Thursday, January 26, 2012

And it was called 'Yellow'

Nearly every month there are fresh articles reporting news of further possible advances in solar technology. Most research aims at either improving the efficiency of the cells by capturing more of the electromagnetic spectrum. Either some modification of the crystal structure modifying the electron band gap or splitting the different wavelengths so that they are captured by semiconductors selective for these specific wavelenghts or by using organic dyes to help capture photons.

Other approaches aim to reduce the cost of solar by using less efficient systems that can be cheaply applied on large easily manufactured surfaces that can be incorporated into existing buildings and construction systems.

However, I'm not so sure that re-marketing Cadmium Yellow as "a paste of semiconducting nanoparticles called solar paint" is the smartest approach.

Solar Cells From A Paintbrush
Chemical and Engineering News Dec 14, 2011.

To fabricate solar cells, engineers may someday trade their clean rooms for a paint smock. Researchers have developed a paste of semiconducting nanoparticles called solar paint that could lead to cheaper and easier-to-produce solar cells.

Many researchers working on solar energy have focused on improving the efficiency of silicon-based solar cells. But silicon devices have a high price tag because of the specialized protocols and equipment needed, says Prashant Kamat of the University of Notre Dame. He points out that some researchers have avoided using silicon and created cells using quantum dots made from materials like lead sulfide. Unfortunately, the fabrication process for these quantum dot solar cells is still expensive and slow. To simplify and lower the costs of solar cell production, Kamat and his colleagues wanted to develop quantum dot materials that someone could paint onto any conductive surface without needing special equipment.

The quantum dots in these so-called solar paints are multi-layered nanoparticles. Each particle has a titanium dioxide nanoparticle core coated with compounds that can absorb photons, either cadmium sulfide or cadmium selenide. When a photon with the right energy hits the cadmium compounds, an electron escapes and TiO2 absorbs it.

The researchers used two methods for coating the TiO2 nanoparticles with the cadmium compounds: They physically mixed the ingredients together or used a process called the pseudo sequential ionic layer adsorption and reaction method to deposit CdS or CdSe onto the cores. To produce electrodes, the scientists suspended the nanoparticles in a water-alcohol mixture and then brushed the paste onto a transparent conducting material. They then tested the light-to-electricity conversion efficiency of cells built from these electrodes, using a cathode made from other materials and additional compounds to replenish the electrons lost by the cadmium compounds.

The best performing cell used paint containing a mixture of the CdS- and CdSe-coated TiO2 nanoparticles. Its light-to-electricity efficiency was 1%. The efficiencies of commercial silicon solar cells are usually between 10 and 15%.

I question this idea because of the potential release of a known toxic carcinogen into the environment as the material leaches out of the roofing materials used to construct these cells. This is especially relevant to Australia as Cd containing roofing materials may preclude rainwater harvesting. As we dig into the lower quality (higher cadmium content) deposits of phosphate to maintain agricultural output, we already face the potential for increased Cd exposure (see this old report).

I also tire of people using the word nanoparticle for common industrial chemicals (unless there is something particularly new about these particular TiO2 particles?).


Anonymous said...

Low efficiency systems/cells have problems later on... it's not supposed to be deployed just for solving the short term problem..

SP said...

Is it worth deploying at all if it creates a longer term health problem that is arguably even more difficult to manage?

US OSHA - Cadmium

Cadmium Banned in Europe in November

Cd is increasingly restricted in application. My guess is that promoters of this coating are going to have be able to guarantee no leaching at all, and probably have to manage the recycling of the product.

Remember, this kind of cell would require very large areas with a surface film containing high levels of Cd.