Experimental Studies of Plasmonic Nanoparticle Effects on Organic Solar Cells - Organic Solar Cells: Materials and Device Physics - page 218

NPs/PEDOT:PSS/P3HT:PCBM/LiF(1 nm)/Al (100 nm). After the insertion of the
pristine PEDOT:PSS layer, the presence of Au NPs in these devices provides no
improvements in device performance. Meanwhile, since optical effects have been
proven to be unlikely to be a major contributor to performance improvement, one
cannot argue that positive LSPR effects are diminished by the alteration of optical
interference profile inside the OSC due to the addition of an extra layer of PE-
DOT:PSS. As a result, the main effect of the extra PEDOT:PSS layer is to
smoothen the rough surface of the PEDOT:PSS:Au NPs layer. Consequently, the
results indicate that the rough PEDOT:PSS: Au NPs layer surface positively
contributes to device performances.
The resistance of PEDOT:PSS from resistive devices with the structure ITO/
PEDOT:PSS:Au NPs/Al(80 nm) has also been reported [ 36 ]. J-V measurements
from these devices indicate a slight reduction in PEDOT:PSS resistance from 1.33
(No NPs) to 0.97 X (0.32 wt%). Although reduction in PEDOT:PSS resistance
reduces the series resistance of OSCs, the small decrease in PEDOT:PSS resis-
tance can only provide minor contributions to device performance improvement.
On the whole, it can be concluded that the improved hole collection at the
roughened interface between PEDOT:PSS and P3HT:PCBM enhance device
performances, while a slight reduction in PEDOT:PSS resistance by the incor-
poration

of Au NPs can also provide minor contribution to performance

improvements.

8.2.5 Effects of Au NPs on Exciton Quenching

A missing picture is the origin of device performance degradation at high NP
concentrations. To investigate this effect, the photoluminescence (PL) spectrum of
PEDOT:PSS:AuNPs/P3HT:PCBM films has been investigated (see Fig. 8.7 ). The
PL spectra show increasing PL intensity upon increasing concentrations of Au
NPs, with a maximum of *10 % increase at *647 nm.
PL intensity changes can be caused by three main reasons: changes in optical
absorption, exciton quenching at metal/organic interfaces [ 25 - 27 ], and exciton
quenching at donor/acceptor (D/A) interfaces [ 28 , 29 ]. The possibility of changes
in optical absorption is eliminated, as we have experimentally and theoretically
shown previously that light absorption of OSCs do not change significantly after
the incorporation of Au NPs. Considering the second reason, exciton quenching at
metal/organic interfaces, it has been reported that capping an insulating layer on
Au NPs can prevent direct contact between the metal and organic layer, hence
preventing exciton quenching on the NP surface [ 30 ]. Furthermore, as shown in
Fig. 8.3 c, Au NPs located near the surface of PEDOT:PSS are well covered by
PEDOT:PSS. Hence, it is expected that the Au NPs are not in direct contact with
the P3HT:PCBM layer and the effect of exciton quenching by Au NPs is

negligible.