Radial junction solar cells prepared on single crystalline silicon wafers by metal-assisted etching

Radial junction solar cells have been proposed as an alternative device geometry to conventional planar solar cells with its remarkable electrical and optical performance. In this geometry, densely packed nano/micropillars allow minority carrier collection in the radial direction and shorten carrier diffusion length to p-n junction. Besides, reduced reflection from surface and increased light trapping in nano/micropillars enhance solar cell efficiency. In this study, photolithography and metal-assisted etching (MAE) techniques are used to form well-ordered silicon micropillar arrays; standard doping, passivation, and metallization steps are followed to form radial junction solar cells. The effect of micropillar length on optical and electrical performance of the solar cells have been investigated. We observed that optical reflection from solar cells surface decreased with increasing micropillar length, hence solar cell short circuit current (J(sc)) and efficiency (eta) increased. Our best solar cell efficiency is 15.6% and this is one of the highest reported values obtained from the radial junction solar cells prepared by MAE technique. (C) 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei

A compact pool boiler utilizing nanostructured plates for microscale cooling applications

A nanostructure based compact pool boiler cooling system consisting of an aluminum base housing the heaters, a pool and four different plates to change the surface texture of the pool is designed. Effects of nanostructured plates of different surface morphologies on boiling heat transfer performance of the system are studied. Three nanostructured plates featuring Si nanowires of diameter 850 nm and of three different lengths, 900 nm, 1800 nm and 3200 nm respectively, which are etched through single crystal p-type silicon wafers using metal assisted chemical etching (MaCE), are utilized to enhance the pool boiling heat transfer. A plain surface Si plate is used as the control sample. C onstant heat flux is provided to the liquid within the pool on the surface of the aluminum base through the plate by boiling heat transfer. Existence of wall superheat gave rise to forming of vapor bubbles near the boiling temperature of the fluid, namely DI-Water. Bubbles emerged from the nanostructured plate along with the phase change. Nucleate boiling on the surface of the plate, bubble formation and bubble motion inside the pool created a heat removal mechanism from the heated surface to the liquid pool. The surface temperature at boiling inception is found to be 104 ºC for plain surface Si control sample, whereas it decreased to near 100 ºC for all the cases where a nanostructured plate was present. Due the enhancement in boiling heat transfer coefficients, this study proves the ability of nanostructure d plates in improving the performance of the cooling system

The effect of nanostructure distribution on subcooled boiling heat transfer enhancement over nanostructured plates integrated into a rectangular channel

In this study, subcooled flow boiling is investigated over nanostructured plates at flow rates ranging from 69 mL/min to 145 mL/min. The first configuration of the nanostructured plate includes ˜600-nm-long, closely packed copper nanorod arrays distributed randomly upon the surface with an average nanorod diameter of ˜150 nm, and the second configuration consists of a periodic structure having ˜600-nm-long copper (Cu) nanorods with an average nanorod diameter of ˜550 nm and a center-to-center nanorod separation of ˜1 μm. The nanorod arrays are deposited utilizing glancing angle deposition (GLAD) technique on the copper thin film (˜50 nm thick) coated on silicon wafer substrates. Dimensions of the test section, heat flux values, and flow rates are chosen to ensure that nanostructured plates remain intact along with their nanorods in their original shape and position, so that the nanostructured plates could be used for many experiments. A consistent increase (up to 30%) in heat transfer coefficients is observed on nanostructured plates compared to the Cu thin film, which is used as the control sample. However, no significant difference in the boiling heat transfer performance between the random and periodic nanorods was observed, which indicates that the distribution of nanostructures may not be very critical in achieving enhanced heat transfer. In light of the obtained promising results, channels with nanostructured surfaces are proven to be useful, particularly in applications such as cooling of small electronic devices, where conventional surface modification techniques are not applicable

A compact pool boiler utilizing nanostructures for microscale cooling applications

A nanostructure based compact pool boiler cooling system consisting of an aluminum base housing the heaters, a pool and four different plates to change the surface texture of the pool is designed. Effects of nanostructured plates of different surface morphologies on boiling heat transfer performance of the system are studied. Three nanostructured plates featuring Si nanowires of diameter 850 nm and of three different lengths, 900 nm, 1800 nm and 3200 nm respectively, which are etched through single crystal p-type silicon wafers using metal assisted chemical etching (MaCE), are utilized to enhance the pool boiling heat transfer. A plain surface Si plate is used as the control sample. C onstant heat flux is provided to the liquid within the pool on the surface of the aluminum base through the plate by boiling heat transfer. Existence of wall superheat gave rise to forming of vapor bubbles near the boiling temperature of the fluid, namely DI-Water. Bubbles emerged from the nanostructured plate along with the phase change. Nucleate boiling on the surface of the plate, bubble formation and bubble motion inside the pool created a heat removal mechanism from the heated surface to the liquid pool. The surface temperature at boiling inception is found to be 104 ºC for plain surface Si control sample, whereas it decreased to near 100 ºC for all the cases where a nanostructured plate was present. Due the enhancement in boiling heat transfer coefficients, this study proves the ability of nanostructure d plates in improving the performance of the cooling system