Swedish researchers have used a new quasi-liquid, polymer-based electrolyte to increase voltage and current of dye-sensitised solar cells.
The material enabled increasing the speed of movement of the oxidised electrolyte inside a dye-sensitised solar cell, improving its efficiency by 20 per cent.
“We now have clear evidence that by adding the ion-conducting polymer to the solar cell’s cobalt redox electrolyte, the transport of oxidized electrolytes is greatly enhanced,” said James Gardner, Assistant Professor of Photoelectrochemistry at Stockholm’s KTH Royal Institute of Technology.
The results of the study have been published in the August issue of Physical Chemistry Chemical Physics journal of the Royal Society of Chemistry.
The new quasi-liquid, polymer-based electrolyte (containing the Co3+/Co2+ redox mediator in 3-methoxy propionitrile solvent) addresses some of the key problems of dye-sensitised solar cells.
In an electrolyte, a cobalt complex redox shuttle acts as a catalyst, providing the internal electrical continuity between the anode and cathode. When the dye releases electrons and becomes oxidised by the titanium dioxide, the electrolyte supplies electrons to replenish the deficiency. This resets the dye molecules back to their original states. As a result, the electrolyte becomes oxidised and electron-deficient and migrates toward the cathode to recover its missing electrons. Electrons migrating through the circuit recombine with the oxidised form of the cobalt complex when they reach the cathode.
In the most efficient solar cells this transport of ions relies on acetonitrile, a low viscosity, volatile organic solvent. In order to build a stable, commercially-viable solar cell, however, a low-volatility solvent has to be used, usually methoxypropionitrile.
But the methoxypropionitrile, apart from being more stable is also more viscous than acetonitrile, inhibiting the flow of ions in the electrolyte. Slowing down the move of electrons does not only reduce the efficiency but also contributes to faster aging as more of the cobalt complexes react with electrons in the semiconductor anode instead of with the electrons at the cathode, resulting in rapid recombination losses.
The new electrolyte used by the Swedish team offers optimal characteristics of both the methoxypropionitrile and acetonitrile.
Dye-sensitised solar cells draw inspiration from photosynthesis. These cells are believed to eventually lead to development of low-cost solar photovoltaics and, when coupled with catalysts, could even generate hydrogen and oxygen, just like plants.
A dye-sensitised solar cell absorbs photons and injects electrons into the conduction band of a transparent semiconductor. This anode is actually a plate with a highly porous, thin layer of titanium dioxide that is sensitised with dyes that absorb visible light. The electrons in the semiconductor diffuse through the anode, out into the external circuit.