Strain Effects on the Energy-Level Alignment at Metal/Organic Semiconductor Interfaces.

Flexible and wearable devices are among the upcoming trends in the opto-electronics market. Nevertheless, bendable devices should ensure the same efficiency and stability as their rigid analogs. It is well-known that the energy barriers between the metal Fermi energy and the molecular levels of organic semiconductors devoted to charge transport are key parameters in the performance of organic-based electronic devices. Therefore, it is paramount to understand how the energy barriers at metal/organic semiconductor interfaces change with bending. In this work, we experimentally measure the interface energy barriers between a metallic contact and small semiconducting molecules. The measurements are performed in operative conditions, while the samples are bent by a controlled applied mechanical strain. We determine that energy barriers are not sensitive to bending of the sample, but we observe that the hopping transport of the charges in flat molecules can be tuned by mechanical strain. The theoretical model developed for this work confirms our experimental observations.

Boron nitride nanoresonators for phonon-enhanced molecular vibrational spectroscopy at the strong coupling limit.

Enhanced light-matter interactions are the basis of surface-enhanced infrared absorption (SEIRA) spectroscopy, and conventionally rely on plasmonic materials and their capability to focus light to nanoscale spot sizes. Phonon polariton nanoresonators made of polar crystals could represent an interesting alternative, since they exhibit large quality factors, which go far beyond those of their plasmonic counterparts. The recent emergence of van der Waals crystals enables the fabrication of high-quality nanophotonic resonators based on phonon polaritons, as reported for the prototypical infrared-phononic material hexagonal boron nitride (h-BN). In this work we use, for the first time, phonon-polariton-resonant h-BN ribbons for SEIRA spectroscopy of small amounts of organic molecules in Fourier transform infrared spectroscopy. Strikingly, the interaction between phonon polaritons and molecular vibrations reaches experimentally the onset of the strong coupling regime, while numerical simulations predict that vibrational strong coupling can be fully achieved. Phonon polariton nanoresonators thus could become a viable platform for sensing, local control of chemical reactivity and infrared quantum cavity optics experiments.

A molecular spin-photovoltaic device.

We fabricated a C60 fullerene-based molecular spin-photovoltaic device that integrates a photovoltaic response with the spin transport across the molecular layer. The photovoltaic response can be modified under the application of a small magnetic field, with a magnetophotovoltage of up to 5% at room temperature. Device functionalities include a magnetic current inverter and the presence of diverging magnetocurrent at certain illumination levels that could be useful for sensing. Completely spin-polarized currents can be created by balancing the external partially spin-polarized injection with the photogenerated carriers.

Graphene as an electrode for solution-processed electron-transporting organic transistors.

Organic field-effect transistors (OFETs) are fundamental building blocks for plastic electronics such as organic photovoltaics or bendable displays with organic light emitting diodes, and radio-frequency identification (RFID) tags. A key part in the performance of OFET is the organic material constituting the channel. OFETs based on solution-processed polymers represent a new class of organic electronic devices. Recent developments in upscale solution-processed polymers have advanced towards high throughput, low-cost, and environmentally friendly materials for high-performance applications. Together with the integration of high performance materials, another enduring challenge in OFET development is the improvement and control of the injection of charge carriers. Graphene, a two-dimensional layer of covalently bonded carbon atoms, is steadily making progress into applications relying on van der Waals heterointerfaces with organic semiconductors. Here, we demonstrate the versatile operation of solution-processed organic transistors both in lateral and vertical geometries by exploiting the weak-screening effect and work function modulation properties of graphene electrodes. Our results demonstrate a general strategy for overcoming traditional noble metal electrodes and to integrate graphene with solution-processed Polyera ActiveInk™ N2200 polymer transistors for high-performance devices suitable for future plastic electronics.

Energy Level Alignment at Metal/Solution-Processed Organic Semiconductor Interfaces.

Energy barriers between the metal Fermi energy and the molecular levels of organic semiconductor devoted to charge transport play a fundamental role in the performance of organic electronic devices. Typically, techniques such as electron photoemission spectroscopy, Kelvin probe measurements, and in-device hot-electron spectroscopy have been applied to study these interfacial energy barriers. However, so far there has not been any direct method available for the determination of energy barriers at metal interfaces with n-type polymeric semiconductors. This study measures and compares metal/solution-processed electron-transporting polymer interface energy barriers by in-device hot-electron spectroscopy and ultraviolet photoemission spectroscopy. It not only demonstrates in-device hot-electron spectroscopy as a direct and reliable technique for these studies but also brings it closer to technological applications by working ex situ under ambient conditions. Moreover, this study determines that the contamination layer coming from air exposure does not play any significant role on the energy barrier alignment for charge transport. The theoretical model developed for this work confirms all the experimental observations.

Synthesis and Properties of a Twisted and Stable Tetracyano-Substituted Tetrabenzoheptacene.

An approach for the synthesis of pyrene-fused acenes that allows the introduction of electron-withdrawing cyano groups in key positions that simultaneously (i) induce twists in the aromatic framework and (ii) stabilize the LUMO level is reported. This combination of steric and electronic features provide a twisted, stable, and n-type tetrabenzoheptacene as confirmed by a combination of theoretical calculations and optical, electrochemical, thermal, and electrical characterization.

K-Conjugated Dibenzoazahexacenes.

The synthesis and properties of two highly stable K-conjugated dibenzoazahexacenes are reported. Single-crystal X-ray, optoelectronic, and electrochemical characterization combined with theoretical studies show a favorable molecular packing and an optimal energy alignment of their frontier orbitals for charge transport. Electrical characterization illustrates that the preferential transport of holes or electrons depends on the number of N atoms in the aromatic framework.

Bis(triisopropylsilylethynyl)-substituted pyrene-fused tetraazaheptacene: synthesis and properties.

The synthesis and characterisation of a pyrene-fused tetraazaheptacene that is constituted of two terminal pyrene units and a central tetraazaanthracene core are reported. The optoelectronic properties (experimental and calculated) of this heptacene derivative are discussed together with its charge transport properties in thin films.

Active Morphology Control for Concomitant Long Distance Spin Transport and Photoresponse in a Single Organic Device.

Long distance spin transport and photoresponse are demonstrated in a single F16 CuPc spin valve. By introducing a low-temperature strategy for controlling the morphology of the organic layer during the fabrication of a molecular spin valve, a large spin-diffusion length up to 180 nm is achieved at room temperature. Magnetoresistive and photoresponsive signals are simultaneously observed even in an air atmosphere.

An electron-conducting pyrene-fused phenazinothiadiazole.

A pyrene-fused phenazinothiadiazole that shows electron mobilities (µe = 0.016 cm(2) V(-1) s(-1)) two orders of magnitude higher than those reported for pyrene-fused pyrazaacenes is described.