Multijunction Concentrator Solar Cells Research Articles

Effect of non-uniformity on concentrator multi-junction solar cells equipped with refractive secondary optics under shading conditions

Non-uniformity is a relevant concern of concentrator photovoltaic systems (CPV) due to its impact on the performance of multi-junction (MJ) solar cells. This work is focused on a state-of-the-art investigation related to the effects of shading on the irradiance profiles and electrical performance of MJ cells. For doing this, three different CPV single-units with a concentration of ≈559x have been experimentally investigated indoors at the CPV laboratory of the University of Jaén. The units are made up of poly(methyl methacrylate) (PMMA) Fresnel lens and standard GaInP/GaInAs/Ge MJ cells. Also, single-lens-optical element (SILO-Pyramid) and refractive truncated pyramid (RTP) homogenizers have been considered. The results indicate that shading produces additional spatial and spectral non-uniformities, and that these are reduced by the use of homogenizers. In addition, the non-uniform spectral illumination has been identified as the main cause of the additional reduction of the performance of MJ solar cells. In this sense, the SILO-Pyramid provides the best results and shows a significantly stable spectral performance under shading conditions. For a no-homogenizer case, the maximum power shows a reduction of 86.3% for a shading factor of 80%, while it is only reduced by 83.2% and by 81.6%, respectively, for the RTP and SILO-Pyramid.

Integration of concentrated multi-junction solar cells with small-scale organic rankine cycle

In this paper, concentrated photovoltaic system using multiple (four) multi-junction solar cells with a refractive system for a single point focus Fresnel lens was developed to achieve high performance in terms of optical efficiency, uniformity (Peak average ratio value) and compactness. Geometric concentration ratio, F-number, focal length and height of homogenizer were investigated showing that at Geometric concentration ratio of 400×, F-number of 1.12, focal length of 632.5 mm and height of homogenizer of 50 mm, the concentrated photovoltaic achieves an optical efficiency and Peak average ratio value of 79.6% and 1.52 respectively. Active cooling was simulated in order to ensure effective operation of the multi-junction solar cells (operating temperature < 110 °C) and to investigate the potential of exploiting the heat generated in the cells to operate an Organic Rankine Cycle to maximize the overall solar energy conversion efficiency. An Organic Rankine Cycle system with a target 1 kW power output is studied at different operating conditions and working fluid in order to be integrated with the Concentrated Photovoltaic Thermal system. Various configurations for integrating the Concentrated Photovoltaic Thermal units with the Organic Rankine Cycle working at various operating conditions and working fluids were investigated. Results showed that the integrated system with surface area of 16.8 m2, Organic Rankine Cycle system using R245fa can produce 6.17 kW of electricity with overall energy conversion efficiency of 46.57%. The developed system has the potential to provide the required electricity for buildings in Saudi Arabia which enjoys abundant solar radiation.

К 125-летию со дня рождения лауреата Нобелевской премии академика Николая Николаевича Семенова Архитектура мезы и эффективность InGaP/Ga(In)As/Ge солнечных элементов

It has been shown that the mesa architecture and achieved quality of the mesa sidewall of concentrator multijunction solar cells ensured increasing their efficiency up to 36.7% at the sunlight concentration before 100 (AMO; 0.136W/cm²). Creation of the mesa structure architecture with following separation of epitaxial plates of monolithic InGaP/GaAs/Ge nanoheterostructure into chips was carried out by single step chemical wet etching in the HBr:H2O2:H2O (8:1:100) through a photoresist mask to the depth of 12-18µm. Conditions of single step etching, which ensure formation of a smooth and even side surface of the InGaP/Ga(In)As/Ge nanoheterostructure mesa containing different in composition and thickness layers have been determined. Determination of the activation energy has shown that etching occurs in the diffusion region of the heterogeneous process. In raising etchant temperature from 2 to 36˚C, a change of the tilt angle in the Ge substrate region from 4.5 to 25 angular degrees is observed, what allows optimizing the amount of concentrator solar cells and their quality at final mechanical separation of the epitaxial plate into chips.

Effect of Non-Uniformity on Concentrator Multi-Junction Solar Cells Equipped with Refractive Secondary Optics Under Shading Conditions

Effect of Non-Uniformity on Concentrator Multi-Junction Solar Cells Equipped with Refractive Secondary Optics Under Shading Conditions

Mathematical power-generation model of a four-terminal partial concentrator photovoltaic module for optimal sun-tracking strategy

High-efficiency solar energy is an emerging technology which can reduce greenhouse gas emissions. A four-terminal (4T) partial concentrator photovoltaic (CPV+) is a promising hybrid concept to maximize the electricity yield by integrating existing photovoltaic technologies. This paper describes the mathematical modeling of a 4T CPV+ module for sun-tracking control optimization. The CPV+ module consists of low-cost auxiliary solar cells placed around the concentrator multijunction solar cells. We derived a mathematical model to predict the generated power of the 4T CPV+ module that incorporates inclination angle of array, location, date/time, lens optical efficiency, and solar irradiance data. The simulation revealed that the 4T CPV+ module with mono-facial auxiliary solar cells should always face toward the sun, similar to the conventional CPV modules, regardless of irradiance conditions. The short-term outdoor experiment using a prototype module validated the mathematical model and stayed within the margin error for generated power-per-unit module area (∼14 W/m2). The derived model will be fundamental for enhanced modeling and design optimization of the 4T CPV+ module.

Non-Curing Thermal Interface Materials with Graphene Fillers for Thermal Management of Concentrated Photovoltaic Solar Cells

Temperature rise in multi-junction solar cells reduces their efficiency and shortens their lifetime. We report the results of the feasibility study of passive thermal management of concentrated multi-junction solar cells with the non-curing graphene-enhanced thermal interface materials. Using an inexpensive, scalable technique, graphene and few-layer graphene fillers were incorporated in the non-curing mineral oil matrix, with the filler concentration of up to 40 wt% and applied as the thermal interface material between the solar cell and the heat sink. The performance parameters of the solar cells were tested using an industry-standard solar simulator with concentrated light illumination at 70× and 200× suns. It was found that the non-curing graphene-enhanced thermal interface material substantially reduces the temperature rise in the solar cell and improves its open-circuit voltage. The decrease in the maximum temperature rise enhances the solar cell performance compared to that with the commercial non-cured thermal interface material. The obtained results are important for the development of the thermal management technologies for the next generation of photovoltaic solar cells.

Performance, limits, and thermal stress analysis of high concentrator multijunction solar cell under passive cooling conditions

Concentration of solar radiation onto the surface of triple-junction solar cells causes high cell temperature and system failure. Recently, several cooling methods were proposed for these systems. However, quantitative evaluation of the essential heat transfer coefficients to maintain stable operation of these systems at different meteorological and operating conditions is not found in the literature. Therefore, in this study, a comprehensive three-dimensional coupled thermal and structural model is proposed for the latest triple-junction AZUR SPACE solar cell. The model is used to investigate the performance of an HCPV system under different solar concentration ratios (CRs), ambient temperature, direct solar irradiance, wind speed, backside heat transfer coefficient, and copper-II substrate area ratios. In addition, a new structure of the solar cell is proposed by modifying the typical solar cell assembly by changing the area of the rear copper layer. The results indicate that by increasing the ambient temperature, CR and direct solar irradiance significantly increase the predicted cell temperature at the same backside heat transfer coefficient. In addition, increasing copper-II substrate area ratios significantly reduces the average cell temperature at the same backside heat transfer coefficient and CR. At the highest backside heat transfer coefficient, when the copper-II substrate area increased, the cell temperature decreased to a certain limit and subsequently remained constant. Critical values of the highest backside heat transfer coefficient were about 200, 600, 1000, and 1600 W/m2 K at CRs of 50, 500, 1000, and 1500 Suns, respectively. In addition, at the highest backside heat transfer coefficient of 1600 W/m2 K, the critical area ratio values were about 2, 3, 4, and 6 at CRs of 50, 500, 1000, and 1500 Suns, respectively.

Series spreading resistance in single- and multi-junction concentrator solar cells

The series resistance of concentrator solar cells is determined by the processes of current spreading under the contact grid. The influence of the number of subcells in multi-junction solar cells on these processes is considered in the paper. Using the tube model of current spreading, it is shown that with the increase in the number of subcells, the current distribution of tubes becomes more uniform. This affects the shape of the IV-curve of the spreading resistance in such a way that the magnitude of the resistive losses at the maximum power point decreases.

Building a Six-Junction Inverted Metamorphic Concentrator Solar Cell

We propose practical six-junction (6J) inverted metamorphic multijunction (IMM) concentrator solar cell designs with the potential to exceed 50% efficiency using moderately high quality junction materials. We demonstrate the top three junctions and their monolithic integration lattice matched to GaAs using 2.1-eV AlGaInP, 1.7-eV AlGaAs or GaInAsP, and 1.4-eV GaAs with external radiative efficiencies >0.1%. We demonstrate tunnel junctions with peak tunneling current >400 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> that are transparent to <;2.1-eV light. We compare the bottom three GaInAs(p) junctions with bandgaps of 1.2, 1.0, and 0.7 eV grown on InP and transparent metamorphic grades with low dislocation densities. The solution to an integration challenge resulting from Zn diffusion in the GaAs junction is illustrated in a five-junction IMM. Excellent 1-sun performance is demonstrated in a complete 6J IMM device with V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> = 5.15 V, and a promising pathway toward >50% efficiency at high concentrations is presented.

High Efficiency Four Junction III-V Bismide Concentrator Solar Cell: Design, Theory, and Simulation

In this paper, the efficiency of a novel GaInP 2 /GaAs/GaAs 0.94 Bi 0.0583 /Ge concentrator multijunction solar cell has been investigated by simulation using indigenously written code for MATLAB software. The materials in this unique cell structure are judiciously selected to ensure the maximum absorption of photons from ultraviolet to deep infrared wavelengths of the solar spectrum. A theoretical model has been developed to channel the same current in all four junctions and to ensure the highest conversion of the sunlight into electricity. The theoretical efficiencies of this quadruple junction solar cell have been simulated to be 49.6% and 60.2% for AM1.5G solar radiation under 1 sun and 500 sun concentration respectively. Finally, these simulated efficiencies have been compared with the theoretical and experimental efficiencies of the reported champion solar cells.

Thermal Management of Concentrated Multi-Junction Solar Cells with Graphene-Enhanced Thermal Interface Materials

We report results of experimental investigation of temperature rise in concentrated multi-junction photovoltaic solar cells with graphene-enhanced thermal interface materials. Graphene and few-layer graphene fillers, produced by a scalable environmentally-friendly liquid-phase exfoliation technique, were incorporated into conventional thermal interface materials. Graphene-enhanced thermal interface materials have been applied between a solar cell and heat sink to improve heat dissipation. The performance of the multi-junction solar cells has been tested using an industry-standard solar simulator under a light concentration of up to 2000 suns. It was found that the application of graphene-enhanced thermal interface materials allows one to reduce the solar cell temperature and increase the open-circuit voltage. We demonstrated that the use of graphene helps in recovering a significant amount of the power loss due to solar cell overheating. The obtained results are important for the development of new technologies for thermal management of concentrated photovoltaic solar cells.

Effect of chromatic aberration on performance of concentrated multi-junction solar cells and their optimization

In order to investigate the influence of the chromatic aberration on the performance of multi-junction solar cells, the performance of the triple-junction GaInP/GaInAs/Ge solar cell under high concentration condition is investigated by a three-dimensional (3D) model based on distributed circuit units. Moreover, the effects of chromatic aberration on the performance of solar cells with different sizes are studied by analyzing the distributions of the voltage, the dark current and the transverse current in each layer. It is indicated that the photo-generated current is mismatched in local region of multi-junction solar cell, which is caused by chromatic aberration. However, the mismatched photo-generated current can be compensated for by the form of transverse current, and the current can be better matched when the size of solar cell is reduced. When the size of solar cell is as big as 20 mm20 mm, the mismatched photo-generated current is large, so are the transverse current and the dark current. But the transverse current is far less than the dark current, only 12% of the mismatched photo-generated carriers can flow from the edge to the center of the cell through the transverse resistance between the sub-cells, the rest of the photo-generated carriers are lost in the form of dark current, and the cell is in a state of current mismatching. Finally, the chromatic aberration gives rise to a reduction in the short-circuit current density, and the efficiency is only 94% as high as that of non-chromatic aberration. When the size of the cell decreases, the mismatched photo-generated current and the transverse current also decrease gradually, but the dark current caused by the chromatic aberration exponentially decreases more quickly, and the ratio of the transverse current to the mismatched photo-generated current increases gradually. Therefore, the overall state of the current mismatching is alleviated, and the short-circuit current density is increased gradually. Moreover, when the size of solar cell is 2 mm2 mm, the transverse current is much larger than the dark current, 99.98% of the mismatched photo-generated carriers can be compensated for in the form of transverse current. Although the photo-generated current of the cell is mismatched in local region, the overall is still in the state of current matching. The short-circuit current densities with and without chromatic aberration are equal, but the filling factor is reduced due to the transverse resistor. When the size of cell is further reduced, the mismatched photo-generated current is very small, and the influence of the transverse series resistance decreases gradually. Therefore, the value of the filling factor gradually approaches to the value without chromatic aberration. Furthermore, the performance of solar cell with and without chromatic aberration is nearly the same when the size of solar cell is as small as 0.4 mm0.4 mm. The efficiencies are both about 34.5% and the effects of chromatic aberration can be ignored.

Exceptionally omnidirectional broadband light harvesting scheme for multi-junction concentrator solar cells achieved via ZnO nanoneedles.

GaInP/GaAs/Ge triple-junction concentrator solar cells with significant efficiency enhancement were demonstrated with antireflective ZnO nanoneedles. The novel nanostructure was attained with a Zn(NO3)2-based solution containing vitamin C. Under one sun AM 1.5G solar spectrum, conversion efficiency of the triple-junction device was improved by 23.7% via broadband improvement in short-circuit currents of 3 sub-cells after the coverage by the nanoneedles with a graded refractive index profile. The efficiency enhancement further went up to 45.8% at 100 suns. The performance boost through the nanoneedles also became increasingly pronounced in the conditions of high incident angles and the cloudy weather, e.g. 220.0% of efficiency enhancement was observed at the incident angle of 60°. These results were attributed to the exceptional broadband omnidirectionality of the antireflective nanoneedles.

Transmissive concentrator multijunction solar cells with over 47% in-band power conversion efficiency

Transmissive concentrator multijunction (TCMJ) solar cells with over 47% in-band power conversion efficiency (PCE) have been designed and realized. These TCMJ solar cells have been characterized under 1 sun and concentrated 500 sun solar spectra, showing that the PCE for in-band light (photon energies above the cell's lowest bandgap) can reach up to 47.6% (29.5% for the full solar spectrum). Temperature coefficients of electrical parameters (Voc, Jsc, fill factor) have been derived from measurements within the temperature range of 20 °C–130 °C, showing linear variations versus temperature change. Optical measurements demonstrate that the cells show 76.5% solar-weighted optical transmission for the out-of-band light (photon energy below the cell's lowest bandgap). This TCMJ solar cell exhibits promising spectrum splitting capability, which has the potential for use in hybrid photovoltaic-solar thermal applications.

Implications of Redesigned, High-Radiative-Efficiency GaInP Junctions on III-V Multijunction Concentrator Solar Cells

Nonradiative recombination in inverted GaInP junctions is dramatically reduced using a rear-heterojunction design rather than the more traditional thin-emitter homojunction design. When this GaInP junction design is included in inverted multijunction solar cells, the high radiative efficiency translates into both higher subcell voltage and high luminescence coupling to underlying subcells, both of which contribute to improved performance. Subcell voltages within two and four junction devices are measured by electroluminescence and the internal radiative efficiency is quantified as a function of recombination current using optical modeling. The performance of these concentrator multijunction devices is compared with the Shockley–Queisser detailed-balance radiative limit, as well as an internal radiative limit, which considers the effects of the actual optical environment in which a perfect junction may exist.

Numerical modeling of loss mechanisms resulting from non‐uniform illumination in multijunction concentrator solar cells

AbstractQuasi‐3D distributed emitter models utilizing a unit cell‐based methodology have been successfully applied to the analysis of lateral current flow in solar cell emitter layers. However, the analysis of the specific loss mechanisms resulting from this flow has not been given adequate attention. In this work, a quasi‐3D model for the simulation of effects related to the lateral flow of current in a solar cell emitter layer, particularly under non‐uniform illumination, is developed. The model is applied to a specific case in which a GaInP/GaAs‐like two‐terminal solar cell is illuminated with a Lorentzian irradiance pattern, which is the expected pattern for parabolic trough concentrator. It is shown that bias‐point loss, which results from the variation in the local operating condition of the cell, is significant under highly non‐uniform illumination at low–moderate optical concentration. Understanding this loss mechanism is useful in considering trade‐offs related to the design of the optical concentrator system, as well as, the design of a grid electrode pattern for maximum power output. Copyright © 2013 John Wiley &amp; Sons, Ltd.

A two subcell equivalent solar cell model for III–V triple junction solar cells under spectrum and temperature variations

While single junction solar cells are mainly influenced by changes in irradiance and temperature, multi junction concentrator solar cells show complex behaviour as their performance is also strongly influenced by changes in spectrum. Despite this, when studying the system, it is possible to reduce the problem to a set of parameters that could easily be measured and fitted. A simple model to obtain the maximum power point of multi junction solar cells under temperature and spectral changes is proposed. This model is based on a single diode model and is described by a simple set of equations that are easy to fit within a computational program. The model could be useful to understand the behaviour of multi junction solar cells and also CPV technology under real conditions. The main purpose of this paper is to define a simple way to estimate the maximum power point of a multi junction solar cell under spectral and temperature changes at one sun conditions.

Transparent Graphene Electrodes for Highly Efficient III–V Multijunction Concentrator Solar Cells

AbstractTransparent graphene electrodes in III–V multijunction concentrator solar cells improved the cell efficiency by reducing the resistive power loss at high irradiance levels. The conversion efficiency of the solar cell increased by 13.8 % as a result of the decreased series resistance. Moreover, the irradiance level at which the maximum efficiency was reached increased upon using the graphene electrodes. This allows the solar cells to operate at higher concentration ratios, which can lead to a better performance and lower cost of the photovoltaic devices.

(Invited) Group-IV Subcells in Multijunction Concentrator Solar Cells

Group-IV solar cells have great value for increasing conversion efficiency when integrated with III-V subcells in multijunction solar cells, as in today's 3-junction GaInP/ GaInAs/ Ge concentrator cells that have demonstrated over 40% efficiency. New types of Ge, Si, SiGe, and SiGeSn subcells show the potential to increase multijunction efficiencies to still higher values. Cells incorporating both dilute nitride GaInNAs and active Ge substrate cells can reach over 46% production average efficiencies at 50.0 W/cm2 (500 suns), 25{degree sign}C under the standard concentrated solar spectrum, with champion efficiencies over 48%.

Extended description of tunnel junctions for distributed modeling of concentrator multi-junction solar cells

One of the key components of highly efficient multi-junction concentrator solar cells is the tunnel junction interconnection. In this paper, an improved 3D distributed model is presented that considers real operation regimes in a tunnel junction. This advanced model is able to accurately simulate the operation of the solar cell at high concentrations at which the photogenerated current surpasses the peak current of the tunnel junction. Simulations of dual-junction solar cells were carried out with the improved model to illustrate its capabilities and the results have been correlated with experimental data reported in the literature. These simulations show that, under certain circumstances, the solar cell's short circuit current may be slightly higher than the tunnel junction peak current without showing the characteristic dip in the J– V curve. This behavior is caused by the lateral current spreading toward dark regions, which occurs through the anode/p-barrier of the tunnel junction.