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ENERGY PHOTOVOLTAICS

# June 17, 2003

Dr. Harin Ullal, Technical Monitor

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, CO 80401

Dear Harin,

Enclosed please find the first quarterly report for Phase II for EPV's
cost-shared subcontract RDJ-2-30630-21 **Advanced CIGS Photovoltaic
Technology** awarded under the Thin Film Partnership Program. The
nominal period covered by the report is 3/1/03 - 5/31/03.

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. Dr. Natko Urli performed the sol-gel
work in this quarter. Dr. Sheyu Guo assists with hollow cathode
sputtering.

This report deals with the following areas of activity:

1)  Optimization of large size CIGS deposition in Zeus, and module
    performance

2)  Mini-module performance and process improvement in Hercules

3)  Other device configurations

4)  CdS process and surface treatment research

5)  Interconnection

6)  AR coating

7)  Plans for next quarter

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1)  **Optimization of large size CIGS deposition in** **Zeus, and module
    performance**

In this quarter, we restored a 1 square foot module process after having
attained better CIGS process control as well as an excellent CIGS recipe
in the Zeus system. Some very encouraging results for small device,
mini-module and intermediate-size module performance will be reported in
this section.

As planned in the last quarterly report, early this quarter we used a
commercial EUROTHERM temperature controller to replace our home-made
(software-based) controller to see if we could improve control of source
temperature in the Zeus. We found that the temperature fluctuation of
both Se boats was lowered to ± 1 ºC at an operating temperature of 320
ºC when the new EUROTHERM controller was used, down from ± 4 ºC when the
old software-based controller was used. For the In source, the
temperature fluctuation was lowered to ± 2 ºC at an operating
temperature at 1090 ºC when the new EUROTHERM controller was used, down
from the original ± 5 ºC. Therefore, process control is considerably
improved. A third EUROTHERM will be installed for the Ga line shortly.

In view of the fact that Cu selenization and the third stage layer
(In,Ga)~2~Se~3~ (here sometimes abbreviated to IGS) play critical roles
in CIGS performance, we designed a sample holder which can hold strips
cut from a big plate deposited with the first stage IGS plus the second
stage sputtered Cu. Through the use of this sample holder, we only need
to load one strip rather than a whole plate to optimize Cu selenization
and/or the third stage IGS layer. Not only can this method accelerate
the pace of optimization many fold, it also makes the optimization
analysis more meaningful since many experiments can be conducted with
strips cut from one large plate, with all strips having the same first
and second layers. Thus far, the sample holder has worked very well.

We started our new CIGS optimization, for simplicity, with flat first
and third IGS profiles, this being similar to the first and third stages
in the NREL three-stage process. Based on the understanding that Ga and
In have a different diffusion mechanism during film growth by NREL's
3-stage process, which could result in a Ga compositional gradient and
beneficial band-gap grading, (*M. Contreras et al.: Proc. 1^st^ WCPEC,
pp. 68*), we realize that we still have two degrees of freedom to
further optimize our recipe by adjusting the Ga profile and ratio in the
first stage and third stage respectively.

Investigation of the Cu selenization conditions was our main focus in
this period. A 38″ x 17.5″ plate (Z1559) was cut into several 3″ x 12.5″
strips, which were then employed to investigate the effects of
selenization conditions on device performance. The main purpose of this
experiment is to find an optimal selenization temperature. Two kinds of
Cu/(In,Ga)~2~Se~3~ precursor, Cu-poor (Cu/(In+Ga) = 0.87 in Z1559-1,2,3)
and Cu-rich (Cu/(In+Ga) = 1.27 in Z1559-5,6,7), as measured after first
IGS layer and sputtered Cu deposition, were used. The third stage IGS
deposition followed Cu selenization for all strips except Z1559-2, on
which no third layer was deposited.

Listed in Table I is a summary of CIGS film composition before and after
the third IGS layer, as measured by ICP, and the I-V parameters of small
cells having an area of \~ 0.2 cm^2^. The results in Table I clearly
show that:

1\) CIGS film (Z1559-2), with Cu selenization but without a third-stage
IGS layer, does not yield good devices. Most of the small devices in
this sample show shunted dark and light I-V behavior which implies that
a conductive Cu-Se phase might emerge on the surface of the CIGS and
grain boundaries, constituting shunting paths even though the total
Cu/(In+Ga) ratio is only 0.86;

2\) Devices on CIGS films made from precursors that passed through a
Cu-rich period ("Cu-rich precursors") (Z1559-5,6,7) demonstrate
noticeably higher V~oc~ even though their Ga ratio is lower;

3\) Devices with Cu-rich precursors (Z1559-5,6,7) selenized at 550ºC
show higher efficiencies than those selenized at 525ºC and 500ºC, while
there was no obvious performance difference for different selenization
temperatures for Cu-poor precursors (Z1559-1,3).

**Table I. Compositions of CIGS films and I-V parameters of th**eir
devices

<table>
<colgroup>
<col style="width: 9%" />
<col style="width: 11%" />
<col style="width: 18%" />
<col style="width: 11%" />
<col style="width: 9%" />
<col style="width: 7%" />
<col style="width: 7%" />
<col style="width: 7%" />
<col style="width: 9%" />
<col style="width: 7%" />
</colgroup>
<tbody>
<tr class="odd">
<td>Z1559</td>
<td><p>Cu/</p>
<p>(In+Ga)</p>
<p>(<strong>before</strong></p>
<p>3<sup>rd</sup> IGS)</p></td>
<td><p>Selenization Conditions</p>
<p>(Temp/Time)</p></td>
<td><p>Cu/</p>
<p>(In+Ga)</p>
<p>(<strong>after</strong> the</p>
<p>3<sup>rd</sup> IGS)</p></td>
<td><p>Ga/</p>
<p>(In+Ga)</p></td>
<td><p>Thick</p>
<p>(μm)</p></td>
<td><p>V<sub>oc</sub></p>
<p>[mV]</p></td>
<td><p>FF</p>
<p>[%]</p></td>
<td><p>J<sub>sc</sub>(QE)</p>
<p>[mA/</p>
<p>cm<sup>2</sup>]</p></td>
<td><p>Eff</p>
<p>[%]</p></td>
</tr>
<tr class="even">
<td>Z1559-2</td>
<td>0.87</td>
<td>550<sup>o</sup>C/30min*</td>
<td>0.86</td>
<td>0.31</td>
<td>1.55</td>
<td>351</td>
<td>52.4</td>
<td>27.2</td>
<td>5.0</td>
</tr>
<tr class="odd">
<td>Z1559-1</td>
<td>0.87</td>
<td>550<sup>o</sup>C/30min</td>
<td>0.59</td>
<td>0.31</td>
<td>2.11</td>
<td>483</td>
<td>64.4</td>
<td>26.5</td>
<td>8.2</td>
</tr>
<tr class="even">
<td>Z1559-3</td>
<td>0.87</td>
<td>525<sup>o</sup>C/30min</td>
<td>0.65</td>
<td>0.30</td>
<td>2.11</td>
<td>499</td>
<td>62.2</td>
<td>26.9</td>
<td>8.4</td>
</tr>
<tr class="odd">
<td>Z1559-5</td>
<td>1.27</td>
<td>550<sup>o</sup>C/30min</td>
<td>0.65</td>
<td>0.28</td>
<td>2.55</td>
<td>558</td>
<td>72.7</td>
<td>26.9</td>
<td>10.9</td>
</tr>
<tr class="even">
<td>Z1559-6</td>
<td>1.27</td>
<td>525<sup>o</sup>C/30min</td>
<td>0.63</td>
<td>0.26</td>
<td>2.56</td>
<td>529</td>
<td>64.7</td>
<td>23.1</td>
<td>7.9</td>
</tr>
<tr class="odd">
<td>Z1559-7</td>
<td>1.27</td>
<td>500<sup>o</sup>C/30min</td>
<td>0.68</td>
<td>0.28</td>
<td>2.70</td>
<td>525</td>
<td>68.9</td>
<td>22.9</td>
<td>8.3</td>
</tr>
</tbody>
</table>

> \* *w.o. 3^rd^ IGS layer*
>
> In a 3-stage process, achievement of a Cu-rich film at the end of the
> second stage Cu-Se delivery onto a (In,Ga)~2~Se~3~ precursor is
> thought to result in a Cu~x~Se liquid-phase that aids grain growth and
> yields large and columnar morphologies and better devices (*M.
> Contreras et al.: Proc. 1^st^ WCPEC, pp. 68*). This requires a Cu-rich
> stage before the third IGS deposition as well as a high substrate
> temperature (\>523ºC for a Cu~x~Se liquid phase). Our results agree
> well with the ideas and reported results of CIGS growth by a 3-stage
> process. This investigation clearly suggests that a Cu-rich precursor
> having a Cu ratio 1.2-1.3 after the sputtered Cu step is needed in
> order to fabricate good devices. Further increase of substrate
> temperature during selenization is planned. An investigation of the
> selenization time is also under way. Right now, the best efficiency
> for small cells made on Zeus material is 10.9%, as plotted in Fig. 1.
>
> ![](media/image2.wmf){width="5.238194444444445in"
> height="3.5791666666666666in"}
>
> **Fig. 1 I-V curve of device Z1559-52 C2**
>
> The distribution of cell performance was carefully checked on a 2″x 4″
> substrate. Excellent uniformity was achieved and the results are given
> in Table II. Disregarding the small fluctuation of J~sc~, the
> non-uniformity of the product V~oc~\* FF is within 10%.

**Table II I-V parameters of CIGS cells (\~0.2 cm^2^) on a 2″ x 4″
plate**

  ---------------- ----------- --------- ------------ --------- -----------
  Sample           Voc\[mV\]   FF\[%\]   Jsc(QE)      Eff \[%\] Voc\*FF

                                         \[mA/cm2\]             

  Z1559-52-A2      552         69.5      26.7         10.2      1.00

  Z1559-52-A1      553         67.1                             0.97

  Z1559-52-B1      553         69.3                             1.00

  Z1559-52-B2      556         70.3                             1.02

  Z1559-52-B5      524         68.3                             0.93

  Z1559-52-C3      541         69.4                             0.98
  ---------------- ----------- --------- ------------ --------- -----------

> Based on the improved CIGS process, a few mini-modules (about 40
> cm^2^) and one-square-foot modules were fabricated in this quarter. So
> far, we are able to produce mini-modules in the 9% range with CIGS
> deposited in the Zeus system and ZnO:Al (RF) sputtered in the Airco
> system. Z1565-31 is the best mini-module we have produced recently. It
> has 15 interconnected cells, each with a segment width of 6.35 mm. Its
> I-V curve is plotted in Fig.2.
>
> ![](media/image3.wmf){width="5.370833333333334in"
> height="3.670138888888889in"}
>
> **Fig.2 I-V curve of Z1565-31 mini-module**
>
> A couple of one-square-foot modules were also produced using CIGS
> films from the Zeus and ZnO:Al from the in-line sputtering system. The
> first module ran, Z1561, was selenized at 550ºC for 30 min, had an
> area of 827 cm^2^ with 44 series-connected cells, and yielded an
> aperture efficiency of about 5%. The efficiency was reduced by a low
> current density due mainly to the poor n-ZnO; however, the high
> V~oc~/segment of 521 mV is very encouraging for further optimization
> of the current CIGS deposition recipe. The I-V curve of Z1561-1 one-
> square-foot module is plotted in Fig. 3.
>
> ![](media/image4.wmf){width="4.8493055555555555in"
> height="3.3201388888888888in"}
>
> **Fig.3 I-V curve of Z1561-1 one square foot module**

Additionally in this quarter we further improved our source delivery
modeling elaborated in our last annual report, where only one source
boat was considered. In our real configuration, two source boats are
connected to a long delivery pipe with 20 tiny holes. The temperatures
of the two boats are designed to be the same since they are heated up
with equal current in a series connection, are of identical design, and
are initially loaded equally. Nevertheless, in practice some temperature
difference between the two boats is observed.

The objective of the new modeling is to see under what conditions vapor
flows will be delivered from both boats even if there is a temperature
difference. For simplicity, we only studied one delivery hole in the
middle of the pipe in this period. However, to extend our modeling to 20
holes is straightforward in concept.

Let P1 be the saturated vapor pressure of source boat 1 at temperature
T1, and P2 that of boat 2 at temperature T2. Suppose T2 is higher than
T1, thus P2 \> P1. We call the gas conductance in a section of pipe
between source and the tiny delivery hole C, and conductance of a hole
α. Assuming F1 and F2 are the vapor flows **out** of the sources**,** we
can write Kirchoff equations:

> F1/C + (F1+F2)/α = P1 (1)
>
> F2/C + (F1+F2)/α = P2 (2)

Solving (1) and (2) for F1, we found the condition for F1 to be positive
(that is, vapor flow **out** of source boat 1):

(P2-P1)/P1 \< α/C (3)

The vapor pressure P can be approximated as an exponential function of
source temperature, so that we have

(P2-P1)/P1 = K \* (T~2~-T~1~)/T^2^ (4)

For Se, (P2-P1)/P1 = 3.5 % at 320 ºC. Assuming conductance ratio of
delivery hole to pipe α/C = 5%, from (3) and (4) we have

(T2-T1) \< 1.43 ºC (5)

The final result (5) indicates that flow F1 will come out of boat 1 as
long as the temperature difference between the two boats is kept small
enough.

2)  **Mini-module performance and process improvement in Hercules**

We processed several excellent mini-modules with CIGS produced from the
Hercules system in this period. The best record run H 175-2 reached
9.63% aperture efficiency with V~oc~ 6.85 V (13 cells), FF 59.6% and Jsc
30.7 mA/cm^2^. We believe the fill factor could be raised to the middle
60s% by reducing cell width to 5 mm from present 6.6 mm. However, we
still don't have a clear clue about low V~oc~ yet (on average about 526
mV per cell, due mainly to CIGS film quality). Thus, we spent most of
our energy and time in this quarter to optimize the CIGS recipe with the
goal of increasing V~oc~ to the 600 mV level in Hercules.

Explored were many directions including:

-   Higher Se flux in the first and third stage of IGS, as well as in
    post-sputter Cu selenization (H176)

-   Elevated substrate temperature in the 3^rd^ stage IGS as well as in
    post-sputter Cu selenization (H177)

-   Slow cool down after the 3^rd^ stage IGS deposition (H179)

-   Change IGS ratio between 1^st^ and 3^rd^ stage (H182 and H183)

-   Reduction of total CIGS thickness (H180 and H181).

Experimental details and device/mini-module performance are summarized
in Tables III and IV, respectively. Our control runs H175 and H178 are
also listed in these tables as references. As the results in Table IV
show, no substantial improvement of Voc was observed in this set of
optimizations, although slow cool down seems to work pretty well.

**Table III Optimization of Hercules CIGS**

+------+------+-------+-------+------+------+------+--------+------+
| *    | *    | **    | **    | **S  |      | **Sl | **CIGS | **   |
| *Run | *IGS | Stage | Stage | tage |      | ow** | thick  | Cu** |
| \#** | s    | 1 Se  | 2**   | 3**  |      |      | ness** |      |
|      | plit | r     |       |      |      | **c  |        | *    |
|      | %**  | ate** | *     |      |      | ool- |        | *rat |
|      |      |       | *Cu** |      |      | do   |        | io** |
|      |      | **(35 |       |      |      | wn** |        |      |
|      |      | 0C)** |       |      |      |      |        |      |
+------+------+-------+-------+------+------+------+--------+------+
|      |      |       |       | **Se | **T  |      |        |      |
|      |      |       |       | ra   | s    |      |        |      |
|      |      |       |       | te** | ub** |      |        |      |
+------+------+-------+-------+------+------+------+--------+------+
| H175 | 7    | 12    | Sput. | 33   | 510  | No   | 2.78   | 0.84 |
|      | 5/25 |       |       |      |      |      |        |      |
+------+------+-------+-------+------+------+------+--------+------+
| H176 | 7    | ***   | Sput. | **   | 510  | No   | 2.78   | 0.88 |
|      | 5/25 | 22*** |       | *\>5 |      |      |        |      |
|      |      |       |       | 8*** |      |      |        |      |
+------+------+-------+-------+------+------+------+--------+------+
| H177 | 7    | 13    | Sput. | 2    | *    | No   | 2.79   | 0.77 |
|      | 5/25 |       |       | 6-38 | **55 |      |        |      |
|      |      |       |       |      | 0*** |      |        |      |
+------+------+-------+-------+------+------+------+--------+------+
| H178 | 7    | 14    | Sput. | 3    | 510  | No   | 2.74   | 0.82 |
|      | 5/25 |       |       | 6-42 |      |      |        |      |
+------+------+-------+-------+------+------+------+--------+------+
| H179 | 7    | 14    | Sput. | 3    | 510  | *    | 2.68   | 0.80 |
|      | 5/25 |       |       | 3-40 |      | **Ye |        |      |
|      |      |       |       |      |      | s*** |        |      |
+------+------+-------+-------+------+------+------+--------+------+
| H181 | 7    | 14    | Sput. | 3    | 510  | *    | 2.33   | **   |
|      | 5/25 |       |       | 6-40 |      | **Ye |        | *0.7 |
|      |      |       |       |      |      | s*** |        | 4*** |
+------+------+-------+-------+------+------+------+--------+------+
| H182 | ***  | 14    | Sput. | 2    | 510  | *    | 2.00   | **   |
|      | 83/1 |       |       | 9-34 |      | **Ye |        | *0.7 |
|      | 7*** |       |       |      |      | s*** |        | 2*** |
+------+------+-------+-------+------+------+------+--------+------+
| H183 | ***  | 14    | Sput. | 2    | 510  | *    | 2.03   | 0.81 |
|      | 83/1 |       |       | 9-38 |      | **Ye |        |      |
|      | 7*** |       |       |      |      | s*** |        |      |
+------+------+-------+-------+------+------+------+--------+------+
| H184 | 7    | 14    | ***   | 3    | 510  | *    | 2.48   | **   |
|      | 5/25 |       | Evap. | 3-30 |      | **Ye |        | *0.6 |
|      |      |       | at    |      |      | s*** |        | 9*** |
|      |      |       | 35    |      |      |      |        |      |
|      |      |       | 0C*** |      |      |      |        |      |
|      |      |       |       |      |      |      |        |      |
|      |      |       | ***no |      |      |      |        |      |
|      |      |       | Se*** |      |      |      |        |      |
+------+------+-------+-------+------+------+------+--------+------+

**Table IV Device and mini-module performance from Hercules**

+---------+-------+--------+-------+--------+------------------------+
| **ID**  | **Voc | **Jsc  | *     | **     | **Comments**           |
|         | (     | (mA/cm | *FF** | Eff.** |                        |
|         | mV)** | ^2^)** |       |        |                        |
|         |       |        | **    | *      |                        |
|         |       |        | (%)** | *(%)** |                        |
+---------+-------+--------+-------+--------+------------------------+
| H175-2  | 526   | 34.9   | 59.6  | 11.0   | Mini-module with 13    |
|         |       |        |       |        | cells                  |
+---------+-------+--------+-------+--------+------------------------+
| H175-4  | 510   | 35.7   | 54.8  | 10.0   | Mini-module with 13    |
|         |       |        |       |        | cells                  |
+---------+-------+--------+-------+--------+------------------------+
| H175-5A | 426   | 32.3   | 53.7  | 7.2    | Unscribed Mo           |
+---------+-------+--------+-------+--------+------------------------+
| H       | 503   | 25.8   | 63.0  | 8.2    | Higher Se flux         |
| 176-3-B |       |        |       |        |                        |
+---------+-------+--------+-------+--------+------------------------+
| H       | 546   | 28.1   | 60.3  | 9.3    | Higher temperature at  |
| 177-4-B |       |        |       |        | 3^rd^ stage            |
+---------+-------+--------+-------+--------+------------------------+
| H       | 515   | 27.9   | 57.5  | 8.3    |                        |
| 178-2-B |       |        |       |        |                        |
+---------+-------+--------+-------+--------+------------------------+
| H1      | 579   | 29.5   | 74.3  | 12.7   | 13 min. slow cool-down |
| 79-2-A2 |       |        |       |        |                        |
+---------+-------+--------+-------+--------+------------------------+
| H1      | 537   | 28.0   | 70.2  | 10.6   | Thinner CIGS           |
| 81-3-C2 |       |        |       |        |                        |
+---------+-------+--------+-------+--------+------------------------+
| H       | 559   | 26.1   | 69.8  | 10.2   | IGS split 83%/17%      |
| 182-5-B |       |        |       |        |                        |
+---------+-------+--------+-------+--------+------------------------+
| H1      | 477   | 27.4   | 62.5  | 8.2    | IGS split 83%/17%      |
| 83-4-B1 |       |        |       |        |                        |
+---------+-------+--------+-------+--------+------------------------+
| H1      | 522   | 28.5   | 67.2  | 10.0   | Evap. Cu without Se    |
| 84-4-B4 |       |        |       |        | present                |
+---------+-------+--------+-------+--------+------------------------+

In our present hybrid process, the first and the third stage IGS are
formed in the Hercules system, while Cu is sputtered in the in-line
sputtering system. Therefore, breaking vacuum twice (before and after
sputtering Cu) is unavoidable at present. Is it possible that lower Voc
in our device is caused by sputtered Cu used in our hybrid process
and/or caused by breaking vacuum? To test these thoughts, we made a run
with evaporated Cu H184 at 350 ºC in the 2^nd^ stage in Hercules instead
of sputtered Cu. We intentionally evaporated Cu without simultaneous Se
co-evaporation to simulate our sputtered Cu process as closely as
possible. Post-Cu selenization was the same as that used in our hybrid
process. Unfortunately, from the test result (experiment details as well
as test results are listed in Tables III and IV) we still didn't find a
big jump in Voc yet. It is worth noting that we still have to break
vacuum once in our all-evaporation process (after the 1^st^ stage IGS in
Run H184) because we evaporate all the material in the open boat in any
given stage. More runs with evaporated Cu are planned for the next
quarter.

3)  **Other device configurations**

One way of designing a next-generation high performance tandem device is
to insert a thin transparent conductive layer between the top and bottom
junctions to serve as contacts and tunnel junction. As the first step,
we would like to study the contact behavior of transparent conductive
layers with CIGS films. We designed an experiment with two variables,
contacts and type of layers, each with two levels. Two levels of
contacts are front and back in the experiment. Two types of transparent
conductive layers are used in the test. One is n-type CTO (SnO~2~) Tech
8 with sheet resistance 8 ohm/sq produced by LOF glass company, and the
other is the same CTO coated with a very thin intrinsic ZnO layer about
500 A thick. CTO sheet resistance does not show any observable increase
after being coated with thin i-ZnO. All four experiment conditions and
their configurations are listed in Table V below.

**Table V Other configuration of devices**

+------+-------+-----------+------------------+------------+--------+
| **   | *     | **CTO**   | **Device         | **Confi    | **     |
| ID** | *Cont |           | Structure**      | guration** | Effici |
|      | act** |           |                  |            | ency** |
+------+-------+-----------+------------------+------------+--------+
| H1   | Back  | Tech8     | Glass/Te         | Substrate  | 4.27 % |
| 78-3 |       |           | ch8/CIGS/CdS/ZnO |            |        |
+------+-------+-----------+------------------+------------+--------+
| H1   | Back  | Tech8     | Glass/Tech8+     | Substrate  | 2.52 % |
| 78-4 |       | +i-ZnO    | i-ZnO/           |            |        |
|      |       |           |                  |            |        |
|      |       |           | CIGS/CdS/ZnO     |            |        |
+------+-------+-----------+------------------+------------+--------+
| H1   | Front | Tech8     | Gla              | S          | 0.1 %  |
| 79-3 |       |           | ss/Tech8/CIGS/Mo | uperstrate |        |
+------+-------+-----------+------------------+------------+--------+
| H1   | Front | Tech8     | G                | S          | 1.7 %  |
| 79-4 |       | +i-ZnO    | lass/Tech8+i-ZnO | uperstrate |        |
|      |       |           | /CIGS/Mo         |            |        |
+------+-------+-----------+------------------+------------+--------+

All CIGS films in the experiment were deposited in Hercules. Device
performance did not show obvious improvement after air annealing at 180
ºC for two hours. As seen from Table V, Tech 8 SnO~2~ yields better back
contact with P-type CIGS than SnO2+i-ZnO, although SnO~2~ of Tech 8 is
doped N-type. However, as a front contact (and junction), Zn in thin
i-ZnO layer seems to diffuse into CIGS and form a weak junction while Sn
does not. It is probably due to the different process temperature for
ZnO and SnO~2~. The former is sputtered at room temperature, while the
latter is produced by APCVD at about 550 ºC, which is thought to be more
stable against the CIGS process.

4)  **CdS process and surface treatment research**

The CBD process is receiving our on-going efforts for improvement and
understanding of the chemistry involved. Recently, studies were done to
evaluate the effects of S/Cd ratio, temperature, and deposition time on
the device performance. Our CdS films were grown in the temperature of
50-60ºC, with deposition times of 5-15 minutes and S/Cd ratios of 100
and 50. We found that at 60ºC the formation of CdS particles in the bath
was very rapid, and such particles were noticed depositing on the CIGS
film surface. However, the deposition temperature of 50ºC and S/Cd ratio
of 100 yielded smooth CdS films on CIGS surfaces, which could be easily
rinsed with water to yield efficient devices. With S/Cd ratio of 100, at
50ºC, and with a deposition time of 15 minutes, reproducible good
quality CdS films could be generated, and this is our routine CdS film
deposition method at present. Some results are listed in Table VI.

**Table VI Recent test results from CdS optimization**

  ---------- --------- -------- ------------- ---------------- --------------- --------- ------------- ---------------
  **ID**     **Voc**   **FF**   **Jsc(QE)**   **Efficiency**   **Jsc(QE-1)**   **CdS**   **Temp./C**   **Container**

  Z1557-11   472.8     64.18    27.45         8.33             29.88           100       50            Beaker

  Z1557-13   456.5     64.44    28.01         8.24             30.01           100       50            Beaker

  Z1557-15   448.5     64.67    29.01         8.41             30.72           100       50            Beaker

  Z1557-12   449.7     64.55    29.42         8.54             30.72           50        50            Beaker

  Z1557-14   437       64.11    29.63         8.30             31.3            50        50            Beaker

  Z1557-16   442.6     64.45    30.77         8.78             32.3            50        50            Beaker
  ---------- --------- -------- ------------- ---------------- --------------- --------- ------------- ---------------

We have initiated some new surface treatments in this quarter including:

(a) Heating in ammonium sulfide solution

(b) Treatment of films with sulfide and ammonium ions electrochemically

(c) Low temperature annealing

Initial studies have resulted in some interesting results and
considerable device improvements. Listed in Table VII below are some of
our test results. We plan to do more work in this area to understand the
effects of these treatments on CIGS surfaces in the next quarter.

**Table VII Some test results from surface treatment**

  ---------- ------------------- --------- -------- ------------ -------------- --------------- ---------
  **ID**     **Pretreatment**    **V~oc~   **FF**   **J~sc~      **Efficiency   **Container**   **Temp.
                                 mV**               mA/cm^2^**   %**                            °C**

  Z1543-30   Am.S 20%            539       64.3     28           9.86           Beaker          52
             80-90C,60min                                                                       

  Z1543-M4   Am.S 20%            548       65.3     27           9.69           Tank            53
             80-90C,60min                                                                       

  Z1543-N4   Am.S 20%            528       58.1     26           8.01           Tank            53
             80-90C,60min                                                                       

  Z1542-25   Electrochem.(+)     568       63.2     23           8.23           Tank            53
             Am.S 20% RT(5V)                                                                    

  Z1542-26   Electrochem.(-)     533       51.7     26           7.22           Tank            53
             Am.S 20% RT(5V)                                                                    
  ---------- ------------------- --------- -------- ------------ -------------- --------------- ---------

5)  **Interconnection**

In our normal module process, we use n-ZnO as interconnection between Mo
back contact and ZnO front contact of the next segment. One disadvantage
of this process is that we have to break vacuum for the 2^nd^ (CIGS)
scribe after i-ZnO and before n-ZnO. As an alternative, we could perform
the 2^nd^ (CIGS) scribe first and simply deposit i-ZnO followed by n-ZnO
without breaking vacuum. This would entail trying to use i-ZnO plus
n-ZnO as interconnection to the Mo. The higher resistance of i-ZnO is a
concern in this case. However, it could conceivably not be a problem
since there is no obvious huge resistance jump after coating i-ZnO on
CTO (mentioned in Section 3 above) due mainly to its thinness (about
500A-1000A). To test this idea, four mini-modules were processed for
comparison. On two of them the n-ZnO/Mo interconnection is used as a
reference, while on the other two the interconnection n-ZnO/i-ZnO/Mo is
tested as an experiment. The performance results are listed in Table
VIII.

**Table VIII Mini-module performance with i-ZnO as interconnction**

+--------+------------+----+------+-----+--------+--------+---------+
| **ID** | **Interco  | *  | *    | **F | **Jsc  | *      | **Eff.( |
|        | nnection** | *V | *Voc | F** | (aper) | *Eff.( | active) |
|        |            | oc | /seg |     | (mA/   | aper.) | %**     |
|        |            | ** | (m   | **  | cm2)** | %**    |         |
|        |            |    | V)** | %** |        |        |         |
|        |            | *  |      |     |        |        |         |
|        |            | *( |      |     |        |        |         |
|        |            | V) |      |     |        |        |         |
|        |            | ** |      |     |        |        |         |
+--------+------------+----+------+-----+--------+--------+---------+
| **Z156 | n-ZnO      | 7. | 537  | 54  | 32.5   | 9.57   | 10.87   |
| 5-41** |            | 52 |      | .76 |        |        |         |
+--------+------------+----+------+-----+--------+--------+---------+
| **Z156 | i          | 6. | 509  | 38  | 31.72  | 6.26   | 7.12    |
| 5-42** | -ZnO+n-ZnO | 62 |      | .78 |        |        |         |
+--------+------------+----+------+-----+--------+--------+---------+
| **Z156 | i          | 8. | 535  | 43  | 30.27  | 7.01   | 7.96    |
| 5-51** | -ZnO+n-ZnO | 03 |      | .23 |        |        |         |
+--------+------------+----+------+-----+--------+--------+---------+
| **Z156 | n-ZnO      | 6. | 499  | 53  | 34.31  | 9.08   | 10.32   |
| 5-52** |            | 99 |      | .02 |        |        |         |
+--------+------------+----+------+-----+--------+--------+---------+

The results clearly show that our normal process using n-ZnO as the
interconnection is superior to i-ZnO+n-ZnO. The noticeably different
module FF indicates that i-ZnO is indeed too resistive for the first
layer of interconnection and leads to unacceptable contact resistance.
However, it remains to be seen how much module FF could be improved
through reduction of the i-ZnO thickness.

6)  **AR coating**

A non-conventional, non-vacuum, low temperature method, basically a
sol-gel process, was used for producing experimental AR coatings on CIGS
devices in this quarter at EPV. We have used a mixture solution of
silicon alkoxides, methanol, and ethanol as porous glass precursors. In
these experiments, completed CIGS devices are first immersed in the
liquid, then the liquid (sol) is drained at a controlled speed so that a
wetting film stays on the ZnO of the CIGS devices and is exposed to the
air. The atmosphere controls the evaporation of the solvent, the
subsequent destabilization of the sol by solvent evaporation leads to
the gelation process, by which a highly porous AR film of silica
nanoparticles is finally formed. The refractive index of this film is an
inverse function of pore volume with a limit value 1.22. The thickness
of the film can be adjusted by change of sol drain speed.

As a first experiment, several CIGS devices were coated by sol-gel at
three different drain speeds. Quantum efficiency was measured before and
after the coating. The short-circuit current density was calculated
based upon the QE data. The Jsc improvement is listed in Table IX below.

**Table IX Jsc improvement by sol-gel AR coating**

  ---------------------- ------------------------ ------------------------
  **Device ID**          **Sol-gel drain speed**  **Jsc improvement**

  Z1559-71C              1.5 cm/s                 +5.2%

  Z1559-71D              1.3 cm/s                 +2.9%

  Z1559-71E              1.0 cm/s                 +2.4%
  ---------------------- ------------------------ ------------------------

7)  **Plans for next quarter (Q2)**

-   Continue recipe optimization to reach V~oc~ of 600mV and higher in
    Hercules.

-   Install 3^rd^ Eurotherm temperature controller in Zeus

-   Improve overall CIGS film properties by process control and
    optimization in Zeus

-   Build and install new heater for in-line sputtering system; improve
    large area ZnO

-   Continue mini-module and one square foot module process, and
    increase their

> efficiency based upon component improvement and module failure
> analysis

-   Make sputtering head in the Zeus operational

-   Complete upgraded fixturing for scribing operations

-   Re-examine non-contact means of Cu/(In+Ga) determination for hybrid
    CIGS

-   Continue to investigate ammonium sulfide and other surface
    treatments

-   Resume alternative junction work with ZIS and HC sputtered ZnO

Note. In Q3 we plan to resume fabrication of 0.43m^2^ modules. This will
require construction of new and improved CBD CdS tanks.

Sincerely,

# Alan E. Delahoy Leon Chen

# Principal Investigator Senior Scientist