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Solar cellsAnd

Nanostrucures

By: M. Asemi

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Outline Solar sells Dye-Sensitized Solar Cells Nanostructures in DSSCs The methods of the nanostructure production1. Nanostructures Offering Large Specific Surface Area

1. ZnO Nanoparticulate Films2. Nanoporous Structured ZnO Films3. Other Nanostructured ZnO Films

2. Nanostructures with Direct Path ways for Electron Transport1. ZnO Nanowires2. ZnO Nanotubes3. ZnO Nanoflowers4. Dendritic ZnO nanowires

3. Core-Shell Structures with ZnO Shell1. The Role of the ZnO Shell2. Influence of Shell Thickness

4. Light Scattering Enhancement Effect1. ZnO Aggregates2. ZnO hollow spheres3. Metal nanoparticles

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Solar cellsThe 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. conversion efficiency2. cost

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Solar cellsOne of the traditional

photovoltaic devices: single-crystalline 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

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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

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Solar cellsAmorphous silicon (a-Si) is a candidate for thin-film solar

cells because:1. Its defect energy level can be controlled by hydrogenation2. The band gap can be reducedSo 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

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Type of solar cellsCrystalline silicon:

1. Monocrystalline silicon2. Polycrystalline siliconThin films

1. Amorphe silicon2. Cadmium telluride solar cell3. Copper indium gallium selenideDye-sensitized solar cells (DSSCs)

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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)

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Dye-Sensitized Solar CellsThe 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)

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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)

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Operation mechanism of the DSSCs:

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Advantages of DSSCs Compared with the conventional solar cells, DSSCs

are possessing:1. practicable high efficiency2. cost effectiveness

The conversion efficiency has been limited due to:

Recombination during the charge-transport process.

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Nanostructures in DSSCsOne 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. electron transport2. light propagation in view of the surface effect

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Nanostructures in DSSCsThose nanostructural forms of ZnO or TiO2

developed during the past several decades mainly include:

1. nanoparticles2. nanowires (nanorods)3. nanotubes4. nanobelts5. nanosheets6. nanotips

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It will show that photoelectrode films with nanostructured ZnO can significantly enhance solar cell performance by offering a:

1. large surface area for dye adsorption2. direct transport pathways for photoexcited electrons3. efficient scattering centers for enhanced light-harvesting

efficiency

Nanostructures in DSSCs

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The methods of the nanostructure production1. Sol–gel synthesis2. Hydrothermal growth3. Chemical bath deposition4. Electrostatic Spray Deposition 5. Electrochemical deposition

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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.

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1.1 ZnO Nanoparticulate FilmsZnO films with nanoparticles for application in

DSSCs have been extensively studied, partially due to:

1. The direct availability of porous structures with assembled nanoparticles

2. The simplicity of synthesis of nanoparticles.

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1.2 Nanoporous Structured ZnO FilmsBesides nanoparticulate films, nanoporous

structured ZnO films were also studied as photoelectrodes in DSSCs due to their high porosity.

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Nanoporous films with nanowalls vertically grown on the substrate by:

1. electrochemical deposition2. chemical bath deposition

Nanoporous films favorable for:1. electron transport from the point of generation to

the collection electrode2. electrolyte diffusion through the intervals between

the nanowalls, resulting in a decrease in both the series resistance and recombination rate of the cell.

1.2 Nanoporous Structured ZnO Films

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1.3 Other Nanostructured ZnO FilmsZnO nanostructures with other morphologies, such

as:NanosheetsNanobeltsNanotetrapods

have also been studied for DSSCs that they also have a large specific surface area.

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1.3 Other Nanostructured ZnO FilmsFor 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. Electron transport2. Light propagation

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2. Nanostructures with Direct Path ways for Electron TransportElectron transport in DSSCs based on

nanoparticulate films has been proposed to occur by:

1. Trapping/detrapping2. Diffusive transport

While an 11% efficiency has been achieved on nanocrystalline films because of deficiencies.

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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.

2. Nanostructures with Direct Path ways for Electron Transport

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2.1 ZnO NanowiresZnO 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.

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2.1 ZnO NanowiresThis is also a confirmation that the nanowires offer

better electron transport when compared to nanoparticles.

The mechanism has been attributed to:1. High crystallinity of nanowires2. An internal electric field

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Charge transport in nanowireThe 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.

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2.2 ZnO NanotubesNanotubes 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.

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2.3 ZnO NanoflowersThis 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.

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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.

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2.4 Dendritic ZnO nanowiresDendritic 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.

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2.4 Dendritic ZnO nanowiresDSSCs 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.

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2.4 Dendritic ZnO nanowiresphotovoltaic efficiency of dendritic ZnO

nanowires is lower than ZnO nanowire.

This is possibly due to:1. The lower nanowire density2. The insufficient nanowire length of dendritic

ZnO nanowires.3. The difference in the crystallinity of nanowires

produced by different fabrication methods.

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3. Core-Shell Structures with ZnO ShellCore-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.

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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. The oxidized dye2. Redox mediator in the

electrolyte.

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3. Core-Shell Structures with ZnO ShellSeveral 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.

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3.1 The Role of the ZnO Shellthe 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.

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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.

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4. Light Scattering Enhancement EffectThe film thickness was:1. expected to be larger than the light-absorption

length so as to capture more photons.2. constrained to be smaller than the electron-

diffusion length so as to avoid or reduce recombination.

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4.1 ZnO aggregatesZnO 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.

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4. Light Scattering Enhancement Effect4.1 ZnO aggregatesaggregates 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.

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4. Light Scattering Enhancement Effect4.2 ZnO hollow spheresZnO 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

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4. Light Scattering Enhancement Effect4.3 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.

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The scattering and absorption cross-sections are given by:

1. α polarizability of the particle

2. V is the particle volume,

3. εp is the dielectric function of the 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)

4. Light Scattering Enhancement Effect4.3 Metal nanoparticles

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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.

4. Light Scattering Enhancement Effect4.3 Metal nanoparticles

S. Pillai, K. R. Catchpole, T. Trupke, and M. A. Green, Surface plasmon enhanced silicon solar cells, Journal of applied physics 101, 093105 (2007)

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4. Light Scattering Enhancement Effect4.3 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 power-conversion 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)

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summaryDye-Sensitized Solar CellsThe methods of the nanostructure productionNanostructures in DSSCs1. Nanostructures Offering Large Specific Surface Area2. Nanostructures with Direct Path ways for Electron

Transport3. Core-Shell Structures with ZnO Shell4. Light Scattering Enhancement Effect

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