ISSN 1063�7850, Technical Physics Letters, 2013, Vol. 39, No. 10, pp. 906–909. © Pleiades Publishing, Ltd., 2013.Original Russian Text © V.M. Emel’yanov, A.V. Bobyl’, E.I. Terukov, O.I. Chesta, M.Z. Shvarts, 2013, published in Pis’ma v Zhurnal Tekhnicheskoi Fiziki, 2013, Vol. 39, No. 20,pp. 40–48.

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One currently important area in the study of pho�toconverters (PhCs) based on α�Si:H and μc�Si:Hfilms is the investigation of processes of photoinduced(light) degradation [1]. This phenomenon is closelyrelated to the effect of decreasing photoconductivityduring long illumination, i.e., the Stabler–Vronskiieffect. Photoinduced degradation is traditionally stud�ied at a temperature of 298 K. However, when solarbatteries are installed in moderate and warm climaticzones, their operating temperature can be substan�tially higher. In this case, the degradation process maypossess some specificities, which should be investi�gated [2]. Experimental study of the photoelectriccharacteristics of PhCs based on α�Si:H during protonirradiation has revealed a substantial decrease in thedegradation rate with elevation of the temperaturefrom 298 to 331 K [3]. A similar temperature effect isalso probable for photoinduced�degradation pro�cesses.

Two�junction PhCs with the structure α�Si:H/μc�Si:H and an initial efficiency of 10.5% were selected asthe study object. These PhCs were produced using themethod developed by Oerlikon Solar Ltd. (Switzer�land). The test specimens were 100 × 100 mm; in thesespecimens, a 60 × 66�mm photoactive portion with anopen surface area of 37.95 cm2 was bounded by laserscribing, which consisted of ten equal�area series�con�nected photoelectric cells. Test specimens aredescribed in more detail in [4].

Photoinduced degradation was studied at tempera�tures of 298, 328, and 353 K, i.e., 25, 55, and 80°C,respectively. At a temperature of 298 K, the character�istics of test specimens were measured under irradia�tion by light flux with equivalent densities of

1000 W/m2 (1X) and 10 000 W/m2 (10X). At tempera�tures of 328 and 353 K, measurements were carried outunder irradiation by light flux with equivalent densityof 5000 W/m2 (5X). To implement the irradiation ofspecimens by 1X light flux, we developed and pro�duced a special bench based on a Philips 13163/5Hhalogen lamp. With 5X and 10X light fluxes, tests wereperformed using an IS�160 solar radiation simulator.During irradiation, test specimens were periodicallydrawn from the apparatuses and their current–voltagecharacteristics (I–V curves) were measured. Thesecharacteristics were measured when specimens wereilluminated by an SS�80AA continuous burning simu�lator (AM1.5G, 1000 W/m2, AAA simulator classaccording to IEC 60904�9). The measurementmethod is described in more detail in [4]. Values ofshort�circuit current and efficiency of specimensobtained from the I–V curves are shown by symbols inFigs. 1 and 2. In these figures, the duration of the lighteffect on PhCs is shown in the time scale reduced to 1Xflux. When reducing the time scale, we allowed for thefact that the dynamics of variations in PhC parameterswas proportional to the squared radiation intensity,i.e., the light�degradation rate increases by 25 timesduring irradiation by light with an equivalent intensityof 5X and 100 times during irradiation by light withequivalent intensity of 10X.

It can be seen from the data in Figs. 1 and 2 that, forPhCs irradiated at temperature of 298 K, photoin�duced�degradation saturation was achieved after 200 hof irradiation in the presented time scale. There is nosubstantial difference between the results for speci�mens irradiated by light with intensities 1X and 10X.For the specimen irradiated at a temperature of 328 K,

Photoinduced Degradation of α�Si:H/μc�Si:H Tandem Photoconvertes at Elevated Temperatures

V. M. Emel’yanov, A. V. Bobyl’, E. I. Terukov, O. I. Chesta, and M. Z. ShvartsIoffe Physical Technical Institute, Russian Academy of Sciences,

St. Petersburg, 194021 Russiae�mail: resso2003@bk.ru

Received May 20, 2013

Abstract—Photoinduced degradation of tandem photoconverters with the structure α�Si:H/μc�Si:H andinitial efficiency of 10.5% at temperatures of 298, 328, and 353 K is experimentally studied. It has been foundthat, at a temperature of 298 K, the efficiency of the photoconverters decreases by 1.0–1.2% during long lightexposure, while an increase in temperature to 328 K leads to a decrease in efficiency to 0.2%; at a temperatureof 353 K, no degradation is observed. To explain these results, a modified H�collision model was used. Ther�mal�activation energy has been determined for the process that hampers the growth of dangling (ruptured)bonds in the i�α�Si:H layer.

DOI: 10.1134/S1063785013100179

TECHNICAL PHYSICS LETTERS Vol. 39 No. 10 2013

PHOTOINDUCED DEGRADATION 907

saturation was achieved after 60 h of irradiation. Nodegradation of the specimen irradiated at 353 K wasobserved. The experimental data obtained were usedto determine the value of the change in the concentra�tion of dangling (ruptured) bonds after the saturationof photoinduced degradation from temperature atwhich it occurred.

Before photoinduced degradation, current–volt�age characteristics of PhCs were approximated by thefollowing formula, which allows for series connectionof ten two�junction single�type cells and the preva�lence of recombination current in PhCs with the p–i–n structure:

(1)

where V is PhC voltage, I is PhC current, ,

and , are densities of photocurrents andrecombination currents of the α�Si:H and μc�Si:Hsubelements, respectively, R is PhC�series resistance, kis the Boltzmann constant, T is absolute temperature,and q is elementary composition.

The densities of photocurrents of the subcells usedto approximate experimental I–V curves were calcu�lated based on experimental spectral dependences ofexternal quantum efficiency for PhC subcells. Valuesof photocurrents of the subcells calculated from spec�tral characteristics are presented in Table 1. To ensurethe best agreement between experimental and calcu�

V 20kTq

����������Iphα�Si I–( ) Iph

μc�Si I–( )

Irα�SiIr

μc�Si( )

��������������������������������������� 1+ln IR,–=

Iphα�Si Ir

α�Si

Iphμc�Si Ir

μc�Si

lated I–V curves of PhCs prior to irradiation, values of

the product of recombination currents and ,as well of series resistance R, were varied. Parametersfound in such a way are given in Table 2.

During photoinduced degradation, densities of

recombination currents change due to an increasein the number of dangling bonds in the i�α�Si:H and i�μc�Si:H layers, which play the role of nonradiativerecombination centers:

(2)

where and are concentrations of recombi�nation centers in the i�α�Si:H and i�μc�Si:H layers

before irradiation and and are their con�centrations after the light effect.

The series resistance of the PhC α�Si:H/μc�Si:H ismainly determined by resistance of the i�α�Si:H layer[4]. After photoinduced degradation, series resistance

can be estimated by the following formula:

(3)

It follows from analysis of Table 1 that the degrada�tion of the μc�Si:H subcell is lower than that of theα�Si:H subcell. For this reason, change in concentra�tion of dangling bonds in the i�μc�Si:H layer can beneglected compared to that in the i�α�Si:H layer.After photoinduced degradation, the I–V curves were

Irα�Si Iμc

μc�Si

Ir

Irα�Si

/Irα�Si Nr

α�Si/Nr

α�Siθ,=

Irμc�Si:H

/Irμc�Si:H Nr

μc�Si/Nr

μc�Siξ,=

Nrα�Si Nr

μc�Si

Nrα�Si

Nrμc�Si

R

R R Nrα�Si

/Nrα�Si

( )≈ θR.=

1 10 100 1000Light soaking time, h

41.5

42.0

42.5

43.0

43.5

44.0

44.5

Short circuit current, mA/cm2

1

3

1'

Fig. 1. Short�circuit current of α�Si:H/μc�Si:H PhCsunder study recorded during photoinduced degradation inreduced time scale at different temperatures and irradia�tion intensities: (1, 1 ') 298, (2) 328, and (3) 353 K; (1) dataobtained at irradiation intensity of 10X; (1 '), at irradiationintensity of 1X.

1 10 100 1000Light soaking time, h

9.0

Efficiency, %

9.5

10.0

10.5

11.0

3

2

11'

Fig. 2. Efficiency of α�Si:H/μc�Si:H PhCs under studyrecorded during photoinduced degradation in reducedtime scale at different temperatures and irradiation inten�sities: (1, 1 ') 298, (2) 328, and (3) 353 K; (1) data obtainedat irradiation intensity of 10X; (1 '), at irradiation intensityof 1X.

2

908

TECHNICAL PHYSICS LETTERS Vol. 39 No. 10 2013

EMEL’YANOV et al.

approximated using formula (1), in which was

replaced by θ and R by θR. To ensure agreementbetween calculated and experimental I–V curves, thecoefficient of θ was varied. The values of this coeffi�cient are presented in Table 2. In the calculations, weused the photocurrents given in Table 1.

One of the most successful photoinduced�degrada�tion models is the modified H�collision model pro�posed in [5]. In this model, it is assumed that a largefraction of hydrogen is present in the semiconductorin the form of metastable Si–H–H–Si complexes.Dangling bonds result from the rupture of the weakSi–Si bond when a metastable Si–H–H–Si complexis located in the close vicinity of this bond. The ruptureof the weak Si–Si bond changes into the disintegrationof the metastable complex, and then the formation ofa pair of dangling bonds and a pair of hydrogenatedSi–H bonds. In this model, the dynamics of changesin concentrations of dangling bonds Nr, weak Si–Sibonds Nw, Si–H–H–Si complexes Np, and Si–Sibonds NH is determined by the following equation:

(4)

where G is irradiation intensity, dr is a coefficient thatcharacterizes the probability of the rupture of a weakbond and the formation of a pair of dangling bonds,

Irα�Si

Irα�Si

dNr/dt dHH/dt 2dNp/dt– 2dNw/dt–= = =

= drG2NwNp/Nr

2 dpNr2NH

2,–

and dp is a coefficient that characterizes the reverseprocess. If we assume that the annihilation process is athermally activated process with activation energy Ea

(dp = dp0exp(–Ea/kT)), then, in the range of the mod�ified H�collision model, we obtain the followingexpression for the saturation stage:

(5)

where the tilde marks the corresponding concentra�tions at photoinduced degradation saturation in thei�α�Si:H layer. It follows from expression (5) that θ ∝

, as θ ∝ .

We calculated the activation energy that ensured achange in coefficient θ when the temperature rosefrom 298 to 328 K; this activation energy was Ea =0.68 eV. The calculated dependence that correspondsto this activation energy is presented in Fig. 3. It can beseen that the theoretical curve passes below the exper�imental point that corresponds to a temperature of353 K. This is indicative of the fact that the completesuppression of photoinduced degradation in the i�α�Si:H layer should occur under lower temperatures,i.e., approximately 345 K.

We should note that the obtained activation energyis less than annealing thermal activation energy ofdangling bonds [6]. In this experiment, the effectrelated to a more complex annealing process, possibly,a photoinduced process was apparently observed. Inparticular, this can also explain the above�notedabsence of differences in light�degradation saturationlevels for various radiation intensities, other authorshaving observed such a difference [7]. These issuesrequire further investigation.

A substantial decrease in light degradation rate wefound at temperature of 55°C that exceeds the stan�dard temperature only slightly and the complete sup�pression of degradation at temperatures of 72–380°Callow us to assert that the dynamics of the gradualdecline of characteristics of PhCs based on α�Si:Hduring their use can be considerably dependent on cli�

Nrα�Si:H drG

2NwNp

dp0 Ea/kT–( )NH2

exp����������������������������������������

4Ea/4kT( ),exp∝=

Ea/kT( ) Nrα�Si

Table 1. Densities of photocurrents of subcells calculated using measured spectral characteristics

Subcells

Photocurrent density for specimens irradiated by lightat different temperatures, mA cm–2

298 (1X) 298 (10X) 328 K 353 K

α�Si:HBefore irradiation 11.20 11.16 11.14 11.16

After degradation saturation 10.07 10.30 11.01 11.14

μc�Si:HBefore irradiation 10.20 10.15 10.33 10.15

After degradation saturation 9.25 9.50 10.22 10.33

Table 2. Modeling parameters that ensured agreementbetween calculated and experimental I–V curves of PhCsbefore and after photoinduced degradation

,

A2 cm–4

PhC series resistance

before degrada�tion R,

Ω

Recombination�center concen�tration enlargement factor

after photoinduced�degradation saturation θ in i�α�Si:H layer

at different temperatures

298 K 328 K 353 K

6.12 × 10–16 18.4 2.41 1.31 1

Irα�Si

Irμc�Si

TECHNICAL PHYSICS LETTERS Vol. 39 No. 10 2013

PHOTOINDUCED DEGRADATION 909

matic conditions in the region where PhCs are used.The degradation process will also be nonlinear in timedue to daily and seasonal variations of PhC tempera�ture. The obtained numerical estimate of activationenergy can be used when refining the light�degrada�

tion model for structures based on amorphous hydro�genated silicon.

Acknowledgments. We are grateful to A.S. Gu�dovskikh for data on PhC degradation at a radiationintensity of 1X and N.Kh. Timoshina for carrying outmeasurements of photoelectric characteristics ofPhCs.

This study was supported by the Ministry of Sci�ence and Education of the Russian Federation, projectno. 16.526.12.6017.

REFERENCES

1. T. Shimizu, Jpn. J. Appl. Phys. 43, 3257 (2004).2. J. Izard et al., in Proc. 26th EPVSEC (2011), p. 2403.3. S. Sato et al., Prog. Photovolt: Res. Appl. (2012).4. V. M. Emel’yanov et al., Semiconductors 47, 679

(2013).5. M. J. Powell, R. B. Wehrspohn, and S. C. Deane,

J. Non�Cryst. Solids 299–302, 556 (2002).6. M. Stutzmann, W. B. Jackson, and C. C. Tsai, Phys.

Rev. B 32, 23 (1985).7. Z. Y. Wu, J. M. Siefert, and B. Equer, J. Non�Cryst.

Solids 137/138, 227 (1991).

Translated by D. Tkachuk

2.4

2.2

2.0

1.8

1.6

1.4

1.2

1.0

0.8

θ, a.u.

300 310 320 330 340 350 360 370T, K

∞exp(Ea/4kT)

Ea = 0.68 eV

Fig. 3. Recombination�center concentration�change fac�tor at photoinduced�degradation saturation as function ofPhC temperature. Symbols correspond to experimentaldata, and line shows calculation results.