BICON: high concentration PV using one‐axis tracking and silicon concentrator cells

BICON is a two‐stage concentrator system developed at Fraunhofer ISE which is one‐axis tracked. The innovation of this one‐axis tracked system is that it enables a high geometrical concentration of 300 × in combination with a high optical efficiency (upto 78%) and a large acceptance angle of ± 23·5° all year through. For this, the system uses a parabolic mirror (40·4 ×) and a three dimensional second stage consisting of compound parabolic concentrators (CPCs, 7·7 ×). For the concentrator concept and particularly for an easy cell integration, rear‐line‐contacted concentrator (RLCC) cells with a maximum efficiency of 25% were developed and a hybrid mounting concept for the RLCC cells is presented. The optical performance of different CPC materials was tested and is analysed in this paper. Finally, small modules consisting of six series interconnected RLCC cells and six CPCs were integrated into the concentrator system and tested outdoor. A BICON system efficiency of 16·2% was reached at around 800 W/m² direct irradiance under realistic outdoor conditions. Copyright © 2006 John Wiley & Sons, Ltd.     

单体聚光太阳电池的热分析和温度测试

A thermal model for concentrator solar cells based on energy conservation principles was designed. Under 400X concentration with no cooling aid, the cell temperature would get up to about 1200 ？C. Metal plates were used as heat sinks for cooling the system, which remarkably reduce the cell temperature. For a fixed concentration ratio, the cell temperature reduced as the heat sink area increased. In order to keep the cell at a constant temperature, the heat sink area needs to increase linearly as a function of the concentration ratio. GaInP/GaAs/Ge triple-junction solar cells were fabricated to verify the model. A cell temperature of 37 ？C was measured when using a heat sink at 400X concentration.     

High‐irradiance degradation tests on concentrator GaAs solar cells

The present work summarises the results of an experiment of light‐soaking high‐concentrator MOVPE‐grown GaAs solar cells under monochromatic light (808 nm). The irradiance level was set so that the short‐circuit current obtained was 1100 times that produced with the AM1ċ5D spectrum at 1 kW/m². This test caused no morphological changes in the devices. The main phenomenon discovered has been a slight increase with time of the reverse current I₀₂. This increase is analogous to that observed in similar degradation experiments based on high forward currents. In general, the results of these tests show that the drop in performance is very limited, supporting the idea that concentrator GaAs solar cells are rugged devices, capable of achieving long lifetimes in field operation. Copyright © 2003 John Wiley & Sons, Ltd.     

Two‐dimensional modeling of front contact silicon solar cells

The modeling of a new type of silicon solar cell intended for operation at very high concentration, with all the contacts at its front face, is presented. The two‐dimensional model developed makes use of the theory of the complex variable, and is able to explain the main features of the operation of these cells. It is shown that if all the parameters reach good state‐of‐the‐art values, and with the appropriate layout, this structure can reach 25% efficiency for a range of concentrations wider than any other known silicon cell. Copyright © 2004 John Wiley & Sons, Ltd.     

Thermal analysis and test for single concentrator solar cells

A thermal model for concentrator solar cells based on energy conservation principles was designed.Under 400X concentration with no cooling aid,the cell temperature would get up to about 1200℃.Metal plates were used as heat sinks for cooling the system,which remarkably reduce the cell temperature.For a fixed concentration ratio,the cell temperature reduced as the heat sink area increased.In order to keep the cell at a constant temperature,the heat sink area needs to increase linearly as a function of the concentration ratio.GaInP/GaAs/Ge triple-junction solar cells were fabricated to verify the model.A cell temperature of 37℃ was measured when using a heat sink at 400X concentratration.     

Explanation for the dark I–V curve of III–V concentrator solar cells

The measurement of the dark I–V curve is one of the most straightforward methods for characterizing solar cells. Consequently, an accurate knowledge of its meaning is of high relevance for the comprehension and technological feedback of these devices. In this paper, an explanation of the dark I–V curve for concentrator III–V solar cells is presented using a 3D (three‐dimensional) model in order to provide a proper data fit that provides meaningful physical parameters that are also compatible and coherent with a data fit from illumination curves. The influence on the dark I–V curve of the most significant series resistance components of concentrator solar cells is also analysed concluding that only the vertical component as well as the front contact‐specific resistance can be assessable by means of this characterization method while both emitter and metal sheet resistances cannot be detected. For comparison purposes, the same experimental data have been fitted by means of a traditional two‐diode model showing that, although an accurate dark I–V curve fitting can be achieved, the extracted parameters are unable to reproduce illumination data since lumped models assume the same ohmic losses distribution for both dark and illumination conditions. Copyright © 2007 John Wiley & Sons, Ltd.     

Evaluation of the reliability of high concentrator GaAs solar cells by means of temperature accelerated aging tests

Evaluating the reliability, warranty period, and power degradation of high concentration solar cells is crucial to introducing this new technology to the market. The reliability of high concentration GaAs solar cells, as measured in temperature accelerated life tests, is described in this paper. GaAs cells were tested under high thermal accelerated conditions that emulated operation under 700 or 1050 suns over a period exceeding 10 000 h. Progressive power degradation was observed, although no catastrophic failures occurred. An Arrhenius activation energy of 1.02 eV was determined from these tests. The solar cell reliability [R(t)] under working conditions of 65°C was evaluated for different failure limits (1–10% power loss). From this reliability function, the mean time to failure and the warranty time were evaluated. Solar cell temperature appeared to be the primary determinant of reliability and warranty period, with concentration being the secondary determinant. A 30‐year warranty for these 1 mm²‐sized GaAs cells (manufactured according to a light emitting diode‐like approach) may be offered for both cell concentrations (700 and 1050 suns) if the solar cell is operated at a working temperature of 65°C. Copyright © 2012 John Wiley & Sons, Ltd.     

Electrons and holes in solar cells with partial rear contacts

When the metal contact of a silicon solar cell is restricted to a fraction of the rear surface, the flow of electrons and holes towards that contact is constricted, which is beneficial for minority charge carriers but detrimental for majority carriers. It is possible to describe their 2D/3D transport and determine their concentration in the vertical and transversal dimensions of the solar cell by separately studying the central region near the contact and the peripheral region surrounding it. A virtue of such geometric approach is that it establishes a link between analytical models and computer simulations, providing both physical insight and sufficient accuracy to optimise partial rear contact devices. In this paper, we extend a previous version of the geometric model to solar cells having a full‐area, locally contacted dopant diffusion on the rear surface. The case for n‐type versus p‐type wafers is considered, point contacts are compared with line contacts, including the impact of the metal/semiconductor resistance and bulk recombination is evaluated. Copyright © 2013 John Wiley & Sons, Ltd.     

太阳能电池栅电极优化设计

本文通过对聚光电池中常用的两种电极结构进行理论模拟,全面具体地分析了电池尺寸以及与电池上电极有关的各种损耗对栅电极优化设计的影响;并且对电极优化设计在不同聚光因子下的影响也进行了模拟,使太阳电池栅电极的优化设计更加完整。     

Optimization of grid design for solar cells

By theoretical simulation of two grid patterns that are often used in concentrator solar cells, we give a detailed and comprehensive analysis of the influence of the metal grid dimension and various losses directly associated with it during optimization of grid design. Furthermore, we also perform the simulation under different concentrator factors, making the optimization of the front contact grid for solar cells complete.     

Wafer bonded four‐junction GaInP/GaAs//GaInAsP/GaInAs concentrator solar cells with 44.7% efficiency

Triple‐junction solar cells from III–V compound semiconductors have thus far delivered the highest solar‐electric conversion efficiencies. Increasing the number of junctions generally offers the potential to reach even higher efficiencies, but material quality and the choice of bandgap energies turn out to be even more importance than the number of junctions. Several four‐junction solar cell architectures with optimum bandgap combination are found for lattice‐mismatched III–V semiconductors as high bandgap materials predominantly possess smaller lattice constant than low bandgap materials. Direct wafer bonding offers a new opportunity to combine such mismatched materials through a permanent, electrically conductive and optically transparent interface. In this work, a GaAs‐based top tandem solar cell structure was bonded to an InP‐based bottom tandem cell with a difference in lattice constant of 3.7%. The result is a GaInP/GaAs//GaInAsP/GaInAs four‐junction solar cell with a new record efficiency of 44.7% at 297‐times concentration of the AM1.5d (ASTM G173‐03) spectrum. This work demonstrates a successful pathway for reaching highest conversion efficiencies with III–V multi‐junction solar cells having four and in the future even more junctions. Copyright © 2014 John Wiley & Sons, Ltd.     

High aspect ratio electrodeposited Ni/Au contacts for GaAs‐based III–V concentrator solar cells

We report on a photolithographic and electro‐deposition process that results in an optimized front grid structure for high efficiency multi‐junction III–V concentrator solar cells operating under flux concentrations up to 1000 suns. Two different thick photoresists were investigated to achieve a 6 µm wide grid line with an aspect ratio of 1:1. A positive photoresist, SPR220 manufactured by Rohm and Haas was compared with a negative photoresist, nXT15 manufactured by AZ. A gold sulfite electrolyte was employed to prevent underplating as well as for environmental and safety considerations. An initial layer of nickel was discovered to be necessary to prevent delamination of the fingers during the removal of the contact layer. When deposited on a purpose grown, heavily doped GaAs contact layer, this Ni/Au contact exhibits an acceptable specific contact resistance in the low 10⁻⁴ to mid 10⁻⁵ Ohm cm² range along with excellent adhesion without sintering. Copyright © 2014 John Wiley & Sons, Ltd.     

Design,fabrication, and analysis of transparent silicon solar cells for multi‐junction assemblies

Transparent silicon solar cells can lead to an increased efficiency of silicon‐based multi‐junction assemblies by transmitting near and below band gap energy light for conversion in a low band gap solar cell. This analysis shows that the maximum efficiency gain for a low band gap solar cell beneath silicon at a concentration of 50 suns is 5.8%, based on ideal absorption and conversion of the photons. This work analyzes the trade‐offs between increased near band edge absorption in the silicon and silicon solar cell transparency. Application of these results to real cases including a germanium bottom solar cell is analyzed, leading to a range of cases with increased system efficiency. Non‐ideal surfaces and real silicon and germanium solar cell device performance are presented. The range of practical system gains may be as low as 2.2 – 1% absolute when compared with the efficiency of a light‐trapped silicon solar cell for 1‐sun operation, based on this work. Copyright © 2012 John Wiley & Sons, Ltd.     

Feasibility study of two‐terminal tandem solar cells integrated with smart stack,areal current matching,and low concentration

The “SMAC module” is a low‐cost, high‐efficiency photovoltaic module that integrates three techniques: a “SM art stack,” “A real current matching,” and “solar C oncentration.” This paper presents the result of a proof‐of‐concept study of the SMAC module conducted using device simulations and indoor experiments. The simulation results show that an SMAC module with a two‐terminal GaAs/Si tandem solar cell can achieve an efficiency of approximately 30% and superior electricity generation per unit top cell area. The performance of the GaAs/Si solar cell developed in this study is similar to that of a GaAs/InGaAsP solar cell under concentrated artificial sunlight and is consistent with the simulation results. © 2016 The Authors. Progress in Photovoltaics: Research and Applications published by John Wiley & Sons Ltd.     

Development of concentrator modules with dome‐shaped Fresnel lenses and triple‐junction concentrator cells

The status of the development of a new concentrator module in Japan is discussed based on three arguments, performance, reliability and cost. We have achieved a 26·6% peak uncorrected efficiency from a 7056 cm² 400 × module with 36 solar cells connected in series, measured in house. The peak uncorrected efficiencies of the same type of the module with 6 solar cells connected in series and 1176 cm² area measured by Fraunhofer ISE and NREL are reported as 27·4% and 24·8% respectively. The peak uncorrected efficiency for a 550× and 5445 cm² module with 20 solar cells connected in series was 28·9% in house. The temperature‐corrected efficiency of the 550 × module under optimal solar irradiation condition was 31·5 ± 1·7%. In terms of performance, the annual power generation is discussed based on a side‐by‐side evaluation against a 14% commercial multicrystalline silicon module. For reliability, some new degradation modes inherent to high concentration III‐V solar cell system are discussed and a 20‐year lifetime under concentrated flux exposure proven. The fail‐safe issues concerning the concentrated sunlight are also discussed. Moreover, the overall scenario for the reduction of material cost is discussed. Copyright © 2005 John Wiley & Sons, Ltd.     

Analysis of the EWT‐DGB solar cell at low and medium concentration and comparison with a PESC architecture

Silicon represents an interesting material to fabricate low‐cost and relatively simple and high‐efficient solar cells in the low and medium concentration range. In this paper, we discuss a novel cell scheme conceived for concentrating photovoltaic, named emitter wrap through with deep grooved base (EWT‐DGB), and compare it with the simpler passivated emitter solar cell. Both cells have been fabricated by means of a complementary metal–oxide–semiconductor‐compatible process in our laboratory. The experimental characterization of both cells is reported in the range 1–200 suns in terms of conversion efficiency, open circuit voltage, short circuit current density and fill factor. In particular, for the EWT‐DGB solar cells, we obtain an encouraging 21.4% maximum conversion efficiency at 44 suns. By using a calibrated finite‐element numerical electro‐optical simulation tool, validated by a comparison with experimental data, we study the potentials of the two architectures for concentrated light conditions considering possible realistic improvements with respect to the fabricated devices. We compare the solar cell figures of merit with those of the state‐of‐the‐art silicon back‐contact back‐junction solar cell holding the conversion efficiency record for concentrator photovoltaic silicon. Simulation results predict a 24.8% efficiency at 50 suns for the EWT‐DGB cell and up to 23.9% at 100 suns for the passivated emitter solar cell, thus confirming the good potential of the proposed architectures for low to medium light concentration. Finally, simulations are exploited to provide additional analysis of the EWT‐DGB scheme under concentrated light. Copyright © 2017 John Wiley & Sons, Ltd.     

The PV‐FIBRE concentrator: a system for indoor operation of 1000X MJ solar cells

This paper describes the overall design and experimental results obtained with the PV‐FIBRE concentrator system (CPV). This system uses a parabolic dish to collect and concentrate the sunlight which is then further guided by a transparent rod and finally by individual short fibres. Eventually, the individual fibres are connected to single cells which are located indoors. Dual‐junction III‐V‐based solar cells with an efficiency of 30% and operating at 1000× are applied in the PV‐FIBRE system. This new system approach allows a suitable optical and electrical interconnection in order to reduce the losses and the indoor operation of the cell receiver. All elements of the CPV system (collector, tracker, transmission rod, cells, fibre bundles, and cooling circuit) have been manufactured according to main design requirements and have been evaluated separately. Finally, the PV‐FIBRE CPV system has been installed in Madrid and tested under real operation conditions. The system has demonstrated an optical efficiency of 62% is feasible, providing homogeneous illumination to the cells. Therefore, this concept can lead to overall efficiencies exceeding 20% when combined with MJ solar cells. In this paper we report on the main achievements, identified problems as well as lessons learned and future research lines to improve the system performance. Copyright © 2007 John Wiley & Sons, Ltd.     

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 (Al_xGa_1−x)_0.51In_0.49P 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.     

Potential of light trapping in microcrystalline silicon solar cells with textured substrates

The real and potential benefits of light trapping in microcrystalline silicon single‐junction solar cells are studied theoretically. Effects of a hypothetical high haze parameter of textured transparent conductive oxide on quantum efficiency and short‐circuit current of the solar cell are analysed by numerical simulation. The role of the angular distribution function of scattered light as a second important scattering parameter is shown. The potential thickness reductions of the intrinsic layer due to enhanced light trapping are demonstrated. Copyright © 2003 John Wiley & Sons, Ltd.     

Validated front contact grid simulation for GaAs solar cells under concentrated sunlight

In a common approach, the electric behavior of a solar cell is modeled by dividing it into smaller sub‐circuits and solving the resulting network by a circuit simulator. In this paper detailed network simulations are presented for a GaAs single‐junction solar cell. All resistive losses and losses influencing the diode saturation currents, such as recombination in the depletion region or at the perimeter are taken into account. With this model the maximum power point of a solar cell can be calculated for one‐sun and for higher illumination intensities. The results were validated experimentally using suitable test structures. This includes solar cell devices with varying dimensions, grid finger spacing and lengths. An excellent agreement between theoretical and experimental results was obtained. The network simulation model allows determining the optimum size and concentration ratio at which a solar cell operates at its maximum efficiency. In the case of a GaAs single‐junction solar cell this global efficiency maximum was found for an area of 1 mm² and at a concentration ratio of 450 suns. Under these conditions the largest loss mechanisms are the finger shading with 36.1% and the emitter resistance losses with 21.5% of the total power losses. Copyright © 2010 John Wiley & Sons, Ltd.