Hydrothermal synthesis of Al-doped ZnO nanowires
and their application for photovoltaic devices
by
MASSACHUSETTS INGTrnTE
OF TECHNOLOGY
Hyoungwon Park
SEP 12 2014
B.S., Materials Science and Engineering
Korea University, Korea (2009)
LIBRARIES
M.E., Advanced Materials Science and Engineering
Korea University, Korea (2011)
Submitted to the Department of Materials Science and Engineering
In Partial Fulfillment of the Requirements for the Degree of
Master of Science
at the
Massachusetts Institute of Technology
September 2014
C 2014 Massachusetts Institute of Technology. All rights reserved
Signature redacted
Signature of Author
Department of MVaterials Science and Engineering
August 14, 2014
Signature redacted
Certified by:
Silvija Gradedak
Thomas Lord Associate Professor in Materials Science and Engineering
Thesis Supervisor
Signature redacted
Accepted by:.
Gerbrand Ceder
R. P. Simmons Professor of Materials Science and Engineering
Chair, Departmental Committee on Graduate Students
1
N
N~.
Hydrothermal synthesis of Al-doped ZnO nanowires and their application
for photovoltaic devices
by
Hyoungwon Park
Submitted to the Department of Materials Science and Engineering
On August 14, 2014 in partial fulfillment of the requirements for the degree of
Master of Science in Materials Science and Engineering
Abstract
Semiconductor nanostructures exhibit distinct properties by virtue of nano-scale
dimensionality, resulting in recent interest in semiconducting nanowires for electronic,
photonic, and energy applications. Along with nanowires, quantum dots are solutionprocessable nanocrystals with tunable band gap energies as a function of their size. Based on
all of these promising properties that nanostructures exhibit, nanowires and quantum dots are
excellent candidates for next-generation optoelectronic devices, including solar cells and
light-emitting diodes. However, the realization of nanostructured materials for solar cell
device applications is limited by the fundamental trade-off between light absorption and
photocarrier collection. Vertically aligned ZnO nanowire arrays can decouple absorption and
collection by acting as highly-conductive channels for extracting photogenerated electrons
from deep within the film.
This thesis illustrates a scheme for the development of ordered bulk heterojunction
photovoltaic devices incorporating solution-based n-type doped ZnO nanowires and PbS
quantum dots. In order to improve the electrical properties of ZnO nanowires, Al doping of
hydrothermally synthesized ZnO nanowires is studied along with the optimization of doping
concentration. The morphology of ZnO nanowire arrays is also studied as a function of the
doping concentration in the growth solution. Finally, photovoltaic devices are fabricated and
the effect of Al-doping of ZnO nanowires is investigated by device characterization
techniques.
Thesis Supervisor: Silvija Gradedak
Title: Thomas Lord Associate Professor in Materials Science and Engineering
3
4
Table of Contents
Abstract
3
List of Figures
7
List of Tables
10
Chapter 1. Introduction
11
Chapter 2. Theoretical Background
13
2.1. Properties of zinc oxide nanowire
13
2.2. Hydrothermal synthesis method of zinc oxide nanowire
15
2.3 N-type doping of ZnO nanowire
18
2.3 Hybrid photovoltaics
19
2.3.1. Basic physics of the solar cell
19
2.3.2. Excitonic solar cell
20
2.3.3. Nanowire / quantum dot hybrid solar cell
21
Chapter 3. Experimental methods
24
3.1. ZnO nanowire growth
24
3.2. Structural characterization
25
3.3. PbS quantum dot synthesis
25
3.4. Quantum dot photovoltaic device fabrication
25
3.5. Device Characterization
26
Chapter 4. Hydrothermal synthesis of Al-doped ZnO nanowire
28
4.1. Morphology and alignment of nanowires
29
4.2. Chemical analysis of Al-doped ZnO nanowires
33
4.3. Photoluminescence analysis of Al-doped ZnO nanowires
36
5
4.4. Electrical property of Al-doped ZnO nanowire arrays
Chapter 5. Application of Al-doped ZnO nanowires to photovoltaic device
5.1. Designed device structure
39
43
44
5.2. Device performance of Al-doped ZnO nanowire-based quantum dot solar cell
device
46
5.3. Morphologies of Al-doped ZnO nanowire/PbS quantum dot solar cell device 49
5.4. Failure analysis of nanowire-based quantum dot solar cell
51
Chapter 6. Conclusion and future work
53
Reference
55
6
List of Figures
Figure 1. (a) Schematic crystal structure of ZnO (Yellow: Oxygen; White: Zinc) (from
http://en.wikipedia.org/wiki/Zincoxide). (b) ZnO crystal planes (from [26]).
-..........13
Figure 2. Typical I-V characteristics of solar cells: no illumination (left) and under
illumination (right) [51]. --------------------------.................................................
20
Figure 3. Schematic diagram of the working principles of excitonic photovoltaics illustrating
charge generation and extraction. ------------------------........................................
21
Figure 4. A Schematic diagram of cross sectional view of nanowire/quantum dot ordered bulk
heterojunction solar cell [24]. ---------------------------..........................................
22
Figure 5. A schematic diagram of hydrothermal synthesis process for ZnO nanowires. ---
25
Figure 6. A schematic diagram of nanowire/quantum dot solar cell device fabrication process.
...................................................
- .-- -- ----- -----............................--26
Figure 7. Tapping-mode AFM image of sol-gel derived ZnO seed layer. Average grain size is
about 46 nm and the root mean square (RMS) roughness is about 1.3 nm. --...-----
29
Figure 8. SEM micrographs of synthesized nanowire array with (a) no Al precursor, (b) 0.2
mol%, (c) 0.4 mol%, (d) 0.6 mol%, (e) 0.8 mol%, and (f) 1.0 mol% of Al precursor,
respectively. -- --------..-----------.........................---
-------------............-.....
30
Figure 9. A plot of average nanowire diameter as a function of Al precursor concentration.
..................................
...................-..-- -.............--..-- - - ..... 31
Figure 10. High resolution bright field TEM images of (a) ZnO nanowire without Al addition
and (b) 0.8mol% of Al precursor addition. Insets correspond to the FFT of each image. -- 32
7
Figure 11. SEM-EDS spectra of undoped and 0.8 mol% Al-doped ZnO nanowire arrays. Inset:
SEM image of a 0.8 mol% Al-doped ZnO nanowire array. Green box in the SEM represents
the area where EDS spectrum was acquired and scale bar is 300 nm. --------................
33
Figure 12. (a) STEM-EDS spectra of a 0.8mol% Al-doped ZnO nanowire and its
corresponding STEM image of the measured nanowire. Scale bar represents 100 nm. (b)
Elemental EDS mapping result of identical sample. Scale bars represent 100 nm. ........ 35
Figure 13. Normalized photoluminescence (PL) spectra from undoped ZnO nanowire array
(control) and Al-doped ZnO nanowire arrays (a) before and (b) after annealing at 350 *C for
30 min. (c) PL spectra from (b) expanded between the wavelengths of 330 to 430 nm. (d)
Summary of PL peak positions of near band-edge emission for (a) and (b).
---............
36
Figure 14. A schematic illustration of band gap widening due to the Burstein-Moss effect in
semiconductors [67]. ---------------------------------...............................................
38
Figure 15. A schematic diagram of the fabrication process for ZnO nanowire-based electrononly device. -----------------------------------......................................................
39
Figure 16. SEM micrographs of etched back PMMA coated (a) undoped, (b) 0.2mol% Aldoped, (c) 0.4mol% Al-doped, and (d) 0.6mol% Al-doped ZnO nanowire arrays after 150sec
of O2-plasma RIE. A Matlab area selection code was used to calculate the exposed area after
the 02-plasma.RIE and the fraction of area is tabulated in each figure. -------...............
40
Figure 17. I-V curves for ZnO nanowire-based electron-only devices. -------...............
41
Figure 18. A line graph illustrating conductivity changes as a function of Al precursor
42
concentration. ------------------------------------....................................................
Figure 19. (a) Schematic of the device architecture and (b) flat-band energy diagram of ZnO
nanowire/PbS quantum dot solar cell. --------------------.. . . . . . . . . . . . . . . . . .--
8
-
-
44
Figure 20. J-V characteristics measured in the dark (dotted lines) and under AM 1.5G
illumination (solid lines) for undoped, 0.4 mol% Al-doped, and 0.8 mol% Al-doped ZnO
nanowire-based quantum dot solar cell devices. ----------------..............................---
46
Figure 21. Average device parameters of nanowire-based quantum dot solar cells showing (a)
Jsc and Rs, (b) Voc and fill factor, and (c) PCE as a function of Al precursor solution. Data is
averaged over 20 different devices. ----------------------......................................---
47
Figure 22. EQE spectra of representative undoped (black) and Al-doped (red) ZnO nanowirebased quantum dot solar cell devices. Integrating the EQE with the AM1.5G solar spectrum
yields predicted photocurrent densities (JSC,EQE) within 10% of the measured values (Jscjv).
.........................................................................
......... 48
Figure 23. Cross-sectional SEM images of (a) undoped and (b) 0.8mol% Al doped ZnO
nanowire/PbS quantum dot solar cells. The cross-sectional surface was prepared with FIB
milling. Nanowire height and quantum dot film thickness is denoted in the images.
-.....
49
Figure 24. Cross-sectional SEM images of non-working devices; (a) too thick and (b) too thin
PbS quantum dot layers result in low efficiency and a shorted device, respectively. (c) Porous
seed layer degrades performance, and (d) large voids between the nanowires causes low Voc.
...........................................................-- .....-- -................. 51
9
List of Tables
Table 1. Measured average nanowire diameters. ------------....................--
-
-.........31
Table 2. Summary of PL peak positions of near band-edge emission for undoped ZnO
nanowire array (control) and Al-doped ZnO nanowire arrays before and after annealing at 350
*C in air for 30 min. ---------------------------------.................................................
37
Table 3. Summary of calculated resistivities and conductivities of each electron-only device
from figures 16 and 17. -------------------------------..............................................
42
Table 4. Summary of averaged device performances figures of merit of nanowire-based
quantum dot solar cells. Data is averaged over 20 different devices and the champion device
performance is shown in the parenthesis. -------------------...................................---
10
47
Chapter 1. Introduction
Global energy demand is increasing drastically and in order to meet this demand,
energy production needs to be in the terawatt range. Since the amount of solar energy incident
on the earth in one hour exceeds current annual global energy demand, [1] photovoltaic (PV)
devices are a viable alternative to fossil fuel-based power generation. Yet, in order to become
major source of power generation, considerable enhancement in power conversion efficiency
of PV devices is required, as well as cost reduction.
Several emerging PV technologies seek to utilize materials that require less
complicated fabrication processes than conventional Si-based PV technologies to achieve
economic viability. These devices utilize an excitonic absorbing material in a donor-acceptor
architecture, which has been demonstrated in several configurations, including organic
photovoltaics (OPV), hybrid organic-inorganic devices [2-5], and more recently, quantum dot
(QD) inorganic semiconductor devices. [6-9] However, the exciton diffusion length is short,
only about 10 nm in organic polymers, [10] and the minority carrier diffusion length is on the
order of 100 nm in PbS QDs. [11] This limits the efficiency of excitonic PV devices. To allow
for efficient carrier extraction, a planar PbS QD film must be thinner than the -250 nm carrier
collection length, which corresponds to the sum of the -150 nm depletion width and the -100
nm minority electron diffusion length. Thus, incorporation of semiconductor nanowires as the
acceptor material to form an ordered bulk heterojunction (OBHJ) solar cell is a convenient
way to overcome the efficiency limit. Ordered semiconductor nanowires may also enhance
exciton dissociation since inorganic semiconductors have high charge carrier mobility and the
nanowire morphology reduces parasitic exciton recombination. This will eventually result in
higher Jsc and Vo.
To design an OBHJ solar cell, the acceptor material should satisfy several
requirements. Unlike classical p-n junction solar cells, light absorption in the acceptor
materials should be minimized, since photo-induced excitons are only generated in the donor
materials. At the same time, semiconductor materials should exhibit good electrical
conductivity to efficiently transport charge carriers. Thus, a wide band-gap material should be
used for the nanowire incorporated into the OBHJ solar cell. Also, spacing between nearest
11
nanowire neighbors should be precisely controlled to maintain superior exciton dissociation
and charge collection efficiency. Most of QD PVs incorporating 1-D nanostructures have
focused on liquid-electrolyte photoelectrochemical cells in which TiO2 nanotubes [12] or ZnO
nanowires [13,14] are sensitized with a thin layer of cadmium chalcogenide QDs. In solidstate OBHJ devices, vertically-oriented nanowire arrays are fully infiltrated with QDs,
orthogonalizing the mechanistic length scales of light absorption and carrier collection, such
that light is absorbed in the QD film parallel to the nanowire major axis, while electrons are
collected laterally to the heterojunction interface. Solid-state OBHJ devices have been
demonstrated with PbS QDs and top-down-patterned TiO 2 nanopillars. [15] In contrast to topdown synthetic techniques such as lithographic patterning [17] or nanoimprint molding, [18]
bottom-up solution growth [16, 19-21] may enable scalable, large-area production of 1-D
nanostructures [22] on various electrode materials [23] for QD solar cell applications.
Nanowire-quantum dot OBHJ PV devices use less materials than conventional Si
solar cells as they only need to be more than 1 pm thick for complete absorption, and their
fabrication is compatible with self-assembly and solution-based fabrication. The potential for
increased efficiency due to the enhancements in absorption and charge collection afforded by
ordered nanowire arrays could make OBHJ PVs competitive with other emerging PV
technologies. Specifically, ZnO is a good candidate for a nanowire acceptor material since it
achieves a balance of high transparency and good conductivity and it has an electron affinity
that is well matched with electron donor materials like poly(3-hexylthiophene) (P3HT) [23]
and PbS QDs with an infrared band gap of about 1.3eV.[24]
This thesis focuses on synthesis of n-type doped ZnO nanowires and their
incorporation into OBHJ QD-based PV devices. The formation of OBHJ between nanowires
and PbS QDs is achieved using solution-based synthesis techniques. In order to improve the
electrical properties of nanowires, the validity of Al doping in hydrothermally synthesized
(HTS) ZnO nanowires is studied along with the optimization of doping concentration in the
growth solution. The morphology of the ZnO nanowire array is also studied with respect to
doping concentration in the growth solution. Finally, photovoltaic devices are fabricated and
the effect of Al doping of ZnO NWs is investigated by device characterization techniques.
12
Chapter 2. Theoretical background
2.1 Properties of zinc oxide nanowire
Zinc oxide (ZnO) is a wide band gap semiconductor material with a direct band gap
of 3.37 eV at room temperature. It has wurtzite crystal structure with lattice parameters of c =
5.207 and a = 3.249 A. As shown in Figure 1, zinc and oxygen atomic planes are alternatively
stacked to form tetrahedra, which cause ZnO to have a non-centrosymmetric crystal structure.
This lack of centrosymmetry enables ZnO to be synthesized as anisotropic microstructures
during the growth process. ZnO consists of two monohedral (0001) and (0001) facets,
hexagonal prism facets of the {1010} and the {1011}, and {1012} hexagonal pyramids
facets (Figure 1.b). Among those facets, Zn 2 -terminated (0001), 02 -terminated (0001) and
non-polar {1010} facets are the most stable [25].
((1011}
(b)
(a)
Figure 1. (a) Schematic crystal structure of ZnO (Yellow: Oxygen; White: Zinc) (from
http://en.wikipedia.org/wiki/Zinc_oxide). (b) ZnO crystal planes (from [26]).
ZnO has a large exciton binding energy of 60 meV, which is several times higher than
kT [27] and typical electron concentrations of1016 -101
cm- yielding n-type semiconductor
behavior. This intrinsic n-type characteristic originates from native point defects, such as
oxygen vacancies and zinc interstitials [27]. Oxygen vacancies and zinc interstitials are
favored under zinc rich synthetic conditions and act as donors, while zinc vacancies are
13
favored under oxygen rich synthetic conditions and act as acceptors. These defects may
observed in a photoluminescence spectra, typically in the form of two major defect peaks
centered around 510 nm and 600 nm, corresponding to oxygen vacancies and oxygen
interstitials, respectively, while possessing strong and sharp near band edge emission at 375
nm (3.2 eV) [28, 29].
Apart from the bulk properties of ZnO, the electrical properties of zinc oxide
nanowires are unique, due to their large surface to volume ratio. Single nanowire currentvoltage measurements show higher resistivity than bulk ZnO, ranging from 0.3 Q cm to 7 K
cm, while maintaining similar carrier concentration value of 1 - 5 x 101 cm 3 [30, 31]. Li et.
al. reported that this decrease in the conductivity is caused by the adsorption of charged
oxygen on the surface of nanowires, acting as surface traps for carriers [31]. Thus, the surface
depletion width of ZnO nanowires is determined by the oxygen partial pressure of the
surrounding environment. Since the typical diameters of nanowires are in the range of 20 to
60 nm, this surface depletion width may cover a substantial cross sectional area resulting in a
large variation in conductivity with oxygen partial pressure.
14
2.2. Hydrothermal synthesis method of zinc oxide nanowire
Because the difference in reactivity between polar and non-polar faces causes shape
anisotropy during the growth process, one-dimensional nanostructures, such as nanowires and
nanorods, can be synthesized with HTS methods. X-ray diffraction (XRD) [32] and
transmission electron microscopy (TEM) studies [33, 34] have shown that ZnO nanowires or
nanorods crystallize in the wurtzite structure with c-plane orientation along their length. The
HTS technique provides a way to utilize a cost-effective and low-temperature synthesis
process while maintaining the beneficial qualities of ZnO nanostructures. Moreover, doping
of ZnO nanowires can be easily achieved by adding dopant molecules to the ZnO growth
solution [35].
The HTS method for zinc oxide nanowire growth was first reported by Vergds et al.
in 1990 and its growth kinetics and mechanisms have been well studied since then [36].
According to Demianets et al. [25, 37], facial states, such as chemical composition, reticular
density and electronic charge, determines the incorporation probability of Zn-complex into
ZnO nanostructures during the growth as well as the physicochemical characteristics of the
growth medium. The atomic structure of any face is determined by an elementary layer. The
elementary layer is the minimum structural unit of the thickness and expressed as dm, where
d is the interplanar spacing and m is the integer dependent on the face symmetry. It represents
the minimum thickness of a layer (A) that can be formed on the surface during the growth.
These layers are repeated in the crystal structure with an interval of d/m. Theoretical
calculations showed that for ZnO crystals, elementary layer thickness of the main
crystallographic planes satisfied the following sequence [37]:
A(1010)> A(0001)--t A(000-)> Agjoij)> A(1012)> A(1010)
Since the elementary-layer thickness is inversely proportional to the growth rate of the faces
(V) [37], under ideal conditions, the (1010) facet has the slowest growth rate and (1120) facet
has the fastest growth rate:
Because charge state of (0001) and (0001) planes and growth rate along dc direction is
15
different, polar growth of zinc oxide crystal is mainly contributed by the ZnO2 2 - [25, 38].
ZnO 22 - can be formed from Zn(OH)42- from following reactions:
Zn(OH) 4 2- -+ Zn(OH) 2 + 20H-
(1)
Zn(OH) 2 -+ 2H+ + Zn0 2 2 -
(2)
Excessive OH- ions from reaction (1) can bind with H ions from reaction (2) to produce H2 0.
Theiefore, the reaction (2) is likely to take place in an alkaline solution environment and
ZnO 2 2 ~ is present in greater concentrations in solutions with high pH. Moreover,
supersaturation of Zn2+ increases the driving force for the crystallization of ZnO, resulting in
an increased growth rate [25]. The positively charged Zn-terminated (0001) surface attracts
more ZnO22- than negatively charged O-terminated (0001) surface, indicating the rate of the
reaction is faster for (0001) plane than (0001) plane.
As mentioned above, ZnO has to be synthesized in a high pH solution since it is
difficult to remove protons from hydrated Zn2+ complexes [32]. Various sources of hydroxyl
ions have been reported such as potassium hydroxide [39], sodium hydroxide [40],
ammonium hydroxide [41], and hexamethylenetetramine [42] (HMTA, chemical formula
C 6H12 N 4 ). Among them, HMTA is the most widely used because it undergoes slow thermal
decomposition at growth temperatures of about 90*C. As HMTA decomposes, it releases
ammonia in a controlled fashion maintaining its supersaturation throughout the growth period
[431. Generally, the HTS growth process of ZnO nanowires with HMTA is expressed as the
five following reactions:
(CH 2 ) 6 N 4 + 6H 2 0
-+
4NH 3 + 6HCHO
(1)
NH 3 -H 2 0
(2)
NH 3 - H20 +-* NH 4 + + OH
(3)
Zn2+ + 20H- +-+ Zn(OH) 2
(4)
Zn(OH) 2 -+ ZnO+ H 2 0
(5)
NH 3 + H20
+
When the substrate is seeded with a ZnO layer, it reduces the required supersaturation
for nucleation and also allows additional control of nanowire alignment, diameter, and density.
Also, the existence of a seed layer provides substrate independence during the growth, which
16
allows a variety of choice in the substrate selection [32, 21]. Typically, the ZnO seed layer is
formed by sol-gel deposition with spin-coating, dip-coating of ZnO nanoparticle solution or
sputtering of ZnO thin film. The density of nucleation points plays an important role in both
the alignment and density of nanowire arrays. The most commonly used seeding method is
sol-gel deposition, which gives well aligned nanowire arrays and allows for control of
nanowire diameter and density. The seed diameter is also directly correlated with the
nanowire diameter and therefore a narrow particle size distribution for ZnO seeds is desirable
to reduce variation in nanowire diameter to have homogenous electrical properties across the
array.
17
2.3. N-type doping of ZnO
Since it is a wide direct band-gap material, ZnO has strong potential for use in
optoelectronic device applications, however, high-quality n- and p-type doping is necessary
for this material to be used. in actual applications. As discussed in the section 2.1, wurtzite
ZnO intrinsically exhibits n-type semiconductor characteristics due to a deviation from
stoichiometry caused by oxygen vacancies and zinc interstitials. Undoped ZnO shows n-type
conductivity with superior charge carrier (electron) density of about 10 21cm- [70]. The first
principles study suggests that high shallow donor concentration caused by, either Zn
interstitials, 0 vacancies or H interstitials, is the main source of this intrinsic n-type behavior
[71-74].
Native n-type characteristics allow further n-type doping of ZnO relatively easily.
Group III elements, such as Al, Ga, and In, act as substitutional elements for n-type doping
[75]. Doping with Al, Ga, and In has been attempted by many groups, resulting in high-quality,
highly conductive n-type ZnO films [76-82]. Myong et al. [76] grew Al-doped ZnO films by
photoassisted MOCVD method and obtained highly conductive films with a minimum
resistivity of 6.2x10-4 fcm. Ataev et al. [77] reported resistivities as low as 1. 2 x10-4
Qcm
for
Ga-doped ZnO films grown by chemical-vapor deposition. Thus, n-type doping of ZnO is
well-developed for the thin-film.
Unlike for thin films, n-type doping of nanowires is still under development due to
the difficulty of measuring electrical characteristics from the nanoscale structures. Many
structural and chemical analyses have been reported in the literature, yet only a few have
reported electrical characteristics of the n-doped nanowires. N-type doping of ZnO nanowires
can be achieved by adding desired amount of dopant during the typical ZnO nanowires
growth methods [35, 83, 84]. Because of their anisotropic structure, electrical properties of
the nanowires are expected to be different than those of a thin film. However, reported values
vary significantly. Noriega et al. [83] synthesized free-standing Al-doped ZnO nanowires via
solution processes and conducted single nanowire characterization. They reported a minimum
resistivity of 2.6x 10- acm. Meanwhile, Ahmad et al. [84] reported resistivities as low as 1.9
Qcm for CVD grown doped nanowires.
18
2.4. Hybrid photovoltaics
2.4.1. Basic physics of the solarcell
Conventional solar cells are generally structured with a photoactive layer praced
between two electrodes. When light is absorbed in a photovoltaic material, electron-hole pairs
are generated. The dissociation of electron-hole pairs in a solar cell occurs mainly at the
interface between the transparent electrode and the active semiconductor layer, as well as at
intrinsic and extrinsic defect sites in the lattice. As long as these pairs do not recombine, the
dissociated charge carriers are then accelerated by an electric field developed in the junction,
resulting in a current out of the device. Without this internal field, it is impossible to
dissociate the photo-generated electron-hole pairs [Error! Reference source not found., 51].
Current-voltage (I-V) characteristics of a solar cell show rectifying behavior under
both dark and illuminated conditions. Under illumination, there is a characteristic open-circuit
voltage (Voc) and a short circuit current (Isc), with a maximum power output of ImAxVMAx. In
order to extract power out of the cell, it is necessary to attach an external load. The opencircuit voltage is determined by the quasi-Fermi level separation of the active materials.
Meanwhile, if the cell is illuminated under short circuit conditions, the current thatruns
through the solar cell reaches maximum value [51].
The power delivered to the external load is P0 ut = IV, which is the area of the rectangle
bounded by the I-V curve under illumination.
Maximum power is delivered to the load
when this rectangular area is maximized. As seen in the Figure 2, the larger the area filled
between Isc and Voc, the higher the maximum power output. The ratio of this area is referred
to as the fill factor (FF). FF is defined as (ImxVmx) / (IsVO), where Imx and VAX
represent the current and voltage at the maximum power point. Generally, the efficiencies of
solar cells are determined by the power conversion efficiency, which is defined as the ratio
between the electric power supplied to the external circuit and the illuminated solar power on
the active area of the device [51].
19
current /11
t
current
[
I
Voltage V
Voftage IV
Figure 2. Typical I-V characteristics of solar cells: no illumination (left) and under
illumination (right) [51].
All these factors, such as energy conversion efficiency, fill factor, open circuit voltage
and short circuit current, constitute figures of merit of device performance. Another important
figure of merit for solar cell devices is the cost since high cost of the solar cell is the main
reason for the low market penetration. The cost consists of material cost and production cost.
The ease of mass production, large area production capability, and long-term stability are also
important.
2.4.2. Excitonic solar cells
Excitonic solar cells are a class of photovoltaic systems that differ from conventional
crystalline solar cells. In conventional p-n junctions, photo-induced electron-hole pairs are
generated near the depletion region and accelerated towards the electrodes by the built-in
electric field created in the junction. In an excitonic solar cell photon absorption typically
takes place in only one component, termed the donor [45]. The donor may be a dye molecule
as in the dye-sensitized solar cell [46] or a conducting polymer as in organic PV (OPV). The
exciton binding energy is proportional to 1, where E is dielectric constant. In contrast to
crystalline semiconductors which have small exciton binding energies, donor materials
typically have low e values in range of 2 to 5 and corresponding exciton binding energies
range between 200 to 800 meV. These values are about an order of magnitude higher than kT
at room temperature. Thus, in these donor materials, the exciton is the primary species
generated upon photoexcitation [45]. Subsequently, generated excitons diffuse to the junction
and separate into an electron and hole due to the built-in field. The other component of these
solar cells is called the acceptor and it generally does not participate in photon absorption.
20
However, it provides a band offset at the junction for efficient charge separation. After the
separation of the exciton at the junction, charge carriers are then extracted to the respective
electrodes for charge collection to the external load circuit.
HOMO
Anode
Donor
Cathode Acceptor
Figure 3. Schematic diagram of the working principles of excitonic photovoltaics illustrating
charge generation and extraction.
Figure 3 shows the working principle of an excitonic solar cell illustrating charge
generation and extraction. Light illumination generates photoexcited electrons in the donor
region (step a)), then the polaron pair binds to form excitons (step Q). The exciton diffuses
towards donor/acceptor junction (step ()
and undergoes charge separation into electron and
hole at the junction (step (4). Once electron and hole are separated, they are transported
through the acceptor and donor respectively (step ().
Finally charge carrier is extracted to
the electrodes for conveyance to the external load circuit (step $).
The external quantum
efficiency (EQE) is the product of all the individual process efficiencies.
2.4.3. Nanowire / quantum dot hybrid solar cell
The quantum dot solar cell is a kind of excitonic solar cell which uses a quantum dot
layer as the light absorbing donor material. Since the band gap of the quantum dots can be
tuned by the size of the dot, it is possible to achieve optimal absorption of sun light by
designing the quantum dot layer to be suited to the solar spectrum. Generally speaking,
quantum dots have high absorption coefficient and can be deposited by simple processes such
as spin-coating, dip-coating or drop-casting [47].
It has been reported that incorporation of these inorganic nanostructures into hybrid
21
solar cells improves device performance. When ordered ZnO nanowire arrays are applied to
the quantum dot device, there is up to 50% enhancement in short-circuit current density (Js,)
observed, improving the overall power conversion efficiency of the device. This enhancement
is mainly attributed to the incorporation of nanowire structures which reduce the probability
of electron back-transfer and recombination within the device [24]. Figure 4 shows a
schematic diagram of the cross-section of the nanowire/quantum dot hybrid solar cell and its
working principle. Basic principle of the device is almost same with general excitonic solar
cells. The only difference is that in quantum dot solar cell, the exciton can spontaneously
dissociate at room temperature.
M003
ZnO nanowire I
PbS OD
Figure 4. A Schematic diagram of cross sectional view of nanowire/quantum dot ordered bulk
heterojunction solar cell [24].
Since quantum dots have a large dielectric constant, the exciton binding energy is
smaller than that of the organic active material used in organic photovoltaics. For instance, the
dielectric constant for lead sulfide quantum dot is about 15 to 20, which corresponds to an
exciton binding energy of 100 meV compared to 200 to 800 meV for organic photovoltaic
materials. This small exciton binding energy allows excitons to be dissociated without an
electric field. This is in contrast to organic donor materials which require a thermodynamic
driving force, such as the band offset mentioned previously, to dissociate excitons into free
electrons and holes.
One of the main disadvantages of the quantum dot donor material is the small
minority carrier diffusion length. Even though charge carriers can be easily formed by
dissociation of the exciton, its minority carrier diffusion length is tens of nanometers. In the
case of a lead sulfide quantum dot film, the depletion width is less than 200 nm [48], which
22
corresponds to about 300 nm of the charge collection radius [15]. In order to maximize the
light absorption, the thickness of the quantum dot layer should be a few micrometers. Yet, the
small minority carrier diffusion length restricts the thickness of the donor materials to the one
third of ideal absorption length. This major drawback of the quantum dot system can be
improved by the incorporation of nanowire structures. Short minority carrier diffusion length
is attributed to the low mobility of quantum dot layer; yet, this can be overcome by the
surface passivation of quantum dots and spacing control between nanowires.
23
Chapter 3. Experimental methods
3.1. ZnO nanowire growth
ZnO nanowires were grown on polycrystalline sol-gel ZnO seed layers according to
literature methods [19, 44, 49]. To prepare the sol-gel precursors, 300 mM solution of zinc
acetate dehydrate (ZnAc, 99.999%, trace metals basis, Sigma-Aldrich) and ethanolamine
(MEA, 99.5+%, redistilled, Sigma-Aldrich) in 2-methoxyethanol (2ME, 99.8%, anhydrous,
Sigma-Aldrich) were mixed in equal proportion and ultrasonicated for 20 min. This mixture
was then spin-cast at 4000 rpm and annealed at 175 *C for 10 min, with two spin-anneal
cycles needed to form a uniform and complete film. To grow the nanowires, patterned
ITO/glass substrates (150 nm, 20 Q/L, TFD Inc.) coated with sol-gel ZnO seed layers were
floated facedown in an aqueous bath containing equal volumes (25 ml) of zinc nitrate
hexahydrate (50 mM, 98%, reagent grade, Sigma-Aldrich) and hexamethylenetetramine
(HMTA, 50 mM, 99+%, ACS reagent, Sigma-Aldrich) in deionized (DI) water at 90 *C. After
a growth period lasting from 30 min to 2 hours, the substrates were thoroughly rinsed with DI
water, dried under flowing nitrogen, and annealed on a hot plate at 200 *C for 5 min to
completely remove water vapor.
Al-doped ZnO nanowires were synthesized in a similar manner. ZnO seed layer was
prepared identically to the undoped case. The substrates were subsequently immersed into
growth solution consisting of 50 mM zinc nitrate hexahydrate (25 ml) and 50 mM,
hexamethylenetetramine (HMTA, 25ml), and DI water. 0.1 ml to 0.5 ml of 25 mM aluminium
sulfate (99.99%, trace metals basis, Sigma-Aldrich) solution in DI water were also added to
growth solution. As-grown nanowires were thoroughly rinsed in DI water and annealed at 350
*C for 30 min. A schematic of this nanowire growth is shown in figure 5.
24
...................
...........
m
ie s 1....
25mM [Zn(IN3) .6H201
eN4}
+ 25mM [CH
in DI
+ 25mM [A12(SO4),] in D1
Seed La e
SpinAcoating with 300mM
[ZnAc + MEAJ In 2ME solution
Anaigal5fr1m
~
HTS growth of ZnO nanowire
got
Rinse with DI
fZOnnwr
Figure 5. A schematic diagram of hydrothermal synthesis process for ZnO nanowires.
3.2. Structural characterization
Scanning electron microscopy (SEM) was performed with a FEI Helios Nanolab 600
Dual Beam in immersion mode at 5 kV. Sample cross-sections for SEM were prepared by
focused ion beam (FIB) milling at 30 kV on the same instrument, with final cleaning
performed at 5 kV. Transmission electron microscopy (TEM) images and the corresponding
electron diffraction patterns of as-prepared ZnO samples were obtained by using JEOL 201 OF
with an accelerating voltage of 200 kV. Low-magnification images were obtained using an
objective aperture to enhance contrast.
3.3. PbS quantum dot synthesis
The PbS QDs used here exhibit a first excitonic absorption peak between 905 nm and
1150 nim, corresponding to an optical bandgap between 1.31 eV and 1.08 eV. Colloidal PbS
QDs capped with oleic acid were synthesized, purified, and redispersed in octane according to
previously-reported literature methods [8, 50]. PbS quantum dot solution used in this study
was provided from professor Moungi Bawendi's group in Department of Chemistry,
Massachusetts Institute of Technology.
3.4. Quantum dot photovoltaic device fabrication
Patterned ITO-coated glass substrates were cleaned by successive ultrasonication in
alkaline cleaning solution, deionized water, and acetone, followed by immersion in boiling
isopropanol for 5 min. Substrates were dried under flowing nitrogen and exposed to oxygen
25
plasma for 120 seconds to remove organic contaminants.
PbS quantum dot films were deposited on the nanowire arrays via layer-by-layer
spin-coating method. Each layer consisted of three steps: quantum dot deposition, ligand
exchange, and solvent rinsing. Two drops (-20 jiL) of PbS quantum dot solution (25 mg/mL)
in octane were spin-coated for 15 seconds at 1500 rpm. Ten drops (-100 pL) of a 1,3benzenedithiol (BDT, 99%, Sigma-Aldrich) solution (1.7 mM in acetonitrile) were then
dropped onto the substrate and spun dry after a 30 seconds wait to ensure complete exchange
with the native oleic acid capping ligands. The substrate was then rinsed three times with
acetonitrile to remove excess ligands and spun dry. For a typical nanowire/quantum dot solar
cell, this process was repeated 10 times, forming a quantum dot film with a thickness of-250
nm. PbS QD and BDT solutions were dispensed through a 0.1 pm PTFE membrane filter. A
25 nm MoO 3 (99.9995%, Alfa Aesar) buffer layer and 100 nm Au (99.999%, Lesker) back
contact were thermally evaporated through a shadow mask at rates of 0.5 A/s and 1 A/s,
respectively, and a base pressure of 1x106 Torr. The active device area was defined by the
overlap of the Au anode with the ITO cathode to be 1.21 mm2. The schematics of quantum
dot/nanowire device fabrication is shown in figure 6.
~X10 timesz....
Au
Annealing at 350*C for 30min
..Spin-coating PbS
QD solution
andigand eange
..-
Deposit MoO3(25nm) I Au
(lonm) by thermal
evaporation
Figure 6. A schematic diagram of nanowire/quantum dot solar cell device fabrication process.
3.5. Device Characterization
Current-voltage characteristics of the solar cell devices were recorded in a nitrogenfilled glovebox using a computer-controlled Keithley 6487 source-meter. 100 mW/cm 2
26
simulated solar illumination was provided by a 150 W Newport 96000 xenon arc-lamp
equipped with an AM1.5G filter and diffuser lens. EQE spectra were measured without bias
illumination.
27
Chapter 4. Hydrothermal synthesis of Al-doped ZnO nanowire
As introduced in Chapter 1, semiconductor nanowires have emerged as promising
nanomaterials for optoelectronic applications. Among them, ZnO has received broad attention
for its wide direct band gap of 3.37 eV and large exciton binding energy of 60 meV at room
temperature [35]. Moreover, its unique anisotropic growth morphology enables it to be widely
used as a building block for nano-scale electronic device applications. Many efforts have been
made to enhance the properties of ZnO nanostructures by doping ZnO with various elements
such as Ga [52 - 55], In [54], Sn [54, 56], Mn [57], Mg [58], Bi [59] and Al [60 - 64]. Among
them, it is reported that Al-doped ZnO nanowires exhibit the highest conductivity while
maintaining their crystallinity. This chapter describes the synthesis of Al-doped ZnO nanowire
arrays with a low-temperature solution-based method. The influences of Al precursor and its
concentration on their structural, chemical, optical and electrical properties are discussed.
28
4.1. Morphology and alignment of nanowires
12 nm
10
-9
8
7
6
5
4
3
2
0
Figure 7. Tapping-mode AFM image of sol-gel derived ZnO seed layer. Average grain size is
about 46 nm and the root mean square (RMS) roughness is about 1.3 nm.
The ZnO seed layer can be deposited by radio-frequency magnetron sputtering or solgel methods. Although sputtered ZnO films are more consistent in electronic behavior, sol-gel
derived ZnO film produces better alignment of nanowire arrays because individual seed
crystals in polycrystalline sol-gel derived film more likely to align in <0001> direction [44].
Morphology of the sol-gel derived ZnO seed layer was characterized by atomic force
microscopy (AFM) in tapping-mode (Figure 7). From the AFM measurement, average gain
size of the seed layer was determined to be 46 nm and the root mean square (RMS) roughness
of the film is about 1.3 nm.
29
Figure 8. SEM micrographs of synthesized nanowire array with (a) no Al precursor, (b) 0.2
mol%, (c) 0.4 mol%, (d) 0.6 mol%, (e) 0.8 mol%, and (f) 1.0 mol% of Al precursor,
respectively.
Figure 8 shows SEM micrographs of hydrothermally grown ZnO and Al-doped ZnO
nanowire arrays with increasing Al precursor concentration from 0.2 mol% to 1.0 mol%. All
the samples were grown simultaneously with an identical growth solution, but varied
quantities of the Al precursor solution. As shown in Figure 8, there are a few obvious trends
with the Al precursor in terms of the morphology and alignment of the nanowires. Firstly, it is
observed that well-aligned ZnO nanowires have grown homogeneously on the seed layer and
they are mostly vertically aligned. Also, the hexagonal prism shape of individual nanowires
confirms that the ZnO nanowires maintain their microstructural characteristics even in the
presence of the doping element in the growth solution. However, as the Al concentration
increases, the alignment of nanowires starts to show some distortion with randomly-oriented
nanowires becoming more dominant. Similar results have been reported in literatures [35, 53],
which showed an increase in misalignment of nanowire arrays with Al precursor present. It
has been reported that the alignment of ZnO nanowires depends on the orientation of the seed
layer [32, 21]. In this study, an identical ZnO seed solution and deposition conditions were
used, so presumably all the seed layers used for the nanowire growths were identical. Thus, it
can be concluded that changes in the nanowire alignment result from the amount of Al
precursor input into the growth solution.
30
50
45-
T
354030-
25
E 20C:
E
15-
10
5-
n
I
i
0.0
0.2
0.6
0.4
1.0
0.8
Samples (mol% Al)
Figure 9. A plot of average nanowire diameter as a function of Al precursor concentration.
Samples
Control
0.2 mol% Al
Diameter 37.7 nm 6.4 36.1 nm 7.3
0.8 mol% Al
1.0 mol% Al
33.5 nm 8.9 26.2 nm 7.0 21.9 nm 7.4
18.5 nm 6.0
0.4 mol% Al
0.6 mol% Al
Table 1. Measured average nanowire diameters.
By measuring diameters of the nanowires from SEM micrographs of 5 different areas
within the same sample, it can be seen that the diameters of nanowires decrease as the Al
precursor concentration increases (Figure 9 and Table 1). Joo et. al. suggested that presence of
Al complexes in the growth solution causes this phenomenon [65]. When Al sulfate is used as
an Al precursor for the hydrothermal growth, it forms negatively charged Al complexes. Since
the (0002) facet of ZnO exhibits negative polarity in the basic growth solution environment, it
repels the negatively charged Al complex to the (1010) facet, which has positive polarity. The
Al complexes therefore suppress the lateral growth of nanowire, resulting in this decrease in
the diameter.
31
(b)
(a)
0002
0150
470110
(30002
0.52 nm
.2m
[0001]
Growth direction
[0001]
Growth direction
5 nmn
5 nmn
Figure 10. High resolution bright field TEM images of (a) ZnO nanowire without Al addition
and (b) 0.8mol% of Al precursor addition. Insets correspond to the FFT of each image.
Bright field TEM micrographs of individual ZnO nanowires grown with and without
Al precursor (Figure 10) show that the single crystalline ZnO structure is preserved regardless
of the presence of the Al precursor in the growth solution, maintaining lattice parameter of c =
0.52 nm and [0001] directional growth. Thus, the Al precursor in the growth solution may
affect the alignment of the nanowire array and lateral growth of the nanowire, yet, it maintains
the structural characteristics of ZnO without having significant lattice distortion.
32
4.2. Chemical analysis of Al-doped ZnO nanowires
0
Al
/Z9~
Control
tf~~i
0.0
0.5
1.0
1.5
-__ __8_-.
2.0
2.5
3.0
.8
m
3.5
l%1 All
4.0
Energy (keV)
Figure 11. SEM-EDS spectra of undoped and 0.8 mol% Al-doped ZnO nanowire arrays. Inset:
SEM image of a 0.8 mol% Al-doped ZnO nanowire array. Green box in the SEM represents
the area where EDS spectrum was acquired and scale bar is 300 nm.
The structural and morphological analysis of Al-doped ZnO nanowires discussed
above does not provide evidence of whether Al is successfully incorporated into the ZnO
nanowires. In order to confirm the Al-doping, energy-dispersive X-ray spectroscopy (EDS) in
SEM was performed with an accelerating voltage of 5 keV for the undoped ZnO nanowire
array and 15 keV for the Al-doped ZnO nanowire array. Since the amount of Al in the growth
solution is small, it is expected that there will be a higher signal from elemental Al with the
higher accelerating voltage. Figure 11 shows SEM-EDS spectra of both undoped and 0.8 mol%
Al-doped ZnO nanowire array. Although the intensity of the signal is small, there is an
obvious peak observed at 1.49 keV, which corresponds to the Ka X-ray of Al. Since high
accelerating voltage was used for the Al-doped ZnO nanowire sample, Si signal from the Si
detector also appeared in the spectra. The EDS spectra indicate that after nanowire growth, a
significant quantity of Al incorporates into ZnO nanowires, yet, it does not provide conclusive
33
evidence of Al doping in the wire. Although the EDS spectrum shows Al signal, there is a
possibility of Al segregation on the nanowire. In order to confirm whether Al segregation
takes place, STEM-EDS was performed on an individual nanowire from a sample identical to
the one used for the SEM-EDS. Figure 12 shows the STEM-EDS spectra of the 0.8 mol% of
Al-doped ZnO nanowire and corresponding elemental EDS map. As shown in Figure 11. a, an
Al signal centered at 1.49 keV was detected from both a line scan along the length of the
nanowire and a point scan on an arbitrary point at the nanowire. Because the total amount of
Al sulfate input into the growth solution is much smaller than the Zn source, the intensity of
Al signal is also small, and thus, the compositional analysis from the EDS measurement may
yield inconclusive quantitative analysis. However, elemental EDS mapping analysis (Figure
11. b) was able to confirm the presence of Al and no significant Al segregation was observed
within the nanowire.
34
(a)
-
n
0
C
072
zn
Line
scan
I
o
-
0
1
2
Point scan
r4 5
10
6
Energy (keV)
(b)
Figure 12. (a) STEM-EDS spectra of a 0.8mol% Al-doped ZnO nanowire and its
corresponding STEM image of the measured nanowire. Scale bar represents 100 nm. (b)
Elemental EDS mapping result of identical sample. Scale bars represent 100 nm.
35
4.3 Photoluminescence analysis of Al-doped ZnO nanowires
(a)
(b)
Control
0 2mol% Al
0 4mol% A
0.6mol% Al
0.8mol% Al
1.Omol% A
Control
I........k
--
500
800
Wavelength (nm)
400
700
800
1
,
i
500
600
Wavelength (nm)
400
A
.Omol% A
.II7II..
~.
.
300
.6mol%
O.8mol% Al
--
1a.'
300
0.2mol% Al
0.4mol% Al
-
--
80
700
3.30
(c)
3.29
0.2mol%
-
Al
0.4mol% Al
O.6mol% Al
O.-mol% Al
0
.Omol%
(d)
3.28
3.273.2?
A
3.20 -
--
a ......................... b.........
O
........
3.24-
Z'
3.23-
3.22
-a- As-grown
-a-Annealed
3.21
3.20
330
340
350
360
370
380
390
400
410
420
430
0.0
0.2
0.4
0.6
0.8
1.0
Sample (mot% Al)
Wavelength (nm)
Figure 13. Normalized photoluminescence (PL) spectra from undoped ZnO nanowire array
(control) and Al-doped ZnO nanowire arrays (a) before and (b) after annealing at 350 C for
30 min. (c) PL spectra from (b) expanded between the wavelengths of 330 to 430 nm. (d)
Summary of PL peak positions of near band-edge emission for (a) and (b).
From the electron microscopy analysis, we confirmed that Al-doping of ZnO
nanowires can be done using a low-temperature solution process. As discussed in chapter 2,
ZnO nanowires synthesized via the solution-based method may exhibit more defect-related
characteristics than those grown with the high-temperature vapor-based methods. In order to
analyze defect related properties, photoluminescence (PL) was conducted at room temperature
with a laser operating at 262 nm and a power density of 60 W/cm 2 for as-grown samples and
samples annealed at 350 C. Figure 13 shows normalized PL spectra for an undoped control
sample and 0.2 mol%, 0.4 mol%, 0.6 mol%, 0.8 mol% and 1.0 mol% Al-doped samples.
Strong near band edge emission at 381 nm characteristic of single-crystalline nanowires was
observed for all samples, with no peak shift as Al concentration increased. Meanwhile, it is
36
obvious from the spectra that broad defect-related emission gets stronger as Al concentration
increases. There are two distinguishable visible emission peaks, one centered near 500 nm
(green) and the other centered around 600 nm (yellow), observed when the Al precursor was
added during growth. A number of hypotheses have been proposed to explain the origin of
these visible emissions in ZnO [28, 29, 66]. Among these hypotheses, it is most widely
accepted that the green emission near 510 nm corresponds to the oxygen vacancies and
yellow emission near 600 nm corresponds to oxygen interstitials. Since both defect-related
emissions increase, it can be concluded that presence of Al in the growth solution contributed
to formation of oxygen-related defects in the nanowires. Specifically, the greater increase of
green emission intensity than yellow emission points to more oxygen vacancies in the higher
Al concentration sample. It is also well reported that heat treatment of the as-grown
nanowires decreases the defect-related emission [28, 29, 66]. Thus, post-annealing of Aldoped ZnO nanowire was conducted at 350 *C for 30 min in air atmosphere and the PL was
re-measured. Figures 13.b and 13.c show PL spectra of Al-doped ZnO nanowire after
annealing. Defect-related visible emission was significantly reduced for both green and
yellow emissions, indicating that post-annealing in air atmosphere reduces oxygen-oriented
point defects in the ZnO crystals. The full width half maximum (FWHM) value of the near
band-edge emission peak at 380 ram also narrowed from 20 nm to 16 nm with annealing,
indicating that the crystalline quality of both undoped- and doped ZnO nanowires is improved
after the post annealing process.
Peak shift
As-grown
Annealed
1.3 nm
380.3nm
381.6 nm
Control
(3.26 eV)
(0.01 eV)
(3.25 eV)
2.1 nm
379.5 nm
381.6 nm
0.2 mol% Al
(3.27 eV)
(0.02 eV)
(3.25 eV)
2.6 nm
379.0 nm
381.6 nm
0.4 mol% Al
(0.02 eV)
(3.27 eV)
(3.25 eV)
2.6 nm
379.0 nm
381.6 nm
0.6 mol% Al
(0.02 eV)
(3.27 eV)
(3.25 eV)
2.6 nm
379.0 nm
381.6 nm
0.8 mol% Al
(0.02 eV)
(3.27 eV)
(3.25 eV)
3.4 nm
378.2 nm
381.6 nm
1.0 MOM Al
(3.25 eV)
(3.28 eV)
(0.03 eV)
Table 2. Summary of PL peak positions of near band-edge emission for undoped ZnO
nanowire array (control) and Al-doped ZnO nanowire arrays before and after annealing at 350
*C in air for 30 min.
37
Moreover, figure 13.c shows that the peak positions of near band-edge emission exhibit a
slight blue-shift after the annealing. The magnitude of this shift increases as Al concentration
in the growth solution increases (Figure 13.d and Table 2). This result indicates that postannealing activates Al dopants inside of the ZnO nanowire and thus increases its band-gap.
This phenomenon could be explained by the Burstein-Moss effect, illustrated in Figure 14.
The apparent band gap of a semiconductor is increased as the absorption edge is pushed to
higher energies as a result of the states close to the conduction band being populated [67].
This shift occurs when the electron carrier concentration is so high that the semiconductor is
degenerate doped. Lu et. al. showed that this explanation is applicable for the Al-doped ZnO
system for low doping concentration regime [68]. In contrast to their result where a shift of
hundreds of meV for 5 at% Al doping was reported, we measure relatively small shift of tens
of meV. This difference may be explained by the small amount of Al that is incorporated in
ZnO for our case.
CBM
VBM4$
Eg
Eg+ABs
Figure 14. A schematic illustration of band gap widening due to the Burstein-Moss effect in
semiconductors [67].
38
4.4. Electrical property of Al-doped ZnO nanowire arrays
In the previous sections, the structural, chemical, and optical properties of Al-doped
ZnO nanowires were discussed. In order to be useful for optoelectronic device applications,
the electrical conductivity of ZnO should be enhanced by Al-doping. It is well established for
Al-doped ZnO thin films that Al-doping enhances the conductivity of the ZnO film
significantly [26, 66-68], and thus it could potentially be used as an alternative transparent
conducting oxide material. However, only a few studies have been reported on the doping of
ZnO nanowires due to the difficulty of measuring electrical properties of nano-scaled
materials. In this section, Al-doped ZnO-based electron-only devices were fabricated and their
corresponding electrical properties, such as conductivity or resistivity, were measured with
current-voltage (I-V) measurement.
PMMA
02
ZnO nanowires
Seed Layer
ITO,'
1. Cover ZnO nanowires
with PMMA
Plasma RIE
Al
Au
FFF
Seed
T
Seed Layer
ITO
2. Expose tips of nanowires
by 02 plasma etching
3. Deposit Al/Au metal
contact (Ohmic contact)
Layer
Figure 15. A schematic diagram of the fabrication process for ZnO nanowire-based electrononly device.
Figure 15 illustrates a fabrication process for a ZnO nanowire-based electron-only
device. 400 nm long ZnO nanowires were grown on ITO substrates using hydrothermal
synthesis method for both undoped and Al-doped samples. In order to prevent shorting of the
device, 600 nm of PMMA (MicroChem, 495PMMA A8) was deposited on the ZnO nanowire
array by spin-coating at 3500 rpm for 60 s followed by baking at 180 *C for 5 min. To expose
the tips of the nanowires, oxygen plasma reactive ion etching (02-plasma RIE) was performed
on the PMMA-coated nanowire substrate for 150 s. The RIE conditions were 20 sccm of 02
flow with 7 mTorr chamber pressure and 100 W of bias power, yielding an etching rate of
about 100-110 nm/min. After the RIE, 300 nm of Al and 50 nm of Au was deposited by
thermal evaporation with rates of 1 A/s in a base pressure of lx 10-6 Torr, followed by I-V
measurement with a Keithley 6487 source-meter.
39
(b
(d)
Figure 16. SEM micrographs of etched back PMMA coated (a) undoped, (b) 0.2mol% Aldoped, (c) 0.4mol% Al-doped, and (d) 0.6mol% Al-doped ZnO nanowire arrays after 150sec
of 02-plasma RIE. A Matlab area selection code was used to calculate the exposed area after
the 0 2 -plasma RIE and the fraction of area is tabulated in each figure.
Figure 16 shows SEM micrographs of the exposed tips of nanowire arrays after 02plasma RIE. To quantify the exposed area of the ZnO nanowire array, Matlab code for area
selection was used and the fraction of exposed area is also tabulated in Figure 15. As shown in
Figure 16, only vertically aligned nanowires were exposed after the 02-plasma RIE and the
rest of the area was covered by PMMA, indicating effective filling of the gaps between
nanowires to prevent shorting of the device.
40
6
54-
3-
0
-2-
Control
0.2 mol% Al
0.4 mol% Al
0.6 mol% Al
-4
-4
-3
-2
-1
0
1
2
3
Voltage (V)
Figure 17. I-V curves for ZnO nanowire-based electron-only devices.
Figure 17 shows I-V measurements of electron-only ZnO nanowire devices. The
ITO/ZnO/Al structures shows linear I-V characteristic behavior, indicating ohmic contact was
made between the ITO/ZnO interface as well as the ZnO/Al interface. Accounting for the
measured I-V data and the actual area of exposed nanowires, corresponding resistivities and
conductivities of each sample were calculated and summarized in Figure Table 3 and Figure
18. There is a clear trend of the Al-doping effect in the electrical properties; as Al
concentration increases, conductivity increases. However, the actual values obtained from the
electron-only devices show lower conductivity values than previously reported
[26].
Generally, the reported values for ZnO nanowires are 10-2 ~ 10' i2cm for resistivity and 10-1 ~
101 S/cm for conductivity, which differ by more than two orders of magnitude from the values
obtained in this study. This may be caused by the design of the electron-only device, since the
electron-only device used in this analysis is based-on the two-point probe method which
cannot avoid parasitic contact resistance at each interface. Still, the trends of enhancement in
41
the conductivity with increasing Al content leads to the conclusion that Al-doping of ZnO
nanowires in solution is successful.
-
1.2x10
-
9.Ox10
-
-3
,X,
1.xl10
7-
7
/
A
/
-
7.Ox10
-
6.Ox10
0
-
3.0x10
3.Oxl&-
-
-
L.) 4.0x10'
2.0x10-
1.OxlO0
0.0
0.2
0.4
0.6
Sample (mol% Al)
Figure 18. A line graph illustrating conductivity changes as a function of Al precursor
concentration
Undoped
Resistivity
0.2 mol% Al
0.4 mol% Al
0.6 mol% Al
(2.57+0.17)x 10 (1.69 0.16)x 10 3 (1.10.26)x 103 (9.98 0.04)x102
3
Conductivity (3.89:0.25)x10' (5.92 0.3 1)x 10' (9.1110.6 1)x 104 (1.0 0.02)x 10 3
(S/cm)
_
__
_
_
I__
_
__
_
_
I__
_
_
_
_
_
_
_
_
_
_
_
I__
Table 3. Summary of calculated resistivities and conductivities of each electron-only device
from figures 16 and 17.
42
Chapter 5. Application of Al-doped ZnO nanowires to photovoltaic device
Recently, semiconductor nanowire-based bulk heterojunction photovoltaic devices
have attracted much interest due to their unique anisotropic structure of nanowires, which
enable efficient charge transport pathway in the device architecture. In addition, onedimensional structures yield a large surface-to-volume ratio, which form large interfacial area,
suitable for well-ordered bulk heterojunctions. Thus, applying nanowire structure for
photovoltaic devices is expected to enhance device performance. When the solar cell
architecture incorporates periodically arranged semiconductor nanowire arrays, there is
improved light absorption and trapping in the active region [23, 24] as well as improved
defect tolerance due to the charge separation along the nanowire radius [69]. Thus, the
application of nanowires in hybrid photovoltaic devices is expected to enhance device
performance. As discussed in chapter 4, Al-doping of ZnO nanowires may enhance the
properties of nanowire arrays while preserving its structural and optical properties. In this
chapter, nanowire-based hybrid photovoltaic devices are fabricated with Al-doped ZnO
nanowire arrays and the effect of the Al-doping is analyzed by comparing with the undoped
ZnO nanowire-based device.
43
5.1. Designed device structure
(a)
(b)
ITO
ZnO
Pk
MoO,
Au
3.8eV
4.6eV
51eV
4,3eeV
5.1eV
Figure 19. (a) Schematic of the device architecture and (b) flat-band energy diagram of ZnO
nanowire/PbS quantum dot solar cell.
Figure 19.a shows a designed device structure of a ZnO nanowire/PbS quantum dot
hybrid solar cell: ITO / ZnO seed layer / ZnO nanowires / PbS QDs / MoO 3 / Au. Also, its
corresponding flat band diagram is shown in Figure 19.b. Since the doping concentration of
Al used in this work was very low, it is assumed that energy levels of both ZnO nanowires
and Al-doped ZnO nanowires are identical. For efficient absorption of light, transparent
conducting oxide (TCO) materials are typically used for the solar cell device. In this study,
ITO is used as the cathode and light is illuminated through the ITO side. A thin ZnO layer is
deposited on the ITO layer to act as both electron-transporting layer and seed layer for the
nanowire growth. Due to the wide band-gap energy of 3.4 eV and high transmittance in the
visible region, absorption in the seed layer can be considered negligible and most of the
incident light is transmitted to the PbS quantum dot active layer. Starting from identical seed
layers, both undoped and Al-doped nanowires were grown via the hydrothermal synthesis
method discussed previously. The growth time of the nanowire arrays was fixed to 40 min
resulting in 200-250 nm long nanowires. In this device design, the thickness of active
quantum dot layer is greater than the maximum reported depletion widths in PbS quantum dot
films (-150 nm). Since the ZnO nanowire array forms an ordered bulk heterojunction, the
effective depletion region in the quantum dot film may extend throughout entire film. The role
of the MoO 3 layer deposited on the quantum dot film is to eliminate the reverse Schottky
barrier at the Au anode [8]. As shown in Figure 19.b, there is an energy difference in the
valence bands of the PbS quantum dots and MoO 3, which will be act as a barrier for the
44
carrier extraction, yet, charge carriers can be extracted to the anode mostly by the tunneling
since thickness of MoO 3 is only about 25 nm.
45
5.2. Device performance of Al-doped ZnO nanowire-based quantum dot solar cell device
5-
0
C,,
)
-15_
-.----DarkControl
Light Control
2..... Dark_0.4mol% Al
-- Light_.0.4mol% Al
-... Dark_0.8mol% Al
-25
-30 -1
-0.1
0.0
0.1
0.3
0.2
11..
0.4
I I
0.5
I
-
Light_0.8mol% Al
0.6
Voltage (V)
Figure 20. J-V characteristics measured in the dark (dotted lines) and under AM 1.5G
illumination (solid lines) for undoped, 0.4 mol% Al-doped, and 0.8 mol% Al-doped ZnO
nanowire-based quantum dot solar cell devices.
In order to investigate the effect of Al-doping of ZnO nanowires on the device
performance, both undoped and Al-doped ZnO nanowire-based quantum dot solar cells were
fabricated and their corresponding photocurrent density-voltage (J-V) characteristics were
measured under AMI.5G illumination. Figure 18 shows the J-V curves obtained from the
measurement. Each curve represents the average performance of 2-4 devices on a single
substrate. As shown in Figure 18, short circuit current density (Jsc) increases as Al doping
concentration increases while open circuit voltage (Voc) does not change greatly. Table 4 and
Figure 21 summarize the average device performances of each nanowire-based quantum dot
solar cell. According to the properties analyzed in the previous chapter, causes for this Jsc
increase would be (1) enhancement of the electrical conductivity of the nanowires and/or (2)
better infiltration of PbS quantum dots into the gaps between nanowires. Since the Jsc
enhancement is a vital factor for enhancement in power conversion efficiency (PCE),
46
modification of nanowire electrical properties by Al-doping is the main cause for the
improvement. If the resistivity of the nanowire array is reduced with Al-doping, there is an
expected decrease in the series resistance of the solar cell. The series resistance of the cell (Rs)
can be estimated from the J-V curves, specifically it is the inverse of the slope at open circuit.
Rs was calculated from the J-V curves in Figure 20 and is listed in Table 4. There is clear
trend of decreasing Rs as Al-doping concentration increases, indicating that conductivity
enhancement of Al-doped ZnO nanowire enhances the device performance.
(b)
(a)
20
30
6040
2
-2
141
12
-.3
040
,
0
2
.
2
.
-32-
8
12
r
oo~~2
01
4
t
1
t
10
m2
-
20
1.-
4
-38
-26
-24
16-
2
10-g0d20
T30
030
5
0.45
o
a28
s
04
2
24
0.05
0
04
0
22
0001
20
00
.6
Sample (mol% A.)
0
.
3
0
2
Sample (mol% Al)
(C)
5.0-
45
4
1.0
~
(21.2)~05-
(.90.1)32(67
0 0010
Sample
(mol% Al)
Figure 2 1. Average device parameters of nanowire-based quantum dot solar cells showing (a)
JsC and Rs, (b) Voc and fill factor, and (c) PCE as a function of Al precursor solution. Data is
averaged over 20 different devices.
FF
0.31 0.01
(0.31)
0.31 0.01
(0.31)
)
Rs (0=m 2
PCE ()
17.1 0.3
2.9 0.2
Udpd
ndpd
(3.2)
(16.7)
13.3 0.3
3.5 0.2
0.45 0.02
l 24.6 1.5
0.ml
0mo Al
(24.5)
(0.48)
(3.7)
(12.9)
12.9 0.5
4.1 0.3
0.32 0.01
0.51 0.02
l 26.2 1.4
0.ml
0mo AI
(26.9)
(0.48)
(0.32)
(4.2)
(12.1)
Table 4. Summary of averaged device performances figures of merit of nanowire-based
Jsc (mA/cm2 ) Voc (V)
0.46 0.02
20.0 1.1
(21.2)
(0.49)
quantum dot solar cells. Data is averaged over 20 different devices and the champion device
performance is shown in the parenthesis.
47
Figure 22 shows representative external quantum efficiency (EQE) spectra for both
undoped and Al-doped ZnO nanowire/PbS quantum dot solar cells. It has been reported that
the MoO 3 hole transporting layer is not stable in air [8, 10, 23] resulting in degradation of Jsc.
Since the EQE apparatus used in this research was set up in an air environment, in contrast
with the J-V measurement set up stored inside a N2 -filled glove box, a different device
architecture, which uses tetrabutylammonium ioidide (TBAI) as the ligand for the PbS
quantum dot instead of BDT, was fabricated for the EQE measurement. When TBAI is used
as a ligand material for the PbS quantum dot device, the MoO 3 layer is no longer needed [7,
15] and thus the device shows better air-stability than the device using BDT. The TBAI-based
devices were fabricated with identical protocol as the BDT-based device except for the ligand
materials (TBAI and BDT) and their corresponding solvent (methanol for TBAI and
acetonitrile for BDT). Improvement of EQE from 400 nm to 1100 nm for Al-doped samples
indicates improvement in collection efficiency for generated photocarriers. Integrating the
EQE spectrum against the AM1.5G solar spectrum gives a predicted photocurrent of 12.75
mA/cm 2 for the Al-doped ZnO nanowire-based device, within 8% of the measured
.
photocurrent of 13.52 mA/cm 2
90
___Control
80
.8mol% Al
-
7060
50
C
407 5 mA
Jsc. v =13.52mA
2
JSC,EE =1 .
30
20-
10
0300
JSC.EQE = 8.97mA
Jsc. jv = 9.33mA
...............................
400
500
600
700
800
900
1000
1100
Wavelength (nm)
Figure 22. EQE spectra of representative undoped (black) and Al-doped (red) ZnO nanowirebased quantum dot solar cell devices. Integrating the EQE with the AM 1.5G solar spectrum
yields predicted photocurrent densities (JSC,EQE) within 10% of the measured values (Jscjv).
48
5.3. Morphologies of Al-doped ZnO nanowire/PbS quantum dot solar cell device
(a)
(b)
Pt
Au (100 nm)XRe
o~~~~
Pb
e(5n
D36
(25 nm)
mPbS QD
340 nm
210 nm
N
ZnOnm)0
(20
NW (200 nm)
nmZnO
Figure 23. Cross-sectional SEM images of (a) undoped and (b) 0.8mol% Al doped ZnO
nanowire/PbS quantum dot solar cells. The cross-sectional surface was prepared with FIB
milling. Nanowire height and quantum dot film thickness is denoted in the images.
As discussed in the previous section, solar cell devices incorporating Al-doped ZnO
nanowires showed better device performance than devices utilizing undoped ZnO nanowires.
In chapter 4, the alignment difference between undoped and Al-doped ZnO nanowire array
was discussed. This alignment difference may affect the device performance as well, since
more vertical alignment will allow better infiltration of the quantum dots. In order to
investigate the effect of the alignment on device performance, inspection of device crosssections was conducted. For investigation of structural effects, focused ion beam (FIB)
milling and subsequent cross-sectional SEM analysis was performed as a primary diagnostic
tool. Figure 23 shows representative undoped and Al-doped ZnO nanowire/PbS quantum dot
solar cell device cross-sections. The SEM analysis reveals individual layer thicknesses and
infiltration of the quantum dots into the nanowire array. For both undoped and Al-doped
nanowires, lengths of nanowires are similar (-220 nm). However, there are more vertically
aligned nanowires in the undoped ZnO nanowire with wire-to-wire spacing of less than 40 nm.
This narrow gap hinders the ability for the quantum dots to fully infiltrate into the nanowire
arrays. As a result, only the top portions of the nanowires are in contact with the quantum dot
active layer so that the structural contribution of nanowires to the charge extraction is limited.
However, for the Al-doped ZnO nanowires, wire-to-wire distance is greater than 60 nm. This
49
relatively wide pitch between nanowires allows for the full infiltration of the quantum dot
layer into the nanowire layer. It can be assumed that the total amount of PbS quantum dot
used in the fabrication process is identical between devices since the total number of spincoating iterations for the quantum dot layer was kept constant. Since the Al-doped ZnO
nanowire arrays show better infiltration by the quantum dots, the total thickness of the device
is reduced from 365 nm to 340 nm. Even though the total thickness of the active layer is
smaller than that of the undoped ZnO nanowire-based device, the Al-doped ZnO nanowirebased device shows improved device performance, which leads to the conclusion that Aldoping of ZnO nanowire enhances device performance.
50
5.4. Failure analysis of nanowire-based quantum dot solar cells
I
Figure 24. Cross-sectional SEM images of non-working devices; (a) too thick and (b) too thin
PbS quantum dot layers result in low efficiency and a shorted device, respectively. (c) Porous
seed layer degrades performance, and (d) large voids between the nanowires causes low Voc.
While performing cross-sectional analysis of the devices, a few criteria for a working
device were uncovered. Figure 24 shows cross-sectional SEM images of non-working devices.
As shown in Figure 24.a and 24.b, both thick and thin layers of PbS quantum dots cause nonfunctional devices. If the quantum dot layer is too thick, minority electronic cannot reach the
PbS/ZnO interface and thus incur recombination losses which results in low Jsc. In contrast, if
the quantum dot layer is too thin, nanowires may create shunting pathways and thus lower the
FF and increase the reverse leakage current. Also as shown in Figure 24.b, thin quantum dot
layers cannot fully cover the nanowires, and thus shorting of device occurs. Therefore, the
thickness of the quantum dot film needs to be optimized and in this study, it is set to about
100 nm above the tips of the nanowires. Another criterion for the device is the porosity of the
51
interfaces. Figure 24. c and 24.d show two representative cases of porosity within the device
structure. If the seed layer is porous, as shown in Figure 24.c, the device shows no
photovoltaic response under illumination. Meanwhile, if the gaps between the nanowires are
not completely filled with quantum dots, leaving voids several tens of nanometer In size, the
device exhibits low Jsc. The existence of voids within the device architecture hinders efficient
charge collection through the electrode, resulting in a non-working device.
52
6. Conclusion and future work
In this thesis, Al doping of ZnO nanowires via the hydrothermal synthesis method
was demonstrated and their properties were analyzed with various characterization methods.
Morphology and alignment of Al-doped nanowires were analyzed with electron microscopy
and chemical analysis was performed by EDS. Relative changes in electrical conductivity of
Al-doped ZnO nanowires were analyzed. Furthermore, the application of doped nanowires to
photovoltaic devices was demonstrated to prove the potential of application to the actual
device. Proposed general criteria for the independent optimization of nanostructures for
photovoltaic applications provide a guide-line to the actual application.
In chapter 4, the detailed synthesis method of the solution-based growth of ZnO
nanowires with Al dopant was demonstrated. The Al doping concentration was controlled by
adding Al sulfate solution to the growth solution in different amounts, and the structural,
chemical and electrical properties of resultant nanowires were investigated. SEM micrographs
confirmed that the diameter of the nanowires decreased with increasing Al dopant
concentration while the alignment of the nanowire array tended to diminish as more Al was
added. EDS analysis with both SEM and TEM showed chemical evidence for existence of Al
present in the nanowires and PL spectroscopy confirmed that post-annealing of as-grown
nanowire reduced defect related visible emission. Also, measuring the electrical properties of
the nanowire array showed an evident trends of improving conductivity with increasing Al
precursor in the growth solution, yet, the values obtained from this measurements could be
improved by performing measurements on the individual nanowire level with four-point
contacts.
In chapter 5, application of the Al-doped ZnO nanowires in photovoltaic devices was
demonstrated by fabricating nanowire-based quantum dot solar cells. A suitable device
architecture for one-dimensional nanostructures was designed with appropriate band
alignment of individual layers. The resultant devices showed clear trends in enhancement in
PCE of the device primarily through improvement in Jsc. Devices made with Al-doped ZnO
nanowires showed photocurrent of over 20 mA/cm 2 and efficiencies of up to 4.1 %, with a
champion device of 4.2 %. Cross-sectional SEM analysis revealed criteria for the device
structures, which requires complete infiltration of quantum dot active materials into vertically
53
aligned nanowire arrays in order to achieve optimal performance.
Although several measurements have demonstrated that Al-doping in ZnO nanowires
can successfully be achieved by the hydrothermal synthesis technique, more direct evidence
will be needed to establish the benefit quantitatively, especially with regard to the electrical
properties. Single nanowire characterization needs to be conducted in order to obtain
quantitative analysis of the doping effect. Resistivity, conductivity, charge-carrier density and
charge carrier mobility need to be addressed as a direct evidence of the effect of Al-doping. In
terms of device application, further performance gains are expected through computational
modeling of the device architecture with one dimensional nanostructures. In addition,
controlling of the length of nanowires and optimizing the device fabrication will lead to the
ultimate goal of a micron-thick, fully-depleted quantum dot active layer, thus achieving the
practical limit of the quantum dot photovoltaic system.
In summary, the controlled growth of Al-doped ZnO nanowires was demonstrated by
a solution-based synthesis method. Direct correlation of structural and chemical properties in
the nanowires reveals that doping of ZnO can be achieved by a low-temperature solutionbased method. Motivated by this information, application of nanowires to photovoltaic
devices was investigated and the role of these doped nanowires in the observed performance
gains of the devices was identified. The hope from this study is that these finding will yield
insights to guide future nanowire growth and application in devices.
54
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