Environment‐sensitive Bragg reflectors are built using functional mesoporous thin films as building blocks. Tuning of optical properties is achieved by changing the composition or porosity of the slabs or the introduction of planar defects. Sorption or capillary condensation of molecules into the pore system results in a 10–40 nm photonic bandgap (PBG) shift. Organic functions added to the pore surface change the response, permitting tailoring of the selectivity towards small‐size molecules.

Herein we report a theoretical analysis of the efficiency of dye sensitized solar cells in which colloidal crystals are introduced in different configurations. We find that piling up different lattice constant crystals leads to light harvesting enhancement in the whole dye absorption range. We provide the optimum structural features of such photonic crystal multilayer needed to achieve a photocurrent efficiency enhancement of around 60% with respect to standard dye sensitized solar cells. We demonstrate that this improvement is the result of the optical absorption amplification effect of slow photon resonant modes partially confined within the absorbing part of the cell by the mirror behavior of the colloidal superlattice. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2200746͔ Dye sensitized solar cells ͑DSSCs͒ constitute an interesting alternative to solid state semiconductor photovoltaic devices, mainly due to their low cost of production. 1,2 Briefly, a DSSC is made by a nanocrystalline titanium oxide layer doped with a ruthenium based dye supported on a conducting electrode and fully immersed in a redox electrolyte ͑typically an iodide solution͒. The whole system is sealed by a counterelectrode. The dye absorbs light in the visible range and injects the photogenerated electrons to the conduction band of the nanocrystalline titania, 3 being the photoelectrons generated transferred to the supporting conducting substrate. At the other end of the circuit, the electrolyte is reduced at the counterelectrode and oxidized back after transferring an electron to the dye.Even though current DSSC still shows lower efficiencies than the p-n junction solar cells, there are many parameters to be investigated yet in order to improve light to current conversion. DSSC photovoltage can be increased by using other metal oxides such as NbO 5 or SrTiO 3 . 4,5 Also higher photocurrents can be achieved by changing the sensitized dye, by combining cells with different dyes which absorb at different wavelength ranges or by means of introducing scattering structures that increase the matter-radiation interaction time, so the probability of photon absorption increases. 6,7 Our work focuses on this latter approach. A modification in the photon collection of the cell affects the photogenerated current as explained in what follows. The light harvesting efficiency ͑LHE͒ or absorptance is the fraction of light intensity absorbed by the dye in the DSSC:It depends on the extinction coefficient of the dye, its concentration, the thickness of the absorbing layer, and the time of flight of photons within that film. The incident photon to current conversion efficiency ͑IPCE͒ is proportional to the LHE and is given by the expressionwhere ⌽ is the quantum yield of charge injection, and is the charge collecting efficiency by the glass supported electrode. From this magnitude its possible to obtain the generated photocurrent density in short-circuit ͑J SC ͒ by using the equation ͑3͒where q is the electron charge, and F͑ ͒ is the incident phot...