the TiO 2 –SnO 2 MHSs is most likely given rise by an enhanced light harvesting due
to multiple light reflecting and scattering in between the hierarchical spherical shells
of the TiO 2 –SnO 2 MHSs, rather than an increased surface area. The JSC value for
the SnO 2 MHSs is 50% higher than that of nano-SnO 2 , suggesting that the enhanced
JSC value for the TiO 2 –SnO 2 MHSs compared with the TiO 2 –nano-SnO 2 is due to
the hierarchical spherical structure rather than the TiCl 4 treatment. The inset in
Fig. 9.11 (right) illustrates the reflecting and scattering of light in a TiO 2 –SnO 2
MHS. It is believed that the shells with the different sizes in a microsphere cannot
only multireflect but also scatter the incident light of different wavelengths in the
range of visible light. The improved photocurrent for the TiO 2 –SnO 2 MHSs is
mainly contributed to its multilayered hollow spherical structure, which provides
an effective way to enhance light-harvesting efficiency.
Photonic-crystal-based optical coupling offers a unique way of light–matter
interaction to increase light harvesting, especially around the absorption edge of a
semiconductor. A photonic crystal is a periodic dielectric structure that can forbid the
propagation of light in a certain crystal direction within a certain spectrum regime,
called a photonic stop-band. The light in a photonic crystal undergoes strong
coherent multiple scattering and travels with very low group velocity near the
photonic stop-band edges, referred to as slow light. Such a slow-light effect can
considerably increase the effective optical path length, therefore leading to a delay
and storage of light in photonic materials. 3D photonic crystal design was utilized to
enhance incident photon-to-electron conversion efficiency (IPCE) of WO3
photoanodes. Large-area and high-quality WO3 photonic crystal photoanodes with
inverse opal structure were prepared (Fig. 9.12). The photonic stop-bands of these
WO3 photoanodes were tuned experimentally by variation of the pore size of inverse
opal structures. It was found that when the red edge of the photonic stop band of
WO3 inverse opals overlapped with the WO3 electronic absorption edge at
Eg ¼ 2.6–2.8 eV, a maximum of 100% increase in photocurrent intensity was
observed under visible light irradiation (λ > 400 nm) in comparison with a disordered porous WO3 photoanode (Fig. 9.13). When the red edge of the stop band was
tuned well within the electronic absorption range of WO3, noticeable but less
amplitude of enhancement in the photocurrent intensity was observed. It was further
shown that the spectral region with a selective IPCE enhancement of the WO3
inverse opals exhibited a blueshift in wavelength under off-normal incidence of
light, in agreement with the calculated stop band edge locations. The enhancement
could be attributed to a longer photon– matter interaction length as a result of the
slow-light effect at the photonic stop-band edge, thus leading to a remarkable
improvement in the light-harvesting efficiency. The present method can provide a
potential and promising approach to effectively utilize solar energy in visible lightresponsive photoanodes [17].
A range of TiO 2 inverse opals with tunable macroporous size were synthesized
using different sized PS arrays as hard templates [18]. After a simple heating
treatment in vacuum, Ti
3+ -doped TiO 2 inverse opals were obtained. The optical
responses of TiO 2 - and Ti
3+ -doped TiO 2 inverse opals could be enhanced by
choosing PS arrays with appropriate size as hard templates due to the slow light
9.4 Multiple Light Scattering
235
to multiple light reflecting and scattering in between the hierarchical spherical shells
of the TiO 2 –SnO 2 MHSs, rather than an increased surface area. The JSC value for
the SnO 2 MHSs is 50% higher than that of nano-SnO 2 , suggesting that the enhanced
JSC value for the TiO 2 –SnO 2 MHSs compared with the TiO 2 –nano-SnO 2 is due to
the hierarchical spherical structure rather than the TiCl 4 treatment. The inset in
Fig. 9.11 (right) illustrates the reflecting and scattering of light in a TiO 2 –SnO 2
MHS. It is believed that the shells with the different sizes in a microsphere cannot
only multireflect but also scatter the incident light of different wavelengths in the
range of visible light. The improved photocurrent for the TiO 2 –SnO 2 MHSs is
mainly contributed to its multilayered hollow spherical structure, which provides
an effective way to enhance light-harvesting efficiency.
Photonic-crystal-based optical coupling offers a unique way of light–matter
interaction to increase light harvesting, especially around the absorption edge of a
semiconductor. A photonic crystal is a periodic dielectric structure that can forbid the
propagation of light in a certain crystal direction within a certain spectrum regime,
called a photonic stop-band. The light in a photonic crystal undergoes strong
coherent multiple scattering and travels with very low group velocity near the
photonic stop-band edges, referred to as slow light. Such a slow-light effect can
considerably increase the effective optical path length, therefore leading to a delay
and storage of light in photonic materials. 3D photonic crystal design was utilized to
enhance incident photon-to-electron conversion efficiency (IPCE) of WO3
photoanodes. Large-area and high-quality WO3 photonic crystal photoanodes with
inverse opal structure were prepared (Fig. 9.12). The photonic stop-bands of these
WO3 photoanodes were tuned experimentally by variation of the pore size of inverse
opal structures. It was found that when the red edge of the photonic stop band of
WO3 inverse opals overlapped with the WO3 electronic absorption edge at
Eg ¼ 2.6–2.8 eV, a maximum of 100% increase in photocurrent intensity was
observed under visible light irradiation (λ > 400 nm) in comparison with a disordered porous WO3 photoanode (Fig. 9.13). When the red edge of the stop band was
tuned well within the electronic absorption range of WO3, noticeable but less
amplitude of enhancement in the photocurrent intensity was observed. It was further
shown that the spectral region with a selective IPCE enhancement of the WO3
inverse opals exhibited a blueshift in wavelength under off-normal incidence of
light, in agreement with the calculated stop band edge locations. The enhancement
could be attributed to a longer photon– matter interaction length as a result of the
slow-light effect at the photonic stop-band edge, thus leading to a remarkable
improvement in the light-harvesting efficiency. The present method can provide a
potential and promising approach to effectively utilize solar energy in visible lightresponsive photoanodes [17].
A range of TiO 2 inverse opals with tunable macroporous size were synthesized
using different sized PS arrays as hard templates [18]. After a simple heating
treatment in vacuum, Ti
3+ -doped TiO 2 inverse opals were obtained. The optical
responses of TiO 2 - and Ti
3+ -doped TiO 2 inverse opals could be enhanced by
choosing PS arrays with appropriate size as hard templates due to the slow light
9.4 Multiple Light Scattering
235