Claudia Piliego

Outlook and Emerging Semiconducting Materials for Ambipolar Transistors

Ambipolar or bipolar transistors are transistors in which both holes and electrons are mobile inside the conducting channel. This device allows switching among several states: the hole-dominated on-state, the off-state, and the electron-dominated on-state. In the past year, it has attracted great interest in exotic semiconductors, such as organic semiconductors, nanostructured materials, and carbon nanotubes. The ability to utilize both holes and electrons inside one device opens new possibilities for the development of more compact complementary metal-oxide semiconductor (CMOS) circuits, and new kinds of optoelectronic device, namely, ambipolar light-emitting transistors. This progress report highlights the recent progresses in the field of ambipolar transistors, both from the fundamental physics and application viewpoints. Attention is devoted to the challenges that should be faced for the realization of ambipolar transistors with different material systems, beginning with the understanding of the importance of interface modification, which heavily affects injections and trapping of both holes and electrons. The recent development of advanced gating applications, including ionic liquid gating, that open up more possibility to realize ambipolar transport in materials in which one type of charge carrier is highly dominant is highlighted. Between the possible applications of ambipolar field-effect transistors, we focus on ambipolar light-emitting transistors. We put this new device in the framework of its prospective for general lightings, embedded displays, current-driven laser, as well as for photonics-electronics interconnection.

5.2% efficient PbS nanocrystal Schottky solar cells

The impact of post-synthetic treatments of nanocrystals (NCs) on the performance of Schottky solar cells, where the active PbS nanocrystal layer is sandwiched directly between two electrodes, is investigated. By monitoring the amount of ligands on the surface of the nanocrystals through Fourier Transform Infrared (FTIR) measurements, we find that optimized processing conditions can lead to high current density and thus to an increase in overall efficiency. Our devices reach an efficiency of 5.2%, which is the highest reported using a PbS nanocrystal Schottky junction. These results demonstrate that even by using the simplest device architecture, accurate post-synthetic treatments result in substantial improvements in the performance. By drawing a direct correlation between ligand-to-NC ratio in the starting PbS solution and the device parameters, we provide important insights on how to gain experimental control for the fabrication of efficient PbS solar cells.

Low Driving Voltage and High Mobility Ambipolar Field-Effect Transistors with PbS Colloidal Nanocrystals

PbS colloidal nanocrystals (NCs) are promising materials for optoelectronic devices, due to their size-tunable properties. However, there is still minimal understanding of their charge transport mechanism. Through a combination of ligand selections, ambipolar transistor structure optimization, and electrochemical gating usage, high carrier mobility is achieved. The outstanding device characteristics open possibility to investigate the intrinsic transport properties of PbS NCs.

Engineering transfer of micro- and nanometer-scale features by surface energy modification.

Micropatterning of surfaces is gaining importance in various applications ranging from biosensors to microfluidic and lab-on-a-chip devices, where the control of the surface chemistry is of great importance for the application. In this paper, we introduce a patterning technique of topographical features, which is applicable on different substrates by modifying their surface energy. The textured surface is obtained via polydimethylsiloxane (PDMS) transfer, and the topographical parameters can be systematically tailored by selective treatment with oxygen plasma of either the PDMS stamp, the substrate, or both. Our approach is an alternative technique to create micro- and nanopatterns of various height and shape over a large area on different substrates. The possibility to control cell behavior on different surfaces tailored with this microtransfer patterning approach was also evaluated. The cell culture on patterned surfaces showed the possibility of modulating cell adhesion. Our method is based on simple transfer of silicone elastomeric patterns to the surface, and therefore, it is very simple and fast compared to other complex techniques. These observations could have implications for tissue-scaffold engineering science in areas such as microfluidic devices and control of cell adhesion.

Bicolor electroluminescent pixels from single active molecular material.

We report on the fabrication of the first bicolor micropixelated OLED from a single molecular material using a single-step bottom up procedure. The implementation of a deposition technique, based on a spatial-switch and conformational-sensitive STD surface-tension-driven lithography, has allowed us to exploit the spontaneous supramolecular properties and the conformational flexibility of a conjugated thiophene-based material, 6-bis-(50-hexyl-[2, 20]bithiophen-5-yl)-3, 5-dimethyl-dithieno[3, 2-b; 20, 30-d]thiophene (DTT7Me). The existence of two regularly alternating emitting regions on a micrometer scale allows obtaining electroluminescent emission at two different wavelengths from a single material.

RANDOM POLY(2, 7‐FLUORENYLENEVINYLENE) COPOLYMERS OBTAINED BY A SUZUKI‐HECK REACTION: SYNTHESIS AND PROPERTIES

The study reports on the synthesis of poly arylene vinylenes by use of a cascade Suzuki‐Heck reaction polymerization. The reaction between suitable aryldibromides with potassium vinyltrifluoroborate in the presence of a Pd(0) catalyst permits the access to a series of poly(fluorenylenevinylene)s. The protocol is characterized by a great versatility, deriving from the use of easily attainable substrates, and yields polymers with low percentages of structural 1, 1‐diarylenevinylene defects. Exploiting this procedure, a series of random poly(9, 9‐dioctyl‐2, 7‐fluorenylenevinylene‐co‐N‐octyl‐3, 6‐carbazolylenevinylene) copolymers at 20÷80% mol/mol of carbazole (PFV2‐6) was achieved. Their properties have been investigated by 1H‐NMR, IR, TGA, DSC, cyclic voltammetry, UV‐Vis and photoluminescence, and compared to those of poly(9, 9‐dioctyl‐2, 7‐fluorenylenevinylene) (PFV1) obtained by the same method. The electroluminescence properties of the materials were tested by constructing OLED devices of ITO/PEDOT‐PSS/P...