Revisiting the Vibrational and Optical Properties of P3HT: A Combined Experimental and Theoretical Study

International audienceWe demonstrate that DFT-based calculations can provide straightforward means to analyze the effect of aggregation on the optical properties of regioregular P3HT oligomers of different lengths (up to 20-mers) and of bioligomers of 8-mers in two different conformations. Our conclusions substantially differ from those obtained previously by applying the exciton model. Indeed, analysis of Huang–Rhys factors has evidenced that two vibrational modes, a collective mode and an effective mode, are combined in the vibronic structure of the absorption spectrum of oligothiophene. Computed spectra match perfectly their experimental counterparts provided we consider that the oligomer and at least the five lowest excited states of bioligomers behave as absorbers, and that both oligomer and bioligomer contribute to the emission spectra. Study of the nature of the Franck–Condon excitation and optimization of the five lowest excited singlet states indicate that high (hot) excited states of the bioligomer may play an important role in the initiation of charge separation and highlight the importance to take into account the relaxation processes in the theoretical modeling of emission properties

Etude des défauts induits par recuit laser excimer sur silicium

National audienceLe recuit par laser (LTA) est une technique bien adaptée pour la réalisation de jonctions ultra minces et ultra dopées. En effet, ce type de recuit est très prometteur car il permet une très forte activation des dopants de manière localisée (en surface et en profondeur) et une distribution uniforme des dopants

Propriétés structurelles et photophysiques du 3-hexylthiophène (P3HT) : études expérimentale et théorique combinées

National audienceL'objectif de ce travail est la mise au point d'une photodiode organique (OPD) intégrée dans un microsystème de détection basé sur les propriétés de luminescence d'une micro-algue. L'OPD est élaborée à partir d'un polymère à faible gap, le PTB7 (efficacité quantique extérieure élevée de 80% lorsqu'il est, associé au PC71BM) [1]. L'optimisation de l'OPD passe par la compréhension théorique des propriétés photophysiques des composés. Aussi, un protocole utilisant des calculs TD-DFT et DFT a été mis au point en s'appuyant sur le P3HT, composé très étudié dans la littérature. Les spectres Raman, infrarouge, d'absorption UV-visible résolu en vibrations (Figure 1) et de photoluminescence (PL) simulés présentent un très bon accord avec les spectres expérimentaux, validant la méthode utilisée. Par ailleurs, nous démontrons que la structure vibronique est due à un mode collectif dominé par l'élongation des liaisons CC inter-cycles et intra-cycles et couplée au mode de vibration des chaînes alkyle. En optimisant et en décrivant les états excités les plus bas de ces oligothiophènes (états singulet S1 et triplet T1), nous montrons également que S1 et T1 présentent une structure quinoïdale avec une distribution de spin localisée sur la partie centrale de la chaîne. Les énergies relatives des deux états excités les plus bas offrent la possibilité à ces systèmes d'être impliqués dans le processus de fission d'excitons de singulet. Ayant démontré que les méthodes ab-initio modernes sont capables de décrire avec de bonnes précisions les propriétés vibrationnelles et optoélectroniques des chaînes de polymères et, d'estimer l'impact de la longueur de chaîne des oligomères, nos efforts théoriques actuels sont axés sur l'étude des propriétés du polymère pour notre photodiode, le PTB7. Références : [1] C

Revisiting the structural and photophysical properties of 3- hexylthiophene (P3HT): a combined experimental and theoretical study

International audienceThe context of our work is to design a lab-on-chip based on the luminescence properties of a micro-algae: Chlamydomonas reinhardtii. The algae luminescence is collected thanks to an organic photodiode (OPD) based on a low band gap polymer, PTB7 (high external quantum efficiency of 80% when mixed with PC71BM) [1]. Before ab-initio calculations simulating those systems, we perform these calculations (TD-DFT and DFT) on the experimentally and theoretically extensively studied P3HT in order to identify the best-adapted simulation's methodology. We will demonstrate here how modern ab-initio methods are able to describe with good precisions vibrational and optoelectronical properties of those polymers chains and particularly to estimate the impact of the chain-length of oligomers (eight units to twenty units). Raman (figure 1), infrared, vibrationally resolved UV-visible absorption (figure 2) and photoluminescence (PL) spectra have been simulated. Their comparison with their respective experimental spectra shows a perfect agreement. We demonstrate that the vibronic structure is due to a collective mode, dominated by interring CC bond stretching, coupled to alkyl chains vibration mode. By optimizing and describing the lowest excited states of these oligothiophenes (singlet S1 and triplet states T1), we find that S1 and T1 present a quinoidal structure with a spin distribution localized on the central part of the chain. Relative energies of the two lowest excited states provide the possibility to these systems to be involved in singlet exciton fission process. As a result of this work, we have checked that our method is able to predicted P3HT photophysical properties. Further theoretical and experimental efforts are now focus on PTB7 studies, and we are confident on our calculation method for various polymers. [1] C

Influence of in-situ x-ray exposure on the magnetotransport properties of NPB and MADN based blue OLED structures

International audienceMagnetic Field Effects (MFE) on Organic Semiconductors (OSC) have a large fundamental and technological interest presently due to the development of organic electronics devices like OLED (organic light emitting diodes) or OPV (organic photovoltaic). Up to now, the largest observed MFE[1] have been reported for OLED-type devices. They are related to the interaction between excited states (singlet, triplet) and charge carriers, either free or trapped.[2] The origin of the magneto-conductance (MC) is still controversial and depends on the device structure. In particular, the materials choice plays an important role, as well as film thickness, fabrication process, temperature. For example, x-ray exposure during the deposition of metal layers with an electron gun may modify optoelectronic properties of OSC and MC, as reported by several authors.[3, 4] They are assumed to cause punctual trap defects in OSC films. We contribute to this issue in this work. We have decided to compare two HTL organic materials, NPB and MADN, since they display almost the same HOMO and LUMO with huge differences in published values for hole mobility which are respectively 10-4 and 3.10-7 cm 2 /V.s for NPB and MADN.[5, 6] We have prepared devices with an OSC single-layer, ITO anode and Al cathode. For photoluminescence (PL) experiments, similar OSC single layers were deposited on bare silicon substrates. All organic layers of one sample set were grown during the same evaporation run, then individually x-ray treated at various acceleration voltages of the e-gun. In the case of NPB, we report a dramatic dependence of current onsets that increase with the e-gun voltage (typically from 4 to 8 V for 100 nm NPB respectively pristine and 5 kV exposed). X-ray exposure is less significant for MADN-based devices with a shift from 4 to 5 V. However, they gave the optimal MFE with up to +6.3 % at 300 mT applied magnetic field for a 4 kV exposed sample. We also report a neat dependence of MFE on the voltage conditioning of the samples, i.e. applying direct polarization to the device. All these results strongly support the fact that MFE is governed by a combination of extrinsic x-rays-induced local defects in OSC layers. We will analyze the observed effects in terms of trap formation, occupancy and polaron-triplet exciton coupling according to present models and the obtained PL spectra

Preparation of GaSb(100) surfaces by ultraviolet irradiation

International audienceThe preparation of GaSb(100) surfaces by ultraviolet (UV) irradiation, prior to molecular beam epitaxy (MBE) growth, has been investigated. After optimization of the UV irradiation parameters (type of lamp, distance between the lamp and the sample, time exposure), GaSb surfaces present no detectable carbon contamination, a stoichiometry close to the MBE surface one, and a very good morphology. The oxide layers grown an the substrate surface have been analyzed by Auger electron spectroscopy and x-ray photoelectron spectroscopy. Large differences in the oxide composition, resulting from variations of the UV irradiation conditions, have been observed and analyzed. We discuss the experimental results and particularly the carbon desorption, in terms of oxide growth mechanisms and we propose a model of oxide growing in the optimal conditions. (C) 1997 American Vacuum Society

UV AND OZONE CLEANING OF GASB (100) SURFACES PRIOR TO MBE GROWTH

International audienceThe formation of sacrificial oxides by UV-ozone exposure on GaAs and InP is a well established procedure in the preparation of clean substrates for MBE growth. We describe a UV-ozone treatment used to prepare GaSb(100) substrates and report results obtained by Auger (Auger electron spectroscopy) which show that the quality of the so prepared substrates is excellent for MBE (molecular beam epitaxy)

Peculiarities of high electric field conduction in p-type diamond

International audienceThe electrical properties of chemical vapour deposited p-type epitaxial diamond layers are studied in high electric field conditions. The quasi-static current-voltage characteristics have been measured using transmission-line pulse method with 100 ns pulses. Reproducible impurity impact ioni-zation avalanche breakdown occurs at a critical electrical field in the range of 100–200 kV cm À1 depending on the acceptor concentration and temperature, leading to complete ionisation of neutral impurities. The current-voltage characteristics exhibit an S-shape with the bi-stable conduction characteristic of impurity impact ionisation. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4946853] Due to their high ionisation energy, dopants in diamond are only partially ionised at room temperature, e.g., less than 0.1% of boron impurities are ionised in a low-doped p-type diamond. As a matter of fact, the temperature range of incomplete ionisation of boron acceptor, i.e., the freeze out region, extends up to about 1000 K. The consequent high re-sistivity severely limits potential applications of diamond as semiconductor. 1,2 In the freeze out regime, the conductivity can be increased by excitation of free carriers bounded to the neutral impurity centres to conduction/valence band by (1) thermal ionisation rising the temperature, (2) optical ionisa-tion with light irradiation of suitable wavelength, and finally (3) impact ionization of neutral centres by the free carriers under a sufficiently high electric field. Under a high electric field, the free carriers can gain enough energy to be able to ionise neutral impurities by inelastic collisions leading to free carrier multiplication. This process results in nonlinear current voltage characteristics.

Influence of boron-interstitials clusters on hole mobility degradation in high dose boron-implanted ultrashallow junctions

International audienceHole mobility degradation has been studied in high-dose boron-implanted ultrashallow junctions containing high concentrations of boron-interstitial clusters BICs, combining an empirical method based on the self-consistent interpretation of secondary-ion-mass spectrometry SIMS and Hall measurements and liquid-nitrogen LN 2 to room temperature RT hole mobility measurements. It has been found that BICs act as independent scattering centers which have a strong impact on hole mobility in addition to the other scattering mechanisms such as lattice and impurities scattering. A mobility degradation coefficient has been introduced, which gives information on the mobility degradation level in the analyzed junctions. In the case of very high concentrations of BICs containing a boron density up to 8 10 14 cm −2 , measured hole mobilities were found to be 40% lower than corresponding theoretical values. BICs dissolution through multiple Flash anneals at high temperature 1300 ° C reduces the observe mobility degradation