ZnO characteristics as transparent electrodes for third generation solar cells

Solar Energy Materials & Solar Cells 100 (2012) 153–161

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Comparison of ITO/metal/ITO and ZnO/metal/ZnO characteristics as transparent electrodes for third generation solar cells Mihaela Girtan n Photonics Laboratory, Angers University, 2, Bd. Lavoisier, 49045 Angers, France

a r t i c l e i n f o

abstract

Article history: Received 4 December 2011 Received in revised form 3 January 2012 Accepted 6 January 2012 Available online 2 February 2012

In this paper we present the physical properties of two types of multilayer structures: ITO/metal/ITO and ZnO/metal/ZnO obtained by successive sputtering depositions of metallic targets (In:Sn, Zn, Ag, Au) in reactive atmosphere (for oxide films) and under inert atmosphere (for metallic interlayer films). Very good quality transparent conducting thin films structures (r ¼ 2  10  5 O cm, T  90%) were obtained. The morphological, optical and electrical properties were analyzed and compared for the multilayer films deposited in identical conditions on glass and PET substrates. The influence of substrate nature on the morphological properties is more pronounced in the case of zinc oxide films. The Haake figures of merit at l ¼550 nm are comprised between 4  10  3 O  1 and 29  10–3 O  1 in function of the nature of the metallic interlayer. The stability of electrical properties with the temperature of the oxide/metal/ oxide films is remarkable in comparison with the usual behavior of single oxide films. & 2012 Elsevier B.V. All rights reserved.

Keywords: Solar cells Transparent conducting electrodes

1. Introduction Transparent conducting thin films are widely used in many thin films devices [1–4]. Since now single thin films of In2O3:Sn, ZnO:Al or SnO2:F deposited on glass substrate were typically used as transparent conducting electrodes [5–7]. The progress, last years, in the fabrication of organic solar cells [8–11] and organic light emitting diodes [12–15], constantly increase the interest for transparent conducting films on flexible substrates as PET or PES [16,17]. One of the first inconvenience which rise at the replacement of glass substrates by plastic substrates, is the one that some of the usually transparent electrode preparation methods such as sol–gel [18], spray pyrolysis [19] or chemical vapor deposition [20,21] involving high deposition temperatures (400 1C–500 1C) cannot be employed anymore. Thin films deposition on plastic substrates supposes deposition at low temperatures and, in this case, more suitable deposition methods are vacuum sublimation [22] sputtering [5,6,23] or pulsed laser deposition [24–26]. The choice of oxide material is also important. At present, the most promising material as transparent electrode seems to be the Indium Tin Oxide (ITO). However the limited resources of Indium on earth and the expensive cost of this material impress the reduction of the quantities employed for films fabrication or even the replacement of ITO by other materials having equivalent properties. The thickness of ITO thin

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films used as transparent electrodes for optoelectronic devices ranges in general, between 150 nm and 700 nm. The reduction of the thickness under 150 nm is not possible in the case of single oxide films due to the increase of the electrical resistivity with the thickness decrease (the classical size effect). Another inconvenient that should be also considered in the case of using plastic substrates, resides in the usual bent mechanical fragility of oxide films. With all these requirements in mind, the realization of transparent electrodes using oxide/metal/oxide multilayer structures has many advantages. First, in the case of ITO, one of the advantage is the reduction of the indium quantity by the reduction of thin films thickness from 150 nm to a total of 50–60 nm for both oxide layers. Second, the mechanical properties are considerably improved due to the ductile metallic interlayer. In the same time the optical and electrical qualities of electrodes are perfectly conserved and even improved if we talk about the electrical conductivity. In this paper we present the physical properties of two types of multilayer structures: ITO/metal/ITO and ZnO/metal/ZnO deposited by reactive sputtering on both glass and PET substrates. The metallic interlayer was: Ag, Au, or Ag/Au.

2. Experimental The oxide/metal/oxide multilayer structures were deposited in identical conditions on glass and PET substrates by successively sputtering depositions. Oxides thin films were deposited in

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reactive atmosphere using In:Sn (90%:10%) and Zn targets from Kurt Lesker. Silver and gold thin films were deposited in argon atmosphere. The glass and PET substrates were placed in a vertical target-substrate configuration onto a rotating disk kept at room temperature. The target-substrate distance was of about 70 mm. The deposition rates were of: 15 nm/min for ITO films (p¼9  10  3 mb, I¼ 40 mA), 40 nm/min for ZnO films (p¼9  10  3 mb, I ¼100 mA), 28 nm/min for gold films (p¼9  10  3 mb, I¼30 mA) and 42 nm/min for silver films (p ¼9  10–3 mb, I¼20 mA).

Films thickness measurements were done by profilometry using a Dektak profilometer and as well by ellipsometry. The total thickness of oxide/metal/oxide multilayer structures ranged between 60 and 90 nm. Samples’ structures were investigated by X-ray diffraction (XRD) ˚ using an D8 Advance Brucker diffractometer CuKa 1,2 (l ¼1.5406 A), equipped with a linear Vantec super speed detector. The thin films morphology was analyzed by contact mode Atomic Force Microscopy (AFM) using a Thermomicroscope Autoprobe LP Research and by scanning electron microscopy (SEM) using a JEOL Microscope.

Fig. 1. XRD patterns for the ITO/metal/ITO multilayer structure deposited on glass substrates.

Fig. 2. XRD patterns for the ZnO/metal/ZnO multilayer structure deposited on glass substrates.

M. Girtan / Solar Energy Materials & Solar Cells 100 (2012) 153–161

The transmittance and reflectance spectra were recorded at room temperature in the 300–1100 nm wavelength range, using a Hitachi UV-4001 spectrophotometer. The measurements of refractive and extinction index were done between 170 nm and 2100 nm using a UVISEL NIR Horiba Jobin Yvon ellipsometer. The electrical measurements were performed in planar geometry during two or three cycles of heatings and coolings between 300 K and 400 K by the four point method.

3. Results and discussions 3.1. Structural and morphological properties

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diffraction angle scans between 151 and 751 on multilayer structures deposited on glass. Fig. 1 shows the results obtained for the ITO/metal/ITO multilayer structure. The absence of ITO peaks indicate an amorphous structure of these oxides films. The small thickness of individual thin films explains in general this amorphous structure, and is frequently remarked that the crystalline phase is improved by the increase of indium tin oxide films thickness [5,6]. Small peaks appear for gold and silver indicating a beginning of crystallization phase of metallic materials, despite their weak thickness. In the case of ZnO/metal/ZnO multilayer structures (Fig. 2) we distinguish the (002) peak for ZnO indicating that in respective films the microcrystallites grow preponderantly with the (002) plane parallel to the substrate surface.

The results of the X-ray diffraction (XRD) analysis are shown in Figs. 1 and 2. The measurements have been done for a 2y

Fig. 3. SEM images for successive deposited layers : ITO/metal/ITO on glass and PET substrates.

Fig. 4. SEM images for successive deposited layers : ZnO/metal/ZnO on glass and PET substrates.

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The morphology of successive layers deposited on glass and on PET substrates was analyzed by scanning electron microscopy. Fig. 3 shows the SEM images for successive deposited layers (ITO/

Table 1 Summary of the root mean square (RMS) and average (RA) roughness values of the oxide top layer of the multilayer structures oxide/metal/oxide deposited on glass and PET substrates. Multilayer structure

Roughness of last (top) layer (nm) Thickness (nm)

RMS values

Substrate 20/7/20 20/8/20 20/7/7/20 25/8/25 25/8/25 25/8/25

Glass

PET

Glass

PET

2.8 1.4 1.2 3.5 1.4 1.9

3.0 1.9 2.4 10.8 5.7 7.8

2.0 0.9 0.9 1.9 0.9 1.4

1.9 1.5 1.8 8.5 4.6 6.2

2

Y[µm]

1.5 1 0.5 0 0

0.5

1 1.5 X[µm]

2

Top indium tin oxide layer 2

Y[µm]

1.5 1 0.5 0 0

0.5

1 1.5 X[µm]

2

Metallic layer 2

Y[µm]

1.5 1 0.5 ITO thin film (20 nm) 0 0

0.5

1 1.5 X[µm]

2

Bottom indium tin oxide layer

Metallic thin film (7nm) ITO thin film (20 nm)

2

PET substrate (175 µm)

1.5 Y[µm]

ITO/Ag/ITO ITO/Au/ITO ITO/Ag/Au/ITO ZnO/Ag/ZnO ZnO/Au/ZnO ZnO/Ag/Au/ZnO

RA values

metal/ITO) on glass and PET substrates. From these images it appears that there are not obvious differences between the indium oxides films deposited on glass and the indium oxides films deposited on PET substrates. On the contrary, the influence of the substrate nature on the crystallite growth is very pronounced in the case of ZnO/metal/ZnO films (see Fig. 4). The PET substrates induce the formation of crystalline grains with larger sizes than in case of same films deposited on glass substrates. The morphology of the first layer it is transmitted to the very thin metallic layer and further to the top zinc oxide layer. The AFM studies confirm these observations and put in evidence that the roughness of top zinc oxides layers deposited on PET is much important as compared to glass deposited films. This observation is also true for indium tin oxide films. However, for indium tin oxide films deposited on glass and PET substrates, the differences in roughness are less important than in the case of zinc oxide films onto the same substrates.

1 0.5 0 0

0.5

1 1.5 X[µm]

2

PET substrate Fig. 5. AFM images obtained for the successive layers of the ITO/metal/ITO films deposited on PET substrates.

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Table 1 resumes the values of the root mean square (RMS) roughness and average roughness (RA) for the top oxide layer of the multilayer structures deposited on both glass and PET substrates. As for examples, Fig. 5 presents the AFM images obtained for the successive layers of the ITO/metal/ITO films deposited on PET substrate and Fig. 6 present, the in comparison, the AFM images of top zinc oxide film for a multilayer structure deposited on glass and PET. The nature of the metallic layer (Ag, Au or Ag/Au) does not influence the morphology of the next oxide layer. That could be explained by the fact that the metallic films thickness is too small to cover the relief of the first layer and consequently, only the bottom oxide layer is responsible for the morphology of the top layer.

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to a decrease of the plasma wavelength value and, hence, to a decrease of the optical window width. The electrical conductivity of the multilayer structures was measured by four point method and the optoelectronic quality of films was evaluated by the figures of merit defined by Fraser and Cook, and by Haake, respectively. The obtained values are given in Table 2. Fraser and Cook figure of merit is defined as [27] F TC ¼

T Rsh

ð1Þ

where Rsh is the sheet resistance and T is the transmission coefficient. The sheet resistance is defined as Rsh ¼ 1=sd

3.2. Optical and electrical properties The transmission and reflection spectra were recorded between 300 nm and 1100 nm. Fig. 7 depicts these spectra for the ITO/metal/ITO structures deposited on glass (Fig. 7a) and PET (Fig. 7b) substrates. Fig. 8 shows the same spectra for ZnO/metal/ ZnO multilayer structures. As one can see, the optical window diminish considerably in the case of structures having two metallic interlayer (Ag/Au). The width of the optical window is controlled by the fundamental absorption wavelength (lg) and the plasma wavelength (lp). The decrease of the optical window width in our case could be easily understood on the basis of the Drude theory which correlate the optical properties with the concentration of free electrons and their mobility. Thus, higher the carrier concentration the lower will be the plasma wavelength. Thereby an increase of metallic layer thickness conduct to an increase of the free charge concentration and by consequence

ð2Þ

where d is the film thickness and s is the electrical conductivity. The figure of merit defined by Haake weights less in favor of sheet resistance and is one of the most employed to evaluate the qualities of transparent conducting films. Haake figure of merit is given by [28]

FTC ¼

T 10 ¼ sd expð10adÞ Rsh

ð3Þ

where a is the absorption coefficient. Table 3 resumes the values of figure of merit for similar structures prepared by different methods. We remark a good agreement with the values obtained by other authors. Most of the studies were done on films deposited on glass substrates and there are only few studies for films deposited on plastic substrates. We can underline the fact that the method employed in this work (reactive sputtering) has the advantage to use in general less energy as compared to sputtering methods using oxide targets.

2

2

1.5

1.5 Y[µm]

Y[µm]

Top zinc oxide layer

1

1

0.5

0.5

0

0 0

0.5

Glass substrate

1 X[µm]

1.5

2

0

0.5

1 X[µm]

1.5

2

Plastic substrate

Fig. 6. AFM images for the top zinc oxide film of a multilayer structure ZnO/metal/ZnO deposited on glass and PET.

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Fig. 7. Transmission and reflection spectra for the ITO/metal/ITO structures deposited on glass (a) and PET (b) substrates, respectively.

The dispersion curves for the refractive index n(l) and extinction k(l) coefficients were recorded by ellipsometry measurements between 170 and 2100 nm on the multilayers structures deposited on glass. As one could remark the refractive index (Fig. 9) is higher in the case of multilayers based on ZnO and in general in the case of multilayers structures having a silver metallic interlayer. The extinction index (Fig. 10) is higher for multilayer structures based on ITO. The plasma resonance wavelengths for the multilayer structures were determined from the intersection of curves n(l) and k(l) and the obtained values are given in Table 4. These results are in good correlation with those obtained from transmission and reflection plots analysis. In order to investigate the stability of the electrical properties of these transparent conducting films the electrical conductivity was measured in open atmosphere during more than two cycles of heatings and coolings. Fig. 11 shows the variation of resistivity in function of temperature, during the first heating, for some of as above mentioned multilayer structures. In these experiments the temperature was varied between room temperature and 200 1C. The upper limit of heating temperature is determined by the plastic nature of the substrates (250 1C for our HIFI PMX739 PET substrates). Beyond this temperatures plastic substrates deformations are irreversible. As could be seen from Fig. 11, the values of the electrical resistivity are perfectly stable between room temperature and 200 1C. These dependencies remains perfectly reversible even, after more than three or four cycles of heatings and

Fig. 8. Transmission and reflection spectra for the ZnO/metal/ZnO structures deposited on glass (a) and PET (b) substrates, respectively.

Table 2 Fraser and Cooke and Haake figures of merit respectively calculated at 550 nm transmittance for the multilayer structures oxide/metal/oxide deposited on glass and PET substrates. Multilayer structure Thickness (nm) substrate ITO/Ag/ITO ITO/Au/ITO ITO/Ag/Au/ITO ZnO/Ag/ZnO ZnO/Au/ZnO ZnO/Ag/Au/ZnO

20/7/20 20/8/20 20/7/7/20 25/8/25 25/8/25 25/8/25

Figure of merit (10  3 O  1) l ¼550 nm Fraser Cook

Haake

Glass

PET

Glass

PET

59 73 125 77 136 174

39 53 127 46 49 83

21 17 24 7 58 26

29 16 28 4 8 15

coolings (not shown here). This is an excellent result comparing to single oxide films when these films are used as transparent electrodes for solar cells. As an example, we present in Fig. 12 the variation of electrical resistivity with the temperature for two commercial ITO thin films deposited on glass and we should precise that the same behavior is generally remarked for other single transparent conducting oxides thin films. Even in the case of high doping level (degenerate semiconductor) the oxide films are semiconductor materials and their resistivity vary with the temperature due to thermal activation, oxygen diffusion or grain size modifications [21,38,39]. Taking into account that the solar

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Table 3 Comparison of Haake figures of merit for different oxide/metal/oxide structures (without annealing) obtained by different methods on glass and plastic substrates. Multilayer structure

Deposition method

Target for oxide films deposition

Figure of merit (10  3 O  1)

Ref.

l ¼ 550 nm Glass substrate ITO/Ag/ITO IZO/Ag/IZO IZO/Au/IZO ITO/Ag/ITO ITO/Cu/ITO AZO/Ag/AZO AZO/Ag/AZO AZO/Au/AZO AZO/Ag/AZO GZO/Ag/GZO ITO/Ag/ITO ITO/Au/ITO ITO/Ag/Au/ITO ZnO/Ag/ZnO ZnO/Au/ZnO ZnO/Ag/Au/ZnO

Sputtering Sputtering Sputtering Sputtering Sputtering e-Beam Sputtering Sputtering Sputtering Sputtering Reactive sputtering Reactive sputtering Reactive sputtering Reactive sputtering Reactive sputtering Reactive sputtering

Oxide (10% SnO2, 90% In2O3) Oxide (10% ZnO, 90% In2O3) Oxide (10% ZnO, 90% In2O3) Oxide (10% SnO2, 90% In2O3) Oxide (10% SnO2, 90% In2O3) Oxide (2% Al2O3, 98% ZnO) Oxide (1% Al2O3, 99% ZnO) Oxide (2% Al2O3, 98% ZnO) Oxide (5% Al2O3, 95% ZnO) Oxide (3% Ga2O3, 97% ZnO) Metallic (In 90%, Sn 10%) Metallic (In 90%, Sn 10%) Metallic (In 90%, Sn 10%) Metallic (Zn 100%) Metallic (Zn 100%) Metallic (Zn 100%)

50 60 25 85 50 25 0.96 6.9 20 32 21 17 24 7 58 26

[29] [30] [30] [31] [31] [32] [33] [34] [35] [35] This This This This This This

work work work work work work

Plastic substrate IZO/Ag/IZO (PET) ITO/Cu/ITO (PC) ITO/Au/ITO (PC) ITO/Ag/ITO (PES) ITO/Ag/ITO (PET) ITO/Au/ITO (PET) ITO/Ag/Au/ITO (PET) ZnO/Ag/ZnO (PET) ZnO/Au/ZnO (PET) ZnO/Ag/Au/ZnO (PET)

Sputtering Sputtering Sputtering Sputtering Reactive sputtering Reactive sputtering Reactive sputtering Reactive sputtering Reactive sputtering Reactive sputtering

Oxide (5% ZnO 95% In2O3) Oxide (10% SnO2, 90% In2O3) Oxide (10% SnO2, 90% In2O3) Oxide (10% SnO2, 90% In2O3) Metallic (In 90%, Sn 10%) Metallic (In 90%, Sn 10%) Metallic (In 90%, Sn 10%) Metallic (Zn 100%) Metallic (Zn 100%) Metallic (Zn 100%)

28 0.14 6.6 75 29 16 28 4 8 15

[17] [36] [37] [16] This This This This This This

work work work work work work

Fig. 9. Dispersion curves for the refractive index n(l) of oxide/metal/oxide films deposited on glass substrates.

Fig. 10. Dispersion curves for the extinction index k(l) of oxide/metal/oxide films deposited on glass substrates.

cells are exposed to periodically temperature variations due to the day–night cycles, the utilization of transparent conducting electrodes insensible to the temperature variations guarantee a better stability of devices performances in time. Figs. 13 and 14 gives the informations on the optical characteristics of two commercial ITO films.

Table 4 Plasma wavelength values obtained from the intersection of n(l) and k(l) dispersion curves for the oxide/metal/oxide multilayer films deposited on glass. ITO/metal/ITO

lp (nm)

ZnO/metal/ZnO

lp (nm)

ITO/Ag/ITO ITO/Au/ITO ITO/Ag/Au/ITO

1040 1170 880

ZnO/Ag/ZnO ZnO/Au/ZnO ZnO/Ag/Au/ZnO

– 1180 1100

4. Conclusions Very good quality transparent conducting thin films structures (r ¼2  10  5 O cm, T  90%) were prepared by sputtering and reactive sputtering of metallic targets. The morphological, optical

and electrical properties were compared for the multilayer films ITO/metal/ITO and ZnO/metal/ZnO deposited both on glass and PET substrates. The influence of substrate nature is more pronounced in the case of zinc oxide films deposition. The Haake

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Fig. 11. The dependence of the electrical resistivity on temperature, during the first heating, for some of studied oxide/metal/oxide multilayer structures.

Fig. 12. The variation of electrical resistivity with the temperature for two commercial ITO thin films deposited on glass.

Fig. 14. Dispersion curves for the refractive index n(l) and the extinction index k(l)for two commercial ITO thin films deposited on glass (a) ITO-1 and (b) ITO-2.

The electrical properties’ stability with the temperature of oxide/metal/oxide structures is remarkable in comparison with the usual behavior of single oxide films. These films are very promising for third generation flexible organic solar cells. Concerning the replacement of ITO with ZnO, from the figure of merit point of view results are comparables, however an inconvenient could rise from the fact that the roughness of zinc oxides films deposited on PET is more important than that of ITO films. Nevertheless, the roughness of indium tin oxide and zinc oxide films deposited on glass substrates are similar. In the same time, even in the case of ITO multilayer structures, the main advantages consist in the reduction of the films thickness from about 150–800 nm to about 40–50 nm and in a much better stability of the electrical resistivity to the temperature variations. The realization of a structure (oxide/metal) without a second oxide layer is not very suitable due to the fragility to scratch of the very thin metallic layer. The second thin oxide layer ensures a very good protective coating. Fig. 13. Transmission and reflection spectra for two commercial ITO thin films deposited on glass.

Acknowledgments figures of merit for l ¼550 nm are comprised between 4  10  3 O  1 and 29  10  3 O  1 in function of the nature of the metallic interlayer Ag, Au or Ag/Au.

Authors are grateful to Romain Mallet, Sonia Georgeault and Guillaume Mabilleau from SCIAM—Microscopy Service for AFM

M. Girtan / Solar Energy Materials & Solar Cells 100 (2012) 153–161

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