Nanostructured Architectures for Solar Energy Conversion

Grätzel Cells

Most dye-sensitized solar cells are based on a thin film of porous, nanocrystalline anatase TiO2, which is sensitized by a dye absorbing in the visible. Absorption of a photon by the dye results in ultra-fast electron injection into the TiO2 conduction band. The electron moves through the TiO2 layer to the external electrode while the dye is regenerated by a liquid I3-/I2 electrolyte. The TiO2 layer is highly disordered and still poorly understood. Its electrical properties are influenced by the porous structure and large surface area as well as by the presence of defects. The goal of this project is (i) to explore whether oxide semiconductors other than TiO2 can be used in Grätzel-type cells, (ii) to elucidate how the surface structure of the semiconductor nanoparticles influences the cell properties and (iii) to ultimately design improved energy conversion devices through the detailed understanding of what controls their microscopic behavior.

Hybrid Chalcogenide Cells

Due to their unique electronic properties, a wide electrochemical stability window and high surface area, carbon nanostructures are among the promising candidates for the development of optical and photovoltaic devices. These developments rely on the idea that carbon nanotubes may easily accept electrons, which, in turn, might be transported under nearly ballistic conditions along the tubular axis. In addition, films of carbon nanotubes cast on optically transparent electrodes respond to visible-light excitation.

Functionalizing chalcogenide nanotubes with electron-donor antenna chromophores may lead to the development of novel nano-conjugate systems that bear promise for converting solar energy into electricity. The combination of a light-harvester/electron-donor molecule, such as porphyrins, together with an electron acceptor, the chalcogenide nanotube, might lead to highly efficient photovoltaic cells. Recently, we could demonstrate that examples of covalent and, to a similar extent, non-covalent porphyrin/chalcogenide nanotube assemblies exhibit the anticipated electron-transfer properties.

Photonic Structures for Photovoltaics

Spectrally-selective photonic structures may be used to improve solar cell systems. These spectrally-selective structures are based on interference effects, representative examples being 3D photonic crystals such as artificial opals or 1D Bragg stacks.

Optical elements such as photonic crystals are used in different ways for improving solar cell efficiencies. Examples range from antireflection coatings, i.e., layers deposited directly on the solar cell to reduce front surface reflections, to lenses or mirrors, or devices separated from the cell to concentrate radiation. We focus on a special class of optical elements, namely 1D Bragg stacks to improve solar cells.

The defining characteristics of spectrally-selective filters for photovoltaic applications are:

  • a specific spectral range where the filter is highly reflective,
  • another spectral range where the filter is highly transparent.

Often, these ranges are adjacent. Thus, an additional requirement is

  • a steep edge between the both ranges

Our approach is to fabricate periodically nanostructured TiO2 films that behave as efficient photoconducting Bragg mirrors. By means of the precisely controlled deposition of TiO2 nanocrystallite layers of alternate porosity, we generate a periodic modulation of the refractive index that confers photonic crystal properties to the film, with well defined and intense Bragg reflections in the visible range. At the same time, the inter- and intralayer connectivity maintains the photoconductive properties of the TiO2 without altering other properties of the oxide, such as its surface chemistry.

Composite materials with periodic variations of density and/or sound velocities, so-called phononic crystals, can exhibit bandgaps where propagation of acoustic waves is forbidden. Phononic crystals are the elastic analogue of the well-established photonic crystals and show potential for manipulating the flow of elastic energy. So far, the experimental realization of phononic crystals has been restricted to macroscopic systems with sonic or ultrasonic bandgaps in the sub-MHz frequency range. In this work, using high-resolution Brillouin spectroscopy we report the first observation of a hypersonic bandgap in face-centred-cubic colloidal crystals formed by self-assembly of polystyrene nanoparticles with subsequent fluid infiltration. Depending on the particle size and the sound velocity in the infiltrated fluid, the frequency and the width of the gap can be tuned. Promising technological applications of hypersonic crystals, ranging from tunable filters and heat management to acousto-optical devices, are anticipated.

Phononic Structures

While photonic crystals are based on strong scattering and destructive interference of optical waves in periodically structured materials, periodic spatial modulations in their density and/or elastic constants can lead to a modification of wave propagation in the acoustic frequency region. Such structured materials are usually termed phononic crystals by complete analogy with the more familiar photonic crystals. The hallmark of a phononic crystal is its capacity to create bandgaps, usually at Bragg frequencies or wavelengths commensurate to its lattice constant, hence preventing acoustic waves with certain frequencies travelling through the crystal at least in certain crystallographic directions. The width of the bandgap, in general, increases with the difference in the densities and sound velocities of the component materials, and the frequency of the gap can be tuned, for example, by changing the lattice parameter.
We have been using the concept of Bragg stacks to fabricate one-dimensional periodic (e.g. SiO2/poly(methyl methacrylate)) multilayer films with phononic band gaps in at gigahertz frequencies. The band gap to midgap ratio of 0.30 occurs for elastic wave propagation along the periodicity direction, whereas for in-plane propagation the system displays an effective medium behavior. The phononic properties are well captured by numerical simulations. The porosity in the silica layers presents a structural scaffold for the introduction of secondary active media for potential coupling between phonons and other excitations, such as photons and electrons

N. Gomopoulos, D. Maschke, C. Y. Koh, E. L. Thomas, W. Tremel, H.-J. Butt, and
G. Fytas, Nano Lett. 2010, 10, 980–984