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SIMPLE DATA ACQUISITION OF THE CURRENT-VOLTAGE AND ILLUMINATION-VOLTAGE CURVES OF SOLAR CELLS

F. Recart1, H. Mckel

2, A. Cuevas

2and R.A. Sinton

3

1 Teknologia Mikroelektronikoaren Institutua, Universidad del Pais Vasco / Euskal Herriko Unibertsitatea, Spain2

Dept. of Engineering, FEIT, The Australian National University, Australia.3

Sinton Consulting, Inc., Boulder, CO, USA.

ABSTRACT

Three electronic circuits that facilitate themeasurement of I-V characteristics of solar cells aredescribed and analyzed. The first circuit enables themeasurement of the one-sun illumination and dark I-Vcurve in a steady state fashion. The second circuit isbased on a large capacitor, which, when discharged,

forces the cell to progress in a quasi-static (QSS)transition through different voltage and current states. Thecircuit can also be used to set transient conditions andcharacterize the cells open circuit voltage decay (OCVD)and series resistance. The third circuit is a logarithmicconverter, which allows, combined with a photodiode, forthe detection of light intensity over eight decades ofmagnitude. It facilitates the measurement of wide rangesof QSS Suns-Voc in a very inexpensive way. The accuracyof these circuits and measurement methods has beenchecked on silicon solar cells, leading to an excellentagreement with established techniques.

INTRODUCTION

The static current-voltage (I-V) curve of solar cellsmeasured under nominal operating illumination (usuallyone-sun) is the definitive indicator of their performance. Inaddition, dark I-V and Isc-Voc (or equivalently, suns-Voc)characteristics assist to better understand the devices andextract information about effective lifetime, seriesresistance or leakage current. The measurement of thestatic I-V characteristics is normally performed whilemaintaining a steady-state illumination (and temperature),biasing the cell at different voltages and measuring thecorresponding current intensities or, in the case of suns-Voc, measuring the open-circuit voltage.

Nevertheless, the characteristic curves can also beobtained in dynamic conditions using flash testers thatavoid the difficulties of producing a steady continuous lightsource. These can be categorized into quasi-steady-stateand peak flash illumination methods with a sub-divisioninto single and multi flash testers. Single flash testerstrace the entire I-V curve in a few milliseconds during aplateau of constant light, ensuring a fast data acquisition(the costs are exacerbated due to the flash requirements).

They can also be afflicted by transient errors due to therelatively fast change in voltage during the measurement.Multi-flash testers are considerably cheaper and usuallyavoid transient errors, but they suffer from the drawback ofrequiring a flash at each point of the I-V curve.

In most commercially available steady-statesystems and single flash testers [1], the cell is biased viaan active load. This electronic load has to handle large

currents and remain stable at any bias conditions. It is arather expensive component that significantly increasesthe entire I-V apparatus cost in small systems and mayalso present stability problems for different cells sizes. Analternative is to use a passive load, usually a largecapacitor that either forces the cell to a constant voltage orsweeps the entire voltage range. The constant voltageflash tester, for example, utilizes a large capacitor to keepthe cell at a constant voltage for each flash, thus widelyavoiding transient errors [2]. Although the use ofcapacitive loads is relatively extended, someconsiderations should be done about the design of thesevery simple circuits and their applications. This issueconstitutes the main part of this paper.

As for the measurement of the suns-Voc (or Isc-Voc)

characteristics, the true SS approach is the use of severallevels of steady illumination and the measurement of theopen circuit voltage and irradiance (or short circuitcurrent). However, it is simpler and equally effective todetermine the cells characteristics by the quasi-steady-state voltage method (QSSVoc) [3], which generates theI-V curve employing a flash lamp. If the short-circuitcurrent is approximately known, or estimated, an I-V curvecan be produced that is similar to the standard curve barseries resistance effects. The detection of the entireintensity-voltage range, however, requires sweepingseveral orders of magnitude of light intensity, which posesa problem for the data acquisition. A satisfactory resolutionof the intensity is usually only guaranteed for two or threeorders of magnitude. Thus in general, three to five flashescovering different intensity ranges are necessary to recordthe entire curve. In this work, we present a circuit thatstrongly facilitates the acquisition of the suns-Voc curve byemploying a direct logarithmic conversion of the intensitysignal. The circuit is able to handle conveniently currentsas low as 1 nA, which makes it useful for detectingluminescence signals avoiding the use of preamplifiers.


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ILLUMINATED I-V CURVE UNDERSTEADY AND QSS CONDITIONS

Conditioning system for SS measurements

State-of-the-art A/D cards allow for thesimultaneous accurate acquisition of signals presentingvery high input impedances, several differential channelsand their acquisition rate is sufficient for many

applications. They can be used, for instance, to apply abias voltage to a device under test and to measure, indifferential mode, the current that flows through a resistorplaced in series with it, so that a point of the steady state(SS) I-V curve is directly measured. If an array of biasvoltages is applied and every voltage is applied for a longtime, static I-V curves are obtained.

However, there is a remaining issue, becausemany devices, including solar cells, require higher currentsthan those provided from the card. This can be solvedusing an auxiliary power supply and a signal conditioningcircuit, like that of Fig. 1. It shows a simple circuit thatenables the measurement of one-sun illumination anddark I-V curves of cells in a steady-state manner changingthe forward voltage step by step, from the data acquisition

card. In our set up, the circuit was built using a 9 V, 5 Asymmetric power supply, an operational amplifier and apush-pull bipolar transistor output stage. The negativefeedback of the operational amplifier forces the voltagefixed at the output of the data acquisition card to appear atcell, providing the necessary current (up to 2.5 A). Stabilityproblems may arise depending on the cell structure orsize, requiring sometimes the use of capacitors to cut offpositive feedback frequencies.

The voltage is measured directly at the cellterminals using separate contacts, while the current is

measured by means of the voltage drop at the 1 W seriesresistor. This leads to good accuracy for high currents,with a lower limit of 0.5 mA. For smaller currents, a moreaccurate current value is obtained from the voltage drop at

the auxiliary 100 W resistor. Note that in this case thebase-emitter voltage is smaller than 100 mV, the currentthrough the transistor base terminals is negligible and the

current flows entirely through the 100 W resistor. Usingboth resistors, an accuracy better than 5% has beenproven in the 2.5 mA 2.5 A range.

Capacitor based circuit for QSS I-V measurements

The measurement of the photovoltaic I-V curveunder quasi-steady state (QSS) conditions is achieved byconnecting the cell in parallel to a passive load, essentiallyconsisting of a large capacitor. The circuit diagram isshown in Fig. 2, with voltage and current (obtained fromthe voltage drop at the shunt) monitored by means of twoinput channels of the A/D card.

Initially, the cell (which is assumed to be underone-sun illumination provided by a continuous steady lightsource) is in open-circuit conditions and the largecapacitor (10 mF) of the circuit is charged presenting avoltage fixed from the power supply in the range of 1.5 V-9 V (negative from the point of view of the cell) that can beobtained from a small battery. The measurement of the I-Vcharacteristics starts when the data acquisition card sendsa trigger signal to the switch, which connects the capacitorto the cell through the shunt. At the very beginning

startCC VV )0( =+

(1)

(0 )Cell junction ocV V

+

=(2)

cellparasiticshunt

ocstartCCell

RRR

VVI

++

+=+ )0( (3)

If the equivalent resistance in series with the celland the capacitor (basically, the shunt resistor) is small,the current will exceed, in these initial moments, the short-circuit current of the cell. If the capacitor is large enough,after a short transition time t1 (the time necessary tostabilize the carrier profiles), the cell voltage will benegative and the current will be directly related to the cellvoltage as per the static conditions:

startCC

VtV )(1

(4)

shuntCellstartCCellRIVtV --=)( 1 (5)

SCCellshcellSCCell IVGItI -+-=)( 1 (6)

From this moment on, the I-V pairs corresponds tothe static I-V characteristics if the voltage evolution is slowenough. The capacitor discharges by means of the cellcurrent intensity, so that the voltage decreases following:

Fig. 1. Schematic diagram of a current-conditioning circuitfor the measurement of steady-state I-V curves using A/Dcards

Fig. 2. Schematic diagram of the solar cell and the passive

load for the quasi-steady-state I-V curve measurement.


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C

I

dt

dV CellC = (7)

For instance, an industrial large solar cell withISC=5 A and a 50 mF capacitor lead to a discharging rateof 100 V/s (10 ms/V), which is slow enough to beconsidered quasi-steady-state conditions. The shunt

resistor value should provide good accuracy for measuringthe current, so one could expect to get about 1 V whenmeasuring the short-circuit current.

2

1 30one-sun

( )shunt

SCCell Cell Cell

VR

J A A cm

W=

(8)

This circuit has been used to measure both smalllaboratory and large industrial type devices. Fig. 3 showsan experimental QSS photovoltaic I-V curve, obtained at

one sun illumination for a 22 cm2 silicon solar cell (cell 1)fabricated on 0.7 Wcm aluminum doped base material.The cell has been measured both with the capacitive-loadmethod and also point by point employing a power supply,

Ampere- and Voltmeter. Fig. 3 shows an excellentagreement between QSS and SS measurements.

Dark I-V curve under QSS conditions

In order to measure the dark I-V curve, thecontacts to the capacitor are reversed to impose a forwardbias on the solar cell, which is initially in equilibrium.

startCC VV )0( =+

(9)

0)0( =+Cell

V (10)

cellparasiticshunt

startCCell

RRR

VI

++

-=+

0)0( (11)

This initial current drives the necessary charge intothe cell so that, after a transition time (t 2), the cell voltagereaches the value that corresponds to that current in staticconditions.

startCCVtV )( 2 (12)

ONparcellshCellstartCCellVRRRIVtV ++-=)( 2 (13)

parasiticcellshunt

ONstartCCell

RRR

VVtI

++

-=)( 2 (14)

As in the previous case the cell voltage fades

towards zero as slowly as the capacitor discharges, andthe transition corresponds to a QSS case. IfVCstart is highand the shunt is low (and minimizing the parasitic seriesresistance of the connections) the current may be veryhigh, exceeding easily 100 A. Note that very high currentsmean fast discharging rates, which compromises the QSShypothesis. At the same time, they also mean highrecombination rates, high injection and low effectivelifetime, favoring the QSS hypothesis.

In order to measure a full I-V curve, a set of shunt

resistors is advisable. A low shunt (10 mW) leads to very

high currents whereas a high shunt (10 W) provides therequired accuracy for the low current range. Fig. 4 showsthe dark I-V data of the same 22 cm2 cell using both QSScapacitive and SS techniques. An excellent agreementhas been observed between both methods.

Characterization through transient effects

In the above sections, transient effects wereconsidered as a problem, but they can sometimes providemeaningful information about the cell. For instance, theseries resistance (Rcell) of the cell can be obtained if,during the measurement of the illuminated QSS I-V curve,at some point t3 between VCstart and Voc, the capacitor isdisconnected, making the current through the cell zero.

Hence, Rcell is obtained from the rapid change in voltagedue to the elimination of the Ohmic voltage drop:

3 3

3

( ) ( )

( )

Cell Cell Cell

Cell

V t V t R

I t

+ -

-

-= (15)

The cell tends rapidly towards open circuit voltagein a transition that is dominated by the light generation rate

Fig. 3. Illuminated QSS and SS measurement of cell 1.

0.0 0.1 0.2 0.3 0.4 0.5 0.60.000

0.005

0.010

0.015

0.020

0.025

0.030

QSS Measurement

SS Measurement

Current(A/cm

2)

Voltage (V)

Fig. 4. Dark QSS and SS measurement of cell1.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

1E-3

0.01

0.1

1

QSS Measurement

SS Measurement

Current(A/cm

2)

Voltage (V)


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and the effective lifetime. In consequence, a fast dataacquisition card is required to detect accurately the veryfast change in voltage (see Fig.5).

In a similar way, the dark series resistance can

also be obtained if the capacitor is disconnected whilemeasuring the dark QSS I-V curve [4]. In that case, therest of the curve corresponds to the OCVD curve whichmay be reconfigured to get the suns-Voc curve [5] (seeFig.5), or analyzed to determine the effective lifetime.

LOGARITHMIC CONVERTER CIRCUIT FORILLUMINATION-VOLTAGE MEASUREMENTS

This last circuit has been developed to facilitate themeasurement of the illumination-voltage curve with theQSSVoc method. The wide range of intensities involved tocover the full voltage curve (6-8 orders of magnitude)usually complicates the data acquisition. If the linearcurrent response of a photodetector is measured by

connecting a resistor in parallel and keeping the voltage atvalues lower than 200 mV, only 2-3 orders of magnitudecan be detected with sufficient accuracy even with state-of-the-art data acquisition cards. Hence, 4-5 differentflashes are necessary to record the entire illumination-voltage curve by decreasing the flash intensity with neutraldensity filters.

To circumvent this issue, it results useful tomeasure the logarithm of the photocurrent while keepingthe photodiode slightly reversely biased. This can indeedbe done using an integrated circuit from Analog Devices,AD8304 [6], which accurately compresses eight decadesof input current into an output voltage ranging from 0.1 to4.5 V. Consequently, this allows the measurement of theillumination-voltage curve in a single flash. The input to the

logarithmic converter is directly the current response of thephotodetector whose bias voltage is maintained at -0.1 Vby a proper feedback. The maximum current is limited toabout 20 mA, meaning that a very small aperture has tobe used in the photodetector to measure high levelilluminations. The main output provides a logarithmicimage of the photodetector current:

log 1 logphotodiode

convref

IV K

I =

(16)

where K1 and Irefare calibration constants.

We performed QSSVoc measurements using thetraditional way of flashing several times while changing themaximum intensity with neutral density filters, and alsoemploying the logarithmic converter circuit. Fig. 6 showsthe illumination-voltage curve of a high-efficiency solar cellproduced on 1.25 Wcm p-type multicrystalline silicon. Theflash employed is a Broncolor flash with a total powerrating of 1600 J, a time constant of ~3 ms and a pulselength of ~12 ms [7]. For the linear measurements, fourflashes were necessary to cover the whole curve. Themeasurement via the logarithmic conversion, in contrast,was done in a single flash. An excellent agreement hasbeen found between both measurements.

CONCLUSION

Simple electronic circuits have been developed thatfacilitate the data acquisition of I-V curves andillumination-voltage curves. The circuits have been testedagainst conventional methods and a good agreement hasbeen found. The simplicity and the low cost of thesecircuits can be beneficial for future design ofcharacterization equipment.

REFERENCES

[1] Photon international, October 2005, pp 68-73 (2005)[2] W. M. Keogh, A. W. Blakers, and A. Cuevas, SolarEnergy Materials and Solar Cells 81, pp. 183-96 (2004).

[3] R. A. Sinton and A. Cuevas, Proc. of the 16thEuropean PVSEC, Glasgow, UK, 2, pp. 1152-5, (2000).[4] D.K. Schrder, Semiconductor material and devicecharacterization, 2

nded, Wiley, pp. 149-150 (1998)

[5] M.J. Kerr et al, J. Appl. Physics 91, pp. 399-404 (2002)[6] Analog Devices, http://www.analog.com/[7] Bron Elektronik Ltd, Hagmattstrasse 7, CH - 4123Allschwil, Switzerland, http://www.bron.ch

Fig. 5. Determining the series resistance of a cell (1 cm )at one-sun and in the dark and measuring its Vocdecay.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

1E-6

1E-5

1E-4

1E-3

0.01

0.1

1

10

100

1000

Linear Measurement

Log. Measurement

Intensity(suns)

Voltage (V)

Fig. 6. Comparison of linear and logarithmic measurementof the illumination-voltage curve.

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