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18 Jun 2012

Volume 100, Issue 25, Articles (25xxxx)

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Appl. Phys. Lett. 100, 252401 (2012); http://dx.doi.org/10.1063/1.4727909 (4 pages)

Ming Yan, Christian Andreas, Attila Kákay, Felipe García-Sánchez, and Riccardo Hertel
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Electron-dependent thermoelectric properties in Si/Si1-xGex heterostructures and Si1-xGex alloys from first-principles

M. Z. Hossain and H. T. Johnson

Appl. Phys. Lett. 100, 253901 (2012); http://dx.doi.org/10.1063/1.4729765 (5 pages) | Cited 1 time

Online Publication Date: 18 June 2012

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Unlike phononic thermal conductivity (which is shown in the literature to be reduced due to alloying and has a nearly constant value over a range of compositional variations), electron-dependent thermoelectric properties are shown here, from first-principles, to vary nonlinearly with composition. Of the Si/Si1−xGex systems considered, the maximum thermopower observed, which is 10% higher than that of crystalline Si, is obtained for a Si0.875Ge0.125 alloy. Also, heterostructuring is shown to reduce thermopower, electrical conductivity, and electron thermal conductivity. Additionally, neither Lorenz number nor Seebeck coefficient shows oscillations for heterostructures, regardless of electron/hole energies, contradicting the conclusions obtained with miniband approximations.
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72.20.Pa Thermoelectric and thermomagnetic effects
66.70.Df Metals, alloys, and semiconductors

An enhancement of a thermoelectric power factor in a Ga-doped ZnO system: A chemical compression by enlarged Ga solubility

Kwang-Hee Jung, Kyu Hyoung Lee, Won-Seon Seo, and Soon-Mok Choi

Appl. Phys. Lett. 100, 253902 (2012); http://dx.doi.org/10.1063/1.4729560 (4 pages)

Online Publication Date: 18 June 2012

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Herein, we report a significant enhancement of the thermoelectric power factor in polycrystalline Ga-doped ZnO. Despite its higher carrier concentration, the Seebeck coefficient of Zn0.985Ga0.015O was larger than that of Zn0.990Ga0.010O benefiting from an enhancement of the density of states (DOS) effective mass. A gradual increase in the compressive stress with Ga substitution gave rise to a higher DOS at the bottom of the conduction band. An enlarged solution limit of Ga in the ZnO matrix due to a lower firing temperature accelerated the chemical compression. A single phase n-type Zn0.985Ga0.015O bulk exhibited a power factor of 12.5 μWcm−1 K−2.
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72.20.Pa Thermoelectric and thermomagnetic effects
71.20.Nr Semiconductor compounds
72.20.Fr Low-field transport and mobility; piezoresistance

Minimizing interfacial losses in inverted organic solar cells comprising Al-doped ZnO

Abay Gadisa, Yingchi Liu, Edward T. Samulski, and Rene Lopez

Appl. Phys. Lett. 100, 253903 (2012); http://dx.doi.org/10.1063/1.4729861 (4 pages) | Cited 1 time

Online Publication Date: 19 June 2012

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We demonstrated a 35% enhancement in the efficiency of inverted solar cells as a result of increased open-circuit voltage and fill factor by adsorbing an ultrathin layer of a ruthenium dye N719 on an aluminum-doped zinc oxide (ZnO-Al) electron collecting interfacial layer. The interface modification with N719 changes the charge injection levels as indicated by ultraviolet photoemission spectroscopy. The efficiency of inverted solar cells comprising a bulk heterojunction photo-active film of poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester has increased from ∼2.80% to 3.80% upon employing the dye modification of the electrode interface.
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88.40.jr Organic photovoltaics
88.40.hj Efficiency and performance of solar cells

A broadband vibrational energy harvester

Louis Van Blarigan, Per Danzl, and Jeff Moehlis

Appl. Phys. Lett. 100, 253904 (2012); http://dx.doi.org/10.1063/1.4729875 (4 pages) | Cited 4 times

Online Publication Date: 20 June 2012

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We propose a design for an energy harvester which has the potential to harvest vibrational energy over a broad range of ambient frequencies. The device uses two flexible ceramic piezoelectric elements arranged in a buckled configuration in the absence of vibrations. Experimental data show that this design allows enhanced harvesting of energy relative to a comparable cantilever design, both for periodic and stochastic vibrations. Moreover, the data suggest that this harvester has its peak energy generation when it responds with chaotic vibrations.
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84.60.-h Direct energy conversion and storage
88.05.-b Energy analysis

Electronic properties of the Cu2ZnSn(Se,S)4 absorber layer in solar cells as revealed by admittance spectroscopy and related methods

Oki Gunawan, Tayfun Gokmen, Charles W. Warren, J. David Cohen, Teodor K. Todorov, D. Aaron R. Barkhouse, Santanu Bag, Jiang Tang, Byungha Shin, and David B. Mitzi

Appl. Phys. Lett. 100, 253905 (2012); http://dx.doi.org/10.1063/1.4729751 (4 pages) | Cited 8 times

Online Publication Date: 21 June 2012

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Admittance spectra and drive-level-capacitance profiles of several high performance Cu2ZnSn(Se,S)4 (CZTSSe) solar cells with bandgap ∼1.0–1.5 eV are reported. In contrast to the case for Cu(In,Ga)(S,Se)2, the CZTSSe capacitance spectra exhibit a dielectric freeze out to the geometric capacitance plateau at moderately low frequencies and intermediate temperatures (120–200 K). These spectra reveal important information regarding the bulk properties of the CZTSSe films, such as the dielectric constant and a dominant acceptor with energy level of 0.13–0.2 eV depending on the bandgap. This deep acceptor leads to a carrier freeze out effect that quenches the CZTSSe fill factor and efficiency at low temperatures.
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88.40.jn Thin film Cu-based I-III-VI2 solar cells

Elastically bounded flapping wing for energy harvesting

C. Boragno, R. Festa, and A. Mazzino

Appl. Phys. Lett. 100, 253906 (2012); http://dx.doi.org/10.1063/1.4729936 (4 pages)

Online Publication Date: 21 June 2012

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In this Letter, we present and discuss an energy harvesting device, based on a wing elastically bounded to a fixed support. Large amplitude and periodic oscillations can be induced when this system is subject to wind, if a few parameters are carefully set. A linear stability analysis as well as two-dimensional numerical simulations confirms the existence of instability regions in the parameter space. In order to harvest energy by using this system, different methods are considered. Preliminary results obtained by an electromagnetic coupling are presented.
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84.60.-h Direct energy conversion and storage
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Plasmon enhanced resonant defect absorption in thin a-Si:H n-i-p devices

F. Lükermann, U. Heinzmann, and H. Stiebig

Appl. Phys. Lett. 100, 253907 (2012); http://dx.doi.org/10.1063/1.4730432 (4 pages) | Cited 2 times

Online Publication Date: 22 June 2012

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By embedding silver nanoparticles (Ag NPs) of approximately 20 nm diameter inside the intrinsic layer of thin hydrogenated amorphous silicon (a-Si:H) n-i-p devices, a photocurrent is measured for photon energies below the a-Si:H bandgap. This is attributed to the excitation of charge carriers from defect states created by the incorporation of the Ag inside the silicon network. The defect location inside the strong electromagnetic fields close to the resonant absorbing NPs enables high transition rates. This is a proof of concept for the use of the impurity photovoltaic effect in a-Si:H devices.
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85.30.-z Semiconductor devices

Fabrication of solution-processed hydrogenated amorphous silicon single-junction solar cells

Takashi Masuda, Naoya Sotani, Hiroki Hamada, Yasuo Matsuki, and Tatsuya Shimoda

Appl. Phys. Lett. 100, 253908 (2012); http://dx.doi.org/10.1063/1.4730614 (4 pages) | Cited 2 times

Online Publication Date: 22 June 2012

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Hydrogenated amorphous silicon solar cells were fabricated using solution-based processes. All silicon layers of the p-i-n junction were stacked by a spin-cast method using doped and non-doped polydihydrosilane solutions. Further, a hydrogen-radical treatment under vacuum conditions was employed to reduce spin density in the silicon films. Following this treatment, the electric properties of the silicon films were improved, and the power conversion efficiency of the solar cells was also increased from 0.01% to 0.30%–0.51% under the AM-1.5G (100 mW/cm2) illumination conditions.
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88.40.jj Silicon solar cells
88.40.hj Efficiency and performance of solar cells
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