WHAT IS SOLAR CELLS?

Solar cells are in fact large area semiconductor diodes. Due to photovoltaic effect energy of light (energy of photons) converts into electrical current. At p-n junction, an electric field is built up which leads to the separation of the charge carriers (electrons and holes). At incidence of photon stream onto semiconductor material the electrons are released, if the energy of photons is sufficient. Contact to a solar cell is realised due to metal contacts. If the circuit is closed, meaning an electrical load is connected, then direct current flows. The energy of photons comes in "packages" which are called quants. The energy of each quantum depends on the wavelength of the visible light or electromagnetic waves. The electrons are released, however, the electric current flows only if the energy of each quantum is greater than WL - WV (boundaries of valence and conductive bands).

FEATURES OF SOLAR CELLS

1.Crystalline Silicon Solar Cells among all kinds of solar cells we describe silicon solar cells only, for they are the most widely used. Their efficiency is limited due to several factors. The energy of photons decreases at higher wavelengths.

The highest wavelength when the energy of photon is still big enough to produce free electrons is 1.15 μm (valid for silicon only). Radiation with higher wavelength causes only heating up of solar cell and does not produce any electrical current. Each photon can cause only production of one electron-hole pair. So even at lower wavelengths many photons do not produce any electron-hole pairs, yet they effect on increasing solar cell temperature.

The highest efficiency of silicon solar cell is around 23 %, by some other semi-conductor materials up to 30 %, which is dependent on wavelength and semiconductor material. Self loses are caused by metal contacts on the upper side of a solar cell, solar cell resistance and due to solar radiation reflectance on the upper side (glass) of a solar cell. Crystalline solar cells are usually wafers, about 0.3 mm thick, sawn from Si ingot with diameter of 10 to 15 cm. They generate approximately 35 mA of current per cm2 area (together up to 2 A/cell) at voltage of 550 mV at full illumination. Lab solar cells have the efficiency of up to 20 %, and classically produced solar cells up to 15 %.

2.Amorphous Silicon Solar CellsThe efficiency of amorphous solar cells is typically between 6 and 8%. The Lifetime of amorphous cells is shorter than the lifetime of crystalline cells. Amorphous cells have current density of up to 15 mA/cm2, and the voltage of the cell without connected load of 0.8 V, which is more compared to crystalline cells. Their spectral response reaches maximum at the wavelengths of blue light therefore, the ideal light source for amorphous solar cells is fluorescent lamp.

MODELS OF SOLAR CELLS

The simplest solar cell model consists of diode and current source connected parallelly. Current source current is directly proportional to the solar radiation. Diode represents PN junction of a solar cell. Equation of ideal solar cell, which represents the ideal solar cell model, is: Where is: IPh - photocurrent (A), IS - reverse saturation current (A) (aproximately range 10-8/m2), V - diode voltage (V), VT - thermal voltage (see equation below), VT = 25.7 mV at 25°C, m - diode ideality factor = 1...5 x VT (-) (m = 1 for ideal diode) Ideal solar cell model Thermal voltage / VT / ( V ) can be calculated with the following equation: Where is: k - Boltzmann constant = 1.38 x 10-23 J/K, T - temperature ( K ), q - charge of electron = 1.6 x 10-19 As Real Solar cell model with serial and parallel resistance Rs and Rp,the consequences of resistances are voltage drop and parasitic currents The working point of the solar cell depends on load and solar insolation. In the picture, I-U characteristics at short circuit and open circuit conditions can be seen. Very important point in I-U characteristics is Maximal Power Point - MPP. In practice we can seldom reach this point, because at higher solar insolation even the cell temperature increases, and consequently decreasing the output power. As a measure for solar cell quality fill-factor - FF is used. It can be calculated with the following equation: Where is: Impp - MPP current ( A ), Vmpp - MPP voltage ( V ), Isc - short cirquit current ( A ), Voc - open cirquit voltage ( V ) In the case of ideal solar cell fill-factor is a function of open cirquit parameters and can be calculated as follows (Stone, see literature below): Where is: voc - voltage calculated with equation below ( V ) Where is: k - Boltzmann constant = 1.38 x 10-23 J/K, T - temperature ( K ), q - charge of electron = 1.6 x 10-19 As, m - diode ideality factor ( - ), Voc - open cirquit voltage ( V ) .