Concentrator Solar Cells Research Articles

High-flux characterization of ultrasmall multijunction concentrator solar cells

A characterization study of ultrasmall ultraefficient multijunction concentrator solar cells is presented, with emphasis on performance sensitivity to light intensity and distribution. Information of this type is essential in the design and optimization of the latest generations of high-flux photovoltaic systems. Cell miniaturization allows increasing the concentration at which efficiency peaks, facilitates passive heat rejection, and permits the use of all-glass optics. However, few device measurements have been published on ultrasmall cells. Extensive measurements, up to ∼5000 suns, on the 1.0mm2 active region within the busbars are reported and analyzed.

Novel materials for high-efficiency III–V multi-junction solar cells

As a result of developing wide bandgap InGaP double hetero structure tunnel junction for sub-cell interconnection, InGaAs middle cell lattice-matched to Ge substrate, and InGaP-Ge heteroface structure bottom cell, we have demonstrated 38.9% efficiency at 489-suns AM1.5 with InGaP/InGaP/Ge 3-junction solar cells by in-house measurements. In addition, as a result of developing a non-imaging Fresnel lens as primary optics, a glass-rod kaleidoscope homogenizer as secondary optics and heat conductive concentrator solar cell modules, we have demonstrated 28.9% efficiency with 550-suns concentrator cell modules with an area of 5445 cm 2. In order to realize 40% and 50% efficiency, new approaches for novel materials and structures are being studied. We have obtained the following results: (1) improvements of lattice-mismatched InGaP/InGaAs/Ge 3-junction solar cell property as a result of dislocation density reduction by using thermal cycle annealing, (2) high quality (In)GaAsN material for 4- and 5-junction applications by chemical beam epitaxy, (3) 11.27% efficiency InGaAsN single-junction cells, (4) 18.27% efficiency InGaAs/GaAs potentially modulated quantum well cells, and (5) 7.65% efficiency InAs quantum dot cells.

40% efficient metamorphic GaInP∕GaInAs∕Ge multijunction solar cells

An efficiency of 40.7% was measured and independently confirmed for a metamorphic three-junction GaInP∕GaInAs∕Ge cell under the standard spectrum for terrestrial concentrator solar cells at 240 suns (24.0W∕cm2, AM1.5D, low aerosol optical depth, 25°C). This is the initial demonstration of a solar cell with over 40% efficiency, and is the highest solar conversion efficiency yet achieved for any type of photovoltaic device. Lattice-matched concentrator cells have now reached 40.1% efficiency. Electron-hole recombination mechanisms are analyzed in metamorphic GaxIn1−xAs and GaxIn1−xP materials, and fundamental power losses are quantified to identify paths to still higher efficiencies.

Erratum: III‐V concentrator solar cell reliability prediction based on quantitative LED reliability data

Erratum: III‐V concentrator solar cell reliability prediction based on quantitative LED reliability data

Assessment of a low-cost gold-free metallization for III–V high concentrator solar cells

A gold-free metallization is proposed to be used as the grid contact in III–V concentrator solar cells. This metallization is based on the Cu/Ge system which has been reported to attain very low specific contact resistances on n-GaAs. In this letter, we show that metal layers with low resistivity (13 μΩ cm) can be obtained if the copper content in the alloy is around 28% in weight for a wide range of annealing temperatures (400–450 °C). Finally, this metallization has been used to manufacture single-junction GaAs high concentrator solar cells. Efficiencies of 26.2% at 1000 suns have been reached.

III‐V concentrator solar cell reliability prediction based on quantitative LED reliability data

AbstractIII‐V Multi Junction (MJ) solar cells based on Light Emitting Diode (LED) technology have been proposed and developed in recent years as a way of producing cost‐competitive photovoltaic electricity. As LEDs are similar to solar cells in terms of material, size and power, it is possible to take advantage of the huge technological experience accumulated in the former and apply it to the latter. This paper analyses the most important parameters that affect the operational lifetime of the device (crystalline quality, temperature, current density, humidity and photodegradation), taking into account experience on the reliability of LEDs. Most of these parameters are less stressed for a III‐V MJ solar cell working at 1000 suns than for a high‐power LED. From this analysis, some recommendations are extracted for improving the long‐term reliability of the solar cells. Compared to high‐power LEDs based on compound semiconductors, it is possible to achieve operational lifetimes higher than 105 hours (34 years of real‐time operation) for III‐V high‐concentration solar cells. Copyright © 2007 John Wiley & Sons, Ltd.

Application of new control strategy for sun tracking

Abstract The application of high concentration solar cells technology allows a significant increase in the amount of energy collected by solar arrays per unit area. However, to make it possible, more severe specifications on the sun pointing error are required. In fact, the performance of solar cells with concentrators decreases drastically if this error is greater than a small value. These specifications are not fulfilled by simple tracking systems due to different sources of errors (e.g., small misalignments of the structure with respect to geographical north) that appear in practice in low cost, domestic applications. This paper presents a control application of a sun tracker that is able to follow the sun with high accuracy without the necessity of either a precise procedure of installation or recalibration. A hybrid tracking system that consists of a combination of open loop tracking strategies based on solar movement models and closed loop strategies using a dynamic feedback controller is presented. Energy saving factors are taken into account, which implies that, among other factors, the sun is not constantly tracked with the same accuracy, to prevent energy overconsumption by the motors. Simulation and experimental results with a low cost two axes solar tracker are exposed, including a comparison between a classical open loop tracking strategy and the proposed hybrid one.

Thermophotovoltaic power generation systems using natural gas-fired radiant burners

Thermophotovoltaic (TPV) power generation in gas-fired furnaces is attracting technical attention. Considerable work has been done in the area of low bandgap GaSb cell-based TPV systems as well as silicon solar cell-based TPV systems. Previous investigations have shown that a radiant burner with a high conversion level of fuel to radiation energy must be developed to realize an efficient TPV system. In our work, we investigated different natural gas-fired radiant burners in order to raise the conversion of fuel energy to thermal radiation. These burners were used as radiation sources to establish and test two TPV prototype systems. It was found that for a non-surface combustion radiant burner, the radiation output can be enhanced using a thermal radiator with a porous structure. Also, we developed a cascaded radiant burner that generates two streams of radiation output. One stream illuminates silicon concentrator solar cells while the other drives low bandgap GaSb cells. In this way, useful radiation output and thus TPV system efficiency are significantly increased due to the cascaded utilization of combustion heat and optimized thermal management.

Photovoltaic characterization of concentrator solar cells by localized irradiation

The ability to determine the macroscopic parameters that characterize photovoltaic performance, including their spatial dependence, especially at high flux, is demonstrated with extensive solar measurements on high-efficiency concentrator solar cells. Two case studies explore (a) the impact of inhomogeneous flux distribution on photovoltaic behavior, (b) establishing how solar cell parameters vary across the cell surface (of particular interest for deployment in high-concentration optical systems), and (c), the sensitivity of photovoltaic parameters to the spatial variation of series resistance that stems from nonuniform cell metallization. In the process, we elucidate current-voltage trends unique to strongly inhomogeneous illumination and to series resistance losses at high flux.

A comparative study of BSF layers for GaAs-based single-junction or multijunction concentrator solar cells

An effective back surface field is a key structural element for a high-efficiency GaAs concentrator solar cell, either in a multijunction or in a single-junction device. In this paper, several BSF materials are analysed, namely: (1) p++GaAs(Zn), (2) p+Ga0.5In0.5P(Zn) and (3) p++Al0.2Ga0.8As(C). The results of the comparison demonstrate that the best option is C-doped Al0.2Ga0.8As, which exhibits a low series resistance and behaves as an excellent minority carrier mirror; p++GaAs(Zn) shows reduced minority carrier mirror properties resulting from Zn diffusion and p+Ga0.5In0.5P(Zn) is shown to produce important series resistance problems because of an unfavourable heterojunction with GaAs.

Super high-efficiency multi-junction and concentrator solar cells

Abstract 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 cm 2 ) 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/m 2 for terrestrial applications and high-efficiency, light-weight and low-cost MJ solar cells for space applications.

Packaging III–V tandem solar cells for practical terrestrial applications achievable to 27% of module efficiency by conventional machine assemble technology

Abstract A new concentrator receiver containing a 7 mm×7 mm 3J concentrator solar cell with a 37.4% peak efficiency was developed. The receiver design includes a homogenizer, heat-handling (epoxy lamination) technologies and a low-resistance soldered connection and can be applied to various concentrator optics, including dish systems. The outdoor efficiency with a combination of a plastic Fresnel lens, made by low-cost injection molding, reached 27% on a hot summer day under 35.0 °C ambient temperature without additional cooling. With this newly developed receiver, mechanical engineers will be able to design their own concentrator module suitable for their environment, using their mechanical knowledge and local industrial resources. A 400X and 7056 cm 2 concentrator module was fabricated with 36 concentrator receivers connected in series and the same number of newly developed dome-shaped, non-imaging Fresnel lenses. The power rating was 200 Wp. The peak outdoor efficiency on a clear sky day was 26.8±1.5%. The integrated efficiency over the course of the day was 25.3±1.4%. This is the highest module efficiency that has been achieved using a practical module size and electrical rating.

06/01815 Development of a silicon concentrator solar cell based TPV power system: Qiu, K. and Hayden, A. C. S. Energy Conversion and Management, 2006, 47, (4), 365–376

06/01815 Development of a silicon concentrator solar cell based TPV power system: Qiu, K. and Hayden, A. C. S. Energy Conversion and Management, 2006, 47, (4), 365–376

Triple-Junction III–V Based Concentrator Solar Cells: Perspectives and Challenges

This paper gives a review of the work performed in the framework of the EC-funded project FULLSPECTRUM aiming for higher photovoltaic (PV) conversion efficiencies by investigating GaInP∕GaInAs∕Ge triple-junction concentrator solar cells. Lattice mismatched structures reached efficiencies beyond 35% at 600 sun concentration level. These cells are now ready to enter the terrestrial PV market. The perspectives and challenges associated with the market introduction of these cells are addressed. Specifically issues of reliability and on-wafer characterization are discussed. A new characterization tool MAPCON was developed and is presented.

Comparison of 1D and 3D analysis of the front contact influence on GaAs concentrator solar cell performance

An extensive analysis of the front contact influence on concentrator GaAs solar cell performance has been carried out. The fill factor, open circuit voltage and efficiency have been calculated by varying the front contact specific resistance and the metal sheet resistance for 500X, 1000X and 2000X. An optimum front grid design has also been developed. The simulations have been carried out using a 3D model based on distributed circuit units, and by a classic lumped model, showing the need to use distributed models to achieve an accurate concentrator solar cell modeling as well as a precise front grid design.

Performance of concentrator solar cells with passive cooling

This paper presents the results of an experimental study regarding the role of passive cooling on silicon-based concentrator solar cell performance. A design is presented for a gravity-dependent copper heat pipe using water or acetone as the working fluid for passively cooling concentrator solar cells operating up to 500×. Heating of the solar cell by illumination and temperature-dependent properties is taken into account. In addition, the performance at high concentrations has been improved using acetone as the working fluid in the gravity-dependent copper heat pipe for passive cooling. The results are promising for reasonable costs for such high efficiency cells working up to 500 suns.

A 3-D Model for Concentrator Solar Cells Based on Distributed Circuit Units

A three-dimensional (3-D) distributed model for high-concentrator solar cells based on elementary units made up of electrical circuits is presented. The recombination mechanisms are dealt with in detail, paying special attention to the perimeter properties. No ohmic effect is omitted making this a powerful simulation tool for concentrator solar cells. A shunt resistance is also included. The model allows the simulation of the external connections and nonuniform illumination profiles making this model very useful for optimizing future structures and technological processes. The proposed 3-D model is compared with a lumped, two-diode model in the simulation of a GaAs solar cell operating from 1 to 2000 suns. It is found that the 3-D distributed model agrees satisfactorily with the experimental data for all concentrations. The agreement cannot be made simultaneously for both low and high concentrations for the lumped model.

Development of a silicon concentrator solar cell based TPV power system

Abstract Most of the previous studies on thermophotovoltaics (TPV) have focused on developing low bandgap TPV devices such as GaSb and GaInAsSb cells. Compared with the low bandgap TPV cells, silicon solar cells are low in cost and commercially available in large quantities. In the present study, a combustion driven TPV power system using silicon concentrator solar cells has been developed. In this system, a novel selective radiant burner was employed. The burner was incorporated with a heat recuperator. A selective radiator with a unique porous structure has been successfully fabricated and used in the burner. A prototype TPV system was designed and built in which a silicon concentrator solar cell array was installed. The electric output characteristics of the combustion driven TPV system have been investigated to demonstrate the feasibility of silicon solar cell based TPV power systems and provide experimental data for the design of larger TPV systems. Experimental results show that a photocurrent with a concentration factor of 20 has been achieved. When the combustion air was preheated to 550 °C, a power density of 0.2 W/cm2 was generated by the silicon concentrator solar cells.

Photovoltaic hysteresis and its ramifications for concentrator solar cell design and diagnostics

We report the observation of a photovoltaic effect with pronounced hysteresis. The phenomenon derives from the sharp transition in the dominant mode of electron transport in the tunnel diodes that regulate multijunction solar cells, and is only observable at high flux. These results emerged from measurements of cell current-voltage characteristics performed with miniature fiber-optic solar concentrators that can deliver flux levels up to 10 000 times that of ambient sunlight in a highly localized fashion. The ramifications of our findings for photovoltaic design, diagnostics, and performance are addressed, and a nondestructive determination of the peak and valley threshold current densities of tunnel diodes is presented.

Degradation study of III-V solar cells for concentrator applications

AlGaAs/GaAs heteroface solar cells with a high aluminium content tend to degrade. The degradation mechanism has been examined and appropriate accelerated ageing procedures have been established. They effectively test the ruggedness of the device against oxidation. Changing the window layer material to (AlxGa1−x)0.51In0.49P with x = 0, 0.5 or 1 leads to stable devices. In addition, III–V tandem solar cells for concentrator applications were subjected to accelerated ageing tests. They proved to be robust against oxidation. The potential degradation due to the high current density involved in concentrator solar cells was assessed in preliminary experiments. Copyright © 2005 John Wiley & Sons, Ltd.