Influence of degree of vacuum during deposition on propertiesof Cu(In,Ga)S2 absorber film and Mo back contact film
R. Kaigawa, R.Fujie, A.Morimoto, K.Funahashi
Department of Electronics and Informatics, Ryukoku University, Seta, Otsu 520-2194, Japan
Keywords: Cu(In,Ga)S2, solar cell, degree of vacuum, orientation, MoAbstract. We have prepared the Cu(In,Ga)S2 absorber film and the Mo back contact film ofthin film solar cell with changing degree of vacuum during deposition, and investigated theorientation, the internal stresses, and the exfoliation. The (112) X-ray diffraction peak was thestrongest in Cu(In,Ga)S2 films deposited under 0.07Pa. Cu(In,Ga)S2 films deposited over0.07Pa have (204) orientation. Mo films which deposited at narrow region of degree of vacuum
from 0.5Pa to 0.6Pa have (211) orientation on tension condition, other Mo films have (110)orientation on compression condition. We have found that Cu(In,Ga)S2 film prepared on Mo
film under tension condition are easily exfoliated from Mo film.

Introduction
Thin film solar cells based on chalcopyrite absorbers are promising candidates for next
generation photovoltaic modules. They have the highest efficiencies of all thin-film solar cells[1], excellent stability and radiation hardness together with competitive projected production
costs. Furthermore the flexible solar cell is practicable on flexible substrate like polyimide [2].
CuInS2 has a band gap of about 1.5eV suitable for optimum conversion efficiency, a lower
toxicity than Se-containing chalcopyrites, and it can be prepared by simple, extremely fast
sequential (two-step) processes [3,4]. Since Cu(In,Ga)S2 solid solutions can have band gaps
from 1.5eV to 2.43eV depending on the ratio of Ga/In, we can employ Cu(In,Ga)S2 absorber forthe top cell of the tandem configuration with wide band gap[5-7]. The highest efficiency ofchalcopyrite sulphide-based (Se-free) Cu(In,Ga)S2 thin film solar cells is 12.3 % (independently
confirmed total area measurements) [8].
We know that Cu(In,Ga)S2 film on Cu-rich condition has (112) orientation(112 plane isparallel to substrate) and that on Cu-poor condition has (204) orientation (204 plane is parallelto substrate) [8]. It was reported that orientation of other materials (such as (Ba,Sr)TiO3[9],AlN[10]) dependent on degree of vacuum during deposition. The relation between theorientation of Cu(In,Ga)S2 film and degree of vacuum during deposition have not been studied
in detail so far.
Cu(In,Ga)S2 solar cell has the substrate / Mo Back contact / Cu(In,Ga)S2 / CdS / ZnO
structure. We occasionally encounter a exfoliation problem of Cu(In,Ga)S2 film from Mo
film[11-13]. Although the exfoliation is great problem for yield, the origin of exfoliation
problem has been not clarified yet. It has been known that the internal stresses of Mo films areaffected by degree of vacuum during deposition [14]. There are possibility that internal stresses
are dependent on a exfoliation of Cu(In,Ga)S2 film from Mo film.
In this study, we have prepared the Cu(In,Ga)S2 film on soda-lime glass(SLG) withchanging degree of vacuum during deposition, and investigated the orientation. Furthermore wehave prepared the Cu(In,Ga)S2 film on polyimide covered with Mo back contact andinvestigated the relations between the internal stresses and the exfoliation.

Experimental
A multi-source 2-stage evaporation has been employed using an EL10 evaporator
manufactured by Eiko engineering. In, Ga, Cu, and S were deposited by means of four Knudsencells. In the first stage, In-Ga-Cu-S precursor layers were deposited at 250°C onto SLG(12.5mm×50mm×1mm). The (Ga+In):Cu ratio was 1:0.9. In the next stage, the substrate washeated to 580°C, and Cu and S were deposited onto the precursor layer. The conversion fromCu-deficiency to Cu-excess is accompanied by a change in the film’s emissivity. We have used
this effect for in-situ monitoring by observing the substrate and heater temperature withseparate thermocouples. The nominal Cu-excess in the film has been adjusted to 10% by
continuing stage 2 evaporation for a certain time after detection of the stoichiometry transition.
A total evaporation period closes to 1.0 hour. Degree of vacuum during deposition isaccidentally different every evaporation, furthermore hydrogen gas was introduce for thepurpose of intentional change the degree of vacuum during deposition. In order to investigate
exfoliation
andstress,Cu(In,Ga)S2 films
weredeposited
onpolyimide
substrate(12.5mm×50mm×0.125mm) by 2-stage evaporation( substrate temperature in second
stage is at 425°C). Compression or tension stresses were estimated from warping of polyimide
substrates.
Mo films were deposited on polyimide substrates using RF-magnetron sputtering methods
flowing Ar. Degree of vacuum during deposition was changed by adjustment of Ar flow.
X-ray diffraction (XRD) spectra have been recorded using Cu-Kα radiation. The thickness ofthe films was estimated from scanning electron microscopy (SEM) cross section images andstylus profiling. The Ga/(In+Ga) and Cu/(In+Ga) ratios were measured using Wavelength
Dispersive X-ray Spectrometry (WDS) in the Electron Probe Micro Analyzer (EPMA).
Resulting films have been analyzed by SEM and optical microscope.

Results and Discussion

X-ray Intensity [a.u.](a)0.027[Pa]

(112)
(204)
(101)

(215,312)

(200)
(211)

(b)

(316,332)

0.072[Pa]

(204)
(112)

(101)

1020(200) (211)

30402 θ [degree](215,312)(316,332)

5060

7080

Fig.1 XRD pattern of Cu(In,Ga)S2 films prepared in degree of vacuum(a) 0.027Pa and (b) 0.072 Pa during deposition

XRD Mo(211)/Mo(110) Intensity ratio

XRD CIGS(204)/CIGS(112) Intensity ratio

Thickness of Cu(In,Ga)S2 and Mo films are 1.7µm and 1.0µm, respectively. Ga/(In+Ga) ofall Cu(In,Ga)S2 thin film are 0.2. Band gap is estimated at 1.69eV. Figure 1 shows XRD patternof Cu(In,Ga)S2 film prepared in degree of vacuum 0.027Pa(a) and 0.072Pa(b) during deposition.
In the case of Cu(In,Ga)S2 film prepared in 0.027Pa, the (112) diffraction peak was thestrongest. On the other hand, in the case of Cu(In,Ga)S2 film prepared in 0.072Pa, the (204)diffraction peak was the strongest, which indicates that Cu(In,Ga)S2 thin film was oriented
along the (204) plane parallel to the substrate. Relative intensities of powder diffraction of(204) and (112) are 25 and 100, respectively (JCPDS 27-159).
Figure 2 shows that (204) XRD intensityof Cu(In,Ga)S2 film normalized to the1.4(112) intensity as a function of degree ofⅠⅡ
vacuum during deposition. Cu(In,Ga)S2films deposited over 0.07Pa have (204)1.2orientation whether with hydrogen orwithout hydrogen. All films were prepared
1.0in Cu-rich condition, namely, it indicatesthat orientations depend on degree of0.8
vacuum during deposition, and are△ with H2
independent of Cu-rich or poor condition.
□ without H20.6
It can be considered that increase of degreeof vacuum may increase oxygen
0.040.08
0.120.16contamination [15]. We presume that andegree of vacuum[Pa]irradiation of hydrogen gas may decrease
oxygen contamination by means of a Fig.2 (204) XRD intensity of Cu(In,Ga)S2 film
reducing process. The mechanism of normalized to the (112) intensity as a functionchange of orientation with degree of of degree of vacuum during deposition
vacuum during deposition is not yet
well-established. However we believe that content of oxygen contamination of films may be
independent on the orientation in this case, since orientations change with degree of vacuum
during deposition in spite of presence or absence of hydrogen.
compression

8Ⅲ Ⅳ
6Mo thin film

tension

ⅤPolyimide
compression

4Mo thin film

2Polyimide
00.4

0.6

0.8

1.0

1.2

1.4

degree of vacuum[Pa]Fig.3 (211) XRD intensity of Mo film
deposited on polyimide substrate normalized tothe (110) intensity as a function of degree ofvacuum during deposition

tensionFig.4 Schematic drawing of the Mo
films on polyimide substrate ofcompression and tension condition.

Figure 3 show that (211) XRD intensity of Mo film deposited on polyimide substratenormalized to the (110) intensity as a function of degree of vacuum during deposition. Relative
intensities of powder diffraction of (211) and (110) are 31 and 100, respectively (JCPDS42-1120). Mo films which deposited at narrow region of degree of vacuum from 0.5Pa to 0.6Pa
have (211) orientation on tension condition, and other Mo films have (110) orientation oncompression condition as shown in Fig.4.
Hoffman et al. have investigated stress dependent of degree of vacuum pressure from 0.07Pa
to 0.5Pa[14]. They reported stress of film due to compression under 0.3Pa, and tension
between 0.3Pa and 0.5Pa. Their data has slight different from our data, however their tendency
is similar to our data. Although they did not investigate degree of vacuum pressure-dependenceof stress above 0.5Pa, we have found that stress of films prepared over 0.6 Pa change again tocompression.
Table 1
The conditions of Cu(In,Ga)S2 film prepared in Ⅰand Ⅱ
region of degree of vacuum in Fig2 on Mo films prepared in Ⅲ, Ⅳ, andⅤ region of degree of vacuum in Fig3. ○ and × show surfaceconditions as shown in Fig.5(a) and (b), respectively.

Ⅲ[0.41Pa] Ⅳ[0.51Pa] Ⅴ[1.33Pa]
Ⅰ[0.036Pa]○×○
Ⅱ[0.099Pa]○×○Cu(In,Ga)S2Mo(a)(b)

0.1mm

0.1mm

Fig.5

Photograph of typical surface of Cu(In,Ga)S2 film in condition shown by○(a) and ×(b) in table 1.
Table 1 shows that conditions of Cu(In,Ga)S2 film prepared inⅠ(0.036Pa) and Ⅱ(0.099Pa)
region in Fig2 on Mo films prepared in Ⅲ(0.41Pa), Ⅳ(0.51Pa), and Ⅴ(1.33Pa) region inFig3. We have confirmed that Cu(In,Ga)S2 film in Ⅰ on polyimide without Mo has
compression condition, however that in Ⅱ doesn’t have almost stress (polyimide substrates
keep flat). Crack in Cu(In,Ga)S2 film were not observed by optical microscope (Fig.5(a)) andSEM in condition shown by ○. However a large amount of cracks were observed on the film
shown as × in Fig.5(b). In the bright parts in Fig.5(b), Cu(In,Ga)S2 film are exfoliated, thenMo appeared on the surface. We have found that Cu(In,Ga)S2 film prepared on Mo film undertension condition are easily exfoliated from Mo film.

Conclusions
We have investigated the relation between the orientation of Cu(In,Ga)S2 and degree ofvacuum during deposition. We have found that the orientation of Cu(In,Ga)S2 film change to(204) orientation over 0.07Pa. Mo back contact films which are only deposited at narrow degreeof vacuum region from 0.5Pa to 0.6Pa have (211) orientation with tension condition. We havefound that Cu(In,Ga)S2 film prepared on Mo film deposited from 0.5Pa to 0.6Pa are easily
exfoliated from Mo film.

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