Stefano Toffanin

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 transparent organic transistor structure for bidirectional stimulation and recording of primary neurons

Real-time stimulation and recording of neural cell bioelectrical activity could provide an unprecedented insight in understanding the functions of the nervous system, and it is crucial for developing advanced in vitro drug screening approaches. Among organic materials, suitable candidates for cell interfacing can be found that combine long-term biocompatibility and mechanical flexibility. Here, we report on transparent organic cell stimulating and sensing transistors (O-CSTs), which provide bidirectional stimulation and recording of primary neurons. We demonstrate that the device enables depolarization and hyperpolarization of the primary neuron membrane potential. The transparency of the device also allows the optical imaging of the modulation of the neuron bioelectrical activity. The maximal amplitude-to-noise ratio of the extracellular recording achieved by the O-CST device exceeds that of a microelectrode array system on the same neuronal preparation by a factor of 16. Our organic cell stimulating and sensing device paves the way to a new generation of devices for stimulation, manipulation and recording of cell bioelectrical activity in vitro and in vivo.

Nanocomposite field effect transistors based on zinc oxide/polymer blends

Significant progress is being made in the realization of thin-film transistors (TFTs) for application in various electronic devices and circuits [1-5]. Currently, one of the important challenges in this area is to design low-cost and stable organic semiconductors that possess high field-effect mobilities for constructing low-power high-speed transistor devices. However, there are only limited stable and cheap organic semiconductors that are applicable for OTFT applications. Here, we report the work in our laboratory that focus on stable, inexpensive and high field-effect mobility nano-composite materials for the potential application in OTFT technologies. Solution processed polymer based nano-composite field effect transistors with wide band gap semi-conducting ZnO nano-tetrapods and nano-crystals dispersed in the polymer matrix were utilized to study the field effect behaviour. The electrical characteristics of polymer based wide band gap nano-crystal or nano-tetrapod composite devices exhibit an increase in the hole mobility up to two orders of magnitude higher than the pristine polymer. The fabricated devices that contained a layer of MEH-PPV only exhibited p-channel behaviour with a hole mobility up to 10-4 cm2/Vs, similar to previously reported.3 Figures la and lb show the TEM (transmission electron microscope) images of ZnO nanocrystals or tetrapods dispersed in MEH-PPV solutions, respectively. The size of the nanocrystals is around 5 nm (Figure la) and the legs of the tetrapods are around 100 nm in width (Figure lb). Figure 2 shows the electrical behaviour of the devices fabricated from MEH-PPV and nanocomposite with ZnO nanocrystals or tetrapods. In Figure 2, the I-V characteristics and the transfer curves of the devices based on MEH-PPV (Figures 2a and 2b), 9 mg of ZnO nanocrystals in 10 mg of MEH-PPV or 47% of ZnO in weight (Figures 2c and 2d) and 9 mg of ZnO tetrapods in 10 mg of MEH-PPV or 47% of ZnO in weight (Figures 2e and 2f) are depicted. A saturation of the hole mobility is observed in the nanocomposite devices when the concentration of ZnO tetrapods or nanocrystal exceeds 40% in weight as shown in Figure 3. From the I-V characteristics, incorporation of ZnO nanocrystals or tetrapods in the polymer enhances the drain current and the mobility. The calculated hole mobility was up to 0.08 cm2/Vs for the ZnO nanocrystals / MEH-PPV devices and up to 0.15 cm2/Vs for the ZnO tetrapods / MEH-PPV devices, at the saturation regime. Where as in the linear regime, the hole mobility was up to 0.071 cm2/Vs for the ZnO nanocrystals / MEH-PPV devices and up to 0.096 cm2/Vs for the ZnO tetrapods / MEH-PPV devices. A decrease in the threshold voltage up to -15 V was found for both nanocomposite devices (ZnO nanocrystals or ZnO tetrapods / MEH-PPV). The sub-threshold swing was found to be 2 V per decade for the ZnO / MEH-PPV nanocomposite devices and up to 10 V per decade for the MEH-PPV devices. The on/off ratio was calculated as 105 for the nanocomposite devices where it was only 103 for MEH-PPV devices. Furthermore, a reduction in density of traps, given by NT = VT Con/q, has been observed, as shown in the inset of Figure 3, while the weight percentage of ZnO increases in the polymer. However, the trap density seems to saturate when the concentration of ZnO tetrapods or nanocrystal in the polymer exceeds 40% in weight. Incorporation of ZnO nanomaterials (nanocrystals or tetrapods) into the MEH-PPV polymer -a p-type semiconductor -did not change the nature of charge transport, as the nanocomposite devices were found to behave as p-channel transistors. However the hole mobility was enhanced in the nanocomposite devices, in addition, the band diagram of MEH-PPV and ZnO are well known. The highest occupied molecular orbital (HOMO, 5.3 eV) and lowest unoccupied molecular orbital (LUMO, 3.0 eV) levels of MEH-PPV and the valence (7.6 eV) and conduction (4.4 eV) bands of ZnO shows clearly that a huge energy barrier exists for holes to be transferred from ZnO to MEH-PPV for transport. Consequently, holes are confined in MEH-PPV and we suggest that the effect of ZnO is to reduce the density of traps in the polymer which probably is a reason for the enhanced mobility and the reduced threshold voltage.

Low-threshold blue lasing from silk fibroin thin films

Silk is a natural biocompatible material that can be integrated in a variety of photonic systems and optoelectronic devices. The silk replication of patterned substrates with features down to tens of nanometers is exploited to realize highly transparent, mechanically stable, and free-standing structures with optical wavelength size. We demonstrate organic lasing from a blue-emitting stilbene-doped silk film spin-coated onto a one-dimensional distributed feedback grating (DFB). The lasing threshold is lower than that of organic DFB lasers based on the same active dye. These findings pave the way to the development of an optically active biocompatible technological platform based on silk.

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.

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.

Photostimulation of Whole-Cell Conductance in Primary Rat Neocortical Astrocytes Mediated by Organic Semiconducting Thin Films

Astroglial ion channels are fundamental molecular targets in the study of brain physiology and pathophysiology. Novel tools and devices intended for stimulation and control of astrocytes ion channel activity are therefore highly desirable. The study of the interactions between astrocytes and biomaterials is also essential to control and minimize reactive astrogliosis, in view of the development of implantable functional devices. Here, the growth of rat primary neocortical astrocytes on the top of a light sensitive, organic polymer film is reported; by means of patch-clamp analyses, the effect of the visible light stimulation on membrane conductance is then determined. Photoexcitation of the active material causes a significant depolarization of the astroglial resting membrane potential: the effect is associated to an increase in whole-cell conductance at negative potentials. The magnitude of the evoked inward current density is proportional to the illumination intensity. Biophysical and pharmacological characterization suggests that the ion channel mediating the photo-transduction mechanism is a chloride channel, the ClC-2 channel. These results open interesting perspectives for the selective manipulation of astrocyte bioelectrical activity by non-invasive, label-free, organic-based, photostimulation approaches.

Integration of a silk fibroin based film as a luminescent down-shifting layer in ITO-free organic solar cells

We report here a study on the integration of the silk fibroin (SF) protein in organic solar cells. The intrinsic low toxicity, natural availability, biodegradability, water processing, good film forming properties and capability to be doped with functional molecules of SF biopolymer inspired us to integrate it as a transparent and inert or functional bottom layer in organic solar cells. Water stable, optically transparent, smooth and homogeneous SF thin films (thickness ∼400 nm) were successfully prepared on glass and characterized. Then ITO-free bulk heterojunction (BHJ) solar cells employing P3HT:PC61BM as a standard active layer and a highly conductive PEDOT:PSS formulation as a semi-transparent anode were deposited over the SF films. As a result, the power conversion efficiency (PCE) of all silk-integrated BHJ solar cells was comparable to the references on bare glass. The ability of SF to act as a host matrix for functional moieties was exploited to give to the SF layer the functionality of a Luminescent Down-shifting film (LDS), as confirmed by the spectral response measurements, by using a water soluble stilbene derivative (Stb). The photovoltaic performance of all SF-based devices was significantly stable over time, overcoming the problems of the ITO-based reference cells after 70 days. Finally, flexible SF-integrated ITO-free solar cells were successfully fabricated on PET substrates.

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.