Photovoltaics : inorganic, organic and hybrid principles and processes

To date, the only inexhaustible, eco-compatible vector is the sun, which provides 178,000 TW of radiation. However, as this lecture will show, the major challenge is to convert this energy efficiently and at low cost. A number of promising solar energy conversion research strategies are currently being pursued, including the transformation of solar energy into the following vectors: i) biofuels obtained through natural or artificial photosynthesis, ii) hydrogen through photo-catalysis reactions, and iii) electricity through photovoltaic conversion. It's this last axis that we've developed, knowing that our energy needs could be met today by covering 0.1% of the earth's total surface with solar panels with a conversion efficiency of 10%. However, this photovoltaic route remains a colossal challenge, despite the numerous existing photovoltaic cell technologies (inorganic, organic or dye-sensitized), which we have described here, outlining their operating principles and efficiencies. Inorganic and organic cells, with respective efficiencies of 20% and 8%, work by absorbing a photon by a semiconductor to create an electron-hole pair, which is dissociated at the pn junction to generate electricity, whereas in dye cells, which mimic plant photosynthesis, the creation of electrons and holes is generated by the absorption of light by a pigmented dye (the equivalent of chlorophyll in plants). The evolution of these different technologies was reviewed, including 1) the development of heterostructures, bipolar cells or concentrating cells enabling yields of up to 40% to be achieved, and 2) dye cells, or organic cells, through the development of new pigments or based on carefully selected organic molecule acceptor-donor couples.

Finally, this lecture also touched on the problems of photovoltaic energy: (i) its cost, which is still 5 or even 7 times higher than that of conventional energies ($0.05/kWh), although it is hoped that this will approach that of nuclear kWh by 2030, and (ii) its intermittency, which requires new energy storage technologies that can be adapted to network applications. However, we couldn't end this lecture without quoting Nathan Lewis's thoughts, which we can easily extend to the global level: " In the United States, on a daily basis, we spend more money buying gas at the pump than we spend annually on solar energy research, while in the same hour, the sun provides us with all the energy our planet uses in a year. Unfortunately, we don't know how to capture, convert and use it efficiently. "