April 2003 • NREL/CP-520-33965 

Investigation of CdZnTe for

Thin-Film Tandem Solar Cell 

Applications 

Preprint 

R. Dhere, T. Gessert, J. Zhou, S. Asher, 

J. Pankow, and H. Moutinho 

To be presented at the 2003 Materials Research 

Society Spring Meeting 

San Francisco, California 

April 21-25, 2003 

National Renewable Energy Laboratory 

1617 Cole Boulevard 

Golden, Colorado 80401-3393 

NREL is a U.S. Department of Energy Laboratory

Operated by Midwest Research Institute • Battelle • Bechtel 

Contract No. DE-AC36-99-GO10337 
NOTICE 

The submitted manuscript has been offered by an employee of the Midwest Research Institute (MRI), a 

contractor of the US Government under Contract No. DE-AC36-99GO10337. Accordingly, the US 

Government and MRI retain a nonexclusive royalty-free license to publish or reproduce the published 

form of this contribution, or allow others to do so, for US Government purposes. 

This report was prepared as an account of work sponsored by an agency of the United States 

government. Neither the United States government nor any agency thereof, nor any of their employees, 

makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, 

completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents 

that its use would not infringe privately owned rights. Reference herein to any specific commercial 

product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily 

constitute or imply its endorsement, recommendation, or favoring by the United States government or any 

agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect 

those of the United States government or any agency thereof. 

Available electronically at http://www.osti.gov/bridge 

Available for a processing fee to U.S. Department of Energy 

and its contractors, in paper, from: 

U.S. Department of Energy 

Office of Scientific and Technical Information 

P.O. Box 62 

Oak Ridge, TN 37831-0062 

phone: 865.576.8401 

fax: 865.576.5728 

email: reports@adonis.osti.gov 

Available for sale to the public, in paper, from: 

U.S. Department of Commerce 

National Technical Information Service 

5285 Port Royal Road 

Springfield, VA 22161 

phone: 800.553.6847 

fax: 703.605.6900 

email: orders@ntis.fedworld.gov 

online ordering: http://www.ntis.gov/ordering.htm 

Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste 
Investigation of CdZnTe for Thin-Film Tandem Solar Cell Applications 

Ramesh Dhere, Tim Gessert, Jie Zhou, Sally Asher, Joel Pankow, and Helio Moutinho, National 

Renewable Energy Laboratory, Golden, CO 80401. 

ABSTRACT 

Modeling of two-junction tandem devices shows that for optimal device performance, the 

bandgap of the top cell should be around 1.6-1.8 eV. CdZnTe alloys can be tailored to yield 

bandgaps in the desired range. In this study, we considered were used to fabricate these films, 

using close-spaced sublimation (CSS) and radio-frequency sputtering (RFS) techniques. In the 

first approach, we used mixed powders of CdTe and ZnTe as the source for film deposition by 

CSS. Even for the ZnTe/CdTe (95:5 ratio) source material, the deposited films were entirely 

CdTe due to higher vapor pressure of CdTe. In the second approach, we used pre-alloyed 

CdZnTe powders (CERAC, Inc.) as the source. Due to the lower sticking coefficient of Zn, even 

for the source composition of 75% Zn, these films contained very low quantities of Zn (~5%). 

We tried unsuccessfully to increase the Zn content in the films by confining Zn vapor by 

enclosing the region between the source and substrate, reducing the substrate temperature to 

400°C, and adjusting the source/substance distance. Finally, we used thin-film couples consisting 

of 300-nm-thick CdTe deposited by CSS and 300-nm-thick ZnTe deposited by RFS; the samples 

were then heat-treated in cadmium chloride vapor. Compositional analysis of the samples 

showed extensive interdiffusion of Cd and Zn for the annealed samples. We will present the data 

on the various stack configurations of CdTe and ZnTe, the effect of different post-deposition 

anneals, the effect of oxygen on the interdiffusion and alloy formation and its possible 

correlation to the device performance degradation. 

INTRODUCTION 

Polycrystalline thin-film solar cells based on II-VI and I-III-VI compounds have been 

quite successful, with efficiencies of laboratory devices reaching 16.5% for CdTe [1] and 19.2% 

for CIGS [2] single-junction solar cells. For further improvement, one needs to look at the 

tandem device option. Device modeling of two-junction tandem devices shows that the optimal 

bandgap of the top cell should be around 1.6-1.8 eV [3]. Ternary alloy CdZnTe is a good 

candidate as it is a well-behaved system and is based on CdTe. 

In this paper, we use three key approaches that have been responsible for improved 

performance of CdTe devices: 1) CSS technique for the deposition of alloy layers because the 

best-performing CdTe devices have resulted from the films prepared by CSS at high deposition 

temperatures. 2) CdCl2 heat treatment, which is a crucial step for the fabrication of high 

efficiency CdTe devices. 3) We explored the use of oxygen during different stages on film 

properties because it is another essential component of high-efficiency CdTe device fabrication. 

One of the objectives of this work is to investigate the differences in CdTe and CdZnTe systems 

that could explain vast difference in the device performance of solar cells fabricated from these 

materials [4,5]. 

1
EXPERIMENTAL DETAILS 

All the samples in this study were prepared on borosilicate glass/SnO2/CdS substrates. Fdoped SnO2 films with a thickness of 0.5 μm were deposited by chemical vapor deposition. CdS 

films (85 nm thick) were deposited by chemical-bath deposition at 90° C. Films with a thickness 

of around 500 nm were deposited from a mixed powder source of CdTe and ZnTe and prereacted CdxZn1-xTe alloys with x values in the range of 0.25 to 0.75 (Cerac, Inc.) by CSS at 

substrate temperatures in the range of 400° to 580° C in He/O2 ambient. ZnTe films (~ 300 nm 

thick) were deposited by RF sputtering at 350° C. Figure 1 is a cross-section of the samples used 

for the interdiffusion experiments. 

CdTe (CSS) 

ZnTe (CSS, RF) 

CdTe (CSS) 

Substrate 

Figure 1. Cross-section of the samples used for interdiffusion studies 

Post-deposition heat treatment of the samples was carried out at temperatures in the range of 

400°-440°C in the presence of CdCl2 vapor in He/O2 ambient. Samples were analyzed using the 

following techniques: X-ray diffraction (XRD), atomic force microscopy (AFM), auger electron 

spectroscopy (AES) and secondary ion mass spectrometry (SIMS). 

RESULTS 

Films deposited from CdTe/ZnTe powders and CdZnTe alloys 

Films deposited using mixed powders of CdTe and ZnTe for different deposition 

parameters mentioned below did not show the presence of Zn, as measured by AES. This could 

be explained by the two factors: First, vapor pressure of ZnTe is almost an order of magnitude 

lower than CdTe in the source temperature range of 650°-700°C; second, both CdTe and ZnTe 

decompose in elemental form, and the sticking coefficient of Zn is considerably lower than Cd at 

substrate temperatures over 400°C. We varied the source composition from a ZnTe:CdTe ratio 

of 1: 1 to 20:1 with no success in incorporating Zn. 

To circumvent the problem due to the vapor pressure difference between CdTe and ZnTe, 

we fabricated films using pre-alloyed CdZnTe powder sources. These alloys were prepared by 

Cerac, Inc., by sintering the powders of CdTe and ZnTe at temperatures above 1000°C. We 

confirmed that these alloys were single phase by XRD analysis. Although we have not measured 

the vapor composition, the deposition of a considerable amount of Zn on the cooler parts of the 

reactor during deposition indicates that the preformed alloys may be evaporating almost 

congruently. We did not see similar deposits when we used mixed powders of CdTe and ZnTe 

with similar Zn content in the source and identical deposition conditions. 

2
We varied the source temperature in the range of 640° to 700°C and the substrate 

temperature from 400° to 580°C and found out that the Zn content in the resulting films was 

always less than 5%. Because there was a considerable amount of Zn evaporating from the 

CdZnTe alloy sources, we tried to confine the vapor by placing spacers on all four sides between 

the source and substrate. We did not see a noticeable increase in Zn content in the film. In the 

future, we will try to increase the partial pressure of Zn during deposition to increase Zn content 

in the films. 

Thin film couples of CdTe and ZnTe 

We attempted to form a CdZnTe alloy by interdiffusion of CdTe and ZnTe layers. We 

used the structure shown in Figure 1 for these experiments, where the substrate was borosilicate 

glass coated with SnO2 and CdS. Here, we present results on two types of samples: In the first, 

the top layers of CdTe were deposited at 580°C (HT sample) whereas in second sample, top 

layer of CdTe was deposited at 400°C (LT sample). In both cases, the bottom CdTe layer was 

deposited at 580°C. We analyzed both as-deposited and CdCl2-treated samples. We show 

representative AFM images of as-deposited and CdCl2-treated samples in Figure 2. 

As-deposited samples, grown at 580°C, show the presence of boulders on the surface that 

disappear after CdCl2 heat treatment. We did not observe these structures on samples deposited 

at lower temperatures. The samples after CdCl2 heat treatment also showed a gradual increase in 

the grain size with increase in the temperature from 400° to 440°C for both sets of samples. 

AES depth profile spectra of HT samples before and after CdCl2 treatment are presented 

in Figures 3 and 4. Even though the top layer of the structure is CdTe, we see intermixing of Cd 

and Zn, as a result of high temperature deposition at 580°C. We also see considerable 

concentration of oxygen in this sample, which is comparable to the quantity of Zn in the sample. 

Another interesting fact is that the profiles of Zn and O and of Cd and Te track each other. We 

also find diffusion of Zn and Te in the CdS layer, with slight diffusion of S in the CdTe layer. 

We do not see the distinct CdTe/ZnTe/CdTe structure for the as deposited sample in Figure 3. 

Figure 2. AFM (1μm) of as deposited (left) and CdCl2-treated (440°C) samples. 

3
0 

10 

20 

30 

40 

50 

60 

70 

0 4 8 10 

Sputering time (min) 

O 

Cd 

S 

Sn 

O 

Zn 

Te 

2 6 

Figure 3. AES data for as-deposited HT sample. 

After CdCl2 heat treatment (Figure 4), there is further interdiffusion, with Zn moving 

toward the front surface and the spreading of Cd and Te toward the center of the sample. We 

still see high concentration of O2 and the profiles of Zn tracking that of O. We also see 

significant Cl concentration toward the front of the sample. 

AES data for the LT samples are presented in Figures 5 and 6. Data for the as-deposited 

sample (Figure 5) shows a distinct 3-layer structure, which is completely different than the data 

for the HT sample (Figure 3). 

In addition, the Cd spectra in the top and bottom layer exhibit different binding energies, 

implying different chemical bonding for CdTe deposited at 580°C (bottom layer) and 400°C (top 

layer). We see similar differences for Te in top CdTe layer and in ZnTe and bottom CdTe layers. 

The Zn spectra in the middle ZnTe layer have different binding energies compared to the 

interdiffused Zn in the top and bottom CdTe layers. AES data for the CdCl2-treated sample 

(figure 6) show extensive interdiffusion, with no signs of distinct layers. There is an 

0 

10 

20 

30 

40 

50 

60 

70 

0 4 6 8 10 

Sputtering time (min) 

Te 

Zn 

Cd 

S 

Sn 

O 

O 

Cl 

2 

Figure 4. AES data for 420°C CdCl2-treated HT sample. 

4
0 

10 

20 

30 

40 

50 

60 

70 

0 

Sputtering time (min) 

Sn 

Cd

Zn 

S 

Cd 

Te Te 

Zn 

O 

2 4 6 8 

Figure 5. AES data for as-deposited LT sample. 

accumulation of Zn in the front with a gradual decrease in the bulk of the film, tailing off at the 

CdS layer. This sample also shows behavior similar to samples deposited at higher temperature; 

the amount of oxygen in the layer is comparable to Zn, and profiles of Zn and O have the same 

shape. In all the samples, we find S confined within the CdS layer in the substrate. 

DISCUSSION 

AES data presented here shows the effects of three factors: deposition temperature, CdCl2 

heat treatment, and oxygen. The as-deposited LT sample shows limited interdiffusion, with clear 

demarcation of individual layers. The bonding energy of Zn in the ZnTe layer, deposited by RF 

sputtering in argon atmosphere, may be representative of Zn to Te bonding. The change of the 

bonding energy of Zn in the top and bottom CdTe layers, both of which are deposited in O2-

containing atmosphere, may be due to Zn bonded to oxygen. The LT sample after CdCl2 

0 

10 

20 

30 

40 

50 

60 

70 

0 5 10 15 

Sputering time (min) 

O 

Zn 

O S 

Te Sn

Cd 

Figure 6. AES data for 420°C CdCl2-treated LT sample. 

5
treatment in He/O2 ambient at 400°C has similar Zn and O profiles indicating that all the Zn in 

this layer is bonded to O forming ZnO. We also see Cd and Te profiles to be similar and the 

concentrations comparable to each other. This indicates that these are present as a CdTe 

compound. For the samples treated at higher temperatures, we find a gradual decrease in Zn 

concentration, with Zn and O having similar features. This may be due to the reaction between 

Cl2 and ZnO, leading to the formation of ZnCl2, thereby losing Zn from the film. The HT 

sample shows the oxidation phenomena more distinctly. Here, even for as-deposited samples, 

we find an accumulation of Zn in front. Thus, during the high-temperature growth of CdTe on 

the ZnTe surface in oxygen ambient (1 torr O2/15 torr He), oxygen reacts with ZnTe at 580°C, 

forming ZnO. After CdCl2 heat treatment, we see further movement of Zn toward the front, as 

well as reduction in Zn/O concentrations, again indicating the loss of Zn by ZnCl2 formation. 

The results show that due to high affinity of Zn to oxygen, the presence of O2 during the 

deposition and/or postdeposition CdCl2 heat treatment results in the oxidation of Zn. Even the 

small amount of oxygen during deposition (1 torr) is sufficient to oxidize all the Zn, particularly 

at high temperature. Thus, the attempt to form the CdZnTe alloy is in fact producing a twophase mixture of CdTe and ZnO. XRD results show the presence of two phases for the samples 

where we have seen significant Zn accumulation at the surface. 

One can explain performance degradation of CdZnTe devices as compared to CdTe 

devices processed under identical conditions. During CdCl2 heat treatment in oxygen 

atmosphere, Zn is oxidized, resulting in a mixed layer of p-type CdTe mixed with n-type ZnO in 

the bulk. In addition, there may be an accumulation of n-type ZnO at the surface, forming a 

junction at the back contact that is opposing the main junction at the CdS interface. 

CONCLUSIONS 

Use of oxygen in any processing step of CdZnTe based devices results in oxidation of Zn, 

thereby decomposing the alloy or preventing the of the alloy. The oxidation process is enhanced 

by high processing temperatures and CdCl2 ambient. CdCl2 vapor also leaches out Zn by the 

reaction with Cl. The principal reason for the poor performance of present-day CdZnTe solar 

cells may be the result of using processing steps that work well for CdTe devices, but are 

inadequate for CdZnTe devices. The presence of ZnO in the absorber and at the back contact of 

these devices can explain severe performance degradation. We need to develop oxygen-free 

device processing to improve device performance of CdZnTe devices. 

ACKNOWLEDGEMENTS 

We thank Dr. Xiaonan Li for SnO2 depositions and Mr. Bobby To for EPMA 

measurements. This work was supported under DOE Contract # DE-AC-99GO10337. 

REFERENCES 

1 X. Wu et al., Proc. Of 17th European PVSEC, p 995 (2001). 

2 K. Ramanathan et al., to be published in Prog. In Photovoltaics (June, 2003). 

3 T. Coutts et al., Technical Digest of 12th PVSEC, p 277 (2001). 

4 B. McCandless, Proc. Of 29th IEEE PVSC, p 488 (2002). 

5 T. Chu et al., J Appl. Phys. 71 (11), p 5635 (1992).

6
REPORT DOCUMENTATION PAGE Form Approved 

OMB NO. 0704-0188 

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources,

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson

Davis Highway, Suite 1204, Arlington, VA 22202-4302, and to the Office of Management and Budget, Paperwork Reduction Project (0704-0188), Washington, DC 20503. 

1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 

April 2003 

3. REPORT TYPE AND DATES COVERED 

Conference Paper 

4. TITLE AND SUBTITLE 

Investigation of CdZnTe for Thin-Film Tandem Solar Cell Applications 5. FUNDING NUMBERS 

PVP34201 

6. AUTHOR(S) 

R. Dhere, T. Gessert, J. Zhou, S. Asher, J. Pankow, and H. Moutinho 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 

National Renewable Energy Laboratory 

1617 Cole Blvd. 

Golden, CO 80401-3393 

8. PERFORMING ORGANIZATION 

REPORT NUMBER 

NREL/CP-520-33965 

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSORING/MONITORING 

AGENCY REPORT NUMBER 

11. SUPPLEMENTARY NOTES 

12a. DISTRIBUTION/AVAILABILITY STATEMENT 

National Technical Information Service 

U.S. Department of Commerce 

5285 Port Royal Road 

Springfield, VA 22161 

12b. DISTRIBUTION CODE 

13. ABSTRACT (Maximum 200 words) Modeling of two-junction tandem devices shows that for optimal device performance, the 

bandgap of the top cell should be around 1.6-1.8 eV. CdZnTe alloys can be tailored to yield bandgaps in the desired 

range. In this study, we considered were used to fabricate these films, using close-spaced sublimation (CSS) and radiofrequency sputtering (RFS) techniques. In the first approach, we used mixed powders of CdTe and ZnTe as the source for 

film deposition by CSS. Even for the ZnTe/CdTe (95:5 ratio) source material, the deposited films were entirely CdTe due to 

higher vapor pressure of CdTe. In the second approach, we used pre-alloyed CdZnTe powders (CERAC, Inc.) as the 

source. Due to the lower sticking coefficient of Zn, even for the source composition of 75% Zn, these films contained very 

low quantities of Zn (~5%). We tried unsuccessfully to increase the Zn content in the films by confining Zn vapor by 

enclosing the region between the source and substrate, reducing the substrate temperature to 400°C, and adjusting the 

source/substance distance. Finally, we used thin-film couples consisting of 300-nm-thick CdTe deposited by CSS and 300-

nm-thick ZnTe deposited by RFS; the samples were then heat-treated in cadmium chloride vapor. Compositional analysis 

of the samples showed extensive interdiffusion of Cd and Zn for the annealed samples. We will present the data on the 

various stack configurations of CdTe and ZnTe, the effect of different post-deposition anneals, the effect of oxygen on the 

interdiffusion and alloy formation and its possible correlation to the device performance degradation. 

14. SUBJECT TERMS 

PV; CdZnTe; tandem device; bandgap; close-spaced sublimation (CSS); radiofrequency sputtering (RFS); thin-film; couple; post-deposition anneal; interdiffusion; 

atomic force microscopy; X-ray diffraction (XRD); auger electron spectroscopy (AES); 

secondary ion mass spectrometry (SIMS); 

15. NUMBER OF PAGES 

16. PRICE CODE 

17. SECURITY CLASSIFICATION 

OF REPORT 

Unclassified 

18. SECURITY CLASSIFICATION 

OF THIS PAGE 

Unclassified 

19. SECURITY CLASSIFICATION 

OF ABSTRACT 

Unclassified 

20. LIMITATION OF ABSTRACT 

UL 

NSN 7540-01-280-5500 Standard Form 298 (Rev. 2-89) 

Prescribed by ANSI Std. Z39-18 

298-102