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Role of Copper in the 

Performance of CdS/CdTe 

Solar Cells 

Preprint 

S. Demtsu and J. Sites 

Colorado State University 

D. Albin 

National Renewable Energy Laboratory

Presented at the 2006 IEEE 4th World Conference on 

Photovoltaic Energy Conversion (WCPEC-4) 

Waikoloa, Hawaii 

May 7–12, 2006 

Conference Paper 

NREL/CP-520-39923 

May 2006 
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ROLE OF COPPER IN THE PERFORMANCE OF CdS/CdTe SOLAR CELLS * 

S.Demtsu1

, D. Albin2, and J. Sites1 

1

 Department of Physics, Colorado State University (CSU), Fort Collins, CO 80523 2

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

ABSTRACT 

The performance of CdS/CdTe solar cells made with 

evaporated Cu as a primary back contact was studied 

through current-voltage (JV) at different intensities, 

quantum efficiency (QE) under light and voltage bias, 

capacitance-voltage (CV), and drive-level capacitance 

profiling (DLCP) measurements. The results show that 

while modest amounts of Cu enhance cell performance, 

excessive amounts degrade device quality and reduce 

performance. The analysis is supported with numerical 

simulations to reproduce and explain some of the 

experimental results. 

Introduction 

The performance of CdTe solar cells relies on the 

formation of a low-barrier back contact. This usually 

involves including Cu as a key element in the contacting 

process. The back-contact behavior and open-circuit 

voltage (Voc) improve with the application of an optimal 

amount of Cu during the process. Unfortunately, rapid 

diffusion of Cu from the back contact toward the main 

junction is believed to contribute to degradation observed 

in long-term stability studies. Cu can form both deep 

interstitial donors Cui and substitutional acceptors Cucd in 

CdTe [1,2]. Cu can also migrate along grain boundaries 

toward the main junction. The standard back-contact 

process at NREL uses a Cu1.4Te and HgTe-doped 

graphite paste as a primary back -contact. This paste is 

a commercial product consisting of multiple compounds, 

carbon, and a polymer binder. This complexity makes it 

difficult to study, control, and reproduce. Nevertheless, 

high-efficiency, CdTe-based solar cells use similar Cudoped graphite pastes [3,4]. In this study, the paste was 

replaced with a relatively simpler one in which Cu metal 

of varying thickness is evaporated on Te-rich CdTe 

surfaces. The effect of Cu on the back contact, the 

CdTe, and the CdS layer as it diffuses from the back -

contact region toward the front of the cell are discussed. 

Device fabrication 

Devices were fabricated by depositing 80-nm-thick 

CdS and 9-μm-thick CdTe on SnO2:F-coated glass 

substrates by chemical-bath deposition (CBD) and closespaced sublimation (CSS), respectively. Prior to the 

application of the back contact, the structures were heat -

treated in a vapor mixture of CdCl2 and O2. After the 

CdCl2 treatment, all devices were etched in nitricphosphoric (NP) acid solution to remove surface oxides 

and to create a Te-rich CdTe surface. Cu metal of 

varying thickness was deposited by electron-beam 

evaporation, without intentional substrate heating, at 

deposition rates of 0.01-0.05 nm/s. The contact was then 

annealed in flowing helium at 280o

C for 25 min to 

promote Cu diffusion and facilitate the formation of CuxTe 

to improve the back contact behavior. Devices were 

completed by evaporating 60 nm of Pd and 300 nm of Al 

as a current-carrying electrode. 

Effect of Cu on the back contact 

Cu is primarily incorporated at the back-contact layer 

to improve the electrical contact. The current-voltage 

characteristics of typical devices made with varying 

amounts of Cu are shown in Fig. 1. 

Fig. 1. JV characteristics of devices made with varying 

Cu amounts. 

The device made with no intentional Cu and those 

made with excess Cu (100 nm) showed significant 

distortion (rollover) in their J-V curves. For devices made 

with optimal amounts of Cu (5-20 nm) the back-contact 

* This work has been authored by an employee or employees of the Midwest Research Institute under Contract No. DE-AC36-99GO10337 with the U.S. Department of Energy. The 

United States Government retains and the publisher, by accepting the article for 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
behavior is significantly improved and no distortion 

(rollover) is observed. JV parameters for the 5-nm Cu 

device are: short-circuit current density, Jsc = 21.5 

mA/cm2; Voc = 803 mV; fill factor, FF = 57.5%; and totalarea efficiency, η% = 10.0%. In general, Cu affects the 

back-contact behavior primarily through its series 

resistance. The high series resistance (low fill factors) 

observed in this set of devices, compared with devices 

completed with a Cu-doped graphite paste, could be due 

to excess Cu diffusion. Cu introduced in elemental form 

may represent a more mobile source of Cu than Cu 

introduced as Cu1.4Te. The paste itself is also believed to 

contain ingredients that hinder Cu diffusion [5]. 

Interestingly, when devices made using Cu-doped paste 

contacts are stressed, the corresponding changes in J-V 

data resembles the J-V data of the unstressed devices 

shown in Fig. 1. 

Effect of Cu on the absorber layer (CdTe) 

The effects of Cu are not limited to the back-contact 

layer where it is deposited. Cu is known to be a fast 

diffuser in p-CdTe, and hence, a significant amount of Cu 

could be present in the CdTe layer in different forms. In 

the CdTe layer, Cu is known to form CuCd acceptor state 

levels at about 0.12-0.3 eV from the valence band, which 

contributes to an increase in the carrier density [1,2]. Cu 

can also exist as Cui, which would compensate the CdTe. 

CV measurements, shown in Fig. 2, show that the 

apparent hole density in CdTe increases after the 

application of modest amounts of Cu (5-20 nm) at the 

back contact. As a result, higher Voc is obtained. A 

further increase in Cu (100 nm) resulted in a higher 

apparent hole density, but a smaller depletion width and 

lower Voc. In general, CV measurements show a wide 

depletion width and a low carrier density in the absence 

of Cu. When excess Cu is used, the depletion width 

becomes smaller, and thus the collection of photogenerated carriers is reduced, resulting in lower Voc and 

FF. 

Fig. 2. Hole density as a function of distance from the 

junction estimated from CV measurement. 

Since room temperature CV measurements may not 

yield the true hole density, DLCP measurements were 

performed to obtain a more accurate assessment of the 

free carrier and trap densities. In Fig. 3, the drive-level 

density (NDL), determined as a function of the distance 

from the junction at different temperatures, is shown. 

Fig. 3. Carrier density as a function of the distance from 

the junction estimated from DLCP measurement for a) no 

Cu and b) 20-nm Cu devices. 

In the low-temperature limit, NDL estimates the free 

carrier density (p), while the difference between high and 

low-temperature levels equals the density of trap states, 

Nt. It was observed that Nt, increased with increasing 

amount of Cu, which could explain at least partially the 

Voc decrease with excess Cu. 

JV measurements at different light intensities show 

apparent intensity dependence in leakage conductance. 

Figure 4 shows JV data at different intensities for devices 

made with and without Cu. All devices shown in Fig. 1 

have dark shunt resistances larger than 10 kΩ-cm2

. 

However, under one sun illumination, the plots of dJ/dV 

vs. V near Jsc are not flat, and the upper-bound estimates 

of the “apparent shunt resistances” were in the range of 

170-600 Ω-cm2

, dependent on the amount of Cu used. 

The apparent shunt resistance, Rsh, was found to 

decrease with increasing Cu thickness. It also decreased 

with increasing intensity. The increase in leakage 

conductance (G = 1/Rsh) with intensity is probably due to 

increased interface defect states whose occupancy is 

changed by the intensity of illumination. 

2
Fig. 4. JV curves as a function of illumination intensity for 

(a) no Cu and (b) 5-nm Cu devices. 

The curved JV region below Vmp is a signature of a 

voltage-dependent collection. In Fig. 5, the QE curve as 

a function of applied voltage shows minimal voltagedependent spectral response for devices made without 

Cu, and increasing voltage dependence when the 

amount of Cu was increased. This indicates that the 

photocurrent, obtained by integrating the QE curves over 

the wavelength, is voltage dependent. Defect levels in 

the CdTe that act as recombination centers play a major 

role in reducing the lifetime of photogenerated carriers. 

This reduces the collection of photogenerated carriers 

and results in a voltage-dependent collection. Timeresolved photoluminescence (TRPL) measurements on 

completed devices confirmed a decrease in bulk lifetime 

with increasing Cu concentration. AMPS-simulated JV 

curves [6], shown in Fig. 6, illustrate the correlation of 

smaller carrier lifetimes with increasing voltagedependent collection. Current techniques for extracting 

JV parameters [7] without consideration of either voltagedependent collection or spectral-dependent shunt 

resistance, may not yield credible results. For example, 

when applied to these results, these techniques predict 

diode quality factors larger than 3, which are not possible 

according to existing recombination theory. 

Fig. 5. QE curves as a function of voltage bias for (a) no 

Cu and (b) 20-nm Cu devices. 

Fig. 6. Effect of lifetime on JV characteristics (simulated). 

Effect of Cu on the window layer (CdS) 

Cu is known to form acceptor states at about 0.34 

and 1.2 eV (midgap) from the valence band in CdS [8]. 

The presence of these states impacts the 

photoconductivity of CdS. Deep acceptor states trap 

most of the free electrons contributed by donors, resulting 

in a low free-electron concentration, and hence, 

increased CdS resistivity in the dark. Under illumination, 

photons absorbed in the CdS layer increases the 

concentration of both free electrons and holes, increasing 

the conductivity of CdS. The photoconductivity of CdS is 

responsible for the light/dark crossover observed in 

devices made with Cu. In the absence of Cu, no 

significant crossover is seen. 

3
Fig. 7. QE curves under white light bias. 

In the blue region (λ < 550 nm), apparent quantum 

efficiency (AQE) larger than unity under white-light 

illumination is observed in the presence of Cu, but not in 

the absence of Cu. A similar AQE observation under redlight illumination in CdS/CdTe solar cells has been 

reported [9]. The modulation of the barrier at the 

CdS/CdTe junction due to charge trapping by acceptorlike defect states in the CdS layer is responsible for the 

observed crossover and AQE effects. The impact on 

actual device performance, however, should be minimal. 

Fig. 8. Comparison of the JV curves of device with 

evaporated-Cu and Cu-doped graphite paste contacts.

Effect of back-contact annealing temperature 

The optimum back-contact annealing temperature for 

our Cu-doped graphite paste contact was previously 

known to equal 280°C. The results from this study 

however show that this may be excessively high for 

contacts using evaporated Cu as the dopant. This was 

confirmed by a subsequent set of devices in which the 

evaporated Cu anneal temperature was reduced to 200 

°C. Figure 8 compares the JV data for devices using 

evaporated Cu annealed at both 200°C and 280°C 

relative to a paste-based contact annealed at 280°C. 

Conclusions 

The results obtained show that Cu clearly enhances 

device performance, but that excess Cu can lead to 

significant collection losses. An optimal amount of Cu 

may increase the hole density of the CdTe by forming 

deep acceptor states. However, Cu also forms midgap 

defect levels that act as recombination centers, which 

lowers the lifetime, and consequently reduces Voc and 

FF. The presence of Cu in the CdS layer is responsible 

for the dark/light crossover and apparent quantum 

efficiencies larger than unity. Under operating conditions 

(one-sun illumination), the presence of Cu in the CdS 

layer seems benign, but could in the extreme case 

contribute to an increased series resistance of the device. 

Though previous experience had indicated that paste 

contacts should be annealed at 280°C it was determined 

that evaporated Cu contacts required a much lower 

anneal temperature of 200°C. Cu in elemental form may 

be more mobile, relative to Cu incorporated as Cu1.4Te in 

our standard graphite paste contacts. 

 

Acknowledgements 

This work was supported by the U.S. Department of 

Energy under contract DE-AC36-99G010337 to the 

National Renewable Energy Laboratory (NREL), and by 

subcontract XXL-5-44025-03 to Colorado State University 

(CSU). We would also like to acknowledge Anna Duda, 

Tom Moriarty, David Young, and Wyatt Metzger. 

REFERENCES 

[1] S. Wei and S. Zhang, Phys. Rev B. 66, 155211(2000). 

[2] A.Balcioglu, R. Ahrenkiel, and F. Hasoon, J. Appl. 

Phys. 88, 7175 (2000). 

[3] C.S. Ferekides, V. Viswanathan, and D.L. Morel, Proc.

26th IEEE PVSC, Anaheim, CA, p. 423 (1997). 

[4] X. Wu, J, Keane, R. Dhere, C. Dehart, D. Albin, A. 

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

Proc. 17th European PVSC, p. 13 (2001). 

[5] S.H. Demtsu, D.S. Albin, J.W. Pankow, and A. Davies, 

Sol. Eng. Matls & Sol. Cells (in press). 

[6] AMPS-1D, software was developed at Pennsylvania 

State University by S. Fonash et al., with support from 

EPRI. 

[7] S.S. Hegedus and W.N. Shafarman, Progress in 

Photovoltaics, 12, p. 155 (2004). 

[8] P. Peka and H. Schulz, Solid State Commun., 89, 225 

(1994). 

[9] S. Hegedus, D. Ryan, K. Dobson, B. McCandless, and 

D. Desai, Mat. Res. Soc. Symp. Proc. 763, p. 447-452 

(2003). 

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