August 2004 • NREL/CP-520-36323 

Conductive Atomic Force 

Microscopy Applied to 

CdTe/CdS Solar Cells 

H.R. Moutinho, R.G. Dhere, C.-S. Jiang, 

M.M. Al-Jassim, and L.L. Kazmerski 

Prepared for the 19th European PV Solar Energy 

Conference and Exhibition 

Paris, France 

June 7–11, 2004 

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CONDUCTIVE ATOMIC FORCE MICROSCOPY APPLIED TO CdTe/CdS SOLAR CELLS 

H.R. Moutinho, R.G. Dhere, C.-S. Jiang, M.M. Al-Jassim, and L.L. Kazmerski 

National Renewable Energy Laboratory

1617 Cole Blvd., Golden CO 80401 USA 

Tel. 303-384-6457, Fax. 303-384-6604, e-mail: helio_moutinho@nrel.gov 

ABSTRACT: In this work we describe for the first time the use of conductive atomic force microscopy (C-AFM) in the 

study of CdTe/CdS solar cells, before and after the etching processes used in device fabrication. C-AFM is a new 

technique that provides information on the electrical properties of the sample in conjunction with topographic images 

with high lateral resolution. At the same time, this technique allows for the generation of I-V curves at very well-defined 

locations. A potential is applied between the sample and a very sharp tip, which scans the sample in contact mode. The 

current images showed that different CdTe grains produce different contrast. Etching the CdTe with a bromine/methanol 

solution enhanced the current along grains boundaries when compared to the intragrain material. Etching with a solution 

of nitric and phosphoric acids did not show this effect. 

surface. 

Keywords: Conductive-AFM, CdTe, etching.

1 INTRODUCTION 

Conductive atomic-force microscopy (C-AFM) is a 

recently developed technique that uses an AFM to apply 

a voltage between a conductive tip and the sample 

surface, and to measure the current [1,2]. Together with 

the topographic image, a current image is obtained. In 

addition, the tip can be placed on any point of the surface, 

and current x voltage (I-V) curves can be obtained. The 

advantage of C-AFM over other techniques is the high 

spatial resolution, which, for instance, allows for the 

isolated study of grain boundaries, as well as intragrain 

regions in polycrystalline materials. 

In the past few years, C-AFM has been used to study 

the properties of many material systems [3-5], but, to the 

best of our knowledge, this is the first time that it has 

been applied for the investigation of CdTe/CdS solar 

cells. We have studied the samples before and after CdCl2 

treatment, and after bromine/methanol (BM) and nitric 

phosphoric (NP) etches. The CdCl2 treatment is an 

essential process during the production of high-efficiency 

devices, promoting recrystallization and grain growth, 

and improving the electrical-optical properties of the 

devices [6-7]. The BM and NP etches create a Te-rich 

highly conductive layer on the top of the CdTe film [8-9], 

important for the fabrication of an ohmic back contact. 

2 EXPERIMENTAL PROCEDURE 

The samples used in this study had the following 

structure: glass\SnO2\CdS\CdTe. The CdS was grown by 

chemical-bath deposition and the CdTe by close-spaced 

sublimation. After the deposition of the CdTe films, the 

whole structure was treated in CdCl2 vapor at 400°C for 5 

minutes. Some cells were then subjected to an etching 

treatment with a 0.5% bromine in methanol solution (BM 

etch), for 2 seconds, followed by a thorough rinse with 

methanol. Finally, these samples were dried with a 

nitrogen gas jet. Other cells were subjected to another 

etching process, in a phosphoric acid:nitric 

acid:deionized water (88:1:35) solution (NP etch), for 30 

to 45 seconds, until the entire surface was covered with 

Instead, it increased the current through the whole sample 

tiny bubbles as a result of the reaction. These samples 

were then thoroughly washed in running DI water, and 

dried with a nitrogen gas jet. 

Four groups of samples were analyzed by C-AFM: 

as deposited, after CdCl2 treatment, after BM etch, and 

after NP etch. The analysis was done in a Digital 

Instruments Dimension 3100 scanning probe microscope, 

using diamond-coated Si tips, which have high 

conductivity and are resistant to wear [10,11]. The 

cantilever spring constant was about 42 N/m. To allow 

the current to pass through the sample, this kind of 

analysis is done in contact mode. The electric contact to 

the sample holder was made on the conductive SnO2 film, 

using silver paint, while the tip was grounded. A positive 

or negative potential was applied to the tip, as well as a 

ramp voltage for the I-V measurements. 

3 RESULTS AND DISCUSSION 

A setup of the experiment is shown in Fig. 1. The 

path of the current is represented by a cone-shaped 

surface starting at the tip and going down the CdTe, 

through the CdS, and ending at the SnO2 film, where the 

current is collected. Notice that the current is higher 

(darker color) in the region below the tip and decreases as 

we move laterally on the film. Also, the current passes 

through a very small cross-sectional area just below the 

tip. This results in a high spatial resolution at the CdTe 

surface, and, consequently, in the detection of variations 

in the electrical properties in very small areas. As the 

current goes into the CdTe, the cross-sectional area 

increases, and the current at these locations reflects the 

average of the properties of the bulk CdTe. So, we expect 

that most of the variation in the C-AFM images will be 

produced by variation in the properties of the CdTe close 

to the surface. Figure 2 shows a typical I-V curve 

obtained from the device. Because the voltage is applied 

to the SnO2 layer, a positive voltage means that the cell is 

polarized in reverse bias, and a negative voltage means 

that it is polarized in forward bias. The behavior in Fig. 2 

is opposite to what we would expect for the CdTe/CdS 

junction. This indicates that the I-V curve is being 

1
(b) 

Figure 1: Set-up for the C-AFM measurements. The 

shaded area under the tip corresponds to the electric 

current. 

controlled by another junction, which has to be the one 

between the tip and the sample surface, and which has the 

opposite polarity as the CdTe/CdS junction. Indeed, the 

boron-doped p-type diamond tip and the p-type CdTe can 

form a rectifying junction depending on the position of 

the Fermi levels of the two materials. Unfortunately, the 

work function and electron affinity of the diamond layer 

are not available, but our results can only be explained by 

a rectifying junction at this interface. So, the current 

when a negative bias is applied to the SnO2 behaves like 

a reverse current, and when a positive bias is applied to 

the SnO2 behaves like a forward current, as observed in 

Fig. 2. It is important to mention that this behavior is 

observed for the as-deposited, CdCl2-treated, and BMetched samples. 

The force used in the analysis varied from about 10-7 

to 10-6 N. Although both forces would produce the same 

results, images taken with the lowest force were, in 

general, more stable. Also, the tips lasted longer and there 

was no degradation of the surface. The degradation effect 

is shown on Fig. 3a, for a force of 5x10-7 N. The image 

Figure 2: I-V curve for a CdTe/CdS sample after CdCl2 

treatment. 

(a) 

(b) 

Figure 3: (a) Topographic image of an as-deposited 

CdTe film after being analyzed at a scan area of 20 μm. 

(b) Corresponding C-AFM image. 

was initially taken at 20 μm, and then at 40 μm. The 

disappearance of sharp features at the center, where the 

original image was taken, is clear. Besides destroying the 

surface, this effect may produce artifacts on the C-AFM 

image, because material is moved around the surface, and 

probably some material is deposited at the grain 

boundaries, which may change the electrical signal. 

Another effect of the analysis is shown in Fig. 3b, where 

the decrease in the current on the previously analyzed 

area is also clear. Indeed, in several analyses, we 

observed a decrease in the current with time, and the 

reason for this has not been determined yet. Nevertheless, 

for the majority of the analyses, there was no significant 

change of the signal with time, and image imprinting, as 

shown in Fig. 3, was not observed. 

Our equipment is able to detect currents down to a 

few pA. Although the qualitative results did not change 

with a change in voltage (and consequently, current), we 

noticed that the images tend to be more stable at higher 

voltages and currents. For this reason, we obtained most 

of the data with voltages from 2 to 10 V, and the current 

in the range of a few to hundreds of nA. Furthermore, 

unless stated otherwise, the C-AFM data shown in this 

work were taken with the CdTe/CdS junction polarized in 

reverse bias. 

Figure 4 shows an AFM image (top) of an asdeposited sample, and the corresponding C-AFM image 

(bottom). There is a very good correspondence between 

both images, and the grain structure is clearly observed in 

both images. The difference in brightness from grain to 

grain in the C-AFM image indicates that the conductivity 

is not uniform. This is probably due to different grains 

having different doping levels and/or different carrier 

mobilities, which are caused by differences in the defect 

2
Figure 4: Topographic image of an as-deposited CdTe 

film (top) and corresponding C-AFM image (bottom). 

structure. Notice also that the current can change even 

inside a grain. No special contrast was observed at grain 

boundaries, which suggests that, for as-deposited 

samples, there is no significant change in conductivity at 

this region. It is important to mention that there is no sign 

of interference of the topography on the current image, 

which is commonly observed in electrical measurements 

done with AFM, such as in scanning capacitance 

microscopy. 

After the CdCl2 treatment, we observed that there is 

a general increase in current levels. Nevertheless, it is 

difficult to correlate this increase in current with changes 

in the properties of the CdTe film, because there may be 

an oxide layer between the film and the tip, and the 

properties of this layer may be changed by the CdCl2 

treatment, consequently changing the magnitude of the 

current. The C-AFM image of a CdTe film after the 

CdCl2 treatment is shown in Fig. 5. The current contrast 

from grain to grain, and even inside a given grain, is still 

observed. Nevertheless, most of the grain boundaries now 

are darker than the intragrain material. This indicates a 

decrease in conductivity after the treatment, and agrees 

with the work from Visoly-Fisher et al., who observed a 

Figure 5: C-AFM image of a CdTe film after CdCl2 

treatment. 

3

lower carrier concentration at grain boundaries for CdTe 

samples after the CdCl2 treatment [12]. 

Figure 6a shows an AFM image of the CdTe surface 

after BM etch. There are no major changes on the 

topographic features, but there is a significant change in 

the relative current going through grains and grain 

boundaries, as seen in Fig. 6b. The enhanced conductivity 

at grain boundaries is probably due to preferential etching 

at these locations. Danaher et al. [13] observed that Br2 

penetrated the CdTe film, presumably through the grain 

boundaries, reaching the indium tin oxide layer. Figure 6c 

shows a linescan from Fig. 6b, where the enhanced 

conductivity along grain boundaries, and the difference in 

conductivity for different grains, can be observed more 

clearly. It is also obvious from the figure that, as pointed 

out before, there is no noticeable effect of topographic 

features on the current image. Still, most of the I-V 

curves showed a rectifying character of the surface 

junction. Nevertheless, the grain boundaries showed 

higher conductivity, providing an equivalent image when 

the dc bias was reversed. This effect indicates that the 

BM treatment was somewhat effective in improving the 

conductivity of grain boundaries, probably by creating a 

Te-rich layer at these regions, but it may not be as 

effective for the other regions of the surface. 

Finally, we discuss the results obtained after the NP 

etch, which are shown in Fig. 7. There is a substantial 

Figure 6: (a) AFM image of a CdTe surface after BM 

etch. (b) Corresponding C-AFM image. The line 

indicates the position of the linescan shown in (c). 
Figure 7: C-AFM image of a CdTe film after NP etch 

(top), and I-V curve taken at a point on the surface 

(bright area). 

increase in the current for these films. Most images had 

to be taken at voltages below 1 V, so that the current did 

not overload the detector. Also, for these conditions, the 

current images lose lateral resolution and no longer match 

the topographic images. This is caused by the conductive 

Te-rich layer, which allows current flowing through a 

larger area on the surface during image acquisition. An IV curve taken on a bright area of the current image is 

shown on the bottom of Fig. 7. Contrary to what has been 

observed for the other samples, the I-V curve is 

characteristic of an ohmic contact. These results agree 

with other studies that show the formation of a highly 

conductive Te-rich layer on the CdTe surface after the 

NP etch [9]. The NP process may also etch grain 

boundaries preferentially. Nevertheless, the C-AFM from 

the surface does not show this effect, possibly because of 

the highly conductive Te layer covering the whole 

surface. To investigate this effect, we will follow this 

work by studying the cross section of CdTe/CdS devices. 

The above results support the observations of our device 

group, which has found that the NP etch is essential for 

the production of high-efficiency CdTe/CdS solar cells, 

and that it is more effective than the BM etch for this 

purpose. 

We have noticed a strong effect of illumination on 

the current images of samples after the NP etch, which 

was not observed in other samples. Even with ambient 

light, there was a photocurrent that increased when the 

sample was submitted to direct illumination. This effect 

will be discussed in detail in a future publication. 

4 CONCLUSIONS 

We applied C-AFM for the first time to CdTe/CdS 

solar cells, and demonstrated that this technique can be of 

great value in investigating the electrical properties of 

this device. High spatial resolution was obtained, which 

allowed the study of the properties of the CdTe films 

close to the surface. 

The C-AFM measurements showed that the asdeposited samples have grains with different 

conductivities, which indicate differences in carrier 

concentration and/or mobility, caused by a different 

defect structure. After the CdCl2 treatment, there was a 

relative decrease in the current at grain boundaries, 

indicating a decrease in carrier concentration at these 

locations. Treatment with BM etch showed a relative 

increase in current at grain boundaries, whereas the NP 

etch resulted in an increase in the current over the entire 

CdTe surface, as well as a change in the contact between 

the tip and the sample surface from rectifying to ohmic. 

Based on our results, we conclude that, when performed 

with the parameters used in this work, the NP etch is 

more suitable than the BM etch for fabricating ohmic 

back contacts in CdTe/CdS solar cells. 

5 ACKNOWLEDGEMENTS 

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

Energy under contract number DE-AC36-99GO10337. 

6 REFERENCES 

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[8] J.-P. Haring, J.G. Werthen, and R.H. Bube, J. Vac. 

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Wolf, W. Vandervorst, L. Hellemans, A. Malave, D. 

Buchel, E. Oesterschulze, W. Kulisch, P. Niedermann, T. 

Sulzbach, and O. Ohlsson, J. Vac. Sci. Technol. B 18 

(2000) 418. 

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Phys. Lett. 82 (2003) 556. 

[13] W.J. Danaher, L.E. Lyons, M. Marychurch, and G.C. 

Morris, Appl. Surf. Sci. 27 (1986) 338. 

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Conductive Atomic Force Microscopy Applied to CdTe/CdS Solar 

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14. ABSTRACT (Maximum 200 Words) 

In this work we describe for the first time the use of conductive atomic force microscopy (C-AFM) in the study of 

CdTe/CdS solar cells, before and after the etching processes used in device fabrication. C-AFM is a new technique 

that provides information on the electrical properties of the sample in conjunction with topographic images with high 

lateral resolution. At the same time, this technique allows for the generation of I-V curves at very well-defined 

locations. A potential is applied between the sample and a very sharp tip, which scans the sample in contact mode. 

The current images showed that different CdTe grains produce different contrast. Etching the CdTe with a 

bromine/methanol solution enhanced the current along grains boundaries when compared to the intragrain material. 

Etching with a solution of nitric and phosphoric acids did not show this effect. Instead, it increased the current 

through the whole sample surface. 

15. SUBJECT TERMS 

PV; conductive atomic-force microscopy (C-AFM); CdTe, etching; 

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