Your calculator probably has a darkish colored panel just above the display. The panel is made up of solar cells that power up your calculator if there is enough light. You may also have seen some solar panels, which people use for charging up their cell phones. Earlier, these solar cells or photovoltaic cells were exclusively used to power the electrical systems of satellites. However, they are now commonly used in less exotic ways as well.
How do photovoltaic cells convert light to electricity? For this, you must understand the way these cells are constructed. A photovoltaic cell has two silicon plates bonded together. Pure silicon is an insulating material and is unable to conduct electricity. This is because of the atomic structure of silicon, which has place for eight electrons in the outermost shell of its atoms. However, there are only four electrons present.
Therefore, when silicon atoms come together, they share their electrons. Each atom shares one electron with its neighbor and they become a pair. That means at any time, four atoms surround each silicon atom, bringing up its catch of electrons to eight on the outer shell. Since all the electrons are now bound up, there is none left free to move about and carry electric charge.
To make the silicon plates able to carry electric charge, one of the two plates must have some free electrons and the other plate must have some holes or lack of electrons. This is done by the process of doping. While making the plates, one of them is given a few phosphorus atoms as impurities. Since phosphorus has five atoms in the outermost shell of its atom, when combining with the silicon atoms, one of its electrons remains unpaired. This makes the silicon plate with the phosphorus impurity have excess electrons and this is called the n-type silicon.
Likewise, the other plate is doped with boron, which has only three electrons in the outermost shell of its atoms. This leaves the combination of silicon and boron atoms with a deficit of electrons and this is called the p-type silicon. This is like a hole, which will readily grab a wandering electron to fill up its vacant space.
Light is essentially a barrage of energetic particles called photons. Photons impart their energy to the surface where they land, which is why you feel warm when you stand in sunlight. If light or photons are allowed to fall on the n-type silicon plate that has extra electrons, they receive the excess energy from the photons. The extra energy allows them to dislodge themselves from their original positions and wander off until they come to the other plate with the holes, where they are eagerly absorbed.
However, the n-type silicon plate that supplied the electrons now has a deficiency of electrons that it must fill up. For electrons to flow, the circuit must be externally completed. This is usually done by connecting a load to the solar cell through external wires. The plate makes up its deficiency of electrons by borrowing them from the connecting wire. In essence, photons drive the electrons through the entire circuit, and that makes the current flow through the solar cell and the load connected to it.
As soon as light falling on the solar cell is removed, the running electrons lose their drive, and the flow of current stops. Although the output from each cell is usually very tiny, by combining them in series and parallel, an impressive amount of power can be generated.