**THE ROLE OF POLYCRYSTALLINE THIN-FILM PV TECHNOLOGIES**

**FOR ACHIEVING MID-TERM MARKET-COMPETITIVE PV MODULES**

B. von Roedern, K. Zweibel, and H.S. Ullal

National Center for Photovoltaics

National Renewable Energy Laboratory (NREL)

1617 Cole Boulevard

Golden, CO 80401-3393 USA

Tel: (303) 384 6480; Fax: (303) 384 6430; E-mail:
bolko_von_roedern@nrel.gov

### ABSTRACT

This paper reviews the current commercial status of CuInSe~2~ alloys
(collectively, CIS) and CdTe-based photovoltaic (PV) modules, comparing
the performance of commercial products with the results achieved for
solar cell and prototype module champions. We provide an update for
these PV cell and module technologies, and also compare CIS and CdTe
performance levels to the results achieved by the crystalline Si PV
industry. This comparison shows that CIS and CdTe module technology
presently offers the best (and perhaps only) approach for significantly
exceeding the cost/performance levels established by crystalline Si PV
technologies. A semi-empirical methodology is used for comparing
"champion" solar cell and prototype module data with performance
achieved on manufacturing lines. Using a conservative assumption that
thin-film technologies will eliminate the 40% of PV module costs arising
from the Si wafer or ribbon, we estimate the future performance of all
established PV module candidates, and conclude that, based on 2004
knowledge about each PV technology, CIS and CdTe should provide
cost-competitive advantages over crystalline Si.

![](media/image1.wmf){width="4.67007874015748in"
height="3.502755905511811in"}**INTRODUCTION**

Figure 1 shows the best verified champion cell performance achieved in
each of the thin-film PV technologies. Table 1 shows champion large-area
modules produced by a number of entities. Note that aperture-area
efficiencies are reported in this table, whereas commercial efficiencies
listed below use total module-area efficiency values. This paper will
provide a measure for estimating mid-term commercial product performance
based on demonstrated and verified champion cell efficiency values.

### TECHNOLOGY --- DRIVER FOR PERFORMANCE

After investing in a commercial manufacturing line, two pathways lead to
increased performance of the manufactured product. The first approach is
to optimize the chosen process for optimum performance and yield. Once
an optimum has been obtained in this manner, one has to decide which
aspects of the manufacturing process should be upgraded to ensure
further increases in module performance. Because such upgrades often
require massive capital investment and "downtime" (i.e., loss of

**Table 1: Best Large-Area, Thin-Film Modules**

***(standard conditions, aperture-area)***

+--------------+------------+--------+-------------+---------+-------+
| **Company**  | **Device** | **Aper | **E         | **      | **D   |
|              |            | ture** | fficiency** | Power** | ate** |
|              |            |        | **(%)**     | **(W)** |       |
|              |            | **Area |             |         |       |
|              |            | (cm    |             |         |       |
|              |            | ^2^)** |             |         |       |
+--------------+------------+--------+-------------+---------+-------+
| **Würth      | #### CIGS  | **     | **13.0**    | *       | **06  |
| Solar**      |            | 6500** |             | *84.6** | /04** |
+--------------+------------+--------+-------------+---------+-------+
| **First      | **CdTe**   | **     | *           | **67.   | **02  |
| Solar**      |            | 6623** | *10.2^\*^** | 5^\*^** | /04** |
+--------------+------------+--------+-------------+---------+-------+
| **Shell      | **CIGSS**  | **     | **13.1**    | *       | **05  |
| Solar GmbH** |            | 4938** |             | *64.8** | /03** |
+--------------+------------+--------+-------------+---------+-------+
| **Global     | **CIGS**   | **     | *           | **71.   | **09  |
| Solar**      |            | 7085** | *10.1^\*^** | 2^\*^** | /04** |
+--------------+------------+--------+-------------+---------+-------+
| **Antec      | **CdTe**   | **     | **7.3**     | *       | **06  |
| Solar**      |            | 6633** |             | *52.3** | /04** |
+--------------+------------+--------+-------------+---------+-------+
| **Shell      | **CIGSS**  | **     | *           | **46.   | **03  |
| Solar**      |            | 3626** | *12.8^\*^** | 5^\*^** | /03** |
+--------------+------------+--------+-------------+---------+-------+
| **Showa      | **CIGS**   | **     | **12.8**    | **      | **05  |
| Shell**      |            | 3600** |             | 44.15** | /03** |
+--------------+------------+--------+-------------+---------+-------+

Revised 09/04 \* NREL Confirmed

manufacturing capacity), it is important that the upgrades will deliver
the expected performance improvement. It appears that during the most
recent years, the activities of the Thin Film Partnership's Technology
Partners (Shell Solar -- CIS, Global Solar -- CIS, EPV -- CIS, and First
Solar -- CdTe) focused on achieving progress via the first approach,
optimizing for high yield and high performance while maintaining a
chosen process. Significant progress was made, resulting in several
megawatts of modules being sold (see Table 1 for "champion" module and
Table 2 for commercial module performance).

To further boost its sales for power generation applications, Shell
Solar announced development of an 80-W CIS module to better compete with
crystalline Si modules that today have power ratings ≥55 W. EPV and ISET
were qualifying and improving their new CIS deposition schemes (using
"hybrid" vacuum deposition or nano-particle precursors, respectively).
At the same time, NREL, through further adjustments to its established
"3-stage" deposition process, achieved a new champion cell with an
efficiency of 19.5%, slightly bettering the previous 19.3% record \[1\].

In 2004, First Solar established a "generation 2" manufacturing line
that eliminates previous bottlenecks in module throughput. The new line
is designed for producing 25 MW of CdTe modules annually and expected to
be in full operation in 2005. Dr. Wu et. al. from NREL demonstrated the
performance enhancements from using cadmium stannate (Cd~2~SnO~4~)
transparent conductive oxide electrodes, in conjunction with tin zinc
oxide (ZnSnO~x~) buffer layers. Using these, the 16.5%-efficient
champion CdTe cell was previously prepared \[2\]. New buffer layers were
also evaluated in conjunction with commercially produced SnO~2~
transparent conductive electrodes, demonstrating superior performance
\[3\]. These processes appear to be the next logical step for
implementation into manufacturing.

An important aspect of CdTe PV technology is to ensure long-term
stability. Experimentally, cells or

coupons from modules are tested, usually under 1-sun intensity
illumination and open-circuit voltage conditions and a device
temperature of about 65^o^C for thousands of hours. Degradation is
typically observed to "stabilize" and can be minimized (in a few
circumstances, there is no degradation, or even an improvement in
performance after stressing), through optimized processing. Presently,
it is not always clear to what degree changes in the device stability
are due to details in cell design and to what degree effects like
corrosion are the cause for degradation. However, published work by the
Colorado State University group and unpublished work from First Solar
have revealed that the details of the CdCl~2~ treatment, essential for
achieving good device performance, can indeed significantly affect the
amount of observed degradation \[4\]. Until better understood, each
change in cell or module process must also be evaluated for long-term
stability. Past work has often focused on eliminating the Cu "doping"
that is applied with many back-contacting recipes. However, recent work
may suggest that rather than eliminating the Cu-content of the back
contact, it may be more important to incorporate Cu in such a way as to
not cause a stability problem \[5\]. CIS technology continues to
experience "transients," i.e., after exposure of the modules to elevated
temperatures in the dark, which happens to be the condition during the
final module lamination process, module power may drop by up to \~20%,
but this loss is restored with subsequent illumination during exposure
\[6\], and has not been observed to lead to degradation in long-term
outdoor exposure experiments.

The Thin Film Partnership has also initiated a program that investigates
module reliability, durability, and module packaging (encapsulation)
issues. This work is useful for identifying manufacturing defects
arising from the module wiring and "package," and is now considered
required to gain confidence that long-term performance can be
guaranteed.

### 

###  

**Table 2: Module Efficiency from Survey of Manufacturers' Websites and
"Performance Ratios***"*

+----+--------------+-----------+-------------+-----------------------+
| ** | # Module     | **Temp.** | **T         | **Performance**       |
| Ef |              | *         | echnology** |                       |
| f. |              | *Coeff.** |             | **Ratio champion      |
| ** |              |           |             | cell-product**        |
|    |              | **(%      |             |                       |
| *  |              | P/^o^C)** |             |                       |
| *( |              |           |             |                       |
| %) |              |           |             |                       |
| ** |              |           |             |                       |
+----+--------------+-----------+-------------+-----------------------+
| 16 | SunPower     | na        | FZ-Si, sp.  | 16.9/24.7\[7\]=       |
| .9 | SPR210       | *('low')* | j.          |                       |
|    |              |           |             | 68%                   |
+----+--------------+-----------+-------------+-----------------------+
| 16 | Sanyo        | -0.33     | CZ-Si,      | 16.1/24.7\[7\]=       |
| .1 | HIP190BA2    |           | "HIT" j.    |                       |
|    |              |           |             | 65%                   |
+----+--------------+-----------+-------------+-----------------------+
| 14 | BP7180       | -0.5      | CZ-Si, sp.  | 14.3/24.7\[7\]=       |
| .3 |              |           | j.          |                       |
|    |              |           |             | 58%                   |
+----+--------------+-----------+-------------+-----------------------+
| 13 | Kyocera      | V~        | MC-Si, s.p. | 13.3/21.2\[8\]=       |
| .3 | KC187G       | OC~=-0.12 |             |                       |
|    |              |           |             | 63%                   |
+----+--------------+-----------+-------------+-----------------------+
| 13 | Shell        | -0.43     | CZ-Si, s.p. | 13.3/21.2\[8\]= 63%   |
| .3 | Ultra175-P   |           |             |                       |
+----+--------------+-----------+-------------+-----------------------+
| 13 | RWESchott    | -0.47     | EFG         | 13.0/21.2\[8\]=       |
| .0 | ASE          |           | (ribbon)    |                       |
|    | -300-DGF/315 |           |             | 61%                   |
|    |              |           | Si, s.p.    |                       |
+----+--------------+-----------+-------------+-----------------------+
| 12 | Sharp        | na        | MC-Si, s.p. | 12.8/21.2\[8\]= 60%   |
| .8 | ND-167-U1    |           |             |                       |
+----+--------------+-----------+-------------+-----------------------+
| 11 | GEPV-165M    | *(up      | CZ-Si       | 11.5/21.2\[8\]=       |
| .5 |              | to)*-0.5  | (           |                       |
|    |              |           | Reclaimed), | 54%                   |
|    |              |           |             |                       |
|    |              |           | s.p.        |                       |
+----+--------------+-----------+-------------+-----------------------+
| 11 | WürthSolar   | -0.36     | CIGS        | 11.0/19.5\[1\]=       |
| .0 | WS31050/80   |           |             |                       |
|    |              |           |             | 56%                   |
+----+--------------+-----------+-------------+-----------------------+
| 9  | Shell Solar  | -0.6      | CIGSS       | 9.4%/19.5\[1\]=       |
| .4 | ST-40        |           |             |                       |
|    |              |           |             | 48%                   |
+----+--------------+-----------+-------------+-----------------------+
| 7  | First Solar  | -0.25     | CdTe        | 7.6/16.5\[2\]=        |
| .6 | FS55         |           |             |                       |
|    |              |           |             | 46%                   |
+----+--------------+-----------+-------------+-----------------------+
| 6  | Mitsubishi   | -0.2      | a-Si,       | 6.4/10.0\[9\]=        |
| .4 | Heavy MA100  |           |             |                       |
|    |              |           | sing        | 64%                   |
|    |              |           | le-junction |                       |
+----+--------------+-----------+-------------+-----------------------+
| 6  | Uni-Solar    | *(-0.21)* | a-Si,       | 6.7/12.1\[7\]=        |
| .3 | US-64        |           |             |                       |
|    |              |           | trip        | 52%                   |
|    |              |           | le-junction |                       |
+----+--------------+-----------+-------------+-----------------------+

s.p. = "standard" diffused junction screen printed cells; sp.j. = cells
employing alternative junctions ("HIT,"

point contacts, buried laser grooves)

### PROJECTIONS OF FUTURE PERFORMANCE

A pragmatic method was developed for predicting future performance and
potential market impact of all technologies.We sought to avoid comparing
favorable projections for one PV technology with less favorable
projections of another one. The method relies on comparing 2004
commercial product efficiency data with 2004 laboratory solar cell
champion data to assess how thin-film PV technologies in the future may
compete with

the mainstream crystalline Si products. The first input to this method
is derived from a survey of manufacturers' Web sites (shown in Table 2).

Table 3: Projected Future Commercial Performance of Commercial Modules
Made With Different PV

Technologies, and Their Relative Performance and Relative Cost
Performance

+-----------+--------------+---------------+--------------------------+
| T         | Future       | Relative      | Relative cost divided by |
| echnology | commercial   | performance   | relative performance     |
|           | module       | rating        | (about proportional to   |
|           | efficiency,  |               | future \$/W~p~ module    |
|           | based on     | (Standard,    | cost differences with    |
|           | 2004         | s.p. silicon  | standard crystalline Si) |
|           | technology   | = 1)          | assuming 40% thin-film   |
|           | knowledge    |               | module cost advantage    |
|           | (%)          |               |                          |
+-----------+--------------+---------------+--------------------------+
| Silicon   | 19.8         | 1.18          | 0.85 (highly             |
| (sp. j.)  |              |               | competitive)             |
+-----------+--------------+---------------+--------------------------+
| Silicon   | 17.0         | 1.00          | 1.00                     |
| (s.p.)    |              |               |                          |
+-----------+--------------+---------------+--------------------------+
| CIGS      | 15.6         | 0.92          | 0.65 (highly             |
|           |              |               | competitive)             |
+-----------+--------------+---------------+--------------------------+
| CdTe      | 13.2         | 0.78          | 0.77 (highly             |
|           |              |               | competitive)             |
+-----------+--------------+---------------+--------------------------+
| a-Si      | 8.0          | 0.47          | 1.28 (not competitive)   |
| (1-j)     |              |               |                          |
+-----------+--------------+---------------+--------------------------+
| a-Si      | 9.7          | 0.57          | 1.05 (about the same)    |
| (3-j)     |              |               |                          |
+-----------+--------------+---------------+--------------------------+

s.p. = "standard" diffused junction screen-printed cells; sp. j. = cells
employing alternative junctions ("HIT,"

point contacts, buried laser grooves)

gradually increases, presumably because (1) champion cell improvements
become harder, and (2) module efficiencies have time to catch up.
However, even for crystalline Si, values \>70% have not yet been
obtained. Based on these trends, we assume that a c/c-ratio of 80% will
likely be a reasonable figure to estimate the best future commercial
module performance based on champion cell performance known today. Thus,
in Table 3, we list expected future commercial product performance based
on today's champion cell performance and a c/c ratio of 80%. In
practice, we expect product performance to improve gradually from
today's values as manufacturing processes are optimized. A value of 80%
is our current best estimate of what could be achieved for a mature
process after advanced process control schemes have been implemented
into manufacturing. This methodology can be upgraded in the future once
new champion cell performance is obtained, or when better
commercial-product-based values for the cc ratios have been established.

The comparison of thin films and crystalline Si PV technology is further
complicated by the fact that it is not clear what market share different
Si technologies will attain in the future. In the past, Si PV
"technology" was usually broken down by type of Si substrate. But, as
Table 2 reveals, the Si wafer type (single-crystal, cast ingot, ribbon)
has resulted in little differentiation regarding the performance of
today's commercial product; rather, what matters is whether the cells
are made by conventional processing (diffusing an emitter and collector
and contacting the emitter with screen-printed finger grids, designated
as 's.p.' in column 4) or by alternative junction-formation processes
currently used only for limited product lines \[10\]. The latter
"special junction" technologies are designated as 'sp. j.' in column 4
of Table 2. We express today's commercial performance as a ratio of
today's champion cell performance for each specific technology, given in
the last column of Table 2.

In the second column of Table 3, we estimate future commercial
performance based on today's cell champion performance, *assuming a c/c*
*ratio of 80% for all technologies*. These efficiency values of future
CIS- and CdTe-based modules appear very reasonable when they are
compared to aperture-area efficiencies already achieved for
one-of-a-kind module prototypes shown in Table 1. The projected
crystalline Si module efficiencies also look very reasonable, showing
that a fair comparison is being made. The third column calculates a
relative module performance ratio, using a value of 1.00 for standard
(s.p.) crystalline Si module technology. The last column in this table
calculates a projected cost ratio for these technologies. It assumes a
40% cost advantage for thin-film modules, based on the knowledge that in
a crystalline Si module, typically 40% of the manufacturing costs are
incurred by manufacturing the silicon wafer or ribbon required for
manufacturing the solar cells. This is a component and materials cost
minimized in thin-film modules. Hence, for the thin-film technologies,
relative cost levels are reduced by multiplying the relative performance
by 0.6, to account for the cost of wafers or ribbons eliminated,
enabling a comparison based on a ratio of \$/W~p~ module manufacturing
cost. That is, the number in the final column should be approximately
proportional to future cost differences between the technology options
and the standard crystalline Si technology.

The 40% figure for thin films may be conservative, since calculations
using existing estimates suggest it could be as high as 60% in some
cases where thin-film capital costs are already low (e.g., for CdTe or
some third-generation thin films). The reader can easily recalculate the
projected cost comparison using a different cost-advantage percentage
based on their own cost estimates. Other nuances are also possible
(e.g., adding some cost disadvantage to the higher performance sp.j.
crystalline Si approach; or assuming that a champion cell efficiency
enhancement will occur faster in one technology than such progress may
occur in the other technologies). One of the possible strengths of our
approach is to figuratively wash our hands of these second-order
perturbations and assume that they just average out. Thus, we are able
to get a single final number (Table 3, far right column) with which to
compare vastly different technologies.

Table 3 suggests that based on today's technology status and
manufacturing know-how, future CIS and CdTe thin-film PV modules will be
able to compete with future crystalline Si modules, even if crystalline
silicon technology successfully transitions to higher cell efficiencies
using cells manufactured with special-junction schemes. The future of
a-Si may depend more on its special value in the building-integrated PV
(BIPV) marketplace or on issues such as silicon feedstock availability.
Third-generation thin films that might be even less efficient may have
even greater problems making a sizable impact.

Lessons to be learned from this are that technology and demonstrated
cell performance remain an important yardstick in determining commercial
product potential. Unless stable laboratory cell efficiencies \>12% are
demonstrated, any existing or new thin-film PV technology has little
chance of being a strong contributor in the mainstream power module PV
markets. We believe that our analysis is valid for standard modules sold
as a commodity. For BIPV products, low performance may constitute
somewhat less of a penalty than for standard modules. On the other hand,
it can be expected that market conditions (including the debit caused by
the need to offset balance-of-systems costs with higher efficiencies)
will be such that higher efficiency modules will command somewhat higher
\$/W~p~ prices.

Although people in the crystalline Si field have a quantitative
understanding of how to go from standard screen-printed processes to
high-efficiency special-junction devices, such information is not yet
available for most thin-film technologies. Development of a better
numerical understanding to be able to predict with much greater
confidence how much a new thin-film layer, deposition process, or
process optimization will contribute to enhanced future commercial
product performance appears desirable, albeit difficult.

Based on performance increases observed in recent years, and investments
for new manufacturing capacity made or announced, we project a rapid
increase in the share of thin-film PV modules in U.S. PV module
manufacturing. From a survey and estimation of the current
manufacturers, which includes Uni-Solar making amorphous silicon roofing
laminates, the following volumes were and are projected to be produced
by U.S.-based manufacturers of thin-film PV modules. The result is shown
in Fig. 2.

![](media/image2.wmf)

**Fig. 2: Past and projected shipment of U.S.-produced thin film PV
modules (MW~p~ per year).**

This paper provides at top-down view on the comparison of different PV
technologies, based on the actual status of today's commercial reality
and established R&D achievements. Another top-down approach is to use
the PV module learning curve and to add either a new slope or an
entirely new line for thin-film technologies \[11\]. There are also many
"bottom-up" studies on the cost of PV systems that have historically
provided ambitious cost projections \[12, 13\]. At some time in the
future, top-down and bottom-up cost estimates will have to approach each
other. Presently, the bottom-up estimates make many assumptions about
expected performance. Only after high-volume manufacturing has been
implemented will it be known what performance levels can be obtained
without compromising low-cost manufacturing techniques. A much better
quantitative understanding of how process optimization and process
changes will affect the performance of commercial modules would shorten
the required development periods to achieve large-scale manufacturing of
high-performance thin-film PV modules.

**CONCLUSIONS**

We showed how cell and prototype module efficiencies will likely remain
important for all PV technologies, as they will be related to commercial
product performance in future years.

Based on 2004 knowledge about the technologies, the performance of CIGS
and CdTe modules should be high enough to allow them to be competitive
with crystalline Si. This includes presumed progress in both thin-film
and crystalline Si PV technologies.

Reliability and customer acceptance remain significant hurdles for
thin-film PV to expand its market share. The NREL Thin Film Partnership
has begun to address these issues as part of its programs.

Forecasting PV costs is not an idle matter, though some of the issues
are much debated. Much of the future of PV as an energy-significant
technology rests on meeting very ambitious cost goals. Establishing a
set of approaches and being able to begin the process of comparing them
is useful within this debate.

**ACKNOWLEDGEMENTS**

This work was supported by the U.S. Department of Energy under Contract
No. DE-AC36-99GO10337. The authors would like to thank the numerous
engineers, scientists, and technicians worldwide whose data have been
included in this paper.

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