Another sunny day in the western side of Europe—I watch my little daughter Maria playing near the sea shore. She is getting more and more tanned every day, a brownish colour that goes so well with her long dark hair. By my side, my son João is complaining as he joins Maria for another sea water bath: “It’s too hot under the sun”. We are enjoying the sun in its entire splendour and in its entire spectrum—Maria is getting tanned by the ultra-violet (UV) part of the sun’s emission spectrum, João feels the heat as the infrared radiation (IR) reaches his skin, and I am taking advantage of the visible (Vis) part of the spectrum to see them playing. As I enjoy this sunny day, I think to myself: is it possible to use the sun’s entire radiation spectrum, all the light reaching earth, and convert it to electricity?
Today, the quest for non-polluting energy sources is a worldwide priority, which we believe can only be reached with investment in knowledge-based energy conversion devices. Although solar radiation is the most widely available and long-term source of renewable energy, only a small fraction of its radiation is currently being used in energy conversion devices. Efficient conversion of light into electricity needs materials that join two characteristics: 1) the material should absorb most of the solar radiation and 2) this absorption should efficiently lead to the creation of electric charges. One can paint a wall with black ink and surely it will get an efficient light absorption, but, unfortunately, no electric charge will be generated.
A Significant Reduction of Cell Cost
So, we have to rely on materials that only partially absorb the solar radiation but are relatively good at creating charges. Most of the currently used solar-to-electricity devices, the ones we can see on the roofs of our houses, are based on monocrystalline silicon photovoltaic. However, those materials are expensive and most of them do not absorb much more of the solar spectrum than the melanin molecules on Maria’s skin, causing her to brown. Recent technological advances in thin-film silicon and wire arrays substantially improve the collection of sun light, but we are looking into yet another direction.
The construction of photovoltaic cells based on materials obtained through chemical organic synthesis, materials mostly made up of carbon and hydrogen, is still relatively immature from the industrial point of view. These materials can be deposited on lightweight flexible supports and are capable of reducing the cells cost by a factor of 10 to 20. However, spectral absorption (photons or wavelength in nanometers [nm]) and durability limitations still need to be solved.
Currently, the best ruthenium-dye-adsorbed titanium dioxide nanocrystallites collect photons up to 650 nm. Extending absorption to 800 nm increases the maximum harvested global solar irradiation from about 25% to 40%. But can this be achieved by an inexpensive and durable dye? We think so. Getting inspiration from nature and with a little help through chemical synthesis, we are getting excellent molecular templates to produce more efficient and durable solar cells that are very well suited to harvest both Vis and near-IR solar photons.
I think we can really make a difference in the visible and near-IR part of the solar spectrum, but what about the IR part? (Remember: this is the radiation that is making my son João sweat). The truth is that the UV and Vis part of the solar spectrum only correspond to 58% of the total available solar energy density. The remaining 42% correspond to the IR contribution, which is normally treated as waste heat in conventional solar cells.
Thermoelectric materials convert heat into electricity, by taking advantage of the Seebeck principle: when the junctions between two different materials are held at different temperatures, a voltage is generated that is proportional to the temperature difference. Efficient thermoelectric conversion is obtained in the presence of large temperature differences (as the one induced by IR solar emission). So, a good idea might be exploring the thermoelectric power of complex cobalt and titanium oxides, as well as of conducting polymers, and investigate their electrical charge and heat transfer properties as a function of structure and chemical composition.
The perfect solution might be in the combination of two worlds: efficiently collecting the light (UV and Vis) and heat (IR) parts of the solar spectrum in a direct solar and thermal energy conversion device. We have a fundamental chemistry perspective, exploring new organic dyes, polymeric and nano-materials for energy conversion applications. We share with other scientists the vision that a system combining solid-state solar cells and thermoelectric materials will efficiently harvest and convert to electricity most of the solar spectrum, and make the widespread use of solar energy economically viable, a promise that urges to be fulfilled.
Meanwhile, at the beach, I think my son João is completely right: it is getting too hot and I am getting as tanned as Maria. Leaving my thoughts at the towel, I join them at the sea, enjoying the good weather and the Atlantic Ocean waves.