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t he concept of DSSC was first presented in 1991 by Michael Grätzel and Brian O’Regan. Grätzel won the Millennium Technology Prize in 2010 for his work in the area of DSSCs. The DSSC is a photoelectron chemical device. Generally, a dye-sensitized solar cell consists of three main components: a dye coated nanocrystalline TiO2 layer on a transparent conductive glass substrate, an iodide/

triiodide redox couple in an organic solvent as an electrolyte, and a platinum film having high electrocatalytic activity coated on conductive glass as counter electrode; its operation is similar to that of photosynthesis. The operation of DSSC can be explained

recent deVelopment In dye-sensItIzed solar cellsDye-sensitized solar cells (DSSCs) are emerging as third generation photovoltaic (PV) device, due to their low production cost and high conversion efficiency. This review describes recent developments in dye-sensitized solar cell research with a focus on development of material and method.

swapna ojah, ranjIth g. naIr and s. K. samdarshI

Dye-sensitized solar cell

RE F e at u r e

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plants and bacterIa use

porphyrIn based chromophores For conVertIng

solar energy Into chemIcal energy.

in three steps: first the dye molecule absorbs the photon and gets excited. The excited dye molecule is then given off to the conduction band of semiconductor and it gets oxidized. After that the transparent conducting oxide (TCO) layer collects the excited electron from the conduction band and the electrons flow from the external load to the counter electrode. Finally the oxidized dye molecule is reduced by gaining electrons from the electrolyte solution (Fig 1). DSSC is the only device that absorbs photon and converts them to electric charge without the need of intermolecular transport of electronic excitation. In conventional solar cells both light absorption and charge carrier transport were performed simultaneously, whereas in DSSC the two operations are performed separately. An energy conversion efficiency of more than 11 per cent has been achieved in DSSCs with an organic liquid-based electrolyte containing I3-/I- as a redox couple.

present dssc research and deVelopment dye: The dye absorbs the photon and creates an electron—it allows the electron to be injected into the conduction band of the semiconductor. There are two classes of dyes: organometallic and organic dyes. Organometallic dyes contain a transition metal in the structure while organic dyes can be indoline-derived dyes, porphyrin-based complexes, benzothiazole merocyanine based dyes, oligoene dyes, and coumarin derivatives. Currently the most efficient DSSCs are ruthenium-based sensitizers such as N3, N719, Z907 and black dye, which have achieved remarkable power conversion efficiency of 10-11 per cent under standard global air mass 1.5 (AM 1.5) illumination. However, disadvantages such as rarity, high cost, relatively low extinction coefficient, hard to be purified and environmental pollution restrict its large scale production. Also Ru dyes do not absorb in the infrared region except black dyes that absorb light up to a wavelength of 900 nm but is very expensive and produces lower VOC due to recombination problems. This has led to the search for cheaper and safer organic based dyes. Organic dyes have many advantages over Ru based dye such as large molar extinction coefficient, control of absorption wavelength, facile design and synthesis and lower cost than Ru complex. Though a dye sensitized electrolyte (DSE) using organic dye does not give high conversion efficiency, it allows the making of thinner DSE.

Plants and bacteria use porphyrin based chromophores for converting solar energy into chemical energy. Campbell et. al. synthesized a novel green porphyrin sensitizer in which the aryl group acts as an electron donor and malonic acid binding group as an acceptor; this gives conversion efficiency upto 7.1 per cent under illumination AM 1.5.

Hwang et. al. synthesized an organic dye TA-St-CA which contains a p-conjugated oligo-phenylenevinylene unit with an electron donor–acceptor moiety for intramolecular charge transfer and a carboxyl group as an anchoring unit for the attachment of the dye onto TiO2 nanoparticles, and found that the solar energy conversion efficiency is 9.1 per cent at 1.5 AM illumination. Under same conditions the efficiency of N719 was 10.1 per cent.

Mater et. al. synthesized a new ruthenium polypyridyl dye TG6, and compared it with N719 dye and found that solar to electricity conversion efficiency is 0.2 per cent higher than N719 DSSC with 12µ m thick TiO2 film and EL01 as electrolyte at a slightly higher voltage. The efficient performance of TG6 dye is attributed to the high absorption extinction coefficient and extended absorption in the visible region of the solar spectrum.

Lin et. al. designed and synthesized two new organic sensitizers (TPCADTS and TP6CADTS) containing coplanar diphenyl-substituted dithienosilole as the central linkage for high-performance dye-sensitized solar cells. By incorporating the diphenyl-substituted dithienosilole (DTS) core, these two dyes exhibited enhanced light-

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capturing abilities and suppressed dye aggregation. These two dyes were used as the organic sensitizers in DSSCs and it was found that solar-cells based on the sensitizer TPCADTS and TP6CADTS yielded a overall conversion efficiency up to 6.65 and 7.6 per cent, reaching about 80 and about 96 per cent respectively of the ruthenium dye N719-based reference cell under the same conditions.

working electrode: The working electrode absorbs dye molecules and conducts photoelectrons. Generally semiconductors with wide band gap like TiO2, SnO2, ZnO, NbO5and SrTiO3 are used as a photoanode. The desirable properties of the working electrode are large surface area to absorb large amounts of dye and high electron diffusion to assure its conduction to the conductive substrate before recombination.

Usually pure TiO2 is used as the electrode which is oxygen deficient; this oxygen deficiency can create electron hole pairs. The oxidizing holes can either react with the dye and destroy it or are scavenged by iodide ions, thus reducing the lifetime of the dye-sensitized solar cell. To solve these problems, Ma et. al. introduced nitrogen-doped titania into the DSSC system to enhance the internal photonic conversion efficiency (IPCE) and to stabilize the solar cell by replacing oxygen deficient titania by visible-light-active, nitrogen-doped titania. An overall conversion efficiency of 8 per cent has been achieved.

For commercial production of DSE the use of TCO layer is not suitable because the cost increases significantly, therefore Park et. al. fabricated a new Ti-mesh electrode for high-efficiency, low-cost solar cell application that replaces the TCO. Thus the cell structure is composed of a glass/dye sensitized TiO2 layer/Ti-mesh electrode/electrolyte/metal counter electrode.

To suppress the recombination and improve the transport of photogenerated electrons across the TiO2 nanoparticle network, Yang et. al. incorporated 2D graphene into the TiO2 nanostructure photoanode to form graphene bridges in DSSCs and found that graphene can enhance the charge transport rate to prevent charge recombination and increase light collection efficiency, so that the total conversion efficiency was increased by 39 per cent, compared to the nanocrystalline titanium dioxide photoanode.

Lee et. al. reported that the charge recombination from TCO to electrolyte can be reduced when Nb-doped TiO2 (NTO) is deposited on a FTO substrate, and the resulting NTO/FTO is used in DSSC as an electrode. The NTO compact layer also reduced the interfacial resistance and enhanced the PV conversion efficiency by 21.2 per cent compared to bare FTO based DSSE.

FIg 1: the working of a dye-sensitized solar cell

Light

Electrolyte

Counter Electrode

Transparent Electrode

3l-l3

-

TiO2

RECEnT DEVELoPmEnT in DyE-SEnSiTizED SoLaR CELLS

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counter electrode: Usually, a thin layer of platinum (Pt) catalyst deposited on transparent conductive oxide substrates is employed as a counter-electrode in DSSCs. Since Pt is expensive, rare and widely in demand as a catalyst in various chemical and electrochemical fields, many researches are looking to find an alternative to replace Pt. Cha et. al. reported Pt-free transparent counter electrodes for DSSC, which are fabricated using carbon nano-tube (CNT) micro-balls with dense CNT packing. When the transparency of a CNT micro-ball transparent counter electrode is more than 70 per cent, the energy conversion efficiency of its DSSC is 80 per cent more than the one prepared using a counter electrode consisting of Pt nanoparticles.Ramasamy et. al. reported highly efficient liquid and quasi-solid DSSCs based on large-pore sized mesoporous carbon counter-electrodes. In combination with dye sensitized TiO2 working electrode, liquid and quasisolid DSSCs show 8.18 and 3.61 per cent, respectively, energy conversion efficiency under one sun condition. Wei et. al. reported a new type of counter electrode that consisted of substrate, aluminium film and catalyzed platinum film. This new counter electrode has many advantages over a Pt counter electrode - simple preparation, low cost, low resistance, high reflectance and so on. This improves the photoelectric conversion efficiency from 3.46 to 7.07 per cent.

electrolyte: The electrolyte reduces oxidized dye and transport holes in the cell. The conventional solvents used in DSC’s electrolyte are some organic solvents such as c-butyrolactone, acetonitrile and 3-methoxypropionitrile. However, all of them are normally poisonous and volatile, which limits the DSSC industrialization. The air and water stable room-temperature ionic liquids (RTILs) are attractive due to their characteristics such as chemical and thermal stability, negligible vapour pressure, no flammability, high ionic conductivity and a wide electrochemical window. Though liquid electrolyte has achieved high efficiency, there are some disadvantages such as evaporation of electrolyte, health hazards, temperature stability problems, sealing problem etc. Generally, ionic liquids based on imidazolium salts are widely used as solvents for DSSCs. But, pure imidazolium iodide/triiodide room temperature ionic liquids are too viscous and obstruct the diffusion of the redox couple (I-/I3-) in the electrolyte, and hamper the device performance. To overcome these problems, many researches are looking to replace the liquid electrolytes with solid or quasi-solid-type charge transport materials.

Bhattacharya et. al. developed a new kind of low-viscosity ionic liquid (1-ethyl 3-methyl imidazolium thiocyanate) solid electrolyte i.e. an ionic liquid doped solid electrolyte (ILSE). Due to an improvement in the number of ionic charge carriers provided by the ILSE it showed better conductivity. The PV performance of the solar cells can be improved by the doping of ionic liquid.

Lan et. al. prepared gel electrolyte by using TiO2 gel to solidify liquid electrolyte which contained an organic iodide salt N-methyl pyridine iodide as I- source. The energy conversion efficiency of light-to-electricity of 3.06 per cent was achieved under irradiation of 60 mW cm-2. Guo et. al. developed a new ionic liquid S-propyltetrahydrothiophenium iodide (T3I) as the solvent and iodide ion source in electrolyte for dye-sensitized solar cells. The photoelectric conversion efficiency of the cell is 3.51per cent under one sun (AM 1.5). Jhong et. al. synthesized eutectic mixture of glycerol and choline iodide (G.CI) as electrolyte for dye-sensitized solar cells. The energy conversion efficiency is found to be 3.88 per cent under AM 1.5, 100 mW/cm2 illuminations. b

The authors are Research Scientist, Researcher, and Professor, respectively, at Solar and Energy Material Laboratory, Department of Energy, Tezpur University, Assam. Email-swapnaojah@gmail.com

In the conVentIonal solar cell both lIght

absorptIon and charge carrIer transport were perFormed sImultaneously,

whereas In dsc the two operatIons are

perFormed separately.