1

Solar cells And Nanostrucures By: M. Asemi 2

Outline Solar sells Dye-Sensitized Solar Cells Nanostructures in DSSCs The methods of the nanostructure production 1. Nanostructures Offering Large Specific Surface Area 1.ZnO Nanoparticulate Films 2.Nanoporous Structured ZnO Films 3.Other Nanostructured ZnO Films 2. Nanostructures with Direct Path ways for Electron Transport 1.ZnO Nanowires 2.ZnO Nanotubes 3.ZnO Nanoflowers 4.Dendritic ZnO nanowires 3. Core-Shell Structures with ZnO Shell 1.The Role of the ZnO Shell 2.Influence of Shell Thickness 4. Light Scattering Enhancement Effect 1.ZnO Aggregates 2.ZnO hollow spheres 3.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. conversion efficiency 2. cost 4

Solar cells One 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 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 Crystalline silicon: 1. Monocrystalline silicon 2. Polycrystalline silicon Thin films 1. Amorphe silicon 2. Cadmium telluride solar cell 3. Copper indium gallium selenide Dye-sensitized solar cells (DSSCs) 8

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 Compared with the conventional solar cells, DSSCs are possessing: 1. practicable high efficiency 2. 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. electron transport 2. 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: 1. nanoparticles 2. nanowires (nanorods) 3. nanotubes 4. nanobelts 5. nanosheets 6. nanotips 15

It will show that photoelectrode films with nanostructured ZnO can significantly enhance solar cell performance by offering a: 1. large surface area for dye adsorption 2. direct transport pathways for photoexcited electrons 3. efficient scattering centers for enhanced light-harvesting efficiency Nanostructures in DSSCs 16

The methods of the nanostructure production 1. Sol–gel synthesis 2. Hydrothermal growth 3. Chemical bath deposition 4. Electrostatic Spray Deposition 5. 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

Nanoporous films with nanowalls vertically grown on the substrate by: 1. electrochemical deposition 2. chemical bath deposition Nanoporous films favorable for: 1. electron transport from the point of generation to the collection electrode 2. 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 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. Electron transport 2. Light propagation 23

2. Nanostructures with Direct Path ways for Electron Transport Electron transport in DSSCs based on nanoparticulate films has been proposed to occur by: 1. Trapping/detrapping 2. Diffusive transport While an 11% efficiency has been achieved on nanocrystalline films because of deficiencies. 24

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 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. High crystallinity of nanowires 2. 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,

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,

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,

2.4 Dendritic ZnO nanowires photovoltaic efficiency of dendritic ZnO nanowires is lower than ZnO nanowire. This is possibly due to: 1. 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. 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.The oxidized dye 2.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,

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 The 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. 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% 41 T. P. Chou, Q. F. Z hang, G.E. Fryxell and G.Z. Cao, Adv. Mater. 2007, 19, 2588.

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,

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

The scattering and absorption cross-sections are given by: 1. α polarizability of the particle 2. V is the particle volume, 3. ε p is the dielectric function of the particle 4. ε m 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, (2007) 4. Light Scattering Enhancement Effect 4.3 Metal nanoparticles 45

We can see that when ε p = -2ε m 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 Effect 4.3 Metal nanoparticles S. Pillai, K. R. Catchpole, T. Trupke, and M. A. Green, Surface plasmon enhanced silicon solar cells, Journal of applied physics 101, (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 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, (2011) 47

summary Dye-Sensitized Solar Cells The methods of the nanostructure production Nanostructures in DSSCs 1. Nanostructures Offering Large Specific Surface Area 2. Nanostructures with Direct Path ways for Electron Transport 3. Core-Shell Structures with ZnO Shell 4. Light Scattering Enhancement Effect 48

49