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Profiling the Built-in Electrical 

Potential in III-V Multijunction 

Solar Cells 

Preprint 

C.-S. Jiang, D.J. Friedman, H.R. Moutinho, 

and M.M. Al-Jassim 

Presented at the 2006 IEEE 4th World Conference on 

Photovoltaic Energy Conversion (WCPEC-4) 

Waikoloa, Hawaii 

May 7–12, 2006 

Conference Paper 

NREL/CP-520-39883 

May 2006 
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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 (NREL), 1617 Cole Blvd., Golden, CO 80401 

ABSTRACT 

We report on a direct measurement of the electrical 

potential on cross-sections of GaInP2/GaAs multiplejunction solar cells by using an ultrahigh-vacuum 

scanning Kelvin probe microscope (UHV-SKPM). The 

UHV-SKPM allows us to measure the potential without air 

molecules being adsorbed on the cross-sectional surface. 

Moreover, it uses a GaAs laser with photon energy of 1.4 

eV for the atomic force microscope (AFM) operation. This 

eliminated the light-absorption-induced bottom-junction 

flattening and top-junction enhancement, which 

happened in our previous potential measurement using a 

1.85-eV laser for the AFM operation. Three potentials 

were measured at the top, tunneling, and bottom 

junctions. Values of the potentials are smaller than the 

potentials in the bulk. This indicates that the Fermi level 

on the UHV-cleaved (110) surface was pinned, 

presumably due to defects upon cleaving. We also 

observed higher potentials at atomic steps than on the 

terraces for both GaInP2 epitaxial layer and GaAs 

substrate. Combining scanning tunneling microscopy 

(STM) and SKPM measurements, we found that the 

potential height at steps of the GaAs substrate depends 

on the step direction, which is probably a direct result of 

unbalanced cations and anions at the steps. 

INTRODUCTION 

The built-in electrical potential in junctions of solar 

cells plays a critical role in photovoltaic action, because it 

collects photo-excited carriers and is a key factor in 

determining the open-circuit voltage of solar cell devices. 

To characterize the built-in potential, we have established 

a nanometer-scale measurement technique of SKPM [1], 

and applied the technique to profiling the electrical 

potential distributions in junctions of III-V, Cu(In,Ga)Se2, 

and hydrogenated amorphous silicon solar cells [1-5]. 

These potential measurements have improved our 

understanding of device physics in the solar cells, and 

provide us a direct method to assess the quality of the pn junctions. We have recently observed a widening and 

movement of the junction in a III-V space cell upon proton 

irradiation [6]. The SKPM technique is based on a noncontact AFM (NC-AFM) [7,8], and it measures the 

electrical potential on sample surfaces. When we 

measure a potential profile in the junction, we cleave the 

solar cell samples and expose the junction on crosssections for the potential measurements. 

Previously, we have reported the potential 

measurement on the III-V multijunction cells by using the 

SKPM based on an air-AFM, in which air molecule 

adsorption on the cross-sectional surface affected the 

surface charge and surface potential distributions [1,2]. 

Moreover, our air-AFM setup uses a laser beam with 

photon energy of 1.85 eV for the AFM operation. 

Absorption of the laser light by the solar cell samples was 

not avoidable in the SKPM measurement, because 1.85 

eV is larger than the bandgaps of most photovoltaic 

materials. In this paper, we will report on the potential 

measurement on the III-V cell by using a UHV-SKPM. 

Moreover, because the UHV-AFM uses a laser with 

photon energy of 1.4 eV, the effect of light absorption 

was avoided. 

MEASUREMENT TECHNIQUE 

 

SKPM is based on NC-AFM. Because the resonant 

oscillation peak of the AFM cantilever is super sharp in 

vacuum (several Hz), frequency modulation that sets the 

frequency shift constant is used for the AFM feedback 

control (Fig. 1), instead of amplitude-modulation mode 

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

Fig. 1. A schematic of the UHV-AFM and SKPM setup. 

*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, paidup, 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
used in the air-AFM. For the Kelvin probe, a highfrequency ac voltage (~400 kHz) at the second resonant 

frequency of the cantilever [Vacsin(ωsrt) in Fig. 1] is 

employed, instead of an off-peak low frequency that is 

used in the air-SKPM. This second resonant frequency 

enhances the energy resolution from ~50 mV of the airSKPM to ~10 mV of the UHV-SKPM. The cantilever 

oscillation signal is separated to low- and high-frequency 

components by low- and high-pass filters, and these 

components are sent to the topographic and the Kelvin 

probe detection circuits, respectively. Comparing the airSKPM, advantages of the UHV-SKPM are: (a) it keeps 

the fresh cross-sectional surface free of air molecule 

adsorption; (b) it uses a GaAs laser with a smaller photon 

energy of 1.4 eV for the AFM operation; and (c) energy 

resolution is improved. However, a disadvantage of the 

UHV-SKPM is that with the NC-AFM it is difficult to image 

a relatively rough thin-film surface with corrugations 

larger than ~50 nm, because it uses the frequencymodulation mechanism for z-feedback control. 

RESULTS AND DISCUSSIONS 

 

Figure 2 shows a SKPM potential and the 

corresponding AFM topographic images taken on the 

cross-section of the GaInP2/GaAs tandem-junction cell. 

Because the potential distribution along the vertical 

direction is uniform, we averaged the potential along the 

direction and display the potential profile in Fig. 2(c). The 

profile shows three potential variations: the p-n junction 

at the top cell, the tunnel junction, and the p-n junction at 

the bottom cell. We note that this potential profile is 

different from the profile we previously measured in air on 

the same sample, where the 1.85-eV laser light was 

absorbed by the bottom cell, and the light absorption 

induced a potential flattening at the bottom junction and a 

potential enhancement at the top junction [1]. The 

potential measured by the air-SKPM is away from the 

thermal equilibrium state. However, in the UHV-SKPM 

measurement, because the bandgap of the bottom cell 

approximately equals the photon energy, the effect of the 

laser light absorption should be weak or negligible. In this 

case, the sample is close to the thermal equilibrium state. 

0

-2

1

4

2

-1 E v

Ec 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-AlGaP 2

2 n-GaInP 

Potential (V)

Distance (μm)

Fig. 3. A calculated band diagram in the bulk of 

GaInP2/GaAs tandem-junction cell by solving Poisson’s 

equation and using parameters of band offsets and 

bandgaps in the literature. 

The band diagram (Fig. 3) in the bulk of the tandem 

cell is calculated by solving Poisson’s equation and using 

parameters of band offsets and bandgaps listed in Table 

1 [9-16]. There are three main potentials at the top, 

tunneling, and bottom junctions. The big potential drop at 

the top junction is close to the value of the GaInP2

bandgap (~1.8 eV). The potential increase (1.56 eV) in 

200

P

otent ai l (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)

Fig. 2. (a) An SKPM potential and (b) the corresponding AFM topographic images taken on the cross-sections of 

GaInP2/GaAs tandem-junction solar cells. (c) shows the averaged potential profile along the vertical direction in Fig. 2(a). 

2
the tunneling junction is slightly larger than the GaAs 

bandgap because of the conduction band offset (0.14 eV) 

between GaAs and the disordered p+-GaInP2 backsurface-field (BSF) layer in the top cell. The potential 

drop in the bottom junction is close to the p-layer’s 

bandgap (GaAs, ~1.42 eV). Although the n-layer of the 

bottom junction has a bigger bandgap (GaInP2, 1.85 eV), 

it does not significantly affect the potential because the 

conduction bands are almost aligned (δEc ~ 60 meV). 

Comparing the measured profile in Fig. 2(c) with the 

calculated potential profile in Fig. 3, we found that the 

measurement qualitatively agrees with the calculation. 

Values of the potentials at the junctions are smaller than 

the calculation. This discrepancy is due to the effect of 

surface Fermi level pinning. Although the sample is 

cleaved in UHV and is free of air molecule adsorption, 

defects on the surface can be responsible for the Fermi 

level pinning [17,18]. From the measured potential 

values, we can propose Fermi level positions in the 

bandgap [1]. However, the potential measurement can 

give us only relative values between the Fermi level 

positions in these epitaxial layers of the cell. To deduce 

the Fermi level pinning positions, at least one Fermi level 

position in the layers has to be known as a reference. For 

GaAs(110), various surface Fermi level positions were 

reported, possibly depending on the nature of surface 

defects and doping types [17-21]. Because the relative 

values of the potential measured by the UHV-SKPM is 

different from the values measured by the air-SKPM [1], 

the Fermi level pinning positions should be different 

between the UHV- and air-cleaved (110) surfaces, due to 

the air molecule adsorptions. 

TABLE 1. Sizes and electronic properties of the multi-layers in the GaInP2/GaAs tandem cell. 

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]

(a) (b) 200 nm

2

0 200 400 600 800

Distance (nm)

Height (Å)

Po et nt ai l (m

V)

40

(c)

2

0 200 400 600 800

Distance (nm)

He gi ht (Å)

Po et nt ai l (m

V)

40

(c)

200 nm

Fig. 4. (a) An SKPM potential and (b) the corresponding 

AFM topographic images taken on the cross-sections of 

GaAs substrate. (c) shows example line profiles along 

the arrows in (a) and (b). 

On the SKPM image [Fig. 2(a)], one sees bright 

lines in the absorber layer of the top cell, which 

correspond to steps, as shown in Fig. 2(b). These lines 

show ~50-mV higher potentials than on the terraces. The 

steps are single or a few atomic steps. The groove 

running along the tunneling junction is ~1 nm in depth, 

and the step along the vertical direction in the top junction 

is ~0.6 nm in height. The steps and the groove were 

created in the sample cleaving. 

We also observed similar potential phenomena at 

atomic steps in the p-type GaAs substrate (Fig. 4), and 

found that the potential height depends on the step 

direction. To investigate the structure of the steps, we 

took STM images on the same sample area (Fig. 5). 

From the STM images, we know that there are angles 

between the atomic steps and the ]011[ direction. The 

−

3
]011[ −

 direction is recognized by the atomic rows in Fig. 

5. The steps in Fig. 4 are in two directions; the steps with 

the larger angle with respect to the direction show 

the larger potential (~100 mV) than do the steps with the 

smaller angle (~30 mV). All the steps are single atomic 

steps [Fig. 4(c)]. This dependence of the potential on the 

step direction may relate to anti-site or vacancy defects at 

the steps, because the unbalance of cation and anion 

concentration on the surface directly results in surface 

charges, which causes the electrical potential at the 

steps. A larger work function at the atomic steps than on 

the GaAs (110) terraces for a n-type GaAs was reported 

by Sommerhalter et al. [21]. 

]011[ −

 

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 a 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 the p-type GaInP2 epitaxial layer and p-type GaAs 

substrate, and found that the potential at steps of the 

GaAs substrate depends on the step directions. 

ACKNOWLEDGEMENT 

This work is supported or funded under DOE 

Contract No. DE-AC36-99GO10337. 

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[001]

[110] -

10 nm

[001]

[110] -

[001]

[110] - [110] -

10 nm

Fig. 5. A STM occupied-state image of the GaAs(110) 

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