Laboratory of Organic Matter Physics

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 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.

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.

Our lab was involved in a discovery of a simple, alternative route towards high-mobility structures of the small-molecular semiconductor 5,11-bis(triethyl silylethynyl) anthradithiophene – TES-ADT that requires one single processing step without the need for any post-deposition processing. The method relies on careful control of the casting temperature of the semiconductor and allows rapid production of transistors with uniform and reproducible device performance over large areas. More information can be found in the published paper.