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Ag–Al alloy thin film on plastic substrate by screen printingfor solar cell back contact application

M.K.M. Ali *, K. Ibrahim, E.M. Mkawi

Nano-Optoelectronics Research and Technology Laboratory, School of Physics, Universiti Sains Malaysia, 11800 Penang, Malaysia

a r t i c l e i n f o

Available online 11 January 2013

Keywords:

Ag-Al alloy

polyethylene terephthalate

screen printing

characterization

a b s t r a c t

Ag–Al alloy thin films with different thicknesses were screen printed onto polyethylene

terephthalate plastic substrates at room temperature. Three different weights of pure Ag

paste were mixed with Al paste to create alloy pastes with different viscosities.

A uniform composition of Ag and Al was obtained with a 70:30 composition ratio.

The variation in the viscosity of the mixed paste enabled production of different

thicknesses of printed layers. The deposited films were annealed at 200 1C for 45 min.

Structural characterization and elemental content analysis of the films were carried out

using X-ray diffraction (XRD), Raman spectroscopy, and energy-dispersive X-ray spectro-

scopy. The surface morphology of the printed films was studied by scanning electron

microscopy and atomic force microscopy. Their electrical properties were investigated by

four-point probe measurements. The crystalline sizes and strain along the a and c axes

were calculated from the XRD patterns. Both were found to increase with increased film

thickness.

& 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Ag and Al can form useful white alloys that are harderthan pure Al. The major advantages of Ag/Al alloys includeoxygen resistance, hardness, polishing ability, and chemi-cal inertness. These alloys are also harder, less expensive,and easier to be stamped or engraved on than Ag [1].Ag–Al alloys are recommended for many applications inthe watchmaking industry. The properties of Ag–Al alloysconsiderably vary according to the percentage of Al. Thealloying process can also induce spatial homogeneity ofcomposition and other morphological changes, such assurface roughness and grain boundaries; especially indeposited films. The effects of these changes on theoptical properties of the alloys have been studied before[2]. The current study examined the effects of alloyingand microstructure on thin films for solar cell back

contact applications. An important note for this applica-tion was that the ratio of Ag to Al should be 70:30 tocreate a back surface field in the solar cell.

Screen printing was introduced in optoelectronicsprocessing during the 1970s to reduce fabrication timeand energy usage [3]. The technique offers the advantageof low-cost production, and can be used to print the frontand back contacts of Si solar cells. Ag paste was used inthe front contact, and a full Al layer was printed on therear of the cell. Subsequent alloying through firing pro-duces a back surface field (BSF). Increasing of Al dopinglevel in the P type region is quite important in describingthe performance of the back surface of solar cell.By the back contact Al atoms, the back–side built-inelectric field opposes the motion of minority carriers(electrons) to the back surface where the recombinationvelocity is high. The collection efficiency is improvedaccordingly [4]. A second print of Al–Ag is required forsolderable contact, which is still cost-effective because Alis inexpensive. In most production, the rear contact issimply made using an Ag–Al grid printed in a single step.

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Materials Science in Semiconductor Processing

1369-8001/$ - see front matter & 2012 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.mssp.2012.10.006

* Corresponding author.

E-mail address: hamofarog@yahoo.com (M.K.M. Ali).

Materials Science in Semiconductor Processing 16 (2013) 593–597

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Nowadays, more than half of commercial Si solar cellcontacts are manufactured by screen printing [5]. In thisstudy, we investigated the properties of Ag–Al alloy thinfilms deposited at a 70:30 ratio on plastic substrates(polyethylene terephthalate, PET) by screen printing. Thecurrent study was cost-effective and reduced the solar cellcost by using Ag–Al alloy as back contact on inexpensiveand flexible plastic substrate (PET) deposited by low cost,simple, and large-area production screen printing. Theproposed original method for printing Ag–Al alloy thinfilms on PET substrate was inexpensive and can beapplied to flexible device holders, back contacts, and backreflectors for optoelectronics.

2. Experimental details

The PET sheet used was purchased from Penfiber Sdn.Bhd. (Penang, Malaysia). The 200-mm thick PET substrate wascut into 4 cm�2 cm pieces and washed ultrasonically inacetone and distilled water for 30 min, dried at 60 1C for20 min, and blown dry with N2 gas. The Ag and Al pasteswere purchased from Ferro Electronic Materials Co.,Germany. Ag and Al pastes with different viscosities weremixed together. The composition was 70% Ag and 30% Al, andthis ratio was fixed in three different viscosities (100, 120,and 140 Pa s) for Ag paste and a constant viscosity (420 Pa s)for Al paste. The viscosities were measured using standardspindles (Visco Basic, UK) and a viscometer. The variation inthe mixed paste viscosity enabled the deposited thin films tohave different thicknesses. Screen has silk meshes of 325 and25 mm emulsion has been used with printing angle of 451 toprint the mixed alloy paste on the PET substrate. Fig. 1 showsthe basic elements, mask design, and fabrication process ofscreen printing. Further details are described elsewhere [6].The obtained thin films with three thicknesses were char-acterized by various techniques. Scanning electron micro-graphs were obtained using a scanning electron microscopy(SEM) system. The surface topography of the alloy thin filmswas examined by SEM. The chemical composition of the filmwas determined by an energy dispersive X-ray (EDX) spectro-scopy system attached to the scanning electron microscope.The topography of the films layers was characterized byatomic force microscopy (AFM). The crystallinity and crystalsizes (D) of the films were calculated by high resolution X-raydiffraction (XRD). The electrical properties were investigatedby four-point probe measurements (Changmin Tech CMT-SR2000N, Northern Korea)

3. Results and discussion

3.1. SEM

The scanning electron micrographs of the films arepresented in Fig. 2a–c for 900 nm, 750 nm, and 600 nm,respectively, with a magnification of 30,000� . Generally,the films exhibited rough surfaces with screen printingships. The grains size of the film in Fig. 2a was bigger thanthat of the films in Figs. 2b and 2c, indicating that grainssize of Ag-Al ally were increased with thin films thickness.To confirm the chemical compositions of the thin films,the elemental compositions of the samples were investi-gated by EDX spectroscopy. Fig. 2 shows a typical EDXmeasurement result displaying Ag and Al peaks as well asother components of the PET substrate, including C and O2

(c), and O2 (b). C and O2 were not detected in (a) withincreased thickness, as confirmed by the XRD results.

3.2. AFM

The surface roughness of the alloy films was probed byAFM in the tapping mode. Fig. 3a–c shows that the surfaceof the Ag–Al alloy thin films on PET substrate wassignificantly rough, which can be attributed to the print-ing effect and the deposition of the films at roomtemperature, which created significant surface roughnessof 161–297 nm. Fig. 3a shows larger alloy clusters on thePET substrate than Figs. 3b and 3c. This increase in clustersize can be attributed to the increased film thickness. Thesize of the Ag–Al clusters deposited on the PET substratesfurther increased to about 950 nm when the coatingthickness reached 900 nm. The root mean square (RMS)surface roughness values of the films were found to be161 nm, 261 nm, and 297 nm for (a), (b), and (c), respec-tively. The surface roughness significantly changed withincreased thickness due to reduced applied pressure onthe squeegee via the screen printing mechanism [7].

3.3. XRD

Fig. 4 shows the typical XRD pattern of Ag–Al alloys at900 nm (a), 750 nm (b), and 600 nm (c) thicknesses,deposited on PET substrate. The XRD patterns of the alloythin films revealed that the films exhibits polycrystallinenature and the structure of Ag and Ag3Al were grown withsame orientation in all samples. Similar diffraction pat-terns were observed for all thin films, confirming thatthey were polycrystalline [8]. In all samples, there was astrong peak at 2y¼38.41 corresponding to Ag (1 1 1) withvariations in intensity due to the amount of material (Ag),which increased with thickness. The peak width of Ag(1 1 1) decreased from (a) to (c). The decreased full-widthat half-maximum (FWHM) of these peaks indicated thatthe crystal size increased with thickness. There wereother small peaks observed, namely at 2y¼34.51, 36.11,and 37.61. The peak at 2y ¼ 79.611 is corresponding toAg2Al (4 4 0). The Al (3 3 1) peak is at 2y¼111.21 in allsamples, which is useful in creating a BSF for solar cellapplications.

Fig. 1. Basic elements and fabrication process of screen-printing

technique.

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Detailed structural characterization revealed that thedeposited films comprised big crystallites or grains welloriented at low heat treatments (200 1C). The crystal sizeD for all phases was also calculated by the Scherrerformula:

D¼kl

b cosyð1Þ

where b is the FWHM, k is a constant equal to 0.94, and lis the incident X-ray wavelength (1.5406 A) [9]. The D

values for the all samples increased with thickness. Theincrease in crystallites with thickness can be attributed tohigher values of paste viscosities resulting from thescreen printing to which the thicker films were subjected.

Lattice parameter a, which was equal to c in this case(cubic), can be calculated from Eq. (2) by the quadraticform of Bragg’s equation. The lattice constant calculatedfrom this pattern was found to be proportional with thefilm thickness and consistent with the standard value ofao of Ag and Ag3Al. The determined lattice constants forthe three thicknesses showed that the values of the lattice

parameters increased with film thickness, in agreementwith the results of Tingting Song and Yulai Gao [10].

d¼ a= h2þk2þ l2

� �1=2ð2Þ

The strains along the a and c axes, ea and ec, werecalculated from the following relations:

ea ¼a�a0ð Þ

a0ð3Þ

where c and a are the calculated lattice parameters, andthe standard and unstrained parameter values areao¼co¼4.086 and 6.946 A for Ag and Ag3Al, respectively.Positive values of ea and ec denote tensile strain, whereasnegative ones denote compressive strain. The strain valuedecreased with increased thickness [11], suggesting thatthe crystal quality can be improved by controlling theincrease in thickness. The values of D, a, ea, and ec werecalculated for the dominant phases, that is, those withthe highest intensities in the XRD patterns. The values ofthe lattice parameter a, in-plane strain ea, and average

Fig. 2. SEM images and EDX of Ag–Al alloy films deposited on PET substrate at a thickness of 900 nm (a), 750 nm (b), and 600 nm (c).

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crystal size determined for the samples are listed inTable 1.

3.4. Raman spectroscopy

The Raman spectrometer module Jobin Yvon HR800UVwith an argon ion laser (514.5 nm) source was usedin this research. The incident laser power was 20 mW.

The grating and the hole size were 50 mm. The Ramanscattering experiments were carried out at room temperaturewith a system resolution of 1 cm�1. Ag–Al alloy thin filmswere studied by Raman spectroscopy, and experiments werecarried out with three different thicknesses. Controlling thethickness and structure is a powerful strategy for modulatingthe electronic structures and optical properties of thin films.Raman spectroscopy is applicable not only to simple thinfilms, but also to complete devices [12–15]. PolycrystallineAg and Ag–Al alloy thin films were examined. The measurednonresonant Raman spectra of the thin films are shown inFig. 5. The strong peak at 1632.6 cm�1 corresponded to Agvibrations for sample of 900 nm. This peak is shifted to 1607cm-1 in sample with thickness of 750 nm and 1627 cm-1 forsamples with thicknesses of 600 nm. The shift in thesesamples is blue shift. Weaker signals were detected at854.2 cm-1, 1158 cm-1, 1737 cm-1, and 1918 cm-1 in samplehas thickness of 900 nm and these peaks were shifted to thelower Raman shift (blue shift) with thickness decreasing to750 nm and 600 nm. These results were in agreement withthose reported by Yang and Sun [16]. The broad and strongpeak at about 1346 cm�1 may correspond to Ag–Al alloybonds vibration, which was much stronger for the 900-nm-thick sample than the other two samples. Raman spectro-scopy revealed the structural changes related to the strainwith the unique properties of thin film. It also explained thechanges in the band gap in the concept of the strainassociated with the structure of thin films with the correla-tion between the increased band gap and decreased latticeconstants of the different thicknesses of thin film. Thenanostructures have advantage properties such as grainboundary and high surface area comparing between thenanostructures and other thin films or bulk material. Theresults were analyzed and compared with other forms of Agand Ag–Al, such as thin film, polycrystalline powder, andsolids, and acceptable values were found. For the Ag3Alsystem, the lattice cell volume expanded, with increasedthickness, contrary to the lattice cell volume variation due tothe paste viscosities and the screen printing mechanism [6].One of the main findings of this study was that the Ramanspectra of the films strongly depended on their thickness.

3.5. Electrical properties of the Ag–Al alloy thin films

Ag has a face-centered cubic crystal structure with abulk electrical resistivity of 15.87 nO �m at 20 1C. Al has asimilar structure but a higher resistivity (28.2 nO �m at20 1C). Ag–Al alloys are widely used materials for elec-trical contacts [17,18]. Room temperature four-pointprobe measurements of Ag–Al films deposited onto PETsubstrate of various thicknesses were accomplished suc-cessfully. A relatively small change in resistance wasobserved for the Ag–Al films; both resistivity and sheetresistance decreased with increased film thickness.Previous studies have suggested that elastically stretch-able metal interconnects undergo increased resistancewith increased mechanical strain. The tensile strain par-tially blocking the way for an influence of direct current,that means the carrier mobility degrade with the increas-ing of the stress which is increases with thickness thinfilms decreasing. The mobility enhancement originates

Fig. 3. AFM images of Ag–Al alloy films deposited on PET substrate at a

thickness of 600 nm (a), 750 nm (b), and 900 nm (c).

Fig. 4. Typical XRD pattern of Ag–Al alloy deposited on PET substrate.

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from the tensile strain partially compensating for aninfluence of direct current stress on mobility degradation[19]. The electrical properties of Ag–Al alloy films depos-ited on PET substrateswith different thicknesses areshown in Table 2.

4. Conclusion

Thin film materials grown by the screen-printingtechnique have been receiving increased attention.In the current study, Ag–Al thin films were screen-printed onto PET substrates using alloy paste with differ-ent viscosities. The structural properties of the filmsimproved with increased thickness; the crystal sizereached 446.11 nm for Ag and 540.15 nm for Ag3Al2 atabout 900 nm film thickness. The maximum surfaceroughness was associated with the greatest thickness,with an RMS value of 297 nm. The resistivity andsheet resistance decreased with increased thickness. Theobtained structure and electrical properties demonstrated

the potential use of the crystalline printed Ag3Al thinfilms in the large-scale manufacturing of solar cells andother optoelectronic applications.

Acknowledgments

This work was supported by the Nano-optoelectronicsResearch Laboratory, School of Physics: Universiti SainsMalaysia.

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Table 1Lattice parameters (a¼c), in-plane strain (ea), out-of-plane strain (ec), and average crystal size determined for the Ag–Al thin films.

Thickness (nm) Phase a (A) d (A) D(nm) ea (%)

(a) 900 Ag(1 1 1) 4.095 2.364 446.11 0.0022

Ag(2 0 0) 4.088 2.044 364.21 0.00049

Ag3.Al(4 4 0) 6.975 1.233 540.15 0.0042

(b) 750 Ag(1 1 1) 4.089 2.361 438.97 0.00073

Ag(2 0 0) 4.086 2.043 298.69 0

Ag3Al(4 4 0) 6.969 1.232 212.73 0.0033

(c) 600 Ag(1 1 1) 4.07 2.35 351.2 �0.1933

Ag(2 0 0) 4.084 2.042 364.2 �0.00049

Ag3Al(4 4 0) 6.958 1.23 360.93 0.0017

Fig. 5. Raman spectra of Ag–Al alloy deposited on PET substrate.

Table 2Summary of electrical properties of typical of Ag–Al alloy films depos-

ited on PET substrate.

Thickness Sheet resistant Resistivity

(nm) Rs (O/&) r (O-cm)

(a) 900 0.09 8.104�10�5

(b) 750 0.14 1.049�10�4

(c) 600 0.182 1.092�10�4

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