Funelling sunlight into solar cells
(appeared in Dec 2017)

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Making the best of solar energy may need changing the form of solar radiation to suit energy gathering devices, says S.Ananthanarayanan.

The earth receives enough solar energy to meet our wildest energy requirements. Plants, “which toil not, neither do they spin”, have adapted to make the best of sunlight. But humans need to use available materials, like silicon, to trap sunlight energy and these are far from optimal.

Given the temperature of the sun, the part of the spectrum which has the largest part of energy is around yellow-orange. There is also high energy in the blue end and near ultra violet, but there is much energy spread over the infra-red region.

While photosynthesis has evolved to make the best of sunlight, there has been no evolution of metals like silicon, which are used to construct solar cells. Solar cells just happen to be most efficient in the near infra-red and red in the visible spectrum. Solar cells, hence, can harvest only part of the energy in solar radiation.

When first developed, solar cells were wonder devices and were used in light meters and to power machinery in remote places. But with today’s need to generate non-polluting energy for ordinary use, the emphasis is to tweak the efficiency of solar cells as much as possible. A paper by Yoichi Murakami, Sudhir Kumar Das, Yuki Himuro and Satoshi Maeda from the Tokyo Institute of Technology and the Raghunathpur College in Purulia, India, published in the journal of the Royal Society of Chemistry, describes an improvement of the process of converting light in the long wave, red or infra-red region, to light in the shorter wave, towards the blue side of the spectrum.

Solar cells typically do not use the longer wave portion of the spectrum. ‘Upconversion’ of long wave light into shorter wave light would hence bring the energy in the long wave part of the spectrum into the solar cells’ net.

As the diagram of energy distribution of sunlight would show, there is energy in the ultra-violet and infra-red region too. The energy in the ultra-violet, apart from being wasted, warms the solar cell (which reduces its efficiency) and causes physical damage. There is hence interest in converting light of shorter wavelength (blue end of the spectrum) to longer wavelength. One place where this happens is in the domestic tube light or the fluorescent lamp. Ultra-violet light emitted when electricity passes through a gas in the tube falls on special materials coated on the inside of the tube. Atoms of these materials absorb the UV light and re-emit the energy at lower, visible wavelengths.

A variation of the process is where a group of atoms passes energy to another group, which then emit energy at a longer wavelength. An improvement of this process, using low-cost and biodegradable materials was reported in 2015 and may be commercialised for making energy in the blue end of the spectrum useful for solar cells. What remains is hence the substantial energy in infra-red end of the spectrum, whose energy is not sufficient to flip the switch within solar cells. Squeezing photons in this band into photons of higher energy would lead to a marked improvement in the output of solar cells.

A method to pump low energy photons into higher energy photons has been developed to enable astronomers view images that are formed in the infra-red. As infra-red light, with its longer wavelength, is scattered less than shorter wavelengths, better details of very distant cosmic objects can be made out in the infra-red. The method developed was somewhat complex, mixing the infra-red light with shorter wavelength light from lasers in special crystals. The method is effective in preserving the image information in the infra-red so that it becomes visible. Our present objective, however, is to preserve the energy in the infra-red in a way that allows the energy to be tapped at shorter wavelengths.

A suitable method for this purpose is to take two, separate low energy photons, add together their energy and then emit a high energy photon. This is implemented with certain molecules that can absorb a pair of low energy photons in succession, to rise to an energy level that is higher by the twice the energy of the photons. The molecule then de-excites to a slightly lower energy state and then de-excites again with the emission of a high energy photon, of nearly twice the energy of the initial photons. This mechanism, which is called two-photon-absorption, however, occurs very feebly and needs excitation with coherent, or laser light.

A more promising method was that of the triplet-triplet-annihilation (TTA). This method depends on how two pairs of electrons can interact. Electrons have a property called spin and a pair of electrons in the ground state need to have opposing spin, and thus only one state, of zero net spin, or a singlet. But if one of the electrons is excited to a higher state, the pair can be again with opposing spin and only the zero net spin state, or they can have the same spin. As the same spin can be in either of two directions, the excited state is in one of three possible states, or a triplet.

Now, if a pair of electrons in the ground, and hence zero net spin state meets a pair of which one electron is excited, there are different kinds of interaction possible. In the first three cases shown in the picture, a pair of singlet states result in a pair of singlet states or a singlet and triplet state result in a triplet and a singlet state. But in the fourth case, a pair of triplet states result in a pair of singlet states – a case of triplet-triplet annihilation. In the first three cases, the number of excited states remains the same. In the TTA case, however, a pair of excited states results in only one excited state, and the sole excited state has all (almost) the energy of the pair. The initial excited states would have started by absorbing one photon each and the final excited state would have the energy of both photons, to emit as one shorter wavelength photon.

TTA is thus a method to get two low energy photons merge into one of almost twice the energy, and this method does not call for lasers, but works with feeble and ordinary sunlight. Attempts to implement the method, however, has called for materials that are expensive, unstable or unsafe, the Royal Society paper says.

The authors of the paper analyse the properties, the electron configuration, not being volatile, responding strongly to feeble incident light, being environment friendly and low cost that, materials for TTA should have, and which current materials do not. The analysis and trials lead the team to a category of transparent dyes that do satisfy these demands and effectively change the colour of incident light towards blue in the spectrum. A welcome discovery that should help solar cells generate a lot more power than they do now, with the same sunlight!

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