Interstitial Halogen Research Articles

Incorporating Electrochemical Halide Oxidation into Drift‐Diffusion Models to Explain Performance Losses in Perovskite Solar Cells under Prolonged Reverse Bias

AbstractPartial shading of a solar module can induce a set of cells within the module to operate under reverse bias. Studies have shown that metal halide perovskite solar cells with a wide variety of compositions and contacts exhibit interesting behavior in reverse bias that includes both reversible performance loss and non‐reversible degradation. In this paper, an advanced drift‐diffusion approach incorporating an electrochemical term to explain the short‐circuit, open circuit and fill factor losses that are experimentally measured after prolonged reverse bias is used. It is shown that holes can tunnel into the perovskite due to sharp band bending near the contact, accumulate within the bulk of the perovskite absorber, and trigger the oxidation of halides to form neutral halogens. The density of neutral halogens is much higher in reverse bias because there are hardly any electrons available to reduce the iodine. The resulting halogens act as bulk recombination centers. While the interstitial halogen density does decay when the cell is operated in forward bias, permanent degradation can occur if the iodine diffuses out of the perovskite layer. Finally, the ways in which changing parameters such as the mobile ion density or the series resistance at the contact can influence device performance and stability are discussed.

The thermostimulated luminescence of radiation defects in KCl, KBr and KI crystals at elastic and plastic deformation

The efficiency of radiation defects formation in alkali halide crystals (AHC) was studied by the method of absorption spectroscopy. However, it is not possible to study the deformation-stimulated processes in detail by the absorption spectrum of radiation defects due to the limited sensitivity compared with luminescent spectroscopy. In this regard, thermally stimulated luminescence (TSL) of radiation defects at elastic and plastic deformation was applied in AHC. In the absence of deformation, the dominant peaks in TSL are -centers. After elastic deformation, low temperature peaks of TSL corresponding to F′-, VK- and VF-centers became dominant. After plastic deformation, the peaks of TSL corresponding to -centers became dominant. The elastic deformation contributes to the increase in concentration of low-temperature F′-, VK- and VF-centers, and the plastic one contributes to that of high temperature -centers (peaks of TSL in KCl at 360K, in KBr at 365K, in KI at 340K), composed by divacancies created by plastic deformation. At elastic deformation, unrelaxed interstitial halogen atoms are converted into VK- and VF-centers, and due to this fact the long-range interaction is absent, the result of which are the -centers.

Vacancy Dipole Currents of Thermostimulated Depolarization in a Plastically Deformed KCl Crystal

A thermostimulated depolarization current of divacancies (υ − υ + ) created due to plastic deformation of a KCl crystal is detected. The thermostimulated depolarization current with a maximum at 205 K is correlated with the increasing degree of relative deformation of the KCl crystal. The mechanism of effective formation of (Cl 3 − ) aca centers is interpreted as a result of interaction of two mobile interstitial halogen atoms in the field of divacancies (υ − υ + ).

Two new barium borate bromides: Ba2BO3Br and Ba3BO3Br3

Two new barium borate bromide crystals, Ba2BO3Br and Ba3BO3Br3, have been obtained by spontaneous crystallization. Ba2BO3Br crystallizes in P−3m1 space group, with cell parameters of a = 5.5157(10) Å, c = 11.019(4) Å, and Z = 2, its structure is build up by alternately stacking along c-axis of [Ba2(BO3)2]2− layers and bromide [Ba2Br2]2+ layers. The solved structure is analog to Ba2(BO3)1−x(CO3)xCl1+x except the interstitial halogen atoms at (0, 0, 1/2) is missing and accordingly the partly CO3 substitution for BO3 has not been observed. Ba3BO3Br3 crystallizes in a new structure type with P−1 space group and cell parameters of a = 9.280(4) Å, b = 9.349(7) Å, c = 13.025(9) Å, α = 92.71(3)°, β = 98.29(3)°, γ = 116.200(18)° and Z = 4. The basic structural unit in Ba3BO3Br3 is the clusters composed of 4 BO3 groups and 12 Ba atoms, which in turn are linked by eight Ba–O bonds with other four clusters to form sheets extend in the (001) plane.

Electron-phonon coupling in halogen-doped carbon clathrates from first principles

Electronic structure and electron-phonon coupling of carbon clathrates ${\mathrm{C}}_{40}$, ${\mathrm{C}}_{34}$, and ${\mathrm{C}}_{46}$ doped with interstitial halogen atoms (fluorine, iodine) in the fullerenic cages of the host, have been investigated from first principles. It turns out that, while in ${\mathrm{C}}_{40}$ and ${\mathrm{C}}_{46}$ the halogen is only partially reduced, in ${\mathrm{FC}}_{34}$ an electron is transferred to the guest ion by injecting holes in the valence band of the host. The calculated electron-phonon coupling constant is as large as $\ensuremath{\lambda}=0.85$. We propose that halogen-doped carbon clathrates might represent another form of tetrahedral carbon, in addition to the recently found B-doped diamond, suitable to turn superconductive at high temperature (${T}_{c}\ensuremath{\sim}77\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ from McMillan formula).

Laser-induced reactions in crystals: Femtosecond pump-probe spectroscopy andab initiocalculations of self-trapped excitons and holes in KBr

We used an embedded cluster Hartree-Fock method that self-consistently accounts for lattice polarization to calculate adiabatic potential energy surfaces of the ground and excited states of the self-trapped exciton and to model its decomposition into Frenkel defect pairs in KBr. The characteristic optical excitation and luminescence energies of the self-trapped exciton and basic Frenkel defects are calculated. We present the experimental results of femtosecond pump-probe spectroscopy, which demonstrate the time evolution of the optical absorption of the KBr crystal excited by an 8-eV pulse at 80 K. These results reveal that Frenkel-defect pairs are formed in KBr prior to the holes relaxing into their most stable ${V}_{K}$ center state. Contrary to these results, the femtosecond spectroscopy of the ${V}_{K}$-center formation in NaBr demonstrates that this is an extremely fast ( 1 ps) process. Theoretical modeling is used to show that the fast process of the F center (electron trapped by a halogen vacancy) and H center (interstitial halogen atom) pair formation in KBr prior to the hole self-trapping can happen in the ground electronic state of the exciton. This process is driven by the interaction of the relaxing hole with electron. We conclude that the speed of the hole vibrational relaxation prior to recombination with an electron and formation of an exciton is an important factor that determines the speed and effectiveness of exciton decomposition into Frenkel defects in alkali halides.

Creation of groups of spatially correlated defects in a KBr crystal at 8 K

The creation spectrum of triplets of spatially correlated defects (F centre + self-trapped hole, centre + interstitial halogen ion, I centre) was measured for the first time in a KBr crystal at 8 K, using synchrotron radiation of 6-25 eV and highly sensitive luminescent methods. The spectrum of spatially correlated F and H centre creation by synchrotron radiation at 8 K was measured as well. The efficiency of triplet creation is especially high at crystal irradiation by photons under the conditions of multiplication of electronic excitations when the absorption of one photon leads, respectively, to the formation of an electron-hole (e-h) pair and a secondary exciton (15.0-16.5 eV), or two e-h pairs (16.7-19.0 eV), or to the formation of a cation exciton (19.5-20.5 eV). It is shown that the formation of an triplet is caused by the recharging of a primary F-H pair by an e-h pair or by a free exciton.

Femtosecond time-resolved spectroscopy of self-trapping processes of holes and electron-hole pairs in alkali bromide crystals.

The dynamics of the lattice relaxation of holes and electron-hole ($e\ensuremath{-}h$) pairs generated by the band-gap excitation in alkali bromide crystals has been studied in femtosecond time regime at temperature range from 6 to 300 K. In KBr including electron-trapping impurities, where relaxation of holes takes place without interaction with electrons, it is found that the self-trapped hole in the form of a halogen-molecular ion, the ${V}_{K}$ center, is generated via a transient localized-hole center as a precursor. This transient hole center shows broad optical absorption bands in the visible region and has a lifetime of about 3 ps at 273 K. It is found that in pure KBr and RbBr, the relaxation processes of ($e\ensuremath{-}h$) pairs have two distinct stages. In the first stage, which terminates within 6 ps after excitation, the transient hole centers are generated as in doped specimens, and their interaction with electrons results in the fast formation of a Frenkel pair consisting of an $F$ center, a halogen vacancy trapping an electron, and an $H$ center, an interstitial halogen atom. Also, an intermediate state is formed in the first stage, and the state is ascribed to the self-trapped exciton (STE) with the on-center configuration on the basis of its spectroscopic features. The second stage of the relaxation, which lasts over 100 ps at low temperatures, comprises the off-center relaxation of the on-center STE formed in the first stage into Frenkel pairs and STE's with the off-center configuration. Relaxation pathways and characteristics including temperature dependence are discussed based on the experimental results and their analysis in terms of a rate-equation model.

Depth profiles of interstitial halogen defects in high-energy ion-bombarded RbI by micro-Raman spectroscopy.

Micro-Raman spectroscopy has been used to measure the depth profiles of halogen aggregates in both x-irradiated and ion-bombarded RbI. In experiments using argon ions of energy 13.6 MeV/A, it is shown that the ${\mathit{I}}_{\mathit{n}}^{\mathrm{\ensuremath{-}}}$ clusters are significantly enhanced in the region of large excitation density near the maximum in electronic energy loss at the expense of the ${\mathit{I}}_{3}^{\mathrm{\ensuremath{-}}}$ defects. The investigations of fluence dependence show that the number of ${\mathit{I}}_{3}^{\mathrm{\ensuremath{-}}}$ defects is progressively enhanced with increasing fluence at all depths within the damaged region. The results are examined in the light of current theoretical models of ion-beam interactions with alkali halides. \textcopyright{} 1996 The American Physical Society.

Multiplication of electronic excitations and formation of radiation defects in alkali halides

The spectrum of F center creation has been measured for KBr:Tl using synchrotron radiation (12–32 eV). The creation efficiency of a stable F center is especially high at 15–17 and 28–30 eV where the absorption of one photon leads to the formation either both of an electron-hole pair and an exciton or two electron-hole pairs and an exciton. In these cases there are favourable conditions for stabilization of mobile radiation defects due to the association of interstitial halogen atoms, holes and cation vacancies.

Depth profiles of interstitial halogen defects in high energy ion bombarded alkali iodides by micro-Raman spectroscopy

Halogen aggregates created in alkali iodides by high energy ions consist of I3−, In− and (I2)n as evidenced by Raman spectroscopy. By extending our studies to micro-Raman measurements, it has been possible to determine the depth profiles of the formation of halogen aggregates. It is shown that In− clusters are created with an enhanced concentration in the region of large excitation density near the Bragg peak at the expense of the I3− defects. Several factors monitor the relationship between I3− and In− clusters. At saturation within the ion tracks, these respective numbers critically depend on their mutually coupled kinetics, their dose dependence and the degree of ion track overlap. It is shown for example that I3− defects are progressively enhanced with increasing fluences all along the ion tracks. Results are compared to those obtained by X-ray irradiations.

Creation of stable Frenkel defects by vacuum uv radiation in KBr crystals under conditions of multiplication of electronic excitations.

The spectrum of F-center creation has been measured for KBr:Tl by means of synchrotron radiation (12--24 eV). Analysis of the spectrum reveals the decay of cation excitons (19.7--20.3 eV) with the formation of a double amount of electron-hole pairs. The creation efficiency of a stable F center is especially high at 15--17 eV where the absorption of one photon leads to the formation both of an electron-hole pair and an exciton. In this case there are favorable conditions for stabilization of mobile radiation defects due to the association of interstitial halogen atoms, holes, and cation vacancies.

Thermally assisted desorption processes in electron bombarded alkali halides

The desorption of alkali and halogen atoms induced by the interaction of energetic electrons with surfaces of alkali halide crystals has been studied by means of an angular-resolved and mass-selected time-of-flight spectroscopy. It has been found that a considerable fraction of halogen atoms was ejected with hyperthermal energies of the order of 0.1 eV. However, alkali atoms and the remaining part of halogen emission had thermal (Maxwellian) spectra of kinetic energies. In this paper we will report on systematic investigations of these thermal desorption processes for single crystal (100) NaCl, KCl, KBr, RbBr, and Kl surfaces. The relative yield of the thermal component has been measured as a function of electron beam energy and beam current density at various sample temperatures. It will be shown that thermal halogen emission can be explained by thermally assisted diffusion of interstitial halogen atoms produced in the bulk of the crystal from decaying self-trapped excitons. The origin of the alkali atom component will be described as due to neutralization and subsequent thermal evaporation of excess alkali atoms from the halogen deficient surface.

Afterglow and Thermoluminescence Response of Europium-Doped Alkali Halides Dosemeters

Abstract It is well known that Eu2+-doped alkali halides show a persistent luminescence, often referred to as afterglow, after being subjected to ionising radiation. Several mechanisms responsible for the observed afterglow and thermoluminescence have been proposed which are dependent on the type of alkali halide crystal, of radiation used and the aggregation - precipitation state of the impurity. We have carried out detailed afterglow and thermoluminescence measurements in europium-doped KC1, irradiated with X rays. Our conclusions support the occurrence of at least two main mechanisms originating in the existence of impurity vacancy dipoles and the recombination of F centres and other defects with interstitial halogen ions.

Afterglow and Thermoluminescence Response of Europium-Doped Alkali Halides Dosemeters

It is well known that Eu2+-doped alkali halides show a persistent luminescence, often referred to as afterglow, after being subjected to ionising radiation. Several mechanisms responsible for the observed afterglow and thermoluminescence have been proposed which are dependent on the type of alkali halide crystal, of radiation used and the aggregation - precipitation state of the impurity. We have carried out detailed afterglow and thermoluminescence measurements in europium-doped KC1, irradiated with X rays. Our conclusions support the occurrence of at least two main mechanisms originating in the existence of impurity vacancy dipoles and the recombination of F centres and other defects with interstitial halogen ions.

Raman study onXi-(Pb+) centers in single crystals of alkali halides doped with lead

Low frequency modes were observed in a series of alkali halides doped with lead after X-irradiation at 77 K. The peak positions of the Raman signals are at 34 cm-1 for NaCl:Pb2+, 30 cm-1 for KCl: Pb2+, 28 cm-1 for RbCl: Pb2+ and 22 cm-1 for KBr:Pb2+. They are assigned to the inelastic light scattering caused by the local vibration of Xi-(Pb+) center, which consists of an interstitial halogen ion stabilized by a substitutional ion Pb+. Polarized Raman measurements and Behavior Type (BT) analysis show that the symmetry of the Xi-(Pb+) center belongs to the point group C3v. A structure model of the Xi-(Pb+) center is proposed based on the Raman data and the result of BT analysis.

Deep Region Emissions of CuGaS2 Crystals

The broad emission bands appearing at 6700 Å, 7500 Å and 8600 Å were discussed in terms of preparation processes of epitaxial films and bulk crystals. For the emission at 7500 Å showing spectral shifts during decay, characteristic to donor-acceptor-pair type recombinations, possible relation is pointed out to sulfur vacancies and interstitial halogen (Cl or I) atoms in films or bulk crystals.

Thermalized and Hot Interstitial Halogen Ions in Alkali Halides

AbstractThe efficiency of α, I pair creation at exciton decay is high at 4.2 K for KCl, KBr, and RbBr crystals for which H centers are oriented along 〈110〉. The efficiency of α, I pair creation by VUV‐irradiation at 4.2 K is extremely low for NaCl crystals for which H centers are oriented along 〈111〉. The process of mobile crowdion‐type interstitial halogen ion (I center) formation in KCl, KBr, RbCl, and RbBr (but not in NaCl) due to the tunneling recharge of primary F, H pairs is discussed. Such an I crowdion can shift by several interanion distances along [110] even at 4.2 K and, as a result, the system leaves the zone of spontaneous annihilation of α and I centers.

Creation of lattice defects in the bulk and at the surface of alkali halide crystals

Abstract When a conduction electron is recaptured in the coulomb field of a self-trapped hole, it is strongly attracted to the site of the halogen molecular ion on which the hole resides, and particularly to the alkali neighbors, but not to the halogen atom core. We will consider how the electron can effectively force the halogen from its site. The interaction thus generates a lattice vacancy which the electron occupies to form an F center, and an interstitial halogen atom. If this occurs very near the surface, ejection of the halogen atom from the crystal may be the result. This defect generation process now accounts for a wide variety of observations.

V centers in irradiated alkali halide crystals

Resonant Raman scattering (RRS) allows a determination of the nature of stable V centers created under X or γ irradiation in alkali halide crystals. In the early stage of irradiation at low temperature, trihalide ions are formed. By increasing the radiation dose, polyhalide ions, characterized by the vibrational mode of the halogen molecule in X − n (X: halogen; n = 5 …), appear. These centers are precursors of halogen clusters created at room temperature (or above). They exhibit the structure of an iodine crystal in KI and that of a “solid like” aggregate of bromine molecules in KBr. Thermal annealing almost restores the crystal except for voids which remain after the disappearance of halogen aggregates. The stabilization of halogen interstitials into V centers is the major process in the production of stable electron and hole defects obtained in alkali halide crystals exposed to ionizing radiations. The nature of the V centers observed after irradiation of pure or doped crystals is discussed in this paper together with effects due to temperature, irradiation dose and thermal annealing.