Relaxation Dynamics Of Charge Carriers Research Articles

Dynamics and relaxation of charge carriers in poly(methylmethacrylate)-lithium salt based polymer electrolytes plasticized with ethylene carbonate

In this paper, we have studied the dynamics and relaxation of charge carriers in poly(methylmethacrylate)-lithium salt based polymer electrolytes plasticized with ethylene carbonate. Structural and thermal properties have been examined using X-ray diffraction and differential scanning calorimetry, respectively. We have analyzed the complex conductivity spectra by using power law model coupled with the contribution of electrode polarization at low frequencies and high temperatures. The temperature dependence of the ionic conductivity and crossover frequency exhibits Vogel-Tammann-Fulcher type behavior indicating a strong coupling between the ionic and the polymer chain segmental motions. The scaling of the ac conductivity indicates that relaxation dynamics of charge carriers follows a common mechanism for all temperatures and ethylene carbonate concentrations. The analysis of the ac conductivity also shows the existence of a nearly constant loss in these polymer electrolytes at low temperatures and high frequencies. The fraction of free anions and ion pairs in polymer electrolyte have been obtained from the analysis of Fourier transform infrared spectra. It is observed that these quantities influence the behavior of the composition dependence of the ionic conductivity.

Dielectric and conductivity relaxation in poly(3,4‐ethylenedioxythiophene) nanotubes

The charge carriers relaxation dynamics in synthesized poly(3,4‐ethylenedioxythiophene) nanotubes has been investigated as a function of frequency and temperature through the framework of dielectric permittivity, modulus formalism and impedance formalism. The fitting of experimental data of real part of dielectric permittivity and imaginary part of modulus with Havriliak‐Negami equation and Bergman modified Kohlrausch‐Williams‐Watts function indicates the presence of non‐Debye type relaxation process in the synthesized poly(3,4‐ethylenedioxythiophene) nanotubes. The scaling of imaginary part of modulus and impedance formalisms have been performed to understand the charge carrier relaxation dynamics in the synthesized samples, which suggests the presence of time temperature superposition principle in the nanotubes. The red shifting of the band from 1440 cm−1 to 1430 cm−1 in micro‐Raman spectra indicates the conformational transition from benzenoid to quinoid structure with increasing dopant concentration. The linear increase of imaginary part of dielectric permittivity with decreasing frequency is ascribed to the higher contribution from dc conductivity as compared to that from the electrode polarization effects. POLYM. ENG. SCI., 56:448–457, 2016. © 2016 Society of Plastics Engineers

Enhancing the carrier thermalization time in organometallic perovskites by halide mixing.

Hybrid metal-organic halide perovskites have recently attracted a great deal of attention because of their interesting electronic, optical and transport properties, which make them promising materials for high-performance, low-cost solar cells. Fundamental understanding of the formation mechanisms and dynamics of photoinduced charge carriers is essential for improving the performance of perovskite solar cell devices. For example, a significant amount of absorbed solar energy is lost as a result of carrier thermalization. This energy could be harnessed by extracting hot carriers before they cool down to the band edges. Although such hot carrier collection is experimentally challenging, theoretical investigations based on time-dependent methods can guide future experimental research by providing insights into the thermalization process. Here, we perform ab initio nonadiabatic molecular dynamics simulations to study non-radiative relaxation dynamics of charge carriers in hybrid halide perovskites. We find that the carrier relaxation time can be considerably increased by mixing halogen atoms in the perovskite materials. These findings show that simple approaches could be adopted to slow down the thermalization process of hot carriers in perovskite materials.

How photon pump fluence changes the charge carrier relaxation mechanism in an organic-inorganic hybrid lead triiodide perovskite.

This study explores the excitation wavelength and fluence dependence of processes occurring in formamidinium lead triiodide (FAPbI3) film using time-resolved transient absorption and terahertz spectroscopies. The results indicate that second-order processes are responsible for charge carrier recombination at low fluences of the absorbed photons (below 8.4 × 1012 ph per cm2). An increase in fluence leads to the appearance and successive reduction of the time component assigned to the Auger recombination of free charge carriers (240-120 ps). Simultaneously, the bimolecular recombination time decreases from ∼1400 to ∼700 ps. Further increasing the pump fluence produces an exciton population that recombines in 6 ps. The comparison of two characteristic bleaching bands located at 480 and 775 nm provides evidence for the validity of the two valence bands model. Excitation with higher fluences results in a marked difference in the probed dynamics at these bands, reflecting the action of two excited states at the conduction band. Our results demonstrate that a single model cannot be applied in characterizing the perovskite absorber transitions at all pump fluences. These findings are relevant in understanding their operating mechanism under specific experimental conditions, which should differ for perovskite based solar cells, lasing media or photon detectors.

Structural Properties, Conductivity, Dielectric Studiesand Modulus Formulation of Ni Modified ZnO Nanoparticles

In the present manuscript, we have reported the structural, ac conductivity, dielectric studies and modulus formulation of Zn$_{1-x}$Ni$_x$O $(x=0.00,0.10,0.20,0.30)$ nanoparticles synthesized by sol-gel technique. The Rietveld refinement of X-ray powder diffraction data reveals that the samples possess hexagonal structure (space group - P63mc) of ZnO. The low intensity diffraction peaks corresponding to NiO comes into existence, it is suggested that phase segregation has occurred in the Ni doped ZnO nanoparticles. Average crystallite size has been estimated from XRD patterns, which is found to increase with increase in Ni content. AC conductivity and dielectric properties of Ni doped ZnO nanoparticles have been studied as a function of frequency (100 Hz to 10 MHz) of the applied ac signal in the temperature range of 323K to 463 K. The results showed that ac conductivity increases while other parameters such as the real dielectric constant $\varepsilon'$ and dielectric loss $\tan\delta$ decreases with frequency of applied field. The values of parameters $\varepsilon'$, $\tan\delta$ and ac conductivity are found to increase with increase temperature. In all the samples small polaron conduction mechanism is predominant. The variation of $\varepsilon'$, $\tan\delta$ and ac conductivity is observed to increase with increase in Ni dopant concentration in ZnO. The results of electrical modulus to study the relaxation dynamics of charge carriers indicate the presence of non-Debye type of relaxation in present samples.

Charge-carrier relaxation dynamics and ac conductivity scaling of poly(3,4-ethylenedioxythiophene) nanoparticles

An investigation of the dielectric characteristics of poly(3,4-ethylenedioxythiophene) nanoparticles has been conducted in the frequency range 20 Hz–2 MHz and the temperature range of 80–300 K through the studies of dielectric permittivity, modulus formalism and ac conductivity. High-resolution transmission electron micrographs confirm the formation of well-ordered spherical nanoparticles. Micro-Raman spectra confirm the conformational transition from benzenoid to quinoid structure with increasing dopant concentration. Linear variation of imaginary permittivity with frequency suggests that the dc conduction process is more significant than interfacial polarisation. The presence of single relaxation peak in the imaginary modulus formalism indicates the Debye-type relaxation. The decrease of frequency exponent with temperature confirms that the charge transport takes place through correlated barrier hopping mechanism. The scaling of ac conductivity spectra confirms that the relaxation dynamics of charge carriers in the poly(3,4-ethylenedioxythiophene) nanoparticles system is temperature independent.

Crystal structure refinement, dielectric and magnetic properties of Ca/Pb substituted SrFe12O19 hexaferrites

SrFe12O19 (SFO), Sr0.5Ca0.5Fe12O19 (SCFO) and Sr0.5Pb0.5Fe12O19 (SPFO) hexaferrites have been synthesized by a conventional solid state reaction technique. Powder X-ray diffraction and Rietveld refinement confirm the presence of M-type hexagonal phase in prepared samples. However in SCFO, secondary phase was also present with main phase. Analysis of Nyquist's plots of SFO hexaferrite revealed the contribution of many electrically active regions corresponding to bulk mechanism, distribution of grain boundaries and electrode processes also. Both conductivity and electric modulus formalisms have been employed to study the relaxation dynamics of charge carriers. A perfect overlapping of the normalized plots of modulus isotherms on a single ‘super curve’ for all the studied temperatures reveals a temperature independence of dynamic processes involved in conduction and for relaxation. In SPFO sample coercivity is reduced effectively but accompanied with increase in magnetization, which is requirement for hexaferrites to be used as magnetic recording media.

The influence of surface passivation on electronic energy relaxation dynamics of CdSe and CdSe/CdS nanocrystals studied using visible and near infrared transient absorption spectroscopy.

Charge carrier relaxation dynamics of electronically excited CdSe and CdSe/CdS core/shell nanocrystals (NCs) were studied using femtosecond time-resolved transient absorption spectroscopy, employing both visible and near-infrared (NIR) probe laser pulses. Following 400 nm excitation, the combination of visible and NIR laser probe pulses were used to determine the influence of surface passivation on electronic relaxation dynamics for nanocrystals overcoated with either organic ligands or inorganic semiconductors. In particular, low-energy NIR photons were used to isolate transient absorption signals due to either electron and hole intraband transitions. Four relaxation components were detected for CdSe NCs passivated by organic molecules: (1) picosecond hole relaxation; (2) electron deep trapping; (3) electron surface trapping; and (4) exciton radiative recombination. Based on TA data collected over a broad energy range, electron deep trapping at Se(2-) sites was suppressed for CdSe NCs passivated by inorganic (CdS) semiconducting materials. By comparing the time-dependent transient absorption data of a series of CdSe/CdS NCs with different shell thicknesses, evidence for the transition from Type-I to quasi Type-II NCs was obtained. These data illustrate the sensitivity of femtosecond time-resolved transient absorption measurements carried out over visible and near infrared probe energies for determining the influence of nanocrystal structure on electronic relaxation dynamics.

Excitation-Dependent Ultrafast Carrier Dynamics of Colloidal TiO2 Nanorods in Organic Solvent

The relaxation dynamics of charge carriers of organic capped TiO2 nanorods dispersed in chloroform was investigated by femtosecond transient absorption in a weak-excitation regime. Anisotropic TiO2 nanocrystals were excited in the UV–vis range, using different pump wavelengths, namely above (300 nm), close to (350 nm), and below (430 nm) the direct band gap of anatase TiO2. We show that the ultrafast dynamics strongly depends on excitation wavelength and determine the time constants of all the processes entering the relaxation. Moreover, we demonstrate that two transient absorption bands at 500 and 700 nm, typically attributed to trapped h+ and e–, respectively, are accessible only when TiO2 is photoexcited well above the band gap, while there is no evidence of such bands when TiO2 is photoexcited close to or below its band gap. In such cases the observed dynamics are attributed to trapped excitons.

Dielectric response of Al-substituted multiferroic TbMnO3 at high temperatures

Multiferroic Tb1−xAlxMnO3 (x=0, 0.1, 0.2) was prepared using the standard solid-state reaction. The dielectric properties of these samples were investigated over wide ranges of frequencies and temperatures (T≥300K) by means of complex impedance spectroscopy. The isovalent substitution of Al3+ for Tb3+ distinctly influences the structural and dielectric properties of the parent TbMnO3. The conductivity data of the undoped and Al-doped samples fit well to Jonscher׳s law σac(ω)=σdc+Aωn. The resulting fitting parameters indicated that the hopping process occured between neighboring sites. The conductivity in the dc regime followed an Arrhenius relation with activation energies of 0.26 and 0.12eV for undoped and Al-doped (x=0.1) samples, respectively. In turn, the ac conductivity was well described by the small polaron hopping model, with energies of 0.2 and 0.14eV for undoped and Al-doped (x=0.1) samples, respectively. The real part of the dielectric permittivity (ε′) increased with increasing temperature and lowering frequency. The value of ε′ also increased with the Al doping. The occurrence of a non-Debye-type relaxation was verified for the studied samples. The relaxation dynamics of charge carriers in the samples was examined within the electric modulus formalism, which allowed determining the most probable relaxation time and the respective activation energy for the dielectric relaxation. In the temperature range 300–425K, the activation energy for the dielectric relaxation was calculated as 0.25eV and 0.16eV for undoped and Al-doped (x=0.1) samples, respectively. The imperfect overlapping of the reduced plots of the modulus curves on a single master curve, particularly at higher frequencies, for all the temperatures and Al concentrations considered, suggests that the behavior of the dynamic processes is slightly temperature- and Al-content-dependent. Finally, the impedance spectra, characterized by the appearance of semicircle arcs, were well modeled in terms of equivalent electrical circuits.

Role of aluminum ions on the dielectric and conducting properties of multiferroic Tb1−xAlxMnO3: Study at high temperatures

Dielectric and conducting properties of Tb1−xAlxMnO3 (x = 0, 0.05) synthesized by the solid–state reaction method have been investigated. The Al ion has the same valence as substituted Tb but is nonmagnetic and its small size gives rise to microstructural strain which affects the multiferroic properties of the parent compound. Samples were characterized by means of complex impedance spectroscopy (CIS) in the frequency range from 40 Hz to 5 MHz, at temperatures above room temperature. The conductivity curves for the two samples are well fitted by the Jonscher law σ(ω) = σdc + Aωn. Results of the fitting procedure indicate that for the studied samples, the hopping motion involves localized hopping without the species leaving the neighbors. Frequency dependence of the dielectric constant (ε″) and tangent loss (tan δ) display a dispersive behavior at low frequencies that can be explained on the basis of the Maxwell–Wagner model and Koop's theory. The relaxation dynamics of charge carriers has been studied by means of the electric modulus formalism. In turn, the variation of the imaginary part of the complex impedance, Z″, shows a peak at a relaxation angular frequency (ωr) related to the relaxation time (τ) by τ = 1/ωr. The complex impedance spectra (Nyquist plots) show well-defined semicircles which are strongly dependent on the Al-doping level and temperature. The complex impedance data have been modeled using electrical equivalent circuits.

Facile and Green Synthesis of CdSe Quantum Dots in Protein Matrix: Tuning of Morphology and Optical Properties

Herein, we have demonstrated a facile and green approach for the synthesis of Cadmium selenide (CdSe) quantum dots (QDs). The process was mediated by bovine serum albumin (BSA) and it was found that BSA plays the dual role of reducing agent as well as a stabilizing agent. The QDs exhibited sharp excitonic absorption features at ~500 nm and subsequently showed reasonably good photoluminescence (PL) at room temperature. The PL is seen to be strongly dependent on the concentration of the precursors and hence, the luminescence of these QDs could be conveniently tuned across the visible spectrum simply by varying molar ratio of the precursors. It can be envisaged from the fact that a red-shift of about 100 nm in the PL peak position was observed when the molar ratio of the precursors ([Cd2+]:[Se2-], in mM) was varied from 10:5 to 10:40. Subsequently, the charge carrier relaxation dynamics associated with the different molar ratio of precursors has been investigated and very interesting information regarding the energy level structures of these QDs were revealed. Most importantly, in conjunction with the optical tuning, the nanomorphology of these nanoparticles was found to vary with the change in molar ratios of Se and Cd precursors. This aspect can provide a new direction of controlling the shape of CdSe nanoparticles. The possible mechanism of the formation as well as for the shape variation of these nanoparticles with the molar ratios of precursors has been proposed, taking into account the role of amino acid residues (present in BSA). Moreover, the QDs were water soluble and possessed fairly good colloidal stability therefore, can have potential applications in catalysis and bio-labeling. On the whole, the present methodology of protein assisted synthesis is relatively new especially for semiconducting nanomaterials and may provide some unique and interesting aspects to control and fine tune the morphology vis-à-vis, their optical properties.

Structural and electric properties of La0.7Sr0.25Na0.05Mn0.9Ti0.1O3 ceramics

This work studies structural and various electrical properties of polycrystalline La0.7Sr0.25 Na0.05 Mn0.9Ti0.1O3 (LSNMTi), which were prepared by standard solid state reaction technique. The formation of a single phase rhombohedral structure of the composition was confirmed by the X-ray diffraction study. The electrical behavior of sintered pellets investigated by impedance spectra has shown frequency dependent behavior. Both conductivity and electric modulus formalisms have been used to study the relaxation dynamics of charge carriers. The variation of ac conductivity with frequency at different temperatures obeys the universal Jonscher׳s power law (σacαw).

Li+ transport properties of W substituted Li7La3Zr2O12 cubic lithium garnets

Lithium garnet Li7La3Zr2O12 (LLZ) sintered at 1230 °C has received considerable importance in recent times as result of its high total (bulk + grain boundary) ionic conductivity of 5 × 10−4 S cm−1 at room temperature. In this work we report Li+ transport process of Li7−2xLa3Zr2−xWxO12 (x = 0.3, 0.5) cubic lithium garnets. Among the investigated compounds, Li6.4La3Zr1.7W0.3O12 sintered relatively at lower temperature 1100 °C exhibits highest room temperature (30 °C) total (bulk + grain boundary) ionic conductivity of 7.89 × 10−4 S cm−1. The temperature dependencies of the bulk conductivity and relaxation frequency in the bulk are governed by the same activation energy. Scaling the conductivity spectra for both Li6.4La3Zr1.7W0.3O12 and Li6La3Zr1.5W0.5O12 sample at different temperatures merges on a single curve, which implies that the relaxation dynamics of charge carriers is independent of temperature. The shape of the imaginary part of the modulus spectra suggests that the relaxation processes are non-Debye in nature. The present studies supports the prediction of optimum Li+ concentration required for the highest room temperature Li+ conductivity in LixLa3M2O12 is around x = 6.4 ± 0.1.

Impedance Spectroscopy Properties of Pr0.67A0.33MnO3 (A = Ba or Sr) Perovskites

We have investigated the dielectric properties of Pr0.67Ba0.33MnO3 (PBMO) and Pr0.67Sr0.33MnO3 (PSMO) perovskites synthesized by the solid-state reaction method at 1473 K. Samples were characterized by complex impedance spectroscopy (CIS) in the frequency range from 40 Hz to 1 MHz, at room temperature. The conductivity curves for the two samples were well fitted by the Jonscher law σ(ω)=σ dc +Aω n . For the PBMO sample, the hopping process occurs at long distance, whereas for PSMO compound it occurs between neighboring sites. Frequency dependence of dielectric constant (e″) and tangent loss (tanδ) show a dispersive behavior at low frequencies that was explained on the basis of the Maxwell–Wagner model and Koop’s theory. Electric modulus formalism has been employed to study the relaxation dynamics of charge carriers. For both compounds, the variation of the imaginary part Z″ shows a peak at a relaxation angular frequency (ω r ) related to the relaxation time (τ) by τ=1/ω r . Nyquist plots of impedance show the presence of two semicircles and an electrical equivalent circuit has been proposed to explain the impedance results.

Rietveld refinement and impedance spectroscopy of calcium titanate

Single phase perovskite CaTiO3 has been synthesized by conventional solid state reaction technique. The ceramic was characterized by XRD at room temperature and its Rietveld refinement inferred orthorhombic crystal structure with the space group Pbnm. The field dependence of dielectric relaxation and conductivity was measured over a wide frequency range from room temperature to 673 K. Analysis of Nyquist plots of CaTiO3 revealed the contribution of many electrically active regions corresponding to bulk mechanism, distribution of grain boundaries and electrode processes. The dc conductivity depicted a semiconductor to metal type transition. Frequency dependence of dielectric constant (ε′) and tangent loss (tan δ) show a dispersive behavior at low frequencies and is explained on basis of Maxwell-Wagner model and Koop's theory. Both conductivity and electric modulus formalisms have been employed to study the relaxation dynamics of charge carriers. The variation of ac conductivity with frequency at different temperatures obeys the universal Jonscher's power law (σacα ωs). The values of exponent ‘s’ lie in the range 0.13 ≤ s ≤ 0.33, which in light of CBH model suggest a large polaron hopping type of conduction mechanism.

Ultrafast Charge Carrier Dynamics and Photoelectrochemical Properties of Hydrogen-treated TiO2 Nanowire Arrays

ABSTRACTHere we report studies of photoelectrochemical (PEC) properties and ultrafast charge carrier relaxation dynamics of hydrogen-treated TiO2 (H:TiO2) nanowire arrays. PEC measurements showed the photocurrent density of the H:TiO2 was approximately double that of TiO2, attributed to increased donor density due to the formation of oxygen vacancies in H:TiO2 due to hydrogen treatment Charge carrier dynamics of H:TiO2, measured using fs transient absorption spectroscopy, showed a fast decay of ∼20 ps followed by slower decay persisting to tens of picoseconds. The fast decay is attributed to bandedge electron-hole recombination and the slower decay is attributed to recombination from trap states. Visible absorption is attributed to either electronic transitions from the valence band to oxygen vacancy states or from oxygen vacancy states to the conduction band of the TiO2, which is supported by incident photon to current conversion efficiency (IPCE) data. H:TiO2 represents a unique material with improved photoelectrochemical properties for applications including PEC water splitting, solar cells, and photocatalysis.

Electrical conductivity and complex electric modulus of titanium doped nickel–zinc ferrites

The samples Ni 1+ x− y Zn y Ti x Fe 2−2 x O 4; y=0.1, 0.0≤ x≤0.5 were prepared in a single-phase spinel structure as indicated from X-ray analysis. Electrical conductivity and dielectric measurements at different temperatures from 300 K to 600 K in the frequency range from 42 Hz to 5 MHz have been analyzed. The relation of conductivity with temperature revealed a semiconductor to semimetallic behavior as Ti 4+ concentration increases. The conduction mechanism depends mainly on the valence exchange between the different metal ions in the same site or in different sites. The dielectric constant as a function of temperature and frequency showed that there is a strong dependence on the compositional parameter x. The electrical modulus has been employed to study the relaxation dynamics of charge carriers. The result indicates the presence of correlation between motions of mobile ion charges. The activation energies extracted from M′( ω) and M″( ω) peaks are found to follow the Arrhenius law. The electrical conductance of the samples found to be dependent on the temperature and frequency.

Charge carrier dynamics and relaxation in (polyethylene oxide-lithium-salt)-based polymer electrolyte containing 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide as ionic liquid

In this paper we report the dynamics of charge carriers and relaxation in polymer electrolytes based on polyethylene oxide (PEO), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMPTFSI) ionic liquid prepared by solution cast technique. It has been observed that the incorporation of BMPTFSI into PEO-LiTFSI electrolyte is an effective way for increasing the amorphous phase to a large extent. It has also been observed that both the glass transition and melting temperatures decrease with the increase of BMPTFSI concentration. The ionic conductivity of these polymer electrolytes increases with the increase of BMPTFSI concentration. The highest ionic conductivity obtained at 25 °C is ~3×10(-4) S cm(-1) for the electrolyte containing 60 wt % BMPTFSI and ethylene oxide (EO)/Li ratio of 20. The temperature dependence of the dc conductivity and the hopping frequency show Vogel-Tamman-Fulcher type behavior indicating a strong coupling between the ionic and the polymer chain segmental motions. The frequency dependence of the ac conductivity exhibits a power law with an exponent n which decreases with the increase of temperature. The scaling of the ac conductivity indicates that relaxation dynamics of charge carriers follows a common mechanism for all temperatures and BMPTFSI concentrations. We have also presented the electric modulus data which have been analyzed in the framework of a Havriliak-Negami equation and the shape parameters obtained by the analysis show slight temperature dependence, but change sharply with BMPTFSI concentration. The stretched exponent β obtained from Kohlrausch-Williams-Watts fit to the modulus data is much lower than unity signifying that the relaxation is highly nonexponential. The decay function obtained from analysis of experimental modulus data is highly asymmetric with time.

Influence of SiO 2 on dispersive conductivity and absorption edge of calcium bismuthate glasses

The dynamics of Ca 2+ ions and the optical properties of calcium bismuth silicate glasses were investigated using relaxation spectroscopy and optical absorption spectra. These glasses were prepared by normal melt quench technique. Density, molar volume and glass transition temperature (T g) have been measured. A large difference in the values of T g and crystalline temperature (T C) suggests the thermal stability of these glasses. The ac and dc conductivities, activation energy for dc conduction (E dc) and for relaxation frequency (E τ) were extracted from the impedance spectra. The compositional variation in conductivity has been attributed to the presence of mixed glass former effect in these glasses. Both electric modulus and the conductivity formalism have been employed to study the relaxation dynamics of charge carriers in these glasses. A single ‘master curve’ for normalized plots of all the modulus isotherms observed for a given composition indicates that the dynamic processes for ions in these glasses are temperature independent. Similar values of E dc and E τ indicate that the ions have to overcome the energy barrier of same height in their conduction as well as relaxation. The observed conductivity spectra follows power law with exponent ‘s’ which increases regularly with frequency and approaches unity at higher frequencies. It has been observed that the position of the absorption edge and cutoff wavelength shift towards blue as Bi 2O 3/SiO 2 ratio decreases. The compositional variation in optical band gap (E g) has been attributed to the influence of the CaO and SiO 2 on conduction band. The values of Urbach energy confirm the existence of phonon-assisted indirect transitions in the present glass system. The average electronic polarizability of oxide ion (α O 2−) and the optical basicity (Λ th) have been estimated from the calculated values of the optical band gap and were found to be dependent directly on Bi 2O 3/SiO 2 ratio.