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16 Jul 2012

Volume 101, Issue 3, Articles (03xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 101, 033301 (2012); http://dx.doi.org/10.1063/1.4734240 (5 pages)

Ting-Gang Chen, Bo-Yu Huang, En-Chen Chen, Peichen Yu, and Hsin-Fei Meng
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Simultaneous enhancement of electronic and Li+ ion conductivity in LiFePO4

Jaekwang Lee, Stephen J. Pennycook, and Sokrates T. Pantelides

Appl. Phys. Lett. 101, 033901 (2012); http://dx.doi.org/10.1063/1.4737212 (4 pages)

Online Publication Date: 16 July 2012

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Enhancing the electronic and ionic conductivity in Li compounds can significantly impact the design of batteries. Here, we explore the influence of biaxial strain on the electronic and Li+ ion conductivities of LiFePO4 by performing first-principles calculations. We find that 4% biaxial tensile strain (BTS) leads to 15 times increase in electronic conductivity and 50 times increase in Li+ ion conductivity at 300 K, respectively. Electronic conductivity is enhanced because BTS softens lattice distortions around a polaron, resulting in a reduction of the activation barrier. The extra volume introduced by tensile strain also reduces the barrier of Li+ ion migration.
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66.30.H- Self-diffusion and ionic conduction in nonmetals
71.38.-k Polarons and electron-phonon interactions

Mechanisms of (NH4)2Sx-treated III-V compound triple-junction solar cells incorporating with hybrid electrode

Chun-Yen Tseng and Ching-Ting Lee

Appl. Phys. Lett. 101, 033902 (2012); http://dx.doi.org/10.1063/1.4737396 (4 pages) | Cited 1 time

Online Publication Date: 17 July 2012

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A hybrid electrode composed of the metal contact pads and the transparent indium-tin-oxide film was incorporated with the (NH4)2Sx-treated InGaP/InGaAs/Ge triple-junction solar cells. The hybrid electrode structure was used to reduce the metal shadow area. The passivation function enabled by the (NH4)2Sx surface treatment was used to enhance the photoluminescence intensity and carrier lifetime of the (NH4)2Sx-treated n-type AlInP window layer. The conversion efficiency of (NH4)2Sx-treated solar cells with the hybrid electrode structure was improved up to 35.73% due to an increase in the absorption of the incident light along with the surface passivation.
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88.40.jp Multijunction solar cells
81.65.Rv Passivation

How grain boundaries in Cu(In,Ga)Se2 thin films are charged: Revisit

C.-S. Jiang, M. A. Contreras, I. Repins, H. R. Moutinho, Y. Yan, M. J. Romero, L. M. Mansfield, R. Noufi, and M. M. Al-Jassim

Appl. Phys. Lett. 101, 033903 (2012); http://dx.doi.org/10.1063/1.4737406 (4 pages)

Online Publication Date: 18 July 2012

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Potential measurements on Cu(In,Ga)Se2 thin films using scanning Kelvin probe force microscopy have been reported extensively to address grain-boundary (GB) recombination by examining GB charging. However, the results are highly inconsistent. We revisit this issue by measuring high- and low-quality wide-bandgap films and using a complementary method of scanning capacitance microscopy. Our results show consistent positively charged GBs in our high-quality films with minimal surface defects, except for the Σ3[112] GBs, which are charge neutral. We discuss possible artifacts due to surface defects when examining the GB charging and the role of GBs in the device performance.
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61.72.Mm Grain and twin boundaries
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
68.55.ag Semiconductors
88.40.H- Solar cells (photovoltaics)

On the energy conversion and efficiency of a dielectric electroactive polymer generator

Huaming Wang, Chengshu Wang, and Tongyan Yuan

Appl. Phys. Lett. 101, 033904 (2012); http://dx.doi.org/10.1063/1.4737439 (5 pages)

Online Publication Date: 18 July 2012

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Dielectric electroactive polymers are studied for their energy generating ability based on a variable capacitance mechanism. Energy conversion for one cycle is analyzed by considering the energy loss in the charging process, where generated energy is defined as the difference between output and total input electrical energy. Experiments are conducted under two charging modes and different stretch rates to investigate their effect on the energy conversion. Energy conversion efficiency of up to 18.3% is achieved with charging-while-stretching mode and a low stretch rate, with an energy density of 18.9 mJ/g and power of 3.775 mW/g per cycle obtained.
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85.50.-n Dielectric, ferroelectric, and piezoelectric devices

High aspect ratio and high breakdown strength metal-oxide capacitors

T. Usui, S. A. Mollinger, A. T. Iancu, R. M. Reis, and F. B. Prinz

Appl. Phys. Lett. 101, 033905 (2012); http://dx.doi.org/10.1063/1.4737641 (4 pages)

Online Publication Date: 18 July 2012

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Thin film capacitors of HfO2 with high energy storage were fabricated using plasma-enhanced atomic layer deposition on a relatively large platform with a lateral area of 0.8-7.1 mm2. An untreated film of 10-nm HfO2 showed a breakdown strength of 0.47 V/nm. Annealing of HfO2 formed a large crystalline phase, which creates electron paths and increases defect-induced currents. Laminate structures of Al2O3 and HfO2 were also fabricated to relate crystallinity, current leakage path, and breakdown behavior. A 7-layer laminate structure exhibited a breakdown strength of 0.58 V/nm with an aspect ratio of 1:300 000 due to suppressed crystallinity.
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84.32.Tt Capacitors
81.05.-t Specific materials: fabrication, treatment, testing, and analysis

CdS buffer-layer free highly efficient ZnO-CdSe photoelectrochemical cells

Rajaram S. Mane, Dipak V. Shinde, Seog Joon Yoon, Swapnil B. Ambade, Joong Kee Lee, and Sung-Hwan Han

Appl. Phys. Lett. 101, 033906 (2012); http://dx.doi.org/10.1063/1.4737865 (3 pages) | Cited 2 times

Online Publication Date: 18 July 2012

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Highly reproducible, wet-chemically processed, CdS buffer-layer free ZnO-CdSe photoelectrochemical cells with 3.38% power conversion efficiency have been fabricated. An enhanced current density is observed due to increase in number of injected photoelectrons with CdSe nanoparticles loading time. Impedance spectroscopy results suggest that interfacial resistance is strongly dependant on CdSe nanoparticles loading time.
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82.47.Jk Photoelectrochemical cells, photoelectrochromic and other hybrid electrochemical energy storage devices
84.60.Jt Photoelectric conversion
72.40.+w Photoconduction and photovoltaic effects

Optimizing performance of layer-by-layer processed polymer solar cells

Hui Li, Yong-Fang Li, and Jizheng Wang

Appl. Phys. Lett. 101, 033907 (2012); http://dx.doi.org/10.1063/1.4737877 (4 pages) | Cited 3 times

Online Publication Date: 18 July 2012

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Layer-by-layer (LL) process has recently been used to fabricate polymer solar cells (PSCs), Power conversion efficiency (PCE) of LL processed PSCs is currently below 4%. In this paper, we fabricated polymer photovoltaic devices with poly(3-hexylthiophene) (P3HT) as donor and indene-C60 bisadduct as acceptor with LL method. By carefully optimizing donor/acceptor layer thickness and related thermal annealing process, PCE of 5.12% was achieved. The device displays open-circuit voltage (Voc) of 0.886 V, short-circuit current density (Jsc) of 8.21 mA/cm2, and fill factor of 70.4%.
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88.40.H- Solar cells (photovoltaics)

Electronic structure and thermoelectric properties of nanostructured EuTi1−xNbxO3−δ (x = 0.00; 0.02)

L. Sagarna, A. Shkabko, S. Populoh, L. Karvonen, and A. Weidenkaff

Appl. Phys. Lett. 101, 033908 (2012); http://dx.doi.org/10.1063/1.4737872 (5 pages) | Cited 1 time

Online Publication Date: 19 July 2012

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The thermoelectric properties of polycrystalline nanostructured EuTiO3−δ samples are improved by a substitution of 2% Nb for Ti. The figure of merit (ZT) was measured to reach ZT(EuTi0.98Nb0.02O3−δ) ≈ 0.4 at T = 1040 K while ZT(EuTiO3−δ) ≈ 0.3 at the same temperature. X-ray photoelectron spectra reveal that the in-gap states at the Fermi level are more pronounced for the Nb-substituted samples, resulting in an improved power factor. The valence band peak below the Fermi level is sharply shaped, therefore fulfilling the condition for a large Seebeck coefficient. The specific porosity of the samples reduces the lattice thermal conductivity with a minor effect on the electron transport.
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71.20.Ps Other inorganic compounds
72.20.Pa Thermoelectric and thermomagnetic effects
72.80.Sk Insulators
79.60.Bm Clean metal, semiconductor, and insulator surfaces
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)
61.46.-w Structure of nanoscale materials

Phase-field modeling of three-phase electrode microstructures in solid oxide fuel cells

Qun Li, Linyun Liang, Kirk Gerdes, and Long-Qing Chen

Appl. Phys. Lett. 101, 033909 (2012); http://dx.doi.org/10.1063/1.4738230 (5 pages)

Online Publication Date: 19 July 2012

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A phase-field model for describing three-phase electrode microstructure (i.e., electrode-phase, electrolyte-phase, and pore-phase) in solid oxide fuel cells is proposed using the diffuse-interface theory. Conserved composition and non-conserved grain orientation order parameters are simultaneously used to describe the coupled phase coarsening and grain growth in the three-phase electrode. The microstructural evolution simulated by the phase-field approach demonstrates the significant dependence of morphological microstructure and output statistic material features on the prescribed kinetic parameters and three-phase volume fractions. The triple-phase boundary fraction is found to have a major degradation in the early evolution.
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88.30.pn Solid oxide fuel cells
82.45.Fk Electrodes
82.47.Ed Solid-oxide fuel cells (SOFC)

Epitaxial Rh-doped SrTiO3 thin film photocathode for water splitting under visible light irradiation

S. Kawasaki, K. Nakatsuji, J. Yoshinobu, F. Komori, R. Takahashi, M. Lippmaa, K. Mase, and A. Kudo

Appl. Phys. Lett. 101, 033910 (2012); http://dx.doi.org/10.1063/1.4738371 (4 pages) | Cited 3 times

Online Publication Date: 19 July 2012

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Epitaxial Rh-doped SrTiO3 thin film photocatalysts were fabricated by pulsed laser deposition under various deposition conditions. The oxygen pressure used during film growth was found to affect the valence of the Rh dopant in the Rh:SrTiO3 films, reproducing the known optical absorption behavior of powder samples. Evaporative Rh loss was observed at low growth pressures and high temperatures. Cyclic voltammetry measurements in a K2SO4 solution showed that Rh-doped SrTiO3 is a highly efficient hydrogen-producing photocathode with a characteristic p-type behavior under visible light irradiation.
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85.60.Ha Photomultipliers; phototubes and photocathodes
81.15.Fg Pulsed laser ablation deposition

GaAsP solar cells on GaP substrates by molecular beam epitaxy

S. Tomasulo, K. Nay Yaung, J. Simon, and M. L. Lee

Appl. Phys. Lett. 101, 033911 (2012); http://dx.doi.org/10.1063/1.4738373 (4 pages) | Cited 1 time

Online Publication Date: 19 July 2012

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We demonstrate molecular beam epitaxy (MBE) of GaAsxP1−x/GaP solar cells over a range of bandgap energies (Eg). Identical GaAs0.66P0.34 cells on GaAs and GaP exhibit similar properties; GaAs0.66P0.34/GaP cells with Eg = 1.82 eV produced an open-circuit voltage (Voc) of 1.24 V, ∼40 mV lower than previous GaAs0.66P0.34/GaAs cells. We then grew GaAs0.56P0.44/GaP cells with Eg = 1.92 eV to investigate their suitability for wide-Eg applications, reaching Voc = 1.27 V. For potential dual-junction integration on Si, we grew Eg = 1.71 eV GaAs0.73P0.27/GaP cells, attaining Voc = 1.15 V. These results indicate that GaAsxP1−x/GaP solar cells by MBE are promising for integration onto Si and for other photovoltaic applications.
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88.40.jm Thin film III-V and II-VI based solar cells
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