Polycrystalline Thin-Film Solar 

Cell Technologies 

Preprint 

H.S. Ullal 

National Renewable Energy Laboratory 

To be presented at the 18th International Photovoltaic Science and 

Engineering Conference and Exhibition 

Kolkata, India 

January 19–23, 2009 

Conference Paper 

NREL/CP-520-44622 

December 2008 
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Polycrystalline Thin Film Solar Cell Technologies*

Harin S. Ullal, Ph.D. 

National Center for Photovoltaics 

National Renewable Energy Laboratory 

1617 Cole Boulevard 

Golden, Colorado 80401 USA 

Tel: 303-384 6486, Fax: 303-384 6430, E-mail: harin_ullal@nrel.gov

1.0 Introduction 

In this paper, we report on the significant progress made worldwide by polycrystalline 

thin-film solar cells, namely, cadmium telluride (CdTe) and copper indium gallium 

diselenide (CIGS). Polycrystalline thin-film PV technology status is also discussed in 

details. In addition, R&D and technology challenges in both areas are elucidated in this 

paper. The worldwide estimated projection for thin-film PV technology production 

capacity announcements (including a-Si/thin Si) which was estimated at 3700 MW in 

2006 [1] is now estimated at more than 8000 MW by 2010. 

1.0 Thin-Film Cadmium Telluride Solar Cell Technologies 

Thin-film CdTe solar cells are one of the most promising thin-film PV devices. With a 

bandgap of 1.45 eV it has an excellent match with the solar spectrum. Since these are 

direct bangap semiconductors with high absorption coefficient, very thin absorber layer 

are needed to absorb the photons. Theoretical efficiencies for these devices are about 

26%. Laboratory efficiencies of 16.5% for thin-film CdTe solar cell has been 

demonstrated by NREL scientists [2]. Historically, thin-film CdTe solar cells were 

referred to as the “dark horse” for thin-film PV devices. Subsequent concerted research 

effort by several groups worldwide has resulted in developing several deposition 

processes for the growth of the absorber layer. The processes are close-space sublimation, 

electrodeposition, vapor transport deposition, spray, screen-printing, sputtering, physical 

vapor deposition, laser ablation, metal organic chemical vapor deposition, molecular 

beam epitaxy, and atomic layer epitaxy [3]. Many of these processes has resulted in thinfilm CdTe solar efficiency of 10% or higher. Five of these processes have demonstrated 

prototype power modules, namely, close-space sublimation, electrodeposition, spray, 

screen printing and vapor transport deposition. 

For thin film CdTe technology, five key research, development, and technology 

challenges include: (1) standardization of equipment for deposition of the absorber layer, 

(2) higher module conversion efficiency, (3) back-contact stability, (4) reduced absorber 

layer thickness to less than 1 micrometer and (5) control of film and junction uniformity 

over large area for power modules. 

 

* Employees of the Alliance for Sustainable Energy, LLC, under Contract No. DE-AC36-08G028308 with the US. 

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1 
The current status of the manufacturing technology is power modules with a conversion 

efficiency of 10.6% made by First Solar (FS). Their manufacturing cost is $ 1.08 per watt 

and installed system price is in the range of $ 4-5 per watt, the lowest in the industry for 

any PV system. Clearly, FS is the world leader in all thin-film PV manufacturing 

technology. They have an installed capacity of more than 140-MW in Perrysburg, Ohio. 

They have also installed a 192-MW manufacturing plant in Germany. In addition, they 

are installing more than 700-MW thin-film CdTe manufacturing line in Malaysia, which 

will be commissioned in phases in 2009 and 2010. Thus FS’s target is to have a 

worldwide installed thin film manufacturing capacity of more than 1100-MW by 

December 2009. FS is also projecting cost to decline to $ 0.92 per watt by Dec 2009. To 

date, FS has installed 500 MW of thin-film CdTe power modules mainly in Germany and 

Spain for commercial roof-top and utility-scale applications. They are also working very 

closely with a system integrator -- Juwi Solar to install a 40-MW solar farm in Germany 

to be completed in December 2008. Like for all PV companies, the German and Spanish 

feed-in-tariff law has clearly helped in the market acceptance of the thin-film CdTe 

power modules. A few more emerging companies are AVA Solar, Colorado, USA; 

Primestar Solar, Colorado, USA; Calyxo, Ohio, USA; Calyxo, Germany; and Arendi, 

Italy. 

3.0 Thin-Film Copper Indium Gallium Diselenide Solar Cell Technologies 

Thin-film PV technology based on copper indium gallium diselenide is another attractive 

option for fabricating high-efficiency, low-cost, and reliable thin-film power modules. 

Thin-film CIS is also a direct bandgap semiconductor and has a bandgap of ~0.95 eV. 

When Ga is added to CIS, the bandgap increases to ~1.2 eV depending on the amount of 

Ga added to the CIGS film. This material has demonstrated the highest total-area, 

conversion efficiency for any thin-film solar cells in the range of 19.3% to 20.0%, 

fabricated by NREL scientists [4,5,6,7]. 

Several challenges still need to be addressed as emerging and new groups develop CIGS 

thin-film PV technologies. The following six challenges are critical for developing lowcost and reliable CIGS products: (1) standardization of equipment for the growth of the 

CIGS absorber films, (2) higher module conversion efficiencies, (3) prevention of 

moisture ingress for flexible CIGS modules, (4) improved processing for CIGS deposited 

by alternative process for high efficiency cells and modules, (5) thinner absorber layers of 

less than 1 micrometer or less and (6) CIGS absorber film stoichiometry, and junction 

and film uniformity over large areas for power module fabrication. 

Worldwide, some 40 companies are actively involved in the technology development of 

thin-film CIGS products. Towards this end, ten deposition processes are being used for 

growing the thin-film absorber layers. In all cases, Mo is used as the back contact 

deposited by sputtering, while majority of the groups use ZnO as the front contact 

deposited by sputtering or chemical vapor deposition. Some companies use ITO instead 

of ZnO for the front contact. Six thin-film CIGS companies: Wurth Solar, Germany; 

Global Solar Energy (GSE), USA; Showa-Shell, Japan; Honda Soltec, Japan; Sulfurcell, 

Germany; and Solyndra, USA are in commercial production. The production capacity 

2 
varies between 5 to 40 MW per year. GSE has recently installed a 40 MW CIGS 

manufacturing plant in Tucson, Arizona, USA and are installing a 35 MW manufacturing 

plant in Germany later in 2008. In addition, GSE is also planning an expansion of 100 

MW plant in Tucson, Arizona that will be completed by December 2010. Thus, GSE will 

have a cumulative production capacity of 175 MW worldwide by December 2010. 

The thin film CIGS absorber layers vary in thickness from 1.5 to 2.5 micrometer and are 

deposited by coevaporation or the two-stage process in which the first step is the growth 

of the precursors, Cu, In, Ga by sputtering followed by selenization in H2Se gas for CIGS 

film formation. 

4.0 Summary 

Rapid progress is being made by CdTe, and CIGS-based thin-film PV technologies in 

entering the commercial markets. In the United States, market share for thin-film 

(includind a-Si) was about 65% in 2007 compared to less than 10% in 2003, and has 

surpassed Si shipments in the year 2007 [8]. Several critical research, development, and 

technology issues need to be addressed by emerging thin-film PV companies as they plan 

to enter the market. The projections for worldwide production capacity announcements 

for all thin-film PV (including a-Si) are estimated at more than 8000 MW in 2010, with 

First Solar’s global target at 1000 MW by Dec 2009 and Sharp’s target of 1000 MW by 

March 2010. These economies-of-scale production capacity should substantially reduce 

the manufacturing price of the thin-film PV products and potentially make solar 

electricity price-competitive with grid-parity electricity and met the U.S. Department of 

Energy’s Solar America Initiative’s levelized cost of electricity goals by the year 2015. 

5.0 Acknowledgment 

This work was supported by the U.S. Department of Energy under Contract No. DEAC36-08G028308. The authors would like to thank the numerous managers, engineers, 

scientists, and technicians whose data are included in this paper. 

6.0 References 

[1] H. S. Ullal and B. von Roedern, “Thin Film CIGS and CdTe Photovoltaic 

Technologies: Commercialization, Critical Issues and Applications” Proceeding 

of the 22nd Photovoltaic Solar Energy Conference, Milan, Italy; September 3-7, 

2007; in press 

[2] X. Wu, “High Efficiency Polycrystalline CdTe Thin-Film Solar Cells, Solar 

Energy, 77, 204, pg. 803-814 

[3] H.S. Ullal, K. Zweibel, and B. von Roedern, “Current Staus of Polycrystalline 

Thin-Film PV Technologies,” 26th IEEE Photovoltaic Specialists Conference, 

Anaheim, CA; September 29-October 3, 1997 

[4] K. Ramanathan, M.A. Contreras, C.L. Perkins, S. Asher, F.S. Hasoon, J. Keane, 

D. Young, M. Romero, W. Metzger, R. Noufi, J. Ward and A. Duda, “Properties 

3 
4 

of 19.2%Efficiency ZnO/CdS/CuInGaSE2 Thin-film Solar Cells,” Progress in 

Photovoltaics: Research and Applications, 11, 2003, pg.225-230 

[5] M.A.Contreras, K. Ramanathan, J. AbuShama, F. Hasoon, D.L. Young, B. Egaas, 

and R. Noufi, “ Diode Characteristics in Stat-of-the-Art ZnO/Cu(In1-xGax) Se2

Solar Cells,” Progress in Photovoltaics: Research and Applications, 13, 2005, pg. 

209-216 

[6] R.N. Bhattacharya, M.A. Contreras, B. Egaas, and R.N. Noufi, “High Eficiency 

Thin-Film CuIn1-x GaxSe2 photovoltaic cells using a Cd1-x ZnxS Buffer Layer,” 

Applied Physics Letters, 89, 2006, pg. 253503-1--23503-2 

[7] I. Repins, M.A.Contreras, B. Egaas, C. DeHart, J. Scharf, C.L. Perkins, B. To, 

and R. Noufi, “19.9%-Efficient ZnO/CdS/CuInGaSe2 Solar Cell with 81.2% Fill 

Factor,” Progress ion Photovoltaics: Research and Applications, 16, 2008, pg. 

235-239 

[8] PV News – 2003 to 2008 
F1147-E(10/2008) 

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14. ABSTRACT (Maximum 200 Words)

In this paper, we report on the significant progress made worldwide by polycrystalline thin-film solar cells, namely, 

cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS). Polycrystalline thin-film PV technology status 

is also discussed. In addition, R&D and technology challenges in both areas are elucidated in this paper. The 

worldwide estimated projection for thin-film PV technology production capacity (including a-Si/thin Si) was estimated 

at 3700 MW in 2006 is now estimated at more than 8000 MW by 2010. 

15. SUBJECT TERMS

PV; polycrystalline; thin film; solar cells; cadmium telluride; copper indium gallium diselenide; grid-parity electricity; 

research and development; market projections; 

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