Raffaella Capelli

Organic light-emitting transistors with an efficiency that outperforms the equivalent light-emitting diodes

The potential of organic semiconductor-based devices for light generation is demonstrated by the commercialization of display technologies based on organic light-emitting diodes (OLEDs). Nonetheless, exciton quenching and photon loss processes still limit OLED efficiency and brightness. Organic light-emitting transistors (OLETs) are alternative light sources combining, in the same architecture, the switching mechanism of a thin-film transistor and an electroluminescent device. Thus, OLETs could open a new era in organic optoelectronics and serve as testbeds to address general fundamental optoelectronic and photonic issues. Here, we introduce the concept of using a p-channel/emitter/n-channel trilayer semiconducting heterostructure in OLETs, providing a new approach to markedly improve OLET performance and address these open questions. In this architecture, exciton-charge annihilation and electrode photon losses are prevented. Our devices are >100 times more efficient than the equivalent OLED, >2x more efficient than the optimized OLED with the same emitting layer and >10 times more efficient than any other reported OLETs.

Luminescent Ethynyl−Pyrene Liquid Crystals and Gels for Optoelectronic Devices

Two functional ethynyl-pyrene derivatives have been designed and synthesized by di- and tetra-substitutions of bromo pyrene derivatives with N-(4-ethynylphenyl)-3,4,5-tris(hexadecyloxy)benzamide fragments. The photoluminescence wavelength of the pyrene core can be tuned by the substitution pattern and the state of matter (solid, solution, gel, or liquid crystal). The disubstituted pyrene derivative 1 is not mesomorphic but produces robust and highly fluorescent gels in DMF, toluene, and cyclohexane. The well-defined fibers and ropes of the gel states were characterized by SEM and laser scanning confocal microscopy, and extended over several micrometers. The gels were integrated as active layers in field-effect transistors, which provided good bulk electron and hole charge mobilities as well as light emission generation. The tetra-substituted pyrene derivative is not a gelator but displays a stable liquid crystalline phase with 2D hexagonal symmetry between 20 and 200 degrees C. The pronounced luminescence properties of the mesophase allow one to observe original mesophase textures with flower-like patterns directly by fluorescence microscopy without crossed-polarizers.

A silk platform that enables electrophysiology and targeted drug delivery in brain astroglial cells

Astroglial cell survival and ion channel activity are relevant molecular targets for the mechanistic study of neural cell interactions with biomaterials and/or electronic interfaces. Astrogliosis is the most typical reaction to in vivo brain implants and needs to be avoided by developing biomaterials that preserve astroglial cell physiological function. This cellular phenomenon is characterized by a proliferative state and altered expression of astroglial potassium (K(+)) channels. Silk is a natural polymer with potential for new biomedical applications due to its ability to support in vitro growth and differentiation of many cell types. We report on silk interactions with cultured neocortical astroglial cells. Astrocytes survival is similar when plated on silk-coated glass and on poly-D-lysine (PDL), a well known polyionic substrate used to promote astroglial cell adhesion to glass surfaces. Comparative analyses of whole-cell patch-clamp experiments reveal that silk- and PDL-coated cells display depolarized resting membrane potentials (-40 mV), very high input resistance, and low specific conductance, with values similar to those of undifferentiated glial cells. Analysis of K(+) channel conductance reveals that silk-astrocytes express large outwardly delayed rectifying K(+) current (K(DR)). The magnitude of K(DR) in PDL- and silk-coated astrocytes is similar, indicating that silk does not alter the resting K(+) current. We also demonstrate that guanosine- (GUO) embedded silk enables the direct modulation of astroglial K(+) conductance in vitro. Astrocytes plated on GUO-embedded silk are more hyperpolarized and express inward rectifying K(+) conductance (K(ir)). The K(+) inward current increases and this is paralleled by upregulation and membrane polarization of K(ir)4.1 protein signal. Collectively these results indicate that silk is a suitable biomaterial platform for the in vitro studies of astroglial ion channel responses and related physiology.

Correlation between Dielectric/Organic Interface Properties and Key Electrical Parameters in PPV-based OFETs

We report on the influence of the dielectric/organic interface properties on the electrical characteristics of field-effect transistors based on polyphenylenevinylene derivatives. Through a systematic investigation of the most common dielectric surface treatments, a direct correlation of their effect on the field-effect electrical parameters, such as charge carrier mobility, On/Off current ratio, threshold voltage, and current hysteresis, has been established. It is found that the presence of OH groups at the dielectric surface, already known to act as carrier traps for electrons, decreases the hole mobility whereas it does not substantially affect the other electrical characteristics. The treatment of silicon dioxide surfaces with gas phase molecules such as octadecyltrichlorosilane and hexamethyldisilazane leads to an improvement in hole mobility as well as to a decrease in current hysteresis. The effects of a dielectric polymer layer spin coated onto silicon dioxide substrates before deposition of the semiconductor polymer can be related not only to the OH groups density but also to the interaction between the dielectric and the semiconductor molecules. Specifically, the elimination of the OH groups produces the same effect observed with hexamethyldisilazane. The hole mobility values obtained with hexamethyldisilazane and polymer dielectrics are the highest reported to date for PPV-based field-effect transistors.

Thienopyrrolyl dione end-capped oligothiophene ambipolar semiconductors for thin film- and light emitting transistors

The design, synthesis and structure-property investigation of a new thienopyrrolyl dione substituted oligothiophene material showing reduced band gap energy, low lying LUMO energy level and ambipolar semiconducting behaviour is described.

Structure–property relationships in multifunctional thieno(bis)imide-based semiconductors with different sized and shaped N-alkyl ends

The relationships between the molecular structure, packing modalities, charge mobility and light emission in organic thin films is a highly debated and controversial issue, with both fundamental and technological implications in the field of organic optoelectronics. Thieno(bis)imide (TBI) based molecular semiconductors provide an interesting combination of good processability, tunable self-assembly, ambipolar charge transport and electroluminescence, and are therefore an ideal test base for fundamental studies on the structure–property correlation in multifunctional molecular systems. Herein, we introduce a new class of thieno(bis)imide quaterthiophenes having alkyl side chains of different shapes (linear, cyclic, branched) and lengths (C1–C8). We found that contrarily to what is generally observed in most molecular semiconductors, the length of the alkyl substituent does not affect the optical, self-assembly and charge transport properties of TBI materials. However, different electroluminescence powers are observed by increasing the alkyl side, this suggesting a potential tool for the selective modulation of TBI functionalities. A deep experimental and theoretical investigation on this new family of TBI materials is provided.

Ambipolar field-effect transistor based on α,ω-dihexylquaterthiophene and α,ω-diperfluoroquaterthiophene vertical heterojunction

Abstract Organic field-effect transistors (OFETs) are alternative emerging device structures for efficient light generation, that could provide a novel architecture to address open issues like exciton-contact and exciton-charge quenching, that still limit the OLEDs efficiency and brightness. Recently, it has been introduced by our research group the model of a tri-layer organic heterostructure implemented in a field-effect configuration, that allows preventing at one time the exciton-metal as well as the exciton-charge quenching in an organic electroluminescence generating device. The device active region is formed by a central optical layer sandwiched between an electron and a hole field-effect conducting film. In order to understand the complex phenomena that happens at the interfaces, with the target to fabricate the most balanced ambipolar structure with high morphological compatibility and high mobility in a vertical heterojunction geometry, we made a preliminary study of a single layer and bi-layer OFET structure composed by α,ω-dihexylquaterthiophene (DH4T) and α,ω-diperfluoroquaterthiophene (DHF4T). By means of this study we showed a new highly balanced ambipolar OFET made of these materials, a first step toward their implementation in a more complex structure as the tri-layer is.

A time-temperature integrator based on fluorescent and polymorphic compounds

Despite the variety of functional properties of molecular materials, which make them of interest for a number of technologies, their tendency to form inhomogeneous aggregates in thin films and to self-organize in polymorphs are considered drawbacks for practical applications. Here, we report on the use of polymorphic molecular fluorescent thin films as time temperature integrators, a class of devices that monitor the thermal history of a product. The device is fabricated by patterning the fluorescent model compound thieno(bis)imide-oligothiophene. The fluorescence colour of the pattern changes as a consequence of an irreversible phase variation driven by temperature, and reveals the temperature at which the pattern was exposed. The experimental results are quantitatively analysed in the range 20–200°C and interpreted considering a polymorph recrystallization in the thin film. Noteworthy, the reported method is of general validity and can be extended to every compound featuring irreversible temperature-dependent change of fluorescence.

Mapping of charge distribution in organic field-effect transistors by confocal photoluminescence electromodulation microscopy.

A novel method for mapping the charge density spatial distribution in organic field-effect transistors based on the electromodulation of the photoluminescence is demonstrated. In field-effect transistors exciton quenching is dominated by exciton-charge carrier interaction so that it can be used to map the charge distribution in different operating conditions. From a quantitative analysis of the photoluminescence quenching, the thickness of the charge carrier accumulation layer is derived. The injection of minority charge carriers in unipolar conditions is unexpectedly evidenced, which is not displayed by the electrical characteristics.

Molecular Host−Guest Energy-Transfer System with an Ultralow Amplified Spontaneous Emission Threshold Employing an Ambipolar Semiconducting Host Matrix

We report on the characteristics of a host-guest lasing system obtained by coevaporation of an oligo(9,9-diarylfluorene) derivative named T3 with the red-emitter 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran dye (DCM). We demonstrate that the ambipolar semiconductor T3 can be implemented as an active matrix in the realization of a host-guest system in which an efficient energy transfer takes place from the T3 matrix to the lasing DCM molecules. We performed a detailed spectroscopic study on the system by systematically varying the DCM concentration in the T3 matrix. Measurements of steady-state photoluminescence (PL), PL quantum yield (PLQY), time-resolved picosecond PL, and amplified spontaneous emission (ASE) threshold are used to optimize the acceptor concentration at which the ASE from DCM molecules takes place with the lowest threshold. The sample with a DCM relative deposition ratio of 2% shows an ASE threshold as low as 0.6 kW/cm(2) and a net optical gain measured by femtosecond time-resolved pump-and-probe spectroscopy as high as 77 cm(-1). The reference model system Alq(3):DCM sample measured in exactly the same experimental conditions presents an one-order-of-magnitude higher ASE threshold. The ASE threshold of T3:DCM is the lowest reported to date for a molecular host-guest energy-transfer system, which makes the investigated blend an appealing system for use as an active layer in lasing devices. In particular, the ambipolar charge transport properties of the T3 matrix and its field-effect characteristics make the host-guest system presented here an ideal candidate for the realization of electrically pumped organic lasers.