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Contract No. DE-AC36-08GO28308

CdS/CdTe Solar Cells 

Containing Directly Deposited 

CdSxTe1-x Alloy Layers

Preprint

Joel N. Duenow, Ramesh G. Dhere, 

Helio R. Moutinho, Bobby To, Joel W. Pankow, 

Darius Kuciauskas, and Timothy A. Gessert

Presented at the 37th IEEE Photovoltaic Specialists Conference 

(PVSC 37)

Seattle, Washington

June 19-24, 2011

Conference Paper

NREL/CP-5200-50755

July 2011
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1 

CdS/CdTe SOLAR CELLS CONTAINING DIRECTLY DEPOSITED

CdSxTe1-x ALLOY LAYERS

Joel N. Duenow, Ramesh G. Dhere, Helio R. Moutinho, Bobby To, Joel W. Pankow, Darius Kuciauskas,

and Timothy A. Gessert

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

ABSTRACT

A CdSxTe1-x layer forms by interdiffusion of CdS and CdTe 

during the fabrication of thin-film CdTe photovoltaic (PV) 

devices. The CdSxTe1-x layer is thought to be important 

because it relieves strain at the CdS/CdTe interface that 

would otherwise exist due to the 10% lattice mismatch 

between these two materials. Our previous work [1] has 

indicated that the electrical junction is located in this 

interdiffused CdSxTe1-x region. Further understanding, 

however, is essential to predict the role of this CdSxTe1-x

layer in the operation of CdS/CdTe devices. In this study, 

CdSxTe1-x alloy films were deposited by radio-frequency

(RF) magnetron sputtering and co-evaporation from CdTe 

and CdS sources. Both RF-magnetron-sputtered and coevaporated CdSxTe1-x films of lower S content (x<0.3) 

have a cubic zincblende (ZB) structure akin to CdTe, 

whereas those of higher S content have a hexagonal 

wurtzite (WZ) structure like that of CdS. Films become 

less preferentially oriented as a result of a CdCl2 heat 

treatment (HT) at ~400°C for 5 min. Films sputtered in a 

1% O2/Ar ambient are amorphous as deposited, but show 

CdTe ZB, CdS WZ, and CdTe oxide phases after a CdCl2

HT. Films sputtered in O2 partial pressure have a much 

wider bandgap than expected. This may be explained by 

nanocrystalline size effects seen previously [2] for 

sputtered oxygenated CdS (CdS:O) films. Initial PV 

device results show that the introduction of a directlydeposited CdSxTe1-x alloy layer into the device structure 

produces devices of comparable performance to those 

without the alloy layer when a CdCl2 HT is performed. 

Further investigation is required to determine whether the 

CdCl2 heat treatment step can be altered or eliminated 

through direct deposition of the alloy layer.

INTRODUCTION

A CdSxTe1-x layer forms by interdiffusion of CdS and CdTe 

during the fabrication of thin-film CdTe PV devices in the 

standard superstrate configuration. High-temperature 

processing steps such as the close-spaced sublimation of 

CdTe and the post-deposition CdCl2 HT contribute to 

formation of this alloy [1]. The CdSxTe1-x layer is thought to 

be important in fabricating high-performance CdTe 

devices because it relieves strain at the CdS/CdTe 

interface that would otherwise exist due to the large lattice 

mismatch (~10%) between these two materials. Our 

previous work indicated that the electrical junction is 

located in this interdiffused CdSxTe1-x region between a 

structurally compatible Te-rich n-type CdSxTe1-x alloy and 

p-type CdTe [1, 3]. 

The CdSxTe1-x alloy has been found to follow Vegard’s law 

[4], such that lattice parameter values obtained from X-ray 

diffraction measurements can be used to calculate the 

mole fraction (x) for different phases of CdSxTe1-x. The 

bandgap (BG) of CdSxTe1-x has been described as a 

quadratic function of x [5], with values decreasing below 

the CdTe BG value of 1.5 eV, to as low as 1.41 eV at 

x~0.3, before increasing at higher x values. The alloy 

system also exhibits a miscibility gap (two-phase region) in 

which both a CdTe-rich zincblende (ZB) phase and CdSrich wurtzite (WZ) phase may be present simultaneously 

at equilibrium. The composition of each phase in the twophase region is the same as in the corresponding singlephase regions at the edges of the miscibility gap, with the 

relative quantity of each phase present varying with x [6]. 

Single-phase films, however, have been grown within the 

miscibility gap, implying non-equilibrium growth methods 

were used. The phases generally separated after a CdCl2

HT [6-8]. We found similar behavior in this study.

Further understanding of CdSxTe1-x alloys is essential to 

predict the role of the CdSxTe1-x layer in the operation of 

CdS/CdTe devices. In this study, we investigate this alloy 

layer by depositing CdSxTe1-x films using two methods. 

We also present preliminary results in which the alloy layer 

has been implemented in PV devices.

EXPERIMENTAL DETAILS

We deposited CdSxTe1-x films by radio-frequency (RF) 

magnetron sputtering using targets of three compositions: 

10/90, 25/75, and 60/40 wt.% CdS/CdTe. Films were 

deposited at room temperature (RT; no intentional 

heating) and at 300°C. Because little difference was seen 

between films grown at RT and at 300°C, only the 300°C 

results are presented here. In addition, two deposition 

ambients were used—100% Ar and 1% O2/Ar. The ratio 

was measured using an ion gauge. Films were also 

deposited by co-evaporation from CdTe and CdS sources 

using Radak II effusion cells. The geometry of the coevaporation system enabled us to obtain a range of 

compositions during a single deposition. The films were 

deposited onto three different substrates—Corning 7059 

glass, Corning 7059 glass/450-nm SnO2:F/150-nm SnO2, 

and Corning 7059 glass/SnO2:F/SnO2/125-nm sputtered 

CdS:O—to be amenable to the different types of 

characterization performed on the films. Reflectance and 

transmittance measurements were performed using a 

Cary 6000i UV-Vis-NIR spectrophotometer. Electronprobe microanalysis (EPMA) was performed using a beam 

energy of 5 kV to obtain film composition. X-ray diffraction 
2 

(XRD; Rigaku Ultima IV) θ-2θ measurements were 

performed using Cu Kα radiation to examine the structure 

of the CdSxTe1-x films. Electron backscatter diffraction 

(EBSD; FEI FEG SEM Nova 630 NanoSEM with an EDAX 

Pegasus/Hikari A40 EDS/EBSD system) measurements 

were performed to examine grain orientation and size for 

selected films. PV devices were grown using the following 

structure: Corning 7059 glass / 450 nm SnO2:F by 

chemical vapor deposition (CVD) / 400 nm SnO2 by CVD / 

80-125 nm CdS / 300 nm CdSxTe1-x / 3 μm CdTe by 

evaporation at 400°C / 500 nm sputtered ZnTe:Cu 

(containing 2 wt.% Cu) at 300°C / 500 nm sputtered Ti.

RESULTS AND DISCUSSION

Composition of the ~150-nm-thick sputtered CdSxTe1-x

alloy films was measured using EPMA (Figure 1). Films 

were grown in 100% Ar and 1% O2/Ar both before and 

after a 5-min vapor CdCl2 HT at 400°C. Films grown from 

the 10/90 wt.% CdS/CdTe target show little compositional 

change between the different deposition ambients and 

after the CdCl2 HT. Films grown from the 25/75 and 60/40 

wt.% CdS/CdTe targets in the 100% Ar ambient display a 

slight enrichment in S (or, equivalently, a loss of Te) after 

the CdCl2 HT. All films appear S-deficient, however, 

compared to the manufacturer’s stated target composition. 

This may be due to differences in sticking coefficient on 

the substrate of the S and Te species. Two ~300-nm-thick 

evaporated CdSxTe1-x films (Figure 2) were deposited 

using different combinations of CdS and CdTe effusion 

cell temperatures. The as-deposited films (solid lines) 

show the expected gradient in x across the substrate. The 

evaporated films also show a significant relative 

enhancement in S (or loss of Te) after the CdCl2 HT.

CdSxTe1-x alloy film structure was investigated using XRD. 

Figure 3 shows θ-2θ scans of films grown from the 10/90 

wt.% CdS/CdTe target in the 100% Ar and 1% O2/Ar 

ambients both before and after a CdCl2 HT. Films 

examined using XRD were deposited on Corning 7059 

glass/450-nm SnO2:F/150-nm SnO2/125-nm sputtered 

CdS:O substrates similar to those used for CdTe PV 

devices. Tetragonal SnO2, cubic ZB CdTe, and hexagonal 

WZ CdS peaks are indicated. The as-deposited film grown 

in 100% Ar shows one prominent peak, CdTe ZB (111). 

Other peaks in this spectrum are due to the SnO2 films on 

the substrate. After a CdCl2 HT is performed on this film, 

the CdTe ZB (111) peak decreases in intensity, while 

small CdTe ZB (220) and (311) peaks, and CdS WZ (100), 

(002), and (101) peaks, appear. This behavior indicates a 

decrease in preferential orientation after the CdCl2 HT. 

The film grown in 1% O2/Ar is amorphous as deposited. 

After the CdCl2 HT, however, many CdTe ZB and CdS WZ 

phases appear, in addition to prominent CdTe oxide 

phases (e.g., CdTeO3, CdTe2O5). Further work is required 

to identify which oxide phases are dominant. A summary 

of XRD results for the sputtered films is shown in Table 1. 

Films grown in 100% Ar from the 60/40 wt.% CdS/CdTe 

target contain only CdS WZ phases, both before and after 

the CdCl2 HT, whereas films of higher CdTe content in this 

ambient show both CdTe ZB and CdS WZ phases. All 

films grown in 1% O2/Ar are amorphous as deposited. The 

CdTe oxide peaks observed after the CdCl2 HT are much 

less intense for the 25/75 and 60/40 wt.% CdS/CdTe films 

than for the 10/90 wt.% film.

XRD measurements were also performed on evaporated 

CdSxTe1-x alloy films both before and after a CdCl2 HT at 

390°C. Measurements were performed at several points 

on the film because of the composition gradient. The CdTe 

ZB (111) peak was observed to shift to an intermediate 

position between it and the adjacent CdS WZ (100) peak 

as the S content increased. After the CdCl2 HT, the CdTe 

ZB (111) peak shifts toward its expected position, while 

other CdTe ZB and CdS WZ phases appear, indicating a

decrease in film preferential orientation. These data are

summarized in Table 2. The separation of the CdTe 

ZB(111) and CdS WZ (100) peaks suggests that phase 

separation occurs as a result of the CdCl2 HT.

Figure 1 Measured x values (obtained using EPMA) 

for sputtered CdSxTe1-x films. Dashed lines indicate 

the manufacturer’s stated target composition.

Figure 2 Measured x values (obtained using EPMA) 

for evaporated CdSxTe1-x films. Evaporated films were 

measured at four positions because of the inherent 

composition gradient across the substrate due to the 

deposition system geometry.
3 

Figure 3 XRD scans for sputtered CdSxTe1-x films grown from the 10/90 wt.% CdS/CdTe target. Growth ambient 

and post-deposition treatment are shown on the plot. Peak positions from the powder diffraction files 

corresponding to these materials are shown at the bottom (with corresponding file numbers shown in the upper 

right). 

Wt.% 

CdS/CdTe

100% Ar 1% O2/Ar

As Deposited After CdCl2 As Deposited After CdCl2

10/90 CdTe ZB CdTe ZB

CdS WZ Amorphous

CdTe ZB

CdS WZ

Strong oxide phases

25/75 CdTe ZB

Minor CdS WZ

CdTe ZB

CdS WZ Amorphous

CdTe ZB

CdS WZ

Oxide phases

60/40 CdS WZ CdS WZ Amorphous CdS WZ

Oxide phases

Table 1 Phases present in sputtered CdSxTe1-x films grown in 100% Ar and 1% O2/Ar before and after a CdCl2 HT.

CdS Content As Deposited After CdCl2

Lower

(x = 0.01 to 0.12 as deposited) CdTe ZB (111) CdTe ZBMinor CdS WZ

Higher

(x = 0.12 to 0.23 as deposited)

CdTe ZB (111), shifted toward

CdS WZ (100) at high S

CdTe ZB

CdS WZ

Table 2 Phases present in evaporated CdSxTe1-x films before and after a CdCl2 HT.

Analysis was performed to extract the lattice constants 

and mole fraction of the CdTe ZB phase from the XRD 

data. The lattice constant aCubic was determined using 

aCubic = d h2 + k

2 + l

2 , where d = λa ( ) 2sinθ is 

calculated from the measured 2θ peak position, λα = 

1.540562 Å is the wavelength of the Cu Kα radiation, and 

h, k, and l are the Miller indices for the diffraction peak. 

When multiple CdTe ZB peaks were present, the aCubic

values were plotted as a function of the Nelson-RileySinclair-Taylor (NRST) function [6],

 NRST = 1

2

cos2θ

sinθ

+

cos2θ

θ

 

  

 

  . 

Fitting a line to these points enabled a lattice constant of 

greater precision to be found by extrapolating to normal 

incidence at NRST = 0 [6]. Vegard’s law, which applies to 

the CdTe-CdS alloy system [4], was then used to 

determine the mole fraction, x, for the cubic ZB phase:

x = aCdSxTe1−x(Cubic) − aCdTe

aCdS − aCdTe

. 

For cubic phases, aCdTe = 6.481 Å and aCdS = 5.818 Å.

Figure 4 compares the x values obtained from EPMA and 

XRD for CdSxTe1-x alloy films sputtered in 100% Ar and 

subjected to a CdCl2 HT. EPMA values measure the 

composition of the whole film. At higher S contents, EPMA 

values strongly exceed those obtained using the XRD 

analysis of the CdTe cubic ZB peaks, which measures the 

composition of the ZB phase only. The difference indicates 

that the film composition is within the CdS/CdTe miscibility 
4 

Figure 4 Mole fraction (x) obtained from EPMA and 

XRD measurements for sputtered CdSxTe1-x films.

Figure 5 EBSD images for a sputtered film (upper 

panel; x = 0.059, grain size 360 nm) and an evaporated 

film (lower panel; x = 0.28, grain size 240 nm). Black 

lines indicate grain boundaries; red lines indicate ∑ 3 

twin boundaries. Both films were imaged after a CdCl2

HT. 

gap for films of the higher two S contents. The miscibility 

gap was measured to lie between x = 0.058 and x = 0.97 

[7] at temperatures of 415°C. Mole fraction 

valuesmeasured by XRD for the Te-rich cubic ZB phase in 

this study are consistent with the x = 0.058 observed by 

Jensen et al. [7]. Similar x values were observed for the 

evaporated films also (not shown).

To obtain information about grain orientation and size in 

these films, EBSD measurements were performed (Figure 

5) on CdSxTe1-x alloy films grown on Corning 

7059/SnO2:F/SnO2/CdS:O substrates. One sputtered film 

and one evaporated film were measured after a CdCl2 HT 

and a 0.5%-concentrated bromine-methanol etch for 2 s. 

Grains of both films are randomly oriented, indicated by 

the different colors of each grain. Pole figures (not shown)

confirm this random orientation. The grain size of the 

sputtered film is 360 nm, whereas that of the evaporated 

film is 240 nm. This difference may be due to the greater 

adatom surface energy of the sputtering process. The 

CdCl2 HT temperature for the sputtered film was also 10°C 

higher (400°C vs. 390°C), which may have contributed to 

increased recrystallization.

The BG of sputtered and evaporated CdSxTe1-x films was 

calculated by measuring the reflectance and 

transmittance, calculating the absorption coefficient (α) [9], 

and plotting (αhν)

2 vs. hν, where hν is the photon energy. 

The evaporated films appear to have similar BG values 

(Figure 6), indicating that these films have phase 

separated due to the miscibility gap. The sputtered films 

deposited in the 100% Ar ambient also appear to have 

values near the 1.5 eV BG of CdTe. An interesting 

phenomenon was observed for the films deposited in the 

Figure 6 Bandgap vs. mole fraction (x) for sputtered 

and evaporated CdSxTe1-x films. The black curve was 

determined by Ohata et al. [5] by fitting experimental 

data.
5 

1% O2/Ar ambient. As deposited, these films had BG 

values much higher than expected. In fact, the 60/40 wt.% 

CdS/CdTe film had a BG value higher than that of CdS 

itself. We suspect this behavior is similar to that observed 

for CdS:O by Wu et al. [2], in which CdS films were 

sputtered in O2 partial pressure. In that study, amorphous 

CdS:O films with BGs of up to 3.1 eV were deposited. The 

increase in BG was attributed to nanocrystalline quantum 

size effect behavior. We believe a similar effect is 

occurring in CdSxTe1-x films deposited in O2 partial 

pressure. After a CdCl2 HT, the low-S film decreases to a 

BG near that of CdTe, whereas the higher-S films 

decrease in BG substantially, but not to the expected 

levels. This decrease is consistent with partial grain 

recrystallization and growth occurring as a result of the 

HT, although it seems to be incomplete.

Figure 7 PV devices incorporating a 300-nm-thick 

sputtered CdSxTe1-x layer deposited from a target of 

composition 25/75 wt.% CdS/CdTe (leading to

measured x = 0.24). Open-circuit voltage (Voc), shortcircuit current (Jsc), fill factor (FF), and efficiency (Eff.)

are shown on the plot. The device in the top panel 

received no CdCl2 HT, while that in the bottom panel 

was treated at 405°C for 5 min.

Preliminary device work was performed using a CdSxTe1-x

alloy layer deposited either by sputtering or coevaporation. For devices utilizing a sputtered alloy layer, 

the film was deposited onto a window layer of 125 nm of 

sputtered CdS:O. The deposition of 3-5 μm of CdTe by 

evaporation onto substrates heated to 400°C followed. 

Devices that received no CdCl2 HT after the CdTe 

deposition all showed poor performance (Figure 7, top 

panel), despite having a pre-formed alloy region. A higher 

CdTe deposition temperature may be required to form a ptype CdTe absorber layer. In future work, we will use the 

close-spaced sublimation technique to deposit CdTe at 

higher temperatures (450-500°C) to investigate this 

possibility. In previous work, it was found that only slight

CdS/CdTe interdiffusion occurred during CdTe deposition 

in this temperature range [3]. Depositing CdTe at these

temperatures, therefore, is not expected to adversely 

affect study of the directly-deposited alloy layer in these 

devices. Devices containing the alloy layer that were 

subjected to a CdCl2 HT after the CdTe deposition (Figure 

7, lower panel) showed performance comparable to that of 

baseline devices with no alloy layer. A baseline device had 

the following parameters: Voc = 763 mV, Jsc = 20.2 

mA/cm2, FF = 59.7%, and eff. = 9.19%. The comparable 

performance of devices containing the alloy layer shows

that introduction of this directly-deposited layer is not 

fundamentally detrimental to device performance. In future 

work, we will further investigate the effects of the CdCl2 HT

on the directly-deposited alloy layer in PV devices, as the 

CdCl2 HT is known to enhance interdiffusion of CdS and 

CdTe in standard devices.

Devices were also fabricated using a CdSxTe1-x alloy layer 

deposited by co-evaporation of CdS and CdTe at a 

substrate temperature of 400°C. The mole fraction of the 

alloy film was x ≈ 0.07. This layer was followed by 

deposition of a 3-5 μm CdTe film at 400°C without 

breaking vacuum. The devices that received no CdCl2 HT

performed poorly (not shown), similar to the device seen in 

the top panel of Figure 7. After a CdCl2 HT at 405°C, 

devices utilizing ≈100 nm of chemical-bath deposited 

(CBD) CdS (Figure 8, top panel) showed performance 

comparable to baseline devices without the alloy layer

(baseline device parameters: Voc = 740 mV, Jsc = 22.6 

mA/cm2

, FF = 66.0%, eff. = 11.0%). The device utilizing 

80 nm of sputtered CdS:O (Figure 8, lower panel), 

however, showed lower performance than that of the CBD 

CdS device and the sputtered CdS:O baseline. Further 

optimization work will be performed to enhance the 

performance of devices utilizing a sputtered CdS:O 

window layer.

CONCLUSIONS

CdSxTe1-x alloy films were deposited by RF magnetron 

sputtering and co-evaporation. As-deposited sputtered 

films grown in 100% Ar from targets containing 10/90 and 

25/75 wt.% CdS/CdTe have a cubic CdTe ZB structure, 

whereas those grown from a target of 60/40 wt.% 

CdS/CdTe have a hexagonal CdS WZ structure. Films
6 

Figure 8 PV devices incorporating a 300-nm-thick 

evaporated CdSxTe1-x alloy layer of x ≈ 0.07. The 

device in the top panel contains a window layer of 

≈100 nm of CBD CdS, while the device in the bottom 

panel contains 80 nm of sputtered CdS:O.

become less preferentially oriented as a result of a vapor 

CdCl2 HT at 400°C for 5 min. Films sputtered in a 1%O2/Ar 

ambient are amorphous as deposited, but show CdTe ZB, 

CdS WZ, and CdTe oxide phases after a CdCl2 HT. 

Evaporated films primarily consist of the CdTe ZB phase 

as deposited, but the CdTe ZB (111) peak is shifted 

significantly toward the adjacent CdS WZ (100) peak for 

films of higher S content. These two peaks become 

distinct after a CdCl2 HT, indicating phase separation has 

occurred. Both sputtered and evaporated films have 

randomly oriented grains after a CdCl2 HT. The grain size 

of the sputtered film is larger than that of the evaporated 

film (360 vs. 240 nm). CdSxTe1-x alloy films sputtered in 

1% O2/Ar are amorphous as deposited and have a much 

higher bandgap than expected. This may be explained by 

nanocrystalline size effects seen previously [2] for CdS:O 

films. PV devices were fabricated using a directlydeposited CdSxTe1-x alloy film between the CdS and CdTe 

layers. Despite the presence of a pre-formed alloy layer, 

devices that received no CdCl2 treatment performed 

poorly. Higher CdTe deposition temperatures that may 

enable higher performance by ensuring p-type CdTe will 

be investigated. Devices containing the alloy layer that 

received a CdCl2 HT after the CdTe deposition in many 

cases showed comparable performance to baseline 

devices without the alloy layer, suggesting that this 

directly-deposited alloy layer is not fundamentally 

detrimental to device performance.

Future work will include additional EBSD measurements to 

identify CdSxTe1-x phases and their intermixing both before 

and after a CdCl2 HT. Auger electron spectroscopy and Xray photoelectron spectroscopy will be used to examine 

the CdTe oxides that result from the CdCl2 HT of films 

grown in 1% O2/Ar. In future PV device work, we first plan 

to replicate the existing superstrate device structure using 

directly deposited CdSxTe1-x layers in PV devices. We also 

expect to design and deposit new superstrate and 

substrate PV device structures using these directly 

deposited layers.

ACKNOWLEDGEMENTS

This work was supported under DOE Contract No. DEAC36-08-G028308 to NREL.

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