ARTICLE IN PRESS

Solar Energy Materials & Solar Cells 92 (2008) 1605–1610

Contents lists available at ScienceDirect

Solar Energy Materials & Solar Cells

0927-02

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/solmat

Highly stable transparent and conducting gallium-doped zinc oxide thinfilms for photovoltaic applications

E. Fortunato�, L. Raniero, L. Silva, A. Gonc-alves, A. Pimentel, P. Barquinha, H. Aguas, L. Pereira,G. Gonc-alves, I. Ferreira, E. Elangovan, R. Martins

CENIMAT/I3N, Materials Science Department, FCT-UNL and CEMOP-UNINOVA, Campus de Caparica, 2829-516 Caparica, Portugal

a r t i c l e i n f o

Article history:

Received 20 April 2008

Accepted 17 July 2008Available online 3 September 2008

Keywords:

Transparent conducting oxides

Radio-frequency sputtering

Electrical properties

X-ray diffraction

48/$ - see front matter & 2008 Elsevier B.V. A

016/j.solmat.2008.07.009

esponding author. Tel.: +351 212948562; fax:

ail address: elvira.fortunato@fct.unl.pt (E. For

a b s t r a c t

Transparent and highly conducting gallium zinc oxide (GZO) films were successfully deposited by RF

sputtering at room temperature. A lowest resistivity of �2.8�10�4O cm was achieved for a film

thickness of 1100 nm (sheet resistance �2.5O/&), with a Hall mobility of 18 cm2/V s and a carrier

concentration of 1.3�1021 cm�3. The films are polycrystalline with a hexagonal structure having a

strong crystallographic c-axis orientation. A linear dependence between the mobility and the crystallite

size was obtained. The films are highly transparent (between 80% and 90% including the glass substrate)

in the visible spectra with a refractive index of about 2, very similar to the value reported for the bulk

material. These films were applied to single glass/TCO/pin hydrogenated amorphous silicon solar cells

as front layer contact, leading to solar cells with efficiencies of about 9.52%. With the optimized

deposition conditions, GZO films were also deposited on polymer (PEN) substrates and the obtained

results are discussed.

& 2008 Elsevier B.V. All rights reserved.

1. Introduction

Transparent conducting oxide (TCO) with an optical transmis-sion exceeding 80% in the visible region (500–650 nm) and aresistivity less than 10�3O cm have been widely used in a varietyof applications for more than a half-century. In recent times, TCOhas become the subject of intense investigation for applications astransparent electrodes in optoelectronic devices such as flat paneldisplays [1], solar cells [2] and organic light-emitting diodes [3].Most of the previous research on TCOs has been focused onindium tin oxide (ITO) [4] and fluorine tin oxide (FTO) [5].However, TCO films based on zinc oxide (ZnO) are receiving muchattention because of their advantages such as low cost, resourceavailability (about a factor of 1000 more abundant than indium),non-toxicity and high thermal/chemical stability [6] over themore commonly used TCOs (indium-based oxides). Undoped ZnOusually presents a high resistivity due to a low carrier concentra-tion. Aluminium (Al), indium (In) and gallium (Ga) have beenreported as effective dopants for ZnO-based films. Although Al isused as a dopant in most of the research works based on ZnO, itpresents a very high reactivity, which may lead to oxidationduring the film’s growth and cause problems. Ga is less reactive

ll rights reserved.

+351 212948558.

tunato).

and more resistant to oxidation than Al [7,8] and it has beendemonstrated that the Ga-doping leads to films with lowresistivity associated with a high transmittance in the visibleregion [9–12]. Several techniques such as metal organic chemicalvapor deposition, evaporation, magnetron sputtering, sol–gel andplasma-assisted molecular beam epitaxy among others have beenutilized [13]. However, most of these techniques require moderatesubstrate temperatures to obtain low resistivity. Among theavailable techniques, RF magnetron sputtering presents severaladvantages including the production of highly transparent andconducting gallium zinc oxide (GZO) without heating thesubstrate [11].

Fig. 1 summarizes the lowest resistivity and highest Hallmobility reported to date using different techniques and processes[14–22]. It is noticed that while the resistivity remains almostconstant (around 2.4�10�4O cm) as the substrate temperatureincreases, the mobility increases, mainly due to an improvementin the crystallinity of the films. As we intend to use inexpensivepolymeric substrates, the deposition process cannot includemoderate or high temperatures. This driving force motivated theauthors to optimize the deposition conditions of the RF magne-tron sputtering technique in order to develop GZO thin filmsprocessed at room temperature (RT) that present high conducti-vity and high transmittance, simultaneously. Further, the addi-tional energy required in this technique for proper coalescence(in order to get compact and dense films) is delivered from the

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1

2

3

4

5

6

7

8

Substrate temperature (°C)

Res

istiv

ity, ρ

(10−4

Ωcm

)

GZO thin films

10

20

30

40

50

60

μ

Hal

l mob

ility

, μ (c

m2 / V

s)

ρ

0 200 400 600 800

Fig. 1. Dependence of the electrical resistivity (r) and Hall mobility (m) as a

function of substrate temperature, for GZO films deposited by several deposition

techniques [4–12].

E. Fortunato et al. / Solar Energy Materials & Solar Cells 92 (2008) 1605–16101606

plasma, a characteristic of the plasma-assisted processes. In thispaper we present some morphological, electrical and opticalproperties of GZO thin films in addition to their application toa-Si:H pin solar cells with improved performance.

2

3

4

5

6

10

12

14

16

18

0.5

1.0

1.5

2.0

μ

Res

istiv

ity, ρ

(x10

-4, Ω

cm)

Thickness (nm)

ρ

N

Mob

ility

,μ (c

m2 /V

s)

Con

cent

ratio

n, N

(x10

21, c

m-3

)

0 200 400 600 800 1000 1200

Fig. 2. Dependence of the electrical resistivity (r), carrier concentration (N) and

Hall mobility (m) as a function of film thickness of the GZO films deposited at room

temperature (in this study).

2. Experimental details

The GZO films were deposited initially on soda lime glasssubstrates by RF (13.56 MHz) magnetron sputtering using a 5 cmplanar ceramic oxide target consisting ZnO (98 wt%):Ga2O3

(2 wt%) from Super Conductor Materials, Inc. with a purity of99.99%. The sputtering was carried out at RT with an argondeposition pressure of 0.15 Pa. The substrate–target distance(10 cm) and the RF power (175 W) were maintained constant forall depositions. The typical growth rate for these depositionconditions is 30 nm/min. The deposition conditions optimizedfrom the GZO films deposited on glass substrates were later usedto deposit the GZO films on polyethylene naphthalate (PEN)substrates. The PEN substrates possess high stiffness, low thermalshrinkage, and high chemical resistance. The film thickness wasmeasured using a surface profilometer (Dektak 3D from SloanTech.). The surface morphology was analyzed using a field effectscanning electron microscope (FE-SEM, S-1400 Hitachi). Theelectrical resistivity (r), free carrier concentration (N) and Hallmobility (m) were inferred by the four point probe method andHall effect measurements in van der Pauw geometry (BioradHL5500) at a constant magnetic field of 0.5 T. X-ray diffractionmeasurements were performed using Cu-Ka radiation (RigakuDMAX III-C diffractometer) in Bragg–Brentano geometry (y/2ycoupled). The optical transmittance measurements were per-formed with a Shimadzu UV/VIS 3100 PC double beam spectro-photometer in the wavelength range from 300 to 2500 nm. Themeasurements were carried out without an uncoated substrate inthe reference path of the beam, i.e. air was used as reference. Thesolar cell was fabricated on GZO coated glass substrate, with thefollowing structure: GZO/p-a-SiC:H/buffer1/buffer2/i-(a/nc-Si:H)/n-a-Si:H/Ag/Al, using the deposition conditions such as substratetemperature ¼ 473 K, power density ¼ 128 mW/cm2, depositionpressure ¼ 187 Pa and total gas pressure ¼ 105 sccm (95% H2+5%SiH4). The buffer1 and buffer2 layers are used to adjust themismatch in optical band gap between the p-(1.94 eV) andi-(1.82 eV) layers. The difference between the buffer1 and buffer2

is the variation in carbon content that was achieved from thedifference in the flow of CH4 gas. The flow of CH4 was fixed as

4 sccm for the buffer1 layer and that was 1 sccm for the buffer2

layer. The band gap of buffer1 is 1.88 eV and that of buffer2 is1.84 eV. Additionally, the buffer layers are useful in controlling thediffusion between the dopants of p- and i- layers. The buffer layerwas used to improve the p/i interface as described elsewhere [23].The back contact was performed with an electrode area of0.07 cm2, in which a thin Ag layer (E30 nm) was used to improvethe light reflectance, followed by an Al layer (E170 nm) [24]. Toavoid interference of the current collected on the regionsurrounding the contact with the electrical device parameters[15], the silicon structure was removed by reactive ion etching(Alcatel RIE system) using CF4 gas with the following conditions:Process temperature-288 K; Power density-135 mW/cm2; Totalpressure-13 Pa; Flow of (80% CF4+20% O2) gas mixture– 20 sccm[25]. To perform the etching, Al was chosen as last layer due to thechemical instability of Ag films in an oxygen environment.

3. Results and discussion

The dependence of the electrical properties of the GZO films onthe film thickness is shown in Fig. 2. A maximum bulk resistivity(r) of �5.7�10�4O cm obtained for the 100 nm thick films wasdecreased with the increasing film thickness to reach a minimumof �2.8�10�4O-cm at 1100 nm. We have observed a continuousincrease on the mobility (m) as the thickness increases (from 11 to18 cm2/V s). This behavior can be attributed to a reduction in theionized impurity scattering and/or an increase in the crystallitesize. However, we notice that r tends to saturate for thicknessesabove 500 nm. Similar results have been obtained for Al-dopedZnO [26]. The low m obtained for the thinner films can beexplained by the scattering at inter-crystalline boundaries. Asthinner films contain more defects than thicker films, an increasedscattering of carriers takes place resulting in low mobility. The lowm values obtained for the produced films may be related with thefact that the deposition took place at RT. It can be noticed from thefigure that r is almost constant irrespective of the variation in thefilm thickness. As r is essentially dependent on the doping level,this may be suggesting that the incorporation of dopant isanalogous irrespective of the film thickness. Besides that, thetheoretical limit of the solid solubility in ZnO is 1.5�1021 cm�3

[6], which is very close to the average carrier concentration (N)value obtained in this work (1.2�1021 cm�3) for all the filmsproduced.

Fig. 3 shows the X-ray diffraction pattern (normalized to thefilm thickness) obtained from the GZO films as a function of film

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E. Fortunato et al. / Solar Energy Materials & Solar Cells 92 (2008) 1605–1610 1607

thickness. For all the films, only the ZnO (0 0 2) peak at2y ¼ 34.311 is observed revealing that the films are polycrystallinewith a hexagonal structure and a preferred orientation along thec-axis perpendicular to the substrate. As the thickness increases

1100 nm

700 nm340 nm

Inte

nsity

(a.u

.)

2θ (degree)

34.42° (JCPDS: 36-1451)

110 nm

(002)

2θ(degree)

(110)(100) (101)

32 33 34 35 36

30 35 40 45 50 55 60

Fig. 3. XRD patterns obtained from the GZO films deposited with different

thicknesses (inset shows the pattern from the thickest sample).

10

12

14

16

18

20

Mob

ility

, μ (c

m2 /V

s)

Crystallite size, dc (nm)

μ = μ0+ 0.5983dc

16 18 20 22 24 26 28 30

Fig. 4. Dependence of Hall mobility of the GZO films with the crystallite size (data

obtained through Scherrer’s formula).

Fig. 5. Surface SEM micrographs (with 401 tilt angle) of GZO film

the peak intensity corresponding to the plane (0 0 2) increasessignificantly, whereas the peak width decreases. This is related toan improvement in the crystallinity and an increase in thecrystallite size, thus agreeing with the electrical behavior shownin Fig. 2.

The Hall mobility and the crystallite size (inferred throughthe Scherrer’s formula), are depicted in Fig. 4. A linear dependencebetween the mobility and the crystallite size was obtainedthrough the equation, m ¼ k+0.5983dc, where k is a constantand dc is crystallite size. This is consistent with the previousresults since the mobility is mainly dependent on thegrain boundary scattering and lattice defects, which decreasewith the increase of the crystallite size. This also suggests thatthe only way to decrease the resistivity is by increasing themobility, since we are close to the limit of the solid solubility ofGa in ZnO.

Fig. 5 shows two typical SEM micrographs of GZO filmswith a low (110 nm) and high (1110 nm) film thicknessesrespectively, with an apparent viewing angle of 401. The surfaceroughness is increased with the increasing film thickness,suggesting an enhancement of the grain size as already confirmedby the electrical and X-ray diffraction measurements.Thus the SEM analysis corroborates the XRD studies. Fig. 6shows the optical transmittance vs. wavelength in the visibleand near-infrared region obtained from the films with thick-nesses ranging from 110 to 1100 nm. The near-infrared transmit-tance decreases as the film thickness increases, whereas theaverage transmittance at the visible range is obtained as 80%and 90% from the low and high thickness films, respectively.These changes in the optical properties are consistent with thechanges observed in the electrical, structural and morphologicalproperties.

Fig. 7 shows the I–V characteristics and power density as afunction of the voltage obtained from the solar cells processedover glass substrates coated with the GZO thin films reported inthis study. Analysis of these characteristics revealed that the GZOfilms sustain well the plasma process, not suffering any chemicalor physical degradation, leading to devices exhibiting a fill factorof 0.67, an open circuit voltage of 0.95 V and a short circuit currentdensity of 14.96 mA/cm2 and so, to solar cell with efficiencies ofabout 9.52%. The high current density value obtained is mainlyattributed to the quality of i-layer and to the p/i interface thatdoes not annihilate the photocarriers generated, to which itcontributes the use of the GZO as the front contact [27]. This isprobably due to its high chemical stability and the improvedtransmittance towards the blue region of the spectrum. The cross-sectional view of the solar cell is presented in Fig. 8. It is seen fromthe figure that the structure is highly compact, showing thetypical dense columnar growth expected for this type of films.Near the glass/GZO interface small grains are observed. On the

s deposited at room temperature, with different thicknesses.

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0

20

40

60

80

100

1100 nm700 nm

340 nm

Tran

smitt

ance

(%)

Wavelength (nm)

110 nm

500 1000 1500 2000 2500

Fig. 6. Optical transmittance obtained from the GZO films as a function of film

thickness.

0.0 0.2 0.4 0.6 0.8 1.0voltage (v)

Cur

rent

den

sity

(mA

/cm

2 )

18

15

12

9

6

3

FF=0.67vo=0.95 vJSC =14.96 mA/cm2

Efficiency =9.52 %

8

6

4

2

Pow

er d

ensi

ty (m

W/c

m2 )

Fig. 7. I–V characteristics obtained from the amorphous silicon solar cells

deposited on glass coated with GZO films.

Fig. 8. Cross-sectional SEM micrographs of the complete solar cell deposited on glass.

Fig. 9. Dependence of the electrical resistivity (r), carrier concentration (N) and

Hall mobility (m) as a function of film thickness of the GZO films deposited at room

temperature on PEN substrates.

E. Fortunato et al. / Solar Energy Materials & Solar Cells 92 (2008) 1605–16101608

other hand, a highly compact structure near the surface makes itimpossible to distinguish between grain boundaries, defects orvoids.

As mentioned in the introduction, the main intention of thepresent study is to deposit the GZO films on the polymersubstrates. Nevertheless, low-temperature processing on thepolymer substrates is very challenging in comparison to that onthe conventional glass substrates. In order to overcome theselimitations, the sputtering parameters were optimized first usingthe glass substrates and then the films were deposited on thecommercially available PEN substrates. PEN substrates have theadvantages such as high stiffness and mechanical strength, lowthermal shrinkage and high chemical resistance. The resultsobtained from the GZO films on PEN substrates are brieflymentioned below. The deposited GZO films were stable andpresent very good adherence to the polymeric substrates. The Hallparameters obtained from these films are shown schematically inFig. 9 as a function of film thickness. r is decreased with theincreasing film thickness to reach a minimum value of �7�10�4

O cm at around 300 nm but then remained almost constant. Onthe other hand, N is increased from 3.5�1020 cm�3 with theincreasing thickness to reach a maximum of �8�1020 cm�3 at300 nm but then decreased slightly. m is increased with theincreasing thickness from �3.2 cm2/V s (�70 nm) to reach amaximum value of �12.1 cm2/V s (�840 nm). Overall, the varia-tion of the electrical parameters follows the similar trendirrespective of whether they are deposited on glass substrates(Fig. 2) or PEN substrates (Fig. 9). However, the GZO films with anaverage thickness of 350 nm show a small variation in theobtained values from the glass and PEN substrates.

The XRD patterns recorded on these samples confirmed thehexagonal crystal structure with a preferred orientation alongthe c-axis (0 0 2) at 2y ¼ 34.11 perpendicular to the substrate. Theobtained 2y value suggests a small shift (difference of �0.021) inthe (0 0 2) orientation to the lower angle size in comparison withthose deposited on glass substrates (Fig. 3). Except for this shift inorientation, no significant difference is observed between the XRDpatterns. The average visible transmittance in the wavelengthranging 400–700 nm is around 80%, which confirmed that there isno significant variation in the transmittance data in comparisonwith those deposited on the conventional glass substrates. It maybe noteworthy that the GZO films were sputtered in a discretemanner on PEN substrate in order to avoid the excess heating ofthe PEN substrate. Continuous sputtering leads to crack, asobserved from SEM analysis, that probably originate from thelarger thermal stress due to the difference in the thermalexpansion coefficients of the PEN substrate and the GZO film.One of such observed cracks is shown in Fig. 10, wherein the cross-sectional view of the deposited film can be seen. It is perceptible

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Fig. 10. Crack observed on the GZO films (�330 nm thick) due the difference in thermal expansion coefficients of GZO films and the PEN substrate: (a) low magnification

(6 K) and (b) high magnification (60 K).

Table 1Electrical properties exhibited by the GZO thin films (with an average thickness of

�350 nm) deposited on different substrates

Sample Sheet

resistance

Rsh (O/&)

Bulk

resistivity,

r (O cm)

Hall

mobility, m(cm2/V s)

Carrier

concentration,

N (cm�3)

GZO/PEN 26 9.0�10�4 8.6 8.0�1019

GZO/SiO2/PEN 15 5.3�10�4 13.7 8.6�1020

GZO/glass 5 1.7�10�4 14.7 2.4�1021

E. Fortunato et al. / Solar Energy Materials & Solar Cells 92 (2008) 1605–1610 1609

that the deposited layer is highly compact and shows a densecolumnar structure.

The foregoing discussions summarize that the sputteringconditions of the GZO films were optimized on glass substratesand then the films were deposited on the PEN substrate with theoptimized sputtering conditions. A comparison of the obtainedresults showed that the electrical properties of the GZO films areslightly deteriorated when deposited on PEN substrates, which ispresumably instigated from the GZO films/PEN substrates inter-face. In order to overcome this barrier, a thin SiO2 layer (�100 nm)was deposited on the PEN substrate prior to the GZO deposition.The aim is to prevent the degradation of the PEN surface duringthe deposition process in addition to the minimization of theresidual thermal stresses. This barrier can also prevent anydiffusion or chemical reactions at the PEN substrate and GZOfilm interface. Table 1 summarizes the electrical propertiesobtained from the GZO films deposited on PEN substrates withand without the SiO2 barrier layer and that deposited on glasssubstrates. It may be noticed from the table that the electricalproperties of GZO films deposited on PEN substrate are inagreement with that deposited on glass substrates when a SiO2

layer is introduced between the GZO film and the PEN substrate.Further work on producing solar cells on PEN substrates is underprogress.

4. Conclusions

The set of data achieved shows that highly conducting andtransparent GZO films can be deposited by RF magnetronsputtering at room temperature. The data show that the crystal-line structure, surface morphology and the electro-optical proper-ties are dependent on the film thickness. Overall, the producedfilms present a resistivity close to �2.8�10�4O cm, Hall mobilityE18 cm2/V s and transmittance 480%. Further work is under wayto increase the Hall mobility without heating the substrate, eitherduring deposition or after deposition, to be compatible with the

emergent plastic electronic industry. The produced solar cellsshow good electrical performances such as current density(14.9 mA/cm2), short circuit voltage (0.95 V) and fill factor (0.67),leading to devices with solar cell efficiencies of about 9.52% underAM1.5 conditions. With the optimized conditions, GZO films weredeposited on PEN substrates and the obtained results arecomparable with those obtained from the glass substrates. Thedevelopment of solar cell based on GZO deposited on PENsubstrates is under progress.

Acknowledgements

This work is financed by FCT-MCTES through CENIMAT-I3Nand in part by the European Commission under Contract NMP3-CT-2006-032231.

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