Organic-based inverters: basic concepts, materials, novel architectures and applications.

While organic materials have demonstrated industry-leading performances in a wide array of electronic applications (including OLEDs and OPVs), their use for integration into electronic circuits has been so far limited, in spite of their potential for portable, flexible, light-weight, low-cost applications. However, recent advances in organic (semi)conductors and novel designs in organic field-effect transistors and hybrid systems have reaffirmed the potential of organic logic circuits. This review article provides an overview of organic-based inverter operation and considers all aspects of such circuits including their active layer, processing methods, hybrid organic/inorganic inverters, novel architectures and potential applications.

Engineering Optically Switchable Transistors with Improved Performance by Controlling Interactions of Diarylethenes in Polymer Matrices.

The integration of photochromic molecules into semiconducting polymer matrices via blending has recently attracted a great deal of attention, as it provides the means to reversibly modulate the output signal of electronic devices by using light as a remote control. However, the structural and electronic interactions between photochromic molecules and semiconducting polymers are far from being fully understood. Here we perform a comparative investigation by combining two photochromic diarylethene moieties possessing similar energy levels yet different propensity to aggregate with five prototypical polymer semiconductors exhibiting different energy levels and structural order, ranging from amorphous to semicrystalline. Our in-depth photochemical, structural, morphological, and electrical characterization reveals that the photoresponsive behavior of thin-film transistors including polymer/diarylethenes blends as the active layer is governed by a complex interplay between the relative position of the energy levels and the polymer matrix microstructure. By matching the energy levels and optimizing the molecular packing, high-performance optically switchable organic thin-film transistors were fabricated. These findings represent a major step forward in the fabrication of light-responsive organic devices.

Direct Photolithography on Molecular Crystals for High Performance Organic Optoelectronic Devices.

Organic crystals are generated via the bottom-up self-assembly of molecular building blocks which are held together through weak noncovalent interactions. Although they revealed extraordinary charge transport characteristics, their labile nature represents a major drawback toward their integration in optoelectronic devices when the use of sophisticated patterning techniques is required. Here we have devised a radically new method to enable the use of photolithography directly on molecular crystals, with a spatial resolution below 300 nm, thereby allowing the precise wiring up of multiple crystals on demand. Two archetypal organic crystals, i.e., p-type 2,7-diphenyl[1]benzothieno[3,2- b][1]benzothiophene (Dph-BTBT) nanoflakes and n-type N, N'-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) nanowires, have been exploited as active materials to realize high-performance top-contact organic field-effect transistors (OFETs), inverter and p-n heterojunction photovoltaic devices supported on plastic substrate. The compatibility of our direct photolithography technique with organic molecular crystals is key for exploiting the full potential of organic electronics for sophisticated large-area devices and logic circuitries, thus paving the way toward novel applications in plastic (opto)electronics.

Tunable Emissive CsPbBr3/Cs4PbBr6 Quantum Dots Engineered by Discrete Phase Transformation for Enhanced Photogating in Field-Effect Phototransistors.

Precise control of quantum structures in hybrid nanocrystals requires advancements in scientific methodologies. Here, on the design of tunable CsPbBr3/Cs4PbBr6 quantum dots are reported by developing a unique discrete phase transformation approach in Cs4PbBr6 nanocrystals. Unlike conventional hybrid systems that emit solely in the green region, this current strategy produces adjustable luminescence in the blue (450 nm), cyan (480 nm), and green (510 nm) regions with high photoluminescence quantum yields up to 45%, 60%, and 85%, respectively. Concentration-dependent studies reveal that phase transformation mechanisms and the factors that drive CsBr removal occur at lower dilutions while the dissolution-recrystallization process dominates at higher dilutions. When the polymer-CsPbBr3/Cs4PbBr6 integrated into a field-effected transistor the resulting phototransistors featured enhanced photosensitivity exceeding 105, being the highest reported for an n-type phototransistor, while maintaining good transistor performances as compared to devices consisting of polymer-CsPbBr3 NCs.

Organic multilevel (opto)electronic memories towards neuromorphic applications.

In the past decades, neuromorphic computing has attracted the interest of the scientific community due to its potential to circumvent the von Neumann bottleneck. Organic materials, owing to their fine tunablility and their ability to be used in multilevel memories, represent a promising class of materials to fabricate neuromorphic devices with the key requirement of operation with synaptic weight. In this review, recent studies of organic multilevel memory are presented. The operating principles and the latest achievements obtained with devices exploiting the main approaches to reach multilevel operation are discussed, with emphasis on organic devices using floating gates, ferroelectric materials, polymer electrets and photochromic molecules. The latest results obtained using organic multilevel memories for neuromorphic circuits are explored and the major advantages and drawbacks of the use of organic materials for neuromorphic applications are discussed.

Covalently linked donor-acceptor dyad for efficient single material organic solar cells.

A novel covalently linked donor-acceptor dyad comprising a dithienopyrrol-based oligomeric donor and a fullerene acceptor was synthesized and characterized. The concomitant effect of favorable optoelectronic properties, energy levels of the frontier orbitals, and ambipolar charge transport enabled the application of the dyad in simplified solution-processed single material organic solar cells reaching a power conversion efficiency of 3.4%.

Boosting and Balancing Electron and Hole Mobility in Single- and Bilayer WSe2 Devices via Tailored Molecular Functionalization.

WSe2 is a layered ambipolar semiconductor enabling hole and electron transport, which renders it a suitable active component for logic circuitry. However, solid-state devices based on single- and bilayer WSe2 typically exhibit unipolar transport and poor electrical performance when conventional SiO2 dielectric and Au electrodes are used. Here, we show that silane-containing functional molecules form ordered monolayers on the top of the WSe2 surface, thereby boosting its electrical performance in single- and bilayer field-effect transistors. In particular, by employing SiO2 dielectric substrates and top Au electrodes, we measure unipolar mobility as high as µh = 150 cm2 V-1 s-1 and µe = 17.9 cm2 V-1 s-1 in WSe2 single-layer devices when ad hoc molecular monolayers are chosen. Additionally, by asymmetric double-side functionalization with two different molecules, we provide opposite polarity to the top and bottom layer of bilayer WSe2, demonstrating nearly balanced ambipolarity at the bilayer limit. Our results indicate that the controlled functionalization of the two sides of the WSe2 mono- and bilayer flakes with highly ordered molecular monolayers offers the possibility to simultaneously achieve energy level engineering and defect functionalization, representing a path toward deterministic control over charge transport in 2D materials.

Asymmetric Injection in Organic Transistors via Direct SAM Functionalization of Source and Drain Electrodes.

The fabrication of organic optoelectronic devices integrating asymmetric electrodes enables optimal charge injection/extraction at each individual metal/semiconductor interface. This is key for applications in devices such as solar cells, light-emitting transistors, photodetectors, inverters, and sensors. Here, we describe a new method for the asymmetric functionalization of gold electrodes with different thiolated molecules as a viable route to obtain two electrodes with drastically different work function values. The process involves an ad hoc design of electrode geometry and the use of a polymeric mask to protect one electrode during the first functionalization step. Photoelectron yield ambient spectroscopy and X-ray photoelectron spectrometry were used to characterize the energetic properties and the composition of the asymmetrically functionalized electrodes. Finally, we used poly(3-hexylthiophene)-based organic thin-film transistors to show that the asymmetric electronic response stems from the different electronic structures of the functionalized electrodes.

Morphology and Electronic Properties of Electrochemically Exfoliated Graphene.

Electrochemically exfoliated graphene (EEG) possesses optical and electronic properties that are markedly different from those of the more explored graphene oxide in both its pristine and reduced forms. EEG also holds a unique advantage compared to other graphenes produced by exfoliation in liquid media: it can be obtained in large quantities in a short time. However, an in-depth understanding of the structure-properties relationship of this material is still lacking. In this work, we report physicochemical characterization of EEG combined with an investigation of the electronic properties of this material carried out both at the single flake level and on the films. Additionally, we use for the first time microwave irradiation to reduce the EEG and demonstrate that the oxygen functionalities are not the bottleneck for charge transport in EEG, which is rather hindered by the presence of structural defects within the basal plane.

Flexible non-volatile optical memory thin-film transistor device with over 256 distinct levels based on an organic bicomponent blend.

Organic nanomaterials are attracting a great deal of interest for use in flexible electronic applications such as logic circuits, displays and solar cells. These technologies have already demonstrated good performances, but flexible organic memories are yet to deliver on all their promise in terms of volatility, operational voltage, write/erase speed, as well as the number of distinct attainable levels. Here, we report a multilevel non-volatile flexible optical memory thin-film transistor based on a blend of a reference polymer semiconductor, namely poly(3-hexylthiophene), and a photochromic diarylethene, switched with ultraviolet and green light irradiation. A three-terminal device featuring over 256 (8 bit storage) distinct current levels was fabricated, the memory states of which could be switched with 3 ns laser pulses. We also report robustness over 70 write-erase cycles and non-volatility exceeding 500 days. The device was implemented on a flexible polyethylene terephthalate substrate, validating the concept for integration into wearable electronics and smart nanodevices.

Solution-processed field-effect transistors based on dihexylquaterthiophene films with performances exceeding those of vacuum-sublimed films.

Solution-processable oligothiophenes are model systems for charge transport and fabrication of organic field-effect transistors (OFET) . Herein we report a structure vs function relationship study focused on the electrical characteristics of solution-processed dihexylquaterthiophene (DH4T)-based OFET. We show that by combining the tailoring of all interfaces in the bottom-contact bottom-gate transistor, via chemisorption of ad hoc molecules on electrodes and dielectric, with suitable choice of the film preparation conditions (including solvent type, concentration, volume, and deposition method), it is possible to fabricate devices exhibiting field-effect mobilities exceeding those of vacuum-processed DH4T transistors. In particular, the evaporation rate of the solvent, the processing temperature, as well as the concentration of the semiconducting material were found to hold a paramount importance in driving the self-assembly toward the formation of highly ordered and low-dimensional supramolecular architectures, confirming the kinetically governed nature of the self-assembly process. Among the various architectures, hundreds-of-micrometers long and thin DH4T crystallites exhibited enhanced charge transport.