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Polycrystalline Thin-Film 

Photovoltaics: From the 

Laboratory to Solar Fields 

Preprint 

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

Presented at the 2006 IEEE 4th World Conference on 

Photovoltaic Energy Conversion (WCPEC-4) 

Waikoloa, Hawaii 

May 7–12, 2006 

Conference Paper 

NREL/CP-520-39838 

May 2006 
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POLYCRYSTALLINE THIN-FILM PHOTOVOLTAICS: 

FROM THE LABORATORY TO SOLAR FIELDS*

Bolko von Roedern, Harin S. Ullal, and Ken Zweibel 

National Center for Photovoltaics (NCPV) 

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 

 We review the status of commercial polycrystalline thinfilm solar cells and photovoltaic (PV) modules, including 

current and projected commercialization activities. 

INTRODUCTION 

 Major technical progress has occurred in the area of 

thin-film PV technologies, particularly those based on 

cadmium telluride (CdTe) and copper indium diselenide 

(CuInGaSe2, CIGS). Some of the many advances made in 

material research, device development, manufacturing 

technology, reliability testing of modules and systems, 

technical R&D issues relating to CdTe and CIS, and 

commercial efforts under way in the United States and 

worldwide are reported in this paper. Perhaps most 

important is the commercial success in transitioning First 

Solar’s CdTe modules from pilot development to a massproduced commercial product. A 1,400-kW thin-film CdTe 

solar field has been installed in Dimbach, Germany, using 

First Solar modules. Several thin-film PV systems varying in 

size from 400 to 1,400 kWp have been installed in Germany 

and the United States using First Solar CdTe modules. 

 CIS module technology has received tremendous 

world-wide interest, and commercial module development 

activities are actively pursued by more than a dozen 

entities. Shell Solar Industries (SSI) is marketing a new 

thin-film power module (80-Wp power, trade-named 

“Eclipse”) for various applications. Edge seals developed in 

collaboration with the National Thin Film Module Reliability 

Team have been used in this product. Global Solar Energy 

has also fabricated a thin-film CIGS power module with an 

aperture-area conversion efficiency of 10.2% and power 

output of 88.9 W. The highest aperture-area conversion 

efficiency of 13.0% for a thin-film CIGS power module has 

been achieved by Würth Solar in Germany with a 

corresponding power output of 84.6 W. 

 To gauge progress with the polycrystalline thin-film cell 

and module technologies, we first look at “champion” cells 

and modules fabricated to date. All of these thin-film cell 

efficiencies reflect “total-area” performance, and module 

aperture-area conversion efficiencies are used. Most 

results have been independently confirmed by 

NCPV/NREL. Table 1 summarizes the results of new highperformance noteworthy CIGS polycrystalline thin-film solar 

cell results. Table 2 shows the results of “champion” 

modules made by the various groups/organizations 

worldwide. Note that aperture-area efficiencies are reported 

in this table, whereas commercial efficiencies use total 

module-area efficiency values. Reviewing these champion 

cell and module results provides some insight as to the 

capabilities of the respective PV technologies. Achieving 

these cell and module results is a testimony to talented 

solar cell researchers who have developed the necessary 

processing know-how for achieving such state-of-the-art 

prototypes. 

STRENGTHENING THE SCIENCE BASE 

 To a very significant degree, processing of highefficiency CIGS and CdTe PV solar cells and modules 

depends on the know-how of experienced researchers. 

Some process details necessary to produce champion-level 

solar cells and modules are published in the scientific 

literature. However, not enough detail is usually provided 

such that the results could be reduced to a commercial 

product by other experienced researchers without further 

experimentation. In part, this may be due to the fact that 

the underlying mechanisms that link cell processing and 

cell performance are not understood in sufficient detail. 

Some research activities are geared toward improving the 

influence of cell processing and material properties on cell 

performance. Many analytical studies of CIGS and CdTe 

cell absorbers focus on characterizing micro- and 

nanostructures of CIGS and CdTe materials [1, 2]. Detailed 

analyses of the cell-junction chemistry is also under way [3]. 

By applying surface analytical methods to actual solar cells, 

much more detailed knowledge of the energy-band 

diagrams and the chemical composition of solar cell 

interfaces is expected. 

 Improved understanding of the solar cell operation is 

particularly important to assist commercial manufacturers 

with improving module performance, manufacturing yields, 

and product stability. Although the leading thin-film PV 

module manufacturers have now established manufacturing 

processes resulting in adequate performance, yield, and 

——————————— 

*This work has been authored by an employee or employees of the Midwest Research Institute under Contract No. DE-AC36-99GO10337

with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, 

acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce 

the published form of this work, or allow others to do so, for United States Government purposes. 

1
stability, the development of such manufacturing processes 

has often taken more time and resulted in somewhat lower 

commercial product performance than was originally 

expected or anticipated. Better understanding of solar cell 

device operation can also assist in helping manufacturers 

decide when to “tweak” established processes and when to 

substitute or add processes to enhance commercial product 

performance. 

 Although greater knowledge of the semiconducting 

CIGS and CdTe properties is very important, one may ask 

the question of how much value can ultimately be derived 

from detailed knowledge of semiconductor material 

properties. While often the case is being made that “much 

more” is known about the silicon semiconductor compared 

to CIGS and CdTe, this begs the question of why—despite 

this greater knowledge base—solar cells and modules 

made from the latter materials have demonstrated generally 

higher efficiencies than devices made from thin-film silicon 

(<50 μm thick). 

CIGS TECHNOLOGY 

 A typical CIS-based solar cell structure is a 

MgF2/ZnO/CdS/CIS/Mo/glass “substrate” configuration. 

Low-cost, soda-lime glass is used for device fabrication. In 

addition, polymers, metal foils, and stainless steel are also 

used for device fabrication. The highest world-record, totalarea efficiency achieved thus far by NREL scientists for a 

thin-film CIGS solar cell is 19.5% measured under the AM 

1.5 global spectrum [4]. Using a single-layer buffer layer of 

ZnS(O,OH) and CdZnS(O,OH), it is remarkable that 

champion efficiency levels (>18%) were attained using 

alternatives to CdS as the heterojunction partner [5]. A 

17.1% total-area conversion efficiency has been 

demonstrated for a thin-film CIGS solar cell using a thin 

absorber layer thickness of 1.1 μm [6]. The device 

parameters of these solar cells are included in Table 1. The 

CIGS film is deposited by the physical vapor deposition 

method, the CdS [CdZnS (O,OH)] by the chemical-bath 

deposition (CBD) technique, and ZnO either by chemical 

vapor deposition or sputtering. Studies why CBD CdS 

results in the highest efficiency cells have been conducted 

[7]. This issue is of technological importance, because 

many module manufacturing processes would prefer 

vacuum deposition of CdS (or alternatives) layers, rather 

than depositing them by a wet-chemical process. Aside 

from evaporating CIGS films, two other deposition 

approaches have been developed. The first one relies on 

vacuum deposition (such as sputtering) of the metal Cu:Ga 

and In precursors that are then reacted into the compound 

semiconductor by annealing in H2Se atmosphere 

(“selenization”). The other method relies on creating 

nanoparticle “inks” (or “paints”) that are applied using nonvacuum coating techniques and then reacted into 

semiconductor films [8, 9]. 

 Some work is being carried out on cells with CuGaSe2

and CuInS2 absorbers. The Florida Solar Energy Center 

has achieved world-class efficiency for a CuInS2 (result 

included in Table 1) [10]. However, for the latter cells, 

efficiencies >13% could not yet be demonstrated; a further 

drawback is the necessity to wet-etch the CuInS2 absorbers 

in a cyanide etch to achieve solar cells and modules with 

reasonable performance. Sulfurcell in Germany has 

developed a manufacturing process for this cell technology, 

and has delivered its first commercial product. A research 

topic receiving significant emphasis is the development of 

CIGS cells with thin absorber layers of 1 μm or less. These 

will reduce the per-watt usage of expensive and rare metals 

(In, Se), and can possibly increase manufacturing 

productivity. It is often found that for absorber thicknesses 

up to 0.7 μm, reasonable cell performance can be 

maintained, while significant losses in VOC, FF, and JSC

have been reported as the thickness is reduced to below 

0.7 μm. However, this is still preliminary, and further 

research is required to ascertain if and how such losses 

may be overcome. 

CdTe TECHNOLOGY 

 For thin-film CdTe solar cells, the typical structure is 

glass/SnO2/CdS/CdTe/contacts. A world-record, total-area 

efficiency of 16.5% has been achieved by NREL scientists 

[11]. In this particular case, the SnO2 front contact is 

replaced by the more promising Cd2SnO4 contact. Also, a 

13.9% thin-film CdTe solar cell has been fabricated using a 

semitransparent back contact [11]. First Solar and NREL’s 

champion cells were deposited by vacuum sublimation onto 

high (>500o

C) substrate temperature, whereas other 

deposition methods such as elecctro-deposition used in the 

past have also resulted in respectable CdTe cell 

performance. Areas of research include micro 

nonuniformity of CdTe films and its impact on device 

performance, thin CdTe absorber layers, interdiffusion at 

the CdS/CdTe interface, where S diffuses into the CdTe 

film, vapor CdCl2 heat treatments, and the role of Cu 

“doping” that is usually used for often proprietary backcontacting procedures. One interesting fundamental 

question requiring an answer is whether the maximum 

open-circuit voltage of CdTe cells could be significantly 

improved (say from 850 mV to greater than 900 mV) without 

degradation of FF and JSC. In manufacturing terms, First 

Solar achieved substantial progress by being able to 

increase their power ratings of its 60 cm x 120 cm module 

from 50 and 55 Wp to new ratings now ranging from 55 to 

65 Wp (9.3% total area) within the last year. 

MODULE RELIABILITY AND COMMERCIALIZATION 

 The Thin Film Partnership has pursued serious studies 

aimed at understanding and improving the reliability of 

CIGS and CdTe PV modules. It is now reasonably well 

established that CIGS and perhaps CdTe are more 

moisture sensitive and often require better module 

encapsulation schemes than crystalline (or amorphous Si) 

PV modules. Without such improved schemes, it has been 

difficult for CIGS and CdTe modules to pass the “damp 

heat” test (1000-hour exposure at 85o

C and 85% relative 

humidity). NREL [12] has embarked on a study to link how 

cell processing affects the performance during continued 

“stressing,” i.e., light exposure at elevated (typically, 65° to 

100o

C) device temperature in various atmospheric 

ambients. It is also well known that electrical bias during 

such exposure affects the changes observed in cells and 

modules. Currently, these studies are carried out on naked 

and encapsulated solar cells. But it is planned to expand 

them to minimodules, as it has become clear that the cell 

interconnects are also affecting long-term performance. For 

CdTe cells, it has been established that the details of the 

entire cell process, as well as the details of back-contacting, 

2
are critical to achieving acceptable device performance and 

reliability. 

 As an additional “real-world” test, the Thin Film 

Partnership has been exposing thin-film PV modules in hot 

and humid climates at the Florida Solar Energy Center 

(Cocoa, FL) and Texas A&M University (College Station, 

TX). The long-term performance of these modules are 

monitored, and, if necessary, the results and corrective 

action are discussed with the respective manufacturers. 

Growth in the sales of U.S. thin-film PV products 

between 2004 and 2005 has about doubled. It is estimated 

that the global thin-film (a-Si, CIS, CdTe, thin-Si) PV 

production capacity could increase from the present 

capacity of 120 MWp to about 435 MWp by 2007, with U.S. 

leading with an estimated 240 MWp capacity by 2007, 

potentially solidifying a U.S. competitive position [13]. In 

2005, U.S. thin-film market share reached 45 MW, or 29% 

of U.S. PV cell/module production, and met the projections 

made one year earlier [14]. This is in contrast to world PV 

production of 1759 MWp, with a worldwide share of thin-film 

PV technologies of only 5.4% in 2005. Thin-film modules 

(including a-Si) are competing directly with crystalline silicon 

in the market place for the first time. Several large thin-film 

solar fields varying in size from 400 to 1,400 kW have been 

installed in Germany and the United States. Table 3 

summarizes the large thin-film solar installations worldwide. 

Figure 1 depicts a grid-connected application. Due to their 

low efficiencies but low costs, thin-film modules are 

especially suited to lower overall cost of large (over 20 kWp) 

systems, e.g., commercial roofs and ground-mounted 

installations. More pictures can be found at [16]. 

 Due to the rapid growth of the world PV market of 

about 45% annually over the past several years, several 

new thin-film PV companies have entered the fray. 

Following is the list of the more established and emerging 

companies in thin-film PV technologies. In the area of CIS 

PV technology, the list of companies are as follows: Shell 

Solar Industries, USA; Global Solar Energy, USA; Würth 

Solar and Sulfurcell, both in Germany; Energy 

Photovolotaics, USA; Miasole, USA; NanoSolar, USA; 

Daystar Technologies, USA; International Solar Electric 

Technologies, USA; Heliovolt, USA; Showa Shell, Japan; 

and Honda Automotive, Japan. In the case of thin-film CdTe 

PV companies, clearly First Solar, USA, is the world leader. 

Other companies are Antec Solar, Germany; Solar Fields, 

USA; and AVA Tech, USA. 

Table 1: Thin-Film CIGS Solar Cells 

No. Area (cm2) Voc (V) Jsc (mA/cm2) FF (%) Efficiency (%) Comment 

1 0.410 0.6975 35.078 79.52 19.5 Standard CIGS cell [4] 

2 0.408 0.7052 35.515 77.90 19.5 CdZnS(O,OH) buffer layer [5] 

3 0.402 0.6698 35.112 78.78 18.5 ZnS (O,OH) buffer layer [5] 

4 0.409 0.6782 31.93 79.20 17.1 1.1-μm CIGS [6] 

5 0.5 0.774 21.6 73.7 12.3 Cu(In,Ga)S2 “champion” [15] 

6 0.409 0.8305 20.88 69.13 11.99 CuInS2 [10] 

Table 2: Polycrystalline Thin-Film PV Modules (Standard Conditions, Aperture-Area) Ranked by Power

Company Device Aperture Area (cm2) Efficiency (%) Power (W) Date 

Global Solar CIGS 8390 10.2* 88.9* 05/05 

Shell Solar CIGSS 7376 11.7* 86.1* 10/05 

Würth Solar CIGS 6500 13.0 84.6 06/04 

First Solar CdTe 6623 10.2* 67.5* 02/04 

Shell Solar GmbH CIGSS 4938 13.1 64.8 05/03 

Antec Solar CdTe 6633 7.3 52.3 06/04 

Shell Solar CIGSS 3626 12.8* 46.5* 03/03 

Showa Shell CIGS 3600 12.8 44.15 05/03 

* NREL Confirmed 

Table 3: Polycrystalline Thin-Film PV Installations 

Location Material Size (kW) Date 

Dimbach, Germany CdTe 1,400 2004–2005 

Reussenkoge, Germany CdTe 1,040 2005 

Fellber, Germany CdTe 800 2005 

Sinzheim, Germany CdTe 800 2005–2006 

Tapfheim, Germany CdTe 778 2005 

Springerville, AZ, USA CdTe 500 2001–2003 

Florsheim, Germany CdTe 440 2005 

Camarillo, CA, USA CIS 245 2003 

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Figure 1: Thin-film CdTe 1,400-kW thin-film solar 

installation. 

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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