Interface Stability of Polymer and Small-Molecule Organic Photovoltaics - Organic Solar Cells: Materials and Device Physics - page 141

Catastrophic failure is primarily associated with defects existing in the organic
active layers, resulting in a sudden decrease or total loss in luminescence as a
result of large leakage currents due to electrical shorts. The underlying mecha-
nisms are ascribed to morphological defects existing in organic layers or
electrodes, or thermally induced during device operation with a temperature
beyond glass transition temperature (T g ) of organic materials. The former two are
considered to be correlated with surroundings. The intrinsic degradation, however,
primarily material dependent, exhibit a progressive decrease in the brightness of
the emissive area of a device with time during operation and no visible changes in
device appearance. Aziz et al. attributed the intrinsic degradation to (1) the indium
migration model, (2) the unstable cationic Alq 3 model, (3) the morphological
instability model, (4) the immobile positive charge accumulation model, and (5)
the mobile ionic impurities model.
For small molecules and polymer OPVs, they have similar device structures
with OLEDs and also suffer similar issues associated with essential materials,
electrodes, layer interface, and environmental conditions. In this section, we will
focus on the discussion about the degradation mechanisms originating from
interface instability at the adjacent layers such as anode and anode buffer layer,
cathode and cathode buffer layer.
6.2.1 Anode and Anode Buffer Layer
ITO is the most commonly used anode for organic optoelectronic devices, which
plays a significant role in injecting or extracting the holes from the active layer.
Although ITO is relatively stable, it still suffers the degradation when exposed to
special environments.
Carter et al. observed that the presence of ITO accelerates a long-term device
failure in polymer OLEDs, induced by photo-oxidation of the light-emitting
polymer via oxygen out of the ITO when polymer was fabricated directly onto ITO
surface [ 24 , 25 ]. The chemical reaction of the vinyl carbon with oxygen from the
ITO anode induces the chain scission of active polymer [ 24 ]. Moreover, Stott et al.
reported two major modes of degradations in ITO/MEH-PPV/Ca when they were
operated in a dry inert atmosphere [ 24 ]: one originating from the oxidization of
polymer MEH-PPV is likely caused by diffusion of the oxygen from ITO, resulting
in luminescence quenching and increased impedance due to formation of aromatic
aldehyde and chain scission, respectively; the other one is attributed to localized
microscopic electrical shorts, however these shorts do not cause immediate failure
of devices since self-induced fusing of the surrounding cathode metal isolates the
shorts, which decreases the effective active area of each device. Only when the
region of damaged cathode starts to coalesce does the ultimate failure happen.
Moreover, Krebs et al. found ITO etching indirectly due to the indium diffusion
into the layers of device Al/C 60 /P3CT/ITO [ 26 ]. The observation is the fact that