Ruth A. Schlitz

Solubility-limited extrinsic n-type doping of a high electron mobility polymer for thermoelectric applications.

The thermoelectric properties of a highperformance electron-conducting polymer, (P(NDIOD-T2), extrinsically doped with dihydro-1H-benzoimidazol-2-yl (NDBI) derivatives, are reported. The highest thermoelectric power factor that has been reported for a solution-processed n-type polymer is achieved; and it is concluded that engineering polymerdopant miscibility is essential for the development of organic thermoelectrics.

Correlating dopant distributions and electrical properties of boron-doped silicon nanowires

Quantitative nonuniform radial doping profiles in vapor liquid solid grown boron-doped silicon nanowires are correlated with axial variations in electrical properties. Boron concentrations measured by atom probe tomography are lower for the core material grown from a gold catalyst than for material deposited on the nanowire surface. Transistors fabricated along a single nanowire exhibit a transition from nonlinear contact-dominated behavior to linear behavior with increasing thickness of the dopant-enriched surface layer. Simple models confirm that the surface is doped to a level that enables the contact resistance to become comparable to the channel resistance, suggesting that unintentional surface doping may play a role in lowering contact resistances in some nanowire devices.

Ligand coupling symmetry correlates with thermopower enhancement in small-molecule/nanocrystal hybrid materials.

We investigate the impact of the coupling symmetry and chemical nature of organic-inorganic interfaces on thermoelectric transport in Cu2-xSe nanocrystal thin films. By coupling ligand-exchange techniques with layer-by-layer assembly methods, we are able to systematically vary nanocrystal-organic linker interfaces, demonstrating how the functionality of the polar headgroup and the coupling symmetry of the organic linkers can change the power factor (S(2)σ) by nearly 2 orders of magnitude. Remarkably, we observe that ligand-coupling symmetry has a profound effect on thermoelectric transport in these hybrid materials. We shed light on these results using intuition from a simplified model for interparticle charge transport via tunneling through the frontier orbital of a bound ligand. Our analysis indicates that ligand-coupling symmetry and binding mechanisms correlate with enhanced conductivity approaching 2000 S/cm, and we employ this concept to demonstrate among the highest power factors measured for quantum-dot based thermoelectric inorganic-organic composite materials of ∼ 30 μW/m · K(2).

Morphology-dependent optical anisotropies in the n-type polymer P(NDI2OD-T2)

Organic semiconductors tend to self-assemble into highly ordered and oriented morphologies with anisotropic optical properties. Studying these optical anisotropies provides insight into processing-dependent structural properties and informs the photonic design of organic photovoltaic and light-emitting devices. Here we measure the anisotropic optical properties of spin-cast films of the

Quantitative statistical analysis of dielectric breakdown in zirconia-based self-assembled nanodielectrics

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Model-blind characterization of thin-film optical constants with momentum-resolved reflectometry

-type polymer P(NDI2OD-T2) using momentum-resolved absorption and emission spectroscopies. We quantify differences in the optical anisotropies of films deposited with distinct face-on and edge-on morphologies. In particular, we infer a substantially larger out-of-plane tilt angle of the optical transition dipole moment in high temperature annealed, edge-on films. Measurements of spectral differences between in-plane and out-of-plane dipoles, further indicate regions of disordered polymers in low temperature annealed face-on films that are otherwise obscured in traditional x-ray and optical characterization techniques. The methods and analysis developed in this work provide a way to identify and quantify subtle optical and structural anisotropies in organic semiconductors that are important for understanding and designing highly efficient thin film devices.

Side-chain effects on the conductivity, morphology, and thermoelectric properties of self-doped narrow-band-gap conjugated polyelectrolytes

Uniformity of the dielectric breakdown voltage distribution for several thicknesses of a zirconia-based self-assembled nanodielectric was characterized using the Weibull distribution. Two regimes of breakdown behavior are observed: self-assembled multilayers >5 nm thick are well described by a single two-parameter Weibull distribution, with β ≈ 11. Multilayers ≤5 nm thick exhibit kinks on the Weibull plot of dielectric breakdown voltage, suggesting that multiple characteristic mechanisms for dielectric breakdown are present. Both the degree of uniformity and the effective dielectric breakdown field are observed to be greater for one layer than for two layers of Zr-SAND, suggesting that this multilayer is more promising for device applications.

Weibull Analysis of Dielectric Breakdown in a Self-Assembled Nanodielectric for Organic Transistors

Determining optical constants of thin material films is important for characterizing their electronic excitations and for the design of optoelectronic devices. Spectroscopic ellipsometry techniques have emerged as the predominant approach for measuring thin-film optical constants. However, ellipsometry methods suffer from complications associated with highly model-dependent, multi-parameter spectral fitting procedures. Here, we present a model-blind, momentum-resolved reflectometry technique that yields accurate and precise optical constants, with quantifiable error estimates, even for film thicknesses less than 50 nm. These capabilities are demonstrated by interrogating an optical absorption resonance in films of the polymer P(NDI2OD-T2). We show that this approach produces exceptional agreement with UV-Vis-NIR absorption measurements, while simultaneously avoiding the need to construct complicated multi-oscillator spectral models. Finally, we use this procedure to resolve subtle differences in the out-of-plane optical properties of different film morphologies that were previously obscured in ellipsometry measurements.