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Fig. 7 a Schematic representation of the spatial distributions of the optical electric field
strengths |E| 2 inside devices prepared with (top) and without (down) an optical spacer. b Incident
monochromatic photon-to-current collection efficiency (IPCE) spectra for devices prepared with
and without a TiO x optical spacer layer. c Current density-voltage characteristics of polymer
solar cells prepared with (circles) and without (squares) a TiO x optical spacer, recorded under
AM1.5 illumination from a calibrated solar simulator (90 mW cm -2 )[ 39 ]
P3HT:PCBM layer was less than approximately 60 nm. Similarly, Andersson et al.
performed simulations, using the transfer matrix method and the finite element
method for OPVs prepared with various band gaps. [ 42 ]. They found that no
beneficial effect could be expected when incorporating optical spacers in devices
prepared with an already-optimized active layer thickness. The effect of an optical
spacer depends strongly on the thickness of the photoactive layer. In other words,
optical spacers can function well only when an optically optimized thickness of the
polymer layer cannot be achieved. For example, the mobilities of the charge car-
riers might limit the use of thick films for certain polymer systems. OPVs fabricated
with these materials might result in lower charge collection efficiencies. In these
cases, optical spacers can be expected to enhance the absorption efficiency.
Another beneficial feature of an optical spacer is that redistribution of the
optical electrical field can decrease the level of exciton quenching near the elec-
trodes [ 39 , 41 ]. For example, our group used ITO as an optical spacer to improve
the efficiency of inverted polymer solar cells [ 41 ]. The thickness of the
P3HT:PCBM layer had been optimized in this case; the best thickness of the active
layer was 180 nm. Figure 8 a presents the electrical characteristics of inverted
devices incorporating ITO spacers of various thicknesses; the inset displays the
device structure of the OPV. Because ITO has high electrical conductivity and
high transparency, it appears to be a suitable candidate for use as an optical spacer.
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