Empa researchers have developed a photoelectrochemical cell made of cheap raw materials that recreates a moth’s eye to drastically increase its light collecting efficiency.

Empa Hydrogen Producing Solar Cell. How the "moth eye solar cell" is created, and how it collects light. Image Credit: Empa. Click image for the largest view.

Empa Hydrogen Producing Solar Cell. How the “moth eye solar cell” is created, and how it collects light. Image Credit: Empa. Click image for the largest view.

World wide researchers are investigating solar cells using sunlight and water that imitate plant photosynthesis to create synthetic fuels such as hydrogen. The Empa cell is made of cheap raw materials – iron oxide and tungsten oxide.

The Empa effort, a Swiss ETH Research Institute project, uses iron oxide or rust as we see it, to revolutionize solar cell technology. Usually an unwanted material, rust can be used to make photoelectrodes which split water and generate hydrogen.  The team’s paper explaining the new technology has been published in the journal Energy & Environmental Science.

The Empa electrode difference is it directly converts sunlight into valuable fuel rather than first being used to generate electricity and then drive electrolysis.

But as a raw material iron oxide has its limitations. Although it is very cheap and absorbs light in exactly the wavelength region where the sun emits the most energy, it conducts electricity very poorly and must therefore be used in the form of an extremely thin film in order for the water splitting technique to work. The disadvantage of this is that these thin-films absorb too little of the sunlight shining on the cell.

Empa researchers Florent Boudoire and Artur Braun have now succeeded in solving this problem. A special microstructure on the photoelectrode surface literally gathers in sunlight and does not let it out again. The basis for this innovative structure are tiny particles of tungsten oxide which, because of their saturated yellow color, can also be used for photoelectrodes. The yellow microspheres are applied to an electrode and then covered with an extremely thin nanoscale layer of iron oxide. When external light falls on the particle it is internally reflected back and forth until finally all the light is absorbed. All the solar energy in the beam is now available to use for splitting the water molecules. Boudoire explained that in principle the newly conceived microstructure functions like the eye of a moth.

Moths are active night creatures that need to collect as much light as possible to see in the dark, and also must reflect as little as possible to avoid detection and being eaten by their predators. The microstructure of their eyes especially adapted to the appropriate wavelength of light.

The Empa photocells take advantage of the same effect. In order to recreate artificial moth eyes from metal oxide microspheres, Boudoire sprays a sheet of glass with a suspension of plastic particles, each of which contains at its center a drop of tungsten salt solution. The particles lie on the glass like a layer of marbles packed close to each other. The sheet is placed in an oven and heated, the plastic material burns away and each drop of salt solution is transformed into the required tungsten oxide microsphere. The next step is to spray the new structure with an iron salt solution and once again heat it in an oven.

The simple process mixing, spraying and burning seems almost too easy and primitive, a series of steps that is eventually successful only by pure chance. However in parallel to their practical experiments, the researchers have been running calculations modeling the process on their computers and have thus been able to simulate the “capturing of light” in the tiny spheres.

Project leader Artur Braun pointed out the results of the simulation agree with the experimental observations. It is clear to see how much the tungsten oxide contributes to the photo current and how much is due to the iron oxide. Also, the smaller the microspheres the more light which lands on the iron oxide underneath the tiny balls.

As a next step the researchers plan to investigate what the effect of several layers of microspheres lying on top of each other might be making the work on moth eye solar cells an effort still in progress.

The concept seems to have very good potential. So far no efficiency notations are available, but its quite early. Just skipping the electric to electrolysis step is going to help as well as greatly simplifying any cell design that might scale up.

The new cell works, and that alone is worthy of congratulations.


1 Comment so far

  1. eamcet on June 20, 2014 7:19 AM

    You are correct really there is New Way to Make Hydrogen with Solar Power thanks for the information

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