Environmental Engineering Reference
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Fig. 8.6 AFM height images for different Au NP concentrations in PEDOT:PSS—0 % (top
left), 0.32 % (top center), 0.64 % (top right). After spin-coating an extra layer of PEDOT:PSS on
top of the PEDOT:PSS:Au NPs layer, the AFM images show a smoothened surface for 0.32 %
(bottom left) and 0.64 % (bottom right)[
18
]
travel to the anode and enhancing hole collection at the anode [
23
]. The increased
interfacial area between PEDOT:PSS and P3HT:PCBM allows the collection of a
larger number of holes in the P3HT:PCBM layer, thus increasing J
sc
of our
devices. In addition, the reduced mean distance of between generated holes and the
PEDOT:PSS interface diminishes the dependence of holes on the external electric
field for collection at the anode, explaining the improvement in FF of our devices.
Besides, it has been suggested that a rough P3HT:PCBM surface creates defect
sites that assist exciton dissociation [
24
]. These effects together account for the
improvements in J
sc
and FF in our devices. The improved forward bias injection
upon addition of Au NPs in our devices further support the claim that the increased
PEDOT:PSS roughness increases PEDOT:PSS/P3HT:PCBM interfacial area.
Considering the optical effects of a rough PEDOT:PSS surface, the transmission of
PEDOT:PSS with or without Au NPs has been studied and observed no discernible
difference. In addition, the refractive index difference between PEDOT:PSS and
P3HT:PCBM is not large and the roughness is nanoscale. Hence, the rough PE-
DOT:PSS surface does not contribute to scatter light significantly.
To further elucidate this effect, an approach has been implemented to remove
the surface roughness of the PEDOT:PSS layer while retaining Au NPs inside the
PEDOT:PSS film. The approach is to spin-coat an extra PEDOT:PSS layer on top
of the PEDOT:PSS:Au NP layer. From AFM images in Fig.
8.6
, this extra layer
smoothens the surface of the PEDOT:PSS to a morphology similar to a regular
PEDOT:PSS layer. Devices are fabricated with the structure ITO/PEDOT:PSS:Au
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