U.S. patent application number 14/176254 was filed with the patent office on 2014-06-05 for secondary treatment of films of colloidal quantum dots for optoelectronics and devices produced thereby.
This patent application is currently assigned to Alliance for Sustainable Energy, LLC. The applicant listed for this patent is Alliance for Sustainable Energy, LLC. Invention is credited to Matthew C. BEARD, Hsiang-Yu CHEN, Joseph M. LUTHER, Octavi Escala SEMONIN.
Application Number | 20140150861 14/176254 |
Document ID | / |
Family ID | 47554918 |
Filed Date | 2014-06-05 |
United States Patent
Application |
20140150861 |
Kind Code |
A1 |
SEMONIN; Octavi Escala ; et
al. |
June 5, 2014 |
SECONDARY TREATMENT OF FILMS OF COLLOIDAL QUANTUM DOTS FOR
OPTOELECTRONICS AND DEVICES PRODUCED THEREBY
Abstract
A method of forming an optoelectronic device. The method
includes providing a deposition surface and contacting the
deposition surface with a ligand exchange chemical and contacting
the deposition surface with a quantum dot (QD) colloid. This
initial process is repeated over one or more cycles to form an
initial QD film on the deposition surface. The method further
includes subsequently contacting the QD film with a secondary
treatment chemical and optionally contacting the surface with
additional QDs to form an enhanced QD layer exhibiting multiple
exciton generation (MEG) upon absorption of high energy photons by
the QD active layer. Devices having an enhanced QD active layer as
described above are also disclosed.
Inventors: |
SEMONIN; Octavi Escala; (New
York, NY) ; LUTHER; Joseph M.; (Boulder, CO) ;
BEARD; Matthew C.; (Arvada, CO) ; CHEN;
Hsiang-Yu; (Castle Rock, CO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Alliance for Sustainable Energy, LLC |
Golden |
CO |
US |
|
|
Assignee: |
Alliance for Sustainable Energy,
LLC
Golden
CO
|
Family ID: |
47554918 |
Appl. No.: |
14/176254 |
Filed: |
February 10, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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13187276 |
Jul 20, 2011 |
8685781 |
|
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14176254 |
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Current U.S.
Class: |
136/255 ; 438/57;
438/98 |
Current CPC
Class: |
H01L 21/02628 20130101;
B82Y 20/00 20130101; H01L 21/02601 20130101; H01L 31/18 20130101;
H01L 21/02568 20130101; H01L 21/02521 20130101; H01L 31/035236
20130101; H01L 21/02491 20130101; H01L 21/02472 20130101; H01L
31/073 20130101; H01L 21/02502 20130101; Y02E 10/543 20130101; H01L
21/02422 20130101; H01L 31/035218 20130101; H01L 31/1836
20130101 |
Class at
Publication: |
136/255 ; 438/57;
438/98 |
International
Class: |
H01L 31/0352 20060101
H01L031/0352; H01L 31/18 20060101 H01L031/18 |
Goverment Interests
CONTRACTUAL ORIGIN
[0001] The United States Government has rights in this invention
under Contract No. DE-AC36-080028308 between the United States
Department of Energy and the Alliance for Sustainable Energy, LLC,
the manager and operator of the National Renewable Energy
Laboratory.
Claims
1. A method of forming an optoelectronic device comprising:
providing a deposition surface; contacting the deposition surface
with a ligand exchange chemical and contacting the deposition
surface with a quantum dot (QD) colloid over one or more cycles to
form an initial QD film on the deposition surface; and contacting
the QD film with a secondary treatment chemical.
2. The method of forming an optoelectronic device of claim 1
wherein the step of contacting the initial QD film with a secondary
treatment chemical comprises contacting the initial QD film with a
secondary treatment chemical and contacting the deposition surface
with a quantum dot (QD) colloid over one or more cycles to deposit
additional QDs on the initial QD film.
3. The method of forming an optoelectronic device of claim 2
wherein the secondary treatment chemical is at least one of
hydrazine, formic acid, mercaptopropionic acid, an organic acid,
methylamine, methanol, ethanol and ethylenediamine.
4. The method of forming an optoelectronic device of claim 2
wherein the initial QD film is formed to a thickness of about 40 nm
to 400 nm.
6. The method of forming an optoelectronic device of claim 2
wherein the initial QD film is thickened by about 1 nm to 50 nm by
the deposition of additional QDs in the secondary treatment
step.
7. The method of forming an optoelectronic device of claim 1
wherein the step of contacting the initial QD film with a secondary
treatment chemical comprises contacting the initial QD film with a
secondary treatment chemical over one or more cycles without
depositing additional QDs on the QD film.
8. The method of forming an optoelectronic device of claim 7
wherein the secondary treatment chemical is at least one of
hydrazine, formic acid, mercaptopropionic acid, an organic acid,
methylamine, methanol, ethanol and ethylenediamine.
9. A method of forming a solar cell comprising: providing a
substrate; forming a first electrode in contact with the substrate;
contacting the first electrode with a ligand exchange chemical and
contacting the first electrode with a quantum dot (QD) colloid over
one or more cycles to form an initial QD film in contact with the
first electrode; contacting the initial QD film with a secondary
treatment chemical; and forming a second electrode in contact with
the initial QD film, not in contact with the first electrode.
10. The method of forming a solar cell of claim 9 wherein the step
of contacting the initial QD film with a secondary treatment
chemical comprises contacting the initial QD film with a secondary
treatment chemical and contacting the deposition surface with a
quantum dot (QD) colloid over one or more cycles to deposit
additional QDs on the initial QD film.
11. The method of forming a solar cell of claim 10 wherein the
secondary treatment chemical is at least one of hydrazine, formic
acid, mercaptopropionic acid, an organic acid, methylamine,
methanol, ethanol and ethylenediamine.
12. The method of forming a solar cell of claim 10 wherein the
initial QD film is formed to a thickness of about 40 nm to 400
nm.
13. The method of forming a solar cell of claim 10 wherein the
initial QD film is thickened by about 1 nm to 50 nm by the
deposition of additional QDs in the secondary treatment step.
14. The method of forming a solar cell of claim 9 wherein the step
of contacting the initial QD film with a secondary treatment
chemical comprises contacting the QD film with a secondary
treatment chemical over one or more cycles without depositing
additional QDs on the QD film.
15. The method of forming a solar cell of claim 14 wherein the
secondary treatment chemical is at least one of hydrazine, formic
acid, mercaptopropionic acid, an organic acid, methylamine,
methanol, ethanol and ethylenediamine.
16. The method of forming a solar cell of claim 9 further
comprising forming a doped window layer in contact with the first
electrode and the initial QD film.
17. A solar cell comprising: a first electrode; a QD active layer
in electrical contact with the first electrode providing for
multiple exciton generation (MEG) upon absorption of photons by the
QD active layer; and a second electrode in electrical contact with
the QD active layer.
18. The solar cell of claim 17 further comprising a doped window
layer in electrical contact with the first electrode and the QD
active layer.
19. The solar cell of claim 17 wherein the QD active layer
comprises at least one of QDs of lead selenide (PbSe), lead sulfide
(PbS), cadmium selenide (CdSe), other semiconductor nanocrystals
(NCs), core-shell and ternary nanocrystals, for example lead
telluride (PbTe), lead selenide sulfide (PbSSe), lead selenide core
with lead sulfide shell, cadmium lead sulfide (CdPbS), cadmium lead
selenide (CdPbSe) tin sulfide (SnS), tin selenide (SnSe), tin
telluride (SnTe), silicon (Si), germanium (Ge), indium arsenide
(InAs), indium phosphide (InP), indium antimonide (InSb), gallium
arsenide (GaAs), indium gallium arsenide (InGaAs) and NC structures
of all other Group IV, II-VI, IV-VI compounds and alloys.
20. The solar cell of claim 19 further comprising a glass
substrate.
Description
BACKGROUND
[0002] An important long range objective of solar energy research
is the discovery and development of photoconversion materials,
processes, and architectures that can produce solar generated
electricity at costs competitive with the cost of electricity
generated from fossil fuels such as petroleum, natural gas, or
coal. In general, solar electricity systems will require relatively
high conversion efficiencies and such systems must be relatively
inexpensive to produce to become cost competitive with fossil
fuel.
[0003] A photovoltaic device, commonly referred to a solar cell or
solar panel, is a type of optoelectronic device that converts
incident sunlight into electrical current which may then be used to
power any type of electrical system or stored in a storage device
such as a battery. Semiconductor materials in bulk form currently
dominate the field of commercial photovoltaic (PV) power. More
sophisticated materials and architectures having higher
efficiencies are being developed.
[0004] Colloidal Quantum Dots (referred to herein as QDs) are one
material being developed for use in solar cells, photovoltaic
devices or other optoelectronics. Colloidal QDs are also known as
nanocrystals (referred to herein as NCs). QDs and/or NCs are
believed to be inherently well suited for the development of
relatively inexpensive higher efficiency solar cells for several
reasons, including but not limited to observations that QD
materials are in certain instances relatively inexpensive and that
these materials exhibit an enhanced capacity for multiple exciton
generation (MEG).
[0005] In particular, the spatial confinement of electrons and
holes in QDs and other NCs causes several important effects: (1)
e.sup.- and h.sup.+ pairs are correlated and thus exist as excitons
rather than free carriers, (2) the rate of exciton cooling can be
slowed because of the formation of discrete electronic states, (3)
momentum is not a good quantum number and thus the need to conserve
crystal momentum is relaxed and (4) Auger processes are greatly
enhanced because of increased e.sup.--h.sup.+ Coulomb interaction.
Because of these factors it has been observed that the production
of multiple e.sup.--h.sup.+ pairs (excitons) from high energy
photons can be enhanced in QDs compared to bulk semiconductors of
the same composition.
[0006] Nonetheless, known solar cells employing a QD active layer
typically exhibit relatively low conversion efficiencies. The
observed lower conversion efficiencies have many causes. One
observed reason for the lower conversion efficiencies observed in
QD solar cells is that solar cells fabricated with a QD active
layer typically do not exhibit MEG.
[0007] The methods and devices disclosed herein are directed toward
overcoming one or more of the problems discussed above. The
foregoing examples of the related art and limitations related
therewith are intended to be illustrative and not exclusive. Other
limitations of the related art will become apparent to those of
skill in the art upon a reading of the specification and a study of
the drawings.
SUMMARY OF THE EMBODIMENTS
[0008] The following embodiments and aspects thereof are described
and illustrated in conjunction with systems, tools and methods
which are meant to be exemplary and illustrative, not limiting in
scope. In various embodiments, one or more of the above-described
problems have been reduced or eliminated, while other embodiments
are directed to other improvements.
[0009] One embodiment includes a method of forming an
optoelectronic device. The method includes providing a deposition
surface and alternately contacting the deposition surface with a
ligand exchange chemical and contacting the deposition surface with
a quantum dot (QD) colloid. This initial process is repeated over
one or more cycles to form an initial QD film on the deposition
surface. The method further includes subsequently contacting the QD
film with a secondary treatment chemical.
[0010] Generally, two types of secondary treatment processes may be
followed. The first type of secondary treatment comprises
alternately contacting the initial QD film with a secondary
treatment chemical and contacting the deposition surface with a QD
colloid. Secondary treatment may be performed over one or more
cycles to deposit additional QDs on the initial QD film. The
secondary treatment chemical can be one of the following:
hydrazine, formic acid, mercaptopropionic acid, another organic
acid, methylamine, methanol, ethanol, ethylenediamine or similar
chemicals. One characteristic of the secondary treatment step is
that these processes restore or impart the capacity of MEG to the
QD film, which capacity or attribute is typically not present after
the initial QD film formation steps.
[0011] The initial QD film may be formed to any desired thickness.
Useful films have been formed to a thickness of about 40 nm to 400
nm. The initial QD film is thickened by about 1 nm to 50 nm by the
deposition of additional QDs in the first type of secondary
treatment step.
[0012] Alternatively or in addition to the above, secondary
treatment of an active layer of an optoelectronic device may
include contacting the initial QD film with a secondary treatment
chemical over one or more cycles without depositing additional QDs
on the QD film.
[0013] An alternative embodiment is a method of forming a solar
cell including the steps of providing a substrate, forming a first
electrode, a conductive window layer or other conductive surface in
contact with the substrate and alternately contacting the first
electrode with a ligand exchange chemical and a quantum dot (QD)
colloid over one or more cycles to form an initial QD film in
contact with the first electrode. The method further includes
contacting the initial QD film with a secondary treatment chemical
and then forming a second electrode in contact with the initial QD
film, with the second electrode not being in contact with the first
electrode.
[0014] The step of contacting the initial QD film with a secondary
treatment chemical may include alternately contacting the initial
QD film with a secondary treatment chemical and contacting the
deposition surface with a quantum dot (QD) colloid over one or more
cycles to deposit additional QDs on the initial QD film. In this
embodiment the initial QD film is thickened by about 1 nm to 50 nm
by the deposition of additional QDs in the secondary treatment
step.
[0015] Alternatively, the secondary treatment step may include
contacting the QD film with a secondary treatment chemical over one
or more cycles without depositing additional QDs on the QD film. In
either case, the secondary treatment chemical can be one of the
following: hydrazine, formic acid, mercaptopropionic acid, another
organic acid, methylamine, methanol, ethanol, ethylenediamine or
similar chemicals. One characteristic of the secondary treatment
chemical is that it restores or imparts the capacity of multiple
exciton generation (MEG) to the QD film, which capacity is
typically not present after the initial film formation steps.
[0016] The method of forming a solar cell may further include
forming any number of additional device layers as desired to make a
functional cell. For example, a doped window layer may be formed in
electrical contact with the first electrode and the QD film.
[0017] An alternative embodiment is an optoelectronic device, for
example a solar cell having a QD active layer formed as described
above. A typical device may include a first electrode; a QD active
layer in electrical contact with the first electrode and a second
electrode in electrical contact with the QD active layer. Any
number of additional layers or structures may be included in the
device to enhance device efficiency or overall performance. In all
cases the QD active layer will provide for multiple exciton
generation (MEG) upon absorption of high energy photons by the QD
active layer.
[0018] Device embodiments may include a QD active layer comprising
at least one of QDs of lead selenide (PbSe), lead sulfide (PbS),
cadmium selenide (CdSe), other semiconductor nanocrystals (NCs),
core-shell and ternary nanocrystals, for example lead telluride
(PbTe), lead selenide sulfide (PbSSe), lead selenide core with lead
sulfide shell, cadmium lead sulfide (CdPbS), cadmium lead selenide
(CdPbSe) tin sulfide (SnS), tin selenide (SnSe), tin telluride
(SnTe), silicon (Si), germanium (Ge), indium arsenide (InAs),
indium phosphide (InP), indium antimonide (InSb), gallium arsenide
(GaAs), indium gallium arsenide (InGaAs) and NC structures of all
other Group III-V, IV, II-VI, IV-VI compounds and alloys.
[0019] In addition to the exemplary aspects and embodiments
described above, further aspects and embodiments will become
apparent by reference to the drawings and by study of the following
descriptions.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] Exemplary embodiments are illustrated in referenced figures
of the drawings. It is intended that the embodiments and figures
disclosed herein are to be considered illustrative rather than
limiting.
[0021] FIG. 1 is a schematic diagram showing Multiple Exciton
Generation (MEG) in a quantum dot.
[0022] FIG. 2 is cross sectional SEM showing the layers of an
exemplary device.
[0023] FIG. 3 is a flowchart representation of a disclosed
method.
[0024] FIG. 4 is a flowchart representation of an alternative
disclosed method.
[0025] FIG. 5 is a graph showing the spectral response of PbSe QD
devices made with coupled-film bandgaps of 0.98 eV.
[0026] FIG. 6 is a graph showing the spectral response of PbSe QD
devices made with coupled-film bandgaps of 0.83 eV.
[0027] FIG. 7 is a graph showing the spectral response of PbSe QD
devices made with coupled-film bandgaps of 0.72 eV.
[0028] FIG. 8 is a graph showing collected IQE curves versus the
photon energy divided by the bandgap, E.sub.ph/E.sub.g, for the
three QD sizes of FIGS. 5-7.
[0029] FIG. 9 is a graph showing peak IQE values from a set of
devices (including those of FIGS. 5-7) made from seven different QD
sizes, plotted versus E.sub.ph/E.sub.g. Also plotted are the peak
IQE values corrected for intrinsic losses in the solar cell.
[0030] FIG. 10 is a graph showing a comparison of current-voltage
characteristics under 100 mW cm.sup.2 illumination for small 0.98
eV QD solar cells prepared with and without the enhanced secondary
treatment disclosed herein.
[0031] FIG. 11 is a graph showing a comparison of current-voltage
characteristics under 100 mW cm .sup.-2 illumination for larger
0.72 eV QD solar cells prepared with and without the enhanced
secondary treatment disclosed herein.
[0032] FIG. 12 is a graph showing a comparison of peak V.sub.oc for
solar cells prepared with and without secondary treatment as
disclosed herein for a range of QD sizes plotted against the
corresponding bandgaps.
[0033] FIG. 13 is a graph showing the effect of aging on
mismatch-corrected power conversion efficiency (PCE) in a N2
atmosphere over 25 days for enhanced devices of FIGS. 10 and
11.
DETAILED DESCRIPTION
[0034] Unless otherwise indicated, all numbers expressing
quantities of ingredients, dimensions, reaction conditions and so
forth used in the specification and claims are to be understood as
being modified in all instances by the term "about".
[0035] In this application and the claims, the use of the singular
includes the plural unless specifically stated otherwise. In
addition, use of "or" means "and/or" unless stated otherwise.
Moreover, the use of the term "including", as well as other forms,
such as "includes" and "included," is not limiting. Also, terms
such as "element" or "component" encompass both elements and
components comprising one unit and elements and components that
comprise more than one unit unless specifically stated
otherwise.
[0036] Optoelectronic devices exploit photovoltaic or photoelectric
effects caused when light energy is absorbed or emitted by certain
semiconducting materials. Optoelectronic devices include but are
not limited to photodiodes including solar cells, phototransistors,
photomultipliers, photoresistors, LEDs, laser diodes and other
types of devices. The methods and embodiments disclosed herein can
be adapted to many types of optoelectronic devices. The specific
embodiments described herein involve photovoltaic solar cell
fabrication methods and devices. The scope of this disclosure
however is intended to cover all suitable optoelectronics whether
or not a type of optoelectronic device is described specifically
herein.
[0037] Electricity can be produced from photovoltaic (PV) cells
also known as solar cells having a semiconductor photoconverter
layer of greater or lesser efficiency. Exemplary methods and device
embodiments disclosed herein describe how relatively higher
conversion efficiencies and multiple exciton generation (MEG) can
be obtained for solar photovoltaic cells using active regions
composed of colloidal quantum dots (QDs), also known as
nanocrystals (NCs). As defined herein a QD or NC is a structure
where the spatial confinement of electrons and holes causes the
e.sup.- and h.sup.+ pairs generated by a photonic effect to be
correlated and thus exist initially as excitons rather than free
carriers. For practical application in a photovoltaic cell the
excitons must be subsequently dissociated into free electrons and
free holes and spatially separated.
[0038] The spatial confinement of electrons and holes in QDs and
other NCs causes several important effects: (1) e.sup.- and h.sup.+
pairs are correlated and thus exist as excitons rather than free
carriers, (2) the rate of exciton cooling can be slowed because of
the formation of discrete electronic states, (3) momentum is not a
good quantum number and thus the need to conserve crystal momentum
is relaxed, and (4) Auger processes are greatly enhanced because of
increased e.sup.--h.sup.+ Coulomb interaction. Because of these
factors it has been observed that the production of multiple
e.sup.--h.sup.+ pairs (excitons) can be enhanced in QDs compared to
bulk semiconductors. Therefore, both the threshold energy
(h.upsilon..sub.th) for electron hole pair multiplication (EHPM)
and its efficiency, .eta..sub.EHPM (defined as the number of
excitons produced per additional bandgap of energy above the EHPM
threshold energy) are expected to be greatly enhanced in QD type
material. The formation of multiple excitons is denominated herein
as Multiple Exciton Generation (MEG). The possibility of enhanced
MEG in QDs was first proposed in 2001. The original concept is
illustrated in FIG. 1, where a single photon 10 is shown creating
an excited e.sup.--h.sup.+ pair 12. This pair generates an exciton
14 and also remains as exciton 13. Thus, one high energy photon has
directly created multiple exciton pairs within the confined
structure of a QD 16.
[0039] Known solar cells produced with QD active layers have
typically not been observed to exhibit MEG. In particular, solar
cells having a QD active layer produced by a layer by layer
deposition process as disclosed in US Published Patent Application
2011/0146766 A1 "Solar Cells Based Upon Quantum Dot or Colloidal
Nanocrystal Films," do not exhibit clearly detectable MEG. The
entire disclosure of the 2011/0146766 A1 application is
incorporated herein by reference for all matters disclosed therein.
Specific secondary treatments and additional processing steps
disclosed in detail herein can be used to create solar cells with a
QD active layer that unambiguously exhibits MEG.
[0040] An exemplary device may employ a QD layer that is the
light-absorbing layer in a photovoltaic solar cell. As used herein
a QD "layer" is synonymous with a QD "film." An exemplary but
highly simplified solar cell may be formed with a first conductive
electrode of any type. In some examples, an indium tin oxide (ITO)
layer may be the first electrode and in other examples an
appropriate thin metal layer or a doped bulk semiconductor layer
may be the first electrode. The first electrode and other layers
may be supported on a substrate such as glass. A second electrode
of some type is required as well. The first and second electrodes
serve to collect electrons and holes formed in the QD active layer
to generate current. Any number of additional layers including
doped window layers, encapsulant layers or other layers may be
included in a working device. In general, a highly simplified QD
solar cell device requires two electrodes and a relatively dense
layer active layer of QDs in contact with the electrodes. As used
herein, "in contact with" means in electrical contact with other
layers, although there may exist any number of intervening layers
between the two layers described as being in contact with each
other.
[0041] The room temperature layer by layer deposition methods
described herein results in the fabrication of a dense QD film
without structural stress, leading to a pinhole and crack-free film
of electronically coupled quantum dots. At least one of the first
and second electrodes must be substantially transparent to
sunlight. All layers may be mechanically supported, typically by a
substrate which may also be substantially transparent to relevant
wavelengths of light.
[0042] Under illumination through one electrode, photons are
absorbed in the QD film thereby generating excitons which are then
dissociated by the electric fields present in the photoactive
quantum dot layer. The separated electrons and holes are
transported to separate cathodic and anodic electrodes to produce a
photovoltaic effect. The electric field in the QD layer could be
produced through a Schottky junction formed between the QD film and
the top contact layer or by a more complex architecture, for
example the use of two different metal layers with different work
functions operating as the two electrodes of the cell, or by doped
semiconductor layers of opposite conductivity type operating as the
electrodes or through a combination of other known solar cell
architectures. More sophisticated electrode arrangements may
improve charge collection and hence the conversion efficiency of a
QD based solar cell. The methods and devices disclosed herein are
not limited to any particular cell architecture configuration.
[0043] An exemplary device having a QD film active layer is
illustrated in the cross-sectional SEM image of a solar cell
structure of FIG. 2. The exemplary device 18 includes a
substantially transparent glass or other suitable substrate 20 with
an indium tin oxide (ITO) electrode 22 deposited on the substrate.
It is important to note that the substrate 20 could be implemented
with any material having the ability to transmit sunlight and
exhibit suitable mechanical characteristics. Similarly the ITO
electrode could be implemented with another type of substantially
transparent material having suitable conductivity or implemented in
multiple layers.
[0044] The illustrated device 18 also includes an n-type window
layer 24 of ZnO or another suitable, substantially transparent,
semiconductor with n-type doping. Alternatively, the electrode and
doped window layer could be the same layer assuming the layer
exhibited the desired electrical characteristics. As described in
detail below, the illustrated architecture works well, but the
methods disclosed herein are not limited to devices having this
architecture. Also shown in FIG. 2 are a PbSe QD layer 26 formed
according to an enhanced deposition method as disclosed herein. The
device also includes a top contact 28. In the FIG. 2 embodiment the
top contact 28 is representative of the second electrode described
above and in particular is an Au layer deposited by known
deposition techniques. Other top contact materials could have been
used. The device 18 could also include any number of additional
layers including but not limited to encapsulant layers or
additional window layers.
[0045] The QD layer 26 could be formed with any QD or NC material
fabricated or treated using the enhanced methods described herein.
For example, the QD layer could be composed of QDs of lead selenide
(PbSe), lead sulfide (PbS), cadmium selenide (CdSe), other
semiconductor NCs, core-shell and ternary nanocrystals, for example
lead telluride (PbTe), lead selenide sulfide (PbSSe), lead selenide
core with lead sulfide shell, cadmium lead sulfide (CdPbS), cadmium
lead selenide (CdPbSe) tin sulfide (SnS), tin selenide (SnSe), tin
telluride (SnTe), silicon (Si), germanium (Ge), indium arsenide
(InAs), indium phosphide (InP), indium antimonide (InSb), gallium
arsenide (GaAs), indium gallium arsenide (InGaAs) and NC structures
of all other Group III-V, IV, II-VI, IV-VI compounds and
alloys.
[0046] Generally, as shown in the flow charts of FIGS. 3 and 4, a
device 18 may be fabricated on a glass substrate 20 with an ITO
electrode 22 deposited thereon (Steps, 30, 32 and 40, 42). The
substrate and electrode may be obtained commercially or
manufactured by known techniques. An n-type window layer 24 is
deposited or otherwise associated with the substrate and ITO layer
by known techniques (Step 34, 44). The substrate, electrode and
window layer may then be cleaned and dried under nitrogen flow and
moved into a glovebox for further room temperature processing.
[0047] The QD film 26 is added to the structure according to an
enhanced layer by layer deposition processes as disclosed herein.
Certain steps included in the enhanced deposition processes are
illustrated in FIG. 3 and FIG. 4. Each enhanced layer by layer
deposition process initially includes contacting the deposition
surface of a device (and the developing QD layer) with a selected
ligand exchange chemical and contacting the device and developing
QD layer with colloidal QDs to deposit an initial QD layer (step 36
and 46). The order in which the device is contacted with the ligand
exchange chemical and colloidal QDs can be varied. In addition, it
is not required that the device or deposition surface be contacted
with each fluid the same number of times or for the same total
time.
[0048] As used herein, the term "contacting" is defined as any
method of physically bringing the ligand exchange chemical or
colloidal QDs into contact with the developing QD layer. The most
basic method of contacting the device with a ligand exchange
chemical or colloidal QDs involves dipping or submerging the device
in a container of the appropriate fluid. Alternative methods of
contacting the device with a ligand exchange chemical or colloidal
QDs include but are not limited to spraying techniques, flowing
fluid across the QD film or jet printing techniques. Any number of
layer by layer deposition cycles may be performed to develop an
initial QD film of a desired thickness, for example, between 40 nm
and 400 nm. 100 nm has been observed to be an initial QD film of
suitable thickness.
[0049] After the initial layer by layer deposition steps have
created the initial QD film, a selected secondary treatment process
is performed. The process of FIG. 3 includes the steps of
contacting the initial QD film with a secondary treatment chemical
and contacting the QD film with additional colloidal QDs in
subsequent layer by layer deposition cycles to deposit additional
QDs to the initial QD layer (step 38). The additional QDs deposited
in secondary treatment may increase the QD layer thickness by any
desired thickness; however the additional thickness is typically
less than the initial layer thickness. For example, the additional
QDs deposited in secondary treatment can add from 1 nm to 50 nm of
additional QD layer thickness.
[0050] An alternative enhanced deposition process is illustrated in
FIG. 4. The alternative enhanced layer by layer deposition process
begins with initial layer by layer deposition using a selected
ligand exchange chemical and colloidal QDs to deposit an initial QD
layer as described above (step 46). In this embodiment however, the
initial deposition steps are followed by the treatment of or
contacting the initial QD layer with at least one selected
secondary treatment chemical without adding additional QDs (step
48). The methods of FIGS. 3 and 4 could be combined so that the
enhanced deposition of QDs includes both the deposition of
additional QDs to the initial QD layer using a secondary treatment
chemical followed by or preceded by secondary treatment without the
addition of supplemental QDs.
[0051] More specifically, the initial layer by layer process begins
by obtaining or preparing a quantity of QDs fabricated to have a
selected absorption peak and colloidally suspended in hexane or
another suitable fluid. For the purpose of initial ligand exchange,
a supply of a ligand exchange chemical, for example 1,2
ethanedithiol (EDT) dissolved in degassed acetonitrile is provided.
Other nucleophillic molecules can be used instead of EDT as a
ligand exchange chemical, including but not limited to methylamine,
benzenedithiol, ethanethiol, ethylenediamine, butylamine, or
benzenediamine.
[0052] In addition, a quantity of a secondary treatment chemical,
possibly also dissolved in degassed acetonitrile or another solvent
is obtained. The secondary treatment chemical may be hydrazine,
formic acid or another larger organic acid, for example
mercaptopropionic acid, methylamine, methanol, ethanol,
ethylenediamine or some other chemical which provides suitable
secondary treatment, as described herein.
[0053] According to one specific process, initial room-temperature
layer by layer deposition occurs by submerging or otherwise
contacting the device with the ligand exchange fluid, for example
ethane dithiol (EDT), for a short time and then removing the
structure. The surface is allowed to dry, which takes a short time.
The device is then submerged or otherwise contacted with the
colloidal QD solution. The device is removed, allowed to completely
dry and then resubmerged in the EDT solution. The process of
dipping the structure in the colloidal QD, removing it, and
treating it with EDT is repeated for a number of times (in some
examples, a total of 8 times, in other examples, up to 20 or more
times), until the substrate is darkly colored with the desired
thickness of initial QD layer. As noted above variations in the
timing, length or order of the dipping or contacting steps are
within the scope of the present disclosure.
[0054] After a suitably thick initial layer of QDs has been
deposited, the disclosed method will include at least one of the
two secondary processes noted above. In one instance, as shown on
FIG. 3, additional QDs may be deposited by submerging or otherwise
contacting the substrate and associated layers with a secondary
treatment fluid, for example hydrazine, formic acid or another
larger organic acid for example mercaptopropionic acid,
methylamine, methanol, ethanol, ethylenediamine. The structure may
then be removed and allowed to dry. Then the structure may be
submerged in the colloidal QD supply, allowed to dry and
re-submerged in the secondary treatment chemical. This process may
be repeated until a quantity of additional QDs is deposited on the
QD layer. The back and sides of the substrate may then be cleaned
and a suitable top contact added (step 39).
[0055] Alternatively, the initially deposited QD layer may be
submerged or otherwise contacted with a secondary treatment
chemical for a selected time and over a selected number of
repetitions, without resubmerging the device in the QD colloid or
otherwise adding additional QDs to the QD layer. After secondary
treatment has been completed, the back and sides of the substrate
may be cleaned and a suitable top contact and any other desired
device layer added (step 49).
[0056] The enhanced room-temperature layer by layer deposition
techniques described above overcome several technical challenges
presented by the fabrication of a suitable QD layer. In particular,
long-chain aliphatic ligands, such as oleic acid, are used in the
synthesis of QDs to control growth kinetics, and allow for stable
colloidal dispersions. In addition, the use of long chain aliphatic
ligands passivates the QD surface states through metal-ligand
chemistry. However, long chain aliphatic ligands, if present in
deposited QD films, create a large barrier to electronic transport.
The use of a ligand exchange chemical or a secondary treatment
chemical as described above during deposition substantially removes
the long chain aliphatic ligand barriers while maintaining or
improving surface passivation and controlling the electrical
properties of the resulting film.
[0057] Previously prepared cells using only ligand exchange
chemicals such as EDT did not unambiguously exhibit MEG however.
The failure of the known devices prepared in a one step, ligand
replacement chemical only, layer by layer deposition process to
exhibit MEG can be deduced from the analysis of the quantum
efficiency of the devices and determined directly through
spectroscopic measurements. In addition, although it is known that
hydrazine treated PbSe QD films exhibit superior electron
mobilities, on the order of 1 cm.sup.2 V.sup.-1 s.sup.-1, no
reports are known showing the successful incorporation of a
hydrazine treated QD layer into a QD solar cell. Previous attempts
using hydrazine as an initial ligand replacement chemical have
produced non-working devices.
[0058] The disclosed enhanced chemical treatments and enhanced
fabrication methods have important effects including but not
limited to producing QD layers which unambiguously exhibit MIG and
other enhanced electrical characteristics. This is an important
breakthrough in the development of QD based devices since
extracting multiple carriers per absorbed high-energy photon could
have a dramatic impact on solar energy conversion technologies.
Quantum confinement can increase the ability of a material to
convert high-energy photons to multiple charge carriers and
ultrafast transient absorption measurements have demonstrated that
MEG has about twice the efficiency in isolated PbSe QDs when
compared to bulk PbSe. The following example provides detailed
analysis of three selected QD solar cells prepared according to the
disclosed methods. The following example is provided for
illustrative purposes only and is not intended to limit the scope
of the embodiments disclosed herein.
EXAMPLE
[0059] Solar cells were prepared having the architecture described
above and shown in FIG. 2. Specifically, the tested structures
consist of a 40-60 nm ZnO n-type window layer deposited on top of a
glass/ITO transparent front electrode, a 50-250 nm thick PbSe QD
layer and a gold anode thermally evaporated onto the PbSe QD layer.
The PbSe QD layer for the tested devices was alternatively prepared
for a given device by selecting from three sizes of QD. The
selected QDs have size dependent bandgaps (Ed. The smallest QDs
have E.sub.g=0.98 eV, the medium sized QDs have E.sub.g=0.0.83 eV
and the larger sized QDs have E.sub.g=0.72 eV. Thus, three
varieties of the same general solar cell architecture were
fabricated and tested with the difference between devices being the
size of the QDs incorporated into the QD active layer creating
devices having three distinct active layer bandgaps for
testing.
[0060] The QD-layer formation protocol for the example cells began
with the initial deposition of 50-250 nm of PbSe QDs with
layer-by-layer EDT treatment as described above, followed by a
secondary treatment comprised of the deposition of approximately 30
nm of additional QDs, using 1M hydrazine in acetonitrile as a
secondary treatment chemical in place of the EDT. The devices were
finished with evaporated gold electrodes. The junction in the
described devices is located close to the n-type window layer. The
selected heterojunction architecture facilitates extraction of
charge-carriers produced from high-energy photons, which are mostly
absorbed about 50 nm into the PbSe film.
[0061] The ZnO n-type window layer was deposited via a sol-gel
spin-coating process using diethylzinc as a precursor in air.
Previous work on ZnO/PbSe heterojunctions has shown that the
valence band maximum is determined by sub-states in the ZnO,
regardless of QD size, and that electron injection into the ZnO
should be blocked for PbSe QDs with bandgaps less than
approximately 0.7 eV. This observation was confirmed herein. Solar
cells made from the QDs with the smallest bandgap show a much lower
V.sub.oc as described below.
[0062] FIGS. 5-7 graphically illustrate the spectral response of
the PbSe QD devices made as described above with a coupled-film
bandgap of 0.98 eV (FIG. 5), 0.83 eV (FIG. 6) and 0.72 eV (FIG. 7).
On each graph, the external quantum efficiency (EQE) is shown with
dotted curves, EQE/(1-R) where R is the reflectance measured using
an integrating sphere, to include diffuse reflectance, is shown
with dashed curves, and EQE/A is illustrated with solid curves,
where A is the modeled absorptance of the PbSe and ZnO layers.
[0063] Both the reflectance and EQE spectra of the devices exhibit
significant interference fringes indicating the buildup of optical
modes within the dielectric stack. These fringes depend upon the
thickness of each layer and a rigorous optical model using
ellipsometric measurements was employed to extract the internal
quantum efficiency (IQE). Despite reflection and absorption by the
glass, ITO, and ZnO layers prior to incident light reaching the QD
layer, as shown on FIG. 7, the EQE reaches 106.+-.3% for the
largest sized QDs (E.sub.g=0.72 eV) at 3.5 eV photon energy which
confirms that the cells exhibit MEG. The accuracy of the foregoing
results was verified by measuring the EQE of a Thorlabs FDS-100-CAL
silicon photodiode, and comparing said measurement against the
calibration given by Thorlabs.
[0064] All photons not absorbed within the described solar cells
are reflected, so an initial determination of the QE of a device
can be determined by dividing the measured EQE by 1-R. Since the
back contact of the tested devices is a gold film, no light is
transmitted through the back. The EQE/(1-R) trace of FIGS. 5-7
represents a lower limit to the actual IQE of a given class of
device since this curve does not account for light absorbed by
other layers that do not contribute to photocurrent. For example,
the large absorption of ITO in the near infrared region leads to
the decrease of the EQE at photon energies of below 2 eV. The
absorption within each layer of the structure was determined by
comparing the measured reflectance to that calculated from an
optical model using the experimentally determined complex
refractive index (N=n+ik) of each component layer as inputs.
[0065] Normalizing the EQE to the calculated absorption yields the
IQE of the active layer:
IQE = EQE A , ##EQU00001##
where the absorptance A=A.sub.PbSe+A.sub.ZnO is produced by the
optical model. The ZnO absorption is included to make a
conservative estimate of the device IQE, since it has not been
determined if photons absorbed within the ZnO layer contribute to
the photocurrent. This IQE, labeled as EQE/A, is plotted for each
of the solar cells shown in FIGS. 5-7. Near the bandgap there is
significant statistical variation, which can lead to improbable IQE
values, such as those observed around 1 eV for the 0.98 eV solar
cell of FIG. 5, however, the variation is significantly smaller in
the visible and ultraviolet portion of the spectrum.
[0066] The IQE curves of FIGS. 5-7 exhibit short-circuit collection
yields of around 85%, until the photon energy surpasses the MEG
threshold, where the IQE rises to a peak efficiency of 130% for the
0.72 eV QD cell (FIG. 7), 108% for the 0.83 eV QD cell (FIG. 6),
and 98% for the 0.98 eV QD cell (FIG. 5). The glass, ITO and ZnO
begin to absorb significant light at photon energies greater than
3.5 eV, and the EQE and IQE drop sharply.
[0067] To assess MEG efficiency, .eta..sub.MEG the IQE curves of
FIGS. 5-7 can be plotted versus the photon energy (E.sub.ph)
normalized to the bandgap (E.sub.g) of the QD layer,
E.sub.ph/E.sub.g. This plot is illustrated in FIG. 8. It is notable
that the IQE curves for different sized QDs are similar on an
E.sub.ph/E.sub.g basis, indicating that the capacity to convert
high-energy photons to multiple excitons is mainly determined by
the excess energy relative to the fundamental energy required to
create an exciton. In coupled QD films, the bandgap energy
red-shifts by 50-100 nm, therefore normalization may be made to the
coupled-film bandgap, as determined by the peak of the first
optical transition seen in the EQE, instead of the first transition
observed in a colloidal solution of isolated QDs.
[0068] The measured peak IQE for each QD size at the appropriate
E.sub.ph/E.sub.g value is illustrated in FIG. 9. A clear trend is
shown that agrees with spectroscopic measurements despite a
difference of about 15% due to intrinsic photocurrent losses. The
IQE curves of the two devices using larger bandgap QDs have their
peak quantum yield (QY) at photon energies below the MEG-threshold
and thus can be used to approximate the intrinsic photocurrent
losses. These losses may be attributed to electron-hole
recombination before carrier separation and collection as
photocurrent, and therefore the measured IQE may be normalized to
these values, as shown by black dots on FIG. 9. These normalized
values compare with a model that accounts for a competition between
MEG and hot-exciton cooling, shown as the solid black curve in FIG.
9. This curve may also be normalized as above for intrinsic
recombination losses, and shown as the dashed black curve. A
least-squares linear fit of a normalized version of the model may
be applied to the IQE for the 0.72 eV cell, yielding
.eta..sub.MEG=0.62.+-.0.01, and an MEG onset threshold, E.sub.th
of:
E th = ( 1 + 1 .eta. MEG ) E g = ( 2.61 .+-. 0.03 ) E g
##EQU00002##
The above analysis shows quantitative agreement with spectroscopic
measurements. This constitutes compelling evidence that MEG is more
efficient in PbSe quantum dots than in bulk PbSe, which exhibits an
MEG efficiency of only 0.31 and a corresponding onset of 4.22
E.sub.g. In addition to being a measurement of greater than 100%
EQE for quantum dots, the above analysis appears to be one of only
two measurements of EQE greater than 100% for any solar cell at
short circuit. An EQE of 128% at a photon energy of 7.7 eV has been
reported in a bulk silicon photodiode exhibiting impact ionization,
corresponding to a relative photon energy of 6.94.
[0069] FIGS. 10-13 show the current-voltage characterization of the
QD solar cells fabricated above using the enhanced secondary
treatment in comparison with devices fabricated without secondary
treatment. The performance characteristics illustrated in FIGS.
10-13 were collected under 100 mW cm.sup.-2 illumination for the
smaller 0.98 eV QD devices (FIG. 10) and larger 0.72 eV QD devices
(FIG. 11).
[0070] For both QD sizes tested, the secondary treatment with
hydrazine yields a dramatic improvement over non-enhanced EDT-only
treated films in all measured performance parameters. For example,
as shown in FIG. 10, the crossover between light and dark currents
in forward bias was eliminated by the hydrazine secondary
treatment, indicating an ohmic anode contact.
[0071] FIG. 12 illustrates a comparison of the dependence of the
open circuit voltage (V.sub.oc) on QD bandgap with and without the
secondary hydrazine treatment. FIG. 13 illustrates the positive
aging effect observed on solar cell performance under air-free
nitrogen storage conditions. In particular, FIG. 13 shows the
mismatch-corrected power conversion efficiency (PCE) as a function
of the device age, with a best PCE of 4.5% after eight days. The
above described secondary treatment techniques are effective with
both PbSe and PbS QDs. Other secondary treatment chemicals such as
formic acid have also been determined to be effective. As the
foregoing data shows, the useful effects of the secondary treatment
allow multiple carriers produced by MEG to be efficiently collected
in a QD solar cell that is both simple to construct and stable
under a nitrogen atmosphere.
[0072] Several embodiments have been particularly shown and
described. It should be understood by those skilled in the art that
changes in the form and details may be made to the various
embodiments disclosed herein without departing from the spirit and
scope of the disclosure and that the various embodiments disclosed
herein are not intended to act as limitations on the scope of the
claims. Thus, while a number of exemplary aspects and embodiments
have been discussed above, those of skill in the art will recognize
certain modifications, permutations, additions and sub combinations
thereof. It is therefore intended that the following appended
claims and claims hereafter introduced are interpreted to include
all such modifications, permutations, additions and
sub-combinations as are within their true spirit and scope.
* * * * *