The photovoltaic phenomenon was discovered by Antoine César Becquerel as early as 1839. However, the first functional solar cell was not built until 1884, ie 45 years after this discovery, by the American inventor Charles Fritts. This first cell was made of selenium semiconductor, which was coated with a very thin layer of gold and had an efficiency of approximately 1 %. However, it was not clear how this solar cell works. The photovoltaic phenomenon had to wait until 1904, when Albert Einstein described the principle of the so-called photoelectric effect, for which he later received the Nobel Prize. At this time, however, these first selenium cells had no chance of being used in power generation due to their low efficiency and high cost, but they began to be used as a light sensor to determine the exposure time of cameras. The inventor of the solar cell as we know it today is the American engineer Russell Ohl. In 1939, he discovered the so-called "P-N junction", which is an area at the interface of a P-type semiconductor and an N-type semiconductor. P-N junctions are used in semiconductor devices such as diodes or transistors. It was during the development of materials for the production of the transistor that a solar cell was created as a by-product, at that time called as a "light-sensitive device" with an efficiency of about 5 %. The current most common monocrystalline cells have an efficiency of about 16 %.
When a photon strikes and the photon is absorbed, a so-called internal photovoltaic phenomenon (also known as the "internal photovoltaic phenomenon") occurs, when a negative electron is released from the atoms of the material when a photon strikes with sufficient energy (at least 1.1 eV) and in its place remains a hole that has a positive charge. There is so-called electron-hole pairs generation. These free electrons and the resulting holes are separated by an electric field, so that an excess of free electrons is formed in the N region of the given P-N junction and an excess of holes in the P region. Thanks to this arises electric voltage between the regions P and N. For silicon cells, it is normally equal from 0.5 V to 0.6 V. An electric charge is equalized and an electric current flows between the electrodes when both electrodes are connected by an external circuit.
The cells are then connected in series and form a so-called (solar) photovoltaic panel. Modern panels are usually equipped with an anti-reflective layer (for example of titanium oxides), which achieves minimal emissivity. This reduces energy losses due to light reflection.
Solar cells can be made of many different materials, but silicon has secured the most important place on the market today due to its low price and well-mastered processing technologies. For efficiency photovoltaic cell is used, however, an important form of this material:
Monocrystalline silicon cell – has been on the market since the seventies. It has a relatively high price, as its production is very energy intensive due to the high consumption of very pure monocrystalline. The main advantage of these cells is high efficiency (12–16 %) and very good durability.
Polycrystalline silicon cell – means an interesting and promising development path. There has been a significant decrease in the energy consumption required to produce polycrystalline silicon of sufficient purity to produce solar cells in recent years. These cells are made by casting pure silicon into molds and then slicing them. Although their efficiency is lower than that of monocrystalline cells, due to the significantly lower price, their efficiency is "satisfactory" 11–16 %.
Amorphous silicon cell – is easier to manufacture and more flexible than the previous two types. Its production takes place by evaporating several thin layers of silicon with an admixture of germanium and several other elements on a plastic, metal or stainless steel surface. This solar cell does not excel much in efficiency (5–7 %), but their advantage is, for example, the just mentioned shape flexibility.
The load characteristic is a standard characteristic providing basic information about the operation of a photovoltaic cell as a source of electricity. Its significant (extreme) points are the "no-load" voltage (electromotive voltage Ue) and the "short-circuit" current (short-circuit current Ik). The "no-load" voltage represents the voltage on the irradiated cell when no appliance is connected to the cell. On the contrary, the "short-circuit" current represents the maximum current that the cell is able to supply under a given lighting. The load characteristic is a graph of the curve of the output voltage of the cell on the current taken (of course under a given constant lighting).
The output of the solar cell is given by the standard relation for the output of electric current on the connected appliance by the relation:
P = U.I
If we consider the solar cell as a source of electricity, it is clear that the important point of the characteristic is the point of maximum power PM. We may also be interested in the course of this power on the current taken. The dependence of both the output voltage and the output power on the current consumed is shown in Figure 1.
Other characteristic values of the solar cell (used as a source) are the fill factor FF (Fill Factor) and efficiency η. The filling factor is characterized as the ratio of the maximum achievable power PM and the power of maximum power (theoretical), which is defined by the voltage "no-load" Ue and the current "short-circuit" Ik:
|FF =||PM||=||UMP IMP|
|Ue Ik||Ue Ik|
It applies to the efficiency of the solar cell, that it is the ratio of the maximum power of the PM cell and the power of the incident solar radiation P in:
|η =||PM||=||UMP IMP|
|P in||P in|
Temperature influence on the operation of the solar cell:
The surface temperature of the cell can reach up to 80 °C with prolonged solar intensity or worsened cooling conditions of the cell (for example no wind). There is a slight increase in photocurrent (increase almost negligible), more significant in this case is a decrease in no-load voltage due to increased temperature. There is a change in the electrical properties of the cell at such temperatures, this leads to a reduction in the load characteristics towards lower voltage, this decrease causes (1) a reduction in power supply P, power factor FF and efficiency η – according to formulas (2) and (3).