High-Performance Bulk-Heterojunction Polymer Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 177

Fig. 12 a J-V curves of P3HT:PCBM solar cells incorporating Ag NPs at various concentra-
tions; inset: device structure. b-e Surface morphologies of active layers containing P3HT:AgNP
ratios of b 1:0 c 16:1 d 8:1, and e 4:1 [ 51 ]
the Au NPs, thereby leading to an improved PCE of 4.29 %. Two possible
mechanisms might be responsible for the enhanced light absorption efficiency of
the OPVs [ 49 ]. As illustrated in Fig. 9 a, the optical path in the active layer might
have increased as a result of the forward scattering; alternatively, excitation of
the LSPR might have led to local enhancement of the electromagnetic field in the
vicinity of the Au NPs, thereby increasing the absorption efficiency (Fig. 9 b).
The second scenario involves the direct placement of plasmonic NPs in the
semiconductor layers, as outlined in Fig. 9 b. For example, in 2004 Kim and
Carroll doped Ag and Au NPs directly into the photoactive layer of BHJ OPVs
[ 50 ]. Although the device efficiency was improved, the authors suggested that the
dominant mechanism behind the enhanced efficiency was the improved electrical
conductivity. In 2010, Xue et al. also incorporated Ag NPs into the P3HT:PCBM
layer; although their device efficiency did not improve (Fig. 12 a) [ 51 ], they found
that the mobility of the active layer increased, while the total number of extracted
carriers decreased. The surface morphology in Fig. 12 b-e reveals that the Ag NPs
tended to phase-segregate from the organic materials, leading to the formation of a
Ag NP sub-network, which was presumably responsible for the increase in
mobility. On the other hand, charge trapping in the sub-network could enhance the
recombination probability, thereby decreasing the degree of charge extraction.
More recently, Sha et al. developed a rigorous electrodynamic approach to
investigate optical absorption in OPVs [ 52 ]. They found remarkable differences
between structures in which the metal NPs had been placed at the interlayer (i.e.,
between the photoactive layer and the anode) and those embedded directly within
the photoactive layer. Theoretical results suggested that the enhancement factors
for the latter were generally greater than those of the former. In other words, direct