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PROFILING THE BUILT-IN ELECTRICAL POTENTIAL IN III-V MULTIJUNCTION 

SOLAR CELLS 

C.-S. Jiang, D. J. Friedman, H. R. Moutinho, and M. M. Al-Jassim 

National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO, USA 

BACKGROUND 

• Built-in potential is a key factor in photovoltaic devices. However, it’s 

characterization has been limited to indirect ways such as I-V and C-V 

measurements. 

• We have recently established and applied scanning Kelvin probe microscopy 

(SKPM), a AFM-based nanometer-scale electrical characterization technique, to 

the photovoltaic devices. 

• SKPM provides us a technique to assess the quality of p-n junctions. 

• We have measured the built-in potential on cross-sectional surface of solar cells: 

• – III-V: GaInP2 single-junction 

• GaInP2/GaAs two-junctions 

• – Thin film: CIGS 

• a-Si 

• We have reported the potential measurement by using a air-SKPM. Air molecules 

are adsorbed on the cross-sectional surface and affect the potential measurement. 

• The air-SKPM uses a laser beam of 1.85 eV for the AFM operation. The photon 

energy is lager than band gaps of most photovoltaic material and the laser is 

absorbed by the solar cells. 

• In this presentation, we report on a potential measurement by using a 

UHV-SKPM. Air molecule adsorption is avoided. The UHV-SKPM uses a 

laser of 1.4 eV. Light absorption in the GaInP2/GaAs cells is minor. 

SUMMARY 

We have observed three electrical potentials at the top, tunneling, and bottom 

junctions of GnInP2/GaAs tandem-junction solar cells, by performing the UHV-SKPM 

measurement. The effect of laser illumination was avoided by using GaAs laser with 

photon energy of 1.4 eV for the AFM operation. We also observed higher potentials at 

the atomic steps than on the terraces for both p-type GaInP2 epitaxial layer and p-type 

GaAs substrate, and found that the potential at steps of GaAs substrate depends on 

the step directions. 

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-520-39864. 

EC 

MEASUREMENT TECHNIQUE 

•SKPM measures contact potential difference (CPD) between the tip and sample 

surface 

•SKPM detects the Coulomb force between the tip and sample, and use it as feedback 

source in determining the Kelvin probe signal. 

•Spatial resolution: ~30 nm; Energy resolution: ~10 meV 

Principle of Scanning Kelvin Probe Microscopy (SKPM) 

EVac 

Semiconductor sample Metal probe

CPD 

EV 

EF 

EVac 

φs 

φm 

~ u=u0cos(ωt) 

2 (2 )

( ) ( )

2 

1 ( ) 2 1 

( ( ) ) 2 

1 

2 

1 

2 2

0 

0 1 2

2 

V COS t

z

C F

Vt Vs V Sin t

z

C F

Vt Vs V

z

C F

F F F F

E C Vt V Sin t Vs

ac

ac

ac

ac

ω

ω

ω

∂

∂ = −

− ∂

∂ =

⎥

⎦

⎤ ⎢

⎣

⎡ − +

∂

∂ =

= + +

= + −

Vb 

− − − Constant

= −

=

= +

Wt

Vb Ws Wt

Vs Ws

Vt Wt Vb

If Vt = Vs

F = 0, 1 

Vb maps Ws

Sample 

AFM tip 

+ 

-

First resonant oscillation of cantilever 

for topography 

Second resonant oscillation of cantilever 

for Kelvin probe 

• SKPM is based on the non-contact mode of atomic force microscopy (NC-AFM) 

• The oscillation signal of the cantilever is separated to low and high frequencies at 

the first and second resonant frequencies 

•The low frequency signal is used for detecting the topography of the sample surface 

•The high frequency signal is used for measuring the Kelvin probe signal. 

Laser 

PSPD 

Cantilever 

Scanner 

Bimorph 

Substrate

+ 

IG PG 

Z Feedback 

controller 

AFM Image 

Lock-in-amp 

+ 

~ V sin( t) ac ωsf

( ) ωsf

Filter 

Frequency 

counter 

LF 

HF 2MHz 

Oscillate 

feedback 

Negative 

feedback 

Vdc 

Kelvin probe 

Signal 

SKPM setup 

RESULTS 

200

P

o

t

e

n

tial (

m

V

)

0 0.4 0.8 1.2 1.6 2.0 

Distance (μm) 

1 μm n-GaInP2 P-GaInP2 P-GaAs 

n-GaInP2 TJ 

(a) (b) 

(c) 

Potential measurement on GaInP2/GaAs tandem cells 

•(a): SKPM potential image; (b): AFM topographic image; (c): Potential profile 

obtained by averaging (a) along the vertical direction. 

•Three potentials at the top, the tunneling, and the bottom junctions. 

• Measured potential values are smaller than the calculated potentials in the bulk, 

presumably due to surface Fermi level pinning. 

• The surface Fermi level may be pinned at defect states, and the defects were 

created upon cleaving. 

•Higher potential at the steps of p-GaInP2 layer than on the terraces. 

0 

-2 

1 

4 

2 

-1 E v 

E c EF 

Evac 

Potential 

0 0.1 0.2 0.3 0.9 1.0 1.1 1.2 1.3 

5 

6 

p-GaAs 

TJ 

2 p-GaInP 

2 n-GaInP P - GaInP2 

+ 

n-AlGaP2 

2 n-GaInP 

Potential (V) 

Distance (μm) 

Name Material Type 

Thickness 

(nm) 

Doping 

(×1017) 

Eg 

(eV) 

Electron 

Affinity 

(eV) 

Window AlInP2 n 25 10 2.35[9] 3.78[10] 

Emitter GaInP2 n 90 10 1.85[11,12] 4.01[13] 

Base GaInP2 p 840 1 1.81 4.01[13] 

Top 

cell 

BSF GaInP2 p 50 10 1.89[11,12] 3.93[13-15] 

p-layer GaAs p 10 100 1.42[16] 4.07[16] Tunneling 

Junction n-layer GaAs n 10 100 1.42[16] 4.07[16] 

Emitter GaInP2 n 100 10 1.85[11,12] 4.01[13] Bottom 

cell Base GaAs p 350 1 1.42[16] 4.07[16] 

Calculated Potential in the thermoequilibrium state in the bulk 

Parameters used in the calculation 

(a) (b) 200 nm 

2 

0 200 400 600 800 

Distance (nm) 

Height (Å)

P

o

t

e

n

tial (

m

V

)

40 

200 nm (c)

Potential measurement on the GaAs substrate 

[001] 

[110] -

10 nm 

Occupied-state STM image; Vs=-2.0V, It=0.1 nA 

•(a): SKPM potential image; (b): AFM topographic image; (c): line profiles in (a) and (b) 

•Steps are single atomic steps. 

•Potential at the steps are higher than on the terraces. 

•Height of the potential depends on the angle of step; the steps with a larger angle 

have a larger potential than the steps with a smaller angle do, presumably due to the 

unbalanced cation and anion at the steps. 

• The angles with the [110] direction is observed from the STM image take on the 

same sample area. 

-

2 

2 

)( 2 

1 

)( 2 

1 

VsVt

z

C

z

E F

VsVtCE

− ∂

∂ = ∂

∂ =

−=

-