Page 267 - A Comprehensive Guide to Solar Energy Systems

270 A COmPreHenSIVe GuIde TO SOlAr enerGy SySTemS

FIGURE 12.10 An ultra-thin noble metal film with a dense array of suboptical wavelength apertures formed by thermal
annealing and imaged using an atomic force microscope (left); transparency of an 11 nm thick silver film electrode
supported on plastic with and without a dense array of suboptical wavelength apertures (middle); incident-photon-to-
charge-efficiency (IPCE) spectra for an OPV device using a silver film electrode with and without apertures (right). These
data show how the incorporation of suboptical wavelength apertures into a silver window electrode used in an OPV,
device reduce the amount of light entering the device by direct transmission but increase photocurrent generation due
to light trapping as plasmonic excitations at the surface of the electrode. The data shown is adapted from reference [60].

One approach has been to use a random array of suboptical wavelength apertures formed
by rapid thermal annealing of an optically thin metal film, which offers the advantage of a
broad band optical response, scalability, and simplicity [60,61].

12.4.2 Improving Long-Term Stability

The stability of OPVs is already suitable for many consumer electronics applications and
for indoor devices with lifetimes of several years (e.g., smart electronic tags). However,
for building integration and automotive applications OPVs must have long-term stability
toward: (1) ultra-violet light—which is a significant proportion of the solar spectrum at
ground level; (2) temperatures up to 60°C—which can be achieved even in relatively tem-
perate climates; and (3) oxygen and moisture ingress into the device—which is inevitable
given sufficient time. For the latter, the encapsulation materials are critically important
and so a great deal of research effort has focused on the development of flexible materi-
als with excellent barrier properties toward water and oxygen, based on widely available
raw materials that can also be processed at low cost [62]. One innovative approach has
been the development of ultra-thin glass, which can be bent through a radius of curvature
small enough to be compatible with roll-to-roll printing [63]. If this type of glass can be
produced at low enough cost it will be very attractive as a substrate for OPVs, as glass offers
outstanding water and oxygen barrier properties even when very thin.
The stability of OPV devices depends not only on the intrinsic chemical stability of the
organic semiconductors used, but also on their morphological stability and the stabil-
ity of the interfaces they make with the electrode materials, as well as the stability of the
electrode materials themselves. One way to extend device lifetime is to integrate into the
electroactive part of the device a material whose function does not change significantly