刊行物
｜ ENGLISH ｜
Menaka De Zoysa
I am focusing on light control to realize next generation highly efficient optoelectronic devices, such as narrow band thermal emission sources, ultra-thin- film solar cells. For light control I utilize an artificially fabricated periodic nano-structures having periodicity of order of light wavelengths, called as “photonic crystals”. Photonic crystals are analogous to semiconductors which are used to control electrons. Using photonic crystals, light emission, absorption, capturing, guiding can be manipulated on demand. Design of appropriate photonic crystal for each application is carried out by using super-computer assisted numerical calculations. Nano-structural photonic crystals are fabricated by making masks using nano-drawing (lithography) by electron beams and patterning by ion-based chemical reactions (etching), in a very clean environment (clean room). Finally device performances are measured by using relevant experimental systems (see the picture below). Experimental results are carefully analyzed and fed back to the design until obtaining an optimal performance. I will discuss below very narrow band thermal emitters and high efficiency ultra-thin-film solar cells realized based on light control by photonic crystal structures.
Aligning an optical measurement system which is used to evaluate device characteristics
Very narrow band thermal emitters
Any heated objects such as the sun, filament of incandescent light bulbs, etc. emit light, which is known as thermal emission. Such light sources have a very broad band emission range. For instance, sun light (solar spectrum) is ranging from ultraviolet to infrared wavelengths. However, in practical applications, only a narrow band wavelength range is used. For example, in mostly used silicon solar cells only ~1/3 of sun light is effectively used and in incandescent light bulbs visible light is only ~1/10 of total emission. Hence, by limiting the emission range to desired wavelength, highly efficient thermal emission source can be realized. To realize such sources, first, we control the movement of electrons in heated object by employing semiconductor structures and then control the emission wavelength- band of light by using a resonator created by photonic crystal. At the first time, we were able to experimentally demonstrate, more than 100 times narrower thermal emission compared to un-controlled emission, in a designed wavelength. We also experimentally demonstrated that power utilization efficiency becomes much higher in narrow band emitters by developing current-driven device (For further details see M. De Zoysa, et al. Nature Photonics 6, 535-539 (2012) ). In addition, we were able to realize such thermal emission device to operate at ultra-fast switching speed which is four orders of magnitude faster than the conventional sources (Further details are discussed in T. Inoue, M. De Zoysa, et al. Nature Materials 13, 928-931 (2014)). Such devices are very useful for high efficient environmental and bio sensing applications. Moreover, they can be applied to convert waste heat (thermal) energy or heat energy collected by solar, to narrow band light emission at the effectively usable wavelength of solar cells, for generation of electricity.
High efficiency ultra-thin-film solar cells
Solar cells have attracted much attention as a renewable clean energy source. In mostly used silicon solar cells, material cost of silicon itself is more than half of the total cost. Hence, cost performance of solar cell remains a lower value. To address this issue it is important to decrease the usage of material. However, reducing material leads to decrease absorption of the light in solar cell, hence efficiency becomes lower. To enhance the light absorption in solar cells, we can successfully employ photonic crystal by realizing light trapping inside the solar cell. Optical path length becomes effectively longer in trapped light and absorption increases, even though using thinner silicon layer. We were able to obtain more than 9% of efficiency (solar power to generated electrical power) by fabricating photonic crystal introduced solar cells having only a ~500 nm of ultra-thin-film silicon (micro- crystalline silicon) layer, which is more than 300 times thinner than usual thickness of solar cells.
In the future, in free-environment of Hakubi period, I would like to realize efficient electricity generation by using waste heat energy as briefly discussed above, which is very useful to address world energy issues. In addition, I am involving to realize high- quality, high-power semiconductor lasers which can be used for numerous applications such as laser processing, sensing, power transmission etc. I am also searching for new concepts to develop unique and efficient optoelectronic devices based on light control, collaborating with researches from various fields.
（めーなか で ぞいさ）
