Byung Tae Ahn

Flexible energy storage devices based on graphene paper

Recently, great interest has been aroused in flexible/bendable electronic equipment such as rollup displays and wearable devices. As flexible energy conversion and energy storage units with high energy and power density represent indispensable components of flexible electronics, they should be carefully considered. However, it is a great challenge to fabricate flexible/bendable power sources. This is mainly due to the lack of reliable materials that combine both electronically superior conductivity and mechanical flexibility, which also possess high stability in electrochemical environments. In this work, we report a new approach to flexible energy devices. We suggest the use of a flexible electrode based on free-standing graphene paper, to be applied in lithium rechargeable batteries. This is the first report in which graphene paper is adopted as a key element applied in a flexible lithium rechargeable battery. Moreover graphene paper is a functional material, which does not only act as a conducting agent, but also as a current collector. The unique combination of its outstanding properties such as high mechanical strength, large surface area, and superior electrical conductivity make graphene paper, a promising base material for flexible energy storage devices. In essence, we discover that the graphene based flexible electrode exhibits significantly improved performances in electrochemical properties, such as in energy density and power density. Moreover graphene paper has better life cycle compared to non-flexible conventional electrode architecture. Accordingly, we believe that our findings will contribute to the full realization of flexible lithium rechargeable batteries used in bendable electronic equipments.

Electronic structure study of lightly Nb-doped TiO2 electrode for dye-sensitized solar cells

To improve the conversion efficiency of dye-sensitized solar cells (DSSCs) it is necessary to understand the electronic structure of the TiO2–dye–electrolyte interface in detail. A sturdy junction at the interface can be provided by modifying the electronic structure of the TiO2 electrode with Nb doping. The Nb-doped TiO2 was prepared by a sol–gel method followed by a hydrothermal treatment; the Nb content was varied from 0.5 to 3.0 mol%. The X-ray photoelectron spectroscopy showed that the Fermi level of TiO2 electrode shifted away from the conduction band minimum (CBM) when the Nb content is low (≤1.5 mol%) and shifted toward the CBM when the Nb content is high (≥2.5 mol%). The shift of Fermi level with low Nb doping was due to the passivation of the oxygen vacancies at the TiO2 nanoparticle surface. Intraband states were formed when dopant content was 1.5 and 2.5 mol%. We have found that the photovoltaic parameters of DSSCs based on doped TiO2 sensitized with a cis-[Ru(dcbpyH)2(NCS)2](NBu4)2, N719 dye, are closely related to the electronic structure of the Nb-doped TiO2 electrode. The changes of short circuit current and open circuit voltage of DSSCs were explained in relation to the electronic structure of the TiO2 electrode. The best efficiency of 8.0% was demonstrated by DSSCs with 2.5 mol% Nb-doped TiO2.

Growth of CuInSe2 thin films by high vapour Se treatment of co-sputtered Cu-In alloy in a graphite container

Abstract The crystallization of CuInSe 2 thin films by high Se vapour selenization of co-sputtered Cu-In alloy precursor within a partially closed graphite container is reported. X-ray diffusion (XRD) analysis of the Cu-In alloy films displayed mainly the CuIn 2 and Cu 11 In 9 phases. A three-fold volume expansion was recorded in all the selenized CuInSe 2 films at 500–550°C. Large and densely packed crystals with sizes of about 5 μm were exhibited by the films irrespective of whether they were Cu-rich or In-rich. Single phase chalcopyrite CuInSe 2 structure with preferential orientation in the (112) direction were obtained. Films with a wide range of compositions (Cu/In of 0.43–1.2 and Se/(Cu+In) of 0.92–1.47) were fabricated. All the films where Se rich, with the exception of samples with very high Cu content. The measured film resistivities varied from 10 −1 to 10 5 Ω-cm in consistence with the increasing Cu content of the alloy precursor during deposition. The alloy films with very high In content yielded the CuIn 2 Se 3.5 or CuIn 3 Se 5 compound as determined from XRD and EDX analyses. A study of the reaction mechanism performed between 250 and 550°C indicated that the crystal growth was assisted by the formation of the CuSe flux agent. The development of a suitable window layer to test the photovoltaic properties of these films is currently in progress.

Effects of Se Flux on the Microstructure of Cu ( In , Ga ) Se2 Thin Film Deposited by a Three-Stage Co-evaporation Process

This work was financially supported by the Ministry of Commerce,Industry and Energy in Korea.

Back contact formation using Cu2Te as a Cu-doping source and as an electrode in CdTe solar cells

Cu2Te was utilized as a Cu source for p + doping in CdTe and as a primary back contact material in CdTe solar cells. A 60 nm-thick Cu2Te layer was deposited on CdTe film by evaporating Cu2Te and the samples were annealed at various temperatures. An amorphous layer was found at the Cu2Te/CdTe interface, while the Cu2Te has both orthorhombic and hexagonal phases. Annealing at 2001C completely crystallized the amorphous interlayer and enhanced the transformation of orthorhombic phase into hexagonal phase that has a coherent interface with CdTe. A good p + contact was formed at 1801C annealing, where the series

Growth of CuIn3Se5 layer on CuInSe2 films and its effect on the photovoltaic properties of In2Se3/CuInSe2 solar cells

Abstract The growth of CuIn3Se5 layer on bulk CuInSe2 films has been studied for the fabrication of CuInSe2 solar cells, using the three-stage process which involved the sequential evaporation of In–Se, Cu–Se, and In–Se elemental sources. After growing CuInSe2 films, the film surface was converted to a defect chalcopyrite (CuIn3Se5) compound. The X-ray diffraction and AES depth analysis indicated the formation of the CuIn3Se5 phase on the CuInSe2 surface. By the formation of the CuIn3Se5 phase, the absorption edge was shifted from 1200 to 1000 nm wavelength and the binding energies of Cu, In, and Se were shifted to higher energies. The current–voltage curves of In2Se3/CuInSe2 cells fabricated with a thick CuIn3Se5 layer on a CuInSe2 film displayed a kink effect which was possibly caused by the increase of series resistance and light absorption in the CuIn3Se5 layer instead of the junction region. The cells with a thin CuIn3Se5 layer at the In2Se3/CuInSe2 interface yielded solar efficiency of 8.46% with an active area of 0.2 cm2.

Cobalt Metallorganic Chemical Vapor Deposition and Formation of Epitaxial CoSi2 Layer on Si(100) Substrate

The Korea Advanced Institute of Science and Technology assisted in meeting the publication costs of this article.

Microwave-induced low-temperature crystallization of amorphous Si thin films

Abstract Microwave heating was utilized for low-temperature crystallization of amorphous Si (a-Si) films. Microwave heating lowered the annealing temperature and reduced the annealing time. By microwave heating the hydrogen in the amorphous films was diffused out long before the nucleation of polycrystalline Si (poly-Si). The combination of NiCl 2 coating on a-Si and microwave heating greatly reduced crystallization temperature. The combination of metal-induced crystallization and microwave-induced crystallization might be a useful technique to develop high-quality poly-Si films at low temperature.

Epitaxial growth of a (100) CoSi2 layer from carbonic cobalt films deposited on (100) Si substrate using an organometallic source

We report the epitaxial growth of a (100) CoSi2 layer on Si (100) substrate by the diffusion of Co from an amorphous carbonic cobalt film. The employment of an intermediate buffer layer, usually required between Si and pure Co, was eliminated in this experiment. The amorphous carbonic cobalt film was prepared by the organometallic chemical vapor deposition of cyclopentadienyl dicarbonyl cobalt, Co(η5–C5H5)(CO)2 at 350 °C. The carbonic cobalt film was capped by a sputtered Ti layer to avoid oxidation of Co during annealing. A CoSi2 layer was epitaxially grown on Si (100) by ex situ rapid thermal annealing at 800 °C in N2 ambient. The supply of Co by diffusion in the carbonic cobalt film seemed to be low enough to form an epitaxial CoSi2 layer.We report the epitaxial growth of a (100) CoSi2 layer on Si (100) substrate by the diffusion of Co from an amorphous carbonic cobalt film. The employment of an intermediate buffer layer, usually required between Si and pure Co, was eliminated in this experiment. The amorphous carbonic cobalt film was prepared by the organometallic chemical vapor deposition of cyclopentadienyl dicarbonyl cobalt, Co(η5–C5H5)(CO)2 at 350 °C. The carbonic cobalt film was capped by a sputtered Ti layer to avoid oxidation of Co during annealing. A CoSi2 layer was epitaxially grown on Si (100) by ex situ rapid thermal annealing at 800 °C in N2 ambient. The supply of Co by diffusion in the carbonic cobalt film seemed to be low enough to form an epitaxial CoSi2 layer.

In situ growth of an epitaxial CoSi2 layer on a Si (100) substrate by reactive chemical-vapor deposition using a cobalt metallorganic source

Uniform epitaxial CoSi2 layers have been grown in situ on a (100) Si substrate at temperatures above 600 °C by reactive chemical-vapor deposition of cyclopentadienyl dicarbonyl cobalt, Co(η5-C5H5)(CO)2. Co-rich phases such as Co2Si and CoSi were suppressed during cobalt metallorganic chemical-vapor deposition at substrate temperatures above 500 °C. A thin carbon layer was found on the top of the epitaxial CoSi2 layer grown on the Si substrate due to incomplete decomposition of the cobalt metallorganic source and diffusion of Co into the Si substrate. In spite of the existence of a surface carbon layer, an ion channeling minimum yield, χmin, of 8% in Rutherford backscattering/channeling spectrometry has been achieved in the epitaxial layer, indicating a nearly perfect epitaxial order. The carbon pileup on the surface of the CoSi2 layer at the initial stage of Co deposition seems to play the role of a cobalt diffusion barrier, avoiding the formation of Co-rich phases.