High-Performance Bulk-Heterojunction Polymer Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 172

ray optics approaches, such as the use of folded device architectures [ 30 , 31 ],
microlenses [ 32 ], antireflection (AR) coatings [ 33 ], and collector mirrors [ 34 ],
have been proposed to effectively improve the light harvesting efficiency. Nano-
structures, including photonic crystals [ 35 , 36 ] and metallic structures for trig-
gering surface plasmons (SP) [ 37 , 38 ], have also been adopted in OPVs to strength
their light absorption ability. In the following section, we focus on the two most
common methods—optical spacers and SPs—developed for OPVs.
4.1 Optical Spacers
Typically, the incorporation of an optical spacer will redistribute the optical elec-
trical field (|E| 2 ) in a thin film device [ 39 - 42 ]. As indicated in Fig. 7 a, the maximum
optical electric field distribution inside a conventional device is located near the
glass-indium tin oxide (ITO) interface, because of the interference effect between
the incoming photons and the reflected ones by the Al electrode. As a result, the
exciton generation rate, which is related directly to the optical electric field of the
photoactive layer, decreases. Furthermore, excitons produced near the ITO/
PEDOT:PSS surface may be quenched by the electrode [ 39 , 41 ]. To overcome these
problems, Kim et al. introduced a layer of optical spacer between the polymer layer
and the Al electrode [ 39 ]. This optical spacer, titanium oxide (TiO x ), was deposited,
using a solution-based sol-gel process, on top of the photoactive layer. After
incorporating the TiO x layer, the optical-electric field changed inside the photo-
active layer, with the maximum strength of the field distribution tuned to fall into
the photoactive layer (Fig. 7 a). The IPCE spectra of these devices exhibited sig-
nificant enhancements (ca. 40 %) over the entire spectral range (Fig. 7 b), attrib-
utable to the increased number of photogenerated charge carriers that resulted from
the optimized redistribution of the light intensity. Figure 7 c displays the device
performance before and after incorporation of the optical spacers. The conventional
device exhibited a value of J sc of 7.5 mA cm -2 , a value of V oc of 0.51 V, and a FF
of 0.54, resulting in a PCE of 2.3 %. The device featuring a TiO x layer achieved a
much higher PCE of 5.0 % (J sc = 11.1 mA cm -2 ; V oc = 0.61 V; FF = 0.66),
suggesting a promising future for optical spacers [ 39 ].
In addition to TiO x , Gilot et al. investigated the insertion of a layer of ZnO
between the active layer and the reflective electrode as the optical spacer [ 40 ].
When the thickness of the P3HT:PCBM layer was 40 nm, insertion of a 39-nm-
thick ZnO layer shifted the position of the maximum into the photon-absorbing
layer, suggesting that a higher photocurrent would be obtained when using this
optical spacer. The authors found, however, that cells made with a larger film
thickness (70-130 nm) exhibited contrasting results. They speculated that the
active layer was already optimized; the incorporation of an optical spacer shifted
the maximum electric field intensity away from the active layer. From both
experimental and calculation results, Gilot et al. concluded that the absorption
efficiency could be enhanced by using optical spacers only if the thickness of the