Photonic crystals for applications in photoelectrochemical processes: Photoelectrochemical solar cells with inverse opal topology

I. Rodriguez; P. Atienzar; F. Ramiro-Manzano; F. Meseguer; A. Corma; H. Garcia

doi:10.1016/j.photonics.2005.09.009

Research · Publication & note

Photonic crystals for applications in photoelectrochemical processes: Photoelectrochemical solar cells with inverse opal topology

Research publication

I. Rodriguez, P. Atienzar, F. Ramiro-Manzano, F. Meseguer, A. Corma and H. Garcia

Photonics Nanostruct. Fundam. Appl. 3, 148-154 (2005) - Available online 14 October 2005

Abstract

Photonic crystal structures, that present strong light localization effects near photonic band gap frequency regions, can be very useful to maximize chemical processes of phototoactive materials. One example is the use of photonic crystals to improve solar energy harvesting in photoelectrochemical solar cells. Here, we describe the optical monitoring synthesis of macroporous materials, with inverse opal topology, made of transition metal and rare-earth oxide nanoparticles. Through the optical properties we can obtain information concerning both infiltration and over layer growth. Finally, we report on the efficiency improvement of photoelectrochemical cells when titania inverse opal topology is used.

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Figures

Complete original Figure 1 composition extracted directly from the embedded PDF image — **Figure 1.** SEM images of the cleft edge of a thin film (30 monolayers) opal made of 615 nm latex particles: (a) before; (b)–(d) after infiltration with ceria nanoparticles. Scale bars: (a) 10 μm; (c) 2 μm; (b) and (d) 5 μm.

Complete original Figure 2 composition extracted directly from the embedded PDF image — **Figure 2.** Optical reflectance of samples, along the (111) direction, during the spin-coating-mediated CeO₂ infiltration process. The number of infiltration cycles increases from spectrum ‘a’ to ‘e’. The numbers of infiltration cycles for ‘a’, ‘b’, ‘c’, ‘d’, and ‘e’ are 0, 2, 6, 8, and 12, respectively. As loading progresses, the peak reflectance decreases, and a dip in the peak is observed when an overlayer forms on the surface (spectrum ‘e’).

Complete original Figure 3 composition extracted directly from the embedded PDF image — **Figure 3.** (a and b) SEM images of inverse ceria structure after the colloidal crystal template removal. (c) Inverse structure with an overlayer of CeO₂ on the top surface. (d) Reflectance spectra of inverse structures with (spectrum ‘b’) and without (spectrum ‘a’) CeO₂ overlayer. Scale bars: 2 μm (inset: 1 μm).

Complete original Figure 4 composition extracted directly from the embedded PDF image — **Figure 4.** (a) Optical reflectance spectra of opal templates before (spectrum ‘a’) and after 12 cycles of infiltration (spectrum ‘b’) with zirconia nanoparticles and after template removal (spectrum ‘c’). (b) Same as (a), but for yttria–opal composites: before infiltration (spectrum ‘a’), after 10 infiltration cycles (spectrum ‘b’), and inverse structure (spectrum ‘c’). (c and d) SEM images of the cleft edges of zirconia and yttria inverse opals with high-magnification image inset. Scale bars: (c) 5 μm (inset: 2 μm); (d) 2 μm.

SEM image of a titania inverse opal electrode with a 2 micrometre scale bar — **Figure 5.** SEM image of a titania inverse opal 8 μm thick electrode used for a photoelectrochemical solar cell. The void size is 170 nm and the bar corresponds to 2 μm.

Complete Figure 6 composition with current-voltage curve and spectral response panels — **Figure 6.** (a) Current–voltage (I–V) characteristic curve for both the titania inverse opal solar cell (continuous curve) and the standard titania (broken curve) photoelectrochemical cells under one-sun illumination (AM1.5). (b) Spectral response of the photonic-crystal photoelectrochemical cell as a function of light wavelength.

Research fields

Bottom-up

Latex opals are infiltrated with TiO₂, CeO₂, ZrO₂ or Y₂O₃ nanoparticles and then removed to leave interconnected inverse-opal frameworks. Repeated infiltration controls wall loading while preserving the three-dimensional macropore topology.

Characterization

Infrared reflectance follows stop-band shifts during infiltration and template removal, providing a non-destructive measure of how the pores fill. SEM and photoelectrochemical current–voltage tests then connect the completed inverse-opal morphology with improved light harvesting in solar-cell electrodes.

RESEARCH TOPICS

HarvestingAn 8 μm TiO₂ inverse-opal electrode is incorporated into a photoelectrochemical solar cell and compared with a conventional nanoparticle electrode under one-sun AM1.5 illumination. Figure 6 shows improvements in both open-circuit voltage and short-circuit current, giving about 35% more total power. Because the reference contains roughly four times more active material at the same thickness, the conversion efficiency per unit TiO₂ volume is about five times higher for the inverse opal. The result isolates a photonic benefit beyond simply adding more semiconductor.

InterferenceDuring infiltration, the opal's Bragg peak shifts to longer wavelength as the average refractive index rises, and a dip develops when the photonic feature overlaps material absorption. Reflectance therefore provides an in-process monitor of pore filling and overlayer growth before the template is removed. In the final titania structure, reduced group velocity and field localisation near the stop-band edge increase interaction with the photoactive material. The spectral photocurrent peaks near TiO₂'s absorption edge, linking the device response to the designed optical structure.

MaterialsLatex-opal films are replicated with nanoparticles of TiO₂ and, in the broader synthesis study, CeO₂, ZrO₂ and Y₂O₃. Figure 5 shows the titania inverse-opal electrode with voids around 170 nm. The connected macropores retain a high internal surface while arranging it periodically, unlike a dense film. Optical monitoring during repeated infiltration cycles helps distinguish successful filling from an unwanted surface overlayer, making structural quality part of the synthesis rather than a post hoc SEM check.

ChemistryLiquid precursors infiltrate the colloidal template, are converted to oxide nanoparticles and are fired at 450 °C to remove polymer, form anatase TiO₂ and improve interparticle electrical contact. The same replication strategy is extended to ceria, zirconia and yttria because each oxide brings different photocatalytic or acid–base properties. For the solar-cell demonstration, dye sensitisation and electrolyte chemistry are combined with the periodic electrode. The study therefore joins sol–gel/precipitation chemistry with optical feedback from the evolving photonic crystal.