Hierarchical Nanostructures for Photo-Electro-Chemical Cells - Hierarchical Nanostructures for Energy Devices - page 176

rapid collection of photogenerated electrons, thereby diminishing the pos-
sibility of charge recombination during interparticle percolation.
In the same context, utilizing hierarchical structures suggests a promising
path towards achieving even higher cell performance. It is not surprising
that the cell eciency has been enhanced by a factor of 4 or 5 through
introducing photoelectrodes with hierarchical nanowire structures in lieu of
straight nanowires. 2,3,10 Hierarchical structures provide a substantially in-
creased surface area, hence enabling higher light harvesting while allowing a
longer effective path for the photons to be absorbed in the photoelectrodes
by stronger scattering and trapping enhancement. Moreover, the resulting
larger surface area boosts the chemical reactions and enhances diffusion of
chemical species (ions or gases) into the interfaces and surfaces in PEC
cells. 11
Hierarchical structures constructed by hybridizing two or more different
materials provide another opportunity. For example, integrating materials of
different band structures enables absorption of photon energy over a
broader range. In addition, each semiconductor conducts the individual half
reactions with effective charge separation in water splitting PEC cells.

d n 3 r 4 n g | 4

8.3 Basic Mechanism of PEC Cells
A PEC cell is a device that directly generates electrical energy and converts
solar energy into chemical fuels such as hydrogen by the water splitting
process. 12 For instance, PEC cells may be referred to as ''PEC cells for water
splitting'' in order to distinguish from ''photovoltaics'' (PVs). 13,14 However,
for the purpose of this chapter, PEC cells include both cases since both
photovoltaics and water splitting cells are devices driven by fundamental
PEC processes.
Both PEC cells for PVs and fuel (hydrogen) production are based on two
key characteristics of semiconductors: (1) electron-hole pairs are generated
when photons are absorbed, (2) space charge, or a built-in electric field near
the surface or interface of a semiconductor eciently separates those pho-
togenerated carriers, thereby preventing recombination. However, the
working mechanisms in PVs and water splitting PEC cells are different in
several aspects. The main difference is that PVs are regenerative cells since
they induce no net chemical change upon converting light directly into
electricity, while water splitting PEC cells involve reduction and oxidation at
each photoelectrode by splitting water into hydrogen and oxygen. More de-
tails are discussed in the next sections.

.

8.3.1 PEC Cell for Electrical Energy Generation
One of the most well-known types of this cell is the dye-sensitized solar cell
(DSSC), also called the dye-sensitized photovoltaic cell or ''Gr¨tzel cell''
named after Michael Gr¨tzel, the pioneer researcher of this type of solar cell.