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layer of LiF, CuO x , CuO x /LiF, LiF/CuO x . The device degradation is strongly
dependent on the decrease of the photocurrent during the storage time. The results
indicate that the CuO x functions not only as a charge transport layer but also as a
protection layer, preventing the formation of thick organic/Al interdiffusion area.
Thus, an air-stable cathode/organic interface is achieved. Wang et al. also found
chromium oxide (CrO x ) as Al cathode interfacial layer to improve the efficiency
and stability in air since the device stability of OPVs depends on the cathode
interfacial layers [ 61 ]. Devices with CrO x exhibit higher PCEs and stability than
those without CrO x , showing improved stability 100 times better than those of
devices without CrO x or with LiF. It is likely attributed to the function of CrO x as
an electronic tunneling layer for electron collection and a protective layer for
minimizing the damages caused by evaporating Al and also blocking diffusion of
oxygen and water.
Insertion of an interfacial layer such as CrO x and CuO x is to effectively prevent
diffusion of Al into the organic layer and thus to eliminate formation of the
insulating layer that makes the device degrade. It is noted that diffusion of Cu
atoms into the organic layer may also occur. However, the oxidation of Cu leads to
Fig. 6.9 Comparison of the a PCE, b (J sc ), c (V oc ), and d FF as a function of storage time for
polymer solar cells without interfacial layer (only Al), with interfacial layer of LiF, CuO x , CuO x /
LiF, LiF/CuO x . Note that the device characteristics are monitored with increasing storage time for
the same device [ 60 ]. (Reprinted with permission from Ref. [ 60 ]. Copyright [2011], American
Institute of Physics)
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