Charge Carrier Separation in Solar Cells

The selective transport of electrons and holes to the two terminals of a solar cell is often attributed to an electric field, although well-known physics states that they are driven by gradients of quasi-Fermi energies. However, in an illuminated semiconductor, these forces are not selective, and they drive both charge carriers toward both contacts. This paper shows that the necessary selectivity is achieved by differences in the conductivities of electrons and holes in two distinct regions of the device, which, for one charge carrier, allows transport to one contact and block transport to the other contact. To clarify the physics, we perform numerical simulations of three different solar cell structures with asymmetric carrier conductivities. Two of them achieve the ideal conversion efficiency limit, despite the fact that the charge carriers flow against an internal electric field, proving that the latter cannot explain carrier separation. A third, i.e., conceptual structure, has no electric field at all but still works ideally as a solar cell. In conclusion, the different conductivities of electrons and holes in two regions of the device can be identified as the essential ingredient for charge carrier separation in solar cells, regardless of the existence of an electric field.