Ankara : Materials Science and Nanotechnology and the Graduate School of Engineering and Science of Bilkent Univ., 2013.Thesis (Master's) -- Bilkent University, 2013.Includes bibliographical references leaves 89-100.Light sensing devices traditionally made from crystalline or amorphous silicon,
operating at the visible and near-infrared wavelengths, have led to a
multibillion-dollar annual market. However, silicon faces various limitations
including weak detection at long wavelengths (insufficient beyond 1.1 µm) with
a cut-off at short wavelengths (in the ultraviolet) and small-area applications. On
the other hand, solution-processed semiconductor nanocrystals (NCs), also
known as colloidal quantum dots, offer large-area light sensing platforms with
strong absorption cross-section. In this thesis we propose and demonstrate a new
class of large-area, semi-transparent, light-sensitive nanocrystal skin (LS-NS)
devices intended for large-surface applications including smart transparent
windows and light-sensitive glass facades of smart buildings. These LS-NS
platforms, which are fabricated over areas up to many tens of cm2 using spraycoating
and several cm-squares using dip-coating, are operated on the basis of
photogenerated potential buildup, as opposed to conventional charge collection.
The close interaction of the monolayer NCs of the LS-NS with the top
interfacing metal contact results in highly sensitive photodetection in the
absence of external bias, while the bottom side is isolated using a high dielectric
spacing layer. In operation, electron-hole pairs created in the NCs of the LS-NS
are disassociated and separated at the NC monolayer - metal interface due to the
difference in the workfunctions. As a result, the proposed LS-NS platforms
perform as highly sensitive photosensors, despite using a single NC monolayer,
which makes the device semi-transparent and reduces the noise generation Furthermore, because of the band gap tunability, it is possible to construct
cascaded NC layers with a designed band gap gradient where the NC diameters
monotonically change. Here we present the first account of exciton funneling in
an active device, which leads to significant performance improvement in the
device. We show highly photosensitive NC skins employing the exciton
funneling across the multiple layers of NC film. To further enhance the device
photosensitivity performance, we demonstrate embedding plasmonic
nanoparticles into the light-sensitive skins of the NCs. In addition, we exhibit
the LS-NS device sensitivity enhancement utilizing the device architecture of
semi-transparent tandem skins, the addition of TiO2 layer for increased charge
carrier dissociation, and the phenomenon of multiexciton generation in infrared
NCs. With fully sealed NC monolayers, LS-NS is found to be highly stable
under ambient conditions, promising for low-cost large-area UV/visible sensing
in windows and facades of smart buildings. We believe the findings presented in
this thesis have significant implications for the future design of photosensing
platforms and for moving toward next generation large-surface light-sensing
platforms.Akhavan, ShahabM.S
