# Quarterly Report October-December, 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:

Table I: samples investigated to date from IEC.

+------+-------+-----+----+-------+-------+-----+--------------+----+
| Nu   | Pr    | Tss | T  | Voc   | Jsc   | FF  | EDS          | E  |
| mber | ocess |     | hi | (V)   | (mA   | (%) | composition  | ff |
|      |       |     | ck |       | /cm2) |     |              | (  |
|      |       |     |    |       |       |     |              | %) |
|      |       |     | -  |       |       |     |              |    |
|      |       |     | ne |       |       |     |              |    |
|      |       |     | ss |       |       |     |              |    |
|      |       |     | (u |       |       |     |              |    |
|      |       |     | m) |       |       |     |              |    |
+------+-------+-----+----+-------+-------+-----+--------------+----+
| 33   | Two-  | 40  | 1  | 0.59  | 33    | 71  | Cu~23.5      | 1  |
| 505. | stage | 0°C | .9 |       |       |     | ~In~18.0~Ga~ | 3. |
| 11(S |       |     |    |       |       |     | 9.3~Se~49.2~ | 7% |
| amp. |       |     |    |       |       |     |              |    |
| 1)   |       |     |    |       |       |     |              |    |
+------+-------+-----+----+-------+-------+-----+--------------+----+
| 33   | Two-  | 55  | 2  | 0.65  | 32    | 76  | Cu~22.7      | 1  |
| 631. | stage | 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.

In the previous two reports, the microstructure and microchemitry of
sample 1 (low temperature, bi-layer) and sample 2 (high temperature,
bi-layer) were studied in view of both plan-view and cross-section. The
main results are listed in Table II, below. This report focuses on the
sample 3 which is processed by a single stage process at 550°C. We have
completed preparation of both plan-view and cross-sectional samples for
comparison as with the previous samples.

1\. Microstructure of IEC sample 3

Fig.1 is a typical plan-view image showing the microstructure of sample
3. It is evident that the CIGS film is polycrystalline, like the other
two samples. No significant second phases were found in the film. Grain
sizes vary with an average size of \~1μm, which is close the sample 2 or
bi-layer sample grown at the same temperature. A significant difference
between sample 3 and the previous two bilayer samples that sample 3 is
very dense. Almost no voids were observed both at grain boundaries and
in the interior of the grains. The preliminary conclusion is that the
bilayer process usually produces voids in these films. Further study
shows that CIGS phase is well ordered, as proved by the sharp
superlattice reflections in the diffraction pattern. The film normal of
many grains are close to \[112\] of CIGS, or the surface is close to the
close packed planes.

Table II: Comparison of two kinds of IEC samples investigated in this
work

+----------+------------------+-------------------+-------------------+
|          | IEC sample1      | IEC sample 2      | IEC sample 2      |
+----------+------------------+-------------------+-------------------+
| Micros   | 1:Grain size     | 1:Grain size:     | 1: Grain size:    |
| tructure | 0.5um            | near 1um          |                   |
|          |                  |                   | 2: almost free of |
|          | 2:High density   | 2: Occasional     | voids             |
|          | of voids, in     | voids in both     |                   |
|          | particular at GB | grains and at     |                   |
|          | areas            | grain boundaries  |                   |
+----------+------------------+-------------------+-------------------+
| Com      | 1:               | 1                 | 1:                |
| position | Cu~24.5~(Ga~7.3  | :Cu~23.5~(In~18.1 | Cu~23.5~(In~17.6  |
|          | ~In~16.2~)Se~52~ | ~Ga~7.6~)Se~51.1~ | ~Ga~7.5~)Se~51.5~ |
|          |                  |                   |                   |
|          | 2: CdS at many   | 2: CdS            | 2: no CdS found   |
|          | GBs              | occasionally at   | at GBs            |
|          |                  | GBs               |                   |
+----------+------------------+-------------------+-------------------+
| Film     | Roughness about  | Better than       | Flat, better than |
| surface  | 100nm            | sample 1          | sample 2          |
+----------+------------------+-------------------+-------------------+
| Grain    | Random           | Random            | Near {112}        |
| ori      |                  |                   |                   |
| entation |                  |                   |                   |
+----------+------------------+-------------------+-------------------+
| I        | Not well defined | Clear in many     | Well defined in   |
| nterface |                  | grains            | many grains       |
+----------+------------------+-------------------+-------------------+

![](media/image1.jpeg){width="3.9583333333333335in"
height="4.093055555555556in"}

Quantitative EDS was applied to study the composition variation within
the samples. Random positions were probed in interior of grains, and
grain boundaries were line-scanned as in previous reports in order to
compare the grain boundary and bulk grain data. The results from the
bulk and grain boundaries are together presented in Figure 2 for the
center part of the ternary phase diagram. The average of about 120 data
points for the bulk grains is 23.5%Cu, 17.6%In, 7.5%Ga, 51.5%Se, which
is quite close the nominal composition provided by IEC, 22.9%Cu,
18.8%In, 6.9%Ga, and 51.5%Se.

> ![](media/image2.jpeg){width="4.917361111111111in"
> height="3.285416666666667in"}

Fig.2. The EDS measurement of compositions of sample 3, showing that
film is slightly Cu-poor and that the grain and grain boundary
compositions overlie one another nearly perfectly.

2.  The cross-sectional study

> Fig. 3 is a typical bright field cross-sectional image of sample 3. In
> agreement with the plan-view sample, the grain size is \~1 μm. Similar
> to the other two samples, the columnar grains span the full thickness
> of the sample. The film is dense and free of voids. Comparing to the
> bilayer sample, the surface of the CIGS is very flat, and consequently
> the CdS and oxide covering layers are flat. The surface of the CIGS
> film is close the {112} close-packed plane of chalcopyrite. This fact
> is further proved by the planes of twins or stacking faults in the
> films, which are marked by arrows. The {112} twin planes are parallel
> to the film surface. High resolution images show the facets more
> clearly.
>
> Fig. 4 presents a STEM bright field image and its corresponding
> annular dark field (ADF) image of the top part of the film. The ADF
> image is sensitive to the average atomic weight of the material in the
> image (Z-contrast). The interface between layers is not sharp or clear
> in the BF image due to similar contrast. However, the flat and sharp
> interfaces were revealed by the ADF technique. The CIGS, CdS, ZnO and
> ITO layers are well defined due to the different atomic numbers (Z) of
> layers. This is a significant difference between the bilayer and
> one-stage samples -- the interfaces are less diffuse due to lower
> nanoscale roughness.
>
> ![](media/image3.jpeg){width="2.4611111111111112in"
> height="2.501388888888889in"}
>
> ![](media/image4.jpeg){width="5.509722222222222in"
> height="2.4347222222222222in"}

Fig. 4. A STEM bright (a) and ADF (b) image reveal the sharp interface
between layers. The sharp interfaces are more apparent in the ADF
images.

> As with the plan-view specimens, the composition of the films were
> investigated by quantitative EDS. The back, center and surface parts
> of the films were scanned separately in order to study the composition
> profile of the films. Each scan ran parallel to the substrate and thus
> gives an idea of the scatter in the compositions at a given depth in
> the sample, while comparisons from one scan to another gives the
> variation in composition with depth. No significant composition
> variations with depth were found in this sample with the exception of
> the surface being slightly In-rich. We further investigated the grain
> and grain boundary compositions as well as shown in Figure 5. As
> before, no significant difference in grain and grain boundary
> composition was found.
>
> Since the carriers are usually collected over a thin area \~50nm wide
> near the heterojunction, line-scanning was performed to study the
> composition variations. Fig.5b is a typical result showing the
> composition variation. It is found Cu and Ga decrease slowly from the
> CIGS towards to CdS, whereas In increases slowly. This fact explains
> why the IEC sample grown with the single-layer process can have good
> performance \-- the surface is slightly In-rich.
>
> ![](media/image5.pct){width="5.393055555555556in"
> height="3.702777777777778in"}![](media/image6.jpeg){width="4.975in"
> height="2.7888888888888888in"}

3\. Summary and future work

In this quarter we investigated the IEC sample 3 which was grown by the
uniform process at 550°C. The performance of this sample is comparable
to bilayer sample grown at the same temperature. However, the
microstructure of sample 3 is different from the bilayer samples: it is
free of voids, and the surface is close to being {112} close packed
planes. EDS measurements show that the film composition is homogeneous,
however, the surface is slightly Cu-depleted.

> The schedule for next quarter is:

1\. To clarify the interfacial structure between CdS and CIGS, we shall
conduct HREM study the interface between CdS and CIGS of samples 2 and
3.

2\. To refine the composition profiles at the CdS/CIGS interface, and to
apply the elemental mapping technique to study possible diffusion at the
interface.

3\. To refine the composition profiles at the grain boundaries. In
particular to focus on the light elements such as O and Na. For this we
shall apply the EELS imaging technique.