Whikun Yi

Enhanced charge collection and reduced recombination of CdS∕TiO2 quantum-dots sensitized solar cells in the presence of single-walled carbon nanotubes

This paper reports the modification of CdS∕TiO2 quantum-dot sensitized solar cells by using single-walled carbon nanotubes (SWCNTs) on indium-doped tin oxide (ITO) electrodes. The presence of the SWCNT layers on an ITO electrode increased the short-circuit current under the irradiation condition and also reduced the charge recombination process under the dark condition. The power conversion efficiency of CdS∕TiO2 on ITO increased 50.0% in the presence of SWCNTs due to the improved charge-collecting efficiency and reduced recombination.

Enhanced charge-collection efficiency of In2S3∕In2O3 photoelectrochemical cells in the presence of single-walled carbon nanotubes

This letter reports on the efficiency of In2S3∕In2O3 photoelectrochemical cells by enhanced charge collection and reduced recombination reaction in the presence of single-walled carbon nanotubes (SWCNTs) on indium-doped tin oxide substrate. Nanocomposite system as In2S3∕In2O3/SWCNTs was assembled using spray-coating and wet chemical processes. Due to enhanced charge collection and reduced recombination in the presence of SWCNTs, 52.9% increment of power conversion efficiency is achieved compared to those without SWCNTs layers.

Growth characteristics of carbon nanotubes using platinum catalyst by plasma enhanced chemical vapor deposition

Abstract In growth of carbon nanotubes (CNTs) using Pt catalyst by plasma enhanced chemical vapor deposition, effects of experimental parameters, such as NH3 plasma pre-treatment, NH3/C2H2 ratio and growth temperature, on the growth characteristics of CNTs were studied in details. It is noteworthy that fine dispersion of numerous nano-sized Pt particles was supported on the wall of a CNT without agglomeration. Application of CNTs with nano-sized Pt particles to the electrodes of the fuel cell can be possible.

Electric-Field Enhancement of Photovoltaic Devices: A Third Reason for the Increase in the Efficiency of Photovoltaic Devices by Carbon Nanotubes

2010 WILEY-VCH Verlag Gmb Carbon nanotubes (CNTs) show unique chemical and physical properties, which have been utilized in the field of nanoscience and nanotechnology. Among their various applications, CNTs have attracted considerable attention as emitters for electron-emission devices because of their high aspect ratio and excellent electrical, mechanical, and thermal properties. CNTs are known to enhance the electrical field in electron-emission devices, which generate high field emission (FE) currents. Furthermore, hybrid composite materials with CNTs have demonstrated a synergy effect in the generation of strong FE currents. CNTs have also been used in photovoltaic devices as conducting electrodes for the replacement of the transparent conducting oxide (TCO) films, as counter electrodes for the reduction of tri-iodide (I3 ), as electron-accepting materials, and as additive materials in n-type semiconductors. Especially, compound semiconductor/CNTs composites have been shown to considerably improve the efficiency of photovoltaic devices, which is often attributed to two different factors. One is the formation of efficient electronic energy cascade structures. In other words, when the conduction band of single-walled carbon nanotubes (SWNTs) is located between the work function of TCO and the conduction band of the semiconductor, excited electrons can easily be transferred to the TCO electrode through the CNTs. The other effect of CNTs on the performance of photovoltaic devices is the decrease of the interfacial resistance. The interfacial resistances in n-type/p-type materials and/or n-type materials/TCO electrodes are reduced by the outstanding electrical properties of CNTs. Both the formation of efficient electronic energy cascade structures and the decrease of the interfacial resistance increase the power-conversion efficiency due to the improved chargecollection efficiency and reduced recombination process. The improvement in the performances of photovoltaic cells afforded by incorporating CNTs has been attributed solely to these two mechanisms. However, they may not be sufficient to fully explain the significant improvement that is observed. In particular, CNTs are well known as materials which enhance the electrical field. Although the CNTs are located on the anode side in photovoltaic cells, it is worthwhile investigating the intrinsic function of the electrical-field enhancement in photovoltaic devices. Therefore, in the present study, we explore the effect of the CNT layers by measuring the reverse-FE currents and the significant increase in the b-value (i.e., the field enhancement factor) that are observed. In addition to these two effects, the enhancement of the electrical field by the CNT layers in photovoltaic devices should be considered as another potentially important factor contributing to the improvement of their efficiency. To confirm this hypothesis, the FE currents were measured using superstrate-type Cu(In,Ga)Se2 (CIGS) with CNTs [F-doped SnO2(FTO)/CNTs/ CdS/CIGS]. The levels of electrical field afforded by the introduction of CNTs into photovoltaic devices were monitored by measuring their reverse-field emission currents. Figure 1 shows a schematic diagram of the measurement of the reverse-FE currents. In general, the systems used to measure the FE currents consist of a sample electrode as the cathode and a conductive electrode as the anode, which tend to have a positive potential. However, the systems employed for investigating the effect of the CNTs in the photovoltaic cells were prepared using CIGS/CdS/SWNTs/ FTO as the anode and amicrotexturized tip silicon (m-tip Si) as the cathode, which tend to have a negative potential. Because a CIGS/ CdS/CNTs/FTO electrode is used as the photoanode in the photovoltaic cells, the system was prepared for the measurement

Control of carbon nanotubes density through Ni nanoparticle formation using thermal and NH3 plasma treatment

Abstract We controlled the size and distribution of catalytic Ni nanoparticles to control the density and diameter of carbon nanotubes (CNTs). With an increase in plasma power from 30 to 90 W for Ni layer of 30 A, the average diameter of particles decreased from 157 to 58 nm due to an enhanced etching effect. Size and distribution of Ni nanoparticles varied by plasma power, pre-treatment time and thickness of catalytic layer, resulting in the change in the density of grown CNTs. Density of CNTs reduced from 1×10 9 to 8×10 6 cm −2 , depending on the pre-treatment condition.

Photodetachment of aryl moieties from covalently functionalized single-walled carbon nanotubes by UV laser irradiation

Photodetachment of aryl moieties from covalently functionalized SWNTs was studied by temperature-dependent and UV Raman spectroscopic tools. The sidewall functionalized SWNTs were prepared via diazonium reactions from 4-bromoaniline and isoamyl nitrite. I–V conductivity measurements were performed for the functionalized SWNTs, after purification from pristine SWNTs using their different solubility. Temperature dependent Raman data appeared to be in line with thermal gravimetric (TGA) data exhibiting removal of aryl moieties above 590 °C. Raman spectra of the functionalized SWNTs were examined using ultraviolet excitation at 244 nm (5.08 eV) and 325 nm (4.82 eV) as well as visible irradiation at 633 nm (1.96 eV). Our experimental results indicated that the disorder-induced D mode should change significantly, whereas the tangential G modes do not become different under UV irradiation. The D mode for the functionalized SWNTs was found weakly at ∼1410 cm−1 at a low power of UV irradiation, whereas it was observed strongly at ∼1310 cm−1 upon 633 nm irradiation. The attached aryl moieties appeared to be removed by UV irradiation as indicated from almost identical spectra with those of pristine SWNTs.

Formation and characterization of poly(acrylic acid) on silica particles irradiated by γ-ray radiation

Organic/inorganic hybrid gels were directly prepared by polymerization on the peroxide surface of silica particles where the particle surface was irradiated by a 60Co γ-ray. These hydrogels have no residues of initiators or cross-linkers, so they can be used in biocompatible gel applications. Wide Raman spectroscopy was used to verify the interaction between the particles and poly(acrylic acid) (PAA). We observed that covalent bonds existing between the peroxide particles and acrylic acid, and the hydrogen bonds between the acrylic acids. For these studies, we prepared hydrogels by varying the particles’ concentration and the size of the silica particles to classify the number of reaction sites, which are the dominant factor for the chemical reaction between the silica particles and PAA.

Electron emission from carbon nanotube-dispersed MgO layer

A simple secondary-electron-emission (SEE)-based source was fabricated using a carbon-nanotube-dispersed MgO precursor solution. This emission source exhibits a high SEE gain as over 103. In addition, the self-sustaining current was observed even after turning off the primary electron beam, then the emission current could be modulated by a small electric field variation (1.2–2V∕μm). The electron energy spectrum of the self-sustaining emission indicates that the major character of the emission is attributed to the field-enhanced SEE rather than direct field emission.

Characteristic features of stone-wales defects in single-walled carbon nanotube; adsorption, dispersion, and field emission

Adsorption behaviors of dodecanethiol (C12H25SH) molecules are investigated on the surface of single-walled carbon nanotubes (SWCNTs) with vibrational and X-ray photoelectron spectrometers. The active adsorption sites are proved as Stone-Wales (SW) defects (5-7 ring defects). The SW defect-removed SWCNTs formed by reacting nanotubes with allyl acrylate molecules are compared with pristine SWCNTs in dispersion and field emission. The former shows higher dispersion and field emission than the latter.

The electric field enhancements by single-walled carbon nanotubes in In2S3/In2O3 photoelectrochemical solar cells

Using reverse-field emission (FE) current measurements, we demonstrate enhancement of the electric field by single-walled carbon nanotubes (SWNTs) in In2S3/In2O3 photoelectrochemical solar cells (PECs). In reverse-FE measurements, anode and cathode consists of In2S3/In2O3/(with or without) SWNTs on indium-doped tin oxide substrate and microtexturized tip silicon, respectively. The enhanced FE results for In2S3/In2O3/SWNTs show an electric field approximately two times more than In2S3/In2O3. The β value (i.e., electric field enhancement factor) of In2S3/In2O3 PECs with SWNT layers is 4950, which is ∼35.2% higher than that of In2S3/In2O3 PECs without SWNTs (3660). In PECs, the enhanced electric field intensifies the power of electron transfer, which accelerates the electron transfer rate in the cell.