Advanced photoactive materials

Significant improvements in the use of solar energy will in any case require new photoactive materials, which react to light and trigger energy-generating processes capable of overcoming the limitations of silicon and multiplying the performance of the panels.

The application of new materials in energy production to improve efficiency and environmental sustainability is one of the great industrial challenges. Especially in photovoltaic development. For conventional solar cells, there is a physical limit to the performance they can provide. That is, the percentage of the amount of light they receive that they can convert into energy. For a normal silicon plate, around 17% or 18% is pretty good.

This physical limit, called the Shockley-Queisser limit [Shockley invented the transistor], has almost been reached in the most efficient cells. You can buy solar panels with an output of up to 25%. Investing in research and development with the intention of improving the possibilities of this technology is very costly, in order to achieve even the smallest favourable difference.

The key is how the materials of which the plates are made react to light. The sun does not send the earth light of a single colour, but a broad spectrum, with invisible extremes from ultraviolet to infrared. Silicon absorbs long-wave light, photons with an energy of around 1.1 electron volts, but at the same time it is like a transparent crystal to those carrying less energy. They pass through it and are lost. Nor do silicon plates manage to capture the energy of the photons that carry more charge, which are transformed into heat without passing through them.

"The sum of the two losses is two-thirds of the total light energy," says veteran chemist Professor Josef Michl, who is still working on 'singlet fission', in search of yields of up to 50% of the solar energy received. The typically organic 'singlet fission' process is based on using molecular material that "absorbs a photon and enters a directly excited state. The excited state has the same spin properties as the fundamental [lowest energy] state. It is called a singlet, and its excitation can be transferred from one molecule to another and shared by several molecules.

The strategy is aimed at absorbing photons with higher energies and those with medium charge, with two separate sheets on the plate, one with a material capable of generating singlet fission and the other of normal silicon, to multiply the absorption.

Michl is working on a second, and also very relevant, issue for the manufacture of its solar cells: it needs to find materials that are efficient in capture and do not deteriorate rapidly under the effect of sunlight, or contact with the atmosphere.

Significant improvements in the use of solar energy will in any case require new photoactive materials (which react to light and trigger energy-generating processes), capable of overcoming the limitations of silicon and multiplying the capacity of the panels.

Research in molecular nanotechnology laboratories proposes to go one step further: the challenge of turning all kinds of surfaces into solar panels, using these photoactive materials. The aim is to modify materials at the molecular level to change their properties.

Third-generation solar cells, based on polymers, are plastic, organic and flexible. This would allow solar cells to be 'painted' and suggests the idea of a coating that can turn a wall, a road or any other surface into a sunlight collector. In this way, the spaces dedicated to photovoltaic collection can be greatly expanded.

Nanotechnology developments in this field are also looking for new materials to create silicon-free solar panels with low-temperature organic solar cells.

Spain's role

Solar panels are not mass-produced industrially in Spain. There are companies that are trying to carve out a niche for themselves with differentiated initiatives in the face of fierce Chinese competition, but most of the activity in this field is dedicated to installation, assembly of some components and maintenance. And the management of photovoltaic farms, i.e. the exploitation of the installed technology.

However, cutting-edge research processes are being developed, which could lead directly to the next generation of solar energy collectors, beyond the silicon module-based processes that still dominate the market and will continue to do so in the years to come. The introduction of new materials that surpass their characteristics is a great disruptive opportunity to enter strongly into a market where we are only customers and users.

The battle is on to overcome the performance efficiency of conventional boards with new technologies, which may continue to bring down prices, but will not be free from the heavy limitations imposed by physics on their performance.