# Energy Photovoltaics, Inc.

276 Bakers Basin Road

Lawrenceville, NJ 08648

# September 6, 2004

Dr. Harin Ullal, Technical Monitor

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, CO 80401

Dear Harin,

This is the third quarterly report for Phase III for EPV's cost-shared
subcontract RDJ-2-30630-21 **Advanced CIGS Photovoltaic Technology**
awarded under the Thin Film Photovoltaics Partnership Program. The
nominal period covered by the report is 6/1/04 -- 8/31/04.

The core CIGS group consists of Dr. Alan Delahoy, Dr. Leon Chen, Dr.
Baosheng Sang, Dr. Masud Akhtar, John Cambridge, Frank Ziobro, Ramesh
Govindarajan, and Renata Saramak. Siân Kleindienst recently left EPV to
start graduate school at MIT.

This report summarizes the main activities conducted in this quarter
(Q3), concluding with plans and other news. It will be recalled that the
Q2 highlight was a 7.5% 26W module verified by NREL. The highlight of
this quarter is a 13.1% cell verified by NREL.

1)  Device optimization

2)  Cell performance vs. i-ZnO conductivity

3)  Uniformity improvement for large area plate

4)  Outdoor sub-module stability

5)  Overall progress with hybrid process

6)  New types of TCO

7)  Plans for next quarter

8)  Other news

```{=html}
<!-- -->
```
1)  **Device optimization**

In the previous quarter (Q2) we worked assiduously on module issues to
achieve the full module performance allowed by the then-available CIGS
quality and uniformity. In the current period (Q3) we recognized that
further improvement of large area module performance required a quantum
jump in the quality and uniformity of the hybrid CIGS films and devices
produced in the Zeus system. Therefore, in Q3, the CIGS team focused
strongly on device optimization, and especially on formation of the CIGS
absorber. In addition, although a hybrid device efficiency of 13.6% was
achieved in 2002 in the Hercules R&D system, we were also eager to
demonstrate that over 13% devices could be produced using the CIGS from
large Zeus plates, and to have this verified by NREL.

Among the many process parameters that are involved in the formation of
CIGS films, we found that substrate moving speed and deposition rate in
the last layer to be deposited (the final IGS layer) are sensitive
parameters for our hybrid process. We believe these parameters affect
the CIGS surface morphology, which we will illustrate below. After
deliberate and careful material optimization, the best cell (Z1699-12
#8) measured in house (using Zeus hybrid CIGS) was 12.9% in efficiency,
with V~oc~ 601 mV, FF 71.0% and J~sc~ 30.3 mA/cm^2^. Some similar
devices were sent to NREL for verification. Two of them, Z1699-1 #5 and
#4, exhibited efficiencies of 13.1% and 12.9%, respectively. The NREL
J-V curve for the 13.1% cell is reproduced in Fig. 1 below.

![](media/image1.png){width="5.0375in" height="6.772916666666666in"}

Fig. 1: J-V curve of Z1699-1 #5 measured at NREL

Selected EPV CIGS samples were sent to NREL for analysis of composition
profile and morphology. Listed in Table I are their deposition
conditions, compositions measured by ICP at EPV, and resulting device
performance.

Table I. Deposition conditions, composition, and device performance

+---------+-------+----------+----+----+------+--------+------+-----+
| Sample  | Se    | Cu ratio | CI |    | V    | J~sc~  | FF   | Eff |
| ID      | Temp  |          | GS |    | ~oc~ |        |      |     |
|         | \[C\] | 1^st^    |    |    |      | \[mA/c | \    | \[  |
|         |       | IGS +Cu  | Cu |    | \[   | m^2^\] | [%\] | %\] |
|         |       |          | Ga |    | mV\] |        |      |     |
+---------+-------+----------+----+----+------+--------+------+-----+
| Z1559-1 | 550   | 0.87     | 0. | 0. | 483  | 26.5   | 64.4 | 8.2 |
|         |       |          | 63 | 31 |      |        |      |     |
+---------+-------+----------+----+----+------+--------+------+-----+
| Z1      | 550   | 0.87     | 0. | 0. | 351  | 27.2   | 52.4 | 5.0 |
| 559-2\* |       |          | 86 | 31 |      |        |      |     |
+---------+-------+----------+----+----+------+--------+------+-----+
| Z1559-6 | 525   | 1.27     | 0. | 0. | 529  | 23.1   | 64.7 | 7.9 |
|         |       |          | 68 | 26 |      |        |      |     |
+---------+-------+----------+----+----+------+--------+------+-----+
| Z1559-8 | 550   | 1.27     | 0. | 0. | 530  | 25.5   | 68.1 | 9.2 |
|         |       |          | 71 | 29 |      |        |      |     |
+---------+-------+----------+----+----+------+--------+------+-----+
| Z1665   | 580   | 1.4      | 0. | 0. | 554  | 30.7   | 70.6 | 1   |
|         |       |          | 89 | 30 |      |        |      | 2.0 |
+---------+-------+----------+----+----+------+--------+------+-----+

> \* Z1559-2 is without a final IGS layer

The Cu ratio and Ga ratio profiles for these samples, calculated from
AES data provided by NREL, are plotted in Fig. 2, while SEM images for
these films are shown in Fig. 3.

![](media/image2.wmf)![](media/image3.wmf)

![](media/image4.wmf)![](media/image5.wmf)

![](media/image6.wmf)

Fig.2: Depth profiles of Cu and Ga ratios

The following are our observations:

-   The Cu ratios and Ga ratios shown in the AES profiles of Fig. 2, but
    averaged over the depth of the film, are basically consistent with
    the values shown in Table I from ICP measurement, which represent a
    bulk (average) composition. However, there do appear to be some
    minor inconsistencies, e.g. Cu ratios for Z1559-8 and Z1665 by ICP
    are 0.71 and 0.89, while the AES data would appear to give a higher
    number for Z1559-8 relative to Z1665.

-   The AES Ga ratios for all samples tend to increase from about 0.2
    near the top surface to about 0.4 near the Mo, except for Z1599-2
    which has a much higher value of 0.6 near the Mo side. It is not yet
    clear if this is related to the process without the last IGS cover
    layer.

-   Samples starting with a Cu-poor precursor (Z1599-1) or starting with
    a Cu-rich precursor but with low selenization and cover layer
    temperatures (Z1599-6) show a strong Ga gradient at the back side
    (near the Mo) and a relatively flat profile in the front. Z1559-2,
    as expected, shows the lowest surface Ga. However, the samples
    starting with a Cu-rich precursor and with higher selenization and
    cover layer temperatures (Z1599-8 and Z1665) show a smaller Ga
    gradient throughout the CIGS layer. The possible benefit of a back
    surface field resulting from a Ga gradient is not addressed by this
    data set.

![](media/image7.png) ![](media/image8.png)

![](media/image9.png) ![](media/image10.png)

![](media/image11.png) ![](media/image12.png)

![](media/image13.png) ![](media/image14.png)

![](media/image15.png) ![](media/image16.png)

Fig. 3: Surface and cross-section views

Observations and problems:

-   We believe SEM images Z1559-1 and Z1599-2 are switched since Z1559-2
    was finished by selenization and is without the 3^rd^ IGS cover
    layer as indicated in the footnote of Table I.

```{=html}
<!-- -->
```
-   A Cu rich precursor (Z1559-6, 8 and Z1665) seems to enhance film
    growth yielding larger grains. A higher selenization temperature
    also promotes grain growth (Z1665 vs Z1559-8, 6). Both observations
    are consistent with device performances shown in Table I.

-   Apparent voids (or cracks) are seen in some images of samples which
    either start with a Cu poor precursor (Z1559-1) or start Cu rich
    with a low selenization temperature (Z1559-6 and 8). It is not
    uncommon that CIGS films formed by selenization of metal precursors
    have voids due to the large volume expansion from metal to compound.
    In our case, a larger-grain top layer is often separated from a
    bottom layer having a different structure. The voids are presumably
    formed during deposition of the last cover layer. However, it is
    hard to see the specific reasons for these physical voids. The voids
    might be related to the moving speeding of plate and/or substrate
    temperature; the moving speed for Z1559 was twice that of Z1665.

-   A rough surface is noticeable in all images. This might be related
    to: 1) the maximum Cu ratio excursion in the precursor (the ratio
    for Z1665 is larger than Z1559); 2) fast moving speed; 3) deposition
    rate in the last stage. Z1665 already shows larger though stacked
    grains, but the surface morphology is still disappointing in that
    grains stand loosely and some crevices can be seen on the top layer.
    We are going to focus on this issue in the coming quarter, and
    hopefully it will lead to improved V~oc~ of the devices. The recent
    improvement of device performance mentioned earlier is one step in
    that direction.

2)  **Cell performance vs. i-ZnO conductivity**

Many research papers have been published that investigate the i-ZnO
layer. Since the resistivity of this layer can range over many orders of
magnitude (0.1 Ωcm to 10^6^ Ωcm) there are certainly numerous issues
that arise, such as: the dependence of device performance on
resistivity; optimal thickness; conditions for it to be eliminated; its
real necessity; and device stability. Some, Ruckh ^\[1\]^, claimed the
best resistivity for i-ZnO to be around 0.2 Ωcm while Contreras ^\[2\]^
has used an apparently very resistive i-ZnO layer (no resistivity value
available) produced using an intrinsic ceramic ZnO target and Ar mixture
gas with 1% oxygen. Others, Kessler ^\[3\]^, were convinced that device
quality is insensitive to i-ZnO resistivity as long as the CdS layer is
thick enough.

We have used 2% wt. Al~2~O~3~-doped ceramic ZnO targets and argon plus
oxygen to deposit i-ZnO in both the Airco (RF) and ILS (pulsed DC) at
EPV. The resistivity of the film could be adjusted via the oxygen
concentration in the gas mixture. Recently we conducted an experiment to
re-examine the sensitivity to i-ZnO resistivity. Two levels of oxygen
concentration, 5% and 1%, were used. Film resistivity for the former is
about 400 Ωcm while for the latter it is 0.6 Ωcm. Plotted in Fig. 4 is a
comparison of device V~oc~ and FF in both cases. No attempt is made to
compare J~sc~ because differences could well be masked in measurement
fluctuation.

![](media/image17.wmf){width="3.245138888888889in"
height="2.2006944444444443in"}![](media/image18.wmf){width="3.13125in"
height="2.2131944444444445in"}

Fig. 4: V~oc~ and FF for high and low resistivity i-ZnO layers

The results shown in Fig. 4 seem to support J. Kessler's assertion that
device quality is largely insensitive to i-ZnO resistivity. However, we
did observe a narrower dispersion of V~oc~ for the higher resistivity
film, and the elimination of the lower V~oc~ tail. Further work in this
area is perhaps justified.

3)  **Uniformity improvement for large area plate**

In this period, we focused on improving the uniformity of Cu sputtering.
Contrary to the non-uniformity of our evaporated IGS in which the IGS is
thicker in the middle (i.e. thinner at the source ends), we found that
sputtered Cu films are thicker near the ends of the cathode. This
results from a combination of cathode magnetic field design and the
particular source-substrate distance in the sputtering system. The net
effect on the finished CIGS film is that the Cu ratio is even smaller in
the middle having been compounded by both effects.

We decided in this quarter to try to correct the sputtered Cu thickness
distribution by modifying the magnetic field distribution of the Cu
cathode. Very encouraging indeed, the uniformity of the sputtered Cu
film after redistribution of the target magnet field is significantly
improved. Plotted in Fig. 5 is a graph of normalized Cu thickness before
and after the improvement. For this figure, the Cu thickness in three
strips along the target direction were measured and normalized. The
strips were taken at 10, 20, and 30 inches from the edge of the plate,
measured along the moving direction. Open symbols in the plot represent
the normalized thickness before target improvement while the solid ones
represent those after target improvement. The thickness uniformity along
the target direction is improved on all three strips, with the overall
level of thickness variation being reduced from 10% to 5%. The edge
thickness is increased and the center dip is largely eliminated.
However, non-uniformity in the moving direction is still apparent. The
cause of this effect is completely different. It appears related to the
pumping port position and the dependence of pressure on plate position.
We will address that problem in the next quarter.

Based upon the successful experience with the Cu target, we intend to
modify the magnet field distribution for the ceramic ZnO target soon. A
small increase of module current can be expected after such improvement
of n-ZnO uniformity. This is because in our present module process we
deliberately make the ZnO film sufficiently thick to keep its sheet
resistance from exceeding 15 ohm per square, even in its thinnest region
on the plate, to minimize the FF loss.

![](media/image19.wmf){width="6.395833333333333in" height="3.99375in"}

Fig. 5: Normalized Cu thickness distribution along the target direction

  ----------------------------------- -----------------------------------
                                      

  ----------------------------------- -----------------------------------

4)  **Outdoor sub-module stability**

Two EVA-laminated CIGS sub-modules Z1658 8-13 and Z1658 14-19 in
dimension of 6" x 17" cut from full size plate Z1658 are being monitored
under continuous exposure outdoor. They have kept excellent stability so
far for about four months. Plotted in Fig. 6 are V~oc~ values translated
to 25°C, J~sc~ values translated to 1000W/m^2^, FF (raw), and efficiency
values translated to 25°C and 1000W/m^2^, based on parameters measured
outdoors without spectrum correction during the spring and the summer
seasons. Fluctuation for V~oc~ and FF is about 1 percent, and for J~sc~
and efficiency about 2 to 3 percent. Long-term tracking of the stability
of these two modules will continue.

![](media/image20.wmf){width="3.127083333333333in"
height="2.3256944444444443in"}
![](media/image21.wmf){width="3.1222222222222222in"
height="2.326388888888889in"}

![](media/image22.wmf){width="3.1222222222222222in"
height="2.3069444444444445in"}
![](media/image23.wmf){width="3.0409722222222224in"
height="2.316666666666667in"}

Fig. 6: Outdoor stability of sub-modules Z1658 8-13 and Z1658 14-19

5)  **Overall progress with hybrid process**

EPV's overall progress to date with the hybrid process is shown in Fig.
7. This illustration was first presented in the Phase II annual report,
and has been updated with the full size module results and the 13.1%
pilot line cell efficiency obtained in this quarter (Q11).

![](media/image24.wmf){width="5.049305555555556in"
height="3.5284722222222222in"}

Fig. 7: Trajectory of progress with hybrid CIGS during the course of
this subcontract.

6)  **New types of TCO**

In the Phase III, Q2 report we showed the IV curve of a 10.7% cell using
a Ti-doped In~2~O~3~ TCO produced by reactive-environment hollow cathode
sputtering (RE-HCS). In Fig. 8 we show the optical transmission of an
ITiO film 545nm in thickness. The carrier concentration is 4.25 x 10^20^
/cm^3^ and the mobility 80.6 cm^2^/Vs, the resistivity being 1.8 x
10^-4^ Ω cm. The latter properties are superior to those generally
exhibited by ZnO:Al. Also shown are modeled curves of transmission for
films having mobilities of 20, 50, and 80 cm^2^/Vs. Note that the
transmission in the near IR increases with increasing carrier
concentration.

![](media/image25.wmf){width="5.115277777777778in"
height="3.584722222222222in"}

Fig. 8: Measured transmission of an ITiO TCO and modeled curves as a
function of mobility

7)  **Plans for next quarter**

We plan to focus on the following areas in the next quarter:

-   Achieve 8% aperture area efficiency in a large area module based
    upon improved device performance as well as plate uniformity.

-   Improve CIGS morphology.

-   Improve sputtered film uniformity along the moving direction.

-   Solve interconnection peeling problem by re-adjusting the thick ZnO
    process conditions or by ordering a new target from the original
    vendor (Target Materials) if it is confirmed that peeling associated
    with the target purchased from the new vendor (Plasmaterials) cannot
    be avoided.

-   Further explore the two-stage hybrid process to simplify CIGS film
    growth.

-   Continue tracking outdoor stability of sub-modules.

-   Further develop new types of TCO (and device process) for devices
    and modules.

-   In view of the steep increase in the price of In over the last year
    due to ever-increasing demand for flat panel displays, making
    ultra-thin CIGS absorber films may become a priority for CIGS
    manufacturers. We intend to start developing a recipe for devices
    with a 1 μm absorber next quarter. We will try optical enhancement
    if it is successful.

8)  **Other news**

The a-Si IMS (Integrated Manufacturing System), a 2.5MW/year facility,
installed by EPV for Tianjin Jinneng Solar Cell Co. in Tianjin, China,
exceeded the capacity test and the requisite progress payment has been
received.

EPV continues to receive orders for both standard EPV-40 modules and a
semi-transparent version. An installation that is under way in Germany
using EPV-40 modules is shown below.

![](media/image26.jpeg){width="5.294444444444444in"
height="3.9784722222222224in"}

Sincerely,

![](media/image27.png)

# Alan E. Delahoy Leon Chen

# Principal Investigator Senior Scientist

References:

\[1\] M. Ruckh, et al. 'Applications of ZnO in CIGS Solar Cells', 25^th^
PVSC, Washington, D.C. (1996) p. 825.

\[2\] M. Contreras, et al. 'High efficiency CIGS-based solar cells:
processing of novel absorber structures', 1^st^ WCPEC, Hawaii (1994) p.
68.

\[3\] J. Kessler, et al. 'Front contact optimization for CIGS sub
modules', 25^th^ PVSC, Washington, D.C. (1996) p. 885.