Team1 Report 4

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Solar Cell Characterization Laboratory

2011/2012

Master in Photovoltaic Solar Energy

LAB SESSION 4: FUNDAMENTALS OF I-V CURVE MEASUREMENTS

Report prepared by: Madrid, 28 October 2011 Maria Crespo Cordo Chen Fei Zhou Edwin Raul Grijalva Campana João Mendes Lopes Ana Pérez Rodríguez Diana Crespo Astudillo

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Contents

CONTENTS 2

INTRODUCTION 3

MATERIAL 4

EXPERIMENTALPROCEDURE 5

PARTI 5CALIBRATIONPROCEDURE 5PRESENTATIONOFRESULTS 6LSM150‐01CELL 7III‐IVN6622CELL 10LSPO2CELL 12COMPARISONBETWEENCELLS 14DISCUSSIONOFRESULTS 14CONCLUSIONS 17

PARTII–INDIVIDUALCOMMENTS 17

PARTIIIBIBLIOGRAPHY 18

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Introduction The goal of this practice is broadly to learn to get in the laboratory the I-V curve of a solar cell and gain fluency in the processes of taking measures necessary to obtain that curve. For that, it is introduced several important concepts: - Reference solar cells. This solar cells are carefully calibrated in laboratories approved for it.

These cells will provide us set the desired irradiance for our works with the solar simulator. - Darkness I-V curve. This curve is obtained experimentally and it can observe its similarity to

the diode curve. In addition a method to rebuild the I-V curve from the darkness I-V curve was built.

- The rules that the standards (IEC 904-1 and ASTM E948-95) set to perform such measures optimally.

- A simple method to determine the polarity of the solar cell. Build i l luminated I-V curves us ing dark I-V curves . The goal of this section is to characterize the solar cell through the rebuilding of it illuminated I-V curve. The data needed are: - The dark I-V curve of the solar cell. In this situation, darkness, the solar cell behaves like a

diode and therefore consume power. Both with light and in darkness the solar cell presents a series resistance and a parallel resistance, because they are inherent to the material with which is built the cell. The measures of current correspond not only to the current that flows through the diode but also correspond to the current that flows through the parallel resistance of the solar cell. This can be easily seen in the circuit. 1.

- The Isc of the solar cell. It was measured at the beginning of the illuminated-measurement using an ammeter.

Circuit 1. Real solar cell

Darkness Illumination

Method to rebuild the illuminated I-V curve: - Subtract to the Isc the values of current obtained in darkness, I’. This method is actually a

simplified method in which is considered the Rs = 0 Ohms and Rp = ∞ Ohms. The current with illumination, I, is the sum of the current of the diode, Id, and the current through the series resistance. So when we subtract to Isc the I’, it obtains the illuminated I-V curve of the solar cell.

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Circuit2. Ideal solar cell.

Darkness Illumination

In graphs, both darkness and illumination, is observed that the section from 0V to the elbow of the curves is perfectly horizontal. This happens when the value of the parallel resistance is very large and therefore the assumption Rp = ∞ is actually true in our cell. Because of this, the current for the range from 0V to the elbow of the curves: in the curve of darkness is almost 0A and therefore, to subtract I’ to Isc has practically no influence. That is why when the illuminated I-V curve is rebuilt, it obtains that the section from 0V to the elbow of the curve coincides with the same section of the illuminated I-V curve. It works like the ideal circuit. However, with the series resistance is different. Looking at both curves, darkness and illumination, it sees that the section right to the elbow of the curves is not vertical but has a significant slope. The characteristic Rs of the solar cell is not ideal and therefore, greater than 0 Ohms. In the rebuilt I-V curve, the Rs is not also 0 ohms but is somewhat lower than in the illuminated I-V curve. The values of the current on the right of the elbow of the curves, darknes and illumination, grow exponentially. However within the same range of variation of V (variation of V of 0.15 from the elbow of the curve to the end of the curve) we have a variation in the range smoother in the darkness I-V curve than in the illuminated I-V curve (variation in the values of current much larger). The Rs in the darkness I-V curve is much lower. Therefore, the values calculated by subtracting to the Isc the I’ are, for each V, higher than the values measured directly with illumination. Finally, both curves will converge to the same point, the short circuit point (same value of Voc). All of this is why the slope and therefore, the Rs of the rebuilt curve is lower than in the illuminated curve. On the other hand, we can see that the rebuilt curve does not reach the value of Voc. This is simply because it was measured points with illumination to a higher voltage than in darkness.1

Material The equipment used for taking measures was: - Solar simulator. Made in the I.E.S. It was used to illuminate the solar cells with 1 Sun =

1000 W/m2. - Programmable load HP 4142B of four quadrants. It was used to measure the values of I

and V of the solar cell. The four-wires method was used to minimize losses in the measures. - Power supply ORIEL 68820. Universal power supply of 400W to 1000W with built-in

starter drive.

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- Multimeters. It were used to measure the Isc of the solar cell used as irradiance sensor and

also to measure the Isc and Voc of the DUT solar cells to choose the maximum and minimum values of I and V suitable for taking measures with the programmable load.

- Solar cells. The TUD solar cells (LSM150, LSP02 and III-V), the reference solar cell and the

solar cell used as irradiance sensor.

Experimental procedure The goal of this laboratory session was to study the fundamental procedures that are required to perform I-V curves measurements of solar cells. Specially, the type of details that one has to be aware when performing experimental measurements and acquisitions. To perform this objective, three PV-cells were characterized, and are presented in Table 1.

Name LSM150-01 LSPO2 N622

Cell type Monocrystalline silicon Polycristalline silicon III-V multijunction cell

Area 1053 mm2 420mm2 1mm2

Table 1 - PV cells that were studied in this laboratory session

Part I

Calibration procedure To perform the calibration of the solar Simulator to standard conditions, a reference cell was used, with the specifications presented in Table 2. Solar cell Connections Housing Monocrystalline silicon 4-wire contact to the solar cell with

standard 2mm connectors

Dimensions:6cmx6cmx6.6mm

n on p polarity White wires with black connectors: Back contac tof the cell (p-side)

Weight: 50g

Circular active area of 1.5cm2 Orange wires with red connectors: Top grid contact of the cell (n-side)

2Electrically and thermally conductive

Electrically insulated from the housing

Mechanically and thermally stable

Back contact and top grid is silver Table 2 - Specifications of the reference cell

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The Standard Conditions for a I-V curve measurement are 1000W/m2, AM 1.5G, T = 25ºC. The temperature was controled using a temperature controlable plate, and kept in T = 25 ± 1ºC. The reference cell, under an irradiance of 1000W/m2 and T = 25ºC, has an ISC value of 55.5mA. So, this value was used to calibrate the Sun Simulator irradiance at the working area level, by adjusting the height of the light source until reach the ideal ISC value. An acquisition of ISC reference cell was made using HP4142B, programmable power supply, using 200 acquisitions in 1 minute. The average value of this acquisition was 55.8 mA, with a standard deviation of 0.2 mA. ISC_Reference cell = 55.8mA ± 0.2 mA. Long term stability source The long term stability of the source was verified by monitoring the ISC value of a sensor cell throughout the entire session. ISC of silicon cells has an excellent linear response to irradiance1 changes was used to test if the irradiance was stable throughout the entire session. The reference cell was the only available cell, and it was used as irradiance sensor, and placed on a corner of the working area, with its entire active area illuminated, to avoid shadowing issues. The cell was placed in short circuit, and the tension in its terminals was measured to determine if the current was sufficiently close to ISC. The value of tension was 90.2 mV, which means this is a relatively good approximation. Assuming that the non-uniformity over the cell is negligible, we use the linear relation between irradiance and ISC. The minimum and maximum registered values of I of the irradiance sensor were, respectively, 52.2 mA and 52.9 mA, which represents a variation of 1.3%. This value is perfectly suitable to the intended objectives. Cell measurement procedure Each DUT measurement followed the specific procedure:

1. Preliminary measurements – Using the standard conditions defined during calibration for irradiance and temperature, preliminary measurements using multimeters were made to determine the limits of tension and current to be used for the HP4142B. Also, the polarity of each cell was determined with this procedure. So, VOC and an approximately ISC value was taken in this step

2. Under illumination and dark measurements using programmable load – Using the limits and polarity determined on the preliminary measurements, the programmable load was used to perform the measurements, changing the number of acquired and integration points. A combination of 20, 50, and 100 acquired points and 1, 5 and 10 integration points for each point was used, to determine the limits of the programmable load acquisition.

Presentation of results General conclusions can be taken from the observation and comparison between the results for the measurements of the three cells. So, the results for the measurements of the three cells are presented together, followed by a joint discussion. For each cell, the preliminary results are shown, followed by a comparison between the curves with different parameters. Also, dark measurements are presented,

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and a comparison between a reconstruction of IV curve from dark measurements, (as explained in the theoretical introduction), and the experimental IV curve is made.

LSM150-01 CELL

1- Results from preliminary measurements

Measurements using multimeter Measurements using HP load Cell ISC VOC Polarity Vmin Vmax Ilimit

LSM150-01 0,2206±0,0001A 0,579±0,001V n/p 0.2 -0.65 0.4 Table 3 – LSM150-01 - Preliminary measurementsl

2- Comparison between measurements under illumination

Figure 2 - LSM150-01 - (on the l e f t ) IV curve for 50 step, for different number of integration points. (On the r i ght ) - Detailed view of the values around Isc. It is possible to observe the different instability with different number of integration points, The most stable signal is the 5 integration points.

Figure1–LSM15001(ontheleft)IVcurvefor20step,fordifferentnumberofintegrationpoints.(Ontheright)DetailedviewofthevaluesaroundIsc.Itispossibletoobservethedifferentinstabilitywithdifferentnumberofintegrationpoints,.

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Figure 3 - LSM150-01 - (on the l e f t ) IV curve for 100 step, for different number of integration points. (On the r i ght ) - Detailed view of the values around Isc. It is possible to observe the different instability with different number of integration points, The most stable signal is the 5 integration points.

3- Comparison between the dark measurement

Figure 4 - LSM150-01 - (on the l e f t ) dark measurement for 20 step, for different number of integration points. (On the r i ght ) - Detailed view of the values around Isc. It is possible to observe that, for dark measurements, the signal has no instability.

Figure 5 - LSM150-01 - (on the l e f t ) dark measurement for 50 step, for different number of integration points. (On the r i ght ) - Detailed view of the values around Isc. It is possible to observe that, for dark measurements, the signal has no instability.

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Figure 6 - LSM150-01 - dark measurement for 50 step, for different number of integration points. As in Figure 4 and

Figure 5, no instability is detected

Figure 7 - LSM150-01 - Comparison between reconstructed IV curve from dark measurements and experimental IV curve. Right - 20 steps. Center - 50 steps. Left - 100 steps. 5 points integration curves

Figure 8 - LSM150-01 - Comparison between reconstructed PV curve from dark measurements and experimental IV curve. Right - 20 steps. Center - 50 steps. Left - 100 steps. 5 points integration curves

Figure 9 – LSM150-01 – Comparison of curves with different number of steps. (On the l e f t ) dark measurements. (On the r i ght ) Experimental IV curves

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Values for the LSM150-01 CELL

Steps Average VOC (±0,0001 V) ISC(±0,00001 A) PMAX (W) PMAX(expected)

1 0,5800 0,30990 - - 5 0,5804 0,30908 0,1218±0,0001 0,1337±0,0002

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10 0,5804 0,30966 - - 1 0,5700 0,30974 - - 5 0,5700 0,30800 0,1229±0,0002 0,1333±0,0002

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10 0,5692 0,30936 - - 1 0,5692 0,31082 - - 5 0,5700 0,30884 0,1221±0,0001 0,1336±0,0002

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10 0,5696 0,30840 - - Table 4 – LSM50-01 Summary table - ISC , VOC, PMAX (W), and the expected value for PMAX (W)

III - IV N6622 Cell

1- Results from preliminary measurements


N6622 - 269 ± 1uA 0,849±0,001V p/n -1mV 0.9 0.3 mA Table 5 – N6622 - Preliminary measurementsl


Figure 10 - NS6622 -(on the l e f t ) IV curve for 20 step, for different number of integration points. (On the r i ght ) - Detailed view of the values around Isc. It is possible to observe the different instability with different number of integration points,.

Figure 11 - N6622 - (on the l e f t ) IV curve for 50 step, for different number of integration points. (On the r i ght ) - Detailed view of the values around Isc. It is possible to observe the different instability with different number of integration points, The most stable signal is the 5 integration points.

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Figure 12 - N6622 -(on the l e f t ) IV curve for 100 step, for different number of integration points. (On the r i ght ) - Detailed view of the values around Isc. It is possible to observe the different instability with different number of integration points, The most stable signal is the 5 integration points.


Figure 13 – N6622 - (on the l e f t ) dark measurement for 20 step, for different number of integration points. (On the r i ght ) - Detailed view of the values around Isc.

Figure 14 - N6622 - (on the l e f t ) dark measurement for 50 step, for different number of integration points. (On the r i ght ) - Detailed view of the values around Isc.

Figure 15 - N6622 - (on the l e f t ) dark measurement for 50 step, for different number of integration points. (On the r i ght ) - Detailed view of the values around Isc.

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Figure 16 - N6622 - Comparison between reconstructed IV curve from dark measurements and experimental IV curve. Right - 20 steps. Center - 50 steps. Left - 100 steps. 5 points integration curves

Figure 17 – N6622- Comparison between reconstructed PV curve from dark measurements and experimental IV curve. Right - 20 steps. Center - 50 steps. Left - 100 steps. 5 points integration curves

Values for the III - IV N6622 Cell


1 0.850 2,68 E-4 - - 5 0.850 2,68 E-4 1,76 E-4 ±0,000001 1,72 E -4±±0,000001

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10 0.850 2,70 E -4 - - 1 0.839 2,68 E -4 - - 5 0.839 2,67 E -4 1,73 E -4±±0,000001 1,76 E-4±±0,000001

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10 0.839 2,70 E -4 - - 1 0.850 2,71 E-4 - - 5 0.850 2,68 E-4 1,78 E -4±±0,000001 1,76 E-4±±0,000001

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10 0.850 2,69 E-4 - - Table 6 – N6622 - Summary table - ISC , VOC, PMAX (W), and the expected value for PMAX (W)

LSPO2 Cell 1- Results from preliminary measurements


LSPO2 38.5±0.1mA 0,61±0,01V n/p 0.2 -0.65 0.4 Table 7 – LSPO2 – Preliminary measurements


Figure 18 – LSPO2 - (on the l e f t ) IV curve for 20 step, for different number of integration points. (On the r i ght ) - Detailed view of the values around Isc. It is possible to observe the different instability with different number of integration points,.

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Figure 19 - LSPO2 - (on the l e f t ) IV curve for 50 step, for different number of integration points. (On the r i ght ) - Detailed view of the values around Isc. It is possible to observe the different instability with different number of integration points, The most stable signal is the 5 integration points

Figure 20 – LSPO2 - on the l e f t ) IV curve for 100 step, for different number of integration points. (On the r i ght ) - Detailed view of the values around Isc. It is possible to observe the different instability with different number of integration points, The most stable signal is the 5 integration points.


Figure 21 – LSPO2 - dark measurements. Right - 20 steps. Center - 50 steps. Left - 100 steps. 5 points integration curves

Figure 22 - Comparison between reconstructed IV curve from dark measurements and experimental IV curve. Right - 20 steps. Center - 50 steps. Left - 100 steps. 5 points integration curves

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Figure 23 – LSPO2 - Comparison of curves with different number of steps. (On the l e f t ) dark measurements. (On the r i ght ) Experimental IV curves

Values for the LSP=2CELL


20 5 0.6216 0.1389 0.0622 ±0,0005 0.0759±0,0003

50 5 0.6132 0.1386 0.0624±0,0003 0.0758±0,0002

100 5 0.6132 0.1387 0.0627±0,0001 0.0757±0,0001

Table 8 – LSPO2 - Summary table - ISC , VOC, PMAX (W), and the expected value for PMAX (W)

Comparison between cells Cell Voc (V) Isc (A) Pmax (W) Area (mm) Efficiency (%) * LSM50-01 0.57 0.30884 0.1221 1053 12% N6622 0.85 2.68E-04 1.78E-04 1 18% LSPO2 0.6132 0.1387 0.0627 420 15%

Table 9 - Comparison between cells

*(assuming 1000W/m2, on the Test Conditions)

Discussion of results The main observations are:

• For all the graphics of cells under radiation, there is a visible difference in the instability of the signal for different averages. In specific, 5 is the average value which stabilitizes the signal the most. This can be explained by observing the irradiance instability of the source, in Figure 24 (from the previous laboratory session). Although it was not possible to calculate the main frequency of the source instability due to a incompatibility with Fast Fourier Transform Algorithm, it is possible to observe that there is a typical frequency associated with the signal. Our best hyphotesis for explaination is that this frequency interfers directly with the sample rating for 5 integration values, thus it is possible to cut the oscilation from the source. Unfortunately, we do not have the necessary data to comprove this theory.

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Figure 24 – example of source instability

• In dark measurements, no oscilation was detected for LSM50-01 cell (Figure 5). For N6622 cell, some oscilation was detected (Figure 14). This is explained by observing the value of currents for this graphic. The value is very small, and the oscilation is caused by parasitic radiation. It can be also verified that in this case, all the integration points oscilate randomsly, and no specific behavior is detected, which gives more strength to our previous theory of source instability.

• Increasing the density of points increases directly the precision of the IV curves, as well as the acquisition time. For the type of cells that we are studying, an acquisition with 100 points and 5 integration points takes around 4 seconds, which is perfectly suitable the goals, thus in this case 100 points is a good choice to measure the performance of a cell.

• The reconstruction of the IV curve from the dark cell measurement allow to have a good idea, and even a number for the quality of a cell. As the reconstruction is based on ideal model (Rs = 0, Rshunt = infinite), then we can consider the distance of the real measurement to the reconstructed curve as quality measurement. Considering the particular cases that were analyzed, LSM50-01 and LSPO2 IV curves do not follow the reconstruction curve, and they deviate from it when arrive near the Pmpp value.(Figure 7 and Figure 22). According to the electrical model for a PV cell, this type of deviation means that the serie resistance value of these two cells is not near the ideality (i.e., it moves away from zero). This non-ideality measurement gets even more obvious when comparing with Figure 16, for N6622 cell, where the experimental IV curve is much closer to the reconstructed curve. The series resistance is nearer the ideal value than for the other two cells. When comparing the efficiencies of the cells on Table 9, N6622 comproves this theory, having a considerable higher efficiency in this range of irradiance than the other two cells. This was also expected considering that N6622 cell is a III-V multijunction cell, thus with an optimized design. In summary, the reconstruction of IV curve from dark measurement can be used in two different ways:

1. Knowing Isc, it is a method that can be used to perform quick (but no precise) IV curves when no calibrated light source is available. Higher the quality of the cell, higher this method will be from the real IV curve

2. It is an excelent method for having a quality measurement for a cell, by measuring the distance between the experimental IV curve and reconstructed IV curve. An alternative for FF factor.

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• The preliminary measurements enabled to have a previous knowledge about the limits of IV curve, that can be used to calibrate the programmable load. Also, the polarity of the cell was used, which is important when interpreting the results and also to ensure that the programmable load and the cell’s earth is the same, to avoid short circuits (for this simulator, the back contact plate is always the earth).

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Conclusions In this laboratory, three different type PV cells were characterized regarding their IV curve and dark measurements. The three cells were successfully characterized, and the main parameters from the I-V curves were extracted. The utility of reconstruction of IV curves from dark measurement was verified as an useful method for measuring the non-ideality of a cell, and the non ideal values of series resistance for LSM50-01 and LSPO2 celll were verified.

1. Knowing Isc, it is a method that can be used to perform quick (but no precise) IV curves when no calibrated light source is available. Higher the quality of the cell, higher this method will be from the real IV curve

2. It is an excelent method for having a quality measurement for a cell, by measuring the distance between the experimental IV curve and reconstructed IV curve. An alternative for FF factor.

Also, this method allowed to have a clear notion of the diference of quality between the three cells (with N6622 having a much higher behaviour). The session also enabled a clear comprehension on the limits of the acquisition instruments, with interference questions between the sample rate and the instability from the source. The difference between the three cells was analyzed and their differences observed. N6622 has a higher efficiency than the other two cells in the Testing Conditions, which was expected, considering is a multijunction cell and, although not optimized to work at 1 sun (i.e. it’s efficiency, using irradiance conditions, is not the highest posssible) still has a higher efficiency of the three studied cells.

Part II – Individual comments The practice has been very interesting, as I learned to reconstruct the curve of the solar cells lighting

and darkness, the difficulties I have found, is to interpret in a better way data obtained from current

and voltage to determine the type of cell, but I think more practice will improve. (Edwin Grijalva)

During this practice I understood more things than the previous session, but the part that even I

have trouble understanding is the polarity of the solar cell, when pn or np and how to connect to the

programmable power supply. (Diana Crespo)

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My difficulty is about the concept of HP 4142B programmable power supply connecting criteria,

Thus, in the experimental process, I connected wrong. And I cannot understand the directions of

current and voltage.（Feizhou Chen）

I did not find very serious difficulties during this session. The only issue is that I still find that two

hours it is not enough time for all the measurements, so we had to hurry at the end and we still

finished half an hour later. (Ana Pérez)

Conceptually speaking, the most difficult and challenging part was to understand the problem

between the interference of the source frequency and the sample rate of the programmable load.

Unfortunately, we were not able to measure the duration time of the acquisitions, and therefore we

were not able to perform a complete analysis to this question. Also, I had quite some fun by start

programming in MatLab for data analysis automation, and I think it will save us a lot of time in

future practices. (João Mendes Lopes)

In the realization of the practice, I had no specially difficulties. I could do all the steps for taking

measures and assimilate the entire procedure. In relation with the theoretical concepts, it has been

interesting for me how to use the reference solar cell to know the irradiation of the solar simulator

and the method for obtain the I-V curve using the dark I-V curve and the Isc. (María Crespo

Cordo).

Part III - Bibliography 1 - EVALUACIÓN DE LA RESISTENCIA SERIE Y PARALELO Y SU INFLUENCIA EN LA PÉRDIDA DE POTENCIA EN UN MÓDULO CON 12 AÑOS DE EXPOSICIÓN. M. T. Montero, H. F. Bárcena, F. R. Farfán, C. A. Cadena Instituto de Investigación en Energías No Convencionales (INENCO - CONICET) 2 - http://itacanet.org/eng/elec/solar/pv.pdf, accessed on 27 October 2011

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