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Investigation of Cd1-XMgxTe 

Alloys for Tandem Solar Cell 

Applications 

Preprint 

R. Dhere, K. Ramanathan, J. Scharf, 

H. Moutinho, B. To, A. Duda, and R. Noufi 

Presented at the 2006 IEEE 4th World Conference on 

Photovoltaic Energy Conversion (WCPEC-4) 

Waikoloa, Hawaii 

May 7–12, 2006 

Conference Paper 

NREL/CP-520-39899 

May 2006 
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INVESTIGATION OF Cd1-xMgxTe ALLOYS FOR TANDEM SOLAR CELL 

APPLICATIONS1

Ramesh Dhere, Kannan Ramanathan, John Scharf, Helio Moutinho, Bobby To, Anna Duda, and Rommel Noufi 

National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401, USA 

ABSTRACT 

Theoretical modeling of two-junction tandem solar 

cells shows that for optimal device performance, the 

bandgap of the top cell should be in the range of 1.6 to 1.8 

eV. Cd1-xMgxTe (CMT) alloys have a lattice constant close 

to that of CdTe, and the addition of a small amount of Mg 

changes the bandgap considerably. In this paper, we 

present our work on developing CMT for solar cell 

applications. CMT films were prepared by vacuum 

deposition with co-evaporation of CdTe and Mg on 

substrates heated to 300°-400°C. Films with a 

composition in the range of x = 0 to 0.66 were fabricated, 

and optical analysis of the films showed that the bandgap 

of the samples ranged from 1.5 to 2.3 eV and varied 

linearly with composition. For the fabrication of devices 

using these alloy films, we also investigated the effect of 

post-depostion CdCl2 heat treatment. We have 

investigated junctions between CdS and CMT alloys in the 

bandgap range of 1.5 to 1.8 eV for tandem cell 

applications. We have also worked on the ohmic contacts 

to the CMT alloy films using Cu/Au bilayers, and the 

preliminary data shows a significant effect of the contact 

processing on the device performance. We present the 

results on the materials properties of these alloys and the 

effect of post deposition treatments on the film properties 

and device performance for different alloy compositions 

and compare them to similar CdTe devices to determine 

their applicability in tandem solar cell applications. 

INTRODUCTION 

 Theoretical modeling of two-junction tandem solar 

cells shows that for optimal device performance, the 

bandgap of the top cell should be in the range of 1.6 to 1.8 

eV [1]. Very few binary compound semiconductors are 

found within this bandgap range, so one needs to look at 

the multinary alloys to find suitable candidates. An 

obvious choice is to explore alloys based on CdTe to 

exploit the knowledge base developed for CdTe solar 

cells. In our previous work[1], we investigated Cd1-xZnxTe 

(CZT) thin films prepared by interdiffusion of CdTe and 

ZnTe thin films. Although it was possible to fabricate CZT 

alloy films in the desired composition range, we found that 

high reactivity of Zn with oxygen and chlorine results in the 

decomposition of CZT films. During post-deposition 

processing steps, such as CdCl2 heat treatment in oxygen 

ambient, Zn from the alloy is leached out to form either 

volatile ZnCl2 or ZnO. Cd1-xMgxTe (CMT) alloys offer a 

wide bandgap range and have not been explored. CMT 

alloys have a lattice constant close to that of CdTe, and 

the addition of a small amount of Mg changes the 

bandgap considerably. A wide range of bandgaps can be 

obtained with CMT alloys with only a minimal perturbation 

to the lattice constant. In addition, this alloy system may 

be useful for understanding the mechanisms that limit the 

open-circuit voltage (Voc) of CdTe to 850 mV. At present, 

CdTe solar cells have achieved efficiencies of 16.5% [2], 

and one obvious avenue towards the efficiency 

improvement is through improved Voc. There is a very 

limited amount of published work available on CMT and 

most of it is related to bulk material and none for 

polycrystalline thin films. 

A number of approaches were tried for the deposition 

of the polycrystalline CMT alloy thin films. Co-deposition 

using CdTe and MgCl2 produced reasonable CMT films, 

but the films were irreproducible due to the inadequate 

control of the MgCl2 source. Deposition from pre-alloyed 

Cd1-xMgxTe source material produced CdTe films with 

negligible Mg due to fractionation of source material. Codeposition of CdTe and MgF2 was attempted, but was 

unsuccessful as MgF2 did not decompose completely at 

the deposition temperatures used and resulted in twophase films of CMT and MgF2. Co-evaporation of CdTe 

and Mg was found to be the most successful approach for 

deposition of CMT films. Due to the relative positioning of 

the substrate and CdTe and Mg sources, Mg 

concentration decreased as the distance from the Mg 

source increased. This facilitated deposition of a wide 

range of compositions in a single deposition. Here we 

report on the development of polycrystalline CMT for solar 

cell applications with an emphasis on alloys in the range of 

1.5 to 1.8 eV. 

________________________________________________________________________________________________ 

1 This work has been authored by an employee or employees of the Midwest Research Institute under Contract No. DE-AC36-

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publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish 

or reproduce the published form of this work, or allow others to do so, for United States Government purposes.

1
 

80

60

40

20

0

Transmission (%)

500 600 700 800

Wavelength (nm)

Fig. 1. Transmission spectra of as deposited 

Cd1-xMgxTe alloy thin films. 

EXPERIMENTAL PROCEDURE 

Two types of substrate were used in this study: 

Glass substrates were used for preparing the samples for 

the analysis of materials properties and glass/SnO2/CdS 

substrates were used for the device fabrication. CMT 

films were deposited by co-evaporation of CdTe and Mg 

using Radak II effusion cells in a vacuum system with a 

base pressure of 2x10-6 torr, at substrate temperature of 

300°-400°C on 3”x3” substrates. CMT films with a 

thickness of 0.5-1 μm were used for the materials analysis 

and films with a thickness of 3-4 μm were used for the 

device fabrication. Post-deposition vapor CdCl2 heat 

treatment was carried out in the He/O2 ambient in the 

temperature range of 380°-420° C for 15 min. Prior to the 

application of the contacts, samples were etched in 0.25% 

bromine/methanol (BM) etch for 2 seconds or nitricphosphoric (NP) etch for 10 seconds. For contacts, 10 

nm of Cu and 180 nm of Au were deposited by 

evaporation in a vacuum system at pressure of 2x10-6 torr. 

Contacts were annealed in a tube furnace with He flow of 

100 cc/min at 175°-250°C. 

Reflectance (R) and transmittance (T) of the films were 

measured using a Cary 5 spectrophotometer equipped 

with an integrating sphere. Absorption coefficient (α) was 

calculated using R and T and the film thickness. Optical 

bandgap was calculated from the x-intercept of (αhν)

2

 vs 

hν plots. Structural analysis was carried out by X-ray 

diffraction measurements using Scintag X-ray 

diffractometer model Xgen-4000. Composition of the films 

was determined by Electron Probe Micro Analysis (EPMA) 

using a JEOL model 8900L Superprobe at 10 and 20 kV. 

Beam size was 20 μm and the beam current was 

maintained at 2.5x10-8 A. Solar cell performance was 

measured using an Oriel AM 1.5 solar simulator. 

RESULTS AND DISCUSSION 

 Transmission spectra of a number of as-deposited 

samples deposited on glass substrates are presented in 

Fig. 1. Due to the positioning of CdTe and Mg sources, 

the samples showed a range of compositions across a 3” 

width. A typical sample covered a range of approximately 

30 nm, deduced from the shift in the absorption edge. 

Thus the data presented here are combined from a 

number of depositions. The absorption edge of the films 

ranged from a CdTe bandgap at 840 to 550 nm. Optical 

bandgaps vs Mg content of the alloy films determined from 

EPMA are presented in Fig. 2. It should be noted that 

when a sample contains a range of compositions, as in the 

case of the samples used in this study, the deduced 

optical bandgap corresponds to the lowest-bandgap alloy 

within measured area. On the other hand, the 

composition measured by EPMA is an average of 

compositions within the examined region. Thus the Mg 

content of the alloy corresponding to the bandgaps 

presented in the Fig. 2 is likely lower than indicated. The 

alloy films cover a wide range of bandgaps from 1.5 eV to 

2.3 eV. The optical properties of the CMT alloys are well 

behaved and can be fitted by a linear fit given by: 

 

2.4

2.2

2.0

1.8

1.6

1.4

Energy gap (eV)

0.0 0.2 0.4 0.6 0.8

Mg/(Cd+Mg)

Fig. 2. Energy gap as a function of Mg content for 

Cd1-xMgxTe alloy thin films. 

Eg = 1.1x + 1.54 eV (1) 

 In addition to the thin samples (0.5–1 μm), we have 

also deposited films with thickness in the 3-4 μm range. 

These thicker films show high subgap transmission 

(>60%) indicating excellent material quality. High subgap 

transmission is essential for the use of these films in 

tandem solar cells. XRD data show that these films are 

single phase with systematic variation of lattice constant. 

2
All the Cd1-xMgxTe alloy films deposited at substrate 

temperature of 300°C exhibit the sphalerite structure 

characteristic of CdTe, and only a small variation in lattice 

constant as expected for these alloys. Although we have 

been able to fabricate alloys in a much wider range, the 

present study is focused on the alloys between 1.5 and 

1.8 eV to determine their usefulness in tandem solar cell 

applications. 

The samples for the device fabrication were 

deposited on glass/SnO2/CdS substrates at a substrate 

temperature of 400°C. Devices fabricated from asdeposited CMT films showed poor device performance, so 

we used the same post-deposition CdCl2 heat treatment 

(HT) used in the fabrication of CdTe-based solar cells. 

CMT films treated with CdCl2 HT up to a temperature 

of 400°C did not show any changes in their properties. 

Samples treated at higher temperatures showed 

recrystallization and grain growth. Transmission spectra 

of a sample before and after CdCl2 HT at 420°C are 

shown in Fig. 3. Heat-treated sample were etched with 

NP etch (top) and BM etch (bottom) prior to the 

measurements. Wavelength-independent transmission 

loss at higher wavelengths results from the absorption in 

the Te-rich layer formed during NP etch. A slight shoulder 

in the spectra seen at 850 nm indicates the loss of Mg 

from top film surface during CdCl2 HT, resulting in a thin 

CdTe layer. Shift of the absorption edge may be the result 

of some Mg loss from the CMT alloy layer during HT. 

Partial decomposition of the CMT layer happens during 

the recrystallization process and is a function of HT 

parameters. Similar features are seen for the sample 

etched with BM etch, except that there is minimal 

transmission loss at higher wavelengths as the BM is a 

more stoichiometric etch and does not create a thick Terich layer. Optimization of HT process and pre-contact 

etching should mitigate these problems. CMT films are 

more resistant to these problems compared to CZT films, 

in which high reactivity of Zn results in a rapid loss of Zn 

during CdCl2 HT [1]. 

0

10

20

30

40

50

60

70

80

750 800 850 900 950 1000

Wavelength, nm

T (%) 

0

10

20

30

40

50

60

70

80

750 800 850 900 950 1000

Wavelength, nm

T (%) 

Fig. 3. Transmission spectra of a CMT device with NP 

(top) and BM (bottom) etch before (solid line) and after 

(dotted line) 420°C CdCl2 HT. 

Formation of ohmic contacts to p-type CdTe is 

problematic, and several processing schemes involving 

the use of Cu have been developed. For the CMT 

devices, we tried several approaches. Contacts using 

graphite paste doped with Hg and Cu resulted in poor 

device performance. Based on the preliminary work 

carried out so far, we have chosen a bilayer of 10-nm Cu 

and 180-nm Au. This worked satisfactorily, and we have 

employed it for the devices processed in this study. We 

processed devices with and without etching prior to 

contact application and found that the BM-etched devices 

performed better, so we have employed BM etch for the 

devices, unless specified otherwise. 

Contact annealing temperature has a major influence 

on device performance. To illustrate this effect, J-V 

characteristics in dark and illumination of a device with 

Cu/Au contact, annealed at various temperatures, are 

presented in Fig. 4 and the device parameters are listed in 

Table 1. The device with no anneal was from the same 

substrate, and anneals were carried out on the same 

device. 

Table 1. Device parameters of a CMT (Eg= 1.55 

eV) device after annealing at different temperatures 

 No 175° C 200° C 225° C 

 anneal 30 min 30 min 30 min 

Voc, mV 224 440 597 629 

Jsc, mA/cm2 19.2 17.7 17.9 17.5 

FF % 28.6 36.2 44.2 49.7 

Eff [%] 1.23 2.81 4.71 5.47 

The diode characteristics of the device before 

contact anneal are poor. There is progressive 

improvement in both the diode characteristics and the 

device performance with successive contact anneals. 

Dark J-V characteristics show that the reverse diode 

saturation current for the device after 225° C anneal is the 

minimum, and the device performance is hampered by 

high series resistance. The performance of the CMT 

devices can be further improved by contact optimization. 

3
We have fabricated devices using CMT alloy films 

with bandgaps in 1.5 to 1.8 eV range. J-V characteristics 

of some of the devices with different Mg contents are 

shown in Fig. 5. We have also included a CdTe device 

deposited in the same system and processed under 

similar conditions for comparison. Device parameters of 

these devices are listed in Table 2. 

Table 2 . Device parameters of CMT cells with different 

compositions 

 CdTe CMT CMT CMT CMT 

 

1.55 

eV 

1.57 

eV 

1.6 

eV 

1.7 

eV 

Voc, 

mV 726 629 636 649 800 

Jsc, 

mA/cm2 22.9 17.5 17.9 17.1 16.4 

FF 

[%] 56.3 49.7 45.5 43.9 31.1 

Eff 

[%] 9.35 5.47 5.17 4.87 4.08 

We have been able to achieve device efficiencies 

over 5% for various alloy compositions. Although the 

performance of CdTe devices processed simultaneously is 

better, CMT devices have consistently given higher Voc,

with the highest Voc of 800 mV resulting from a CMT 

device with a 1.7-eV bandgap. With further improvements 

in the post-deposition HT and improved back contact, we 

should be able to enhance the device performance of the 

CMT devices. 

Fig. 5. Light J-V plots of CMT devices with different 

optical bandgaps. 

Fig. 4. Light (top) and dark (bottom) J-V plots of a 

CMT device after different contact anneals. 

CONCLUSIONS 

We have demonstrated the fabrication of 

polycrystalline CMT thin films in a wide composition range. 

Optical and structural properties of these samples show 

that CMT alloy film are well behaved in the composition 

range covered in this study. Devices fabricated using 

these films show promising device performance and attest 

to the device quality of polycrystalline CMT thin films 

ACKNOWLEDGEMENTS 

This work is supported and funded under DOE 

Contract No. DE-AC36-99GO10337. We would like to 

thank T. Moriarty for QE measurements, X. Li for SnO2

thin film depositions, and X. Wu, T. Coutts, D. Albin, and 

B. McCandless for useful discussions. 

REFERENCES 

[1] R. Dhere, T. Gessert, J. Zhou, J. Pakow, S. Asher, and 

H. Moutinho, Phys. Stat. Sol. (b), vol. 241, no. 3, p.771 

(2004). 

[2] X. Wu, J. Keane, R.G. Dhere, C. Dehart, D.S. Albin, A. 

Duda, T.A. Gessert, S. Asher, D.H. Levi, and P. Sheldon, 

Proceedings of 17th European PVSEC, Munich, Germany, 

p. 595 (Oct. 2001) 

4