Page 254 - A Comprehensive Guide to Solar Energy Systems

Chapter 12 • Organic Photovoltaics 257

are low temperature, rapid fabrication methods that are scalable to large area, and so offer
a large cost advantage over the high temperature batch-to-batch processes used to fabri-
cate convention PV [15].
An important consequence of the low processing temperature for OPVs is that the
energy required to fabricate an OPV module is expected to be ∼5% of that needed to
produce a multicrystalline silicon PV module. Additionally, due to the very low thickness
of semiconductor needed, less than 1 g of organic semiconductor is sufficient to produce
2
1 m of OPV cells. As a result, OPVs have an intrinsically low carbon footprint both in
terms of the manufacturing process and the materials used, and can return the energy
used in their fabrication within a few months of installation [15]. For example, in 2013
an ePBT of ∼180 days was reported for a large-scale installation of solution-processed
OPVs with a power conversion efficiency of only 1.6%–1.8% when operated in a southern
european setting [16], and so it is realistic to expect that an ePBT of <100 days is reach-
able with today’s high performance OPVs. An ePBT of <100 days would be the lowest of
any renewable source of electricity, being comparable to that of land-based wind tur-
bines [8,16]. remarkably, a fundamental analysis suggests that an ePBT of the order of
1 day could ultimately be achieved for solution-processed OPVs, through innovations in
materials and manufacture [17].
Outside of the physical sciences the term organic is very often associated with living
things, although in the context of OPVs this prefix refers to the use of carbon-based semi-
conducting molecules as the light-harvesting element, in place of conventional inorganic
semiconductors. Interestingly however, two of the most important molecules for life; heme
and chlorophyll are both based on the porphyrin macrocycle, which is a type of organic
semiconductor. Indeed, porphyrin derivatives were used in early stages of the develop-
ment of OPVs owing to their strong absorption in the visible spectrum [18]. Importantly,
like heme and chlorophyll, the organic semiconductors used in high performance OPVs
contain no toxic heavy metals, such as lead or cadmium, or rare earth metals such as ru-
thenium, which are essential elements of other types of emerging PVs [19,20]. For practical
applications the possibility of catastrophic failure of the materials separating the active
electronic materials from the environment must be considered. The use of toxic elements
such as lead and cadmium, which are used in lead halide perovskite [19] and quantum dot
PVs [20] respectively, presents a significant barrier to market entry for these PV technolo-
gies in many parts of the world, not least because contamination of the natural environ-
ment by toxic metals is well recognized as a major global problem [21]. Indeed, in europe
and the uSA, regulations on lead already restrict the shipping of lead perovskite PVs in
these areas. In contrast OPVs are almost certainly the most environmentally sustainable
class of emerging PV technology in terms of the toxicity and sustainability of the materials
used in their production. At the end of life, OPV modules can also be safely incinerated and
the metals used in the electrodes recovered, which is made easy by simple deinstallation to
form a compact roll [16]. Since the thickness of the organic semiconductor layer used in an
OPV is less than 1 µm, the total CO 2 produced by incineration per unit area of PV module
is also very small [8,16].