Semiconductor Nano-Morphologies for Photovoltaics

Titanium dioxide is a semiconductor with great potential in dye-sensitized solar cells, energy storage and photocatalysis. These applications depend on control of surface area, porosity and morphology. We have recently developed a highly flexible new preparation strategy for the formation of various titania nano-morphologies, based on fusing preformed ultrasmall titania nanocrystals with surfactant-templated sol-gel titania acting as a structure-directing matrix and as a chemical glue. In this “brick & mortar” approach, the “mortar” acts as a reactive precursor for the further growth of the crystalline phase seeded by the nanocrystalline “bricks”. This synergy leads to a significantly lowered temperature needed for crystallization and the preservation of the mesoporous structure. It also allows us to build various hierarchical structures such as titania inverse opals penetrated by titania mesopores, because we can significantly reduce shrinkage effects. Coatings with a broad variety of periodic mesostructures can be tuned by varying the surfactant and the fraction of the “bricks”, and thicknesses ranging from a few nanometers to several micrometers are accessible. These mesostructured and crystalline films are being employed as active layers in thin dye-sensitized solar cells exhibiting high conversion efficiency due to short diffusion paths. We also investigate additional sensitizing approaches such as extremely thin absorber layers (ETA) and alternative solid hole conductors. Moreover, we find that the ultrathin crystalline walls of the mesoporous brick & mortar titania feature extremely fast lithium insertion kinetics.

Key publications:

  • Ultrasmall Titania Nanocrystals and Their Direct Assembly into Mesoporous Structures Showing fast Lithium Insertion: J. M. Szeifert, J. M. Feckl, D. Fattakhova-Rohlfing, Y. Liu, V. Kalousek, J. Rathousky, T. Bein, J. Am. Chem. Soc. 2010, 132, 12605–12611.
    Ultrasmall and highly soluble anatase nanoparticles were synthesized from TiCl4 using tert-butyl alcohol as a new reaction medium. This synthetic protocol widens the scope of nonaqueous sol−gel methods to TiO2 nanoparticles of around 3 nm with excellent dispersibility in ethanol and tert-butanol. Microwave heating was found to enhance the crystallinity of the nanoparticles and to drastically shorten the reaction time. The advantages of the retention of the mesoporous order with extremely thin nanocrystalline walls were shown by electrochemical lithium insertion. The films made using microwave-treated nanoparticles showed supercapacitive behavior with high maximum capacitance due to quantitative lithiation with a 10-fold increase of charging rates compared to a standard reference electrode made from 20 nm anatase particles.

  • Niobium-Doped Titania Nanoparticles: Synthesis and Assembly into Mesoporous Films and Electrical Conductivity: Y. Liu, J. M. Szeifert, J. M. Feckl, B. Mandlmeier, J. Rathousky, O. Hayden, D. Fattakhova-Rohlfing, T. Bein, ACS Nano 2010, 4, 5373–5381.
    Crystalline niobium-doped titania nanoparticles were synthesized via solvothermal procedures using tert-butyl alcohol as a novel reaction medium, and their assembly into mesoporous films was investigated. The solvothermal procedure enables the preparation of crystalline doped and undoped nonagglomerated titania nanoparticles, whose size can be controlled from 4 to 15 nm by changing the reaction temperature and time. The anatase lattice of these particles can incorporate more than 20 mol % of Nb ions. The nanoparticles can be easily dispersed at high concentrations in THF to form stable colloidal suspensions and can be assembled into uniform porous mesostructures directed by the commercial Pluronic block copolymer F127. The resulting mesoporous films show a regular mesostructure with a d spacing of about 17 nm, a uniform pore size of about 10 nm with crystalline walls, a high porosity of 43%, and a large surface area of 190 m2 cm−3. Substitutional doping with niobium ions drastically increases the electrical conductivity of the titania particles. The electrical conductivity of as-prepared nanoparticles containing 20 mol % of Nb is 2 × 10−5 S cm−1; it increases to 0.25 S cm−1 after treatment at 600 °C in nitrogen.

  • Ultrafast terahertz photoconductivity in nanocrystalline mesoporous TiO2 films: H. Nemec, P. Kuzel, F. Kadlec, D. Fattakhova-Rohlfing, J. Szeifert, T. Bein, V. Kalousek, J. Rathousky, Appl. Phys. Lett. 2010, 96, 062103.
    Terahertz time-resolved spectroscopy is used to investigate the transport of photoexcited electrons in nanocrystalline mesoporous TiO2 films prepared by the recently proposed “brick and mortar” technology with a variable fraction of nanocrystalline titania “bricks” and amorphous titania “mortar”. Both long- and short-range conductivity is significantly enhanced upon calcination. After an ultrafast (subpicosecond) regime where the intrananograin conductivity dominates, the electron conductivity becomes limited by the interaction of electrons with the amorphous mortar. Comparison of the experimental results with Monte Carlo simulations of the electron motion allows us to determine the crystalline grain size after calcination and the yield of mobile photocarriers.

  • “Brick and Mortar” Strategy for the Formation of Highly Crystalline Mesoporous Titania Films from Nanocrystalline Building Blocks: J. M. Szeifert, D. Fattakhova-Rohlfing, D. Georgiadou, V. Kalousek, J. Rathousky, D. Kuang, S. Wenger, S. M. Zakeeruddin, M. Grätzel, T. Bein, Chemistry of Materials 2009, 21, 1260-1265.
    We present a novel “brick and mortar” strategy for creating highly efficient transparent TiO2 coatings for photocatalytic and photovoltaic applications. Our approach is based on the fusion of preformed titania nanocrystalline “bricks” through surfactant-templated sol−gel titania “mortar”, which acts as a structure-directing matrix and as a chemical glue. The similar chemical composition of both bricks and mortar leads to a striking synergy in the interaction of crystalline and amorphous components, such that crystallization is enhanced upon thermal treatment and highly porous and highly crystalline structures are formed at very mild conditions. Coatings with a broad variety of periodic mesostructures and thicknesses ranging from few nanometers to several micrometers are accessible using the same organic template, and the final structures are tunable by varying the fraction of the “bricks”. The beneficial combination of crystallinity and porosity leads to greatly enhanced activity of the films in photocatalytic processes, such as the photooxidation of NO. Acting as the active layers in dye-sensitized solar cells, films of only 2.7 μm in thickness exhibit a conversion efficiency of 6.0%.