Some common examples of such polymers are polymethyl methacrylate (PMMA),

polyamide (PA), and polyvinyl alcohol (PVA). Some performances of the PENGs based

on the mentioned polymers are listed in Table 20.1.

20.3.1.3 TEG

TEG nanogenerators are based on the generation of thermoelectricity due to the Seebeck

effect. The Seebeck effect usually tells us that a temperature gradient established between

two dissimilar electrical conductors or semiconductors leads to a potential difference be­

tween the two substances. The potential difference is due to the migration of charge carriers

(electrons or holes) from one junction (hot) to the other (cold) [14]. The efficiency of a TEG is

determined by the figure of merit (FOM) of a thermoelectric material (which is practically a

dimensionless constant), and is mathematically expressed as [14]

FOM

S

T

k

k

=

+

e

lat

2

(20.5)

where ke and klat is the thermal conductivity of the electron and lattice, respectively. is the

electrical conductivity of the materials used in the TEG, is the Seebeck coefficient, and T is the

absolute temperature. It is clear from the above equation that to obtain a high conversion rate

of thermal to electrical energy, materials with a very high value of S and

are required [8].

20.3.1.4 PyNG

The pyroelectric nanogenerators are typically based on the pyroelectric effect, which

refers to the production of voltages by certain classes of materials when there is a change

in temperature [7]. When a temperature gradient is established, the voltages are produced

due to the difference in the polarization (in certain materials that naturally possess

spontaneous polarization). The temperature change thus leads to a current in the external

circuit. From the theory of pyroelectricity, the potential across the electrodes connected to

a pyroelectric material is given by [15]

U

P

T

=

×

(

1)

r

0

(20.6)

where P denote the change in the polarization, T is the change in the temperature, r is

the permittivity of the pyroelectric material, and 0 is the permittivity of the vacuum. The

technology based on PyNG was first demonstrated in 2012 in which a lead zirconium

titanate-based PyNG was used for monitoring the temperature changes [16]. However,

the toxicity associated with the PZT led to the development of KbNO3 nanowire-based

PyNG to harvest the energy from sunlight illumination. These PyNGs were able to

generate an output voltage and current of 2.5 mV and 25 pA for a temperature change of

295 to 298 K. It is expected that a higher temperature change shall lead to higher pyro­

electric potential, as shown in equation (20.6).

20.3.2 Photovoltaic Energy Harvesting

Light energy is a source of energy that can power portable energy devices. Photovoltaic

(PV) cells are sources that can convert light energy into electricity. The photovoltaic effect

in this context refers to the transformation of the sunlight to useful voltages or currents

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Bioelectronics