Nano-Gap Thermophotovoltaic Generation of Electricity using Spectral Controlled Near-Field Radiation

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Effect of near-field radiation (evanescent wave) to enhancement of thermophotovoltaic (TPV) generation of electricity has been investigated in the current study. A tungsten emitter and a GaSb TPV cell were put together in face to face in vacuum chamber. The emitter was heated by CO_2 laser and kept at 950K. The TPV cell was mounted on a water-cooled copper block and kept at atmospheric temperature. Under the condition of the gap less than 50μm, the output power does not change since the view factor is almost unity for the propagating radiation. On the other hand, on further decrease in gap to less 1 pm, the open circuit voltage and the short circuit current increase by a factor of 1.4 and 4, respectively due to the near-field effect. As a result, the maximum power in the case of near-field radiation is 5.6 times higher than that of usual propagating radiation. In this case, the commercial TPV cell has finger electrodes on its surface; so that it is not suitable for the nano-gap TPV energy-conversion system. Therefore, using the MBE equipment, we tried to fabricate an original cell, in which the electrode will be submerged in the p-type semiconductor to make a flat surface. In a high vacuum chamber, p-type GaSb layer was grown on the n-type GaSb substrate. The carrier density and the mobility of cell were 5.73x10^7cm^<-3> and 1.35x10^2cm^2/Vs, respectively. Furthermore, the GaSb TPV cell is active for the wavelength from 0.8 to 1.8μm. So, the radiation come from the emitter is concentrated in this wavelength range. A surface with micro-cavities is useful for the spectral control of emission of radiation. A surface with micro-cavities of 500x500x500nm^3 has a high emittance in the wavelength range from 0.8μm to 1.5μm. Consequently, it will be expected that, provided that those are combined, a new sophisticated energy conversion system from thermal energy to electricity through near-field effect will be developed successfully in near future.