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This PDF file contains the front matter associated with SPIE Proceedings Volume 6651, including the Title Page, Copyright information, Table of Contents, Plenary Papers, and the Conference Committee listing.
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Lifetimes of minority carriers in semiconductors are very important parameters for devices, especially for solar cells.
For germanium and silicon, it was discovered that the actually measured lifetimes were by orders of magnitude smaller
than the theoretical limit, which can be deduced from the principle of detailed balance of black-body radiative
absorption and emission. The thermodynamic limit of solar cell efficiency was also derived based on this principle. The
development of solar cells with silicon and its non-ideal junction behavior are reviewed. The contemporary suggested
trends to overcome this detailed balance limit are being discussed.
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Essentially loss-less all-dielectric micro-fabricated optics can be tailored to completely eliminate the shadowing losses metallization grids create on the surface of concentrator solar cells. The nonimaging micro-concentrator exploits total internal reflection to redistribute the elevated flux from available macro-concentrators, rather than increasing overall concentration. The optical designs permit widening the metal fingers toward lessening series resistance losses, which can also finesse the need for the intricate metallization patterns of some high-flux cells. Realistic net efficiency gains of ~15% (relative) are achievable in a wide variety of concentrator cells.
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CuIn1-xGaxS2 (CIGS2) has a bandgap of ~1.5 eV making it an ideal candidate for space applications. CIGS2 thin films
were prepared by sulfurizing CuGa/In precursor on Mo-coated glass/ stainless steel (SS) substrates in N2:H2S (4% or
8%) mixture at 475°C. PV parameters measured under AM1.5 conditions at NREL were as follows: the first cell on
stainless steel substrate, Voc = 763.3 mV, Jsc = 20.26 mA/cm2, FF = 67.04% and η = 10.4% and for the second cell on
Mo-coated glass substrate, Voc = 830.5 mV, Jsc = 20.88 mA/cm2, FF = 69.13% and η = 11.99%. A detailed comparative
study of PV parameter of the two cells showed that the increase in the efficiency from 10.4% to 11.99% was made
possible by an increase of shunt resistance Rp in the dark from 1160 Ω-cm2 to 2500 Ω-cm2; a slight reduction of series
resistance Rs; and a reduction of the diode factor, A and reverse saturation current density, Jo respectively from ~2.1 and
~2.6x10-8 A cm-2 to ~1.72 and ~1.41x10-10 A cm-2.
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The temperature dependence of silicon (Si)-based thin film single junction solar cells whose intrinsic absorbers were
fabricated near the phase boundary of hydrogenated amorphous silicon (a-Si:H) to hydrogenated microcrystalline silicon
(μc-Si:H) was investigated. By varying the hydrogen dilution ratio, wide bandgap protocrytalline silicon (pc-Si:H) and
the mixed-phase of a-Si:H and μc-Si:H absorber layers were obtained. Photo J-V characteristics were measured under
AM1.5 illumination at ambient temperature in the range of 25-75 °C. We found that the pc-Si:H solar cells which exist
below the a-Si:H and µc-Si:H transition boundary exhibited the lowest temperature coefficient (TC) for conversion
efficiency (η) and open-circuit voltage (Voc), while the solar cells fabricated at the mixed-phase of a-Si:H and μc-Si:H
revealed a relatively high TC for η and Voc. Experimental results indicated that pc-Si:H which fabricated at the silane
concentration (SC), SC = [SiH4]/([SiH4]+[H2]), of 5.75% showed the highest initial η, low TC for η and degradation
ratio. This material at this condition is a promising for using as an absorber layer of single junction or top cell for tandem
solar cells which operating in high temperature regions.
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We suggest an energy selective and diffractive optical element as intermediate layer in thin-film tandem solar cells. By
adjusting the lattice constant of this photonic crystal, we fitted the optical properties to match a silicon tandem pair. Our
device enhances the pathway of incident light within an amorphous silicon top cell in its spectral region of low
absorption. In this spectral overlap region of the tandem-junction's quantum efficiencies, photons are being transferred
towards the amorphous cell, which leads to an increase in the short-circuit current of the limiting top cell. From our
simulations we expect a current increase of 1.44mA/cm2 for an - amorphous/microcrystalline - silicon tandem cell,
corresponding to improvement of the tandem's absolute efficiency of about 1.3%.
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Methods of spectroscopic ellipsometry (SE) have been applied to investigate the growth and properties of the material
components used in the three major thin film photovoltaics technologies: (1) hydrogenated silicon (Si:H); (2) cadmium
telluride (CdTe); and (3) copper indium-gallium diselenide (CuIn1-xGaxSe2 or CIGS). In Si:H technology, real time SE
(RTSE) has been applied to establish deposition phase diagrams that describe very high frequency (vhf) plasmaenhanced
chemical vapor deposition (PECVD) processes for hydrogenated silicon (Si:H) and silicon-germanium alloy
(Si1-xGex:H) thin films. This study has reaffirmed that the highest efficiencies for a-Si:H and a-Si1-xGex:H component
solar cells of multijunction devices are obtained when the i-layers are prepared under maximal H2 dilution conditions. In
CdTe technology, the magnetron sputter deposition of polycrystalline CdTe, CdS, and CdTe1-xSx thin films as well as
the formation of CdS/CdTe and CdTe/CdS heterojunctions has been studied. The nucleation and growth behaviors of
CdTe and CdS show strong variations with deposition temperature, and this influences the ultimate grain size. The
dielectric functions ε of the CdTe1-xSx alloys have been deduced in order to set up a database for real time investigation
of inter-diffusion at the CdS/CdTe and CdTe/CdS interfaces. In CIGS technology, strong variations in ε of the Mo back
contact during sputter deposition have been observed, and these results have been understood applying a Drude
relaxation time that varies with the Mo film thickness. Ex-situ SE measurements of a novel In2S3 window layer have
shown critical point structures at 2.77±0.08 eV, 4.92±0.005 eV, and 5.64±0.005 eV, as well as an absorption tail with an
onset near 1.9 eV. Simulations of solar cell performance comparing In2S3 and the conventional CdS have revealed
similar quantum efficiencies, suggesting the possibility of a Cd-free window layer in CIGS technology.
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A new method for optical process control of the three-stage co-evaporation of Cu(In,Ga)Se2 thin films is presented.
Precise control of the deposition process is desirable as the field of process parameters is rather complex. In an
enhancement to laser light scattering (LLS) with a single photo-detector, the diffuse part of the scattered laser light is
now used to a larger extent. In consequence, it is possible to deduce compositional information (e.g., the Ga/III-ratio) for
the deposited layer with high accuracy. This is demonstrated in a series of experiments on Mo-coated float glass and
titanium foil substrates where the final Ga content of the Cu(In,Ga)Se2 thin film has been intentionally varied. As an
additional benefit of the enhanced LLS system, the new system can also be used for process control, in cases where
previously the intensity of scattered component of light has not been sufficient for reliable interpretation. The
information from this new monitoring technique was used to set up an optical model for semitransparent, coevaporated
InxGaySez-layers of various compositions. Using this model, an evaluation of phases formed during the process and
adjustment of deposition parameters is possible. The knowledge of phases formed on glass and titanium substrates is
important since the Cu(In,Ga)Se2 formation depends on properties of the InxGaySez-layer evaporated in stage 1 of the
three-stage process. Break-off experiments at different points within stage 1 were carried out to test and improve the
model. Depth profiling by means of x-ray fluorescence (XRF) and microstructural studies by means of x-ray diffraction
(XRD) also deliver valuable information for the optical model.
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The properties of a transparent conductive oxide (TCO) used as a front electrode for thin-film solar cells and modules
play a major role in determining the maximum attainable conversion efficiency. Doped ZnO is an important TCO that is
widely used in amorphous/nanocrystalline silicon (a-Si/nc-Si) and CIGS thin-film solar cells. In the case of a-Si/nc-Si
cells, the ZnO thin film should be textured to promote light trapping to increase the short-circuit current density Jsc. In
this work, textured, aluminum-doped ZnO (ZnO:Al) thin films have been directly deposited by a sputtering-based
method and without the need for post-deposition etching. The morphology, optical properties and electrical properties of
the films have been studied. SEM micrographs show that feature sizes around 0.2 - 0.4μm have been achieved at a film
thickness of 1μm, and that the morphology can be controlled by the deposition conditions. AFM images were analyzed
to extract a set of topographic parameters (amplitude, spatial, and hybrid). The optical transmission, haze, and angle-resolved
light scattering of the textured ZnO:Al films were measured and compared to properties of commercially-available
textured SnO2:F thin films on glass. Higher haze and reduced absorption could be obtained with the textured
ZnO:Al films. Hall effect measurements on these films yielded a carrier concentration and mobility of 2.75 x 1020cm-3
and 24.1cm2/Vs, respectively. We also report that the use of these textured ZnO:Al films as the top TCO for CIGS solar
cells results in reduced cell reflectance and increased Jsc. The novel deposition method provides a potential pathway to
large area and cost effective production of a textured ZnO TCO for thin-film PV manufacturing operations.
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ECN is aiming at the development of fabrication technology for roll-to-roll production lines for high efficiency thin film
amorphous and microcrystalline silicon solar cells. The intrinsic layer will be deposited with high deposition rate
microwave plasma enhanced chemical vapour deposition. This plasma source, however, is not suitable for the deposition
of doped layers. Therefore, we use a novel, linear RF source for the deposition of doped layers. In this RF source, the
substrate is electrically disconnected from the RF network. As a result, the ion bombardment onto the substrate is very
mild, with ion energies typically < 10 eV. The low ion energies make this source very attractive for surface treatments
like passivation of crystalline silicon wafers by thin SiNx or a-Si layers. In this contribution, we will introduce the novel
RF source and discuss the deposition of device quality amorphous and microcrystalline intrinsic Si layers with the novel
linear RF source.
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The nanostructure of hydrogenated amorphous silicon-germanium alloys, a-Si1-xGex:H (x=0.62 to 0.70), prepared by the
hot-wire deposition technique applying different substrate and filament temperatures was analyzed by anomalous small-angle
x-ray scattering experiments. For all alloys the Ge-component was found to be inhomogeneously distributed. The
results from the structural and quantitative analysis have been correlated to the material photoconductivity. A clear
improvement of the photoconductivity was achieved by optimizing the substrate temperature (between 130 and 360 °C)
due to the reduction of hydrogen containing voids in coincidence with the formation of mass fractal structures of Ge
with the fractal dimension p < 1.6 and a size of about 40 nm. The two processes cause the structural re-organization of
Hydrogen from voids into Ge-fractals with enhanced Ge-H bonding, thereby improving the material photoconductivity.
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Based on Crosslight APSYS, two-dimensional simulations have been performed on Si-based solar cell devices especially
those with V-grooved surface texture. These Si-based solar cells include rear-contacted cells and passivated emitter, rear
totally diffused cells etc. The APSYS simulator is based on drift-diffusion theory with many advanced features. It can
enable an efficient computation across the whole solar spectra by taking into account the effects of multiple layer optical
interference and photon generation. The integrated ray-tracing module can compute optical absorption through the
complex texture surface with multiple antireflection coating layers. Basic physical quantities like band diagram, optical
absorption and generation can be demonstrated. The I-V characteristics with short-circuit current density and open-circuit
voltage agree with the published experimental results and enhanced cell efficiency is shown with the V-grooved
texture. The results are analyzed with respect to surface recombination, antireflection coating, bulk doping/resistivity and
lifetime etc. Modeling capabilities for polycrystalline silicon and amorphous silicon cells are also discussed.
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Large multicrystalline cast silicon ingots (>310 kg) are cost effective in the photovoltaic industry and attenuate
the feedstock shortage. The bulk lifetime τn and diffusion length Ln of minority carriers vary through the height due to the segregation of metallic impurities during the directional solidification. The native impurity
concentrations increase from the bottom to the top of the ingot, which is solidified last, while the ingot bottom,
which is solidified first, is contaminated by the contact with the crucible. It was found that τn and Ln are the smallest in the top and in the bottom of the ingot. In solar cells, the evolution is similar, however in the central
part of the ingot Ln is strongly increased due to the in-diffusion of hydrogen from the SiN-H antireflection
coating layer. The variations along the ingot height of the conversion efficiency η and of τn in raw wafers are
well correlated, that can predict the values of η, allowing an in-line sorting of the wafers, before solar cells are
made. If τn is smaller than 1 μs, as observed at the extremities of the ingot, η will be limited to 10% only; if τn is higher than 2.5 μs η achieve 15 % at least. In addition, impurity segregation phenomena around grain boundaries are observed at the extremities of the ingots, linked to the long duration of the solidification process. Reducing
the height of the ingots could suppress these phenomena and not much material must be discarded. Another problem can come from the use of upgraded metallurgical silicon feedstock in which the densities of
boron and phosphorus are very close. Due to the difference in the segregation coefficients, ingots may be entirely
or partly p or n type, suggesting that a purification step tawards the dopants is required.
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An analysis of the monolithical series connection of silicon thin-film modules with metal back contact fabricated by high
speed laser ablation will be presented. Optically pumped solid state lasers with wavelengths of 1064 nm and 532 nm
were used for the patterning steps. The near infrared laser is applied to pattern the TCO (P1) while the green laser is used
for the ablation of the silicon layer stack (P2) and the back contact layer stack (P3). The influence of various laser
parameters on the performance of amorphous and microcrystalline silicon modules was studied. In particular the back
contact patterning and the Si removal can significantly affect the module efficiency. Non-optimized patterning
conditions for P2 can lead to a high contact resistance, while the ablation of the ZnO/Ag back contact system can
introduce shunts at the laser scribed line. Therefore, a criterion for flakeless patterning will be briefly introduced and the
influence of flakeless back contact patterning on the electrical behavior of silicon single junction cells will be discussed.
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Cu(In,Ga)Se2 (CIGS) solar cells are leading candidates for low-cost and high-efficiency solar cells. A band gap energy
(Eg) of CIGS can be controlled from 1.0 eV (CuInSe2) to 1.7 eV (CuGaSe2). The Eg of CIGS can be adjusted to the
theoretically estimated optimum value of 1.4 eV. However, maximum efficiencies for CIGS solar cells were achieved at
Eg=1.1~1.2 eV. A higher-Ga addition degrades the electronic properties of CIGS films. Compared to CIGS,
Cu(In,Al)Se2 (CIAS) can be adjusted the same Eg by a small Al addition. We report on the fabrication of the CIAS film
on Mo/soda-lime glass (SLG) substrate by a three-stage evaporation process. The film composition was
Cu/(In+Al)=0.89, Se/Metal=0.99 and Al/(In+Al)=0.15. The Eg of the film was 1.15 eV from the quantum efficiency
measurement. The cross-sectional scanning electron microscope image of the film showed a grain size of approximately
1μm. The composition depth profile by secondary ion mass spectroscopy showed the V-shape distribution of Al in the
depth direction. The CIAS solar cell consisted of Al/ITO/ZnO/CdS/CIAS/Mo/SLG was fabricated. The active cell area
was 0.12 cm2. A current-voltage measurement under illumination (AM1.5, 100mW/cm2) at 25°C showed the area
efficiency of 13.1% without antireflection coating.
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