As a prelude to the detailed examination of the design and performance of multijunction cells, it is useful to review briefly the fundamental factors that limit the efficiency of single-junction cells. Consider an ideal single-junction cell with characteristic bandgap Eg. A photon incident on this cell with photon energy hv > Eg will be absorbed and converted to electrical energy, but the excess energy hv — Eg will be lost as heat. The greater the excess, the lower the fraction of that photon’s energy will be converted to electrical energy. On the other hand, a photon of energy hv < Eg will not be absorbed and converted to electrical energy. Thus, the efficiency of photon conversion is a maximum efficiency at hv = Eg. Note that this maximum efficiency is less than 100%; the maximum work per absorbed photon is calculated by Henry .
Since the solar spectrum is broad, containing photons with energies ranging from near 0 to 4eV, single-junction solar cell efficiencies under the unconcentrated spectrum of a black body at the temperature of the sun are limited to 31%, called the Shockley-Queisser limit (see Chapter 4). The solution to this problem is (in principle) simple: rather than trying to convert all the photon energies with one cell with one bandgap, divide up the spectrum into several spectral regions and convert each with a cell whose bandgap is tuned for that region. For instance, suppose the spectrum is divided up into three regions ж – hv1, hv1 – hv2, and hv2 – hv3, where hv1 > hv2 > hv3.
The light from these spectral regions would be converted by cells with bandgaps Eg1 = hv1, Eg2 = hv2, and Eg3 = hv3, respectively. The more spectral regions allowed, the higher the potential overall efficiency.