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Lifetime, mobility, and diffusion of photoexcited carriers in ligand-exchanged lead selenide nanocrystal films measured by time-resolved terahertz spectroscopy.

ACS nano (2015-02-04)
Glenn W Guglietta, Benjamin T Diroll, E Ashley Gaulding, Julia L Fordham, Siming Li, Christopher B Murray, Jason B Baxter
ABSTRACT

Colloidal semiconductor nanocrystals have been used as building blocks for electronic and optoelectronic devices ranging from field-effect transistors to solar cells. Properties of the nanocrystal films depend sensitively on the choice of capping ligand to replace the insulating synthesis ligands. Thus far, ligands leading to the best performance in transistors result in poor solar cell performance, and vice versa. To gain insight into the nature of this dichotomy, we used time-resolved terahertz spectroscopy measurements to study the mobility and lifetime of PbSe nanocrystal films prepared with five common ligand-exchange reagents. Noncontact terahertz spectroscopy measurements of conductivity were corroborated by contacted van der Pauw measurements of the same samples. The films treated with different displacing ligands show more than an order of magnitude difference in the peak conductivities and a bifurcation of time dynamics. Inorganic chalcogenide ligand exchanges with sodium sulfide (Na2S) or ammonium thiocyanate (NH4SCN) show high mobilities but nearly complete decay of transient photocurrent in 1.4 ns. In contrast, ligand exchanges with 1,2-ethylenediamine (EDA), 1,2-ethanedithiol (EDT), and tetrabutylammonium iodide (TBAI) show lower mobilities but longer carrier lifetimes, resulting in longer diffusion lengths. This bifurcated behavior may explain the divergent performance of field-effect transistors and photovoltaics constructed from nanocrystal building blocks with different ligand exchanges.

MATERIALS
Product Number
Brand
Product Description

Sigma-Aldrich
Oleic acid, ≥99% (GC)
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Oleic acid, technical grade, 90%
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1-Octadecene, technical grade, 90%
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Oleic acid, suitable for cell culture, BioReagent
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Selenium, powder, −100 mesh, 99.99% trace metals basis
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Tetrabutylammonium iodide, reagent grade, 98%
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Trioctylphosphine, 97%
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Oleic acid, analytical standard
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Selenium, powder, −100 mesh, ≥99.5% trace metals basis
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Diphenylphosphine, 98%
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Tetrabutylammonium iodide, ≥99.0% (AT)
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Trioctylphosphine, technical grade, 90%
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Oleic acid, meets analytical specification of Ph, Eur., 65.0-88.0% (GC)
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1-Octadecene, ≥95.0% (GC)
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Selenium, pellets, <5 mm, ≥99.99% trace metals basis
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Selenium, pellets, <5 mm particle size, ≥99.999% trace metals basis
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1-Octadecene, analytical standard
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Oleic acid, Selectophore, ≥99.0%
Oleic acid, European Pharmacopoeia (EP) Reference Standard
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Tetrabutylammonium iodide, suitable for ion pair chromatography, LiChropur, ≥99.0%
Selenium, foil, 25x25mm, thickness 3mm, 99.95%
Selenium, pellets, < 5mm, ≥99.999%