Hidekazu Arase

A Novel Thin Concentrator Photovoltaic With Microsolar Cells Directly Attached to a Lens Array

We propose a novel concept of thin and compact CPV modules in which submillimeter solar cells are directly attached to lens arrays without secondary optics or an extra heat sink. With this small cell size, the optical path length of the module can be brought down to one-twentieth that of conventional CPV modules. To achieve precise alignment of the microsolar cells at the lens focal points, we have developed a fluidic self-assembly technique that utilizes surface tension. This novel CPV module with triple junction microsolar cells demonstrated an efficiency of 34.7% under sunlight in the particular measured condition.

Interfacial-energy-controlled deposition technique of microstructures using blade-coating.

A novel blade-coating technique for the fluidic self-assembly of microstructures on large-scale substrates is presented. In our blade-coating technique, water and microstructures dispersion, which includes chemically modified microstructures and water-insoluble solvent, are continuously blade-coated on a substrate on which surface hydrophilic areas are surrounded by a hydrophobic self-assembled monolayer. In the process studied, first, water is selectively placed on the hydrophilic areas; second, the water-insoluble solvent covers the water to create a solvent/water interface; third, fluidic self-assembly of microstructures onto the water takes place by a capillary force between the water and the microstructures; and finally, the microstructures are deposited onto the hydrophilic areas after the evaporation of the water and the solvent. SiO(2) plates sized 10 x 50 x 0.3 microm(3) were used to verify the feasibility of our technique. About 40,000 SiO(2) plates were selectively deposited on the hydrophilic areas on a substrate with an area of 20 cm(2) with a deposition probability of 0.52 by utilizing dispersion consisting of plates chemically modified with 1-chloroethyltrichlorosilane and a mixture of 1,4-dichlorobutane and n-hexane. The deposition probabilities of the plates primarily depended on the type of solvent for plate dispersions and increased with an increase in the value of free energy change of the plate/solvent/water system by the movement of the plate from the solvent to the solvent/water interface during the blade-coating process. These results indicate that the deposition probabilities are governed directly by the capillary force acting on the plates. Our deposition technique for microstructures using blade-coating is potentially applicable to the deposition of micrometer-size electronic devices on large-scale substrates.

High efficiency micro solar cells integrated with lens array

We demonstrate high efficiency triple junction solar cells with submillimeter dimensions in an all-back-contact architecture. 550 × 550 μm2 cells flash at 41.3% efficiency under the air mass 1.5 direct normal spectrum at 50 W/cm2 at 25 °C. Compared to standard size production cells, the micro cells have reduced performance at 1-sun due to perimeter recombination, but the performance gap closes at higher concentrations. Micro cells integrated with lens arrays were tested on-sun with an efficiency of 34.7%. All-back-contact architecture and submillimeter dimensions are advantageous for module integration and heat dissipation, allowing for high-performance, compact, lightweight, and cost-effective concentrated photovoltaic modules.

High-efficiency thin and compact concentrator photovoltaics using micro-solar cells with via-holes sandwiched between thin lens-array and circuit board

We have developed a compact concentrator photovoltaic (CPV) module that comprises micro-solar cells with an area of ≈0.6 × 0.6 mm2 sandwiched between a 20-mm-thick lens array and a 1-mm-thick circuit board with no air gap. To establish electrical connections between the circuit board and the micro-solar cells, we developed a micro-solar cell with positive and negative electrodes on the lower face of the cell. In this study, we demonstrated the photovoltaic performance of the micro-solar cell closely approaches that of the standard solar cell measuring ≈5 × 5 mm2 commonly used in conventional CPVs under concentrated illumination. Our study showed that the negative effect on PV performance of perimeter carrier recombination in the micro-solar cell was insignificant under concentrated illumination. Finally, we assembled our micro-solar cells into a CPV module and achieved the module energy conversion efficiency of 34.7% under outdoor solar illumination.

Fluidic Self-Assembly of Microstructures Using a Blade-Coating Technique: Influence of Volume of Water Droplets on Probability of Microstructure Placement

Fluidic self-assembly (FSA) is a promising technique for fabricating devices that are composed of large numbers of small electronic components. We have previously proposed a printing method that utilizes the FSA principle. In our method, by simply blade-coating first water and then a dispersion liquid of microstructures on a substrate, the microstructures are automatically placed on pre-patterned hydrophilic areas by means of water/solvent interfacial force. To improve the placement probability of microstructures on the intended hydrophilic areas, in the present study we investigate the influence of the volume of water droplets on the probability of microstructure placement. We prepared various sizes of hydrophilic patterns on a glass substrate to vary the volume of water droplets in hydrophilic areas, and placed square SiO2 plates, measuring 50 ×50 ×0.3 µm3, using FSA. The probability and accuracy of placement was evaluated using a high-speed microscope, and the results were interpreted using a simple model based on capture coefficients and the collision cross section of the water droplets. We verified that the model closely fitted the experimentally obtained probability of placement as observed using the high-speed microscope. We found that the capture coefficient increased with increasing area of the water droplet. These results indicate that the size of the hydrophilic area is one key to improving the probability and accuracy of our placement technique.

Interfacial-Force-Controlled Placing Technique of Microstructures of Sub- to One Hundred Micrometer Size Using Blade Coating

The surface mounting technology of electronic devices using pick-and-place machines is commonly used to fabricate functional electronic appliances, such as motherboards, flat panel displays, and mobile phones. However, the pick-and-place method begins to encounter difficulties in mounting electronic devices when devices shrink to a few hundreds of micrometers or less. We propose a new blade-coating method of placing microstructures smaller than several hundreds of micrometers on a substrate. The method comprises three steps: (1) preparing a microstructure dispersion consisting of chemically modified microstructures and a water-insoluble organic solvent, (2) continuous blade-coating of water and the dispersion on a chemically patterned substrate on which hydrophilic areas are surrounded by a hydrophobic self-assembled monolayer, and (3) spontaneous placing of the microstructures on the hydrophilic areas by a water/solvent interfacial force that acts on the microstructures. Using this method, we have been able to place microstructures ranging in length from submicrometer to one hundred micrometers, including silicon nanowires and SiO2 microstructures of various sizes. However, our blade-coating method for placing microstructures can be realized with successful combinations of chemical modifiers for the microstructures and water-insoluble solvents. We present a simple method of assessing dispersion using a chemically modified glass test tube filled with water and a solvent for the dispersion.

A biomimetic strategy for designing easily-installable CPV tracking system with high wind resistivity

Due to its higher photovoltaic efficiency, CPV has the advantage of needing a smaller panel footprint than fixed PV systems. This characteristic makes it ideal for use as an independent power supply in remote locations where transportation and site procurement present a challenge. The overall system needs to be compact and light without sacrificing weather resistance. Skilled personnel are scarce in remote areas, so easy setup and maintenance are also important. We have developed two techniques based on the notion of bio-mimicry. One is machine learning to counteract errors in installation position without the need for human intervention, and the other is a three-dimensional panel structure that cuts the wind load by up to 20%. We describe our experimental results gained in field tests.