Dye-Sentitised Solar Cells - Essay Example

? Flinders Faculty of Science and Engineering School of Chemical and Physical Sciences Nano8711: Advanced Na chnology Topic Coordinator: Professor. Joe Shapter Assignment 1-Essay Writing: Dye-Sensitized Solar Cells Author: Yousef Abdullah AlBaharnah Student Number: 2083102 Contents 1.0 Introduction 5 2.0 Historical Background 7 3.0 Structural Components 9 3.1 Nanocrystalline Semiconductor Film Electrode 10 3.2 Dye Sensitizer 11 3.3 Electrolyte 18 4.0 Principle 20 5.0 Energetics of DSC 24 6.0 Present Research 25 6.1 Dye 25 6.2 Electrode 27 6.3 Electrolyte 30 7.0 Commercial Applications 32 7.0 Future Prospects 32 8.0 Conclusion 33 9.0 References: 34 List of Abbreviations PV Cells Photovoltaic cells CIGS Cu(In, Ga) Se2 or copper indium gallium selenide DSC Dye sensitized solar cells PEN Polyethylene tetraphthalate IPCE Incident photon to current conversion efficiency Bchlorine 1 Methyl trans 32 carboxy 8 deethyl 7 ethyl 8 oxo pyropheophorbide a Bchlorine 2 Methyl trans 32 carboxy 7 deethyl 8 ethyl 7 oxo pyropheophorbide a CDCA Chenodeoxycholic acid FRET Forster resonance energy transfer DCM 4-(Dicyanomethylene)-2methyl-6-(4-dimethylaminostyryl)-4H-pyran GBL Gamma-butyrolactone TBP 4-tert-butylpyridine DMII 1,3-dimethylimidazolium iodide CSIRO Commonwealth Scientific and Industrial Research Organization Table of Figures Figure 1: Basic structure of dye sensitized solar cell (DSC), and their equivalence to process of photosynthesis5 6 Figure 2: Components of DSC11 10 Figure 3: common organic dyes with their conversion efficiency in DSC6 13 Figure 4: Molecular structures of Ruthenium based inorganic dye complexes18 14 Figure 5: Structure of Ru(II) NCS complex19 15 Figure 6 Photocurrent action spectra of bipyridine and Terpyridine dye along with photocurrent response of plain TiO2 films4 15 Figure 7: General structure of main betalain dyes extracted from red turnip and wild prickly pear. 1. Betacyanin, 2. Indicaxanthin. R1 and R2 = H (betanidin) or R1= ?-D-glucose and R2= H (betanin)14 17 Figure 8 Photoaction spectra of betalains from red turnip and wild prickly pear on transparent titania19 17 Figure 9: Photoaction spectra of Bchlorin 1 and Bchlorin 224 18 Figure 10: Iodide triodide redox couple25 19 Figure 11: Spirobifluorene4 20 Figure 12: Operation principle of DSC26 20 Figure 13 Current generation in DSC, step 1 21 Figure 14: Current generation in DSC, step 2 22 Figure 15: Current generation in DSC, Step 3, light absorption 22 Figure 16: Current generation in DSC, step 4 22 Figure 17: Current generation in DSC, step 5 23 Figure 18: Current generation in DSC, step 6 23 Figure 19 : Maximum voltage in DSC25 24 Figure 20: Kinetics of DSC operation. 25 Figure 21: A: Action spectra of Energy relay dye DCM alone and in combination with near IR sensitizing dye TT1; B: Molecular structure of DCM and TT128 27 Figure 22: porphyrin chromophore linked to dye29 27 Figure 23: Photocurrent density of TiO2 nanowires photoelectrode film as a function of measured potential (IPCE spectrum in inset)2 28 Figure 24: Absorptance spectrum (?) calculated from measured diffuse reflectance (Rd) and diffuse transmittance (Td) spectra of a cellulose template dye sensitized 6µm thick film of titania hollow fibers. APCE (absorbed photon to current conversion efficiency calculated from IPCE plot and ? plot 30 29 Figure 25: Hybrid nanosheets31 30 Figure 26: Spectral response of photocurrent DSCs using T2/T- and I3-/I- as the electrolyte34 31 1.0 Introduction Energy is the basic need of all economies, whether developed or developing. For the past two centuries electricity requirements have been provided mostly by fossil fuels, which have two basic limitations: being non renewable and environmentally damaging . Considering the ever increasing demand of electricity that is expected to peak in coming years1 it has become imperative to explore alternative sources of energy which are not only renewable, but also eco-friendly. Of these, use of solar energy has been found to be practical and viable. The solar energy available to Earth is 3?1024 J/year, which is 104 times of present global energy requirements.2 Hence, devices to enable capture and conversion of solar energy in to electrical energy, termed as solar or photovoltaic (PV) cells, have been a field of ongoing research interest. As a result, recent years have witnessed considerable advances in the development of the design and efficiency PV cells. PV cells are based on the principle of natural means of solar energy fixation or photosynthesis followed by green plants as illustrated by figure 1. The fixed chemical energy is utilized by the plant for various energy requiring processes. Thus, the photovoltaic effect discovered in 1839 by Bequerel, refers to the generation of electricity due to light irradiation upon two electrodes attached to a solid or liquid system.3 PV cells are therefore based on charge separation at the interface of two materials having distinct conduction. Developments in the field of PV cells has until now been concentrated in the use of junction devices made of inorganic solid materials, primarily silicon and other semiconductor mediums such as cadmium telluride, copper indium gallium selenide (CIGS) etc. However, the third generation of PV cells utilize organic materials, such as thin films of nanocrystalline, and conductive polymers, which provide two key advantages - higher efficiency and lower cost of fabrication, along with several others. These cells utilize a solid, liquid or gel phase electrolyte forming the contacting phase between the thin films of electrodes, and are therefore classified as photo-electrochemical cells.4 Figure 1: Basic structure of dye sensitized solar cell (DSC), and their equivalence to process of photosynthesis5 Further advancement in the field has lead to the incorporation of dyes as a sensitizer in order to facilitate light absorption, the cell now referred to as Dye Sensitized Solar Cells (DSC). A DSC is a solar cell with three major components: two conductive glasses, one which has a titania (TiO2) film interspersed with a dye; the other conducting glass acting as the opposite electrode. The second major component is the intervening electrolyte.6 Despite the many advantages of DSC, a number of drawbacks still demand continuous research in the field, mainly finding panchromatic dye sensitizers for energy exploitation in the entire visible, as well as, most of near IR range, a less volatile electrolyte to enable long term application of the device, and reduction of corrosive action of iodide redox complex.7 This paper has three primary objectives; first, to provide a historical background of how incident light has been converted to electricity in the past; and the developments that have been made through advancements in technology towards DSC in recent times; second, to investigate the three main components of DSC, while illustrating through graphical representation their energy levels, its underlying mechanisms and fundamental principles; third, to illustrate the current state of research within DSCs through support from empirical evidence in the literature to outline the usefulness of the techniques and methods used. Lastly, recommendations will be made to suggest the prospective of large scale commercialization of DSC and the uprising trend towards the application of DSC in energy markets. 2.0 Historical Background The study of the technique of converting solar energy into electrical energy dates back to 1839, when Edmond Bequerel demonstrated the production of electric current upon illumination of a set up - comprising of two brass plates immersed in a liquid. Towards the end of the nineteenth century C. E. Fritts, an American, reported the production of electric current using selenium array exposed to sunlight3. In 1873, the first panchromatic film capable of reproducing a black and white image of a captured scene was reported in which dyes were used with silver halide. This was shortly followed by the first report of sensitization of a photoelectrode. However, these and many similar reports remained obscure, and the correlation between the above two processes could not be derived until the development of quantum mechanics and study of semiconductor behaviour.4 The first single crystal solar cell with an efficiency of 6% was demonstrated in 19548. By the 1960s it was realized that the dye sensitized photo cells operate on the mechanism of photoexcitation of dye, leading to emission and introduction of electrons into the conduction band of semiconductors.9 By the late 1960s, liquid junction photoelectrochemical, cells using single silicon crystal and yielding up to 15% conversion efficiency, were developed.10 However, one major drawback of these cells was found to be photocorrosion, which results due to the oxidation of semiconductor materials that have narrow band-gap by the holes generated; since their photo-response corresponds to the solar spectrum. To overcome this deficit, wide band gap oxide semiconductors were used as electrodes. Even so, due to the large band gap, these semiconductors were unable to use solar energy for generation of electron hole pairs, since this required higher energy light for this purpose. To overcome this problem these oxide semiconductors were sensitized by dyes, thus enabling them to utilize sunlight for electron hole pair generation.10 Following this, the concept built that the efficiency of dye could be enhanced by its chemisorbtion on the semiconductor surface was established.4 Initially, this lead to the use of dispersed particles to allow sufficient interface and subsequently particulate photoelectrodes to come into use. By now titanium dioxide had become the preferred semiconductor because of being inexpensive, easily available, nontoxic and biocompatible. The dye of choice was tris(2’2’-bipyridyl, 44’-carboxyalte) ruthenium(II), in which the carboxylate group was responsible for chemisorptions on the titania electrode.7 By 1970s silicon based PV cells with conversion efficiency of more than 25% had been built. The first dye sensitized nanocrystalline photo cell was demonstrated in 1991 by Michael Gratzel, which displayed a conversion efficiency of 7.1%.7 While silicon based cells were already being used successfully in aircraft10, the third generation of photoelectrochemical cells, one of which was DSC, popularly known as Gratzel cells, were being developed. Since this development, much research has been targeted to improve the performance of DSC, by understanding the titania, dye, electrolyte interface with respect to dye layer formation, and electrolyte properties. Improving the efficiency and cost effectiveness of dye, using dyes extracted from natural materials and using two or more dyes is being researched. Nanotechnology is being increasingly employed in improving electrode efficiency and many other electrolytes besides the iodide redox electrolytes such as polymer gel electrolyte and organic electrolytes are being explored to obtain higher efficiency of DSC, a discussion of which will be covered in forthcoming sections. 3.0 Structural Components The major components of DSC as shown in figure 2 are electrodes made of nanocrystalline semiconductor oxide film; dye sensitizers, the electrolytes, counter electrode and the transparent conducting substrate.6 Figure 2: Components of DSC11 3.1 Nanocrystalline Semiconductor Film Electrode The semiconductor oxide film acts as carrier of sensitizer monolayer, and is also the medium of electron transfer to conducting substrate. Thus, it is important to have a high efficiency semiconductor electrode with uniform size particles and high surface area, along with a porous structure and an orientation at right angles to conducting surface to enable efficient photogeneration, separation and regeneration of photoelectron.6 The commonly used materials for electrode film are TiO2, ZnO2, SnO2, etc; however, due to various advantages mentioned, TiO2 has been the preferred oxide for this purpose. The surface of the conducting substrate is covered with a thin film of the oxide prepared as colloidal dispersions. These colloids can be made using either of the two methods: Method A: sol-gel procedure of titania deposition Method B: using commercial titania, composed of nanoparticles 25nm in size The next step involves the deposition of oxide on to the conducting surface. This can be done using either of the two processes called doctor blading or screen printing. The preparation of oxide films involves usage of high temperatures (400-450?C), which does not allow the use of flexible organic substrates as a conducting surface, which is important for enhancing the portability of DSC. On the other hand, low temperature annealing methods with a conversion efficiency of 3.0%, compression methods with a conversion efficiency of 4.1%; and a ‘liftoff’ technique using polyethylene terephthalate and a conversion efficiency of 5.8% have been reported.12 An efficiency level of 7.1% with flexible DSC was achieved by Gratzel by 19919, using a titanium metal foil substrate as photoanode and a platinum electrodeposited counter electrode on ITO-polyethylene tetraphthalate (PEN). The major advantage of semiconductor film is that larger number of dye molecules can be adsorbed on its surface, enabling larger number of electrons released, thus allowing for higher rate of solar energy exploitation. However, the large surface area enhances the losses due to interfacial recombination between the electrons of conduction band of semiconductor oxide and of electrolyte electron acceptors; the reduction of these losses has been an area of interest in ongoing research.6 3.2 Dye Sensitizer Dye sensitizers are the solar energy driven electron pumps of DSC and are therefore, the determining factors of DSC efficiency. Efficiency of these sensitizers is determined in terms of their capacity to enable conversion of visible and near infrared photons to electrical energy, hence the prerequisites for an efficient dye sensitizer are: Should be able to absorb all wavelengths below 920nm.9 Should be anchored on to the semiconductor oxide surface.13 Efficiency of transfer of electron to conduction band should be high.6 Should have a high redox potential in order to allow its regeneration by electrolyte or hole conductor donated electrons.4 Should allow for 108redox turnovers, remaining stable throughout.7 Two classes of sensitizers are primarily used in dye sensitized solar cells: a. Organic dye including natural as well as synthetic dyes with organic structures (e.g. chlorophyll derivatives, carotenoids and anthocyanins).14 Some of the common organic dyes with their conversion efficiencies are illustrated in figure 3. b. Inorganic dyes including metal complexes (e.g. ruthenium and osmium polypyridyl complexes15, metal porphyrin16, phthalocyanine17 etc.) Among these, ruthenium(II) complexes which allow diverse modifications at 4, 4’ positions of one of the two bipyridyl moieties, not only enhance molar extinction coefficients, but also exhibit high thermal stability, light utilization and compatibility with common ionic electrolytes.13 Structure of some of the common ruthenium based dye complexes are depicted in figure 4. Figure 3: common organic dyes with their conversion efficiency in DSC6 Figure 4: Molecular structures of Ruthenium based inorganic dye complexes18 The highest conversion efficiency to date has been reported for polypyridyl complexes of ruthenium13 and osmium19. Among these, the thiocyanate derivatives, especially Ru(II) NCS complex N3, the structure of which is shown in figure 5, has shown exceptionally good results.7 The comparative spectral response of two dye sensitizers bipyridine (L) and terpyridine (L’) are shown in figure 6. X-axis represents incident photon to current conversion efficiency (IPCE), where y-axis represents wavelength. While both exhibit high IPCE values in visible range, that of L’ extends more into the near IR range. However, the monodentate NCS impairs the stability of N3 complex; hence, attempts have been made to replace it, one of which has resulted in a cyclometalated ruthenium complex containing 2-(2, 4-Difluorophenyl) pyridine, with a conversion efficiency of 83%.19 Figure 5: Structure of Ru(II) NCS complex19 Figure 6 Photocurrent action spectra of bipyridine and Terpyridine dye along with photocurrent response of plain TiO2 films4 Organic dyes on the other hand, have been found to be more cost effective and easier to design, the basic design being a donor (D)-? conjugated with bridge acceptor (A)6. These dyes not only exhibit intense bands in the visible range, but also have suitable ground and excited states, favouring the functioning of DSC.7 Even though the conversion efficiency of organic dyes is lower than metal based dyes, significant success has been reported in the designing of synthetic organic dyes in recent years.6 Dyes based on chlorophyll, porphyrins20 and phthalocyanins21 have been designed, and have been able to exploit the near IR regions of the solar energy spectrum. However, since the synthesis of organic dyes is a tedious procedure, involving multiple steps of synthesis and purification, alternatives are being explored in the extraction of natural dyes from easily available fruits, vegetables, flowers etc.14 The natural dyes obtained from fruits and vegetables can be studied as possible molecular sensitizers because of their ease of availability and inexpensive extraction procedures.19 The efficiency of natural dyes cyanins and betalains are already established.14 The structures of these natural dyes are shown in figure 7. Red turnip, wild prickly pear and eggplant are now being identified as sources for these dyes.14, 22 Both red turnip and wild prickly pear performed well as molecular sensitizers. With an underlying compact TiO2 layer, a short circuit photocurrent up to 9.4mA/cm2 and a photovoltage of 0.48V was obtained for red turnip extracts. The results were comparable for prickly pear extracts as well, with a with a short circuit photocurrent of 8mA/cm2, which rose to 108mA/cm2 in presence of the blocking layer. Moreover, it was found that the natural dyes performed better than pure synthetic dyes, which can possibly be due to presence of ancillary molecules such as alcohols, organic acids; that improve the efficiency of dyes as molecular sensitizers.14 Figure 8 shows the absorption spectra of dye extracted from red turnip and prickly pear with peak absorption spectra between 450 and 470nm, with maximum IPCE of approximately 60%. Figure 7: General structure of main betalain dyes extracted from red turnip and wild prickly pear. 1. Betacyanin, 2. Indicaxanthin. R1 and R2 = H (betanidin) or R1= ?-D-glucose and R2= H (betanin)14 Figure 8 Photoaction spectra of betalains from red turnip and wild prickly pear on transparent titania19 Sensitization with a single dye, no matter how efficient, is unmatched to cosensitization with two or more dyes since varied spectral responses of these dyes enable harvesting of different regions of the solar spectrum. However, using multiple dyes with different absorption spectra are present, also leads to electron transfer between dyes, thus reducing electron injection to photoelectrode.23 A series of combination of squaraine dyes and ruthenium polypyridyl dyes has been found to exhibit 13% higher efficiency than the latter alone. A combination of two dye sensitizers methyl trans 32 carboxy 8 deethyl 7 ethyl 8 oxo pyropheophorbide a (Bchlorine 1) and methyl trans 32 carboxy 7 deethyl 8 ethyl 7 oxo pyropheophorbide a (Bchlorine 2) has been used by Wang and colleagues. Both dyes when simultaneously used on titania film, possibly due to higher aggregation of B-chlorine 1 compared to B-chlorine 2 - higher annihilation of excited electron, lead to lower current output of the cell. Coadsorption with chenodeoxycholic acid (CDCA) disrupts the dye aggregates of B-chlorine 1 and thereby raises the current output.24 The photoaction spectra of the two dyes shown in figure 9, reveals that both together are able to absorb sunlight throughout the visible region. Figure 9: Photoaction spectra of Bchlorin 1 and Bchlorin 224 3.3 Electrolyte The third component determining efficiency and sustainability of DSC is the electrolyte, which can be of varied types, such as liquid (organic and ionic), solid, and quasi solid electrolyte. Liquid Electrolytes can be either organic solvent or ionic liquid electrolytes.6 The organic solvent electrolytes have been preferred for their high efficiency, easy design, low viscosity and high ionic diffusion rates. They are comprised of three components: the organic solvent such as nitriles (acetonitrile, valeronitrile etc), esters (ethylene carbonate propylene carbonate etc); redox couple such as I3-/I- shown in figure 10, Br-/Br2 etc; and the additive such as 4-tert-butylpyridine, N-methylbezimidazole etc. Of these the I3-/I- has shown the maximum efficiency, with alkylimidazolium cation and lithium cation as its counterions. Additives on the other hand enhance the photoelectric conversion efficiency by reducing the dark current and increasing the photovoltaic fill factor.9 Organic electrolytes however; due to their volatility and instability are gradually being replaced by ionic electrolytes such as 1-alkyl-3methylimidazolium iodides, alkyl pyridinium salts based liquids, trialkylmethylsulfonium salt based liquids etc. Liquid electrolytes, irrespective of their composition, suffer from the major drawback of sealing and leakage. To overcome these shortfalls, solid and quasi solid DSCs are being designed. In these, the redox liquid is replaced by a solid p type semiconductor on which the nanocrystalline titania is adsorbed. This allows the dye molecules to remain neutral even after losing electrons due to the hole transport characteristics of the semiconductor. This is made possible by an interpenetrating network of semiconductor and a charge transporting conducting material spin, coated from the liquid phase on the presensitized nanostructure. The charge transfer material in use at present is spirobifluorene the molecular structure of which is depicted in figure 11. The solid state DSC, first reported in 1998 is still being researched and efficiency up to 3.8% have been obtained till date.4 4.0 Principle The operation principle of DSC, diagrammatically represented in figure 12, is similar to that of photosynthesis involving dyes, which like chlorophyll of green plants, absorb light from the visible region of solar spectrum. In a DSC, dye is placed over the nanostructured semiconductor film deposited on a conductive glass. The dye molecules get excited upon irradiation, and as a result, free electrons are generated. These electrons are injected in to the conducting band of the semiconductor, from where they go to an anode. Simultaneously, there is a charge transfer to the dye from the redox electrolyte, thus leading to regeneration of dye. Herein the cycle is completed; resulting in the conversion of light energy to electrical energy. A voltage equal to the difference of Fermi levels of solid titania and redox potential of electrolyte is produced.26 The stepwise description of the operation of DSC is represented in figures 13 to 18.25 Figure 13 Current generation in DSC, step 1 Figure 14: Current generation in DSC, step 2 Figure 15: Current generation in DSC, Step 3, light absorption Figure 16: Current generation in DSC, step 4 Figure 17: Current generation in DSC, step 5 Figure 18: Current generation in DSC, step 6 5.0 Energetics of DSC The two steps that involve the loss of free energy are introduction of electrons from the excited dye to the titania conduction band, and thereafter, the regeneration of dye by the electrolyte redox couple as shown in figures 19 and 20. The resulting output voltage is equivalent to the difference between the titania Fermi level and redox electrolyte chemical potential.6 Figure 19 : Maximum voltage in DSC25 Figure 20: Kinetics of DSC operation. Blue lines represent forward processes leading to generation of electricity. Gray lines represent energy loss pathways. Vertical line represents the free energy (G) associated with different states.9 6.0 Present Research Due to widespread applicability and attractive future prospects of the DSC, a lot of research has been devoted to improvement of the technology with respect to dye, electrolyte and cathode performance. The major fields of research in these areas are listed in the table below. 6.1 Dye Present researches aim to develop dyes which would enable light harvesting at longer wavelengths. This involves molecular engineering of ruthenium dyes; on one hand to make it capable of harvesting light up to 900nm, and on the other, the use of combinations of organic and ruthenium dyes to achieve absorption up to 920nm, which would increase the short circuit photocurrent from the present limit of 20.5mA cm-2 to approximately 28mAcm-2.4 The major research studies directed at enhancing light harvesting ability of dyes are: Use of Forster resonance energy transfer (FRET) among two or more dye materials. It is found that with suitable dyes the energy transfer efficiency can be as high as 90%. However the performance of suitable dyes with high diffusibility and fluorescence lifetime is lowered due to quenching by the redox couple. Hence further research should aim to develop combinations of electrolyte and dyes which can give high efficiencies in single set up.27 Designing of energy relay dyes with high molar extinction coefficients and solubility in acetonitrile based electrolyte. An energy relay dye 4-(Dicyanomethylene)-2methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) is used in combination with a near IR sensitizing zinc phthalocyanin dye TT1, so that the combined energy efficiency of the two is raised from 3.5 to 4.5%. The molecular structures of the two dyes are shown in figure 21 B and their action spectra are exhibited in figure 21 A.28 The limiting factor in this set up was the solvent acetonitrile. Hence, although the use of energy relay dyes holds promise, research studies are needed for the development of highly soluble energy relay dyes with high molar extinction coefficients. A B Figure 21: A: Action spectra of Energy relay dye DCM alone and in combination with near IR sensitizing dye TT1; B: Molecular structure of DCM and TT128 A porphyrin chromophore, when linked to a donor acceptor dye as a ? conjugated bridge, is found to exhibit a conversion efficiency of 11%, and thus provides a much better cell performance structure of a pophyrin chromophore linked to metal ion Zn as is depicted in figure 22. Metal free dyes with complementary spectral response to porphyrin chromophore, utilizing broader spectral range, can be investigated to further enhance efficiency of DSCs.29 Figure 22: porphyrin chromophore linked to dye29 6.2 Electrode High efficiency electrodes are imperative for enhancing the performance of DSC and the list of major research studies devoted to this are: Designing of mesoporous submicron size titania spheres with high internal surface area. Nanostructures such as nanoparticles, nanowires and nanotubes are usage in order to facilitate higher efficiency, by providing larger surface area for dye absorption and more light scattering ability. Moreover, a thinner photoelectrode to provide lesser chances of recombinations. A saturated photocurrent is obtained for titania nanowires at approximately -0.25 V, and an IPCE of 90% as shown in figure 23. Thus, if titania nanoparticles are of appropriate size, porosity can be synthesized to create a DSC of much higher efficiency than the present limit of 10-11%.2 Figure 23: Photocurrent density of TiO2 nanowires photoelectrode film as a function of measured potential (IPCE spectrum in inset)2 Use of nanostructured titania hollow fibers designed using cellulose fibers for enhanced electron collection efficiency. The photocurrent action spectrum of a DSC with a cellulose template titania, which is hollow fiber sensitized with C101 dye, clearly shows that the conversion yield is above 90% for the spectral range 550-650nm as depicted in figure 24.30 Since the conversion yield is equivalent to calculated yield, the recombination loss is almost negligible. The studies of cellulose fiber templates have a lot of significance considering the easy availability and low cost of cellulose. Figure 24: Absorptance spectrum (?) calculated from measured diffuse reflectance (Rd) and diffuse transmittance (Td) spectra of a cellulose template dye sensitized 6µm thick film of titania hollow fibers. APCE (absorbed photon to current conversion efficiency calculated from IPCE plot and ? plot 30 Designing of hybrid nanosheets with anthraquinone sulfonate anions immobilized onto the surface of nanosheets, which are then applied on to titania spheres in a DSC, as illustrated in figure 25. This provides higher conversion efficiency than a single dye cell, due to the strong photochromic function in the anthraquinone sulfonate present on the hybrid nanosheet.31 Figure 25: Hybrid nanosheets31 6.3 Electrolyte The major research in the field has been dealing with replacement of iodide complex with coordination compounds, with cobalt complexes being the most researched. A list of recently reported research includes: Developments in the field of DSC/CIGS tandem solar cells involving the use of Cu(In, Ga) Se2 cells in combination with DSC. Conversion efficiency up to 20% is obtained using this combination, opening up possibilities of future research to design dual junction solar cells with higher efficiencies.32 Use of polymer gel electrolyte made of poly(ethylene oxide) copolymers including gamma-butyrolactone (GBL) and LiI and I2 exhibiting efficiency up to 4.4%. The intermediate iodide I5- in this set up is a more efficient electron acceptor. The major drawback of this set up is higher rate of charge recombination, which further increases with increase in levels of GBL. The increase in levels of LiI and I2 increase the cation regeneration but also raise the dark current. Thus, considerably more research is needed to optimize the use of polymer gel electrolyte in DSCs.33 An organic redox electrolyte containing disulfide/thiolate redox couple has been investigated to exhibit a conversion efficiency of 6.4% in DSCs. Equimolar concentration of 5-mercapto-1methyltetrazole ion (T-) and its dimer (T2) is taken. Figure 26 shows the spectral response of this new electrolyte in comparison to iodide complex.34 Figure 26: Spectral response of photocurrent DSCs using T2/T- and I3-/I- as the electrolyte34 Device B: 0.4 M T2 and 0.4 M T- Device D: 0.4 M T2 and 0.4 M T- along with additive 4-tert-butylpyridine or TBP (0.4) and LiClO4 (0.05) Device E: 0.4 M T2 and 0.4 M T- along with TBP (0.5M) and LiClO4 (0.05) Device F: DMII/I2(0.8M/0.4M) The IPCE shown by the left ordinate is highest for device E and much more than device F containing the iodide complex as the electrolyte. For the visible range device E gives the highest photocurrent as well.34 Hence with new additives this organic dye can be used to obtain higher efficiency from DSCs. 7.0 Commercial Applications DSC is appropriate for both small scale and large scale commercial applications. The prime application of DSC has been found in building constructions - allowing for integration as walls, windows, and roofs, in varying color and transparency. In these settings, DSCs are able to generate electricity even in diffuse and low light availability. Commonwealth Scientific and Industrial Research Organization (CSIRO) energy centre in Australia has walls with a DSC system built by Dyesol.26 Another application is solar power for mobiles for extra battery life, built by G24i.9 Exhibition house of Toyota at Aichi 2005 expo had two large DSC modules; the efficiency off which was higher than that of a Si panel.2 In fact the DSC modules have been found to have many definite advantages compared to Si panels: In the temperature range of 25-65? C the efficiency of Si PV cells has been found to undergo a decline of approximately 20%. The efficiency of DSC on the other hand remains constant at least for this temperature range. Variations in the angle of incidence do not have much effect on light harvesting efficiency of DSC, though this point needs further substantiation. Even in diffuse light or cloudy weather conditions, DSC has higher output than Si PV cells.9 7.0 Future Prospects Up to 20% efficient laboratory sensitized DSC are expected to be in operation by 2015, with innovative dyes such as those already designed by Mitsubishi. Stable and up to 10% efficient cells are expected to be soon available well within affordable prices.26 Large scale applications of the technology are expected to be practically viable and cost effective with the new advances. Engineering at the molecular levels can be used to manipulate interfacial charge recombinations in favor of higher conversion efficiency. There is also scope of improvement in open circuit voltage gains by alterations in meso structures of electrode oxides. Hybrid cells, nanostructured ETA cells and many others yet to be developed technologies make the future of this still nascent technology bright.9 Finally, the ultimate success of this technology would be a device that can successfully split water to hydrogen and oxygen, and use the energy of this photoconversion in production of high energy chemicals.7 The theoretical possibility of this technology and its remarkable impact on energy scenario will definitely make it the focus of researches in years to come. 8.0 Conclusion Using the much studied principle of photosynthesis, a unique solution to forthcoming energy problems is expected to emerge. Researchers have successfully demonstrated a conversion efficiency of 11% for single junction DSC and up to 15% for tandem liquid electrolyte DSC. After thorough investigation of DSC, this paper is compiled to provide a conclusion based around three chief concepts. Through examination of the history of renewable solar energy in the past, it is evident that recent developments in science and technology indicate that DSC may become the next revolution of renewable, sustainable and economical energy resources. The second theme of the paper discussed the mechanisms and underlying principles of Nanocrystillaline Semiconductor Film Electrodes, Dye Sensitizers and Electrolytes to provide insight to the three key, though not all features of the DSC. Further enhancements of current research of these primary elements in the near future suggest that higher conversion efficiency of DSC is possible and may have greater prospects for commercialization. However, some of these components need improvisations due to their limited performances. 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