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

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.

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

This contribution reports a series of anionic narrow-band-gap self-doped conjugated polyelectrolytes (CPEs) with π-conjugated cyclopenta-[2,1-b;3,4-b']-dithiophene-alt-4,7-(2,1,3-benzothiadiazole) backbones, but with different counterions (Na(+), K(+), vs tetrabutylammonium) and lengths of alkyl chains (C4 vs C3). These materials were doped to provide air-stable, water-soluble conductive materials. Solid-state electrical conductivity, thermopower, and thermal conductivity were measured and compared. CPEs with smaller counterions and shorter side chains exhibit higher doping levels and form more ordered films. The smallest countercation (Na(+)) provides thin films with higher electrical conductivity, but a comparable thermopower, compared to those with larger counterions, thereby leading to a higher power factor. Chemical modifications of the pendant side chains do not influence out of plane thermal conductivity. These studies introduce a novel approach to understand thermoelectric performance by structural modifications.

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.

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).

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

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 ß ≈ 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.