Screen-Printed Dye-Sensitized Large Area Nanocrystalline Solar Cell
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film solar cells available in the market with efficiencies - 15-17%) [4]. The main difficulties in achieving higher efficiencies with these screen printed PEC cells are the large dark currents arising mostly from the semiconductor-electrolyte junction [5]. The photoelectrode of a PEC cell consists of a 12-18 prm screen printed film of nanocrystalline TiO 2 particles (5-25 nm in diameter) with a monolayer of adsorbed dye molecules. An electrolyte is applied on the dye-soaked TiO, by screen printing. Platinum deposited conducting glass serves as a counter electrode. Upon photoexcitation, the dye molecules inject electrons efficiently into the TiO 2 conduction band, effecting charge separation. The injected electrons traverse the nanocrystalline film and are collected at the conducting surface of the glass substrate. These electrons then traverse through the external load and re-enter the cell at the counter electrode and reduce the electrolyte, which then diffuses into the pores of the dye-soaked TiO 2 film to reduce the photoexcited dye back to its original state. Figure 1 illustrates the operating principle of the dye sensitized cell. In this paper we report the fabrication of a large 15 cm x 15 cm cell by a commercially viable screen printing method using nanocrystalline films of TiO 2 in aqueous solution. The efficiency of these large cells are compared to the laboratory made small cells.
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Mat. Res. Soc. Symp. Proc. Vol. 581 ©2000 Materials Research Society
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Figure 1. Dye-sensitized operation principle of TiO 2 solar cell. The photoexcited dye (S/S*) transfers an electron to the semiconducting TiO 2 layer via electron injection. The electron is then transported through the rough, porous TiO 2 layer and collected by the conductive layer on the glass. Within the electrolyte, the mediator (I-/13) undergoes oxidation (and regeneration). The electrons lost by the dye to the TiO 2 are replaced by the iodide, resulting in iodine or triiodide, which in turn obtains an electron at the catalyst-coated counter electrode as current flows through the load [8]. EXPERIMENTAL (a) Preparation of the TiO 2 Slurry (ink) for screen printing: Nanocrystalline TiO 2 powders were procured from MIT, Cambridge, Massachusetts and Degussa, Germany. 12 g TiO 2, procured either from Degussa or from MIT, was ground in a porcelain mortar with a small amount of water containing acetylacetone. The product was diluted with de-ionized water and ground again. The final ground material was thoroughly mixed with ethylene glycol and the viscosity of the material was adjusted by thoroughly mixing with a low-melting point glass. Finally, a detergent was added to make the final slurry ready for screen printing. The slurry thus prepared was screen printed using a 230 mesh silk screen. The screen printed films were air dried for 30 minutes followed by sintering - 480' C. Film thickness was measured with a digital gauge and with a Dek Tak profilometer [Dek Tak 8000] after firing. (b) Physical Characterization Surface morpho
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