Harvesting dim light with synthetic photosynthesis

Think of solar power, and what comes immediately to mind are large, fixed arrays of photovoltaic cells operating in bright sunshine. But can useful amounts of energy be extracted on smaller scales in dim light conditions? Plants and some bacteria seem to manage it, so perhaps there is a way of mimicking the photosynthesis that turns light into chemical energy.

We are still at a very early stage in our understanding of plant photosynthesis, but a number of researchers are looking at an altogether more tractable problem: how some bacteria manage to harvest light, even deep under water.

Bacteriochlorophyll-c molecule

Deep-water bacteria such as Chlorobium tepidium and Chloroflexus aurantiacus grow rod-like structures that contain thousands of light-gathering pigment molecules, or chromophores, in stacks around 100 nanometres long. Silviu Balaban of the Institute of Nanotechnology in Karlsruhe, Germany, is one of a number of researchers studying such phototropic bacteria.

“We are concerned with mimicking self-assembly of chromophores with completely synthetic molecules, and this self-assembly principle operates in some early photosynthesising bacteria,” explains Balaban, who with others intends to incorporate synthetic chromophores into new kinds of solar cells, including thin films that could form the basis of low-power, portable power generators. Balaban’s group have so far identified a number of molecules that act as a glue between the pigment molecules.

Balaban adds: “We took the same recognition algorithm from photosynthetic bacteria, and applied this to molecules which can be made synthetically in the lab on a large scale.”

Although they are unlikely to achieve the 97% absorption rate of deep-water bacteria, the researchers hope to significantly improve on existing organic cells, and need only 5% efficiency for the devices to be profitable. The record to date for organic, plastic-based solar cells is around 4.5%, so we could soon see the practical exploitation of such devices.

Figure: Bacteriochlorophyll-c molecule – which builds up chlorosomes by self-assembly. Colour coding: carbon-green, oxygen-red, nitrogen-blue and magnesium-white. Note that the magnesium atom is bound to oxygen, which could be from a water molecule, or another oxygen atom from a different bacteriochlorophyll molecule. This is the basis for the self-assembly process. The R group is a long chain from a fatty alcohol such as farnesol or stearol. The asterisks denote the chiral centres which make this molecule ‘smart’.

Article first published in Nanomaterials News.