Transfer of preformed three-dimensional photonic crystals onto dye-sensitized solar cells.

Photonic crystals are materials that exhibit periodicities in their refractive index on the order of the wavelength of light, and thus provide many interesting possibilities for “photon management”. Applications for photonic crystals include light bending, inhibition of spontaneous emission, and amplified photon absorption or emission. A major limitation, however, for the incorporation of photonic crystals, in particular self-assembled three-dimensional photonic crystals, in optoelectronic devices are incompatibilities between the fabrication routes for the photonic structures and the active device. Most notably, the ideal substrate for the self-assembly of colloidal photonic crystals is a planar, nonporous, and chemically homogeneous surface, yet most active devices have rough surfaces, are chemically heterogeneous, and are often porous. Three-dimensional photonic colloidal crystals are of particular interest for enhancing light harvesting in dyesensitized solar cells (DSSCs) because these crystals can both be porous and significantly enhance light–matter interactions. Diffraction, dielectric mirror effects, and resonant modes are some of the phenomena that are exhibited by photonic crystals and can greatly enhance the effective light optical path within the active layer. In DSSCs, the light absorption takes place in an organic dye adsorbed onto a porous conductive network. The overall efficiency is limited because in most cases the dye does not exhibit a strong optical absorption towards the red part of the visible spectrum. This limitation is particularly dramatic when the typical titania matrix is substituted by a material with improved electron mobility but lower surface area, such as ZnO nanowires (NWs). Increasing the light–matter interaction, for example through the use of a porous photonic crystal coupled to the working electrode, would increase the overall efficiency. Artificial opals are good candidates for this role because of their inherent porosity, which allows the liquid electrolyte present in DSSCs to regenerate the oxidized dye. However, growth of a photonic crystal on a rough and porous DSSC working electrode is nearly impossible. Additional issues arise if higher-refractive-index photonic crystals, which are formed by using a colloidal crystal as a template for a high-dielectriccontrast material such as titanium oxide or silicon, are desired because of their potential to exhibit wider or even full photonic bandgaps. The inversion steps used to form such structures will clog the pores of the DSSC electrode, thus preventing adsorption of the sensitizing dye. Previous attempts to form photonic crystals on DSSCs include spincoating of colloidal crystals, use of an intermediate polymer layer that coats the titania layer, and substitution of the nanocrystalline titania layer with a mesoporous film to provide better surface properties for opal growth. However, these methods provide photonic crystal films with low optical quality compared with what can be achieved on flat substrates, thus reducing the benefits of the optical coupling between the photonic structure and the photovoltaic device. Herein we demonstrate the general concept of transferring preformed 3D photonic crystals onto various substrates, and in particular, the coupling of preformed photonic crystals with independently processed porous DSSCs. We fabricated three-dimensional colloidal, inverse opal silicon, and inverse opal titania photonic crystals, embedded them in a polycarbonate matrix, and transferred them onto several different types of porous electrodes used in DSSCs. The excellent optical properties of the photonic crystal films are maintained and an enhancement in the efficiency of the DSSCs is observed. The key step in the fabrication and transfer of the preformed photonic films is the infiltration of the photonic structure with a polycarbonate (PC) matrix that provides mechanical stability to the film after it is released from its original substrate yet can be cleanly thermally removed. This polymer has been used previously to enable the formation of free-standing flexible porous inorganic one-dimensional Bragg stacks. The first step is the growth of a silica colloidal crystal by evaporation-induced self-assembly on an oxidized silicon substrate (Figure 1a). The oxide layer acts as a sacrificial release layer when the substrate is immersed in hydrofluoric acid. The release step is only necessary for the transfer of inverted photonic crystals (e.g., Si or TiO2). If desired, the colloidal crystals are subsequently infiltrated with 20 nm of TiO2 (n= 2.4) by atomic layer deposition (ALD) [16] or 48 nm of amorphous silicon (n= 3.5) by chemical vapor deposition (CVD; Figure 1b). The silica colloidal crystals, [*] Dr. A. Mihi, C. Zhang, Prof. P. V. Braun Department of Materials Science and Engineering Frederick Seitz Materials Research Laboratory Beckman Institute for Advanced Science and Technology University of Illinois at Urbana-Champaign Urbana, IL 61801 (USA) Fax: (+1)217-333-2736 E-mail: [email protected] Homepage: http://braungroup.beckman.illinois.edu/