Thorough investigation of fast charge transport through large single-crystal organic semiconducting layers elucidated a new type of charge transport. Together with colleagues from Belgium, Ukraine, Slovenia and Germany, we demonstrate that the degree of charge delocalization has a strong impact on polarization energy and thereby on the position of the transport band edge in organic semiconductors. This gives rise to long-range potential fluctuations, which govern the electronic transport through delocalized states in organic crystalline layers. This concept is employed to formulate an analytic model that explains a negative field dependence coupled with a positive temperature dependence of the charge mobility observed by a lateral time-of-flight technique in a high-mobility crystalline organic layer. This has important implications for the further understanding of the charge transport via delocalized states in organic semiconductors. More in PRB here.
Role of transport band edge variation on delocalized charge transport in high-mobility crystalline organic semiconductors
We have studied submonolayer coverages of N,N-1H,1H-perfluorobutyl dicyanoperylenecarboxydiimide (PDIF-CN2) on mechanically exfoliated graphene transferred onto SiO2 substrates. Our atomic force microscopy (AFM) data show that PDIF-CN2 forms irregularly-shaped 1.4 nm-high islands. From the selected area diffraction performed with transmission electron microscope (TEM) we conclude that this height corresponds to stacks of molecules, which are inclined for 43° relative to the graphene surface. AFM also showed complete absence of PDIF-CN2 on single-layer graphene (SLG). Electric force microscopy revealed a marked difference in surface charge density between a single-layer graphene and bilayer graphene, with a higher surface charge on SLG than on the bilayer graphene. We associate this behavior with type doping of graphene due to the electrostatic dipole induced by the molecular water layer present at the graphene/SiO2 interface. The crucial role of the graphene/SiO2 interface in determining growth of PDIF-CN2 was further confirmed by TEM examination of PDIF-CN2 deposited onto unsupported SLG. The results are published in Surface Science.
Fast-Response Photonic Device Based on Organic-Crystal Heterojunctions Assembled into a Vertical-Yet-Open Asymmetric Architecture
Organic semiconductor, due to their excellent interaction with light, could be made into photosensors. The scientists of the Laboratory of Organic Physics have been working together with colleagues from University of Strasbourg and CNR Bologna on novel forms of photonic devices. In these, crystalline dioctyl-3,4,9,10-perylenedicarboximide nanowires and 6,13-bis(triisopropylsilylethynyl) pentacene microplates are integrated into a vertical-yet-open asymmetrical heterojunction for the realization of a high-performance organic photovoltaic detector, which shows fast photoresponse, ultrahigh signal-to-noise ratio, and high sensitivity to weak light. The results are published in Advanced Materials.
We have performed a systematic study of dependence of time-resolved photocurrent on the point of charge excitation within the organic semiconductor channel formed by two coplanar metal electrodes. The results confirm that spatial variation of electric field between the electrodes crucially determines transport of photogenerated charge carriers through the organic layer. Time-of-flight measurements of photocurrent demonstrate that the transit time of photogenerated charge carrier packets drifting between the two electrodes decreases with increasing travelling distance. Such counterintuitive result cannot be reconciled with the spatial distribution of electric field between coplanar electrodes, alone. It is also in contrast to expected role of space-charge screening of external electric field. Supported by Monte Carlo simulations of hopping transport in disordered organic semiconductor layer, we submit that the space-charge screens the external electric field and captures slower charge carriers from the photogenerated charge carrier packet. The remaining faster carriers, exhibit velocity distribution with significantly higher mean value and shorter transit time. The work is published in Organic Electronics.
Time-dependence measurement of photoconductivity crucial for the new type of organic optical components
On 25th of July 2016, the prestigious international journal Nature Nanotechnology has published an article by an international group of scientists from the University of Strasbourg (France) and the University of Nova Gorica (Slovenia). The article presents the results of an experimental study of a novel two dimensional nanostructured photo-sensor. The photo-sensor comprises two-dimensionally aligned semiconducting polymer nanowires, which results to be highly efficient system for the detection of the visible light. This discovery opens the door to the development of fast light photo-sensors, which will be entirely made of organic materials. The sensors exhibit response times below 10 ns and extremely high signal to noise ratios. More can be found here or here (in slovenian language).
Researchers from the University of Nova Gorica (Slovenia), University of Strasbourg & CNRS (France) and the Humboldt University of Berlin (Germany) have shown that a carefully chosen blend of a small photoswitchable molecule and a semiconducting polymer can be used to fabricate high-performance memory devices that can be written and erased by light. Such multilevel (8-bit!) optical memories have also been implemented on flexible substrates, paving the way to applications in wearable electronics, E-papers, and smart devices. These results have been published in Nature Nanotechnology.
Our researchers with collaboration of colleagues from MRL laboratory and IJS scientists, found a novel and efficient method used to improve carrier mobilities of poly(3-hexylthiophene) (P3HT)-based organic field effect transistors by means of nanowire formation. The treatment, termed solvation, consists of depositing a small quantity of a solvent directly on top of a previously deposited semiconducting film, and allowing the solvent to evaporate slowly. Such treatment results in an increase of the saturation mobility by more than one order of magnitude, from 1.3 × 10−3 up to 3.4 × 10−2 cm2/Vs, while devices preserve their high ON/OFF ratio of ∼104. The atomic force and scanning electron microscopy studies show that solvated P3HT layers develop a network of nanowires, which exhibit increased degree of structural order, as demonstrated by micro Raman spectroscopy. The time-of-flight photoconductivity studies suggest that higher hole mobility stems from a reduced energy disorder of the transporting states in these structures. More can be found in this paper.
Graphene flakes at the SiO2/organic-semiconductor interface for high-mobility field-effect transistors
Our group published new research in collaboration with colleagues from Max Planck Institute from Mainz. We demonstrated that high-transconductance organic thin film transistors can be achieved by depositing electrochemically exfoliated graphene flakes at the gate-dielectric/organic semiconductor (OS) interface. This effect is applicable to both, solution processed, polymer-based and vacuum-evaporated small-molecule OS-based transistors. Poly(3-hexylthiophene) (P3HT) transistors exhibit a factor of seven higher charge carrier mobility, while pentacene transistors exhibit a fourfold increase in charge carrier mobility, if graphene flakes are present at the dielectric/OS interface. More…
Atomic force microscopy has been used to examine early stages of pentacene growth on exfoliated single-layer graphene transferred to SiO2 substrates. We have observed 2D growth with mean height of 1.5 ± 0.2 nm on as-transferred graphene. Three-dimensional islands of pentacene with an average height of 11 ± 2 nm were observed on graphene that was annealed at 350 °C prior to pentacene growth. Compellingly similar 3D morphology has been observed on graphene transferred onto SiO2 that was treated with hexamethyldisilazane prior to the transfer of graphene. On multilayer graphene we have observed 2D growth, regardless of the treatment of SiO2. We interpret this behavior of pentacene molecules in terms of the influence of the dipolar field that emerges from the water monolayer at the graphene/SiO2 interface on the surface energy of graphene. This work was published in Langmuir.
In collaboration with the University of Strasbourg and ISOF-CNR Bologna, Italy, our group published results of a study of graphene/organic semiconductor composites. Organic thin-film transistor transfer characteristics and time-of-flight (TOF) photoconductivity measurements were used to investigate the effect of the addition of liquid-phase exfoliated graphene nanoflakes (GNs) on the electron mobility in thin films of N,N’-1H,1H-perfluorobutyl dicyanoperylene-carboxydi-imide (PDIF-CN2). Transfer characteristics measurements reveal that the charge carrier mobility of PDIF-CN2 increases by almost three orders of magnitude via blending with GNs. TOF photocurrent measurements confirm that the GNs improve the charge carrier transport in PDIF-CN2. We have found a strong dependence of the TOF-determined electron mobility on the excitation wavelength and obtained a maximum mobility of 0.17 cm2/Vs for charge carriers produced in GN: PDIF-CN2 blends, using photon energy of 5.9 eV. This value is in good agreement with the field-effect mobility of 0.2 cm2/Vs determined from data recorded in the low-voltage region of transfer characteristics. More information can be found in the article.