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Photovoltaics: Concepts and Challenges

1.1. Brief description of the different photovoltaic cell technologies

Edmond Becquerel (Figure 1.1) was a French physicist who discovered the voltaic effect. He was the son of the physicist Antoine Becquerel and the father of Henri Becquerel, who discovered radioactivity with Pierre and Marie Curie. Edmond Becquerel demonstrated for the first time that certain materials could produce small amounts of electricity when exposed to light. A photovoltaic cell is composed of two silicon-based semiconductor layers.

Figure 1.1. Photograph of Edmond Becquerel by Nadar

The first layer is made of silicon, which is enriched by traces of phosphorus. The phosphorus atom has one more external electron than the silicon atom. This fifth additional electron can thus circulate easily between the atoms of Si (in blue) and P (in orange). A semiconductor material known as being of type N is thus formed (negative charges in excess) (Figure 1.2).

Figure 1.2. Photovoltaic effect

(source: Moine (2016)). For a color version of this figure, see www.iste.co.uk/grison/ecology.zip

The second layer is made of silicon doped with traces of boron (or aluminum, in green). This time, boron (or aluminum) has one external electron less than silicon. The global atomic network thus has some electronic gaps, assimilated with a positive charge. A material known as type P is thus formed (positive charges in excess).

Following the shock of photons from the sunlight, the excess electrons (N layer) enter the P zone. This movement of electrons creates an electric field. By connecting the two layers with an electric circuit, the current can circulate (Figure 1.2). Thus, under the effect of solar radiation, the energy of the photons is transmitted to the electrons, which then transform it into electric energy.

The semiconductor materials are generally silicon and cadmium telluride. The first type is the most used. Silicon can be amorphous, semi-crystalline or crystalline depending on its mode of preparation, giving different color tones to the photovoltaic cells. The photovoltaic cells are then, respectively, gray, blue or blue and dotted with patterns left by the crystals. The structure of the semiconductor directly affects the performance of the voltaic cell and ultimately that of its assembly in solar panels. A comparative summary of the main technologies is presented in Table 1.1.

In France, the government finances up to 43% of the research and development of photovoltaic panels. Numerous studies are carried out to improve the yield of solar panels, using increasingly efficient technologies. This is one of the major challenges that academic research is currently facing in the field of photovoltaic energy. To achieve this, many efforts are being made in the production of new semiconductor materials, new cells and modules.

ADEME (Agence de la transition écologique, French Environment and Energy Management Agency) has defined several areas of research that are priorities in the strategic roadmap of photovoltaic solar energy in order to achieve the objectives set by the legislation. These areas of development for the photovoltaic industry are:

• - technology/innovation/industry;
• - building integration;
• - services and network optimization;
• - interdisciplinary themes.

The last area reflects the desire to develop a sector that generates an acceptable environmental footprint. This is a key issue, which reminds us that new technologies must be developed taking into account life cycle analysis. For example, high-performance CIGS1 technology requires the use of cadmium sulfide. Cadmium is a highly toxic metallic element, regulated by REACH (Registration, Evaluation and Authorization of Chemicals). Research is underway to replace cadmium with less toxic compounds (Zn, Mg, O, S) or indium sulfide (a rare metal).

Table 1.1. Different technologies of photovoltaic cells

Technology Description Performance Maturity Amorphous silicon cell The amorphous silicon photovoltaic cell is composed of a thin layer of silicon, much thinner than the monocrystalline or polycrystalline silicon cells. 7% With poor performance, it is rather intended for solar watches, garden lighting and solar calculators Monocrystalline silicon cell (1st generation photovoltaics) This is the historical sector of photovoltaics. Monocrystalline cells are the first generation of solar cells. They are made from a block of silicon crystallized in a single piece. 16-20% 35% of the global market Polycrystalline silicon cell (1st generation photovoltaics) Polycrystalline cells are made from a block of silicon composed of multiple crystals. They are often found in domestic, agricultural and industrial installations. Approximately 15% 55% of the global market (best value for money) Thin layers (2nd generation photovoltaics) The cells are composed of layers of semiconductor and photosensitive materials on a support made of glass, plastic, steel, etc. Different materials can be used, the most common being amorphous silicon. 5-15% 10% of the global market Decrease in production cost Organic cells (2nd generation photovoltaics) The use of polymeric materials aims to replace mineral materials with organic semiconductors, that is, plastics, for the manufacture of photovoltaic cells. These are cheap, have good absorption properties, are easy to deposit and are much lighter than the 1st and 2nd generation materials. 20-30% In demonstration Concentration cell (CPV technology) This technology uses optical lenses that focus light onto small, high-performance photovoltaic cells.
It is necessary to always be positioned facing the sun (mobile system). 5-10% Experimental stage Cadmium telluride cells The manufacturing process hermetically seals a single cadmium telluride absorption layer between two glass supports. 10% - CIGS cells The semiconductor material is an alloy of copper, indium, selenium and gallium. This mixture is arranged in a very thin layer on a support.
In high-performance technologies, the CIGS structure forms a complex junction made of materials of different natures (heterojunction) of the CIGS(p)/CdS(n)/ZnO(n) type. 13-23% > 10% Hybrid perovskite cells Perovskites are crystals formed by molecules containing an organic and an inorganic part. They are emerging as a new class of semiconductors that show exceptional performance (Deleporte 2017). 22% Research stage

Figure 1.3. Global depletion of mineral resources (Hunt et al. 2015). For a color version of this figure, see www.iste.co.uk/grison/ecology.zip

The same is true for cadmium telluride technology, the nanoparticles of which are highly toxic. In the case of release of the Cd2+ cation, genotoxic, reprotoxic and endocrine-disrupting effects may arise (Cadmium 2010).

The elements zinc, indium and gallium pose the problem of the depletion of mineral resources. A review published in Green Chemistry (Hunt et al. 2015) confirms this major problem for the photovoltaic sector in a short-term shortage of widely used metal species.

This problem echoes Guillaume Pitron's book (Pitron 2018). Of particular interest is an investigation by the director of the Canadian company Uragold, Bernard Tourbillon, that produces the materials needed for the solar industry. A detailed calculation of the ecological impact of photovoltaic panels suggests that the production of a silicon-based solar panel generates 70 kg of CO2. Assuming that the number of photovoltaic panels will increase by 23% per year in the coming years, solar installations will produce 10 gigawatts (10 GW) of additional electricity per year, and generate 2.7 billion tons of CO2, equivalent to the pollution generated by the annual activity of 600,000 automobiles (Pitron 2018).

These data prove that photovoltaics must be thought through in terms of life cycle analysis (LCA): from the design of semiconductor materials to the recycling of these same materials after use. Photovoltaic industry players must adopt a dual LCA and circular economy approach and integrate it into their development.

This is the only way that photovoltaics can be considered as green energy and meet the great energy challenge, which, let us remember, is also ecological. This consistency of objectives is important, especially since France has real potential in solar energy.

The sunshine statistics of the French territory are indeed favorable for the development of solar energy (Figure 1.4) (Kalyanpur et al. 2013). Solar power plants are therefore a possible...