2.1. Plasmonic enhancement of DSCs

Plasmonic enhancement or light entrapment in DSCs by means of the plasmonic resonance of metal nanostructures has been a topic of intense research in the last decade. Since the first report in 2000 of metal nanostructured mediated enhancements in DSCs, many successful studies has been published outlining the role of size, shape and composition of metal nanoparticles on DSCs performance and working mechanisms [59–61]. Metal nanostructures capable of surface plasmon such as Au and Ag has been systematically introduced with TiO2 NPs. Such as these nanoparticles can be designed and integrated in TiO2 NPs in a way to offer light entrapment from visible to NIR region [62].

In plasmonic materials the coupling of incident photons to conduction band electrons upon excitation give rise to collective oscillations of electrons defined as localized surface plasmon resonances (LSPR) [63, 64]. By the engineering of plasmonic nanostructure's geometry, dimensions and composition LSPR's radiative (hot electron transfer, plasmon resonant energy transfer) and non-radiative (far-field scattering >50 nm size, near field coupling 3–50 nm size) processes can be tailored (Figure 5) [55, 56]. Out of four processes summarized in Figure 5 far-field scattering and near field coupling are easily observed for DSCs such as by improvement in IPCE, whereas role of hot electron transfer and PRET to improve DSCs is thus far poorly explored [62]. Detailed discussion of each process and its implications for DSCs are beyond the scope of this chapter [62, 65].

In radiative effects metal nanostructure acts as a secondary light source and in non-radiative effects absorbed energy is subsequently transferred to neighboring semiconductor NPs (Figure 5).

Figure 5. Summary of radiative (a) and (b) and non-radiative LSPR-based processes and their features, modified from reference [62], with permission from the Royal Society of Chemistry.

was blended with TiO2 paste to constitute the active layer (Figure 6). With most prominent current report, DSCs employing organic dye (FNE29) compared to Ru (II) dyes for plasmonic enhanced DSCs are rare. With the survey of reports published so far it is clear that though plasmonics has led to increased device efficiencies however, reports lack (1) application of nonprecious metal plasmonics for enhanced DSCs such as Al, graphene and semiconducting nanocrystal plasmonics (2) application of metal nanoparticles in conjunction with high photovoltage redox shuttles such as Co (III/II) or Cu (II/I) and organic sensitizers (3) long term stability studies particularly against redox shuttles for corrosion and chemical resistance of

Figure 6. Effect of ~2 nm sized inlaid Au NPs with different wt% in TiO2 paste on IPCE, adopted from Ref. [66], with

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NP based mesoporous TiO2 shows excellent features for DSCs however it suffers from low electrical conductivity and charge recombination losses [75–78]. Additionally, enhanced light scattering and dye adsorption can be achieved by modifying the shape of NPs or mixing nanotubes, nanowires, nanospheres, and hollow TiO2 [58, 77–81]. On the same note, 2D and 3D structures of TiO2 such as nanoribbons, nanodisks, nanoleaves, nanoflowers, nanorods, hedgehog nanostructure and dendritic hollow structures have also be explored for DSCs [82–87]. The studies focused on the morphological modification of TiO2 have demonstrated marginal increase in DSC performance with scattered results, however, due to structural complexity and reproducibility issues such investigations has not resulted in wide spread

application for DSCs [76]. For details please refer to the cited work.

plasmonic DSCs [69–74].

2.2. Morphological investigation on TiO2

permission from the Royal Society of Chemistry.

For LSPR based enhancement of DSCs the size of the metal nanoparticles plays huge role. Such as at 5 nm regime of metal nanostructure non-radiative processes are dominant, near field coupling for 45 nm size regime and far field scattering for 120 nm size metal nanostructures [62]. In terms of application, some reports evidence the beneficial effect of topical presence of plasmonic nanoparticles on TiO2 film, however, homogenous integration of plasmonic nanoparticles throughout active layer have proven more efficient, particularly in enhancing photocurrent response of devices (Figure 6) [67, 68].

The highest PCE achieved through plasmonic enhancements so far (2013) is 10.8% which is 30% higher (8.3% PCE) than the control device employing N719 sensitizer by Belcher et al.. with 0.01–0.32% core shell particles mixed with regular TiO2 [61]. In this study, oxide-metaloxide multiple core-shell nanostructured spheres were blended with already available photoactive materials to achieve balanced light harvesting in panchromatic fashion. Kamat et al.. in a pioneering work (2012) identified the plasmonic and charging effect based on the composition of Au@SiO2 and Au@TiO2, leading to achieve a higher photocurrent and photovoltage with overall PCE of 10.2 and 9.7%, respectively [60]. Au nanoparticles of 5 nm size were used in the core with shell as passivation layer of either SiO2 or TiO2 and mixed with Solaronix T/SP paste in 0.7 wt%. Wang et al in 2013, in a unique study employed the organic sensitizer FNE29 and I/I3 , along with TiO2 inlaid 2 nm sized Au nanoparticles resulting in 10.1% PCE improved from 5.5% PCE (84% increase) [66]. Au nano colloid in 0–0.8 wt% ratio

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Figure 6. Effect of ~2 nm sized inlaid Au NPs with different wt% in TiO2 paste on IPCE, adopted from Ref. [66], with permission from the Royal Society of Chemistry.

was blended with TiO2 paste to constitute the active layer (Figure 6). With most prominent current report, DSCs employing organic dye (FNE29) compared to Ru (II) dyes for plasmonic enhanced DSCs are rare. With the survey of reports published so far it is clear that though plasmonics has led to increased device efficiencies however, reports lack (1) application of nonprecious metal plasmonics for enhanced DSCs such as Al, graphene and semiconducting nanocrystal plasmonics (2) application of metal nanoparticles in conjunction with high photovoltage redox shuttles such as Co (III/II) or Cu (II/I) and organic sensitizers (3) long term stability studies particularly against redox shuttles for corrosion and chemical resistance of plasmonic DSCs [69–74].
