***ENERGY PHOTOVOLTAICS***

## *Thin-Film Photovoltaics Partnership Program*

### Advanced CIGS Photovoltaic Technology

**Final Technical Report**

covering the period

November 15, 2001 -- February 13, 2005

Subcontract No. ZDJ-2-30630-21

under

Prime Contract No. DE-AC36-99-GO10337

Prepared by:

A.E. Delahoy and L. Chen

*Energy Photovoltaics, Inc.*

*P.O. Box 7456, Princeton, NJ 08543*

Submitted to:

H.S. Ullal, Contract Technical Monitor

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, CO 80401-3393

February 28, 2005

*Preface*

Technically, thin-film PV technologies have advanced considerably in the
last few years. The leading thin film technologies are, broadly, a-Si
(and variations), CdTe, CIGS, c-Si film, and dye-sensitized. At the time
of writing, the leading commercially-available thin film technologies
have demonstrated the following record aperture area efficiencies and
powers for large area modules:

CIGSS 13.1% 64.8W Shell Solar GmbH (glass)

CIGS 13.0% 84.6W Würth Solar (glass)

CdTe 10.2% 67.4W First Solar (glass)

a-Si/μc-Si 10.0% 38.0W Kaneka (glass)

CIGS 10.1% 71.2W\* Global Solar (ss)

a-Si/a-SiGe/a-SiGe 7.6% 70.8W\* United Solar (ss)

CdTe 7.3% 52.3W Antec Solar (glass)

a-Si 6.4% 100W Mitsubishi Heavy Industries (glass)

a-Si/a-Si 6.1% 33.3W RWE (glass)

a-Si 6.0% 48.6W Kaneka (glass)

a-Si/a-si 5.8% 43.3W Energy Photovoltaics (glass)

\* assembly of cells (not monolithic)

Even higher module efficiencies have been demonstrated by some companies
that currently do not use the technology commercially, e.g. 13.4% for
CIGS by Showa Shell, and 11% for CdTe by BP Solar and Matsushita. Within
each thin-film semiconductor technology category, various deposition
methods have been devised, and many are represented in the above table.
While a high module efficiency is desirable, module efficiency figures
do not tell the whole story. The long term commercial success of the
various approaches is not automatically assured, but is dependent on a
combination of module efficiency, manufacturing cost and market niche.
For example, the manufacture of thin-film c-Si modules in the efficiency
range 8-10% can be approached by depositing and recrystallizing a-Si:H.
Indeed, prototype modules have been produced and the process used has
been described in the literature. It is instructive to analyze this
thin-film c-Si process, and to compare it to CIGS processing. The
analysis reveals more complex processing than is required for CIGS
(about 20 steps versus 12, exclusive of encapsulation), a high cost for
the borosilicate glass, high capital costs, and extensive use of
indirect materials in multiple etching processes. It is not clear that
it offers a viable pathway to cost-effective manufacturing. Amorphous
silicon, on the other hand, is of lower efficiency, but can be
manufactured with high yield and at the lowest \$/W of all the
technologies mentioned. CIGS continues to hold the efficiency record,
but the technology, although having entered the realm of manufacturing,
is arguably not yet sufficiently evolved to be cost-competitive for
production of standard power modules.

One further factor that will eventually emerge as a strong driver of
success for PV technologies in the energy market is the specific energy
for module production. For PV to continue growing at 30% per year for
the next 30 years so that it can take its place as a significant energy
source on the world stage, modules will have to be made in a more
energy-efficient manner. At this growth rate, for a new PV factory to
generate a positive energy return in less than 10 years, the specific
energy for module production must be less than 18MJ/W~p~ \[1\]. If a
particular PV technology cannot meet this condition, it may be
questioned whether large quantities of energy will in practice be
expended to manufacture modules using such a technology. The published
range of total energy requirements to produce wafer-based modules is
20-100 MJ/W~p~. For a-Si the figure is 12-15MJ/W~p~ (EPV), while for
CIGS the figure is 11MJ/W~p~ (Shell Solar).

From the above discussion we see that the driving forces for CIGS are
compelling: potentially high efficiency and low specific energy for
production. To these we may add the broadly advantageous properties of
most thin-film PV processes relative to wafer-based PV: monolithic
design and large substrates (leading to reduced parts handling), low
consumption of both direct and indirect materials, and fewer process
steps.

Energy Photovoltaics, Inc. ("EPV") is a solar energy company that
primarily designs, develops, manufactures, and markets thin-film
photovoltaic (PV) modules and Integrated Manufacturing Systems to serve
the growing international PV marketplace. The strategy being pursued by
EPV is premised on a fundamental belief that, for PV to be successful as
more than a specialty source of electricity (with growth stimulated by
government incentive programs), it must deliver electricity at the
lowest possible cost. In this vein, EPV continues to ship its EPV-40
tandem junction, amorphous silicon PV modules manufactured at its
headquarters in Lawrenceville, NJ. The modules are UL-listed and have
IEC 61646 certification. The production is fully sold out for 2005. On
the IMS front, EPV completed a 2.5MW a-Si module manufacturing plant for
the Tianjin Jinneng Solar Cell Corp. in China in April 2004. The plant
met all production rate and quality deliverables and is in full
production. EPV is also supplying an a-Si IMS rated at 11MW to
Heliodomi, S.A. of Thessaloniki, Greece.

In parallel with a-Si production, Energy Photovoltaics, Inc. is also
developing technology to be able to cost-effectively manufacture much
higher efficiency CIGS modules. EPV has consistently pursued a
vacuum-based approach to CIGS production, and has developed novel linear
thermal source technology to supply materials to heated, moving,
soda-lime glass substrates. It has also deliberately chosen to develop
processing methods with worker safety in mind. These
strategically-important choices offer a low-cost substrate, control over
layer homogeneity and purity, and production without significant
hazards. Although such approaches help to minimize the processing costs
of CIGS, further advances appeared necessary in order to improve both
ease of production and yield. One such advance, the introduction of Cu
sputtering, was accomplished under the NREL **Thin-Film Photovoltaics
Partnerships Program (TFPPP).**

To facilitate the development of CIGS, CdTe, and Si-based thin-film
technologies, NREL operates the Thin-Film Photovoltaics Partnerships
Program. The long-term objective of the TFPPP is to demonstrate
commercial, low-cost, reproducible modules of 15% aperture-area
efficiency \[2\]. As a Technology Partner within this program, EPV has
performed research under a three-phase, cost-shared subcontract entitled
"Advanced CIGS Photovoltaic Technology" and participates in the National
CIS Team Meetings. One of the main objectives of this subcontract
(RDJ-2-30630-21) was for EPV to demonstrate its capability to produce
reasonably efficient CIGS modules at a substrate size of 4300 cm^2^. The
processing also needed to be reproducible with good controllability.
This goal was successfully accomplished by the development and
utilization of a new hybrid process for CIGS growth during the three
year contract period. This final technical report mentions highlights of
the first and second phases of the subcontract, and describes in detail
results obtained during the third phase of the subcontract.

The benefits accruing from the hybrid process raises the obvious
question of whether process and equipment development does more to
advance PV technology than the more traditional research into materials
and devices.

**CONTENTS**

Preface
................................................................................................
ii

Table of
Contents....................................................................................
v

List of Tables
.........................................................................................
vi

List of Figures
........................................................................................
vi

1.0 **Introduction**
.....................................................................................
1

2.0 **Highlights of Phases I and
II**................................................................. 2

3.0 **CIGS Optimization and Device
Results**................................................... 3

3.1 Composition and Cu rich
regime...................................................... 3

3.2 Selenization
temperature...............................................................
4

3.3 Ga distribution and bandgap
profile................................................\... 5

3.4 Other
parameters.........................................................................
5

3.5 Device
performance.....................................................................
5

4.0 **CIGS Film
Analysis**............................................................................
6

4.1 Scanning electron microscopy
......................................................\... 6

4.2 Depth profiling by Auger electron spectroscopy
................................... 7

4.3 X-ray
diffraction.........................................................................
8

4.4 Activation
energy.....................................................................\...
9

5.0 **Full Size Module Process and Performance**
.............................................. 9

5.1 CBD CdS made in full size
tank....................................................... 9

5.2 Investigation of contact resistance between ZnO and
Mo...\.....................\... 10

5.3 Uniformity improvement for large area
plates....................................\... 11

5.4 Module
performance....................................................................
15

5.5 Long-term stability of EPV
module................................................\... 16

5.6 Process control and
database........................................................... 17

6.  **Surface
    Treatment**...........................................................................\...
    17

7.  **Development of New TCO Window
    Layers**............................................... 18

8.0 **Future
Work**......................................................................................
19

###### 9.0 Phase III Summary ............................................................................. 20 

Acknowledgments.....................................................................................
21

References..............................................................................................
22

#### TABLES

Table 3.1. Deposition conditions and resulting composition and device
performance

Table 5.1. Device FF mapping to compare beaker and tank CBD process

Table 5.2. Transmission distribution (at 420 nm) of CTO glass coated
with CdS film

Table 5.3. Performance of modules made during the Final Phase

Table 6.0. Device performance with various surface treatments

Table 7.0. Properties of TCO films made by RE-HCS

Table 7.1. J-V parameters for cells with RE-HCS window layers

**FIGURES**

Fig. 3.1. QE curves for devices made at different CIGS temperatures.

Fig. 3.2. J-V curve of Z1699-1 #5 measured at NREL (efficiency 13.1%).

Fig. 4.1. SEM cross-section images of Z1559-1 (left) and Z1730 (right).

Fig. 4.2. Ga/(In+Ga) *vs* film depth for three films shown in Table 3.1.

Fig. 4.3. XRD patterns for CIGS film Z1699.

Fig. 4.4. Activation energy for CIGS of 0.36 V calculated from linear
fitting.

Fig. 5.1. I-V curve for exceptional poor ZnO-Mo contact.

Fig. 5.2. Contour plot of device efficiency for run Z1653 (deposited
early in Q2).

Fig. 5.3. Ga/(In+Ga) ratio versus segment number before and after Ga
boat adjustment.

Fig. 5.4a. Distribution of ZnO sheet resistance before ILS
modifications.

Fig. 5.4b. Distribution of ZnO sheet resistance after ILS modifications.

Fig. 5.5. Normalized thickness distribution along Cu target direction.

Fig. 5.6. Device performance distribution from plate Z1677.

Fig. 5.7. I-V curve for a 26W, 7.5% module, as measured by NREL.

Fig. 5.8. Outdoor stability of CIGS sub-module Z1658 8-13.

Fig. 7.0. Spectral transmission of a high mobility ITiO film and modeled

transmission curves as a function of mobility.

Fig. 9.0 Trajectories of progress with hybrid CIGS during subcontract
ZDJ-2-30630-21

**1.0 Introduction**

This is the Final Technical Report for EPV's cost-shared subcontract
ZDJ-2-30630-21 **Advanced CIGS Photovoltaic Technology** awarded under
the Thin Film Photovoltaics Partnership Program (TFPPP). The work period
covered by this final report is from November 15, 2001 to February 13,
2005.

As part of the Thin Film Photovoltaics Partnership Program, EPV has
conducted research to help generate a technology base for production of
CIGS PV modules using vacuum deposition of CIGS onto glass. This
strategy is consistent with the observation that, despite there being
several approaches to forming device quality CIGS, vacuum deposition has
maintained the world record for the highest efficiency CIGS device. This
record currently stands at 19.5% (692mV, 35.2 mA/cm^2^, FF 79.9%) for
0.41 cm^2^ device grown at NREL by the three-stage process \[3\]. To
overcome barriers to manufacturing CIGS by vacuum deposition, EPV has
developed vacuum equipment incorporating novel linear evaporation
sources and magnetron sputtering cathodes designed for uniform coating
of large (0.43m^2^) moving substrates \[4\]. The use of elemental
selenium avoids the need for gaseous H~2~Se.

**Project Objectives**

The principal objective has been to develop the best CIGS large area
module process based upon capability of implementation on EPV's large
scale processing equipment. The first requirement was to develop recipes
for CIGS, junction formation and a high quality ZnO window that together
are capable of producing small area devices with efficiencies in excess
of 13%. The second requirement was to significantly improve the
uniformity of all layers (Mo, CIGS, CdS and ZnO) on large area
substrates. Thirdly, patterning procedures needed to be improved to
generate an interconnection with the lowest possible contact resistance,
to eliminate possible shunting paths, and to reduce the dead area. The
processes should have good reproducibility and therefore be easily
controllable. Finally, having assembled all of these aspects mentioned
above, the goal was to fabricate large area, monolithic CIGS modules
with efficiencies in the range 7 - 10%. A study of module reliability
and long-term stability would be conducted to establish the foundation
and confidence for embarking on future manufacturing.

**Approach**

Process development and process verification for CIGS are conducted
initially in the Hercules 4-source system (an R&D system with a
capability of six 5cm x 10cm stationary substrates per run) for the
purpose of proof of concept. As soon as good device and mini-module
performance are achieved and repeated, the basic process concept will be
transferred to our large area system Zeus that is fitted with a 4
linear-source head and that handles a single 4300 cm^2^ moving substrate
per run. In the latter system, which is load locked, source materials
are delivered downwards to the moving glass using custom-built, linear
evaporative sources, the source axes being perpendicular to the
direction of glass travel. This approach allows a wide range of
vacuum-based CIGS recipes to be implemented. Copper can be supplied
either by thermal evaporation or by planar-magnetron sputtering in the
Zeus system, or by sputtering in a separate in-line deposition system.
During the period covered by this Final Report, we focused on a
so-called hybrid process utilizing evaporation of In, Ga and Se and
sputtering of Cu to take advantage of improved plate uniformity and the
flexibility of forming a Ga profile. After verification of process
transfer, all further optimization of the CIGS process was conducted in
the Zeus system. Film and device characterization were realized through
in-house laboratories containing ICP-AES, SEM, computerized QE, and
indoor/outdoor IV facilities. Some samples were sent out to NREL and IEC
for further analysis.

For junction formation, EPV relies on CBD CdS on a day-to-day basis, and
from time to time explores alternative methods in parallel. Other
methods include buffer layers applied by evaporation (e.g. ZnIn~2~Se~4~
or other materials), by spray deposition, or by hollow cathode
sputtering. A proprietary surface treatment of the CIGS is routinely
used in our device and module processing and new processes for surface
treatment are being explored. For ZnO deposition, two planar-magnetron
sputtering systems are available (Airco and large area, in-line system
or ILS) and are fitted with ceramic ZnO targets. New transparent
conductors produced by reactive-environment hollow cathode sputtering
are also being explored. Module encapsulation is accomplished using
glass-glass vacuum lamination with EVA, with processing similar to that
used in the commercial EPV-40 amorphous silicon module. For long-term
outdoor testing, a Campbell Scientific datalogger is available.

**2.0 Highlights of Phases I and II** \[5,6\]

-   A hybrid CIGS process (involving Cu deposition by sputtering) was
    explored in the R&D Hercules system in Phase I, with the goal of
    improving the uniformity of both CIGS thickness and composition. The
    remaining elements, In, Ga, and Se, continued to be delivered by
    evaporation.

-   An improved thickness distribution on large area plates produced in
    Zeus was realized in Phase I for CIGS produced by the hybrid
    process.

-   An extensive survey of post-deposition treatments of CIGS was made.
    A useful wet treatment was identified in Phase I that improved the
    performance of devices.

-   Optimization of EPV's procedure for CBD CdS was conducted. Improved
    device performance was realized in Phase I after a change from Cd
    acetate to Cd sulfate.

-   Using the hybrid CIGS process, post-deposition treatment, and the
    improved CBD CdS recipe, cells up to 13.5% efficiency (569 mV, 32.3
    mA/cm^2^ without ARC, FF 73.5%) were produced in the R&D Hercules
    system in Phase I. The devices were found to improve upon light
    soaking.

-   Using EPV CIGS prepared by evaporation of all elements, a 10.1% cell
    was produced in Phase I using an evaporated zinc indium selenide
    (ZIS) buffer layer, and an 8.6% cell using evaporated In~2~S~3~.

-   A mini-module with laser-scribed Mo and mechanically-scribed CIGS
    and ZnO was produced using Hercules CIGS with an aperture area
    efficiency of 9.0% in Phase I.

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-   The concept of CIGS prepared by a hybrid process that involves
    evaporation of In and Ga and sputtering of Cu was successfully
    transferred to large area Zeus system in Phase II. The process shows
    good reproducibility, and shows promise for being manufacturable.

-   Both in-situ and ex-situ sputtering of the Cu have been successfully
    utilized to form high quality CIGS by the hybrid process in Phase
    II.

-   Diagnostic devices formed on large area (0.43m^2^) CIGS plate in
    Zeus by the hybrid process have been produced in Phase II with
    V~oc~s as high as 659mV, and efficiencies up to 11.8%.

-   10.1% devices were prepared by an exploratory process that involves
    evaporation of IGS compound and Cu sputtering in Phase II.

-   The CBD process for CdS was improved through adoption of a slow
    reaction in Phase II. This improved uniformity.

-   Improved ZnO properties were obtained at elevated substrate
    temperatures in Phase II.

-   The type of ZnO best suited to the ZnO/Mo interconnection has been
    investigated and important rules have been developed in Phase II.

-   Large area module processing was restored, and reasonably
    reproducible 1ft^2^ modules were produced relatively quickly in the
    5.0 -5.5% efficiency range in Phase II.

-   A 15.5% cell (with CdS) was fabricated by EPV on bare NREL CIGS, and
    a 13.3% cell was fabricated using no CdS but with hollow cathode
    sputtering of the ZnO.

-   An 8.2% cell (with CdS) was fabricated on EPV CIGS using IMO instead
    of ZnO.

-   Water immersion tests indicated superior stability of IMO relative
    to ZnO.

**3.0 CIGS Optimization and Device Results**

**3.1 Composition and Cu rich regime**

During the period of Phase III, the CIGS team at EPV mainly focused on
device optimization, and especially on the formation of the CIGS
absorber. This focus follows from the realization that CIGS quality is
the single most important factor contributing to module efficiency.
Optimization of the hybrid process was conducted in the large area,
pilot-line system using 0.43 m^2^ substrates. We examined the necessity
of letting the CIGS formation process pass through a Cu-rich
(Cu/(In+Ga)\>1) stage. This had been reported to enhance the growth of
high-quality, large-grained films via the utilization of Cu~x~Se as a
liquid fluxing agent at sufficiently high substrate temperatures (\>523
C) to promote lateral grain growth \[7\], but remained controversial. We
found that processing a Cu-rich precursor (Cu/(In,Ga)~2~Se~3~) is
clearly more robust in repeatedly producing high-quality CIGS films and
yielding better devices. Listed in Table 3.1 are some samples produced
from Zeus during CIGS optimization.

Table 3.1. Deposition conditions and resulting composition and device
performance

+--------+-------+----------+-----+-----+------+--------+-----+-----+
| Sample | Se    | Cu ratio | Cu  |     | V    | J~sc~  | FF  | Eff |
| ID     | Temp  |          | &   |     | ~oc~ |        |     |     |
|        | \     | 1^st^    | Ga  |     |      | \[mA/c | \[  | \[  |
|        | [°C\] | IGS +Cu  | ra  |     | \[   | m^2^\] | %\] | %\] |
|        |       |          | tio |     | mV\] |        |     |     |
|        |       |          | of  |     |      |        |     |     |
|        |       |          | C   |     |      |        |     |     |
|        |       |          | IGS |     |      |        |     |     |
|        |       |          |     |     |      |        |     |     |
|        |       |          | Cu  |     |      |        |     |     |
|        |       |          | Ga  |     |      |        |     |     |
+--------+-------+----------+-----+-----+------+--------+-----+-----+
| Z15    | 550   | 0.87     | 0   | 0   | 351  | 27.2   | 5   | 5.0 |
| 59-2\* |       |          | .86 | .31 |      |        | 2.4 |     |
+--------+-------+----------+-----+-----+------+--------+-----+-----+
| Z      | 550   | 0.87     | 0   | 0   | 483  | 26.5   | 6   | 8.2 |
| 1559-1 |       |          | .63 | .31 |      |        | 4.4 |     |
+--------+-------+----------+-----+-----+------+--------+-----+-----+
| Z      | 525   | 1.27     | 0   | 0   | 529  | 23.1   | 6   | 7.9 |
| 1559-6 |       |          | .68 | .26 |      |        | 4.7 |     |
+--------+-------+----------+-----+-----+------+--------+-----+-----+
| Z      | 550   | 1.27     | 0   | 0   | 558  | 26.9   | 7   | 1   |
| 1559-5 |       |          | .71 | .28 |      |        | 2.7 | 0.9 |
+--------+-------+----------+-----+-----+------+--------+-----+-----+
| Z      | 580   | 1.4      | 0   | 0   | 554  | 30.7   | 7   | 1   |
| 1665-3 |       |          | .89 | .30 |      |        | 0.6 | 2.0 |
+--------+-------+----------+-----+-----+------+--------+-----+-----+
| Z      | 600   | 1.43     | 0   | 0   | 560  | 32.3   | 6   | 1   |
| 1728-5 |       |          | .92 | .30 |      |        | 8.6 | 2.3 |
+--------+-------+----------+-----+-----+------+--------+-----+-----+
| Z      | 600   | \-       | \-  | 0   | 543  | 33.4   | 7   | 1   |
| 1730-5 |       |          |     | .32 |      |        | 0.1 | 2.7 |
+--------+-------+----------+-----+-----+------+--------+-----+-----+

> \*Z1559-2 was made in a two-step process without the last IGS
> finishing layer

The third column indicates the ratio of Cu to In+Ga measured with ICP
AES after the first IGS and Cu depositions. The values in the third
column are all greater than one (Cu-rich precursor) before the last IGS
layer is deposited except for sample Z1559-1 & 2. A comparison of the
V~oc~ and FF of Z1559-5 versus those of Z1559-1 (both have the same
selenization temperature T~ss~ = 550°C), reveals the benefit of passing
through a Cu-rich regime in the three-stage hybrid process.

**3.2 Selenization temperature**

Despite the improvement of both V~oc~ and the overall efficiency by
passing through a Cu-rich regime, the QE response for Z1559-5 (550°C)
shown in Fig 3.1 was clearly unsatisfactory, especially the poor
collection at long wavelengths.

![](media/image1.wmf){width="4.522916666666666in"
height="3.7319444444444443in"}

Fig. 3.1. QE curves for devices made at different CIGS temperatures.

It was thought that poor bulk properties of the CIGS film limited the
current collection. Therefore, the influence of selenization temperature
T~s~ on CIGS films and devices was studied. By increasing the
selenization temperature from 525°C to 600°C, the short-circuit current
density J~sc~ and the collection of photo-generated electrons at long
wavelengths were indeed significantly improved (see Table 3.1 and Figure
3.1). It is worth noting that the real substrate temperature might be
slightly lower than the values in the Table 3.1 since the substrate is
moving during the deposition and the temperature sensor is static and
located closer to the heaters.

**3.3 Ga distribution and bandgap profile**

That the Ga distribution in a CIGS film, and thus the bandgap profile,
will affect CIGS device performance has been known for a long time
\[8\]. (Indeed, a similar concept has been applied to a-SiGe cells for
more than a decade. Various graded in, graded out, and notch structures
were tested and modeled \[9\].) Creating a higher bandgap material in
the rear part of the CIGS layer by increasing the Ga concentration is
thought to form a back-surface field, which will tend to drive
photo-generated electrons (minority carriers) away from the rear
interface and towards the front, which should be beneficial to J~sc~ and
FF. Device Z1730-5 in the last row of Table 3.1, compared with Z1728-5,
is an example of such a design. Its improved performance is consistent
with the concepts just described. A further refinement might increase to
some degree the Ga ratio (and thus bandgap) within the depletion region
and towards the junction which should boost V~oc~ without presenting a
conduction-band barrier to electron collection. A lower bandgap region
deeper within the absorber can help absorb long-wavelength photons to
retain high J~sc~.

**3.4 Other parameters**

Among the many other process parameters pertaining to the formation of
CIGS films, we found the substrate moving speed and deposition rate for
the final evaporated IGS layer are sensitive parameters for the hybrid
process. As currently implemented, the substrate undergoes multiple
scans under the linear sources. It seems that the CIGS surface
morphology can in this way be influenced, which we will illustrate
below.

**3.5 Device performance**

After deliberately optimizing everything mentioned above, we were able
to achieve an efficiency of over 13% for hybrid CIGS devices made in the
large area system Zeus \[10\]. One such device, Z1699-1, was sent to
NREL for verification. Plotted in Fig. 3.2 is the JV curve for cell #5,
as measured at NREL. The PV parameters are: 598mV, 32.2mA/cm^2^, FF
68.3%, efficiency 13.1%.

![](media/image2.png){width="4.0125in" height="5.370138888888889in"}

Fig. 3.2. J-V curve of Z1699-1 #5 measured at NREL (efficiency 13.1%).

**4.0 CIGS Film Analysis**

**4.1 Scanning electron microscopy**

Some of the samples listed in Table 3.1 were sent to NREL for morphology
analysis. SEM micrographs show that the grain structure of the CIGS
films changes dramatically as T~ss~ and the Cu ratio of the precursor
are changed (see Fig. 4.1). The film Z1730 formed at T~ss~ = 600°C from
a Cu-rich precursor (right-hand micrograph) shows columnar grains with
typical grain size around 1.0 μm \[11\]. But for the film Z1559-1
(mislabeled as S1559-2 in the left-hand micrograph) formed at T~ss~ =
550°C from a Cu-poor precursor, the grains are smaller and do not grow
continuously from the Mo to the top layer. Thus, photo-generated
electrons may need to pass through several grain boundaries, affecting
their collection. This observation concurs with device performance, and
confirms that a Cu-rich precursor and a sufficiently high substrate
temperature are important in processing.

![](media/image3.jpeg){width="2.925in" height="2.375in"}

Fig. 4.1. SEM cross-section images of Z1559-1 (left) and Z1730 (right).

**4.2 Depth profiling by Auger electron spectroscopy**

Depth-profile analyses of CIGS films were carried out by AES. Figure 4.2
shows the Ga/(In+Ga) ratio *versus* film depth for three films listed in
Table 3.1. Z1559-2 was made at T~ss~ = 550°C from a Cu-poor precursor
without the In, Ga, Se finishing layer. A very sharp transition is seen
with Ga considerably depleted in the surface region. Z1665 was made from
a Cu-rich precursor at T~ss~= 580°C, and the Ga distribution is fairly
uniform. For these two films, the In and Ga fluxes were delivered
simultaneously.

![](media/image5.wmf){width="3.692361111111111in" height="3.05in"}

Fig. 4.2. Ga/(In+Ga) *vs* film depth as determined by AES for three
films shown in Table 3.1.

There are two possible phenomena regarding the In, Ga diffusion during
the conversion of the Cu/(In,Ga)~2~Se~3~ precursor to CIGS. One is the
out-diffusion of In, Ga accompanied by in-diffusion of Cu. The
out-diffusing In, Ga reacts with Cu and Se to form new unit cells,
contributing to the growth in thickness. As In has a larger atomic
radius than Ga, the strain energy of the film is lowered by preferential
In out-diffusion relative to Ga, and the additional film become Ga
depleted. On the other hand, when excess Cu and Se and high temperatures
result in liquid-phase Cu~x~Se, the diffusion due to the concentration
gradients of In and Ga are increased via the Cu~x~Se at grain
boundaries. Consequently, the Ga/(In+Ga) gradient decreases. The uniform
Ga distribution of our films under those conditions might indicate that
the time spent in the Cu-rich regime is too long.

In run Z1730 the conditions were adjusted to form a precursor with a
higher initial Ga content by intentionally changing the Ga flux during
the first (In,Ga)~2~Se~3~ layer. From Fig. 4.2, a graded Ga distribution
for this sample is seen. The fabricated device exhibited increased
efficiency with superior current density and QE (see Table 3.1 and Fig.
3.1). The improved electron collection, especially for electrons
generated by long wavelength photons, can be attributed to the graded Ga
distribution.

**4.3 X-ray diffraction**

Although a graded Ga distribution with a higher Ga content towards the
Mo back contact was achieved, the Ga content at the surface of the CIGS
films is still low. This might be one of the reasons that the devices do
not have exceptional V~oc~. Another factor contributing to lower V~oc~
might be from CIGS surface morphology. In some samples, for example
Z1699, we found flake-like morphology with some crevices at the top. It
was reported that the hexagonal Cu(InGa)~5~Se~8~ film also has a layered
structure \[12\]. In order to examine if there is another phase in the
top layer of the film, XRD (Cu-Kα, 35kV, 20mA) and GIXRD (Cu-Kα, 40kV,
40mA, Ω =1 deg, sampling depth \~300 nm) measurements were carried out,
and the patterns are shown in Fig. 4.3.

![](media/image6.wmf){width="3.452777777777778in"
height="2.5493055555555557in"}

Fig. 4.3. XRD patterns for CIGS film Z1699.

The GIXRD pattern is the same as that from XRD, and all peaks can be
indexed based on a tetragonal Cu(In~0.7~Ga~0.3~)Se~2~ unit cell (JCPDS
card #35-1102). This means that no additional phase could be identified,
and the cause of the layered morphology is not clear, nor is it known
how it affects junction properties. It is hypothesized that the crevices
in the top layer result from migration due to a non-uniform Cu~x~Se
distribution during the conversion of Cu~x~Se into CIGS during the final
In, Ga, Se delivery \[13\]. The total extent of Cu-richness during
growth would then be a factor that could improve this, so the maximal
tolerable Cu ratio for a Cu-rich precursor needs to be investigated.

**4.4 Activation energy**

Another possible cause for unexceptional V~oc~ of devices could be
reduced band bending resulting from a higher than desirable activation
energy of the CIGS film. To investigate this possibility, measurement of
the activation energy E~a~ for conduction was carried out during this
Phase. Plotted in Fig 4.4 is conductivity vs. reciprocal of temperature
for a CIGS film from a Zeus run. The activation energy extracted from
this data shows a noticeably high value of 0.36 V. So far it is not
clear if there is a strong correlation between E~a~ and device V~oc~. It
should be noted that the measured E~a~ was deduced from the lateral
conductivity of the CIGS film. Further measurements and comparisons are
planned for the next contract.

![](media/image7.wmf){width="4.261111111111111in" height="2.90625in"}

Fig. 4.4. Activation energy for CIGS of 0.36 V calculated from linear
fitting.

**5.0** **Full Size Module Process and Performance**

**5.1 CBD CdS process in full size tank**

A module aperture area efficiency of 5.5% in an area of 1 ft^2^ was
achieved in Phase II. At that time, the only missing equipment required
for processing full size (0.43 m^2^) modules was a new full size tank
for CBD CdS. (Use of an earlier full size tank had been discontinued on
safety grounds.) We were able to complete installation of a new large
area dipping tank for full size CBD CdS processing at an early stage in
Phase III. Some of the features of the set up are as follows. The
chemicals are contained in a stainless steel tank. Heaters are attached
along the length outside the tank in an insulated compartment. A
temperature controller is used to obtain and monitor the desired bath
temperature. The chemical solution is re-circulated by a liquid pump
driven with a digital module to control solution flow. The tank is easy
to maintain and clean up. The process area is well vented for safe
operation.

Two kinds of verification tests were conducted for acceptance of the
process transferred from a well-characterized beaker set up. First,
device performance was compared using CdS deposited in the beaker (as
reference) and in the big tank. Listed in Table 5.1 is one of numerous
comparison experiments conducted to verify that the tank CBD process is
as good as that of the beaker.

Table 5.1. Device FF mapping to compare beaker and tank CBD process

  --------------- -------------- --------------- --------------- --------------
  **Positions**   **Column 1     **Column 2 from **Column 3 from **Column 4
                  from Beaker**  Beaker**        Tank**          from Tank**

  **Row 1**       68             67              68              69

  **Row 2**       70             68              69              69

  **Row 3**       71             70              67              67
  --------------- -------------- --------------- --------------- --------------

Second, the tank CdS process was further optimized for plate uniformity,
which is checked by measuring the transmission at 420 nm and 440 nm of a
CdS film deposited on a large area glass with CTO coating. Listed in
Table 5.2 is a typical distribution of CdS film transmission.

Table 5.2. Transmission distribution (at 420 nm) of CTO glass coated
with CdS film

  --------------- ----------- ----------- ----------- ----------- -----------
  Width\\Length   3"          11"         19"         27"         **35"**

  3"              0.50        0.50        0.50        0.45        **0.45**

  9"              0.50        0.50        0.50        0.43        **0.50**

  **15"**         **0.46**    **0.50**    **0.49**    **0.45**    **0.45**
  --------------- ----------- ----------- ----------- ----------- -----------

**5.2 Investigation of contact resistance between ZnO and Mo**

From time to time in our large area module production, we cut a small
glass sample from a full size CIGS plate and process it into a 2" x 4"
mini-module (using manual patterning and Airco ZnO) to cross-check the
processing. For a particular period, we found that mini-modules with
Airco ZnO showed normal IV curves while those made with ZnO in the ILS
exhibited extremely large series resistance. What puzzled us was that
ZnO co-deposited on plain glass in both systems showed almost the same
low value of sheet resistance. After a careful series of measurements,
we found that while the ZnO resistance on each cell (on CdS) was almost
same in both cases, the ZnO/Mo contact resistance on the mini-modules
was totally different. Zinc oxide deposited in the ILS yielded a much
larger contact resistance than that deposited in the Airco.

This observation led us to thoroughly investigate various issues that
might influence contact resistance. In principle, high contact
resistance could be caused by:

-   Improper CIGS scratch

-   Contaminated Mo surface

-   Improper ZnO

-   Poor step coverage

At first, we paid much attention to the first two potential causes
since, having low sheet resistance, we thought we had good ZnO. We
adjusted the CIGS scribing speed and pressure to ensure that the CIGS
layer was totally removed and the Mo exposed; we tried different
scribing tips with various hardnesses and shapes; we increased the CIGS
cutting width to increase the contact area; we also tested ultrasonic
cleaning of the substrate in place of the usual glass washer; we
annealed the CIGS after CdS deposition to desorb water to prevent
contamination. After extensive experimentation, none of these factors
was found to be the root cause we were searching for, although contact
resistance was influenced by many variables. Finally, we realized our
low sheet resistance ZnO deposited in ILS performed well as a front
window layer but performed poorly as a contact. Indeed, sheet resistance
describes the lateral resistance of a thin film, but does not give any
information about resistance homogeneity in the perpendicular direction.
This simplified picture is not adequate to control the properties of the
ZnO/Mo contact. Once this was recognized, we were able to modify our
deposition procedures to routinely obtain good ZnO/Mo contacts with
specific contact resistivity \< 1 x 10^-3^ Ω-cm \[4\]. Along the way, we
demonstrated that an unsuitable ZnO layer could even give rise to
diode-like behavior at the ZnO/Mo contact. This is shown in Fig. 5.1
below.

![](media/image8.wmf){width="5.000694444444444in"
height="3.411111111111111in"}

Fig. 5.1. I-V curve for an exceptionally poor ZnO-Mo contact

**5.3 Uniformity improvement for large area plates**

The uniformity of the various layers deposited on large area plates is a
central issue in module production. From time to time, rather than
completing a module, we measure and map device performance to check the
uniformity over the plate. Fig. 5.2 is a contour plot of test device
efficiency from Plate Z1653 produced in the beginning of this Phase.
Immediate observations are that, while about one half of the plate shows
efficiencies in the range 8-10%, a strip of the plate along the bottom
edge shows a very significant drop in efficiency to the range 4-7%. As
we diagnosed in our previous report, one of the main causes of
non-uniformity across the short direction of the plate is non-uniformity
of the Ga. The root cause, we found, was due to an asymmetry in the
temperature of the two Ga boats.

![](media/image9.wmf){width="6.25in" height="3.2395833333333335in"}

Fig. 5.2. Contour plot of device efficiency for run Z1653 (deposited
early in Q2).

In Fig. 5.3 we show the Ga/(In+Ga) ratio in the direction of the linear
source. Curve A (data from run Z1660) represents the problematic
distribution. Fortunately, instead of rebuilding the whole delivery line
and boat assembly, which would have been expensive and time consuming,
we found an easier way to balance the temperatures of the Ga boats by
adjusting the thermal insulation of one of the boats. Subsequent to that
adjustment, the Ga distribution was found to be substantially improved.
Curve B (data from run Z1673) is the new Ga distribution after boat
temperature adjustment.

![](media/image10.wmf){width="4.877777777777778in"
height="2.7180555555555554in"}

Fig. 5.3. Ga/(In+Ga) ratio versus segment number before and after Ga
boat adjustment.

Another factor leading to device non-uniformity arises from the ZnO
sputtered in the in-line sputtering system. By mapping out the sheet
resistance of the ZnO we obtained the contour plot shown in Fig. 5.4a.
This revealed a surprisingly large range for the sheet resistance, from
8 to 14 ohms/square. In an attempt to improve the situation, we
installed a new gas manifold adjacent to the ZnO target to deliver a
more uniform gas flow and also adjusted the pumping configuration. The
two-dimensional distribution of ZnO sheet resistance resulting from
these hardware changes is plotted in Fig. 5.4b. In this run, most of the
plate exhibits sheet resistances in the range 12 to 15 ohms/square,
representing a tighter distribution.

![](media/image11.wmf){width="4.836805555555555in"
height="2.6083333333333334in"}

Fig. 5.4a. Distribution of ZnO sheet resistance before ILS
modifications.

![](media/image12.wmf){width="4.833333333333333in"
height="2.6055555555555556in"}

Fig. 5.4b. Distribution of ZnO sheet resistance after ILS modifications.

Finally, thickness mapping of sputtered Mo and sputtered Cu films both
show a similar trend in that the films are a little thinner in the
middle than at the long edges. This would seem to be due either to
weaker plasma in the middle of target or to enhanced deposition near the
ends of the racetracks. Therefore, we redesigned the magnetic field
distribution for the Cu cathode. Very encouraging indeed, the uniformity
of sputtered Cu films after redistribution of target magnetic field is
significantly improved. Plotted in Fig. 5.5 is a graph of normalized
thickness of Cu film versus position before and after the improvement.
In this figure, the thickness measured on three strips oriented along
the target direction, but at different locations in the moving direction
(namely 10, 20 and 30 inches from one edge), are measured and
normalized. Hollow symbols (and light lines) in the plot represent the
normalized thicknesses before improvement, while solid symbols (and
heavy lines) represent those after improvement. The uniformity along the
target direction is improved on all three strips, with the dip near the
center of the target being reduced to 5% from 10%. However,
non-uniformity along the moving direction still remains. This effect is
related to the pumping port position and the gas delivery. We hope to
work on this problem in the near future.

![](media/image13.wmf){width="5.209722222222222in"
height="3.2881944444444446in"}

Fig. 5.5. Normalized thickness distribution along Cu target direction

After these many adjustments, modules recently made at EPV show improved
uniformity. For example, plotted in Fig. 5.6 are the diagnostic cell
distributions of V~oc~ and FF for plate Z1677, measured in the direction
of the sources. The distribution of V~oc~ is very good, while that for
the FF is acceptable, but with some variations whose origin has yet to
be pinned down.

![](media/image14.wmf){width="4.488888888888889in"
height="2.828472222222222in"}

Fig. 5.6. Device performance distribution from plate Z1677.

**5.4 Module performance**

During Phase III, after incorporating the above process improvements and
better process control, we were able to produce full size modules almost
all of which have STC power of 20 W and above after lamination. The most
reliable performance measurements were taken in sunlight with the module
mounted on an orientable outdoor rack at EPV, and with 4-wire I-V curves
acquired by an indoor computer. The short-circuit current I~sc~ is
normalized by measured light intensity to a one-sun value, while V~oc~
is normalized to a 25°C value by measuring module temperature and
applying a -2 mV/degree/cell temperature coefficient. Listed in Table
5.3 is part of our module inventory. Two of the modules, Z1652 and
Z1668, were sent to NREL for efficiency verification, and both NREL
results were very close to or a little higher than EPV's outdoor
measurement.

Table 5.3. Performance of modules made during the Final Phase

+--------+-----+-----+----+-------+---------+--------+-------+------+
| **ID** | **V | **I | ** | **Po  | **Effic | **Seg  | **V~  | *    |
|        | ~oc | ~sc | FF | wer** | iency** | ment** | oc~/c | *Ap. |
|        | ~** | ~** | ** |       |         |        | ell** | ar   |
|        |     |     |    | **    | **(%)** | **\#** |       | ea** |
|        | *   | *   | *  | (W)** |         |        | **(   |      |
|        | *(V | *(A | *( |       |         |        | mV)** | **(  |
|        | )** | )** | %) |       |         |        |       | cm^2 |
|        |     |     | ** |       |         |        |       | ^)** |
+--------+-----+-----+----+-------+---------+--------+-------+------+
| Z1652  | 4   | 1   | 49 | 20.9  | 6.2     | 71     | 582   | 3343 |
|        | 1.3 | .02 | .4 |       |         |        |       |      |
+--------+-----+-----+----+-------+---------+--------+-------+------+
| Z1655  | 4   | 1   | 48 | 21.7  | 6.1     | 71     | 568   | 3550 |
|        | 0.3 | .10 | .8 |       |         |        |       |      |
+--------+-----+-----+----+-------+---------+--------+-------+------+
| Z1657  | 3   | 1   | 48 | 19.6  | 5.8     | 71     | 531   | 3355 |
|        | 7.7 | .08 | .1 |       |         |        |       |      |
+--------+-----+-----+----+-------+---------+--------+-------+------+
| Z1667  | 3   | 1   | 54 | 25.1  | 6.7     | 71     | 534   | 3740 |
|        | 7.9 | .23 | .1 |       |         |        |       |      |
+--------+-----+-----+----+-------+---------+--------+-------+------+
| Z1668  | 3   | 1   | 56 | 26.0  | 7.5     | 71     | 542   | 3454 |
|        | 8.5 | .20 | .4 |       |         |        |       |      |
+--------+-----+-----+----+-------+---------+--------+-------+------+
| Z1670  | 3   | 1   | 47 | 23.7  | 6.3     | 71     | 531   | 3770 |
|        | 7.7 | .33 | .2 |       |         |        |       |      |
+--------+-----+-----+----+-------+---------+--------+-------+------+
| Z1673  | 3   | 1   | 47 | 20.8  | 5.9     | 71     | 508   | 3500 |
|        | 6.1 | .23 | .1 |       |         |        |       |      |
+--------+-----+-----+----+-------+---------+--------+-------+------+
| Z1674  | 3   | 1   | 52 | 22.6  | 6.0     | 71     | 507   | 3737 |
|        | 6.0 | .20 | .4 |       |         |        |       |      |
+--------+-----+-----+----+-------+---------+--------+-------+------+
| Z1677  | 3   | 0   | 58 | 21.7  | 6.5     | 71     | 549   | 3366 |
|        | 9.0 | .95 | .5 |       |         |        |       |      |
+--------+-----+-----+----+-------+---------+--------+-------+------+
| Z1678  | 3   | 1   | 50 | 20.1  | 5.5     | 71     | 492   | 3650 |
|        | 4.9 | .14 | .5 |       |         |        |       |      |
+--------+-----+-----+----+-------+---------+--------+-------+------+
| Z1683  | 3   | 1   | 52 | 20.0  | 5.4     | 71     | 485   | 3660 |
|        | 4.4 | .11 | .0 |       |         |        |       |      |
+--------+-----+-----+----+-------+---------+--------+-------+------+

While accumulating the inventory represented by Table 5.3, we continued
to experiment with CIGS process parameters. Regardless of this
experimentation, all modules were functional, and the results in Table
5.3 demonstrate that EPV now has a controllable, large area hybrid CIGS
process. A relatively high yield was also achieved, but this has not yet
been consistently demonstrated over long periods of time. It does,
however, lay down an excellent foundation for further systematic
improvement. Once a module efficiency in excess of 9% is achieved, it
will become worthwhile to examine the manufacturing potential of the
technology. Fig. 5.7 is a copy of the I-V data for module Z1668, as
measured by NREL. The P~max~ of this module was 26.0W, with an average
voltage per cell of 542mV.

![](media/image15.jpeg){width="4.0625in" height="4.729861111111111in"}

Fig. 5.7. I-V curve for a 26W, 7.5% module, as measured by NREL.

**5.5 Long-term stability of EPV module**

A laminated sub-module, Z1658 8-13, having a dimension of 6" x 17" and
cut from full size plate Z1658, has been mounted on a south-facing,
outdoor rack. It is periodically measured using the four-wire scheme
mentioned above. The module has so far kept excellent stability for more
than nine months. Plotted in Fig. 5.8 are normalized V~oc~ (by
temperature), normalized short-circuit current I~sc~ (by one-sun
irradiance), FF, and normalized efficiency (by both temperature and
irradiance) derived from outdoor measurements without spectrum
correction. The observed fluctuation in V~oc~ and FF is about 2 percent,
and that in I~sc~ and efficiency is about 3 percent, which seems
reasonable given that spectrum and wind speed are not monitored. It is
interesting to observe that the normalized efficiency after long-term
outdoor exposure shows a slight upward trend resulting mostly from
improvement of fill factor and to a lesser extent current. It is not
uncommon for CIGS devices and modules produced at EPV to improve in
performance after light soaking, and indeed, with the current hybrid
process this is the general rule. However, this is the first time we
have done long-term outdoor exposure with a consistent measurement
protocol.

![](media/image16.wmf){width="3.175in" height="1.9666666666666666in"}
![](media/image17.wmf){width="3.1756944444444444in"
height="1.9583333333333333in"}

![](media/image18.wmf){width="3.1756944444444444in"
height="1.9583333333333333in"}
![](media/image19.wmf){width="3.1756944444444444in"
height="1.9583333333333333in"}

Fig. 5.8. Outdoor stability of CIGS sub-module Z1658 8-13.

**5.6 Process control and database**

Process controls for module production have been implemented at each
individual station for quite a while. However, process parameters and
measurable control values were separately located in individual
databases. To better organize information, and to be able to grasp a
picture for the entire process, we developed in the later period of
Phase III a whole-process database linked to all of the databases for
the individual process steps. In this way, all historical processing
information can be easily accessed. Such a database is extremely useful
to help identify problems and to analyze data for statistical studies.

6.  **Surface Treatment**

At EPV, we have found that post-deposition treatment of the CIGS is an
important step to improve device and module performance. Therefore, a
'standard treatment' has been implemented as one of EPV's routine
processing steps. Nevertheless, we have continued searching for better
and easier ways to treat CIGS plates. Some recently developed treatments
show encouraging results. Listed in Table 6.0 are some data measured
lately. The data in Table 6.0 show that newly developed test treatments
are comparable with, and in some cases better than, our standard
process.

Table 6.0. Device performance with various surface treatments

  ---------- --------------- ---------- ---------- -------------- --------------
  **Sample   **Treatment**   **V~oc~    **FF (%)** **J~sc~        **Efficiency
  ID**                       (mV)**                (mA/cm^2^)**   (%)**

  Z1730-25   Standard        504        65.4       32.8           10.8

  Z1730-26   Test 1          550        70.8       31.9           12.4

  Z1730-27   Test 2          557        70.0       33.0           12.9

  Z1730-28   Standard        545        70.0       32.0           12.2

  Z1730-29   Test 1          539        68.2       32.0           11.7

  Z1730-30   Test 2          501        67.9       34.0           11.5

  Z1730-31   Test 1          550        71.3       31.8           12.5

  Z1730-32   Test 3          556        71.3       31.1           12.3

  Z1730-33   Test 1          543        70.2       31.6           12.0

  Z1730-34   Test 3          522        66.7       32.3           11.2

  Z1730-35   Standard        545        68.2       32.7           12.2

  Z1730-36   Test 3          540        71.5       32.4           12.5

  Z1730-37   Test 1          546        70.0       30.9           11.8

  Z1730-38   Test 3          547        71.2       31.7           12.4
  ---------- --------------- ---------- ---------- -------------- --------------

7.  **Development of New TCO Window Layers**

The application of new types of TCO produced by reactive-environment
hollow cathode sputtering to CIGS devices and modules is one of our
innovative efforts during this final Phase. Using RE-HCS, we have
produced high-quality films of ZnO:B, In~2~O~3~:Mo (IMO), In~2~O~3~:Zr,
and In~2~O~3~:Ti (ITiO) \[4, 13-15\]. Representative properties of these
materials are shown in Table 7.0. While conventional ZnO:Al has an
electron mobility of 20-25 cm^2^/Vs, ZnO:B produced by RE-HCS has a
mobility of 38 cm^2^/Vs, and IMO and ITiO films produced by RE-HCS have
mobilities up to 80 cm^2^/Vs. All three of these latter films have an IR
transmission superior to that of ZnO:Al.

Table 7.0. Properties of TCO films made by RE-HCS

+--------------+-------+-------+-----+-------+-----------+----------+
| **Material** | **T   | **t** | **  | **R~  | **ρ**     | **μ**    |
|              | ~s~** |       | T** | sh~** |           |          |
|              |       | *     |     |       | **10^-4^  | **cm     |
|              | *     | *μm** | **  | **Ω   | Ωcm**     | ^2^/Vs** |
|              | *°C** |       | %** | /sq** |           |          |
+--------------+-------+-------+-----+-------+-----------+----------+
| ZnO:B        | 110   | 0.44  | 96  | 17.8  | 7.8       | 37.8     |
+--------------+-------+-------+-----+-------+-----------+----------+
| In~2~O~3~:Mo | 254   | 0.44  | 85  | 3.6   | 1.6       | 70.2     |
+--------------+-------+-------+-----+-------+-----------+----------+
| In~2~O~3~:Zr | 250   | 0.56  | 87  | 4.0   | 2.3       | 63.3     |
+--------------+-------+-------+-----+-------+-----------+----------+
| In~2~O~3~:Ti | 300   | 0.54  | 87  | 3.3   | 1.8       | 80.6     |
+--------------+-------+-------+-----+-------+-----------+----------+

In Fig. 7.0 we show the optical transmission of an ITiO film (produced
by RE-HCS) that is 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 exhibited
by ZnO:Al. Also shown in the figure 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 mobility.

Using films of ZnO:B and ITiO produced by RE-HCS as window layers on
CIGS devices, we have so far achieved device efficiencies approaching
11% (see Table 7.1). The potential current gain with ITiO was not fully
realized as the deposition temperature had to be reduced below its
optimal value (300°C or higher) in order to not damage the CIGS
junction.

![](media/image20.wmf){width="4.68125in" height="3.2354166666666666in"}

Fig. 7.0. Spectral transmission of a high mobility ITiO film

and modeled transmission curves as a function of mobility.

Table 7.1. J-V parameters for cells with RE-HCS window layers

+-------------------------+-----------+----------+---------+----------+
| **Structure**           | **V~oc~** | *        | **FF**  | **Eff**  |
|                         |           | *J~sc~** |         |          |
| **CIGS/\...**           | **mV**    |          | **%**   | **%**    |
|                         |           | **mA     |         |          |
|                         |           | /cm^2^** |         |          |
+-------------------------+-----------+----------+---------+----------+
| CdS/i-ZnO/ZnO:B         | 564       | 27.3     | 70.7    | 10.9     |
+-------------------------+-----------+----------+---------+----------+
| CdS/i-ZnO/In~2~O~3~:Ti  | 565       | 28.4     | 66.8    | 10.7     |
+-------------------------+-----------+----------+---------+----------+

A new, scaled-up hollow cathode system which can handle substrates
having widths up to 45 cm is under construction. Most of the
installation has been completed, and initial tests with a 12 cm cathode
will take place soon.

**8.0 Future Work**

It is hoped that future work can address certain limitations of current
CIGS technology. These limitations include module performance and
production cost. Module performance is most strongly impacted by basic
cell efficiency, and therefore further work to achieve high cell
efficiency and more uniform large area deposition is warranted. Cost
factors also cast a shadow over future high volume production. Cost can
be reduced by simplifying the CIGS growth process, by reducing the
thickness of the CIGS from 2.5 μm to 0.5 - 1.0 μm, by invention of a
CIGS process that produces device-quality material at lower
temperatures, by improved Se utilization, and by many lesser factors
e.g. eliminating the need for ceramic targets for TCO production. The
use of ultra-thin CIGS would also benefit from the development of a
practical light-trapping scheme for CIGS.

#### 9.0 Phase III Summary

> Energy Photovoltaics, Inc.

Advanced CIGS Photovoltaic Technology

Subcontract No. ZDJ-2-30630-21

-   Film qualities and device performances for CIGS films produced by
    the hybrid process (involving both thermal sources and sputtering)
    have been significantly improved by process optimization including
    adoption of a Cu-rich precursor, increase of selenization
    temperature, adjustment of bandgap profile and other parameters. As
    a result, 0.4 cm^2^ devices with efficiency higher than 13%
    (NREL-verified) have been produced using the large area system Zeus.

-   Materials property analysis of CIGS films including ICP, SEM, AES
    depth-profile, XRD, and activation energy were performed to help
    understand process parameters and to direct optimization.

-   The large area deposition uniformity of the following layers was
    substantially improved: co-evaporated (In, Ga)~2~Se~3~, sputtered
    Cu, CBD CdS, and sputtered ZnO:Al.

-   A thorough investigation of factors influencing the contact
    resistance between ZnO and Mo led to improved understanding and
    excellent interconnections.

-   Further reduction of area loss due to patterning operations was
    realized.

-   Dozens of large area modules (up to 4300 cm^2^) were produced in the
    power range of 20 W and above. The best module had a power of 26 W
    (under SRC), corresponding to an efficiency of 7.5%, as measured by
    NREL.

-   Process controls are being emphasized at every process step, and a
    whole process and performance database is being established.

-   New CIGS surface treatments are yielding very encouraging results.

-   New types of TCO deposited by reactive-environment hollow cathode
    sputtering (RE-HCS) show excellent optical and electrical
    properties. In particular, very effective Ti-doping was demonstrated
    in RE-HCS In~2~O~3~, and electron mobilities of 80 cm^2^/Vs were
    achieved. The new TCOs have been applied to CIGS devices and show
    good potential.

-   A laminated CIGS module has demonstrated excellent long-term outdoor
    stability and has slightly improved in power output after more than
    nine months exposure.

-   Overall progress in devices and modules produced by the hybrid
    process during this subcontract is shown in the figure below.

![](media/image21.wmf){width="5.605555555555555in"
height="3.8826388888888888in"}

Fig. 9.0 Trajectories of progress with hybrid CIGS during subcontract
ZDJ-2-30630-21

**ACKNOWLEDGMENTS**

This research was carried out by EPV's CIGS group: Dr. M. Akhtar, J.
Cambridge, Dr. L. Chen (Group Leader), Dr. A. Delahoy (Principal
Investigator), R. Govindarajan, Dr. S. Guo, S. Kleindienst, Dr. B. Sang,
R. Saramak, and F. Ziobro. The Company wishes to thank all members for
their sterling work. Notable technical support was also provided by A.
Foustotchenko, R. Zhang, R. Lyndall, and J. Allen (consultant), as well
as by manufacturing, equipment development, facilities, and the machine
shop.

The following individuals (and institutions) are also acknowledged for
their analyses, advice, or interest: J. Keane, J. Pankow, S. Asher, K.
Ramanathan, R. Noufi, H. Ullal, K. Zweibel (National Renewable Energy
Laboratory); J. Sites (Colorado State University); A. Rockett
(University of Illinois); B. McCandless, W. Shafarman (Institute of
Energy Conversion/University of Delaware); T. Anderson, S. Li
(University of Florida); Neelkanth Dhere (University of Central
Florida).

Thanks are due also to all members of the National CIS Team for their
collaborations and discussions.

Finally, EPV thanks the US DOE for support under subcontract
ZDJ-2-30630-21 with NREL.

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