domingo, 25 de julio de 2010

Use of sol–gel hybrids for photovoltaic cell application

Up to now, the main photovoltaic (PV) devices are based on solid-state junctions, usually made of silicon and are profiting from the development of the semiconductor industry. In this situation, a challenging new generation of solar cell is emerging, based on the use of interpenetrating network of nanocrystalline oxide and conducting electrolyte. The so-called dye-sensitized solar cell (DSC) constitutes a promising technical and economical alternative to p–n junction photovoltaic devices. Unlike the conventional silicon systems in which the semiconducting material supports both the light absorption and charge carrier transportation functions, in the DSC the two tasks are split. Light is absorbed by a sensitizer, which is anchored to the surface of a wide band gap oxide semiconductor. Charge separation takes place at the interface via photon-induced electron injection from the dye into the conduction band of the solid. Charge carriers are then transported in the conduction band of the semiconductor to the charge collector. The use of sensitizers having a broad absorption band in conjunction with oxide films of nanocrystalline structure allows the collection of a large fraction of sunlight. The intimate penetrating network of mesoscopic oxide and conducting electrolyte competes today with the conversion efficiency of conventional solid-state devices. They reach in the case of TiO2 as semiconductor, ruthenium-based complex as dye and redox couple-containing liquid as electrolyte, an overall solar (standard AM 1.5) to current conversion efficiency of 10.4%.189 Although the efficiency oflight conversion is spectacular in these systems, one limitation is the liquid electrolyte-based DSC approach using low viscosity, highly volatile solvents (e.g. acetonitrile) which hinder commercialization spreading. A number of solutions have been proposed to solve this problem. One includes replacement of the liquid junction with a conducting ion gelelectrolyte,190 with inorganic materials such as p-type CuI CuSCN or with organic hole conductors such as triphenyldiamine (TPD) or polypyrrole191–195 or substituted polythiophenes.

196,197 This third generation of PV cell is very promising and overall efficiencies from 3.2 up to 3.8% have been obtained.198,199 Such all-solid-state organic–inorganic hybrid dye-sensitized solar cells intend to eliminate practical problems of liquid electrolyte use like degradation with time, cell sealing, handling, technological aspects of module production. Nevertheless, one needs to completely understand and control the interface between the organic and the inorganic portions of the PV cell in order to facilitate reactions occurring at the interface. Among the critical parameters, a major one is the spreading of the interaction between each working layer of the PV cell stack. Especially, the inorganic layer plays an important role in hybrid DSC, accepting an electron from the excited state of the dye and preventing recombination of the newly formed electron–hole pair. In order to increase the efficiency of the sunlight harvesting area, a fractal TiO2 semiconductor oxide layer prepared by the sol–gel chemical method has been replaced with a nanostructured thin film first deposited from colloidal suspensions. This route evidently provides a much more reproducible and controlled interacting porous high surface area. The optimized material used in hybrid solar cells is prepared using the hydrothermal synthesis of Brooks,200 which enables the preparation of size-controlled nanoparticles impacting onto the nanostructure and porosity of the semiconductor layer. In this approach, there is no need to dope the oxide film since the injection of one single electron from the surface adsorbed sensitizer into a TiO2 nanoparticle is enough to make it conductive. Moreover, such mesoporous thin films having high surface roughness do not promote charge carrier loss due to recombination. Among new trends, new developments are directed towards synthesis of structured new materials having desirable morphology like mesoporous channels or nanorods aligned perpendicular to the transparent conducting oxide glass.201,202

A new promising work reports the development of mesostructured materials using the evaporation-induced selfassembly of surfactant templates in the reaction medium.203,204 Porous ordered structures of mesostructured semiconductor oxides compared to former random particle networks are supposed to simultaneously optimize the P–N surface area, the pore filling with the hole conductor polymer, charge carrier transportation and percolation. The use of an inorganic mesophase structure in the PV cell opens a large field of investigation in relation with the numerous accessible organized structure of the porosity (like the 3-D cubic Im3m phase) which can be optimized for enhanced energy conversion efficiency and therefore solar cell performance.
Asignatura: CRF
Fuente: Journal of Materials Chemistry

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