Interface Stability of Polymer and Small-Molecule Organic Photovoltaics - Organic Solar Cells: Materials and Device Physics - page 143

6.2.2 Cathode and Cathode Buffer Layer

Low work function (LWF) metals, such as Ca, Mg, and Al, are used to inject or
collect electrons as cathode. It is known that LWF metals are prone to be oxidized
and to cause device degradation due to the diffusion of oxygen and water into itself
or organic active layer. Moreover, at the organic/metal interface, instability might
exist due to the intrinsic or extrinsic properties of metals. In order to achieve
matched energy levels of organic layer and metal, a few interfacial layers can be
incorporated in between organic active layer and metal to improve the charge
injection or collection.
Aziz et al. studied the main mechanisms of degradation in OLEDs (ITO/PPV/
Al), concluding that the electrochemical reaction between two electrodes is found
to be a major cause, resulting in corrosion and microstructural changes in both
electrode materials [ 20 , 31 , 32 ]. Therefore, the electrode materials can degrade
significantly forming a complex with electrolyte-like polymer, leading to the
increase of threshold voltage. At the same time, the additional conditions such as
moisture or impurities in polymers will enhance the ionic conductivity of the active
layer and accelerate the corrosion of the electrode at the affected locations. Sub-
sequently, Aziz et al. further investigated the degradation mechanism in Alq 3 based
OLEDs [ 32 ]. The results indicate that the short lifetime is mainly caused by the
injection of holes into Alq 3 , leading to a decrease in fluorescence quantum effi-
ciency due to unstable cationic Alq 3 species as well as fluorescence quenching sites.
Therefore, many stabilized OLEDs by doping the hole-transporting layer, or using a
buffered hole-injection contact, or forming alternating hole and electron trans-
porting based emitting layers, can be explained reasonably and correspondingly.
For OPVs, as investigated by Reese et al., the conclusion has been drawn that

the organic/metal interface is a major source of device degradation for

P3HT:PCBM OPVs [ 33 , 34 ].
Krebs et al. systematically investigated the stability of MEH-PPV:PCBM-based
devices in terms of atmosphere, handling, electrode treatment, mode of preparation,
and barrier layers by the dependence of J sc on time [ 15 ]. The authors separate
various degradation processes responsible for the decay and carry on the study with
the model of I sc (t)/I sc (0) = Ae -bt ? Ce -dt ? , where b and d are the time con-
stants, A and C are the weighting of the individual exponential functions [ 15 ]. It is
expected that individual responsibility for the device degradation could be linked to
the experimental parameters by making changes correspondingly. These experi-
mental parameters can be selected separately and in parallel. Then, the integrated
charge could be achieved by Q total = A/b ? C/d ? when extrapolated to infinity
[ 15 ]. Similarly, the degradation of P3HT:PCBM-based OPVs can be represented by
an exponential term describing the fast initial decay and a second exponential term
describing the long-term degradation, as g ¼ g 0 ½ ae ct þ be dt , where g 0 is the
initial efficiency and a ; b ; c ; d are curve fitting parameters [ 35 , 36 ]. The time
constant c indicates the fast decay process of degradation and c is related to a slow
decay. Schuller et al. developed various models for dynamic degradation, including