Page 253 - A Comprehensive Guide to Solar Energy Systems

256 A COmPreHenSIVe GuIde TO SOlAr enerGy SySTemS

Again, for consumer acceptance the PV module must not add significantly to the weight of
the device or detract from the ergonomics or its aesthetics. A further critically important
constraint for both of the aforementioned application areas is the need for the PV module
to be comprised of nontoxic materials, as consumers and regulatory bodies demand that
catastrophic failure of the device does not present a significant risk to the user or natural
environment.
The dominant commercial PV technologies of today are fabricated using batch-to-
batch processes, which are slow compared to continuous production processes such as
roll-to-roll printing. Conventional PVs are also fabricated with at least one high tem-
perature-processing step, which limits the choice of supporting substrate and is energy
intensive. The latter is important because it is a key determinant of the energy payback
time (ePBT), which is the time taken for the PV module to produce the energy that goes
into its production, and therefore its CO 2 mitigation potential [6]. For example, silicon
production requires a processing temperature of ∼1500°C and so this energy intensive
processing step, combined with the relatively thick layer of semiconductor used and
slow batch-to-batch fabrication process, means that its ePBT is typically 1.5–2.7 years
[7,8].
In contrast, PVs based on organic semiconductors (organic photovoltaics—OPVs),
use a semiconductor thickness of less than 1 µm, which is less than 1% of the thickness
used in conventional inorganic PVs. Additionally, OPVs are processed at temperatures
below 150°C, making them compatible with low cost flexible plastic substrates, such as
poly(ethylene terephthalate) [9], and continuous roll-to-roll processing by printing from
solution [10] or vacuum evaporation [11,12] (Fig. 12.1). Consequently, OPV modules can
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be extremely light weight at <0.5 kg m [13,14] and with a thickness of <1 mm [13,14]
making them very well matched to applications in transportation and portable consumer
electronics. While the fabrication cost is likely to be lowest for wholly printed OPVs (since
all processes are performed at ambient pressure) both printing and vacuum evaporation

FIGURE 12.1 Vacuum processed small molecule OPVs produced by Heliatek GmbH (photographer: Tim Deussen) (left);
solution processed polymer/small molecule OPVs produced by InfinityPV ApS (right: upper and lower).