When it comes to understanding the main operating principle of solar cells, it suffices to investigate what is known as a homogeneous junction, i. e. the junction between p – and n-conducting semiconductors made of the same base material. A homogeneous junction automatically engenders a space charge zone and thus a
space charge region,
barrier zone, p-n-junction
strong electric field, which can be used to separate the electron-hole pair resulting from the internal photoelectric effect – as can of course the space charge zones and electric fields associated with the p-n junction between different base materials. In this scenario, known as a heterogeneous junction, the p – and n-doped parts are composed of chemically heterogeneous materials, or a Schottky junction can occur, i. e. a junction between a semiconductor and a metal.
Figure 3.7 illustrates such a p-n junction without an external voltage source. Here, electrons are diffused into the p-zone from the n-zone, where they fill holes. The positively charged donor atoms left behind engender a positive space charge in the n-zone, while the now negatively charged acceptor atoms engender a negative space charge in the p-zone. These space charges create an electric field in the
boundary layer that initially impedes further electron diffusion and ultimately brings it to a halt. The barrier layer thus created at the boundary between the n – and p-material is now devoid of freely moving charge carriers. Diffusion voltage Vd is created via the diffusion zone, thus also engendering a potential difference.
Although what we have said thus far accounts for the creation of a diffusion voltage VD, we are still in the dark as to its level, which is a key factor for solar cells as it determines the maximum possible open – circuit voltage Voc. In all solar cells, Voc is lower than Vd.
When the effects of electron diffusion from the n-zone to the p-zone in the band model are taken into consideration, it becomes clear that the consequent lower potential Von the p-side induces an increase in the p-side energy bands (the electrons have more energy on account of their negative charge resulting from lower potential). The electrons diffused from the n-zone to the p-zone can gain energy until the lower valence band edge in the p-zone increases to the point where there is no longer a substantial difference between the energy level of the donor electrons on the n-side and that of the acceptor holes on the p-side. Inasmuch as the energy of the donor electrons is ED lower than the bottom conduction band edge, and the energy of the acceptor holes is EA higher than the upper valence band edge, eVD must be somewhat lower than band gap energy EG.
The following applies to homogeneous p-n junctions at ambient temperature:
Hence the diffusion voltage is roughly 0.35-0.5 V lower than the so-called theoretical photovoltage VPh (see Equation 3.16), which is determined by dividing the band gap energy EG by the electron charge (e = 1.602 ■ 10—19As).
If a metal contact is integrated into the n-zone and p-zone, a semiconductor diode results (see Figure 3.8). If such a diode is briefly short-circuited, despite the diffusion voltage at the p-n junction the flow of current is still blocked. In such a case, space and contact charges are immediately created at the contact points between the metal and semiconductor, and this exactly compensates for the diffusion voltage.