As we have seen majority carriers (electrons in n-type material holes in p-type) are the main players in a conventional semiconductor diode By initial diffusion across the p-n junction they set up a depletion layer and create a potential barrier Forward-biasing the diode reduces the height of the barrier marking it easier for them to cross the junction and produce substantial current in reverse bias the barrier increases and current flow is severely inhibited Diode action is principally due to the behaviour of majority carriers under the influence of an applied external voltage
With solar cells however it is light-generated minority carriers that take center stage the basic reason may be simply stated a potential barrier that inhibits transfer of majority carriers across a p-n junction positively encourages the transfer of minority carriers Whereas majority carriers
Experience a hill to climb minority carriers see a hill to roll down With luck they are swept down this hill collected at the cell terminals and produce an output current proportional to the intensity of the incident light
Let us consider the three photons in Figure 2.8 that successfully create electron-hole pairs in the crystal lattice Number 4 produces a pair in the p-type region close to the junction Its free electron a minority carrier in p-type material is easily swept across the junction and collected So is the hole produced in the n-type region by number 5 which is swept across the junction in the opposite direction Both these minority carriers should contribute to the light-generated current
Photon 6 also creates an electron-hole pair but well away from the junction and its associated electric field The free electron does not immediately experience a hill to roll down but instead starts wandering randomly through the silicon lattice In the figure it is shown eventually reaching the junction and being swept away to success But the journey is a dangerous one it may instead encounter a hole and be annihilated Although such reco bination is not illustrated in the figure unfortunately it occurs not only in the main body of the cell (bulk recombination) but even more importantly at the edges and metal contacts due to defects and impurities in the crystal
The longer a minority carrier wanders around the greater the distance travelled through the crystal and the more likely it is to be lost by