Experimental system for current–voltage curve measurement of photovoltaic modules under outdoor conditions

This paper describes an experimental system developed in the photovoltaic laboratory at the University of Málaga (Spain) to measure the current–voltage curve of photovoltaic modules under outdoor conditions. The measurement is performed in an automated way by employing commercial instruments controlled by a computer using the GPIB standard. Several modules, selected sequentially through a set of relays, are biased by a four‐quadrant power supply while a function generator synchronizes two multimeters in order to acquire voltage and current values. The measurement uncertainties were also estimated. The proposed method for synchronizing the voltage and current measurements ensures that these measurements are performed simultaneously; this means that the estimated uncertainty is lower than those obtained using other previously proposed methods. The main electrical parameters are estimated. A user‐friendly application allows the user to configure several parameters, such as bias rate, voltage, and current limits and the number of points of the curve. Copyright © 2011 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.     

Current‐voltage characteristics of high‐concentration,photovoltaic arrays

The current–voltage ( I–V ) characteristics of photovoltaic (PV) systems have always been a good indicator of the overall performance of a system. The aim of this paper is to give an overview and elucidate the use of the I–V characteristics of concentrator PV (CPV) modules and arrays as an important diagnostic tool to identify factors that lower a system's performance and the types of mismatch that exist between series‐connected single‐junction cells within a module. Possible causes for mismatch between cells include factors such as; misalignment of optical elements and cells, nonuniform cell material parameters, uneven cell illumination due to dew, dust or degradation of the secondary and main optical elements. The different types of mismatch typically found in CPV are categorized and their effects on the resultant module I–V curves are discussed and shown. The effect of bypass diodes on the module's I–V curves is also illustrated. This paper also reports on, and interprets I–V measurements that were recorded for a commercially available point‐focus concentrator module under various real outdoor conditions. Copyright © 2004 John Wiley & Sons, Ltd.     

Dark I–V curve measurement of single cells in a photovoltaic module

A simple method of obtaining single cell dark I–V curves in a photovoltaic module was developed. The method does not require disassembling the module and was verified experimentally. From the dark I–V curves, the cell characteristic parameters were obtained. By following the time evolution of the characteristic parameters it is possible to determine the main degradation mechanisms and predict the mean life time of the module before failure. Copyright © 2005 John Wiley & Sons, Ltd.     

A 500‐kW PV generator I–V curve

This paper presents the measurement of the I–V curve of a 500‐kW PV generator by means of an own‐made capacitive load. It is shown that I–V curve analysis can also be applied to big PV generators and that when measuring the operation conditions with reference modules and taking some precautions (especially regarding the operation cell temperature), it is still a useful tool for characterizing them and therefore can be incorporated into maintenance procedures. As far as we know, this is the largest I–V curve measured so far. Copyright © 2013 John Wiley & Sons, Ltd.     

Design of a twin capacitive load and its application to the outdoor rating of photovoltaic modules

This paper describes the design of an original twin capacitive load that is able of tracing simultaneously the I–V characteristics of two photovoltaic modules. Besides, an example of the application of this dual system to the outdoor rating of photovoltaic modules is presented, whose results have shown a good degree of repeatability. Copyright © 2013 John Wiley & Sons, Ltd.     

Obtaining small photovoltaic array operational curves for arbitrary cell temperatures and solar irradiation densities from standard conditions data

The paper presents a simple approach to deriving I–V curves of photovoltaic panels and small arrays for arbitrary environmental conditions on the basis of three points of a single operating curve data and short current temperature coefficient only. The proposed method does not employ fitting of any type and is solely based on a numerical solution of a system of transcendental equations. The equations are expressed in a dimensionless form, simplifying both the solution and photovoltaic panel parameters' representation. The solution is used to find the values of normalized equivalent circuit elements for the available data and then perform an appropriate adjustment to obtain the operating curves for arbitrary conditions. The proposed method was applied to monocrystalline and polycrystalline commercial solar panels and was compared with both manufacturer‐provided and experimentally measured operating curves to analyze the approach applicability and accuracy. Copyright © 2012 John Wiley & Sons, Ltd.     

Identifying parasitic current pathways in CIGS solar cells by modelling dark J–V response

The non‐uniform presence of shunting defects is a significant cause of poor reproducibility across large‐area solar cells, or from batch‐to‐batch for small area cells, but the most commonly used value for shunt parameterisation (the shunt resistance) fails to identify the cause for shunting. Here, the use of equivalent circuit models to describe dark current–voltage characteristics of ZnO:Al/i‐ZnO/CdS/CIGS/Mo devices in order to understand shunting behaviour is evaluated. Simple models, with a single shunt pathway, were tested but failed to fit experimental data, whereas a more sophisticated model developed here, which includes three shunting pathways, yielded excellent agreement throughout the temperature range of 183–323 K. The temperature dependence of fitting parameters is consistent with known physical models. Activation energies and contact barriers are determined from the model, and extracted diode factors are unique across the voltage range. A case study is presented whereby the model is used to diagnose poor reproducibility for CIGS devices (efficiency ~3–14% across a 100 cm² plate). It's shown that lower efficiencies correlated with greater prevalence of Ohmic and non‐Ohmic shunt currents, which may form due to pinholes in absorber and buffer layers respectively, whereas the quality of the main junction was constant for all cells (diode factor ~1.5–2). Electron microscopy confirmed the presence of ZnO:Al/i‐ZnO/Mo and ZnO:Al/CIGS/Mo regions, supporting the multi‐shunt pathway scheme disclosed by modelling. While the model is tested with CIGS cells here, this general model is a powerful diagnostic tool for process development for any type of thin‐film device. Copyright © 2015 John Wiley & Sons, Ltd.     

Method for photovoltaic parameter extraction according to a modified double‐diode model

The variables presented in the current–voltage equation of a photovoltaic (PV) device are usually called PV parameters. There are several different methods for PV parameter extraction from measured data according to different models. However, many of these methods provide results that do not represent I–V curves of thin films devices correctly. This can occur because either the applied model or the PV parameter extraction methods are not suitable. It is also possible that the extracted parameters provide a good mathematical representation of the curves but without physical meaning (e.g. negative series resistance). This work presents a method for PV parameter extraction based on a modified double‐diode model. In this model, the ideality factor related to the recombination of the charge carriers in the space‐charge region is assumed as a variable. This method has been tested for different I–V curves of different PV module technologies providing very good results and parameters with physical meaning in all the cases. Copyright © 2012 John Wiley & Sons, Ltd.     

Translating outdoor CPV I–V measurements to a CSTC power rating and the associated uncertainty

A complete procedure is presented for translating outdoor concentrator photovoltaic (CPV) I–V measurements to the Concentrator Standard Test Conditions (CTSC) (1000 W/m² and 25 °C cell temperature). Methods are demonstrated for measuring all the necessary input parameters for the translation, including outdoor thermal transient measurements and indoor dark I–V curves. Four modules are subjected to the translation method based on multiple months of outdoor data, one module measured at National Renewable Energy Laboratory and three at Fraunhofer ISE. The modules are also characterized under a sun simulator to provide a comparison to the translation approach. The results show that translated CSTC efficiencies are in good agreement with the efficiencies from the solar simulator. Two of the modules agreed within 1%, whereas the other two modules agree within approximately 4%. An uncertainty analysis of the input parameters is discussed in the context of the total uncertainty associated with the translation to CSTC. The reference voltage and efficiency temperature coefficient are the key parameters impacting the translation uncertainty, whereas uncertainty in the outdoor data is driven by spectral and meteorological parameters. Copyright © 2015 John Wiley & Sons, Ltd.     

Influence of cracks on the local current–voltage parameters of silicon solar cells

The existence of cracks in silicon solar cells can drastically reduce the electrical performance of an individual cell and even of an entire photovoltaic module. An in‐depth understanding of the influence of cracks on solar cells enables therefore calculations of the crack impact and other following effects on module level. This paper shows a detailed analysis of the electrical influence of cracks with two different spatially resolved methods including global and local current–voltage characteristics. The main influence of cracks is an increased recombination current density in the depletion region, which is clearly shown by spatially resolved dark lock‐in thermography measurements with local current–voltage investigation. This increased recombination current density affects further cell parameters such as the efficiency, which is confirmed also by the global current–voltage characteristics. The additionally used ratio image technique based on electroluminescence measurements is in comparison with the local current–voltage method, the more reliable and faster method for the crack detection itself, and allows on cell‐level and module‐level a continuous inspection of cracks. Copyright © 2013 John Wiley & Sons, Ltd.     

Modelling photovoltaic modules with neural networks using angle of incidence and clearness index

A neural network for modelling photovoltaic modules using angle of incidence and clearness index is proposed. Engineers require methods to estimate the output of a photovoltaic plant depending on meteorological conditions. Therefore, models for the grid inverter and the generator must be provided, and their outputs must be combined. The connection between both models is related to the maximum power point of the generator and how it is tracked by the inverter. That maximum power point under specific conditions of irradiance and module temperature is determined by the I–V curve of the module, which must be simulated under those conditions. Algebraic procedures were used to simulate the I–V curve. Recently, neural networks have been used for the same purpose. Previous methods only take into account the irradiance and the module temperature. The model proposed is based on neural networks, and it uses not only the irradiance and the module temperature but also the angle of incidence and the instantaneous clearness index as additional inputs. The normalised clearness replaces the standard clearness index because it allows the removal of the hourly trend found in this last index. This new model improves the results obtained with previous ones as it can distinguish amongst samples with the same solar irradiance and temperature values but with different angle of incidence and instantaneous clearness index. Results show that this model could be used to improve the accuracy of the tools used to forecast the output of photovoltaic plants. Copyright © 2014 John Wiley & Sons, Ltd.     

Organic–Inorganic Perovskite Light‐Emitting Electrochemical Cells with a Large Capacitance

While perovskite light‐emitting diodes typically made with high work function anodes and low work function cathodes have recently gained intense interests. Perovskite light‐emitting devices with two high work function electrodes with interesting features are demonstrated here. Firstly, electroluminescence can be easily obtained from both forward and reverse biases. Secondly, the results of impedance spectroscopy indicate that the ionic conductivity in the iodide perovskite (CH₃NH₃PbI₃) is large with a value of ≈10^?8 S cm^?1. Thirdly, the shift of the emission spectrum in the mixed halide perovskite (CH₃NH₃PbI_3?xBr_x) light‐emitting devices indicates that I^? ions are mobile in the perovskites. Fourthly, this work shows that the accumulated ions at the interfaces result in a large capacitance (≈100 μF cm^?2). The above results conclusively prove that the organic–inorganic halide perovskites are solid electrolytes with mixed ionic and electronic conductivity and the light‐emitting device is a light‐emitting electrochemical cell. The work also suggests that the organic–inorganic halide perovskites are potential energy‐storage materials, which may be applicable in the field of solid‐state supercapacitors and batteries.     

Change in I–V characteristics of thin‐film photovoltaic (PV) modules induced by light soaking and thermal annealing effects

The performance of photovoltaic (PV) modules is generally rated under standard test conditions (STC). However, the performance of thin‐film photovoltaic modules is not unique even under STC, because of the “metastability”. The effects of the light soaking and thermal annealing shall be incorporated into an appropriate energy rating standard. In this study, the change in I–V characteristics of thin‐film PV modules caused by the metastability was examined by repeated indoor measurements in addition to round‐robin outdoor measurements. The investigated thin‐film modules were copper indium gallium (di)selenide (CIGS), a‐Si : H, and a‐Si : H/µc‐Si : H (tandem) modules. The increase in the performance of the CIGS module between the initial and final indoor measurements was approximately 8%. Because of light‐induced degradation, the indoor performance of the a‐Si : H and a‐Si : H/µc‐Si : H modules decreased by approximately 35% and 20%, respectively. The performance was improved by about 4–6% under high temperature conditions after the initial degradation. The results suggest that the performance of thin‐film silicon modules can seasonally vary by approximately 4–6% only due to thermal annealing and light soaking effects. The effect of solar spectrum enhanced the outdoor performance of the a‐Si : H module by about 10% under low air mass conditions, although that of the a‐Si : H/µc‐Si : H modules showed a little increase. The currents of these a‐Si : H/µc‐Si : H modules may be limited by the bottom cells. Therefore, it is required to optimize the effect of solar spectrum in addition to the effects of light soaking and thermal annealing, in order to achieve the best performance for thin‐film silicon tandem modules. Copyright © 2013 John Wiley & Sons, Ltd.     

Two‐terminal metal‐inter‐connected multijunction III–V solar cells

A novel bonding approach with an interface consisting of a metal and dielectric is developed, and a “pillar‐array” metal topology is proposed for minimal optical and electrical loss at the interface. This enables a fully lattice‐matched two‐terminal, four‐junction device that consists of an inverted top two‐junction (2J) cell with 1.85 eV GaInP/1.42 eV GaAs, and an upright lower 2J cell with ~1 eV GaInAsP/0.74 eV GaInAs aimed for concentrator applications. The fabrication process and simulation of the metal topology are discussed along with the results of GaAs/GaInAs 2J and (GaInP + GaAs)/GaInAs three‐junction bonded cells. Bonding‐related issues are also addressed along with optical coupling across the bonding interface. Copyright © 2014 John Wiley & Sons, Ltd.     

Analytic treatment of the distributed resistance of the window layer in solar cells with parallel grids

A formalism is presented to understand the effect of distributed resistance of the window layer on the current–voltage characteristics of solar cells with parallel grids. Along the window/active‐layer interface, the current density and the electric potential are calculated iteratively from the current density–voltage relation and the Laplace equation, respectively. The former property is approximated in a series expansion of sinusoidal functions, which leads to an analytic solution to the electric potential in the window layer. The total current and the average current density are derived in analytic forms. A few calculated results are presented to show the influence of the distributed resistance of the window layer on the device performance. Copyright © 2011 John Wiley & Sons, Ltd.     

Industrial solar cell tester: study and improvement of I‐V curves and analysis of the measurement uncertainty

Determination of solar cell parameters by illuminated I–V measurement is a standard characterisation technique in the photovoltaic industry. These measurements are carried out under standard conditions (STC: 25 °C, 1000 W/m² AM1.5G spectrum). It can be considered as the most crucial in‐line test for solar cells as it provides the industry with the conversion efficiency, and it is also a reliable quality control test. Reference cells are mainly used in testing equipment to set irradiance and working conditions in the tester/sorter, the rest of the cells being measured and classified by comparison with that reference. An accurate calibration of the irradiance at STC in cell testers and high precision in determining the main parameters of the I–V curve are required; a suitable design of the mechanical components and an adequate selection of different programme options should be made (distribution of the points measured, temperature correction or classification method). Here, we have studied the accuracy of an industrial solar simulator whose mechanical, electrical, electronic and software components were analysed with an individual solar device and a production sample. An uncertainty analysis was carried out in order to determine the power uncertainty and which components to improve in order to reduce it. Copyright © 2015 John Wiley & Sons, Ltd.     

High temperature current–voltage characteristics of InP‐based tunnel junctions

In this paper, we present temperature‐dependent current–voltage measurements of tunnel junctions lattice matched to InP at temperatures ranging from room temperature to 220 °C. Temperature‐dependent tunneling properties were extracted by fitting the current–voltage characteristics using a simple analytical formula. Three different designs of tunnel junction were characterized, including a bulk InAlGaAs tunnel junction, an InAlGaAs tunnel junction with InAlAs cladding layers and an InGaAs/InAlGaAs quantum‐well tunnel junction. Each device exhibited different temperature dependence in peak tunnel current and excess current, with the quantum‐well tunnel junction exhibiting the greatest temperature sensitivity. We use a non‐local tunneling model, in conjunction with a numerical drift‐diffusion solver, to explain the performance improvement available by using double heterostructure cladding layers around the junction region, and use the same model to explain the observed temperature dependence of the devices. Copyright © 2014 John Wiley & Sons, Ltd.     

Practical method for estimating the power and energy delivered by photovoltaic modules operating under non‐standard conditions

This work describes a method developed for estimating the energy delivered by building integrated photovoltaics systems operating under non‐standard conditions of irradiance and temperature. The method is based on calculation of the maximum power (P_Gmax) supplied by the modules array as a function of irradiance and ambient temperature, achieved by simulating its I–V and P–V curves using an algorithm which needs only the performance parameters supplied by the manufacturers. The energy generated by the PV system is estimated from monthly average values of P_Gmax calculated for using monthly average values of ambient temperature and irradiance obtained from data measured during 2 years. The method is applied to crystalline Si modules and tested by comparing the simulated I–V and P–V curves with those obtained by outdoor measurements as well as for comparing the energy produced during the years 2009 and 2010 with a 3.6 kWp building integrated photovoltaics system installed at the Universidad Nacional located in the city of Bogotá, Colombia, at 4°35′ latitude and 2.580 m altitude. The contrast of the simulated I–V and P–V curves for two different types of commercial Si‐modules with those experimentally obtained under real conditions indicated that the simulation method is reliably. Copyright © 2012 John Wiley & Sons, Ltd.