Role of Copper in the performance of CdS/CdTe solar cells 

Samuel Demtsu1, David Albin2, and James Sites1 

1Colorado State University, Department of Physics, Fort Collins, CO 80523 

2National Renewable Energy Laboratory • Golden, CO 80401 

The information contained in this poster is subject to a government license. 

2006 IEEE 4th World Conference on Photovoltaic Energy Conversion • Waikoloa, Hawaii • May 7-12, 2006 • NREL/PO-520-39981 

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. 

Pd/Al (~ 0.4 μm) 

Cu (0-100 nm) 

p-CdTe (~ 9 μm) 

n-CdS (~ 0.08μm) 

SnO2: F ( ~0.5μm) 

Glass substrate 

Te 

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 valuable assistant from Anna Duda, Tom 

Moriarty, David Young, and Wyatt Metzger. 

Device structure 

Cux Te 

JV Characteristics 

Evaporated-Cu back-contact 

- No Cu and excess Cu (100 nm) devices showed 

rollover, and hence low FF. 

- Parameters of the 5-nm Cu device are: Jsc= 21.5 

mA/cm2, Voc= 803 mV, FF=57.5%, and η = 10%. 

Apparent carrier density profile 

(estimated from CV ) 

- Low carrier density and a wide depletion width with no Cu. 

- Higher carrier density and smaller depletion width with 

increasing Cu. 

DLCP 

- Density of trap states increased with increasing Cu amount 

QE Curves as a function of voltage bias 

- Minimal voltage dependence when Cu is not used; 

increasing voltage dependence with increasing Cu. 

JV curves as a function of illumination 

Comparison of JV curves 

- Comparison of devices made with evaporated-Cu 

contact (annealed at 2000 C and 2800 C) relative to a 

Cu-doped graphite contact (annealed at 2800 C). 

- For devices with Cu, the shunt conductance is intensity 

dependent, probably due to interface defect states whose 

occupancy is changed by intensity of illumination. 

-Cu clearly enhances device performance, but excess 

Cu can lead to significant current losses. 

- Cu increases the acceptor density in CdTe, 

however, Cu also forms defects that lower the lifetime, 

and hence reduced Voc and FF. 

- The presence of Cu in the CdS layers is responsible 

for the crossover and AQE effects. The impact on 

actual device performance, however, should be 

minimal. 

- The performance of device made with evaporatedCu contact (annealed at 2000 C) is comparable to 

that of devices with the standard Cu-doped graphite 

paste when the same absorber is used. 

QE curves under white light bias Conclusions

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

efficiency (AQE) larger than unity under white light 

illumination is observed in the presence of excess Cu.