Environmental Engineering Reference
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Fig. 8.7 Photoluminescence
spectra of P3HT:PCBM for
different Au NP
concentrations in
PEDOT:PSS [ 18 ]
For the third reason, the reduction in exciton quenching at D/A junctions
indicates reduced D/A interface area, and hence a change in the morphology of the
active layer [ 31 ]. It should be noted that studies have shown that PL increase can
be a result of an increase in crystallinity of P3HT in the P3HT:PCBM blend,
causing an increase in hole mobility and PCE [ 28 , 32 , 33 ], which apparently
cannot address our results. However, it should be noted that the increase in
crystallinity of P3HT has also been associated with an enhancement of absorption
of P3HT:PCBM in the red region, which is not observed in the results of Ref. [ 18 ].
Hence, the increase in PL is attributed to changes in phase separation in the
P3HT:PCBM blend leading to reduced D/A junction interfacial area, instead of
changes in crystallinity of P3HT. The reduced D/A junction area causes exciton
dissociation efficiency to decrease and reduce PCE. In addition, the AFM images
of the top surface of the P3HT:PCBM film show no significant change in top
surface morphology. This implies that the rough PEDOT:PSS surface, caused by
the incorporation of NPs, could only cause a change in internal networking of the
subsequently spin-coated P3HT:PCBM film near the PEDOT:PSS side.
Further evidence of the roughness effects on the change of phase separation can
be found from the PL spectrum of PEDOT:PSS:Au NPs/PEDOT:PSS/
P3HT:PCBM film (see Fig. 8.7 ) that no significant difference in PL intensity
between the samples with single PEDOT:PSS layer (without Au NPs) and double
PEDOT:PSS layer (with Au NPs in first layer) is observed. In this case, no clear
PL intensity change is observed as the smoothened PEDOT:PSS surface, caused
by the addition of an extra PEDOT:PSS layer, does not alter the internal net-
working of the active layer. Therefore, it can be concluded that incorporation of
Au NPs modifies PEDOT:PSS surface morphology and leads to the reduced
exciton quenching, which can be explained by alterations in the internal net-
working of the P3HT:PCBM film near the PEDOT:PSS side.
In terms of the impact on device performances, on the one hand, the reduction
in exciton quenching after addition of Au NPs implies a reduction in free carrier
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