Page 106 - A Comprehensive Guide to Solar Energy Systems

Chapter 5 • Sustainable Solar Energy Collection and Storage 101

PV require less materials and energy for manufacturing, and offer lower cost electricity
generation, short energy payback time, and reduced emissions associated with electric-
ity generation [48]. In addition, flexible devices can be created. However, manufacturing
involves costly vacuum processes, and devices contain toxic materials (such as Cd) and
‘critical raw materials’ (e.g., In, Ga, Te), the use of which may limit widespread deployment
of these technologies [25]. The only PV manufacturing from raw materials to product
within Africa to date is carried out by PTiP Innovations in Stellenbosch, South Africa. PTiP
is producing thin-film copper-indium-gallium-selenium-sulphite (CIGSSe) modules on
glass [49–52].
In light of these issues, we are now witnessing the emergence of printable PV
(PPV), which are thin-film devices based on molecular photoactive layers, potentially
manufactured from earth abundant materials using cheap roll-to-roll production. Ear-
ly versions of dye-sensitized solar-cells (dSSC) and organic photovoltaic (OPV) devices
for niche applications are now commercially available, and research into new materials,
improved device performance, and superior manufacturing processes is ongoing. Flex-
ible OPV products have emerged on the market including Heliatek’s Heliafilms for use
in building incorporated PV applications, and in the automotive sector for integration
with car roofs [53]. Solar phone chargers, solar adhesive tapes, and flexible solar foils
are commercially available [54]. Such products are suitable for retrofitting of buildings,
windows, and consumer electronics. Perovskite solar cells [55], the newest of the PPV
technologies, are yet to emerge on the market as issues with device stability have yet to be
fully addressed. However, power conversion efficiencies of lab-based perovskite devices
have already reached 22.1% [56], which is comparable to record cell efficiencies for com-
peting thin-film technologies. PPV can be applied vertically to walls and windows, for
example, the building incorporated PV dSSCs in the façades of the SwissTech Conference
Centre. Additional possibilities arise from the transparent nature of PPV, allowing combi-
nations with existing PV technologies in tandem devices for higher efficiency. The earliest
perovskite products will probably be ‘tandem cells’, in which a perovskite device is com-
bined in tandem with existing PV technologies [51]. This is the goal of Oxford PV who are
developing and commercialising thin-film perovskite solar cells for printing directly onto
Si or CIGS modules.
PPV is cheap because it can be made using roll-to-roll production on flexible substrates
[57] using solution deposition of materials. In such processes, rolls of substrate are run
through a series of sequential deposition techniques in which each of the layers of solar
cells are deposited as thin films (10 nm–10 µm), with the final coated product recoiled at
the end of the line (Fig. 5.12). The result is rapid production at relatively low cost. Substrates
include metals such as steel for functionalized building envelopes, or ITO on polyethylene
terephthalate (PET) for transparent devices.
The low cost of PPV in comparison to other technologies will make it an interesting
option for the African market. Its lightweight nature will also be useful for retrofitting rural
homes. However, lifecycle challenges must be addressed for these technologies before they