Experimental Studies of Plasmonic Nanoparticle Effects on Organic Solar Cells - Organic Solar Cells: Materials and Device Physics - page 234

Fig. 8.22 The schematic
pattern for the plasmon
resonance and charge
distribution of an Au NP, as
well as the near-field
distributions for the vertically
incident light with TE
polarization in P3HT:PCBM
[ 56 ]

detrimental to the photovoltaic performance [ 29 ]. The increase in hole mobility
allows more balanced charge transport in the active layer, thus improving the J sc

and FF of the device.

8.4.4 Effects of Au NPs Incorporated in All Polymer Layers

When Au NPs are doped into both PEDOT:PSS and the active layer, the PCE of
our OSCs further increases to 3.85 %. From hole-only devices, the hole mobility
of P3HT:PCBM is determined to be *1.21 9 10 -3 cm 2 /V.s, which is similar to
the case when NP is doped in the active layer only (1.19 9 10 -3 cm 2 /V.s). From
the cross-section SEM images are shown in Fig. 8.23 , Au NPs doped in the active
layer are mostly located at the bottom of P3HT:PCBM (near to the interface with
PEDOT:PSS), while the Au NPs doped to PEDOT:PSS are well embedded in the
PEDOT:PSS layer. However, despite the fact that the active layer is adjacent to the
PEDOT:PSS layer, the highly similar absorption spectra of Devices C (Au NPs in
P3HT:PCBM only) and D (Au NPs in both P3HT:PCBM and PEDOT:PSS) shown
in Fig. 8.20 show that there is no clear interaction between the Au NPs in the
active layer and those in the PEDOT:PSS layer. From the principle of optics, the
absence of couplings between Au NPs in PEDOT and Au NPs in P3HT:PCBM is
reasonable, because the polarization direction of the electric field of vertically
incident light is parallel to each device layer. Furthermore, we have shown that the
improvement mechanisms when incorporating NPs in PEDOT:PSS mainly origi-
nates from PEDOT:PSS/P3HT:PCBM interfacial effects, while incorporating NPs
in the active layer mainly improves optical absorption and charge transport in the
bulk of the active layer. As interfacial and bulk effects are two individual effects
and optical coupling between NPs is not observed, we expect that the improvement
of PCE from 3.16 to 3.85 % is the accumulated improvements of addition of NPs
into the individual layers.