Interface Stability of Polymer and Small-Molecule Organic Photovoltaics - Organic Solar Cells: Materials and Device Physics - page 140

It is possible to classify the likelihoods of each mechanism as HIGH, MED-
IUM, and LOW [ 9 ]. Diffusion of oxygen and water as well as electrodes (Indium
Tin Oxide (ITO) and metals) possesses ''HIGH'' likelihood compared to other
degradation mechanisms. It is obvious that the ''HIGH'' degradation has close
relation with various interfaces. The ''MEDIUM'' likelihood of photo-oxidation of
organic semiconductors and interfacial layer are followed. Furthermore,
mechanical degradation has the relatively ''LOW'' likelihood. Therefore, the
approaches to improve device stability can be conducted on the basis of likeli-
hoods to overcome the degradation from the most significant level (HIGH) and to
the least significant level (LOW).
Specifically, the efficient approaches to prevent the diffusion of oxygen and
water are to develop mature device encapsulation to control the ability of O 2
oxygen transmission rate (OTR) and water water vapor transmission rate (WVTR)
to cross an encapsulating membrane within a certain value, i.e., it is generally
accepted that the lifetime of OPVs above 10, 100 h requires the upper limits of
OTR of 10 -3 cm 3 m -2 day -1 atm -1 and WVTR of 10 -6 g m -2 day -1 , which
are six to eight orders lower than the corresponding values of commercially
available polymer films [ 18 , 19 ]. The adhesives between layers should also be
enhanced. Moreover, for electrode degradation, an alternative to ITO is remark-
ably required and interfacial engineering (hole-transporting layer (HTL)) is nec-
essary correspondingly. On the other side, optimization of metal electrode such as
electron-transporting layer (ETL) is of great importance to remain a good contact
and insensitive to environment.
So far, there are many reports and reviews on study of device degradations [ 8 ,
20 - 23 ]. However, few reviews are concentrated on the effect of interface degra-
dation on the device stability. In this chapter, we will mainly discuss the interface
degradation mechanisms, followed by possible characterization techniques for
detecting the change of interface structure. The understanding of interface deg-
radation should serve as potential routes for improving interface stability and also
entire device stability; therefore, highly promising improvement approaches of
interface stability will be summarized. Finally, one of the challenging efforts on
technology development is device encapsulation with low-cost and high-quality
barrier. Thus, the encapsulation techniques will be introduced.

6.2 Mechanisms for Interface Degradation

For small-molecule organic light-emitting diodes (OLEDs), the luminescence
degradation mechanisms have been systematically and comprehensively reviewed
[ 20 ], which are mainly attributed to three individual and distinct modes as (i) dark-
spot degradation, (ii) catastrophic failure, and (iii) intrinsic degradation. ''Dark
spots'' refers to the formation of visible nonemissive defects or regions, leading to
a decrease in device luminance due to the losses of emissive area. The underlying
mechanisms

might be electrochemical in nature or be thermally activated.