As Figure 3.10 shows, a monocrystalline or polycrystalline silicon solar cell (see Figure 3.11) is composed of a large diode with a barrier layer that is exposed to light. In order for as many light quanta as possible to arrive at a point near the barrier layer, the (usually n-Si) semiconductor zone facing the light must be ultrathin (e. g. 0.5 pm). Moreover, the metallic front electrical contacts that are needed to achieve low internal resistance cannot be allowed to shade more than a minute portion of the active solar cell area. An anti-reflection coating is applied to the outside of the solar cell area to minimize reflection.
When light quanta arrive at the solar cell, those light quanta where h ■ n > EG can be absorbed by the crystal lattice, thus allowing for creation of an electron-hole pair secondary to the internal photoelectric effect. The strong electric field E in the barrier layer quickly separates this electron-hole pair before it can recombine. On account of the electrons’ negative charge, they are subject to a force whose direction is opposite to that of the field direction, and thus accumulate in the n-zone. The holes migrate in the field direction and accumulate in the space charge-free portion of the p-zone.
Since the charges have been separated, the space charge (and thus the electric field strength in the barrier layer) is reduced until it is no longer strong enough to separate electron-hole pairs. It is at this point that the solar cell reaches its open-circuit voltage Voc. In this process, the voltage conducted through the barrier layer becomes far weaker than diffuse voltage VD, but does not quite reach 0. Hence solar cell open-circuit voltage VOC is always somewhat lower than Vd.
Figure 3.10 Basic structure of a crystalline silicon solar cell (aggregate thickness roughly 150-300 pm)
But if, on the other hand, the front and back contacts are briefly shorted via a conductive connection, the load carriers engendered by the internal photoelectric effect immediately flow out of the relevant zones. Neither the volume charge nor the electric field is attenuated, diffusion voltage Vd continues to flow through the barrier layer, and a maximum current for the defined irradianceis conducted, i. e. short-circuit current ISC, which in a solar cell is proportional to irradiance G.
In most cases the n-layer on the cell area is doped considerably more than the p-layer, a difference that is sometimes indicated by marking the n-layer n+. Inasmuch as a solar cell must be electrically neutral as a whole, the volume charge zone extends much further into the p-zone than is suggested by the simplified depiction in Figure 3.10.