faycal Kouki | University of Tunis El Manar (original) (raw)
Papers by faycal Kouki
A B S T R A C T The impacts of the Oleic Acid capped ZnSe (OA-ZnSe) nanoparticles content variati... more A B S T R A C T The impacts of the Oleic Acid capped ZnSe (OA-ZnSe) nanoparticles content variation on the microstructural and electrical properties of the ITO/PVK:OA-ZnSe/Al hybrid cell have been investigated by using a combination between the FTIR and the electrical measurements. Based on the J-V characteristics of studied hybrid cell in dark, the conduction mechanism for voltages lower than 1.5 V follows the Richardson–Schottky law. On voltages above 1.5 V, the J–V characteristics can be successfully modeled by other theories such as the space-charge limited current (SCLC) or the trap charge limited current (TCLC). Under white light illumination, the J–V characteristics are clearly affected by the incorporation of OA-ZnSe nanoparticles, indicating a significant improvement of the photovoltaic cell parameters, in particular the conversion efficiency which increases 12-fold compared to the cell based on non-doped PVK. The effective photocurrent density Jph of the hybrid cells with different OA-ZnSe contents are also considered.
Many experimental studies have established the substantial impact of the sound structure of the o... more Many experimental studies have established the substantial impact of the sound structure of the organic thin films on optoelectronic device performances. This study is a collection of experimental and theoretical investigation of the radiative recombination kinetics in an organic light emitting diode based on spin cast a-4T. The structural properties of the organic thin films (a-4T) deposited by the spin coating technique are discussed. The electrical characterization of the diode-like samples (ITO/a-4T/Al) have revealed a very low effective mobility of charge carriers in the spin cast a-4Tand the radiative recombination current was extracted. The luminescence-current characterization of the diode-like samples (ITO/a-4T/Al) was investigated and modeled.
Lancet
The photoluminescence spectra were recorded, in ␣-quaterthiophene (␣-4T) single crystal, for visi... more The photoluminescence spectra were recorded, in ␣-quaterthiophene (␣-4T) single crystal, for visible and UV excitation light. The latter exhibits a blueshift of about 5 meV with respect to the former. This value is comparable to the Van Der Waals energy. Delayed Fluorescence (DF), in ␣-quaterthiophene (␣-4T) single crystal, was observed at room temperature. The delayed fluorescence emitted is explained in terms of a triplet exciton fusion model or the so called triplet-triplet excitons annihilation. Triplet excitons were created using direct excitation in their absorption band (1.91 eV). From the involved spectrum of delayed fluorescence the Davydov Splitting was experimentally evaluated.
Materials Science and Engineering: C, 2002
Evidence for photovoltaic behaviour of sexithiophene sandwich cell is shown through I -V characte... more Evidence for photovoltaic behaviour of sexithiophene sandwich cell is shown through I -V characterisation in dark and under illumination. Then transient measurements were performed under short laser pulse excitation. Transient photocurrent, which vanishes rapidly, could not be accounted for by a simple exponential decay. We used a theoretical model in which we resolved the kinetic equations, which govern the time evolution of carriers in the device. The best agreement of the model with the experiment allows us to reach the life time of the carriers (s n c 1 As, s p c 10 As) and the interaction coefficient between electrons plus holes, R c 2.5 Â 10 À 9 cm 3 s À 1 .
Thin Solid Films, 1998
Electronic structures of quinquethiophene (5T) and sexithiophene (6T) polycrystalline films depos... more 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 → T n 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.
Thin Solid Films, 1998
ABSTRACT
Organic Electronics, 2003
Using Volterra series we resolve analytically the non-linear rate equations describing the time e... more 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 s n and s p 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.
Synthetic Metals, 1999
Low quantum efficiency of electroluminescence (EL) from solid state evaporated thin films of cc-s... more Low quantum efficiency of electroluminescence (EL) from solid state evaporated thin films of cc-sexithiophene (cr-6T) was accounted for by quenching due to inter-chain interaction. The substitution of cx-6T by voluminous substituants at the end of the chains led to an increase of the fluorescence in the solid state with a concomitant decrease in conductivity down to IO-" S cm-' due to the increase in spacing between molecules. A bilayer device a-6T/DPS6T improved hole blocking in the recombination region between organic layers leading to creation of excitons away from the Al electrode. By dispersing the molecules in a highly conducting hole transporting host matrix of PVK the efficiency was further improved. Transient EL response of devices indicate that poor electron injection and mobility limits output efficiencies.
Synthetic Metals, 2009
In this paper, we study the effect of self-assembled monolayers (SAMs) on the electric behavior o... more In this paper, we study the effect of self-assembled monolayers (SAMs) on the electric behavior of organic diodes based on sexithiophene (6T) sandwiched between indium tin oxide (ITO) and aluminum. We have used molecules of SAMs based on a thiol with functional groups of oligothiophene (3T(CH 2 ) 6 SH). Wettability measurements have been performed to characterize ITO surface energy and its modification upon deposition of SAMs. The results of contact angle measurements and surface energies demonstrate the homogeneity and rigidity of grafting surface. The current vs. applied voltage characteristics of devices show that conduction in weak biasing follow Richardson-Schottky behavior. Beyond 1.5 V, J-V characteristics can be successfully modeled by space-charge limited current (SCLC) theory followed by a trap charge limited current (TCLC). The electrical as well as optical characteristics of 6T layer are clearly affected by the presence of the SAM. The differences between ITO/SAM and bare ITO samples are interpreted in terms of structural effect induced by the self-assembled monolayer of 3T(CH 2 ) 6 SH.
Synthetic Metals, 2002
Optically detected magnetic resonance (ODMR) experiments are performed at room temperature on cry... more 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 triplettriplet 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. #
Temperature dependent steady state and transient photoluminescence (PL) measurements were perform... more Temperature dependent steady state and transient photoluminescence (PL) measurements were performed on sexithiophene 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 lowest energy set is attributed to transitions from the lower Au excited state, resulting from the Davydov splitting of the 1Bu excited state of the isolated molecules. The highest energy set of peaks is ascribed to an excited Ag level. At low excitation levels, the transient PL follows a single exponential decay, with a time constant of 1.8 ns. When the excitation energy is increased beyond to 35 (mu) J/cm2, a second much faster transient is superimposed, and the PL spectrum narrows to a single line, centered at the 0-1 transition of the low energy set of the steady PL spectrum. The width of the emission line is 13 cm-1. These observations are explained in terms of a stimulated emission process.
Applied Physics Letters, 1998
Single crystal of conjugated sexithiophene oligomer shows stimulated emission when excited with a... more 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. © 1998 American Institute of Physics. ͓S0003-6951͑98͒03817-0͔
Synthetic Metals, 2002
We have studied photovoltaic cells using sexithiophene in (ITO/6T/Al) structure, and we have meas... more We have studied photovoltaic cells using sexithiophene in (ITO/6T/Al) structure, and we have measured the action spectrum obtained by illumination through the aluminum side which is compared to the measured absorption spectrum. Two models were used to interpret this experiment: the Ghosh model using the carriers diffusion property in the bulk, and the kinetic model describing the dynamic behavior of the charge carriers in the device. The confrontation of these models with the experimental spectra allows us to reach the diffusion length of electron-hole pair L ¼ 2 Â 10 À6 cm. We also give the mobility of carriers (m n % 1:5 Â 10 À4 cm 2 /V s, and m p % 1:5 Â 10 À5 cm 2 /V s) and their diffusion coefficients (D n % 4 Â 10 À6 cm 2 /s, and D p % 4 Â 10 À7 cm 2 /s). #
Lancet
Photogeneration process in organic materials may occur via multiple mechanisms depending on exper... more Photogeneration process in organic materials may occur via multiple mechanisms depending on experimental conditions. In this work we investigate how self-assembled monolayers of two types of benzoic acid molecules (ABA and NBA) affect photocarrier generation in a sexithiophene based photovoltaic cell. Photogeneration of free carriers in organic photovoltaic cells results from dissociation of Frenkel excitons. This dissociation is either thermally assisted process and/or results from the interaction with structural defects in the bulk or with the surface states. A theoretical model is used for quantitative description of the open circuit voltage as a function of generation rates at the electrodes. The orientation and the magnitude of dipole moment of SAM molecules are the crucial factors that affect the organic photovoltaic parameters.
Synthetic Metals, 2002
In this paper we report X-ray diffraction measurements and UV-visible spectra of thin film of 8T ... more In this paper we report X-ray diffraction measurements and UV-visible spectra of thin film of 8T oligothiophene grown at two substrate temperatures 25 and 175 8C. We also report the scanning electron microscopy pictures of 8T films deposited at different temperatures (25, 120, 150 and 175 8C). These experimental results account for a different structural organization of organic molecules in the thin films. We also show that the increase of substrate temperature enhances the charge transport in 8T-based field-effect transistors. UV-visible spectroscopy shows that heating the substrate during 8T film deposition enhances the molecular order and leads to large crystallites. The last result is confirmed by scanning electron microscopy. Carrier mobility in 8T field-effect transistor increases rapidly when the substrate temperature exceeds 120 8C, up to a value of 0.33 cm 2 V À1 s À1 .
Chemical Physics, 2000
The effects of intermolecular interactions on the optical spectra of oligothiophenes are examined... more 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 components lies a broadband that is tentatively attributed to charge-transfer states or to a noninteracting molecular transition reminiscent of isolated matrix spectra. The Davydov splitting of the first optically allowed transition is determined to be about 10 000 cm Ϫ1 and increases slightly with chain length. The Herzberg-Teller region in 6T reveals a dominant coupling mode of 340 cm Ϫ1 . Vibronic structure in excitation and emission spectra shows peaks of 1460 cm Ϫ1 spacing.
Advanced Materials, 1996
... By Gilles Horowitz,* Francis Garnier, Abderrahim Yassar, Riadh Hujhoui, and Fuygal Kouki ... ... more ... By Gilles Horowitz,* Francis Garnier, Abderrahim Yassar, Riadh Hujhoui, and Fuygal Kouki ... temperature of 210 "C. The crystals have been fully characterized by X-ray diffraction.[211 The structure of the field-effect transistor (FET) is shown in Figure 1. The gate electrode (a 100 ...
A B S T R A C T The impacts of the Oleic Acid capped ZnSe (OA-ZnSe) nanoparticles content variati... more A B S T R A C T The impacts of the Oleic Acid capped ZnSe (OA-ZnSe) nanoparticles content variation on the microstructural and electrical properties of the ITO/PVK:OA-ZnSe/Al hybrid cell have been investigated by using a combination between the FTIR and the electrical measurements. Based on the J-V characteristics of studied hybrid cell in dark, the conduction mechanism for voltages lower than 1.5 V follows the Richardson–Schottky law. On voltages above 1.5 V, the J–V characteristics can be successfully modeled by other theories such as the space-charge limited current (SCLC) or the trap charge limited current (TCLC). Under white light illumination, the J–V characteristics are clearly affected by the incorporation of OA-ZnSe nanoparticles, indicating a significant improvement of the photovoltaic cell parameters, in particular the conversion efficiency which increases 12-fold compared to the cell based on non-doped PVK. The effective photocurrent density Jph of the hybrid cells with different OA-ZnSe contents are also considered.
Many experimental studies have established the substantial impact of the sound structure of the o... more Many experimental studies have established the substantial impact of the sound structure of the organic thin films on optoelectronic device performances. This study is a collection of experimental and theoretical investigation of the radiative recombination kinetics in an organic light emitting diode based on spin cast a-4T. The structural properties of the organic thin films (a-4T) deposited by the spin coating technique are discussed. The electrical characterization of the diode-like samples (ITO/a-4T/Al) have revealed a very low effective mobility of charge carriers in the spin cast a-4Tand the radiative recombination current was extracted. The luminescence-current characterization of the diode-like samples (ITO/a-4T/Al) was investigated and modeled.
Lancet
The photoluminescence spectra were recorded, in ␣-quaterthiophene (␣-4T) single crystal, for visi... more The photoluminescence spectra were recorded, in ␣-quaterthiophene (␣-4T) single crystal, for visible and UV excitation light. The latter exhibits a blueshift of about 5 meV with respect to the former. This value is comparable to the Van Der Waals energy. Delayed Fluorescence (DF), in ␣-quaterthiophene (␣-4T) single crystal, was observed at room temperature. The delayed fluorescence emitted is explained in terms of a triplet exciton fusion model or the so called triplet-triplet excitons annihilation. Triplet excitons were created using direct excitation in their absorption band (1.91 eV). From the involved spectrum of delayed fluorescence the Davydov Splitting was experimentally evaluated.
Materials Science and Engineering: C, 2002
Evidence for photovoltaic behaviour of sexithiophene sandwich cell is shown through I -V characte... more Evidence for photovoltaic behaviour of sexithiophene sandwich cell is shown through I -V characterisation in dark and under illumination. Then transient measurements were performed under short laser pulse excitation. Transient photocurrent, which vanishes rapidly, could not be accounted for by a simple exponential decay. We used a theoretical model in which we resolved the kinetic equations, which govern the time evolution of carriers in the device. The best agreement of the model with the experiment allows us to reach the life time of the carriers (s n c 1 As, s p c 10 As) and the interaction coefficient between electrons plus holes, R c 2.5 Â 10 À 9 cm 3 s À 1 .
Thin Solid Films, 1998
Electronic structures of quinquethiophene (5T) and sexithiophene (6T) polycrystalline films depos... more 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 → T n 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.
Thin Solid Films, 1998
ABSTRACT
Organic Electronics, 2003
Using Volterra series we resolve analytically the non-linear rate equations describing the time e... more 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 s n and s p 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.
Synthetic Metals, 1999
Low quantum efficiency of electroluminescence (EL) from solid state evaporated thin films of cc-s... more Low quantum efficiency of electroluminescence (EL) from solid state evaporated thin films of cc-sexithiophene (cr-6T) was accounted for by quenching due to inter-chain interaction. The substitution of cx-6T by voluminous substituants at the end of the chains led to an increase of the fluorescence in the solid state with a concomitant decrease in conductivity down to IO-" S cm-' due to the increase in spacing between molecules. A bilayer device a-6T/DPS6T improved hole blocking in the recombination region between organic layers leading to creation of excitons away from the Al electrode. By dispersing the molecules in a highly conducting hole transporting host matrix of PVK the efficiency was further improved. Transient EL response of devices indicate that poor electron injection and mobility limits output efficiencies.
Synthetic Metals, 2009
In this paper, we study the effect of self-assembled monolayers (SAMs) on the electric behavior o... more In this paper, we study the effect of self-assembled monolayers (SAMs) on the electric behavior of organic diodes based on sexithiophene (6T) sandwiched between indium tin oxide (ITO) and aluminum. We have used molecules of SAMs based on a thiol with functional groups of oligothiophene (3T(CH 2 ) 6 SH). Wettability measurements have been performed to characterize ITO surface energy and its modification upon deposition of SAMs. The results of contact angle measurements and surface energies demonstrate the homogeneity and rigidity of grafting surface. The current vs. applied voltage characteristics of devices show that conduction in weak biasing follow Richardson-Schottky behavior. Beyond 1.5 V, J-V characteristics can be successfully modeled by space-charge limited current (SCLC) theory followed by a trap charge limited current (TCLC). The electrical as well as optical characteristics of 6T layer are clearly affected by the presence of the SAM. The differences between ITO/SAM and bare ITO samples are interpreted in terms of structural effect induced by the self-assembled monolayer of 3T(CH 2 ) 6 SH.
Synthetic Metals, 2002
Optically detected magnetic resonance (ODMR) experiments are performed at room temperature on cry... more 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 triplettriplet 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. #
Temperature dependent steady state and transient photoluminescence (PL) measurements were perform... more Temperature dependent steady state and transient photoluminescence (PL) measurements were performed on sexithiophene 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 lowest energy set is attributed to transitions from the lower Au excited state, resulting from the Davydov splitting of the 1Bu excited state of the isolated molecules. The highest energy set of peaks is ascribed to an excited Ag level. At low excitation levels, the transient PL follows a single exponential decay, with a time constant of 1.8 ns. When the excitation energy is increased beyond to 35 (mu) J/cm2, a second much faster transient is superimposed, and the PL spectrum narrows to a single line, centered at the 0-1 transition of the low energy set of the steady PL spectrum. The width of the emission line is 13 cm-1. These observations are explained in terms of a stimulated emission process.
Applied Physics Letters, 1998
Single crystal of conjugated sexithiophene oligomer shows stimulated emission when excited with a... more 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. © 1998 American Institute of Physics. ͓S0003-6951͑98͒03817-0͔
Synthetic Metals, 2002
We have studied photovoltaic cells using sexithiophene in (ITO/6T/Al) structure, and we have meas... more We have studied photovoltaic cells using sexithiophene in (ITO/6T/Al) structure, and we have measured the action spectrum obtained by illumination through the aluminum side which is compared to the measured absorption spectrum. Two models were used to interpret this experiment: the Ghosh model using the carriers diffusion property in the bulk, and the kinetic model describing the dynamic behavior of the charge carriers in the device. The confrontation of these models with the experimental spectra allows us to reach the diffusion length of electron-hole pair L ¼ 2 Â 10 À6 cm. We also give the mobility of carriers (m n % 1:5 Â 10 À4 cm 2 /V s, and m p % 1:5 Â 10 À5 cm 2 /V s) and their diffusion coefficients (D n % 4 Â 10 À6 cm 2 /s, and D p % 4 Â 10 À7 cm 2 /s). #
Lancet
Photogeneration process in organic materials may occur via multiple mechanisms depending on exper... more Photogeneration process in organic materials may occur via multiple mechanisms depending on experimental conditions. In this work we investigate how self-assembled monolayers of two types of benzoic acid molecules (ABA and NBA) affect photocarrier generation in a sexithiophene based photovoltaic cell. Photogeneration of free carriers in organic photovoltaic cells results from dissociation of Frenkel excitons. This dissociation is either thermally assisted process and/or results from the interaction with structural defects in the bulk or with the surface states. A theoretical model is used for quantitative description of the open circuit voltage as a function of generation rates at the electrodes. The orientation and the magnitude of dipole moment of SAM molecules are the crucial factors that affect the organic photovoltaic parameters.
Synthetic Metals, 2002
In this paper we report X-ray diffraction measurements and UV-visible spectra of thin film of 8T ... more In this paper we report X-ray diffraction measurements and UV-visible spectra of thin film of 8T oligothiophene grown at two substrate temperatures 25 and 175 8C. We also report the scanning electron microscopy pictures of 8T films deposited at different temperatures (25, 120, 150 and 175 8C). These experimental results account for a different structural organization of organic molecules in the thin films. We also show that the increase of substrate temperature enhances the charge transport in 8T-based field-effect transistors. UV-visible spectroscopy shows that heating the substrate during 8T film deposition enhances the molecular order and leads to large crystallites. The last result is confirmed by scanning electron microscopy. Carrier mobility in 8T field-effect transistor increases rapidly when the substrate temperature exceeds 120 8C, up to a value of 0.33 cm 2 V À1 s À1 .
Chemical Physics, 2000
The effects of intermolecular interactions on the optical spectra of oligothiophenes are examined... more 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 components lies a broadband that is tentatively attributed to charge-transfer states or to a noninteracting molecular transition reminiscent of isolated matrix spectra. The Davydov splitting of the first optically allowed transition is determined to be about 10 000 cm Ϫ1 and increases slightly with chain length. The Herzberg-Teller region in 6T reveals a dominant coupling mode of 340 cm Ϫ1 . Vibronic structure in excitation and emission spectra shows peaks of 1460 cm Ϫ1 spacing.
Advanced Materials, 1996
... By Gilles Horowitz,* Francis Garnier, Abderrahim Yassar, Riadh Hujhoui, and Fuygal Kouki ... ... more ... By Gilles Horowitz,* Francis Garnier, Abderrahim Yassar, Riadh Hujhoui, and Fuygal Kouki ... temperature of 210 "C. The crystals have been fully characterized by X-ray diffraction.[211 The structure of the field-effect transistor (FET) is shown in Figure 1. The gate electrode (a 100 ...