NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Sodium-Doped Molybdenum 

Targets for Controllable 

Sodium Incorporation in 

CIGS Solar Cells

Preprint

Lorelle M. Mansfield, Ingrid L. Repins, Stephen 

Glynn, Joel W. Pankow, Matthew R. Young, 

Clay DeHart, Rajalakshmi Sundaramoorthy, 

Carolyn L. Beall, and Bobby To 

National Renewable Energy Laboratory

Michael D. Carducci and David M. Honecker 

Climax Molybdenum Technology Center

Presented at the 37th IEEE Photovoltaic Specialists Conference 

(PVSC 37)

Seattle, Washington

June 19-24, 2011

Conference Paper

NREL/CP-5200-50722

July 2011
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1 

SODIUM-DOPED MOLYBDENUM TARGETS FOR CONTROLLABLE SODIUM 

INCORPORATION IN CIGS SOLAR CELLS

Lorelle M. Mansfield1, Ingrid L. Repins1, Stephen Glynn1, Michael D. Carducci2, David M. Honecker2, Joel W. Pankow1, 

Matthew R. Young1

, Clay DeHart1, Rajalakshmi Sundaramoorthy1, Carolyn L. Beall1, and Bobby To1

1

National Renewable Energy Laboratory, Golden, Colorado, USA 2

Climax Molybdenum Technology Center, Sahuarita, Arizona, USA

ABSTRACT

The efficiency of Cu(In,Ga)Se2 (CIGS) solar cells is 

enhanced when Na is incorporated in the CIGS absorber 

layer. This work examines Na incorporation in CIGS 

utilizing Na-doped Mo sputtered from targets made with

sodium molybdate-doped (MONA) powder. Mo:Na films 

with varying thicknesses were sputtered onto Mo-coated 

borosilicate glass (BSG) or stainless steel substrates for 

CIGS solar cells. By use of this technique, the Na content 

of CIGS can be varied from near-zero to higher than that 

obtained from a soda-lime glass (SLG) substrate. Targets 

and deposition conditions are described. The doped Mo 

films are analyzed, and the resulting devices are 

compared to devices fabricated on Mo-coated SLG as well 

as Mo-coated BSG with NaF. Completed devices utilizing 

MONA exceeded 15.7% efficiency without anti-reflective 

coating, which was consistently higher than devices 

prepared with the NaF precursor. Strategies for minimizing 

adhesion difficulties are presented.

INTRODUCTION

Laboratory-scale Cu(In,Ga)Se2 (CIGS) solar cells have 

now reached over 20% conversion efficiency [1]. Na plays 

a crucial role in the fabrication of the world-record devices. 

Therefore, high-efficiency CIGS solar cells are typically 

deposited on soda-lime glass [2]. At the high growth 

temperatures (~600 ̊C), Na from the glass diffuses through 

the molybdenum back contact and into the forming CIGS 

film. This makes the solar cell efficiency very dependent 

on the properties of the molybdenum back contact [3, 4]. 

The advent of high-volume CIGS manufacturing

necessitates Na incorporation methods that are precise, 

manufacturable, and controllable over a wide range of Na 

concentrations. A reliable Na delivery system will increase 

the reproducibility of CIGS device performance. 

Additionally, another source of Na is required in order to 

fabricate solar cells on Na-free substrates such as 

stainless steel or polymer film. One proven Na precursor is 

NaF [5-8]. However, CIGS films deposited on NaF can 

display adhesion problems, particularly when some portion 

of the NaF precursor is not consumed during CIGS 

growth, such as at lower deposition temperatures or large 

NaF thicknesses. The adhesion issues limit the amount of 

Na supplied from NaF to lower than that supplied by sodalime glass. Climax Molybdenum Technology Center has 

developed sodium-containing molybdenum (MONA) that

eliminates the need to extract Na from glass. We have 

produced solar cells with 16.6% efficiency (total-area, with 

anti-reflective coating) using these Na-doped molybdenum 

targets. 

EXPERIMENTAL 

Sputtering target fabrication involves several steps. First, 

MONA powder was produced by mixing sodium molybdate

(~15%) and molybdenum powder (~85%) in deionized 

water and spray-drying the resulting slurry. The powder 

used in this fabrication contained approximately 3%[wt]

sodium (9.6%[at]), as measured by ICP-OES. The powder 

was consolidated into a suitable form to fabricate 

sputtering targets using cold isostatic pressing followed by 

an encapsulated hot isostatic pressing process. Targets 

with the stated composition and fabricated using this 

proces, densify to a range of 94.8% to 98.4% (theoretical 

maximum density 8.23 g/cc) depending on the conditions 

used.

CIGS solar cells were fabricated with three different 

sodium sources including NaF, Mo:Na, and soda-lime 

glass (SLG). Sodium-free borosilicate glass (BSG) with 

the addition of a 500 Å chromium barrier-layer was used 

as the substrate for the NaF samples to ensure that no 

additional source of Na existed. The back contact for the 

NaF samples included an approximately 8000 Å

molybdenum layer, and NaF was deposited on top in an ebeam evaporation system. This configuration is shown in 

Fig. 1a and Fig. 1b. Mo:Na samples were prepared on 

chromium-coated BSG (see Fig. 1d and 1e) or chromiumcoated stainless steel (SS) (see Fig. 1f) with ~8000 Å Mo

and Mo:Na on top. Mo:Na was DC-sputtered to the 

desired thickness, ranging from 20 nm to 1000 nm, with a 

chamber pressure of 10 mtorr. In some cases a ~500 Å 

Mo cap layer was added to promote CIGS adhesion. This 

configuration is shown in Fig. 1e and Fig. 1f and is known 

as a “Mo:Na sandwich.” Comparison samples were also 

prepared on the laboratory-standard SLG with a Mo back 

contact between 5000 Å and 10000 Å thick (see Fig. 1c). 

In general, CIGS depositions contained a 1 in. by 3 in.

piece of substrate with Mo:Na (see Fig. 1d, 1e, or 1f), with 

NaF (see Fig. 1a or 1b), and standard Mo on SLG (see 

Fig. 1c). The CIGS absorber layer was deposited via 

NREL’s three-stage process with temperatures reaching

approximately 600 ̊C [9-13], as measured with a 

thermocouple placed on the backside of the glass. 

Devices were finished with standard processing described 

elsewhere [11, 12, 14], including CdS, bi-layer ZnO, and 

Ni/Al grids. An anti-reflective (AR) coating, 1000 Å of 

MgF2, was applied to only two selected samples. Quoted 
2 

efficiencies in this paper are without AR coating, except 

where specifically designated otherwise.

Figure 1 Substrate and back contact configurations. 

The layers are not drawn to scale.

RESULTS AND DISCUSSION

Sodium Content

Sodium content in the CIGS absorber layer as a function 

of Mo:Na thickness was investigated using secondary ion 

mass spectroscopy (SIMS). Fig. 2 shows an average of 

the positive Na counts within the bulk of the CIGS film vs. 

the Mo:Na thickness. (Here, the “bulk” of the film is 

defined by excluding 0.5 um from the top of the film and 

0.5 um at the CIGS/Mo interface.) For comparison, 

horizontal lines on the graph represent Na counts from a 

high-efficiency CIGS film on SLG (dashed black line), from 

a CIGS film deposited on 150 Å of NaF (dashed pink line), 

and from CIGS on 85 Å of NaF (dashed purple line). CIGS 

samples grown directly on Mo:Na films (see Fig. 1d) are 

indicated with blue diamonds in Fig. 2. A blue line guides 

the eye to show that by using 1000 nm of Mo:Na, the Na 

content is more than double the Na content measured in 

high-efficiency CIGS on SLG. At the high Mo:Na 

thicknesses, however, adhesion issues inhibited device 

performance and will be discussed later. Note that the 500 

Å Mo cap (see Fig. 1e) slightly decreases the amount of 

Na in the CIGS for a given Mo:Na thickness as indicated 

by the red circles in Fig. 2. The level of Na in the CIGS of 

Mo:Na samples surpasses that of 85 Å NaF in all but the 

thinnest (20 nm) Mo:Na layer. Na content in CIGS is 

higher than that obtained with 150 Å NaF in Mo:Na 

thicknesses over 200 nm.

Device Performance

To determine the efficiency of each solar cell device, 

current density-voltage (J-V) measurements were 

performed under one-sun conditions. Fig. 3 includes 

example J-V curves for illuminated solar cells with NaF 

(green dotted line), Mo:Na (pink line), and SLG (purple 

dashed line) as the Na source. The device parameters are 

shown in Table 1. At lower Mo:Na thicknesses, the opencircuit voltages (Voc) were not as high as the Voc of devices 

fabricated on SLG. However, fill factor (FF), Voc, and 

efficiency values were higher than those of the devices 

that had NaF as their Na source. The best device 

efficiency on Mo:Na, 16.6%, was obtained on a 160-nm 

Mo:Na sandwich with AR coating. The other parameters of 

note for this champion device (from a different CIGS 

deposition that is not shown on the graph) are Voc = 0.634 

V, Jsc = -35.3 mA/cm2, and FF = 74.2%.

Figure 2 Average SIMS Na counts in CIGS films versus 

Mo:Na thickness. Horizontal lines represent the 

average SIMS Na counts in a high-efficiency CIGS film 

(black), CIGS grown on 150 Å NaF (pink), and 85 Å 

NaF (purple).

The efficiencies of devices fabricated on Mo:Na were 

compared to those on SLG. Since the individual CIGS 

depositions on SLG varied in efficiency from 13.3% to 

17.9%, we calculated for each Mo:Na piece the 

percentage of the SLG companion piece efficiency that it 

achieved. The most efficient Mo:Na device for each CIGS 

deposition was divided by the most efficient device on 

SLG for that same deposition, and then multiplied by 100 

to obtain the percentage on the y-axis in Fig. 4. As Mo:Na

thickness increased, so did the performance of the 

devices. This finding is consistent with observations in the 

SIMS data in Fig. 2. The trend is displayed in Fig. 4, along 

with a green shaded area representing the range of 

compared efficiencies (to devices on SLG) for devices on 

85 Å NaF, and a blue shaded area for devices on 150 Å 

NaF.
3 

Figure 3 Current-density vs. voltage curves of sample 

devices under one-sun illumination. Included are a 

cell fabricated on a NaF-coated substrate (green 

dotted line), on Mo:Na (pink line), and on SLG (purple 

dashed line). Notice that the Mo:Na sample has a 

higher open-circuit voltage than the NaF sample.

Table 1 Parameters for devices shown in Fig. 3. Fill 

factor (FF), open-circuit voltage (Voc), and efficiency 

(Eff) are higher for devices on Mo:Na than devices on 

NaF. Short-circuit current (Jsc) is comparable for NaF 

and Mo:Na devices, but higher for devices on SLG.

Substrate Voc

(V) 

Jsc

(-mA/cm2)

FF

(%)

Eff 

(%)

150 Å NaF 0.620 30.9 71.3 13.7

160 nm Mo:Na 0.664 30.6 77.5 15.8

Mo/SLG 0.694 33.1 77.4 17.8

The efficiencies of devices fabricated on Mo:Na were also 

compared to those in the same deposition on NaF. The yaxis in Fig. 5 is the best Mo:Na device efficiency divided 

by the best NaF device efficiency from the same CIGS 

deposition, multiplied by 100. Most of the points in the 

graph fall above the dotted line at 100%, indicating that for 

the individual CIGS depositions, devices on thicker Mo:Na

consistently reach or exceed the devices on 150 Å NaF.

Figure 4 Percentage of SLG companion efficiency 

versus Mo:Na thickness. The y-axis is the best Mo:Na 

device efficiency divided by the best SLG device 

efficiency from the same CIGS deposition, multiplied 

by 100. The light blue shaded area represents the 

range of efficiency comparison for devices fabricated 

on 150 Å NaF. The light green shaded area shows the 

same for devices fabricated on 85 Å NaF.

Figure 5 Percentage of NaF companion efficiency 

versus Mo:Na thickness. The y-axis is the best Mo:Na 

device efficiency divided by the best NaF device 

efficiency from the same CIGS deposition, multiplied 

by 100. Samples with higher Mo:Na thicknesses easily 

reach or exceed the efficiency of devices on 150 Å 

NaF.

Adhesion Issues

Device results for the thickest Mo:Na films are not 

reported because producing a fully adherent CIGS layer 

on these substrates was challenging. CIGS deposited 

directly on Mo:Na (substrate depicted in Fig. 1d) was 
4 

prone to peeling (see right side of Fig. 5 inset) unless a 

three-minute pre-deposition selenization was performed at 

~400 ̊C. For Mo:Na thicknesses above 100 nm, 

selenization did not mitigate the adhesion issue. Energydispersive x-ray spectroscopy (EDX) performed on the 

backside of peeled CIGS flakes (see Fig. 5) did not reveal 

Mo, indicating that the films delaminated at the 

CIGS/Mo:Na interface.

The CIGS adhesion was enhanced for Mo:Na thicknesses 

up through 200 nm by adding a thin layer of Mo on top of 

the Mo:Na as shown in Fig. 1e. However, for 400 nm 

Mo:Na on BSG with a Mo cap layer, scanning electron 

microscopy (SEM) (see Fig. 7a) revealed nodules on the 

substrate surface. By performing Auger electron 

spectroscopy (AES) on the nodules (see Fig. 7b), we 

detected much more Na and O than off the nodules (see 

Fig. 7c). It is likely that the nodules cause poor adhesion 

due to too much Na at the surface, similar to what is 

observed when using thick NaF precursor layers. We 

understand within bounds (up to 400 nm Mo:Na, just 

below the amount of Na obtained using SLG) how to 

achieve adherent device stacks. However, we have not 

used the thickest Mo:Na films in high-performing devices 

because of adhesion and related difficulties.

Figure 6 The EDX graph does not show Mo on the 

back of the CIGS flakes, indicating that the peeling 

occurred at the CIGS/Mo:Na interface. Inset is a 

photograph of the CIGS film on all three substrates. 

The peeling film on the right is CIGS deposited 

directly on Mo:Na, as in Fig. 1d. 

Figure 7 SEM of nodules on the surface of 400-nm 

Mo:Na sandwich (a). AES on (b) and off (c) one of the 

nodules shows that Na is aggregating and oxidizing 

on the surface.
5 

To further promote CIGS adhesion, Mo:Na was deposited 

on stainless steel (SS) substrates (see Fig. 1f) with the 

reasoning that the rougher surface might inhibit the 

formation of the large Na nodules of Fig. 7a. In general, 

the CIGS did adhere better to the Mo:Na on SS. We were 

able to fabricate devices on Mo:Na thicknesses up to 600 

nm. However, average Na SIMS counts for the CIGS on 

SS substrates with Mo:Na were lower than those on 

Mo:Na on glass (see green triangles in Fig. 2.) This may 

be due in part to the difficulty in obtaining SIMS depth 

resolution for these rougher samples, or it may be a real 

physical effect. The devices on SS were less efficient than 

those on Mo:Na and glass, which is expected, because 

SS is a rougher substrate and the steel may contribute 

some harmful impurities to the CIGS [15]. While initial 

efficiency results from Mo:Na on SS were very promising 

(see the green triangles above 100% in Fig. 4), the SLG 

efficiency in these CIGS depositions was lower than 

normal (~13%). Subsequent CIGS depositions yielded 

better results on SLG substrates, but the efficiencies on 

SS remained lower. J-V measurements for the best CIGS 

cell on Mo:Na/SS and its SLG and NaF companion 

devices are shown in Fig. 8. The device parameters are 

shown in Table 2. As for the case of BSG, CIGS solar 

cells on SS with Mo:Na performed better than those with 

NaF as the sodium source. 

Figure 8 Current-density vs. voltage curves of sample 

devices under one-sun illumination. Included are a 

cell fabricated on a NaF-coated substrate (green 

dotted line), on Mo:Na on SS (pink line), and on SLG 

(purple dashed line). An AR coating was applied to the 

Mo:Na on SS device (red dash and dotted line).

Table 2 Parameters for devices shown in Fig. 8. Fill 

factor (FF), open-circuit voltage (Voc), and efficiency 

(Eff) are higher for devices on Mo:Na than devices on 

NaF. Short-circuit current (Jsc) is comparable for NaF 

and Mo:Na devices, but higher for devices on SLG.

Substrate Voc

(V) 

Jsc

(-mA/cm2)

FF

(%)

Eff 

(%)

150 Å NaF 0.576 32.0 71.3 13.2

400 nm Mo:Na

on SS 0.601 30.5 74.6 13.7

400 nm Mo:Na 

on SS with AR 0.601 33.3 74.3 14.9

Mo/SLG 0.671 34.0 77.3 17.6

CONCLUSION

High-efficiency CIGS solar cells were fabricated using Nadoped Mo targets. Devices produced on the Mo:Na had 

higher Voc, FF, and efficiency than those on NaF. Our best

device with 16.6% efficiency was fabricated on 160 nm of 

Mo:Na with a 500-Å Mo cap layer and an anti-reflective 

coating. SIMS revealed that CIGS grown on 400 nm 

Mo:Na had higher Na incorporation than CIGS grown on 

NaF-coated substrates. SIMS data from the thickest 

Mo:Na layers show that Na content in the CIGS can be 

made to exceed that in high-efficiency devices on SLG. 

However, additional effort is needed to obtain acceptable 

adhesion for these thicker Mo:Na layers. Based on these 

data, Mo:Na may provide a manufacturing option for 

controllable Na incorporation in CIGS solar cells.

ACKNOWLEDGEMENTS

Thanks to Miguel Contreras and Rommel Noufi for 

valuable CIGS discussions, and Miguel for suggesting SS 

substrates. The Climax Technology Center acknowledges 

Chris Michaluk for his initiation of and contributions to this 

project. This work was supported by the U.S. Department 

of Energy under Contract No. DE-AC36-08-GO28308 with 

the National Renewable Energy Laboratory.

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