Thermal conductivity of porous alumina ceramics prepared using starch as a pore-forming agent

International audienc

Electron-Selective Layers for Dye-Sensitized Solar Cells Based on TiO2 and SnO2

Titanium dioxide (anatase, rutile) and quasi-amorphous tin dioxide are prepared on F-doped SnO2 in the form of dense thin films, which can serve as electron-selective layers in perovskite solar cells and dye-sensitized solar cells (DSSCs). The present study brings new data about electronic and electrochemical properties of these films at the authentic conditions occurring in a dye-sensitized solar cell (DSSC). Hydrolysis of TiCl4 provides pure rutile TiO2 at low temperatures, but TiO2 (anatase) grows in these layers upon calcination. In acetonitrile medium, the flat band potential of TiO2 (rutile) is more negative than that of TiO2 (anatase). This is opposite ordering to that observed in aqueous media. The energy of conduction band minimum of TiO2 (anatase) equals -4.15 +/- 0.07 eV at the conditions mimicking the DSSC's environment. Electrochemical reductive doping of SnO2 provides a material with the most negative flat band potential and the largest overpotential for the reduction of I-3(-), Co(bpy)(3)(3+), and Cu(tmby)(2)(2+). Voltammetric screening of all the electrode materials in six different electrolyte solutions, relevant to DSSC applications, gives salient information about the mediator type and effects of calcination and the addition of 4-tert-butylpyridine. These data provide novel inputs for optimization of DSSCs and for perovskite photovoltaics, too

Dye-sensitization of boron-doped diamond foam: Champion photoelectrochemical performance of diamond electrodes under solar light illumination

Diamond foams composed of hollow spheres of polycrystalline boron-doped diamond are chemically modified with two donor-acceptor type molecular dyes, BT-Rho and CPDT-Fur, and tested as electrode materials for p-type dye-sensitized solar cells with an aqueous electrolyte solution containing methyl viologen as a redox mediator. Reference experiments with flat polycrystalline diamond electrodes evidence full blocking of the methyl viologen redox reaction by these dyes, whereas only partial blocking is observed for the diamond foams. This is ascribed to sp(2)-carbon impurities in the foam, viz. trans-polyacetylene and graphite-like carbon. Cathodic photocurrents under solar light illumination are about 3 times larger on foam electrodes compared to flat diamond. Long-term (1-2 days) illumination of the sensitized foam electrodes with chopped light at 1 sun intensity causes an increase of the cathodic photocurrent density to ca. 15-22 mu A cm(-2). These photocurrent densities represent the largest values reported so far for dye-sensitized diamond electrodes. The photoelectrochemical activation of the sensitized diamond electrodes is accompanied with characteristic changes of the dark voltammogram of the MV2+/MV+ redox couple and with gradual changes of the IPCE spectra

Electrochemical Characterization of CuSCN Hole-Extracting Thin Films for Perovskite Photovoltaics

CuSCN thin films (optimized previously for perovskite photovoltaics) are deposited on glass, F:SnO2 (FTO), Au, glass-like carbon (GC), and reduced graphene oxide (rGO). They exhibit capacitive charging in an electrochemical window from ca. -0.3 to 0.2 V vs Ag/AgCl. Outside this window, CuSCN film is prone to chemical and structural changes. Anodic breakdown (at ca. 0.5 V) causes restructuring into submicrometer particles and denuding of the substrate. The natural p-doping is demonstrated by both the Hall effect and Mott-Schottky plots from electrochemical impedance. The corresponding flatband potentials (in V vs Ag/AgCl) varied with the substrate type as follows: 0.12 V (CuSCN@FTO), 0.08 V (CuSCN@Au), -0.02 V (CuSCN@GC), and 0.00 V (CuSCN@rGO). The acceptor concentrations determined from electrochemical impedance spectroscopy are by orders of magnitude larger than those from electrical conductivity and the Hall effect, the latter being regarded correct. Raman spectra confirm that thiocyanate is the dominating structural motif over the isomeric isothiocyanate. In situ Raman spectroelectrochemistry discloses substrate-specific intensity changes upon electrochemical charging. The blocking function is tested by a newly designed redox probe, Ru(NH3)(6)(3+/2+). It not only has the appropriate redox potential for testing of the CuSCN films but also avoids complications of the standard "ferrocyanide test" which is normally used for this purpose. The perovskite solar cells exhibit better solar conversion efficiency, fill factor, and open-circuit voltage for the rGO-containing devices, which is ascribed to a larger driving force for the hole injection from CuSCN into rGO

Large area heavily boron doped nano-crystalline diamond growth by MW-LA-PECVD [Póster]

Diamond is a unique semiconductor with a wide bandgap which usually is easily doped with boron and is acknowledged as one of the best materials for electrochemical applications. Heavily boron doped, high quality single crystal synthetic diamond can reach electrical conductivity as high as 103 S.cm, whereas polycrystalline material usually reaches c.a. 102 S.cm. However, many potential applications are restricted by the deposition temperature and limited coating area of conventional MW PECVD systems. Deposition of boron doped nano-crystalline diamond (BNCD) layers using a microwave PECVD system with linear antenna delivery (MW-LA-PECVD), enabling large area coating, was first reported in 2014 [1]. However, layers showed lower electrical conductivity in comparison to BNCD layers deposited using conventional PECVD systems. In addition, deposition of BNCD by MW-LA-PECVD is complicated by the necessity for the addition of oxygen species, which are known to limit boron incorporation and the competitive growth of silicon carbide at low CO2 concentrations [2, 3]. In this work, we further study the effect of deposition conditions on the synthesis of BNCD using the MW-LA-PECVD technique. In order to produce highly conductive BNCD with a low sp2 fraction, we have investigated in greater detail the effect of deposition temperature, from 250 °C up to 750 °C, using temperature controlled substrate stages and the effect of precursor gas compositions

Heavily boron doped nano-crystalline diamond growth by MW-LA-PECVD [Póster]

Diamond is a unique semiconductor with a wide bandgap which is easily doped with boron and is acknowledged as one of the best materials for electrochemical applications. Heavily boron doped, high quality single crystal synthetic diamond can reach electrical conductivity of c.a. 103 S.cm, whereas polycrystalline material can reach c.a. 102 S.cm. However, many potential applications are restricted by the deposition temperature and limited coating area of conventional MW PECVD systems. Deposition of boron doped nano-crystalline diamond (BNCD) layers using a microwave PECVD system with linear antenna delivery (MW-LA-PECVD), enabling large area coating, was first reported in 2014. However, layers showed lower electrical conductivity in comparison to layers deposited using conventional PECVD systems. In addition, deposition of BNCD by MW-LA-PECVD is complicated by the necessity for the addition of oxygen species, which are known to limit boron incorporation and the competitive growth of silicon carbide at low CO2 concentrations. In this work, we further investigate the effect of deposition conditions on the synthesis of BNCD using the MW-LA-PECVD technique. In order to produce highly conductive BNCD, we have investigated the effect of CO2 concentration, boron to oxygen ratio and boron to carbon ratio (to well above standard values). The effect of deposition temperature was also studied (from 250 °C up to 750 °C) using temperature controlled substrate stages