The black pallets you see on roofs or the grey little roofs you can see over meteorological stations in remote areas are solar cells. The solar cell works making use of the photovoltaic effect, that is generating voltage from the energy of the sun. For this we need two semi-conductors, e.g. two kinds of silicon. If a junction of such semiconductors is lighted, the energy of the light will separate the charge carriers from one of them and enable them to travel to the other one. In one of the semiconductors there is electron deficiency whereas in the other there is excess of them. Thus electric voltage is created.
The operation of such a junction is not easy, not to mention its cost. The silicon has to be of a specific kind, with the admixture of boron ( p-type) or with the admixture of phosphorus ( n-type). The light has to possess enough energy and the resulting voltage is always lower than the light energy. Cheap silicon cells produce only 0.4 volt.
Solar cells are used to power artificial satellites, space probes, calculators and watches.
On a small scale a photoelectrical panel can be built in a very simple way. Take a common photoluminescent diode and invert its function. Instead of generating light by attaching it to a battery, the electric current (or more precisely the voltage) can be generated when the diode is exposed to light. Connect the diode to the entrance of a voltmeter (with a high entrance resistance) and illuminate the diode with a lamp or solar light. A voltage of about 1 V can be read, depending not on the light intensity but on the colour of the diode used. Clearly, this is a photoelectric effect, or more precisely internal photoelectric effect.
In the external photoelectrical effect, electrons are ejected out of a metal surface by impinging photons. The energy of photons (E = hv = hc/λ) must be sufficiently high to overcome so-called work function. In other words, the wavelength l of the light must be sufficiently short, or its colour must be " blue enough". The work function corresponds to the minimum energy needed to extract an electron from the metal and is a characteristic feature of the metal. The photoelectric effect was explained by A. Einstein in 1905.
Ekin = h v - W
Ekin ... is the kinetic energy for the emitted electron, h ..... is Planck's constant (6.626.10-34 Js), v ..... is the frequency of light impinging on metal, W ..... work function for this metal.
In the internal photoelectric effect, an electron is moved from a lower band (called a valence band) to a higher one, called a conduction band, leaving a hole in the conduction band. The electron and the hole drift in opposite directions. The voltage created corresponds to the band gap between the valence and the conduction bands, with c.a. 0.8 V subtracted (due to so-called polarization of the junction). The colour of a photoluminescent diode is related to the band gap (which depends on the type of semiconductors used). For example, the red colour at the 680 nm wavelength corresponds to the energy of 1.8 eV. Therefore, the red diode in our experiment produces 1.0 V and the green one (520 nm) 1.5 V.