S. Romdhane

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 .

Evidence of triplet–triplet interaction in crystalline sexithiophene

Abstract Optically detected magnetic resonance (ODMR) experiments are performed at room temperature on crystalline sexithiophene (6T) photoluminescence (PL). The observed spectra are interpreted in terms of microwave induced transitions between spin sublevels of triplet–triplet pairs states. These triplets are produced by intersystem crossing and their mutual annihilation which leads to delayed fluorescence (DF) is modulated by the high power microwave and enhance the PL process.

Role of recombination, dissociation, and competition between exciton-charge reactions in magnetoconductance of polymeric semiconductor device

Magnetoelectrical measurements were performed on a diode structure, based on an anthracene-containing poly(arylene-ethynylylene)-alt-poly(arylene-vinylene) denoted AnE-PVstat, to clarify the role of the recombination and dissociation of electron-hole (e-h) pairs in the magnetoconductance (MC). We report the observed MC under a weak magnetic field (<1 T) at room and low temperatures. Positive MC is observed and reaches up to 2% at a magnetic field of 450 mT at room temperature. It is found that with the increase of the voltage, the MC effect decreases. We also report the difference in MC between perpendicular (θ = 90°) and parallel (θ = 0°) alignment of magnetic field with respect to the current direction. The experimental data were analyzed in the context of the e-h pair model, based on the Stochastic Liouville Equation. To interpret the experimental results on magnetoconductance measurements, anisotropic hyperfine interaction has been introduced through an anisotropic hyperfine field. The dissociation ra...