snvhome.netsnvhome.net/ee-braude/devices-lab/labs/Solar cell lab... · Web viewAdvanced Laboratory for Characterization of Devices – 31820 Dr. Radu Florescu Dr. Vladislav Shteeman

Solar Cell(photovoltaic cell)

May 19, 2023

Department of Electrical & Electronic Engineering Braude College of Engineering

Advanced Laboratory for Characterization of Semiconductor Devices - 31820

Dr. Radu Florescu Dr. Vladislav Shteeman

Department of Electrical and Electronic Engineering Braude College of Engineering

Advanced Laboratory for Characterization of Devices – 31820

The goal.In this experiment, you will measure the Current-Voltage (I-V) characteristics of a

photovoltaic cells’ module and extract its main physical parameters using computerized

parameter analyzer Keithley SCS 4200.

The following characteristics will be extracted from the I-V measurements:

1. Dark I-V characteristics

1.1. Shunt resistance R shunt and saturation current I sat

1.2. Series resistance R series

1.3. Ideality factor n

2. Illuminated I-V characteristics

2.1. Maximum power point Pmax .

2.2. Maximum Power Voltage and Current values Vmax and Imax such as

Pmax=ImaxVmax .

2.3. Cell efficiency η=

PmaxPin where Pin is the power of the incoming light.

2.4. Open Circuit Voltage V OC (output current I=0 ).

2.5. Short Circuit Current I SC (output voltage V=0 ).

2.6. Fill Factor FF=ImaxVmax / ( I SC V OC )

Dr. Radu Florescu Dr. Vladislav Shteeman 2



Short theoretical background.

Solar cell[1] (also called photovoltaic cell) is a specific kind of semiconductor diode, that

directly converts the energy of light into the electrical energy through photovoltaic effect

(see Figure 1).

Figure 1. Sketch of principle of operation of solar cell.

Typical I-V characteristics of a solar cell is shown on Figure 2.

Figure 2. Sketch of I-V characteristics of a solar cell (after [2]).




Today, the majority of solar cells are fabricated from silicon. Unlike batteries or fuel cells,

solar cells do not utilize chemical reactions or require fuel to produce electric power, and,

unlike electric generators, they do not have any moving parts.

Equivalent circuit of solar cell is shown on Figure 3:

In solar cell, there are 2 parasitic resistances: series resistance R seriesand shunt resistance

R shunt , which impact the performances of photovoltaic device (see Figure 3 and List of

definitions in for definitions of R series and R shunt ). Ideally, series resistance should be zero (

Rseries=0 ), while shunt resistance should be infinite (R shunt=∞ ).

1. Model of solar cell I-V characteristics [2],[3].

At the 1 st approximation , I-V characteristics of solar cell (being a specific kind of diode),

measured in the dark, can be described by the modified Shockley equation, corrected for

series resistance R series .


Figure 3. Equivalent circuit of solar cell (after [2]).



As opposite to a “standard” diode, R series affects I-V characteristics of solar cell all along, not

only for V D>V 0 (where V 0 is a built-in voltage). R series here is a model, representing (arising

from) the combination of three majors resistances:

1) the intrinsic silicon resistance

2) the contact resistance between the metal contact and the doped silicon regions

3) the resistance of the top and the rear metal contacts

Thus,

I D=I ( dark )=I sat ( dark )(e (V D−I D Rseries)

kT /q −1)⏟dark current (1)

(where I D , V D - measured current and applied voltage, I (dark ) - current measured in the dark,

I sat ( dark ) - saturation current of ideal pn-junction (from Shockley modes), q - electron charge,

k - Boltzmann constant, T [ K ] - temperature)

At the 2 nd approximation , due to the large surface and the complex association of non-ideal

materials, generation-recombination current in the depletion layer should be accounted for:

I D=I ( dark )+ I ( gen )=I sat ( dark )(e (V D−I D Rseries)

kT /q −1)⏟dark current

+ I sat ( gen)(e

(V D−I D R series)2kT /q −1)⏟

generation current (2)

(I sat ( gen ) - saturation current due to generation-recombination only)




The usual practice is to introduce an ideality factor, n , which allows to account for both

currents in a single expression:

I D=I sat(e

(V D−I D Rseries )nkT /q −1)

(3)

(I sat - saturation current due to both generation current and “dark” current)

At the 3 rd approximation , one should account for influence of shunt (parallel) resistance,

R shunt . Presence of R shunt may cause significant power losses in a solar cell. Those losses are

typically due to the manufacturing defects, rather than because of poor solar cell design. Low

R shunt causes power losses by providing an alternate current path for the light-generated

current. This reduces the current, flowing through the solar cell pn-junction and reduces the

voltage, acquired from the solar cell.

The influence of R shunt is particularly strong at low light intensities, since there will be less

light-generated current. In addition, at lower voltages where the effective resistance of a

solar cell is high, the impact of a resistance in parallel is large.

Thus, accounting for R shunt gives:

I D=V D−I D R series

R shunt+ I sat

(e (V D−I D Rseries)nkT /q −1)

(4)

At the 4 th approximation , under illumination conditions, the additional current, I (light ) , is

generated by the electron-hole photoemission. Thus, the total current via the photovoltaic

cell will have a form:




I D=V D−I D R series

R shunt+ I sat

(e (V D−I D Rseries )nkT /q −1)

⏟different currents in the dark

− I ( light )⏟light current

(5)

2. Evaluation of different physical parameters of solar cell from

the I-V measurements.

2.1. Evaluation of Imax ,Vmax ,I SC , V OC , Pmax ,FF (see List of symbols in Appendix 1 for

details).

All the physical parameters above can be estimated from I-V characteristics of forward biased

solar cell (see Figure 4 for details).

Figure 4. Typical forward bias I-V characteristics of a solar cell (after [2]).




2.2. Evaluation of the shunt resistance R shunt and saturation current I sat .

R shunt and I sat can be derived from the graph of reverse-bias I-V measurements (in range

from 0 to ~ −1÷−2 [ V ] ) (see Figure 5). The test should be performed in the dark.

Figure 5. Explanation to calculation of R shunt and I sat of solar cell (after [2]).

R shunt and I sat can be calculated from the linear fit of the I-V graph:

R shunt=1

slope, I sat=free term

(6)

The expected value of R shunt in this experiment is~400 [Ω ] .

2.3. Evaluation of series resistance R series .

We’ll estimate series resistance R series using so-called Slope method. This method is based on the measurements of I –V characteristics of the cell at two different light intensities giving

the short-circuit currents I SC (1 ) and I SC (2 ), respectively (see Figure 6).




Figure 6. Explanation to the series resistance measurement by the Slope method (after [2]).

The current δ I below the I SC , I=I SC−δ I , is picked on both I–V curves. The currents I(1)=I SC (1 )−δ I and I(2)=I SC (2 )−δ I correspond to the voltages V (1 ) and V (2 ). The series resistance is then:

R series=ΔVΔI

=V (1 )−V (2)

I (2 )−I (1)=

V (1)−V (2 )

I SC (2)−I SC (1)= 1

slope(7)

By using more than two light intensities, more than two points are generated. Drawing a line through all of the points gives the series resistance by the slope of this line (see Figure7).

Figure 7. Example of series resistance measurements by the Slope method (after [2]).

The expected value of Rseries in this experiment is~0 .8−1 .2 [Ω ] .


I SC (1 )

I SC (2 )

R series=ΔVΔI

=V (1 )−V (2)

I (2 )−I (1)



2.4. Evaluation of ideality factor n .

Knowledge of R series , R shunt and I sat allows one to estimate (from the dark forward bias

measurements) the ideality factor n . As it follows from the Eq. (4), for the region of

voltages V D>0 .15 [V ] (i.e. when exp( (V D−ID Rseries )

nkT /q )>>1 holds),

ID=V D−I D R series

Rshunt+ I sat e

(V D−I D Rseries )nkT /q

(8)

Or

ln ((I D−V D−I D Rseries

R shunt )/I sat)=V D−I D Rseries

0 .027n(9)

(where kT /q at the room temperature was replaced with 0.027 [V]).

One can make graph ln ((I D−

V D−ID Rseries

Rshunt )/ I sat) vs (V D−I D Rseries ). The acquired

curve should be approximately linear. The slope of this curve should be equal to

10 .027n (see Figure 8 for details).

Figure 8. Explanation to computation of ideality factor n.




Assignments and analysisPreparation of the experimental setup

1. Check (and change if necessary) the cables connections on the rare panel of the

switching matrix and S4200 analyzer (see for details about the cables connection).

2. Place the box with the solar cell and light power sensor the chunk table in the

Shielded probe station.

3. Connect the “+” contact of the solar cell to the 1st and 2nd cables and the “-” contact

to the 3rd and 4th cables.

4. Connect the light power sensor to the Power meter.

I-V characteristics measurementsNote: Before executing the measurements and processing the acquired data, save this

Excel template to the Desktop of the Keithley computer (double click on the Excel icon File Save as … ). During the measurements, fill in the data in the B - D columns of the template. After finishing the measurements, copy their results (located in the measurements folder of Keithley in the subdirectory “tests/data”), namely, data from the files “rev-ivsweep#[email protected]” and “fwd-ivsweep#[email protected]” to the Excel template.

Dark I-V characteristics

1. Open the Keithley Solar cell program. Manually connect pins (A1 – B2 – D3

– E4) using the switching matrix and the light pen (see ).

2. Close the doors of the Shielded probe station and run the measurements of

the forward and backward biased I-V characteristics.

SAVE ALL THE RESULTS in the Keithley program.




Light I-V characteristics

IMPORTANT: in this experiment you use Solar simulator lamp. DO NOT operate

the simulator under power supply 150 W and above 200 W. Normal operational

mode of the lamp, as defined by the manufacturer, is 150 W – 200 W.

1. Switch on the Solar simulator lamp (using the lamp controller) in the

Shielded probe station. Use “set” button on the controller panel

(press and hold 2 seconds) and arrows buttons to set the power,

supplied to the lamp, to 190 W.

2. Wait for 5 minutes to let the lamp warm up.

3. Check if the light spot from the lamp fully covers the area of the

photovoltaic cell. If it is necessary – move the lamp upward or

downward to change the size of the spot.

4. Switch on the power meter and zero its meterage (press “zero” button

when the device’ detector is inside the Shielded probe station).

5. Measure the incident light power on the light spot, using the Power meter.

Write the measured value into the Excel table.

6. Run the Keithley measurement program and acquire I-V characteristics of

the photovoltaic cell for forward bias only . (PAY ATTANTION: Use the yellow-greed

button (“append”) to run the measurement. DO NOT use the green button

(“override”): it overrides your previous measurements.)

SAVE ALL THE RESULTS in the Keithley program.

7. Repeat the measurements (steps 5 - 6) for additional 4 lamp’ supplied powers (e.g.

180 W, 170 W, 160 W, 150 W). (Use “set” button on the controller panel (press and

hold 2 seconds) and arrows buttons to set the power, supplied to the lamp, to the

required value, and wait for 2 minutes until the lamp’ radiation will be stabilized.)

SAVE ALL THE RESULTS in the Keithley program.Dr. Radu Florescu Dr. Vladislav Shteeman 12

light spot from the lamp

photovoltaic cell



Use Table 1 to convert the electrical power, supplied to the Solar emulator, into the

optical power (per unit area) of the outgoing light.

Evaluation of physical parameters from I-V measurements.

1. Shunt resistance and saturation current R shunt and I sat . From the dark I-V

measurement at the reverse bias, evaluate the shunt resistance R shunt and I sat of the

solar cell, as explained in the subsection “Evaluation of the shunt resistance R shunt and

saturation currentI sat “ (see p. 8).

2. Series resistance R series. From the light I-V measurements at the forward bias,

evaluate the series resistance R series of the solar cell, as explained in the subsection

“Evaluation of series resistance R series “ (see p. 8).

3. Ideality factor n . From the dark I-V measurements at the forward bias in the voltage

region V D≥0 .15 [V ] , evaluate the ideality factor, n , as explained in the subsection “Evaluation of ideality factor n ” (see p.10).

4. Different physical parameters of the solar cell. For each of the I-V measurements

(both light and dark) at the forward bias, calculate Imax ,Vmax ,I SC , V OC , Pmax ,FF , as

explained in the subsection “Evaluation of Imax ,Vmax ,I SC , V OC , Pmax ,FF ” (see p. 7).

Fill in the following Table (in the Excel file):

Pin [Watt /cm2 ] Imax [ A ] Vmax [ V ] ISC [ A ] V OC [ V ] Pmax [Watt ] FF ηIntensity 1Intensity 2Intensity 3Intensity 4Intensity 5




(Here, Pin [ Watt ] is a multiplication of the incoming optical power per unit area (see Table

1 in Appendix 4) and the area of the photovoltaic cell (4.25 cm2) ).

5. Present graphically the data, acquired in the Table above. Namely, build in Excel (and

include in the Final report) the following graphs:

a) Imax vs Pin b) Vmax vs Pin c) I SC vs Pin d) V OC vs Pin

e) Pmax vs Pin f) FF vs Pin g) η vs Pin

Final Report content.

Final Report must include the following solar cell’ parameters and graphs with explanations :

[1] I-V characteristics of solar cell in the dark (2 graphs: I D (V D ) for forward and for reverse bias)

[2] A set of I-V characteristics of solar cell under different lightening intensities|ID (V D )| (a single

graph)

[3] Shunt resistance R shunt (a single value)

[4] Series resistance R series (a single value)

[5] Saturation current I sat (a single value)[6] Ideality factor n (a single averaged value)[7] A filled table:

Pin [Watt /cm2 ] Imax [ A ] Vmax [ V ] ISC [ A ] V OC [ V ] Pmax [Watt ] FF ηIntensity 1Intensity 2Intensity 3Intensity 4Intensity 5

[8] The following graphs (2 graphs, including each 3-4 subplots):




a) Imax vs Pin b)Vmax vs Pin c) I SC vs Pin d)V OC vs Pin

e) Pmax vs Pin f) FF vs Pin g)η vs Pin




Experimental set-up and sample to be studied

The experimental setup includes

Keithley switching matrix and SCS 4200 I-V and Parameter analyzer (Figure 9)

Shielded probe station (SPS) with the Solar simulator lamp (Figure 10)

Photovoltaic cell (Figure 11)

Portable power meter (Figure 12)


Keithley 708A Switching Matrix

Monitor

Keithley SCS

4200 I-

Figure 9. Keithley measurement setup

simulator lamp

Lamp controller

Photovoltaic cell

Figure 10. Shielded probe station (SPS) with the Solar simulator lamp and controller.

Figure 11. Photovoltaic cell.

Figure 12. Power meter



AcknowledgementElectrical Engineering Department of Braude College would thank to Alex Cherchun for his extensive help in the preparation of this laboratory work.

Several parts of this brochure were adapted from the Amorphous Silicon Solar Module manual of the Advanced Semiconductor Devices Lab (83-435) of School of Engineering of Bar-Ilan University. We would like to thank Dr. Abraham Chelly for the granted manual.


Keithley SCS

4200 I-



Appendix 1 : List of symbols and definitions List of symbols

R series- series resistance [Ω ]

R shunt - shunt resistance [Ω ]

Pmax - maximum power point [ W ] I D - current through the pn-junction[ A ] I sat - a total saturation current, including

different currents in the dark[ A ]

I ( dark) - dark current [ A ]

I sat (dark ) - saturation dark current (saturation

current of ideal diode) [ A ]

I ( gen ) - generation current

in the depletion region[ A ]

I sat ( gen ) - saturation current of generation current [ A ]

I (light ) - light current [ A ] q=1 .6×10−19

- electron charge [ C ] n - ideality factor [dimensionless]

I SC - short circuit current (output voltage V D=0 ) [ A ] V OC - open circuit voltage (output current I D=0 ) [ V ] V D - voltage (bias) applied to the diode (solar cell) [ V ] V 0 - Built-in voltage

Imax - maximum power current (corresponding to the max. power point Pmax ) [ A ] Vmax - maximum power voltage (corresponding to the max. power point Pmax ) [ V ] FF - fill factor: FF=ImaxVmax / ( I SC V OC ) [dimensionless]. % of efficiency vs. an ideal cell.

A - pn-junction’ cross-section area [cm2] kT - thermal energy (i.e. energy, associated with the temperature of the object,T )

kTq - thermal voltage [ V ] . For the room temperature: T=300∘ K kT /q=0 .026 [ V ]

Pin incoming light’ power [ W ] . Can be computed as a multiplication of the incoming optical power per unit area (see Table 1 in Appendix 4) times the area of the solar cell.




η - cell efficiency [dimensionless] . η=

PmaxPin : power output as a ratio of power input to the

cell.

List of definitions

Series resistance R series- resistance due to the resistance of the metal contacts, ohmic losses in the front surface of the cell, impurity concentrations, and junction depth. Ideally, the

series resistance should be zero (R series=0 ).

Shunt resistance R shunt - resistance, representing the losses due to surface leakage along the edge of the cell or due to crystal defects. Ideally, the shunt resistance should be infinite

(R shunt=∞ ).







Appendix 2 : Cable connection of the switching matrix and SCS 4200 I-V analyzer for I-V measurements of Solar

cell.

Figure 13. Standard cable connection (all the experiments except Solar Cell).




Figure 14. Solar cell cable connection.




Appendix 3 : Kite settings for I-V measurements.

1.Making Connections to the Solar Cell for I-V Measurements:

Figure below illustrates a solar cell connected to the Model 4200-SCS for I-V measurements. One side of the solar cell is connected to the Force and Sense terminals of SMU1; the other side is connected to the Force and Sense terminals of the ground unit (GNDU) as shown.

Using a four-wire connection eliminates the lead resistance that would otherwise affect this measurement’s accuracy. With the four-wire method, a voltage is sourced across the solar cell using one pair of test leads (between Force HI and Force LO), and the voltage drop across the cell is measured across a second set of leads (across Sense HI and Sense LO). The sense leads ensure that the voltage developed across the cell is the programmed output value and compensate for the lead resistance.




2.pin connection scheme:

3. I-V Keithley settings

Connect pins

Note that because of simultaneous usage of “force” and “sense” inputs, pin connection must be done manually.


SMU 1 (cables 1 and 2) GND (cables 3 and 4)



In order to connect pins (as shown on the figure above), do the following steps:

press “local” button on the switching matrix panel take the light pen, connected to the matrix, bring it to the close proximity of the A1

cell of the matrix, and press once the button on the pen (A1 cell will light up). repeat for B2, D3 and E4 matrix cells press “copy” button to save the connections

forward bias settings




Expected results – forward bias

Use the yellow-greed button (“append”) to run the series of measurements of I-V

characteristics under different lightening conditions. DO NOT use the green button (“override”): it overrides your previous measurements.




Appendix 4 : Solar Simulator A solar simulator (also artificial sun) is a device that provides illumination approximating natural sunlight. The purpose of the solar simulator is to provide a controllable indoor test facility under laboratory conditions, used for the testing of solar cells, sun screen, plastics, and other materials and devices.

In this experiment, you will use Newport Corp. Solar simulator with Xenon Short Arc Lamp. This Solar Simulator provides close spectral match to solar spectra. The match is not exact but better than needed for many applications.

For the supplied (electrical) power of 80 W, this lamp produces light with the intensity (per unit

area) of ~0 .04 [W cm2 ], i.e. approximately ½ of the intensity of Solar light. Incoming electrical

power and outgoing optical power of the Solar simulator are shown in Table 1.

Table 1. Incoming electrical power and outgoing optical power of the Solar simulator.

Electrical power supplied to the Solar

simulator [W]

Output optical power per unit area [W/cm2]

Pin [W], optical power, which can be transformed into the

electrical power

100 0.065 0.276100 + ND2 0.019 0.08100 + ND 4 0.0125 0.053

90 0.056 0.23880 0.04 0.17


Newport Solar simulator (Xenon lamp) with controller unit.

Cut – away view of a Newport Solar simulator

Output optical power [W/cm2] times photovoltaic cell area (4.25 cm2)



Appendix 5 : Neutral Density (ND) optical filters. Neutral density (ND) filters are used to attenuate the intensity of a light beam. An ideal neutral density filter reduces intensity of all wavelengths of light equally.

The number after ND abbreviation means the “reduction power” of the filter. For example, ND2 reduces twice the incoming power (transmittance 50%), ND4 reduces fourth the incoming power (transmittance 25%), ND8 reduces eights the incoming power (transmittance 12.5%), etc.

In this experiment you will use a set of simple photo ND filters. Unfortunately, since those filters are NOT scientific grade, they strongly cut the incoming optical power at the

near IR wavelengths (λ>0 .65 [ μm ] ). Nevertheless, since we do not issue the question “what part of the solar spectrum produces the photocurrent”, they still can be used to attenuate the incoming optical power.




Bibliography 1 Solar cell – wikipedia: http:// en.wikipedia.org/wiki/Solar_cell

2 “Photovoltaic measurements: testing the electrical properties of today’s solar cells”. Keithley

Instruments, 2009.

“Making I-V and C-V measurements on solar / photovoltaic cells using the model 4200 SCS

Semiconductor Characterization System”. Keithley Instruments – Application Note series (No

2876).

3 A. Chelly, “Amorphous Silicon Solar Module”, Lab manual - Advanced Semiconductor Devices

Lab (83-435), School of Engineering of Bar-Ilan University.

4 B. Van Zeghbroeck, “Principles of semiconductor devices”, Lectures – Colorado University,

2004.

5 B. Streetman, S. Banerjee, “Solid state electronic devices” (6th edition), Prentice Hall, 2005.

6 R.F. Pierret, "Semiconductor Device Fundamentals", Addison-Wesley 1996.


http://en.wikipedia.org/wiki/Solar_cell

http://en.wikipedia.org/wiki/Solar_cell



Preparation Questions

1. Explain (in short) the principle of operation of solar cell2. Plot equivalent circuit of solar cell3. Plot (on a single figure) qualitative graphs of a dark and illuminated solar cell I-V

characteristics (Hint: see “Expected results” in )4. Plot a single qualitative “upside down” graph (i.e. graph –I vs V) of illuminated solar cell I-

V characteristics. On the graph, indicate the following parameters:

Imax - maximum power current

I SC - short circuit current

Vmax - maximum power voltage

V OC - open circuit voltage

Pmax - maximum power point

(Hint: see the figure in Appendix 1)

5. Why must a solar cell be operated in the 4th quadrant of the junction I-V characteristics ? (Hint: see Streetman “Solid State Electronic Devices”, 6th edition, Chapter 8, problem 8.7)


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snvhome.netsnvhome.net/ee-braude/devices-lab/labs/Solar cell lab... · Web viewAdvanced Laboratory for Characterization of Devices – 31820 Dr. Radu Florescu Dr. Vladislav Shteeman