Lighting-environment-adjustable block-type 3D indoor PV for wireless sensor communication.

Demand is increasing for photovoltaics (PVs) as a result of the development of the Internet of Things and edge computing technologies. As the lighting environment is different for the applications, thus, PVs must be adjustable to various light environments in which systems are installed. PVs should therefore be capable of easily changing their morphology without damaging the cells. To address this problem, in this work, a three-dimensional (3D) structure that increases power output under omnidirectional light was applied to a crystalline silicon solar cell array using a block-type method. The resultant block-type 3D indoor PV could operate a Bluetooth low-energy module in conjunction with a power management integrated circuit when the illuminance was 532 lx and 1620 lx and each PV installation area was 129.9cm2 and 32.48 cm2 respectively.

Small area high voltage photovoltaic module for high tolerance to partial shading.

The urban application of photovoltaics is necessary to achieve carbon-free electricity production. However, the serial connections within modules cause problems under partial shading conditions, which is inevitable in urban applications. Therefore, a partial shading-tolerance photovoltaic module is needed. This research introduces the small-area-high-voltage (SAHiV) module with rectangle and triangle shapes for high partial shading tolerance and compares its performance with conventional and shingled modules. We tested it with discrete and continuous shading shape groups to represent unpredictable shading by simulations using LTspice with Monte Carlo simulation combined with latin hypercube sampling that were validated by comparison with experimental results. The SAHiV triangle module exhibited the best partial shading tolerance under most scenarios. Both types of SAHiV modules (rectangular and triangular) were robust against all types of shading patterns and angles, as indicated by their stable shading-tolerance values. These modules are thus suitable for use in urban areas.

Kirigami-inspired automatically self-inclining bifacial solar cell arrays to enhance energy yield under both sunny and cloudy conditions.

The application of photovoltaics (PVs) is expanding in various locations ranging from industrial facilities to residential housing. The emphasized concept in the PVs field is shifting from "watt-per-cost" to "energy-yield-per-watt." To attain a high energy yield, fixed modules are not well suited to capture both direct and omnidirectional light. To achieve a high energy yield under both light conditions, we propose a self-inclining bifacial solar cell array fabricated by integrating a photothermal actuator, which senses incident light by itself, and actuating solar cells that incline at the appropriate angle to maximize captured light. In the vertical illumination state, the specific power of the self-inclining bifacial two-cell array is 11% higher than a fixed-angle aligned array. In an outside environment with a large proportion of diffused light, the self-inclining bifacial two-cell array also shows higher performance. We expect this work to enable PVs to be applied without regard to weather conditions.

Automated shape-transformable self-solar-tracking tessellated crystalline Si solar cells using in-situ shape-memory-alloy actuation.

Photovoltaic energy systems in urban situations need to achieve both high electricity production and high capacity in restricted installation areas. To maximize power output, solar-tracking systems tilt solar arrays to track the sun's position, and typically flat modules are used to maximize the cross-sectional area. Such tracking systems are complex and expensive, and flat modules cannot utilize omnidirectional incident light. For solar systems in urban environments, we have developed two-dimensional (2D) or three-dimensional (3D) tessellated solar-cell modules that use shape transformation, and combine solar tracking and an arch structure for use in restricted areas. The modules can use scattered and omnidirectional incident light. Simply by attaching shape-memory alloy strips to the surface of the solar panels, the shape of the array can be transformed in response to heat from sunlight. Compared to a perfect solar-tracking system, our simulation results indicate that the modules present a large cross-sectional area perpendicular to the direction of sunlight and provide superior tracking performance, resulting in a 60% increase in electricity production over the course of 1 day. In addition, by using different designs for the tessellation units, dome shaped or other 3D structures are possible, for specific applications and to meet aesthetic requirements.

Highly stretchable large area woven, knitted and robust braided textile based interconnection for stretchable electronics.

With the rapid development of stretchable and wearable technologies, stretchable interconnection technology also demanded along it. Stretchable interconnections should have high stretchability and stable conductivity for use as an electrode. In addition, to develop to commercialization scale from research scale, a simple fabrication process that can be scaled up, and the stretchable interconnection should be able to be electrically connected to devices or modules directly. To date, printable conductor inks, liquid metals and stretchable structured interconnections have been reported for stretchable interconnections. These approaches have demonstrated high stretchability and conductivity, but in aspect of scale, it is appropriate to apply in micro-scale devices. For requirements of stretchability, conductivity and direct integration into meso- or centimeter-scale electronic devices or modules, here we introduce stretchable interconnections with a textile structure composed of metal fibers. The stretchable woven and knitted textiles show 67% strain and stable conductivity, and the cylindrical textile shows more than 700% strain with high strength. The stretchable textiles were fabricated using a weaving, knitting and braiding machine that can be used to produce textiles without any limit to length or area. These textiles exhibit high and stable conductivity even under deformation, and can be directly integrated into devices or modules by soldering. These high-performance stretchable textiles have great potential for commercial applications.

Fractal solar cell array for enhanced energy production: applying rules underlying tree shape to photovoltaics.

Plants and photovoltaics share the same purpose as harvesting sunlight. Therefore, botanical studies could lead to new breakthroughs in photovoltaics. However, the basic mechanism of photosynthesis is different to semiconductor-based photovoltaics and the gap between photosynthesis and solar cells must be bridged before we can apply the botanical principles to photovoltaics. In this study, we analysed the role of the fractal structures found in plants in light harvesting based on a simplified model, rotated the structures by 90° and applied them to fractal-structured photovoltaic Si solar cell arrays. Adoption of botanically inspired fractal structures can result in solar cell arrays with omnidirectional properties, and in this case, yielded a 25% enhancement in electric energy production. The fractal structure used in this study was two-dimensional and symmetric; investigating and optimizing three-dimensional asymmetric fractal structures would further enhance the performance of photovoltaics. Furthermore, this study represents only the first step towards the development of a new type of photovoltaics based on botanical principles, and points to further aspects of botanical knowledge that could be exploited, in addition to plant fractal structures. For example, leaf anatomy, phyllotaxis and chloroplastic mechanisms could be applied to the design of new types of photovoltaics.

Omni-direction PERC solar cells harnessing periodic locally focused light incident through patterned PDMS encapsulation.

Photovoltaic panels based on crystalline Si solar cells are the most widely utilized renewable source of electricity, and there has been a significant effort to produce panels with a higher energy conversion efficiency. Typically, these developments have focused on cell-level device modifications to restrict the recombination of photo-generated charge carriers, and concepts such as back surface field, passivated emitter and rear contact (PERC), interdigitated back contact, and heterojunction with intrinsic thin layer solar cells have been established. Here, we propose quasi-Fermi level control using periodic local focusing of incident light by encapsulation with polydimethylsiloxane to improve the performance of solar cells at the module-level; such improvements can complement cell-level enhancements. Locally focused incident light is used to modify the internal quasi-Fermi level of PERC solar cells owing to the localized photon distribution within the cell. Control of the local focusing conditions induces different quasi-Fermi levels, and therefore results in different efficiency changes. For example, central focusing between fingers enhances the current density with a reduced fill factor, whereas multiple local focusing enhances the fill factor rather than the current density. Here, these effects were explored for various angles of incidence, and the total electrical energy production was increased by 3.6% in comparison to a bare cell. This increase is significant as conventional ethylene vinyl acetate-based encapsulation reduces the efficiency as short-wavelength light is attenuated. However, this implies that additional module-scale studies are required to optimize local focusing methods and their synergy with device-level modifications to produce advanced photovoltaics.

Omni-directional light capture in PERC solar cells enhanced by stamping hierarchical structured silicone encapsulation that mimics leaf epidermis.

Conventional crystalline silicon solar cell photovoltaic module technology requires much more development due to the challenges of efficiency loss and reliability problems such as browning damage. As an alternative to conventional ethylene-vinyl acetate (EVA)-glass encapsulation, silicone-based encapsulation is a promising innovation. Added to the many advantages of silicone based encapsulation for Si solar cells, here we present surface modification of silicone encapsulation with hierarchical structures inspired by leaf epidermis structures that improve light capture and hydrophobicity of the module surface using a simple, large-area silane and ozone treatment technique. The hierarchical structures comprise tens-of-micrometer-scale hills, valleys, and bump structures and sub-micrometer-scale wave patterns; the combination of these surface structures improved light transmission, light haze, and the wetting angle. These synergistic structures improve efficiency under vertical illumination compared to a bare cell, which is significant considering the efficiency loss in conventional EVA-glass encapsulation from those of bare cells. Furthermore, the enhancement increased the angle of incidence and improved the omni-directional performance so that electrical energy was generated more efficiently. We demonstrated that the modification of module surfaces by mimicking leaf epidermis structures yields considerable benefits, and further studies are expected to optimize this structure and identify the underlying principles for technological innovations based on silicone encapsulation.

Omnidirectional Light Capture by Solar Cells Mimicking the Structures of the Epidermal cells of Leaves.

It is important to develop solar cells that can capture and utilize omnidirectional light in urban environments, where photovoltaic (PV) devices are installed in fixed directions. We report a new design for such light capture, which mimics the structure of a leaf epidermis. First, we analyzed the epidermal structures of different plant species in detail so that we could copy them and fabricate light-trapping layers with different shapes: as lens arrays, pillars, and lens arrays with rough surfaces. Then we analyzed the results of two-dimensional ray-tracing simulations of perfectly aligned and Gaussian-scattered incident light in terms of light-trapping capabilities. Based on these results, we prepared high-performance dye-sensitized solar cells with light-trapping layers that exhibited omnidirectional light capturing functionality. Our layers enhanced the efficiency of obliquely incident light capture by 70%. Therefore, we expect that new possibilities for next-generation PVs, extending beyond the current rigid concepts, will arise upon the application of these results and from findings that build on these results.

Leaf Anatomy and 3-D Structure Mimic to Solar Cells with light trapping and 3-D arrayed submodule for Enhanced Electricity Production.

Plant leaves are efficient light scavengers. We take a 'botanical approach' toward the creation of next-generation photovoltaic cells for urban environments. Our cells exhibit high energy conversion efficiency under indirect weak illumination. We used two features of leaves to improve dye-sensitized solar cells (DSSCs). Leaves feature a cuticle, a covering epidermis, and palisade and spongy cells. Leaves are also carefully arrayed within the plant crown. To mimic these features, we first created a light-trapping layer on top of the solar cells and microscale-patterned the photoanodes. Then we angled the three-dimensional DSSCs to create submodules. These simple mimics afforded a 50% enhancement of simulated daily electricity production. Our new design optimizes light distribution, the photoanode structure, and the DSSC array (by creating modules), greatly improving cell performance.

Three-Dimensional Textile Platform for Electrochemical Devices and its Application to Dye-Sensitized Solar Cells.

The demand for easy-to-use portable electric devices that are combined with essential items in everyday life, such as apparel, has increased. Hence, significant research has been conducted into the development of wearable technology by fabrication of electronic devices with a textile structure based on fiber or fabric. However, the challenge to develop a fabrication method for wearable devices based on weaving or sewing technology still remains. In this study, we have proposed and fabricated a 3-D textile with two electrodes and one spacer in a single sheet of fabric, utilizing a commercial weaving machine. The two electrodes fulfil the role of electron transfer and the spacer between the electrodes circulates electrons and prevents electrical shorting. Hence, the 3-D textile could be applied to a wide range of electrochemical devices. In addition, it is possible to control the textile structure, size and quantity and change the electrode or spacer materials by replacing the thread. We applied the 3-D textile to dye-sensitized solar cells (DSSCs) which has distinctive advantages such as low manufacturing cost, esthetic appearance for interior or exterior application and high power output under relatively weak light illuminations. The 3-D textile DSSCs were fabricated through a continuous process, from manufacturing to encapsulation, using a non-volatile electrolyte and demonstrated a specific power of 1.7% (1 sun, 1.5 A.M.). The 3-D textile DSSCs were electrically connected in parallel and series by twisting, stainless steel wires, which were used as the weft, and a light-emitting diode lamp was turned on using 3-D textile DSSCs connected in series. This study represents the first stage in the development and application of wearable textile devices.

Improvement in Energy Conversion Efficiency by Modification of Photon Distribution within the Photoanode of Dye-Sensitized Solar Cells.

The dye-sensitized solar cell (DSSC) is a potential alternative to the widely used Si-based solar cell, with several advantages including higher energy conversion efficiency under weak and indirect illumination conditions, and the possibility of practical application in urban life due to their exterior characteristics. However, despite these advantages, the energy conversion efficiency of DSSCs has remained low at ∼10%. To improve the efficiency of DSSCs, research has been done on modifying the materials used in DSSC component parts, such as the photoanode, electrolyte, and counter electrode. Another approach is to modify the photoanode to increase the diffusion coefficient, reduce the recombination rate, and enhance the light behavior. One of the most popular methods for improving the efficiency of DSSCs is by trapping and dispersing the incident light using a scattering layer. Use of a scattering layer has shown various and interesting results, depending on the application, but it is currently used only in a simple form and there has been no deep research on the further potential of the scattering layer. In this study, the scattering center was introduced to maximize the effect of scattering. Light distribution near the scattering center, within or on the photoanode, was investigated using finite differential time domain (FDTD) numerical methods. Based on the FDTD analysis, an optimized, dome-shaped three-dimensional modified structure of a transparent photoanode with minimized scattering centers was introduced and indicated the possibility of modifying the photon distribution in the photoanode to enhance the performance of DSSCs. In addition to using the scattering center, we have introduced the structure of the dome-shaped three-dimensional structure to utilize the light distribution within the photoanode. This novel three-dimensional transparent photoanode and scattering center design increased the energy conversion efficiency of DSSCs from 6.3 to 7.2%. These results provide a foundation for investigating the role of the scattering center via further in-depth research. This new three-dimensional photoanode design provides a means to overcome the previous limitations on DSSC performance.

High Energy Conversion Efficiency with 3-D Micro-Patterned Photoanode for Enhancement Diffusivity and Modification of Photon Distribution in Dye-Sensitized Solar Cells.

Dye sensitize solar cells (DSSCs) have been considered as the promising alternatives silicon based solar cell with their characteristics including high efficiency under weak illumination and insensitive power output to incident angle. Therefore, many researches have been studied to improve the energy conversion efficiency of DSSCs. However the efficiency of DSSCs are still trapped at the around 10%. In this study, micro-scale hexagonal shape patterned photoanode have proposed to modify light distribution of photon. In the patterned electrode, the appearance efficiency have been obtained from 7.1% to 7.8% considered active area and the efficiency of 12.7% have been obtained based on the photoanode area. Enhancing diffusion of electrons and modification of photon distribution utilizing the morphology of the electrode are major factors to improving the performance of patterned electrode. Also, finite element method analyses of photon distributions were conducted to estimate morphological effect that influence on the photon distribution and current density. From our proposed study, it is expecting that patterned electrode is one of the solution to overcome the stagnant efficiency and one of the optimized geometry of electrode to modify photon distribution. Process of inter-patterning in photoanode has been minimized.