Solar Cell -- The P-N Junction

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A conventional monocrystalline solar cell has a silvery top surface surmounted by a fine grid of metallic fingers forming one of its electrical contacts. What is less obvious is that the cell actually consists of two different layers of silicon that have been deliberately doped  with very small quantities of impurity atoms, often phosphorus and boron, to form a p–n junction. The addition of such dopants  is absolutely crucial to the cell’s operation and provides the mechanism which forces electrons and holes generated by sunlight to do useful work in an external circuit.

The p–n junction may be regarded as the basic building block of the semiconductor revolution that began back in the 1950s. It is perhaps a little surprising that an invention normally associated with mainstream electronics should also form the basis of PV technology; but a silicon solar cell is essentially a form of p–n junction specially tailored to the task of converting sunlight into electricity.

We have already noted that heating or shining light on pure silicon can alter its electrical properties, progressively converting it from an insulator into a conductor. Another extremely important way of modifying its properties is to add small amounts of dopants. For example if phosphorus is added to molten silicon, the solidified crystal contains some phosphorus atoms in place of silicon. While the latter has four valence electrons able to form bonds with neighboring atoms, phosphorus has five. The extra one is only weakly bound to its parent atom and can easily be enticed away, as shown in Figure 2.5 (a). In other words silicon doped with phosphorus provides plenty of free electrons, known as the majority carriers. Generally there are also a few holes present due to thermal generation of electron – hole pairs, as in intrinsic silicon, and these are called   minority carriers.  The material is a fairly good conductor and is referred to as negative - type or n - type.

A complementary situation arises if silicon is doped with boron, which has only three valence electrons loosely bound to its nucleus, illustrated in part (b) of the figure. Each boron atom can only form full bonds with three neighboring silicon atoms, so boron introduces broken bonds into the crystal. In this case holes are the majority carriers and electrons the minority carriers. Once again, the material becomes a conductor; it is referred to as positive - type or p - type.

We see that n  - type material has many surplus electrons and p  - type material has many surplus holes. The next step is to consider what happens when the two materials are joined together to form a p–n junction, illustrated in Figure  2.6 (a).

Near the interface, free electrons in the n - type material start diffusing into the p - side, leaving behind a layer that is positively charged due to the presence of fixed phosphorus atoms. Holes in the  p - type material diffuse into the n - side, leaving behind a layer that is negatively charged by the fixed boron atoms. This diffusion of the two types of majority carriers, in opposite directions across the interface, has the extremely important effect of setting up a strong electric field, creating a potential barrier to further flow. Equilibrium is established when the tendency of electrons and holes to continue diffusing down their respective concentration gradients is offset by their difficulty in surmounting the potential barrier. In this condition there are hardly any mobile charge carriers left close to the junction and a so called depletion region is formed.

The depletion region makes the p – n  junction into a diode, a device that conducts current easily in one direction only. Figure  2.6 (b) shows an external voltage V applied to the diode, making the p-type material positive with respect to the   n  - type, referred to as   forward bias.  In effect the external voltage counteracts the  ‘ built - in ’  potential barrier, reducing its height and encouraging large numbers of majority carriers to cross the junction – electrons from the n - side and holes from the p - side. This results in substantial forward current flow (note that conventional positive current is actually composed of negatively charged electrons flowing the other way; we may think of them as going right round the circuit through the battery and back into the n - type layer). Conversely if the external voltage is inverted to produce a reverse bias, the potential barrier increases and the only current flow is a very small dark saturation current (I0 ). This is because a bias that increases the potential barrier for majority carriers decreases it for minority carriers  –  and at normal temperatures there are some of these present on both sides of the junction due to thermal generation of electron-hole pairs.

The practical result of these movements of electrons and holes is summarized by the diode characteristic in Figure 2.7. Diode current I increases with positive bias, growing rapidly above about 0.6 V; but with negative bias the reverse current ‘saturates’ at a very small value I0 . Clearly this device only allows current flow easily in one direction. Mathematically the curve is expressed as: 

where q is the charge on an electron,   k  is Boltzmann’s constant, and T is the absolute temperature.

You are perhaps beginning to wonder what all this has to do with solar cells, because we have not so far discussed the effects of shining light on the diode and it is not obvious what these will be. However, rest assured that understanding the above discussion of electrons and holes, majority and minority carriers, and potential barriers is essential for unraveling the mysteries of photovoltaics!