Page 142 - Multifunctional Photocatalytic Materials for Energy

128 Multifunctional Photocatalytic Materials for Energy

they will fall short of fulfilling the burgeoning global demand for energy. Because of
increasing global warming, air pollution, and environmental concerns, greater efforts
are being directed toward the development of energy storage and energy conversion de-
vices with high energy and power densities [1]. Therefore, over recent years, the quest
has steadily increased to find alternative, unconventional energy sources. Moreover, the
production of “clean and green energy” as an alternative to nuclear energy has become
a factor of utmost importance in energy production. However, the development of al-
ternative strategies for the production of clean energy is one of the biggest challenges
facing the scientific community. In this regard, several alternative energy sources have
been extensively explored, including wind, hydro, and solar energies [2,3]. Among the
various energy sources, solar energy is considered to be the most outstanding alterna-
tive source of renewable energy owing to its unlimited energy supply of ~10000 TW,
which is much greater than total worldwide energy consumption [4]. Generally, solar
cells convert the incident photon energy into useful electrical energy through the gen-
eration and subsequent collection of electron-hole pairs. Based on their performance
and cost-effectiveness, solar cells are classified in three main types: (i) silicon-based
solar cells, (ii) thin film-based solar cells, and (iii) dye-sensitized solar cells (DSSCs).
Silicon-based solar cells were the precursors of photovoltaic cells and formed the
first-generation solar cells. These types of solar cells are favored mostly because of an
abundance of silicon and a large industrial infrastructure that facilitates bulk produc-
tion of solar cells. The majority of commercially available solar cells are fabricated by
using a solid state p-n heterojunction semiconductor first reported by Chapin et al. in
1954 [5]. When the photon is incident on the semiconductor surface, an electron-hole
pair is created and a separation occurs at the junction and charge carrier, which is
collected through the p-n terminal of the semiconductor. Light absorption and charge
carrier transport are established by the same semiconductor materials. However, sili-
con types of photovoltaic cells are expensive and require high-purity material, which
leads to an unfriendly environmental manufacturing process [5].
In the second type of solar cells, thin film-based solar cells, semiconducting ma-
terial such as CuInSe, CdTe, CdS, and Cu(In,Ga)Se 2 [6–8] are utilized to create a
p-n junction. However, thin film-based solar cells exhibit low efficiency, increased
toxicity, and low cost-effectiveness, which are of concern in terms of their large-scale
production [4]. Therefore much interest is focused on the development of an alterna-
tive to conventional solar cells that is highly efficient, that is not expensive to produce,
and that is also ecofriendly.
The third type of solar cells include, dye-sensitized solar cells (DSSCs), organic
solar cells (OVCs), perovskite solar cells (PSCs), and polymer-based solar cells
that are different from previous types of solar cells and are considered to be third-
generation solar cells [9–11]. These types of solar cells are not affected by the Shockley-
Queisser limit and do not rely on p-n heterojunctions to separate photogenerated
charge carriers. These types of solar cells generally require low-cost materials and easy
device-fabrication and are capable of achieving versatile material synthesis. However,
the performance of these types of solar cells still needs to be developed because they
still suffer from lower efficiency, stability, and reproducibility as compared with p-n
junction solar cells. The literature reveals that efforts have focused primarily on the