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Phase transitions, screening and dielectric response of CsPbBr 3
Abstract
Cesium–lead–bromide (CsPbBr3) is the simplest all inorganic halide perovskite. It serves as a reference material for understanding the exceptional solar cell properties of the organic–inorganic hybrid halide perovskites and is itself discussed as an alternative absorber material. Broadband dielectric spectroscopy has proven to yield an in depth understanding of charge screening mechanisms in the halide solar cell absorbers based on methylammonium and modifications hereof. For a deeper understanding of charge carrier screening, we have investigated CsPbBr3 across wide temperature (120 K–450 K) and frequency ranges. Besides the two known phase transitions at 403 K and 361 K, the dielectric data show another anomaly around 220 K, which can be interpreted as another phase transition. XRD and EPR studies confirm the presence of this anomaly, but Raman scattering spectra do not show any lattice anomalies in the vicinity of 220 K. This additional anomaly is of first order character (different transition temperatures upon cooling and heating) but hardly influences the lattice dynamics. Our broadband dielectric investigations of CsPbBr3 display the same microwave limit permittivity as for MAPbX3 (εr ≈ 30, X = Cl, Br, I, MA = CH3NH3⁺) but do not afford a second permittivity relaxation up to this frequency. Our prior assignment of the second contribution in the methylammonium compounds being due to the relaxation dynamics of the methylammonium ion as a dipole is herewith proven. Nevertheless, CsPbBr3 shows large charge carrier screening up to very high frequencies which can still play a vital role in charge carrier dynamics and exciton behaviour in this material as well.
... As the tested temperature increases to 400 • C, no XRD signals can be detected, indicating the amorphous nature of the material. The temperature-dependent phase evolution of CsPbBr 3 is very interesting and is also verified by differential scanning calorimetry (DSC) measurements [31][32][33]. ...
... As the tested temperature increases to 400 °C, no XRD signals can be detected, indicating the amorphous nature of the material. The temperature-dependent phase evolution of CsPbBr3 is very interesting and is also verified by differential scanning calorimetry (DSC) measurements [31][32][33]. The PL spectrum of the CsPbBr 3 single crystal is shown in Figure 3a, in which the wavelength of the excited light is 360 nm (one-photon excitation). ...
... A strong emission centered at 527 nm was detected, which is consistent with the bandgap of the CsPbBr 3 single crystal [34,35] and a little longer than that of CsPbBr 3 nanowires (519 nm) [36] and nanocrystals (511 nm) [37]. The time-resolved PL decay trace of the CsPbBr 3 single crystal is presented in Figure 3b; its PL lifetime is determined to be as short as 1.86 ns, indicating the fast surface recombination of the photogenerated carrier, assisted by its trap state [32]. This PL lifetime is shorter than other reported values of CsPbBr 3 single crystals [24,25,34,35]. ...
A facile and low-cost strategy to fabricate CsPbBr3 single crystals is essential for developing perovskite optoelectronic devices. Herein, we have presented a room temperature anti-solvent precipitate method for growing sub-centimeter-sized CsPbBr3 single crystals. The as-prepared CsPbBr3 single crystal has an orthorhombic structure, and phase transition occurs as the measured temperature increases. The as-grown CsPbBr3 single crystal also shows abundant surface morphologies including footsteps, precipitated crystals, cracks, and pits. Subsequently, a metal–semiconductor–metal (MSM)-structured photodetector was fabricated based on the CsPbBr3 single crystal. Under 525 nm green light illumination, the photodetector exhibits an obvious response and the photocurrent linearly increases with the increase in the light intensity. The rise time of the photodetector increases from 0.82 s to 2.19 s as the light intensity is enhanced from 15 mW/cm2 to 160 mW/cm2, indicating that more time is required to reach to a stable photocurrent. However, the decay time is as fast as ~0.82 ms, irrelevant of the light intensity. The photocurrent, under continuous light illumination, was further studied and this indicates that a stronger light intensity can accelerate the attenuation of the device.
... The promising first principle-based density functional theory (DFT) tools have been utilized to extract the optoelectronic properties of the ETL, HTL and perovskite structure [71][72][73][74][75][76]. Nowadays, researchers with experimental work have produced high-quality simulation/theoretical analysis-based study articles [33][34][35][36]. The theoretical models can process PSCs without a highly assisted laboratory. ...
... The literature has been mined for all of the FTO parameters [27]. According to the literature [11,15,27,33,34,46,47], the parameters of perovskite are as follows: dielectric permittivity ϵ r = 27, energy band gap E g = 2.34 eV, thickness 650 nm and electron affinity 3.4 eV. Table 1 provides a summary of the structured materials' input parameters. ...
... It is important to note that the operating temperature of PSCs is one of the main parameters influencing the device's PCE. CsPbBr 3 is an inorganic perovskite material with high thermal stability [33,34,65]. The temperature of the simulated cell structure was altered from 300 to 500 K to investigate the influence of temperature on PSC performance. ...
In this work, the improved efficiency of an all-inorganic perovskite solar cell (PSC) is anticipated by utilizing theoretical and computational quantum mechanical approaches, including first-principle density functional theory (DFT) and the solar cell capacitance simulator (SCAPS). In order to redesign and improve the performance of the device, the influence of thickness, energy bandgap, operating temperature, defect density and doping concentration of the perovskite layer; electron affinity of ETL; thickness and doping concentration of ETL and HTL, was investigated. For the electron affinity of 3.1 eV of ETL-ZnO; the perovskite layer's defect density of 1E+11 (1/cm3), bandgap of 2.25 eV and thickness of 2000 nm; ETL and HTL optimized thickness of 50 nm and 200 nm; and doping concentration of ETL, HTL and perovskite layer of 1E+19 (1/cm3) were found to generate the greatest performance from the PSC. In order to optimize the bandgap of CuX and further use it to improve the performance of the device, accurate optoelectronic properties of the structure HTL-CuX (X = I, Cl, and Br) were also derived using DFT. This paper details improved device performance to 25.93% with HTL-CuCl and optimized photovoltaic parameters. By maximizing the photovoltaic parameters, this discovery confirmed the photovoltaic potential of PSC and provided a trustworthy, established and reliable way to assess PSC performance by combining device modeling software with first principles DFT.
... Studying its crystal structure is crucial for understanding its properties and enhancing its performance in optoelectronic devices. To investigate the changes in the physical properties of MAPbBr 3 induced by structural phase transitions, numerous researchers have employed various methods to characterize the structural phase transitions of MAPbBr 3 , including X-ray diffraction (XRD), photoluminescence, Raman spectroscopy, dielectric response, grain size, etc. [6][7][8][9][10][11][12][13]. Hata used a developed classical interatomic potential model, and Poglitsch combined XRD and temperature-dependent complex permittivity measurements to determine the phase transition temperature of MAPbBr 3 [14,15]. ...
Over the past few years, lead halide perovskites have demonstrated significant potential in the field of solar cells, light-emitting diodes, and field-effect transistors. We investigated the transient photoconductivity and temperature-induced phase transitions in thin films, using ultrabroadband optical pump-terahertz probe spectroscopy with varying excitation fluences and temperature conditions. As the temperature drops from 293 to 123 K, undergoes two phase transitions, resulting in a significant extension of exciton lifetime in the tetragonal phase near 150 K, where exhibits a longer exciton lifetime. Our research contributes to a better understanding of the temperature-induced phase transition mechanism of , enhancing its potential applications in optoelectronic devices.
... Despite powder samples, the measured LT value of the dielectric permittivity remained rather high (∼20), suggesting the lattice polarizability by the lone-pair electrons of the lead cations recently proposed by Fabini et al. 63 In addition, the high permittivity value is expected to provide efficient screening of the photogenerated carrier and defect states as in the related 3D lead halide perovskites. 29,64 NLO Studies. The primary aim of the NLO study was to spectroscopically examine the symmetry of all crystal phases of AZRPbX 3 perovskites in a broad temperature range, i.e., to see whether any of the described crystal phases generate a second harmonic of radiation, which would indicate its noncentrosymmetric nature. ...
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.
... The calculated free energy in Glazer space contains the correct phase transitions as observed in experiments [16][17][18][65][66][67][68]. They occur, however, at slightly lower temperatures, which can be attributed to the exchangecorrelation functional used in the reference DFT calcula- tions [46]. ...
Halide perovskites have emerged as a promising class of materials for photovoltaic applications. A challenge in these applications is how to prevent the crystal structure from degradation to photovoltaically inactive phases, which requires an understanding of the free energy landscape of these materials. Here, we uncover the free energy landscape of two prototypical halide perovskites, CsPbBr and MAPbI via atomic scale simulations using umbrella sampling and machine-learned potentials. For CsPbBr we find very small free energy differences and barriers close to the transition temperatures for both the tetragonal-to-cubic and the orthorhombic-to-tetragonal transition. For MAPbI, however, the situation is more intricate. In particular the orthorhombic-to-tetragonal transition exhibits a large free energy barrier and there are several competing tetragonal phases. Using large-scale molecular dynamics simulations we explore the character of these transition and observe latent heat and a discrete change in structural parameters for the tetragonal-to-cubic phase transition in both CsPbBr and MAPbI indicating first-order transitions. We find that in MAPbI the orthorhombic phase has an extended metastability range and furthermore identify a second metastable tetragonal phase. Finally, we compile a phase diagram for MAPbI that includes potential metastable phases.
... 2(a) and 2(b) (see also Fig. S1 in the supplementary material) and are fully consistent with earlier x-ray diffraction (XRD) measurements. 49,85,86 At high temperatures, the GIWAXS data contain a limited number of scattering peaks, indicating that the structure is in the highly symmetric α phase. By decreasing the temperature, peak splitting is observed around 403 K. ...
Metal-halide perovskites (MHPs) exhibit excellent properties for application in optoelectronic devices. The bottleneck for their incorporation is the lack of long-term stability such as degradation due to external conditions (heat, light, oxygen, moisture, and mechanical stress), but the occurrence of phase transitions also affects their performance. Structural phase transitions are often influenced by phonon modes. Hence, an insight into both the structure and lattice dynamics is vital to assess the potential of MHPs. In this study, GIWAXS and Raman spectroscopy are applied, supported by density functional theory calculations, to investigate the apparent manifestation of structural phase transitions in the MHP CsPbBr3. Macroscopically, CsPbBr3 undergoes phase transitions between a cubic (α), tetragonal (β), and orthorhombic (γ) phase with decreasing temperature. However, microscopically, it has been argued that only the γ phase exists, while the other phases exist as averages over length and time scales within distinct temperature ranges. Here, direct proof is provided for this conjecture by analyzing both theoretical diffraction patterns and the evolution of the tilting angle of the PbBr6 octahedra from molecular dynamics simulations. Moreover, sound agreement between experimental and theoretical Raman spectra allowed to identify the Raman active phonon modes and to investigate their frequency as a function of temperature. As such, this work increases the understanding of the structure and lattice dynamics of CsPbBr3 and similar MHPs.
... The systematic investigation of all three phases of CsPbBr 3 , considering both renormalization effects and fourphonon scattering, as well as the contribution of off-diagonal terms, remains largely unexplored. Previous studies have primarily focused on phase transitions, vibrational mechanisms, and dielectric properties [12,13]. To the best of our knowledge, no systematic studies have been conducted to date that encompass all three phases while considering renormalization, four-phonon scattering, and off-diagonal contributions. ...
We investigate the influence of three- and four-phonon scattering, perturbative anharmonic phonon renormalization, and off-diagonal terms of coherent phonons on the thermal conductivity of CsPbBr3 phase change perovskite, by using advanced implementations and first-principles simulations. Our study spans a wide temperature range covering the entire structural spectrum. Notably, we demonstrate that the interactions between acoustic and optical phonons result in contrasting trends of phonon frequency shifts for the high-lying optical phonons in orthorhombic and cubic CsPbBr3 as temperature varies. Our findings highlight the significance of wavelike tunneling of coherent phonons in ultralow and glasslike thermal conductivity in halide perovskites.
... Moreover, a marked quenching of the PL is found. We observe that in the explored temperature range perovskite microcrystals maintain the orthorhombic phase, 24 and therefore, the observed effects cannot be ascribed to a phase change which occurs at 350 K. The blue shift of the emission with the increase in temperature is in agreement with the variation of the bandgap energy of CsPbBr 3 . ...
Halide perovskites are extremely interesting semiconductors for innovation in optoelectronics and photovoltaics. In particular, they are efficient emitters of both incoherent and coherent light and, therefore, of interest as active materials in lasers, light amplification systems (resonators and waveguides), and other photonic devices. Here, we present a set of experimental results concerning nonlinear effects in the radiative emission of CsPbBr3 films deposited by spin-coating on a silicon substrate and on metasurfaces realized by microspheres having a core of SiO2 and a shell of TiO2 (T-Rex). We evidence the presence of amplified spontaneous emission which, depending on the sample structure, shows different behavior as a function of the excitation power. In particular, we distinguish between two processes: light amplification by randomly arranged emitters and amplification by a quite ordered distribution of nanocrystals around a resonator.
... The systematic investigation of all three phases of CsPbBr 3 , considering both renormalization effects and four-phonon scattering, as well as the contribution of off-diagonal terms, remains largely unexplored. Previous studies have primarily focused on phase transitions, vibrational mechanisms, and dielectric properties [12,13]. To the best of our knowledge, no systematic studies have been conducted to date that encompass all three phases while considering renormalization, four-phonon scattering, and off-diagonal contributions. ...
We investigate the influence of three- and four-phonon scattering, perturbative anharmonic phonon renormalization, and off-diagonal terms of coherent phonons on the thermal conductivity of CsPbBr3 phase change perovskite, by using advanced implementations and first-principles simulations. Our study spans a wide temperature range covering the entire structural spectrum. Notably, we demonstrate that the interactions between acoustic and optical phonons result in contrasting trends of phonon frequency shifts for the high-lying optical phonons in orthorhombic and cubic CsPbBr3 as temperature varies. Our findings highlight the significance of wave-like tunneling of coherent phonons in ultralow and glass-like thermal conductivity in halide perovskites.
... In the crystal structure of CsPbBr 3 , Pb 2+ occupies the lattice center, Cs + occupies the vertex and Br 2− is located in the plane center [8,9]. Since photoelectric properties are closely related to the phase transition and structural stability, CsPbBr 3 has been brought into research, and experiments have shown that the structural phase of CsPbBr 3 changes from an orthorhombic system to a tetragonal system at 373 K and to a cubic system at 403 K [10,11] due to the Jahn-Teller effect. In addition, given the high experimental cost and considerable computational time required, atomic simulations with little loss of accuracy provide a valid measure for exploring the internal mechanism of structural phase transformations. ...
CsPbBr3 perovskite has excellent optoelectronic properties and many important application prospects in solar cells, photodetectors, high-energy radiation detectors and other fields. For this kind of perovskite structure, to theoretically predict its macroscopic properties through molecular dynamic (MD) simulations, a highly accurate interatomic potential is first necessary. In this article, a new classical interatomic potential for CsPbBr3 was developed within the framework of the bond-valence (BV) theory. The optimized parameters of the BV model were calculated through first-principle and intelligent optimization algorithms. Calculated lattice parameters and elastic constants for the isobaric-isothermal ensemble (NPT) by our model are in accordance with the experimental data within a reasonable error and have a higher accuracy than the traditional Born-Mayer (BM) model. In our potential model, the temperature dependence of CsPbBr3 structural properties, such as radial distribution functions and interatomic bond lengths, was calculated. Moreover, the temperature-driven phase transition was found, and the phase transition temperature was close to the experimental value. The thermal conductivities of different crystal phases were further calculated, which agreed with the experimental data. All these comparative studies proved that the proposed atomic bond potential is highly accurate, and thus, by using this interatomic potential, the structural stability and mechanical and thermal properties of pure inorganic halide and mixed halide perovskites can be effectively predicted.
... CsPbBr3 was stable up to 465 °C, and about 40% mass loss was observed at the melting temperature [9,13] (567 °C) (Figure 3a). DSC analysis showed tetragonal (P4/mbm) ↔ cubic (Pm3m) phase transformation at 132 °C and orthorhombic (Pbnm) ↔ tetragonal (P4/mbm) phase transformation ( Figure 3c) at 88 °C, which agrees with the literature observation [9,13,[47][48][49][50]. The other peaks at 332 and 335 °C might be due to the melting of PbBr2. ...
Inorganic lead-halide perovskite, cesium lead bromide (CsPbBr3), shows outstanding optoelectronic properties. Both solution- and melt-based methods have been proposed for CsPbBr3 crystal growth. The solution-based growth was done at low-temperature, whereas the melt-based growth was done at high-temperature. However, the comparison of optical, physical, and defect states using these two different growth conditions has been scarcely studied. Here, we have compared the thermal and optical properties of solution-grown and melt-grown single crystals of CsPbBr3. Positron Annihilation Lifetime Spectroscopy (PALS) analysis showed that melt-grown crystal has a relatively smaller number of defects than the chemical synthesis method. In addition, crystals grown using the chemical method showed a higher fluorescence lifetime than melt-grown CsPbBr3.
... Note that such symmetry-preserving transitions are rather frequent in hybrid perovskites and related materials. 19,39,64 Our dielectric experiments also allowed us to capture and distinguish different dipolar relaxation processes in the studied compounds. Based on the previous studies, the rotational dynamics of the MA cations in the tetragonal phase of MAPbI 3 occur in the GHz frequency range 65 with the activation (barrier) energy E a of 100 meV. ...
Mixing of molecular cations in hybrid lead halide perovskites is a highly effective approach to enhance stability and performance of the optoelectronic devices based on these compounds. In this work, we prepare and study novel mixed methylammonium (MA)-ethylammonium (EA) MA1-xEAxPbI3 (x < 0.4) hybrid perovskites. We use a suite of different techniques to determine the structural phase diagram, cation dynamics and photoluminescence properties of these compounds. Upon introduction of EA, we observe a gradual lowering of the phase transition temperatures indicating stabilization of the cubic phase. For mixing levels higher than 30%, we obtain a complete suppression of the low-temperature phase transition and formation of a new tetragonal phase with different symmetry. We use the broadband dielectric spectroscopy to study the dielectric response of the mixed compounds in an extensive frequency range, which allows us to distinguish and characterize three distinct dipolar relaxation processes related to the molecular cation dynamics. We observe that mixing increases the rotation barrier of the MA cations and tunes the dielectric permittivity values. For the highest mixing levels, we observe signatures of the dipolar glass phase formation. Our findings are supported by the density functional theory calculations. Our photoluminescence measurements reveal a small change of the band gap upon mixing indicating suitability of these compounds for optoelectronic applications.
... The existence of this lowfrequency feature is supported by various observations, such as over-damped modes and critical inelastic neutron scattering, [2,5,73,[75][76][77] central peak in Raman scattering, [11,16,17,23,28,29,41] soft modes, [78,79] short phonon coherence length [80][81][82] and a high dielectric function at low-frequencies. [1,74,83,84] Our interpretation is disencumbered by contradictions with Raman selection rules or complementary measurements. The model similarly applies to oxide and halide perovskites, emphasizing the similarities between the crystal families and their interaction with light. ...
The anharmonic lattice dynamics of oxide and halide perovskites play a crucial role in their mechanical and optical properties. Raman spectroscopy is one of the key methods used to study these structural dynamics. However, despite decades of research, existing interpretations cannot explain the temperature dependence of the observed Raman spectra. We demonstrate the non-monotonic evolution with temperature of the scattering intensity and present a model for 2nd-order Raman scattering that accounts for this unique trend. By invoking a low-frequency anharmonic feature, we are able to reproduce the Raman spectral line-shapes and integrated intensity temperature dependence. Numerical simulations support our interpretation of this low-frequency mode as a transition between two minima of a double-well potential surface. The model can be applied to other dynamically disordered crystal phases, providing a better understanding of the structural dynamics, leading to favorable electronic, optical, and mechanical properties in functional materials.
... The electrical and dielectric properties of homogeneous materials can be fully ) 1 . The spectral and temperature dependence of * directly reveal phenomena taking place at the molecular scale such as dipolar relaxation processes [2][3][4] , phase transitions [5][6][7] and charge transport processes [8][9][10][11][12][13] . The direct correlation existing between the spectral dependence of * and molecular phenomena is drastically impacted by the presence of interfaces and interphases, when the materials under study are heterogeneous (composite materials, multiphase materials, etc.). ...
An experimental and theoretical investigation of the scaling laws governing the phenomenon of Maxwell-Wagner-Sillars interfacial polarization in composite materials in dependence on morphology, volume fraction, orientation of fillers, form factor and the presence of interphases is presented in the current study. By considering the complex dielectric function of the matrix and of the fillers, the dielectric spectra are calculated in the frequency range from 10 ⁷ Hz to 10 ⁻² Hz and compared to dielectric measurements by Broadband Dielectric Spectroscopy, carried out in the frequency range from 10 ⁷ Hz to 0.5Hz and between -90 o C and 150 o C. The characteristic frequencies of the global dielectric response are reported to strongly vary with the conductivity value of the conductive phase, while a much weaker dependence is observed upon varying the volume fraction, the form factor and the orientation of fillers. The value of permittivity at low frequency does not change with the conductivity value, whereas a significant variation is observed in dependence on the composite morphology, form factor, orientation of fillers and presence of interfaces with different gradients of properties. Two possible applications of our analysis are reported: (i) measuring the conductivity of materials without employing a direct electrical contact between the electrodes and the sample and (ii) discriminating different phenomena of electrical polarization in complex materials by analyzing the scaling laws. Our study delivers thus a useful and necessary analysis of the dielectric behavior of composite materials, where interfacial polarization effects play a major role.
... 72 The resulting dependence, calculated using optical data from ref 71, is shown by the blue line in Figure 5a. Concurrently, a more efficient electron−hole scattering at higher N results in an asymptotic ∼1/N dependence, 13 shown by the orange line in Figure 5a, which was calculated using reported values of the static permittivity 78 and of the reduced carrier effective mass. 79 The value combined via Matthiessen's rule, as 1/D theor (N) = 1/ D deg (N) + 1/D eh (N), is plotted as the red line in Figure 5a. ...
Describing the nanoscale charge carrier transport at surfaces and interfaces is fundamental for designing high-performance optoelectronic devices. To achieve this, we employ time- and angle-resolved photoelectron spectroscopy with ultraviolet pump and extreme ultraviolet probe pulses. The resulting high surface sensitivity reveals an ultrafast carrier population decay associated with surface-to-bulk transport, which was tracked with a sub-nanometer spatial resolution normal to the surface, and on a femtosecond time scale, in the case of the inorganic CsPbBr3 lead halide perovskite. The decay time exhibits a pronounced carrier density dependence, which is attributed via modeling to enhanced diffusive transport and concurrent recombination. The transport is found to approach an ordinary diffusive regime, limited by electron-hole scattering, at the highest excitation fluences. This approach constitutes an important milestone in our capability to probe hot-carrier transport at solid interfaces with sub-nanometer resolution in a theoretically and experimentally challenging, yet technologically relevant, high-carrier-density regime.
... XRD patterns of the FAPbBr 3 powder recorded at different temperatures are shown in Fig. 4. According to the most recent literature data [8,9], FAPbBr 3 undergoes two successive structural transformations when cooling below room temperature: from a cubic structure to a tetragonal one and from tetragonal to orthorhombic. The sequence of the transitions is the same as that observed in CsPbBr 3 [14], which is an inorganic relative of FAPbBr 3 . However, it should be noticed that as opposed to cesium lead bromide, the FA-containing perovskite is significantly less distorted, since the angles of the oxygen octahedral tilts that induce the tetragonal and the orthorhombic distortions are much smaller [9]. ...
The high power conversion efficiency of perovskite solar cells reaching 25.5% today [1], raises the question on the relation between the photovoltaic performance and physical nature of these materials. To get a better understanding of the properties, good quality single crystals are highly demanded. In this paper we report on large single crystals of formamidinium lead halide grown by a combination of inverse temperature crystallization and seed growth. Structure, phase transitions, dielectric and optical properties of the single crystals are shown.
... When including the changes of the electron clouds around atomic centers, the rotational and translational motions of ions and molecules, as well as the transport of charged species (ions and electrons), they recommend the following values of dielectric permittivity for the representative MAPbI 3 : e optic = 4.5-6.5 for high-frequency processes and e ionic = 16.5 [195,196,199,[201][202][203] for the ionic contribution, which leads to a bulk static dielectric response e r = 22.0 [197,199,201,204]. The broadband dielectric investigations across a wide temperature (120-450 K) range, and frequency ranges of CsPbBr 3, display a similar microwave permittivity of about 30 [197,205,206]. An huge increase of low-frequency dielectric permittivity was found for illuminated lead halide perovskites [205]. ...
Hybrid organic–inorganic perovskites (HOIPs) constitute unique coordination polymers with a huge potential of transforming modern materials chemistry due to their wide applications, mainly in electronics, optoelectronics and photovoltaics. Yearly, thousands of works on HOIPs are published in various fields of science, technology and industry. Many of these studies are devoted to structural and structure-related properties. Molecular spectroscopy methods (optical and broadband dielectric spectroscopy, IR, Raman, EPR, NMR) are often used to understand their magnetic, optical and luminescent properties; energy conversion; basic molecular dynamics; hydrogen bond-associated guest molecule-framework interactions; stability; flexibility; mechanisms of temperature- and pressure induced phase transitions; and other more sophisticated physical phenomena, such as polar properties, ferroelectricity, multiferroicity, etc. The goal of this review is to create a comprehensive and versatile phase-related guide for researchers working in the field of HOIPs using a wide range of molecular spectroscopy techniques. This review presents a systematical description of very recent achievements obtained using modern molecular spectroscopy methods. It also summarizes the current state of knowledge on HOIPs, including metal formates, hypophosphites, azides, cyanides, dicyanamides, and halides templated by protonated amines and phosphines that adopt a perovskite architecture.
Perovskite nanocrystal (PNC) solids are promising materials for optoelectronic applications. Recent studies have shown that exciton diffusion in PNC solids occurs via alternate exciton hopping (EH) and photon recycling (PR). The energy disorder induced by the size distribution is a common factor in PNC solids, and the impact of this energy disorder on the exciton diffusion remains unclear. Here, we investigated the exciton diffusion in CsPbBr3 NC solids with a Gaussian size distribution of 11.2 ± 6.8 nm via steady and time-resolved photoluminescence (PL) spectroscopy with multiple detection bands in transmission mode. Our results indicated that exciton diffusion was controlled by a downhill transfer among the different energy sites through the disordered energy landscape, as confirmed by the accompanying low-temperature PL analysis. A detailed examination revealed that the acceptor distribution in tandem with the reabsorption coefficient determined the contribution of EH and PR to exciton transfer between different energy sites. Consequently, the exciton diffusion mechanism varied in PNC solids of different thicknesses: in a thin solid with a thickness of several hundred nanometers, the exciton transfer was dominated by efficient EH and PR from the high-energy sites to the lower-energy sites; in a few-micrometer-thick solid, transfer from the medium-energy sites toward the lower-energy sites also became prominent and occurred mainly through PR. These findings enhance the understanding of the vital role that the acceptor distribution plays in the exciton diffusion process in PNC solids, providing important insights for optoelectronic applications based on PNC solids. Our work also exploits the use of commonly available tools for in-depth exciton diffusion studies, which reveals the interior diffusion information that is usually hidden in surface sensitive PL imaging methods.
Ytterbium-doped cesium lead halide perovskites exhibit unusual down-conversion with quantum efficiencies exceeding 100%. This phenomenon has been attributed to the quantum-cutting (QC) when a perovskite exciton is converted into two...
Understanding the recombination lifetime of charge carriers (τc) is essential for the diverse applications of photovoltaic materials, such as perovskites. The study on the inorganic perovskite, CsPbBr3, reveals recombination dynamics exceeding 1 ms below 200 K and τc approaching 100 μs at room temperature. Utilizing time‐resolved microwave‐detected photoconductivity decay in conjunction with injection dependence, it is found that τc is dominated by impurity charge trapping. The observed injection dependence is well corroborated by modeling of the trap mechanism. The ultralong decay time is also consistent with photoconductivity measurements with a continuous‐wave excitation at powers corresponding to around 1 Sun irradiation. While charge‐carrier trapping may, in theory, impose limitations on the photovoltaic efficiency of single‐cell devices, it can also contribute to increased efficiency in tandem cells and find applications in photodetection, photocatalysis, and quantum information storage.
Halide perovskites have gained relevance in the field of solar cells due to their remarkable electro-optical properties, which enable efficient conversion of solar energy into electricity. Despite their promising characteristics, challenges such as long-term stability and structural complexity demand exceptional attention and dedication in the research on these materials. Their inherent soft nature, high atom mobility (especially of the halides) and the unconventional dynamics of structural motifs (halide octahedra) make them interesting from a fundamental point of view as well. The study focuses on understanding phase transitions in CsPbBr3 perovskite, considering the importance of the dynamic properties it exhibits. The phase transitions of the CsPbBr3 perovskite were studied through ab initio NPT molecular dynamics simulations considering several different temperatures. By taking into account the average structures over a simulation time of 45 ps after thermalization, we predict phase transitions between 300 and 325 K, as well as between 400 and 450 K, in line with previous experimental findings reported in the literature. Furthermore, through the analysis of the angles within the octahedron (Br–Pb–Br) and between octahedra (Pb–Br–Pb), the mechanism underlying the phase transitions is understood, and the structural anomalies previously reported [Svirskas et al., J. Mater. Chem. A, 2020, 8, 14015–14022] near 220 K are identified. In addition to the results obtained by performing long-time averages, we also conducted an analysis of the implications of using different time windows when calculating the average properties of interest. In this case, we observed a more complex pattern, where the material exhibits various structures depending on the exposure time and temperature, which aligns with the polymorphic nature of these materials. Our results show that, depending on the type of experiment that is being performed, different analysis, with averages considered over different times, must be performed. Long-time averages can be compared to X-ray diffraction experiments, while short-time averages should be compared to experiments that track the local structure of the material, such as PDF or Raman. Our results also indicate that phase transitions in CsPbBr3 are not as abrupt as previously considered, posing new challenges for the experimental observation of these features.
Hybrid methylammonium (MA) lead halide perovskites have emerged as materials exhibiting excellent photovoltaic performance related to their rich structural and dynamic properties. Here, we use multifrequency (X-, Q-, and W-band)...
Lead halide perovskites are extensively investigated as efficient solution-processable materials for photovoltaic applications. The greatest stability and performance of these compounds are achieved by mixing different ions at all three sites of the APbX3 structure. Despite the extensive use of mixed lead halide perovskites in photovoltaic devices, a detailed and systematic understanding of the mixing-induced effects on the structural and dynamic aspects of these materials is still lacking. The goal of this review is to summarize the current state of knowledge on mixing effects on the structural phase transitions, crystal symmetry, cation and lattice dynamics, and phase diagrams of three- and low-dimensional lead halide perovskites. This review analyzes different mixing recipes and ingredients providing a comprehensive picture of mixing effects and their relation to the attractive properties of these materials.
Halide perovskites have been studied very intensively by researchers during the last decade. Development of these materials has improved their unique optoelectrical properties reaching even higher standards, making them promising candidates for photovoltaic applications. It should be noted that most inorganic halide perovskites obtained to date have been synthesized using organic solvents in the form of nanosized colloids. Here, a low‐temperature synthesis protocol for the preparation of microcrystalline CsPbBr3 perovskite powder doped with Yb³⁺ ions is proposed. The structural and photoluminescence features of the studied material are thoroughly investigated and described. It turns out that the excitation of the CsPbBr3:Yb³⁺ perovskite with a 375 nm wavelength leads to spontaneous luminescence of excitons and Yb³⁺ ions. Hence, the use of CsPbBr3:Yb³⁺ as a luminescent thermometer or an additional absorbing layer on a solar cell surface is possible. The latter application may result in an increase in the conversion efficiency of the cell. In order to verify this, such a layer is prepared and installed on a commercial silicon solar cell. Its photovoltaic properties are investigated by measurements of the current–voltage characteristics with 1‐sun illumination and the spectral characteristics of the external quantum efficiency.
Pixelated semiconductor detectors with superior energy and spatial resolution have broad applications in the field of ionizing radiation detection. Recently, radiation detectors based on CsPbBr
are gradually gaining acceptance due to their favorable properties. However, the energy resolution (ER) of the CsPbBr
pixelated detector is constrained because of the unmatched electrode structure and immature electrode preparation. In this work, we report that the pixel configuration coupled with the CsPbBr
detector thickness to achieve dynamic regulation of the
Cs 662-keV
-rays both theoretically and experimentally, which is beneficial to eliminate potential discrepancies in performance associated with the variations in crystal quality. The simulation results yield an ER that is weakly correlated with the fundamental properties of CsPbBr
but strongly depends on the device structure. The theoretical resolution of the CsPbBr
pixelated detector is
1% by adjusting the pixel-width-to-thickness ratio in the range of 0.15–0.4. Furthermore, with an optimized pixel width-to-thickness ratio, the CsPbBr
pixelated detectors achieve a resolution of 7.31% (without any signal processing) at 662 keV and show excellent stability under a large bias up to 700 V (2892
). This study is an extension of pixelated geometry fabrication and provides a strategy for the CsPbBr
detector to achieve better resolution.
It is necessary to improve the action cross section (η × σn) of high-order multiphoton absorption (MPA) for fundamental research and practical applications. Herein, the core-shell FAPbBr3/CsPbBr3 nanocrystals (NCs) were constructed, and fluorescence induced by up to five-photon absorption was observed. The value of η × σ5 reaches 8.64 × 10-139 cm10 s4 photon-4 nm-3 at 2300 nm, which is nearly an order of magnitude bigger than that of the core-only NCs. It is found that the increased dielectric constant promotes modulation of MPA effects, addressing the electronic distortion in high-order nonlinear behaviors through the local field effect. Meanwhile, the quasi-type-II band alignment suppresses the biexciton Auger recombination, ensuring the stronger MPA induced fluorescence. In addition, the core-shell structure can not only reduce the defect density but also promote the nonradiative energy transfer though the antenna-like effect. This work provides a new avenue for the exploitation of high-performance multiphoton excited nanomaterials for future photonic integration.
Using muon spin relaxation measurements on formamidinium lead iodide [FAPbI3, where FA denotes HC(NH2)2], we show that, among the five structurally distinct phases of FAPbI3 exhibited through two different temperature hysteresis, the reorientation motion of FA molecules is quasi-static below ≈50 K over the time scale of 10−6 s in the low-temperature (LT) hexagonal (Hex-LT, <160 K) phase, which has a relatively longer photo-excited charge carrier lifetime (τc∼10−6 s). In contrast, a sharp increase in the FA molecular motion was found above ≈50 K in the Hex-LT phase, LT-tetragonal phase (Tet-LT, <140 K), the high-temperature (HT) hexagonal phase (Hex-HT, 160–380 K), and the HT-tetragonal phase (Tet-HT, 140–280 K), where τc decreases with increasing temperature. More interestingly, the reorientation motion is further promoted in the cubic phase at higher temperatures (>380/280 K), while τc is recovered to comparable or larger than that of the LT phases. These results indicate that there are two factors that determine τc, one related to the local reorientation of cationic molecules that is not unencumbered by phonons and the other to the high symmetry of the bulk crystal structure.
Metal-halide perovskites exhibit excellent optoelectronic properties, making them suitable for various applications. In this work, we propose a novel approach, employing the message-passing neural networks (MPNNs), to model the potential energy surface (PES) of metal-halide perovskites. The MPNN model can learn the PES of perovskites with an error of less than 1 meV per atom, which is comparable to ab initio calculations. With the accurate and fast prediction of the MPNN model, we perform large-scale molecular dynamics simulations for CsPbBr3 and Cs2AgBiBr6, successfully identifying their temperature-dependent octahedron tilting patterns, ion displacement, and structural phases that are crucial to obtaining their electronic structure correctly. We also observe significant Ag migration and vacancy defects in Cs2AgBiBr6, which are further investigated using ab initio calculations, offering insights into its ambient stability. This work provides a comprehensive understanding of the structural phase transition in metal-halide perovskites, as well as information regarding ion migration.
Using muon spin relaxation (SR) measurements on formamidinium lead iodide [FAPbI, where FA denotes HC(NH] we show that, among the five structurally distinct phases of FAPbI exhibited through two different temperature hysteresis, the reorientation motion of FA molecules is quasi-static below K over the time scale of 10 s in the low-temperature (LT) hexagonal (Hex-LT, K) phase which has relatively longer photo-excited charge carrier lifetime (10 s). In contrast, a sharp increase in the FA molecular motion was found above K in the Hex-LT phase, LT-tetragonal phase (Tet-LT, K), the high-temperature (HT) hexagonal phase (Hex-HT, 160-380 K), and the HT-tetragonal phase (Tet-HT, 140-280 K) where decreases with increasing temperature. More interestingly, the reorientation motion is further promoted in the cubic phase at higher temperatures ( K), while is recovered to comparable or larger than that of the LT phases. These results indicate that there are two factors that determine , one related to the local reorientation of cationic molecules that is not unencumbered by phonons, and the other to the high symmetry of the bulk crystal structure.
Knowledge of the charge-carrier recombination lifetime, tau, is crucial for the various applications of photovoltaic perovskites. We studied the novel inorganic perovskite, CsPbBr3 and we observe recombination dynamics beyond 1 ms below 200 K and tau approaching 100 us at room temperature. Time-resolved microwave-detected photoconductivity decay (TRMCD), used in combination with injection dependence, evidence that tau is dominated by impurity charge trapping. The observed injection dependence is well corroborated by modeling of the trap mechanism. The ultra-long decay time is also consistent with photoconductivity measurements with a continuous-wave excitation at powers corresponding to around one Sun irradiation. While in principle charge-carrier trapping may limit the photovoltaic efficiency in single-cell photovoltaic devices, it could also lead to enhanced efficiency in tandem cells as well as for alternative applications including photodetection and quantum information storage.
Caesium-based mixed halide perovskite single crystals, CsPbBr3-xIx, are shown to be good candidates for many optoelectronic applications under ambient conditions. The partial substitution of iodine into the bromine sites of CsPbBr3 single crystal makes attractive effects in optical and structural properties as well as their thermal and phase stability. Herein, the Inverse Temperature Crystallization (ITC) method has been utilised to grow high-quality, large, and single-phase single crystals of CsPbBr2.4I0.6 perovskites. The iodine doping is found to result in improved purity, thermal, phase, and atmospheric stability of the single crystal and helped to tune the band gap. Further, there is a conspicuous decrease in the defect density with iodine doping. Thus the present study establishes iodine doping as a facile route to grow CsPbBr3 single crystals with overall stability and improved defect density with a broad absorption spectrum, that point toward extended optoelectronic and photovoltaic applications.
The anharmonic lattice dynamics of oxide and halide perovskites play a crucial role in their mechanical and optical properties. Raman spectroscopy is one of the key methods used to study these structural dynamics. However, despite decades of research, existing interpretations cannot explain the temperature dependence of the observed Raman spectra. We demonstrate the nonmonotonic evolution with temperature of the scattering intensity and present a model for second-order Raman scattering that accounts for this unique trend. By invoking a low-frequency anharmonic feature, we are able to reproduce the Raman spectral line shapes and integrated intensity temperature dependence. Numerical simulations support our interpretation of this low-frequency mode as a transition between two minima of a double-well potential surface. The model can be applied to other dynamically disordered crystal phases, providing a better understanding of the structural dynamics, leading to favorable electronic, optical, and mechanical properties in functional materials.
Metal halide perovskites (MHPs) have been one of the most promising materials for the photoelectric, electro‐optical, and all‐optical conversion devices, thanks to their excellent energy conversion. To investigate the energy conversion induced by structural symmetry, herein, the carrier dynamics and dielectric properties in MHPs are surveyed and reviewed as the microscopic and macroscopic bridge, respectively. The intrinsic correlations between the structural symmetry broken, carrier dynamics, and energy conversion performance are systematically analyzed by considering the energy band structures and dielectric constants. The internal energy conversion mechanisms are elucidated to pave the way for the perovskite advanced applications.
Herein we show that dispersing inorganic cesium lead bromide (CsPbBr3) perovskite quantum dots (QDs) in optical quality films, possessing an accessible and controlled pore size distribution, gives rise to fluorescent materials with a controlled and highly sensitive response to ambient changes. A scaffold-based synthesis approach is employed to obtain ligand-free QDs, whose pristine surface endows them with high sensitivity to the presence of different vapors in their vicinity. At the same time, the void network of the host offers a means to gradually expose the embedded QDs to such vapors. Under these conditions, the luminescent response of the QDs is mediated by the mesostructure of the matrix, which determines the rate at which vapor molecules will adsorb onto the pore walls and, eventually, condensate, filling the void space. With luminescence quantum yields as high as 60%, scaffold-supported ligand-free perovskite nanocrystals display intense photoemission signals over the whole process, as well as high photo- and chemical stability, which allows illuminating them for long periods of time and recovering the original response upon desorption of the condensed phase. The results herein presented open a new route to explore the application of perovskite QD-based materials in sensing.
Mixing molecular cations in hybrid lead halide perovskites is a highly effective approach to enhance the stability and performance of optoelectronic devices based on these compounds. In this work, we prepare and study novel mixed 3D methylammonium (MA)-ethylammonium (EA) MA1-x EA x PbI3 (x < 0.4) hybrid perovskites. We use a suite of different techniques to determine the structural phase diagram, cation dynamics, and photoluminescence properties of these compounds. Upon introduction of EA, we observe a gradual lowering of the phase-transition temperatures, indicating stabilization of the cubic phase. For mixing levels higher than 30%, we obtain a complete suppression of the low-temperature phase transition and formation of a new tetragonal phase with a different symmetry. We use broad-band dielectric spectroscopy to study the dielectric response of the mixed compounds in an extensive frequency range, which allows us to distinguish and characterize three distinct dipolar relaxation processes related to the molecular cation dynamics. We observe that mixing increases the rotation barrier of the MA cations and tunes the dielectric permittivity values. For the highest mixing levels, we observe the signatures of the dipolar glass phase formation. Our findings are supported by density functional theory calculations. Our photoluminescence measurements reveal a small change of the band gap upon mixing, indicating the suitability of these compounds for optoelectronic applications.
Methylammonium (MA) lead halide perovskites have been widely studied as active materials for advanced optoelectronics. As crystalline semiconductor materials, their properties are strongly affected by their crystal structure. Depending on their applications, the size of MA lead halide perovskite crystals varies by several orders of magnitude. The particle size can lead to different structural phase transitions and optoelectronic properties. Herein, we investigate the size effect for phase transition of MA lead bromide (MAPbBr3) by comparing the temperature-dependent neutron powder diffraction patterns of microcrystals and nanocrystals. The orthorhombic-to-tetragonal phase transition occurs in MAPbBr3 microcrystals within the temperature range from 100 to 310 K. However, the phase transition is absent in nanocrystals in this temperature range. In this work, we offer a persuasive and direct evidence of the relationship between the particle size and the phase transition in perovskite crystals.
Understanding and predicting lattice dynamics in strongly anharmonic crystals is one of the long-standing challenges in condensed matter physics. Here, we propose a first-principles method that gives accurate quasiparticle (QP) peaks of the phonon spectrum with strong anharmonic broadening. On top of the conventional first-order self-consistent phonon (SC1) dynamical matrix, the proposed method incorporates frequency renormalization effects by the bubble self-energy within the QP approximation. We apply the developed methodology to the strongly anharmonic α−CsPbBr3 that displays phonon instability within the harmonic approximation in the whole Brillouin zone. While the SC1 theory significantly underestimates the cubic-to-tetragonal phase transition temperature (Tc) by more than 50%, we show that our approach yields Tc=404–423 K, in excellent agreement with the experimental value of 403 K. We also demonstrate that an accurate determination of QP peaks is paramount for quantitative prediction and elucidation of the phonon linewidth.
Inorganic halide perovskites (IHPs) have provoked intense research efforts because of their superior stability, excellent optoelectronic properties, cost-effectiveness, and striking optoelectronic applications. Recently, the doping of lanthanide ions in IHPs has opened new avenues, particularly for emerging applications like NIR and white light-emitting diodes, NIR emitters, NIR cameras, optical temperature sensing, optical data encoding, etc. Besides, lanthanide doping has also significantly improved the stability (thermal, photo, and phase), structure and optical properties of IHPs, which has resulted in improved device performance. However, a comprehensive review of this development for IHPs is rare. This review article is an attempt to fill this gap and is designed to provide important fundamental aspects as well as recent developments in the field. It comprises all the basics, synthesis strategies, crystal structure (including phase transition and phase stability), and optical properties (absorption, emission, lifetime, quantum yield, exciton binding energy, and anisotropy) for pristine IHPs with special reference to CsPbX3 (X = Cl, Br and I). The effects of lanthanide doping on the above-listed properties for IHPs are explored, and the doping of non-lanthanide metal ions such as alkali metals, alkaline earth metals, transition metals, and post-transition metals in all-IHPs are also covered for comparison. Furthermore, the review specifically outlines a few novel applications, which are due to the inherent merits of lanthanide doping in CsPbX3. Potential challenges and future perspectives are also discussed.
The phonon dispersion relations of crystal lattices can often be well described with the harmonic approximation. However, when the potential energy landscape exhibits more anharmonicity, for instance, in the case of a weakly bonded crystal or when the temperature is raised, the approximation fails to capture all crystal lattice dynamics properly. Phonon-phonon scattering mechanisms become important and limit the phonon lifetimes. We take a novel approach and simulate the phonon dispersion of a complex dynamic solid at elevated temperatures with machine-learning force fields of near-first-principles accuracy. Through large-scale molecular dynamics simulations the projected velocity autocorrelation function (PVACF) is obtained. We apply this approach to the inorganic perovskite CsPbBr3. Imaginary modes in the harmonic picture of this perovskite are absent in the PVACF, indicating a dynamic stabilization of the crystal. The anharmonic nature of the potential makes a decoupling of the system into a weakly interacting phonon gas impossible. The phonon spectra of CsPbBr3 show the characteristics of a phonon liquid. Rattling motions of the Cs+ cations are studied by self-correlation functions and are shown to be nearly dispersionless motions of the cations with a frequency of ∼0.8THz within the lead-bromide framework.
Metal halide perovskites are the first solution processed semiconductors that can compete in their functionality with conventional semiconductors, such as silicon. Over the past several years, perovskite semiconductors have reported breakthroughs in various optoelectronic devices, such as solar cells, photodetectors, light emitting and memory devices, and so on. Until now, perovskite semiconductors face challenges regarding their stability, reproducibility, and toxicity. In this Roadmap, we combine the expertise of chemistry, physics, and device engineering from leading experts in the perovskite research community to focus on the fundamental material properties, the fabrication methods, characterization and photophysical properties, perovskite devices, and current challenges in this field. We develop a comprehensive overview of the current state-of-the-art and offer readers an informed perspective of where this field is heading and what challenges we have to overcome to get to successful commercialization.
A recently published report on a new low-temperature phase transition in CsPbBr 3 was verified. We provide strong evidence that the room-temperature orthorhombic phase of the crystal is stable at least up to 95 K.
In this contribution, more evidence is provided to support the presence of low-temperature anomaly in CsPbBr 3 single crystals.
Triboelectric nanogenerators (TENGs) are a widely researched type of energy harvester suited to powering mobile micro-electronic devices. In this study, we present a hybrid MAPbIxCl3-x perovskite-based triboelectric nanogenerator (P-TENG) based on the coupling of triboelectric and photoelectric conversion mechanisms for simultaneous vibrating energy and solar energy harvesting. By optimizing the device structure, for the first time, planar TiO2 as electron transport layer (ETL) and ultrathin pentacene as hole transport layer (HTL) are combined together to photoenhance the output of a TENG. Experimental results reveal that P-TENG has achieved the optimal photoinduced enhancement due to the most effective charge separation that relies on the joint of HTL and ETL. As a result, the optimized P-TENG with ~ 0.7 cm² effective area, the open-circuit voltage (Voc), short-circuit current (Isc) and the maximum transfer charge amount (Qsc) are increased by 55.7%, 50.8% and 58.2% upon illumination, respectively. Besides, the P-TENG shows fast response on both the full-spectrum simulated sunlight and monochromatic light extending from ultraviolet to entire visible region which enhances the potential application in photodetection. Our work presents a route to designing high-performance P-TENG via interfacial engineering to further boost the output ability of this photoelectric hybrid TENG.
Lead halide perovskites belong to a broad class of compounds with appealing optoelectronic and photovoltaic properties. Doping with transition metal ions such as Mn²⁺ and Co²⁺ has recently been reported to substantially enhance luminescence and stability of these materials. However, so far atomic-level evidence for incorporation of the dopants into perovskite phases has been missing. Here, we introduce a general and straightforward method for confirming the substitutional doping of bulk perovskite phases with paramagnetic dopants. Using ¹³³Cs and ¹H solid-state MAS NMR relaxation measurements we provide for the first time direct evidence that, consistent with current understanding, Mn²⁺ is incorporated into the perovskite lattice of CsPbCl3 and CsPbBr3 and does not form clusters. We also show that, contrary to current conviction, Co²⁺ is not incorporated into the perovskite lattice of MAPbI3.
Lead halide perovskites have been demonstrated as high performance materials in solar cells and light-emitting devices. These materials are characterized by coherent band transport expected from crystalline semiconductors, but dielectric responses and phonon dynamics typical of liquids. This “crystal-liquid” duality implies that lead halide perovskites belong to phonon glass electron crystals, a class of materials believed to make the most efficient thermoelectrics. We show that the crystal-liquid duality and the resulting dielectric response are responsible for large polaron formation and screening of charge carriers, leading to defect tolerance, moderate charge carrier mobility, and radiative recombination properties. Large polaron formation, along with the phonon glass character, may also explain the marked reduction in hot carrier cooling rates in these materials.
Solution-processable metal halide perovskites (MHPs) offer great promise for efficient light-harvesting and -emitting devices because of their long carrier lifetime and superior carrier transport properties. Similar to traditional oxide perovskites, MHPs exhibit strong dynamic disorders that may further impact their electronic properties. Here we investigate the coherent acoustic phonons of inorganic and organic–inorganic (hybrid) MHP single crystals (CsPbCl3, MAPbCl3, CsPbBr3, MAPbBr3, FAPbBr3, MAPbI3, and FAPbI3) using pump–probe reflection spectroscopy. We show significant phonon softening in the cubic phase of all compositions close to the cubic-to-tetragonal phase transition temperature. Such phonon softening in conjunction with strong acoustic damping is attributed to pretransitional polar fluctuations. Comparison of MHPs with different compositions show that the degree of pretransitional fluctuations is correlated with the size rather than the dipole moment of the A-site cations, and further with the size of the anions.
Cesium lead bromide (CsPbBr3) was recently introduced as a potentially high performance thin-film halide perovskite (HaP) material for optoelectronics, including photovoltaics, significantly more stable than MAPbBr3 (MA = CH3NH3⁺). Because of the importance of single crystals to study relevant material properties per se, crystals grown under conditions comparable to those used for preparing thin films, i.e., low-temperature solution-based growth, are needed. We show here two simple ways, antisolvent-vapor saturation or heating a solution containing retrograde soluble CsPbBr3, to grow single crystals of CsPbBr3 from a precursor solution, treated with acetonitrile (MeCN) or methanol (MeOH). The precursor solutions are stable for at least several months. Millimeter-sized crystals are grown without crystal-seeding and can provide a 100% yield of CsPbBr3 perovskite crystals, avoiding a CsBr-rich (or PbBr2-rich) composition, which is often present alongside the perovskite phase. Further growth is demonstrated to be possible with crystal seeding. The crystals are characterized in several ways, including first results of charge carrier lifetime (30 ns) and an upper-limit of the Urbach energy (19 meV). As the crystals are grown from a polar aprotic solvent (DMSO), which is similar to those used to grow hybrid organic-inorganic HaP crystals, this may allow growing mixed (organic and inorganic) monovalent cation HaP crystals.
Recently, lead halide perovskite quantum dots have been reported with potential for photovoltaic and optoelectronic applications due to their excellent luminescent properties. Herein excitonic photoluminescence (PL) excited by two-photon absorption in perovskite CsPbBr 3 quantum dots (QDs) has been studied at a broad temperature range, from 80 to 380 K. Two-photon absorption has been investigated and the absorption coefficient is up to 0.085 cm/GW at room temperature. Moreover, the PL spectrum excited by two-photon absorption shows a linear blue-shift (0.32 meV/K) below the temperature of 220 K. However, for higher temperatures, the PL peak approaches a roughly constant value and shows temperature-independent chromaticity up to 380 K. This behavior is distinct from the general red-shift for semiconductors and can be attributed to the result of thermal expansion, electron–phonon interaction and structural phase transition around 360 K. The strong nonlinear absorption and temperature-independent chromaticity of CsPbBr 3 QDs observed in temperature range from 220 to 380 K will offer new opportunities in nonlinear photonics, light-harvesting, and light-emitting devices.
The past two years have seen the unprecedentedly rapid emergence of a new class of solar cell based on mixed organic-inorganic halide perovskites. Although the first efficient solid-state perovskite cells were reported only in mid-2012, extremely rapid progress was made during 2013 with energy conversion efficiencies reaching a confirmed 16.2% at the end of the year. This increased to a confirmed efficiency of 17.9% in early 2014, with unconfirmed values as high as 19.3% claimed. Moreover, a broad range of different fabrication approaches and device concepts is represented among the highest performing devices-this diversity suggests that performance is still far from fully optimized. This Review briefly outlines notable achievements to date, describes the unique attributes of these perovskites leading to their rapid emergence and discusses challenges facing the successful development and commercialization of perovskite solar cells.
Organolead trihalide perovskites are shown to exhibit the best of both worlds: charge-carrier mobilities around 10 cm2 V−1 s−1 and low bi-molecular charge-recombination constants. The ratio of the two is found to defy the Langevin limit of kinetic charge capture by over four orders of magnitude. This mechanism causes long (micrometer) charge-pair diffusion lengths crucial for flat-heterojunction photovoltaics.
We report a detailed high-resolution powder neutron diffraction investigation of the structural behavior of the multiferroic hexagonal polymorph of YMnO3 between room temperature and 1403 K. The study was aimed at resolving previous uncertainties regarding the nature of the paraelectric-ferroelectric transition and the possibilities of any secondary structural transitions. We observe a clear transition at 1258±14 K corresponding to a unit-cell tripling and a change in space group from centrosymmetric P63/mmc to polar P63cm. Despite the fact that this symmetry permits ferroelectricity, our experimental data for this transition (analyzed in terms of symmetry-adapted displacement modes) clearly support previous theoretical analysis that the transition is driven primarily by the antiferrodistortive K3 mode. We therefore verify previous suggestions that YMnO3 is an improper ferrielectric. Furthermore, our data confirm that the previously suggested intermediate phase with space group P63/mcm does not occur. However, we do find evidence for an isosymmetric phase transition (i.e., P63cm to P63cm) at ≈920 K, which involves a sharp decrease in polarization. This secondary transition correlates well with several previous reports of anomalies in physical properties in this temperature region and may be related to Y-O hybridization.
Pressure-temperature phase relations of CH3NH3PbX3 (X = Cl, Br, I) crystals were studied by using a high pressure DTA apparatus in the range between 0.1 Pa and 200 MPa. A triple point was found in each compound below 100 MPa. By pressurization, the low pressure phases disappeared at the triple point in the chloride and bromide while a new high-pressure phase appeared in the iodide. The pressures and temperatures of the triple points are 75.1 MPa, 175.7 K for CH3NH3PbCl3, 43.2 MPa, 152.9 K for CH3NH3PbBr3, and 84.8 MPa, 176.2 K for CH3NH3PbI3. All the boundaries between the cubic and tetragonal phases are upward convex and that of the iodide has a maximum at about 120 MPa. Other phase boundaries are essentially straight lines in the measured pressure and temperature ranges. By the use of the Clausius-Clapeyron equation, the transition volumes were calculated from the slopes of the phase boundaries and the transition entropies obtained in a previously published calorimetric experiment (J. Phys. Chem. Solids51, 1383 (1990)).
In spite of intrinsic limitations, neutron powder diffraction is, and will still be in the future, the primary and most straightforward technique for magnetic structure determination. In this paper some recent improvements in the analysis of magnetic neutron powder diffraction data are discussed. After an introduction to the subject, the main formulas governing the analysis of the Bragg magnetic scattering are summarized and shortly discussed. Next, we discuss the method of profile fitting without a structural model to get precise integrated intensities and refine the propagation vector(s) of the magnetic structure. The simulated annealing approach for magnetic structure determination is briefly discussed and, finally, some features of the program FullProf concerning the magnetic structure refinement are presented and discussed. The different themes are illustrated with simple examples.
The perovskite film with large grain size and low defect states density is technically important to improve the power conversion efficiency and environmental stability of perovskite solar cells (PSCs). In the present work, we raise a compositional engineering strategy of inorganic CsPbBr3 perovskite films by substituting partial Pb²⁺ with divalent transition metal ions having smaller ionic radius (TM²⁺ = Mn²⁺, Ni²⁺, Cu²⁺ and Zn²⁺). The photovoltaic performances of hole transporting layer-free all-inorganic CsPbBr3 PSCs are markedly improved through maximizing grain size with few grain boundaries as well as low trap states density and minimizing energy loss in charge carrier transfer. A champion power conversion efficiency as high as 9.18% is achieved for the all-inorganic CsPb0.995Zn0.005Br3 PSC free of encapsulation, which shows a remarkable long-term stability over 760 h in 80% humidity air atmosphere at 25 °C. The high efficiency and improved stability demonstrate compositional engineering is a promising strategy to forward high-quality perovskite films and high-performance of all-inorganic CsPbBr3 PSCs.
The full-wave analysis was applied for a coaxial line (i.e., transmission line) that has a “short-circuited” discontinuity. The discontinuity has a radius less than or equal to the inner radius of the coaxial line. The “sample region” can be considered as a partially filled circular waveguide. Such a structure is very practical and is of particular interest for the dielectric spectroscopy applications. It takes into account the inhomogeneous field distribution, which is the limiting factor for the determination of high dielectric permittivity values at microwave frequencies. The direct problem was solved by using the mode-matching technique, and the relationship between the complex reflection coefficient and the dielectric permittivity of the cylindrical sample was obtained. By solving the inverse problem, it is possible to obtain the complex dielectric permittivity from the experimental values of the scattering matrix. The results were verified by the finite element modeling of the system and applied for particular materials. The correspondence between these approaches is excellent. This method is very suitable for the determination of permittivity, which exceeds several thousands (it is applicable for any type of material). It extends the frequency range where the permittivity can be determined reliably. There is no necessity to prepare samples with different geometries (i.e., surface area and thickness).
Lead halide perovskites are promising materials for optoelectronic applications because of their exceptional performances in carrier lifetime and diffusion length; however, the microscopic origins of their unique characteristics remain elusive. The organic–inorganic hybrid perovskites show unique dielectric functions, i.e., ferroelectric-like phonon responses in the 0.1–10 THz region and liquid-like rotational relaxation in the 1–100 GHz range. To reveal the role of the dielectric responses is of primal importance because the dielectric screening is a key to understanding the optoelectronic properties governed by polarons in the perovskites. Here, we conducted comparative studies of broadband dielectric spectroscopy on both all-inorganic CsPbBr3 and organic–inorganic hybrid (CH3NH3)PbBr3 single crystals to uncover the origin of the liquid-like dielectric relaxation in the 1–100 GHz range. We confirmed the absence of the dielectric response in the range of 10⁶–10¹⁰ Hz in CsPbBr3, which was clearly present in the hybrid (CH3NH3)PbBr3. This suggests that the response is almost purely due to the rotational motions of the organic dipoles in the hybrid perovskites. We evaluated the lifetimes of the polarons using surface-free transient photoluminescence. The lifetime in CsPbBr3 was up to 1.6 µs, while the lifetime in (CH3NH3)PbBr3 was 18 µs. The lifetime in the hybrid (CH3NH3)PbBr3 was significantly longer than in CsPbBr3, also confirmed by transient infrared spectroscopy. We concluded that the liquid-like dielectric response inhibits polaron recombination due to the efficient separation of opposite charges by the additional dynamic disorder.
Triboelectric nanogenerators (TENGs) have attracted considerable interests because of their high conversion efficiency from low-frequent mechanical energy to electricity. Robust dielectric materials are critical in enhancing the contact electrification charge density of TENGs. Herein, all-inorganic cesium lead halide (CsPbX3, X = I, Br, Cl) perovskites with tunable morphologies and dielectric properties are synthesized through a facile solvent etching method and applied as the triboelectric materials in perovskite TENGs. The influences of effective contact area and the dielectric properties associated with electron donating capacity of perovskite films are systematically investigated through solvent and compositional engineering. Compared with the all-inorganic perovskite free of etching, the crystal growth conditions are well controlled. In addition, the doping of Cl⁻ or I⁻ ions is in favor of regulating the polarity and electronic structure of the [PbBr6]⁴⁻ octahedral. Benefitting from the physical surface engineering approaches for enhancing the triboelectric charge density and the chemical doping for regulating dielectric property, an open-circuit voltage of 192 V, a short-circuit current of 16.7 μA and maximum power density of 1.2 W m⁻² at a contact frequency of 0.5 Hz are achieved in the champion CsPbBr2.6I0.4 TENG. The discovery of extraordinary dielectric behavior of the perovskite halides may pave the way of perovskite TENGs with boosting triboelectric charge density for mechanical energy harvesting.
Inorganic cesium lead tri-bromine (CsPbBr3) perovskite is a star material in modern optoelectronic and electronic nanodevices due to its fascinating optical and electronic properties and excellent stability in atmosphere. However, the unique dielectric behavior still remains to be exploited. Herein, inorganic CsPbBr3 perovskite is introduced into triboelectric nanogenerators for the first time in view of its tremendous dielectric and electrical properties. Electron binding energy, dielectric properties and surface potentials are systematically optimized through doping Ba²⁺ into CsPbBr3 lattice to form CsPb1-xBaxBr3 (x = 0.01–0.13) perovskites. The output performances are significantly improved on account of enhanced space charge polarization and increased work function. An open-circuit voltage of 220 V, short-circuit current density of 22.8 mA m⁻² and maximized power density of 3.07 W m⁻² are registered by the champion CsPb0.91Ba0.09Br3 based TENG with lighting up over 80 LED lights. Finally, impacts of temperature and relative humidity on the output performance of perovskite TENG are investigated, high durability and stability of the perovskite TENGs are presented indicating the remarkable reversibility and adaptability of all-inorganic CsPbBr3 perovskites in TENG for mechanical energy harvesting.
We report that luminescence of Eu3+ ion incorporated into Ruddlesden-Popper phases allows monitoring phase transition in powders (instead of single crystals), in a time-efficient manner (compared to neutron diffraction), and importantly, with greater sensitivity than previous methods. Crystal structure and dielectric response of undoped and 0.5%Eu3+-doped Sr3Sn2O7 ceramics were studied as a function of temperature over the temperature range of 300-800 K. The luminescence studies of 0.5%Eu3+-doped Sr2SnO4 and Sr3Sn2O7 samples were performed in the temperature range of 80-500 K. These results were compared with the respective dependences for the undoped compounds. The structural transformations in 0.5%Eu3+-doped Sr3Sn2O7 were found at 390 and 740 K. The former is associated with the isostructural atomic rearrangement that resulted in a negative thermal expansion along two of three orthorhombic crystallographic axes, while the latter corresponds to the structural transition from the orthorhombic Amam phase to the tetragonal I4/mmm one. A similar temperature behavior with the structural transformations in the same temperature ranges was observed in undoped Sr3Sn2O7, although the values of lattice parameters of the Eu3+-doped and undoped compounds were found to be slightly different indicating an incorporation of europium in the crystal lattice. A dielectric anomaly associated with a structural phase transition was observed in Sr3Sn2O7 at 390 K. Optical measurements performed over a wide temperature range demonstrated a clear correlation between structural transformations in Eu3+-doped Sr2SnO4 and Sr3Sn2O7 and the temperature anomalies of their luminescence spectra, suggesting the efficacy of this method for the determination of subtle phase transformations.
Owing to its nice performance, low cost, and simple solution-processing, organic-inorganic hybrid perovskite solar cell (PSC) becomes a promising candidate for next-generation high-efficiency solar cells. The power conversion efficiency (PCE) has boosted from 3.8% to 25.2% over the past ten years. Despite the rapid progress in PCE, the device stability is a key issue that impedes the commercialization of PSCs. Recently, all-inorganic cesium lead halide perovskites have attracted much attention due to their better stability compared with their organic-inorganic counterpart. In this progress report, we summarize the properties of CsPb(IxBr1−x)3 and their applications in solar cells. The current challenges and corresponding solutions are discussed. Finally, we share our perspectives on CsPb(IxBr1−x)3 solar cells and outline possible directions to further improve the device performance.
All-inorganic CsPbBr3 perovskite solar cells (PSCs) have attracted tremendous attentions in the photovoltaic field these days in view of their outstanding stability, especially thermal stability. However, the bromide-rich perovskite, such as CsPbBr3, always suffer from a low phase-purity and poor morphology synthesized by traditional two-step deposition route. Herein, we demonstrate a facile multistep spin-coating strategy to fabricate high-quality CsPbBr3 films on the low-temperature processed compact TiO2 (c-TiO2) electron transport layer (ETL) of the carbon-based PSCs. As-prepared films exhibit more homogeneous with higher CsPbBr3-phase purity and larger average grain sizes up to 1 µm, compared to those prepared through traditional two-step deposition process. The champion power conversion efficiency (PCE) of the planar CsPbBr3 PSC is boosted from 7.05% to 8.12%, getting an increase by 15.2%, due to the increased crystallinity and light-harvesting ability as well as reduced trap states of the CsPbBr3 film. To further enhance the device performance, a SnO2 thin layer with much higher carrier mobility than TiO2 is introduced to passivate the c-TiO2 ETL. It is found that the SnO2 layer can not only improve the surface morphology of the ETL, but also reduce the current shunting pathways in the c-TiO2. The TiO2/SnO2 bilayered ETL possesses a superior electron extraction capability, beneficial to the charge transport and suppression of the interfacial trap-assisted recombination. The best-performing TiO2/SnO2-based CsPbBr3 PSC delivers an excellent fill factor of 0.817 and a high PCE of 8.79%, which is the highest efficiency for planar CsPbBr3 PSCs reported to date. More importantly, the unencapsulated all-inorganic PSCs show a promising humidity and thermal stability with no decline in efficiency when stored in ambient air at room temperature (25 °C) for over 1000 h and 60 °C for one month, respectively. Our work pave the ways for practical applications of cost-effective, highly efficient and stable all-inorganic PSCs.
All-inorganic perovskite solar cell (PSC) is regarded as a promising solution to improve the environmental tolerances of emerging photovoltaics, however the narrow light response (< 550 nm) of cesium lead bromide (CsPbBr3) perovskite markedly drag power conversion efficiency (PCE) output. We present here experimental realization of physical proof-of-concept mixed light harvester from CsPbBr3 halide and inorganic SnS:ZnS photoactive layer (PAL) to broaden spectral absorption from 550 to 700 nm. Owing to the reduced surface defects of perovskite film, improved charge extraction and broadened light response, the all-inorganic PSC with a structure of FTO/c-TiO2/m-TiO2/CsPb0.97Tb0.03Br3/SnS:ZnS/NiOx/carbon delivers a short-circuit current density of 8.21 mA cm-2, an open-circuit voltage of 1.57 V, a fill factor of 79.6%, and a champion PCE as high as 10.26% under one sun illumination. The preliminary results also suggest that these devices are relatively stable under persistent attacks by 80% RH or temperature of 80 oC in air. The physical proof-of-the-concept mixed perovskite/PAL absorber provides new opportunities to wide-spectral response and therefore markedly increased PCE outputs.
Interfacial charge recombination seriously drags efficiency enhancement of all-inorganic perovskite solar cells (PSCs) because of large energy-level differences. Although CsPbBr3-xIx light-harvesters markedly reduce interfacial charge recombination, the long-term stability of solar cell devices is still unsatisfactory to meet practical requirements. Here we present the improved charge extraction by setting an intermediated energy-level at CsPbBr3/carbon interface with colorful CsSnBr3-xIx quantum dots (QDs). The charge extraction is maximized by tuning Br:I ratio, yielding a power conversion efficiency as high as 9.13% for CsSnBr2I QDs-tailored CsPbBr3 solar cell. Moreover, all the devices show high stability in 80%RH or 80 °C over 720 h. The interfacial decoration of all-inorganic perovskite solar cells by lead-free perovskite QDs provides new opportunities of enhancing solar cell efficiency and reducing environmental impacts.
Organic–inorganic hybrid halide perovskites materials have inspired enormous interest in the photovoltaic research community; however, they are currently plagued by insufficient environmental stability. To overcome this problem, all-inorganic halide perovskites have been developed and exhibit significantly improved stability. Here, vacuum thermal evaporation deposition of CsPbBr3 perovskite films is developed by dual-source coevaporation of CsBr and PbBr2 precursors. The evaporation rate ratio of CsBr to PbBr2 determines the stoichiometry of the deposited CsPbBr3 films, and the substrate temperature and post annealing temperature affect the crystallinity of the CsPbBr3 films. Through systematic optimization of the deposition parameters, high-quality CsPbBr3 films with good crystallinity and uniformity are obtained. Planar CsPbBr3 perovskite solar cells with high efficiency of 6.95% in small size (0.09 cm²) and 5.37% in large size (1 cm²) are fabricated. Moreover, the CsPbBr3 solar cells show good long-term stability under ambient conditions without encapsulation.
The high power conversion efficiency of the hybrid CH3NH3PbX3 (where X = I, Br, Cl) solar cells is believed to be tightly related to the dynamics and arrangement of the methylammonium cations. In this Letter, we propose a statistical phase transition model which accurately describes the ordering of the CH3NH3⁺ cations and the whole phase transition sequence of the CH3NH3PbI3 perovskite. The model is based on the available structural information and involves the short-range strain-mediated and long-range dipolar interactions between the cations. It is solved using Monte Carlo simulations on a three-dimensional lattice allowing us to study the heat capacity and electric polarization of the CH3NH3⁺ cations. The temperature dependence of the polarization indicates the antiferroelectric nature of these perovskites. We support this result by performing pyrocurrent measurements of CH3NH3PbX3 (X = I, Br, Cl) single crystals. We also address the possible occurrence of the multidomain phase and the ordering entropy of our model.
The emergence of perovskite solar cells (PSCs) has generated enormous interests in the photovoltaic research community. Recently, cesium metal halides (CsMX3, M = Pb or Sn; X = I, Br, Cl or mixed halides) as a class of inorganic perovskites showed great promise for PSCs and other optoelectronic devices. However, CsMX3-based PSCs usually exhibit lower power conversion efficiencies (PCEs) than organic-inorganic hybrid PSCs, due to the unfavorable bandgaps. Herein, a novel mixed-Pb/Sn mixed-halide inorganic perovskite, CsPb0.9Sn0.1IBr2, with a suitable bandgap of 1.79 eV and an appropriate level of valence band maximum, was prepared in ambient atmosphere without glovebox. After thoroughly eliminated labile organic components and noble metals, the all-inorganic PSCs based on CsPb0.9Sn0.1IBr2 and carbon counter electrodes exhibit a high open-circuit voltage of 1.26 V and a remarkable PCE up to 11.33%, which is record-breaking among the existing CsMX3-based PSCs. Moreover, the all-inorganic PSCs show good long-term stability and highly-improved endurance against heat and moisture. This study indicates a feasible way to design inorganic halide perovskites through energy-band engineering for the construction of high-performance all-inorganic PSCs.
Due to the unprecedented rapid increase of their power conversion efficiency, hybrid organic–inorganic perovskites CH3NH3PbX3 (X = I, Br, Cl) can potentially revolutionize the world of solar cells. However, despite tremendous research activity, the origin of the exceptionally large diffusion length of their photogenerated charge carriers, that is, their low recombination rate, remains elusive. Using frequency and temperature-dependent dielectric measurements across the entire frequency spectrum, it is shown that the dielectric constant conserves very high values (>27) for frequencies below 1 THz in all three halides. This efficiently prevents photocarrier trapping and their recombination owing to the strong screening of charged entities. By combining ultrasonic and Raman spectroscopy with dielectric analysis, similarly large contributions to the dielectric constant are attributed to the dipolar disorder of the CH3NH3 + cations as well as lattice dynamics in the gigahertz range yielding dielectric constants of εstat = 62 for the iodide, 58 for the bromide, and about 45 for the chloride below 1 GHz at room temperature. Disorder continuously reduces for decreasing temperature. Dipole dynamics prevail in the intermediate tetragonal phase. The low-temperature orthorhombic state is antipolar. No indications of ferroelectricity are found.
Hybrid lead-halide perovskites have emerged as an excellent class of photovoltaic materials. Recent reports suggest that the organic molecular cation is responsible for local polar fluctuations that inhibit carrier recombination. We combine low-frequency Raman scattering with first-principles molecular dynamics (MD) to study the fundamental nature of these local polar fluctuations. Our observations of a strong central peak in the cubic phase of both hybrid (CH3NH3PbBr3) and all-inorganic (CsPbBr3) lead-halide perovskites show that anharmonic, local polar fluctuations are intrinsic to the general lead-halide perovskite structure, and not unique to the dipolar organic cation. MD simulations indicate that head-to-head Cs motion coupled to Br face expansion, occurring on a few hundred femtosecond time scale, drives the local polar fluctuations in CsPbBr3.
The device instability has been an important issue for hybrid organic-inorganic halide perovskite solar cells (PSCs). This work intends to address this issue by exploiting inorganic perovskite (CsPbBr3) as light absorber, accompanied by replacing organic hole transport materials (HTM) and metal electrode with carbon electrode. All the fabrication processes (including those for CsPbBr3 and carbon electrode) in the PSCs are conducted in ambient atmosphere. Through a systematical optimization on the fabrication processes of CsPbBr3 film, carbon-based PSCs (C-PSCs) obtained the highest power conversion efficiency (PCE) of about 5.0 %, a relatively high value for inorganic perovskite-based PSCs. More importantly, after storage for 250 h at 80 ºC, only 11.7 % loss in PCE is observed for CsPbBr3 C-PSCs, significantly lower than that for popular CH3NH3PbI3 C-PSCs (59.0 %) and other reported PSCs, which indicated a promising thermal stability of CsPbBr3 C-PSCs.
A charge carrier in a lead halide perovskite lattice is protected as a large polaron responsible for the remarkable photophysical properties, irrespective of the cation type. All-inorganic-based APbX3 perovskites may mitigate the stability problem for their applications in solar cells and other optoelectronics.
Halide perovskites are a rapidly developing class of medium-bandgap semiconductors which, to date, have been popularized on account of their remarkable success in solid-state heterojunction solar cells raising the photovoltaic efficiency to 20% within the last 5 years. As the physical properties of the materials are being explored, it is becoming apparent that the photovoltaic performance of the halide perovskites is just but one aspect of the wealth of opportunities that these compounds offer as high-performance semiconductors. From unique optical and electrical properties stemming from their characteristic electronic structure to highly efficient real-life technological applications, halide perovskites constitute a brand new class of materials with exotic properties awaiting discovery. The nature of halide perovskites from the materials' viewpoint is discussed here, enlisting the most important classes of the compounds and describing their most exciting properties. The topics covered focus on the optical and electrical properties highlighting some of the milestone achievements reported to date but also addressing controversies in the vastly expanding halide perovskite literature.
Organic and inorganic hybrid perovskites (e.g., CH3NH3PbI3), with advantages of facile processing, tunable bandgaps, and superior charge-transfer properties, have emerged as a new class of revolutionary optoelectronic semiconductors promising for various applications. Perovskite solar cells constructed with a variety of configurations have demonstrated unprecedented progress in efficiency, reaching about 20% from multiple groups after only several years of active research. A key to this success is the development of various solution-synthesis and film-deposition techniques for controlling the morphology and composition of hybrid perovskites. The rapid progress in material synthesis and device fabrication has also promoted the development of other optoelectronic applications including light-emitting diodes, photodetectors, and transistors. Both experimental and theoretical investigations on organic-inorganic hybrid perovskites have enabled some critical fundamental understandings of this material system. Recent studies have also demonstrated progress in addressing the potential stability issue, which has been identified as a main challenge for future research on halide perovskites. Here, we review recent progress on hybrid perovskites including basic chemical and crystal structures, chemical synthesis of bulk/nanocrystals and thin films with their chemical and physical properties, device configurations, operation principles for various optoelectronic applications (with a focus on solar cells), and photophysics of charge-carrier dynamics. We also discuss the importance of further understanding of the fundamental properties of hybrid perovskites, especially those related to chemical and structural stabilities.
Hybrid organic-inorganic lead halide perovskite photovoltaic cells have already surpassed 20% conversion efficiency in the few years that they have been seriously studied. However, many fundamental questions still remain unanswered as to why they are so good. One of these is “Is the organic cation really necessary to obtain high quality cells?” In this study, we show that an all-inorganic version of the lead bromide perovskite material works equally well as the organic one, in particular generating the high open circuit voltages that are an important feature of these cells.Keywords: CsPbBr3; photovoltaic cells; all-inorganic lead halide perovskite cells
Solution-processable metal-halide perovskites recently opened a new route towards low-cost manufacture of photovoltaic cells. Converting sunlight into electrical energy depends on several factors among which a broad absorption across the solar spectrum and attractive charge transport properties are of primary importance. Hybrid perovskites meet such prerequisites but, despite foremost experimental research efforts, their understanding remains scanty. Here we show that in these materials, the appropriate absorption and transport properties are afforded by the multibandgap and multi-valley nature of their band structure. We also investigate the nature of the photoexcited species. Our analysis suggests exciton screening by collective orientational motion of the organic cations at room temperature, leading to almost free carriers. Molecular collective motion is also expected to couple to carrier diffusion at room temperature. In mixed-halides, our interpretation indicates that doping might hinder collective molecular motions, leading to good transport properties despite alloying and local lattice strain.
The hybrid organic–inorganic perovskite (CH3NH3)PbI3 may find application in next generation solid-state sensitised solar cells. Although this material and related perovskites were discovered many decades ago, questions remain concerning their diverse structural chemistry and unusual properties. The article presents a review of previous work and provides a detailed description of the preparation, structural characterisation and physical characteristics of (CH3NH3)PbI3. The phase changes exhibited by (CH3NH3)PbI3 have been probed using variable temperature powder and single crystal X-ray diffraction, combined with differential scanning calorimetry, thermogravimetric analysis and phase contrast transmission electron microscopy. The optical band gap for (CH3NH3)PbI3 determined by UV-Visible spectroscopy was compared to values obtained from density-of-state simulation of the electronic band structure.
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