Urbach Band Tail Research Articles

Absorption edge shift and broadening in nanostructured Al doped ZnO thin films

Samples of ZnO were prepared by sol-gel and deposited by dip-coating. The influence of synthesis conditions on the optical properties were studied in different sets of ZnO films, synthesized by alternately varying the following parameters: addition of additives to the precursor solution, Al doping percentage and number of layers. The optical properties of the obtained films were studied by transmittance, being typically >90 % at 600 nm. The absorption edge for undoped samples showed a structure due to exciton formation at room temperature. The bandgap energy Eg, was between (3.227 ± 0.010) eV and (3.275 ± 0.010) eV for undoped samples, increasing to (3.352 ± 0.010) eV for Al doped ones (10 % Al/Zn in solution with additives). For intermediate 5 % doping the mean bandgap energy was (3.315 ± 0.015) eV. A similar value (3.320 eV ± 0.010) eV was obtained for 10 % Al/Zn when no additives were included. Doped samples showed a smoother absorption edge. This edge shape evolution was studied by Urbach band tail analysis. The Urbach band tail parameter EU increased with doping, varying from 30 meV to 90 meV and increasing as Eg increases. This correlation describes the influence of impurity states in the structure and optical properties of the material.

Influence of Ancillary Ligand on the Photosensitization and Photophysical Properties of Red Luminescent Tri-fluorinated Β-diketonate Eu(III) Complexes.

Highly emissive ternary Eu(III) complexes were synthesized with a tri-fluorinated β-diketone as principal ligand and heterocyclic aromatic compounds as ancillary ligands to assess their utility as an illuminating material for display devices and other optoelectronics.The general characterizations, regarding the coordinating facets of complexes were accomplished via various spectroscopic techniques. Thermal stability was investigated via TGA/DTA. Photophysical analysis was accomplished by PL studies, Band gap value, color parameters and J-O analysis. DFT calculations were performed adopting geometrically optimized structure of complexes.Superb thermal stability has been achieved in complexes, which decides their concrete candidature for display devices. The bright red luminescence of complexes is ascribed to 5D0 → 7F2 transition of Eu(III) ion. Colorimetric parameters unlocked the applicability of complexes as warm light source and J-O parameters adequately summarized the coordinating surrounding around the metal ion. Various radiative properties were also evaluated which suggested the prospective use of complexes in lasers and other optoelectronic devices. The band gap and Urbach band tail, procured from absorption spectra, revealed the semiconducting behavior of synthesized complexes. DFT studies rendered the energies of FMO and various other molecular parameters.It can be summarized from the photophysical and optical analysis of synthesized complexes that these complexes are virtuous luminescent materials and possess potentiality to be used in diverse domain of display devices.

Utilization of Judd-Ofelt theory to assess the photophysical properties of β-keto carboxylate Tb(III) complexes with heterocyclic secondary sensitizer

One binary and five ternary green glowing Tb(III) complexes with β-keto acid as primary sensitizer and heterocyclic aromatic system as secondary sensitizer, were synthesised by an eco-friendly method and characterized via various spectroscopic techniques. The assessment of photoluminescent properties in solid complexes and in solution phase with DMSO as solvent, revealed the bright green emission of complexes on irradiation of UV light due to 5D4 → 7F5 transition, whose intensity persistently increased on insertion of secondary sensitizer. Higher relative quantum yield in ternary complexes communicated the contribution of secondary sensitizers in enhancement of luminescence. Judd-Ofelt parameters (Ω2, Ω4, Ω6) were evaluated from NIR absorption spectra of complexes. Higher Ω2 value in complexes demonstrated the higher covalence character and asymmetry around Tb(III) ion. Radiative, nonradiative transition probabilities, radiative life time, branching ratios and luminescence efficiency were also estimated. Complex T6 reported to have 72.5% efficiency which signalizes superb luminescence performance of this complex. Colorimetric parameters were obtained in solid as well as solution form. Exquisite color purity and CCT value > 4000 K established these complexes as cold green light source. Band gap energy and Urbach band tail, obtained via nonlinear fitting of absorption coefficient, turned out to be in semiconductor range thus can have materiality in various photonic devices. The large value of stimulated emission cross section and narrow full width at half maxima (FWHM) of 5D4 → 7F5 emission peak suggested the prospective use of synthesised complexes as laser media for green color lasers.

Judd-Ofelt, optical and photophysical analysis of β-ketocarboxylate Sm(III) complexes with N-donor aromatic system as secondary sensitizers

Reddish orange, Sm(III) complexes were synthesised by means of environmentally sustainable grinding method with β-keto carboxylic acid and heterocyclic aromatic systems. The photoluminescent studies revealed that 4G5/2 → 6H7/2 transition (608 nm) is responsible for the reddish orange emission of complexes when irradiated with UV light. Judd-Ofelt theory was applied to NIR absorption spectra to obtain three intensity parameters (Ω2, Ω4, Ω6) from oscillator strength. The higher Ω2 values obtained in complexes indicate greater covalence character. To assess luminescence performance, the radiative, nonradiative transition probability, radiative life time and intrinsic quantum yield were also evaluated. High quantum yield obtained for complexes signalises the superior luminescent behaviour. Exquisite color purity (89.8–96.2%) and CCT <3200 K establish these complexes as warm orange light source. The experimental data of absorption coefficient was fitted nonlinearly with Tauc's relation to obtain band gap energy. Two band gaps were reported in each complex hence the synthesised complexes can have materiality in solar cell and photovoltaic devices. The Urbach band tail width was also obtained through fitting with a new empirical relation which affirms the semiconducting properties of complexes. The investigation of antimicrobial and antioxidant properties of complexes ascertained the complexes as potent antibacterial and antioxidant agents.

Optical and photophysical portrayal of Sm3+ complexes possessing two band gaps for relevance in solar cells and photovoltaic devices

Octa-coordinated samarium (III) complexes were synthesised using a fluorinated β-keto carboxylic acid (L) as prime ligand and N-donor systems as auxillary ligands, by ecologically desirable grinding method. The photoluminescent properties were investigated in powder and solution form. The characteristic emission at ∼605 nm (solid state) and ∼599 (solution phase) relative to 4G5/2 → 6H7/2 transition is accountable for the bright vermillion color of complexes. Quantum yield and colorimetric parameters were obtained for solid and solution state. CCT and color purity value characterise these complexes as warm orange light source. Reflectance spectra of solid complexes were fitted nonlinearly to procure optical band gap and Urbach band tail width. Two band gaps were reported in each complex, hence can have materiality in solar cell and photovoltaic devices. The FWHM obtained for most emissive transition (4G5/2 → 6H7/2) is found to be comparable with Nd-doped fluorophosphates glass. Energy transfer pathway studied for all the synthesised complexes, affirmed the effective energy transfer from ligands to Sm3+ ion. Investigation of antimicrobial behaviour of complexes revealed either higher or similar activity with respect to the ligand.

Vanadyl doped Li-zinc borate glasses: Optical and ESR study

Glasses with the composition xV2O5-(15-x) Li2O-70B2O3-15ZnO, for x varies from 0 to 4 mol. %, were synthesized using standard quenching technique. The amorphous nature for selected samples (x= 0, 1 and 4 mol. %) was ensured from X-ray diffraction (XRD) measurements. The samples were found to compose mainly from BO3 and BO4 basic structural units as inferred from Fourier transform infrared (FTIR) data. The number of BO4 (BO3) structural units was found to decrease (increase) with increasing the number of NBOs. These structural modifications within the glass network are responsible for the here observed composition dependence optical energy gap (Eotp) and Urbach band tail (ΔE). The electron spin resonance (ESR) results revealed a tetragonal compression for VO2+ ions present in octahedral coordination, with distinct optical transitions in the visible spectral regime.

Optical absorption of Nd2O3-Doped polyvinylidene fluoride films

Optical absorbance measurement is a powerful tool for studying the optical properties of polymer materials. In this work, the optical properties of neodymium-doped polyvinylidene fluoride (PVDF/Nd) samples were studied at room temperature. The results show that addition of the Nd to the PVDF matrix enhances the optical transmittance up to 8.0%, and decreases it in the UV region for PVDF/Nd 1.0%. The absorption edge changes with an increase of neodymium oxide (Nd) content in the PVDF matrix. For pure PVDF, the extinction coefficient decreases from 200 to 230 nm and, after increasing, presents a peak at 347 nm, which is related to F–C bonds. For other compositions, Nd addition diminishes the intensity of the characteristic peak of PVDF. For all samples, the extinction coefficient value reduces quickly till 230 nm, and after that increases with an increase of wavelength. This effect might be related to optical absorption by spherulites and the Nd grain boundary. Skin depth decreases as a function of photon energy for all PVDF samples. The bandgap energy presents an oscillatory behavior that might be related to the defects formed by the addition of Nd. The Urbach energy and band tailing parameter follow the behavior of the bandgap energy. In summary, the results obtained in this work are in good agreement with those reported in the literature, and they are essential for the optical and photonic application of PVDF/Nd doped samples.

Enhancing Optical Properties of Modified PVC and Cr2O3 Nanocomposite

In this work, a novel grafting of chromium (III) oxide Cr2O3 nanoparticles (NPs) within polyvinyl chloride (PVC) was conducted to obtain high conductivity and stability toward the light absorption for the polymer. The optical properties of pure and doped PVC thin films were investigated with different content to embed chromium (III) oxide. The measurements of spectrophotometer reflectance in the wavelengths range of 250–1300 nm and UV–Visible in the wavelengths range of 200–800 nm were reported. Adding 0.05 g of Cr2O3 NPs leaded to improve all the optical properties of PVC–Cr2O3 films. The optical features calculations were determined, namely absorbance, reflectance, transmittance, extinction coefficient, refractive index, optical density, real and imaginary of dielectric constant, direct and indirect energy gaps, Urbach energy, and optical conductivity. Two Schiff bases were synthesized and used for doping, identified as a, and b. The energy gaps of modified PVC films were calculated and found to decrease from 5.5 eV of pure PVC to 2.4 and 3.2 eV for a and b samples, respectively, in the direct allowed transition. In the indirect allowed transition, lower values were obtained, where the pure PVC gab was 4.2 eV and decreased to 1.75 and 3.3 eV for a and b, respectively. The Urbach band tail was also computed for thin films and concluded in a table. For our best knowledge, doping Cr2O3 NPs in the PVC is firstly reported here, which aimed to reinforced the polymer and reduced its energy gap. The results detected that the photostability of doped PVC is better than the pure PVC.

Control of the Urbach band tail and interband dephasing time with post-growth annealing in low-temperature-grown GaAs

Femtosecond four-wave mixing experiments on low-temperature-grown (LT-) GaAs for a range of post-growth annealing temperatures indicate that the Urbach band tail abruptly diminishes above 550°C due to the conversion of As-related point defects to As clusters and that the interband dephasing time is limited by scattering with As point defects for annealing temperatures below 550°C. In addition, we observe a complex interplay of polarization source terms associated with the exciton and Urbach band tail for annealing temperatures below 550°C. These experiments shed light on the carrier dynamics and ultrafast nonlinear optical properties of LT-GaAs.

Radiative Efficiency Limit with Band Tailing Exceeds 30% for Quantum Dot Solar Cells

Thin films of colloidal quantum dots (QDs) are promising solar photovoltaic (PV) absorbers in spite of their disordered nature. Disordered PV materials face a power conversion efficiency limit lower than the ideal Shockley–Queisser bound because of increased radiative recombination through band-tail states. However, investigations of band tailing in QD solar cells have been largely restricted to indirect measurements, leaving their ultimate efficiency in question. Here we use photothermal deflection spectroscopy (PDS) to robustly characterize the absorption edge of lead sulfide (PbS) QD films for different bandgaps, ligands, and processing conditions used in leading devices. We also present a comprehensive overview of band tailing in many commercial and emerging PV technologies—including c-Si, GaAs, a-Si:H, CdTe, CIGS, and perovskites—then calculate detailed-balance efficiency limits incorporating Urbach band tailing for each technology. Our PDS measurements on PbS QDs show sharp exponential band tails, w...

Observation of the exciton and Urbach band tail in low-temperature-grown GaAs using four-wave mixing spectroscopy

Four-wave mixing (FWM) spectroscopy reveals clear signatures associated with the exciton, free carrier inter-band transitions, and the Urbach band tail in low-temperature-grown GaAs, providing a direct measure of the effective band gap as well as insight into the influence of disorder on the electronic structure. The ability to detect (and resolve) these contributions, in contrast to linear spectroscopy, is due to an enhanced sensitivity of FWM to the optical joint density of states and to many-body effects. Our experiments demonstrate the power of FWM for studying the near-band-edge optical properties and coherent carrier dynamics in low-temperature-grown semiconductors.

Spray pyrolysed Cu2ZnSnS4 absorbing layer: A potential candidate for photovoltaic applications

Cu2ZnSnS4 thin films have been successfully deposited onto ultra clean glass substrates using spray pyrolysis technique. The characterizations of these films regarding structure, morphology, optical absorption and electrical transport were carried out using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM, Optical absorption spectroscopy and current–voltage measurements. XRD, TEM and HRTEM showed that the samples have a tetragonal structure and a polycrystalline nature with an average crystallite size of ∼3.4nm. SEM analysis reveals a compact surface morphology with slightly rough surface. Optical measurements showed a direct band gap of 1.56eV and absorption coefficient >104cm−1, which are optimal for low cost solar cells. The weak absorption below the band gap edge was observed and described by Urbach band tail rule. Dc electrical resistivity of films, in temperature range 10–300K, was found to arise due to variable range hopping conduction mechanism. Using the percolation theory and the diffusion model, the density of states near the Fermi level was calculated. Photoluminescence (PL) spectra exhibit the presence of broad emission band at 1.33eV.

Investigations on electrical, magnetic and optical behaviors of five-layered Aurivillius Bi6Ti3Fe2O18 polycrystalline films

Five-layered Aurivillius Bi6Ti3Fe2O18 (BTF2) polycrystalline thin films were prepared by a chemical solution deposition route. The crystalline structures and microstructures were characterized by X-ray diffractometer, micro-Raman spectrometer and atomic force microscopy. Its electrical and magnetic properties were investigated, and especially optical properties were addressed in detail. Coexistence of the ferroelectric and weak ferromagnetic properties indicated the multiferroic behavior of the BTF2 films. The optical dielectric functions in the photon energy range of 1.13–4.13eV have been extracted by fitting the experimental data in the light of the Tauc–Lorentz dispersion model. A direct interband transition with bandgap energy ~3.72eV was determined for the BTF2 films. The weak absorption below band-gap edge was explained by the Urbach band tail rule. Finally, the optical dispersion behaviors of the BTF2 films in the transparent oscillating region were described using a single oscillator model.

Optical investigation on cadmium-doped zinc oxide nanoparticles synthesized by using a sonochemical method

Cadmium-doped zinc oxide CdxZn1−xO (x = 0, 0.05, 0.15 and 0.40) nanoparticles were synthesized by using a sonochemical method. The X-ray diffraction (XRD) patterns show that CdxZn1−xO nanoparticles keep a wurtzite structure up to x = 0.40. However, the lattice parameters a, c, and internal parameter u gradually increase with increasing x. The Raman scattering investigation indicates that the Raman mode shows a significant softening in Cd-doped ZnO nanoparticles with increasing x, which suggests an expansion of the Zn–O bond. We also observe a significant red-shift of the band gap for CdxZn1−xO nanoparticles as x increases, i.e., the gap energy Eg ∼ 3.21 eV for x = 0 shifts to 2.92 eV for x = 0.40. In addition, the Urbach band tail gradually increases with x-value, which indicates an enhancement of the defect states in samples with Cd incorporation. In CdxZn1−xO nanoparticles, we further find that the near band-edge (NBE) emission band shows a red-shift with increase in x-value. A broad photoluminescence (PL) band is also observed at around 590 nm, which enhances from x = 0 to 0.15. With further increases in the x-value to 0.40, a weak PL band appears around 450 nm while the broad band around 590 nm is suppressed. The variation of PL intensity is ascribed to the change of the Cd-doping induced defect states and the nonradiative energy transfers.

Optical properties and structural evaluation of Li 2O–Al 2O 3–SiO 2–TiO 2 glassy semiconductor containing passive agent CeO 2

In the present work, the effect of CeO 2 as the passive agent on glassy structure of Li 2O–Al 2O 3–SiO 2–TiO 2 (LAST) material for production of a new amorphous semiconductor has been investigated. Optical properties and vibrational spectroscopy of the samples have been studied using UV–Vis and FTIR absorption spectra, respectively. Accordingly, structural variations of 0–1.5% CeO 2:LAST glasses were evaluated by calculation of density, molar volume, Urbach band tailing, direct and indirect optical band gaps, Fermi energy level and metallization criterion. Eventually for the analysis of the variation of energy level in presence of passive agent CeO 2, Licciardello's model for glassy semiconductors was utilized. Results of the investigation illustrates that while Ce 3+ ions could be regarded as the color agents due to spin allowed f–f transition, Ce 4+ ions might have a destructive influence on negative-U centers (color centers) of the glass and hence, reduce the recombination centers in the band gap of the glass.

Electrical, magnetic, and optical properties in multiferroic Bi5Ti3FeO15 thin films prepared by a chemical solution deposition route

Perovskite Aurivillius structure Bi5Ti3FeO15 (BTF) polycrystalline thin films were prepared by a chemical solution deposition route. The microstructure, electrical, and magnetic properties were investigated and in particular the optical properties of the BTF films were addressed. The coexistence of the ferroelectric and weak ferromagnetic properties at room temperature demonstrates the multiferroic behavior of the BTF films. The optical dielectric functions in the photon energy range of 1.13–4.13 eV have been extracted by fitting the experimental data in terms of the Tauc-Lorentz and Cauchy models. A direct interband transition is allowed and the energy bandgap is determined to be ∼3.3 eV for the BTF films. The weak absorption below the bandgap edge was observed and described by the Urbach band tail rule. Finally, the optical dispersion behaviors of the BTF films in the transparent region were explained using a single oscillator model.

Temperature dependent optical properties of Mn doped (Pb,Sr)TiO3 ferroelectric films in absorption region: Electron–phonon interaction

The 2 mol % Mn doped Pb0.5Sr0.5TiO3 films were fabricated on sapphire substrates by chemical solution deposition. The complex dielectric functions and optical parameters of the films determined by transmittance spectroscopy in a broad temperature range of 80–500 K were investigated. Apparent thermally-driven band-gap energy shrinkage, line-width of interband electron transition broadening and the Urbach band tail behaviors were observed. The results can be mainly correlated with the renormalization of electronic band structure and band-gap energy due to the electron–phonon interaction, which is well established in terms of the Bose–Einstein model.

Temperature dependence of the optical properties of AlInN

Measurements of the temperature dependence of transmission have been carried out on AlInN thin films grown by reactive radio-frequency magnetron sputtering on (0001) sapphire substrates. By following a detailed procedure developed for analyzing the transmission spectra, we obtained more reliable data for the effects of temperature on the optical properties of AlInN. Two sets of temperature and photon-energy dependence of empirical formulas have been established for the absorption coefficient, bandgap, Urbach bandtail, and refractive index, which not only unify various experimental data reported in the literature but also provide a database of the optical properties of AlInN based on experimental results. It was found that the shift in the temperature dependence of the bandgap increases with the Al content and that the Urbach bandtail parameter is closely related to the structural characteristics of the AlInN thin films. These optical properties provide an experimental basis for further theoretical investigation and the design of AlInN-based devices.

Structural, electrical, and optical properties of thermally evaporated nanocrystalline PbTe films

Nanocrystalline PbTe films are deposited on different substrates at room temperature through thermal evaporation. The films are stoichiometric single-phase polycrystalline with (200) texture. Electrical properties of the films are analyzed in the framework of a grain boundary channel conduction model. The index of refraction and extinction coefficient of PbTe films are extracted from infrared spectroscopic ellipsometry measurement in the wavelength range of 2–8 μm, yielding an optical band gap of 0.386 eV and evidence for the presence of an Urbach band tail. The optical band gap is larger than the typical value for bulk material due to quantum confinement effect.

Correlation between the stress in ZnO thin films and the Urbach band tail width

AbstractA comparison of the changes in the optical and structural properties of ZnO thin films, undoped and doped with Al, Er and Ta, deposited by magnetron co‐sputtering in Ar is performed in this work. A set of ZnO samples deposited by sputtering in Ar+H2 is studied, as well. The films are deposited at different substrate temperatures and with different concentrations of the doping elements. The band gap energy and the Urbach band tail parameter are calculated from the spectral dependence of the coefficient of absorption. The stresses in the films are calculated from X‐ray diffraction patterns. A correlation between the band tail widths and the stress in the ZnO thin films is observed. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)