1
Solar cells
And
Nanostrucures
By: M. Asemi
2
Outline




1.
Solar sells
Dye-Sensitized Solar Cells
Nanostructures in DSSCs
The methods of the nanostructure production
Nanostructures Offering Large Specific Surface Area
1.
2.
3.
ZnO Nanoparticulate Films
Nanoporous Structured ZnO Films
Other Nanostructured ZnO Films
Nanostructures with Direct Path ways for Electron Transport
2.
1.
2.
3.
4.
ZnO Nanowires
ZnO Nanotubes
ZnO Nanoflowers
Dendritic ZnO nanowires
Core-Shell Structures with ZnO Shell
3.
1.
2.
The Role of the ZnO Shell
Influence of Shell Thickness
Light Scattering Enhancement Effect
4.
1.
2.
3.
ZnO Aggregates
ZnO hollow spheres
Metal nanoparticles
3
Solar cells

The conversion from solar
energy to electricity is
fulfilled by solar cell devices
based on the photovoltaic
effect.

Many photovoltaic devices
have already been developed
over the past five decades.

However, wide-spread use is
still limited by two significant
challenges:
1.
2.
conversion efficiency
cost
4
Solar cells

One of the traditional
photovoltaic devices: singlecrystalline silicon solar cell.

which was invented more
than 50 years ago and
currently makes up 94% of
the market.

Single-crystalline silicon
solar cells operate on the
principle of p–n junctions
5
Solar cells



theoretical efficiency of the single-crystalline silicon: 92%
commercial designs efficiency: 20%
Because of the considerably high material costs, thin-film
solar cells have been developed.
Q. Zhang, C.S. Dandeneau, ZnO Nanostructures for Dye-Sensitized Solar Cells, Adv. Mater.
2009, 21, 4087–4108
6
Solar cells
Amorphous silicon (a-Si) is a candidate for thin-film solar
cells because:
1. Its defect energy level can be controlled by hydrogenation
2. The band gap can be reduced
 So that the light-absorption efficiency is much higher than
crystalline silicon.
 The problem is that amorphous silicon tends to be unstable.
and can lose up to 50% of its efficiency within the first
hundred hours.
 Efficiency of the a-Si thin films: 15%
 Efficiency of the polycrystalline-silicon film: 18%

Q. Zhang, C.S. Dandeneau, ZnO Nanostructures for Dye-Sensitized Solar Cells, Adv. Mater.
2009, 21, 4087–4108
7
Type of solar cells

1.
2.

1.
2.
3.

Crystalline silicon:
Monocrystalline silicon
Polycrystalline silicon
Thin films
Amorphe silicon
Cadmium telluride solar cell
Copper indium gallium selenide
Dye-sensitized solar cells (DSSCs)
8
Dye-sensitized solar cells (DSSCs)

To aim at further lowering the production costs,
DSSCs based on oxide semiconductors and organic
dyes have recently emerged as promising approach to
efficient solar energy conversion.

Efficiency of the DSSCs: 13%
J. Qu and C.Lai, One-Dimensional TiO2 Nanostructures as Photoanodes for, Journal of
Nanomaterials (2013)
9
Dye-Sensitized Solar Cells

The DSSCs are a photoelectrochemical system.
1. a porous-structured oxide film
with adsorbed dye molecules as
the photosensitized anode.
2. a platinized FTO glass acts as the
counter electrode (cathode).
3. a liquid electrolyte that
traditionally contains I-/I-3 redox
couples serves as a conductor to
electrically connect the two
electrodes.
N.Park, Dye-Sensitized Metal Oxide Nanostructures and Their Photoelectrochemical Properties,
Journal of the Korean Electrochemical Society Vol. 13, (2010)
10
Operation mechanism of the DSSCs:
photons captured by the dye
 monolayer create excitons
that are rapidly split at the
nanocrystallite surface of the
oxide film.
 Electrons are injected into
the oxide film and holes are
released by the redox
couples in the liquid
electrolyte.

N.Park, Dye-Sensitized Metal Oxide Nanostructures and Their Photoelectrochemical Properties,
Journal of the Korean Electrochemical Society Vol. 13, (2010)
11
Operation mechanism of the DSSCs:
12
Advantages of DSSCs

1.
2.
Compared with the conventional solar cells, DSSCs
are possessing:
practicable high efficiency
cost effectiveness
The conversion efficiency
has been limited due to:
Recombination during the
charge-transport process.
13
Nanostructures in DSSCs

One of the defining features of nanostructures is
their basic units on the nanometer scale. This,
first of all, provides the nanostructures with a
large specific surface area.

It may also result in many particular behaviors
such as:
1.
2.
electron transport
light propagation in view of the surface effect
14
Nanostructures in DSSCs

Those nanostructural forms of ZnO or TiO2
developed during the past several decades
mainly include:
nanoparticles
2. nanowires (nanorods)
3. nanotubes
4. nanobelts
5. nanosheets
6. nanotips
1.
15
Nanostructures in DSSCs

1.
2.
3.
It will show that photoelectrode films with
nanostructured ZnO can significantly enhance
solar cell performance by offering a:
large surface area for dye adsorption
direct transport pathways for photoexcited electrons
efficient scattering centers for enhanced light-harvesting
efficiency
16
The methods of the nanostructure
production
1.
2.
3.
4.
5.
Sol–gel synthesis
Hydrothermal growth
Chemical bath deposition
Electrostatic Spray Deposition
Electrochemical deposition
17
1. Nanostructures Offering Large
Specific Surface Area

Nanomaterials can satisfy this requirement due
to the formation of a porous interconnected
network in which the specific surface area may
be increased by more than 1000 times when
compared with bulk materials.
18
1.1 ZnO Nanoparticulate Films

ZnO films with nanoparticles for application in
DSSCs have been extensively studied, partially
due to:
1. The direct availability of
porous structures with
assembled nanoparticles
1. The simplicity of synthesis
of nanoparticles.
19
1.2 Nanoporous Structured ZnO Films

Besides nanoparticulate films, nanoporous
structured ZnO films were also studied as
photoelectrodes in DSSCs due to their high
porosity.
20
1.2 Nanoporous Structured ZnO Films

1.
2.

1.
2.
Nanoporous films with nanowalls vertically
grown on the substrate by:
electrochemical deposition
chemical bath deposition
Nanoporous films favorable for:
electron transport from the point of generation to
the collection electrode
electrolyte diffusion through the intervals between
the nanowalls, resulting in a decrease in both the
series resistance and recombination rate of the cell.
21
1.3 Other Nanostructured ZnO Films

ZnO nanostructures with other morphologies, such
as:
Nanosheets
 Nanobelts
 Nanotetrapods

have also been studied
for DSSCs that they
also have a large
specific surface area.
22
1.3 Other Nanostructured ZnO Films

For these nanostructures, the specific surface area
is not the only factor that determines the
photovoltaic efficiency of the DSSCs.

Solar cell performance is also believed to be
affected by the geometrical structure of the
photoelectrode films, which provides particular
properties in terms of the:
1.
2.
Electron transport
Light propagation
23
2. Nanostructures with Direct Path
ways for Electron Transport

1.
2.
Electron transport in DSSCs based on
nanoparticulate films has been proposed to occur by:
Trapping/detrapping
Diffusive transport
While an 11% efficiency
has been achieved on
nanocrystalline films
because of deficiencies.
24
2. Nanostructures with Direct Path
ways for Electron Transport

one-dimensional nanostructures serve to
increase the electron diffusion length by
providing a direct pathway for electron transport.

the transport of electrons occurs in the interior of
a continuous crystal and the electrons would not
suffer any grain boundary scattering.

Typical one-dimensional ZnO nanostructures
used for DSCs involve types of nanowires,
nanotubes, nanotips.
25
2.1 ZnO Nanowires

ZnO nanowire arrays were first used in DSSCs by
Law et al. in 2005 with the intention of replacing
the traditional nanoparticle film with a
consideration of increasing the electron diffusion
length.
26
2.1 ZnO Nanowires

This is also a confirmation that the nanowires offer
better electron transport when compared to
nanoparticles.

The mechanism has been attributed to:
1.
2.
High crystallinity of nanowires
An internal electric field
27
Charge transport in nanowire
The internal electric field is thought to be able
to:
(1) corral the injected electrons so as to reduce the
recombination rate.
(2) accelerate the diffusion of injected electrons towards the
interior of the nanowires.
28
2.2 ZnO Nanotubes

Nanotubes differ from nanowires in that they
typically have a hollow cavity structure. An array
of nanotubes possesses high porosity and may
offer a larger surface area than that of nanowires.

It yields a relatively low conversion efficiency of
1.6%, primarily due to the modest roughness factor
of commercial membranes.
A. B. F. Mart inson, J. W. Elam, J. T. Hupp, M. J. Pellin, Nano Lett.2007, 7 , 2183.
29
2.3 ZnO Nanoflowers

This is based on the consideration that the
nanowires alone may not capture the photons
completely due to the existence of intervals inherent
in the morphology.
 Nanoflower structures, however, have nanoscale
branches that stretch to fill these intervals and,
therefore, provide:
A larger surface area
 A direct pathway for electron transport along the channels
from the branched ‘‘petals’’ to the nanowire backbone.

30
2.3 ZnO ‘‘Nanoflowers’’

The solar-cell performance of ZnO nanoflower
films was characterized by an overall conversion
efficiency of 1.9%,This value is higher than the
1.0% for films of nanorod arrays.
J. B. Baxter, E. S. A ydil, Appl. Phy. Lett.2005, 86, 053114.
31
2.4 Dendritic ZnO nanowires

Dendritic ZnO nanowires,
which possess a fractal
structure more complicated
than that of nanoflowers,
are formed by a nanowire
backbone with outstretched
branches, on which the
growth of smaller-sized
nanowire backbones and
branches is reduplicated.
32
2.4 Dendritic ZnO nanowires

DSSCs characterization showed that the short-circuit
density increased with increasing growth generation
due to the larger surface area, which in turn led to
increased adsorption of dye molecules.

The dendritic ZnO nanowires are intentionally developed
to increase the specific surface area of the
photoelectrode film by multiple-generation growth of
ZnO nanowires.
J. B. Baxter, E . S. A ydil, Sol . E nerg. Mat. Sol. C 2006, 90, 607.
33
2.4 Dendritic ZnO nanowires

photovoltaic efficiency of dendritic ZnO
nanowires is lower than ZnO nanowire.

This is possibly due to:
The lower nanowire density
2. The insufficient nanowire length of dendritic
ZnO nanowires.
3. The difference in the crystallinity of nanowires
produced by different fabrication methods.
1.
34
3. Core-Shell Structures with ZnO Shell

Core-shell structures are a configuration designed
for electrode films in DSSCs to reduce the
recombination rate at the electrode/electrolyte
interface.
A core-shell nanostructured
electrode usually consists of a
nanoporous TiO2 matrix that
is covered with a shell of
another metal oxide or salt.
35
3. Core-Shell Structures with ZnO Shell

The conduction-band potential of the shell
should be more negative than that of the core
semiconductor (TiO2).
This establishes an energy
barrier which hinders the
reaction of electrons in the
core with:
1.
2.
The oxidized dye
Redox mediator in the
electrolyte.
36
3. Core-Shell Structures with ZnO Shell

Several shell materials such as:

ZnO

Al2O3

SiO2

Nb2O5

WO3

MgO

SrTiO3

CaCO3
have been reported to form an energy barrier on TiO2,
enhancing the solar cell performance by providing a 35%
increase in the overall conversion efficiency.
S. J. Roh, R. S. Mane, S. K . M in, W . J. Lee, C. D. Lokhande, S. H. Han, Appl. Phy. Lett.
2006, 89, 253512.
37
3.1 The Role of the ZnO Shell

the role of the ZnO shell has most often been
demonstrated to provide an energy barrier at the
interface between the TiO2 and the dye or electrolyte,
so as to reduce the recombination of electrons with
oxidized dye molecules or those accepting species in
the electrolyte.
38

Those photogenerated electrons in the dye
molecules with high kinetic energy can readily
tunnel through the ZnO shell and inject into the
TiO2. However, the transport of electrons in the
reverse direction may be blocked due to the
presence of an energy barrier provided by the ZnO
shell, thereby reducing the recombination rate of
photogenerated electrons.
39
4. Light Scattering Enhancement Effect

1.
2.
The film thickness was:
expected to be larger than the light-absorption
length so as to capture more photons.
constrained to be smaller than the electrondiffusion length so as to avoid or reduce
recombination.
40
4.1 ZnO aggregates
ZnO aggregates offer a large surface area and
light scattering ability simultaneously.
 DSSCs efficiency with ZnO nanocrystallites: 2.4%
 DSSCs efficiency with ZnO aggregates: 5.4%

T. P. Chou, Q. F. Z hang, G.E. Fryxell and G.Z. Cao, Adv. Mater. 2007, 19, 2588.
41
4. Light Scattering Enhancement Effect
4.1 ZnO aggregates

aggregates are spherical in shape with
diameters ranging from several tens to
several hundred nanometres, consisting of
15 nm ZnO nanocrystallites.
T. P. Chou, Q. F. Z hang, G.E. Fryxell and G.Z. Cao, Adv. Mater. 2007, 19, 2588.
42
4. Light Scattering Enhancement Effect
4.2 ZnO hollow spheres
ZnO hollow spheres are a structure similar to the ZnO
aggregates mentioned above, however, the assemblies of
nanocrystallites form a shell structure with a hollow interior.
 conversion efficiency is
 4.33% for ZnO hollow spheres
 3.12% for ZnO nanoparticles

43
4. Light Scattering Enhancement Effect
4.3 Metal nanoparticles

Many solar cells are currently being doped with metallic
nanoparticles that scatter light.

This increases the path length of the photon, and hence,
increases the chance for an electron excitation.

Metal nanoparticles are strong scatterers of light at
wavelengths near the plasmon resonance, which is due
to a collective oscillation of the conduction electrons in
the metal.
44
4. Light Scattering Enhancement Effect
4.3 Metal nanoparticles

The scattering and absorption
cross-sections are given by:
1.
α polarizability of the particle
V is the particle volume,
εp is the dielectric function of the
2.
3.
particle
4.
εm is the dielectric function of the
embedding medium
S. Pillai, K. R. Catchpole, T. Trupke, and M. A. Green, Surface plasmon enhanced silicon solar cells, Journal
of applied physics 101, 093105 (2007)
45
4. Light Scattering Enhancement Effect
4.3 Metal nanoparticles



We can see that when εp= -2εm the
particle polarizability will become
very large.
This is known as the surface
plasmon resonance.
At the surface plasmon resonance
the scattering cross-section can
well exceed the geometrical cross
section of the particle.
S. Pillai, K. R. Catchpole, T. Trupke, and M. A. Green, Surface plasmon enhanced silicon solar cells, Journal
of applied physics 101, 093105 (2007)
46
4. Light Scattering Enhancement Effect
4.3 Metal nanoparticles

The thinner TiO2 films (4μm) than the typical thicknesses of
DSSCs (10μm) were employed to clarify the effects caused by
metal nanoparticles. These factors led to the lower powerconversion efficiency of 2.7% for the bare DSSC, compared with
typical DSSCs exhibiting over 5%
The effects of 100 nm-diameter Au nanoparticles on DSSCs, Applied physics letters 99, 253107 (2011)
47
summary
Dye-Sensitized Solar Cells
 The methods of the nanostructure production
 Nanostructures in DSSCs

1.
2.
3.
4.
Nanostructures Offering Large Specific Surface Area
Nanostructures with Direct Path ways for Electron
Transport
Core-Shell Structures with ZnO Shell
Light Scattering Enhancement Effect
48
49