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We attempted to clarify the impact of the compositional distribution on recently reported improvement in the conversion efficiency of solar cells based on bulk multicrystalline SiGe. For this purpose, Si1-xGex/Si1-yGey multiple quantum well structures on heavily doped Si-on-insulator were employed as model crystals. The combination of x and y, the width of each layer, and the number of repetitions were systematically changed to study the influence of the introduction of Ge on photocarrier generation and carrier transport while keeping the average Ge composition as 0.03. Spatial modulation of the band structure leads to formation of quantum wells for holes and gives negative impact especially in the photocarrier collection from the n-type region. When the depth of wells was designed to be constant, short-circuit carrier density was found to show a maximum at appropriate compositional distribution due to the competition between the increase in the photocarrier generation and the decrease in the minority carrier diffusion length. Within a limited compositional range, the overall performance of the solar cell was revealed to be improved by the introduction of the compositional distribution compared with that based on uniform Si0.97Ge0.03. Therefore, intentional introduction of the compositional distribution is concluded to be useful for improvement in the solar cell performance if appropriate dispersion is chosen. [ABSTRACT FROM AUTHOR]

A sub-10 nm, high-density, periodic silicon nanodisk (Si-ND) array with a SiC interlayer has been fabricated using a new top-down process that involves a 2D array of a bio-template etching mask and damage-free neutral beam etching. Optical and electrical measurements were carried out to clarify the formation of mini-bands due to wavefunction coupling. We found that the SiC interlayer could enhance the optical absorption coefficient in the layer of Si-NDs due to the stronger coupling of wavefunctions. Theoretical simulation also indicated that wavefunction coupling was effectively enhanced in Si-NDs with a SiC interlayer, which precisely matched the experimental results. Furthermore, the I-V properties of a 2D array of Si-NDs with a SiC interlayer were studied through conductive AFM measurements, which indicated conductivity in the structure was enhanced by strong lateral electronic coupling between neighboring Si-NDs. We confirmed carrier generation and less current degradation in the structure due to high photon absorption and conductivity by inserting the Si-NDs into p-i-n solar cells. [ABSTRACT FROM AUTHOR]

Abstract: We propose a new concept of growing a polycrystalline Si ingot suitable for solar cells by casting based on the directional growth behavior of polycrystalline Si investigated using an in situ observation system. The cooling conditions for the Si melt were crucial for controlling growth in the initial stage. At a high cooling rate, dendrite growth occurred along the crucible wall. This growth mechanism was found to be useful for obtaining a polycrystalline structure with large oriented grains. In fact, using the dendrites grown in the initial stage of casting, a polycrystalline Si ingot with large oriented grains was obtained. The solar cell properties of such structure-controlled polycrystalline Si were as good as those of single-crystalline Si. [Copyright &y& Elsevier]

Hemisphere-shaped crystal wafers can be prepared by the plastic deformation of Si crystal wafers. To obtain hemispherical Si wafers, graphite convex and concave dies were used. A Si wafer was set between dies and pressed at high temperatures. The Si wafer was pressed by an overweight of 200 N at various temperatures. The deformation regions in which well-shaped (100) and (111) wafers can be obtained by plastic deformation were determined using parameters of thickness and temperature. In order to demonstrate that the shaped wafers are of sufficiently high quality to be used in the preparation of devices, solar cells were fabricated using the hemispherical Si wafers pressed at 1,120°C and 1,200°C. The conversion efficiency of the hemispherical solar cells is 8.5-11.5%. It was clarified from the conversion efficiency of solar cells that the quality of the shaped crystal wafers can be improved by a proper annealing process. Thus, the hemispherical shaped wafers are of high quality to be used in the preparation of devices. [ABSTRACT FROM AUTHOR]

Abstract: We investigated structural properties of polycrystalline SiGe grown by directional growth method for solar cells. The average Ge composition was systematically changed in the range between 0 and 10at%. Distributions of concentration and crystallographic orientation in the SiGe crystal were measured and the preferential growth orientation was found to change from (111) to (110) with increasing average Ge composition. Misfit dislocation was observed using transmission electron microscope. There was few dislocations in SiGe crystals when the average Ge composition was between 0 and 5at% though a lot of dislocations existed at the average Ge composition of 10at%. We concluded that the optimum Ge composition is around 5at% for solar cells. [Copyright &y& Elsevier]

Quantum dot solar cells consisting of a sub-10-nm highly ordered and dense 2-dimensional (2D) array of Si nanodisks (Si-NDs) as a quantum dot superlattice with a SiC interlayer are achieved. The Si-NDs are fabricated with an original top-down process involving a 2D array of bio-templates with a 4.5-nm-diameter iron core and damage-free neutral beam etching (Si-ND diameter: 6.4 nm). The Si-ND array has a high optical absorption coefficient due to miniband formation. As a result, the cells achieve open-circuit voltage of 0.556 V, short-circuit current density of 31.3 mA/cm2, a 72% fill factor, and 12.6% conversion efficiency. [ABSTRACT FROM AUTHOR]

Abstract: We report on proof-of-concept experiments to demonstrate the advantages of “floating cast method” as a growth method to realize high-quality Si bulk multicrystals for solar cells. The fundamental concept is to use the top center of the melt as the nucleation site and to avoid contact between the crystal and the inner wall of the crucible during crystal growth. In contrast to the conventional casting method to start with inhomogeneous nucleation at the bottom of the crucible, the size of crystal grains is expected to become larger by limiting the number of nuclei. In addition, minimization of the contact of the crystal with the inner wall of the crucible is considered to result in low contamination of diffused impurities from the lining material as well as fewer defects by decreasing residual strain. These features are confirmed by comparing properties of crystals grown by the floating cast and the conventional methods. Furthermore, relative increase of the conversion efficiency of solar cells and the improvement of the material yield applicable to solar cells are presented, which certifies the advantage of the floating cast method. [Copyright &y& Elsevier]

Abstract: The dark current–voltage (J–V) characteristics and the conversion efficiency of the solar cells with embedded stacked Ge islands in the intrinsic layer were investigated. These islands were grown by molecular beam epitaxy on Si substrates. We used a two-diode model to analyze the dark J–V characteristics of Ge island solar cells. Results showed that the minority carrier diffusion and the recombination current components increase as a function of the stacked Ge island layers. This increase of the minority carrier diffusion current was due to an increase of the intrinsic carrier density as a function of the number of stacked layers. Similarly, the increase in the recombination current components was due to the enormous recombination of carriers in the intrinsic region as the number of stacked layer increases. This phenomenon could lead to a decrease of the open circuit voltage, V oc. The decrease of V oc should be overcompensated by the increase of photocurrent, due to the presence of stacked Ge islands with higher absorption coefficient, in order to attain an optimum value of the conversion efficiency. [Copyright &y& Elsevier]

Abstract: We demonstrated a solar cell system comprising a concave-Si-mirror solar cell and a conventional small Si solar cell set at the focal point of the concave-mirror solar cell. The concave-Si-crystal mirror was prepared by mechanically polishing a single-crystal Si wafer. It was used both as a solar cell and mirror. This system can make effective use of the photons reflected from the concave-mirror solar cell because the number of total photons is the sum of photons from both the mirror solar cell and the small solar cell set at the focal point. When the conventional small solar cell with an efficiency of 13.0% was used, the total conversion efficiency of the present system increased to 12.2% compared with the conversion efficiency of 11.5% for only the concave-Si-mirror solar cell. These results show that the present solar cell system has the capability to achieve high efficiency through the effective use of the photons reflected from conventional solar cells. [Copyright &y& Elsevier]