Renewable energies: Sunset on silicon? Progress and prospects.

    A provisional electricity generation balance in France in 2014 highlights the slow rise of the contribution of renewable sectors, solar and wind power. In September 2014, the wind power available was 8.8 GW and one observes that a minimum of 600 MW of new wind power plants have been connected to the grid in 2014 while, between 2010 and 2013, the connection rate had steadily declined from one year to another. Wind energy has produced 3% of electricity in France in 2014. The installed capacity in photovoltaic is also clearly growing, it accounted for 5.5 GW in September 2014 with a production representing 1.4% of the national production (Commissariat Général au Développement Durable, Figures and statistics No. 584, November 2014 The steady growth of renewable energy is also manifest in the world. Thus, according to a report from the Network REN21 (Renewable Energy Network for the 21st Century, Report 2014) it accounted, all techniques being combined, for 19% of the energy consumed in the world in 2012. The installed electrical power, excluding hydropower, accounted for 560 GW in 2013 with a strongly dominant wind energy (318 GW), the Photovoltaic Solar coming in second with 140 GW but with 3 GW concentrated solar power only (photography representing Themis experimental sation in France, near Perpignan) , the weight of bioenergy (use of biomass) being not negligible ( 88 GW). A group of six countries stands out clearly in the development of renewable electricity : China, the United States, Germany, Spain, Italy and India. In Spain and Italy solar photovoltaic electricity contributed 8% to national electricity production in 2013 and in Denmark a third of the electricity came from wind power. The situation in Europe is quite mixed; while renewables accounted in 2012 for 14% of total final energy consumption with a drop of nearly 20% of investments in the sector between 2011 and 2012. It is found on the other hand that consumption and production of biofuels increased only slightly since 2010, consumption in the EU was 14 Mtoe in 2012 with two leading countries: Germany with 3 Mtoe and France with 2.7 Mtoe.

The increase in renewable sectors is undeniable. Investments in solar photovoltaics which had lagged significantly behind wind energy seem to catch up since 2013, during that year they indeed represented 53% of total investment in the field of renewable energy in the world. The total production of solar cells (mostly silicon) represented about 40 GW of power with a strong dominance of China (a production of 22 GW in 2012). The decrease in cell cost is continuous since ten years, the price of peak watt for silicon solar cells is now well below $ 1 on world markets (see JRC PV Status report 2014, It is true that the cells represent only 40% at most of the total cost of a solar panel for installation on a building. The lower price per kWh produced, while being real, is not yet sufficient to guarantee for the short term, solar competitiveness in most European countries for the short term, (the average tax free cost of kWh production is between 12 c€ and 18 c€ for plants with a life of 20 years).

If silicon cells are overwhelmingly dominant (cells with light concentration devices having yields are in the range of 20-25%) meanwhile,  they are not the only one. Inorganic semiconductors such as cadmium telluride or copper selenide and indium (or gallium) have a 20% yield. As far as organic compounds are concerned, their yields are unfortunately still low, barely above 10%. As we pointed out in our blog on October 2014, other perspectives have been opened recently with the so-called perovskites materials family. Perovskites are « classical » materials: Bimetallic oxides constituted, for example, with barium and titanium. Their electrical properties are well known and used since a long time, in particular in capacitors and piezoelectric devices for detecting mechanical shocks. New perovskites were synthesized in Japan, in 2009, combining inorganic and organic compounds in which oxygen is replaced by a halogen, chlorine, iodine or bromine, one of the metals being either lead or tin, an organic radical, such as methyl ammonium, being substituted to the second metal in the crystal. Perovskites, including these new materials, are semiconductors absorbing solar radiation; they are tested in solar cells (they are deposited on a substrate which can be glass) and since 2014, publications announcing an improvement in yields « rain down ». The best one, announced in early 2015, are close to 20% (see Dong Shi et al. « Low trap-state density and long carrier diffusion in perovskite organolead trihalide single crystal », Science, Vol. 347, No. 6221, p. 519, 30 January 2015 ). Energie solaire nanoCombining these materials with silicon is possible as they do not absorb exactly the same spectral lengths. The potential advantage of perovskites being that, although they have probably a yield equivalent to that of silicon, their production cost is likely much lower, their synthesis being carried out at a temperature not exceeding 200 ° C. Their disadvantage, for now, is the use of lead, a toxic metal (tin being an alternative). The long term stability of the cells must be tested (see a review of prospects Prachi Patel and David Mitzi « Perovskite in the spotlight, » Energy Quartley MRS Bulletin Vol. 39, p.768, September 2014 ).

Interesting perspectives can also be opened by materials made of monoatomic layers (photography represents solar cells with nano structures). Graphene being one of them. It is constituted by carbon, it is not semiconductor but its conductivity is excellent, it might be used in electrodes or batteries, or solar cells Bonnacorso F. et al. « Graphene, related two-dimensional crystals, and hybrid systems for energy conversion and storage », Science, vol. 347, No. 6217, p. 41, 2 January 2015, ). The silicene discovered more recently (especially by a Marseille laboratory materials, G. Lay et al.), consisting of a silicon monolayer is also, a semiconductor and might be used in solar cells (see M . Peplow « Silicene Makes its transistor debut, » Nature, Vol. 518, p.18, February 5, 2015, ), germanene consisting of germanium monolayers might be another possibility but it is too early to tell.

Strategic thinking is required. Silicon is certainly not on the eve of its decline but serious tracks are opening the possibility to implement completely new synthetic materials with lower manufacturing costs than silicon. In other words, European countries like France which have lost the silicon industrial battle might be interested in conceiving a strategy probably consisting in investing less heavily in the current silicon generation of photovoltaics cells but more strongly in R & D to prepare a technical and industrial alternative to silicon.








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