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by Midwest Research Institute • Battelle 

Contract No. DE-AC36-99-GO10337 

C.-S. Jiang, R. Noufi, K. Ramanathan, 

J.A. AbuShama, H.R. Moutinho, and 

M.M. Al-Jassim 

Prepared for the 31st IEEE Photovoltaics Specialists 

Conference and Exhibition 

Lake Buena Vista, Florida 

January 3–7, 2005 

February 2005 • NREL/CP-520-37338 

AFM-Based Microelectrical 

Characterization of Grain 

Boundaries in Cu(In,Ga)Se2

Thin Films 
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AFM-BASED MICROELECTRICAL CHARACTERIZATION OF GRAIN BOUNDARIES IN 

Cu(In,Ga)Se2 THIN FILMS 

C.-S Jiang, R. Noufi, K. Ramanathan, J. A. AbuShama, 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 twodimensional potential distribution on the surface of 

Cu(In,Ga)Se2 thin films using a nanoscale electrical

characterization of scanning Kelvin probe microscopy both

in air and in ultra-high vacuum. The potential 

measurement reveals a higher surface potential or a

smaller work function on grain boundaries (GBs) of the 

film than on the grain surfaces. This demonstrates the 

existence of a local built-in potential on GBs, and the GB

is positively charged. The role of the built-in potential in

device performance was further examined and found to be

positive, by tuning Ga content or band gap of the film. With 

increasing Ga content, the potential drops sharply in a Ga

range of 28%~38%. Comparing the change in the built-in

potential to the theoretical and experimental

photoconversion efficiencies, we conclude that the

potential plays a significant role in the device conversion

efficiency of NREL’s three-stage Cu(In,Ga)Se2 device. 

INTRODUCTION 

One of the most surprising achievements for

Cu(In,Ga)Se2 (CIGS) polycrystalline solar cell material is

its high conversion efficiency (19.2%) [1]. In general,

defects in grain boundaries (GBs) in polycrystalline

materials form effective recombination centers. However,

this is not the case for the CIGS film, in which state-of-theart cell efficiency of >19% is higher than that

demonstrated for single-crystal cells (~13%) [2]. Two 

controversial arguments on the role of recombination in

GBs suggest that the GB is either electrically active or

inactive as a recombination center for photogenerated

carriers, which may be due to the different GB structures 

generated by the different fabrication processes of the

films [3-5]. However, even if the GB is electrically inactive,

this property alone is not sufficient to explain the high

performance of the polycrystalline cell compared to its

single-crystal counterpart. 

A classical model of interface states suggests a local

built-in potential around a GB as a result of trapped

charges at the interface states on the GB [6]. This local

built-in potential can be expected to spatially extend the

minority-carrier collection area and thus benefit the

photovoltaic effect. However, the deep level of the

interface states may also act as recombination centers for 

the minority carriers, thus reducing their lifetime. In

addition, the local built-in potential on the GB does not

necessarily exist in all thin-film materials. It remains 

unclear whether or not the local built-in potential exists on

GBs of the CIGS material and whether it plays a positive

or negative role in the photovoltaic performance of the

device. 

In this paper, we report on a direct measurement of

the local built-in potential on GBs of CIGS films using a 

microelectrical characterization technique, namely

scanning Kelvin probe microscopy (SKPM). Furthermore,

the built-in potential on the GB was examined to be

beneficial to photoconversion efficiency of the CIGS

devices. 

MEASUREMENT TECHNIQUE 

 

We have established the SKPM technique both in air

and in ultra-high vacuum (UHV) [7]. SKPM is based on

Laser PSPD

Cantilever

Scanner

Bimorph

Substrate

+

IG PG

Z Feedback

controller

AFM Image

Lock-in-amp

+

~ Vac sin( ω t) sf

( ω sf )

Filter

Frequency

counter

LF

HF 2MHz

Oscillate 

feedback

Laser

PSPD

Cantilever

Z Feedback controller

+

+

~ Scanner

Substrate

AFM Image

Lock-in-amp

V ac sin( ω t)

( ω l)

Bimorph ωfr ( )

IG PG

(a) In air

(b) In UHV

~ Vac sin( t) ωfr

Negative

feedback

Negative

feedback

l ωfr ( )

VdcKelvin probe 

Signal

Vdc

Kelvin probe 

Signal

Fig.1. Schematics of SKPM setups (a) in air and (b) in 

UHV. 

1
the non-contact mode of atomic force microscopy (NCAFM). Because the resonant oscillation peak of the AFM

cantilever is super sharp in UHV (several Hz), amplitude

modulation mode of the NC-AFM, which is used in the airAFM, cannot be used in the UHV-AFM. In contrast to that 

the cantilever in the air-AFM is driven by an ac voltage at

a fixed frequency [Vacsin(ωfrt) in Fig. 1(a)], the cantilever

oscillation in UHV is excited and controlled by a amplifier 

that keeps the cantilever oscillating at the strongest (the

first) resonant oscillation frequency [ the “Oscillate 

feedback” box in Fig. 1(b)]. The real shift of the resonant 

frequency by the atomic force between the sample surface

and the tip is counted and this shift is used to control the Z

feedback loop and generate the topographic image. This 

mode is called frequency modulation of NC-AFM. 

(a) AFM 5 μm

(b) SKPM 5 μm

Distance (μm)

0 2 4 6 8 10

100

Potential (

mV)

(c)

Fig. 2. (a) AFM and (b) the corresponding SKPM images

of the CIGS film taken by air-SKPM; (c) a line cut along 

the arrows in (b). 

Based on the NC-AFM difference between in air and

in UHV, the setups of SKPM in air and in UHV are also

different. The air-SKPM uses a relatively low frequency ac

voltage [Vacsin(ωlt) in Fig. 1(a) and ωl ≈20 kHz] to oscillate

the cantilever, and the Coulomb force between the tip and

the sample in this frequency can be described by F(ωl)∝

(φs-φt) Vacsin(ωlt), where φs and φt are work functions of the

sample and the tip and (φt-φs) is called contact potential 

difference. The oscillation in this frequency (note it is

different from the frequency ωfr ≈ 70 kHz that is used in

probing the sample topography) is detected by the lock-in

amplifier, and the detected signal is sent to the negative

feedback loop that is applied to the tip. The voltage Vdc

from the feedback loop is the Kelvin probe signal Vdc=(φtφs), because in this case the contact potential difference or 

the work function difference is compensated by the Kelvin

probe signal and the coulomb force F(ωl) is zero. Because 

of the convolution of topography into the Kelvin probe 

signal, frequency of the ac voltage ωl ≈20 kHz has to be

set far away from the first resonant frequency ωfr ≈70 kHz

that is used in probing the topography. The deviation of ωl

from ωfr lowers the energy sensitivity of the potential

measurement (~50 mV). 

In UHV, because the attenuation of cantilever 

oscillation in high frequency is not significant, the second

resonant frequency [Vacsin(ωsrt) in Fig. 1(b) and ωsr ≈400 

kHz] is used in the Kelvin probe [8], and this enhances the

energy sensitivity to ~10 mV. 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. Because the second 

resonant frequency, ωsr ≈400 kHz, is far away from the 

first resonant frequency, ωfr ≈70 kHz, the convolution of

the topography into the Kelvin probe signal is reduced.

However, the setup of our UHV-NC-AFM is difficult to scan 

in a large size (<3 m) on a relatively rough polycrystalline

sample (~ 100 nm). 

RESULTS AND DISCUSSIONS 

 

The CIGS films being investigated were deposited by

NREL’s three-stage co-evaporation process on Mo-coated

soda lime glass substrates. The film was rinsed in highpurity water before the potential measurement. Waterrinsing was used to remove Na residing on the film

surface, which diffused from the soda lime glass substrate

onto the surface. The surface Na makes the measured

potential peak on the GB broad due to the Na-induced

surface dipoles [9]. Figure 2 shows an AFM topographic

image of the film surface and the simultaneously obtained

SKPM image, measured by the air-SKPM. Comparing the 

AFM [Fig. 2(a)], SKPM [Fig. 2(b)] images, and the 

example line profiles [Fig. 2(c)], one see that the electrical

potential is higher on the GBs than on the surface of 

grains. Both topographic and potential images measured

in UHV (Fig. 3) are similar to those measured in air (Fig.

2). However, the noise level or the energy resolution is

improved in the UHV-SKPM, from ~50 mV to ~10 mV [Fig.

3(c)]. The primary purpose for measuring the potential

distribution in UHV is to avoid the effect of air-molecule 

absorption and surface contamination. However,

annealing the sample at 110 °C in UHV for 30 minutes to 

desorbe water molecules from the sample surface did not

significantly change the distribution of the surface 

potential. Further Ar ion sputtering at 600 V for several

2
seconds likely damaged the surface structure and caused

disappearance of the potential contrast on the GB. 

The higher potential on GB demonstrates a downward

band bending around the GB in the p-type material [Fig.

4(a)], and that the GB is positively charged. Such local

built-in potential makes the local work function smaller on

the GB than on the grain surface. Our measured potential

height (~150 mV) seems to be small compared to the

band gap (~1.15 eV) of CIGS. Most likely, the depletion on

the grain surface reduces the contrast of potential

between the GB and the grain surface and makes the

measured potential peak on the GB smaller than the

potential barrier on the GB in the bulk [Fig. 4(b)]. Such

surface depletion exists widely on semiconductor 

surfaces. Therefore, the local built-in potential in the bulk 

should be between the measured height (~150 mV) and

the band gap (~1.15 V). 

The local built-in potential on GBs of the CIGS films 

was further examined to play a significant role in

photoconversion efficiency of the device. Band gap of the

CIGS films can be tuned from 1.01 to 1.68 eV by adjusting

Ga content of Ga/(In+Ga) from 0% to 100% [10]. Although

a Ga content of ~65% provides the optimal band gap

value for optimal conversion efficiency, as theoretically

predicted from the solar spectra [11], the actual Ga

content in the current record-efficiency device is ~28% [1]. 

This difference between the theoretical optimal band gap 

and the experimental Ga content of the highest-efficiency

device implies that, in addition to the band gap, other 

factors play major roles in conversion efficiency.

Therefore, identifying the factors that determine

conversion efficiency provides an opportunity to address 

the question of whether the built-in potential on GBs of

CIGS film is beneficial to photovoltaic performance. 

Ev

Ec

Evac

Potential

Without surface depletion

EF GB

Surface Bulk

(a)

Ev

Ec

Evac

Potential

Without surface depletion

EF GB

Surface Bulk

(a)

With surface depletion

GB

Surface Bulk

Ev

EF

Ec

Evac

Potential

(b) With surface depletion

GB

Surface Bulk

Ev

EF

Ec

Evac

Potential

(b)

Fig. 4. Schematics of two-dimensional band diagram

around the GB in cases of (a) without and (b) with

consideration of surface band bending. 

0

100

P

otential (

m

V)

Distance (μm)

2 4 6 8 10

(c)

Air-SKPM

UHV-SKPM

1 μm (a) AFM

(b) SKPM 1 μm

Fig. 3. (a) AFM and (b) the corresponding SKPM images

of the CIGS film taken by UHV-SKPM; (c) a line cut along 

the arrows in (b) together with the line profile in Fig. 2(c) 

for comparison. 

With varying the Ga content in the film, we found that

the potential on the GB drops sharply in a range of

28%~38% Ga [Fig. 5(a)]. The theoretical curve in Fig. 5(b)

is predicted solely from the band gap consideration [11]. 

By comparing the plot of built-in potential in Fig. 5(a) to the 

theoretical and experimental plots of the conversion

efficiency in Fig. 5(b), one sees that the potential on the

GB correlates well with the measured device efficiency.

The increase of efficiency in the range of 0%~28% Ga is

due mainly to the increase of the band gap, and the

potential on the GB in this Ga range stays strong.

However, at the higher Ga content, the absence of the

potential on the GB seems to be significant compared to

the effect of the band gap widening. Thus, the drop in the 

measured efficiency is slower compared to the precipitous 

drop in the potential at the GB. Even though the 

3
measured efficiency of a thin-film device is affected by

other factors in addition to band gap and built-in potential

at the GB, such as the nature of the junction partner and

its related band alignment with that of the absorber (e.g.,

the CIGS in this study), the condition stated above is

strong enough to demonstrate a strong benefit of the builtin potential on GBs in thin-film CIGS-based devices. 

Although the physics underlying why the GB potential

benefits the photovoltaic performance cannot be directly

deduced from the potential measurement, a possible

mechanism is the GB potential-assisted minority-carrier

collection, because the GB potential attracts electrons and

repulses holes. If there is a electron path on the GB 

through the cathode of the device (CdS and ZnO layers)

and a hole path in the grain interior through the anode (Mo

contact), the potential on the GB can be expected to

increase the collection area of minority carriers and

change the one-dimensional collection in the p-n junction

to a three-dimensional collection configuration. A study of

CIGS devices by tuning Ga content pointed out that the

low conversion efficiency at high Ga content results mainly

from voltage-dependent minority-carrier collection and the

low value of fill factor [12], which is consistent with the GB

potential-assisted minority-carrier collection mechanism. 

CONCLUSIONS 

Hei

g

ht of p

otential

peak (mV) Efficie

n

c

y

η (%)

0

50

100

150

200

0 20 40 60 80 100

Ga Content Ga/(In+Ga) (%)

8

12

16

20

24

(a)

(b)

28

32

FIG. 5. (a) Measured potential height on the GB versus Ga 

content of Ga/(In+Ga); (b) Theoretical (solid line) and

experimental (squares) conversion efficiencies of CIGS 

devices versus Ga content. 

We have demonstrated the existence of local built-in

potential at the GB that arises from the positively charged

GB, and that the GB potential plays a positive role in

device conversion efficiency. This points out an approach 

for cell design in which the band gap or Ga content is 

increased while the built-in potential on the GB has to be 

kept strong. In addition, our finding that the GB potential

drops sharply in the Ga range of 28%~38% may open

both theoretical and experimental research to investigate

what happens to the atomic and electronic structures of 

the GB in this Ga content. 

ACKNOWLEDGEMENT 

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

Energy under contract number DE-AC36-99GO10337. 

REFERENCES 

[1] K. Ramanathan, M. Contreras, C. Perkins, S. Asher, F.

Hasoon, J. Keane, D. Young, M. J. Romero, W. Metzger, 

R. Noufi, J. Ward, and A. Duda, Prog. Photovolt. Res. 

Appl. 11, 225 (2003). 

[2] C. H. Champness, Proceedings of the 29th IEEE 

Conference (IEEE, Piscataway, 2002), P. 732; L. S. Yip 

and I. Shih, Proceedings of the 1st World Conference on

Photovoltaic Energy Conversion (IEEE, Piscataway,

1994), P. 210. 

[3] B. M. Keyes, P. Dippo, W. K. Metzger, J. AbuShama,

and R. Noufi, J. Appl. Phys. 94, 5584 (2003). 

[4] O. Lundberg, M. Bodegard, and L. Stolt, Thin Solid 

Films 431-432, 26 (2003). 

[5] K. Bothe, G. H. Bauer, and T. Unold, Thin Solid Films

403-404, 453 (2002). 

[6] J. Bardeen, Phys. Rev. 71, 717 (1947). 

[7] C.-S. Jiang, H. R. Moutinho, D. J. Friedman, J. F. 

Geisz, and M. M. Al-Jassim, J. Appl. Phys. 93, 10035

(2003). 

[8] A. Kikukawa, S. Hosaka, and R. Imura, Appl. Phys.

Lett. 66, 3510 (1995). 

[9] C.-S Jiang, R. Noufi, J. A. AbuShama, K. Ramanathan, 

H. R. Moutinho, J. Pankow, and M.M. Al-Jassim, Appl. 

Phys. Lett. 84, 3477 (2004).

[10] D. S. Albin, J. J. Carapella, J. R. Tuttle, and R. Noufi, 

Mat. Res. Soc. Symp. Proc. Vol. 228, 267 (1992). 

[11] M. A. Green, Solar Cells (The University of New

South Wales, P.O. Box 1, Kensington, NSW 2033,

Australia), p. 89.

[12] W. N. Shafarman, R. Klenk, and B. E. McCandless, 

Proceedings of the 25th IEEE photovoltaic specialists 

conference, 1996, p. 917

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4. TITLE AND SUBTITLE

AFM-Based Microelectrical Characterization of Grain Boundaries in 

Cu(In,Ga)Se2 Thin Film 

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NREL/CP-520-37338 

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C.-S. Jiang, R. Noufi, K. Ramanathan, J.A. AbuShama, 

H.R. Moutinho, and M.M. Al-Jassim 

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14. ABSTRACT (Maximum 200 Words)

We report on a direct measurement of two-dimensional potential distribution on the surface of Cu(In,Ga)Se2 thin films 

using a nanoscale electrical characterization of scanning Kelvin probe microscopy both in air and in ultra-high 

vacuum. The potential measurement reveals a higher surface potential or a smaller work function on grain 

boundaries (GBs) of the film than on the grain surfaces. This demonstrates the existence of a local built-in potential 

on GBs, and the GB is positively charged. The role of the built-in potential in device performance was further 

examined and found to be positive, by tuning Ga content or bandgap of the film. With increasing Ga content, the 

potential drops sharply in a Ga range of 28%~38%. Comparing the change in the built-in potential to the theoretical 

and experimental photoconversion efficiencies, we conclude that the potential plays a significant role in the device 

conversion efficiency of NREL’s three-stage Cu(In,Ga)Se2 device. 

15. SUBJECT TERMS

PV; AFM-based microelectrical characterization; nanoscale electrical characterization; scanning Kelvin probe 

microscopy; grain boundaries (GBs); device conversion efficiency; 

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