Lee J. Richter

Molecular Packing of High-Mobility Diketo Pyrrolo-Pyrrole Polymer Semiconductors with Branched Alkyl Side Chains

We describe a series of highly soluble diketo pyrrolo-pyrrole (DPP)-bithiophene copolymers exhibiting field effect hole mobilities up to 0.74 cm(2) V(-1) s(-1), with a common synthetic motif of bulky 2-octyldodecyl side groups on the conjugated backbone. Spectroscopy, diffraction, and microscopy measurements reveal a transition in molecular packing behavior from a preferentially edge-on orientation of the conjugated plane to a preferentially face-on orientation as the attachment density of the side chains increases. Thermal annealing generally reduces both the face-on population and the misoriented edge-on domains. The highest hole mobilities of this series were obtained from edge-on molecular packing and in-plane liquid-crystalline texture, but films with a bimodal orientation distribution and no discernible in-plane texture exhibited surprisingly comparable mobilities. The high hole mobility may therefore arise from the molecular packing feature common to the entire polymer series: backbones that are strictly oriented parallel to the substrate plane and coplanar with other backbones in the same layer.

Laser-excited hot-electron induced desorption: A theoretical model applied to NO/Pt(111)

Abstract We present a theoretical model for stimulated desorption due to the interaction of energetic substrate carriers with molecular adsorbates. The model is based on the premise that optically excited hot electrons scatter into an unoccupied valence electron resonance of the adsorbate, thus forming a temporary negative molecular ion which then experiences an enhanced attraction towards the substrate. Neutralization of the ion returns the adsorbed molecule to one of the continuum states of the molecule/substrate potential energy surface, possibly in an internally excited state. The consequences of such a model are worked out using semiclassical wave packet dynamics which, in the short time limit relevant to the present situation, can be brought to an analytic realization. The model provides considerable insight into recent experiments on the laser-induced desorption of NO from Pt(111).

Molecular origin of high field-effect mobility in an indacenodithiophene–benzothiadiazole copolymer

One of the most inspiring and puzzling developments in the organic electronics community in the last few years has been the emergence of solution-processable semiconducting polymers that lack significant long-range order but outperform the best, high-mobility, ordered semiconducting polymers to date. Here we provide new insights into the charge-transport mechanism in semiconducting polymers and offer new molecular design guidelines by examining a state-of-the-art indacenodithiophene-benzothiadiazole copolymer having field-effect mobility of up to 3.6 cm(2) V(-1) s(-1) with a combination of diffraction and polarizing spectroscopic techniques. Our results reveal that its conjugated planes exhibit a common, comprehensive orientation in both the non-crystalline regions and the ordered crystallites, which is likely to originate from its superior backbone rigidity. We argue that charge transport in high-mobility semiconducting polymers is quasi one-dimensional, that is, predominantly occurring along the backbone, and requires only occasional intermolecular hopping through short π-stacking bridges.

Vibrationally resolved sum-frequency generation with broad-bandwidth infrared pulses.

We present a novel procedure for vibrationally resolved sum-frequency generation (SFG) in which a broad-bandwidth IR pulse is mixed with a narrow-bandwidth visible pulse. The resultant SFG spectrum is dispersed with a spectrograph and detected in parallel with a scientific-grade CCD detector, permitting rapid and high signal-to-noise ratio data acquisition over a 400-cm(-1) spectral region without scanning the IR frequency. Application to the study of a self-assembled monolayer of octadecanethiol is discussed.

Substrate-dependent interface composition and charge transport in films for organic photovoltaics

The buried interface composition of polymer-fullerene blends is found by near-edge x-ray absorption fine structure spectroscopy to depend on the surface energy of the substrate upon which they are cast. The interface composition determines the type of charge transport measured with thin film transistors. These results have implications for organic photovoltaics device design and the use of transistors to evaluate bulk mobility in blends.

State‐resolved evidence for hot carrier driven surface reactions: Laser‐induced desorption of NO from Pt(111)

State‐specific diagnostics are used to characterize the laser‐induced desorption of NO from Pt(111). Two desorption channels are observed; one is consistent with thermal activation, while the other is driven by adsorbate interactions with hot carriers. For this latter channel, the observed dependence of the desorption yield on the wavelength of the incident laser pulse (1907, 1064, 532, and 355 nm) and the wavelength dependence of the kinetic energy distributions establish the nonthermal nature of the excitation process. The inverted spin–orbit population, the non‐Boltzmann rotational state distributions, and the vibrational state population are interpreted in terms of a desorption mechanism involving a temporary ion resonance.

Molecular Characterization of Organic Electronic Films

Organic electronics have emerged as a viable competitor to amorphous silicon for the active layer in low-cost electronics. The critical performance of organic electronic materials is closely related to their morphology and molecular packing. Unlike their inorganic counterparts, polymers combine complex repeat unit structure and crystalline disorder. This combination prevents any single technique from being able to uniquely solve the packing arrangement of the molecules. Here, a general methodology for combining multiple, complementary techniques that provide accurate unit cell dimensions and molecular orientation is described. The combination of measurements results in a nearly complete picture of the organic film morphology.

Correlations between Mechanical and Electrical Properties of Polythiophenes

The elastic moduli of polythiophenes, regioregular poly(3-hexylthiophene) (P3HT) and poly-(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene) (pBTTT), are compared to their field effect mobility showing a proportional trend. The elastic moduli of the films are measured using a buckling-based metrology, and the mobility is determined from the electrical characteristics of bottom contact thin film transistors. Moreover, the crack onset strain of pBTTT films is shown to be less than 2.5%, whereas that of P3HT is greater than 150%. These results show that increased long-range order in polythiophene semiconductors, which is generally thought to be essential for improved charge mobility, can also stiffen and enbrittle the film. This work highlights the critical role of quantitative mechanical property measurements in guiding the development of flexible organic semiconductors.

Use of X-Ray Diffraction, Molecular Simulations, and Spectroscopy to Determine the Molecular Packing in a Polymer-Fullerene Bimolecular Crystal

The molecular packing in a polymer: fullerene bimolecular crystal is determined using X-ray diffraction (XRD), molecular mechanics (MM) and molecular dynamics (MD) simulations, 2D solid-state NMR spectroscopy, and IR absorption spectroscopy. The conformation of the electron-donating polymer is significantly disrupted by the incorporation of the electron-accepting fullerene molecules, which introduce twists and bends along the polymer backbone and 1D electron-conducting fullerene channels.

The populations of bridge and top site CO on Rh(100) vs coverage, temperature, and during reaction with O

The intensities of the stretch modes of CO adsorbed in bridge and top sites on Rh(100) are presented as a function of coverage, temperature, and during reaction with O. Following adsorption at 90 K, the top site is predominantly occupied at low coverages, and at 0.5 atomic monolayers (ML) only the top site is occupied and a c(2×2) low energy electron diffraction (LEED) pattern is observed. With higher coverages the bridge site is increasingly occupied, and at saturation a p(4(2)1/2×(2)1/2)R45° LEED pattern is observed. The occupations of bridge and top sites at a fixed (0.5 ML) total coverage are observed to vary reversibly when the temperature is linearly ramped from 87 to 371 K and back down to 90 K; the difference in the binding enthalpy of bridge and top sites is determined to be 1.10±0.06 kcal mol−1. The enthalpy difference between bridge and top sites is also obtained from temperature programmed EELS (TP‐EELS) which follows the site occupations as the temperature is increased past desorption. Surfac...