Environmental Engineering Reference
In-Depth Information
layers were used: 0.05 ± 0.03 eV with PEO, 0.22 ± 0.05 eV with Cs
2
CO
3
, and
0.33 ± 0.03 eV with WPF-6-oxy-F (Fig.
4.4
d). As transparent cathode in inverted
OPV device, WPF-6-oxy-F doping MLG film was found to be the best material
because its work function had been reduced to be close to the LUMO of PCBM. It
is to note that WPF-6-oxy-F had a negligible effect on the absorption properties of
the MLG film in the visible wavelengths because the energy gap of WPF-6-oxy-F
lies in the UV wavelength range. BHJ device with an inverted structure (Fig.
4.4
e)
MLG/WPF-6-oxy-F/P3HT:PCBM/PEDOT:PSS/Al gave PCE 1.23 %, which was
almost half of the PCE 2.23 % of cells with WPF-6-oxy-F functionalized ITO
electrodes.
Although the initial results demonstrate that graphene films prove to be effective
and exhibit great potential as transparent electrodes, significant improvement is
needed for organic electronic devices employing graphene as the transparent
electrodes. In future studies, improvement of the conductivity without sacrifice of
the transparency should be first considered. In general, defects in the graphene
sheets for both from rGO and from CVD are the main factors for low conductivity.
Thus, reducing defects in the preparation process as well as combining with
post-treatment are necessary. On the other hand, for device fabrication, the
hydrophility of graphene films has to be improved or modified to allow for spin
coating with hole-transporting layers such as PEDOT:PSS. Finally, for commer-
cialization consideration, low-cost and large-scale production is preferred for the
graphene-based transparent electrodes. In addition, roll-to-roll process is preferable
for low-cost and large-scale production for OPVs. How to fulfill the requirement of
roll-to-roll process is also a challenge for graphene-based transparent electrodes.
4.3 Graphene for Acceptor Material in OPVs
Currently, the most successful organic solar devices are fabricated in BHJ
structure, with low band-gap polymers as the donor and fullerene derivatives such
as PCBM as the acceptor. In contrast to the widely focused attentions on the design
and synthesis of low band-gap donor materials [
43
,
44
], unfortunately, few
acceptor materials other than the fullerene derivatives have been developed. As the
most widely used acceptor, C
60
based acceptors have some limits [
45
-
47
], e.g.,
very weak absorption in the visible range, low LUMO energy level which is hard
to tune for high V
oc
in BHJ devices. Although much efforts have been made on
modifying fullerene and its derivatives, only limited improvement has been
achieved. This has prompted studies for new acceptor materials with energy levels
different from those of C
60
derivatives, and wide versatility in terms of derivati-
zation and functionalization [
48
,
49
]. In view of its excellent electronic properties
such as high mobility, tunable energy level, well dispersion ability in organic
solution, etc., it is expected that graphene and its derivatives should be a good
alternative acceptor material in OPVs.
Search WWH ::
Custom Search