Effects of rapid thermal annealing (RTA) and dual compression-pulse-laser sintering (compression-PLS) on
photovoltaic, CdTe nanowire (NW) and quantum dot (QD) films are investigated. Unlike regular furnace annealing,
RTA involves raising the temperature of a substrate’s atmosphere by several hundred degrees in a matter of seconds,
letting it sit for 30 to 120 seconds, then cooling it back to T<sub>0</sub>. To the best of our knowledge, such treatments of CdTe
nanocrystal (NC) films have not been documented. In compression-PLS, a large pressure (MPa) is applied to a film
through a laser-pulsing mechanism. Next, a high-energy, high frequency laser beam is pulsed onto it for sintering.
During the compression, we used a single pulse of 5 nanoseconds. For the sintering, we used a 7.05 mJ beam for two
pulses, at 25 ns per pulse. Such parameters were determined from SEM and other preliminary film characterization
results. Morphology, material content, and conductivity of the films are analyzed before and after treatment using
tunneling and scanning electron microscopy, EDS, and two-probe measurements, respectively.
This study provides new knowledge regarding the morphological and structural outcomes of RTA and
compression-PLS on CdTe nanoparticle films. Furthermore, RTA and compression-PLS can increase the film electrical
conductivity by improving their contact with each other. We found that RTA partially sinters the film and enhances inplane
current density by a factor of ~1.7, for a values on the order of ~10<sup>-7</sup>A/cm<sup>2</sup>. Compression-PLS successfully sinters
the NW film and improves current density up to a factor of ~167, for values on the order of ~10<sup>-5</sup> A/cm<sup>2</sup>. On the other
hand, QD films do not exhibit current density improvement with treatments. These values remain on the order of ~10<sup>-7</sup>
The resistivities of the sintered NW films reach as low as 6.7*10<sup>6</sup> Ω*cm, while the RTA’d NW film has a
resistivity on the order of 10<sup>8</sup> Ω*cm. These values are comparable to values of bulk and thin-film CdTe: single
crystalline, undoped CdTe resistivity values range from 10<sup>5</sup> to 10<sup>8</sup> Ω*cm,<sup>8,9</sup> while polycrystalline thin-film values range
from 10<sup>4</sup> to 10<sup>6</sup> Ω*cm.<sup>11,12</sup> The QD films also have comparable resistivities to these results, albeit on the higher side.
Time-domain non-adiabatic ab initio simulations are performed to study the phonon-assisted hot electron relaxation
dynamics in CdSe spherical quantum dots (QDs) and elongated quantum dots (EQDs). EQDs have a narrower band gap
and denser electron and hole energy states than QDs. As temperature increases, band gap values will become smaller due
to thermal expansion effect. Also more phonons are excited to scatter with electrons and thus result in a higher relaxation
rate for hot electrons. Besides, it is also found in our simulation that hot electron relaxation rate in EQDs has a weaker
temperature dependence than in QDs, which could be attributed to the larger thermal expansion in EQDs.
The optical properties of ordered and disordered vertical silicon nanowire arrays, including random position,
diameter, and length, are investigated using the finite-difference time-domain method. The ordered array with
diameter of 100 nm shows overall small reflection and large absorption. An absorption peak is located at 2.4
eV, which is due to the optical resonance effect. Both randomly-positioned and random diameter arrays remain
unreflective as the ordered array. The absorptance of randomly-positioned nanowire arrays has similar frequency
dependence as ordered array, while it is enhanced due to the enhanced scattering. Random diameter array has
a different absorptance profile and no evident absorption peak is observed, which is explained by the different
resonant frequencies of the inclusion nanowires. Random length can create a random rough top surface on the
nanowire arrays, which can reduce reflection and enhance absorption compared to uniform top surface.
Temperature dependent dynamics of phonon-assisted relaxation of hot carriers, both electrons and holes, is
studied in a PbSe nanocrystal using ab initio time-domain density functional theory. The electronic structure
is first calculated, showing that the hole states are denser than the electron states. Fourier transforms of the
time resolved energy levels show that the hot carriers couple to both acoustic and optical phonons. At higher
temperature, more phonon modes in the high frequency range participate in the relaxation process due to their
increased occupation number. The phonon-assisted hot carrier relaxation time is predicted using non-adiabatic
molecular dynamics, and the results clearly show a temperature-activation behavior. The complex temperature
dependence is attributed to the combined effects of the phonon occupation number and the thermal expansion.
Comparing the simulation results with experiments, we suggest that the multiphonon relaxation channel is
efficient at high temperature, while the Auger-like process may dominate the relaxation at low temperature.
This combined mechanism can explain the weak temperature dependence at low temperature and stronger
temperature dependence at higher temperature.