Intense Optical Absorption Research Articles

Insights on the Optical Properties of Poly(3,4-Ethylenedioxythiophene):Poly(styrenesulfonate) Formulations by Optical Metrology.

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is among the most widely used polymers that are used as printed transparent electrodes for flexible Organic Electronic (OE) devices, such as Organic Photovoltaics (OPVs). The understanding of their optical properties and the correlation of the optical properties with their electronic properties and metallic-like behavior can lead to the optimization of their functionality as transparent electrodes in multilayer OE device architectures. In this work, we study the optical properties of different PEDOT:PSS formulations by non-destructive Spectroscopic Ellipsometry (SE), from the infrared to the far ultraviolet spectral regions. The optical response of PEDOT:PSS includes an intense optical absorption originated from the conductive part (PEDOT) at lower photon energies, whereas the electronic transition energies of the non-conductive PSS part have been measured at higher photon energies. Based on the different PEDOT:PSS formulations, the optical investigation revealed significant information on the relative contribution of conductive PEDOT and insulating PSS parts of the PEDOT:PSS formulation in the overall optical response, which can strongly impact the final device functionality and its optical transparency.

Optical response in a laser-driven quantum pseudodot system

We investigate theoretically the intense laser-induced optical absorption coefficients and refractive index changes in a two-dimensional quantum pseudodot system under an uniform magnetic field. The effects of non-resonant, monochromatic intense laser field upon the system are treated within the framework of high-frequency Floquet approach in which the system is supposed to be governed by a laser-dressed potential. Linear and nonlinear absorption coefficients and relative changes in the refractive index are obtained by means of the compact-density matrix approach and iterative method. The results of numerical calculations for a typical GaAs quantum dot reveal that the optical response depends strongly on the magnitude of external magnetic field and characteristic parameters of the confinement potential. Moreover, we have demonstrated that the intense laser field modifies the confinement and thereby causes remarkable changes in the linear and nonlinear optical properties of the system.

The Role of the Axial Substituent in Subphthalocyanine Acceptors for Bulk‐Heterojunction Solar Cells

AbstractFour hexachlorosubphthalocyanines SubPcCl6‐X bearing different axial substituents (X) have been synthesized for use as novel electron acceptors in solution‐processed bulk‐heterojunction organic solar cells. Subphthalocyanines are aromatic chromophoric molecules with cone‐shaped structure, good solution processability, intense optical absorption in the visible spectral region, appropriate electron mobilities, and tunable energy levels. Solar cells with subphthalocyanines as the electron acceptor and PTB7‐Th as the electron donor exhibit a power conversion efficiency up to 4 % and an external quantum efficiency approaching 60 % due to significant contributions from both the electron donor and the electron acceptor to the photocurrent, indicating a promising prospect of non‐fullerene acceptors based on subphthalocyanines and structurally related systems.

The Role of the Axial Substituent in Subphthalocyanine Acceptors for Bulk-Heterojunction Solar Cells.

Four hexachlorosubphthalocyanines SubPcCl6‐X bearing different axial substituents (X) have been synthesized for use as novel electron acceptors in solution‐processed bulk‐heterojunction organic solar cells. Subphthalocyanines are aromatic chromophoric molecules with cone‐shaped structure, good solution processability, intense optical absorption in the visible spectral region, appropriate electron mobilities, and tunable energy levels. Solar cells with subphthalocyanines as the electron acceptor and PTB7‐Th as the electron donor exhibit a power conversion efficiency up to 4 % and an external quantum efficiency approaching 60 % due to significant contributions from both the electron donor and the electron acceptor to the photocurrent, indicating a promising prospect of non‐fullerene acceptors based on subphthalocyanines and structurally related systems.

(Invited) Utilization of Photoluminescence from Semiconductor Nanoparticles to Prove Electron Transfer Reactions Influenced By Their Surface Conditions

Semiconductor nanoparticles, or quantum dots, have many advantages as photoluminescent materials such as the broad and intense optical absorption and single, narrow, and size-tunable emission. However, they have different types of fragility when compared to other organic/inorganic fluorophores. The importance of the surface on photoluminescence property is well-known by nanoparticle chemists but has hardly been investigated from both qualitative and quantitative points of view. The band edge emission can be observed when photogenerated excitons are confined in a particle having no defect levels: the whole structure including particle core, shell, and insulating ligands bound to the nanoparticle surface is essential to a high luminescence. The present study demonstrates a simple and efficient method to investigate the condition of ligands, which typically consist of organic molecules like alkylamines, carboxylates, and thiols. The emission quenching of these nanoparticles were observed when small electron acceptors like quinones were introduced to the nanoparticle solution, because these small molecules unexpectedly penetrated the protecting ligand layers. The mathematical analyses revealed that there were many defects in the ligand layer even if they are commonly used molecules and believed to be appropriate for the synthesis and storage of nanoparticles. Figure 1a shows the photoluminescence intensity ratio (I 0/I – 1) when the CdSe nanoparticles capped with n-alkylamine ligands were quenched by 1,4-benzoquinone introduced as an electron acceptor. The magnitude of quenching became greater with the alkyl moiety of the ligands became smaller. Although it is generally expected to be straight when the quenching mechanism follows the Stern-Volmer relationship that describes the photoinduced electron transfer between the species both dissolved and moving freely, they were all concave up. To reveal the origin of the curvatures, a model of quenching was developed. A nanoparticle has several voids of ligands on its surface, and through which the quenchers (1,4-benzoquinone) approach to the nanoparticle core to the distance that the electron transfer is possible (Figure 1b). A mathematical analysis with taking statistical distributions into account, was conducted following the proposed model, and they provided good fits with curvatures with three important parameters; the number of accessible sites existing on a nanoparticle, electron transfer rate, and the adsorption coefficients of quenchers to the nanoparticles. None of these parameters are available with conventional analysis methods, and all of them should be very import to address the intrinsic weakness of semiconductor nanoparticle fluorophores.Time variations on the magnitude of quenching were also observed during the storage of as-prepared nanoparticles. The nanoparticle surface became gradually damaged during the storage in a pure chloroform solution, while the nearly-perfect surface was formed when only a small amount of additional ligands given in the storage solution, which were proved quantitatively for the first time by our methods. Figure 1

Solid-state Effects in the Cyclovoltammetric HOMO-LUMO Determination: The case of Dinitrophenyl-hydrazone α,Ͽ-substituted Oligothiophenes

New acceptor⿿donor⿿acceptor (A-D-A) small molecules consisting of a central thiophene-based donor building block, i.e. 3-dodecyl-thiophene (for NTN) and 3,3⿲⿿-dihexyl-terthiophene (for NT3N), and dinitrophenyl-hydrazone acceptor ends were synthesized. The materials display intense optical absorption spectra with absorption edge down to 1.65eV. Cyclic voltammetry in solution shows oxidation and reduction processes with HOMO and LUMO energy levels at -5.6 and -3.5eV. Cyclic voltammetry of NT3N cast films displays an unusual splitting of both oxidation and reduction peaks into two components with the same potential separation (0.45V). Films of tetrahexyl-sexithiophene, selected as a model compound of NT3N without polar ends, show an analogous albeit much lower (0.13V) splitting of the first one-electron oxidation process. The co-presence of a crystalline and an amorphous phase in the cast films is believed to be responsible for the splitting, which corresponds to the crystallization energy.

Hydrothermal synthesis of α-Fe2O3/g-C3N4 composite and its efficient photocatalytic reduction of Cr(VI) under visible light

α-Fe2O3/g-C3N4 composites were fabricated by a facile hydrothermal method. The prepared composites were characterized by XRD, XPS, TEM, DRS, PL and photoelectrochemical measurement. Under visible light irradiation, α-Fe2O3/g-C3N4 composite displays higher photocatalytic activity than pure g-C3N4 and α-Fe2O3 for Cr(VI) reduction. The enhanced photocatalytic activity should be attributed to the well-matched band structure and intimate contact interfaces between g-C3N4 and α-Fe2O3, which lead to the effective transfer and separation of the photogenerated charge carriers. The decoration of α-Fe2O3 does not affect the morphology and size of g-C3N4. The lower photoluminescence intensity and higher photocurrent density indicate the effective transfer and separation of photogenerated charge carriers. α-Fe2O3/g-C3N4 composite shows more intense optical absorption compared with pure g-C3N4. The addition of hole scavengers can greatly facilitate the photocatalytic reduction of Cr(VI) and the α-Fe2O3/g-C3N4 composite shows excellent stability during the photocatalytic reduction of Cr(VI).

Activatable two-photon fluorescence nanoprobe for bioimaging of glutathione in living cells and tissues.

Glutathione (GSH) serves vital cellular biological functions, and its abnormal levels are associated with many diseases. To better understand its physiological and pathological functions, efficient methods for monitoring of GSH in living systems are desired. Although quite a few small molecule-based and nanomaterial-based one photon fluorescence probes have been reported for GSH, two-photon (TP) probes, especially nanoprobes with good membrane permeability, are more favorable for bioimaging applications, since TP fluorescence imaging can provide improved spatial localization and increased imaging depth. In this work, we for the first time reported a "turn-on" TP fluorescence nanoprobe for efficient detection of GSH in aqueous solutions and TP excited fluorescence imaging of GSH in living cells and tissues. The nanoprobe consists of two-photon mesoporous silica nanoparticles (TP-MSNs) with a large TP excitation action cross-section (Φδ) value of 103 GM and MnO2 nanosheets, which show intense and broad optical absorption and could act as efficient quenchers for TP fluorescence. In the sensing system, the negatively charged MnO2 nanosheets are adsorbed on the positively charged MSNs through electrostatic interaction, resulting in efficient quenching of their fluorescence, with very low background fluorescence observed. The addition of GSH could reduce MnO2 into Mn(2+), lead to the decomposition of the MnO2 nanosheets, and thereby result in remarkable enhancement of both one photon and TP excited fluorescence of the nanosystem. The nanoprobe shows a highly sensitive response to GSH in aqueous solutions, with a detection limit of 200 nM achieved. It also exhibits a high selectivity toward GSH relative to other biomolecules and electrolytes, with good membrane permeability and excellent biocompatibility. The nanoprobe was successfully applied in monitoring the change of the intracellular GSH in living cells and tissues via TP fluorescence imaging, demonstrating its value of practical application in biological systems.

π‐Extended Narrow‐Bandgap Diketopyrrolopyrrole‐Based Oligomers for Solution‐Processed Inverted Organic Solar Cells

A series of narrow‐bandgap π‐conjugated oligomers based on diketopyrrolopyrrole chromophoric units coupled with benzodithiophene, indacenodithiophene, thiophene, and isoindigo cores are designed and synthesized for application as donor materials in solution‐processed small‐molecule organic solar cells. The impacts of these different central cores on the optoelectronic and morphological properties, carrier mobility, and photovoltaic performance are investigated. These π‐extended oligomers possess broad and intense optical absorption covering the range from 550 to 750 nm, narrow optical bandgaps of 1.52–1.69 eV, and relatively low‐lying highest occupied molecular orbital (HOMO) energy levels ranging from −5.24 to −5.46 eV in their thin films. A high power conversion efficiency of 5.9% under simulated AM 1.5G illumination is achieved for inverted organic solar cells based on a small‐molecule bulk‐heterojunction system consisting of a benzodithiophene‐diketopyrrolopyrrole‐containing oligomer as a donor and [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC71BM) as an acceptor. Transmission electron microscopy and energy‐dispersive X‐ray spectroscopy reveal that interpenetrating and interconnected donor/acceptor domains with pronounced mesoscopic phase segregation are formed within the photoactive binary blends, which is ideal for efficient exciton dissociation and charge transport in the bulk‐heterojunction devices.

Size Affects the Stability of the Photoacoustic Conversion of Gold Nanorods

Gold nanorods exhibit intense optical absorption bands in the near-infrared region of principal interest for applications in biomedical optics, which originate from sharp plasmon resonances. This high absorbance, combined with the biochemical inertness and targetability of gold nanoparticles, makes these materials excellent candidates to provide contrast in photoacoustic imaging and for other applications such as the selective hyperthermia of cancer. One issue demoting the potential of gold nanorods as contrast agents in photoacoustic applications is their limited photostability, which falls below relevant permissible exposure limits. In particular, when gold nanorods are resonantly excited by laser pulses in the nanosecond duration regime, there may occur phenomena like reshaping into rounder nanoparticles as well as fragmentation and sublimation, which modify their optical absorption bands and hinder their efficiency of photoacoustic conversion. Here we investigate the influence of nanoparticle size on the photostability and reproducibility of photoacoustic conversion of gold nanorods embedded in biomimetic phantoms. We compare samples containing gold nanorods with different sizes but the same shapes and overall optical densities. We demonstrate clear size effects as the thresholds of optical fluences for nanoparticle deformation improve from below 2 to above 6 mJ/cm2 with nanoparticle miniaturization from 22 to 5 nm effective radii. We interpret these results in terms of a better thermal coupling and faster heat dissipation from smaller nanoparticles to their environment, originating from their larger specific surface area.

Invited Presentation: One-Dimensional Plasmon Resonance in Single-Walled Carbon Nanotubes

Single-walled carbon nanotubes (SWCNTs) are attractive one-dimensional (1D) carbon materials. Confinement of electrons in the radial direction, which is provided by the monolayer thickness of SWCNTs, leads to the unusual electronic properties of these 1D structures. For instance, SWCNTs show very unique optical properties associating with their low-dimensionality over wide spectral ranges. In the ultraviolet (UV)-visible (vis)-near infrared (NIR) region, sharp transition peaks are observed, which is associated with the interband transition between 1D van Hove singularities [1]. In addition, a broad absorption peak is observed in the UV region, which can be attributed to the π-plasmon collective excitation of SWCNTs [2,3].Even in the far infrared (FIR) and terahertz (THz) frequency ranges, there is a broad absorption peak around a few 100 cm-1 [4]. This feature was firstly reported more than 10 years ago and interpreted as the narrow gap of metallic SWCNTs. The finite radial curvature of SWCNTs induces the shift of the Fermi point from the original K point, resulting in the small energy gaps near the Fermi level of metallic SWCNTs. The induced narrow-gap was actually observed by the low-temperature scanning tunneling spectroscopy in the energy region of ~10 meV which corresponds to the FIR frequencies (a few 100 cm-1) [5]. Importantly, this narrow gap exhibits strong diameter dependence, i.e., the gap energy is inversely proportional to the square of the tube diameter.An alternative interpretation of the optical response in the FIR-THz region is the plasmon excitation of SWCNTs with the finite tube length [6,7]. The confined free carriers in metallic SWCNTs interact with the low energy photons, resulting in the intense optical absorption. However, the detailed mechanisms of the phenomena have been unclear so far because of the luck of the systematic study on the FIR spectroscopy of well-prepared samples. Especially, because plasmonic motion in 1D nanostructures should be sensitive to the path length, it is highly desired that the spectroscopic measurements by using SWCNTs with the same tube diameter, but different tube length.Here we report the FIR and THz study of various SWCNT samples having different tube length. Each SWCNT was cut by strong sonication with different duration time. Importantly the sonication processes did not affect the diameter distribution, which was confirmed by resonance Raman and UV-vis-NIR spectroscopy. The obtained FIR-THz absorption frequency show a linear correlation with the inverse of the tube length, which is consistent with the 1D dispersion relation. Moreover, the different doping effects on the FIR-THz spectra between metallic and semiconducting SWCNTs indicate that the motion of the photo-induced carriers is dominantly accounts for the optical response. The observed resonance peaks were very sensitive to the presence of the defects on the tube wall, and rationally correlated to the electric resistance of the specimens. H. Kataura, Y. Kumazawa, Y. Maniwa, I. Umezu, S. Suzuki, Y. Ohtsuka, Y. Achiba, Synth. Met. 103, 2555 (1999).J-S. Lauret, C. Voisin, G. Cassabois, C. Delalande, Ph. Roussignol, O. Jost, L. Capes, Phys. Rev. Lett. 90, 057404 (2003).Brian J. Landi, Herbert J. Ruf, Chris M. Evans, Cory D. Cress and Ryne P. Raffaelle, J. Phys. Chem. B, 109, 9952 (2005).A. Ugawa, A. G. Rinzler, D. B. Tanner, Phys. Rev. B, 60, R11305, (1999).M. Ouyang, J.-L. Huang, C. L. Cheung, C. M. Lieber, Science, 292, 702, (2001).N. Akima, Y. Iwasa, S. Brown, A. M. Barbour, J. Cao, J. L. Musfeldt, H. Matsui, N. Toyota, M. Shiraishi, H. Shimoda, O. Zhou, Adv. Mater, 18, 1166 (2006).T. Nakanishi, T. Ando, J. Phys. Soc. Jpn., 78, 114708 (2009).

Electrical and Optical Properties of Sb-Doped BaSnO3

The electronic and optical properties of the cubic perovskite, BaSnO3, are compared with the well-known transparent conducting oxides (TCOs), SnO2 and In2O3. The optical band gaps of the undoped compounds, as measured by diffuse reflectance spectroscopy on powdered samples, are 3.1 eV for BaSnO3 and 3.8 eV for SnO2. Electronic structure calculations show that both compounds possess a large conduction band dispersion, which suggests that BaSnO3, like SnO2, should be a good TCO if it can be n-doped. To explore this possibility the properties of substitutionally doped, BaSn1–xSbxO3 samples were investigated. The electrical conductivity increases drastically for BaSn1–xSbxO3 samples with x ≤ 0.05, showing a transition from an insulating to a metallic state. For higher doping levels, 0.05 < x < 0.15, the conductivity saturates at 4 S cm–1. This is accompanied by saturation in the expansion of the cubic lattice parameter. The color of the pellets changed from white (x = 0) to bluish black (x = 0.15). This darkening originates from the formation of an intense and broad optical absorption band centered at 1200–1300 nm for the more highly doped samples. This absorption band spans both the visible and the near-infrared regions, resulting in a loss of transparency. Various spectroscopic techniques were used to elucidate the observed behavior. Mössbauer spectroscopy reveals the presence of mixed valent antimony, Sb3+(5s2)/Sb5+(5s0), as x increases. ESR spectra collected on a BaSn0.99Sb0.01O3 sample indicate that only ∼5% of unpaired electrons are present as delocalized carriers. The other 95% are localized to form a Sn3+ trapped electron center. To explain the tendency for carrier localization in doped BaSnO3 a mechanism based on strong electron–phonon interactions in perovskites containing cations with (n – 1)d10ns0 electronic configuration is proposed. This mechanism explains why BaSn1–xSbxO3 possesses relatively poor TCO characteristics, in sharp contrast to widely used TCO materials Sn1–xSbxO2 and In1–xSnxO1.5.

Structure–property correlations in lead silicate glasses and crystalline phases

Lead silicate glasses containing 40–65 mol% of PbO were prepared at two melt-quenching rates and characterized by X-ray diffraction, UV-Visible absorption spectroscopy, density, microhardness, thermo-mechanical analysis, differential scanning calorimetry and Raman scattering studies. On increasing the PbO concentration, density increases, glass transition temperature decreases and the optical absorption edge shifts towards longer wavelength. An intense optical absorption band was observed just below the absorption edge in glasses with 55 mol% and higher concentration of PbO. Dilatometric measurements show an unusual property that glasses do not show any abrupt increase in volume near the glass transition temperature but transform directly into the liquid state. Raman spectroscopy confirmed that the concentration of SiO4 tetrahedra containing one or more NBOs increase with PbO mol%. Devitrification studies on lead silicate glasses found that samples with 40–45 mol% of PbO do not crystallize, whereas samples with higher PbO concentration produce multiple crystalline phases like PbSiO3, Pb33Si24O81, Pb2SiO4 and Pb3Si2O7 on heat treatment.

A novel photofunctional g-C3N4/Ag3PO4 bulk heterojunction for decolorization of Rh.B

A novel photofunctional graphitic carbon nitride/silver phosphate (g-C3N4/Ag3PO4) bulk heterojunction has been reported for photodecolorization of Rhodamine B (Rh.B), which was synthesized step-by-step by the ion-impregnating method. The UV–vis diffuse reflectance spectra showed that the g-C3N4/Ag3PO4 bulk heterojunction has two intense optical absorption edges corresponding to g-C3N4 and Ag3PO4 in the visible light region. Photoluminescence spectra confirmed that the separation efficiency of photogenerated charges in the heterojunction was more intensive than pristine g-C3N4. Furthermore, the vectorial charge transfer at the g-C3N4/Ag3PO4 interface could be confirmed by photocurrent response. The g-C3N4/Ag3PO4 heterojunction with 5:1 weight ratio exhibited the highest photocatalytic activity for decolorization of Rh.B which was higher than that of pristine g-C3N4 and Ag3PO4.

The synthesis and spectral investigation of a novel highly water-soluble, aggregation-free antimony(V)-phthalocyanine absorbing light in optical therapeutical window

A novel antimony–phthalocyanine, [Sb(H3tsppc)(OH)2], where H3tsppc denotes monodeprotonated tetrakis{(2′,6′-dimethyl-4′-sulfonic acid)phenoxyl}phthalocyaninate, has been synthesized through sulfonation of [Sb(tppc)(OH)2]+ (tppc denotes tetrakis{(2',6'-dimethyl)phenoxyl}phthalocyaninate) in concentrated sulfuric acid. This compound is highly soluble in water (ca. 4×10−2M) without surfactant or alcohol. Moreover, it has been found free from aggregation in water up to almost 10−4M, unlike its copper and metal-free analogues, and show an intense optical absorption and emission band in optical therapeutical window (700–800nm). The axial hydroxyl groups play a crucial role in disaggregation of the antimony derivative in water.

Hybrid nanocomposite films for laser‐activated tissue bonding

We report new advancements in the biomedical exploitation of plasmonic nanoparticles as an effective platform for the photothermal repair of biological tissue. Chitosan films are loaded with gold nanorods with intense optical absorption in the "therapeutic window" of deepest light penetration through the body, and then activated by near infrared laser excitation to give adhesion with adjacent connective tissues. The adhesion consists of 0.07 mm(2) welds of ~20 kPa tensile strength at the film/tissue interface, which are obtained by administration of pulses with duration in the hundreds of millisecond timescale from a diode laser at ~130 J cm(-2). We investigate the adhesive effect as a function of pulse power and duration and identify an optimal operative window to achieve effective and reproducible welds with minimal detrimental superheating. These results may prove valuable to standardize laser bonding techniques and meet current needs for new knowledge which is urged by the penetration of nanotechnology into biomedical optics.

Intense optical absorption of defects created in Er3+-diffused layer in MgO (5 mol%)-doped LiNbO3crystal by local Er3+diffusion under Li-poor atmosphere

Abstract

Optical (DRUV-VIS) and magnetic (EPR) behavior of synthetic melanins

The properties of melanins prepared from L-dopa oxidation by both chemical and electrochemical methods are reported, searching for a material with more intense optical absorption in the visible region. The characterization of the samples have been done by cyclic voltammetry (CV), cronoamperometry (CA), Fourier transformed infrared spectroscopy (FTIR), electron paramagnetic resonance spectroscopy (EPR) and diffuse reflectance ultraviolet-visible spectroscopy (DRUV-VIS) techniques. The electrochemical method enhanced the organic free radical (spin) concentration and the absorption intensity in the visible region of the spectrum. The DRUV-VIS technique along with mathematical tools, such as the Kubelka-Munk remission function, were good options for the characterization of the final products.

Graphene oxide modified g-C3N4hybrid with enhanced photocatalytic capability under visible light irradiation

Graphene oxide modified g-C3N4 (GO/g-C3N4) with efficient photocatalytic capability under visible light irradiation was fabricated by sonochemical approach. Transmission electron microscopy images demonstrated that GO was two-dimensional sheets with chiffon-like ripples and the g-C3N4 possessed a layered structure. GO was overlaid on the surface of g-C3N4 in the GO/g-C3N4 hybrids. The UV–vis diffuse reflectance spectra showed that the GO/g-C3N4 hybrid had intense optical absorption in the visible light region. Photoluminescence spectra confirmed that the separation efficiency of photogenerated charge in GO/g-C3N4 was more intensive than pristine g-C3N4, indicating the GO acts as a separation centre and electron acceptor in the GO/g-C3N4 hybrid. The effective photogenerated charge separation efficiency lead to a remarkable improvement in the visible light photocatalysis. The pseudo-first-order kinetic constants of photocatalytic degradation of rhodamine B and 2, 4-dichlorophenol under visible light irradiation with GO/g-C3N4 were 3.80 and 2.08 times as large as that with pristine g-C3N4, respectively. This work indicates that the metal-free GO/g-C3N4 hybrid photocatalyst is a promising material in waste control, and GO could be an excellent material to combine with other semiconductors to make composites.

Holes bound as small polarons to acceptor defects in oxide materials: why are their thermalionization energies so high?

Holes bound to acceptor defects in oxide materials usually need comparatively highenergies, of the order of 0.5–1.0 eV, to be ionized thermally to the valence band maximum.It is discussed that this has to be attributed to the stabilization of such holes by mainlyshort range interactions with the surrounding lattice, leading to the formation of smallO − polarons. This is tantamount to the localization of the hole at only one of several equivalentoxygen ions next to the defect. The hole stabilizing energies can be determinedexperimentally from the related intense optical absorption bands. This paperexploits previous phenomenological studies of bound-hole small polarons in orderto account for the large hole stabilization energies on this basis. A compilationdemonstrates that bound-hole small polarons occur rather often in oxides and alsoin some related materials. The identification of such systems is based on EPRand optical studies and also on recent advanced electronic structure calculations.