In 2003, Mallouk and co-workers proposed the use of a novel type of optical elements to improve the efficiency of DSSC. Their approach was based on the coupling of a particular type of 3DPC to dye-sensitized nc-TiO2 films (Nishimura et al., 2003). This approach consisted of integrating a 3 pm layer of a TiO2 ordered porous structure, known as inverse opal (see figure 5). By doing so, IPCE was shown to increase with respect to that of a standard cell used as reference (Figure 6a). Although the origin of this enhancement was first attributed to the reduction of the group velocity of photons near the edge of a stop band or photonic pseudogap, which implies the increase of the probability of absorption, it was later found that the absorption enhancement effect was partly due to the diffuse scattering caused by the imperfections present in the opal film (Halaoui et al., 2005) and, mainly, to the coupling between a standard electrode and the ordered structure, as explained next.,
Fig. 5. (a) SEM cross section image of an inverse TiO2 opal. 300 nm latex spheres were used as template, and then removed by thermal treatment. (b) Scheme and (c) SEM cross section image of a TiO2 3D inverse opal deposited on top of a TiO2 nanocrystalline electrode.
The theoretical analysis of the phenomena (Mihi & Miguez, 2005) demonstrated that the enhancement of the photocurrent in bilayer structures formed by a dye sensitized nanocrystalline TiO2 film coupled to an inverse titania opal is mainly due to the surface resonant modes confined within the overlayer (figure 6b). This is a consequence of the mirror effect of the photonic crystal. The theoretical results identified an increase in the absorptance spectrum of the modelled bilayer and an estimated photocurrent enhancement factor very similar to that reported in Mallouk experiments (Figure 6c). Later experimental results confirmed convincingly the model proposed by Mihi & Miguez (Lee et al., 2008; Mihi et al., 2008) and as they predicted, the improvement was found for frequencies comprised within the photonic pseudogap of the inverse opal structure. In that way, the dependence of the increased IPCE with the position of the forbidden interval range can be employed to enhance absorption at desired spectral ranges. The selection of this region is made through the lattice parameter of the photonic crystal which is controlled as well by the size of the templating spheres and the infiltration degree. In these specific experiments, DSSC required an opal with a lattice parameter that gives red photocurrent enhancement.
Additionally it is possible to couple two or more 3DPC with different lattice parameters to enhance photocurrent in a wider spectral range (Mihi et al., 2006). Since this effect was confirmed theorically, we can mention that the disadvantage of these materials is the several steps involved to achieve these types of structures. This leads to defects within the 3D photonic crystal, and therefore a lower reflectivity is obtained.