Environmental Engineering Reference
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on the surfaces of MDMO:PPV and PCBM substrates, showing that AlF 3 is absent
at the Al/LiF/MDMO-PPV and Al/LiF/PCBM interfaces, proving that the occur-
rence of the reaction is missing, which is used to explain the improved electron
injection and collection. Moreover, a reaction of Al with carboxylic oxygen of
PCBM actually takes place, proved by XPS measurements; however, this reaction
is inhibited by LiF layer at the PCBM/Al interface.
6.3 Characterization Techniques for Interface Degradation
Highly efficient characterization techniques for interface degradation are very
significant in order to understand the variance of interfaces during the device
operation and their correlation with device degradation [ 8 ].
In OLEDs, Aziz et al. [ 31 ] employed characterization techniques, such as
impedance spectroscopy (IS), electrochemical measurement, and scanning
electron microscopy (SEM) to provide a mechanistic picture for degradation
process. Experimental observations are analyzed and correlated to reveal the
presence of an electrochemical reaction which causes device failure.
Jorgensen et al. summarized the characterization techniques and classified them
into two typical ones [ 8 ]: one is including the approaches which could offer
morphological and chemical information from specific locations in the device; the
other one is covering the approaches which provide information from nonspecific
locations in the device, i.e., information from all layers or the whole averaged
analysis over a large area. The characterization techniques from the specific
locations are typically time-of-flight secondary ion mass spectrometry (TOF-
SIMS), depth profiling, XPS, atomic force microscopy (AFM), SEM, interference
microscopy, fluorescence microscopy, and RBS. Meanwhile, the characterization
techniques from the nonspecific locations are usually IS, near-field scanning
microscopy (NSOM), infrared spectroscopy (IRAS), X-ray reflectometry (XRR).
In addition, various ambient conditions such as white light irradiation, in dark,
exposures to air, dry oxygen, and humid nitrogen atmospheres, can be applied to
support the characterization of device degradation [ 30 ].
In this section, we will emphasize the characterization techniques for interface
degradation and their corresponding analysis divided into optical and electrical
aspects.
6.3.1 Optical Techniques
It is easily understood that optical techniques are based on the optical character-
ization and measurement of degradation and provide information about the cor-
relation between interface variance and device degradation.
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