Porous TiO2: Electron Transport and Application to Dye Sensitized Injection Solar Cells

Abstract

Porous TiO 2 (anatase, rutile) belongs to the class of materials with very low drift mobility of electrons (10 ± ±4 ±10 ± ±7 cm/Vs). The drift mobility is limited by traps being responsible for the slow interparticle charge transfer. The deep traps are generated at the TiO 2 surface in oxygen deficient atmosphere and should be avoided in dye sensitized injection solar cells since they increase the saturation current. Another group of defects is related to disorder in the bulk TiO 2 . The disorder is much stronger for anatase than for rutile. The respective optical bandgap of the undisturbed anatase is 3.45 eV. The disorder related defects do not limit the efficiency of the dye sensitized injection solar cell. The current dependent barrier height has been shown to be important as a limiting factor for the energy conversion efficiency in dye sensitized injection solar cells at moderate light intensity.Introduction Nanoporous semiconductors belong to the class of materials with very low drift mobilities (m) of charge carriers. For example, m is in the order of 10 ± ±4 ±10 ± ±1 and 10 ± ±4 ±10 ± ±6 cm 2 /Vs for electrons in nanoporous Si [1] and porous TiO 2 [2], respectively, what is lower by four to six orders of magnitude than m for the single crystalline Si and TiO 2 [3], an overview of electron Hall mobilities is given in [4]. The main reasons for the strong decrease of m are the complex network of a nanoporous structure, see e.g.[5] and the huge internal surface area of a porous semiconductor, see e.g. [6] which may lead to hopping transport on a fractal [7] or trap limited transport [8]. However, the microscopic picture of the charge transport is more complicated, especially in the presence of injected carriers of charge.The electronic transport in porous semiconductors is extremely sensitive to adsorbed molecules which change the distribution of electronic states. This bears a large potential for applications in sensors, especially for porous metal oxides, see e.g. [9,10]. The electronic states below the bandgap are not well investigated in porous metal oxides and their nature is not well understood up to now. Metal oxides are very sensitive to nonstoichiometry. A deficit of oxygen causes n-type doping (high temperature (T), low partial pressure of oxygen (p O 2 )) while additional incorporation of oxygen into the metal oxide leads to p-type doping, see e.g. [11]. TiO 2 is of n-type [12]. By using reduction and oxidation reactions one can reversibly cycle or refresh the electronic properties of metal oxides.TiO 2 is a wide bandgap semiconductor (E g 3.06 eV for the rutile phase [13]) and it can be easily produced as a nanocrystalline powder. (TiO 2 nanoparticles are widely used, for example, in white paint.) These two properties make TiO 2 nanoparticles quite interesting for application in dye sensitized injection solar cells, the so-called ªGra È tzelcellsº [14]. The idea of the injection solar cell is the separation in space of the steps of

Electron transport inTiO2probed by THz time-domain spectroscopy

“…While anatase-type TiO 2 is more commonly used in solar cell devices, 1 rutile has been shown to behave very similarly in such devices. 17,22,23 The band gap of rutile is 2.9 eV at room temperature, increasing slightly at lower temperatures. 24 The 1-mm-thick single-crystal samples, grown commercially by Crystal-GmbH, are mounted in a helium cryostat.…”

Polyelectrolyte Multilayer Template Assisted in-Situ Synthesis of the Inorganic Nanostructures

“…With the increasing thickness of the film, larger TiO 2 crystallites were observed in the film, and hence the TiO 2 film had a greater roughness, whereas the absorption edge is red shifted. Therefore, as previously reported, an increase in the crystallite diameter, the thickness increment and the higher roughness of the TiO 2 films could be responsible for the red shift of the absorptionmaxima peak position as the number of PE bilayers in the organic template is increased from 7 to 17 [41][42].…”

Highly efficient charge collection in dye-sensitized solar cells based on nanocomposite photoanode filled with indium-tin oxide interlayer

“…The global carrier mobility for the ITO granular film reflects the average value throughout the film and is therefore significantly affected by interparticle transfer, surface scattering, and trapping. Even so, the ITO interlayer with high conductivity yields a significantly larger carrier mobility than the mesoporous TiO 2 layer (10 −4 -10 −6 cm 2 /Vs) [13,31]. The dynamic process of the charge transfer through TIT photoanode is illustrated in Fig.…”

“…However, the undesirable interfacial charge recombination occur when an electron recombines with a hole located in either an oxidized dye molecule or the electrolyte (e.g., iodide/ triiodide redox couple), ultimately resulting in a reduced charge collection efficiency [11]. It is worth noticing that a trap adjacent to the cation (e.g., oxygen vacancy) also creates charge recombination centers, which can be remarkably eliminated by minimizing electron scattering at the TiO 2 NP surface and grain boundaries [12,13]. In order to reduce charge recombination rates caused by TiO 2 [17], nanotubes [18], nanofibers [19][20][21][22], and nanorods [23], which would lead to fast electron transport and suppressed charge recombination owing to their straightforward electron transport pathways.…”