Electric field and charge distribution imaging with sub-micron resolution in an organic Thin-Film Transistor

Organic Electronics (Impact Factor: 3.68). 01/2012; 13:66-70. DOI: 10.1016/j.orgel.2011.09.023

ABSTRACT Here we show how Stark spectroscopy, coupled with confocal microscopy, is able to directly map the electric field in an n-type Copper-Fluorinated Phthalocyanine Thin-Film Transistor (TFT) under different operating conditions. To this extent, we locally probe Electro-Reflectance, with a nominal spatial resolution better than 500 nm, exploiting the fact that the detected signal is directly proportional to the square of the local field on the probe volume. This electric field imaging technique has unique advantages because it is non-invasive, since it exploits low incident power and because it probes the existing field in the bulk rather than the surface. Combining the experimental data with numerical modeling, it is possible not only to reconstruct the space charge profile in the few-nanometer thick accumulation layer, but also to extract the AC electron mobility.

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    ABSTRACT: Structure-property relationships are of fundamental importance to develop quantitative models describing charge transport in organic semiconductors based electronic devices, which are among the best candidates for future portable and lightweight electronic applications. While microstructural investigations, such as those one based on X-rays, electron microscopy or polarized optical probes, provide necessary information for the rationalization of transport in macromolecular solids, a general model predicting how charge accommodates within structural maps is not yet available. Therefore techniques capable of directly monitoring how charge is distributed when injected into a polymer film and how it correlates to structural domains can help filling this gap. Supported by Density Functional Theory (DFT) calculations, here we show that polarised Charge Modulation Microscopy (p-CMM) can unambiguously and selectively map the orientational order of the only conjugated segments which are probed by mobile charge in the few nanometers thick accumulation layer of a high-mobility polymer-based Field-Effect Transistor (FET). Depending on the specific solvent-induced microstructure within the accumulation layer, we show that p-CMM can image charge-probed domains that extend from sub-micron to tens of microns size, with markedly different degree of alignment. Wider and more ordered p-CMM domains are associated with improved carrier mobility, as extracted from device characteristics. This observation evidences the unprecedented opportunity to correlate, directly in a working device, electronic properties with structural information of those conjugated segments involved in charge transport at the buried semiconductor-dielectric interface of a field-effect device.
    ACS Nano 05/2014; · 12.03 Impact Factor
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    ABSTRACT: Charge transport in an ambipolar organic field-effect transistors (OFETs) is discussed in accordance to the potential profiles reconstructed from the electric-field induced second-harmonic generation experiment. The Maxwell-Wagner model based on drift-diffusion equation in OFET is used for the potential profile analysis. A good agreement between dielectric model and the experiment suggests importance of the space-charge field effects in the design of the ambipolar light-emitting OFETs. Further, the highest enhancement of the electric field is on zero-potential position in the channel, which represents the meeting point of electrons and holes and is an origin of the electroluminescence.
    Applied Physics Letters 12/2012; 5(12). · 3.52 Impact Factor
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    ABSTRACT: Charge-modulated optical spectroscopy is used to achieve dynamic two-dimensional mapping of charge carrier distributions in poly(3-hexylthiophene) thin film transistors. The resulting in-channel distributions evolve from uniformly symmetric to asymmetrically saturated as the devices are increasingly biased. Furthermore, physical, chemical, and electrical defects are spatially resolved in cases where their presence is not obvious from device performance.
    Advanced Materials 05/2014; 26(26). · 15.41 Impact Factor

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