Carrier Dynamics in Nanocrystals and Nanocrystal Solids: Experiment and Simulation
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Autor(in)
Datum
2017Typ
- Doctoral Thesis
ETH Bibliographie
yes
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Abstract
Films of semiconductor nanocrystals (NC-solids) hold great promise
as low-cost, solution-processable semiconductors with electronic and
optical properties that can be tuned by varying the size and composition
of the constituent nanocrystals (NCs), as well as their surfaceterminating
ligands. Further increase in the performance parameters
of devices incorporating NC-solids will require a robust understanding
of the effects of the composition, size, and surface termination of the
individual NCs, on the charge carrier dynamics resulting from their
assembly into densely packed films.
Here we investigate the phononic structure of the NCs, the resulting
electron-phonon interactions, and their influence on charge
carrier transport in NC-solids. We choose lead sulfide (PbS) NCs
as a model system, due to the extensive literature and their use in
a wide range of applications from LEDs, photodetectors, solar cells,
and thermoelectrics.
Knowledge of the phononic properties of NCs, and how the phonons
couple to the electronic states of the NCs is crucial for understanding
energy gain, loss, and transport processes in NC-solids. Using inelastic
neutron scattering measurements and Ab-Initio Molecular Dynamics
(AIMD) simulations, we quantify the phonon density of states of
nano-sized crystallites as a function of crystallite size. We demonstrate
that the mechanical softening of the surface of nanocrystalline
domains results in low and high energy phonon modes with reduced
symmetry and large thermal displacements. Using simulation, we
show that these modes couple strongly to the electronic states in
NCs, and cause strong thermal broadening of optical transitions and
efficient phonon-mediated electronic transitions, effects which have been observed experimentally. We study the impact of thiol versus
halide terminations of the NC surface, and demonstrate that the
electron-phonon coupling in the NCs can be tuned through the choice
of surface termination.
To investigate charge carrier transport in NC-solids, we perform
Time of Flight (TOF) photocurrent transient measurements on PbS
NC-solids. The TOF technique provides a robust approach to probe
carrier dynamics in disordered, low-mobility materials. We find that
the carrier mobility extracted via TOF in NC-solids is temperature
activated, and that both the activation energy and temperature independent
prefactor of the carrier mobility scale strongly with NC
size. Our analysis indicates that the distribution of carrier hopping
times is power-law distributed with divergent mean values. This is
characteristic of disordered solids and explains our finding that the
extracted carrier mobilities from TOF depend on the device thickness.
Indeed, this implies that the series resistance will depend superlinearly
on the device thickness.
Finally, we motivate a model for charge carrier transport in NCsolids
based on phonon-mediated electron transfer. Using AIMD, we
calculate the wavefunction overlap between neighboring NC and the
reorganization energies associated with charge transfer for a series
of NC sizes. Our model explains the scaling of the temperature
dependent mobility with NC size measured with TOF. We further
demonstrate that the activation energy of the transport can additionally
be associated with the size dispersion of the NCs.
Our work here on PbS NCs suggests a future path for systematically
tuning charge carrier dynamics in arbitrary NC systems. Namely,
NC size and surface chemistry can be selected to determine the electronic
and phononic properties of NCs, electron-phonon interactions
in them, and charge transfer rates between them. Mehr anzeigen
Persistenter Link
https://doi.org/10.3929/ethz-b-000219042Publikationsstatus
publishedExterne Links
Printexemplar via ETH-Bibliothek suchen
Verlag
ETH ZurichOrganisationseinheit
03895 - Wood, Vanessa / Wood, Vanessa
ETH Bibliographie
yes
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