# Quarterly Report March-June 2003

## **Understanding the Structural and Chemical Basis of**

### **Chalcopyrite Solar Cell Operation**

> A. Rockett and I.M. Robertson

Previously, we have reported on the micro-structure and micro-chemistry
of *industrial* samples provided by Global Solar Energy (GSE) and Shell
Solar Industries. Differences between the two materials were found, and
the initial results were summarized in a paper published in MRS spring
meeting (copy attached).

Recently, we received and began to study three kinds of samples from IEC
at University of Delaware. The growth procedure and resulting
efficiencies of devices produced from each sample are different, and are
different from the *industrial* films. All samples except the Shell
Solar sample were received as completed devices with CdS junctions
pattern and transparent oxide coatings. The work of this quarter
concentrated on IEC sample 1 (No: 33505) and the second GSE sample
(Sample No. 809SA). Because of weak adhesion between the film and
Mo-buffer layer sputtered on glass, a careful sample preparation
involving the use of a low-speed wire saw was developed in order to
prepare the plan-view and cross-sectional samples.

1\. Microstructure & microchemistry of IEC sample 1 (No: 33505)

Figure 1a is a typical TEM bright-field image. It gives an overview of
the IEC film morphology. The film is not fully dense and shows voids of
different sizes. The typical grain-size varies from 0.1μm to 0.8μm,
mostly being around 0.4μm, proving once again that the large grain sizes
are not necessary to obtaining a good device performance (near 14% in
this case). Both the number and volume of voids in IEC sample 1 are
higher than those of GSE sample 1 previously studied. Inside the grains
some parallel fringes were observed due to twins or stacking faults.
These planar defects are frequently observed in CIGS samples.
Dislocations are occasionally found in a few grains. The number of
dislocations varies from several to many within these grains. Figures 1b
and 1c show two grains with different dislocation densities. It is hard
to determine any correlation between grain size and dislocation density.
Ongoing studies will also examine any compositional dependence.

The chemical measurements of more than 120 probes (with the typical
beam-diameter of about 5nm) are summarized in figure 2a. No significant
impurities such as O or Na were found inside the grains. The average
composition (atomic percentage) is 24.5%Cu, 7.3%Ga, 52.1%Se, 16.2%In,
which is quite close to the result of 23.5%Cu, 9.3%Ga, 49.2%Se, 18.0%In
provided by IEC. Figure 2b is an enlargement of the data plotted in
figure 2a, and shows the composition of this CIGS film is slightly
off-stoichiometry. Fitting of the results shows that the data points lie
along a line connecting stoichiometric Cu(InGa)Se~2~ and CuSe~2~. This
is not a normal tie line of ternary phase diagram and indicates a loss
of In without change in the Cu/Se ratio. This off-stoichiometry may be
attributed to an in-homogeneous microstructure caused by an inefficient
annealing treatment, or to the loss of In during the sample preparation
(In is easily removed during ion milling). Further microchemical
analysis on cross-sectional samples will be performed in the next
funding period.

![](media/image1.png){width="2.7527777777777778in"
height="2.2888888888888888in"}

![](media/image2.png){width="1.9520833333333334in"
height="1.7756944444444445in"}
![](media/image3.png){width="1.7840277777777778in"
height="1.7840277777777778in"}

Figure 1. The microstructure of IEC sample 1 (33505). (a) An overview
image showing the high density of voids, at grain boundaries. White
arrows mark the possible twins and stacking faults. Two enlarged images
showing the grains containing of (b) many of and (c) several
dislocations.

![](media/image4.jpeg){width="3.2875in"
height="2.49375in"}![](media/image5.jpeg){width="2.472916666666667in"
height="1.6347222222222222in"}

EDS line scans were performed to study the local composition variation
at the grain boundaries. Figures 3a and 3b present two typical images in
which the microchemistry crossing grain boundaries was probed. Figures
3c and 3d show the elemental variation along the probe line. CdS was
obviously found at the grain boundary of figure 3c, but not in figure
3d. No significant CdS was found inside the grains in both figures 3a
and 3b, indicating that these elements do not diffuse into the grains
significantly even when a thin CdS volume was present in grain
boundaries.

![](media/image6.wmf){width="2.8333333333333335in"
height="1.9965277777777777in"}![](media/image8.png){width="2.7215277777777778in"
height="1.7916666666666667in"}
![](media/image9.png){width="2.7215277777777778in"
height="1.7819444444444446in"}

Figure 3 Line scan cross the grains and grain boundaries. (a) and (b)
are two typical images. (c) and (d) are corresponding composition. CdS
was found in the GBs of (a) and (c) whereas not in (b) and (d).

Figure 4 presents the charts revealing correlations between Cd and S,
and other elements (Ga, In, Se, Cu). No strong correlations between Cd
and other elements (Ga, In, Se, Cu) were revealed. In addition, no
correlations between S and other elements were revealed. However, it was
found that there is a significant correlation between S and Cd: the Cd L
percentage increases with the S K percentage almost linearly but with a
ratio of 2 Cd atoms per S atom. There is no significant correlation of
Cd with Se, indicating that Se is not compensating for the loss of S. A
further study of the microchemistry on the cross-sectional samples, in
particular at the areas close to the CdS/CIGS interface is required to
understand the CdS at grain boundaries.

![](media/image10.wmf){width="3.02992125984252in"
height="2.502755905511811in"}

![](media/image12.wmf){width="3.0200787401574805in"
height="2.495275590551181in"}

![](media/image14.wmf){width="3.0in" height="2.538976377952756in"}

Figure 4. The correlation of (a) Cd L and SK, (b) Cd L and Cu K, (c) Cd
L and In L, (d) Cd L and Ga K, (e) Cd L and Se K, and (f) S K and In L.

2\. Microstructure of GSE sample 2 (sample number 809SA)

Figure 5a presents a plan-view image of sample 809SA. This
microstructure is similar to the GSE sample 1 that was reported
previously. The film is much denser than that of the IEC sample 1.
Moreover, the typical grain-size varies from 0.5μm to 1.2μm, most being
close to the 1μm level. Hence, the grains in the GSE sample are bigger
than those of IEC sample 1. Inside the grains twins and stacking faults
are found, as marked with white arrows. These observations are
consistent with those reported previously for sample 1. Electron
diffraction reveals that the film is well ordered. The evidence is shown
in figure 5b, which is the \[112\] pattern (the index is based on
diamond structure) of CIGS. The superlattice spots in figure 5b are
strong and sharp, revealing good ordering of CIGS grains.

3\. Summary and future work

It is of great importance to compare these samples, to find the
difference between them. Table 1 summarizes the information of three
kinds of films we are investigating.

![](media/image16.png){width="2.920138888888889in"
height="2.598611111111111in"} ![](media/image17.jpeg){width="1.95in"
height="1.95in"}

Figure 5. (a) A TEM bright field image showing the microstructure of
CIGS films. (b) is an electron diffraction pattern recorded from one
grain. The white frame marks the unit cell of diamond structure while
arrows mark the superlattice spots resulting from the order of CIGS. The
strong and sharp superlattice diffraction spots reveal the good ordering
degree.

Table1: Comparison of three kinds of samples investigated in this work

+----------+------------------+--------------------+------------------+
|          | Shell Sample     | GSE sample1        | IEC sample1      |
|          |                  | (823SB)            | (33505)          |
+----------+------------------+--------------------+------------------+
| S        | CIS/Mo/Glass     | Ox                 | ITO/C            |
| tructure |                  | ide/CdS/CIGS/Mo/SS | dS/CIGS/Mo/Glass |
+----------+------------------+--------------------+------------------+
| Ef       | 14%              | 11%                | 13.7%            |
| ficiency |                  |                    |                  |
+----------+------------------+--------------------+------------------+
| A        | 550ºC            | 550ºC              | 400ºC            |
| nnealing |                  |                    |                  |
| T        |                  |                    |                  |
+----------+------------------+--------------------+------------------+
| Pre      | Single stage     | Two stages         | Two stages       |
| paration |                  |                    |                  |
+----------+------------------+--------------------+------------------+
| Micros   | 1:Grain size     | 1:Grain size: near | 1:Grain size     |
| tructure | near 1 um        | 1um                | 0.5um            |
|          |                  |                    |                  |
|          | 2:Dense film     | 2: Occasional      | 2:High density   |
|          | free of voids    | voids in both      | of voids, in     |
|          |                  | grains and at      | particular at GB |
|          |                  | grain boundaries   | areas            |
+----------+------------------+--------------------+------------------+
| Com      | 1:CuInSe~2~      | 1:Cu~19.7~(I       | 1:               |
| position |                  | nGa)~28.2~Se~52.1~ | Cu~24.5~(        |
|          | 2: Impurities: O |                    | Ga~7.3~In~16.2~) |
|          |                  | 2:Impurities: Na,O | Se~52~           |
|          |                  |                    |                  |
|          |                  |                    | 2: CdS at GBs    |
+----------+------------------+--------------------+------------------+

Based on the results we have obtained and questions encountered, our
plans in next funding period are as follows.

> \(1\) To repeat to compositional analysis of plan-view &
> cross-sectional samples of the Shell Solar Industries material in
> order to verify the different behavior of oxygen, sodium and selenium
> between grain interiors and grain boundaries. Microchemical analysis
> of additional grain boundaries in both plan and cross-sectional view
> samples of the Global Solar Energy will be performed to confirm the
> initial observations.
>
> \(2\) To prepare the plan-view and cross-sectional samples of the
> other two IEC samples, and determine the microstructure and
> microchemistry.
>
> \(3\) To compare the composition gradients of IEC samples and GSE
> samples in order to understand how the growth temperature and
> preparation methods can affect the equilibrium of film microstructure.

\(4\) To study the interfacial microchemistry between CdS and CIGS.