Micro-solar Cells Research Articles

Realistic Multidimensional Optoelectrical Modeling Guide for Copper Indium Gallium Diselenide Solar Cells

Herein, an optoelectrical model is presented for copper indium gallium diselenide (CIGSe) solar cells in COMSOL Multiphysics, capable of multidimensional simulations, and it is applied to ultrathin (500 nm absorber thickness) solar cells. First, the modeling approach is shown. Special attention is paid to back contact materials, interface states, and defect application and their impact on the current–voltage (J–V) characteristics. To address whether the back contact is Schottky or Ohmic, the influence of the Schottky barrier height, recombination velocity, and interface states is shown. Then, the additional application of an acceptor defect gradient at the absorber back and a donor defect density distribution at the p–n junction is investigated. The results of these parameter adjustments are discussed, and the trends are shown to enable fast fitting of experimental J–V curves. Finally, the results are compared to the experimental J–V curves for indium tin oxide and Mo back contact, and challenges encountered are discussed while fitting. The optoelectrical model for CIGSe solar cells, established in two dimensions here, paves the way for comprehensively describing 2D and 3D solar cell structures, e.g., nanotextured or microsolar cells, as well as for considering different absorber thicknesses.

Inverted pyramid structure on monocrystalline silicon processed by wet etching after femtosecond laser machining in air and deionized water

The micro-structured silicon has been preliminarily applied to improve the surface and optical properties of micro-solar cells, micro-optical systems, and aerospace engineering. However, the morphological changes and optical properties of monocrystalline silicon during femtosecond laser machining assisted with chemical etching are not clear, especially in a liquid environment. In this paper, monocrystalline silicon was ablated by femtosecond laser in air and deionized water respectively and then etched by alkaline potassium hydroxide (KOH). The reflectivity, absorptivity, and transmissivity of monocrystalline silicon were tested. Experimental results showed that inverted pyramid (IP) structure could be processed on a monocrystalline silicon wafer by ablating the surface with a high-power laser under the air environment and a low-power laser under the deionized water environment, followed by KOH etching. Meanwhile, the optical properties of the monocrystalline silicon with IP structure were effectively improved.

Array of micro multijunction solar cells interconnected by conductive inks

Micro-concentrator photovoltaics (micro-CPV) consists of the reduction in size of the components of the conventional concentrator photovoltaic (CPV) technology, attaining equally high efficiencies and reducing material costs and manufacturing costs. In this publication we focus on the implementation of high throughput manufacturing methods for the interconnection of solar cells. The goal is to enable large area interconnection of thousands of micro-solar cells with a low cost of 3 €/m 2 . A first prototype was manufactured, interconnecting multiple 1 mm 2 solar cells via directly printing onto the front contact pads. The result is a glass substrate, with a size of 190 × 145 mm 2 , with a fully printed circuit interconnecting 12 rows each containing 11 multijunction solar cells in parallel. The fill factor ( FF ) of the full board is 76% at 214 suns. • High throughput interconnection method for of micro-CPV solar cells on a large area. • Expensive wire bonding is avoided by directly printing onto the contacts of micro-solar cells. • The goal is to enable large area interconnection of thousands of micro-solar cells with a low cost of 3€/m 2 . • 132 1 mm 2 solar cells were interconnected on a glass substrate of 190 × 145mm 2 in dimension. • An overall fill-factor of 76% was achieved for the full board and single cells on the board reached 83% at 200 suns.

(Nickel) Substrate Nano-ZnO Dye-sensitized Solar Cell Preparation and Performance Research

With the rapid growth and development of the world and global economy, human material and living standards continue to improve and improve, people’s demand for renewable energy has become more and more, and many traditional energy sources, such as oil, natural gas, coal, etc., due to their limited storage capacity, serious environmental pollution during mining, and many other reasons, no longer fully meet the human demand for sustainable use of renewable energy in our era. Therefore, the development of a clean industrial energy is gradually becoming an important solution for contemporary Chinese enterprises to solve the problem of clean industrial energy utilization. Solar cells are an inexhaustible clean and green energy. It has great research and development and application value. It has attracted extensive research attention from social scientists. Among them, solar energy dye-sensitized battery solar plastic cells mainly have The dye process is simple, the cost of the preparation process is low, the conversion efficiency is high, and the large-scale production can be achieved throughout the year. Therefore, it has gradually become a hot spot in my country’s solar power battery technology research in recent years. This article aims to study the preparation and performance of (nickel) substrate nano-ZnO dye-sensitized solar cells. The optimal conditions for the synthesis of ZnO and the influence of different reaction conditions on the growth of ZnO were explored, and the growth mechanism of ZnO was speculated. The photoelectric synthesis catalytic conditions of the microspheres agglomerated between different sizes of ZnO are explored, and the direct influence of the small dye-sensitized micro-solar synthesis cells currently used in the synthesis on the synthesis of small dye-sensitized micro-solar cells is directly affected. Laboratory research results show that in order to use more reactive dyes for dye adsorption in order to improve the efficiency of the conversion using dye sensitizers and solar fuel cells, the dye film needs to pass through ZnO and be adsorbed by the dye at the same time. The morphological structure should not be too dense and the film should not be too thick; at the same time, the absorption of the dye film should be proportional to the overall coating thickness of the film. Therefore, the overall thickness of the film should be moderate when used in ZnO.

Substitutionally Doped MoSe2 for High-Performance Electronics and Optoelectronics.

2D materials, of which the carrier type and concentration are easily tuned, show tremendous superiority in electronic and optoelectronic applications. However, the achievements are still quite far away from practical applications. Much more effort should be made to further improve their performance. Here, p-type MoSe2 is successfully achieved via substitutional doping of Ta atoms, which is confirmed experimentally and theoretically, and outstanding homojunction photodetectors and inverters are fabricated. MoSe2 p-n homojunction device with a low reverse current (300 pA) exhibits a high rectification ratio (104 ). The analysis of dark current reveals the domination of the Shockley-Read-Hall (SRH) and band-to-band tunneling (BTB) current. The homojunction photodetector exhibits a large open-circuit voltage (0.68 V) and short-circuit currents (1 µA), which is suitable for micro-solar cells. Furthermore, it possesses outstanding responsivity (0.28 A W-1 ), large external quantum efficiency (42%), and a high signal-to-noise ratio (≈107 ). Benefiting from the continuous energy band of homojunction, the response speed reaches up to 20 µs. Besides, the Ta-doped MoSe2 inverter exhibits a high voltage gain (34) and low power consumption (127 nW). This work lays a foundation for the practical application of 2D material devices.

Micro-sized thin-film solar cells via area-selective electrochemical deposition for concentrator photovoltaics application

Micro-concentrator solar cells enable higher power conversion efficiencies and material savings when compared to large-area non-concentrated solar cells. In this study, we use materials-efficient area-selective electrodeposition of the metallic elements, coupled with selenium reactive annealing, to form Cu(In,Ga)Se2 semiconductor absorber layers in patterned microelectrode arrays. This process achieves significant material savings of the low-abundance elements. The resulting copper-poor micro-absorber layers’ composition and homogeneity depend on the deposition charge, where higher charge leads to greater inhomogeneity in the Cu/In ratio and to a patchy presence of a CuIn5Se8 OVC phase. Photovoltaic devices show open-circuit voltages of up to 525 mV under a concentration factor of 18 ×, which is larger than other reported Cu(In,Ga)Se2 micro-solar cells fabricated by materials-efficient methods. Furthermore, a single micro-solar cell device, measured under light concentration, displayed a power conversion efficiency of 5% under a concentration factor of 33 ×. These results show the potential of the presented method to assemble micro-concentrator photovoltaic devices, which operate at higher efficiencies while using light concentration.

Micro-Solar Cells By Electrodeposition into a Microelectrode Array – Effect of Dot Diameter

Next generation concentrator micro-solar cells offer enhanced power conversion efficiencies whilst using less precious semiconductor material than normal photovoltaic modules, as less device area is required to harvest the same amount of light. Vacuum-based deposition processes are typically used to produce large-area thin film semiconductors. However, to make micro-sized semiconductors for solar cells, area selective electrodeposition (ASED) onto well-defined microelectrodes is an attractive strategy. Here, we investigate the electrodeposition of Cu, In, and Ga onto variable sized microelectrodes in order to produce working micro solar cells based on copper indium gallium diselenide, Cu(In,Ga)Se2. To template arrays of microelectrodes, soda lime glass/molybdenum (SLG/Mo) substrates were patterned by plasma enhanced chemical vapour deposition of a 2 µm thick SiO2 insulating layer. Direct laser lithography coupled with reactive ion etching was used to define circular wells – referred to as dots – with diameters ranging from 20 to 300 µm. The template enables area selective electrochemical growth of metallic layers – copper (Cu), indium (In) and gallium (Ga) – from a combination of aqueous and ionic liquid (IL) media, which are the precursor layers required to produce, the Cu(In,Ga)Se2 semiconductor by means of a subsequent reactive selenium annealing. Here we investigate the Cu electrodeposition in detail. The arrays’ electrochemical behaviour was characterized with cyclic voltammetry (CV) using the ruthenium hexamine (Ru2+/3+(NH3)6) redox couple at different scan rates, keeping the working microelectrodes stationary. Qualitatively, sigmoidal shaped CVs recorded at 150 mVs-1 were observed for dots with diameters up to 100 µm, indicating that a characteristic hemispherical diffusion layer is formed, leading to a steady-state limiting current that appears to depend on dot diameter. At a slower scan rate of 10 mVs-1, sigmoidal shape is observed on dot diameters up to 300 µm. Furthermore, the current observed for an array is equal to the sum of the currents of the individual dots, indicating that no overlapping of the individual diffusion layers is occurring and that each dot is behaving as an individual microelectrode. Using chronoamperometric techniques, potentiostatic continuous and pulsed electrodepositions of Cu were performed on all dot sizes, keeping a constant deposition charge per unit area, using an aqueous Cu2+ basic bath at room temperature. Characterization using scanning electron microscopy showed the deposited layers in the dots are continuous throughout the array and energy-dispersive X-ray spectroscopy data showed no difference in the amount of material deposited in each dot across the array. Surprisingly, the electrodeposition plating efficiency does appear to depend upon the dot size. Overall, large area samples prepared using a continuous potentiostatic method have shown RMS roughness of about 70 nm, while the pulsed method has produced layers with lower roughness of around 10 nm, as measured by profilometry for Cu layers with thicknesses between 150 < d (nm)< 350. Several micro solar cell proof-of-concept devices were successfully prepared from electrodeposited precursors and showed a maximum power conversion efficiency of 4.8% at 1 sun illumination by obtaining current-voltage curve from a single dot with a diameter of 200 µm. Figure 1

Thermal management approaches of Cu(Inx,Ga1−x)Se2 micro-solar cells

Concentrator photovoltaics (CPV) is a cost-effective method for generating electricity in regions that have a large fraction of direct solar radiation. With the help of lenses, sunlight is concentrated onto miniature, highly efficient multi-junction solar cells with a photovoltaic performance above 40%. To ensure illumination with direct radiation, CPV modules must be installed on trackers to follow the sun’s path. However, the costs of huge concentration optics and the photovoltaic technology used, narrow the market possibilities for CPV technology. Efforts to reduce these costs are being undertaken by the promotion of Cu(Inx,Ga1−x)Se2 solar cells to take over the high cost multi-junction solar cells and implementing more compact devices by minimization of solar cell area. Micrometer-sized absorbers have the potential of low cost, high efficiencies and good thermal dissipation under concentrated illumination. Heat dissipation at low (<10×) to medium (10 × to 100×) flux density distributions is the key point of high concentration studies for macro- and micro-sized solar cells (from 1 µm2 to 1 mm2). To study this thermal process and to optimize it, critical parameters must be taken in account: absorber area, substrate area and thickness, structure design, heat transfer mechanism, concentration factor and illumination profile. A close study on them will be carried out to determine the best structure to enhance and reach the highest possible thermal management pointing to an efficiency improvement.

Planar Light Concentration in Micro-Si Solar Cells Enabled by a Metallic Grating–Photonic Crystal Architecture

A one-dimensional photonic crystal (1D PC) on top of a metal grating can efficiently couple incoming broadband, unpolarized light into multiple guided-wave modes (surface-plasmon-polariton waves and waveguide modes) with relatively long propagation lengths. These properties make this plasmonic architecture potentially attractive for solar concentration over length scales of tens to hundreds of micrometers. For proof-of-concept purposes, we have experimentally demonstrated a solar concentration device based on this plasmonic approach. The fabricated plasmonic concentrators were constructed as 1D-PC-loaded 1D metallic gratings, and microsolar cells were attached to the concentrators to test their performance. We found the plasmonic concentrators can have optical transfer efficiency of 24% at a concentration factor around 2×. In addition, light collection by the plasmonic concentrators was found to be insensitive to the angle of incidence of the light. Although the plasmonic concentrator structures realized ...

Nonuniformity Sunlight-Irradiation Effect on Photovoltaic Performance of Concentrating Photovoltaic Using Microsolar Cells Without Secondary Optics

We propose a compact 20-mm-thick concentrating photovoltaic (CPV) module that is much thinner than conventional CPV modules. To achieve this structure, we assembled a CPV module by placing microsolar cells with an area smaller than 1 mm2 directly on the back of a lens array. Since our CPV module has no secondary optics, the incident sunlight is focused through the lens array onto the solar cell so that the light intensity is nonuniformly distributed due to chromatic and comatic aberration. In this paper, we analyze the dependence of the short current ( $I_{{\rm sc}}$ ) and the fill factor of our CPV module on the area, the position, and the intensity distribution of the irradiated sunlight on the solar cell. Our results indicate that the main factor that affects the PV performance of our CPV module is not the sunlight's nonuniform irradiation in the three-junction subcells due to chromatic and comatic aberration but the resistance loss between the emitter layer of the top cell and the grid electrodes.

High-efficiency thin and compact concentrator photovoltaics with micro-solar cells directly attached to a lens array

We propose a thin and compact concentrator photovoltaic (CPV) module, about 20 mm thick, one tenth thinner than those of conventional CPVs that are widely deployed for mega-solar systems, to broaden CPV application scenarios. We achieved an energy conversion efficiency of 37.1% at a module temperature of 25 °C under sunlight irradiation optimized for our module. Our CPV module has a lens array consisting of 10 mm-square unit lenses and micro solar cells that are directly attached to the lens array, to reduce the focal length of the concentrator and to reduce optical losses due to reflection. The optical loss of the lens in our module is about 9.0%, which is lower than that of conventional CPV modules with secondary optics. This low optical loss enables our CPV module to achieve a high energy conversion efficiency.

A Low-Power and Low-Voltage Power Management Strategy for On-Chip Micro Solar Cells

Fundamental characteristics of on-chip micro solar cell (MSC) structures were investigated in this study. Several MSC structures using different layers in three different CMOS processes were designed and fabricated. Effects of PN junction structure and process technology on solar cell performance were measured. Parameters for low-power and low-voltage implementation of power management strategy and boost converter based circuits utilizing fractional voltage maximum power point tracking (FVMPPT) algorithm were determined. The FVMPPT algorithm works based on the fraction between the maximum power point operation voltage and the open circuit voltage of the solar cell structure. This ratio is typically between 0.72 and 0.78 for commercially available poly crystalline silicon solar cells that produce several watts of power under typical daylight illumination. Measurements showed that the fractional voltage ratio is much higher and fairly constant between 0.82 and 0.85 for on-chip mono crystalline silicon micro solar cell structures that produce micro watts of power. Mono crystalline silicon solar cell structures were observed to result in better power fill factor (PFF) that is higher than 74% indicating a higher energy harvesting efficiency.

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.

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.

Improved performance of GaAs-based micro-solar cell with novel polyimide/SiO2/TiAu/SiO2 structure

A novel interconnected structure consisting of a conventional polyimide layer (PM) and an additional SiO2/TiAu/SiO2 sandwich multilayer has been firstly proposed for improving the performance of GaAs micro-solar cell array. The specific experimental investigations on the array with 108 unit cells have demonstrated that the novel structure of PM/SiO2/TiAu/SiO2 can effectively enhance the open-circuit voltage from 75 to 84 V and the fill factor from 35% to 57%. The performance improvement of our devices can be attributed to two aspects. Firstly, the TiAu shielding film can prevent the incident light from penetrating into the GaAs substrate so as to induce the serious substrate leakage current. Secondly, the compact texture of SiO2 in the novel structure can reduce the perimere recombination due to the crackle or shrinkage of PM. It is indicated that the novel structure can distinctly reduce the leakage current and improve output characteristic of micro-solar cells.

Analysis of leakage current in GaAs micro-solar cell arrays

The output characteristics of micro-solar cell arrays are analyzed on the basis of a modified model in which the shunt resistance between cell lines results in current leakage. The modification mainly consists of adding a shunt resistor network to the traditional model. The obtained results agree well with the reported experimental results. The calculation results demonstrate that leakage current in substrate affects seriously the performance of GaAs micro- solar cell arrays. The performance of arrays can be improved by reducing the number of cells per line. In addition, at a certain level of integration, an appropriate space occupancy rate of the single cell is recommended for ensuring high open circuit voltages, and it is more appropriate to set the rates at 80%-90% through the calculation.

Photovoltaic Energy Harvester with Power Management System

We present a photovoltaic energy harvester, realized in 0.35-μm CMOS technology. The proposed system collects light energy from the environment, by means of 2-mm2on-chip integrated microsolar cells, and accumulates it in an external capacitor. While the capacitor is charging, the load is disconnected. When the energy in the external capacitor is enough to operate the load for a predefined time slot, the load is connected to the capacitor by a power management circuit. The choice of the value of the capacitance determines the operating time slot for the load. The proposed solution is suitable for discrete-time-regime applications, such as sensor network nodes, or, in general, systems that require power supply periodically for short time slots. The power management circuit includes a charge pump, a comparator, a level shifter, and a linear voltage regulator. The whole system has been extensively simulated, integrated, and experimentally characterized.

Modelling of the perimeter recombination effect in GaAs-based micro-solar cell

Abstract Results obtained using one-dimensional simulator SCAPS–1D are modified to account for perimeter recombination which is a two-dimensional effect. The modification consists mainly of adding a perimeter contribution to the dark current of the cell. The perimeter recombination current in micro-solar cells is analysed on the basis of a model in which the Fermi level pinning reduces the electric field at the surface of the space-charge region. Consequently, the surface recombination effective width is increased. A simple analytical form of the perimeter current at the space-charge region surface is developed, surface recombination outside this region is neglected. The obtained results agree well with reported experimental results of a 0.5 cm × 0.5 cm GaAs solar cell. The calculation demonstrates that perimeter current affects seriously the performance of small area solar cells. For solar cells with a perimeter to area ratio (P/A) greater than 100 cm−1 the expected bulk recombination current is two to three orders of magnitude too small to account for. The performance of small area solar cells are reduced by the perimeter effect, the most important deterioration is recorded in the theoretical energy conversion efficiency η .

A GaAs Micro Solar Cell with Output Voltage over 20 V

Twenty-four micro-solar cells connected in series have been fabricated on semi-insulating (SI) GaAs substrates for application in the fields of micro-electromechanical systems. The array was formed in an area of 0.8×1.0 mm2, and exhibited an open-circuit voltage of 22.5 V under illumination of 5 mW at the wavelength of 815 nm. Calculation and experiment has demonstrated that, unlike conventional solar cells, the shunt resistance deteriorates the output characteristics far more seriously than the series resistance in the case of micro-solar cells. Leakage currents in both the unit diode and the substrate were evaluated separately. A quantitative estimate based on the measurements revealed that photocurrents generated in the surface of the SI substrate could function as a vital shunt resistance. Light-blocking metal films were successfully employed to obtain high output voltage.