Mujeeb Ullah

Intermolecular hydrogen-bonded organic semiconductors—Quinacridone versus pentacene

Quinacridone is a five-ring hydrogen-bonded molecule analogous in structure and size to the well-known organic semiconductor pentacene. Unlike pentacene, quinacridone has limited intramolecular π-conjugation and becomes highly colored in the solid state due to strong intermolecular electronic coupling. We found that quinacridone shows a field-effect mobility of 0.1 cm2/V·s, comparable to mobilities of pentacene in similarly prepared devices. Photoinduced charge generation in single-layer quinacridone metal-insulator-metal diodes is more than a hundred times more efficient than in pentacene devices. Photoinduced charge transfer from quinacridone to C60 is not effective, as evidenced by measurements in heterojunctions with C60. Hydrogen-bonded organic solids may provide new avenues for organic semiconductor design.

Simultaneous enhancement of brightness, efficiency, and switching in RGB organic light emitting transistors.

An innovative design strategy for light emitting field effect transistors (LEFETs) to harvest higher luminance and switching is presented. The strategy uses a non-planar electrode geometry in tri-layer LEFETs for simultaneous enhancement of the key parameters of quantum efficiency, brightness, switching, and mobility across the RGB color gamut.

All solution-processed, hybrid light emitting field-effect transistors

All solution-processed, high performance hybrid light emitting transistors (HLETs) are realized. Using a novel combination of device architecture and materials a bilayer device comprised of an inorganic and organic semiconducting layer is fabricated and the optoelectronic properties are presented.

Strain induced anisotropic effect on electron mobility in C60 based organic field effect transistors

The electron mobility was found to increase (decrease) upon applied compressive (tensile) strain, respectively, when a high-performance flexible C60-based organic field-effect transistor (OFET) was subjected to different bending radii. The observed almost twofold relative change in the electron mobility is considerably larger than that reported before for pentacene-based OFETs. Moreover, the strain dependency of electron mobility in C60 films is strongly anisotropic with respect to the strain direction measured relative to the current flow. Analysis within a hopping-transport model for OFET mobility suggests that the observed strain dependency on electron transport is dominated mostly by the change of inter-grain coupling in polycrystalline C60 films.

Electrical response of highly ordered organic thin film metal-insulator-semiconductor devices

We report a detailed investigation of the electrical properties of organic field-effect transistors (OFETs) and metal-insulator-semiconductor (MIS) capacitors formed from highly ordered thin films of C60 as the active semiconductor and divinyltetramethyl disiloxane-bis(benzocyclobutene) (BCB) as the gate dielectric. Current-voltage measurements show the OFETs to be n-channel devices characterized by a high electron mobility (∼6 cm2/V s). An equivalent circuit model is developed which describes well both the frequency and voltage dependences of the small-signal admittance data obtained from the corresponding MIS capacitors. By fitting the circuit response to experimental data, we deduce that increasing gate voltages increases the injection of extrinsic charge carriers (electrons) into the C60. Simultaneously, the insulation resistance of the BCB decreases, presumably by electron injection into the insulator. Furthermore, the admittance spectra suggest that the capacitance-voltage (C-V) behavior originates ...

Electric field dependent activation energy of electron transport in fullerene diodes and field effect transistors: Gill’s law

The electric field and temperature dependence of the electron mobility is studied comparatively in the bulk of fullerene (C60) diodes and at the interface with dielectric of organic field effect transistors (OFETs). Electron mobility values follow a Poole–Frenkel-type electric field dependence in both types of devices. The activation energy for electron transport is electric field dependent and follows the square root law of field in both devices as predicted by Gill’s law. The same Gill’s energy EGill=34 meV is measured in diodes and OFETs, which corresponds well to Meyer–Neldel energy (EMN=35 meV). It is shown that both the electric field and charge carrier concentration must be accounted for the description of disordered charge transport.

Dependence of Meyer–Neldel energy on energetic disorder in organic field effect transistors

Meyer–Neldel rule for charge carrier mobility was studied in C60-based organic field effect transistors (OFETs) fabricated at different growth conditions which changed the degree of disorder in the films. The energetic disorder in the films was found to correlate with a shift in the Meyer–Neldel energy, which is in excellent agreement with the predictions of a hopping-transport model for the temperature dependent OFET mobility in organic semiconductors with a Gaussian density-of-states (DOS). Using this model the width of the DOS was evaluated and it was found to decrease from 88 meV for the films grown at room temperature to 54 meV for films grown at 250 °C.

High‐Performance, Fullerene‐Free Organic Photodiodes Based on a Solution‐Processable Indigo

A solution-processable dibromoindigo with an alkyoxyphenyl solubilizing group is developed and used as a new electron acceptor in organic photodiodes. The solution-processed fullerene-free organic photodiodes show an almost spectrally flat response with a high responsivity (0.4 A W(-1)) and a high detectivity (1 × 10(12) Jones). These values are comparable to silicon-based photodiodes.

Effect of source-drain electric field on the Meyer–Neldel energy in organic field effect transistors

We studied the influence of the lateral source-drain electric field on the Meyer–Neldel phenomenon observed for the charge mobility measured in C60-based organic field effect transistors (OFETs). It was found that the characteristic Meyer-Neldel temperature notably shifts with applied source drain electric field. This finding is in excellent agreement with an analytic model recently extended to account also for the field dependence of the charge carrier mobility in materials with a Gaussian density-of-states distribution. As the theoretical model to predict charge carrier mobility is not limited to zero-electric field, it provides a more accurate evaluation of energetic disorder parameters from experimental data measured at arbitrary electric fields.

Hybrid Area‐Emitting Transistors: Solution Processable and with High Aperture Ratios

Area emission is realized in all-solution-processed hybrid light-emitting transistors (HLETs). A new HLET design is presented with increased aperture ratio, and optical and electrical characteristics are shown.