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Fig. 14 Chemical structures of PBDTTT-C-T and PC
71
BM (left). Schematic representation of the
device structure: NP device (top), grating device (bottom), and dual metallic structures (right)[
54
]
Poly{[4,8-bis(2-ethylhexylthien-5-yl)benzo[1,2-b:4,5-b
0
]dithien-2,6-diyl]-alt-[2-(2
0
-
ethylhexanoyl)thieno[3,4-b]thien-4,6-diyl]} (PBDTTT-C-T), a polymer having a
low band gap, was used as the p-type polymer. The flat (control) device fabricated
without any nanostructures exhibited a PCE of 7.59 ± 0.08 %. The authors used
vacuum-assisted nanoimprinting at room temperature to form the nanostructure of
the Ag grating. Under the optimized conditions, the PCE increased to
8.38 ± 0.20 %. Furthermore, when Au NPs were added into the active layer, the
PCE increased to 8.79 ± 0.15 %. Li et al. suggested that the Au NPs offered
enhanced absorption in the region 480-600 nm, whereas the Ag grating had a greater
impact in the absorption regions below 400 and above 600 nm. Their study provided
a general method for achieving enhanced broadband absorption [
54
].
Nevertheless, plasmonic-enhanced OPVs incorporating periodic nanostructures
and exhibiting pronounced enhancement remain rarely reported. In addition, the one-
dimensional (1-D) grating structures often exhibit high polarization-dependence,
making it quite difficult to simultaneously optimize both polarization modes. More
recently, two-dimensional (2-D) structures possessing higher-order symmetries
have been proposed, potentially overcoming the polarization dependence [
55
]. In
the near future, we foresee the fabrication of many more periodic nanostructures
that will effectively improve device efficiencies.
5 Low-Band-Gap Materials
Organic materials usually absorb only a limited amount of the solar spectrum. For
example, Fig.
15
displays the spectral response (SR) of the well-known
P3HT:PCBM polymer blend. The response range is limited at approximately
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