Dye Sensitized Solar Cells-Working Principles, Challenges and Opportunities

Even before the industrial revolutions human life quality is greatly affected by the availability of energy. The escalated and savage consumption of conventional sources of energy are leading to forecasted energy and environmental crises. Renewable energy sources such as solar energy are considered as a feasible alternative because “More energy from sunlight strikes Earth in 1 hour than all of the energy consumed by humans in an entire year.”(Lewis, 2007). Facilitating means to harvest a fraction of the solar energy reaching the Earth may solve many problems associated with both the energy and global environment (Nansen, 1995). Therefore, intensive research activities have resulted in attention-grabbing to the different classes of organic and inorganic based solar cells. A major study by IntertechPira stated that “The global Photovoltaic (PV) market, after experiencing a slow period, is expected to double within the next five years, reaching US$ 48 billion. Waferbased silicon will continue as the dominant technology, but amorphous thin-film and Cadmium Telluride (CdTe) technologies will gain ground, and are expected to account for a combined 22% of the market by 2014” (www.intertechpira.com). A solar cell is a photonic device that converts photons with specific wavelengths to electricity. After Alexandre Edmond Becquerel (French physicist) discovered the photoelectrochemical (photovoltaic) effect in 1839 (Becquerel, 1839) while he was investigating the effect of light on metal electrodes immersed in electrolyte, research in this area continued and technology developed to produce many types and structures of the materials presently used in photovoltaic (PV) technology. First and second generations photovoltaic cells are mainly constructed from semiconductors including crystalline silicon, III-V compounds, cadmium telluride, and copper indium selenide/sulfide (Hara & Arakawa, 2003; Hoffert, 1998; Zhao et al., 1999). Low cost solar cells have been the subject of intensive research work for the last three decades. Amorphous semiconductors were announced as one of the most promising materials for low cost energy production. However, dye sensitized solar cells DSSCs emerged as a new class of low cost energy conversion devices with simple manufacturing procedures. General comparison between semiconductor based solar cells and dye sensitized solar cells is presented in Table 1. Incorporation of dye molecules in some wide bandgap semiconductor electrodes was a key factor in developing photoelecrochemical solar cells. Michael Gratzel and coworkers at the Ecole Polytechnique Federale de Lausanne (Gratzel, 2003; Nazerruddin et al., 1993; O' Regan & Gratzel, 1991) succeeded for the first time to produce what is known as “Gratzel Cell” or


Introduction
Even before the industrial revolutions human life quality is greatly affected by the availability of energy. The escalated and savage consumption of conventional sources of energy are leading to forecasted energy and environmental crises. Renewable energy sources such as solar energy are considered as a feasible alternative because "More energy from sunlight strikes Earth in 1 hour than all of the energy consumed by humans in an entire year." (Lewis, 2007). Facilitating means to harvest a fraction of the solar energy reaching the Earth may solve many problems associated with both the energy and global environment (Nansen, 1995). Therefore, intensive research activities have resulted in attention-grabbing to the different classes of organic and inorganic based solar cells. A major study by IntertechPira stated that "The global Photovoltaic (PV) market, after experiencing a slow period, is expected to double within the next five years, reaching US$ 48 billion. Waferbased silicon will continue as the dominant technology, but amorphous thin-film and Cadmium Telluride (CdTe) technologies will gain ground, and are expected to account for a combined 22% of the market by 2014" (www.intertechpira.com). A solar cell is a photonic device that converts photons with specific wavelengths to electricity. After Alexandre Edmond Becquerel (French physicist) discovered the photoelectrochemical (photovoltaic) effect in 1839 (Becquerel, 1839) while he was investigating the effect of light on metal electrodes immersed in electrolyte, research in this area continued and technology developed to produce many types and structures of the materials presently used in photovoltaic (PV) technology. First and second generations photovoltaic cells are mainly constructed from semiconductors including crystalline silicon, III-V compounds, cadmium telluride, and copper indium selenide/sulfide (Hara & Arakawa, 2003;Hoffert, 1998;Zhao et al., 1999). Low cost solar cells have been the subject of intensive research work for the last three decades. Amorphous semiconductors were announced as one of the most promising materials for low cost energy production. However, dye sensitized solar cells DSSCs emerged as a new class of low cost energy conversion devices with simple manufacturing procedures. General comparison between semiconductor based solar cells and dye sensitized solar cells is presented in Table 1. Incorporation of dye molecules in some wide bandgap semiconductor electrodes was a key factor in developing photoelecrochemical solar cells. Michael Gratzel and coworkers at the Ecole Polytechnique Federale de Lausanne (Gratzel, 2003;Nazerruddin et al., 1993;O' Regan & Gratzel, 1991) succeeded for the first time to produce what is known as "Gratzel Cell" or the dye sensitized solar cell (DSSC) to imitate photosynthesis -the natural processes plants convert sunlight into energy-by sensitizing a nanocrystalline TiO 2 film using novel Ru bipyridl complex. In dye sensitized solar cell DSSC charge separation is accomplished by kinetic competition like in photosynthesis leading to photovoltaic action. It has been shown that DSSC are promising class of low cost and moderate efficiency solar cell (see Table 2 and Figure 1) based on organic materials (Gratzel, 2003;Hara & Arakawa, 2003).

Semiconductor solar cells DSSC Transparency
Opaque Transparent

Normal
High High Great Low Normal Color Limited Various Table 1. Comparison between semiconductor based solar cell and the dye sensitized solar cell DSSC.
In fact, in semiconductor p-n junction solar cell charge separation is taken care by the junction built in electric field, while in dye sensitizes solar cell charge separation is by kinetic competition as in photosynthesis (Späth et al., 2003). The organic dye monolayer in the photoelectrochemical or dye sensitized solar cell replaces light absorbing pigments (chlorophylls), the wide bandgap nanostructured semiconductor layer replaces oxidized dihydro-nicotinamide-adenine-dinucleotide phosphate (NADPH), and carbon dioxide acts as the electron acceptor. Moreover, the electrolyte replaces the water while oxygen as the electron donor and oxidation product, respectively (Lagref. et al., 2008;Smestad & Gratzel, 1998). The overall cell efficiency of dye sensitized solar cell is found to be proportional to the electron injection efficiency in the wide bandgap nanostructured semiconductors. This finding has encouraged researchers over the past decade. ZnO 2 nanowires, for example, have been developed to replace both porous and TiO 2 nanoparticle based solar cells (Law et al., 2005). Also, metal complex and novel man made sensitizers have been proposed (Hasselmann & Meyer, 1999;Isalm et al., 2000;Yang et al., 2000). However, processing and synthesization of these sensitizers are complicated and costly processes (Amao & Komori 2004;Garcia et al., 2003;Hao et al., 2006;Kumara et al., 2006;Polo & Iha, 2006;Smestad, 1998;Yanagida et al., 2004). Development or extraction of photosensitizers with absorption range extended to the near IR is greatly desired. In our approach, the use of natural dye extracts, we found that our environment provides natural, non toxic and low cost dye sources with high absorbance level of UV, visible and near IR. Examples of such dye sources are Bahraini Henna (Lawsonia inermis L.) and Bahraini raspberries (Rubus spp.). In this work we provide further details about the first reported operation of Henna (Lawsonia inermis L.) as a natural dye sensitizer of TiO 2 nanostructured solar cell (Jasim & Hassan, 2009;Jasim et al. in press 2011). We have experienced the usefulness of commercialized dye sensitized solar cell kits such as the one provided by Dyesol  to "illustrates how interdisciplinary science can be taught at lower division university and upper division high school levels for an understanding of renewable energy as well as basic science concepts." (Smestad, 1998;Smestad & Gratzel 1998) Furthermore, it aids proper training and awareness about the role of nanotechnology in modern civilization.  Green & Emery, 2002).
In this chapter, we overview some aspects of the historical background, present, and anticipated future of dye sensitized solar cells. Operation principle of the dye sensitized solar cell is explained. Some schemes used in preparation and assembly of dye sensitized solar cell are presented with few recommendations that might lead to better performance and stability of the fabricated cell. The structural, optical, electrical, and photovoltaic performance stability of DSSC are discussed. The performance of nanocrystalline solar cell samples can be appreciably improved by optimizing the preparation technique, the class of the nanostructured materials, types of electrolyte, and high transparent conductive electrodes. Challenges associated with materials choice, nanostructured electrodes and device layers structure design are detailed. Recent trends in the development of nano-crystalline materials for DSSCs technology are introduced. Manufacturability and different approaches suggested for commercialization of DSSC for various applications are outlined. We believe that the availability of efficient natural dye sensitizers, flexible and inkprintable conductive electrodes, and solid state electrolyte may enhance the development of a long term stable DSSCs and hence the feasibility of outdoor applications of both the dye sensitized solar cells and modules.

Structure of dye sensitized solar cell
The main parts of single junction dye sensitized solar cell are illustrated schematically in Figure 2. The cell is composed of four elements, namely, the transparent conducting and counter conducting electrodes, the nanostructured wide bandgap semiconducting layer, the dye molecules (sensitizer), and the electrolyte. The transparent conducting electrode and counter-electrode are coated with a thin conductive and transparent film such as fluorinedoped tin dioxide (SnO 2 ). per square at room temperature. The nanostructured wide bandgap oxide semiconductor (electron acceptor) is applied, printed or grown on the conductive side. Before assembling the cell the counter electrode must be coated with a catalyzing layer such as graphite layer to facilitates electron donation mechanism to the electrolyte (electron donor) as well be discussed later. One must bear in mind that the transparency levels of the transparent conducting electrode after being coated with the conductive film is not 100% over the entire visible and near infrared (NIR) part of the solar spectrum. In fact, the deposition of nanostructured material reduces transparency of the electrode. Figure 3 shows a typical transmittance measurement (using dual beam spectrophotometer) of conductive glass electrode before and after being coated with nanostructured TiO 2 layer. Fig. 3. Transmittance of conductive glass electrode before and after being coated with nanostructured TiO 2 layer.

Nanostructured photoelectrode
In the old generations of photoelectrochemeical solar cells (PSC) photoelectrodes were made from bulky semiconductor materials such as Si, GaAs or CdS. However, these kinds of photoelectrodes when exposed to light they undergo photocorrosion that results in poor stability of the photoelctrochemical cell. The use of sensitized wide bandgap semiconductors such as TiO 2 , or ZnO 2 resulted in high chemical stability of the cell due to their resistance to photocorrosion. The problem with bulky single or poly-crystalline wide bandgap is the low light to current conversion efficiency mainly due to inadequate adsorption of sensitizer because of limited surface area of the electrode. One approach to enhance light-harvesting efficiency (LHE) and hence the light to current conversion efficiency is to increase surface area (the roughness factor) of the sensitized photoelectrode. Due to the remarkable changes in mechanical, electrical, magnetic, optical and chemical properties of nanostructured materials compared to its phase in bulk structures, it received considerable attention (Gleiter, 1989). Moreover, because the area occupied by one dye molecule is much larger than its optical cross section for light capture, the absorption of light by a monolayer of dye is insubstantial. It has been confirmed that high photovoltaic efficiency cannot be achieved with the use of a flat layer of semiconductor or wide bandgap semiconductor oxide surface but rather by use of nanostructured layer of very high roughness factor (surface area). Therefore, Gratzel and his coworkers replaced the bulky layer of titanium dioxide (TiO 2 ) with nonoporous TiO 2 layer as a photoelectrode. Also, they have developed efficient photosensitizers (new Ru complex, see for example Figure 16) that are capable of absorbing wide range of visible and near infrared portion of the solar spectrum and achieved remarkable photovoltaic cell performance (Nazerruddin et al., 1993;O' Regan & Gratzel, 1991;Smestad & Gratzel, 1998). Nanoporusity of the TiO 2 paste (or colloidal solution) is achievable by sintering (annealing) of the deposited TiO 2 layer at approximately 450 C in a well ventilated zone for about 15 minutes (see Figure 4). The high porosity (>50%) of the nanostructured TiO 2 layer allows facile diffusion of redox mediators within the layer to react with surface-bound sensitizers. Lindström et al. reported "A method for manufacturing a nanostructured porous layer of a semiconductor material at room temperature. The porous layer is pressed on a conducting glass or plastic substrate for use in a dye-sensitized nanocrystalline solar cell." (Lindström et al., 2001)  Because it is not expensive, none toxic and having good chemical stability in solution while irradiated, Titanium dioxide has attracted great attention in many fields other than nanostructured photovoltaics such as photocatalysts, environmental purification, electronic devices, gas sensors, and photoelectrodes (Karami, 2010). The preparation procedures of TiO 2 film is quite simple since it is requires no vacuum facilities. Nanostructured TiO 2 layers are prepared following the procedure detailed in (Hara & Arakawa, 2003;Nazerruddin et al., 1993;O' Regan & Gratzel, 1991;Smestad, 1998) "A suspension of TiO 2 is prepared by adding 9 ml of nitric acid solution of PH 3-4 (1 ml increment) to 6 g of colloidal P25 TiO 2 powder in mortar and pestle. While grinding, 8 ml of distilled water (in 1 ml increment) is added to get a white-free flow-paste. Finally, a drop of transparent surfactant is added in 1 ml of distilled water to ensure coating uniformity and adhesion to the transparent conducting glass electrode. The ratio of the nitric acid solution to the colloidal P25 TiO 2 powder is a critical factor for the cell performance. If the ratio exceeds a certain threshold value the resulting film becomes too thick and has a tendency to peel off. On the other hand, a low ratio reduces appreciably the efficiency of light absorption" (Jasim & Hassan, 2009). Our group adopted the Doctor blade method to deposit TiO 2 suspension uniformly on a cleaned (rinsed with ethanol) electrode plate. The TiO 2 layer must be allowed to dry for few minutes and then annealed at approximately 450C (in a well ventilated zone) for about 15 minutes to form a nanoporous, large surface area TiO 2 layer. The nanostructured film must be allowed to cool down slowly to room temperature. This is a necessary condition to remove thermal stresses and avoid cracking of the glass or peeling off the TiO 2 film. Scanning electron microscopy SEM (see Figure 5-a) or X-ray diffraction measurements (XRD) (see Figure 5-b) is usually used to confirm the formation of nanostructured TiO 2 layer. Analysis of the XRD data (shown in Figure 5-b) confirmers the formation of nanocrystalline TiO 2 particles of sizes less than 50 nm (Jasim & Hassan, 2009). The nanoporous structure of the TiO 2 layer suggests that the roughness factor of 1000 is achievable. In other words, a 1-cm 2 coated area of the conductive transparent electrode with nanostructured TiO 2 layer actually possessing a surface area of 1000 cm 2 (Hara & Arakawa, 2003). The formation of nanostructured TiO 2 layer is greatly affected by TiO 2 suspension preparation procedures as well as by the annealing temperature. We found that a sintered TiO 2 film at temperatures lower than the recommended 450C resulted in cells that generate unnoticeable electric current even in the A level. Moreover, nanostructured TiO 2 layer degradation in this case is fast and cracks form after a short period of time when the cell is exposed to direct sunlight. Recently Zhu et al. investigated the effects of annealing temperature on the charge-collection and light-harvesting properties of TiO 2 nanotubebased dye-sensitized solar cells (see Figure 6) and the reported "DSSCs containing titanium oxide nanotube (NT) arrays films annealed at 400 °C exhibited the fastest transport and slowest recombination kinetics. The various structural changes were also found to affect the light-harvesting, charge-injection, and charge-collection properties of DSSCs, which, in turn, altered the photocurrent density, photovoltage, and solar energy conversion efficiency" (Zhu et al. 2010). One of the important factors that affect the cell's efficiency is the thickness of the nanostructured TiO 2 layer which must be less than 20 m to ensure that the diffusion length of the photoelectrons is greater than that of the nanocrystalline TiO 2 layer. TiO 2 is the most commonly used nanocrystalline semiconductor oxide electrode in the DSSC as an electron acceptor to support a molecular or quantum dot QD sensitizer is TiO 2 (Gratzel, 2003). Other wide bandgap semiconductor oxides is becoming common is the zinc oxide ZnO 2 . ZnO 2 possesses a bandgap of 3.37 eV and a large excitation binding energy of 60 meV. Kim et al. reported that the nanorods array electrode showed stable photovoltaic properties and exhibited much higher energy conversion efficiency . Another example, Law and coworkers have grown by chemical bath deposition ZnO 2 nanowires 8-m long with 100 nm diameters as photoelectrod (see Figure 7) the efficiency of a ZnO 2 nanowire photoelectrode DSSC is about 2.4%. This low efficiency level compared to that of nanostructured TiO 2 photoelectrode DSSC is probably due to inadequate surface area for sensitizer adsorption (Baxter et al., 2006;Boercker et al., 2009;Law et al., 2005). Other research groups suggested that the growth of longer, thinner, denser ZnO 2 nanowires is a practical approach to enhance cell efficiency (Guo et al., 2005). Investigations show that ZnO 2 nanorod size could be freely modified by controlling the solution conditions such as temperature, precursor concentration, reaction time, and adopting multi-step growth. Nanorod structured photoelectrode offers a great potential for improved electron transport.
It has been found that the short circuit current density and cell performance significantly increase as nanorods length increases because a higher amount of the adsorbed dye on longer nanorods, resulting in improving conversion efficiency . Because titanium dioxide is abundant, low cost, biocompatible and non-toxic (Gratzel & Hagfeldt, 2000), it is advantageous to be used in dye sensitized solar cells. Therefore, nanotube and nanowire-structured TiO 2 photoelectrode for dye-sensitized solar cells have been investigated (Mor et al., 2006;Pavasupree et al., 2005;Pavasupree et al., 2006;Shen et al., 2006;Suzuki et al., 2006). Moreover; SnO 2 , or Nb 2 O 5 employed not only to ensure large roughness factor (after nanostructuring the photoelectrode) but also to increase photgenerated electron diffusion length (Bergeron et al., 2005;Sun et al. 2006). Many studies suggest replacing nanoparticles film with an array of single crystalline nanowires (rods), nanoplants, or nanosheets in which the electron transport increases by several orders of magnitude (Kopidakis et al., 2003;Law et al., 2005;Noack et al., 2002;Tiwari & Snure, 2008;Xian et al., 2006). Incorporation of vertically aligned carbon nanotube counter electrode improved efficiency of TiO 2 /anthocyanin dye-Sensitized solar cells as reported by Sayer et al. They attributed the improvement to "the large surface area created by the 3D structure of the arrays in comparison to the planar geometry of the graphite and Pt electrodes, as well as the excellent electrical properties of the CNTs." (Sayer et al., 2010). investigated the cis-Ru(dcbH 2 ) 2 (NCS) (Shoute et al., 2003), and Kleverlaan et al. worked with OsIII-bpa-Ru (Kleverlaan et al 2000). Sensitizations of natural dye extracts such as shiso leaf pigments (Kumara et al., 2006), Black rice (Hao et al., 2006), Fruit of calafate (Polo and Iha, 2006), Rosella (Wongcharee et al., 2007), Natural anthocyanins (Fernando et al., 2008), Henna (Lawsonia inermis L.) (Jasim & Hassan, 2009;Jasim et al., in press 2011), and wormwood, bamboo leaves (En Mei Jin et al., 2010)   Gratzel group developed many Ru complex photosensitizers (examples are shown in Figure  16). One famous example is the cis-Di(thiocyanato)bis(2,2'-bipyridyl)-4,4'-dicarboxylate) ruthenium(II), coded as N3 or N-719 dye it has been an outstanding solar light absorber and charge-transfer sensitizer. The red dye or N3 dye (structure is shown in Figure 8-a and Figure 16) is capable of absorbing photons of wavelength ranging from 400 nm to 900 nm (see Figure 16) because of metal to ligand charge transfer transition. Theoretical Study of new ruthenium-based dyes for dye sensitized solar cells by Monari et al., states "The UV/vis absorption spectra have been computed within the time-dependent density functional theory formalism. The obtained excitation energies are compared with the experimental results." (Monari et al., 2011) In fact, for dye molecule to be excellent sensitizer, it must possess several carbonyl (C=O) or hydroxyl (-OH) groups capable of chelating to the Ti (IV) sites on the TiO 2 surface as shown in Figure 8 (Tennakone et al., 1997). Extracted dye from California blackberries (Rubus ursinus) has been found to be an excellent fast-staining dye for sensitization, on the other hand, dyes extracted from strawberries lack such complexing capability and hence not suggested as natural dye sensitizer (Cherpy et al., 1997;Semistad & Gratzel, 1998;Semistad, 1988  Commercialized dye sensitized solar cells and modules use ruthenium bipyridyl-based dyes (N3 dyes or N917) achieved conversion efficiencies above 10% (Nazerruddin, et al., 1993). However, these dyes and those chemically engineered are hard to put up and are expensive (Cherepy et al., 1997). Therefore, in attempt to develop green solar cells; our group at the University of Bahrain used Soxhlet Extractor in the extraction of natural dye solutions from abundant natural dye sources such as Bahraini Henna (Lawsonia inermis L.), Yemeni Henna, pomegranate, raspberries, and cherries after being dried( Jasim, submitted for publication 2011). We used methanol as solvent in each extraction process. The absorbance of the extracted dye solution has been measured using dual beam spectrophotometer (see Figure 9). Different concentrations of Henna (Lawsonia inermis L.) extracts have been prepared from the original extract. The light harvesting efficiency (LHE) for each concentration has been calculated from the absorbance (see Figure 10). The light harvesting efficiency is given as: where A () is the absorbance of the sample at specific wavelength. The absorbance and hence the LHE increases with concentration of dye extract. Also, as shown in Figure 10, as Henna extract concentration increases the absorbance increases and covers broader range of wavelengths.
Since not all photons scattered by or transmitted through the nanocrystalline TiO 2 layer get absorbed by a monolayer of the adsorbed dyes molecules, the incorporation of energy relay dyes might help enhancing the light harvesting efficiency. A remarkable enhancement in absorption spectral bandwidth and 26% increase in power conversion efficiency have been accomplished with some sensitizers after energy relay dyes have been added (Harding et al., 2009). Metal free organic sensitizers such as metal free iodine reported by Horiuchi et al. demonstrated remarkable high efficiency "The solar energy to current conversion efficiencies with the new indoline dye was 6.51%. Under the same conditions, the N3 dye was 7.89%" (Horiuchi et al., 2004). Semiconductor quantum dots QDs are nanostructured crystalline semiconductors where quantum confinement effect due to their size results in remarkable optical linear and nonlinear behaviors. Excitonic absorption edge of quantum dots is size dependent as shown in Figure 11 for lead sulfide PbS quantum dots suspended in toluene. It is anticipated that quantum dots are alternatives of dyes as light-harvesting structures in DSSC. Light absorption produces excitons or electron-hole pairs. Excitons have an average physical separation between the electron and hole, referred to as the Exciton Bohr Radius. Usually, Bohr radius is greater the QD diameter (e.g., for PbS Boher radius is 20 nm) leading to quantum confinement effect (discrete energy levels = artificial molecule). Excitons dissociate at the QD TiO 2 interface. The electron is subsequently injected in the semiconductor oxide conduction band, while the hole is transferred to a hole conductor or an electrolyte. Efficient and rapid hole injection from PbS QDs into triarylamine hole conductors has been demonstrated, and IPCE (Incident Photon to Current Conversion Efficiency) values exceeding 50% have been obtained. QDs have much higher optical cross sections than molecular sensitizers, depending on their size. However, they also occupy a larger area on the surface of the nanostructured photoelectrode, decreasing the QD concentration in the film. Thus, the value of the absorption length is similar to that observed for the dye-loaded nanostructured photoelectrode. Investigations show that multiple excitons can be produced from the absorption of a single photon by a QD via impact ionization if the photon energy is 3 times higher than its band gap (Ellinson et al., 2005;Nozik, 2004;Nozik, 2005). The issue to be confronted is to find ways to collect the excitons before they recombine get lost in the cell. Unlike dyes that absorb over relatively narrow region, semiconductor quantum dots such as PbS (see Figure 11-b) absorb strongly all photons with energy greater than the bandgap,

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Dye Sensitized Solar Cells -Working Principles, Challenges and Opportunities 183 thus a far higher proportion of light can be converted into useful energy using nanocrystals compared to dyes. Perhaps most important, dyes are disgracefully unstable and tend to photobleach over a relatively short amount of time. Quantum dots prepared with a properly designed outer shell are very stable and hence long lasting solar cells without degradation in performance are feasible. Quantum dots-sensitized solar cell produces quantum yields greater than one due to impact ionization process (Nozik, 2001). Dye molecules cannot undergo this process. Solar cells made from semiconductor QDs such as CdSe, CdS, PbS and InP showed a promising photovoltaic effect (Hoyer & Konenkamp, 1995;Liu & Kamat 1993;Plass et al., 2002;Vogel & Weller 1994;Zaban et al., 1998;Zweible & Green, 2000). Significant successes have been achieved in improving the photo-conversion efficiency of solar cells based on CdSe quantum dote light harvesters supported with carbon nanotube this is accomplished by incorporating carbon nanotubes network in the nanostructured TiO 2 layer, and accordingly assisting charge transport process network (Hasobe et al., 2006;Robel et al., 2005). Consequently, appreciable improvement in the photo-conversion efficiency of the DSSC is attainable. Recently Fuke et al., reported CdSe quantum-dot-sensitized solar cell with ~100% internal quantum efficiency. A significant enhancement in both the electron injection efficiency at the QD/TiO 2 interface and charge collection efficiency at the QD/electrolyte interface" were achieved (Fuke et al., 2010).

Redox electrolyte
Electrolyte containing I  /I 3  redox ions is used in DSSC to regenerate the oxidized dye molecules and hence completing the electric circuit by mediating electrons between the nanostructured electrode and counter electrode. NaI, LiI and R 4 NI (tetraalkylammonium iodide) are well known examples of mixture of iodide usually dissolved in nonprotonic solvents such as acetonitrile, propylene carbonate and propionitrile to make electrolyte. Cell performance is greatly affected by ion conductivity in the electrolyte which is directly affected by the viscosity of the solvent. Thus, solvent with lower viscosity is highly recommended. Moreover, counter cations of iodides such as Na + , Li + , and R 4 N + do affect the cell performance mainly due to their adsorption on nanostructured electrode (TiO 2 ) or ion conductivity. It has been found that addition of tert-butylpyridine to the redoxing electrolyte improves cell performance (Nazeeruddin et al., 1993) (see Figure 19). Br  /Br 3  redox couple was used in DSSCs and promising results were obtained. The V oc and I sc increased for the Eosin Y-based DSSC when the redox couple was changed from I  /I 3  to Br  /Br 3  (Suri & Mehra, 2006).
The redoxing electrolyte needs to be chosen such that the reduction of I 3  ions by injection of electrons is fast and efficient (see Figure 13). This arise from the fact that the dependence of both hole transport and collection efficiency on the dye-cation reduction and I  /I 3  redox efficiency at counter electrodes are to be taken into account (Yanagida, 2006). Besides limiting cell stability due to evaporation, liquid electrolyte inhibits fabrication of multi-cell modules, since module manufacturing requires cells be connected electrically yet separated chemically (Matsumoto et al., 2001;Tennakone et al., 1999). Hence, a significant shortcoming of the dye sensitized solar cells filled with liquid state redoxing electrolyte is the leakage of the electrolyte, leading to reduction of cell's lifespan, as well as the associated technological problems related to device sealing up and hence, long-term stability (Kang, et al., 2003). Many research groups investigate the use of ionic liquids, polymer, and hole conductor electrolytes (see Figure 12) to replace the need of organic solvents in liquid electrolytes. Despite the reported relative low cell's efficiency of 4-7.5% (device area < 1 cm 2 ) , these kind of electrolyte are promising and may facilitate commercialization of dye sensitize solar modules (Kawano, et al., 2004;Kuang et al., 2006;Schmidt-Mende & Gratzel, 2006;Wang et al., 2004). Addition of polymer gel to quasi-solidify electrolytes has been investigated by many research groups (Ren et al., 2001;Kubo et al., 2001;Nogueira et al., 2001). It has been found that the addition of Poly(viny1idene fluoride-co-hexafluoropropylene) to the KI/I 2 electrolyte has improved both the fill factors and energy conversion efficiency of the cells by about 17% (Kang, et al., 2003). Gel electrolytes also are very attractive from many perspectives such as: Efficiency is a compromise between electrolyte viscosity and ionic mobility; gelled ionic liquids have an anomalously high ionic mobility despite their high viscosity, and particularly for realization of monolithic arrays inter-cell sealing . Innovative classes of electrolytes such as p-type, polymeric conductor, PEDOT or PEDOT:TMA, which carries electrons from the counter electrode to the oxidized dye encouraging further investigations to optimize and/or design new ones. Recently one of the first systematic study of charge transport and recombination in solid state dye sensitized solar cell SDSCs using conjugated polymer hole transporter has been reported by Zhang et al., in this investigation organic indoline dye D131 as the sensitizer and poly(3hexylthiophene) (P3HT) as the hole transporter a power conversion efficiency of 3.85% have been recorded. Therefore, this class of solar cells is expected to represent one of the most efficient SDSCs using polymeric hole transporter (Zhang et al, 2011).

How dye sensitized solar cell works
In this section we overview the following: Process during which light energy get converted to electric one, photovoltaic performance, charge injection, charge transport in the nanostructured electrode, charge recombination, and cell dark current.

Operating principle of dye sensitized solar cell
Nanocrystalline TiO 2 is deposited on the conducting electrode (photoelectrode) to provide the necessary large surface area to adsorb sensitizers (dye molecules). Upon absorption of photons, dye molecules are excited from the highest occupied molecular orbitals (HOMO) to the lowest unoccupied molecular orbital (LUMO) states as shown schematically in Figure  13. This process is represented by Eq. 2. Once an electron is injected into the conduction band of the wide bandgap semiconductor nanostructured TiO 2 film , the dye molecule (photosensitizer) becomes oxidized, (Equation 3). The injected electron is transported between the TiO 2 nanoparticles and then extracted to a load where the work done is delivered as an electrical energy, (Equation 4). Electrolytes containing I  /I 3  redox ions is used as an electron mediator between the TiO 2 photoelectrode and the carbon coated counter electrode. Therefore, the oxidized dye molecules (photosensitizer) are regenerated by receiving electrons from the I  ion redox mediator that get oxidized to I 3  (Tri-iodide ions). This process is represented by Eq. 5. The I 3  substitutes the internally donated electron with that from the external load and reduced back to I  ion, (Equation 6). The movement of electrons in the conduction band of the wide bandgap nanostructured semiconductor is accompanied by the diffusion of charge-compensating cations in the electrolyte layer close to the nanoparticle surface. Therefore, generation of electric power in DSSC causes no permanent chemical change or transformation (Gratzel, 2005  As illustrated in Fig. 13, the maximum potential produced by the cell is determined by the energy separation between the electrolyte chemical potential (E redox ) and the Fermi level (E F ) of the TiO 2 layer. The small energy separation between the HOMO and LUMO ensures absorption of low energy photons in the solar spectrum. Therefore, the photocurrent level is dependent on the HOMO-LUMO levels separation. This is analogous to inorganic semiconductors energy bandgap (E g ). In fact, effective electron injection into the conduction band of TiO 2 is improved with the increase of energy separation of LUMO and the bottom of the TiO 2 conduction band. Furthermore, for the HOMO level to effectively accept the donated electrons from the redox mediator, the energy difference between the HOMO and redox chemical potential must be more positive (Hara & Arakawa, 2003). Figure 14 presents examples of the I-V characteristics of natural dye sensitized solar cell NDSSC with Bahraini Henna (Lawsonia inermis L.), pomegranate, Bahraini raspberries, and cherries. We found that nature of the dye and its concentration has a remarkable effect on the magnitude of the collected photocurrent. Under full solar spectrum irradiation with photon flux I 0 = 100 mW/cm 2 (Air Mass 1.5), the photon energy -to-electricity conversion efficiency is defined as (Gratzel, 2003):

Photovoltaic performance
where J sc is the short circuit current, V oc the open circuit voltage, and FF is the fill factor of the solar cell which is calculated by multiplying both the photocurrent and voltage resulting in maximum electric power delivered by the cell.  Table 3 shows the electrical properties of some assembled NDSSCs. Photocurrent and voltage drop on a variable load have been recorded instantaneously while the cell is exposed to direct sun illumination. Due to light reflection and absorption by the conductive photoelectrode and the scattering nature of the nanostructured TiO 2 , the measured transmittance of the photoelectrode (see Figure 3) shows an average of 10% of the solar spectrum (Air Mass 1.5) may reach the sensitizers. Since TiO 2 past is applied on the conductive electrode using doctor blade method the effective area of the irradiated part of the cell is 1.5 cm  2 cm. Despite the variation of Bahraini Henna extract concentration the cells produced almost the same open circuit voltage V oc . On the other hand, the short circuit current I sc varies with Henna extract concentration. Highly concentrated Bahraini Henna extracts results in non-ideal I-V characteristics even though it possesses 100% light harvesting efficiency in the UV and in the visible parts of the electromagnetic spectrum. The dye concentration was found to influence remarkably the magnitude of the collected photocurrent. High concentration of Henna extract introduces a series resistance that ultimately reduces the generated photocurrent. On the other hand, diluted extracts reduces the magnitude of the photocurrent and cell efficiency.   Gratzel and coworkers reported cell efficiency of 10.4% using black dye (RuL ' (NCS) 3 complexes) and as shown in Figure 15, cells with solar to electric power conversion efficiency of the DSSC in full AM 1.5 sun light validated by accredited PV calibration laboratories has reached over 11% (Chiba et al., 2006). Jiu et al., (Jiu, et al., 2006)

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Dye Sensitized Solar Cells -Working Principles, Challenges and Opportunities 189 synthesized highly crystalline TiO 2 nanorods with lengths of 100-300 nm and diameters of 20-30 nm. The rod shape kept under high calcination temperatures contributed to the achievement of the high conversion efficiency of light-to-electricity of 7.29%. Reported efficiencies of nanostructured ZnO 2 photoelectrodes based cells are encouraging and many research groups are dedicating their efforts to provide cells with efficiency close to that reported for sensitized nanostructured TiO 2 photoelectrodes. The short circuit current magnitude affects directly the incident photon-to-current conversion efficiency IPCE which is defined using the photoresponse and the light intensity as: where  is the wavelength of the absorbed photon and I is the light intensity at wavelength . Figures 15 and 16 present IPCE examples of some commonly used sensitizers by Gratzel and coworkers.
In terms of light harvesting efficiency LHE, quantum yield of electron injection quantum yield  inj , and collection efficiency  c of the injected electrons at the back contact IPCE is given by: Therefore, IPCE equals the LHE if both  inj and  c are close 100%. However, Charge injection from the electronically excited sensitizer into the conduction band of the nanostructured wide bandgap semiconductor is in furious competition with other radiative and non-radiative processes. Due to electron transfer dynamics (see Figure 17), if electron injection in the semiconductor is comparable to, or slower than, the relaxation time of the dye,  inj will be way below 100%. This can be deduced from the definition of the quantum yield  inj (Cherepy et al., 1997): The quantum yield approaches 100% only when the radiative and nonradiative rates (k rad , k nrad ) (paths shown in Figure 13) are much smaller than the injection rate k inj . The rate constant for charge injection k inj is given by Fermi golden rule (Gratzel, 2001;Hara & Arakawa, 2003): where h is Planck's constant, |V| is the electron-coupling matrix element and ρ(E) is the density of electronic acceptor states in the conduction band of the semiconductor. Equation (11) assumes that electron transfer from the excited dye molecules into the semiconductors is activationless and hence exhibits a temperature-independent rate. Some representative examples of electron injection rate constants k inj and electronic coupling matrix elements |V| measured by laser flash photolysis for some sensitizers adsorbed onto nanocrystalline TiO 2 , t f and Φ ιnj (the excited-state lifetime and the injection quantum yield, respectively) are presented in Table 4 (Gratzel, 2001;Hara & Arakawa, 2003). The shown values of |V| on Table 4 credited to the degree of overlapping of photosensitizer excited states wavefunction and the conduction band of the nanostructured photoelectrode. The distance between the adsorbed sensitizer and the nanostructured photoelectrode affect the value of the electronic coupling matrix elements.
Advantages of tandem structure have been investigated both theoretically and experimentally as approaches to improve the photocurrent of DSSC (Durr et al., 2004). "The

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Dye Sensitized Solar Cells -Working Principles, Challenges and Opportunities 191 tandem structured cell exhibited higher photocurrent and conversion efficiency than each single DSSC mainly caused by its extended spectral response." (Kubo et al., 2004) 3.3 Charge injection, transport, recombination, and cell dark current Kinetics of electron injection into the semiconductor photoelectrod after being excited from the photosensitizer has been investigated by many researchers using time-resolved laser spectroscopy (Hara & Arakawa, 2003). It has been found that both the configuration of the photosensitizer material and the energy separation between the conduction band level of the wideband gap semiconductor and the LUMO level of the photosensitizer are greatly affecting the electron transfer rate to the wideband gap semiconductor. Figure 17 shows a schematic illustration of kinetics in the DSSC. The shown arrows indicate excitation of the dye from the HOMO to the LUMO level, relaxation of the exited state (60 ns), electron injection from the dye LUMO level to the TiO 2 conduction band (50 fs -1.7 ps), recombination of the injected electron with the hole in the dye HOMO level (ns -ms), recombination of the electron in the TiO 2 conduction band with a hole (I 3  ) in the electrolyte (10 ms), and the regeneration of the oxidized dye by I  (10 ns). (Hagfeldt & Gratzel, 2000).  (Hara & Arakawa, 2003). To achieve good quantum yield, the rate constant for charge injection should be in the picosecond range. In conclusion, Fast recovery of the sensitizer is important for attaining long term stability. Also, long-lasting charge separation is a very important key factor to the performance of solar cells. Thus, new designs for larger conjugated dye-sensitizer molecules have been reported by investigators ,for example, Haque et al., (Haque et al., 2004) studied hybrid supermolecules that are efficiently retard the recombination of the charge-separated state and therefore assure enhanced energy conversion efficiency by extending the lifetime of light-induced charge-separated states as illustrated in Figure 18, "Hybrid supermolecule: This is the structure of the redox triad that gave the most efficient charge separation in the report by Haque and colleagues . The triad is made of a ruthenium complex anchored to nanocrystalline TiO 2 (the electron acceptor) and covalently linked to polymeric chains of triphenyl-amine groups (the electron donor). Arrows represent the direction of the electron transfer process. The first step of the electron transfer is the light-induced excitation of the chromophore (process 1). Following this an electron is readily injected from the sensitizer excited state into the conduction band of the TiO 2 semiconductor (process 2). The direct recombination of primarily separated charges (process 3) would degrade the absorbed energy into heat. In this supermolecule this is avoided through the fast reduction of the ruthenium by the linked triphenyl-amine electron donor groups (process 4). The secondary recombination process (process 5) between the injected electron and the oxidized amine radical is made increasingly slow because the positive charge can hop from one triphenylamine function to the adjacent one along the chain (process 6) and the hole moves away from the TiO 2 surface. The overall photoinitiated process thus results in unidirectional electron flow from the end of the polymeric chains to the oxide (from right to left) and a very long-lived charge-separated state" (Moser, 2005). In TiO 2 nanoparticle DSSCs, the electrons diffuse to the anode by hopping 103-106 times between particles (Baxter et al., 2006). With each hop there is a considerable probability of recombination of the photoexcited electron with the electrolyte since both the diffusion and recombination rates are on the order of milliseconds. Hence, this allows recombination to limit the cell efficiency. On the other hand, nanowire or tube structured photoelectrode (e.g., ZnO 2 ) provide a direct path (express highway) to the anode, leading to increased diffusion rate without increasing the recombination rate and thus increases cell efficiency.  where k B is the Boltzmann constant, T is the absolute temperature of the cell, and q is the magnitude of the electron charge. In fact, dark current mainly occurs at the TiO 2 /electrolyte interface where no photosensitizer got adsorbed. One successful way to suppress dark current is to use one of pyridine derivatives (e.g., tert-butylpyridine TBP) as coadsorbates on the nanostructured TiO 2 surface. Figure 19 shows the current-voltage characteristics obtained for NKX-2311-sensitized TiO 2 solar cells .

Applications of DSSC
Because of the physical nature of the dye sensitized solar cells, inexpensive, environmentfriendly materials, processing, and realization of various colors (kind of the used sensitizing dye); power window and shingles are prospective applications in building integrated photovoltaics BIPV. The Australian company Sustainable Technologies International has produced electric-power-producing glass tiles on a large scale for field testing and the first building has been equipped with a wall of this type (see for example, Figure 20-a). The availability of lightweight flexible dye sensitized cells or modules are attractive for applications in room or outdoor light powered calculators, gadgets, and mobiles. Dye sensitized solar cell can be designed as indoor colorful decorative elements (see Figure 20-b). Flexible dye sensitized solar modules opens opportunities for integrating them with many portable devices, baggage, gears, or outfits (Pagliaro et al., w w w. pv-te ch.org) (see Figure  20-c and Figure 20-d). In power generation, dye sensitized modules with efficiency of 10% are attractive choice to replace the common crystalline Si-based modules. In 2010 Sony announced fabrication of modules with efficiency close to 10% and hence opportunity of commercialization of DSSC modules is attainable.

Commercialization of DSSC
Commercialization of dye sensitized solar cells and modules is taking place on almost all continents (Lenzmann & Kroon, 2007). In Asia, specifically in Japan: IMRA-Aisin  Figure 21) to demonstrate various sizes and colors in a series-connected dye solar cell module in many international exhibitions and conferences reflects the potential role of dye sensitized solar cells systems in the PV technology. In fact, Toyota has installed in their dream house walls a similar kind of DSSCs panels shown in Figure 21-b. Glass substrate is robust and sustains high temperatures, but it is fragile, nonflexible, and pricey when designed for windows or roofs. Flexible DSSCs have been intensively investigated. Miyasaka et. al. (Miyasaka & Kijitori, 2004) used the ITO (indium tin oxide) coated on PET (polyethylene terephatalate) as the substrate for DSSCs. Generally, the conducting glass is usually coated with nanocrystallineTiO 2 and then sintered at 450  C-500  C to improve the electronic contact not only between the particles and support but also among the particles. Plastics films have a low ability to withstand heat. The efficiency of plastic-based dye sensitized solar cells is lower than that of using glass substrate (η = 4.1%, J sc = 9.0mA/cm 2 , V oc = 0.74V, FF = 0.61) because of poor necking of TiO 2 particles. Kang et al., (Kang et al.2006), used the stainless steel as the substrate for photoelectrode of DSSCs (see Figure 22). The cell illuminated through the counter electrode due to the non-penetration of light through metal substrate. In their system, the SiOx layer was coated on stainless steel (sheet resistance ~ 1 m per square) and separated ITO from stainless steel, for preventing photocurrent leakage from stainless steel to the electrolyte. The constructed cells resulted in J sc = 12 mA/cm 2 , V oc = 0.61 V, FF = 0.66, and η = 4.2 %. Recently, Chang et al. fabricated flexible substrate cell that produced conversion efficiency close to 2.91%. The photoelectrode substrates are flexible stainless steel sheet with thickness 0.07mm and titanium (Ti) sheet with thickness 0.25mm (Chang et al., 2010). Also, the reported approach by Yen et al. in developing a low temperature process for the flexible dye-sensitized solar cells using commercially available TiO 2 nanoparticles (such as P25 ) is interesting since it yielded a conversion efficiency of 3.10% for an incident solar energy of 100 mW/cm 2 (Yen et al. 2010). Because Titanium has extremely high corrosion resistance, compared with stainless steel, Titanium is still the privileged substrate material. Fig. 22. A prototype of a flexible dye sensitized solar cell using stainless steel substrate (From Kang et al., 2006).
Availability of nonvolatile electrolyte is another issue toward commercialization of single or multi-junction modules. Polymer (solid) electrolyte, hole conductor, and solidified ionic liquids are solvent free choices with high electronic conductivity and chemical stability  The key to high power heterojunction DSSC is to increase the effective diffusion length of electron within the nanostructured electrode by increasing the mobility of hole conductor or the extinction coefficient of the sensitizer to ensure more efficient light harvesting action. Since heat and UV light degrade cells performance, development of heat sink and optimized low cost UV coating is a must for outdoor applications. The successes in development of flexible substrate, solid electrolyte, and spectrally broad absorption range inexpensive nontoxic dyes will potentially open the possibility of role-to-role mass production of dye sensitized solar cells and modules (see Figure 23). Molecular engineering of efficient and stable organic sensitizers is an open invitation for many research groups, the successes in this area is expected to advance production and commercialization of DSSC

Conclusions
In This chapter we have discussed one example of the third generation solar cells, called photoelectrochemical cell and now called nanocrystalline dye sensitized solar cells DSSC or Gratzel cell. Nanocrystalline dye sensitized solar cell DSSC is classified as a low cost, environmental friendly, and capable of being highly efficient cell mainly due to materials, charge carriers generation and transport within the cell structure. The nanostructured dyesensitized solar cell (DSSC) is going to provide economically credible alternative to present day p-n junction photovoltaics. In fact, dye sensitized solar cell are green solar cell mimicking the green leave. In dye sensitized solar cell electricity is generated as a result of electron transfer due to photoexcitation of dye molecules adsorbed to nanostructured wide bandage material photoelectrode. The oxidized dye molecules regenerated by gaining electrons from electrolyte which is reduced by the electrons reaching the counter-electrode of the cell. In dye sensitized solar cells light absorption is separated from carrier transport. From educational point of view, since nanostructured dye sensitized solar cell DSSC is mimicking photosynthesis in plants, it provides an interdisciplinary context for students learning the basic principles of biological extraction, chemistry, physics, environmental science and electron transfer. The requirements of practical sensitizers are: broad band and high level absorption of visible and near infrared region of the electromagnetic spectrum, exhibit thermal and photochemical stability, definitely chelating to the semiconductor oxide surface and inject electrons into the conduction band with a quantum yield of unity, and owning suitable ground-and excited state redox properties. Investigations of solvent free electrolyte such as polymer based, and ionic liquid are promising. In order to commercialize dye sensitized solar cell in low power applications, flexible DSSCs have been intensively investigated. The solar to electric power conversion efficiency of the DSC in full AM 1.5 sun light validated by accredited PV calibration laboratories has reached over 11 % and modules with efficiency close to 10% has been exhibited in 2010. Nanowires and quantum dots QDSSCs may be a promising solar cell design. The search for green sources or generators of energy is considered one of the priorities in today's societies and occupies many policy makers' agendas. We at the University of Bahrain are the first to start the investigation of Dye sensitized solar cells DSSCs in the Arabian Gulf. Dye sensitized solar cells using natural organic dies were prepared using low cost materials and natural dyes. Sensitization of wide gap oxides semiconductor materials was accomplished with the growth of nanocrystalline TiO 2. The natural dyes extracted from Henna (Lawsonia inermis L.), pomegranate, cherries, and raspberries (Rubus spp.). We found that the nanocrystalline material based solar cell system exhibits an excellent optical absorption parameters for visible and near infrared portion of the electromagnetic spectrum. The performance of natural dye extract sensitized nanocrystalline solar cells can be appreciably enhanced by optimizing preparation technique, using different types of electrolyte, reported additives, and sealing. In short, compared to Si based solar cells dye sensitized solar cells are of low cost and ease of production, their performance increases with temperature, possessing bifacial configuration -advantage for diffuse light, have transparency for power windows, color can be varied by selection of the dye, invisible PV-cells based on near-IR sensitizers are feasible, and they are outperforms amorphous Si. Moreover, DSSC shows higher conversion efficiency than polycrystalline Si in diffuse light or cloudy conditions. It is believed that nanocrystalline photovoltaic devices are becoming viable contender for large scale future solar energy converters.

Acknowledgments
The author is greatly indebted to the University of Bahrain for financial support. I would like to express my thanks to Prof. Dr. Shawqi Al Dallal for being keen in providing fruitful