7 research outputs found
Quantum Dot Solar Cell Fabrication Protocols
Colloidally
synthesized quantum-confined semiconducting spherical
nanocrystals, often referred to as quantum dots (QDs), offer a high
degree of chemical, optical, and electronic tunability. As a result,
there is an increasing interest in employing colloidal QDs for electronic
and optical applications that is reflected in a growing number of
publications. In this protocol we provide detailed procedures for
the fabrication of QD solar cells specifically employing PbSe and
PbS QDs. We include details that are learned through experience, beyond
those in typical methodology sections, and include example pictures
and videos to aid in fabricating QD solar cells. Although
successful solar cell fabrication is ultimately learned through experience,
this protocol is intended to accelerate that process. The protocol
developed here is intended to be a general starting point for developing
PbS and PbSe QD test bed solar cells. We include steps for forming
conductive QD films via dip coating as well as spin coating. Finally,
we provide protocols that detail the synthesis of PbS and PbSe QDs
through a unique cation exchange reaction and discuss how different
QD synthetic routes could impact the resulting solar cell performance
Quantum Dot Solar Cell Fabrication Protocols
Colloidally
synthesized quantum-confined semiconducting spherical
nanocrystals, often referred to as quantum dots (QDs), offer a high
degree of chemical, optical, and electronic tunability. As a result,
there is an increasing interest in employing colloidal QDs for electronic
and optical applications that is reflected in a growing number of
publications. In this protocol we provide detailed procedures for
the fabrication of QD solar cells specifically employing PbSe and
PbS QDs. We include details that are learned through experience, beyond
those in typical methodology sections, and include example pictures
and videos to aid in fabricating QD solar cells. Although
successful solar cell fabrication is ultimately learned through experience,
this protocol is intended to accelerate that process. The protocol
developed here is intended to be a general starting point for developing
PbS and PbSe QD test bed solar cells. We include steps for forming
conductive QD films via dip coating as well as spin coating. Finally,
we provide protocols that detail the synthesis of PbS and PbSe QDs
through a unique cation exchange reaction and discuss how different
QD synthetic routes could impact the resulting solar cell performance
Air-Stable and Efficient PbSe Quantum-Dot Solar Cells Based upon ZnSe to PbSe Cation-Exchanged Quantum Dots
We developed a single step, cation-exchange reaction that produces air-stable PbSe quantum dots (QDs) from ZnSe QDs and PbX<sub>2</sub> (X = Cl, Br, or I) precursors. The resulting PbSe QDs are terminated with halide anions and contain residual Zn cations. We characterized the PbSe QDs using UV–vis–NIR absorption, photoluminescence quantum yield spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. Solar cells fabricated from these PbSe QDs obtained an overall best power conversion efficiency of 6.47% at one sun illumination. The solar cell performance without encapsulation remains unchanged for over 50 days in ambient conditions; and after 50 days, the National Renewable Energy Laboratory certification team certified the device at 5.9%
Electron–Phonon Coupling and Resonant Relaxation from 1D and 1P States in PbS Quantum Dots
Observations
of the hot-phonon bottleneck, which is predicted to
slow the rate of hot carrier cooling in quantum confined nanocrystals,
have been limited to date for reasons that are not fully understood.
We used time-resolved infrared spectroscopy to directly measure higher
energy intraband transitions in PbS colloidal quantum dots. Direct
measurements of these intraband transitions permitted detailed analysis
of the electronic overlap of the quantum confined states that may
influence their relaxation processes. In smaller PbS nanocrystals,
where the hot-phonon bottleneck is expected to be most pronounced,
we found that relaxation of parity selection rules combined with stronger
electron–phonon coupling led to greater spectral overlap of
transitions among the quantum confined states. This created pathways
for fast energy transfer and relaxation that may bypass the predicted
hot-phonon bottleneck. In contrast, larger, but still quantum confined
nanocrystals did not exhibit such relaxation of the parity selection
rules and possessed narrower intraband states. These observations
were consistent with slower relaxation dynamics that have been measured
in larger quantum confined systems. These findings indicated that,
at small radii, electron–phonon interactions overcome the advantageous
increase in energetic separation of the electronic states for PbS
quantum dots. Selection of appropriately sized quantum dots, which
minimize spectral broadening due to electron–phonon interactions
while maximizing electronic state separation, is necessary to observe
the hot-phonon bottleneck. Such optimization may provide a framework
for achieving efficient hot carrier collection and multiple exciton
generation
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Revisiting the Valence and Conduction Band Size Dependence of PbS Quantum Dot Thin Films
We use a high signal-to-noise X-ray
photoelectron spectrum of bulk
PbS, GW calculations, and a model assuming parabolic bands to unravel
the various X-ray and ultraviolet photoelectron spectral features
of bulk PbS as well as determine how to best analyze the valence band
region of PbS quantum dot (QD) films. X-ray and ultraviolet photoelectron
spectroscopy (XPS and UPS) are commonly used to probe the difference
between the Fermi level and valence band maximum (VBM) for crystalline
and thin-film semiconductors. However, we find that when the standard
XPS/UPS analysis is used for PbS, the results are often unrealistic
due to the low density of states at the VBM. Instead, a parabolic
band model is used to determine the VBM for the PbS QD films, which
is based on the bulk PbS experimental spectrum and bulk GW calculations.
Our analysis highlights the breakdown of the Brillioun zone representation
of the band diagram for large band gap, highly quantum confined PbS
QDs. We have also determined that in 1,2-ethanedithiol-treated PbS
QD films the Fermi level position is dependent on the QD size; specifically,
the smallest band gap QD films have the Fermi level near the conduction
band minimum and the Fermi level moves away from the conduction band
for larger band gap PbS QD films. This change in the Fermi level within
the QD band gap could be due to changes in the Pb:S ratio. In addition,
we use inverse photoelectron spectroscopy to measure the conduction
band region, which has similar challenges in the analysis of PbS QD
films due to a low density of states near the conduction band minimum
Revisiting the Valence and Conduction Band Size Dependence of PbS Quantum Dot Thin Films
We use a high signal-to-noise X-ray
photoelectron spectrum of bulk
PbS, GW calculations, and a model assuming parabolic bands to unravel
the various X-ray and ultraviolet photoelectron spectral features
of bulk PbS as well as determine how to best analyze the valence band
region of PbS quantum dot (QD) films. X-ray and ultraviolet photoelectron
spectroscopy (XPS and UPS) are commonly used to probe the difference
between the Fermi level and valence band maximum (VBM) for crystalline
and thin-film semiconductors. However, we find that when the standard
XPS/UPS analysis is used for PbS, the results are often unrealistic
due to the low density of states at the VBM. Instead, a parabolic
band model is used to determine the VBM for the PbS QD films, which
is based on the bulk PbS experimental spectrum and bulk GW calculations.
Our analysis highlights the breakdown of the Brillioun zone representation
of the band diagram for large band gap, highly quantum confined PbS
QDs. We have also determined that in 1,2-ethanedithiol-treated PbS
QD films the Fermi level position is dependent on the QD size; specifically,
the smallest band gap QD films have the Fermi level near the conduction
band minimum and the Fermi level moves away from the conduction band
for larger band gap PbS QD films. This change in the Fermi level within
the QD band gap could be due to changes in the Pb:S ratio. In addition,
we use inverse photoelectron spectroscopy to measure the conduction
band region, which has similar challenges in the analysis of PbS QD
films due to a low density of states near the conduction band minimum
Targeted Ligand-Exchange Chemistry on Cesium Lead Halide Perovskite Quantum Dots for High-Efficiency Photovoltaics
The
ability to manipulate quantum dot (QD) surfaces is foundational
to their technological deployment. Surface manipulation of metal halide
perovskite (MHP) QDs has proven particularly challenging in comparison
to that of more established inorganic materials due to dynamic surface
species and low material formation energy; most conventional methods
of chemical manipulation targeted at the MHP QD surface will result
in transformation or dissolution of the MHP crystal. In previous work,
we have demonstrated record-efficiency QD solar cells (QDSCs) based
on ligand-exchange procedures that electronically couple MHP QDs yet
maintain their nanocrystalline size, which stabilizes the corner-sharing
structure of the constituent PbI<sub>6</sub><sup>4–</sup> octahedra
with optoelectronic properties optimal for solar energy conversion.
In this work, we employ a variety of spectroscopic techniques to develop
a molecular-level understanding of the MHP QD surface chemistry in
this system. We individually target both the anionic (oleate) and
cationic (oleylÂammonium) ligands. We find that atmospheric moisture
aids the process by hydrolysis of methyl acetate to generate acetic
acid and methanol. Acetic acid then replaces native oleate ligands
to yield QD surface-bound acetate and free oleic acid. The native
oleylÂammonium ligands remain throughout this film deposition
process and are exchanged during a final treatment step employing
smaller cationsî—¸namely, formamidinium. This final treatment
has a narrow processing window; initial treatment at this stage leads
to a more strongly coupled QD regime followed by transformation into
a bulk MHP film after longer treatment. These insights provide chemical
understanding to the deposition of high-quality, electronically coupled
MHP QD films that maintain both quantum confinement and their crystalline
phase and attain high photovoltaic performance