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.

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.

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.

CuIn_1-xGa_xS₂ (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, V_oc = 763.3 mV, J_sc = 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 R_p 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, J_o respectively from ~2.1 and ~2.6x10^-8 A cm^-2 to ~1.72 and ~1.41x10^-10 A cm^-2.

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 (V_oc), 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 V_oc. 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.

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%.

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 (CuIn_1-xGa_xSe² 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 (Si_1-xGe_x:H) thin films. This study has reaffirmed that the highest efficiencies for a-Si:H and a-Si_1-xGe_x: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 CdTe_1-xS_x 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 CdTe_1-xS_x 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.

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 In_xGa_ySe_z-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 In_xGa_ySe_z-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.

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 J_sc. 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 J_sc. 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.

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 SiN_x 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.

The nanostructure of hydrogenated amorphous silicon-germanium alloys, a-Si_1-xGe_x: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.

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.

Large multicrystalline cast silicon ingots (>310 kg) are cost effective in the photovoltaic industry and attenuate the feedstock shortage. The bulk lifetime τ_n and diffusion length L_n 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 τ_n and L_n 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 L_n 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 τ_n 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 τ_n is smaller than 1 μs, as observed at the extremities of the ingot, η will be limited to 10% only; if τ_n 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.

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.

Cu(In,Ga)Se₂ (CIGS) solar cells are leading candidates for low-cost and high-efficiency solar cells. A band gap energy (E_g) of CIGS can be controlled from 1.0 eV (CuInSe₂) to 1.7 eV (CuGaSe₂). The E_g 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 E_g=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 E_g 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 E_g 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.