Page 574 - Polymer-based Nanocomposites for Energy and Environmental Applications

Hybrid materials based on polymer nanocomposites for environmental applications 527

19.3.2 Energy and environment issues

In this section, we present the second approach for preserving the environment
through the energy aspect. As mentioned in the introduction, the utilization of the
available fossil and fission energies has a negative impact on the environment that
needs to be neutralized by treatments of air and water to improve the human living
conditions. Besides the threat of shortage of primary energy sources, it is also nec-
essary to limit the pollution due to their transformation and use. There are three
ongoing strategies for the energy development for these objectives: (i) to use natural
and clean energy sources such as solar, wind, and ocean energies; (ii) to store the
excess of produced energy in order to reuse it later; and (iii) to control the energy
consumption and make saving by appropriate techniques. Hybrid polymer-based
composites have been developed to address these problems and have shown their
competitive efficiency with conventional techniques. We review and discuss in this
section the different aspects of the energy/environment relation and the solutions by
using materials and technologies to improve and preserve nature and our living
conditions.

19.3.2.1 Hybrid nanocomposites for solar cells
The first option for improving the environmental conditions is to use clean energy
sources to replace fossil and fission fuels. Such energy sources are abundant all over
the world and can be exploited almost without limit. They include solar, wind, and
ocean energies.
Solar energy can be exploited by two ways: solar thermal and solar photovoltaics.
With the solar thermal, the sunlight energy is directly converted to heat, which will be
then used for heating or to operate steam generators to produce electricity. With solar
photovoltaics, the sunlight is directly converted to electricity, which will be then used
for powering devices or electric stations. We will focus in this section on the hybrid
polymer composites that are used as absorbers in solar cells.

Operating principle and performance of solar cells
A solar cell converts light into electricity by a five-step process as illustrated in
Fig. 19.10. The absorber or photoactive layer is composed of two components, which
are donors (D) and acceptors (A). For a conventional semiconductor solar cell, an inci-
dent photon of appropriate energy will create an electron in the CB of the donor, which
is associated with a hole in the VB (step 1). The electron-hole pair or exciton can move
through the material (step 2). At the D-A interface, the created photon will be disso-
ciated by the offset in energy levels of the CB and the VB between the donor and
acceptor (step 3). The electrons will be transferred to the CB of the acceptor and
the holes to the VB of the donor (step 4). Finally, electrons are collected at the cathode
and holes at the anode of the solar cell (step 5).
According to this scheme, the performance of solar cells depends strongly on the
creation of excitons, on the charge separation and the collection of the charge