**4. Technological aspects from solar energy to photovoltaic electricity**

The PV effect consists in the direct conversion of solar energy into electricity (Fig.1). Three interdependent and successive physical phenomena are involved: a) the optical absorption of light rays, b) the transfer of the energy from the photons to the electrons in the form of potential energy; c) the collection of the electrons excited in this manner so that they recover their initial energy. The ideal converter is still the semi-conductor, since both the conductivity and the collection method are both sufficient and efficient. However, there are two major obstacles with respect to PV conversion. The first one is related to the photons and electrons. In fact, not all the photons are absorbed and not all of the excited electrons are collected. This impacts the energy performance of a semi-conductor, one of the key parameters for the PV industry. In practical terms, the performance of a solar cell is the maximum power produced, expressed in Watts-peak (Wp) and the higher the Wp is, the better the performance of the cell is (Goetzberger & Hoffman, 2005 ; Labouret & Villoz, 2009). The other major obstacle is the price of the solar module. Development of the technologies and the PV materials is continuing while the two goals are to increase energy performance and reduce the cost of the Wp beneath the symbolic threshold of \$1 US/Wp (Krauter, 2006; Xakalashe & Tangstad, 2011).

Photovoltaic Conversion: Outlook at the Crossroads

**5. Practical applications** 

al., 2010).

2009).

2009).

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research is focussing on the use of hybrid organic-inorganic cells with a great deal of load

The solar modules consist of cells assembled in series, encapsulated between supports made of tempered glass, a special Tedlar® type plastic or "solar" resin, and then framed. In order to amplify their power, the modules may be grouped in voltaic panels or even voltaic fields with power output ranging from 1 kWp (kilowatt peak) to more than 100 kWp (Antony et

The two types of PV systems in use are autonomous (off-grid system) systems and those connected to the public electrical network (on-grid system); they differ in terms of their finality and the nature of their components. The electricity produced by the autonomous systems is consumed on site whereas that generated by facilities connected to the network is intended to

Moreover, there is a hybrid system, an intermediary and emerging form of the PV market that allows connection to another source of energy. Efforts to combine sources of energy are continuing particularly as a complementary source of energy although this type of system remains complex, laborious and onerous (Goetzberger & Hoffman, 2005; Labouret & Villoz,

There are many applications for autonomous systems such as internal market for solar gadgets (calculators, clocks, etc.), solar home systems and water pumps. These systems are still a preferred solution for developing countries where more than two billion people are not connected to an electrical network and have no hope of being connected to one someday (Goetzberger & Hoffman, 2005; Labouret & Villoz, 2009). Nevertheless, despite their appeal as sources of energy and their potential for development, these systems are still the source of major concerns requiring intense consideration so as to ensure both their sustainability and

In this case, it would be possible to enhance their tangible added value in the global energy landscape. First, apart from the internal market and "sun-related" applications such as pumping or ventilation, the autonomous systems would have to include judicious storage batteries in order to accumulate excess electricity, but these batteries are problematic. The financing for the autonomous generators is the first negative element since, even if only 20%-30% of the initial investments are for storage, the reduced lifespan of the batteries (batteries have to be replaced every 2, 5 or 10 years) results in a final cost that could amount

It is a fact that the positive development of individual solar systems in the developing countries is having pernicious effects since that easier access to electricity could lead to an increase in the acquisition of electrical appliances and, consequently, to the overuse of batteries, thereby reducing their lifespan (Goetzberger & Hoffman, 2005). Moreover, the scarcity of training on autonomous systems, aggravated by the high rate of illiteracy in the developing countries, could result in difficulties in maintaining the batteries which, obviously, influences their durability. Thus, the integration of batteries, although essential for autonomous systems, will have an impact on their costs, already high (\$500 to \$1500 US), thereby handicapping, to a certain extent, their generalization in terms of rural electrification in developing countries (Goetzberger & Hoffman, 2005; Labouret & Villoz,

mobility that uses cadmium selenide as the inorganic material (Freitas et al., 2010).

fully or partially supply that network (Labouret & Villoz, 2009; Antony et al., 2010).

their wide-scale generalization in developing countries.

to 70% of the total costs (Labouret & Villoz, 2009).

The material currently used preponderantly in the design of PV cells is silicon, which is abundant in nature, accounting for 90% of the global market for the production of the modules. More than 80% of the silicon used is in crystalline form with an energy performance between 14% and 22% for a solar cell, compared to 12%-19% once assembled in modules (Labouret & Villoz, 2009; EPIA/Greenpeace, 2011; Xakalashe & Tangstad, 2011).

There are currently three generations of photovoltaic cells. Those referred to as the first generation are made of crystalline silicon. The cells are provided in plates or wafers and have to be made from very pure silicon, using a manufacturing process that is still very onerous (Goetzberger & Hoffman, 2005; Labouret & Villoz, 2009; Jaeger-Waldau, 2010). The price of the solar module based on first generation cells is estimated at close to \$2 US/Wp (Xakalashe & Tangstad, 2011; SolarServer, 2011).

The second-generation solar cells, so-called thin layer cells, require less material and should cost less to design. Their development is more and more promising since their market share grew from 5% in 2005 to 16%-20% in 2009. Their production capacity, estimated at about 10 GW in 2010, could grow to 20 GW in 2012 and 70GW in 2015 (Jaeger-Waldau, 2010). The thin-layer solar cells include, first and foremost, amorphous silicon, with a very uncompetitive performance of between 4% and 8% although the price per Wp is advantageous, approximately \$1.3 US in 2011 (EPIA/Greenpeace, 2011; SolarServer, June 2011). The second generation also includes other polycrystalline thin-layer films, particularly those based on cadmium telluride (CdTe), copper indium selenide (CIS) and its alloy copper indium gallium selenide (CIGS). The average performance of the CdTe cells is between 8% and 10%. The price per Wp was \$0.81 US in the first quarter of 2010 and at the end of the same year, CdTe modules contributed to the production of almost 14% of the PV solar electricity generated by thin-layer cells (Jaeger-Waldau, 2010).

In theory, the CIS and CIGs cells have the highest performance for thin-layer cells, which is estimated at 20% in laboratory tests. However, the modules installed yield only 7% to 12%. Nevertheless, this technology is in the early stages of development and the manufacturing process is still complex, particularly since indium is a material that is in high demand in the flat screen (LCD) industry, which makes its use in PV systems problematic (Labouret & Villoz, 2009; EPIA/Greenpeace, 2011).

The objective set for the third generation cells is in the vicinity of 30% and these cells rely on innovative technologies. This group includes primarily: a) multi-junction cells with a thin layer of silicon or gallium arsenide combined with a solar concentrator, b) organic polymer cells or poly-electrochemical cells, also called Grätzel cells; c) thermophotovoltaic cells, primarily with an indium arsenide base (EPIA/Greenpeace, 2011). The multi-junctions, equipped with solar concentrators with a factor of up to 1000, are by far the most performing, with a record performance of 35.8% announced in 2009. However, the applications remain limited since they are confined to the space and military fields (Chataing, 2009; Guillemoles, 2010). While the performances of the organic cells are lower, from 8% to 12%, interest in such cells and particularly the Grätzel cells is growing since the production costs are constantly declining with an interesting price outlook estimated at \$0.73 dollars US (0.5 Euros) per Wc in 2020 (Chabreuil, 2010; EPIA/Greenpeace, 2011).

One of the emerging technologies in the field of PVs is nanotechnology, which uses nanocrystalline particles or quantum dots, which would significantly increase the efficiency of the conversion compared to conventional semi-conductors (Nozik et al., 2010). Current research is focussing on the use of hybrid organic-inorganic cells with a great deal of load mobility that uses cadmium selenide as the inorganic material (Freitas et al., 2010).
