Abstract-Lifetime, Mobility, and Diffusion of Photoexcited Carriers in Ligand-Exchanged Lead Selenide Nanocrystal Films Measured by Time-Resolved Terahertz Spectroscopy

Glenn W. Guglietta †, Benjamin T. Diroll ‡, E. Ashley Gaulding §, Julia L. Fordham §, Siming Li†, Christopher B. Murray *‡§, and Jason B. Baxter*†

† Department of Chemical and Biological Engineering,Drexel University, 3141 Chestnut Street, Philadelphia, Pennsylvania 19104, United States

‡ Department of Chemistry, University of Pennsylvania231 South 34th Street, Philadelphia, Pennsylvania 19104, United States

§ Department of Materials Science, University of Pennsylvania, 3231 Walnut Street, Philadelphia, Pennsylvania 19104, United States

ACS Nano, Article ASAP

DOI: 10.1021/nn506724h

Publication Date (Web): February 2, 2015

Copyright © 2015 American Chemical Society

*Address correspondence to jbaxter@drexel.edu, cbmurray@sas.upenn.edu.

http://pubs.acs.org/doi/abs/10.1021/nn506724h

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.