AVS2013 Session EN-TuP: Energy Frontiers Poster Session

One of the actual major societal challenges is the reduction of the CO₂ emission. This can be achieved either by controlling the production of this greenhouse gas, or by converting the ejected CO₂ into other molecule(s) which then can used in other industrial processes, or by the sequestration of the gas.

In this fundamental study, pure CO₂ is transformed in an Inductively Coupled RF plasma at low pressure, under different experimental conditions (pressure, flow, power, pulsing…), in order to optimize its conversion into CO. The challenge of this approach consists into quantifying in situ the reaction yield. We show that by calibrating a quadrupole mass spectrometer with reference gases, it is possible to reach this goal. We present CO₂ -> CO conversion rate reaching up to 70%, for different plasma and pressure conditions, with a reactor energy efficiency up to 2%.

The measurements were compared with the results from a 2D plasma simulation based on the drift and diffusion approximation.

Funded through the Greenwin Gazton project (Région wallonne, Belgium)

We here present deposition of Ge nanoparticle films for quantum dot solar cell applications. Semiconductor nanoparticle films have attracted much at tention because of the unique characteristics such as tunable band gap and multiple exciton generation effects of quantum dots [1]. Ge nanoparticles have more outstanding quantum confinement effects than Si nanoparticles due to the larger excitonic Bohr radius of 24.3 nm for bulk Ge than 4.9 nm for bulk Si. In this study, we deposited Ge nanoparticle films by radio frequency (rf) magnetron sputtering in argon and hydrogen gas mixture at 20 % H₂ dilution ratio under a high pressure condition of 1.5 Torr. At such high pressure, Ge nanoparticle formation in gas phase is possible, because the mean free path of Ge atoms is as short as an order of micrometer.

We measured Raman spectra of Ge thin films and bulk Ge crystal. We obtained the full width at half maximum (FWHM) and peak frequency of each peak of Raman spectra, as a function of gas flow rate (R_{gas flow}). The film deposited at R_{gas flow} = 80 sccm shows a peak at 298 cm^-1, and this peak is assigned to Ge crystal. As R_{gas flow} increases, the peak slightly shifts to higher frequency, and FWHM increases from 7.2 cm^-1 at R_{gas flow} = 80 sccm to 11.7 cm^-1 at R_{gas flow} = 250 sccm. FWHM and peak frequency strongly depend on the particle size [2]. Form the dependence, the Ge particle size at R_{gas flow} = 250 sccm is deduced to be 6-7 nm. We also fabricated in Ge quantum-dot sensitized solar cells, and succeeded in carrier generation in Ge nanoparticles.

[1] G. Uchida, et al., “Effect of nitridation of Si nano-particles on the performance of quantum-dot sensitized solar cells ”, Jpn. J. Appl. Phys, Vol. 51, pp. 01AD01-1 – 01AD01-5, 2011.

Dye-sensitized solar cell (DSSC) has been expected to be an alternative to the conventional silicon solar cell due to simple manufacturing process and low fabrication cost [1]. The condition of dye-coating process is one of the important factors in the fabrication process. In order to enhance dye adsorption on the TiO₂ layer of large scale DSSC, the condition was optimized for the dye-coating process such as temperature, time and concentration of base on references paper [2].

In this work, we report dye-coating conditions to enhance the efficiency of which was made in the area of 0.25, 6 and 15 Cm². The experiment shows the optimal coating condition with the coating temperature of 70 ℃, the dye concentration of 10 mM and the coating time of 3 min. We confirmed that the energy conversion efficiencies of DSSCs with area 6Cm² (3X2) and 15Cm²(3X5) were about 5% and 4.5%, respectively.

REFERENCES

[1] B. O'Regan, M. Gratzel, Nature, 353, 737, 1991

[2] Electrochimica 66(2012)121-125

Hydrogen represents a promising energy carrier that may simultaneously offer a good transportation fuel as well as large-scale energy storage for variable energy systems. Photoelectrochemical splitting of water is a good option for hydrogen generation via solar energy usage [1]. Present report deals with the synthesis of nitrogen doped (1 % at.) nanostructured zinc oxide thin films by sol-gel and electrodeposition on to conducting glass (ITO) substrate for possible use in photoelectrochemical (PEC) splitting of water. Films were characterized for crystallographic, optical and surface properties by XRD, SEM, UV-visible absorption and AFM analysis. High intensity peaks observed at 2θ angles, 31.7, 34.4, and 36.2⁰ revealed dominant presence of hexagonal wurtzite ZnO [2] in all the samples, with lattice parameters a= 3.19, c= 5.21 and a= 3.45, c= 5.63 in electrodeposited and sol-gel prepared thin films, respectively. Scherrer’s calculations [3] have been used to determine the average crystallite size of the samples and it was found to lie between 34 to 56 nm. The UV-Visible absorption measurements and the Tauc plots yielded band-gap energy of samples ranging 2.9-3.1 eV. SEM and AFM images indicated distinct grain boundaries with partial c-axis orientation of nanocrystallites. The PEC measurements involved determination of current-voltage (I-V) characteristics, both under darkness as well as under illumination. PEC cell was filled with 0.1 M NaOH (pH 13) solution as electrolyte. The photocurrent of 0.49 mA/cm^-2 and 1.30 mA/cm^-2 at 0.5 V applied voltage (vs. saturated calomel electrode), was recorded using, respectively, electrodeposition and sol-gel derived films as photo anode in PEC splitting of water under the illumination of 150 Wcm^-2 (Xenon arc lamp; Oriel, USA). The photocurrent values were utilized further to calculate applied bias photon-to-current efficiency (% ABPE), which was estimated to be 0.38 and 1.05 % at 0.5 V bias.

References

1)U. Sahaym, E M. Grant Norton, J Mater Sci, 43 (2008) 5395–5429.

2)V. Sharma, P. Kumar, N. Singh, S. Upadhyay, V.R. Satsangi, S. Dass, R. Shrivastav, Int. J. of Hydrogen Energy, 17 (2012) 12138 – 12149.

3)P. Scherrer, Nachr. Ges. Wiss. Gottingn, 2 (1918) 89.

For the use of promising new technologies such as electric cars and smart power grids to become more widespread, batteries must be developed with the ability to store larger amounts of energy. Increases in energy density are often accomplished by substituting different compounds into already successful commercial battery architectures. As one of the most common types of portable power cells, conventional lithium ion batteries rely upon intercalation to store charge in the cathode:

Li⁺ + e^- + CoO₂ --> LiCoO₂

This process can be improved by replacing lithium cobalt oxide with a conversion reaction material, whose structure and composition are instead chemically altered by lithiation. In particular, a cathode composed of FeF₂ can react with lithium ions to produce metallic iron, storing an additional electron per formula unit:¹

2Li⁺ + 2e^- + FeF₂ --> 2LiF + Fe

However, the mechanism by which this reaction progresses is not well understood, so it is important to investigate the chemical phase and morphological changes caused by the lithiation process in a model system. This has been accomplished by working with well-characterized samples outside of the electrolytic environment of a typical battery, thereby isolating the fundamental properties of the reacting materials. My poster describes the spectroscopic analysis of epitaxial iron (II) fluoride (110) thin films in ultrahigh vacuum, before and after a series of lithium exposures. Angle-Resolved X-Ray Photoelectron Spectroscopy (ARXPS) was used to determine the chemical concentrations of the material as a function of depth in the film. Evidence supports a uniform reaction front progression downwards from the surface of the film, but with the presence of a barrier to full conversion of the fluoride. Funding for this project was provided by the National Science Foundation grant PHY-1263280.

¹ Rangan, S.; Thorpe, R.; Bartynski, R. A.; Sina, M.; Cosandey, F.; Celik, O.; Mastrogiovanni, D. D. T. J. Phys. Chem. C 2012, 116, 10498-10503.

The research on quaternary compound semiconductors has been increased in particular due to their photovoltaic applications and the increasing price and scarcity of many elements (Cd, Te). Currently, the semiconducting kesterite-type Cu₂ZnSnSe₄ (CZTSe) arises as an important and interesting alternative to play an important role in the energy conversion domain in the future. This work shows the development of CZTSe thin films obtained by a dc sputtering equipment. First, multiple thin layers were deposited on glass substrates by alternate sputtering of single metallic sources: (Cu/Zn/ Sn). Samples had a selenisation process at different temperature and time in order to study the effect on the optical, structural and electrical properties. The transmittance and reflection properties were determined using a UV-Vis Spectrophotometer. The absorption coefficient of the films was estimated to be around 10⁵ cm^-1 and the band gap energy was in the range between 1.35 and 1.65 eV. The Cu₂ZnSnSe₄ films were characterized using X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). The XRD data showed clear peaks corresponding to Cu₂ZnSnSe₄, the main growth orientations were [112], [213] and [312]. Grain size and lattice parameters were calculated and related with the selenization temperature. The SEM images indicated that selenisation temperature controls the nanocluster size and density on surface. Photoconductivity measurements of the CZTSe samples showed an increase on photogeneration of charge carriers. Band gap energies, stochiometry and photocurrent of CZTSe films obtained by sputtering of single metallic sources are suitable for photovoltaic applications.