Published on September 5th, 2013 | by James Ayre2
Solar Cell Energy Efficiency And Lifespan Improved With Ion-Conducting Polymer
Dye-sensitized solar cells will soon receive a big boost to their energy efficiency and durability/lifespan thanks to new research from Stockholm’s KTH Royal Institute of Technology. The improvements are thanks to the discovery of a previously unknown quasi-liquid, polymer-based electrolyte — one that works to notably increase a dye-sensitized solar cell’s voltage and current, while at the same time lowering the resistance between its electrodes.
The new findings emphasize the advantages/possibilities of speeding up the movement of oxidized electrolytes in a dye-sensitized solar cell (DSSC) — leaving open the possibility for further improvements through similar means.
“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,” states James Gardner, Assistant Professor of Photoelectrochemistry at KTH. “The fast transport increases solar cell efficiency by 20 percent.”
The press release from KTH the Royal Institute of Technology has more:
A dye-sensitized 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 sensitized with dyes that absorb visible light. The electrons in the semiconductor diffuse through the anode, out into the external circuit.
In the 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 oxidized by the titanium dioxide, the electrolyte supplies electrons to replenish the deficiency. This “resets” the dye molecules, reducing them back to their original states. As a result, the electrolyte becomes oxidized and electron-deficient and migrates toward the cathode to recovers its missing electrons. Electrons migrating through the circuit recombine with the oxidized 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. But in order to build a stable, commercially-viable solar cell, a low volatility solvent is used instead, usually methoxypropionitrile. The problem is that while methoxypropionitrile is more stable, it is also more viscous than acetonitrile, and it impedes the flow of ions.
But with the introduction of a new quasi-liquid, polymer-based electrolyte (containing the Co3+/Co2+ redox mediator in 3-methoxy propionitrile solvent), the research team has overcome the viscosity problem. At the same time, adding the ion-conducting polymer to the electrolyte maintains its low volatility. This makes it possible for the oxidized form of the cobalt complex to reach the cathode, and get reduced, faster.
And the faster that this transport occurs, the less that the cobalt complexes can react with the electrons in the semiconductor anode — leading them to instead react with the electrons at the cathode. This lowers the resistance, as well as increasing the voltage and the current in the dye-sensitized solar cell.
The new research was just published in the Royal Society of Chemistry’s journal, Physical Chemistry Chemical Physics.