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Cross-Sectional Conductive 

Atomic Force Microscopy of 

CdTe/CdS Solar Cells: Effects 

of Etching and Back-Contact 

Processes 

Preprint 

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

T. Gessert, A. Duda, M. Young, W.K. Metzger 

X. Li, 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-520-39802 

May 2006 
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CROSS-SECTIONAL CONDUCTIVE ATOMIC FORCE MICROSCOPY OF CdTe/CdS 

SOLAR CELLS: EFFECTS OF ETCHING AND BACK-CONTACT PROCESSES1

H.R. Moutinho, R.G. Dhere, C.-S. Jiang, T. Gessert, A. Duda, M. Young, W.K. Metzger, X. Li and M.M. Al-Jassim 

National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401, USA 

ABSTRACT 

We investigated the effects of the etching processes 

using bromine and nitric-phosphoric acid solutions, as well 

as of Cu, in the bulk electrical conductivity of CdTe/CdS 

solar cells using conductive atomic force microscopy (CAFM). Although the etching process can create a 

conductive layer on the surface of the CdTe, the layer is 

very shallow. In contrast, the addition of a thin layer of Cu 

to the surface creates a conductive layer inside the CdTe 

that is not uniform in depth, is concentrated at grains 

boundaries, and may short circuit the device if the CdTe is 

too thin. The etching process facilitates the Cu diffusion 

and results in thicker conductive layers. The existence of 

this inhomogeneous conductive layer directly affects the 

current transport and is probably the reason for needing 

thick CdTe in these devices. 

INTRODUCTION 

Over the years there has been numerous studies on 

the formation of ohmic back-contacts to solar cells using 

CdTe [1,2]. However, many issues still require a better 

understanding, such as the exact role of the etching 

processes and Cu incorporation. It is generally accepted 

that the etching of the CdTe surface is an important step in 

fabricating good back contacts. The most common etches 

use bromine/methanol (BM) or nitric/phosphoric acids 

(NP) solutions, and are supposed to leave a rich 

conductive Te layer on the surface of the CdTe [3] that 

facilitates the formation of an ohmic contact. In addition, 

in one way or another, all back contacts used in CdTe 

cells use Cu. Some back-contact configurations use 

Au/Cu, graphite paste, or ZnTe:Cu. Copper is believed to 

dope the CdTe close to the surface [4], increasing the 

carrier concentration and allowing for tunneling, and/or 

favoring the creation of Cu2-xTe at the surface, which 

lowers the potential barrier at this location [5]. It is also 

known that Cu diffuses into the CdTe, even reaching the 

CdS film [6]. 

In this work, we study the conductivity of the bulk of 

CdTe/CdS solar cells using conductive atomic force 

microscopy (C-AFM), which is a new analytical technique 

that uses the sharp tip of an AFM to produce images of 

the electrical current going through the sample [7]. In this 

technique, a conductive tip is scanned over the surface of 

a sample in contact mode, while an electrical potential is 

maintained between the tip and the sample. The position 

of the cantilever, as well as the current, are both 

measured at the same time, and topographic and current 

images are generated from the same area. Also, the 

voltage can be ramped, giving rise to current versus 

voltage curves. The advantage of this technique over 

standard electrical measurements is the high spatial 

resolution that can detect variations in the electrical 

properties of the sample in very small regions. For 

instance, in this work, we can detect differences in electric 

current going through the material when the tip is placed 

inside grains or at grain boundaries. 

In previous work [8], we have applied C-AFM to 

study CdTe/CdS solar cells after NP and BM etching 

processes. At that time, we applied a potential between 

the tip and the SnO2 film, while the tip was scanned over 

the film surface. Among other findings, we observed that 

the BM etch created a more conductive layer at the grainboundary regions, whereas the NP etch created a 

conductive layer on the whole surface of the film. In the 

present work, we investigated the effect of these two 

etching processes in the bulk of CdTe by performing CAFM on cross sections of the CdTe/CdS solar cells. We 

also studied the cells with and without a back-contact layer 

containing Cu, which was shown to be the most important 

factor for creating a conductive layer inside the CdTe film. 

EXPERIMENTAL PROCEDURE 

In this work, we used the following solar cell 

structure: CdTe/CdS/i-SnO2/SnO2/substrate. The CdTe 

was deposited by close-spaced sublimation, with a 

thickness of about 8 μm. The CdS was grown by 

chemical-bath deposition and was about 100 nm thick. 

The SnO2 bi-layer (conductive and insulating) was 

________________________________________________________________________________________________ 

1 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 

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or reproduce the published form of this work, or allow others to do so, for United States Government purposes.

1
deposited by chemical-vapor deposition. We used two 

types of substrates: glass and GaAs. Although the real 

devices are prepared on glass, it is difficult to prepare flat 

cross-sections on samples deposited on this substrate. 

Often, the glass and the CdTe film do not break at the 

sample level, resulting in a step between them, which 

makes imaging with the AFM difficult. GaAs is very easy 

to cleave, and, in general, there was no step between the 

GaAs and the CdTe film. In the beginning of this work, we 

compared samples using both substrates, and the results 

of the C-AFM analyses were very similar. For this reason, 

we used GaAs in the subsequent depositions. After 

deposition, the films were treated in CdCl2 vapor at 400°C 

for 5 minutes. Some films were then etched with 

nitric/phosphoric or bromine/methanol solution. We varied 

the etching time to verify its effect on the formation of the 

conductive layer inside the CdTe film. We used times 

from 15 to 120 s for the NP etch and from 1 to 5 s for the 

BM etch. Some samples received the deposition of an 

Au/Cu layer to investigate the effects of Cu diffusion into 

the film. The thickness of the Au film was about 150 nm, 

and the thickness of the Cu film was 5 and 10 nm; both 

films were deposited at room temperature by electronbeam deposition. Several samples were heat treated at 

150°C for 90 minutes to study the effect of temperature on 

the observed conductive layer. For comparison, we also 

analyzed films with ZnTe:Cu, instead of the Au/Cu layer. 

Finally, we analyzed standard solar cell devices, which 

use graphite paste or ZnTe:Cu to form the back contact. 

Because the objective of the measurement was to 

investigate the resistivity of the bulk CdTe, we placed one 

contact on the sample surface while the other contact was 

provided by the tip. We also contacted the SnO2 to allow 

current through this layer, making it visible in the current 

images. 

The C-AFM analysis was performed in a Veeco 

Dimension 3100 scanning probe microscope with 

Nanoscope IV electronics. The analysis was performed in 

contact mode, and we used doped-diamond-coated tips, 

which have low resistivity and are resistant to wear [9]. 

RESULTS AND DISCUSSION 

The C-AFM analysis showed a highly conductive 

layer inside the CdTe films with Au/Cu contact and after 

the etching process, as shown in Fig. 1. Figure 1a shows 

the topography of the solar cell. On the left is the GaAs 

substrate, then the SnO2/CdS layers (the individual CdS 

film is not visible), then the CdTe film. Figure 1b shows a 

current image from the same area, with negative bias 

applied to the SnO2 and Au films, and Fig. 1c shows the 

same image with positive bias. The conductive SnO2 layer 

and part of the CdTe film close to the sample surface are 

clearly seen in both current images. The CdTe deeper in 

the film is much more resistive and practically no current is 

observed for the applied dc bias. By applying opposite 

polarities (Figs. 1b and 1c) and performing I-V 

measurements, we found an ohmic behavior between the 

conductive region and the doped-diamond tip. Comparing 

the topographic and current images, we observe that the 

higher conductivity region is concentrated on the grains 

close to the surface, whereas there is no significant 

current on the grains located deeper into the CdTe. We 

notice the formation of a high-current region inside most 

CdTe films that had a Cu film on the surface and that were 

subjected to NP or BM etches. However, in general, the 

conductive layer is not as uniform as in Fig. 1. The 

penetration can be deeper in a given area, while it is 

shallower in another. An example of this behavior is 

shown in Fig. 2. In this figure, we traced the conductive 

area on the CdTe (Fig. 2b) and placed the trace over the 

topographic image (Fig. 1a). It is clear that the conductive 

area extends up to a grain close to the CdS film and that it 

is highly non uniform. The depth and distribution of the 

high-current area depend on many factors, such as 

treatment and back contact, but also on the type of crosssection fracture. A difficulty in interpreting our results lies 

in the fact that we are studying the conductivity of the bulk 

of the sample, a three-dimensional property, while we are 

analyzing cross sections of the sample, which are twodimensional slices of the film. The analysis of the 

topographic images indicates that the CdTe fracture 

occurs in a complex way, with a mixture of intragrain 

 

Fig. 1- Topographic (a) and current images (b,c) of a 

CdTe/CdS solar cell with Au/Cu back contact after NP 

etch for 120 s. Figures b and c were taken with dc 

bias equal to -100 mV and 100 mV, respectively. 

2
(through the grain) and intergrain (through grain 

boundaries) fractures. Figure 3 shows a sample with a 

grain having a mixture of intragrain and intergrain 

fractures. The large grain on the bottom shows several 

terraces that are extremely flat and clearly indicate 

intragrain fracture. Notice that the area with high 

conductivity stops at this grain, where the interior is 

exposed. This feature was observed in several images, 

and, in general, grains with intragrain fracture did not 

showed significant conductivity. On the contrary, grains 

with high conductivity, in general, have a rougher 

topography, which is characteristic of intergrain fracture. 

These results indicate that most of the observed high 

conductivity was observed at grain-boundary regions, 

whereas the interior of the grains have still high resistivity. 

This agrees with the work of Chou et al., who observed 

that Cu diffuses faster in polycrystalline CdTe than in 

single-crystal CdTe [10], and associated this fact with 

faster Cu diffusion at grain boundaries. 

We did not notice any major differences in the 

current images in samples etched with BM or NP 

solutions. We also changed the etching times for both 

processes, but could not establish a clear correlation 

between this parameter and the depth of the conducting 

layer in the CdTe. A clear example is shown in Figs. 1 

and 2, where the high-conductivity area is thicker in the 

sample etched for a lower time. To investigate relevance 

of the etching process and to determine the most 

important factor in creating the high-conductivity layer 

inside the CdTe film, we analyzed etched samples without 

Cu film on the surface, and samples with Cu film but with 

no etching. The results showed that there is no highconductivity area inside the CdTe film if there is no Cu 

deposited on the film surface, independent of the etching 

process. To confirm that the high conductivity is caused 

by Cu, we deposited only Cu, without the Au film, and the 

results were equivalent (Fig. 3). The correlation of Cu 

contents and the high-current areas was confirmed by 

secondary-ion mass spectrometry. Due to lack of space 

here, these results will be reported in a future publication. 

Similarly, we found that if there is Cu on the CdTe surface, 

but the film is not subjected to etching, in general, the 

high-conductivity layer is shallow or non-existent. This 

indicates that, besides helping the formation of an ohmic 

contact, the etching process facilitates the diffusion of Cu 

into the material. The mechanics of this process may 

include the generation of defects that facilitate the 

diffusion of Cu through the grain boundaries. However, 

further studies will be needed before an explanation for 

the process can be provided. 

 

Fig. 2 – Topographic (a) and current (b) images of a 

CdTe/CdS cell with Au/Cu back contact after NP etch 

for 60 s. The dc bias applied in b was 100 mV. 

To verify if the creation of the high-current area is 

present in regular devices, we analyzed standard solar 

cells with two different types of back contacts: 

HgTe/CuTe-doped graphite paste and ZnTe;Cu. In the 

case of the cells using ZnTe:Cu, there is no wet-etching 

process, but an ion-beam milling before the ZnTe:Cu 

deposition. The objective of this treatment is to clean and 

create a nearly stoichiometric surface [11]. The current 

image for a cell with graphite paste back-contact is shown 

 

Fig. 3 – Topographic (a) and current (b) images of a 

Cu/CdTe/CdS cell after NP etch for 60 s. 

3
in Fig. 4. In this device, the observed high-current region 

is shallow in all the analyzed areas. This cell had an 

efficiency (η) equal to 10.6 %, open-circuit voltage (Voc) 

equal to 0.813 V, short-circuit current (Jsc) equal to 21.2 

mA/cm2

, and fill factor (FF) equal to 61.3%. In contrast, 

we analyzed another cell that showed a deep highconductivity layer in all analyzed areas, with the highcurrent areas approaching the junction in some spots. 

This cell had η equal to 4.6 %, Voc equal to 0.588 V, Jsc

equal to 17.7 mA/cm2

, and FF equal to 43.8. It is possible 

that the poor performance of the latter cell is due to local 

shunts caused by the deep penetration of the conductive 

areas. Solar cells with the ZnTe:Cu back contact also 

presented high-conductivity areas inside the CdTe layer. 

The above results indicate that the effect of the 

etching process is to enhance the diffusion of Cu into the 

material, mainly at grain boundaries. Cu is a faster 

diffuser into CdTe, and the observed penetration 

happened without any intentional heating of the substrates 

during film deposition. Due to the relatively high currents 

observed here, we believe that Cu forms a highconductivity region inside the CdTe that may cause micro 

shunts if it reaches the junction. For our samples, in over 

more than a hundred measurements, we observed very 

deep penetration in only a few cases, as seen in Fig. 2, 

including shunting of the device twice. This potential 

short-circuit explains why these cells need a thick layer of 

CdTe, although only about 1 μm is needed to absorb the 

solar radiation. For our base line devices, using a wetetch process, the optimal thickness of the CdTe is about 8 

μm. 

CONCLUSIONS 

 

Fig. 4 – Topographic (a) and current (b) images of a 

standard CSS CdTe/CBD CdS solar cell using graphite 

paste in the back contact. 

We have used C-AFM to study cross-sections of 

CdTe/CdS solar cells with high spatial resolution. We 

observed that Cu, present in the back contact, diffuses 

into the material, creating high-conductivity regions inside 

the CdTe film. This diffusion process is enhanced by both 

etching processes, using nitric-phosphoric acids or 

bromine/methanol. The conductive layer inside the 

material is concentrated at grain boundaries and may 

reach the junction in a few areas of the sample, causing 

short circuits. These results may explain the need for 

thick CdTe layers used in our devices. 

ACKNOWLEDGEMENTS 

This work is supported or funded under DOE 

Contract No. DE-AC36-99GO10337. 

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