Electrolyte improves dye-sensitised solar cells

Drawing their inspiration from photosynthesis, dye-sensitised solar cells are said to offer the promise of low-cost solar photovoltaics and – when coupled with catalysts – present the possibility of generating hydrogen and oxygen.

The team - including KTH’s James Gardner, assistant Professor of Photoelectrochemistry, Lars Kloo, Professor of Inorganic Chemistry and researcher Muthuraaman Bhagavathi Achari - reported the success of a new quasi-liquid, polymer-based electrolyte that increases a dye-sensitised solar cell’s voltage and current, and lowers resistance between its electrodes. 

The study is claimed to highlight the advantages of speeding up the movement of oxidised electrolytes in a dye-sensitised solar cell (DSSC). Their research was published in the Royal Society of Chemistry’s journal, Physical Chemistry Chemical Physics on August 19.

‘We now have clear evidence that by adding the ion-conducting polymer to the solar cell’s cobalt redox electrolyte, the transport of oxidised electrolytes is greatly enhanced,’ Gardner said in a statement. ‘The fast transport increases solar cell efficiency by 20 per cent.’

A dye-sensitised solar cell absorbs photons and injects electrons into the conduction band of a transparent semiconductor. This anode is 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.

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 oxidised 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 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 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.

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, Gardner said.

At the same time, adding the ion-conducting polymer to the electrolyte maintains its low volatility, making it possible for the oxidised form of the cobalt complex to reach the cathode, and get reduced, faster.

Speeding up this transport is important because when slowed down, 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. Speeding up the cobalt lowers resistance and increases voltage and current in the solar cell, Gardner said.