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Apr 2, 2012 - thin Mo layer which allows us to fabricate a double side CIGS solar cell, in which the lights ... case, the Mo layer should be as thin as possible to let the sun light go through while still ... rough interface might result in an island growth in variation of ... The sheet resistance was measured by a four point probe.

CHINESE JOURNAL OF PHYSICS

VOL. 50, NO. 2

April 2012

The Thickness Dependent of Optical Properties, Resistance, Strain and Morphology of Mo Thin Films for The Back Contact of CIGS Solar Cells Chih-Hao Lee,1, 2 Fong-Gang Guo,1 and Chia-Chin Chu1, 3 1

Department of Engineering and System Science, National Tsing Hua University, Hsinchu, 30013, Taiwan 2 National Synchrotron Radiation Research Center, Hsinchu 30077, Taiwan 3 Department of Biomedical Engineering and Environment Science, National Tsing Hua University, Hsinchu, 30013, Taiwan (Received June 19, 2011) The Mo layer was used as conducting back contact on a CIGS solar cell. Mo thin films were sputtering deposited on glass substrates. The morphology, stress, resistance and optical properties were measured by X-ray reflectivity, X- ray diffraction and uv-vis spectrometer as functions of film thicknesses in this work. The Mo layers on the glass substrates exist some tensile stress of about 5 ∼ 7 GPa. These tensile stresses make the Mo layers easy to peel off from the glass. The sheet resistance is larger than the inversed thickness law, which might due to a significant natural oxidation layer on the top surface and small grain sizes. With the higher series resistance of thinner Mo thin films, the concept of using double side CIGS solar cell is not feasible. PACS numbers: 61.10.-i, 68.55.Jk, 73.50.-h

I. INTRODUCTION

In the structure of Cu(In,Ga)Se2 (CIGS) solar cell, the CIGS absorbed layer connects to a back contact thin film, such as Mo film. The resistivity, transmittance and stress in Mo thin film are important factors in producing a good CIGS solar cell [1–9]. To reduce the cost of a solar cell, the thinner Mo and CIGS layer are better. Nevertheless, the Mo layer should be thick enough not to sacrifice the solar cell efficiency. Thickness dependent of those physical parameters is important information for people to fabricate an optimum CIGS solar cell. For the Mo layer to be a good conducting layer, it should be thick enough so that the series resistance of the solar cell can be low. In addition, the thickness of Mo should be much longer than the extinction length in order to have stronger reflection light from the interface of CIGS and Mo layer. On the contrary, thinner Mo layer is better to enhance the Na diffusion from the soda lime glass. The Na atoms migrating to the CIGS layer is thought to be a factor to enhance the cell efficiency [4]. In the some special applications, a thin Mo layer which allows us to fabricate a double side CIGS solar cell, in which the lights can enter the cell from both front side and back side to harvest more sun lights. In this case, the Mo layer should be as thin as possible to let the sun light go through while still keep the resistance as low as practical possible. Furthermore, different thickness of Mo films always result in building up of the strain depending on the preparation of the thin films, which makes the Mo layer easily to peel off from the glass substrate. Probing the interface roughness is also important. Typically, rougher surfaces usually form at thicker deposited

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c 2012 THE PHYSICAL SOCIETY

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films. For the interface structure, a rough interface might enhance the contact between two conjunction layers to avoid the cracks along the interface during further annealing process. For the light harvest, a thick and rough interface although is not good for the light to penetrate through, it is excellent for producing haze light to shorten the thickness of active material and creating a non-reflected surface for more lights to enter the active layers. A rough interface might result in an island growth in variation of crystalline orientations and grain sizes in growing the consecutive CIGS layer, which affects the solar efficiency [3, 4]. Therefore, in this study, we used X-ray reflectivity to probe the surface roughness of this first layer of CIGS solar cell, that is, the Mo on the sola lime glass which is the foundation base of a CIGS solar cell. We further illustrate the thickness dependent of physical properties. In other words, different thicknesses of Mo films were prepared and the electrical, mechanical strain, morphology and optical properties of the Mo layer were measured.

II. EXPERIMENTAL

The Mo thin film was deposited on pre-cleaned Corning glass substrates using a RFsputtering machine under a base pressure of 10−7 Torr. The sputtering process was under an Ar pressure of 10 mTorr with a flow rate of 60 cm3 /min. The sputtering process was monitored by a quartz crystal microbalance (QCM). Several samples with nominal thickness of 1, 5, 10, 20, 50, 100, and 500 nm were prepared and measured by the X-ray diffraction (XRD) for the crystalline structure. The morphology of samples was characterized by Xray reflectivity (XRR) measurement and atomic force microscopy (AFM, Digital MultiMode ElectroChemical AFM running in contact mode). AFM usually is not a good tool because it only probes the surface [10, 11], while buried interface is a very important factor. The XRR and XRD measurements were performed at BL17A and BL17B beam lines of Taiwan Light source [12]. A recursive method [10, 11] was used to analyze the XRR data which reveal accurately the film density, thickness, surface and interface roughness. During the parameter fitting of the Mo thin film, we kept the substrate density and surface roughness essentially fixed for all the Mo films. The transmittance and reflectance of Mo thin film was done by a uv-vis spectrometer together with an integral sphere to collect also the diffuse scattering light. The sheet resistance was measured by a four point probe.

III. RESULT AND DISCUSSION

Fig. 1 shows the X-ray diffraction curve of Mo thin film with 8 keV of incident Xrays. We can observe that the thin film has the preferred orientation of (110) when the film thickness larger 100 nm. Since Mo(110) is the most dense surface of a bcc structure, the preferred textured with this orientation is quite natural. With this crystal orientation of Mo thin film, the solar cell efficiency was reported to be the best one [9]. The XRD patterns also show a strain developed in the Mo layer. The diffraction peak (110) shifts from 39.89◦ for the 100 nm thin film to 40.34◦ for the 500 nm film, which means a tensile

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stress inside the thin film developed. By measuring the strains at different tilt angles, the resulting stress is listed in the last column of Table 1. We can see that when the thickness is increased from 100 nm to 500 nm, the tensile stresses of thin film decrease from 7.3 GPa to 5.5 GPa. This tensile stress can be released at thick film when the dislocations and defects appear. Because the sample has high tensile stress, the Mo thin film is easy to peel off from the glass substrate after further heat treatment in the growth of CIGS layer and transparent conducting layer. From the published documents [1, 6, 7], different processes of preparing the Mo thin film on glass result in different tensile stresses. For example, Vink et al. found the stress is 2 GPa [6]. John H. Scofield et al. [7] showed that the Mo thin film has low resistivity but poor adhesion when deposited at lower Ar pressure (< 2 mTorr), and high resistivity and good adhesion with higher Ar pressure (> 5 mTorr). In the future, we can use a bilayer Mo structure to take the advantage of both low resistivity and high adhesion by depositing at higher Ar pressure at first, and then depositing another low resistance Mo layer at lower Ar pressure.

Relative Intensity (a.u.)

15000

Mo (110) MoO3 (-211)

12000

500 nm 9000

300 nm 100 nm

6000

50 nm 3000

10 nm 5 nm

0 20

30

40

50

60

2Theta (deg.)

FIG. 1: XRD of Mo thin films. The preferred orientation is Mo(110) and the peak intensity is gradually increased from 50 nm to 500 nm with peak position shifted. The incident X-Ray is 1.55 ˚ A.

Fig. 2 shows the X-ray reflectivity curve of Mo thin films. The fitting result is also listed in Table 2. The nominal thickness obtained from the QCM is not as accurate as that obtained by the X-ray reflectivity method. Typically, 20% of error for QCM is quite common due to the change of Z-ratio when the density of thin film is varied and the stress is building up during the thin film growth. The fitted density profile of the Mo thin film is shown in Fig. 3. The density profiles show a lower density layer was formed at the surface of glass substrate which was verified using a bare glass for X-ray reflectivity measurement. A

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TABLE I: The tensile stress of 100, 300, 500 nm thin films. Measurements of the diffraction peaks (110) at ψ = 0◦ , 15◦ , 30◦ to get the slope (σ(1 + v)/E), where v is Poisson ratio, v = 0.31 and E is the Young’s modulus, E = 329 GPa. peak (110)

2θ (Ψ = 0◦ )

2θ (Ψ = 15◦ )

2θ (Ψ = 30◦ )

slope of sin2 ψ − (d − d0 )/d0 plot

σ (Gpa)

100 nm

39.89◦

39.56◦

39.78◦

0.0291

7.3 ± 0.2

300 nm

40.26◦

39.91◦

40.10◦

0.0269

6.7 ± 0.2

500 nm

40.34◦

40.01◦

40.18◦

0.0220

5.5 ± 0.2

low density layer was also found on the top of Mo layers. The third column of Table 2 shows the root-mean-squared roughness, σ, of the top Mo layers deduced from the density profiles. We can see that the top surface roughness increases as the thickness of the film increases as shown in Fig. 4. By assuming the film growth model following the σ ∝ tβ , where the parameter β can be named as the growth exponent. This growth exponent β is 1 for the first four data points, and becomes 0.56 at later stage. The different values of β may be due to different growth mechanisms at beginning and later stage of thin film growth. These β exponent values do not fit the kinetic roughening theory of Kardar, Parisi and Zhang (KPZ), where the surface growth exponent β = 0.1 − 0.25 [13]. It can be better fitted to a random deposition growth theory in which β = 0.5 at later stage. For the initial growth stage, the high value of β is better interpreted as the island growth of the thin film. When the islands coalesce, the surface roughness becomes smoother and the β approaches 1/2. This growth mechanism can be similar to the sputtering growth mode of Ta2 O5 , [14, 15] or LaNiO3 [16]. Or even very similar to the growth mode of vacuum evaporation of Pt thin films on silicon wafer [17]. However, in this experiment, a small oxidation peak can be seen in the XRD patterns. This natural oxidation layer can be formed because the samples were kept in the dry air for one or two weeks before the XRD and XRR experiments. If the oxidation layer is grown conformably, then the surface roughness still follows the original surface morphology. In this way, the interpretation of growth mechanism similar to that of sputtering oxide thin films [14–16] is still holds probably. At early stage of growth, the low density profile on the top layer of the samples might be contributed from both the true interface roughness with sharp interface boundary and the oxidation layer with diffuse interface. In order to distinguish the island growth and oxidation layer formation, the top surface was viewed with SEM and the result is shown in Fig. 5. We can see the grains grow larger on the surface as the film thickness increases. The larger grain leads to a rougher film surface. At early stage of growth the island might be too small to be seen using the traditional SEM. In the later stage, the coarsen islands give a rougher surface resulting an average low density profile at the top of surface. The contribution of the oxidation layer should be less important in the depth profile. In a real CIGS solar cell, a rough Mo surface, in fact, is a better morphology to increase the adhesion of the contact between Mo layer

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and consecutive CIGS layer.

FIG. 2: The experimental and fitted X-ray reflectivity data for the Mo thin films on glass substrates. Each curve is shifted vertically for clarity.

TABLE II: The thickness and surface roughness obtained from QCM, X-ray reflectivity and AFM.

Nominal Thickness

*

Thickness*

Surface Roughness

QCM (nm)

XRR (nm)

XRR (nm)

AFM (nm)

1

3.2

0.4



5

5.2

0.7



10

11.1

1.2

10

20

17.6

1.9



50

37.1

2.4

12.4

100

76.8

2.9

9.9

The thickness includes oxidation layer.

A few selected samples were also probed with a contact mode AFM, the result of surface roughness was also listed in the last column of Table 2. We can see the roughness of AFM is almost ten times larger than that obtained from XRR. It is not too surprised to see the inconsistency result between the AFM and XRR measurements as seen in a previous study [10, 11]. In our samples, many spikes were found in the AFM image which

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FIG. 3: The density profile of Mo thin films obtained from XRR.

Surface Roughness (nm)

1

Mo/Glass X-ray Reflectivity

0.1

0.01 1

10

100

Thickness of Mo Layers (nm)

FIG. 4: The surface roughness as a function of film thickness.

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FIG. 5: SEM pictures of the surface morphology of Mo thin films on glass substrates at different thicknesses.

results in a high surface roughness value. Those spike peaks might come from low density contaminants which cannot be seen by XRR method. The inconsistency can be partly remedied by the selection of calculating area to avoid high spike locations during the AFM image processing. This shows the tricky part of using the AFM in contact mode which is only capable of plotting out the surface height profile without knowing the density of the contaminants on surface. In this case, XRR is an important complementary tool to study this problem. The results of transparency measurements and sheet resistance measurements of the Mo layers are listed in Table 3. The calculated sheet resistance following the inversed thickness law and deduced from the bulk Mo resistivity(5.34 × 10−8 Ω m) is also listed in the column 4 of Table 3. The result of all the measured sheet resistances being higher than the calculated values is consistent with previous published works [1, 6, 18, 19]. From Fig. 6, we can see that the experimental sheet resistance is not inversely proportional to the thickness of Mo film, especially, at thinner Mo films. The largest resistance at thinnest film might be attributed to a discontinuous film in the island growth at beginning. Once

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the Mo is thick enough, the islands coalesce resulting in a quick drop of resistance. Another contribution is due to the natural oxidation layer in the thinner layers. Less significant fraction of oxidation layer occurred for the thicker Mo layer resulting in a lower resistance. Additional resistance drop is due to the reduction of grain boundary scattering [20], which is evident from the grain coarsen of the SEM observation as shown in Fig. 5. TABLE III: The optical properties and sheet resistance of the Mo thin films. The fill factor is calculated by assuming ideality factor is 1. Nominal Thickness (nm)

Reflectance Average (%)

Transmission Calculated Tx (%)

@600 nm

@600 nm

Exp.

Calcn.

Calcn.

Calcn.

5

8.6

69.4

176.2

10.68

0.459

0.779

10

43.9

48.2

75.3

5.34

0.713

1.056

20



23.2



2.67

0.853

1.050

50

60.9

2.6

12.6

1.068

0.940

0.964

100

65.1

0.067

3.9

0.534

0.970

0.970

300

64.5

3.0E-8

0.72

0.178

0.990

0.990

500

64.1



0.23

0.107

0.994

0.994

QCM

Sheet resistance (Ω/)

F F/F F0

Gain

Fig. 7 shows the reflectance of the Mo thin film with uv-vis spectrometer. When we increase the thickness over 50 nm, the reflectance of Mo films tends to a saturated value about 63 %. When the thickness is less than 50 nm, more than 40 % of incident light is lost by penetrating these films. A high reflection light from Mo layer back to the CIGS layer is highly desired in order to reduce the thickness of expensive CIGS film. Thus, the thickness of Mo layer higher than 50 nm is requested to reflect light from the interface. However, a drawback is that the sodium atoms from the substrate are not easy to diffuse through the thick Mo film to CIGS layer to contribute more solar cell efficiency. On the contrary, reducing the thickness of Mo thin film, increase the series resistance. It seems that 200 nm of Mo layer is the best thickness, which is also suggested by Kamikawa-Shimizu et al. [1], who makes the thickness between 100 ∼ 300 nm in a bilayer structure to decrease the manufacturing cost without sacrificing too much efficiency. In the case of designing a double side CIGS solar cell, thinner Mo layer which allows a higher light transmittance from the backside will be better. However, the thinner Mo thicknesses result in a higher series resistance which also reduces the fill factor of solar cell and causes a drop of solar cell efficiency. One-diode model calculation of the fill factor, F F , as a function of the series resistance of Mo layer is listed in the Table 3 (column 6). This calculation model assumes a highly ideal case with no shunt resistance and no series resistance from the top transparent conducting layer and active layers (CIGS and CdS layers). We also assumed the series resistance of Mo is equal to the calculated sheet

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FIG. 6: The measured sheet resistance (solid squares) as a function of Mo thickness. The solid line with blank squares is the predicted sheet resistances following the inversed thickness law using bulk Mo resistivity.

FIG. 7: The reflectance of Mo thin films on glass substrates as a function of incident wavelength at different thicknesses.

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resistance as listed in the column 5 of Table 3. By considering the light penetration, Tx, through the nominated Mo thickness and generated more current, the light harvest gain can be expressed as Gain = (1+Tx )F F/F F0 , where F F0 is the ideal fill factor when the ideality factor is equal to one. For a best CIGS cell with an open circuit voltage, Voc =0.690 V and an short circuit current, Jsc = 35.5 mA [21]; the F F0 = 0.8445 in the one-diode model. The final gain at sunlight harvest is listed in the last column of Table 3. From this calculated result, we can see the series resistance prevents us from using the double side CIGS design to increase the light harvest unless we can further reduce the series resistance of Mo layer significantly. For example, a thin Mo layer of 10 nm would allows about 48% of sunlight penetrating through Mo layer to the active CIGS layer, which could increase the sunlight harvest from the other side. The solar cell efficiency may increase 48% if both sides of the cell are illustrated by the same amount of light. However, the increase of series resistance results in a 72% decrease of the fill factor. Thus, the gain increase only less than 6% and did not reach 48% as original excepted. In reality, the resistance goes up to 75Ω/ instead of the calculated 5.4Ω/ used in our calculation using a highly ideal model. It is evident that the idea of a double side CIGS solar cell is not feasible if the series resistance is still too high. The resistance can be further reduced by choosing the right dimensions of segments and pitch of fingers on the interconnect or using a thick mesh design which can reduce the series resistance while still allow most of the light penetrating through. Keeping the Mo layer in vacuum or inert gas environment all the time before the deposition of sequential CIGS layer can solve the problem of oxidation formation on the surface. Sputtering Mo layer together with the aid of ion bombardment and applying bias on the substrates might also reduce the thin film resistance approaching the bulk Mo value [22].

IV. CONCLUSION

In conclusion, a thickness dependent study of physical properties of Mo layer for the CIGS solar cell was investigated. We found the tensile stress of about 5 ∼ 7 GPa for the Mo films on glass substrates. To solve this problem, Mo bilayer fabrication under different Ar pressures should be adopted. The X-ray reflectivity results show that: when the thickness more than 20 nm, the surface roughness increase following the random thin film growth model. The high sheet resistance at thinner film may be contributed from the oxidation layer and island growth mode. The high series resistance prohibits a design of double side CIGS solar cell unless a further design of lower series resistance of Mo layer can be prepared.

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