A stream of photons containing minute packets or quanta of energy shines on the cell. Their numbers are staggering: in strong sunlight a 6 inch (15cm) cell receives more than 1019 photons every second. Various possible fates await them, some productive, others fruitless, and we show a few important examples in the figure.Unfortunately, there is some loss of photons by optical reflection back from the conducting fingers, top surface, and rear surface (nos. 1, 2, and 3 in the figure). The rest enter the cell body, but only those with a certain minimum energy, known as the bandgap, have any chance of creating an electron-hole pair and contributing to the cell’s electrical output. The most productive ones, for reasons explained below, create electron–hole pairs in the n-type layer or in the p-type layer very close to the junction (4 and 5). Less productive, on average, are the ones that travel further into the p-type material (6). Successful cell design involves producing as many electron–hole pairs as possible, preferably close to the junction. But even high quality cells are subject to theoretical limits dictated by the spectral distribution of sunlight, the nature of light absorption in silicon, and quantum theory. We shall discuss these topics a little later.
First comes the big question: what happens to the electron–hole pairs generated within the cell by sunlight, and how do they produce current flow in an external circuit?
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, making 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 behavior of majority carriers under the influence of an applied external voltage.
With solar cells, however, it is light generated minority carriers that take centre 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 recombination 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 traveled through the crystal and the more likely it is to be lost by recombination. Two measures are used to describe the risk. The carrier lifetime is the average amount of time between electron–hole generation and recombination (the bigger the better) which for silicon is typically 1μs. The diffusion length is the average distance a carrier moves from the point of generation until it recombines, for silicon typically 0.2 mm which is comparable with the thickness of the monocrystalline wafer. This again emphasizes the value of electron–hole pairs generated close to the junction.
We have now covered some fundamental aspects of solar cell operation, including the key role played by light generated minority carriers. The next task is to consider the voltage–current characteristics of the cell as measured at its output terminals.
