Chemistry Reference
In-Depth Information
Molecular materials, such as 2-(2-hydroxyphenyl)benzoxazole and 2-(2-hydroxyphenyl)-
benzothiazole, which contain intramolecular hydrogen bonds are known to undergo excited state
(charge transfer) intramolecular proton transfer upon photo-excitation.
10.8 Polymer-Based Solar Cells
Polymer-based solar cells, also known as organic photovoltaic cells, have been around since the
1990s. But their performance, and their efficiency by end of 2010, to convert light to electricity, after
much research, has reached only approximately 8%. This is not good enough to compete with
inorganic solar cells, like those based on cadmium teluride, that convert 10-15% of light to
electricity. This single digit value of organic solar cells pales even further when compared with
some highly specialized, high-priced state-of-the-art inorganic devices with conversion efficiencies
topping 40%. The promise of low-cost organic solar cells, however, has encouraged intense research
in many laboratories in efforts to improve the efficiency. Such research usually focuses on solution-
processable organic polymers that can be converted to semiconductors [
262
]. The polymers used are
regarded as intrinsic wide band gap semiconductors, where the band gaps are above 1.4 eV. This can
be compared to insulators, where the band gaps are below 3 eV. Doping of the film forming materials
is done to introduce extrinsic charge carriers and convert them into organic semiconductors. Such
charge carriers, as explained in Sect.
10.2
, can be positive, p-type, or negative, n-type.
Originally, a donor-acceptor bilayer device of two films was used as an n-p junction in solar cells.
Thus, they were fabricated as sandwich structures. An example would be one where a transparent
substrate is first coated with a conductor, like indium-tin oxide. A conducting polymer like, poly
(ethylene dioxythiphene), doped with polystyrene-sulfonic acid, would then be applied from and
aqueous solution. The indium-tin oxide acts as an electrode for hole injection or extraction. The polymer
is then covered with a conductor, an aluminum foil. The doped polymer can be illustrated as follows:
n
O
O
n
S
SO
3
H
semi conducting polymer
doping material
The construction of the above-described solar cell can be illustrated as follows:
aluminum foil
poly(ethylene dioxythiophene)-polystyrene sulfonic acid
indium tin oxide electrode
transparen layer
The donor material, containing a chromophore, absorbs the light energy and generates excitons.
Excitons are high-energy couples where the energetic electrons are bound to positively charged
