Super high-efficiency multi-junction and concentrator solar cells

III–V compound multi-junction (MJ) (tandem) solar cells have the potential for achieving high conversion efficiencies of over 50% and are promising for space and terrestrial applications.We have proposed AlInP–InGaP double hetero (DH) structure top cell, wide-band gap InGaP DH structure tunnel junction for sub cell interconnection, and lattice-matched InGaAs middle cell. In 2004, we have successfully fabricated world-record efficiency concentrator InGaP/InGaAs/Ge 3-junction solar cells with an efficiency of 37.4% at 200-suns AM1.5 as a result of widening top cell band gap, current matching of sub cells, precise lattice matching of sub cell materials, proposal of InGaP–Ge heteroface bottom cell, and introduction of DH-structure tunnel junction. In addition, we have realized high-efficiency concentrator InGaP/InGaAs/Ge 3-junction solar cell modules (with area of 7000 cm²) with an out-door efficiency of 27% as a result of developing high-efficiency InGaP/InGaAs/Ge 3-junction cells, low optical loss Fresnel lens and homogenizers, and designing low thermal conductivity modules.Future prospects are also presented. We have proposed concentrator III–V compound MJ solar cells as the 3rd-generation solar cells in addition to 1st-generation crystalline Si solar cells and 2nd-generation thin-film solar cells. We are now challenging to develop low-cost and high output power concentrator MJ solar cell modules with an output power of 400 W/m² for terrestrial applications and high-efficiency, light-weight and low-cost MJ solar cells for space applications.     

Third generation multi-layer tandem solar cells for achieving high conversion efficiencies

Different ways of connecting solar cell structures to form multi-layer tandem solar cells have been considered by re-visiting relevant device designs. It is found that the present use of a series connection or tunnel junction approach is detrimental to charge-carrier collection in the tandem cells. Each tunnel junction introduced to the solar cell structure decelerates the charge carriers and allows them to recombine at the vicinity of the tunnel junction. The adoption of parallel connections has several advantages over series connections and there is high potential for achieving enhanced efficiencies in third generation tandem solar cells. In these devices, charge carriers are continuously accelerated across the whole device and collected in the external circuit. Multi charge-carrier production and impurity photo-voltaic mechanisms are also built into this system to enhance its performance by increasing the short-circuit current density.     

Possible cost advantage of a primary paraboloid/secondary hyperboloid tandem solar concentrator

We report a comparison between an efficiency-optimised parabaloidal concentrator and an efficiency-optimised tandem concentrator consisting of a primary paraboloid and focus-mounted secondary hyperboloid. The performance improvement (or otherwise) of a tandem collector (concentrator plus cavity receiver) is expressed as a relaxed paraboloid slope error and translated into a manufacturing cost advantage. At 900°C a cost advantage of a few percent is predicted. We conclude that a tandem collector based on a Brayton-cycle engine operating at 1200°C could be economically attractive.     

A fresnel-winston tandem concentrator system

Some geometrical concentration characteristics of a solar concentrator system employing a Fresnel reflector concentrator and a compound parabolic concentrator in tandem are discussed.     

SOLAR FIBER-OPTIC MINI-DISHES: A NEW APPROACH TO THE EFFICIENT COLLECTION OF SUNLIGHT

A new concept for efficient solar energy concentration and power delivery is proposed — one that offers substantial advantages in efficiency, compactness, reduced mechanical loads, and ease of fabrication and installation relative to conventional solar designs. The design exploits the availability of low-attenuation optical fibers, as well as the practical advantages of mass producing highly accurate very small parabolic dishes. The system's building block is a miniature (e.g. 0.2 m diameter) solar dish which concentrates sunlight into a single optical fiber. The fiber transports power to a remote receiver. A second-stage concentrator can boost flux levels to those approaching the thermodynamic limit and can be performed either in each individual dish or collectively in one or more larger devices at the entrance to the remote receiver. Collector modules, close-packed with mini-dishes, are mounted on individual trackers close to the ground. Systems are modular and can be employed in central power generation ranging from a few kilowatts to tens of megawatts. Designs for maximum efficiency attaining collection efficiencies as high as 80%, and maximum-concentration designs realizing flux levels of 30 000 suns, are achievable.     

Physical models for predicting the performance of Si/Si, AlGaAs/GaAs, and Si/SiGe solar cells

Analytical and physical models for homojunction and heterojunction solar cells are developed, and the performances of solar cells made by the Si/Si homojunction and made by the increasingly important and popular AlGaAs/GaAs and Si/SiGe heterojunctions compared. The models developed, which include relevant device physics such as the effective surface recombination velocity at the high-low junction and band discontinuities associated with heterojunctions, correctly explain the solar cell characteristics experimentally observed. Our calculations suggest that the highest efficiencies attainable for AlGaAs/GaAs, Si/Si, and Si/SiGe cells, with optimized doping concentrations but without surface passivation and geometry optimization, are 21.25%, 17.8% and 13.5%, respectively, under 1 AM1.5 sun condition. For concentrator cell applications, the efficiencies improve to about 24.5%, 22.2%, and 22.0% for AlGaAs/GaAs, Si/Si, and Si/SiGe cells, respectively, under 100 AM1.5 suns. While the AlGaAs/GaAs cell possesses the highest efficiency among the three cells, the Si/Si and Si/SiGe cells can achieve a satisfactory conversion efficiency at high sun concentration (22% at 100 suns), making them attractive for concentrator cell applications because their processing is the same as or is compatible with existing silicon technology. Model predictions for two Si/Si and one AlGaAs/GaAs cells compare favorably with data reported in the literature.     

Geometrical designs and performance analysis of a linear fresnel reflector solar concentrator with a flat horizontal absorber

Two different approaches for designing a linear Fresnel reflector solar concentrator (LFRSC) with a flat horizontal absorber are described. The performance characteristics of both the designs are studied in detail. The distribution of local concentration ratio on the surface of the absorber, for each design, is investigated using the ray trace technique. Results of some typical numerical calculations are presented graphically and discussed.     

Quantum dot solar concentrators: Electrical conversion efficiencies and comparative concentrating factors of fabricated devices

A novel, non-tracking concentrator is described, which uses nano-scale quantum dot technology to render the concept of a fluorescent dye solar concentrator (FSC) a practical proposition. The quantum dot solar concentrator (QDSC) comprises quantum dots (QDs) seeded in materials such as plastics and glasses that are suitable for incorporation into building façades. Photovoltaic (PV) cells attached to the edges convert direct and diffuse solar energy collected into electricity for use in the building. Small scale QDSC devices were fabricated. Devices have been characterised to determine current, voltage and power readings. Electrical conversion efficiencies, fill factors and comparative concentrating factors are reported.     

Tandem gallium solar cell voltage-matched circuit performance projections

Using tandem cell test units with GaAs and GaSb concentrator cells, we have achieved a NASA verified conversion efficiency of 30.8% for space applications. Here, we describe tandem gallium cell assemblies and flex circuit tape interconnect concepts for use in practical power generating concentrator panels. The forward and reverse characteristics of tandem cell voltage-matched circuits are described. It is noted that the GaSb IR cell doubles as a bypass diode, providing shading protection for the GaAs cell.     

Integrated tandem solar cells

The performance of integrated tandem solar cell (ITSC) devices made from pn homojunctions has been investigated. The intrinsic electrical interconnection present in an ITSC device places important restrictions on the choice of the energy gaps of the semiconductors to be used. Total conversion efficiencies are calculated and they are compared both with single solar cells and with independent tandem solar cell systems. It is concluded that ITSC devices are attractive solar energy converters provided that proper care is taken in the choice of materials used.     

Numerical and experimental research of the characteristics of concentration solar cells

The development of automatic tracking solar concentrator photovoltaic systems is currently attracting growing interest. High concentration photovoltaic systems (HCPVs) combining triple-junction InGaP/lnGaAs/Ge solar cells with a concentrator provide high conversion efficiencies. The mathematical model for triple-junction solar cells, having a higher efficiency and superior temperature characteristics, was established based on the one-diode equivalent circuit cell model. A paraboloidal concentrator with a secondary optic system and a concentration ratio in the range of 100X–150X along with a sun tracking system was developed in this study. The GaInP/GalnAs/Ge triple-junction solar cell, produced by AZUR SPACE Solar Power, was also used in this study. The solar cells produced by Shanghai Solar Youth Energy (SY) and Shenzhen Yinshengsheng Technology Co. Ltd. (YXS) were used as comparison samples in a further comparative study at different concentration ratios (200X–1000X). A detailed analysis on the factors that influence the electrical output characteristics of the InGaP/lnGaAs/Ge solar cell was conducted with a dish-style concentrating photovoltaic system. The results show that the short-circuit current (I_sc) and the open-circuit voltage (V_oc) of multi-junction solar cells increases with the increasing concentration ratio, while the cell efficiency (η_c) of the solar cells increases first and then decreases with increasing concentration ratio. With increasing solar cell temperature, I_sc increases, while V_oc and η_c decrease. A comparison of the experimental and simulation results indicate that the maximum root mean square error is less than 10%, which provides a certain theoretical basis for the study of the characteristics of triple-junction solar cell that can be applied in the analysis and discussion regarding the influence of the relevant parameters on the performance of high concentration photovoltaic systems.     

AB based solar cells and concentrating optical elements for space PV modules

This paper presents briefly the results of the development of , and GaSb cells manufactured for tandem solar cells, as well as tandems designed for point and line-focus concentrator modules. The maximum efficiency 23–23.8% (25°C, AM0) under 20–100 suns has been reached in the infrared transparent cells with prismatic cover. The efficiency 27.5% under AM1.5, 140 suns conditions has been reached as well. The bottom cells are based on lattice-matched or GaSb homo-junction Zn-diffused structures. The summation of the highest efficiencies measured in the top and bottom cells gave the values 28.8%–29.4% (AM0, 20–70 suns, 25°C) and 33.2% (AM1.5, 100 suns, 30°C). Two types of concentrator photovoltaic modules employing the reflective optical elements have been developed. The first type is based on compound parabolic concentrators, the second one on line-focus parabolic troughs. The estimated specific parameters of these modules with single-junction solar cells are the following: 230–240 W · m⁻² (AM0) and 3 kg · m⁻². The usage of tandem cells will allow to increase specific power of these modules on the value of 20–25%.     

A graphical method of measuring the performance characteristics of solar collectors

A graphical method to measure average and instantaneous efficiencies of a solar concentrator used for heating and boiling liquids and a flat plate collector is presented. The overall heat loss coefficient for the collectors and the optical loss factors: γ(τa)_b—the product for a concentrator and (τa)—the product for a flat plate collector, are also obtained. The method involves measuring the temperature of stagnated liquid in the absorber/collector as a function of time at noon. The efficiencies obtained are correct to within 5% of the efficiencies obtained from accurate measurements involving solar radiation data, the design parameters of collectors and the physical characteristics of the materials used in the fabrication of collectors.     

Gallium antimonide infrared solar cells with improved efficiency and manufacturability

Mechanically-stacked gallium arsenide (GaAs) and gallium antimonide (GaSb) concentrator solar cells have achieved the highest photovoltaic receiver efficiency to date. This paper describes a new process for GaSb infrared cells that has produced cells with higher efficiency, manufacturability and yield. Cells with efficiency of 6.6% under a GaAs filter have been demonstrated at 50X AM0. These cells were exposed to 1 MeV protons, and were shown to have higher end-of-life efficiencies than the previous cells. V-grooving the front surface resulted in s slightly higher radiation resistance.     

Operating characteristics of multijunction solar cells

Multijunction solar cells produced by Spectrolab are the most efficient solar cells in the world, with a record efficiency of over 40%. Cell designs have been modified for high performance in concentrator photovoltaic (CPV) systems with the potential for low-cost, high-volume manufacturing. High-performance CPV cells have been designed, tested, and entered into production for field testing in CPV systems. Performance under variable concentrations and temperatures has been characterized and compared to semiconductor theory. The cell response has been applied to a spectral irradiance model to predict field performance at reference locations. Cell qualification has been completed for the current-generation C1MJ design.     

Effect of finite angular width of the sun on design and concentration characteristics of a fresnel-winston tandem concentrator system

The effect of the finite angular width of the sun on the design and concentration characteristics of a Fresnel reflector concentrator and a compound parabolic concentrator in tandem is investigated.     

High-flux photovoltaic solar concentrators with kaleidoscope-based optical designs

We propose, analyze and offer sample designs and results for a high-flux photovoltaic concentrator comprised of a large-aperture paraboloidal-dish primary concentrator, and a second-stage kaleidoscope flux homogenizer. The following key design aims are all satisfied: (1) highly uniform irradiance on the solar cell absorber; (2) maximum collection efficiency; and (3) not exceeding the prescribed target flux level (for illustrative purposes here taken to be 500 suns), despite the dish being capable of much higher concentration. As a result of recent advances in the low cost and ease of production of large dish concentrators, the kaleidoscope-based design offers an intriguing alternative to other high-concentration optical designs developed to date. Admissible kaleidoscope geometries are identified. We generate quantitative results for a compact practical design that incurs low optical losses, and produces a highly homogeneous flux map.     

Multi-junction III–V solar cells: current status and future potential

Our recent R&D activities of III–V compound multi-junction (MJ) solar cells are presented. Conversion efficiency of InGaP/InGaAs/Ge has been improved up to 31–32% (AM1.5) as a result of technologies development such as double hetero-wide band-gap tunnel junction, InGaP–Ge hetero-face structure bottom cell, and precise lattice-matching of InGaAs middle cell to Ge substrate by adding indium into the conventional GaAs layer. For concentrator applications, grid structure has been designed in order to reduce the energy loss due to series resistance, and world-record efficiency InGaP/InGaAs/Ge 3-junction concentrator solar cell with an efficiency of 37.4% (AM1.5G, 200-suns) has been fabricated. In addition, we have also demonstrated high-efficiency and large-area (7000 cm²) concentrator InGaP/InGaAs/Ge 3-junction solar cell modules of an outdoor efficiency of 27% as a result of developing high-efficiency InGaP/InGaAs/Ge 3-junction cells, low optical loss Fresnel lens and homogenizers, and designing high thermal conductivity modules.Future prospects are also presented. We have proposed concentrator III–V compound MJ solar cells as the 3rd generation solar cells in addition to 1st generation crystalline Si solar cells and 2nd generation thin-film solar cells. We are now developing low-cost and high output power concentrator MJ solar cell modules with an output power of 400 W/m² for terrestrial applications.     

Characteristics of GaAs-based concentrator cells

GaAs-based cells, including GaAs single-junction cells, AlGaAs/GaAs two-junction cells, and InGaP/GaAs two-junction cells grown on GaAs substrates by metal-organic chemical vapor deposition (MOCVD) are examined in various levels of concentration and backside cooling temperature. All types of cells have shown boost of efficiency in low and medium ranges of concentration. The cell efficiencies obtained are 31.5% at 20-suns of AM1.5 for InGaP/GaAs tandem cell, and 29.2% at 7-suns of AM1.5 for AlGaAs/GaAs tandem cell, respectively. The GaAs single-junction cell is also examined as the reference. A new equivalent circuit model reveals that increase of apparent leakage current is responsible for a rapid efficiency drop in the high-concentration region. It is possible to improve it by reducing contact resistance and using uniform concentrated illumination.     

Calculations of efficiencies achievable using gallium arsenide and silicon cells mounted together in cavities illuminated by single-stage concentration

Detailed calculations have been performed to predict the efficiencies achievable using gallium arsenide and silicon cells mounted in cavities with concentrated light entering. Prototype devices with power output of the order of 2 W are proposed and analysed. The concept of the elliptical cavity with restriction of the angle of the escaping rays is exploited in order to avoid having to use a secondary concentrator. The light first falls on the gallium arsenide cell, with a concentration of 720 suns, before being reflected onto a silicon cell. Arrangements using both one and two silicon cells are studied. Depending on the type of cells used, and whether a dichroic filter is included, the possible efficiencies (relative to the light entering the cavity) are in the range 30.4–36%. The effect of the cavity is to increase thepower output by about 5% when compared to a similar bandsplitting arrangement not using a cavity. It is concluded that the cavity effect should be of interest for practical photovoltaic converters.