Environmental Engineering Reference
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the surface chemistry with a probe depth of 1-10 nm away from the top. It is also
used to analyze the elements giving information on the chemical state [ 8 ]. Gennip
et al. [ 45 ] used XPS to detect and confirm the reaction of Al with the carboxylic
group (O 1 s carboxylic oxygen peak) present in PCBM so that a LiF layer at the
PCBM/Al interface blocks this reaction.
SIMS is the most sensitive surface analysis technique to analyze the compo-
sition of solid surfaces and thin films by sputtering surface of the specimen with a
focused primary ion beam and collecting and analyzing ejected secondary ions.
The measured secondary ions with a mass spectrometer are used to determine the
elemental, isotopic, or molecular composition of the surface. TOF-SIMS is a
method of mass spectrometry in which an ion's mass-to-charge ratio is determined
via a time measurement. More importantly, TOF-SIMS technique is superior to
XPS in view of the imaging capabilities and the sensitivity [ 8 ]. Gennip et al. [ 45 ]
measured SIMS to show that LiF does not react with aluminum to form AlF 3 , nor
does it react in any other way to liberate Li, as mentioned in Sect. 2.2 . Much more
details on optical techniques for degradation characterization can be found in
ref [ 8 ].
6.3.2 Electrical Techniques
Electrical techniques are based on the information provided by the characterization
and measurements of electrical properties, which can be able to reveal the change
of interface state and their influence on the interface stability.
Impedance spectroscopy (IS), also known as electrochemical impedance
spectroscopy, measures the dielectric properties of a medium as a function of
frequency. It is based on the interaction of an external field with the electric dipole
moment of the sample. It is also widely applied in a broad field of inorganic,
organic, and biological systems based devices.
In BHJ OPVs, IS is commonly used to study the electrical transport properties
[ 8 ]. An equivalent circuit, as shown in Fig. 6.5 , is modeled to translate the fre-
quency response of the circuit to the correspondingly individual interfaces and
consequent layers in the devices. In this case, the dielectric constants for various
materials could be found via changing the film thickness and fitting the experi-
mental data [ 53 ], such as the distributed resistors r t (representing the electron
transport), the distributed chemical capacitance c n = C l and r rec (representing the
electron recombination resistance), a series resistance R s (representing the contact
and wire effects), a capacitor C g ee 0 A = L (representing dielectric contribution of
the diode), and two corresponding characteristic times: electron diffusion C g ¼ r t c n
(transit time) and effective lifetime C n ¼ r rec c n , respectively [ 53 , 54 ]. In typical
OPVs, due to special conditions during deposition of Al layer, the nature of the Al/
polymer interface plays a key role in the electron and hole transfer. IS can also be
used to monitor the effect of the annealing progress.
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