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24 Sep 2012

Volume 101, Issue 13, Articles (13xxxx)

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Appl. Phys. Lett. 101, 133101 (2012); http://dx.doi.org/10.1063/1.4752467 (5 pages)

Yen Husn Su and Wei-Yu Chen
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Snow cleaning of substrates increases yield of large-area organic photovoltaics

Nana Wang, Jeramy D. Zimmerman, Xiaoran Tong, Xin Xiao, Junsheng Yu, and Stephen R. Forrest

Appl. Phys. Lett. 101, 133901 (2012); http://dx.doi.org/10.1063/1.4754690 (4 pages)

Online Publication Date: 25 September 2012

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We demonstrate large-area organic photovoltaic cells (OPVs) based on boron-subphthalocyanine chloride (SubPc)/C60 and 2,4-bis[4-(N,N-diphenylamino)-2,6 dihydroxyphenyl]squaraine/C60 heterojunctions on substrates “snow-cleaned” with a jet of mixed-phase CO2. Snow cleaning reduces particulates on the indium-tin-oxide (ITO)-coated glass substrates, thereby reducing device shorts and shunt paths. Snow cleaning improves yield of 1.44 cm2 SubPc/C60 OPV cells from zero for conventionally solvent-cleaned substrates to ∼70%. The standard deviation of power conversion efficiency for a population of 19 snow-cleaned devices is ≤4.0%. By using a sub-electrode structure, we obtain a power conversion efficiency of 2.21% ± 0.05% for 6.25 cm2 SubPc/C60 devices, compared to 2.69% ± 0.03%, for 0.008 cm2 devices, with the efficiency decrease due to series resistance of the ITO.
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85.60.-q Optoelectronic devices

Phase-field modeling of stress generation in electrode particles of lithium ion batteries

Magalie Huttin and Marc Kamlah

Appl. Phys. Lett. 101, 133902 (2012); http://dx.doi.org/10.1063/1.4754705 (4 pages)

Online Publication Date: 25 September 2012

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Many cathode materials in lithium ion batteries show capacity fade due to particle crackings even at low applied charge and discharge current (C-rate). The promising candidate material LixMn2O4 exhibits such effects on the 4 V-plateau when the state of charge 0<x<1, hence the crystalline host remains cubic spinel. Single particle failure in this range of values indicates the existence of very large stresses, which may occur due to phase segregation. In this letter, we employ a phase-field model coupled to mechanics to demonstrate the relationship between phase segregation and high values of the mechanical stresses thus explaining numerous experimental results.
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64.75.Gh Phase separation and segregation in model systems (hard spheres, Lennard-Jones, etc.)
66.30.H- Self-diffusion and ionic conduction in nonmetals
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.de Elastic moduli

Plasmon enhanced polymer solar cells by spin-coating Au nanoparticles on indium-tin-oxide substrate

H. L. Gao, X. W. Zhang, Z. G. Yin, H. R. Tan, S. G. Zhang, J. H. Meng, and X. Liu

Appl. Phys. Lett. 101, 133903 (2012); http://dx.doi.org/10.1063/1.4754839 (4 pages) | Cited 1 time

Online Publication Date: 25 September 2012

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To enhance light absorption in polymer solar cells, the Au nanoparticles (NPs) with different sizes were incorporated to the devices by spin-coating Au colloid solution on the indium-tin-oxide substrates prior to deposition of buffer layer. It has been found that the power conversion efficiency of bulk heterojunction cells can be increased from 3.50% to 4.07% after incorporating the 60 nm Au NPs, corresponding to an improvement of 16%. The improved device performance is ascribed to the localized surface plasmon excitation of the Au NPs. The method we report herein is a kind of simple and quick solution process.
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88.40.H- Solar cells (photovoltaics)

Sandwiching intermediate reflectors in tandem solar cells for improved photon management

Stephan Fahr, Carsten Rockstuhl, and Falk Lederer

Appl. Phys. Lett. 101, 133904 (2012); http://dx.doi.org/10.1063/1.4755873 (4 pages)

Online Publication Date: 25 September 2012

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In tandem solar cells, intermediate reflectors are employed to increase light absorption in the top cell. Thus far, the use of photonic crystals for this purpose was not optimal since side-lobes in the reflection spectrum reduced the absorption in the bottom cell. To compensate this reduction, the bottom cell thickness had to be excessively increased; nullifying the main advantage of thin-film solar cells. Here, we suggest to solve this issue by reducing the impedance mismatch between photonic crystal and bottom cell using anti-reflection layers. The concept is even validated for solar cells comprising random textures.
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88.40.hj Efficiency and performance of solar cells

Enhanced carrier extraction from Ge quantum dots in Si solar cells under strong photoexcitation

Takeshi Tayagaki, Noritaka Usami, Wugen Pan, Yusuke Hoshi, Kazufumi Ooi, and Yoshihiko Kanemitsu

Appl. Phys. Lett. 101, 133905 (2012); http://dx.doi.org/10.1063/1.4756895 (4 pages)

Online Publication Date: 27 September 2012

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We report studies of the carrier extraction mechanism in Si solar cells with Ge quantum dots (QDs), which enable the optical absorption of photons with energies below the band gap of the host. Photocurrent measurements revealed that the photocurrent in the QD solar cells increased superlinearly with increasing excitation intensity under strong photoexcitation, which differed greatly from the behavior of Si solar cells without Ge QDs. This nonlinear photocurrent generation indicates that the carrier extraction efficiency from QDs is enhanced under strong photoexcitation by nonlinear carrier extraction processes, such as two-step photon absorption and hot carrier generation via Auger recombination.
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88.40.jj Silicon solar cells
88.40.hj Efficiency and performance of solar cells

Tapered and aperiodic silicon nanostructures with very low reflectance for solar hydrogen evolution

Siah Hong Tan, Chew Beng Soh, Wei Wang, Soo Jin Chua, and Dongzhi Chi

Appl. Phys. Lett. 101, 133906 (2012); http://dx.doi.org/10.1063/1.4754614 (5 pages)

Online Publication Date: 27 September 2012

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We introduce a facile method to generate silicon nanostructures with superior anti-reflectance (AR) properties. These nanostructures possess the ideal tapered structure and aperiodic distribution required for low reflectance over a broad range of wavelengths. Consideration of effective medium theories might explain the behavior between structure and reflectance, and suggest the advantage of aperiodicity in affording the material broadband AR properties. We then implemented these nanostructures as photocathodes to drive the hydrogen evolution reaction in AM 1.5 illumination. These nanostructures showed a significant improvement in photoelectrochemical performance over their planar counterpart, with the best performances corresponding to nanostructures which possessed AR properties matching the solar spectrum output.
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81.16.-c Methods of micro- and nanofabrication and processing
81.65.Cf Surface cleaning, etching, patterning
61.46.-w Structure of nanoscale materials

Determination of electron and hole lifetimes of rutile and anatase TiO2 single crystals

Yasuhiro Yamada and Yoshihiko Kanemitsu

Appl. Phys. Lett. 101, 133907 (2012); http://dx.doi.org/10.1063/1.4754831 (4 pages) | Cited 4 times

Online Publication Date: 27 September 2012

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The dynamical behavior of photoexcited states of TiO2 governs the activities of TiO2-based solar cells and photocatalysts. We determined the lifetimes of photoexcited electrons and holes in rutile and anatase TiO2 single crystals by combining advantages of time-resolved photoluminescence, photoconductance, and transient absorption spectroscopy. Electrons and holes in rutile show exponential decays with the lifetime of a few tens of nanoseconds, while non-exponential decays are observed in anatase, indicating the presence of multiple carrier trapping processes. We revealed the generic features of the carrier recombination processes in rutile and anatase TiO2.
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72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
72.40.+w Photoconduction and photovoltaic effects
78.55.Hx Other solid inorganic materials
82.50.Hp Processes caused by visible and UV light
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
61.66.Fn Inorganic compounds

Enhanced photoanode properties of epitaxial Ti doped α-Fe2O3 (0001) thin films

H. Magnan, D. Stanescu, M. Rioult, E. Fonda, and A. Barbier

Appl. Phys. Lett. 101, 133908 (2012); http://dx.doi.org/10.1063/1.4755763 (4 pages) | Cited 1 time

Online Publication Date: 27 September 2012

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The growth, crystal and electronic structures, and photo-electrochemical properties of undoped and Ti doped hematite epitaxial films were studied. We evidence that Ti4+ substitutes Fe3+ in the hematite lattice inducing a slight modification of the oxygen octahedron. Ti doping is shown to induce a shift of the valence band toward higher binding energy due to a movement of the Fermi level toward the conduction band. The resulting modification of electrical conductivity appears as a possible origin of the improvement of photo-electrochemical properties in the doped sample.
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68.55.-a Thin film structure and morphology
73.20.At Surface states, band structure, electron density of states
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
81.15.Aa Theory and models of film growth
82.45.Fk Electrodes

The influence of quantum dot size on the sub-bandgap intraband photocurrent in intermediate band solar cells

Alexander Mellor, Antonio Luque, Ignacio Tobías, and Antonio Martí

Appl. Phys. Lett. 101, 133909 (2012); http://dx.doi.org/10.1063/1.4755782 (4 pages) | Cited 4 times

Online Publication Date: 27 September 2012

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The effect of quantum dot (QD) size on the performance of quantum dot intermediate band solar cells is investigated. A numerical model is used to calculate the bound state energy levels and the absorption coefficient of transitions from the ground state to all other states in the conduction band. Comparing with the current state of the art, strong absorption enhancements are found for smaller quantum dots, as well as a better positioning of the energy levels, which is expected to reduce thermal carrier escape. It is concluded that reducing the quantum dot size can increase sub-bandgap photocurrent and improve voltage preservation.
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88.40.J- Types of solar cells
88.40.hj Efficiency and performance of solar cells

Front contact layer of multiphase silicon-carbon in thin film silicon solar cell

Sun Ho Kim, Dong Joo You, Jin Hee Park, Sung Eun Lee, Heon-Min Lee, and Donghwan Kim

Appl. Phys. Lett. 101, 133910 (2012); http://dx.doi.org/10.1063/1.4756798 (4 pages)

Online Publication Date: 28 September 2012

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In order to increase the quantum efficiency as well as conversion efficiency, we propose the boron doped hydrogenated multiphase silicon-carbon (called as “multiphase silicon-carbon”) as the front contact layer in thin film silicon solar cells. The multiphase silicon-carbon consists of amorphous carbon, amorphous silicon, and crystalline silicon-like clustering phase. We achieved a high conductivity and a low optical absorptance of multiphase silicon-carbon and compared it with the existing boron doped microcrystalline silicon. Applying this layer between transparent conductive oxide and the p layer, the amorphous silicon and silicon-germanium (a-Si/a-SiGe) double junction cell showed an increase of quantum efficiency in short wavelength and an improvement of the conversion efficiency by about 0.6% in 1 cm2 area.
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88.40.jj Silicon solar cells
88.40.hj Efficiency and performance of solar cells
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