Is conversion efficiency still relevant to qualify advanced multi‐junction solar cells?

For better conversion of sunlight into electricity, advanced architectures of multi‐junction (MJ) solar cells include increasing numbers of subcells. The Achilles' heel of these cells lies in their increased sensitivity to the spectral distribution of sunlight, which is likely to significantly alter their performance during real working operation. This study investigates the capacity of MJ solar cells comprising up to 10 subcells to accommodate a wide range of spectral characteristics of the incident radiation. A systematic study is performed, aimed at a realistic estimation of the energy output of MJ‐based concentrating photovoltaic systems at characteristic locations selected to represent a large range of climatic conditions. We show that optimal MJ architectures could have between 4 and 7 subcells. Beyond seven subcells, the slight gains in peak efficiency are likely outweighed by detrimental increases in dependence on local conditions and in annual yield variability. The relevance of considering either conversion efficiency or modeled energy output as the most appropriate indicator of the cell performance, when considering advanced architectures of MJ solar cells, is also discussed. Copyright © 2016 John Wiley & Sons, Ltd.     

Measuring the External Quantum Efficiency of Two‐Terminal Polymer Tandem Solar Cells

Tandem configurations, in which two cells are stacked and connected in series, offer a viable approach to further increase the power conversion efficiency (PCE) of organic solar cells. To enable the future rational design of new materials it is important to accurately assess the contributions of individual subcells. Such accurate measurement of the external quantum efficiency (EQE) of the subcells of two‐terminal organic or polymer tandem solar cells poses specific challenges, caused by two characteristics of these cells, i.e. a sub‐linear light intensity dependence of the current and a field‐assisted charge collection. These properties necessitate that EQE experiments are carried out under representative illumination conditions and electrical bias to maintain short‐circuit conditions for the addressed subcell. We describe a method to determine the magnitudes of the bias illumination and bias voltage during EQE measurements, based on the behavior of single junction cells and optical modeling. The short‐circuit current densities of the subcells obtained by convolution of the EQE with the AM1.5G solar spectrum are consistent with those obtained from optical modeling and correctly predict the current density–voltage characteristics of the tandem cell under AM1.5G conditions.     

>35% 5-junction space solar cells based on the direct bonding technique

Multijunction solar cells are the highest efficiency photovoltaic devices yet demonstrated for both space and terrestrial applications. In recent years five-junction cells based on the direct semiconductor bonding technique (SBT), demonstrates space efficiencies >35% and presents application potentials. In this paper, the major challenges for fabricating SBT 5J cells and their appropriate strategies involving structure tunning, band engineering and material tailoring are stated, and 4-cm² 35.4% (AM0, one sun) 5J SBT cells are presented. Further efforts on detailed optical managements are required to improve the current generating and matching in subcells, to achieve efficiencies 36%–37%, or above.     

Determination of spectral variations by means of component cells useful for CPV rating and design

A methodology is presented to determine both the short‐term and the long‐term influence of the spectral variations on the performance of multi‐junction (MJ) solar cells and concentrator photovoltaic (CPV) modules. Component cells with the same optical behavior as MJ solar cells are used to characterize the spectrum. A set of parameters, namely spectral matching ratios (SMRs), is used to characterize spectrally a particular direct normal irradiance (DNI) by comparison to the reference spectrum (AM1.5D‐ASTM‐G173‐03). Furthermore, the spectrally corrected DNI for a given MJ solar cell technology is defined providing a way to estimate the losses associated to the spectral variations. The last section analyzes how the spectrum evolves throughout a year in a given place and the set of SMRs representative for that location are calculated. This information can be used to maximize the energy harvested by the MJ solar cell throughout the year. As an example, three years of data recorded in Madrid shows that losses lower than 5% are expected because of current mismatch for state‐of‐the‐art MJ solar cells. Copyright © 2015 John Wiley & Sons, Ltd.     

Japanese programs on novel concepts in PV

Japanese R&;D activities in photovoltaics (PV) and our R&;D activities with III-V compound multijunction (MJ) solar cells are presented. We have realized high-efficiency InGaP/InGaAs triple-junction solar cells with an efficiency of 36.5–37% (AM1.5G, 200 suns) and concentrator triple-junction solar cell modules with an outdoor efficiency of 27% as a result of designing a grid structure, developing low optical loss Fresnel lens and homogenizers, and designing low thermal conductivity modules. Our challenge now is to develop low-cost and high output power concentrator MJ solar cell modules with an output power of 400 W/m² for terrestrial applications.     

Current‐matching estimation for multijunction cells within a CPV module by means of component cells

An indoor method is presented for the quantification of the current‐matching ratio of a multijunction cell within a concentrator under arbitrary spectral irradiance conditions. The cell current is measured across a very large spectral sweep to force the relevant subcells into a limiting condition. The light spectrum is monitored using component cells to avoid the need for a spectroradiometer and spectral response measurements. The method also provides an estimation of the current losses beyond the overall current mismatch, for example, losses produced in concentrators with chromatic aberration by the non‐uniformity of the incident spectrum across the cell. The method has been applied to a pair of refractive point‐focus concentrator systems; first, a 300X single‐stage Fresnel lens over a lattice‐matched GaInP/Ga(In)As/Ge triple‐junction cell and second, a 1000X two‐stage system with the same Fresnel lens over a homogenizing secondary lens that encapsulates a triple‐junction cell of the same kind but smaller. The experiment demonstrates that the single‐stage concentrator exhibits a higher sensitivity of the current mismatch to variations in the focal distance. Copyright © 2012 John Wiley & Sons, Ltd.     

A monolithic three‐terminal GaInAsP/GaInAs tandem solar cell

We describe the design and performance of a three‐terminal tandem solar cell for low‐concentration terrestrial applications. Designed for operation under a GaAs filter, the tandem demonstrates cumulative conversion efficiencies of 10.2 and 11.9% at 1 sun and 45 suns, respectively, under the concentrated direct spectrum. The middle terminal is shared between the two subcells and allows them to be operated independently at their respective maximum power points. Copyright © 2009 John Wiley & Sons, Ltd.     

Electrical performance evaluation of low‐concentrating non‐imaging photovoltaic concentrator

Second generation prototype photovoltaic facades of reduced costs incorporating devices with optically concentrating elements (PRIDE) incorporate 6 mm wide ‘Saturn’ solar cells at the absorber of the dielectric concentrator. The concentrators were made using injection moulding technique with potential to manufacture in large‐scale applications. Four different concentrator panels have been experimentally verified at outdoors to identify the non‐identical current–voltage (I–V) curves. The I–V curve, fill factor and solar to electrical conversion efficiency of four PRIDE concentrator modules have been evaluated from the 24 manufactured in the ‘IDEOCONTE’ project. The maximum solar to electrical conversion efficiency and the fill factor of the PRIDE concentrator were 9·1 and 70%, respectively. The mismatch loss of the ‘unit concentrators’ has been identified that occurred due to the lack of bonding between the concentrator unit and the solar cell and the rear glass. The average power concentration ratio of PRIDE concentrators manufactured by the improved method was 2·10 compared to a similar non‐concentrating panel and the optical efficiency of the PRIDE system was 83%. Copyright © 2008 John Wiley & Sons, Ltd.     

Concentration photovoltaic optical system irradiance distribution measurements and its effect on multi‐junction solar cells

This paper proposes an indoor procedure based on charge‐coupled device camera measurements to characterize the non‐uniform light patterns produced by optical systems used in concentration photovoltaic (CPV) systems. These irradiance patterns are reproduced on CPV solar cells for their characterization at concentrated irradiances by using a concentrator cell tester and placing high‐resolution masks over the cells. Measured losses based on the masks method are compared with losses in concentrator optical systems measured by using the Helios 3198 solar simulator for CPV modules. Copyright © 2011 John Wiley & Sons, Ltd.     

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.     

A high‐efficiency LPE GaAs solar cell at concentrations ranging from 2000 to 4000 suns

III–V solar cells for terrestrial concentration applications are currently becoming of greater and greater interest. From our experience, concentrations higher than 1000 suns are required with these cells to reduce PV electricity cost to such an extent that this alternative could become cost competitive. In this paper, a single‐junction p/n GaAs solar cell, with efficiencies of 23ċ8 and 22ċ5% at concentration ratios of 2700 and 3600 suns respectively, is presented. This GaAs solar cell is well suited for use with non‐imaging optical concentrators, which possess a large aperture angle. Low‐temperature liquid phase epitaxy (LTLPE) has been the growing technique for the semiconductor structure as an attempt to use a simplified, cheap and clean technique, within a renewable energy perspective. The GaAs solar cell presented is compared with the highest efficiency tandem solar cells at concentration levels exceeding 1000 suns. The GaAs solar cell performance maintains high efficiencies up to 4000 suns, while tandem cells seem to drop very quickly after reaching their maximum. Therefore, single‐junction GaAs solar cells are a good candidate for operating at very high concentrations, and LPE is able to supply these high‐quality solar cells to work within terrestrial concentration systems, the main objective of which is the reduction of PV electricity costs. Copyright © 2003 John Wiley & Sons, Ltd.     

Radiation degradation characteristics of component subcells in inverted metamorphic triple‐junction solar cells irradiated with electrons and protons

The radiation response of In_0.5Ga_0.5P, GaAs, In_0.2Ga_0.8As, and In_0.3Ga_0.7As single‐junction solar cells, whose materials are also used as component subcells of inverted metamorphic triple‐junction (IMM3J) solar cells, was investigated. All four types of cells were prepared using a simple device layout and irradiated with high‐energy electrons and protons. The essential solar cell characteristics, namely, light‐illuminated current–voltage (LIV), dark current–voltage (DIV), external quantum efficiency (EQE), and two‐dimensional photoluminescence (2D‐PL) imaging were obtained before and after irradiation, and the corresponding changes due to the irradiations were compared and analyzed. The degradation of the cell output parameters by electrons and protons were plotted as a function of the displacement damage dose. It was found that the radiation resistance of the two InGaAs cells is approximately equivalent to that of the InGaP and GaAs cells from the materials standpoint, which is a result of different initial material qualities. However, the InGaAs cells show relatively low radiation resistance to electrons especially for the short‐circuit current (I sc). By comparing the degradation of I sc and EQE, data, It was confirmed that the greater decrease of minority‐carrier diffusion length in InGaAs compared with InGaP and GaAs causes severe degradation in the photo‐generation current of the InGaAs bottom subcells in IMM3J structures. Additionally, it was found that the InGaP and two InGaAs cells exhibited equivalent radiation resistance of V oc, but radiation response mechanisms of V oc are thought to be different. Further analytical studies are necessary to interpret the observed radiation response of the cells. © 2016 The Authors. Progress in Photovoltaics: Research and Applications published by John Wiley & Sons Ltd.     

Highly conductive p + + ‐AlGaAs/n + + ‐GaInP tunnel junctions for ultra‐high concentrator solar cells

Tunnel junctions are key for developing multijunction solar cells (MJSC) for ultra‐high concentration applications. We have developed a highly conductive, high bandgap p ^{+ +} ‐AlGaAs/n ^{+ +} ‐GaInP tunnel junction with a peak tunneling current density for as‐grown and thermal annealed devices of 996 A/cm ² and 235 A/cm ², respectively. The J–V characteristics of the tunnel junction after thermal annealing, together with its behavior at MJSCs typical operation temperatures, indicate that this tunnel junction is a suitable candidate for ultra‐high concentrator MJSC designs. The benefits of the optical transparency are also assessed for a lattice‐matched GaInP/GaInAs/Ge triple junction solar cell, yielding a current density increase in the middle cell of 0.506 mA/cm ² with respect to previous designs. Copyright © 2014 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.     

Temperature accelerated life test on commercial concentrator III–V triple‐junction solar cells and reliability analysis as a function of the operating temperature

A temperature accelerated life test on commercial concentrator lattice‐matched GaInP/GaInAs/Ge triple‐junction solar cells has been carried out. The acceleration of the aging has been accomplished by subjecting the solar cells at temperatures markedly higher than the nominal working temperature inside a concentrator, and the nominal photo‐current condition (820 X) has been emulated by injecting current in darkness. Three tests at different temperatures have been carried out. The failure distributions across the three test temperatures have been fitted to an Arrhenius–Weibull model. An Arrhenius activation energy of 1.59 eV was determined from the fit. The reliability functions and parameters of these solar cells at two nominal working conditions (80 and 100 °C) have been obtained. In both cases, the instantaneous failure rate function monotonically increases, that is, the failures are of the wear‐out kind. We have also observed that the reliability data are very sensitive to the nominal temperature condition. In fact, at a nominal working condition of 820 X and 80 °C, assuming that the concentration module works 5 h per day, the warranty time obtained for a failure population of 5% has been 113 years. However, for a nominal working condition of 820 X and 100 °C, the warranty time obtained for a failure population of 5% has been 7 years. Therefore, in order to offer a long‐term warranty, the working temperature could be a key factor in the design of the concentration photovoltaic systems. Copyright © 2014 John Wiley & Sons, Ltd.     

Estimation of multi‐junction solar cell parameters

The paper deals with the parameter estimation of InGaP/GaAs/Ge multi‐junction solar cell and is based on minimizing the difference between the measured I–V and the theoretical I–V characteristics—the objective function. The parameter estimation was first performed on a multi‐junction solar cell represented by a single‐diode model containing eight parameters: five conventional parameters and three additional parameters for the negative diode breakdown voltage. An extended model is also presented for detailed analysis of the multi‐junction cell containing three subcells connected in series. In this model, each subcell is represented by eight parameters, and therefore a total of 24 parameters describe the cell. The parameter estimation procedure requires derivatives of the first and the second order of an objective function, filtering of noisy measurements, iteration algorithm, guessing of initial parameters, zero finding, and stopping criteria. The paper presents a mathematical method and a procedure for extracting solar cell parameters based on I–V measured data. The parameters' values may be used for analysis of the current mismatch of the subcells, the power loss, the output power of the multi‐junction cell for different environmental conditions, and to some extent, for cell fabrication. Copyright © 2012 John Wiley & Sons, Ltd.     

Performance prediction of concentrator solar cells and modules from dark I–V characteristics

The indoor performance of concentrator solar cells and modules at operating conditions is a complex task, owing to the required illumination and temperature conditions, and even more so during extensive procedures, such as on a production line. The solution proposed throughout this paper consists of predicting the illumination I–V characteristic of the solar cells, with the dark I–V curve and the photogenerated current as the only input data. As well as this, the technology‐dependent components of the series resistance are obtained from the dark characteristics for quality control. Theory and experiments on several types of concentrator cell have been carried out to validate the method. The equipment to be used on a production line has been developed by IES and used by BP Solar to test up to 25 000 cells and 2000 modules for the 480 kW_p power plant using the EUCLIDES^TM concentrator. Copyright © 2003 John Wiley & Sons, Ltd.     

Validation of energy prediction method for a concentrator photovoltaic module in Toyohashi Japan

III–V concentrator photovoltaic systems attain high efficiency through the use of series connected multi‐junction solar cells. As these solar cells absorb over distinct bands over the solar spectrum, they have a more complex response to real illumination conditions than conventional silicon solar cells. Estimates for annual energy yield made assuming fixed reference spectra can vary by up to 15% depending on the assumptions made. Using a detailed computer simulation, the behaviour of a 20‐cell InGaP/In_0.01GaAs/Ge multi‐junction concentrator system was simulated in 5‐min intervals over an entire year, accounting for changes in direct normal irradiance, humidity, temperature and aerosol optical depth. The simulation was compared with concentrator system monitoring data taken over the same period and excellent agreement (within 2%) in the annual energy yield was obtained. Air mass, aerosol optical depth and precipitable water have been identified as atmospheric parameters with the largest impact on system efficiency. Copyright © 2012 John Wiley & Sons, Ltd.     

Activity and current status of R&D on space solar cells in Japan

Japan's Research and Development (R&D) activities on high‐performance III–V compound space solar cells are presented. Studies of new CuInGaSe₂ thin‐film terrestrial solar cells for space applications are also discussed. Performance and radiation characteristics of a newly developed InGaP/GaAs/Ge triple‐junction space solar cell, including radiation response, results of a flight demonstration test of InGaP/GaAs dual‐junction solar cells and CuInGaSe₂ thin‐film solar cells, and radiation response of three component sub‐cells are explained. This study confirms superior radiation tolerance of InGaP/GaAs dual‐junction cells and CuInGaSe₂ thin‐film cells by space flight experiments. Copyright © 2005 John Wiley & Sons, Ltd.     

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.