The efficiency limits discussed in this section are based on a concept that has been widely accepted for decades, to the effect that each photon generates only a single electron-hole pair. However, more than a decade ago the idea was developed that this unshakeable law may not apply in certain cases [Gre95], which means that (a) efficiency zt could conceivably be closer to spectral efficiency Zs and (b) efficiency in the neighbourhood of 40% may be within reach. The operant concepts here are the impact ionization effect, quantum well structure cell and subband absorption cell. That said, it should also be noted that, to date, using these various principles, no one has made an actual functioning solar cell with an efficiency exceeding the theoretical efficiency for single-junction solar cells (see Figure 3.25). Hence such concepts should be regarded with a certain amount of scepticism.
Figure 3.36 Cross-section of a two-terminal tandem cell with optical and series connection. The front cell’s band gap energy Eg is similar to that of amorphous silicon (roughly 1.75 eV), while the back cell is made of CIS with Eq about 1 eV. At the contact area between the front and back cell, the holes generated in the front cell recombine with the electrons generated in the back cell. Helmholtz-Zentrum Berlin