No abstract

We present experimental evidence of a transverse thermopower, or planar Nernst effect, in ferromagnetic metal thin films driven by thermal gradients applied in the plane of the films. Samples of 20 nm thick Ni and Ni(80)Fe(20) were deposited on 500 nm thick suspended Si-N thermal isolation platforms with integrated platinum strips designed originally to allow measurement of thermally generated spin currents (the spin Seebeck effect). The low thermal conductivity of the thin supporting Si-N structure results in an essentially 2D geometry that approaches the zero substrate limit, dramatically reducing the contribution of thermal gradients perpendicular to the sample plane typically found in similar experiments on bulk substrates. The voltage on the platinum strips generated transverse to the applied thermal gradient (V(T)) is linear with increasing ΔT and exhibits a sign reversal on hot and cold sides of the sample. However, V(T) is always even in applied magnetic field and shows a sinθ cosθ angular dependence, both key indicators of the planar Nernst effect. Within the 5 nV estimated error of our experiment there is no evidence of a signal from the spin Seebeck effect, which would have cosθ angular dependence, suggesting a reduced spin Seebeck coefficient in a planar, entirely thin-film geometry.

Semiconducting single-walled carbon nanotubes (s-SWCNTs) have recently attracted attention for their promise as active components in a variety of optical and electronic applications, including thermoelectricity generation. Here we demonstrate that removing the wrapping polymer from the highly enriched s-SWCNT network leads to substantial improvements in charge carrier transport and thermoelectric power factor. These improvements arise primarily from an increase in charge carrier mobility within the s-SWCNT networks because of removal of the insulating polymer and control of the level of nanotube bundling in the network, which enables higher thin-film conductivity for a given carrier density. Ultimately, these studies demonstrate that highly enriched s-SWCNT thin films, in the complete absence of any accompanying semiconducting polymer, can attain thermoelectric power factors in the range of ∼400 μW m −1 K −2 , which is on par with that of some of the best single-component organic thermoelectrics demonstrated to date.

Semiconducting single-walled carbon nanotubes (s-SWCNTs) are promising candidates as the active layer in photovoltaics (PV), particularly for niche applications where high infrared absorbance and/or semi-transparent solar cells are desirable. Most current fabrication strategies for SWCNT PV devices suffer from relatively high surface roughness and lack nanometer-scale deposition precision, both of which may hamper the reproducible production of ultrathin devices. Additionally, detailed optical models of SWCNT PV devices are lacking, due in part to a lack of well-defined optical constants for high-purity s-SWCNT thin films. Here, we present an optical model that accurately reconstructs the shape and magnitude of spectrally resolved external quantum efficiencies for ultrathin (7,5) s-SWCNT/C60 solar cells that are deposited by ultrasonic spraying. The ultrasonic spraying technique enables thickness tuning of the s-SWCNT layer with nanometer-scale precision, and consistently produces devices with low s-SWCNT film average surface roughness (Rq of <5 nm). Our optical model, based entirely on measured optical constants of each layer within the device stack, enables quantitative predictions of thickness-dependent relative photocurrent contributions of SWCNTs and C60 and enables estimates of the exciton diffusion lengths within each layer. These results establish routes towards rational performance improvements and scalable fabrication processes for ultra-thin SWCNT-based solar cells.