Alternatives are set to make the solar cell environment friendly, says S.Ananthanarayanan.

Renewable energy is the only visible solution to the crisis that the world finds itself in. While wind and hydroelectric energy can be used where we have the right coastlines or river valleys, solar panels have greater scope as they can be widely deployed.

The trouble is that solar panels use pure silicon, and mining and purifying silicon has a large carbon footprint, apart from social and other environmental cost. Although they provide electricity with no emissions, solar cells take over fifteen years to work off the emissions produced in their manufacture. Solar panels now provide some 3% of the world’s energy. Raising this to a level that materially helps reduce emissions would call for massive silicon extraction.

The most efficient use of solar energy, of course, is in green vegetation, where chlorophyll helps sunlight break down CO¬2 and produce hydrocarbons. Ways to use synthetics to mimic the process, using bacteria, are being developed and could lead to some cleaning of the environment. To turn this technology to generate electricity, however, would be a game changer.

That organic dyes can generate electricity when light shines on them was discovered in the late 1060s and the first dye-sensitised solar cell, or the DSSC, was developed by Brian O’Reagan and Michael Grätzel at University of California at Berkley in 1988, and published from École Polytechnique Fédérale at Lausanne, in 1991. In the DSSC, a molecular dye absorbs sunlight, like the chlorophyll in green leaves, leading to collection of electric charge, rather than promoting a chemical reaction. The charge then drives an electric current, on the way back to the dye, where it gets the dye ready to absorb more sunlight. The current issue of the journal, Nature Communications, contains a paper by Michael Grätzel, Dan Zhang, Marko Stojanovic, Yameng Ren, Yiming Cao, Felix T. Eickemeyer, Etienne Socie, Nick Vlachopoulos, Jacques-E. Moser, Shaik M. Zakeeruddin and Anders Hagfeldt, from the institute in Lausanne, that reports advances that improve the performance of the DSSC, for greater relevance as a source of green energy.

The functioning of the conventional solar cell is thanks to the nature of the silicon atom. The atom of silicon, has four electrons in its outermost shell, which is half way between metals, which are good conductors, and non-metals. This enables a uniform, ‘hand-holding’ structure of the silicon crystal. If there is a trace impurity with one more outer shell electron than silicon, however, this can result in the extra electron being unpaired and ‘free’. If the trace impurity is of atoms with one less outer shell electron than silicon, this creates a ‘shortage of an electron’ or a ‘hole’, which can also move over the crystal lattice. In a junction of silicon with the two kinds of impurity, some of the ‘free’ electrons would cross over to other side, where there are ‘holes’. This would create electrical tension. Now, if light falls on the ‘holes’ side of the junction, an electron gets ‘freed’, and the electrical tension drives it to the other side through a wire that connects to the two sides of the junction.

In the DSSC, in place of silicon, the source of electrons is an organic dye, which behaves like chlorophyll, releasing an electron when excited by light. While the principle had been discovered in the 1960, O’Reagan and Grätzel worked out the arrangement where this electron is snared and sent out as an electrical current, with a method of sending it back to the organic molecule.

The principle is illustrated in the picture. Photons, or particles of light pass through the transparent conductor on the right and strike titanium oxide nanoparticles, which carry molecules of the organic dye. The nanoparticles are for assuring large surface area for dye molecules. Photon knock electrons out of the dye molecules, and the electrons are collected by the conducing electrode. The electrons form a current, to do work and reach the opposite, electrode, which is of inert platinum. The medium between the two electrodes contains iodine, which can change form, to pick up electrons from the platinum electrode and carry them to the titanium oxide particles, which are left with a positive charge when they lost electrons. When the electrons reach the particles, they get back into the dye, and the dye molecules are ready for use again.

In practice, a glass sheet is coated with a thin layer of tin dioxide and then a layer of titanium dioxide, as a porous structure, with large surface area. This layer is then coated with the photosensitizer dye. The second plate is a sheet of platinum, coated with a thin layer of the iodide electrolyte. And the two plates are joined together and sealed.

The conventional solar cell, apart from being based on silicon, needs a thick layer of silicon, to be reasonably effective. Although ‘thin film’ solar cells have been developed, the DSSC films can be far thinner and are substantially cheaper. This is despite the use of titanium and platinum, for which, again, replacements are rapidly becoming available.

The negative, however, has been that DSSCs are less efficient in converting light into electricity and that organic dyes degrade. What the Laussane group has done is to use molecular engineering and develop a dye for use in conjunction with the current one, to enable the device to react to more parts of the spectrum of the incident light. The additive also prevents electrons that are emitted from being captured by neighbouring dye molecules before they move through the circuit. The result is greater production of electrons, a voltage of 1.24 V or power conversion efficiency of 13.5%. The DSSC is hence suitable for use in cloudy conditions or to power indoor devices, a press release says.

Existing PV devices convert only part, the lower frequency part of sunlight, into electricity. As the higher frequency range, even the ultra violet, contains much of the energy in sunlight, not using this part of the spectrum amounts to wasted energy. What is more, this energy heats the PV device, which reduces its efficiency. Sheathing, with organic materials, has been developed, for use with conventional PV devices, to absorb blue and UV light and ‘step it down’ to frequency that the device can use. Another initiative has been to absorb lower frequency energy, in the infra-red, and ‘step it up’ to be useful for the PV cell.

Although the conventional solar cell dominates in the market and it is viewed as a possible solution to the danger of pollution by fossil fuels, however, the cost of silicon based solar cells makes this unlikely. Alternatives to silicon, like cadmium telluride, have been developed, but there are limitations in because of pollution concerns or performance. In the context, the rise of the organic, high performance and easily assembled, DSSC holds out another branch of hope to achieving power generation without environmental damage.

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