# Quarterly Report July-September 2003

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

### **Chalcopyrite Solar Cell Operation**

> A. Rockett and I.M. Robertson

The objective of this project is to link microstructural and
microchemical features of device-quality Cu(In,Ga)Se~2~ to the
performance of resulting solar cells. At this point we are focusing
primarily on three samples from IEC with properties as follows:

+------+-------+-----+----+-------+-------+-----+--------------+----+
| Nu   | Pr    | Tss | T  | Voc   | Jsc   | FF  | EDS          | E  |
| mber | ocess |     | hi | (V)   | (mA   | (%) | composition  | ff |
|      |       |     | ck |       | /cm2) |     |              | (  |
|      |       |     |    |       |       |     |              | %) |
|      |       |     | -  |       |       |     |              |    |
|      |       |     | ne |       |       |     |              |    |
|      |       |     | ss |       |       |     |              |    |
|      |       |     | (u |       |       |     |              |    |
|      |       |     | m) |       |       |     |              |    |
+------+-------+-----+----+-------+-------+-----+--------------+----+
| 33   | Bi-   | 40  | 1  | 0.59  | 33    | 71  | Cu~23.5      | 1  |
| 505. | layer | 0°C | .9 |       |       |     | ~In~18.0~Ga~ | 3. |
| 11(S |       |     |    |       |       |     | 9.3~Se~49.2~ | 7% |
| amp. |       |     |    |       |       |     |              |    |
| 1)   |       |     |    |       |       |     |              |    |
+------+-------+-----+----+-------+-------+-----+--------------+----+
| 33   | Bi-   | 55  | 2  | 0.65  | 32    | 76  | Cu~22.7      | 1  |
| 631. | layer | 0°C | .4 |       |       |     | ~In~18.2~Ga~ | 6. |
| 23(S |       |     |    |       |       |     | 8.1~Se~51.1~ | 0% |
| amp. |       |     |    |       |       |     |              |    |
| 2)   |       |     |    |       |       |     |              |    |
+------+-------+-----+----+-------+-------+-----+--------------+----+
| 33   | Un    | 55  | 2  | 0.65  | 33    | 74  | Cu~22.9      | 1  |
| 621. | iform | 0°C | .3 |       |       |     | ~In~18.8~Ga~ | 5. |
| 23(S |       |     |    |       |       |     | 6.9~Se~51.4~ | 9% |
| amp. |       |     |    |       |       |     |              |    |
| 3)   |       |     |    |       |       |     |              |    |
+------+-------+-----+----+-------+-------+-----+--------------+----+

Samples were received as completed devices with CdS junctions and
transparent oxide top contacts. These samples provide comparisons of
single and bilayer device behaviors at standard deposition temperatures
as well as comparing high and low deposition temperatures. Therefore, it
is of interest to compare the microstructure and microchemistry of these
samples, in order to understand how the temperature and process
determine the microstructure and microchemistry, and to correlate this
with the performance of the solar cell devices. Details of the
preparation and characterization of these devices may be found in W.N.
Shafarman and J. Zhu, Proc. Mat. Res. Soc. Symp., Spring 2001, San
Francisco.

For comparison with these samples, data have also been obtained on
polycrystalline samples from Global Solar Energy, Shell Solar
Industries, the Uppsala University, and the Hahn Meitner Institute.
Single crystalline samples from the Hahn Meitner Institute (CuGaSe~2~)
and the University of Illinois have been examined as well.

In the previous report, we described the micro-structure and
micro-chemistry of IEC sample 1(No: 33505.11). The plan-view
microstructure image from sample 1 showed that the film contains a high
density of voids. The average grain size is about 0.4 microns.
Micro-chemistry from the plan-view sample showed that there is no
significant compositional variation within the grains, however, CdS was
detected at the some grain boundaries. The following sections describe
the ongoing analyses of the above samples. We have completed preparation
of cross-sectional samples for comparison with the plan view data in the
previous report. In addition, we have conducted extensive microchemical
analyses of the samples.

**1. Microstructure & microchemistry of IEC sample 1 in cross-section**

Figure 1a is a typical TEM bright-field image that gives an overview of
the cross-sectional morphology of IEC sample 1. Most grains are columns,
growing from the Mo layers to the surface. The boundaries between the
neighboring columns are not dense. Voids exist in the film, creating
weak connections the between grains. The typical column-size varies from
0.1μm to 0.6μm, with most being around 0.4μm. Inside some columns a high
density of parallel fringes were observed, (a) as shown in figure 1b.
These parallel fringes show the presence of twins or stacking faults.
Planar defects are frequently observed in CIGS and are not obviously
correlated with device performance. These observations are consistent
with the plan-view observations. The CIGS is capped with CdS and a
transparent conductive oxide layer. The cross-sectional image shows that
the surface of the CIGS film is not flat, which leads to a rough ITO
surface as well. The roughness of the surface is estimated to be about
100nm. The interfaces between the layers are not well defined, mainly
because of the rough surface, as shown in figure
1c.![](media/image1.jpeg){width="5.506944444444445in"
height="6.529166666666667in"}

![](media/image2.png){width="3.0743055555555556in"
height="2.3618055555555557in"}![](media/image3.jpeg){width="5.235416666666667in"
height="2.8631944444444444in"}

The chemical measurements of bottom, central and surface areas (with the
typical beam-diameter of about 5nm) are summarized in the ternary plot,
figure 2a. Figure 2b is the enlargement of the central part of figure
2a. No significant impurities such as O or Na were found inside the
grains. The film composition is nearly homogenous for growth at 400°C.
No significant composition variations are found between the bottom of
the film (Mo side) and the center of the film. The scatter in the data
shown in Figure 2 is primarily due to noise, although it appears that
all compositions are significantly Se-rich. The scatter in the data at
the rear of the film is somewhat greater than in the center of the film,
possibly due to the presence of the substrate or to the initial
formation of the film on the Mo back contact. By contrast, significant
composition variations occur at the surface regardless of whether one
measures parallel to or perpendicular to the surface. The average
composition (atomic percentage) obtained from the central areas is
24.0%Cu, 7.25Ga%, 51.8%Se, and 16.83%In, which is slightly different
from the EDS result of 23.5%Cu, 9.3%Ga, 49.2%Se, 18.0%In provided by IEC
(obtained by SEM analysis of the surface of the film). The higher In
fraction in the IEC analysis is consistent with the group III rich
surface composition. Comparing the EDS data from the bottom, central and
surface areas obtained by TEM shows that the surface is Ga-rich, while
the bottom is In-rich, in spite of the fact that Ga and In were
deposited together at a fixed flux ratio. This suggests that in the
bilayer process the Ga tends to move more toward the surface at this low
deposition temperature.

EDS measurements were performed to study the local composition variation
at the grain boundaries with the line-scanning technique. Similar to the
results obtained in the plan-view samples, CdS was found at some grain
boundaries. No significant CdS was found inside the grains, indicating
that these elements do not diffuse into the grains significantly even
when a thin CdS volume was present in grain boundaries. The results
suggest that CdS deposited by CBD can penetrate deep into the grain
boundaries when they are not fully dense. A significant result of the
analysis is that there is no change in the stoichiometry of the film as
a function of CdS content. The Cu/(In+Ga) ratio remains constant with
variations in Cd atomic percentage from 0 to 40 atomic percent. Thus,
the small amount of CdS in the boundary is not associated with a
chemical change in the CIGS in this
sample.![](media/image4.jpeg){width="4.285416666666666in"
height="4.520138888888889in"}

**2. Microstructure & microchemistry of IEC sample 2 (No: 33631.23)**

Sample 2 is identical to sample 1 in deposition conditions except for
the temperature (550°C for sample 2 and 400°C for sample 1). Figure 3
presents a typical TEM bright field image, which was recorded from the
plan-view specimen of IEC sample 2. It was found that grain size in IEC
sample 2 varies from 0.6μm to 1.2μm, which is much larger than in IEC
sample 1. Voids can occasionally be found in the interior of grains and
at the grain boundaries but the number and volume of the voids are
significantly lower than those found in IEC sample 1. The grain
boundaries are very clear, and the grains contain few defects, although
twins and stacking faults are still present in the grains. No second
phase was observed in the films. Electron diffraction techniques show
that the grains are well ordered, see the inset.

About 130 micro-EDS measurements were obtained from the interior grains
of the plan-view specimen and are summarized in figure 4a. The average
composition (atomic percent) of these is 23.5%Cu, 7.6Ga%, 50.8%Se, and
18.1%In, which is quite close to the EDS result of 22.7%Cu, 8.1%Ga,
51.1%Se, 18.2%In provided by IEC (by SEM). It is evident that these data
in the ternary plot are close to the stoichiometric point, though
slightly Cu-poor, indicating a very uniform composition as a function of
position in the film. A comparison of Figure 4 with Figure 2 shows that
the bilayer films are not well mixed when grown at 400°C while at 550°C,
complete mixing occurs.

![](media/image5.png){width="2.910416666666667in"
height="2.140972222222222in"}![](media/image6.pct){width="3.167361111111111in"
height="2.2895833333333333in"}

The line-scanning technique was used to study the local composition
variation of the grain boundaries. A plot of the EDS data is shown in
figure 4b. Similar to the results obtained from sample 1, CdS was
occasionally found at a few grain boundaries. No significant CdS was
found inside the grains, indicating that these elements do not diffuse
into the grains even when a thin CdS volume was present at grain
boundaries.

Figure 5a is a typical TEM bright-field image that gives an overview of
the cross-sectional morphology of IEC sample 2. Unlike the columnar
grains found in the low temperature film (sample 1), these grains do not
show the long narrow column shape anymore. The grains are larger and
more symmetric than the grains in the low temperature sample, having a
grain diameter similar to the film thickness and penetrating completely
through the film. As in the plan-view sample, twins and stacking faults
exist in the grains. The cross-sectional image shows that the CIGS film
surface is not flat, which leads to a rough ITO surface. However, the
roughness of the surface is much smaller than the low temperature film.
In some grains the interface between the CIGS and the CdS and oxide
layers is well defined. Some CdS is found in the grain boundaries or
intergranular surface grooves, as shown in figure 5b.

![](media/image7.jpeg){width="6.202083333333333in" height="2.6875in"}

**3. Summary and future work**

Samples 1 and 2 were deposited by the bilayer (Cu~2~Se followed by
III-selenides) process but at two different temperatures. Table 1
summarizes our results to date comparing the materials. It is found that
the increase of the growth temperature increases grain size and
intermixing of the Cu and III selenide layers. The higher temperature
also reduces the voids in the film, makes the film surface flatter,
seals the grooves between grains, and consequently produces a
well-defined interface between layers.

For the next period, we will focus on

> \(1\) Chemistry determination from the cross-sectional specimen of IEC
> sample 2, in particular at the CIGS/CdS interface.
>
> \(2\) Preparation of plan-view and cross-sectional samples of the
> third IEC sample, and determination of the microstructure and
> microchemistry.
>
> \(3\) Comparison of the composition gradient of IEC samples, in order
> to understand how the annealing treatment and preparation can affect
> the equilibrium of film microstructure.

\(4\) The interfacial structure and microchemistry between CdS and CIGS.

Table1: Comparison of two kinds of IEC samples investigated in this work

+-----------+-------------------------+-------------------------------+
|           | IEC sample1 (33505.11)  | IEC sample 2 (33631.23)       |
+-----------+-------------------------+-------------------------------+
| Structure | IT                      | ITO/ZnO/CdS/CIGS/Mo/Glass     |
|           | O/ZnO/CdS/CIGS/Mo/Glass |                               |
+-----------+-------------------------+-------------------------------+
| E         | 13.7%                   | 16%                           |
| fficiency |                         |                               |
+-----------+-------------------------+-------------------------------+
| Annealing | 400ºC                   | 550ºC                         |
| T         |                         |                               |
+-----------+-------------------------+-------------------------------+
| Process   | Two stages              | Two stages                    |
+-----------+-------------------------+-------------------------------+
| Micro     | 1:Grain size 0.5um      | 1:Grain size: near 1um        |
| structure |                         |                               |
|           | 2:High density of       | 2: Occasional voids in both   |
|           | voids, in particular at | grains and at grain           |
|           | GB areas                | boundaries                    |
+-----------+-------------------------+-------------------------------+
| Co        | 1:                      | 1:Cu~2                        |
| mposition | Cu                      | 3.5~(In~7.6~Ga~18.1~)Se~51.1~ |
|           | ~24.5~(Ga~7.3~In~16.2~) |                               |
|           | Se~52~                  | 2: CdS occasionally at GBs    |
|           |                         |                               |
|           | 2: CdS at many GBs      |                               |
+-----------+-------------------------+-------------------------------+
| Film      | Roughness about 100nm   | Better than sample 1          |
| surface   |                         |                               |
+-----------+-------------------------+-------------------------------+
| Grain     | Random                  | Random                        |
| or        |                         |                               |
| ientation |                         |                               |
+-----------+-------------------------+-------------------------------+
| Interface | Not well defined        | Clear in many grains          |
+-----------+-------------------------+-------------------------------+