INVESTIGATION OF JUNCTION PRO

CdS/CdTe SOLAR CELLS AND 

CORRELATION TO DEVICE PRO

R.G. Dhere, Y. Zhang, M.J. Ro

Asher, M. Young, B. To, R. No

Gessert.

Acknowledgements- K. Ramanath

DOE contract # DE-AC36-99G

NREL/PR-520-43286Presented at the 33rd IEEE Photovoltaic Specialist Conference held May 11-16, 20
Objective - Junction Stud

•Understand the nature of the junctio

CdTe/CdS device 

•Correlate the device fabrication para

the junction formation 

•Develop a self consistent device mo

explain the device properties

Detailed analysis of CdS/CdTe and S

devices prepared using CSS CdTe.
Gl

Sn

B

Cd

Cd

CdT

CoCu

CdTe Device Structure
n+-p device model for CdS/CdTe dev

based on blue QE loss:

– One sided junction with depletion width 

– Only field assisted collection.
Problems with the n

+-p m

•Phenomenological Model – can explain tperformance but without physical basis.•CBD CdS has carrier concentration arouwhich is even less than CdTeHere we present our interface/junction anSecondary Ion Mass Spectrometry (SIMSreflectance techniques and Electron Beam Current (EBIC) to elucidate the junction p
SIMS Results

• Roughness of the samples (R

μm) makes it impossible to refeatures at CdS/CdTe interfac

• NREL SIMS and Microscopy 

developed sample preparatiopolishing to improve the interfresolution.
10

0

10 110

2

10

3

10

4

10

5

Secondary Ion Counts

0 1 2

3

4

Depth (μm)

S - as deposite

S - CdCl

2 treated

Te - as dep,CdCl2 treated
Observations

• Interdiffusion at CdS/CdTe int

increases with Tsub and CdCl

2 

• Accumulation of Cl at CdS/Cd

interface after CdCl

2 HT. Lev

increases with level of HT

• Cl is a n-type dopant in both C

CdTe; also in the intermixed a
Photo- or Electro-Modul

Reflectance (PR or ER

CdTe/CdSSolar Cell

DC Light 

Modulate

ΔR

Dete

AC voltage
R n n

n n

a

a

=

−

+

Δ Δ R

R

≈ α ε1

Reflectance modu

n2 = ε1 + i ε2, na2 = εa 

Near band-gap ⇒ major contribution 
Fitting Modulation Reflectance 

FAA i i ( ) [ ( ) ( )] ' ηπ ηη η = − 2

2

G AB AB ii ii ( ) [ ( ) ( ) ( ) ( )]

' '

η π η η ηη η = −

⎜⎜⎝⎛

−

Ω

− ⎟ Ω

⎠⎞ ⎜

⎝

⋅ ⎛ =Δ )( 

2

)(

||2

0

0

2/3

2

0

22

22

2

1 

h

h

h

hh

r

r

h

μ

ω

ω

ε cv

g

E

E

G

m

Pee

Shen & Pollak, Phys. Rev. B 42, 7097 (1990)
Photo-reflectance

1.35 1.40 1.45 1.50 1.55

-10

-5

05

1015

Eg = 1.4480 eV

F = 31.9 kV/cm

1.5122 eV

 (n = 4)

1.4994 eV

 (n = 3)

1.4850 eV

 (n = 2)

1.4692 eV

 (n =1)

CdTe-A1 300 K 532nm

1.4486 eV

 (Eg

)

 experimental data

fitting results

ΔR/R (106)

Energy (eV)

900 880 860 840 820 800

Wavelength (nm)

FroEma

E

g = 
Electro-reflectance

1.35 1.40 1.45 1.50 1.55

-50-40-30-20-10

0

1020304050

1.524 eV

 (n = 5)

Eg = 1.4482 eV

F = 34.6 kV/cm

1.514 eV

 (n = 4)

1.5012 eV

 (n = 3)

1.4868 eV

 (n = 2)

1.4709 eV

 (n =1)

CdTe-A1 300 K 100 mV P-P

1.4486 eV

 (Eg

)

 experimental data

fitting results

ΔR/R (106)

Energy (eV)

900 880 860 840 820 800

Wavelength (nm)

FroEma

E

g = 
Effect of CdCl

2 treatment 

1.2 1.3 1.4 1.5 1.6 1.7

-8-6-4-2

0246

1.505 eV

 CdTe300 K 532nm

CdTe, as depositedCdTe, CdCl treated

ΔR/R (106)

Energy (eV)

1000 950 900 850 800 750

Wavelength (nm)
Modulated Reflectan

• Modulated electro-reflectance an

reflectance studies identify a regelectric field (~32-35 kV/cm) for hefficiency CdS/CdTe devices. Thpresent in the region of 1.45 eV m

• SnO

2/CdTe devices do not show 

region

The high field region corresponds 

CdSTe alloy.
2.4

2.22.01.81.61.41.21.0

Optical Bandgap, eV

0.0 0.2 0.4 0.6 0

Composition Parameter, x

CdTe 1-x Sx alloy Bandgaps

 D. Albin, et al. Ohata, et al., 

 Jap. J. Appl. Phys 12(10) 1

Zincblende Wurtzite
From Emax = 32 kV and 

depletion width on p-side = 3 μm 

C-V and EBIC results)

Using Emax = qNAXp/εs

Gives NA = 5.5x1014 cm-3Evaluation of ND based on SIMS 

EBIC results
EBIC 

• SEM and HR-EBIC measurem

performed on high Voc (835 m

• Measurements on the cross-

the device. Shows EBIC respclose to CdS/CdTe interface. 
substrate

SnO

2

CdS

Re

pico

e-beam

CdTe

e-h
Electron-b

CdS CdTe

200 nm 200 nm

SnO

2
CdSxTe1-x

SnO2 CdTe

SnO2 CdS Cd

Hetero Quasi-homo

Voc,mV 600 750

850 890

n-p

n+-p

Device model
Device structure Voc, mV

SnO2/CdTe 600-650

CdS/CdTe as dep 720-750CdS/CdTe w/ CdCl2 840-850 

• Lower Voc devices are true hetero-junctions, whereas th

CdCl2 treatment have a junction between n+ Te-rich CdS

with Cl) and p-type CdTe with compatible cubic structure homojunction.• A true hetero-junction CdS/CdTe device performance 

by interface defects at the hetero-interface which will be wdepletion region. This may be the case for as deposited at lower temperatures and SnO2/CdTe devices giving low 

• Role of CdS is mainly to produce Te rich alloy layer that 

type during CdCl2 process and passivation of the surface