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.
Using atomic force microscopy we have examined morphology of sub-monolayer thick pentacene layers, deposited on exfoliated single-layer and bilayer graphene, transferred onto SiO2. We observe two-dimensional island growth in the range of substrate temperatures from 10 to 60 °C. We find that island size distributions on single-layer graphene are broader and centered at higher average island areas than on bilayer graphene. We have found activation energy for molecule aggregation of Ea = 427 ± 25 and Ea = 537 ± 46 meV for single layer and bilayer graphene, respectively. We associate the observed differences in pentacene layer morphology with the influence of the dipole field that can be associated with the water layer at the graphene/SiO2 interface. More can be found in this paper
Together with Imperial College, Holst Centre, ICMM Madrid, University of Kentucky and ETH Zürich we have been investigating electric transport properties of 5,11-bis(triethylsilylethynyl) anthradithiophene (TES ADT). TES ADT– one of the most promising, solution-processable small-molecular organic semiconductors – was analyzed, revealing the highest performing polymorph among four solid-state phases, opening pathways toward the reliable fabrication of high-performance bottom-gate/bottom-contact transistors. The results are published in Chemistry of Materials.
Influence of transfer residue on the optical properties of chemical vapor deposited graphene investigated through spectroscopic ellipsometry
Graphene is the material of the future. Our group in collaboration with Serbian partners have examined the effects of transfer residue and sample annealing on the optical properties of chemical vapor deposited graphene, transferred onto a sapphire substrate. The optical absorption of graphene was obtained from point-by-point inversion of spectroscopic ellipsometry measurements in the visible and ultraviolet ranges (250-800nm). Measured spectra were analyzed by optical models based on the Fresnel coefficient equations. The optical models were supported by correlated Raman, scanning electron microscopy, and atomic force microscopy measurements. The obtained data were phenomenologically described by a Fano model. The results show that a residue layer left on graphene can significantly increase its optical absorption in the visible range, compared to an annealed sample. These results were published in the paper.
Photo-induced intramolecular charge transfer in an ambipolar field-effect transistor based on a π-conjugated donor–acceptor dyad
Our research team collaborated in a photoconductivity characterization of an interesting dyad molecule. A π-conjugated tetrathiafulvalene-fused perylenediimide (TTF-PDI) molecular dyad is successfully used as a solution-processed active material for light sensitive ambipolar field-effect transistors with balanced hole and electron mobilities. The photo-response of the TTF-PDI dyad resembles its absorption profile. Wavelength-dependent photoconductivity measurements reveal an important photo-response at an energy corresponding to a PDI-localized electronic π–π* transition and also a more moderate effect due to an intramolecular charge transfer from the HOMO localized on the TTF unit to the LUMO localized on the PDI moiety. This work clearly elucidates the interplay between intra- and intermolecular electronic processes in organic devices. More details can be found in original paper.
We have examined the effect of submonolayer coverage of 1-pyrene butyric acid on charge carrier transport in reduced graphene oxide. We have modeled the interaction of 1-pyrene butyric acid molecules with graphene and determined the amount of charge transfer at the interface between the two materials. The effect of 1-pyrene butyric acid as electron acceptor was determined by transfer characteristics measurements on thin film transistors for thick layers. By using time-resolved photocurrent measurements we were able to detect a reduction of electron mobility in reduced graphene oxide for coverage as low as 0.08%. These results were published in Organic Electronics.
We are proud to announce Graphene Workshop 2013 – from fundamental properties to application, which will be hosted in our Laboratory from 8th to 11th April 2013.
Folds in graphene – grafolds are frequently present even after transferring graphene layers, which were fabricated by chemical vapor deposition (CVD), onto SiO2 substrates. As we are showing in the present Letter, grafolds have a crucial role in determining the morphology of pentacene layers on graphene, and should be as such considered in designing fabrication protocols of pentacene-graphene devices. Our atomic force microscopy investigation of the initial stages of growth of vacuum-evaporated pentacene on CVD graphene shows three-dimensional growth at submonolayer coverages of pentacene. Moreover, islands were found to preferentially nucleate at the location of the grafolds. We interpret this behavior in terms of increased chemical reactivity of grafolds that was predicted in previous theoretical works. These results were published here.
We explained the difference between charge mobility of time-of-flight measurement and field-effect measurement in a recent paper. Time-of-flight measurements of the photocurrent in thin organic semiconductor layers represent an effective way to extract charge carrier mobility. A common method to interpret the time-dependence of the photocurrent in these material systems assumes a position-independent electric field between two coplanar electrodes. In this letter, we compare time-dependence of the photocurrent, measured in the samples comprising thin layers of poly-3-hexylthiophene, with the Monte Carlo simulations. In the simulations, we have used both, a position-independent and a position-dependent electric field. We obtained a favorable agreement between the simulations and the measurements only in the case of position-dependent electric field. We demonstrate that the charge carrier mobility may be underestimated by more than one order of magnitude, if a position-independent electric field is used in the calculations of the mobility.
Scientific publication in the journal Nature reinforces the important role of Nova Gorica University in the field of organic electronics.
The researchers of Laboratory for Organic Matter Physics, University of Nova Gorica dr. Egon Pavlica and prof. dr. Gvido Bratina, together with colleagues from the University of Strasbourg, France, Humboldt University in Berlin, Germany and Stanford University, USA, have published an article in a prominent scientific journal Nature Chemistry on the June 24th. In the published work, they have presented the principle of operation and fabrication of a bi-stable photo-activated switch, which is based on a blend of two different organic semiconductor materials (Nature Chemistry, doi: 10.1038/nchem.1384). The discovery joins together the functionality of electric charge transport and the sensitivity to light into a single active layer, reducing the size and the complexity of a photo-modulated switch. The researches in the published work have prepared the blend of two different organic semiconductors: a derivative of the diarylethene molecule and a polyhexylthiophene polymer. Such blends exhibit bi-stable nature of electrical conductivity, which can be modulated with the light illumination. The conductivity can be switched from non-conducting to conducting state, when the blend layer is illuminated with the light of specific wavelength. When the blend is illuminated with the light of different wavelength, the conductivity switches from conductive to non-conductive state. In order to prove the principle of operation, a photo-switchable organic thin-film transistor was fabricated. The device photoresponse was found to be in the microsecond range, and thus on a technologically relevant timescale. This modular blending approach allows for the convenient incorporation of various molecular components, which opens up perspectives on multifunctional devices and logic circuits.