Stability Of Polarons Research Articles

Thermodynamic evidence for polaron stabilization inside the antiferromagnetic order of Eu5In2Sb6

Materials exhibiting electronic inhomogeneities at the nanometer scale have enormous potential for applications. Magnetic polarons are one such type of inhomogeneity which link the electronic, magnetic and lattice degrees of freedom in correlated matter and often give rise to colossal magnetoresistance. Here, we investigate single crystals of Eu5In2Sb6 by thermal expansion and magnetostriction along different crystallographic directions. These data provide compelling evidence for the formation of magnetic polarons in Eu5In2Sb6 well above the magnetic ordering temperature. More specifically, our results are consistent with anisotropic polarons with varying extent along the different crystallographic directions. A crossover revealed within the magnetically ordered phase can be associated with a surprising stabilization of ferromagnetic polarons within the global antiferromagnetic order upon decreasing temperature. These findings make Eu5In2Sb6 a rare example of such coexisting and competing magnetic orders and, importantly, shed new light on colossal magnetoresistive behavior beyond manganites.

Elucidating phonon dephasing mechanisms in layered perovskites with coherent Raman spectroscopies.

Organic-inorganic hybrid perovskite quantum wells exhibit electronic structures with properties intermediate between those of inorganic semiconductors and molecular crystals. In these systems, periodic layers of organic spacer molecules occupy the interstitial spaces between perovskite sheets, thereby confining electronic excitations to two dimensions. Here, we investigate spectroscopic line broadening mechanisms for phonons coupled to excitons in lead-iodide layered perovskites with phenyl ethyl ammonium (PEA) and azobenzene ethyl ammonium (AzoEA) spacer cations. Using a modified Elliot line shape analysis for the absorbance and photoluminescence spectra, polaron binding energies of 11.2 and 17.5meV are calculated for (PEA)2PbI4 and (AzoEA)2PbI4, respectively. To determine whether the polaron stabilization processes influence the dephasing mechanisms of coupled phonons, five-pulse coherent Raman spectroscopies are applied to the two systems under electronically resonant conditions. The prominence of inhomogeneous line broadening mechanisms detected in (AzoEA)2PbI4 suggests that thermal fluctuations involving the deformable organic phase broaden the distributions of phonon frequencies within the quantum wells. In addition, our data indicate that polaron stabilization primarily involves photoinduced reorganization of the organic phases for both systems, whereas the impulsively excited phonons represent less than 10% of the total polaron binding energy. The signal generation mechanisms associated with our fifth-order coherent Raman experiments are explored with a perturbative model in which cumulant expansions are used to account for time-coincident vibrational dephasing and polaron stabilization processes.

Achieving ultrahigh electrochromic stability of triarylamine-based polymers by the design of five electroactive nitrogen centers

A shortage of long-term stability of electrochromic materials has been hindering their practical application. A series of novel polyamides (PAs) (4) with five electroactive nitrogen atoms within triphenylamine (TPA)-containing structures was synthesized via phosphorylation polyamidation. Polyamide 4b exhibited highly integrated electrochromic performances, including multiple color changes, high contrast of optical transmittance change, and the highest electrochromic stability (only 11.2 and 9.6 % decay of its coloration efficiency (CE) at 450 nm after 50000 and 16000 switching cycles, respectively) compared to all other triarylamine-based polymers to date. This record-high electrochromic cycling is attributed to the enhanced stability of polaron, which was studied through the electron-donating or electron-withdrawing effect of substituents and the resonance effect of electron delocalization over the electroactive nitrogen centers. Importantly, our core design of five electroactive nitrogen centers with electron-donating methoxy groups is the key factor to increase stability of polaron in the resulting polymers 4. More electroactive nitrogen centers are able to create a weaker electronic coupling due to a charge of cation radical dispersed by resonance successfully in-between the different redox states, which is evidenced clearly by the observed longer wavelength absorption in the NIR region. Our research confirms that the key to determining polaron stability is the resonance by the electrons delocalized over all the redox centers, rather than the electronic coupling between the different redox centers. And the resonance leads to increasing electrochromic stability of the polymer.

Effect of surface polarization and structural deformation on the formation and stabilization of polarons in two-dimensional Ruddlesden–Popper metal halide perovskites

First-principles density-functional theory calculations were performed to reveal the effect of surface polarization and structural deformation on the formation and stabilization of the polaron in two-dimensional Ruddlesden–Popper perovskites. Our results revealed that the orientational distribution of organic cations induces surface polarization. The surface dipole moment can be well featured by the c axis distances between N and the nearest I atoms. Structural deformation and surface dipole moments result in separate real-space distributions of hole and electron polarons. Our results also reveal that the structural deformation of the [PbI6] sublattices and surface polarization are closely related to the reorientation of organic cations and can be effectively modulated by it. This reorientation significantly impacts the stabilization of polarons. Our understandings provide insight into the nature of polarons in two-dimensional Ruddlesden–Popper perovskites and general guidance for the proper selection of organic cations in two-dimensional perovskites for suitable applications in photovoltaic and optoelectronic devices.

Polarons in perovskite solar cells: effects on photovoltaic performance and stability

Organic–inorganic hybrid perovskites manifest unique photophysical properties in terms of their long carrier lifetime, low recombination rate, and high defect tolerance, enabling them to be promising candidates in optoelectronic devices. However, such advanced properties are unexpected in perovskite materials with moderate charge mobility. Recent investigations have revealed that these appealing properties were endowed due to the formation of large polarons in the perovskite crystals, resulting from the coupling of photogenerated carriers and a polarized crystal lattice, which largely affected the carrier-transport dynamics and structural stability of perovskite solar cells (PSCs). In this review, first the crystal structure of the perovskite lattice and the formation mechanism of polarons are elucidated. Then, the modulation of polaron states in PSCs, including large polaron stabilization, polaron-facilitated charge transport, hot-carrier solar cells, and polaron-related stability issues such as polaron-induced metastable defects, polaronic strain, and photostriction are systematically investigated. Finally, the prospect of further understanding and manipulating polaron-related phenomena, working toward highly efficient and stable PSCs, is suggested.

Accelerated polaron formation in perovskite quantum dots monitored via picosecond infrared spectroscopy

Picosecond mIR-spectroscopy reveals polaron stabilization processes in perovskite quantum dots on a few ps time-scale, considerably faster than the bulk analogue.

First-principles study on small polaron and Li diffusion in layered LiCoO2.

Li-ion conductivity is one of the essential properties that influences the performance of cathode materials for Li-ion batteries. Here, using density functional theory, we investigate the polaron stability and its effect on the Li-ion diffusion in layered LiCoO2 with various magnetic orderings. We show that the local magnetism promotes the localized Co4+ polaron with the Li-diffusion barrier of ∼0.34 eV. While the Li-ion diffuses, the polaron migrates in the opposite direction to the Li movement. In the non-magnetic structure, on the other hand, the polaron does not form, and the Li diffusion barrier is lowered to 0.21 eV. Although the presence of the polaron raises the diffusion barrier, the magnetically ordered structures are energetically more stable during the migration than the non-magnetic case. Thus, our work advocates the hole polaron migration scenario for Li-ion diffusion. We further demonstrate that the strong electron correlation of Co ions plays an essential role in stabilizing the Co4+ polaron.

(Invited, Digital Presentation) Doping Gallium Oxide and Competing Ultra-Wide Bandgap Oxides

Although Ga2O3 is widely believed to be the among most promising ultrawide-bandgap semiconductors, its inability to be p-type doped hampers future applications. Moreover, alloying with aluminum increases the Ga2O3 band gap, but it is unclear if electrical conductivity of AlGO alloys can be controlled. The properties of group-IV (C, Si, Ge, and Sn) and transition metal (Hf, Zr, and Ta) substitutional dopants in AlGO alloys are systematically explored here using first-principles hybrid functional calculations. In Ga2O3, all dopants act as shallow donors. However, in Al2O3 all are deep defects, characterized by DX behavior or the emergence of positive-U (+/0) levels. Combining our calculations of dopant charge-state transition levels with information of the AlGO alloy band structure, the critical Al composition at which each dopant transitions from being a shallow to a deep donor is estimated. Si is identified as being the most efficient dopant to achieve n-type conductivity in high Al-content AlGO alloys, acting as a shallow donor over the entire predicted stability range for AlGO solid solutions. Compensation by other unintentional impurities (such as H and C) is also assessed.Other oxide materials have also emerged as potential competitors to Ga2O3, but their propensity for hole conductivity is less well known. Here the stability of hole polarons in a set of ultrawide-bandgap oxides (Ga2O3, Al2O3, ZnGa2O4, MgGa2O4, LiGaO2 and GeO2) is examined and compared, both in pristine material and in the presence of acceptor impurities. Holes spontaneously self-trap in all oxides investigated, with varying stabilities. Acceptor impurities further stabilize these trapped holes, leading to large acceptor ionization energies. Hole trapping also leads to characteristic distortions and distinct optical transitions, which may explain some experimentally-observed signals. These results indicate that achieving p-type conductivity in any of these oxides is unlikely, with the possible exception of GeO2.This work was performed in collaboration with Darshana Wickramaratne (NRL), Joel Varley (LLNL), Sai Mu (UCSB), Mengen Wang (UCSB) and Chris Van de Walle (UCSB), and was supported by the ONR/NRL 6.1 Basic Research Program.

Autogenous Oxidation/Reduction of Polyaniline in Aqueous Sulfuric Acid

In this work, we have shown through open circuit potential experiments that in aqueous sulfuric acid solutions, a thick polyaniline film undergoes autogenous oxidation when reduced below a threshold potential and autogenous reduction when oxidized above the threshold potential. This phenomenon is associated with the high resonance stability of polarons in long polyaniline chains present in thicker films. We have determined the rates of these reactions using a linear sweep chronopotentiometry technique. We propose that the oxidation reaction of polyaniline produces polarons with a concomitant reduction of hydrogen ions to hydrogen radicals, which further combine with each other to produce the hydrogen molecule in the absence of dissolved oxygen. On the other hand, at high potentials polarons are reduced with the concomitant oxidation of water to hydroxyl radicals. Both the radicals are stabilized due to the interaction of their unpaired electrons with pi-electrons of the aromatic rings of the polymer backbone. At the equilibrium value of the open circuit potential, both the hydrogen radicals and hydroxyl radicals are generated at equal rates and react with each other to form water.

“Self-trapping” in solar cell hybrid inorganic-organic perovskite absorbers

In the simplest picture, a “self-trapped” polaron forms when an excess electron or hole deforms a crystal lattice, creating a potential well with bound states. Properties of self-trapped polarons in methylammonium lead iodide perovskite (MAPbI3), which is widely used as solar cell absorber, are of great interest, and are a subject of ongoing investigations and debates concerning the existence of large polarons with the co-presence of metastable self-trapping. Herein, we employ a self-interaction-free density functional theory method to investigate the stability of small polarons in tetragonal MAPbI3 phase. The electron small polaron is found to be unstable, while the hole small polaron is found to be metastable at realistic operation temperatures of solar cells. Further, the hole polaron is found to have a hole band close to the conduction band, which in conjunction with its metastability suggests that small polarons will have an appreciable effect on charge-carrier recombinations in MAPbI3. Further, we posit that the existence of the metastable polarons in addition to the large polarons may explain the experimentally observed non-monotonic temperature dependence of bimolecular charge-carrier recombination rate in tetragonal MAPbI3 phase.

Self-trapped holes and polaronic acceptors in ultrawide-bandgap oxides

Although Ga2O3 is widely believed to be one of the most promising ultrawide-bandgap semiconductors, its inability to be p-type doped hampers its future applications. Other oxides have recently emerged as potential competitors to Ga2O3, but their propensity for hole conductivity is less well known. Here, the stability of hole polarons is examined in pristine material and in the presence of impurities for a set of ultrawide-bandgap oxides (Ga2O3, Al2O3, ZnGa2O4, MgGa2O4, LiGaO2, and GeO2). Holes spontaneously self trap in all oxides investigated here. Acceptor impurities (such as group-I elements, N, and F) further stabilize these trapped holes, leading to large acceptor ionization energies. Hole trapping also leads to characteristic distortions and distinct optical transitions, which may explain some experimentally observed signals. These results indicate that achieving p-type conductivity in any of these oxides is unlikely, with the possible exception of GeO2.

A subnanometric material reveals new quantum-chemical insights into surface polarons

The recent advent of cutting-edge experimental techniques allows for a precise synthesis of monodisperse subnanometric metal clusters composed by just a few atoms, and opens new possibilities for subnanometer science. The decoration of titanium dioxide surfaces with Ag5 atomic clusters has enabled the stabilization of surface polarons and provided new quantum-chemical insights into an electron polarization phenomenon revealed by their formation.

Poly(N-methylaniline) vs. polyaniline: An extended pH range of polaron stability as revealed by Raman spectroelectrochemistry

A comparative study of polyaniline (PANI) and poly(N-methylaniline) (PNMA) has been performed by means of Raman spectroelectrochemical technique at 633 nm and 785 nm laser line excitations. The excitation wavelengths used fall into a resonance with the blue colored semi- and full-oxidized forms of these conducting polymers. The dependence of Raman features on electrode potential and solution acidity was studied, and relative content of polaronic and bipolaronic states was evaluated. In an acidic solution, the semioxidized emeraldine form of either PANI or PNMA exists in equilibrium between their polaronic and bipolaronic states. In a neutral or even slightly alkaline solution, this equilibrium for PANI shifts to bipolaron state, resulting in loss of its conductance. For PNMA, however, the relative content of polaron state appears high enough even in pH-neutral soulions, thus determining a higher conductivity of PNMA in pH-neutral environment as compared to that of PANI. A mechanistic interpretation for this, based on differences in the chemical structures of these polymers, is also presented.

Investigation of electrochemical oxidative coupling of 3 and 6 substituted carbazoles

• Investigation of carbazole reactivity and electropolymerization mechanism. • Combination of fast electrochemistry experiments and molecular orbital calculations. • Investigation of the stability of radical cations and polarons formed by electro-oxidation of carbazoles. • Comparison of the stability of the dimers depending on the coupling positions. In this study, electrochemical behavior of different functionalized carbazoles has been studied through classical cyclic voltammetry, computational calculations, electrochemical simulations and more originally fast cyclic voltammetry. Indeed, fast electrochemistry is rarely used to study reactivity of such compounds even if it can bring very interesting information as shown in this work. Carbazoles were substituted with ethyl, tert -butyl and phenyl groups at 3 and 6 positions. Oxidation of 3-ethylcarbazole leads to very fast dimerization of formed radical cations (k dim = 1.5.10 7 M −1 .s −1 ) and electrochemical polymerization of this compound leads to a thick adherent conducting polymer film. This was also the case for 3-phenylcarbazole and 3,6-diphenylcarbazole when potential was brought over 1.7 V/SCE because of formation of very reactive radicals after oxidation process involving phenyl groups. For 3,6-diethylcarbazole, 3- tert -butylcarbazole, 3,6-ditert-butylcarbazole, as well as 3-phenylcarbazole and 3,6-diphenylcarbazole when potential is kept under 1.4 V/SCE, dimerization of radical cations is slower (k dim ≤ 1.5.10 6 M −1 .s −1 ) and no efficient electropolymerization occurs. A theoretical study of the same substituted carbazoles in their neutral and oxidized states was done using molecular orbital calculations (DFT method) to obtain additional information about carbazole reactivity. These numerical calculations were in perfect accordance with results of fast electrochemistry and electrochemical simulations regarding stability of radical cations. They demonstrated that monosubstituted carbazole dimers are probably obtained through 6-6′ coupling while 1-1′ coupling is the most favorable for disubstituted carbazole dimers.

Improved positive polaron stability of the p-type host for long lifetime in phosphorescent organic light-emitting diodes

The device lifetime of phosphorescent organic light-emitting diodes was improved by employing a hole-transport-type host derived from carbazole with phenyl blocking groups at the 3- or 6-positions.

Molecular design tactics enhancing the negative polaron stability of a p-type host for long device lifetime by fusion of carbazole with furan

A negative polaron stabilizing p-type host was developed by fusing two carbazole units with furan, which improved the external quantum efficiency and device lifetime of blue phosphorescent organic light-emitting diodes.

Ag5-induced stabilization of multiple surface polarons on perfect and reduced TiO2 rutile (110).

The recent advent of cutting-edge experimental techniques allows for a precise synthesis of subnanometer metal clusters composed of just a few atoms, opening new possibilities for subnanometer science. In this work, via first-principles modeling, we show how the decoration of perfect and reduced TiO2 surfaces with Ag5 atomic clusters enables the stabilization of multiple surface polarons. Moreover, we predict that Ag5 clusters are capable of promoting defect-induced polarons transfer from the subsurface to the surface sites of reduced TiO2 samples. For both planar and pyramidal Ag5 clusters, and considering four different positions of bridging oxygen vacancies, we model up to 14 polaronic structures, leading to 134 polaronic states. About 71% of these configurations encompass coexisting surface polarons. The most stable states are associated with large inter-polaron distances (>7.5 Å on average), not only due to the repulsive interaction between trapped Ti3+ 3d1 electrons, but also due to the interference between their corresponding electronic polarization clouds [P. López-Caballero et al., J. Mater. Chem. A 8, 6842-6853 (2020)]. As a result, the most stable ferromagnetic and anti-ferromagnetic arrangements are energetically quasi-degenerate. However, as the average inter-polarons distance decreases, most (≥70%) of the polaronic configurations become ferromagnetic. The optical excitation of the midgap polaronic states with photon energy at the end of the visible region causes the enlargement of the polaronic wave function over the surface layer. The ability of Ag5 atomic clusters to stabilize multiple surface polarons and extend the optical response of TiO2 surfaces toward the visible region bears importance in improving their (photo-)catalytic properties and illustrates the potential of this new generation of subnanometer-sized materials.

Formation and stability of small polarons at the lithium-terminated Li4Ti5O12 (LTO) (111) surface.

Zero strain insertion, high cycling stability, and a stable charge/discharge plateau are promising properties rendering Lithium Titanium Oxide (LTO) a possible candidate for an anode material in solid state Li ion batteries. However, the use of pristine LTO in batteries is rather limited due to its electronically insulating nature. In contrast, reduced LTO shows an electronic conductivity several orders of magnitude higher. Studying bulk reduced LTO, we could show recently that the formation of polaronic states can play a major role in explaining this improved conductivity. In this work, we extend our study toward the lithium-terminated LTO (111) surface. We investigate the formation of polarons by applying Hubbard-corrected density functional theory. Analyzing their relative stabilities reveals that positions with Li ions close by have the highest stability among the different localization patterns.

Small Electron Polarons in CsPbBr3: Competition between Electron Localization and Delocalization

We study the nature of excess electrons in CsPbBr3 and identify several single and double polaronic states. We emphasize the importance of proper inclusion of the self-interaction corrections for the stability of small electron polarons in this material. We demonstrate that spin–orbit coupling (SOC) has a significant impact on the energetics of the polaronic states. In particular, we find that SOC disfavors electron localization and leads to different polaronic geometries. Additionally, by carrying out thermodynamic integration, we show that small electron polarons are thermally stabilized in CsPbBr3. The small energy differences between the localized and delocalized electronic states could possibly reconcile the apparently conflicting properties of high charge-carrier mobilities and low recombinations rates.

Evolution of thermoelectric properties in EuxYb1−xB6 family

Seebeck effect in the crystalline samples of EuxYb1−xB6 (x = 0, 0.082, 0.127, 0.9, 1) was investigated at temperatures 2–300 K. For all the compounds thermopower is shown to be well described by the sum of diffusion (Sd = AT) and phonon drag components. The latter contribution is induced by quasilocal (Einstein) modes of ytterbium and europium ions with characteristic temperatures ΘE(YbB6) ≈ 91 K and ΘE(EuB6) ≈ 122 K. The estimation of effective mass m* of the charge carriers proves that increasing of Eu content induces crossover from ‘heavy’ holes with mh*(x ⩽ 0.127) ≈ 0.3–0.36 m0 to ‘light’ electrons with me*(x ⩾ 0.9) ≈ 0.12–0.13 m0 (m0−free electron mass). For the Eu-rich compounds we propose the existence of additional point on the phase diagram, which corresponds to short-range magnetic order with enhanced spin fluctuations preceding the stabilization of magnetic polarons.