Light emitting microalgae have been surviving in all sorts of environment for billions of years and in the process, it picked up a few tricks for harvesting light as efficiently as possible. Little light reaches algae that resides at a depth of one meter or more in seawater, as light is absorbed by seawater. These fluorescent algae are able to capture up to 95 per cent of available light using a novel method, a way much better than our most efficient solar panels are able to do. Researchers are finally getting an idea of how they do it and how we potentially can do the same. The technique these light emitting microalgae use, could help us develop the next generation of solar panels.
Using advanced mass spectrometry methods, where ionisation is used to analyse chemical and structural properties of the organisms, scientists could get more insightful look at two types of microalgae namely cyanobacteria and red algae. So how are they such efficient solar converters? The surfaces of these microorganisms are anchored with a mass of light harvesting antenna called phycobilisomes which are protein complexes. The geometrical arrangement of a phycobilisome is very elegant and results in 95 per cent efficiency of energy transfer. Each phycobilisome consists of a central core of one type of protein called allophycocyanin which sits above a photosynthetic reaction centre (RC), from which several outwardly rods made of stacked discs of three other types of proteins branch out. The central triangular core is composed of allophycocyanin and that the peripheral rods contain phycocyanin and phycoerythrin (types of protein). The triangular core is assembled from three stacks of disc (size – 12 nm diameter and about 6-7 nm thick) shaped subunits. Radiating from the triangular core are several rods of about 12 nm in diameter. Each outwardly rod consists of stacks of about 6 discs, each disc of about 6 nm thick. They act as the bridging pigment between phycobilisome and photosynthetic lamella. This increases the surface area of the absorbing section and helps focus and concentrate light energy down to the reaction centre to the chlorophyll lamella in less than 100 ps time. With these phycobilisomes or antennae, the microalgae are able to capture 95 per cent of the light that reaches them. Absorption of a photon by a molecule lead to electronic excitation when the energy of the captured photon matches that of an electronic transition. The fate of such excitation can be a return to the ground state or another electronic state of the same molecule. When the excited molecule has a nearby neighbouring molecule, the excitation energy may also be transferred, through electromagnetic interactions, from one molecule to another. This process is called resonance energy transfer and the rate depends strongly on the distance between the energy donor and energy acceptor molecules. Light harvesting complexes have their pigments specifically positioned to optimise these rates and they are located around the reaction centre.
One factor that makes photosynthesis so efficient is that it occurs at incredibly high speeds. During the process, sunlight hits a light trapping pigment ‘chlorophyll lamella’ that energises an electron, causing it to fly across the cell’s membrane in billionths of a second. The electrons make its journey via a series of specially located pigments referred above, which, evolution has finely tuned to create what is essentially a one-way path. In contrast, in case of conventional solar cells, the electron can easily bounce back across the membrane, losing its energy and rendering the whole process very inefficient.
If we are able to use the same technique, then the potential boost for renewable energy would be huge, considering the majority of mainstream photovoltaic cells we have at the moment operate in 10 – 20 per cent efficiency range. The ingenious control panel that algae use to convert sunlight into energy is more complicated than a swiss watch. Armed with the knowledge of how light harvesting algae and other plants harness power from sun, a team of researchers from Vanderbilt University started work to make a solar cell mimicking its mechanism. More than 40 years ago, scientists discovered that one of the proteins involved in photosynthesis, called photosystem 1 (PS1), continued to function when it was extracted from plants like spinach. Then they determined PS1 converts sunlight into nearly 100 per cent efficiency. When a PS1 protein is exposed to sunlight, it absorbs the energy in the photons and uses it to free electrons and transport them to one side of the protein. That creates regions of positive charge, called holes, which moves to the opposite side of the protein.
To make the prototype, Vanderbilt team extracted PS1 from spinach or microalgae into an aqueous solution and poured the mixture on the surface of a p-doped silicon wafer. Then they put the wafer in a vacuum chamber in order to evaporate water away leaving a film of protein. They found that the optimum thickness was about one micron, about 100 PS1 molecule thick. Protein alignment is very important. In a plant, all the PS1 are perfectly aligned. In earlier prototypes made by the researchers, PS1 were oriented randomly and that was a major problem. Those PS1 that are oriented in one direction provide electrons, while those that are oriented in opposite direction pull electrons out of the matrix. As a result, both positive and negative currents are produced that cancel each other out to leave a very small net current flow. The secret to overcome this problem is doping silicon wafer. The p-doped silicon overcomes this problem to a great extent, because it allows electrons to flow into PS1 but will not accept them from proteins. In this manner, the electrons flow through the circuit in a common direction. It is reported that the prototype PS1/p-doped combination silicon solar cell produces about one milliamp of current per square centimetre at 0.3 volts. The reason the combo works well is because the electrical properties of the silicon substrate have been tailored to fit those of the PS1 molecule. This is done by implanting electrically charge atoms in the silicon matrix to alter its electrical properties: a process known as “doping”. In this case, the protein worked extremely well with silicon doped with positive charges and worked poorly with negatively doped silicon. A prototype of size 1,800 mm x 1,800 mm solar panel with about 1,000 numbers of this centimetre sized cells connected in series has been made.
Studies on photosynthetic protein based solar cells and quite a number of such cells have been made, show that initially they achieved high photocurrents, but they are short-lived and they can hardly produce any photocurrent after a week which is indicative of some kind of degradation with time. As the major photovoltaic components in these solar cells are biomolecular complexes, it is vital to understand their vulnerability in a foreign environment. The biomolecular components often lack a protective environment in the solar cells which deteriorates their functionalities, ensuing short-lived solar cells. The in-vitro stability of reaction centres (RC) needs to be improved. As RCs are isolated from their native environment, they are prone to conformational changes as the stabilizing effect offered by the membrane lipids is lost. Lipids play an important role in affecting the biophysical and electron transfer properties and promote structural stability and flexibility. Further much work will have to be done to make this route commercially viable.
