Α-sexithienyl Thin Films Research Articles

Molecular orientations and adsorption structures of α-sexithienyl thin films grown on Ag[formula omitted] and Ag[formula omitted] surfaces

Molecular orientations and adsorption structures of α-sexithienyl (6T) thin films grown on Ag(110) and Ag(111) surfaces were studied by near edge X-ray absorption fine structure (NEXAFS) and reflection high energy electron diffraction (RHEED). The out-of-plane (perpendicular to the surface) and in-plane (parallel to the surface) orientations of the 6T molecules were determined by analyzing the polarization dependence of S 1s to π∗ or σ∗ resonance intensity. 6T molecules grew on Ag(110) and Ag(111) with their molecular planes parallel to the substrate surface irrespective of film thickness. On Ag(110), a novel pseudo-one-dimensional structure of 6T molecules oriented toward [001] direction could be observed. The models for adsorption structures of 6T on Ag(110) and Ag(111) are proposed by taking account of the lattice commensurability.

Temperature dependent optical emission efficiency in vacuum sublimed α-sexithienyl thin films

The photoluminescence spectra and quantum yield of α-sexithienyl (T 6 ) thin films grown in high vacuum have been measured at different temperatures in the range 30K-300K. The measurements have been performed with a home-built experimental apparatus based on an integrating sphere, which allows luminescence spectra and quantum yield measurements in the temperature range 5K - 400K. The analysis of the photoluminescence spectra allows to identify the spectral contribution from molecular aggregates and excitons at each temperature. The quantum yield of the molecular aggregates placed below the exciton band span from 0.1% to 5% in the temperature range 300K - 30K. In the same temperature range, the quantum efficiency of the intrinsic excitons increases by a factor of two from 0.4% to 1%. Thus we reconcile the multitude of photoluminescence spectra and quantum yield values previously reported for T 6 films grown in different conditions and therefore containing different concentration of aggregate states.

Time-resolved stimulated emission in an α-sexithienyl thin film

We have investigated the stimulated emission of an evaporated thin film of α-sexithienyl at 20 K. We have performed femtosecond differential transmission measurements with excitation energies around 2.35 eV and probing the spectral region from 2 to 2.65 eV. The results are compared to time-resolved photoluminescence spectra. We observe amplification of the probe light at the spectral position of a sharp peak measured in time-resolved photoluminescence. The stimulated emission decays with a time constant of 2.5 ps.

Luminescence quantum yield of molecular aggregates and excitons in α-sexithienyl thin films at variable temperature

The energy transport and photoluminescence quantum yield in vacuum sublimed thin films of the α-sexithienyl (T6) model system, are investigated as a function of temperature in the range 30–300 K. The emission from the intrinsic bulk excitons and from the molecular aggregate states, is identified in the photoluminescence spectrum at each temperature. The corresponding absolute quantum yield of photoluminescence is measured with a home-built experimental apparatus based on an integrating sphere, which allows photoluminescence and electroluminescence quantum yield measurements in the temperature range 5–400 K. The photoluminescence quantum yield of the molecular aggregates placed below the exciton band span from 0.1% to 5% in the temperature range 300–30 K. In the same temperature range, the quantum efficiency of the intrinsic excitons increases by a factor of 2 from 0.4% to 1%. Therefore, in organic thin films both the spectral and quantum yield properties depend on the relative concentration of molecular aggregates formed during the growth process. We reconcile in this way the wealth of photoluminescence spectra and quantum yield values reported in the literature for T6 films grown under different conditions and with different thickness, i.e., containing different concentration of aggregate states.

Observation of Phonon Resonances in the Optical Nonlinearity in an α-Sexithienyl Thin Film

Observation of Phonon Resonances in the Optical Nonlinearity in an α-Sexithienyl Thin Film

Femtosecond Differential Transmission Spectroscopy of α-Sexithienyl Thin Film at Low Temperature

Femtosecond Differential Transmission Spectroscopy of α-Sexithienyl Thin Film at Low Temperature

Femtosecond differential transmission spectroscopy of α-sexithienyl thin film

We have investigated the nature of primary excitations in α-sexithienyl thin film by comparing excitations above and below the charge transfer exciton. We performed femtosecond differential transmission spectroscopy at low temperature. In the probed spectral region from 2 to 2.8 eV, we found a bleaching of the first excited singlet with a superimposed absorption into higher excited gerade states. During the first few picoseconds, the dynamics of the photoexcitations in the range of the first excited singlet manifold are independent of the excitation photon energy. When exciting below the origin of the first excited singlet, we observe an optical Stark effect. Thermal effects dominate the change in transmission for longer delay times for all excitation energies.

Morphology dependent fluorescence in α-sexithienyl thin film at 4.2k

We investigated by fluorescence spectroscopy at 4.2K high vacuum sublimated thin films of α-sexithienyl grown on mica upon a systematic variation of the growth parameters. We studied how the film morphology, induced by different conditions of growth (thickness and substrate temperature), acts on the emission properties at the early stages of growth. We show how the increased molecular order corresponds to a less efficient excitation energy transfer to low energy levels (due to the presence of defects and aggregates) and to an enhanced excitonic character of the fluorescence.

The Origin of Photoluminescence from α-Sexithienyl Thin Films

We have studied photoluminescence (PL) from sublimed films of α-sexithienyl (T6) in order to understand the nature of the excited species which give rise to radiative emission. Temperature-dependen...

Study of α-Sexithienyl Thin Film by Polarized Near Edge X-ray Absorption Fine Structure

An ultrathin α-Sexithienyl (6T) film prepared by an organic molecular beam deposition method on a silver (Ag) film was studied by polarized near edge X-ray absorption fine structure (NEXAFS) spectroscopy using synchrotron radiation and IR-reflection absorption spectroscopy. The carbon K-edge NEXAFS spectrum of 6T was similar to those reported for poly-(3-methylthienylene) and thiophene. Polarized NEXAFS spectra of 6T exhibit strong angular dependence of 1 s→π* resonance intensity, showing that 6T molecules in the film deposited on the Ag film at room temperature had a highly oriented structure. To obtain quantitative information about molecular orientation, this dependence is analyzed by a comparison with theoretical calculation, indicating that the molecular axis is inclined by about 71° to the substrate surface. This angle is almost the same as that reported for 6T film on quartz.

The Influence of Film Morphology on the Photovoltaic Effect in α-Sexithienyl Thin Film Devices

ABSTRACTWe report measurements made on the photovoltaic effect in devices fabricated with thin films of α-sexithienyl (αT6) as the active layer. aT6 was sublimed onto glass/ITO substrates (as the positive contact), and the substrate temperature was varied between room temperature and 156°C during sublimation to control the film morphology. As the substrate temperature increases, the average size of the αT6 grains increases, from ˜200nm at 22° C to ˜1500nm at 150°C, and order within the grains increases. Aluminium metal was evaporated onto the αT6 as the top (negative) contact.The dark current-voltage (I-V) curve is strongly rectifying, whereas under illumination it becomes almost symmetrical about ˜0.7V on the voltage axis, which represents the maximum open-circuit voltage. This behaviour indicates that the efficiency of charge separation is strongly bias dependent, and that charges are separated by the bulk field rather than at, for example, a Schottky barrier. The spectral response of the photocurrent is independent of the film morphology, which is surprising since the environment in which excitons recombine radiatively in αT6 is known to depend on the morphology of the film. The spectral response is consistent with previous measurements on photoconductivity, indicating that the efficiency of charge separation increases with increasing photon energy. There is also a significant photocurrent at energies well below the absorption edge of αT6, suggesting an exponential distribution of defect states extending at least 0.6eV into the optical gap, which may be a distribution of X-traps below the absorption edge. As the substrate temperature during growth increases from 28C to 150C the efficiency of charge separation decreases by an order of magnitude, suggesting that exciton dissocation is assisted by traps or defects which are present in smaller numbers in the more ordered films.