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Responsive Field-Effect Transistors: A Life-Long Training Career Development action

Final Report Summary - RESPONSIVE (Responsive Field-Effect Transistors: A Life-Long Training Career Development action)

Tremendous developments have been taking place in the field of electronic materials with the emergence of organic electronics. The use of small conjugated molecules and polymers, and very recently graphene as semiconductors in electronic devices has already come to fruition in flat panel displays. An intrinsic property of organic materials is the ability to blend two or more components to create hybrid materials in which properties from both components are retained. This concept enables the construction of organic electronic devices integrating simultaneously more than one function, which strongly increases the value of the material.
The Marie Curie IEF project RESPONSIVE was targeted at exploiting tailor-made interfaces between organic semiconductors and photochromic molecule via either blending or decoration of interfaces with metallic electrodes in order to create light-responsive organic field-effect transistors (OFETs). To add the light-responsive function to the transistor, the photochromic molecule plays a key role. A photochromic molecule, i.e. a molecule with the capability of reversibly changing its structure with a light stimulus, is used for example to turn on and off charge traps in the transistor. The presence (or absence) of traps for charges fundamentally changes the charge mobility in the device, and thus enables to turn it off and on with light stimulus. Through blending the photochromic molecule with semiconductors, RESPONSIVE has successfully shown that it is possible to achieve photoswitchable transistors that retain the high mobility of the semiconductor, thus using the intrinsic properties of both parts of the blend to the maximum of their capability.
Particular attention has been paid on blending diarylethenes (DAEs) together with different fullerene derivatives (small molecules). Fullerenes are n-type semiconductors, and have together with a pair of newly developed photochromic molecules (DAEs) matched energy levels for n-type field-effect transistor (FET) switching, proving for the first time photoswitching in n-type blended FETs. Nanoscale/multiscale characterization of these blends has been performed. Using Atomic Force Microscopy (AFM), 2D Grazing Incidence X-ray Diffraction and Auger spectroscopy, it has been revealed that the blended films are amorphous, with phase-segregated domains of the two film components, which is as expected since fullerenes phase segregate in other binary mixtures, such as together with P3HT.
The ability of the photochromic molecule to photoisomerize in the blended systems is critical for photoresponsive devices and much effort has been put down to examine the switching efficiency in blended materials. The switching efficiency of photochromic molecules is known to be environment-dependent, and a method to study the switching efficiency in thin blended films has been developed. Using this developed technique, the switching efficiency of DAE/small molecule blends, of DAE with bulky substituents, and of DAE/P3HT blends post-processed at different temperatures and with different molecular weights of P3HT, has been determined.
Moreover, within RESPONSIVE, the cooperative nature of azobenzene isomerization when chemisorbed on Au planar and non-planar surfaces has been explored by making use of spectroscopical approaches. This is key in order to optimize the integration of such SAMs in working organic based transistors.
The developed organic semiconductor/photochromic molecule blended system possesses great potential to find ways into applications such as optically controllable memory switching for light-assisted programming and high-sensitive photosensors by the choice of proper device configurations.
Through RESPONSIVE, a Europe-based first class training experience was provided to a very promising young researcher. RESPONSIVE gave the Fellow the possibility to strengthen his background in the interdisciplinary and intersectorial field of organic electronics and to further develop his research career in Europe by performing cutting-edge science.

Contact information:
Prof. Paolo Samorì
E-mail: samori@unistra.fr