Interface Stability of Polymer and Small-Molecule Organic Photovoltaics - Organic Solar Cells: Materials and Device Physics - page 138

Therefore, the analysis on the interface degradation and development of
characterization techniques would contribute to the understanding of interface
stability and further enhance the entire device lifetime.
6.1 Introduction
Polymer and small-molecule organic photovoltaics (OPVs) have been attracting
great attention in the past few years due to their unique properties of low cost, easy
process at low temperature, and flexibility. They are becoming one of the future
photovoltaic (PV) technologies for the lowest cost clean energy production. The
rapid increase of OPV efficiency from 1 % in 1999 to 8.33 % in 2010 has made the
commercial products realistic [ 1 - 6 ].
The light harvesting of organic semiconductors can be tuned to match solar
irradiative spectrum by chemical design and synthesis. The capability of being
solution processed is the most potential for manufacture by printing or coating
techniques, which significantly enhances possibility of realizing large-area and
low-cost cells. To enter commercial PV market, OPVs have to satisfy a few basic
and standard requirements in cost, efficiency, and lifetime. Actually, the low
efficiency and short lifetime of OPVs are compensated by their low module cost,
which provides a strong impetus to invest in OPV technology [ 7 ]. The low effi-
ciency could be solved by organic semiconductor materials, transparent electrode,
light management, device physics to drive the OPV technology toward much more
developed.
One of the limitations to commercialization is the short lifetime or instability of
OPVs. Typically, investigations on stability of OPVs have been focused on the
effect of the water and oxygen on the degradation of photoactive layers and
electrodes in encapsulated or unencapsulated devices under different environment
conditions such as continuous illumination, oxygen, nitrogen, moisture, and vac-
uum [ 8 ], i.e., factors that affect stability are primarily oxygen, humidity, temper-
ature, cycling of temperature, light, loading conditions, pre-treatment, packaging,
electrodes, and so on. Figure 6.1 displays the effect of three main factors influ-
encing device stability [ 9 ]. In general, most studies on degradation of OPVs are
correlated with oxidative damage to the photoactive layer associated with illu-
mination of the device in presence of molecular oxygen, and several light
dependent degradation pathways have been identified [ 10 - 16 ]. It is well known
that OPVs have to suffer a relatively high temperature during operation, which
could also degrade organic photoactive materials and electrodes to a great degree.
Since organic semiconductors are highly sensitive to photo-oxidation or photo-
bleaching, encapsulation of OPVs is necessary, preventing the exposure of active
materials and electrodes to oxygen and water. Additionally, polymerization-
induced impurities have to be extracted as they are likely to form free radicals
upon photolysis [ 8 , 17 ].