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Contract No. DE-AC36-99-GO10337 

T.A. Gessert, S. Smith, T. Moriarty, M. Young, 

S. Asher, S. Johnston, A. Duda, and C. DeHart 

National Renewable Energy Laboratory 

A.L. Fahrenbruch 

Colorado State University 

Prepared for the 31st IEEE Photovoltaics Specialists 

Conference and Exhibition 

Lake Buena Vista, Florida 

January 3–7, 2005 

February 2005 • NREL/CP-520-36472 

Evolution of CdS/CdTe Device 

Performance During Cu Diffusion
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EVOLUTION OF CdS/CdTe DEVICE PERFORMANCE DURING Cu DIFFUSION

T.A. Gessert, S. Smith, T. Moriarty, M. Young, S. Asher, S. Johnston, A. Duda, and C. DeHart 

NREL, 1617 Cole Blvd., Golden, CO 80401

A.L. Fahrenbruch, Colorado State University, Fort Collins, CO 80523 

ABSTRACT

Cu diffusion from a ZnTe:Cu/Ti back contact onto

CdS/CdTe thin-film solar cells is studied. We find if Cu 

diffusion is insufficient, the entire CdTe layer is depleted.

However, if Cu diffusion is excessive, the depletion width 

can become too narrow to provide optimum current 

collection. This analysis suggests that most contact 

processes used for CdS/CdTe devices are optimized

(often unknowingly) to result in a depletion width that

extends just far enough into the CdTe to yield the highest

possible field in the region where light absorption occurs.

Analysis of the samples with very high Cu concentration

also suggests that Cu doping of CdS may affect carrier

collection from the CdS. 

INTRODUCTION 

Understanding effects that diffusion of contact 

constituents into CdTe and CdS have on the operation

and stability of CdS/CdTe thin-film photovoltaic (PV) solar 

cells remains an important area of study. Previous

research has shown that a significant amount of Cu enters

the CdTe layer during the 400°C CdCl2 treatment, followed 

by additional Cu diffusion during either graphite/HgTe:Cu

paste or ZnTe:Cu contact processes [1,2]. Although it is

generally believed that too much Cu diffusion from the

contact is detrimental to device stability, it has also been 

shown that acceptor formation in CdTe, required for high

performance, is also linked to Cu diffusion. 

Previous studies have shown that the use of a

ZnTe:Cu contact interface layer can produce not only

high-performance CdS/CdTe devices, but also allows for

controlled diffusion of Cu. In earlier studies, this amount

of Cu diffusion was increased from very low

concentrations (consistent with uncontacted devices) to

concentrations consistent with devices demonstrating 

optimum performance. These studies showed that device 

performance improved with decreasing depletion (spacecharge) width in the CdTe. The reduction in depletion

width is expected from increasing CdTe acceptor 

concentration. 

In this work, we extend the study of Cu diffusion to

include CdS/CdTe devices expected to have considerably

more Cu than required for optimum performance. This is 

accomplished by using a ZnTe:Cu contact layer containing

more Cu than used previously, by making this layer

thicker, and depositing it at higher temperatures. These

parameters combine to produce higher Cu concentrations 

in both the CdTe and CdS, allowing operational

characteristics consistent with excessive Cu to be studied. 

EXPERIMENTAL

The CdS/CdTe material used in this study was

produced by vapor-transport deposition (VTD) outside of

NREL [3]. CdS and CdTe layers were ~0.3 and ~4 μm

thick, respectively. The ZnTe:Cu/Ti contact was produced

at NREL as follows: Samples were placed into a multisource vacuum processing chamber and preheated for

120 min. to the contact-fabrication temperature (samples 

were maintained at the substrate temperature throughout 

the contacting process). Substrate temperature was 

controlled using a resistive heater at constant voltages of

0, 20, 24, 28, 30, 32, 34, or 36 Volts, corresponding to

temperatures of ~25°, ~200°, ~240°, ~280°, ~300°, ~320°,

~340°, or ~360°C, respectively. Substrate temperature

was calibrated using an Al reference sample with an

embedded thermocouple. Prior to ZnTe:Cu deposition,

ion-beam milling was performed with a 3-cm Kaufmantype ion gun, operating at a beam energy and current of 

500 eV and 6 mA, respectively, and operated using UHPgrade Ar at a chamber pressure of 2x10-5 torr. A 1-μm 

thick ZnTe:Cu layer (~9 at.% Cu, measured with electron

microprobe) was deposited by r.f. sputter deposition,

followed by d.c. sputter deposition of ~0.5 μm Ti. To

ensure that any parameters linked to Ti diffusion were

comparable for samples with different ZnTe:Cu deposition

temperatures, the time interval between completion of 

ZnTe:Cu and Ti deposition was varied to assure that the

heater temperature at the start of Ti deposition was fixed 

at ~185°C. Following deposition and sample cooling, a

pattern of four individual 0.25-cm2

 cells were defined

photolithographically on each sample. Cell definition was 

by two-step chemical etching, first using TFT Ti Etchant 

(Transene Co. Inc., Rowley, MA) to remove the Ti, 

followed by an aqueous solution containing 39% FeCl3 to 

remove the ZnTe:Cu and CdTe. Following photoresist

removal, unwanted areas were mechanically removed,

and a perimeter contact to the SnO2 layer was formed with 

ultrasonically soldered In. 

Electrical analysis included light and dark currentvoltage (LIV/DIV) measurements using an XT-10 solar

simulator adjusted to approximate Global AM1.5 current

from a CdS/CdTe reference cell. Capacitance-voltage (CV) measurements were performed using an HP 4274 LCR

meter at a frequency of 100 kHz. Voltage-dependent 

quantum efficiency (QE[V]) measurements were

performed at a chopping frequency of 159 Hz, for both

white-light bias (tungsten lamp set to produce a device

current of ~1-sun) and dark bias configurations. Bias

voltages used were –0.5, 0, 0.2,0.4, and 0.6 volts. 

1
The same devices used for the above electrical 

analyses were also used for compositional analysis

(secondary ion mass spectrometry, SIMS) following

chemical removal of the Ti layer with the TFT etchant. 

SIMS was performed from the contacted side of the

devices using a Cameca IMS-3F instrument tuned for a

mass resolution (M/∆M) of ~4000 to allow for separation of 63Cu+

 from 126Te2+ species. 

Near-field scanning optical microscopy (NSOM) 

measurements incorporated an atomic-force microscope

(AFM) using a probe tip that emits 635-nm light within a

50-100-nm spot [4]. For NSOM studies, the thickness of 

the ZnTe:Cu was reduced to 50-200 nm, and the Ti layer

was selectively patterned using photolithography [4]. 

RESULTS AND DISCUSSION 

Figure 1 shows SIMS depth profiles of Cu for devices 

contacted at various heater voltages. As observed 

previously [5], higher contact temperature leads to increased

Cu incorporation into the CdS layer. This study also reveals

a systematic increase in Cu concentration in the CdTe layer 

with heater voltage. This trend may have been present in

the earlier work, but is here more obvious due to thicker

ZnTe:Cu layers and/or higher contact temperatures. Figure

1 also indicates the devices produced at 30 and 32 volts 

(near optimum conditions) have very similar compositional 

profiles, whereas devices contacted at 24 volts have lower 

Cu concentrations, and devices contacted at 34-36 volts 

have noticeably higher Cu concentrations. 

1017

1018

1019

1020

1021

Cu Concentration (c

m

-3)

0 1 2 3 4 5

Depth (μm)

ZnTe:Cu CdTe CdS

 UC697AQCu, 24V

 UC702AQCu, 30V

 UC696AQCu, 32V

 UC695AQCu, 34V

 UC698AQCu, 36V

Figure 1. Quantified, high-resolution SIMS depth profiles of Cu

concentration in CdTe/CdS devices contacted at various heater

voltages. Figure shows approximate location of layer interfaces. 

Analysis is preformed from the CdTe side of device. 

LIV/DIV measurements indicate that optimum device

performance (shown on Figure 2) results for heater

voltages of ~30-32 Volts. Further, because losses in

voltage, fill factor and current are similar for devices

contacted at lower (Fig. 2a) or higher (Fig. 2b)

temperatures, LIV/DIV alone is a poor indicator of Cu

concentration. Also, because increased Cu concentration

with contact temperature is observed in both the CdTe and 

CdS layers, this analysis does not indicate if Cu in one or

the other layer dominates performance changes. 

Nevertheless, Figure 1 together with Figure 2 clearly 

indicate that devices with either insufficient or excessive

Cu concentration produce non-optimum performance.

40

20

0

-20

Current Density (mA cm-2)

-0.2 0.0 0.2 0.4 0.6 0.8

Voltage (Volts)

(a) Contact Deposition Temperatures 

 Less Than Optimum

 ~240oC (24V)

 ~280oC (28V)

 ~320oC ( 32V, Optimum)

 (821 mV, 20.3 mA/cm2

, 73.3%, 12.1%)

40

20

0

-20

Current Density (

mA cm-2)

-0.2 0.0 0.2 0.4 0.6 0.8

Voltage (Volts)

(b) Contact Deposition Temperatures

Greater Than Optimum

 ~320oC (32V, Optimum) 

 (821 mV, 20.3 mA/cm2

, 73.3%, 12.1%)

 ~340oC (34V)

 ~360oC (36V)

Figure 2. LIV/DIV of devices produced for this study that are

expected to contain (a) insufficient or (b) excessive Cu. Optimum 

LIV values were measured at NREL using the X25 Simulator. 

Arrows show change in Voc with increasing heater temperature. 

10 12

10 14

10 16

10 18

NA-N D (cm -3

)

1 2 3 4

Depletion Width ( μm)

 UC697A3, 24V

 UC702A1, 30V

 UC696A2, 32V

 UC695A2, 34V

 UC698A1, 36V

34 V 32V

24 V

36 V 30 V

Figure 3. C-V analysis of CdS/CdTe devices contacted at various 

temperatures. Arrows indicate location of depletion width at zero

bias for devices contacted at stated heater voltages. 

C-V analysis (Figure 3) provides additional insight into

how the contact affects the electrical properties of the 

device. Key points revealed are: 1) The depletion width

decreases systematically with increasing contact 

temperature. This would be consistent with incorporation of

Cu increasing the net defect concentration (NA-ND) in the 

CdTe. 2) Devices contacted at higher than optimum

temperatures (34 and 36 volts) show significant increase in

NA-ND (from 5x1013 cm-3 to ~1x1015 cm-3). This suggests

that the considerable increased Cu concentration observed

in Figure 1 at high contact temperature translates into

increased net defect concentration in the CdTe. 3) Contact 

deposition at 34 and 36 volts also show a significant

reduction in depletion width at zero-bias (from ~3 μm to <1

μm). This reduction is due partly to increased net defect 

concentration in the CdTe, but may be impacted by the 

requirement of space-charge neutrality following Cu

compensation of CdS donors (to be discussed below). The 

2
depletion width for these devices is so narrow that photogenerated carriers may increasingly be created outside the 

space-charge region in forward bias, and voltage-dependent

collection (JL[V]) should be observed. 4) Although the

depletion width at zero bias for the optimum device (32 V) is

relatively wide, it reduces significantly in forward bias so that

at +0.6 V (about the maximum-power point for this device)

the depletion width is similar to devices with much more Cu.

Therefore, this device also could show JL[V].

To investigate if effects of JL[V] are limiting device

performance, QE[V] analysis was performed on three 

devices contacted at heater voltages of 24 V, 32 V, and 36 V 

(i.e., insufficient, optimum, and excessive Cu, respectively). 

In all cases, QE[V] results were scaled so that each QE

curve integrates to the appropriate device current (at the

stated bias voltage) as measured under standard conditions

at NREL. Figure 4 shows a comparison of the devices 

measured without voltage or light bias. This comparison

reveals that the device with insufficient Cu demonstrates 

relatively uniform collection across its responsive range, as 

expected from a fully depleted device. In contrast, the

device produced with excessive Cu shows reduction

primarily in its red response, consistent with the depletion

width being too narrow. Interestingly, the band edge of this 

device also shows a slight increase. As suggested from C-V 

analysis, the device produced near optimum conditions also 

shows some reduction in its red response, but not as severe

as for the device with excessive Cu. QE response from the 

CdS shows very slightly reduced collection for the sample

with insufficient Cu. 

100

80

60

40

20

Quant

0

um Efficiency (%)

400 500 600 700 800 900

Wavelength (nm)

 24 Volt Heater, Insuficient Cu

 32 Volt Heater, Optimum Cu

 36 Volt Heater, Excessive Cu

0 Volt Bias, No light Bias

Figure 4. QE[V] analysis of CdS/CdTe/ZnTe:Cu/Ti devices 

measured with no light bias and 0 volts voltage bias. 

100

80

60

40

20

Quant

0

um Efficiency (%)

400 500 600 700 800 900

Wavelength (nm)

 24 Volt Heater, Insufficient Cu

 32 Volt Heater, Optimum Cu

 36 Volt Heater, Excessive Cu

 0 Volts Bias

1-Sun Light Bias

Figure 5. QE[V] analysis of CdS/CdTe/ZnTe:Cu/Ti devices 

measured with ~1-sun light bias and 0 Volts voltage bias. 

Figure 5 shows the same three devices measured

without voltage bias, but now adding ~1-sun white-light bias

during the QE measurement. Results in the CdTe

responsive range (500-850 nm) are similar to measurements

performed without light bias. However, the CdS region (350-

500 nm) now shows a more pronounced increase in

response with increasing contact temperature. 

100

80

60

40

20

Quantum Effic

0

iency (%)

400 500 600 700 800 900

Wavelength (nm)

 24 Volt Heater, Insufficient Cu

 32 Volt Heater, Optimum Cu

 36 Volt Heater, Excessive Cu

+0.4 Volts Bias

1 Sun Light Bias

Figure 6. QE[V] analysis of CdS/CdTe/ZnTe:Cu/Ti devices 

measured with ~1-sun light bias and +0.4 Volts voltage bias. 

Figure 6 compares the same three devices measured

using both white-light bias and +0.4 volts bias (forward bias).

Although not shown, the +0.4 volts bias reduces the spectral

response from the CdTe region for samples containing

insufficient or excessive Cu. Both these reductions are 

expected because of insufficient field strength within the

depletion width (low Cu) and insufficient depletion width

(high Cu). The spectral response for the optimum device is 

not altered appreciably by the +0.4 volts bias. We suggest

this is because the depletion width remains sufficiently wide

and the field sufficiently strong as the device enters forward 

bias conditions. Indeed, we propose this is an underlying

criterion for many “optimum” contact processes. An

important remaining feature of Figure 6 is the apparent

increase in response from the CdS spectral region for the

sample with excessive Cu. 

100

80

60

40

20

Quantum Effic

0

iency (%)

400 500 600 700 800 900

Wavelength (nm)

 36 Volt Heater

Excessive Cu Device

 -0.5 Volts Bias

 0 Volts Bias

 +0.2 Volts Bias

 +0.4 Volts Bias

 +0.6 Volts Bias

Figure 7. QE(V) analysis of CdS/CdTe/ZnTe:Cu/Ti devices with 

measured with ~1-sun light bias and stated voltage bias. 

The apparent increased CdS response under forward-,

light-bias measurement conditions is illustrated more clearly

in Figure 7, where a full set of voltage-bias conditions (1-Sun

white-light bias) are shown for the sample containing

excessive Cu. The figure shows a systematic decrease in

response for the CdTe spectral region with forward bias

voltage. We suggest this reduction is due to the depletion

width being too narrow to collect all the current (i.e., forward

bias reduces already poor collection). In contrast, the CdS 

region shows a systematic increase in spectral response for

forward-bias conditions.

Changes in CdS spectral response under various bias

conditions have been studied historically in CuxS/CdS

devices [6,7], and more recently in CdS/CdTe devices [8,9].

While our results also reveal changes in the apparent

3
response from the CdS region, some interesting differences 

exist. The most significant difference may be that enhanced

CdS response is observed for white-light bias at a chopping

frequency of 159 Hz. The small increase in CdS response 

shown in Figure 5, and the larger increase shown in Figures. 

6 and 7 implies, for high contact temperatures, Cu may 

compensate donors in CdS, forming a more intrinsic material

and allowing the formation of a stronger drift field within the

CdS. Indeed, using these assumptions, preliminary 

modeling using AMPS-1D suggests that moderate collection

of carriers from light absorbed in the CdS appears possible

(i.e., QE of ~20% at 400 nm). Alternatively, Cu could be

passivating interface recombination pathways, enhancing

minority-carrier transport at the CdS/CdTe interface.

In addition to providing insight into how the junction 

evolves in a CdS/CdTe device during back contacting, the

previous analysis suggests that, for low Cu concentration, it

may be possible to collect light incident from the back of the 

device (because the device is completely depleted, a drift

field will be present near the back contact). In addition to

demonstrating device response from backside illumination

(note the ZnTe:Cu is transparent to light >550 nm),

depletion-width control has also allowed the use of NSOM to 

spatially map collection near grain boundaries at the back of

the device.

1.2

0.8

0.4

0.0

Peak Photocurrent (nA)

0 50 100 150 200 250

ZnTe:Cu Thickness (nm)

This study has shown that a CdS/CdTe PV device

undergoes significant changes during formation of a back

contact that uses a ZnTe:Cu contact interface layer.

These changes not only involve formation of lowresistance barriers at the CdTe/ZnTe:Cu and

ZnTe:Cu/metal interfaces, but also considerable changes

in the net defect level within the CdTe bulk. This work

also shows the CdS layer of a CdTe/CdTe device

undergoes significant changes when sufficient Cu is

allowed to diffuse into it at high temperatures. Specifically,

changes in the apparent spectral response are revealed

when the CdS contains sufficient Cu. Although these

changes are similar to those reported by others, some

interesting differences exist. At this time, we are 

investigating if these observations can be used to benefit

Figure 8. Peak NSOM-generated photocurrent from a the overall performance of thin-film CdS/CdTe devices.

CdS/CdTe/ZnTe:Cu/Ti device, measured as a function of ZnTe:Cu 

thickness. 

Figure 8 shows that the highest NSOM photocurrent is 

produced when the ZnTe:Cu layer is only about 50 nm thick.

This is also consistent with previous EBIC analysis that

indicated an enhanced field near the back contact for

devices with 50-nm ZnTe:Cu contact layers [3]. Figure 8 

also shows the NSOM-generated device current decreases 

as the ZnTe:Cu layer thickness increases. Because the 

ZnTe:Cu layer (Eg = 2.25 eV) is transparent to the NSOM 

illumination source (1.95 eV), the systematic current 

reduction for ZnTe:Cu thickness > 50 nm is not due to

absorption in the thicker ZnTe:Cu. Rather, we conclude Cu

diffusion from the thicker ZnTe:Cu layers produces more 

acceptors in the CdTe, and these cause the outer edge of

the space-charge layer to move away from the back surface,

producing a non-depleted region (i.e., higher recombination)

where the NSOM illumination is generating most of its 

carriers. Collection could be limited further by a potential

barrier at the back contact forming a second depletion layer.

The field of this second depletion layer would be in the

opposite direction of the main junction field, causing 

electrons to drift away from the main junction. 

NSOM analysis using a 50-nm-thick ZnTe:Cu film (as 

described above) is shown in Figure 9. In this figure, a 

comparison of the resulting AFM images (left side) and

NSOM images (right side) suggests, for this structure,

collection is enhanced near grain boundaries. Further

discussion of this NSOM analysis is presented elsewhere 

[4]. 

0 nm (1.5 nA) 

451 nm (7 nA)

Figure 9. Left: AFM image of polycrystalline CdTe/CdTe/ZnTe:Cu

solar cell. Right : NSOM image of the same region.

1 μm

CONCLUSIONS 

ACKNOWLEDGEMENTS 

This research was supported by the U.S. Department

of Energy under Contract #DE-AC36-99GO10337.

REFERENCES 

1. T. Gessert et al., “Analysis of the ZnTe:Cu Contact on 

CdS/CdTe Solar Cells,” Mat. Res. Soc. Sump. Proc. 763, pp.

133-138 (2003). 

2. S. Asher et al., “Determination of Cu in CdTe/CdS Devices 

Before and After Accelerated Stress Testing,” 28th IEEE 

PVSC, 2000, pp. 479-482. 

3. T. Gessert, et al., “Spectroscopic Cathodolumine-scence 

Studies of the ZnTe:Cu Contact Process for CdS/CdTe Solar

Cells, 29th IEEE PVSC, 2002, pp. 535-538. 

4. S. Smith et al., “Near-field Optical Beam-induced Currents in 

CdTe/CdS Solar Cells.” Appl. Phys Lett, 85 (17) (2004). 

5. T. Gessert et al., “Effects of Cu From ZnTe:Cu Contacts in 

CdS/CdTe Cells, 28th IEEE PVSC, 2000, pp. 479-482. 

6. A. Fahrenbruch and R. Bube, Fundamentals of Solar Cells 

(Academic Press, New York, 1983), Ch. 10. 

7. W.D Gill and R.H. Bube, J. Appl. Phys. 41 3731 (1970). 

8. S. Hegedus et al., “Photoconductivity CdS:How Does it 

Affect CdTe/CdS Solar Cell Performance,” Mat. Res. Soc.

Symp. Proc. 763, pp. 447-452 (2003). 

9. M. Gloeckler and J. R. Sites, J. Appl. Phys. 95 pp. 38-45

(2004). 

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Evolution of CdS/CdTe Device Performance During Cu Diffusion 

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14. ABSTRACT (Maximum 200 Words)

Cu diffusion from a ZnTe:Cu/Ti back contact onto CdS/CdTe thin-film solar cells is studied. We find if Cu diffusion is 

insufficient, the entire CdTe layer is depleted. However, if Cu diffusion is excessive, the depletion width can become 

too narrow to provide optimum current collection. This analysis suggests that most contact processes used for 

CdS/CdTe devices are optimized (often unknowingly) to result in a depletion width that extends just far enough into 

the CdTe to yield the highest possible field in the region where light absorption occurs. Analysis of the samples with 

very high Cu concentration also suggests that Cu doping of CdS may affect carrier collection from the CdS. 

15. SUBJECT TERMS

PV; device; performance; Cu diffusion; optimum current collection; optimum current collection; 

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