TCO/Ag/TCO transparent electrodes for solar cells application

S. Boscarino • I. Crupi • S. Mirabella •

F. Simone • A. Terrasi

Received: 9 September 2013 / Accepted: 3 January 2014

� Springer-Verlag Berlin Heidelberg 2014

Abstract Among transparent electrodes, transparent

conductive oxides (TCO)/metal/TCO structures can achieve

optical and electrical performances comparable to, or better

than, single TCO layers and very thin metallic films. In this

work, we report on thin multilayers based on aluminum zinc

oxide (AZO), indium tin oxide (ITO) and Ag deposited by

RF magnetron sputtering on soda lime glass at room tem-

perature. The TCO/Ag/TCO structures with thicknesses of

about 50/10/50 nm were deposited with all combinations of

AZO and ITO as top and bottom layers. While the electrical

conductivity is dominated by the Ag intralayer irrespective

of the TCO nature, the optical transmissions show a

dependence on the nature of the top and bottom TCOs,

mainly due to the change in the reflectivity of the multi-

layers. Structural, electrical and optical properties are

studied to optimize the structure for very thin transparent

electrodes suitable for photovoltaic applications.

1 Introduction

Transparent electrodes are essential components in many

optoelectronic technologies such as touch screens, LCDs,

OLED and solar cells [1–8]. Doped and undoped metal

oxides such as indium tin oxide (ITO), aluminum zinc

oxide (AZO) or indium zinc oxide (IZO) are mostly used

for this aim. These compounds along with many others are

known as transparent conductive oxides (TCO) and they

are deposited with typical thicknesses of the order of 1 lm

to reach a good compromise between optical transparency

and electrical conductivity. Lower thicknesses, in fact,

would increase the sheet resistance [8], with detrimental

consequences for the device performances. To meet the

expectations for growing demand and lower production

costs for photovoltaic and electronic applications, there is a

need to develop alternative low-cost, but high-quality,

transparent electrodes with properties comparable to the

conventional TCOs [8–12]. Although new transparent

electrodes such as polymer films, metal nanowires network,

single or few layers of grapheme and carbon nanotube have

already matched the electro-optical properties of today’s

best ITO films, they are still too far from real applications

on large scale [12].

Over the past decades, transparent electrode based on a

sequence of very thin TCO/metal/TCO films has been

rediscovered as a good alternative to standard thick TCO.

Multilayers with different kinds of TCO and metals have

been reported in the literature [8–15], mostly with a single

type of TCO encapsulating a very thin metallic film. The

TCO/metal/TCO multilayer, with a low thickness

(*100 nm) compared to single TCO layer, typically shows

low-sheet resistance, high-optical transparency in the vis-

ible range, and room temperature deposition processes. The

latter point, in particular, avoids the problem related to

TCOs, for which a heat treatment above 250 �C during or

after deposition is necessary to achieve high optical and

electrical performance [9]. Lowering the thermal budget

has the double advantage of reducing production costs and

being compatible with plastic substrates, which is neces-

sary for the development of flexible devices.

Although to date TCO/metal/TCO multilayer has been

deeply investigated [3, 4, 8, 14], there is still a lack of

S. Boscarino (&) � F. Simone � A. Terrasi

Dipartimento di Fisica e Astronomia, Universita di Catania, via

S. Sofia 64, 95123 Catania, Italy

e-mail: stefano.boscarino@ct.infn.it

S. Boscarino � I. Crupi � S. Mirabella � A. Terrasi

MATIS, CNR-IMM, via S. Sofia 64, 95123 Catania, Italy

123

Appl. Phys. A

DOI 10.1007/s00339-014-8222-9

knowledge to transfer these materials in a real fabrication

process for thin film solar cells. In our work, we have

studied and compared the structural, electrical and optical

properties of several sequences of TCO/Ag/TCO multi-

layers, in particular ITO/Ag/ITO, AZO/Ag/AZO, ITO/Ag/

AZO and AZO/Ag/ITO grown on glass substrates by RF

magnetron sputtering at room temperature, to investigate

the role, if any, of the sequence of the top and bottom TCO

layers. The thickness of the top and bottom TCO films was

fixed at about 50 nm, while the Ag intralayer film was

10-nm thick. We show that the presence of the Ag accounts

for a low sheet resistance in spite of the very thin TCO

layers. On the other hand, the reduced overall thickness of

the multilayered structure maintains a high optical trans-

mittance through the electrode. Finally, by changing the

type of top and bottom TCO, it is possible to vary the

reflectivity of the whole structure, with possible important

applications for solar cells.

2 Experimental

AZO/Ag/AZO, ITO/Ag/ITO, AZO/Ag/ITO and ITO/Ag/

AZO multilayers were deposited on soda lime glass sub-

strates by RF magnetron sputtering at room temperature. A

bottom/Ag/top notation is used, so that AZO/Ag/ITO

indicates that AZO is the bottom layer while ITO is the top

one; similarly for ITO/Ag/AZO, where AZO is the top

layer. Glass substrates were cleaned in acetone, rinsed in

deionized water and in absolute ethanol, dried with flowing

nitrogen gas, then kept in the sputtering chamber 70 mm

away from the targets. The sputtering system allows to

deposit the different materials in sequence without break-

ing the vacuum.

TCO and Ag sources were targets of ITO (10 wt%

SnO2-doped In2O3), AZO (2 wt% Al2O3-doped ZnO) and

Ag (99.999 % purity), respectively. Plasma was obtained

in a pure Ar gas atmosphere, with a gas flow of 6 sccm, at a

process pressure of 1 Pa. The sputtering powers and cor-

responding deposition rates were 225 W and 0.71 nm/s,

175 W and 1 nm/s, 30 W and 0.18 nm/s for AZO, ITO and

Ag, respectively. For comparison, samples of 100 nm of

AZO and ITO were also prepared.

Rutherford backscattering spectrometry (RBS) mea-

surements were employed to quantify the doses of Ag, Zn,

In in the TCO/Ag/TCO structures. A 2.0 MeV He? ion

beam hits the sample surface at normal incidence, while

backscattered He? atoms are detected at an angle of 15�out of the normal to the surface. Once Ag, Zn, In doses are

known, the layer thickness is derived by assuming a proper

atomic density (5.84 9 1022 Ag/cm3, 4.22 9 1022 Zn/cm3

for AZO, 2.7 9 1022 In/cm3 for ITO). Structural charac-

terization of the samples was also performed using a field

emission scanning electron microscope (FE-SEM, Zeiss

supra 25).

The sheet resistance of the samples was measured by a 4

points probe in the Van der Pauw configuration at room

temperature.

A Varian Cary 500 double-beam scanning UV/Vis-NIR

spectrophotometer was used to measure the optical direct

transmittance (T) and specular reflectance (R) in the

wavelength range of 300–1,200 nm. Direct transmittance

spectra were normalized to a 100 % baseline obtained by

mounting the empty sample holder, while the reflectance

spectra have been detected in specular geometry using a

calibrated standard sample as reference.

3 Results and discussion

Figure 1 shows the RBS spectra of our TCO/Ag/TCO

samples. The peak at about 1.70 MeV is related to He?

backscattered by the Ag layers, the two peaks at 1.57 and

1.51 MeV are related to He? backscattered by Zn in the

AZO top and bottom layers, respectively, while the peak at

1.73 MeV is related to He? backscattered by In in the ITO

top layer. Due to their close atomic numbers, it was not

possible to distinguish between In and Sn signals, as well

as between In from bottom ITO layer and Ag signals. RBS

measurements and spectra simulation allowed to determine

the thickness of the films in the multilayer. The top and

bottom TCO layers were 50 ± 5 nm, while the embedded

Ag layer was estimated to be 10 ± 1 nm.

Figure 2 shows the cross-section SEM image of the

AZO/Ag/AZO sample, where the typical polycrystalline

columnar structure of the AZO can be clearly observed.

The thicknesses calculated by the RBS data were con-

firmed by the SEM analyses. Figure 2 shows a continuous

Ag layer with a homogeneous thickness of *10 nm as

Fig. 1 Rutherford backscattering spectra of TCO/Ag/TCO multilay-

ers grown on glass substrate

S. Boscarino et al.

123

given by the FWHM of the peak obtained using the image

analysis software (inset of the figure). The sharp boundary

among the layers also reveals no diffusion of Ag into TCO

films during the deposition, which is in agreement with our

previous work [13] showing that the AZO acts as an

excellent barrier against the diffusion of Ag, even at tem-

peratures as high as 400 �C.

Table 1 reports the thickness, resistivity, average value

of the optical transmittance Tvis in the 400–800 nm range,

and that measured at 550 nm wavelength, T550, for all

samples. These data will be now compared and discussed.

As far as the electrical characterization is concerned,

AZO and ITO single layers of thicknesses in the range of

50–100 nm exhibit a resistivity of about 10-2 Xcm or

higher, depending on the nature of TCO (see Table 1) and

deposition process parameters. Moreover, the conduction

through the thin TCO or metal layer depends on the Ag

morphology. In a previous study [13], we found, in

agreement with the literature, that a thickness of about

10 nm for the Ag layer is the best compromise in between

optical transparency and electrical conductivity. For lower

thickness, in fact, it has been reported a discontinuous,

clustered Ag layer [1, 3, 15, 16]. Therefore, the insertion of

10 nm of Ag between the top and bottom TCO layers leads,

for all samples, to a sheet resistance, suitable for solar cell

application, of about 10 X/sq.

In the case of TCO/Ag/TCO, the sheet resistance of the

whole structure, RT, can be expressed considering the

single layers as resistors connected in parallel:

1

RT

¼ 1

RTCObottom

þ 1

RAg

þ 1

RTCOtop

ffi 1

RAg

ð1Þ

The sheet resistance value of the stack, taking into

account Eq. (1) and TCO resistivity, confirms that the Ag

film controls the whole conduction of these three-layer

electrodes, with a resistivity of about 10-5 Xcm, lower

than a conventional and thicker ITO single layer,

1–2 9 10-4 Xcm [12]. Our data indicate a resistivity of the

Ag layer seven times higher than the bulk value

(1.51 9 10-6 Xcm), which can be explained by the very

low thickness of our Ag films of the same order of mag-

nitude of the mean free path of conduction electrons in

good metals (10-8 m).

Despite the very similar electrical properties, the optical

behavior of our multilayer electrodes depends on the TCO

nature and, in particular, on the sequence of the TCO films.

The low absorption of the silver thin film in the visible-

NIR range [15] is essential to increase the light transmis-

sion of the multilayer structure. To maximize the trans-

mittance in the visible range, TCOs’ thickness was set at

50 nm (TCO thicknesses in the range of 30–60 nm are

commonly used [9]). Figure 3a, b reports the optical

transmittance and reflectance of our samples along with

AZO and ITO single layers for comparison. The trans-

mittance of the glass substrate is also reported. All the

optical data include the substrate contribution. The absor-

bance, calculated as A = 100 % - (T ? R), is displayed

in Fig. 3c.

As far as the transmittance is concerned, the wavelength

region around 650 nm shows very similar T values for all

Fig. 2 Cross sectional SEM

image of the AZO/Ag/AZO

multilayer. The inset is the

output of the software used to

estimate the thickness of the Ag

intralayer. The FWHM of the

peak is 10.5 nm

Table 1 Thickness, resistivity, visible average transmittance and

transmittance at 550 nm wavelength of all samples

Sample Thickness

(nm)

Resistivity

(Xcm)

Tvis (%) T550 (%)

AZO 100 3 9 10-1 79 79

ITO 100 3 9 10-2 79 83

AZO/Ag/AZO 50/10/50 *1 9 10-5 75 79

ITO/Ag/ITO 68 74

AZO/Ag/ITO 76 78

ITO/Ag/AZO 80 86

TCO/Ag/TCO transparent electrodes for solar cells application

123

samples. The symmetric ITO/Ag/ITO structure exhibits the

worst performance all over the wavelength range, followed

by the AZO/Ag/AZO structure. Concerning the asymmet-

ric structures, we observe a higher T with respect to the

symmetric ones, at least in one of the regions below or

above the 650 nm reference point. The best transmittance

below 650 nm is observed for the ITO/Ag/AZO sequence

which, above this wavelength value, reproduces the same

transmittance of AZO/Ag/AZO. On the other hand, the best

T above 650 nm is obtained by the AZO/Ag/ITO structure

which, below this wavelength value, has the same T of the

AZO/Ag/AZO sample. When compared to the single TCO

(AZO or ITO) films, the multilayer structure exhibits a

more structured behavior of the T curves. Single TCO

samples have a higher T in between 350 and 450 nm, and

above 900 nm wavelengths.

Reflectance curves are reported in Fig. 3b, showing that

most of the variations observed in the T spectra are related

to the R behavior rather than to a different light absorption.

All multilayer structures have a clear minimum around

650 nm wavelength, in some cases extended to a wide

region of the visible range (450–750 nm). On the contrary,

TCO single layer has an almost constant R value of about

20 %. However, if we look at the absorbance of Fig. 3c, all

multilayer samples show an almost constant trend above

450 nm, with higher values with respect to single TCOs.

An overlook of the transmittance, reflectance and absor-

bance spectra of Fig. 3 allows to highlight some important

points. It is clear that the TCO single layers have a much

lower absorbance with respect to the structure containing

the Ag film in the whole spectral range, but the very low

reflectance of the TCO/Ag/TCO samples in the 450–

750 nm wavelength window produces similar, or even

higher, transmittance values. Moreover, the presence of

ITO or AZO as top and bottom TCO in the asymmetric

multilayer samples has strong consequences on the trans-

mittance of the whole structure, allowing a selective tai-

loring of this parameter in different wavelength ranges (in

our case, below and above 650 nm). The observed behavior

is clearly related to the variation of the reflectivity

(Fig. 3b). Depending on the sequence of the TCO top and

bottom layers in our samples, we changed the R values in

the different wavelength regions and, as a consequence, the

T values. In a multilayer thin film structure, this effect can

be ascribed to small differences in the thickness and/or in

the refractive indices ‘‘n’’ of each single layer, In our case,

although the thickness of TCO and Ag films calculated by

RBS is affected by an error of about 5 and 1 nm, respec-

tively, the systematic difference between symmetric and

asymmetric structures suggests that the refractive indices

of ITO and AZO thin films [16, 17] play a major role. In

particular, the ‘‘n’’ curves of ITO and AZO have an

intersection at around 570 nm [18], almost in correspon-

dence with the observed crossing of reflectance (and

transmittance) in our samples. Moreover, further data in the

literature show a similar behavior for ITO/ZnO combina-

tion [19]. This peculiarity of the multilayered samples

could be employed to decrease or increase the reflectivity

of the front and back transparent electrodes, respectively,

in a proper range of the solar spectrum, with possible

advantages in the case of semi-transparent photovoltaic

solar cells. To compare the electrical and optical perfor-

mances for different transparent electrodes, we also con-

sidered the Haacke’s figure of merit [20] defined as:

FTC ¼T10

RT

ð2Þ

Using the value of T at 550 nm, we found FTC values in

between 4.9 9 10-3 X-1 (for ITO/Ag/ITO) and

22 9 10-3 X-1 (for ITO/Ag/AZO), depending on the

sequence of the TCOs. These values, except for the lowest

Fig. 3 Optical transmittance (a), reflectance (b) and absorbance

(c) spectra of TCO single layers and TCO/Ag/TCO multilayers,

expressed in percentage

S. Boscarino et al.

123

one, are comparable to those reported in the literature [8, 9,

12]. From the previous results, the asymmetric multilayer

structures seem to have the best performances as trans-

parent electrodes and, moreover, they offer the possibility

of tuning the optical transmittance in selected wavelength

ranges.

4 Conclusion

In summary, we have studied structural, optical and elec-

trical behaviors of thin transparent electrodes deposited by

RF magnetron sputtering sequential deposition of TCO/Ag/

TCO. These structures have been proposed as a valid

alternative to thicker TCO single layers currently used in

several industrial applications. In our case, we report on

several samples, with thickness of about 100 nm, made of a

single AZO or ITO film, or TCO/Ag/TCO with all possible

combinations of AZO and ITO as external TCO layers. Our

data demonstrate that a single TCO layer of 100 nm has

good optical properties but a very high electrical resistivity

not suitable for practical applications. On the other hand,

the TCO/Ag/TCO samples, with the same thickness, have

shown good optical properties coupled with an excellent

electrical conductivity. In particular, the multilayered

structures have shown better performances when the top

and bottom TCOs are of different types and the possibility

of tuning the reflectivity in a proper range of wavelengths,

which is very attractive for applications in devices requir-

ing transparent electrodes such as, for example, front and

back contacts in semitransparent solar cells.

Moreover, our samples have been deposited at room

temperature, while standard thick TCO layers often require

a thermal process to improve final electro-optical perfor-

mances. The absence of a thermal budget and the very low

thickness of our samples are very promising also for

applications on flexible devices fabricated on plastic

substrates.

Acknowledgments The authors are grateful to Carmelo Percolla for

technical assistance. One of the author, S. Boscarino, acknowledges

the financial support by the MIUR project PON01_01725.

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