Ionic Defects Research Articles

Behavior Investigation of Oxide Electrodes for Li Ion Batteries Viewing from Charge Compensation

Charge compensation resulted defect reactions have played important role on affecting electrical and structural properties of oxides. Depending upon the types of ions available, ionic defects and/or electronic defects are created. For instance, ionic defects tend to enhance the migration of cations/anions. Electronic defects including electrons or hole defects may enhance the electronic conductivity of oxides. For Li batteries, the lithiation of transition metal oxides may be viewed as substitution of Li for transition metal ions. As a result, the lithiation induces the oxidation of Ni, Co ions. Further lithiation may also cause the structural transformation from rock-salt oxide to layered oxides. During a charging/delithiation process at cathode, electron extraction and Li removal from the Li sublattice may be viewed as the creation of electronic/ionic defects. On the contrary, opposite defect reactions including electron injection (or reduction) and lithiation may be observed from the anode. For a high-capacity cathode, Li-rich layered oxides formulated as xLi2MnO3-(1-x)LiMO2(M = Mn, Ni, Co, etc.) show a unique charging plateau around 4.5V. The extraction of electrons is given by rearranged oxygen ions during 1st charging process. In the following discharging as seen in Fig. 1, reduction of Mn+4 from accepting e’ gave a reversible capacity as high as 250 mAh/g. Interestingly, a backward defect reaction was observed in Li4Ti5O12 (LTO) spinel anode during the charging/lithiation process. A self-improving behavior may be explained through accompanying defect reaction. During low-pO2 synthesis, lattice oxygen near surface of LTO tends to be removed with the introduction of Ti+3. As a result, the electron conduction and electrochemical properties of LTO were significantly improved. Finally, the structure transformation of cycled Li-rich oxides from Layered to Spinel structure may be illustrated due to the formation high concentration electronic defects.

Exploring point defects and trap states in undoped SrTiO3 single crystals

The defect chemistry and electronic trapping energies in undoped single crystalline SrTiO3 were examined by electrochemical impedance spectroscopy at low (25–160 °C) and intermediate (500–700 °C) temperatures. Electronic and ionic conductivity as well as chemical capacitance values were obtained with a transmission line equivalent circuit. Impedance spectroscopy at low temperatures was used to quantify trapping energies of main ionic defects. Particularly the chemical capacitance is shown to be a highly valuable, though hardly used tool for establishing a defect model based solely on electrochemical measurements. It is very sensitive for minority charge carriers and can thus unveil otherwise hardly accessible defect concentrations. The chemical capacitance analysis yields a valence dependent acceptor concentration in the ppm range for the investigated samples. Complementary positron annihilation lifetime spectroscopy (PALS) suggests existence of Ti vacancies and both methods (chemical capacitance and PALS) agree in their quantification of the corresponding vacancy concentration (6 ppm). Beyond successfully predicting acceptor defect concentrations in undoped SrTiO3, the method is sensitive for electronically relevant defects in sub-ppm concentrations.

Metal–Insulator Transition via Ion Irradiation in Epitaxial La0.7Sr0.3MnO3−δ Thin Films

Complex oxides provide rich physics related to ionic defects. For the proper tuning of functionalities in oxide heterostructures, it is highly desired to develop fast, effective, and low‐temperature routes for the dynamic modification of defect concentration and distribution. Herein, the use of helium implantation to efficiently control the vacancy profiles in epitaxial La0.7Sr0.3MnO3−δ thin films is reported. The viability of this approach is supported by lattice expansion in the out‐of‐plane lattice direction and dramatic change in physical properties, i.e., a transition from ferromagnetic metallic to antiferromagnetic insulating. In particular, a significant increase of resistivity up to four orders of magnitude is evidenced at room temperature, upon implantation of highly energetic He ions. The result offers an attractive means for tuning the emergent physical properties of oxide thin films via strong coupling between strain, defects, and valence.

Ion migration in halide perovskite solar cells: Mechanism, characterization, impact and suppression

Metal halide perovskites are emerging as the most promising candidate for the next-generation Photovoltaics (PV) materials, due to their superior optoelectronic properties and low cost. However, the resulting Perovskite solar cells (PSCs) suffer from poor stability. In particular, the temperature and light activated ionic defects within the perovskite lattice, as well as electric-field-induced migration of ionic defects, make the PSCs unstable at operating condition, even with device encapsulation. There is no doubt that the investigation of ion migration is crucial for the development of PSCs with high intrinsic stability. In this review, we first briefly introduce the origin and pathways of ion migration, and also the essential characterization methods to identify ion migration. Next, we discuss the impact of ion migration on the perovskite films and cells with respect to photoelectric properties and stability. Then, several representative strategies to suppress ion migration are systematically summarized in the context of composition engineering, additive engineering and interface engineering, with an in-depth understanding on the underlying mechanisms which may provide more clues for further fabrication of PSCs with improved stability. Finally, a perspective with some suggestion on future research directions and chemical approaches are provided to alleviate ion migration in perovskite materials and the entire devices.

Manipulating crystallization dynamics through chelating molecules for bright perovskite emitters

Molecular additives are widely utilized to minimize non-radiative recombination in metal halide perovskite emitters due to their passivation effects from chemical bonds with ionic defects. However, a general and puzzling observation that can hardly be rationalized by passivation alone is that most of the molecular additives enabling high-efficiency perovskite light-emitting diodes (PeLEDs) are chelating (multidentate) molecules, while their respective monodentate counterparts receive limited attention. Here, we reveal the largely ignored yet critical role of the chelate effect on governing crystallization dynamics of perovskite emitters and mitigating trap-mediated non-radiative losses. Specifically, we discover that the chelate effect enhances lead-additive coordination affinity, enabling the formation of thermodynamically stable intermediate phases and inhibiting halide coordination-driven perovskite nucleation. The retarded perovskite nucleation and crystal growth are key to high crystal quality and thus efficient electroluminescence. Our work elucidates the full effects of molecular additives on PeLEDs by uncovering the chelate effect as an important feature within perovskite crystallization. As such, we open new prospects for the rationalized screening of highly effective molecular additives.

Complementary study of anisotropic ion conduction in (110)-oriented Ca-doped BiFeO3 films using electrochromism and impedance spectroscopy

Oxygen vacancies are ubiquitous in oxides, and taking advantage of their mobility is the cornerstone for a variety of future applications. The visualization and quantification of collective defect flow based on electrochromism is a powerful approach to explore oxygen kinetics and electrochemical reaction even in cases that electronic conduction is considerably mixed, but whether or not the measured kinetic properties harmonize with those obtained by the conventional impedance spectroscopy remains veiled. Here, we identify complementary relationships between the two methods by investigating the oxygen vacancy transport in Ca 30%-doped bismuth ferrite thin films epitaxially grown on SrTiO3 (110) substrates. We find that the activation energy of ionic hopping is 0.78 (or 0.92 eV) for the application of an electric bias along [001] (or [11¯0]) due to the grain elongation along [001]. We anneal the films in an N2 gas environment at high temperatures to suppress the electronic contribution for access to standard impedance spectroscopy. The oxygen kinetic properties obtained from the two methods are consistent with each other, complementarily revealing the collective phase evolution as well as the ionic impedance of the bulk, grain boundary, and interfacial regions. These comparative works provide useful insights into ionic defect conduction in oxides in an intuitive and quantitative manner.

Aerosol Assisted Solvent Treatment: A Universal Method for Performance and Stability Enhancements in Perovskite Solar Cells

AbstractMetal‐halide perovskite solar cells (PSCs) have had a transformative impact on the renewable energy landscape since they were first demonstrated just over a decade ago. Outstanding improvements in performance have been demonstrated through structural, compositional, and morphological control of devices, with commercialization now being a reality. Here the authors present an aerosol assisted solvent treatment as a universal method to obtain performance and stability enhancements in PSCs, demonstrating their methodology as a convenient, scalable, and reproducible post‐deposition treatment for PSCs. Their results identify improvements in crystallinity and grain size, accompanied by a narrowing in grain size distribution as the underlying physical changes that drive reductions of electronic and ionic defects. These changes lead to prolonged charge‐carrier lifetimes and ultimately increased device efficiencies. The versatility of the process is demonstrated for PSCs with thick (>1 µm) active layers, large‐areas (>1 cm2) and a variety of device architectures and active layer compositions. This simple post‐deposition process is widely transferable across the field of perovskites, thereby improving the future design principles of these materials to develop large‐area, stable, and efficient PSCs.

Alleviating halide perovskite surface defects

Alleviating halide perovskite surface defects

Shape‐Dependent Kinetics of Halide Vacancy Filling in Organolead Halide Perovskites

AbstractHalide perovskites show high photoluminescence quantum yields and tunable bandgap. While perovskites’ optical properties significantly degrade due to the ionic and electronic defects, a correlation among their structure, size, defects, and degradation rate remains concealed. The authors report the crystal shape‐ and halide vacancy‐dependent stability of methylammonium lead bromide single crystals. The vacancies are filled in the cubic‐, plate‐, and rod‐shaped crystals by the halide‐soaking and light‐soaking processes. The differences in the stability, photoluminescence intensity, lifetime, and vacancy filling rates in these geometrical shapes derive from their specific surface‐to‐volume ratios. The shape‐dependent vacancy filling helps to correlate the electron microscope images, elemental compositions, and the photoluminescence intensity and lifetime.

Ionic additive engineering for stable planar perovskite solar cells with efficiency >22%

High crystallization quality and low defect state density of perovskite films are essential for high-performance devices. Herein, we develop a facile and effective method using potassium hexafluorophosphate (KPF6) to fabricate perovskite solar cells (PSCs) with highly improved power conversion efficiency (PCE) and outstanding stability. The corresponding mechanisms of the improved crystallization process, grain boundaries (GBs) passivation, and internal ionic defect passivation of the perovskite films are systematically studied. The results indicate that the introduced PF6− can promote crystallization by increasing fully coordinated intermediate groups in the perovskite solution and compensate for halide vacancies, which can modify ionic point defects in the perovskite films, resulting in reduced non-radiative recombination. Meanwhile, K+ ions are predominantly distributed at the GBs, which not only benefits the device efficiency but also significantly reduces the hysteresis due to the passivation effect. The champion passivated device presents a PCE of 22.04%, with a significantly improved PCE of around 16% compared with that of the control device (18.99%), retaining 80% of its initial PCE after 1000 h of light soaking without any encapsulation. Our work provides a defect passivation method for further improving both the PCE and stability of PSCs and can also be extended to other devices.

Halide Ion Mobility of Mixed Halide Perovskites in Light and Dark

The intrinsic ionic defects, specifically halide ion vacancies, often dictate the mobility of halide species within the perovskite film during the operation of solar cells. Of particular interest is the halide ion mobility in metal halide perovskites, which plays an important role in determining the performance of perovskite solar cells. Photoinduced phase segregation seen in mixed halide perovskite films under steady state irradiation offers a convenient way to visualize halide ion segregation. Interestingly, upon storage in dark, the process is reversed and the original mixed halide composition gets restored. Whereas entropy of mixing explain the thermally activated mixing of halide ions to yield mixed halide perovskite, the opposite trend observed during photoirradiation remains an intriguing phenomenon. The threshold energy of incident light to observe halide segregation increases with increasing temperature. The diffusion of these halide species, which is tracked through changes in the absorption spectra at different temperatures, offers a direct measurement of thermally activated halide diffusion in perovskite films. The thermally activated halide exchange shows the challenges of stabilizing metal halide perovskites with varying bandgap in tandem solar cells.

Synergistic Defect Passivation for Highly Efficient and Stable Perovskite Solar Cells Using Sodium Dodecyl Benzene Sulfonate

Polycrystalline perovskite films with a lot of ionic defects and pinholes are the key factors to both the efficiency and stability of organic–inorganic halide perovskite devices. Here, an efficient additive modification process is reported for passivating defects in the perovskite film with sodium dodecyl benzene sulfonate (SDBS), leading to valid defect passivation of perovskite films and conspicuous improvement in both power conversion efficiency (PCE) and stability of mixed-cation perovskite solar cells (PSCs). Upon incorporating SDBS in perovskite precursor, the PCE of planar structure mixed-cation PSCs shows a best PCE of 20.88% with negligible hysteresis behavior (1.27), which are outstanding and better than that of the control PSCs (PCE = 19.15% and H-index = 8.18). The benzene sulfonic acid group of SDBS attracts lead ions of the perovskite and attaches on the surfaces of the perovskite crystal, which obviously slows down the process of perovskite crystallization and benefits from the high-quality film with lower defect density preparation. Also, the long-chain alkyl groups of SDBS with hydrophobicity benefit from the perovskite film with better water stability. In addition, SDBS modification decreases the electron trap density, improving charge utilization, suppressing charge recombination, and resulting in an increase of the efficiency of the mixed-cation PSC devices. Moreover, PSCs with optimal SDBS passivation show a better ambient stability compared with the control PSCs. These results indicate that the synergistic interfacial passivation of SDBS in perovskites is beneficial in fabricating highly efficient and air stable PSCs.

Enhancing the formation of ionic defects to study the ice Ih/XI transition with molecular dynamics simulations

Ice Ih, the common form of ice in the biosphere, contains proton disorder. Its proton-ordered counterpart, ice XI, is thermodynamically stable below 72 K. However, the formation of ice XI is kinetically hindered, and experimentally it is obtained by doping with KOH. Doping creates ionic defects that promote the migration of protons and the associated change in proton configuration. In this article, we mimic the effect of doping with a bias potential that enhances the formation of ionic defects in molecular dynamics simulations. The recombination of the ions thus formed proceeds through fast migration of the hydroxide along hydrogen bond loops, providing a physical and expedite way to change the proton configuration. A key ingredient of this approach is a machine learning potential trained with density functional theory data and capable of modelling molecular dissociation. We exemplify the usefulness of this idea by studying the order-disorder transition using an appropriate order parameter that distinguishes the proton environments in ice Ih and XI. We calculate the changes in free energy, enthalpy, and entropy associated with the transition. Our estimated entropy agrees with experiment within the error bars of the calculation.

Polarization-Controlled Surface Defect Formation in a Hybrid Perovskite

Hybrid perovskites have two properties that are absent in traditional inorganic photovoltaic materials, namely, polarization and mobile ionic defects, the interaction between which may introduce new features into the materials. By using the first-principles calculations, we find that the formation energies of the vacancy defects at a tetragonal MAPbI3(110) surface are highly related to the surface polarization. The positive total polarization and local polarization of MA facilitate the formation of surface MA vacancies, whereas the negative total polarization and local polarization of MAI are favorable for the formation of surface iodine vacancies. The phenomena can be explained quantitatively on the basis of the two kinds of Coulomb interactions between the charged defect and the polarization-induced electrostatic field. The comprehensive insights into the interaction between the polarization and the ionic defects in hybrid perovskites can provide a new avenue for defect control for high-performance perovskite solar cells via surface polarization.

Synergistic Engineering of Natural Carnitine Molecules Allowing for Efficient and Stable Inverted Perovskite Solar Cells

Efficient control of the perovskite crystallization and passivation of the defects at the surface and grain boundaries of perovskite films have turned into the most important strategies to restrain charge recombination toward high-performance and long-term stability of perovskite solar cells (PSCs). In this paper, we employed a small amount of natural vitamin B (carnitine) with dual functional groups in the MAPbI3 precursor solution to simultaneously passivate the positive- and negative-charged ionic defects, which would be beneficial for charge transport in the PSCs. In addition, such methodology can efficiently ameliorate crystallinity with texture, better film morphology, high surface coverage, and longer charge carrier lifetime, as well as induce preferable energy level alignment. Benefiting from these advantages, the power conversion efficiency of PSCs significantly increases from 16.43 to 20.12% along with not only a higher open-circuit voltage of 1.12 V but also an outstanding fill factor of 82.78%.

Mixed ionic-electronic transport in the high-entropy (Co,Cu,Mg,Ni,Zn)1-xLixO oxides

A series of the high-entropy (Co,Cu,Mg,Ni,Zn)1-xLixO oxides with a lithium substitution level of x = 0, 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30 is evaluated in terms of the crystal structure, morphology and transport properties, with thorough studies aimed at elucidation of the nature of different contributions to the total electrical conductivity. It is found that cubic Fm-3m structure is preserved in the whole investigated series, with (Co,Cu,Mg,Ni,Zn)0.8Li0.2O composition showing a high internal strain, supporting to some degree one of the so-called core effects, anticipated for the high-entropy materials. For samples with Li content x > 0.20 the strain is relaxed by formation of the oxygen vacancies. As unambiguously evidenced by DC polarization experiments and measured impedance spectroscopy data with ionically-blocking Au and reversible Li electrodes used, the previously reported in the literature transition to the lithium superionic conductivity in the Li-rich compounds, up to σi ≈ 1–10⋅10−3 Scm−1, is more complex, with emergence of the electronic conduction as well, reaching similar magnitude for (Co,Cu,Mg,Ni,Zn)0.7Li0.3O. The observed behavior upon increase of lithium concentration (x) can be explained by a qualitative change of the nature of the electronic and ionic defects present in (Co,Cu,Mg,Ni,Zn)1-xLixO series, with initial oxidation of 3d metals (mainly Co), followed by possible formation of the interstitial lithium, and final emergence of the oxygen vacancies. Furthermore, the recorded electrochemical properties of (Co,Cu,Mg,Ni,Zn)0.7Li0.3O lithium cell electrode, suggesting presence of intercalation-like behavior at the initial stages of lithiation, confirm the proposed mixed ionic-electronic conductivity.

Defect engineering of rutile TiO2 ceramics: Route to high voltage stability of colossal permittivity

Donor-acceptor co-doped rutile TiO2 ceramics with colossal permittivity (CP) have been extensively investigated in recent years due to their potential applications in modern microelectronics. In addition to CP and low dielectric loss, voltage stability is an essential property for CP materials utilized in high-power and high-energy density storage devices. Unfortunately, the voltage stability of CP materials based on co-doped TiO2 does not catch enough attention. Here, we propose a strategy to enhance the voltage stability of co-doped TiO2, where different ionic defect clusters are formed by two acceptor ions with different radii to localize free carriers and result in high performance CP materials. The (Ta + Al + La) co-doped TiO2 ceramic with suitable La/Al ratio exhibits colossal permittivity with excellent temperature stability as well as outstanding dc bias stability. The density functional theory analysis suggests that La3+Al3+VO••Ti3+ defect clusters and Ta5+-Al3+ pairs are responsible for the excellent dielectric properties in (Ta + Al + La) co-doped TiO2. The results and mechanisms presented in this work open up a feasible route to design high performance CP materials via defect engineering.

Impact of Oxygen Non‐Stoichiometry on Near‐Ambient Temperature Ionic Mobility in Polaronic Mixed‐Ionic‐Electronic Conducting Thin Films

AbstractEnhanced ionic mobility in mixed ionic and electronic conducting solids contributes to improved performance of memristive memory, energy storage and conversion, and catalytic devices. Ionic mobility can be significantly depressed at reduced temperatures, for example, due to defect association and therefore needs to be monitored. Measurements of ionic transport in mixed conductors, however, proves to be difficult due to dominant electronic conductivity. This study examines the impact of different levels of quenched‐in oxygen deficiency on the oxygen vacancy mobility near room temperature. A praseodymium doped ceria (Pr0.1Ce0.9O2–δ ) film is grown by pulsed laser deposition and annealed in various oxygen partial pressures to modify its oxygen vacancy concentration. Changes in film non‐stoichiometry are monitored by tracking the optical absorption related to the oxidation state of Pr ions. A 13‐fold increase in ionic mobility at 60 °C for increases in oxygen non‐stoichiometry from 0.032 to 0.042 is detected with negligible changes in migration enthalpy and large changes in pre‐factor. Several factors potentially contributing to the large pre‐factor changes are examined and discussed. Insights into how ionic defect concentration can markedly impact ionic mobility should help in elucidating the origins of variations seen in nanoionic devices.

Efficient and Stable Perovskite‐Based Photocathode for Photoelectrochemical Hydrogen Production

AbstractAlthough organometal halide perovskites (OHPs) have desirable photovoltaic properties, their photoelectrochemical (PEC) water‐splitting application for hydrogen production is limited by the instability originating from their intrinsic ionic defects and hygroscopic vulnerability. Herein, a highly efficient and stable OHP‐based photocathode achieved by a new zwitterion (L‐proline) passivation and a eutectic gallium indium alloy (EGaIn) encapsulation method is described. The zwitterion, which has both cations and anions, can simultaneously passivate both positively and negatively charged defects in OHPs. The resulting OHP photovoltaic cells with passivated shows an over 20% power conversion efficiency with an open‐circuit voltage of 1.13 V and a short‐circuit current of 22.13 mA cm−2. The EGaIn‐incorporated Ti foil provides complete encapsulation from the external environment while maintaining good transport of photogenerated charges from OHPs. Thus, these photocathodes exhibit a remarkable average photocurrent density of 21.2 mA cm−2 which has less than 5% current loss between PV cells and PEC cells. More admirably, the photocathode has the highest stability over 54 hours under continuous full sunlight illumination in a sulfuric acid electrolyte.

Real-Time Blinking Suppression of Perovskite Quantum Dots by Halide Vacancy Filling.

Despite the excellent optoelectronic properties of halide perovskites, the ionic and electronic defects adversely affect the stability and durability of perovskites and their devices. These defects, intrinsic or produced by environmental factors such as oxygen, moisture, or light, not only cause chemical reactions that disintegrate the structure and properties of perovskites but also induce undesired photoluminescence blinking to perovskite quantum dots and nanocrystals. Blinking is also caused by the nonradiative Auger processes in the photocharged quantum dots or nanocrystals. Herein, we find real-time suppression of halide vacancy-assisted nonradiative exciton recombination and photoluminescence blinking in MAPbBr3 and MAPbI3 perovskite quantum dots by filling the vacancies using halide precursors (MABr and MAI). Also, halide vacancy filling increases the photoluminescence quantum efficiencies and lifetimes of the quantum dots. We estimate the rates of halide vacancy-assisted nonradiative recombination at 1 × 108 s-1 for MAPbBr3 and 1.9 × 109 s-1 for MAPbI3 quantum dots. The real-time blinking suppression using the halide precursors and statistical analysis of the ON/OFF blinking time reveal that the halide vacancies contribute to the type-A blinking through charging and discharging. Conversely, the blinking of the quantum dots after halide vacancy filling is dominated by the type-B mechanism.