August 2004 • NREL/CP-520-36241 

Polycrystalline Thin-Film 

Photovoltaic Technologies: 

Progress and Technical Issues 

H.S. Ullal 

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POLYCRYSTALLINE THIN-FILM PHOTOVOLTAIC TECHNOLOGIES: 

PROGRESS AND TECHNICAL ISSUES 

Harin S. Ullal 

National Center for Photovoltaics (NCPV) 

National Renewable Energy Laboratory (NREL)

1617 Cole Boulevard 

Golden, Colorado 80401 USA 

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

ABSTRACT: Polycrystalline thin-film materials based on copper indium diselenide (CuInSe2, CIS) and cadmium 

telluride (CdTe) are promising thin-film solar cells for various power and specialty applications. Impressive results 

have been obtained in the past few years for both thin-film copper indium gallium diselenide (CIGS) solar cells and 

thin-film CdTe solar cells. NCPV/NREL scientists have achieved world-record, total-area efficiencies of 19.3% for a 

thin-film CIGS solar cell and 16.5% for thin-film CdTe solar cell. A number of technical R&D issues related to CIS 

and CdTe have been identified. Thin-film power module efficiencies up to 13.4% has been achieved thus far. 

Tremendous progress has been made in the technology development for module fabrication, and multi-megawatt 

manufacturing facilities are coming on line with expansion plans in the next few years. Several 40-480 kW 

polycrystalline thin-film, grid-connected PV arrays have been deployed worldwide. Hot and humid testing is also under 

way to validate the long-term reliability of these emerging thin-film power products. The U.S. thin-film production 

(amorphous silicon [a-Si], CIS, CdTe) is expected to exceed 50 MW by the end of 2005. 

Keywords: Cadminum telluride, copper indium diselenide, thin films. 

1 INTRODUCTION 

Remarkable progress has been made by emerging 

thin-film solar cells based on CIS and CdTe absorber 

materials. NCPV/NREL scientists have achieved a 

world-record, total-area efficiency of 19.3% for a thinfilm CIGS solar cell using the NREL-patented, 3-stage 

physical vapor deposition technique for a Ga/(Ga+In) 

ratio of 26:64 [1,2]. When the Ga/(Ga+In) ratio was 

changed to 35:65 the total-area efficiency achieved was 

18.4%. Researchers at NCPV/NREL has also achieved a 

total-area efficiency of 16.8% for a thin-film CIGS solar 

cell using a single-step chemical bath deposition (CBD) 

ZnS buffer layer. The advantage of the ZnS (3.6 eV) 

buffer layer is that it has a larger bandgap as compared to 

CdS (2.4 eV), which potentially could result in a higher 

short-circuit current (Jsc). For a thin-film CdTe solar cell, 

the total-area efficiency is 16.5% as demonstrated by 

NCPV/NREL scientist. A novel transparent conductor, 

namely, cadmium stannate (Cd2SnO4, CTO) has been 

used in the device fabrication. For thin-film CIS- based 

solar cells, technical research issues such as junction 

formation, chemical, electronic, optical and structural 

properties studies; effect of heat; moisture ingress; 

alternate substrates, such as stainless steel, polymers, and 

metal foils; and new encapsulation schemes continue to 

be investigated. In the case of thin-film CdTe solar cells, 

technical issues such as micro-nonuniformity and its 

impact on cell and module performance, vapor CdCl2 

heat treatments, accelerated life testing, contact stability, 

defect chemistry of the absorber films, and edge sealant 

for module packaging are research topics of intense 

investigation. 

In the area of technology development for thin-film 

CIS-based power modules, aperture-area conversion 

efficiencies in the range of 12.2% to 13.4% have been 

achieved by several groups worldwide. The module 

efficiencies for thin-film CdTe are in the range of 7.0% 

to 11.0%. Hot and humid testing of thin-film modules is 

currently under way at the Florida Solar Energy Center, 

Cocoa, Florida, to validate the long-term reliability of 

these emerging thin-film products. Several large gridconnected thin-film arrays varying in size from 40 to 480 

kW have been installed worldwide, and they are 

delivering electricity to the utility grid. Based on the 

progress made to date by the U.S. thin-film industry, 

more than 50 MW of thin-film products (a-Si, CIS, 

CdTe) are expected to be fabricated by the end of 2005. 

2 THIN-FILM CIS AND CDTE MATERIALS 

AND DEVICES - R&D 

For the past 20 years thin-film CIS and CdTe devices 

have demonstrated steady progress. The Understanding 

the chemical, electronic, optical, and structural properties 

of the materials, and the development of novel device 

design for solar cell fabrication and processing 

techniques have contributed to the success of the thinfilm solar cell technology to about 20% total-area 

conversion efficiency for laboratory devices. 

2.1 Thin-Film CIS Devices 

CIS is a direct-bandgap semiconductor with a 

bandgap of ~0.9 eV. When Ga is added to the absorber 

film, the bandgap can be increased depending on the Ga 

content in the absorber film. The universally accepted 

device design for fabricating high-efficiency, thin- film 

CIGS solar cells is the following: MgF2/ZnO/CdS/ 

CIGS/Mo/glass and is shown in Fig. 1. Low-cost sodalime glass is used as the substrate. NCPV/NREL 

scientists have achieved world -record, total-area 

efficiency of 19.3%. The solar cell parameters are Jsc = 

34.6 mA/cm2

, open-circuit voltage (Voc) = 0.70 V, and 

filll factor (FF) = 0.796. The light current-voltage (I-V) 

curve is shown in Fig. 2. On an active-area basis the solar 

cell conversion efficiency is over 20%. This is the first 

time that Voc of 0.70 V has been achieved for the device 

with a Ga/(Ga+ In) ratio of 26:76 with a bandgap of 1.13 

eV and high FF of 0.80. The high Voc and FF values are 

attributed to the superior junction formation of the 

device. The composition of the absorber film is measured 

1 
Table 1: Advantages of CBD CdS Process 

Figure 1: Thin Film CIS solar cell structure 

Figure 2: I-V curve for a 19.3% thin-film CIGS solar 

cell 

by the wavelength-dispersive spectroscopy (WDS) 

technique. When the ratio of Ga/(Ga+In) is changed to 

35:65, a total-area conversion efficiency of 18.4% was 

achieved with a band gap of 1.18 eV. The cell parameters 

are Jsc = 31.72 mA/cm2

, Voc = 0.74 V, and FF = 0.785. 

Scanning electron microscopy (SEM) micrographs show 

significant changes in the structural properties of the 

absorber films with varying Ga content. For the highefficiency device (19.3%), columnar structure has 

consistently been observed, whereas for the lower 

efficiency device (18.4%), smaller grains are observed 

due to varying growth kinetics and lattice -mismatch due 

to the higher Ga content in the absorber films. The role of 

the diffusion of the Na from the soda-lime glass 

substrates into the CIGS absorber film and its impact on 

device performance has been reported elsewhere [3]. 

One of the important processing steps to fabricating 

high-efficiency devices is the use of CBD CdS. The 

advantages of the CBD CdS are listed in Table 1. The 

CBD CdS is a dependable, reliable, reproducible, and 

low-cost process for the manufacture of thin-film CISbased power modules. Several buffer layers have also 

been used to fabricate thin-film CIGS solar cells in work 

reported previously [3]. However, the performance 

characteristics do not compare well with the CBD CdS. 

NCPV/NREL scientists have also achieved a total-area, 

conversion efficiency of 16.8% for a thin-film CIGS 

solar cell using single-step CBD ZnS as the buffer layer 

[4]. The cell parameters are as follows: Jsc = 33.8 

mA/cm2

, Voc = 0.66 V, and FF = 0.754. The Voc is lower 

as compared to the CBD CdS, presumably because of 

interfacial defects due to ZnS precipitates and nonoptimum junction formation with the CBD ZnS. The 

impetus to develop the CBD ZnS bandgap (Eg = 3.6 eV) 

buffer layer is to allow higher-energy photons to reach 

the junction to improve the Jsc above that obtained for 

the CBD CdS (Eg = 2.4 eV). In a joint collaborative 

research effort between International Solar Electric 

Technologies (ISET) and NCPV/NREL, an NRELverified, total-area conversion efficiency of 13.2% was 

obtained for a thin-film CIGS solar cell using ISET’s 

patented ink-based nanotechnology process for absorber 

formation [5]. 

2.1 Thin-Film CdTe Devices 

With an optimum bandgap of 1.45 eV, CdTe is one 

of the most promising thin film solar cells because of the 

excellent match with the solar spectrum. In addition, only 

a few microns of CdTe are required for solar cell 

fabrication, because it is a direct-bandgap semiconductor 

and has a high absorption coefficient of >105

 cm-1. Thus, 

better than 90% of the solar spectrum is absorbed within 

1 micron at the top of the solar cell very close to the 

junction region. A typical solar ell structure is 

glass/SnO2/CdS/CdTe/contact and is shown in Fig. 3. 

NCPV/NREL scientists have achieved a world-record 

efficiency of 16.5% using a modified device structure. 

The front transparent contact used in this structure is 

CTO, which replaces the conventional SnO2. There are 

several advantages of this new structure compared to the 

conventional solar cell. CTO has higher optical 

transmission, lower resistivity, and smoother surface 

roughness, as depicted from atomic force microscopy 

(AFM) micrographs [6]. A vapor CdCl2 heat treatment 

Figure 3: Thin-film CdTe solar cell structure 

2
done at about 400°C for 20 minutes is a crucial 

processing step needed to fabricate high-efficiency 

devices. This processing step results in grain boundary 

passivation, enhanced grain growth, increased CdS/CdTe 

interface alloying, and reduced lattice mismatch between 

the CdS buffer and CdTe absorber layers. Numerous 

contacting schemes have been investigated by researchers 

and has been reported elsewhere [7]. 

Contact stability is an important aspect for long-term 

and reliable device performance. Accelerated life testing 

(ALT) is an active area of research to validate the longterm reliability of solar cells and power modules. Details 

of ALT has been reported previously [8]. Other research 

issues include micro nonuniformity and its impact on cell 

and module performance, vapor CdCl2 heat treatments 

and defect chemistry (DC). DC is being investigated 

using several characterization techniques such as, 

cathodoluminescence, electroluminescence, light-beaminduced current, photoluminescence, time-resolved 

photoluminescence, and thermal admittance spectroscopy 

[9]. 

3 TECHNOLOGY DEVELOPMENT AND 

MODULE FABRICATION 

Thin-film CIS and CdTe power module fabrication 

by several organizations worldwide is progressing at a 

steady pace. The major industrial players for thin-film 

CIS module processing are Energy Photovoltaics, USA; 

Honda Engineering, Japan; Global Solar Energy, USA; 

Shell Solar Industries (SSI), USA; Showa Solar, Japan; 

and Wurth Solar, Germany. Power module efficiencies 

for thin-film CIS-based material is in the range of 12.2% 

to 13.4%. In the area of thin-film CdTe manufacturing, 

First Solar (FS), USA, is the only company actively 

involved in multi-megawatt production of power 

modules. The best power module efficiency achieved by 

FS is 10.2%. Table 2 summarizes the champion aperturearea, polycrystalline thin-film module conversion 

efficiencies and power outputs made by various 

manufacturers worldwide. Figure 4 depicts the steady 

progress made in the aperture-area efficiency for the two 

thin-film module technologies (CIS and CdTe) for the 

past several years. 

The Thin Film PV Partnership Program within the 

NCPV has undertaken a project to test the various thinfilm PV modules in a hot and humid environment. Figure 

5 shows the thin-film modules being tested at the Florida 

Solar Energy Center, Cocoa, Florida, to validate the longterm reliability of the thin-film PV power modules in 

such a harsh environment. 

Table 2: Polycrystalline Thin Film Photovoltaic 

Modules 

Figure 4: Progress of aperture-area, thin-film module 

efficiencies 

FSEC 

Figure 5: Hot and humid testing of thin-film modules 

at FSEC, Cocoa, Florida 

4 THIN FILM PV SYSTEMS 

As the production capacity of thin-film 

manufacturing continue to grow in MW production, a 

number of grid-connected thin-film PV systems have 

been installed worldwide. The U.S. thin-film production 

(a-Si, CIS, CdTe) is expected to exceed 50 MW by the 

end of 2005. Figure 6 shows an 85-kW buildingintegrated PV (BIPV) thin-film CIS-based solar facade 

deployed in North Wales, UK. The BIPV array uses 

SSI’s Model ST36 thin-film CIS modules, which are 

mounted on a custom-engineered framing structure 

supplies by UniRac, USA. There are 2400 thin-film CISbased modules in this facade, which is the largest thinfilm CIS-based array installed in Europe. Another 245-

kW thin-film CIS-based rooftop solar array shown in Fig. 

7 has been deployed by SSI at their manufacturing 

facilities in Camarillo, California. This is the largest thinfilm, grid-connected rooftop solar array in the world. The 

array consists of 6144 of SSI’s ST40 thin-film CIS-based 

modules arranged in 13 rows that are wired into 256 

module strings of 24 modules each [10]. Lightweight, 

flexible thin-film CIGS power modules are being used by 

the U.S. Army and Marines [11]. 

Figure 6: 85-kW thin-film CIS-based BIPV facade, 

North Wales, UK 

3 
Figure 7: 245-kW rooftop, thin-film CIS-based solar 

electric array, Camarillo, California 

The number of thin-film CdTe arrays deployed in 

Europe and the United States has increased significantly 

over the past few years. Figure 8 shows 80-kW thin-film 

CdTe array deployed in Dimbach, Germany, using FS 

modules. The electricity generated is sold to the local 

utilities. The world’s largest polycrystalline thin-film 

480-kW solar field shown in Figure 9 is deployed in 

Springerville, Arizona, by the local utility company 

Tucson Electric Power using thin-film CdTe modules 

supplied by FS. Typically, thin-film PV systems have 

better energy delivery, that is, kWh/kW installed, as 

compared to crystalline silicon systems because of the 

spectral response of thin-film modules. The thin-film 

arrays come online earlier in the day and are the last to 

turn-off at the end of the day. In addition, losses due to 

temperature coefficients are lower for thin film modules 

as compared to crystalline silicon modules. 

5 CONCLUDING REMARKS 

NCPV/NREL scientists have demonstrated worldrecord thin-film CIS solar cells with a total-area 

conversion efficiency of 19.3%, with a high Voc of 0.7 V 

and FF of 0.80. Both these device parameters indicate 

excellent junction formation at the CIS/CdS interface and 

Figure 8: 80-kW thin-film CdTe solar array, Dimbach, 

Germany 

Figure 9: 480-kW thin-film CdTe solar field, 

Springerville, Arizona 

4 

excellent chemical, electronic, optical and structural 

properties of the CIS absorber material. NCPV/NREL 

scientists have also achieved a world-record, total-area 

conversion efficiency of 16.5% for a thin-film CdTe solar 

cell using a CTO front-contact for device fabrication. 

Technological improvements have resulted in an 

aperture-area conversion efficiency of 13.4% for a thinfilm CIGS module. Several thin-film grid-connected 

arrays varying in size from 40-480 kW have been 

deployed worldwide. The U.S. thin-film production (a-Si, 

CIS, CdTe) is expected to exceed 50 MW by the end of 

2005. 

6 ACKNOWLEDGMENTS 

This work was supported by the U.S. Department of 

Energy under Contract No. DE-AC36-99GO10337. The 

author would like to thank the numerous engineers, 

scientists, and technicians whose data are included in this 

paper. Special thanks to Dr. Kannan Ramanathan, 

NCPV/NREL, for fabricating the world-record, totalarea, thin-film CIGS solar cell having a conversion 

efficiency of 19.3%. 

7 REFERENCES 

[1] K. Ramanathan, M.J. Romero, C.S. Jiang, and R. 

Noufi, Proceedings of the European Materials 

Research Society 2004 Spring Meeting, Strasbourg, 

France; May 24-28, 2004; in press. 

[2] M.A. Contreras, B. Egaas, K. Ramanathan, J. 

Hiltner, A. Swartzlander, F. Hasoon, and R. Noufi, 

Progress in Photovoltaics: Research and 

Applications, Vol. 7, (1999) 311. 

[3] H.S. Ullal, K. Zweibel, and B. von Roedern, 

Proceedings of the 26th IEEE Photovoltaic Specialists Conference, Anaheim, California; (1997) 301. 

[4] R. Bhattacharya, Private Communications, NREL, 

Golden, Colorado; (2004). 

[5] V.K. Kapur, A. Bansal, P. Le, and O. Asenio, 

Proceedings of the 29th IEEE Photovoltaic 

Specialists Conference, New Orleans, Louisiana; 

(2002) 688. 

[6] X.Wu, R. Ribelin, R.G. Dhere, D.S. Albin, T.A. 

Gessert, S. Asher, D.H. Levi, R. Matson, H.R. 

Moutinho, and P. Sheldon, Proceedings of the 28th 

IEEE Photovoltaic Specialists Conference, 

Anchorage, Alaska; (2000) 470. 

[7] H. S. Ullal, K. Zweibel, and B. von Roedern, 

Proceedings of the 28th IEEE Photovoltaic Specialists Conference, Anchorage, Alaska; (2000) 418. 

[8] H.S. Ullal, K. Zweibel, and B. von Roedern, 

Proceedings of the 29th IEEE Photovoltaic Specials 

Conference, New Orleans, Louisiana; (2002) 523. 

[9] P.V. Meyers and H.S. Ullal, Editors, National CdTe 

R&D Team Meeting, Perrysburg, Ohio (2004). 

[10] K. Mackamul and R. Wieting, Proceedings of the 

19th European Photovoltaic Solar Energy 

Conference, Paris, France (2004); in press. 

[11] H.S. Ullal, Proceedings of the 14th Photovoltaic 

Science and Engineering Conference, Bangkok, 

Thailand (2004) 417. 
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Polycrystalline Thin-Film Photovoltaic Technologies: Progress and 

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14. ABSTRACT (Maximum 200 Words) 

Polycrystalline thin-film materials based on copper indium diselenide (CuInSe2, CIS) and cadmium telluride (CdTe) 

are promising thin-film solar cells for various power and specialty applications. Impressive results have been obtained 

in the past few years for both thin-film copper indium gallium diselenide (CIGS) solar cells and thin-film CdTe solar 

cells. NCPV/NREL scientists have achieved world-record, total-area efficiencies of 19.3% for a thin-film CIGS solar 

cell and 16.5% for thin-film CdTe solar cell. A number of technical R&D issues related to CIS and CdTe have been 

identified. Thin-film power module efficiencies up to 13.4% has been achieved thus far. Tremendous progress has 

been made in the technology development for module fabrication, and multi-megawatt manufacturing facilities are 

coming on line with expansion plans in the next few years. Several 40-480 kW polycrystalline thin-film, gridconnected PV arrays have been deployed worldwide. Hot and humid testing is also under way to validate the longterm reliability of these emerging thin-film power products. The U.S. thin-film production (amorphous silicon [a-Si], 

CIS, CdTe) is expected to exceed 50 MW by the end of 2005. 

15. SUBJECT TERMS 

PV; cadmium telluride; copper indium diselenide; thin films 

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