Carrier Lifetime Enhancement in a Tellurium Nanowire/PEDOT:PSS Nanocomposite by Sulfur Passivation
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Carrier Lifetime Enhancement in a Tellurium Nanowire/PEDOT:PSS Nanocomposite by Sulfur Passivation James N. Heyman1, Ayaskanta Sahu2, Nelson E. Coates2, Brittany Ehmann1, Jeffery J. Urban2 1. Physics Department, Macalester College, St. Paul, MN 55105, USA 2. The Molecular Foundry, Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720, USA ABSTRACT We report static and time-resolved terahertz (THz) conductivity measurements of a highperformance thermoelectric material containing tellurium nanowires in a PEDOT:PSS matrix. Composites were made with and without sulfur passivation of the nanowires surfaces. The material with sulfur linkers (TeNW/PD-S) is less conductive but has a longer carrier lifetime than the formulation without (TeNW/PD). We find real conductivities at f = 1THz of σTeNW/PD = 160 S/cm and σTeNW/PD-S = 5.1 S/cm. These values are much larger than the corresponding DC conductivities, suggesting DC conductivity is limited by structural defects. The free-carrier lifetime in the nanowires is controlled by recombination and trapping at the nanowire surfaces. We find surface recombination velocities in bare tellurium nanowires (22m/s) and TeNW/PD-S (40m/s) that are comparable to evaporated tellurium thin films. The surface recombination velocity in TeNW/PD (509m/s) is much larger, indicating a higher interface trap density. INTRODUCTION Improvements in the efficiency of thermoelectric materials would open the door to a wide range of significant applications in energy production, waste energy capture and thermal management. The efficiency of a thermoelectric generator or refrigerator is controlled by the thermoelectric figure of merit of its constituents, ZT = S 2σ T / κ . Nanocomposite materials created by embedding nanoparticles in a suitable matrix have attracted great interest as possible thermoelectric materials1-‐3. In these materials, a high density of interfaces is available to scatter phonons so as to effectively suppress κ. Nanocomposites also offer many material parameters that can be adjusted to optimize the thermoelectric power factor S2σ. From a practical standpoint, many nanocomposites can be fabricated by solution processing and other techniques that are inherently inexpensive and scalable. However, efficient thermoelectric materials must also be good conductors. It is critical to engineer efficient carrier transport in nanocomposite materials by building them from conductive components and suppressing electron scattering at nanoparticle/matrix interfaces. We investigated two nanocomposite materials consisting of tellurium nanowires coated with PEDOT:PSS. The first material (TeNW/PD) was produced by a one-pot synthesis based on a previously established nanowire synthesis with the substitution of PEDOT:PSS as the structure-directing agent 4-‐6. Our PEDOT:PSS (PD) was purchased from Clevios™ and is the PH1000 formulation. Samples investigated in this work had nanowire diameters of ~12nm and a tellurium volume fraction of approximately 50%. The second material
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