arXiv subjects

Naoya Miyashita

Publications and source records attributed to Naoya Miyashita.

3 recordsLinked to original sources

arXiv2022

Thinned Germanium Substrates for III-V Multijunction Solar Cells

Multijunction solar cells are usually grown on Ge substrates. This implies several disadvantages that hinder the performance of the whole multijunction and limit their possible applications. The drawbacks caused by the substrate are: heavier devices, higher operation temperatures, lower performance and lack of photon confinement. In this work we propose thinning the substrate as a valid solution to the aforementioned challenges. The influence of the substrate thickness on the Ge subcell performance inside a multijunction is simulated using 2D TCAD tools. Simulation results point to the back surface recombination as the key parameter to enhance the development of thinned Ge subcells. Ge substrates have been thinned down, achieving 115μm thick samples. Finally, solar cells have been manufactured out of the thinned substrates proving a limited degradation and showing the feasibility of this process to manufacture Ge subcells thinned down up to 115μm.

physics.app-ph↗

arXiv2021

Resonant absorption in multilayer quantum-well and quantum-dot solar cells

Epitaxially-grown quantum well and quantum dot solar cells suffer from weak light absorption, strongly limiting their performance. Light trapping based on optical resonances is particularly relevant for such devices to increase light absorption and thereby current generation. Compared to homogeneous media, the position of the quantum layers within the device is an additional parameter that can strongly influence resonant absorption. However, this effect has so far received little attention from the photovoltaic community. In this work, we develop a theoretical framework to evaluate and optimize resonant light absorption in a thin slab with multiple quantum layers. Using numerical simulations, we show that the position of the layers can make the difference between strong absorption enhancement and completely suppressed absorption, and that an optimal position leads to an absorption enhancement twice larger than average. We confirm these results experimentally by measuring the absorption enhancement from photoluminescence spectra in InAs/GaAs quantum dot samples. Overall, this work provides an additional degree of freedom to substantially improve absorption, encouraging the development of quantum wells and quantum dots-based devices such as intermediate-band solar cells.

physics.app-ph↗

arXiv2020

Theoretical and Experimental Assessment of Thinned Germanium Substrates for III-V Multijunction Solar Cells

Solar cells manufactured on top of Ge substrates suffer from inherent drawbacks that hinder or limit their potential. The most deleterious ones are heavy weight, high bulk recombination, lack of photon confinement and an increase of the heat absorption. The use of thinned Ge substrates is herein proposed as a possible solution to the aforementioned challenges. The potential of a thinned Ge subcell inside a standard GaInP/Ga(In)As/Ge triple-junction solar cell is assessed by simulations, pointing to an optimum thickness around 5-10 μm. This would reduce the weight by more than 90 %, whereas the available current for the Ge subcell would decrease only by 5 %. In addition, the heat absorption for wavelengths beyond 1600 nm would decrease by more than 85 %. The performance of such a device is highly influenced by the front and back surface recombination of the p-n junction. Simulations remark that good back surface passivation is mandatory to avoid losing power generation by thinning the substrate. In contrast, it has been found that front surface recombination lowers the power generation in a similar manner for thin and thick solar cells. Therefore, the benefits of thinning the substrate are not limited by the front surface recombination. Finally, Ge single-junction solar cells thinned down to 85 μm by wet etching processes are demonstrated. The feasibility of the thinning process is supported by the limited losses measured in the current generation (less than 6 %) and generated voltage (4 %) for the thinnest solar cell manufactured.

physics.app-ph↗