Tomonori Nagashima

A Germanium Back Contact Type Thermophotovoltaic Cell

A Ge back contact type photovoltaic cell has been proposed to reduce resistance loss for high current densities in thermophotovoltaic systems. The back contact structure requires less surface recombination velocities than conventional structures with front grid contacts. A SiO2/SiNx double anti‐reflection coating including a high refractive index SiNx layer was studied. The SiNx layer has an enough passivation effect to obtain high efficiency. The quantum efficiency of the Ge cell was around 0.8 in the 800–1600 nm wavelength range. The conversion efficiency for infrared lights was estimated at 18% for a blackbody surface and 25% for a selective emitter by using the quantum efficiency and a simulation analysis.

A Novel Method Based on Oblique Depositions to Fabricate Quantum Dot Arrays

We propose a novel method to fabricate quantum dot (QD) arrays, utilizing nanometer-sized columnar structures of obliquely deposited thin films arising from the self-shadowing effect. We found that columns consisting of alternating arrays of QDs and barriers can be formed by means of alternate oblique depositions of the semiconductor and the dielectric onto rotating substrates. The deposition conditions were optimized by both computer simulations and experiments. The proposed method was demonstrated by depositing Si (QDs) and SiO2 (barriers) alternately onto rotating Si wafers or SiO2 glass plates at a deposition angle of 85deg, followed by annealing at 1000 degC to promote crystallization. The formation of crystalline Si QD arrays was confirmed by scanning electron microscopy, Raman scattering, and photoluminescence. This method can be utilized for various materials and has a significant advantage over the self-assembly of QDs based on the Stranski-Krastanow growth mode, and other methods, as these limit the materials that can be grown

Three-terminal tandem solar cells with a back-contact type bottom cell

In this paper, the authors analyze three-terminal tandem solar cells with a back-contact type bottom cell, in which III-V compound materials are stacked on a substrate of IV materials. This structure reduces loss resulting from current mismatch of the Ge bottom cell and resistance loss caused by tunnel junction at the GaAs/Ge interface. It is expected that the structure will achieve a high efficiency. They carried out calculations to find the relationship between carrier transportation characteristics and energy bands and to obtain I-V curves of the tandem solar cells. They found that the structure fulfills solar cell functions.

Surface Passivation for Germanium and Silicon Back Contact Type Photovoltaic Cells

Back contact type cells have large area contacts at the back, and have no front grid contact preventing light absorption. The cells are able to simultaneously offer reduced resistance loss for high current densities and high light absorption. However, the structure requires low surface recombination loss to obtain high efficiency. We studied various structures to reduce the loss for Si and Ge back contact type cells. A floating emitter at the surface of the Si cell is fabricated to reduce recombination loss with a SiO2 passivation layer. The Ge cell for thermophotovoltaics has a high refractive index SiNx passivation layer at the front surface. The layer also serves as a bottom layer of the anti reflection coating. In a GaInP/GaAs/Ge triple-junction cell with a Ge back contact type bottom cell, the energy band at the front of the Ge cell is bent due to the diffusion layer, and is effective in reducing recombination loss

A germanium back contact type cell for thermophotovoltaics

A Ge back contact type photovoltaic cell has been proposed to reduce resistance loss for high current densities in thermophotovoltaic systems. This structure requires low surface recombination velocities. A SiO/sub 2//SiN/sub x/ double AR coating including a high refractive index SiN/sub x/ layer was studied. The SiN/sub x/ layer has a high enough passivation effect to obtain high efficiency. The quantum efficiency was around 0.8 in the 800-1600 nm wavelength range. The efficiency for emitted infrared lights was estimated at 18% for a blackbody surface and 25% for a selective emitter. A higher efficiency will be obtained by using a low resistance substrate after the recombination loss is decreased, due to the improved SiN/sub x/ passivation layer and the floating emitter. It is considered that the Ge cell efficiency will increase in excess of 30% for infrared light by using these structures.