LOW-POWER PHOTOVOLTAIC CELLS BATTERIES USED AS … · in particle accelerators related experiments,...

Romanian Journal of Physics XX, XYZ (2017)

LOW-POWER PHOTOVOLTAIC CELLS BATTERIES USED ASGAMMA RADIATION DOSE ESTIMATORS

V. FUGARU1, R. O. DUMITRESCU2,3, C. D. NEGUT1

1Horia Hulubei National Institute for Nuclear Physics and Engineering,30 Reactorului, P.O.Box MG-6, RO-077125 Bucharest-Magurele, Romania

2Doctoral School of Physics – University of Bucharest, P.O.B. MG-11, Romania3CBRN Defence and Ecology Scientific Research Centre, 041309 Bucharest, Romania

Corresponding author: [email protected]

Received October XX, 2017

Abstract. In this paper, the possibility of using commercial low-power photovoltaic cells batteriesas sensors for the estimation of the gamma radiation dose in a certain point in space and for certain energyin particle accelerators related experiments, when the occurred nuclear reactions are involving the presenceof higher or lower gamma radiation fields, was studied. Initially, three low-power photovoltaic cellsbatteries were exposed to three different well-known gamma radiation doses (33.45 kGy, 35.61 kGy and37.9 kGy) and one was kept as blank (0 kGy), and then, an analysis of some of their functional parameters(short circuit current, open circuit voltage, external quantum efficiency) was performed. The functionalparameters were studied with respect to the illuminance conditions and to the absorbed dose values in orderto determine which of them can be used in gamma radiation dosimetry. Five different artificial lightingconditions were used, and, in each case, the values of sensitivity of the sensors (expressed in nA/Gy)and their external quantum efficiencies were determined. As it was shown in this study, low-powerphotovoltaic cells batteries can indeed be used as gamma radiation dose estimators, and even more, can beused in complex gamma radiation fields distribution studies.

Key words: Low-power photovoltaic cells battery, illuminance, gamma radiation, solid statedosimetry.

1. INTRODUCTION

Nuclear radiations are emissions of waves or particles, carrying energy.These energies induce some changes to the structure of exposed materials (inparticular on electrical and optical properties) [1-10]. Using this “cause-effect”relation, the estimation of an unknown gamma radiation dose induced to anexposed material can be done.

In this paper, the possibility of using commercial low-power photovoltaicbattery cells as sensors for the estimation of the gamma radiation dose in a certain

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Diab et al, Rom J Biophys (Bucharest), Vol. 22, No. 2, Pp. 125–135 - an Article created by double publication, mentioned in the bodytext of this item at page 7 and in the bibliography at page 15

Article no. XYZ V. Fugaru, R. O. Dumitrescu and C. D. Negut 2

point in space and for certain energy in particle accelerators related experiments,when the occurred nuclear reactions are involving the presence of higher or lowergamma radiation fields, was studied.

A photovoltaic cells battery is a group of photovoltaic cells which provideelectrical power based on the conversion of light into electricity usingsemiconducting materials that exhibit the photovoltaic effect. It is defined as adevice whose electrical characteristics, such as current, voltage, or resistance, varywhen exposed to light, and it is based on the following aspects: absorption of lightgenerating either electron-hole pairs or excitons, the separation of charge carriersof opposite types and the separate extraction of those carriers to an external circuit[11-17]. A photovoltaic cell consists of two or more layers of semiconductormaterials deposited on a surface made of a wide range of materials (FR4 type 0.5mm glass textolite sheets in the case of the ones used in this paper). The depositedsemiconductor layers have usually thicknesses between 10-9 m and 2 x 10-7 m andcontain in their base matrix certain chemical elements in order to form type “p”and type “n” junctions. A photovoltaic cell is basically a p-n junction. On theirentire exterior surface, the photovoltaic cells are coated with epoxy resins in orderto provide a satisfying mechanical resistance to them but not significantly affectingthe entrance of the light in their volume. Usually, photovoltaic cells are builthaving relatively small surfaces, providing small quantities of electric current. Bycombining them in various serial or parallel connections (photovoltaic cellsbatteries), their current/voltage can be increased in order to be high enough forpractical usability.

2. MATERIALS AND METHODS

The functional parameters (short circuit current, open circuit voltage,external quantum efficiency) of four photovoltaic cells batteries exposed to gammaradiation were studied with respect to the illuminance conditions and to theabsorbed dose values in order to determine which of them can be used in gammaradiation dosimetry.

Three amorphous Si based low-power photovoltaic cells batteries wereexposed to a Co-60 gamma-ray source inside the “hot cells” found inRadioisotopes and Radiation Metrology Department (DRMR) from Horia HulubeiNational Institute for Nuclear Physics and Engineering (IFIN-HH), ROMANIA, inorder to obtain three different well-known gamma radiation doses (33.45 kGy,35.61 kGy and 37.9 kGy) and one was kept as blank (0 kGy). The Co-60 gammarays source provided a constant dose rate of 395 Gy/h at 10 cm distance from it, inthe place where the exposures were performed. The dose debit was measured usinga 30006 type “Farmer” ionization chamber, a waterproof standard chamber for

3 Low-power photovoltaic cells batteries used as dose estimators Article no. XYZ

absolute dosimetry (0.65 % combined standard uncertainty for a coverage factor k= 2).

The mean value of energies of the gamma rays emitted by a Co-60 sourceis 1.25 MeV, way higher than the bandgap energy of amorphous Si (a-Si), of 1.12eV.

Each photovoltaic cells batteries was made of 12 photovoltaic cells(connected in serial), placed on 2 rows of six cells, as it can be seen in Fig 1. Thephotovoltaic cells used in this paper were based on amorphous Si, which provideincreased sensitivity in low light conditions. The cells batteries had the followingcharacteristics: sensitive area – 12 cells x (6 x 10) x 10-6 m2, sensibility - 350 nA/lx, Vout/klx - 5v (idle voltage), Isc - 350 µA (short circuit current), r = 12 (numberof p-n junctions/elementary cells in a battery).

Fig. 1 – Photovoltaic cells battery.

In order to locate each cell in a battery, a matrix approach was proposed, meaningeach battery was represented as a matrix with l lines and c columns (l = 1…n, c =1…m). A certain dose value corresponding to a certain cell can be defined as Dl,c,if individual cell measurements are performed. These types of individual cellmeasurements are important if gamma ray field uniformity studies are performed[18].

In order to measure each of the functional parameters of the irradiatedphotovoltaic cells batteries, an experimental device was built, as it can be seen inFig 2.


Fig. 2 – Experimental set-up.

Practically, the experimental set-up was a dark box ((60 x 60 x 110) mm)containing the exposed photovoltaic cells batteries, a stabilized source of light anda high quality electrometer. The source of light used was a multispectralincandescent E10 type light bulb (6.3V, 2.0W). The illuminance inside theexperimental device was varied by modifying the voltage provided to the lightsource. The voltages used were 2.1 V, 2.9 V, 3.7 V, 4.4 V and 5.2 V correspondingto the following illuminance values: 33 lx ± 0.7 %, 79.8 lx ± 0.7 %, 196.8 lx ± 0.6%, 422.3 lx ± 0.6 % and 813 lx ± 0.7 %. The illuminance measurements wereperformed using a MOBILE-CASSY LD 524009 type lux-meter. For each of thefour photovoltaic batteries the currents in dark, Id, were measured: Id1 = 0.81 x 10-9

A, Id2 = 1.4 x 10-9 A, Id3 = 1.48 x 10-9 A and Id4 = 0.1862 x 10-9 A.When no light source is present, the electric current inside the short circuit

is the sum of the currents provided by the major and minor carriers. Locally, thecurrent is depending to the electric potential difference for each p-n junction,which depends to the temperature.

When a light source is involved, on the exterior circuit of the photovoltaiccells battery a photocurrent occurs [19-22], as follows (Equation 1):

fI

TB

k

Vre

es

II

]1

)0

(

[ (1)

where: I – electric current on the exterior circuit,Is – saturation current of unirradiated p-n junctions,If – photocurrent (photo-electric current),


V0 – electric potential difference for a p-n junction (open circuit),e – electron change,r – number of p-n junctions/elementary cells in a battery,kB – Boltzmann constant,T – temperature.

Short circuit current is increasing in a linear manner with the increase ofthe intensity of the light. The intensity of the light source (incandescent light bulb,multispectral emission) was modified by modifying the voltage to it. The obtainedphotocurrent is practically proportional to illuminance.

By assigning the zero value to the electrical current thru the exteriorcircuit, the expression of the electric potential difference (open circuit) for an rnumber of p-n junctions (r x V0) is the following, Equation (2):

1ln

0s

If

I

e

kTVr (2)

Equation (2) also shows the dependence between the electric potential difference –open circuit voltage (V0) and the temperature. The influence of the temperaturewas diminished as much as possible in the experiment performed in this paper byusing the photovoltaic cells batteries in current generator regime. This way, theserial circuit of the p-n junctions is closed on a negligible resistance related to theinternal one, for which 0

0Vr . The short circuit current becomes

fI

shortI and it is not depending to the temperature anymore (ambient

temperature during experiment was 220C).The dependence of the short circuit current value to the photovoltaic cell

parameters, dose, time and illuminance is given by the Equation (3), [23]:

)(

,

jLgelS

iR

jishort

I

(3)

where:ji

shortI

,

- measured short-circuit current expressed in A,

ti

D

iR - dose rate (Di), expressed in Gy/s,

ApS - cross-section area expressed in m2,l - minority-carrier diffusion length expressed in m,ρ - density of the material (2330 kg/m3 for Si),


e - electronic charge (1.6 x 10-19 A s),ε - average energy required to produce one electron-hole pair (5.76 x 10-

19 J),

The Equation (3) becomes Equation (4):

)(81047.6,

jLglS

iD

jishort

I (4)

where: g(Lj) - photo-generation rate,Lj - illuminance expressed in lx.

The illuminance is expressed as follows, Equation (5):

LS

PCj

jL

(5)

where: Фj – luminance per lamp expressed in lm;C - utilization coefficient;P - light loss factor;SL - area per lamp expressed in m2.

Equation (4) shows the linear dependence between short circuit current and theabsorbed dose, but also the influence of the illuminance.

For a photovoltaic cell, in normal functioning conditions, a very importantfactor of its efficiency is the filling factor (FF). This factor is a measure of thequality of a cell, and it is defined by determining its current-voltage characteristic.The filling factor is dependent to the internal resistance of the photovoltaic cellsbattery (FF is higher when the internal resistance is lower). Unlike the short-circuitcurrent, the open circuit voltage (V0) is not depending to the internal resistance ofthe photovoltaic cells battery.

By exposing a photovoltaic cells battery to increasing gamma radiationdose, its internal resistance is increasing, leading to a decreasing short circuitcurrent. This phenomenon is the base of the possibility of using photovoltaic cellsbattery as instruments for gamma radiation doses estimations.

The evaluation of the absorbed dose is a very important task to be fulfilled,especially in the cases of high dose values (> 100 Gy). In this direction, manymaterials and devices were studied in order to determine their behaviour whenexposed to ionizing radiation [24]. As an example, the glass based dosimetersshowed their functionality on a wide range of absorbed dose values. Their usabilitywas proven in literature [25-28] by using various types of photo-spectrometricmeasurements (optical densitometry). These types of dosimeters already showed


their potential in different fields as: applied physics experiments, nuclear medicine,industry etc. [29-33].

In the case of photovoltaic cells batteries, their usability was shown forhigher energy gamma radiation (hundreds of keV) and for higher dose values.

Due to the fact that in this paper the short circuit current measurement wasperformed (unaffected by temperature variations), its dependence to absorbed doseis linear, as Equation (6) shows:

shortImn

fI

shortIfD )( (6)

where: Ishort – short circuit photocurrent of photovoltaic cells battery ( 0short

I ),

n, m – fitting parameters.

In order to determine the variance associated to the estimated dose, theUncertainties Propagation Law is applied to Equation (6). In this case, the generalanalytical relation is a function depending on three variables, D = D( If, n, m),leading to Equation (7):

22

22

22

2

fI

fI

Dmm

Dnn

DD

(7)

The concrete form of Equation (7) is given by Equation (8), which contains onlyknown quantities:

222

222 mf

IfI

mnD

(8)

By solving Equation (8), the uncertainty associated to a determined dose value canbe estimated using Equation (9):

2DD

(9)

3. RESULTS AND DISCUSSIONS

The accumulation of radiation induced defects in the structure of irradiatedphotovoltaic cells batteries is the cause of the reduction of their functionparameters. The graphical representation of functional parameters as a function of

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absorbed dose value (i = 1, 2, 3, 4) and illuminance, Lj (j = 1, 2, 3, 4, 5), allows usto determine the relation between them and to decide if any of them can be used asdosimetric parameter.

In Fig. 3, the short circuit current as a function of illuminance, for each ofthe four studied photovoltaic batteries, is presented.

Fig. 3 – Short circuit current as a function of illuminance, for each absorbed dose value.

As it can be seen from Fig. 3, the fitting relations have the following form,Equation (10):

LbaI iishorti

(10)

It can also be seen that the short circuit absolute values are increasing with theincrease of the illuminance and are lower for higher absorbed dose values. As itcan be seen in Fig. 3, the square of the sample correlation coefficient (R2) takesvalues over 94 % in all cases, showing the goodness of fits.

The variation of the open circuit voltage as a function of illuminance, foreach of the four studied photovoltaic batteries is shown in Fig. 4. It can be seen thatthe open circuit voltage value is decreasing with the increase of the absorbed dose.An increase of the open circuit voltage value with the increase of illuminance can


be also seen. For values of illuminance over 200 lx, a saturation effect is starting tobe observed.

Fig. 4 – Open circuit voltage as a function of illuminance, for each absorbed dose value.

The sensibility of photovoltaic cells batteries reported to absorbed dose andilluminance are shown in Table 1.

Table 1

Sensibility of photovoltaic cells batteries reported to absorbed dose and illuminance

Sensibility [Gy/nA] 0.52 0.32 0.21 0.12 0.11Illuminance [lx] 33.0 79.8 196.8 422.3 813.0

From Table 1 in can be observed that with the increase of the illuminance alower dose value is needed to obtain a nA of short circuit current, meaning anincreased sensibility [34]. A saturation trend can also be observed, starting fromaround a 422.3 lx illuminance value. Table 1 also shows that the five illuminancevalues chosen in this paper are optimal.

The variation of the external quantum efficiency with illuminance, for eachabsorbed dose value, η (%), is presented in Fig. 5. The external quantum efficiency


represent the fraction of the incident light that is converted in electrical energy, andit is given by Equation (11):

100,

0,

,

ji

P

ijV

jishort

I

ji (11)

where Pi represents the power of the optical radiation reaching the surface of thephotovoltaic sensor.

Fig. 5– External quantum efficiency as a function of illuminance, for each absorbed dose value.

From Fig. 5 it can be seen that with the increased of the absorbed dose value, adecrease of the efficiency of the battery is occurring. For the studied photovoltaiccells batteries, the external quantum efficiency was placed between 0.2 % and 1.8%, on the entire range of artificial illuminating conditions. The external quantumefficiency in artificial illuminating conditions is inferior to the one provided bySun, (5 - 7) %.


From the three functional parameters studied, the one chosen to be used asdosimetric parameter was the short circuit current, due to the fact that it is notaffected by other external parameters as the temperature. In Fig. 6, the variation ofthe short circuit current with the absorbed dose value, on the entire doses range ispresented.

Fig. 6 – Short circuit current as a function of absorbed dose, for each illuminance conditions.

As it can be seen in Fig. 6, the short circuit current absolute value is decreasingwith the increase of the absorbed dose value and increasing with the increase ofilluminance. For a certain illuminance value, the analytical relation between theshort circuit current and absorbed dose is a relation of the type of Equation (6).

Short circuit current as a function of absorbed dose, for each illuminanceconditions on the (0 – 35.61) kGy interval is presented in Fig. 7. This interval waschosen for its higher linearity.


Fig. 7 – Short circuit current as a function of absorbed dose ((0 – 35.61) kGy), for each illuminanceconditions.

By using the fitting relations from Fig. 7, the analytical relations between theabsorbed dose and short circuit current, for each illuminance conditions, weredetermined (calibration curves). The results are presented in Table 2.

Table 2

Analytical relations between absorbed dose and short circuit current on (0 – 35.61) kGy interval

Calibration curve for eachilluminance value

Ishort

for (0 – 35.61) kGyinterval

[A] x 10-5

Illuminance[lx]

R2

[%]

D1 [kGy] = 333.3 + 1.6 x107 x Ishort (-2.06) ÷ (-1.75) 33.0 64.8D2 [kGy] = 341.7+ 4.2 x 106 x Ishort (-8.1) ÷ (-7.0) 78.9 75.2D3 [kGy] = 366.7+ 3.3 x106 x Ishort (-11.1) ÷ (-9.7) 196.8 73.1D4 [kGy] = 383.0+ 2.1 x 106 x Ishort (-18.0) ÷ (-16.0) 422.3 88.0D5 [kGy] = 280.0+ 106 x Ishort (-27.8) ÷ (-24.0) 813.0 99.6


By measuring the short circuit value and using the relations from Table 2according to the chosen illuminance conditions, an unknown absorbed dose valuecan be determined. These relations are valid only on the studied (0 – 35.61) kGyinterval due to the fact that the calibration “short circuit current – absorbed dose”was performed only on this interval. Having more dose values spread on a largerinterval, new calibration curves can be obtained. As it can be seen from R2 values,the calibration curve corresponding to the 813.0 lx illuminance fits best theexperimental points due to the fact this value is closest to the one recommended bythe producer of this type of photovoltaic cells batteries (working regime).

4. CONCLUSIONS

In this paper, the functional parameters (short circuit current, open circuitvoltage, external quantum efficiency) of low-power photovoltaic cells batteries aftertheir exposure to gamma radiation were studied. It was shown that all of theseparameters are affected by the energy deposited in their volume. Do to this fact, itwas shown that low-power photovoltaic cells batteries can be used as sensor ingamma radiation dosimetry. It was also determined which of these parameters isthe best parameter to be use in dosimetry. The best parameter to be use indosimetry was found to be the short circuit current.

By studying the short circuit current variation with the absorbed dosevalue, calibration curves were obtained. Using these calibration curves, anyunknown dose value can be determined. The calibration curves are valid only onthe studied (0 – 35.61) kGy interval due to the fact that the calibration “shortcircuit current – absorbed dose” was performed only on this interval. Having moredose values spread on a larger interval, new calibration curves can be obtained.

The uncertainty associated to the proposed gamma radiation dosimetrymethod was also estimated. The main qualities of this dosimetry method are that itis fast, easy to be applied in various situations and economically efficient.

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