Photophysics of Self-Assembled Monolayers of a π-Conjugated Quinquethiophene Derivative

The photophysics of fully and partially covered self-assembled monolayers (SAMs) of a quinquethiophene (5T) derivative have been investigated. The monolayers behave as H-aggregates. The fluorescence of fully covered SAMs is weak and red-shifted, and the extinction is blue-shifted as compared to that of single molecules. The fluorescence of partially covered SAMs is dominated by that of single molecules on the surface. The extinction spectra are similar for fully and partially covered monolayers, which show that even the smallest islands are H-aggregates. The extinction spectra furthermore closely resemble those for 5T single crystals, which demonstrates that in oligothiophene crystals the intermolecular interactions within one layer molecules are stronger than the interlayer electronic coupling.

Gate-Bias Controlled Charge Trapping as a Mechanism for NO2 Detection with Field-Effect Transistors

Detection of nitrogen dioxide, NO2, is required to monitor the air-quality for human health and safety. Commercial sensors are typically chemiresistors, however field-effect transistors are being investigated. Although numerous investigations have been reported, the NO2 sensing mechanism is not clear. Here, the detection mechanism using ZnO field-effect transistors is investigated. The current gradually decreases upon NO2 exposure and application of a positive gate bias. The current decrease originates from the trapping of electrons, yielding a shift of the threshold voltage towards the applied gate bias. The shift is observed for extremely low NO2 concentrations down to 10 ppb and can phenomenologically be described by a stretched-exponential time relaxation. NO2 detection has been demonstrated with n-type, p-type, and ambipolar semiconductors. In all cases, the threshold voltage shifts due to gate bias induced electron trapping. The description of the NO2 detection with field-effect transistors is generic for all semiconductors and can be used to improve future NO2 sensors.

Small band gap copolymers based on furan and diketopyrrolopyrrole for field-effect transistors and photovoltaic cells

Four small band gap semiconducting copolymers based on electron deficient diketopyrrolopyrrole alternating with electron rich trimers containing furan and benzene or thiophene have been synthesized via Suzuki polymerization. The polymers have optical band gaps between 1.4 and 1.6 eV, optimized for solar energy conversion, and exhibit ambipolar charge transport in field-effect transistors with hole and electron mobilities higher than 10-2 cm2 V-1 s-1. In solar cells the polymers are used as electron donors and provide power conversion efficiencies up to 3.7% in simulated solar light when mixed with [70]PCBM as acceptor.

Revealing Buried Interfaces to Understand the Origins of Threshold Voltage Shifts in Organic Field-Effect Transistors

The semiconductor of an organic field-effect transistor is stripped with adhesive tape, yielding an exposed gate dielectric, accessible for various characterization techniques. By using scanning Kelvin probe microscopy we reveal that trapped charges after gate bias stress are located at the gate dielectric and not in the semiconductor. Charging of the gate dielectric is confirmed by the fact that the threshold voltage shift remains, when a pristine organic semiconductor is deposited on the exposed gate dielectric of a stressed and delaminated field-effect transistor.

Manipulating the Local Light Emission in Organic Light-Emitting Diodes by using Patterned Self-Assembled Monolayers

In organic light-emitting diodes (OLEDs), interface dipoles play an important role in the process of charge injection from the metallic electrode into the active organic layer.[1,2] An oriented dipole layer changes the effective work function of the electrode because of its internal electric field. The differences between the work functions of the electrodes and the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the light-emitting polymer determine the injection barriers and, thus, the electron/hole current balance and light emission of an OLED.[3] Therefore, the local light emission can be enhanced or suppressed by changing the work function on a local scale. This control of local emission is ideally suited for OLED-based signage applications, for example

Gas sensing with self-assembled monolayer field-effect transistors

A new sensitive gas sensor based on a self-assembled monolayer field-effect transistor (SAMFET) was used to detect the biomarker nitric oxide. A SAMFET based sensor is highly sensitive because the analyte and the active channel are separated by only one monolayer. SAMFETs were functionalised for direct NO detection using iron porphyrin as a specific receptor. Upon exposure to NO a threshold voltage shift towards positive gate biases was observed. The sensor response was examined as a function of NO concentration. High sensitivity has been demonstrated by detection of ppb concentrations of NO. Preliminary measurements have been performed to determine the selectivity.

Microstructure and Phase Behavior of a Quinquethiophene-Based Self-Assembled Monolayer as a Function of Temperature

The self-assembly of monolayers is a highly promising approach in organic electronics, but most systems show weak device performances, probably because of a lack of long-range order of the molecules. The present self-assembled monolayer was formed by a molecule that contains a dimethylchlorosilyl group combined with a quinquethiophene unit through an undecane spacer. This system is the first reported self-assembled monolayer on silicon oxide surfaces that forms two-dimensional crystals. A detailed structural solution is presented based on grazing-incidence X-ray scattering experiments and theoretical packing analysis. By transverse shear microscopy, the shape and size of the crystallites were determined: polygonal shapes with lateral sizes of several micrometers were observed. In situ temperature studies revealed gradual changes of the molecular packing that were irreversible. Melting of the crystal structure was found at 520 K, whereas the self-assembled monolayer remained stable up to 620 K. This work presents unknown structural properties of a self-assembled monolayer revealing insights into layer formation and irreversible evolution upon temperature treatment.