Experimental Studies of Plasmonic Nanoparticle Effects on Organic Solar Cells - Organic Solar Cells: Materials and Device Physics - page 227

Fig. 8.16 Effects of Au NP
concentration on the decay
rate of bound electron-hole
pair (k F )(square) and exciton
dissociation probability (P exc )
under short-circuit condition
(circle)[ 35 ]
therefore compete with each other in our devices. At low Au NPs concentration,
the blend morphology does not have clear changes from our AFM results (i.e., rms.
roughness only increases to *1.204 nm for 1 wt% Au NP incorporation, and
therefore only plays a less important role in modifying carrier mobility. As a
result, with low Au NPs concentration, the increases of carrier mobility should be
explained by the introduction of hopping sites for holes. These hopping sites are
expected to have greater influence on hole mobility than electron mobility, which
agrees well with the experimental results. Compared with the control devices
(without Au NPs), incorporation with 2 wt% Au NPs contributes to an improve-
ment of hole mobility by *237 % (from 1.26 9 10 -4 to 4.25 9 10 -4
cm 2 V -1 s -1 ), and an improvement of electron mobility only by *28 % (from
0.93 9 10 -3 to 1.2 9 10 -3 cm 2 V -1 s -1 ). With high Au NPs concentration, the
NP-induced morphology change dominates the charge transport process, and thus
both the hole and electron mobility are expected to degrade, which is well con-
sistent with the experimental results as shown in Fig. 8.16 . Therefore, the
enhanced carrier mobilities with the proper amount of Au NPs can partly account
for the improved photocurrent generation and FF due to the improved carrier
collection and the reduced bulk resistance. However, when the carrier mobility
maintains increment until Au NPs concentration reaching 2 wt%, device perfor-
mances, i.e., J sc and PCE decrease when Au NPs concentration C1 wt%. This
indicates that besides the carrier mobility, Au NPs should affect other operation
processes of OSCs. One process likely to be affected is the dissociation of excitons
to free carriers as described below.
8.3.4 Effects on Exciton Dissociation
The efficiency of exciton dissociation can be investigated by fitting photocurrent
(J ph ) as a function of effective voltage (V EFF ) as reported in Mihailetchi et al. [ 46 ].
The maximal generation rate of excitons (G max ) for all the devices with different