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Electrochimica Acta 106 (2013) 333– 341

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

jo u r n al hom ep age: www.elsev ier .com/ locate /e lec tac ta

nfluence of oxygen on Ag ionization in molten lead borosilicate glassuring screen-printed Ag contact formation for Si solar cells

o-Mook Chunga,b, Sung-Bin Choa, Jung-Woo Chuna, Young-Sik Kimc,uninori Okamotod, Joo-Youl Huha,∗

Department of Materials Science and Engineering, Korea University, Seoul 136-713, Republic of KoreaDepartment of Research and Development, KPM TECH, Ansan 425-090, Republic of KoreaSchool of Advanced Materials Engineering, Andong National University, Andong 760-749, Republic of KoreaProcess Materials R&D Center, Cheil Industries Inc., Uiwang 437-711, Republic of Korea

a r t i c l e i n f o

rticle history:eceived 2 March 2013eceived in revised form 21 May 2013ccepted 22 May 2013vailable online 3 June 2013

eywords:rystalline Si solar cellcreen-printed Ag contact

a b s t r a c t

In order to gain further insight into the formation mechanism of fire-through Ag contacts of Si solarcells, the ionization of Ag during the dissolution of Ag powder into a lead borosilicate glass melt waselectrochemically investigated at 800 ◦C under various ambient conditions with different oxygen partialpressures (PO2 ). Voltammetric analyses of the Ag-free and Ag-containing glass melts confirmed that someof the Ag powder dissolved into the molten glass as Ag+ ions through interaction of the powder withoxygen in the ambient atmosphere. The concentration of Ag+ in the molten glass significantly increasedwith increasing PO2 . The dependence of the Ag+ solubility in the molten glass on PO2 was estimatedfrom chronoamperometric measurements for a series of glass melts containing different amounts of

+

ead borosilicate glassg ionizationiring ambience

Ag powder. The chronoamperometry results clearly demonstrated that the solubility limit of Ag in themolten glass at 800 ◦C also increased significantly with increasing PO2 . The present results strongly supportthe mechanism proposed recently for fire-through Ag contact formation in which Ag+ ions dissolved inthe molten glass play a crucial role. The present study also suggests that the reaction kinetics duringthe fire-through Ag contact formation is effectively controlled by adjusting PO2 in the ambient firingconditions as well as by modifying the glass chemistry to alter the solubility of Ag+ ions.

© 2013 Elsevier Ltd. All rights reserved.

. Introduction

Screen-printed Ag thick-film metallization is the predominantechnique used for the fabrication of the front-side contacts of com-

ercial Si solar cells primarily because of its cost-effectivenessnd high throughput. In this technique, Ag paste, which consistsf Ag powder and glass frit, together with some organics, is firstcreen-printed on the n+-Si emitter covered with an antireflectionoating (ARC) and then fired under ambient air at a peak tempera-ure normally below the Ag–Si eutectic temperature of 835 ◦C [1].fter the burn out of the organics at the heating stage of the firingrocess, the remaining mixture of molten glass with Ag powdereacts first with the dielectric ARC to punch through and then withhe emitter Si to form ohmic contacts. After the firing process, the

icrostructure of the contact interface consists of Ag crystallitesmbedded onto the emitter surface and a thin glass layer betweenhe Si emitter and the sintered Ag bulk. The formation of fine and

∗ Corresponding author. Tel.: +82 2 3290 3278; fax: +82 2 928 3584.E-mail address: jyhuh@korea.ac.kr (J.-Y. Huh).

013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2013.05.109

densely distributed Ag crystallites on the emitter surface is desir-able for achieving high-quality ohmic contacts while retaining theglass layer as thin as possible. Therefore, it is important to carefullycontrol the reaction kinetics by optimizing the firing scheme andthe glass chemistry, especially when a lightly doped, shallow emit-ter is adopted to improve the cell conversion efficiency [2–11]. Theneed for careful control has motivated extensive studies in order tounderstand the nature of the reactions involved in the firing pro-cess [1,12–18], but the detailed mechanism of the fire-through Agcontact formation still appears to be a source of controversy in theliterature. Moreover, the recent development of n-type Si solar cellsrequires the formation of high-quality ohmic contacts to the p+-Siemitter [19–23], which also necessitates a better understanding ofthe firing reactions.

Recently, Hong et al. [16,24] proposed a new model for the reac-tions involved in the fire-through Ag thick-film contact formation,in which Ag+ ions dissolved in molten glass, not the glass compo-

nent(s) as proposed previously [12,13,18], are the active speciesparticipating in the redox reactions with the SiNx ARC layer andthe Si emitter. According to their model, Ag powder dissolves intomolten glass as Ag+ ions through the interaction of the powder with

3 imica Acta 106 (2013) 333– 341

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34 B.-M. Chung et al. / Electroch

xygen in the ambient atmosphere at the heating stage of the firingrocess:

Ag(s) + O2(g) → 4Ag+(in glass) + 2O2−

(in glass) (1)

he Ag+ and O2− ions dissolved in the molten glass then reactequentially with the SiNx ARC layer and the Si emitter throughhe following redox reactions:

iNx(s) + 4Ag+(in glass) + 2O2−

(in glass)

→ SiO2(in glass) + 4Ago(in glass) + x

2N2(g) (2)

i(s) + 4Ag+(in glass) + 2O2−

(in glass) → SiO2(in glass) + 4Ago(in glass) (3)

t the cooling stage of the firing process, the neutral Ago atomseduced by redox reactions (2) and (3) become oversaturated inhe molten glass and precipitate as Ag crystallites at the pits cre-ted on the emitter surface by reaction (3) and as nanosized Agolloids within the glass. According to this model, the reaction rateor the Ag paste to etch out the SiNx ARC and form Ag crystallitesn the Si emitter should strongly depend on the concentration ofg+ ions in the molten glass and thus on PO2 in the ambient firingtmosphere. Indeed, in both cases of Ag pastes containing PbO- andi2O3-based glass frits, it was recently confirmed that the amount ofg crystallites precipitated on the Si emitter significantly increasedith increasing PO2 in the firing atmosphere [25,26]. When firednder pure N2 atmosphere, there was no observable Ag crystalliteormation on the emitter surface [25,26]. Although these observa-ions strongly support the claim that oxygen in the ambient firingtmosphere plays a crucial role in the firing reactions, they are stillndirect evidence for the assertion that the Ag+ ions are the activeomponent involved in the firing reactions.

Electrochemical analysis techniques such as voltammetry andhronoamperometry are known to be effective methods for detec-ing metallic ions in glass melts and for understanding the reactionsn which ionic species participate. In a previous study using cyclicoltammetry [27], it was reported that Ag+ ions were formed atemperatures over 900 ◦C in a sodium borosilicate melt by addingg powder to the glass. Although many studies have reported the

onization of Ag in various oxide glass melts [28–31], there haveeen no reports on the ionization of Ag in the melt of a lead borosil-

cate glass, which is the typical glass contained in Ag paste for Siolar cells. Moreover, the effect of ambient oxygen on the solubilityf Ag+ ions in molten lead borosilicate glass has not been reported.

The aim of this study is to examine electrochemically whetherg is ionized in molten lead borosilicate glass with the aid of ambi-nt oxygen at 800 ◦C, which is the typical peak temperature of thering process. The effect of oxygen in the ambient atmosphere onhe solubility of Ag+ ions in the molten glass at 800 ◦C was alsonvestigated. For this purpose, glass frit with a composition (wt%)f 71PbO–19SiO2–10B2O3 and Ag powder with a mean particleiameter of 0.3 �m were employed. Voltammetry and chronoam-erometry were conducted on the glass melts with and without theddition of Ag powder at 800 ◦C under various ambient conditionsith different PO2 values. Using voltammetry, the effect of PO2 on

he ionization reaction of Ag at 800 ◦C was examined. Chronoam-erometry was used to reveal the effect of PO2 on the solubility ofg+ ions in the molten glass at 800 ◦C.

. Experimental procedures

Lead borosilicate glass and Ag powder are the two main ingre-

ients retained in the Ag paste after the burn out of organics duringhe firing process. The lead borosilicate glass used in this studyad a composition (wt%) of 71PbO–19SiO2–10B2O3 and was pre-ared by melting a powder mixture of PbO, SiO2, and H3BO3 at

Fig. 1. Schematic diagram showing the experimental setup used for voltammetryand chronoamperometry analyses.

1200 ◦C for 40 min and then quenching it in deionized water. Theas-produced glass was pulverized to a glass frit with a mean particlesize of 1.8 �m. The glass transition temperature of the glass frit wasmeasured to be 423 ◦C by differential scanning calorimetry (DSC).The glass frit was mixed with Ag powder (spherical grains witha mean diameter of 0.3 �m) to prepare uniform powder mixturescontaining different fractions (0–17 wt%) of Ag. The grain size of theAg powder was somewhat smaller than those of powders used incommercial Ag pastes, but it was chosen in order to increase thereaction area during the dissolution of Ag powder into the moltenglass.

Both melting and electrochemical analyses of the glass frit andits mixtures with Ag powder were carried out under a controlledambient atmosphere using the apparatus shown schematically inFig. 1. The apparatus consisted of an electric furnace, a quartz cham-ber, and components for electrochemical measurements. The valueof PO2 in the quartz chamber was controlled by the steady flowof a 1 atm gas mixture comprising O2 and N2 at various volumefractions of O2 ranging from 0% to 100%. For melting, a Pt cru-cible (inner diameter: 25 mm) was filled with 80 g of the glassfrit or the glass frit/Ag powder mixture. The Pt crucible was thenplaced inside the quartz chamber that had been separated fromthe furnace. After vacuum-evacuating and purging the chamberwith a predetermined gas mixture three times, the gas mixturewas continuously blown into the chamber at 0.5 L/min until theelectrochemical analysis was completed. The quartz chamber withthe Pt crucible inside was plugged into the furnace that was presetto 800 ◦C so that the melting of the glass frit and the dissolutionof the Ag powder into the molten glass could occur under con-trolled ambient conditions with a fixed PO2 . The glass melt or itsmixture with Ag powder was held at 800 ◦C for 12 h to removeany gas pockets in the melt and establish equilibrium beforethe electrodes were immersed into the melt for electrochemicalmeasurements.

Not all the Ag powder initially mixed with the glass frit wasdissolved into the molten glass; rather, some of it submerged to thebottom of the crucible and remained even after holding for 12 h at800 ◦C. In particular, when the ambient condition was pure N2, theAg powder exhibited poor wettability with the molten glass, and

thus, most of the Ag powder floated on the melt surface, causing ashort between the electrodes in the subsequent analysis. Therefore,the electrochemical analyses of the Ag-containing glass melts werecarried out under ambient atmosphere containing at least 5% O2.

imica Acta 106 (2013) 333– 341 335

uFiittcwPatemofe

t−ocotfwmtatou

3

3

gipecpbacpTtft

O

buTiddsSpm

Fig. 2. Voltammograms measured from the molten glasses with and without theaddition of 3 wt% Ag at 800 ◦C under ambient conditions with PO2 = 0.21 atm. Thepotential applied to the WE was regulated against the electrode potential of the RE.

B.-M. Chung et al. / Electroch

The experimental setup for the electrochemical measurementssing voltammetry and chronoamperometry is also illustrated inig. 1. Pt wires with diameters of 0.3 mm were used as the work-ng electrode (WE) and the reference electrode (RE) because Pts not attacked by molten glass or ambient oxygen even at highemperatures [27]. The electrodes were connected to a poten-iostat/galvanostat (Reference 600, Gamry, USA). The Pt crucibleontaining the melt was used as the counter electrode (CE) andas electrically connected to the potentiostat/galvanostat using

t wire. The RE and WE were immersed in the melt to a depth ofpproximately 1 cm below the melt surface and held there for 3 ho stabilize the electrode potential in the melt at 800 ◦C prior to thelectrochemical measurements. The potential applied to the WE byeans of the potentiostat/galvanostat was regulated against that

f the RE [27]. To improve reliability in the comparison among dif-erent measurements, the WE was replaced with a new Pt wire forach electrochemical measurement.

For the voltammetry measurements, the potential applied tohe WE was cathodically swept from the rest potential (E = 0) to0.9 V at a scan rate of −0.1 V/s to obtain current–potential curvesr voltammograms. For chronoamperometry measurements, theurrent was recorded for 5 min at a fixed potential of −0.3 V tobtain current–time curves or chronoamperograms. After each ofhe electrochemical measurements, the WE and RE were removedrom the melt, and the surface area of the WE immersed in the meltas calculated from the immersion depth in order to convert theeasured current into the current density per unit surface area of

he WE. The cross-sectional microstructures of the WE and RE werelso examined by scanning electron microscopy (SEM) in backscat-ered electron (BSE) imaging mode to detect any reaction productsn the electrode surfaces. The phase compositions were analyzedsing energy-dispersive X-ray spectrometry (EDX) at 20 keV.

. Results and discussion

.1. Voltammetric analyses of Ag ionization

Voltammetric measurements were conducted on the moltenlasses with and without the addition of Ag at 800 ◦C under var-ous ambient conditions having different values of PO2 . Since theotential was cathodically applied to the WE in the present study,lectrolysis of the melt was expected to result in the reduction ofations at the WE and the oxidation of anions at the CE. The cationsresent in the glass melt could be Si4+, B3+, Pb2+, which were formedy the dissociation of the oxide components in the glass [32,33],nd Ag+, in the case of the Ag-added melt. The anions in the meltould be O2− ions that were either dissociated from the oxide com-onents of the glass or incorporated through redox reaction (1).herefore, as the applied potential on the WE was swept from E = 0,he electrode reaction at the WE was expected to shift sequentiallyrom the most noble reaction to less noble reactions, while that athe CE remains as

2−(in glass) → O2(g) + 2e−. (4)

Fig. 2 compares the voltammograms obtained from the leadorosilicate glass and its mixture with 3 wt% Ag powder at 800 ◦Cnder the ambient condition of N2 + 21% O2 (i.e., PO2 = 0.21 atm).he voltammogram for the Ag-free glass melt exhibited only onenflection at E = −0.594 V below which the reduction current wasetected, implying that only one of the cationic species was reduceduring the potential sweep. In aqueous solution systems, the

tandard electrode potential of Pb is nobler than those of B andi. Assuming similar ionization tendencies, the reduction current isrobably attributed to the reduction of Pb2+. However, the voltam-ogram for the Ag-containing glass melt exhibited two inflections,

The current density (i) is per unit surface area of the WE immersed into the moltenglass.

one at E = −0.164 V and the other at E = −0.418 V. This suggeststhat two different cationic species were reduced during the poten-tial sweep. Since the only difference between the latter melt andthe former is the addition of 3 wt% Ag and the standard electrodepotential of Ag is even nobler than that of Pb in aqueous solu-tion systems, the first inflection at −0.164 V may be attributed tothe onset of the reduction of Ag+, whereas the second inflectionat the less noble potential reflects the onset of the reduction ofPb2+.

In order to identify the ionic species reduced at different stagesof the potential sweep, the glass mixed with 3 wt% Ag powderwas electrolyzed potentiostatically at 800 ◦C under the ambientcondition of N2 + 21% O2. A constant potential of either −0.3 V or−0.5 V was applied to the WE for 5 min. After the electrolysis,the cross-section of the WE was examined by SEM and EDX. Asshown in Fig. 3a, the electrolysis at −0.3 V resulted in only a binaryAg–Pt layer containing approximately 64 wt% Ag at the glass/WEinterface. This confirms that the reduction current detected at thepotentials between −0.164 V and −0.418 V in the voltammogramshown in Fig. 2 can be attributed solely to the reduction of Ag+

in the melt. However, when electrolyzed at −0.5 V, as shown inFig. 3b, a mixed layer composed of binary Ag–Pt and Pb–Pt phaseswas observed at the glass/WE interface, indicating that both Ag+

and Pb2+ were reduced at −0.5 V. It is noted in Fig. 2 that theonset potential for the reduction of Pb2+ in the Ag-containing glassmelt (−0.418 V) was higher than that in the Ag-free glass melt(−0.594 V). In the case of the Ag-containing glass melt, Ag+ isreduced first and then Pb2+ is reduced during voltammetry. There-fore, Ag deposited on the Pt electrode prior to the reduction of Pb2+

should change the electrode potential of the WE to a less noblevalue, leading to the onset of Pb2+ reduction at a higher (i.e., nobler)potential.

Concerning with the reactions during the firing process for theformation of Ag thick-film contacts to Si solar cells, two salientpoints can be made from the results shown in Figs. 2 and 3. Thefirst is that at least a part of the Ag powder initially mixed with thelead borosilicate glass frit dissolves into the molten glass as Ag+ ionsunder ambient air atmosphere. The second point is that the redoxpotential of Ag+ is nobler than that of Pb2+ in the lead borosili-

cate melt. Therefore, when a Ag-containing lead borosilicate meltis in contact with a SiNx ARC or a Si emitter, the redox reactioninvolving Ag+ will occur more readily than that involving Pb2+. This

336 B.-M. Chung et al. / Electrochimica Acta 106 (2013) 333– 341

Fig. 3. BSE micrographs of the cross-section of the glass/WE interface after the elec-trolysis of the molten glass with the addition of 3 wt% Ag at (a) −0.3 V and (b) −0.5 Vf

eidn

ovitwtsvtcmt

i

oagP

mads

cg

P

Fig. 4. (a) Voltammograms of the molten glass without Ag addition under ambient

or 5 min at 800 ◦C under ambient conditions with PO2 = 0.21 atm.

xplains why only Ag precipitates could be observed at the glass/Sinterface after the firing process when a small amount of Ag pow-er was added to the glass frit but Pb-containing precipitates couldot [13,24].

Before examining the effect of PO2 in the ambient atmospheren the ionization of Ag in the glass melt, the effect of PO2 on theoltammetric characteristics of the Ag-free glass melt was exam-ned. Fig. 4a shows the voltammograms of the lead borosilicate melthat were measured at 800 ◦C under various ambient conditionsith different PO2 values (0–1 atm). The glass was melted under

he same ambient conditions as those for the voltammetric mea-urements, and it was found that, regardless of the PO2 value, theoltammograms exhibit only one inflection reflecting the onset ofhe Pb2+ reduction. This suggests that the dissociation of the glassomponent PbO into Pb2+ and O2− in the molten glass was onlyinimally influenced by PO2 in the ambient atmosphere. However,

he onset potential of the Pb2+ reduction, EPb2+

∣∣Pbo , decreased with

ncreasing PO2 . EPb2+

∣∣Pbo is plotted as a function of the logarithm

f PO2 in Fig. 4b; PO2 in pure N2 atmosphere was assumed to bes low as PO2 = 10−10 atm, accounting for a trace amount of oxy-en in highly pure N2 gas. It is noted that the assumed value ofO2 = 10−10 atm in pure N2 atmosphere is still greater than theaximum value of PO2 ≈ 1.84 × 10−11 atm for PbO to be reduced

t 800 ◦C [34], in accordance with no direct reduction of PbOuring holding the molten glass at 800 ◦C under the pure N2 atmo-phere.The dependence of E

Pb2+∣∣Pbo on PO2 can be understood by

onsidering the overall cell reaction in the electrolysis of the moltenlass:

b2+(in glass) + O2−

(in glass) = Pbo(l) + 1

2O2(g). (5)

conditions with various PO2 values and (b) the corresponding plot of the reductionpotential of Pb2+ (EPb2+∣Pbo ) as a function of the logarithm of PO2 . PO2 in pure N2

ambient was assumed to be 10−10 atm.

The onset potential EPb2+

∣∣Pbo corresponds to the cell potential at

which reaction (5) is in equilibrium. Therefore, the Nernst equationfor reaction (5) gives

EPb2+

∣∣Pbo = Eo − RT

4Fln PO2 + RT

2Fln(aPb2+ aO2− ) (6)

where Eo is the standard cell potential, which depends only ontemperature; R, the gas constant; T, the absolute temperature; F,the Faraday constant; aPb2+ and aO2− are the activities of Pb2+ andO2−, respectively, in the molten glass. Assuming that aPb2+ andaO2− are minimally influenced by the ambient conditions, E

Pb2+∣∣Pbo

depends linearly on the logarithm of PO2 with a proportional con-stant of −RT/4F. The proportional constant −RT/4F is calculated tobe −0.023 at 800 ◦C, which is in good agreement with the slope of−0.025 in Fig. 4b. This signifies that the reduction of Pb2+ at theWE occurred simultaneously with reaction (4) at the CE during thevoltammetry and that the dissociation of PbO into Pb2+ and O2−

in the molten glass was indeed independent of PO2 in the ambientatmosphere.

Fig. 5a shows the voltammograms of the Ag-containing glassmelts that were prepared from the glass mixture with 7 wt% of Agpowder under ambient conditions with PO2 values of 0.05, 0.21,and 1 atm. All the voltammograms, which were measured at 800 ◦C

imica

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wt

oo

TFd

Fap

Pb Agbe a strong function of PO2 because the Ag+ concentration (Co

Ag+ )

in the glass melt strongly depends on PO2 , as discussed previously.

B.-M. Chung et al. / Electroch

nder the same ambient conditions as those for the preparation ofhe melts, exhibited two distinct steps in the reduction current. Therst step at a potential of approximately −0.07 V must be attributedo the onset of the Ag+ reduction, and the second step at a less nobleotential must be attributed to the onset of the Pb2+ reduction. Mostotably, the reduction current due to the Ag+ reduction increasedignificantly with increasing PO2 , as is also shown in Fig. 5b for theeak current density due to the Ag+ reduction, ip,Ag+ , as a functionf PO2 .

The absolute value of ip,Ag+ during the voltammetry can beescribed by the Randle–Sevcik equation [35]:

ip,Ag+∣∣ = 1.475

(�F3DAg+

RT

)1/2

× CoAg+

{0.0235

rWE

(DAg+

�

)1/2

+ 0.306

}(7)

here v is the potential scan rate; rWE, the radius of the WE; DAg+ ,he diffusivity of Ag+ in the melt; and Co

Ag+ , the initial concentration

f Ag+ in the bulk of the melt. According to Eq. (7), ip,Ag+ dependsnly on Co when v, r , and T are fixed as in the present study.

Ag+ WE

herefore, the increase in |ip,Ag+ | with increasing PO2 , as shown inig. 5b, indicates that the melt prepared under the ambient con-itions with a higher PO2 had a greater concentration of Ag+. This

ig. 5. (a) Voltammograms of the molten glass with the addition of 7 wt% Ag undermbient conditions with various PO2 values and (b) the corresponding plot of theeak current density (ip,Ag+ ) due to the reduction of Ag+ as a function of PO2 .

Acta 106 (2013) 333– 341 337

result signifies that the ionization of Ag during the dissolution ofAg powder into the molten glass was enhanced by increasing PO2in the ambient atmosphere, which supports the model of the Agionization process described in reaction (1).

Although the onset potential of the Ag+ reduction, EAg+

∣∣Ago , in

Fig. 5a appeared to decrease with increasing PO2 from 0.05 atm to1 atm, its dependence on PO2 was not as distinct as that of E

Pb2+∣∣Pbo

in the Ag-free glass melt shown in Fig. 4. Similar to the reductionof Pb2+, the overall cell reaction during the reduction of Ag+ shouldbe

2Ag+(in glass) + O2−

(in glass) = 2Ago(s) + 1

2O2(g), (8)

which corresponds to the backward reaction of reaction (1) for theAg ionization. The Nernst equation for reaction (8) gives

EAg+

∣∣Ago = Eo − RT

4Fln PO2 + RT

F

(ln aAg+ + 1

2ln aO2−

). (9)

In contrast to a 2+ in Eq. (6), the activity of Ag+, a + , in Eq. (9) can

Therefore, an increase in aAg+ with increasing PO2 should attenuate

Fig. 6. (a) Chronoamperograms of the molten glasses with different amounts(0–9 wt%) of Ag powder under ambient conditions with PO2 = 0.05 atm and (b) thecorresponding plot of the charge transfer (QAg+ ) due to the Ag+ reduction as a func-tion of the amount of Ag powder added (Co

Ag). The chronoamperometric analyseswere conducted for 5 min at a fixed potential of −0.3 V at which only the Ag+ ionswere reduced at the WE.

338 B.-M. Chung et al. / Electrochimica Acta 106 (2013) 333– 341

F (g–l) tg der ad

t

h

wp

t

w

si

3

A(cotaa8ta−gmvup

manmh

o

ig. 7. Cross-sectional BSE micrographs showing (a–f) the glass/RE interfaces and

) 0 wt%, (b, h) 1 wt%, (c, i) 3 wt%, (d, j) 5 wt%, (e, k) 7 wt%, and (f, l) 9 wt% of Ag pow

he PO2 dependence of EAg+

∣∣Ago by the second term on the right-

and side of Eq. (9), leading to only a minor change in EAg+

∣∣Ago

ith PO2 , as shown in Fig. 5a. Eq. (9) also suggests that the onsetotential E

Ag+∣∣Ago for a given PO2 would shift toward a noble poten-

ial as aAg+ (i.e., CoAg+ ) in the melt increases. In fact, this explains

hy EAg+

∣∣Ago under the ambient conditions with PO2 = 0.21 atm

hifted from −0.164 V to −0.07 V as the amount of Ag powder wasncreased from 3 wt% (Fig. 2) to 7 wt% (Fig. 5a).

.2. Chronoamperometric analyses of Ag+ ion solubility

The voltammetry results confirmed that the ionization ofg powder into the molten glass occurred through reaction

1) involving oxygen in the ambient atmosphere. Therefore,hronoamperometry was employed to examine the dependencef the solubility limit of Ag+ in the molten glass at 800 ◦C on PO2 inhe ambient atmosphere. A series of glass mixtures with differentmounts of Ag powder were melted under ambient conditions with

fixed PO2 . Chronoamperometric measurements of these melts at00 ◦C were then conducted at a potential of −0.3 V for 5 min underhe same ambient conditions. It is noted from the voltammetricnalyses (Fig. 5a) that the reduction current at the potential of0.3 V was solely attributed to the reduction of Ag+ in the moltenlass. In the following, the weight fraction of Ag powder initiallyixed with the glass frit is denoted as Co

Ag. An increase in CoAg pro-

ides more reaction area for the molten glass to dissolve Ag powdernder the influence of the ambient conditions during the meltingrocess.

Fig. 6a shows the chronoamperograms measured from the glasselts with different values of Co

Ag (0–9 wt%) under N2 + 5% O2

mbient conditions (PO2 = 0.05 atm). As expected at E = −0.3 V,o meaningful current was detected from the Ag-free glasselt. However, the Ag-containing glass melts exhibited initially

igh reduction current densities iAg+ that gradually decreased to

he glass/WE interfaces after the chronoamperometric analyses shown in Fig. 6: (a,ded (Co

Ag).

saturation values with time. Both the initial and the saturationvalues of the reduction current density increased with increasingCo

Ag up to 5 wt% and showed no appreciable change upon furtherincreasing Co

Ag. The total reduction current during the chronoam-perometry for 5 min was calculated by integrating iAg+ with time,which should correspond to the total charge transfer (QAg+ ) perunit surface area of the WE due to the reduction of Ag+. Thevalue of QAg+ is plotted as a function of Co

Ag in Fig. 6b, showing

that QAg+ became saturated, reaching Q satAg+ ≈ 0.166 C/cm2 when

CoAg ≥ 5 wt%. According to the Cottrell equation [36], QAg+ in the

present study depends linearly only on the concentration of Ag+

(CoAg+ ) in the bulk melt. Therefore, the result in Fig. 6b implies

that the glass melts with CoAg ≥ 5 wt% were saturated with Ag+

when they were melted under ambient conditions with PO2 =0.05 atm.

Fig. 7 compares the cross-sectional microstructures of the WEand RE after the chronoamperometric measurements shown inFig. 6. Regardless of Co

Ag, the glass/RE interfaces (Fig. 7a–f) wereclean and sharp, indicating no chemical or electrochemical reac-tions at the RE during the chronoamperometry. On the other hand,a binary Ag–Pt layer was observed at the glass/WE interfaces(Fig. 7h–l), indicating that only the Ag+ ions were reduced at theWE and then reacted with Pt to form the Ag–Pt layer during thechronoamperometry at 800 ◦C. In accordance with the change inQAg+ with Co

Ag (Fig. 6b), the average thickness of the Ag−Pt layerincreased with increasing Co

Ag only up to 5 wt%. EDX revealed thatthe Ag content in the Ag–Pt layer was approximately 62 wt% andthat it increased only slightly with increasing Co

Ag.Analyses similar to those for Figs. 6 and 7 were also carried

out for the melts that were prepared under the ambient condi-tions of N2 + 21% O2 (PO2 = 0.21 atm) and 100% O2 (PO2 = 1 atm).

Fig. 8a compares the changes in QAg+ with CAg under various ambi-ent conditions with different PO2 values. In contrast to the case ofPO2 = 0.05 atm, QAg+ in the cases of PO2 = 0.21 atm and PO2 = 1 atmincreased with increasing Co

Ag up to 7 wt% and 11 wt%, respectively.

B.-M. Chung et al. / Electrochimica Acta 106 (2013) 333– 341 339

Fig. 8. Plots of (a) the charge transfer (QAg+ ) due to the Ag+ reduction and (b) thethickness (L) of the Ag–Pt layer formed at the WE by the chronoamperometry as afunction of the amount of Ag powder (Co ) under various ambient conditions withda

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s

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Fig. 9. Log–log plots showing the dependences of (a) the saturated charge transfer(Q sat

Ag+ ) and (b) the saturated amount of Ag reduced at the WE (W satAg ) on PO2 in the

Ag+.

Ag

ifferent PO2 values. The chronoamperometric analyses were conducted for 5 mint a fixed potential of −0.3 V, at which only the Ag+ ions were reduced at the WE.

he average values of QAg+ at CoAg ≥ 7 wt% and Co

Ag ≥ 11 wt% were

aken as the saturation values of QAg+ (Q satAg+ ) under the respective

O2 conditions, as indicated in Fig. 8a. It is clear that Q satAg+ strongly

epended on PO2 in the ambient atmosphere, increasing by ∼5.8imes as PO2 increased from 0.05 atm to 1 atm. According to the Cot-rell equation [36], the increase in Q sat

Ag+ reflects an increase in the

aturated concentration of Ag+ (CsatAg+ ) in the molten glass. There-

ore, the present result implies that the solubility limit of Ag+ in theolten glass was also increased by ∼5.8 times with increasing PO2

rom 0.05 atm to 1 atm in the ambient atmosphere.After each of the chronoamperometric measurements shown in

ig. 8a, the glass/WE interface was examined using SEM and EDX.he average thickness (L) of the Ag–Pt layer formed at the glass/WEnterface was measured by dividing the area of the Ag–Pt layer inhe cross-sectional micrograph by the linear length of the glass/WEnterface. As shown in Fig. 8b, the change in L with Co

Ag under theiven ambient conditions exhibited a trend similar to that of QAg+ ,pproaching a saturation value with increasing Co

Ag. EDX revealed

hat the Ag content in the Ag–Pt layer, CAg−Pt, increased not only

Agith increasing Co

Ag but also with increasing PO2 in the ambienttmosphere. When L reached its saturation values (Lsat) of 0.43,.62, and 1.99 �m under the respective PO2 conditions (0.05, 0.21,

ambient atmosphere when the glass melt became saturated with Ag+ ions underthe respective PO2 conditions, as shown in Fig. 8. The slopes denoted in (a) and (b)represent the dependences of ln Q sat

Ag+ and ln W satAg on ln PO2 , respectively.

and 1 atm), CAg−PtAg was measured to be 62.2, 67.1, and 79.1 wt%,

respectively.The amount of Ag reduced at the WE, WAg, after the chronoam-

perometry was calculated from the thickness (L), Ag content(CAg−Pt

Ag ), and weight density (�Ag−Pt) of the Ag–Pt layer:

W satAg = �Ag−PtL

satCAg−Pt

Ag

100(10)

where W satAg represents the amount of Ag reduced at the WE when

the thickness L reached its saturation value Lsat for a given PO2 .From the literature [37], �Ag−Pt of Ag–Pt alloys with 62.2, 67.1, and79.1 wt% Ag was taken to be 11.8, 12.57, and 13.19 g/cm3, respec-tively. The calculated values of W sat

Ag for the ambient PO2 conditions

of 0.05, 0.21, and 1 atm were 0.352, 0.521, and 1.835 mg/cm2,respectively. The dependence of W sat

Ag on PO2 was in good agree-

ment with that of Q satAg+ , as shown by the log-log plots in Fig. 9. This

further confirms that Q satAg+ is solely attributed to the reduction of

The chronoamperometry results shown in Figs. 8 and 9 clearlydemonstrate the strong dependence of Csat

Ag+ on PO2 in the ambi-

ent atmosphere, which supports the model of the ionization of Ag

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40 B.-M. Chung et al. / Electroch

escribed in reaction (1). Although the electrochemical measure-ents were carried out after holding the melts at 800 ◦C for 15 h,

he influence of PO2 on the ionization of Ag should be predominantn the dissolution stage of the Ag powder into the molten glass. Thisonclusion is also supported by the previous observation that theeaction kinetics to form Ag crystallites at the contact interface ofi solar cells were significantly enhanced by increasing PO2 , evenn the spike-firing process [25,26] where the molten glass was inontact with Ag powder less than 1 min.

. Conclusions

In an attempt to confirm the mechanism proposed for fire-hrough Ag contact formation, in which Ag+ ions dissolved in the

olten glass play a crucial role, the effect of PO2 in the ambientn the ionization of Ag powder in the molten lead borosilicatelass was electrochemically examined using voltammetry andhronoamperometry at 800 ◦C. The voltammetry demonstratedhat, in contrast to the formation of Pb2+ by the dissociation of PbOontained in glass, the concentration of Ag+ formed by the dissolu-ion of Ag powder increased significantly with increasing PO2 in thembient atmosphere. In the molten glass, the reduction potentialf Ag+ was much nobler than that of Pb2+. The chronoamperometryesults demonstrated that the solubility limit of Ag+ in the moltenlass at 800 ◦C also strongly depended on PO2 in the ambient atmo-phere, increasing by ∼5.8 times as PO2 increased from 0.05 atm to

atm.Together with previous reports [25,26] demonstrating that the

mount of Ag crystallites formed at the contact interface increasedignificantly with increasing PO2 in the ambient firing atmosphere,he present study strongly supports the mechanism in which theg+ formed by reaction (1), and not PbO or Pb2+ contained in thelass itself, reacts sequentially with the SiNx ARC and the Si emit-er during the firing process. This is because Ag+ has much greaterendency to be reduced by redox reactions with the SiNx ARC and Simitter than does Pb2+, as shown by the difference in the reductionotential between Ag+ and Pb2+ in the molten glass. As a con-equence, the reaction kinetics during the firing process dependritically on the Ag+ concentration in the molten glass and thus cane controlled by PO2 in the ambient firing atmosphere. Althoughhe molten glass itself does not participate directly in the firingeactions, it plays an important role as a dissolution and transportedium for the Ag+ and O2− ions. Therefore, the glass chemistry

an also have a strong influence on the reaction kinetics duringring through the solubility and diffusivity of Ag+ ions.

cknowledgments

The authors gratefully acknowledge the support by Cheil Indus-ries Inc. This work was supported by the National Researchoundation of Korea (NRF) Grant (No. 2010-0014480) and Humanesources Development of the Korea Institute of Energy Technol-gy Evaluation and Planning (KETEP) Grant (No. 20104010100640)unded by MEST and MKE, respectively, of the Korea Government.

eferences

[1] G. Schubert, F. Huster, P. Fath, Physical understanding of printed thick-filmfront contact of crystalline Si solar cell – review of existing models and recentdevelopments, Solar Energy Materials and Solar Cells 90 (2006) 3399.

[2] K. Shirasawa, Mass production technology for multicrystalline Si solar cells,Progress in Photovoltaics: Research and Applications 10 (2001) 107.

[3] M.M. Hilali, M.M. Al-Jassim, B. To, H. Moutinho, A. Rohatgi, S. Asher, Under-

standing the formation and temperature dependence of thick-film Ag contactson high-sheet-resistance Si emitters for solar cells, Journal of the Electrochem-ical Society 152 (2005) G742.

[4] G. Grupp, D.M. Huljic, R. Preu, G. Willeke, J. Luther, Peak firing temperaturedependence of the microstructure of Ag thick-film contacts on silicon solar cells

[

[

Acta 106 (2013) 333– 341

– a detailed AFM study of the interface, in: Proceedings of the 20th EuropeanPhotovoltaic Solar Energy Conference, Barcelona, Spain, 6–10 June, 2005, p.1379.

[5] M.M. Hilali, K. Nakayashiki, C. Khadilkar, R.C. Reedy, A. Rohatgi, A. Shaikh, S. Kim,S. Sridharan, Effect of Ag particle size in thick-film Ag paste on the electrical andphysical properties of screen printed contacts and silicon solar cells, Journal ofthe Electrochemical Society 153 (2006) A5.

[6] M.M. Hilali, S. Sridharan, C. Khandilkar, A. Shaikh, A. Rohatgi, S. Kim, Effect ofglass frit chemistry on the physical and electrical properties of thick-film Agcontacts for silicon solar cells, Journal of Electronic Materials 35 (2006) 2041.

[7] M. Hörteis, S.W. Glunz, Fine line printed silicon solar cells exceeding 20% effi-ciency, Progress in Photovoltaics: Research and Applications 16 (2008) 555.

[8] Y. Zhang, Y. Yang, J. Zheng, W. Hua, G. Chen, Thermal properties of glass frit andeffects on Si solar cells, Materials Chemistry and Physics 114 (2009) 319.

[9] D. Kim, S. Hwang, H. Kim, Effect of the thermal properties of frits on the electricalproperties of screen-printed silicon solar cells, Journal of the Korean PhysicalSociety 55 (2009) 1046.

10] S. Kontermann, M. Hörteis, M. Kasemann, A. Grohe, R. Preu, E. Pink, T. Trupke,Physical understanding of the behavior of silver thick-film contacts on n-typesilicon under annealing conditions, Solar Energy Materials and Solar Cells 93(2009) 1630.

11] Z.G. Li, K.R. Mikeska, L. Liang, A. Meisel, G. Scardera, L.K. Cheng, P.D. VerNooy,M.E. Lewittes, M. Lu, F. Gao, L. Zhang, A.F. Carroll, C.S. Jiang, Microstructuralcharacterization of front-side Ag contact of crystalline Si solar cells with lightlydoped emitter, in: Proceedings of the 38th IEEE Photovoltaic Specialists Con-ference, Austin, USA, 3–8 June, 2012, p. 2196.

12] C. Ballif, D.M. Huljic, G. Willeke, A. Hessler-Wyser, Silver thick-film contacts onhighly doped n-type silicon emitters: Structural and electronic properties ofthe interface, Applied Physics Letters 82 (2003) 1878.

13] G. Schubert, F. Huster, P. Fath, Current transport mechanism in printed Agthick film contacts to an n-type emitter of a crystalline silicon solar cell, in:Proceedings of the 19th European Photovoltaic Solar Energy Conference, Paris,France, 7–11 June, 2004, p. 813.

14] B. Sopori, V. Mehta, P. Rupnowski, J. Appel, M. Romero, H. Moutinho, D. Domine,B. To, R. Reedy, M. Al-Jassim, A. Shaikh, N. Merchant, C. Khadilkar, D. Carlson,M. Bennet, Fundamental mechanisms in the fire-through contact metallizationof Si solar cells: a review, in: Proceedings of the 17th Workshop on CrystallineSilicon Solar Cells and Modules: Materials and Processes, Vail, USA, 5–8 August,2007, p. 93.

15] C.H. Lin, S.Y. Tsai, S.P. Hsu, M.H. Hsieh, Investigation of Ag-bulk/glassy-phase/Siheterostructures of printed Ag contacts on crystalline Si solar cells, Solar EnergyMaterials and Solar Cells 92 (2008) 1011.

16] K.K. Hong, S.B. Cho, J.Y. Huh, H.J. Park, J.W. Jeong, Role of PbO-based glass fritin Ag thick-Film contact formation for crystalline Si solar cells, Metals andMaterials International 15 (2009) 307.

17] D. Pysch, A. Mette, A. Filipovic, S.W. Glunz, Comprehensive analysis of advancedsolar cell contacts consisting of printed fine-line seed layers thickened by sil-ver plating, Progress in Photovoltaics: Research and Applications 17 (2009)101.

18] M. Hörteis, T. Gutberlet, A. Reller, S.W. Glunz, High-temperature contact for-mation on n-type silicon: Basic reactions and contact model for seed-layercontacts, Advanced Functional Materials 20 (2010) 476.

19] R. Lago, L. Pérez, H. Kerp, I. Freire, I. Hoces, N. Azkona, F. Recart, J.C. Jimeno,Screen printing metallization of boron emitters, Progress in Photovoltaics:Research and Applications 18 (2010) 20.

20] Y. Veschetti, R. Cabal, P. Brand, V. Sanzone, G. Raymond, A. Bettinelli, High effi-ciency on boron emitter n-type Cz silicon solar cells with industrial process,IEEE Journal of photovoltaics 1 (2011) 118.

21] A. Kalio, A. Richter, M. Hörteis, S.W. Glunz, Metallization of n-type silicon solarcells using fine line printing techniques, Energy Procedia 8 (2011) 571.

22] J.A. Silva, M. Gauthier, C. Boulord, C. Oliver, A. Kaminski, B. Semmache, M. Lemiti,Progress in the metallization of n-type multicrystalline silicon solar cells, SolarEnergy Materials and Solar Cells 95 (2011) 3333.

23] J.A. Silva, M. Gauthier, C. Boulord, C. Oliver, A. Kaminski, B. Semmache, M. Lemiti,Improving front contacts of n-type solar cells, Energy Procedia 8 (2011) 625.

24] K.K. Hong, S.B. Cho, J.S. You, J.W. Jeong, S.M. Bae, J.Y. Huh, Mechanism for theformation of Ag crystallites in the Ag thick-film contacts of crystalline Si solarcells, Solar Energy Materials and Solar Cells 93 (2009) 898.

25] S.B. Cho, K.K. Hong, J.Y. Huh, H.J. Park, J.W. Jeong, Role of the ambient oxygen onthe silver thick-film contact formation for crystalline silicon solar cells, CurrentApplied Physics 10 (2010) S222.

26] J.Y. Huh, K.K. Hong, S.B. Cho, S.K. Park, B.C. Lee, K. Okamoto, Effect of oxygen par-tial pressure on Ag crystallite formation at screen-printed Pb-free Ag contactsof Si solar cells, Materials Chemistry and Physics 131 (2011) 113.

27] K. Yata, N. Hanyu, T. Yamaguchi, Voltammetric study of silver in a glass melt,Journal of the American Ceramic Society 78 (1995) 1153.

28] M. Maric, M.P. Brungs, M. Skyllas-Kazacos, Voltammetric and chronopotentio-metric reactions of silver ions in molten disilicate glass, Physics and Chemistryof Glasses 30 (1989) 12.

29] O. Claußen, C. Rüssel, A voltammetric study of the Ag+/Ago-equilibrium in soda-alumina-silicate melts, Journal of Molecular Liquids 83 (1999) 295.

30] T. Yano, T. Nagano, J. Lee, S. Shibata, M. Yamane, Cation site occupation byAg+/Na+ ion-exchange in R2O–Al2O3–SiO2 glasses, Journal of Non-CrystallineSolids 270 (2000) 163.

31] G.F. Kessinger, T. Huett, J.E. Delmore, Ag ion formation mechanisms in moltenglass ion emitters, International Journal of Mass Spectrometry 208 (2001) 37.

imica

[

[

[

[

B.-M. Chung et al. / Electroch

32] R. Didtschenko, E.G. Rochow, Electrode potentials in molten silicates, Journalof the American Chemical Society 76 (1954) 3291.

33] O. Claußen, C. Rüssel, Self diffusion of polyvalent ions in a borosil-icate glass melt, Journal of Non-Crystalline Solids 215 (1997)68.

34] D.R. Gaskell, Introduction to the Thermodynamics of Materials, 5th ed., Taylor& Francis, New York, 2008, p. 582.

[

[

Acta 106 (2013) 333– 341 341

35] H.V. Drushel, J.F. Miller, Anodic polarographic estimation of aliphatic sulfidesin petroleum, Analytical Chemistry 29 (1957) 1456.

36] N.P. Bansal, J.A. Plambeck, An aid to the interpretation of electrochemical datameasured with spherical and cylindrical electrodes: Corrections to the Cottrellequation, Canadian Journal of Chemistry 56 (1978) 155.

37] J.F. Thompson, E.H. Miller, Platinum silver alloys, Journal of the American Chem-ical Society 28 (1906) 1115.