Mark C. Lonergan

6,12-Diarylindeno(1,2-b)fluorenes: Syntheses, Photophysics, and Ambipolar OFETs

Herein we report the synthesis and characterization of a series of 6,12-diarylindeno[1,2-b]fluorenes (IFs). Functionalization with electron donor and acceptor groups influences the ability of the IF scaffold to undergo two-electron oxidation and reduction to yield the corresponding 18- and 22-π-electron species, respectively. A single crystal of the pentafluorophenyl-substituted IF can serve as an active layer in an organic field-effect transistor (OFET). The important finding is that the single-crystal OFET yields an ambipolar device that is able to transport holes and electrons.

Electron-accepting 6,12-diethynylindeno[1,2-b]fluorenes: synthesis, crystal structures, and photophysical properties.

Polycyclic hydrocarbons that possess extended p conjugation are of significant interest because of their potential use in optical and electronic devices such as light emitting devices, field-effect transistors, and photovoltaics. While a majority of studies have focused on acenes such as pentacene and its derivatives (e.g., 1; Scheme 1), these systems are susceptible to oxidative and photolytic degradation; thus, there is a need for alternative, acene-like molecules. One avenue in this search has explored compounds containing five-membered rings, rather than the more traditional six-membered rings. Prime examples of such molecules are dibenzopentalene (2) and derivatives thereof, wherein the groups of Saito, Kawase, and Tilley have recently described improved methods for their construction.

Current transport and the role of barrier inhomogeneities at the high barrier n-InP | poly(pyrrole) interface

A detailed study of current transport at the Schottky-type n-InP | poly(pyrrole) interface is presented. At room temperature, this interface exhibits an average quality factor of n=1.02±0.02, a C–V barrier height of qφbCV=0.78±0.01 eV, and a surface recombination velocity over two orders-of-magnitude slower than at ideal n-InP metal interfaces. These latter two parameters imply an effective barrier height of 0.9 eV, which is among the highest values ever reported for an n-InP Schottky-type diode. The quality factor increases monotonically with decreasing temperature reaching a value of 1.23 at 98 K. Substantial curvature is also observed in a Richardson plot at reduced temperature. These temperature dependencies can be quantitatively modeled using thermionic emission theory in the presence of barrier inhomogeneities. Standard models, including thermionic emission with image force effects, interfacial layer models with and without surface states, and tunneling, do not adequately explain the temperature dep...

Coupling semiconductor nanocrystals to a fused-silica microsphere: a quantum-dot microcavity with extremely high Q factors

We demonstrate a quantum-dot microcavity by coupling core-shell semiconductor nanocrystals to a fused-silica microsphere. We show that the composite microcavity can feature Q factors of the order of 10(8), providing a model system for investigating cavity QED and microlasers at the level of single quantum dots.

Current transport at the p-InP|poly(pyrrole) interface

The interface between the inorganic semiconductor p-type InP and the conjugated polymer poly(pyrrole) exhibits the electrical characteristics of a Schottky diode. Capacitance–voltage measurements yield an average barrier height of 0.62 ± 0.01 eV at temperature T =298 K. At the same temperature, the empirical quality factor, extracted from current–voltage measurements, is near unity. However, the current–voltage measurements show a deviation from thermionic emission theory as the temperature is reduced, as witnessed by the increase of the quality factor and the curvature in the Richardson plot. Such deviation is best explained by the barrier inhomogeneity model, in which the barrier becomes voltage dependent due to the interaction of a small low-barrier region with a higher surrounding potential, termed the “pinch-off” effect. Traditional current–voltage models, including image force lowering or an interfacial layer, cannot predict the temperature dependence of the current–voltage data, although thermionic...

Coherent two-dimensional photocurrent spectroscopy in a PbS quantum dot photocell

Recently there has been growing interest in the role of coherence in electronic dynamics. Coherent multidimensional spectroscopy has been used to reveal coherent phenomena in numerous material systems. Here we utilize a recent implementation of coherent multidimensional spectroscopy--two-dimensional photocurrent spectroscopy--in which we detect the photocurrent from a PbS quantum dot photocell resulting from its interactions with a sequence of four ultrafast laser pulses. We observe sub-picosecond evolution of two-dimensional spectra consistent with multiple exciton generation. Moreover, a comparison with two-dimensional fluorescence spectra of the quantum dots demonstrates the potential of two-dimensional photocurrent spectroscopy to elucidate detailed origins of photocurrent generating electronic state coherence pathways. Since the measurement is based on detecting the photocell current in situ, the method is well suited to study the fundamental ultrafast processes that affect the function of the device. This opens new avenues to investigate and implement coherent optimization strategies directly within devices.

Solution phase n-doping of C60 and PCBM using tetrabutylammonium fluoride

The solution phase n-doping of C60 and PCBM with tetrabutylammonium fluoride is shown to occur via an initial chemical reaction followed by electron transfer to a second fullerene molecule. The formation of ionic and radical intermediate species has significant implications for the use of ionically functionalized materials as electron-selective interface layers in OPVs.

Calculation of transmission coefficients at nonideal semiconductor interfaces characterized by a spatial distribution of barrier heights

The use of temperature-dependent current–voltage and capacitance–voltage measurements in the determination of transmission coefficients, which can be related to various other heterogeneous charge-transfer rate constants, is analyzed for semiconductor interfaces characterized by a spatial distribution of barrier heights. The level of error introduced by the unknowing neglect of heterogeneity is analyzed using a discrete parallel network of regions with potentially voltage and temperature dependent effective areas and potential barriers. In general, the unknowing neglect of heterogeneity results in an overestimation of the transmission coefficient calculated from barrier heights based on capacitance–voltage measurements and an underestimation when based on barrier heights from temperature dependent current–voltage measurements (Richardson plots). Of particular focus is the calculation of transmission coefficients at semiconductor interfaces that exhibit anomalous behavior, most notably ideality or quality f...

Synthesis, Properties, and Design Principles of Donor-Acceptor Nanohoops.

We have synthesized a series of aza[8]cycloparaphenylenes containing one, two, and three nitrogens to probe the impact of nitrogen doping on optoelectronic properties and solid state packing. Alkylation of these azananohoops afforded the first donor–acceptor nanohoops where the phenylene backbone acts as the donor and the pyridinium units act as the acceptor. The impact on the optoelectronic properties was then studied experimentally and computationally to provide new insight into the effect of functionalization on nanohoops properties.

Gate electrode processes in an electrolyte-gated transistor: Non-Faradaically versus Faradaically coupled conductivity modulation of a polyacetylene ionomer

The significance of gate electrode processes to the transfer characteristics of electrochemical transistors is demonstrated using a device based on a polyacetylene ionomer. Two regimes of operation are identified. The first involves a conventional redox process where two Faradaic processes balance: oxidative doping of the polymer to modulate conductivity is coupled to reduction at the gate electrode. The second involves a nonconventional hybrid mode of operation: doping is coupled to the charging of an ionic double layer at the gate electrode, which is a non-Faradaic process. The implications of these two regimes on the design and performance of electrochemical transistors are discussed.