Effect of ionomer content and relative humidity on polymer electrolyte membrane fuel cell (PEMFC) performance of membrane-electrode assemblies (MEAs) prepared by decal transfer method

Jeon, Sunyeol,Lee, Jisun,Rios, Gema M.,Kim, Hyoung-Juhn,Lee, Sang-Yeop,Cho, EunAe,Lim, Tae-Hoon,Hyun Jang, Jong Elsevier 2010 International journal of hydrogen energy Vol.35 No.18

<P><B>Abstract</B></P><P>Membrane-electrode assemblies (MEAs) were fabricated by the decal transfer method with various Nafion ionomer contents (10–40 wt%) and their single cell performance and electrochemical characteristics were examined in atmospheric air at relative humidities of 25–95%. At high humidity (95%), the MEA performance was the highest with a cathode ionomer content of 30 and 20 wt% at 0.6 and 0.4 V, respectively. The optimum ionomer content of the decal MEAs increased with decreasing humidity, because of the change in the oxygen transport rate (water flooding) and number of active sites (ionic resistance). The concentration overpotential gradually increased with relative humidity up to about 0.4 V at 0.8 A/cm<SUP>2</SUP>, which was not considered in previous studies using pressurized air and oxygen. The combined effect of the electrochemical active surface area and ionic resistance of the cathodes on the activation overpotential was also investigated, focusing on intermediate and low humidity levels, using a newly developed impedance analysis method.</P>

Structural and Electrical Properties of ZnO Thin Films Deposited by Atomic Layer Deposition at Low Temperatures

Jeon, Sunyeol,Bang, Seokhwan,Lee, Seungjun,Kwon, Semyung,Jeong, Wooho,Jeon, Hyeongtag,Chang, Ho Jung,Park, Hyung-Ho The Electrochemical Society 2008 Journal of the Electrochemical Society Vol.155 No.10

Characteristics of Zinc-Oxide-Sulfide-Mixed Films Deposited by Using Atomic Layer Deposition

전형탁,Sunyeol Jeon,방석환,이승준,권세명,정우호,장호정,박형호 한국물리학회 2008 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.53 No.6

Mixed thin films of ZnO and ZnS were deposited by using atomic layer deposition (ALD) to achieve film properties for active channel materials of transparent electronic devices and the characteristics of ZnO, ZnS and their mixed films with various composition ratios were examined. The chemical bond states of mixed films consisting of ZnO and ZnS phases were analyzed and no sulfate or sulfite phases were observed. The structures of the ZnO and the ZnS films exhibited wurtzite hexagonal crystalline structures. However, the mixed films did not show any specic preferred orientation; the amorphous character of the mixed films was due to the large lattice mismatch between the ZnO and the ZnS films. The ZnO thin film showed a low resistivity with a higher carrier concentration (up to ∽1019 cm-3) than the ZnS thin film which showed ∽1012 cm-3 carrier concentration. The mixed films exhibited carrier concentrations of 1015 - 1018 cm-3 and a resistivity range of 102 - 103 Ωcm, depending on the composition. Furthermore, no significant changes in the electrical properties of the mixed films were observed with respect to the post-heat treatments.

Al₂O₃ buffer in a ZnO thin film transistor with poly-4-vinylphenol dielectric

Bang, Seokhwan,Lee, Seungjun,Jeon, Sunyeol,Kwon, Semyung,Jeong, Wooho,Kim, Honggyu,Shin, Iksup,Chang, Ho Jung,Park, Hyung-ho,Jeon, Hyeongtag Institute of Physics 2009 Semiconductor science and technology Vol.24 No.2

<P>We compared the characteristics of bottom-gate ZnO-thin film transistors using poly-4-vinylphenol (PVP) and PVP/Al<SUB>2</SUB>O<SUB>3</SUB> dielectrics. The PVP dielectric is more hydrophobic than the PVP/Al<SUB>2</SUB>O<SUB>3</SUB> dielectric and is not useful for TFT devices because of its high leakage current density, but this leakage current density can be significantly reduced by inserting Al<SUB>2</SUB>O<SUB>3</SUB>. We deposited ZnO and Al<SUB>2</SUB>O<SUB>3</SUB> films by atomic layer deposition (ALD) because it is a low-temperature process. The ZnO-TFTs with either a PVP or a PVP/Al<SUB>2</SUB>O<SUB>3</SUB> dielectric exhibit typical field-effect transistor characteristics with n-channel properties. The ZnO-TFT containing PVP/Al<SUB>2</SUB>O<SUB>3</SUB> exhibits clear pinch-off and excellent saturation with an enhanced mode operation. The on/off ratio of 7.9 × 10<SUP>4</SUP> for the device containing the hybrid dielectric is about three orders of magnitude higher than the ratio of 47 for the device containing PVP. The subthreshold gate swings are 12 V/decade for the TFT containing PVP and 1.2 V/decade for the TFT containing PVP/Al<SUB>2</SUB>O<SUB>3</SUB>. The density of the interface trap state is significantly lower in the device containing PVP/Al<SUB>2</SUB>O<SUB>3</SUB> than in the ZnO-TFT containing PVP. The saturation mobility was 0.05 and 0.8 cm<SUP>2</SUP> V<SUP>−1</SUP> s<SUP>−1</SUP>, respectively, in the TFTs containing PVP and PVP/Al<SUB>2</SUB>O<SUB>3</SUB>.</P>

Characteristics of the ZnO thin film transistor by atomic layer deposition at various temperatures

Kwon, Semyung,Bang, Seokhwan,Lee, Seungjun,Jeon, Sunyeol,Jeong, Wooho,Kim, Hyungchul,Gong, Su Cheol,Chang, Ho Jung,Park, Hyung-ho,Jeon, Hyeongtag Institute of Physics 2009 Semiconductor science and technology Vol.24 No.3

<P>ZnO thin films were deposited by atomic layer deposition (ALD) at various temperatures and the resulting electrical and chemical properties were examined. The fraction of O–H bonds in ZnO films decreased from 0.39 to 0.24 with increasing processing temperatures. The O/Zn ratio decreased from 0.90 at 70 °C to 0.78 at 130 °C. The carrier concentration and resistivity changed sharply with decreasing temperature. The ZnO thin film transistors (TFTs) were fabricated at processing temperatures of 70 to 130 °C and the electrical properties of the TFT were as follows: the field-effect mobility ranged from 8.82 × 10<SUP>−3</SUP> to 6.11 × 10<SUP>−3</SUP> cm<SUP>2</SUP> V<SUP>−1</SUP> s<SUP>−1</SUP>, the on/off current ratio ranged from 1.28 × 10<SUP>6</SUP> to 2.43 × 10<SUP>6</SUP>, the threshold voltage ranged from −12.5 to 14.7 V and the subthreshold swing ranged from 1.21 to 24.1 V/decade. The electrical characteristics of the ZnO TFT were enhanced as the processing temperature decreased.</P>

Characteristics of Atomic-Layer-Deposited HfO2 Films by Using a Remote Plasma on Pre-Deposited Hf Metal Layer

Hyungseok Hong,전형탁,Honggyu Kim,Hyungchul Kim,Sanghyun Woo,Seokhoon Kim,Seokhwan Bang,Seungjun Lee,Sunyeol Jeon,Wooho Jeong 한국물리학회 2008 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.52 No.4

We investigated the physical and the electrical properties of HfO₂ films grown by using a remote-plasma atomic layer deposition technique on a pre-deposited Hf metal layer. The Hf metal pre-deposited film retarded effectively the growth of an interfacial layer while the HfO₂ film without a Hf metal layer showed a growth of an interfacial layer. The as-deposited HfO₂ layers on thick Hf metal layers are crystallized while those on thin Hf metal layers remain amorphous. The pre-deposited Hf metal layer decreased the equivalent oxide thickness while it increased the crystallization temperature. The flat-band voltage (VFB) shifted in the negative direction with increasing pre-deposited Hf metal thickness and the effective fixed oxide charge density corresponding to the △VFB of the HfO₂ films increased with increasing pre-deposited Hf metal thickness. After annealing at 800℃, the VFB for the HfO₂ films shifted toward the ideal VFB and the effective fixed oxide charge decreased. The Hf pre-deposition technique was found to be effective in reducing the EOT by suppressing interfacial layer growth while the electrical properties, such as the leakage current density and the effective fixed oxide charge density, were degraded. We investigated the physical and the electrical properties of HfO₂ films grown by using a remote-plasma atomic layer deposition technique on a pre-deposited Hf metal layer. The Hf metal pre-deposited film retarded effectively the growth of an interfacial layer while the HfO₂ film without a Hf metal layer showed a growth of an interfacial layer. The as-deposited HfO₂ layers on thick Hf metal layers are crystallized while those on thin Hf metal layers remain amorphous. The pre-deposited Hf metal layer decreased the equivalent oxide thickness while it increased the crystallization temperature. The flat-band voltage (VFB) shifted in the negative direction with increasing pre-deposited Hf metal thickness and the effective fixed oxide charge density corresponding to the △VFB of the HfO₂ films increased with increasing pre-deposited Hf metal thickness. After annealing at 800℃, the VFB for the HfO₂ films shifted toward the ideal VFB and the effective fixed oxide charge decreased. The Hf pre-deposition technique was found to be effective in reducing the EOT by suppressing interfacial layer growth while the electrical properties, such as the leakage current density and the effective fixed oxide charge density, were degraded.