Between hopes, disappointments and spectacular advances, research around photovoltaic solar energy is progressing fast. Overview of the most promising trends with Daniel Lincot
and Pere Roca i Cabarrocas, CNRS research directors and photovoltaic specialists.
The photovoltaic effect, discovered in the nineteenth century, is based in part on the properties of semiconductor materials present in solar panels. How do you work to improve them?
Daniel Lincot1: Research on solar photovoltaic energy is a team effort. The goal is to make more efficient solar cells that better convert sunlight into electricity at lower cost. To increase the efficiency of the photovoltaic cells, it is necessary to improve the semiconducting properties of each of the materials present in the cell, such as silicon, but also their combinations. A photovoltaic cell is a bit like a football team! It must form a whole: there is the individual and the collective. The materials must be better at personal titles, but also able to pass electrons between them.
Pere Roca i Cabarrocas2: When trying to improve performance or cost, every small detail counts. Photovoltaic solar research is truly an open and dynamic branch that integrates physics, electronics, chemistry, materials science and optics. To improve solar cells, we need this multidisciplinary approach.
D. L.: And research is moving fast. When the first silicon cells were developed in the 1950s, their efficiency dropped in the space of a few years from almost 0% to more than 10%. Today, theoretically, it is estimated that it could one day reach 85%! This proves that, in terms of photovoltaics, we are still under foot …
Every day, the sun supplies Earth with an average of 3 kilowatt hours per square meter. This energy seems inexhaustible. So what are the locks that still limit its development on a large scale?
P. R. C.: Contrary to what we think, solar energy is already viable and competitive. In France, EDF is pushing its consumers to become themselves producers of solar electricity. And in many other countries, solar is competitive with other energies.
D. L .: Locks are almost all lifted on a global scale. With Solar Impulse alone, we have shown that we can alternate day and night, and solve some energy storage problems. All these locks simply stand up by the demonstration. The only obstacle is that we would have to go even further … In France, we are still confronted with economic, political and cultural obstacles.
to what we think,
is already viable
P.C .: Solar Impulse has illustrated that there are solutions and that with intelligent energy management, we can do it. There are even more convincing examples, but less media, that prove the potential of photovoltaics. With solar, we can imagine creating wealth in the countries of the South, which would sell their solar energy. It is a utopia that becomes achievable.
Silicon, used since the 1960s to power space satellites, is still today the flagship material of solar panels. Why is he still not perfect?
D.L .: The silicon industry accounts for 90% of the solar energy market. Its principle is simple. It consists of cutting silicon ingots into wafers to then form photovoltaic cells capable of transforming the energy of the sun into electricity. The problem is that silicon needs a lot of thickness to fully absorb sunlight.
P. R. C.: The record yield of silicon is 26.33% in the laboratory, we must do more! We know that we can go beyond 60% yield with advanced concepts such as multijunctions. In any case, no principle of physics prevents us. The better the photons are trapped, the better the yield is.
Silicon wafer, solar energy, photovoltaic
Silicon wafer which, after various operations, will be transformed into a photovoltaic cell.
C.FRESILLON / CNRS PHOTOTHEQUE
To improve the performance of solar panels, should we consider finer materials?
D. L.: It’s not really a question of performance but rather of process and cost. This is the challenge of the so-called thin film sector, which accounts for around 10% of the market share. It is composed of three types of semiconductor materials: cadmium telluride, Cigs3 and silicon in thin layers. This consists of covering a support with a thin layer of one of these materials. Photovoltaic cells become 100 times thinner
Have some lines of research been abandoned over time?
D. L .: Yes, there are almost extinct channels. A few years ago, it was believed, for example, copper sulphide, because it is abundant in nature, non-toxic and absorbs light in a remarkable way. In the 1980s, it was the Grail. Except that the cells were not stable enough and degraded in a few days.
P. R. C.: Other areas are a little overdue, like that of organic photovoltaic cells. There was a boom in the 2000s, which quickly fell. In fact, the researchers who worked in this branch abandoned it in favor of the perovskites. It is true that some organic cells still have problems of stability, yield and cost. But this is not a sector to avoid, because it concerns specific markets.
Are the recycling and lifespan of photovoltaic cells also part of the specifications for solar research?
D. L.: This is very important. We analyze the life cycle of cells, from the mine to recycling. We expect lifetimes of twenty to thirty years or more. So we could even consider bequeathing solar panels legacy!
P.C .: The recycling market is developing very well. The “energy return time” of a cell is only one or two years. That is, in that time you will have produced the same amount of energy that you spent on making the cell. You will have “paid off” the energy. The twenty or thirty years of life after, it’s a bonus!
You are both involved in the IPVF, which will bring together more than 200 researchers around solar energy. What are his ambitions for the future?
P. R .: Research on solar photovoltaic in France is among the best in the world, but we have little visibility. IPVF brings us a spotlight. And it also makes it possible to bring together the different sectors: thin layers, silicon, perovskites …
D. L.: The creation of the IPVF makes it possible to prepare the future for our country. We believe that the key decade is 2020-2030. It will be the solar years, the switchover. It is estimated that, during this period, the solar power generation capacity will exceed the terawatt! The flagship “30/30/30” goal of the IPVF would be to achieve 30% return at 30 cents per watt in 2030. For this, this structure offers remarkable opportunities for public-private synergies, in associating at the same time academic partners such as the CNRS or the École Polytechnique, major industrialists, such as EDF, Total and Air Liquide, but also smaller ones like Horiba Jobin Yvon or Riber. The CNRS is at the heart of this construction, it is a great pride for us and also a great responsibility.
Solar panels covering roofs of homes in the Deer Valley neighborhood, in Phoenix, Arizona.
J.LOTT / New York Times-REDUX-REA
ShareToday, solar energy represents only 1% of the energy produced in the world. In time, will solar manage to surpass fossil energies?
P. R. C.: Of course, yes! While coal seems cheaper, it is because we do not take into account all the long-term costs of health and the environment. Solar is already largely competitive in many parts of the world, like Arizona. The future is not in fossil energy, although the transition will obviously not happen next year. Solar energy can charge the battery of a laptop as well as provide the energy needed for an entire village, it is extremely flexible. And solar is an unlimited and cheap resource.D.L .: In about twenty years, solar energy will most likely supplant conventional energy systems. Already today, photovoltaics is approaching in some countries the 10% of electricity supply. We will learn how to adapt our energy production to the seasons, for example by promoting solar in summer and wind or hydro in winter. The system is finally balanced … When we work on solar energy, we are necessarily guided by the idea that it is good for humanity.