Experimental Studies of Plasmonic Nanoparticle Effects on Organic Solar Cells - Organic Solar Cells: Materials and Device Physics - page 236

enhancement due to incorporation of Au NPs is insignificant and provides only
minor contribution to PCE improvement. It is found that the reason is due to the
lateral distribution feature of the strong near-field of plasmonic resonance around
the metallic NPs. Importantly, the finding can also be applied to other cases with
metallic NPs (such as Ag and Pt) incorporated into the buffer layer adjacent to
active layers of typical organic thin film solar cells. These studies [ 35 , 58 ], that the
optical properties of OSCs such as the direction dependence of absorption
enhancement cannot but fully described by traditional physical quantities, such as
scattering cross-section. It is highly necessary to account for near-field physics in
order to provide a full picture for the effective optical design of photovoltaics.
Considering electrical characteristics, it is found [ 18 ] that the incorporation of
an appropriate amount of Au NPs reduces the resistance of PEDOT:PSS layer.
AFM images of NP incorporated PEDOT:PSS show that there is an increase in the
interfacial roughness between P3HT:PCBM and PEDOT:PSS after incorporation
of Au NPs. The roughened interface contributes to the improvement of hole col-
lection efficiency and leads to J sc and FF enhancements. PL measurements show
that incorporation of Au NPs lead to reduced exciton quenching at D/A junctions
at high NP concentrations due to change in internal networking of the active layer.
The report indicates [ 18 ] that the competition between the effects of hole collec-
tion improvements and reduced exciton quenching, instead of LSPR effects, lead
to the performance peak at 0.32 wt%.
In Sect. 8.3 , the effects of PEG-capped Au NPs (0.5 wt%) on OSCs have been
theoretically and experimentally discussed by introducing the Au NPs into a blend
of a newly synthesized polymer of PFSDCN and PCBM [ 35 ]. The results show
that due to the interesting feature of the strong lateral distribution of LSPR near
field along the active layer, light absorption is enhanced by incorporating Au NPs
into the active layer. This is in stark contrast with the case of NPs incorporated into
PEDOT:PSS where LSPR cannot contribute to absorption enhancement.
Meanwhile, our discussion shows that electrical properties are also strongly
affected by the NPs. Carrier mobility was found to increase which is beneficial to
device performance. In particular, hole mobility increases at a quicker rate than
electron mobility which may contribute to reducing carrier mobility imbalance.
Exciton dissociation probability is also found to increase slightly then decrease
rapidly upon increasing NP concentration. The morphology of the active layer is
also strongly altered by high concentrations of Au NPs. The interplay of these
various factors showed that enhancement in electrical properties can initially
improve OSC performance at low NP concentrations. At higher concentrations,
electrical effects can counter-diminish the optical enhancement from LSPR which
reduces the overall performance improvement. Hence, it is very important that
both optical and electrical properties need to be studied and optimized simulta-
neously. After optimization, power conversion efficiency can be improved by
*32 % [ 35 ].
The studies in Sects. 8.2 and 8.3 have provided us with insights into the device
mechanisms of OSCs. The role of LSPR is found to be not as important as many
studies claimed to be in improving PCE of solar cells. In particular, only NPs