Photoelectrochemical cells for hydrogen production from solar energy - Solar Energy Sciences and Engineering Applications - page 328

Figure 10.4.3 Energetic diagram of a PEC cell under dark (a) and illumination (b) conditions.

density, N D , can be obtained from the slope of Equation 10.4.23 while, with extrap-
olation for 1 /C SC = 0 it is possible to obtain the flat band potential, FB , using the

following relation:

SC = Appl . − FB (10.4.24)

10.4.2 Continuity and kinetic equations
The net charge transfer from the semiconductor to the redox species in the electrolyte
solution occurs via majority charge carriers in dark conditions and, via minority charge
carriers under illumination (Nozik and Memming, 1996). In the present section, we
will focus our attention in the light-induced process of the PEC cells. Figure 10.4.3
shows the energy diagram for a PEC cell in the dark and under illumination conditions.
The efficient separation of the photogenerated electron-hole pairs in the semiconductor
highly depends on the electrical field formed at the semiconductor/electrolyte interface.
As mentioned, this field-induced charge separation is a key parameter to high energy-
conversion efficiencies. The strength of the electric field depends on the doping level of
the semiconductor and on other energy features of the systems (Nozik and Memming,
1996). At an n-type semiconductor/electrolyte interface, the electric field drives the
minority carriers (i.e. holes) towards the solid/liquid interface and drives the majority
carriers (i.e. electrons) into the bulk of the semiconductor. The energy storage pro-
cess is completed if the holes undergo interfacial charge transfer to the donors in the
electrolyte prior to any recombination events (Nozik and Memming, 1996). These
recombination events can occur by way of: bulk recombination via bandgap states or
direct electron loss to holes in the valence band; photocorrosion of the semiconduc-
tor; and dissolution reactions (Nozik and Memming, 1996). The last two processes
are responsible for the degradation of the electrode and for the consequent stability
issues.
Optical excitation of a PEC cell leads to the perturbation of the equilibrium
attained when the semiconductor is placed into contact with an electrolyte solution