Organic-semiconductor-based all-solid-state photoelectro chemical cells
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Organic-semiconductor-based all-solid-state photoelectrochemical cells Robert Hudej, Egon Pavlica and Gvido Bratina Nova Gorica Polytechnic, Vipavska 13 SI-5001 Nova Gorica, Slovenia Urška Lavrencic- Štangar, Angela Šurca Vuk and Boris Orel National Institute for Chemistry, Hajdrihova 19, SI-1000 Ljubljana, Slovenia ABSTRACT The solid-state solar cells comprising dye-sensitized nanostructured SnO and vacuum-evaporated 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) layers exhibit significant photoresponse. Despite of unfavorable electronic energy level alignment at the dye/PTCDA interface, photon-to-electron conversion efficiencies as high as 1% were observed. INTRODUCTION Dye-sensitized photochemical solar-cells (DPSCs) based on highly porous nanocrystalline films of titanium dioxide [1] have attracted attention due to an alternative low-cost, high-efficiency compared to conventional silicon-based solar cells. A typical DPSC is supported by a SnO -coated glass substrate onto which a several micrometer thick nanocrystalline metal-oxide film comprising adsorbed Ru-bipyridyl-based dye is deposited. The dye-coated metal oxide crystallites are in contact with the electrolyte solution containing iodide and triiodide ions. A second electrical contact is provided with Pt-coated SnO deposited on a glass support. The light/electrical current conversion proceeds via light absorption by the dye molecules. For an adequate energy offset between the excited levels of the dye and the minimum of the metal oxide conduction band, the excited electrons are injected from dye into the metal-oxide conduction band and collected by a transparent SnO electrode. The electrolyte reduces the oxidized dye molecules by I ions completing thereby the electric charge current. _
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Despite of impressive conversion efficiencies, such design suffers from a major drawback related to difficulties of encapsulation of the liquid electrolyte. Therefore, increasing effort is being invested in replacing the liquid electrolyte with a solid-state equivalent. Currently, organic semiconductors (OS) and ionic conducting polymers appear to be the most promising materials [2, 3, 4], although the efficiency of all solid state solar cells remain lower than their liquid-electrolyte-based counterparts. The principle of operation of a solid-state DPSC is based on dye excitation and electron injection into a mesoporous wide-band-gap semiconductor. The OS layer serves as a dye-regeneration medium. The excited dye molecules are regenerated by hole injection into the OS layer. The hole mobility in OS layer must be threfore relatively high, and the energy offset at the dye-OS interface favoring hole injection into the OS layer. The mesoporous structure of metal oxide plays an important role in increasing
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