Environmental Engineering Reference
In-Depth Information
NSOM is used to establish the relation between morphology of BHJ and
photocurrent generation. It is a potential technique to characterize the solar cells at
the nanoscale level although it does not conclude the influence of device mor-
phology on the device degradation and only shows the possibility [
8
,
46
].
X-ray reflectometry or X-ray reflectivity (XRR) technique is used to probe the
interfacial properties of layered samples such as films on substrates, multiple
layers, and superlattices, which is based on Snell rule [
47
]. In the energy dispersive
(ED) mode, a polychromatic primary X-ray beam is used and the reflection
patterns are collected at a fixed angle, by an ED solid-state detector [
48
,
49
]. The
in situ EDXRR could overcome the issues associated with probable intrinsic errors
in removing and repositioning the samples and allow the high-accuracy recording
of the film morphology with the time evolution [
50
]. It is said that the in situ
EDXRR technique is a powerful nondestructive tool to study the aging effect at the
interface of OPVs under working conditions.
Paci et al. studied the evolution of the morphology in OPVs of glass/ITO/
MDMO-PPV:PCBM/Al during operation by using EDXRR, in which three steps
are included [
50
]. The first step is a preliminary set of XRR measurements on three
different samples ((1) Glass/ITO/MDMO-PPV:PCBM; (2) Glass/ITO; (3) Glass/
ITO/MDMO-PPV:PCBM/Al), representing the successive stages of cell con-
struction, which identifies the contributions from each layer to the overall cell
reflection patterns and provides accurate data regarding the electronic densities of
the different layers. The second step as shown in the inset of Fig.
6.3
is the
verification of morphological stability of the device under ambient condition,
representing repeatable measurements even after a few months. The third step
shown in Fig.
6.3
is a collection of XRR results and patterns on an OPV, measured
under controlled atmospheric conditions, i.e., both in the dark and during 15 h
illumination, leading to systematically morphological variation at the electrode/
organic layer interface. Therefore, it is concluded that the direct correlation
between progressive thickening of this interface and the decrease in device
performance, explained by a ''real'' effect—the formation of an Al oxide layer at
the film surface and at the interface with organic active layer due to a photo-
oxidation reaction, or a possible but indistinguishable change of the formation of a
layer at the Al/organic interface due to indium diffusion from the ITO.
Paci et al. further confirmed the photoinduced oxidation of the metal electrode
at the organic/metal interface by using EDXRR technique in cells without (Cell A)
and with (Cell B) protection layer of LiF at the organic/metal interface [
51
]. It was
suggested that the Al oxidation in Cell A is correlated with the first aging process
exhibited by the exponential decay in Fig.
6.4
. Meanwhile, Cell B does not show
the same two-step process, exhibiting no loss of efficiency in the first 15 h due to
the stable morphology at the LiF/Al interface and also the absence of oxidation
process. Therefore, the insertion of LiF is beneficial to the protection of the fast
photoinduced degradation in the first few hours. As highlighted by the authors, the
conclusion that the formation of oxidization layer between organic layer and metal
is prevented by inserting a LiF buffer layer has been drawn under the real-time in
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