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Dark and illuminated characteristics of photovoltaic solar modules. Part I: Influence of dark electrical stress Jean Zaraket, Michel Aillerie, and Chafic Salame

Citation: AIP Conference Proceedings 1758, 020016 (2016); doi: 10.1063/1.4959392 View online: https://doi.org/10.1063/1.4959392 View Table of Contents: http://aip.scitation.org/toc/apc/1758/1 Published by the American Institute of Physics

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Dark and Illuminated Characteristics of Photovoltaic Solar Modules. Part I: Influence of Dark Electrical Stress Jean Zaraket 1, 2, 3, a,), Michel Aillerie 2, 3, b), and Chafic Salame1,4, c) 1

CEER, Faculty of Sciences II, Lebanese University, B.P 90656 Jdeidet El Mten, Lebanon 2 Université de Lorraine, LMOPS, EA 4423, 57070 Metz, France. 3 CentraleSupelec, LMOPS, 57070 Metz, France 4 CNRSL, National Council for Scientific Research, Beirut, Lebanon a)

[email protected] [email protected] c) [email protected]

b)

Abstract.The purpose of this paper is to discuss the effect of electric reverse stress currents on the performance of photovoltaic solar modules.The effect of the reverse stress current induced into the solar cell structure on the IV characteristics and parameters in the dark and illuminated conditions at room temperature for several common periods of time. A digital double exponential model was used to analyze the experimental measurements. The changes in characteristics which are caused from the effect of a reverse current introduced for different stress levels simulated the effect of accumulated extreme reverse currents that can occur in the solar cells and modules as result of shading and other different reasons. The paper contributes to the research on the adverse effects of reverse currents on the normal functioning of cells and solar modules. Keywords: Solar cell/module, Photovoltaic, Reverse current, Current–voltage characteristics, Diffusion and Recombination current, Shunt.

INTRODUCTION Photovoltaic research has been attributing a lot of results and recommendations during the past decade or so due to the increase demand on the use of photovoltaic solar cells in producing clean and productive energy. This type of research tackled mainly the effect of a reverse current on the dark [1] and / or the illuminated electrical properties of solar cells. The previously mentioned reverse current may occur as a result of many factors. These factors are closely related to certain defects in the solar cell which may be related to material, design or manufacturing defects or even such defects related to a kind of shading or even temperature changes of the solar cells itself [2,3] .In brief, most types of defects, which will be discussed in what follows, will be the cause of a reverse current having certain stress levels for a certain period of time intervals. A solar cell is treated as a diode of larger area silicon p-n junction forward bias with a photovoltage. This photovoltage is created from the dislocation of the electrons as a result of incident photons within the junction or diode. Any disturbance of this electron flow, mainly through the silicon crystal /crystals or the cell junction, is due to what is considered as material defects, such as grain boundaries, dislocations, or any other inhomogeneity in the microstructure, will have a large impact on a part or on the over whole performance of the solar cell.[4,5] Shading or partial shading on a solar module or array has demonstrated to influence the I-V characteristics and reduce the total output of the solar cell. This problem may become more serious when the shaded cells get reverse biased, leading to a high resistance diode, which will get overheated when the difference in illumination is high enough. The overheating of the diode will eventually lead to serious damage in the photovoltaic cell. [6,7] Technologies and Materials for Renewable Energy, Environment and Sustainability AIP Conf. Proc. 1758, 020016-1–020016-10; doi: 10.1063/1.4959392 Published by AIP Publishing. 978-0-7354-1416-7/$30.00

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The reverse current is also referred to certain fault currents and environmental conditions in photovoltaic arrays with several strings connected in parallel to form a PV array with a direct-current output equal to the sum of the PV string outputs. The panel circuitry can be then referred to as the PV generator. Standards, such as the Australian Standard AS/NZ5033 “Installation of PV Arrays”, recommend fuses to protect both cabling and PV modules in case of the occurrence of these fault conditions. The relationship between acceptable reverse current levels and exposure durations of reverse currents on PV modules are compared with trip current of fuses and typical time delays experienced with fuse tripping. [8] An extensive investigation about the effect of a reverse current on the performance and efficiency of a PV solar module, on the dark as well as on the illuminated I-V characteristic of solar cells is needed. This work focuses on the experimental quantitative measurement of the effect of reverse currenton dark and illuminated characteristics by applying a high level of electric stress in the opposite electron flow direction of the solar module to simulate a damaging reverse bias affecting the module over a certain period of time. The results and discussion offered aimed to contribute to material, performance and efficiency of photovoltaic solar module research.

EXPERIMENTAL A series of small commercial off the shelf, amorphous silicon, solar photovoltaic modules 6x6 (36 cells) 3.8 v and 85 mA were collected from the local consumer market. These modules and are generally used to power up calculators and other small electronic gadgets. In this experiment, the photovoltaic modules were placed under stress in the dark in order to compare their behavior under dark and later under illuminated conditions of 87 klux. To simulate the reverse current that occurs in the solar module when partially shaded, modules underwent several quantities of reverse current stress levels induced in the laboratory through the pn junction. The reverse stress current values ranged from 10 mA up to 80 mA by adjusting 10 min time intervals. Data was collected for each procedure and presented in this paper. The testing module of the work done is presented in figure 1.

FIGURE 1. Testing module

The results presented contain both experimental electrical characterization along with some mathematical calculation used to determine the junction parameters through an equivalent circuit model of the module. The general mathematical description of the output characteristics for a PV cell has been studied for over the past four decades. The equivalent circuit of the general model consists of a photo current, a diode, a parallel resistor expressing a leakage current, and a series resistor describing an internal resistance to the current flow [9].An even more precise mathematical description of a solar cell, which is called the double exponential model, as shown in Figure 2, is derived from the physical behavior of solar cells constructed from polycrystalline silicon. This model is composed of a two ideal diodes, a series resistance R s and a parallel shunt resistance Rsh. It considers the calculation of both series and shunt resistances along with the junction ideality factor A, and the components of the diode diffusion I01 and recombination I02 saturation currents. [4]

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FIGURE 2.Idealized equivalent circuit of a double exponential model [7]

Experimentally collected I-V curves were introduced into specially designed software developed under Matlab/Simulink environment that performs numerical calculations based on the double exponential model of a p-n junction formulated by the following equation [10]: ܸ െ ܴ‫ ݏ‬Ǥ ‫ܫ‬ ܸ െ ܴ ‫ ݏ‬Ǥ ‫ܫ‬ ܸ െ ܴ ‫ݏ‬Ǥ ‫ܫ‬  ሺͳሻ ൰െ ͳൠ ൅ ൰െ ͳൠ ൅ ‫ܫ‬02 ൜ ൬ ܴ‫ݏ‬ ‫ܶܣ‬ ܸܶ ݄ ܸ where: I: represents the intensity of the total cell (A) ; I02 is the reverse saturation current corresponding to generation and recombination of electrons and holes in the depletion region; I01 is the reverse saturation current corresponding to the diffusion and recombination of electrons and holes in the p- and n-side, respectively; V is the applied voltage, VT=KT/q is the thermal voltage, q is the elementary electron charge = 1.6 × 10−19 C, k is the Boltzmann constant =1.38 ×10-23 J/K, T is the absolute cell temperature, A is the ideality factor >1 and RSh is the shunt resistance, RS is the parasitic series resistance [11] ‫ ܫ‬ൌ ‫ܫ‬01 ൜ ൬

RESULT AND DISCUSSION The reverse current and I-V characteristics The reverse stress currents of values 10 mA up to 80mA were injected by setting the time for 10 minutes in each case. At each step, the current was interrupted to collect the I-V characteristics. The shape of the I-V curve does not change from that of a cell, for a module or array of PV cells. However, it is scaled based on the number of cells connected in series and in parallel. When m is the number of cells connected in parallel, and n is the number of cells connected in series, and Isc (short circuit current) and V oc (open circuit voltage) are values for individual cells; an IV curve having n.Voc value of intersection with the x-axis and m.Isc value of intersection with the y-axis may be created. The results of the forward log I-V characteristics are shown in figure 3 before and after the application of the stress for 10 min reverse current from 10 up to 80 mA in the dark condition. Whereas figure 4 represents the forward log IV characteristics under the same conditions of stress levels in the illuminated condition. The reverse current is maintained by a programmable current source. It was noticed that the cells in the module went out of order in the dark case when the current was greater or equal to 80 mA, while the cells in the module went out of order at 70 mA in the illuminated case. When plotted in logarithmic scale, the obtained dark I-V curves could be divided in two parts. The first extending from 0V to approximately 2V, the nominal operation voltage V oc as defined by the manufacturer, and is related to a leakage current within the module (V oc) is hardly changing for a reverse current of 10 mA through 80 mA. These curves are only experiencing shifting up in current in certain orders as the reverse current is been increased. The previous mentioned order is seen to have a certain pattern directly related to the value of the reverse current. This means that the altering in the module’s function and performance is directly related to the amount of reverse current encountered. This issue maybe of crucial importance mainly when researching the amount of degradation in the photovoltaic modules, when the possibility of them getting reverse biased. The above mentioned results may be seen in a more dramatic presentation when compared to those of the illuminated condition. This is due to the fact that the module is in the operating condition. The values of the current as well as the voltage are seen to be shifted into much higher orders but the same explanation of the current leakage, due to some heated spots, seems to be still applicable and valid.

1

T=10min reverse stress current caracteristic in dark

0.1

current I (A)

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0.001 100mA 10 mA 20mA 40mA 60mA 80mA without stress

1E-4

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FIGURE 3.Log I vs V for different reverse current stress levels in the dark condition

Current I (A)

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T=10min reverse stress current characteristics in light

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20mA 30mA 40mA 50mA 60mA without stress

1E-3 5.4

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voltage V (V)

FIGURE 4. Log I vs V for different reverse current stress levels in the illuminated condition

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The reverse characteristic of the module is measured by applying a reverse voltage through the junction to verify that no current is flowing, a large current means that the module is out of order and is no more functioning. Figures 5 and 6 show the reverse characteristics of the used modules, before and then when fused by a 10 mA up to 80 mA current, in the dark and illuminated conditions respectively, and plotted in a log I-V scale.

Reverse current (A)

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T=10min reverse stress current characteristics in dark

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10mA 20mA 40mA 60mA 80mA without stress

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Reverse voltage (v)

FIGURE 5. Log reverse I vs reverse V for different reverse current stress levels (dark)

0.08 0.07 0.06

T=10min reverse stress current characteristics in light

Reverse current (A)

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0.02 10mA 40mA 60mA 80mA without stress

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Reverse voltage (V)

FIGURE 6. Log reverse I vs reverse V for different reverse current stress levels (illuminated)

A considerable increase in the reverse current is observed as soon as the stressing current is applied even during the first 10 minutes, after which this variation becomes stable. This observation is applicable for both the dark and the illuminated states with the difference in the measured reverse current levels. The major degradation in the module, in fact, was produced right after the application of the stressing current producing some burned and destroyed zones in the module, and forming current passages, forcing the charges to escape through them without the need to damage other possible undamaged areas. This clearly explains the previously mentioned observations in the figures 5 and 6 respectively, where junction seems to still function as the stressing currents are intensified. .

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Shunt and series resistances vs. stress current The shunt resistance and the series resistance for both the dark and illuminated conditions,shown if Figures 7 and 8 respectively, were calculated based on a double exponential model, each I-V curve was uploaded to a special software that calculates the electrical parameters. The shunt resistance is in fact the inverse of the slope of the linear part of the curve IV of the reverse bias which is calculated as [12-14]:Rsh = ΔVreverse polarization / Δ Ireverse polarization. This parameter should be higher for an unstressed device and as the stressing currents are applied the slope of the I-V curves increases resulting in the drop in shunt resistance. The previously mentioned observation is seen in Figure 7 where the shunt resistance values decrease to about 30x103 ohms in the dark and to about 300 ohms in the illuminated condition, and then stabilize especially after the application of 20 mA stressing current. A higher value of the leakage current means that a smaller shunt resistance is obtained. As the leakage is stabilized, due to the fact that no more parts of the cells are subjected to destruction, the values of both the shunt and series resistance changes no more.

Dark illuminated

Rsh (ohm)

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Current (mA) reverse stress current

FIGURE 7. Log Shunt resistance (Rsh) vs Current (in Dark and light)

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Series Resistance Rs (ohm)

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Current (mA) reverse stress current

FIGURE 8. Log Series resistance (Rs) vs Current (in Dark and light)

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For the sake of comparison, and may be useful information, the ratio of the shunt resistance in the dark to that in the illuminated condition Rsh (dark) / Rsh (light), in the stressed case came to be in the order of 100 times. On the other hand, the same ratio of the series resistance came to be in the order of 5 times. These ratios show a quantitative value of the effect of degradation, as the result of applying stress currents, on the performance of the modules. To prove that these ratios will have any meaning, further testing should be performed on different photovoltaic modules. If these ratios will remain the same or somewhat similar, then they will be creating a platform to the degradation prediction of the effect of reverse current stresses in both the dark and illuminated conditions. This information may be also useful in the production and future design of photovoltaic solar cells and modules.

Measurement of the Ideality Factor The value of the ideality factor can be estimated directly from the data of I-V by calculating the slope of the straight regions dark lnIvs V, in the absence of the effects of series and shunt resistance. Figure 9 shows the plot of the ideality factor vs the stress currents in both the dark and illuminated conditions. The ideality factor of 2.5 in the dark did not show any changes or deviations with increasing the reverse stress current of values 10, 20,40,60 and 80mA stress. The previously mentioned value of the ideality factor is considered relatively high, providing additional information about the permanent damage in the solar photovoltaic modules due to the deterioration of the internal junction structure. On the other hand, the ideality factor decreased to a good value of about 1 when the module was illuminated. The illumination in this case decreased the ideality factor which reduced the effect of the deterioration of the internal junction structure.

Dark illuminated

A Ideality Factor

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0 0

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current (mA)

FIGURE9. Variation of the ideality factor vs I (mA) (in Dark and light)

The reverse recombination and diffusion currents behavior The diffusion current is caused by the diffusion of carriers across the junction of the PV cell. In ideal operating conditions of equilibrium, the net current from the PV cell is zero, because of the balancing effect of the diffusion, drift and the recombination currents. Upon applying a current in the reverse direction of electron flow, this balance is disturbed and due to overheating of the p-n junction and expanding the junction width, diffusion as well as recombination curren...