Page 268 - A Comprehensive Guide to Solar Energy Systems

Chapter 12 • Organic Photovoltaics 271

when oxidized, such that it serves as a sink for water and oxygen [64]. An important ad-
vance in OPV research came with the discovery that thin films of the partially reduced
transition metal oxides WO 3−x , moO 3−x , or V 2 O 5−x , deposited either by vacuum evaporation
or solution processing, can be used as high performance hole-extracting materials at the
interface between the light harvesting organic semiconductors and the hole-extracting
electrode [65,66]. What is special about these materials is that they are intrinsically n-type
due to oxygen array vacancies, with a large work function and wide bandgap. Consequent-
ly, these oxides are transparent and can be used to ensure optimized alignment of the
electrode Fermi level with the HOmO of the donor phase in a bulk heterojunction. The
high work function is important for OPV stability, because it enables the use of organic
semiconductors with a high ionization potential (i.e., a deep lying HOmO), which offer
improved stability toward oxidation in air.
Accelerated stability testing of Heliatek’s [33] double junction cells (efficiency ∼7.7%)
using conventional foil encapsulates have shown less than 10% degradation after 3000 h
in damp heat (85°C/85% relative humidity) under 1 sun continuous illumination. This re-
markable stability, particularly given the complexity of the device, bodes very well for the
prospects of achieving OPV module lifetimes suitable for building integration applications
in the near future.

12.4.3 Minimizing the Cost of Materials and Device Fabrication

To realize the full cost advantage and potential environmental benefits of OPVs over oth-
er types of emerging PV technologies, there are several areas in which developments are
needed. It is essential that the organic semiconductors used in an OPV module can be
synthesized using inexpensive, easily accessible starting materials in a small number of
synthetic steps, as the production cost (and embodied energy) scales rapidly with num-
ber of synthesis steps. For example, P3HT (Fig. 12.5), which was the work horse of OPV
research for many years, can be produced in as few as three synthetic steps using low cost
precursor materials [67]. Given that less than 1 g of organic semiconductor is needed to
2
fabricate 1 m of OPV devices, the volume of materials required for large-scale production
of OPVs is relatively small, which gives scope for flexibility in the synthetic methods used
for scale-up [68].
An important aspect that has received surprisingly little attention to date, is the need
to move away from using chlorinated solvents for deposition of solution-processed or-
ganic semiconductors [69,70]. Chlorinated solvents are toxic and unsustainable, and so
the precautionary and containment measures needed to enable their use for large-scale
roll-to-roll production will inevitably add to the production cost of solution-processed
OPVs while also undermining the green credentials of this type of OPV. notably, this is
not a problem that needs to be addressed for small molecule OPVs fabricated by vacuum
processing.
A key difference between the design of silicon PVs and OPVs, is that the latter are typi-
cally fabricated on a glass (or plastic) substrate coated with a wide bandgap conducting