(608b) Ultrafast Photoexcited Carrier Dynamics in Ligand-Exchanged PbSe Nanocrystal Films: Lifetime, Mobility, Diffusion, and Interfacial Charge Transfer

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 films of monodisperse lead selenide (PbSe) nanocrystals prepared with five common ligand-exchange reagents. Non-contact 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 THz mobilities above 25 cm²V^-1s^-1, which is desirable for transistors, but nearly complete decay of transient photocurrent within 1.4 ns. The high mobility with NH₄SCN and Na₂S exchanges is more than offset by their short lifetimes and results in diffusion lengths of only ~200 nm. In contrast, ligand exchanges with 1,2-ethylenediamine (EDA), 1,2-ethanedithiol (EDT), and tetrabutylammonium iodide (TBAI) show ~5x lower mobilities but much longer carrier lifetimes, with ~30% of photoexcited carriers remaining for >10 ns. The long lifetimes with EDA, EDT, and TBAI yield diffusion lengths of at least 500 nm, which is approaching the film thickness desired for strong light absorption in solar cells. This bifurcated behavior may explain the divergent performance of field-effect transistors and photovoltaics constructed from nanocrystal building blocks with different ligand exchanges. Moreover, we have investigated ligand-exchanged PbSe films comprised of two different nanocrystal sizes. We will report on the effects of band alignment, ligand chemistry, and structural order on interfacial charge transfer.

Reference: Guglietta, Diroll et al., ACS Nano, 2015.