to β- and β- to γ-phases of CsPbX3 are driven by anharmonic vibrations. These structural dynamics are strongly coherent with the phonon diffuse scattering, substantially affecting the thermal conductivity and carrier relaxation. This work provides guidelines for maintaining favorable ILHP phases through delicately manipulating their lattice dynamics.

Sorption-enhanced steam reforming of toluene using multifunctional perovskite phase transition sorbents in a chemical looping scheme

Sorption-enhanced steam reforming (SESR) of toluene (SESRT) using catalytic CO2 sorbents is a promising route to convert the aromatic tar byproducts formed in lignocellulosic biomass gasification into hydrogen (H2) or H2-rich syngas. Commonly used sorbents such as CaO are effective in capturing CO2 initially but are prone to lose their sorption capacity over repeated cycles due to sintering at high temperatures. Herein, we present a demonstration of SESRT using A- and B-site doped Sr1−x A’ x Fe1−y B’ y O3−δ (A’ = Ba, Ca; B’ = Co) perovskites in a chemical looping scheme. We found that surface impregnation of 5–10 mol% Ni on the perovskite was effective in improving toluene conversion. However, upon cycling, the impregnated Ni tends to migrate into the bulk and lose activity. This prompted the adoption of a dual bed configuration using a pre-bed of NiO/γ–Al2O3 catalyst upstream of the sorbent. A comparison is made between isothermal operation and a more traditional temperature-swing mode, where for the latter, an average sorption capacity of ∼38% was witnessed over five SESR cycles with H2-rich product syngas evidenced by a ratio of H2: CO x > 4.0. XRD analysis of fresh and cycled samples of Sr0.25Ba0.75Fe0.375Co0.625O3-δ reveal that this material is an effective phase transition sorbent—capable of cyclically capturing and releasing CO2 without irreversible phase changes occurring.

Effect of temperature on structural phase transition and photoluminescence in organic-inorganic hybrid CH3NH3PbI3-xClx perovskite

Organic-inorganic hybrid (CH3NH3PbI3-xClx) perovskite is a useful light accumulating material. Its characteristic features are found to be very sensitive to synthesis and environmental conditions. We studied its structural phase transformation, inter band transitions, and photoluminescence as a function of temperature, which are key parameters governing its light absorbing characteristics. We found that photoluminescence peaks exhibit several finer features, which are identified, deconvoluted, and discussed in detail. Our analyses revealed the presence of multiple phases, defects (mainly Cl), excitonic binding energies, and radiative recombination of trap states in the perovskite. Based on the results, a schematic energy band diagram has been proposed, and it is concluded that the trap states will become less active at elevated temperatures. Moreover, increased post annealing temperature will decrease the recombination at interfaces due to improved crystallinity and large grain sizes. Control over the surface and interface trapping by using the appropriate working and annealing temperature for the device may be key to increasing the efficiency of perovskite based solar cells.