Fayçal Kouki

Experimental determination of excitonic levels in α-oligothiophenes

The effects of intermolecular interactions on the optical spectra of oligothiophenes are examined. Absorption spectra of isolated molecules are calculated and experimentally recorded in a rigid host matrix whereby molecules are distributed randomly or organized unidirectionally. Absorption spectra of thin films in an ordered and disordered state are given. Ordered films have three principal spectral regions which are discussed in terms of classical exciton theory. Absorption spectra in transmission of single crystals of quinquethiophene and sexithiophene are analyzed. The lowest optically allowed transition in the crystal corresponds to the lowest Davydov component. It consists of a sharp peak that is observed for even-numbered oligothiophenes in b polarization, and absent for odd-numbered rings due to the perfect alignment of the transition dipole moment with the long molecular axis. The upper Davydov component is viewed in both thin film and single crystal spectra. In between the two principal Davydov c...

Photoinduced spontaneous and stimulated emission in sexithiophene single crystals

Abstract Temperature dependent steady-state and transient photoluminescence (PL) measurements were carried out on sexithiophene (6T) single crystals. As the temperature is lowered, three different sets of equally spaced peaks appear on the steady state PL spectrum. This behavior is interpreted in the frame of the molecular exciton theory. The first energy set is attributed to transitions to the lower level of the Davydov splitting of the 1B u excited state of the isolated molecule, whereas the two other sets are ascribed to the crystal defects. An analysis of the PL emission and excitation spectra allowed us to establish a general exciton energy scheme of sexithiophene crystal. At low excitation levels, the transient PL follows a single exponential decay, with a decay time of 1.8 ± 01 ns. When the excitation energy is increased, a second, much faster component is added. Concurrently, the PL spectrum narrows to a single line centered at the origin of the low energy set of the steady state PL. The width of the emission line is 13 cm −1 at 10 K, and tends to widen and shift towards lower energies when the temperature is raised. All these observations are accounted for in term of stimulated emission.

THE FOUR-LEVEL STIMULATED EMISSION IN SEXITHIOPHENE SINGLE CRYSTALS

Single crystal of conjugated sexithiophene oligomer shows stimulated emission when excited with a low-energy photonic pulse. This phenomenon is interpreted in terms of the excitonic energy diagram of the crystal, which presents a four-level pathway for the photoexcitation and emission, similar to the one observed in classical inorganic-based laser materials.

Transient electroluminescence of monolayer and bilayer sexithiophene diodes

Abstract The transient electroluminescence of monolayer and bilayer sexithiophene-based diodes has been measured. The delay time of the luminescence onset of the monolayer diode corresponds to a hole mobility of 5 × 10−6 cm2 V−1 s−1 which is considerably lower than that obtained by field-effect measurements. This is interpreted in terms of strong transient trapping. The bilayer diode presents a twofold time-resolved response which is attributed to the different mobility of its constituent layers.

Optoelectronic properties of sexithiophene single crystals

Abstract UV—Vis absorption, photoluminescence (PL) and photoconductivity (PC) measurements have been carried out on sexithiophene (α6T) single crystals grown from the vapor phase. UV—Vis absorption data were used to determine the absorption coefficient and refraction index under polarized light. The spectrum at polarization perpendicular to the unique axis b is similar to that measured on polycrystalline films, whereas that at light polarized parallel to b presents a weak structured absorption. The refractive index is 1.656, independent of the wavelength, under parallel polarization, and follows the Sellmeier equation, with n ∞ = 1.867, under perpendicular polarization. The PL excitation spectrum with parallel polarization follows exactly the corresponding weak absorption, whereas, at perpendicular polarization, it saturates when the absorption length compares the crystal thickness, and then decreases down to a minimum at 3.5 eV. This lowering mirrors a decrease of the PL yield at higher energies. The PC action spectrum shows an opposite behavior: it presents first a rise at 2.4 eV, parallel to the absorption onset, then a second rise at 3.0 eV up to a maximum at 3.3 eV. The first rise corresponds to the generation of singlet excitons, which then give birth to charges, through either exciton breaking, or more likely charge detrapping. The second rise could correspond to a direct ionization process. The magnetic field effect on PC has been studied. A relative decrease of up to 6% at a field of 4000 G has been detected. The decrease is attributed to the lowering of the triplet—doublet interaction constant R .

Performances of effective medium model in interpreting optical properties of polyvinylcarbazole:ZnSe nanocomposites

The effective medium model is applied to investigate the optical properties of hybrid nanocomposite layers of Polyvinylcarbazole (PVK) and nanoparticles of Zinc Selenide (ZnSe). Thin films of PVK:ZnSe nanocomposites show a porous microstructure with pore diameters of 500 nm. Numerical calculations led to the determination of optical constants such as the refractive index n, the extinction coefficient k, the dielectric permittivity e, and absorption coefficient α. Using common theoretical models, we have determined the Cauchy parameters of the refractive index, namely, static es and lattice e∞ dielectric constants as well as the plasma frequency ωp, carrier density to effective mass ratio Nme*, and the optical conductivity σoc. We show that the optical band gap energy Eg of the nanocomposite structure decreases slightly upon the increase of the nanoparticles volume fraction and is in good agreement with the Vegard law.

Directional dispersion of exciton levels in α-oligothiophenes

Angle dependence of the upper Davydov transition of α-quinquethiophene (5T) is presented in terms of macroscopic dielectric theory. Optical constants are derived from ellipsometry measurements and used in the model to analyse the directional dispersion of excitons with incidence angle. The observed transition around 3.6 eV lies between the frequencies corresponding to the transverse optic (TO) at 3.35 eV and longitudinal optic (LO) mode at 3.8 eV.

Characterization of oligothiophene films by high resolution electron energy loss spectroscopy

Electronic structures of quinquethiophene (5T) and sexithiophene (6T) polycrystalline films deposited by evaporation on gold substrates were studied by high resolution electron energy loss spectroscopy (HREELS). Vibrational spectra were compared to infrared spectroscopy and Raman scattering results. They reveal that surfaces are not rough and are free from contaminants. Energy gap values of 2.38 ∠ 0.02 and 2.27 ∠ 0.02 eV for 5T and 6T films, respectively, were evaluated from electronic spectra. Comparison with optical spectroscopy enabled the assignment of losses corresponding to optically allowed and forbidden transitions. Some of the latter ones located at 1.2, 1.7 and 2.0 eV were observed in 5T films and were assigned to S 0 φι Tn transitions. Differential cross section analysis reveals the presence of resonance processes in the excitation mechanism. Ionization potentials were estimated to be 4.7 ∠ 0.5 eV for both films.© 1998 Elsevier Science S.A. All rights reserved

Volterra series analysis of the photocurrent in an Al/6T/ITO photovoltaic device

Abstract Using Volterra series we resolve analytically the non-linear rate equations describing the time evolution of the charge carrier concentration in an Al/6T/ITO cell when the device is illuminated and charge carriers are uniformally generated in the bulk at a known rate. We present the analytic expressions of the charge carrier densities in the bulk as a function of time and the expression for the photocurrent across the device. We show that the response contains a transient part and a permanent one. Using the first and the second order permanent response the photocurrent action spectrum of the device is obtained. We use these analytic results to suggest a method to evaluate experimentally the carrier lifetimes τn and τp and the Langevin bimolecular recombination rate. The system of rate equations has also been resolved numerically, using the Runge–Kutta method. Finally we compare successfully the measured photocurrent action spectrum with those obtained numerically and analytically.

Optical characterisation of 6t and 4t single crystals by ellipsometry; anisotropy and crystalline structure

Abstract Ellipsometry and absorption spectroscopies have been carried out on sexithiophene (α-6T), and quaterthiophene (α-4T) single crystals grown at High Temperature (HT) or Low Temperature (LT). Effective Optical Indices (EOI) neff and keff have been obtained in two positions of the crystals: b axis being either perpendicular to the yz incidence plane (α=0 °) or parallel to Oy (α=90 °).The spectra contain only a strong absorption feature around 3.4eV and 3.7eV for 6T and 4T respectively. We have compared experimental data and those obtained by simulation of EOI considering a biaxial semi-infinite medium with its three orthogonal principal axis parallel to Ox, Oy, Oz directions. The strong anisotropy of the 4T and 6T LT forms induces an anomalous dispersion curve of neff and a blue shift of keff (~0.1–0.3eV) regardless of the crystal set. The weak anisotropy of the HT form, leads to an anomalous dispersion and a blue shift for keff (~0.1–0.3eV) only for the crystal set α=0 °. The absorption anisotropy or (kz-kx,y) is related to the angle between the L molecular axis and the crystal plane ( b, c ).