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R. Dhere, K. Ramanathan, J. Keane, J. Zhou, 

H. Moutinho, S. Asher, and R. Noufi 

Prepared for the 31st IEEE Photovoltaics Specialists 

Conference and Exhibition 

Lake Buena Vista, Florida 

January 3–7, 2005 

February 2005 • NREL/CP-520-37368 

Effect of Na Incorporation on 

the Growth and Properties of 

CdTe/CdS Devices 
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EFFECT OF Na INCORPORATION ON THE GROWTH AND PROPERTIES 

OF CdTe/CdS DEVICES 

Ramesh Dhere, Kannan Ramanathan, James Keane, Jie Zhou, Helio Moutinho, Sally Asher, and Rommel Noufi 

National Renewable Energy Laboratory (NREL), Golden, CO 80401 

ABSTRACT 

Sodium is known to enhance p-type doping in copper 

indium diselenide (CIS)-based devices fabricated on sodalime glass substrates, and similar amounts of Na are present in commercial cadmium telluride (CdTe) devices. We

present the results on the effects of Na incorporation on 

the properties of CdTe/CdS solar cells prepared on borosilicate glass substrates. A NaF layer 10 to 30 nm thick 

was incorporated at either the CdS/CdTe interface or on 

the CdTe surface, as a source of Na. CdTe layers were 

deposited by close-spaced sublimation at substrate temperatures from 425°C (LT) to 620°C (HT), followed by

heat-treatment in the presence of CdCl2 vapor. Atomic 

force microscopy analysis showed that the samples with 

NaF at the CdS/CdTe interface deposited in He ambient 

have larger grains with a sub-grain structure that disappears after CdCl2 heat treatment accompanied by an increase in grain size. Samples deposited in O2 ambient 

have smaller grains without a sub-grain structure. For 

samples with NaF deposited on the CdTe surface, LT

samples with CdCl2 heat treatment showed a morphology

similar to samples without NaF layers; but samples heattreated in He ambient at 500°C prior to CdCl2 treatment 

showed a different microstructure with platelets on the 

surface. HT samples with a NaF layer at the CdTe surface showed an additional layer at the surface with both 

types of heat treatments, indicating that NaF does not 

react readily for these samples. In addition to the discussion of the effect of Na incorporation on the material properties, we will include preliminary data on the effect of Na 

incorporation at the interface or surface on device properties. 

INTRODUCTION 

Best device performance for CdTe solar cells have been 

achieved for devices fabricated on borosilicate 7059 glass 

substrates [1]. These substrates differ from soda-lime 

glass substrates used in the industry in two main aspects, 

namely, the difference in thermal coefficient of expansion 

(TCE) and the Na in soda-lime glass. The TCE of borosilicate glass is comparable to that of CdTe, whereas the 

TCE of soda-lime glass substrates is nearly twice that of 

CdTe. In addition, a significant amount of Na is found in 

the cells fabricated on soda-lime glass substrates. Both of 

these factors are considered to be responsible for inferior 

performance of the devices fabricated on soda-lime glass 

substrates. In the case of CIS- based devices, the situation is exactly opposite and record-efficiency devices have 

been obtained only on soda-lime glass substrates. Improved the device performance is attributed to the enhancement of p-type doping in CIGS absorber material 

and grain growth. In the case of CdTe-based devices, 

there is limited knowledge on the influence of Na on the 

material and device properties [1]. Because considerable 

amounts of Na are present in the devices prepared on the 

soda-lime substrates used by the industries, it is necessary to determine its effect on device properties. In addition, the open-circuit voltage (Voc) of CdS/CdTe thin-film 

solar cells is limited to 850 mV, and it is desirable to explore ways of increasing it. To first order, the Voc is directly proportional to the carrier concentration in the CdTe

absorber. It is well known that it is difficult to achieve appreciable p-type conductivity in CdTe because of the selfcompensation and low solubility of dopants. Substitution 

of Group V impurities (As, P, Sb) on the anion site has 

been studied in detail [2]. Group IB (Cu, Ag) impurities 

introduce deep levels and are unsuitable for achieving an 

increase in conductivity. One avenue that has not been 

explored in sufficient detail is the doping of CdTe by Group 

IA impurities (Li, Na). Interestingly, the ionization energy

of acceptor levels introduced by Li and Na doping are reported to be about 58 meV, and this value is very close to 

the effective mass limit [3]. Hence, there is ample justification for performing a controlled experimental study of ptype doping of CdTe with Na impurities. Deliberate incorporation of Na in CuInSe2 alloys has led to remarkable 

improvements in device properties [4]. CuInSe2 is a derivative of the binary II-VI family, and the defect physics is 

expected to translate readily. 

In this work, we have incorporated thin layers of NaF

(10–30 nm) at the interface region of the CdS/CdTe structures. NaF was deposited by electron-beam evaporation 

onto CdS surfaces prepared by chemical-bath deposition 

(CBD) onto tin oxide-coated glass substrates. The NaF

layer is expected to decompose at high substrate temperature and release the Na and F. For low substrate temperatures, the layer may remain intact. The presence of a 

thin, foreign layer alone can influence the growth habit of 

CdTe. 

EXPERIMENTAL 

Corning borosilicate 7059 glass coated with F-doped 

SnO2 with sheet resistivity of 8 ohms/sq and 80-nm CdS 

deposited by CBD were used as the substrates in this 

study. NaF layers with thicknesses in the range of 10–30 

1 
nm were deposited at room temperature by vacuum 

evaporation. CdTe layers were deposited by close-spaced 

sublimation (CSS) at substrate temperatures of 425°C 

(LT) and 620°C (HT), with thicknesses in the range of 6–8 

μm. Two types of structures were fabricated for this study: 

glass/SnO2/CdS/NaF/CdTe—to study the effect of Na at 

the interface, and glass/SnO2/CdS/CdTe/NaF—to examine 

the influence on the properties of CdTe. Two types of 

heat treatments were used in this study: post-deposition 

heat-treatment was performed in He ambient and/or in the 

presence of CdCl2 vapor. Cu-doped graphite paste was 

used to make contact to CdTe. Morphology of the samples was analyzed by atomic force microscopy (AFM). 

RESULTS AND DISCUSSION 

AFM analysis was used to study the effect of a NaF

layer at the CdS/CdTe interface and the deposition conditions on the microstructure of CdTe films. The LT sample 

with NaF at the CdS/CdTe interface showed a significantly

different microstructure as compared to the samples without the presence of Na. Figure 1a shows the AFM image 

of an as-deposited LT CdTe sample deposited on 

glass/SnO2/CdS/NaF substrate in He ambient. These 

films exhibit larger grains compared to the films deposited 

directly on CdS under identical conditions. Films on NaF

substrate are also more compact and show a sub-grain 

structure not observed in the films on CdS. 

Fig. 1b. AFM image of CdCl2-treated LT CdTe sample 

deposited on glass/SnO2/CdS/NaF in He ambient.

Figure 1b shows the AFM image of a LT CdTe sample 

deposited on glass/SnO2/CdS/NaF substrate in He ambient and CdCl2-treated at 425°C for 5 min. The sub-grain 

structure disappears after CdCl2 heat treatment, and there 

is an increase in grain size similar to CdCl2-treated films 

deposited on CdS. The grain size of the LT sample deposited on a substrate without NaF layer, i.e., directly on 

CdS, is around 0.5 μm and increases to 2 μm after CdCl2

Fig. 1a. AFM image of as-deposited LT sample deposited on glass/SnO2/CdS/NaF in He ambient. 

heat treatment, which is almost comparable to the asdeposited LT sample deposited on NaF.

The presence of O2 in the deposition ambient during 

CSS deposition of CdTe has a significant effect on the 

grain size, as well as on the microstructure of the films. 

As-deposited LT samples deposited in a He/O2 ambient 

with NaF at the CdS/CdTe interface have smaller grains 

as compared to samples deposited in an O2-free ambient 

(Fig. 1a), but do not show any sub-grain structure. Figure 2a shows the morphology of an as-deposited LT CdTe

sample deposited on glass/SnO2/CdS/NaF surface in a 

He/O2 ambient. 

2 
This sample showed morphology similar to the LT CdTe

samples deposited directly on CdS. In these samples, the 

presence of a NaF layer mainly influences the grain size of 

the CdTe layer. Figure 2b is an AFM micrograph of CdCl2-

treated LT sample deposited on glass/SnO2/CdS/NaF 

surface in a He/O2 ambient. There is an increase in the 

grain size of this sample, and it also shows a loss of faceting seen in the as-deposited sample (Fig. 2a). Again 

these results are similar to samples deposited directly on 

CdS films. 

In the second set of experiments, we looked at the microstructure of samples with a NaF layer deposited on the 

CdTe surface after two types of heat treatments. Figure 

3a shows an AFM image of a HT CdTe sample with a 30-

nm-thick NaF layer after a 400°C CdCl2 heat treatment. 

The image is similar to the one for a sample before the 

treatment. In both cases, we see the NaF layer on the 

surface. Pre-heat-treatment in He at 500°C before CdCl2

heat treatment also did not have any effect on the microstructure. 

Figure 3b shows an AFM image of a LT CdTe sample 

with a 30-nm-thick NaF layer after 425°C CdCl2 HT. This 

sample shows significant change in grain size, as well as 

in microstructure. A similar sample with a pre- heattreatment in He at 500°C before CdCl2 heat treatment 

shows a different microstructure, as well as platelets on 

the surface. It appears that the heat treatment in He, even 

at 500°C, induces recrystallization in LT samples with the 

NaF layer. 

In summary, our microstructural investigation shows that 

the effect of a NaF layer at the CdS/CdTe interface is significant for the CdTe samples deposited at lower substrate 

temperatures, particularly in an O2-free ambient. The 

grain size of the samples is considerably larger than for 

films deposited directly on CdS films. 

Fig. 2b. AFM image of CdCl2-treated LT CdTe sample 

deposited on glass/SnO2/CdS/NaF in He/O2 ambient.

Fig. 3b. AFM image of LT CdTe sample with a 30-nmthick NaF layer after 425°C CdCl2 heat treatment.

Fig. 2a. AFM image of as-deposited LT CdTe sample 

deposited on glass/SnO2/CdS/NaF in He/O2 ambient.

3 
Devices were fabricated using these samples with 

CuTe-doped graphite paste. The samples were etched in 

NPH or Br/methanol prior to application of the paste. 

Baseline devices (deposited at 620°C, with 400°C/5 min 

CdCl2 anneal) were also fabricated in the same batch to 

serve as a reference. The baseline device had a Voc = 

809 mV, short-circuit current density (Jsc) = 21.3 mA/cm2

, 

fill factor (FF) = 66.8%, and efficiency of 11.5%. 

The devices fabricated from as-deposited LT samples in 

an O2-free ambient with NaF at the interface showed had 

a reasonable Voc of 660 mV, but suffered mainly from low

Jsc (7 mA/cm2

) and low FF (27%). A similar sample using 

CdTe deposited in an O2 ambient had some improvement 

in Jsc (11 mA/cm2

) and FF (37%). It is noteworthy that 

although the device performance of these devices is poor, 

devices prepared from similar samples deposited on CdS 

show negligible photoactivity. Improvement in the device 

performance is possibly related to the increased grain size 

of samples deposited on NaF substrates. CdCl2 heat 

treatment did not improve the device performance for 

these devices. 

The devices prepared from samples deposited in an O2

ambient on a NaF layer at 500°C gave a Voc of 630 mV, 

Jsc of 15.2 mA/cm2

, and FF of 42%. A similar sample deposited in an O2 ambient had a Voc of 695 mV, Jsc of 19.5 

mA/cm2

, and FF of 44%. 

A possible explanation for the inferior performance of LT

CdTe devices with NaF at the CdS/CdTe interface may be 

the relative stability of the NaF layer at 425°C. Such a 

high-resistivity layer at the junction would deteriorate the 

FF of the devices. With an increase in the substrate temperature to 500°C, there is an improvement in all device 

parameters, possibly due to a partial decomposition of the 

NaF layer at the interface. Devices deposited at both 

temperatures show a beneficial effect of O2 on the device 

performance. 

Finally, the devices fabricated using samples with a NaF

layer at the back contact showed interesting results. For 

HT samples in this set, the samples with a Br/methanol 

etch had lowered device parameters; but the performance 

of devices with a NPH etch was comparable to the baseline devices. These results are due to the difference in the 

etching action of the two types of etches. From the AFM 

image of Fig. 3a, we know that the NaF layer remains intact after a 500°C heat treatment in He, as well as in the 

CdCl2 heat treatment. The Br/methanol etch removes a 

very thin layer from the sample, and thus leaves the highresistivity NaF layer intact at the back contact, thereby

affecting the device parameters. On the other hand, NPH 

etch attacks the top 100 nm of the sample, thus creating a 

conducting Te-rich region and possibly eliminating the 

remaining NaF layer from the back surface. This situation 

can explain the improved performance of NPH-treated 

devices. In the case of devices prepared from LT samples 

in this set, performance of devices prepared using both 

types showed poor performance. Microstructural analysis 

of these samples (Fig. 3b) showed recrystallization for 

both. During the process, the NaF layer may have been 

mixed with the bulk of CdTe. Thus, removal of the top 

layer with the NPH etch did not eliminate the NaF layer, as 

in the case of HT samples. 

Fig. 3a. AFM image of HT CdTe sample with a 30-nmthick NaF layer after 400°C CdCl2 heat treatment.

CONCLUSIONS

A NaF layer at the CdS/CdTe interface is helpful for 

improving the structural quality of CdTe layers. The CdTe

films show grain growth, are compact, and the grain size 

of the films is significantly larger than the films deposited 

directly on the CdS surface. The effect of the inclusion of 

a NaF layer at the interface is more pronounced for the 

CdTe films grown in an O2-free ambient. In the case of 

glass/SnO2/CdS/CdTe/NaF samples with NaF deposited 

on CdTe, HT samples showed that CdCl2 and He heat 

treatments did not produce any structural changes and the 

NaF layer remained intact. On the other hand, LT samples with NaF deposited on CdTe showed significant structural changes with change in morphology and grain growth 

during heat treatments, resulting in the inclusion of a NaF

layer in the bulk of CdTe during the process. 

In summary, we have shown that the inclusion of a thin 

NaF layer results in an increase in grain size of CdTe deposited at low temperatures. This effect might be helpful 

in recrystallizing the interface region. Electronic effects of 

Na may be realized fully when NaF dissociates and the Na 

atoms are allowed to diffuse. 

Devices with a NaF layer at the CdS/CdTe interface 

gave poor results for the LT samples deposited at 425°C, 

because NaF did not dissociate at this temperature. The 

device performance improved for the samples deposited at 

500°C, which may be due to partial decomposition of the 

NaF layer at 500°C, thus reducing the effect of the highresistivity NaF layer at the interface. We will work on the 

devices fabricated at higher substrate temperatures, 

where a complete decomposition of NaF will be possible, 

thus eliminating the effect of the high-resistivity NaF layer 

on the device properties. 

4 
ACKNOWLEDGEMENTS REFERENCES 

[1Wu etal. This work was supported by the U.S. Department of Energy under Contract No. DE-AC36-99GO10337. [2] 

[3] E. Molva, J.L. Pautrat, K. Saminadayar, G. Michlchberg, and N.Magnea, Phy. Rev B 30,1984, p 3344. 

[4] Stolt, IEEE Kentucky;Contreras, Anaheim IEEE 

5 
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Effect of Na Incorporation on the Growth and Properties of 

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14. ABSTRACT (Maximum 200 Words)

Sodium is known to enhance p-type doping in copper indium diselenide (CIS)-based devices fabricated on soda-lime 

glass substrates, and similar amounts of Na are present in commercial cadmium telluride (CdTe) devices. We 

present the results on the effects of Na incorporation on the properties of CdTe/CdS solar cells prepared on 

borosilicate glass substrates. A NaF layer 10 to 30 nm thick was incorporated at either the CdS/CdTe interface or on 

the CdTe surface, as a source of Na. CdTe layers were deposited by close-spaced sublimation at substrate 

temperatures from 425°C (LT) to 620°C (HT), followed by heat-treatment in the presence of CdCl2 vapor. Atomic 

force microscopy analysis showed that the samples with NaF at the CdS/CdTe interface deposited in He ambient 

have larger grains with a sub-grain structure that disappears after CdCl2 heat treatment accompanied by an increase 

in grain size. Samples deposited in O2 ambient have smaller grains without a sub-grain structure. For samples with 

NaF deposited on the CdTe surface, LT samples with CdCl2 heat treatment showed a morphology similar to samples 

without NaF layers; but samples heat-treated in He ambient at 500°C prior to CdCl2 treatment showed a different 

microstructure with platelets on the surface. 

15. SUBJECT TERMS

PV; p-type doping; copper indium diselenide (CIS); devices; soda-lime glass substrates; cadmium telluride (CdTe); 

atomic force microscopy; 

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