Modelling of trapping/detrapping processes in organic semiconductors and implementation of a trap characterization experiment on organic transistors.

Le sujet de thèse s’inscrit dans le cadre de l’étude de la fiabilité des transistors organiques. La limitation majeure des transistors organiques est due à la présence de pièges dans le composant. Notre étude porte sur la modélisation des phénomènes de piégeage dans les semi-conducteurs organiques (SCO) et la mise en place d’un dispositif de caractérisation de pièges dans les transistors organiques. Le modèle prend en compte des densités d’états gaussiennes pour les porteurs libres et piégés et a été utilisé avec succès pour rendre compte de mesures de spectroscopie de défauts issues de la littérature. Les résultats obtenus montrent que les énergies d’activation de pièges extraites avec le modèle classique sont sous-estimées, de plus il nous faut moins de contributions de pièges pour rendre compte des mesures. Nous proposons ainsi un nouveau modèle permettant de décrire les niveaux de piège dans les SCO. La seconde partie de nos travaux porte sur la mise en place d’un dispositif de caractérisation de pièges dans les transistors organiques. Nous étudions l’effet du stress électrique sur les performances de transistors à base de P3HT en mesurant des transitoires de courant à partir desquels nous déterminons l’évolution temporelle de la tension de seuil (UT) des transistors. Un modèle utilisant une fonction exponentielle étirée permet de rendre compte de l’évolution de UT et d’obtenir une estimation des paramètres de piège. Nous avons montré que dans un transistor à base de P3HT les trous peuvent être capturés par trois contributions de pièges, confirmées par d’autres auteurs, ce qui montre l’intérêt du dispositif pour l’étude de la fiabilité des transistors organiques.This thesis aims to study the reliability of organic transistors. The main limitation of organic transistors is their instability due to the presence of traps able to lower their electrical performances. Our work is about the modelling of trapping/detrapping processes in organic semiconductors and the implementation of a trap characterization experiment on organic transistors. Our model takes into account adapted energetic distributions of both free and trapped carriers in emission and capture processes. It was used on some trap determination measurements on organic semiconductors from the literature using the Deep Level Transient Spectroscopy (DLTS). Our results show that considering relevant DOS for the HOMO/LUMO and for the trap distribution is not only more relevant for organic semiconductors but also allows one to better fit the measures with less contributions. A new model is then proposed to describe defect states in organic semiconductors considering relevant distributions for both free and trapped carriers. Good agreement with experimental defect data is obtained by the DLTS technique. The second theme of our study is about the implementation of a trap characterization experiment on organic transistors. To do so, we studied bias stress effects on the electrical characteristics of our P3HT based transistors. The principal effect observed in our transistors is a shift of the threshold voltage with the bias stress. We found that three trap contributions are responsible of instabilities noticed in our tested transistor, they are confirmed by other authors in the literature. Hence the interest of our experiment in the study of organic transistors reliability

Generation-recombination in disordered organic semiconductor: Application to the characterization of traps

International audienceThe presence of traps in organic semiconductor based electronic devices affects considerably their performances and their stability. The Shockley-Read-Hall (SRH) model is generally used to extract the trap parameters from the experimental results. In this paper, we propose to adapt the SRH formalism to disordered organic semiconductors by considering a hopping transport process and Gaussian distributions for both mobile and trapped carriers. The model is used to extract multiple trap parameters from charge based Deep Level Transient Spectroscopy (QDLTS) spectrum. Calculation of the charge transients are given in detail. The model predicts that the activation energy of the trap should not follow an Arrhenius plot on large temperature ranges. Also, the charge transients are no longer exponential when considering Gaussian trap distributions, enlarging the Q-DLTS peaks. The model fits the Q-DLTS spectra measured on organic diodes with a limited number of trap contributions with a good agreement. It is found that an increase of the material rate of disorder reduces the extracted trap energy distances to the LUMO but has no influence on the extracted trap distribution widths. This work shows the importance of considering the specific properties of organic materials to study their properties and their trap distributions

Electrode Confined Acoustic Wave (ECAW) devices for Ultra High Band applications

International audienceThis paper presents a new approach to increase the operational frequency of Surface Acoustic Wave devices. The excitation of a new type of mode called ECAW ( Electrode Confined Acoustic Wave) exhibiting a phase velocity much higher than the one of a standard SAW is demonstrated. In this context, a new composite substrate is considered to guarantee a perfect spectral purity. Therefore, Piezo-On-Glass substrates are manufactured and used for the manufacturing of ECAW device prototypes. Preliminary results show a frequency at 2.5 GHz with an IDT pitch equal to 2 μm. In terms of power handling, widest tested components successfully handle up to 36 dBm without destruction of the devices