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M. Symko-Davies 

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February 2005 • NREL/CP-520-37379 

Status of High Performance 

PV: Polycrystalline Thin-Film 

Tandems 
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STATUS OF HIGH PERFORMANCE PV: 

POLYCRYSTALLINE THIN-FILM TANDEMS 

Martha Symko-Davies 

National Renewable Energy Laboratory (NREL), Golden, CO 

ABSTRACT 

The High-Performance Photovoltaic (HiPerf PV) 

Project was initiated by the U.S. Department of Energy to 

substantially increase the viability of photovoltaics (PV) for 

cost-competitive applications so that PV can contribute 

significantly to our energy supply and our environment. 

The HiPerf PV Project aims at exploring the ultimate 

performance limits of existing PV technologies, 

approximately doubling their sunlight-to-electricity

conversion efficiencies during its course. This work 

includes bringing thin-film cells and modules toward 25% 

and 20% efficiencies, respectively; and developing 

multijunction concentrator cells and modules able to 

convert more than one-third of the sun’s energy to 

electricity (i.e., 33% efficiency). This paper will address 

recent accomplishments of the NREL in-house research 

effort involving polycrystalline thin-film tandems, as well as 

the research efforts under way in the subcontracted area. 

INTRODUCTION 

The HiPerf PV Project directs Federal resources 

toward some of the most critical barriers to the widespread 

use of photovoltaics for energy-significant applications. 

This addresses one of the highest-priority goals for applied 

research in the U.S. Photovoltaics Industry Roadmap [1]: 

“developing high-efficiency, low-cost materials and 

devices.” 

This paper will describe progress on exploring critical 

pathways for a PV technology having a high potential to 

reach cost-competitiveness goals: low-cost polycrystalline 

thin-film tandems for large-area, flat-plate modules. The 

concept was introduced to increase efficiency, but its 

potential for reducing cost also became apparent many

years ago [2]. 

This technology has the potential to reach the 

installed system cost goal of about $1/Wp with continued 

progress in efficiency, reliability, and manufacturing cost. 

PROJECT GOALS AND OBJECTIVES

The NCPV at NREL directs in-house and subcontracted research in high-performance polycrystalline 

thin-film and multijunction concentrator devices. During 

the project period, extensive collaboration and the work 

performed to push the research toward established goals 

should produce significant contributions to the entire PV 

industry. A roadmap of the High-Performance PV Project 

approach is shown covering approximately the next 

decade (Fig. 1). 

Exploring and Accelerating

Ultimate Pathways, Phase IB

Identifying Critical Pathways,

Phase IA

Implementation, Phase II

Commercial Prototype

Products, Phase III

2001 2003 2006 2009

Exploring and Accelerating

Ultimate Pathways, Phase IB

Identifying Critical Pathways,

Phase IA

Implementation, Phase II

Commercial Prototype

Products, Phase III

2001 2003 2006 2009

Fig. 1. Roadmap of the High-Performance PV Project. 

The first phase of the project is critical, because it 

provides a means to identify, explore, and accelerate the 

most promising paths for implementation, followed by

commercial prototype products. These latter efforts 

constitute the second and third phases of this planned 

research program. The first of a two-part phase, 

“Identifying Critical Pathways,” investigated a wide range 

of complex issues in both the polycrystalline thin-film 

tandems and III-V multijunctions for the longer-term 

development and application of high-performance PV 

technologies. The current Phase IB, “High-Performance 

PV—Exploring and Accelerating Ultimate Pathways,” is a 

continuation of Phase I and addresses exploring and 

accelerating ultimate pathways to reach the project’s longterm goals. It is thought that several promising 

approaches will be explored in each category during this 

phase, which will lead to Phase II, “Implementation of 

Pathways.” Seven companies and universities were 

competitively selected and have received awards for the 

HiPerf PV Phase IB (see Table 1). 

Subcontractor Title

Georgia Institute of 

Technology

Thin-Film Si Bottom 

Cells for Tandem Device 

Structures 

University of Delaware 

(IEC) 

High-Performance PVPolycrystalline Thin-Film 

tandem Cells 

1 
University of Toledo Sputtered II-VI Alloys and 

Structures for Tandem PV 

University of Florida Identification of Critical 

Paths in the 

Manufacturing of LowCost High-Efficiency

CGS/CIS Two-Junction 

Tandem Cells 

University of Oregon Identifying the Electronic 

Properties Relevant to 

Improving the Performance of 

High-Bandgap Copper-Based 

I-III-VI2 chalcopyrite Thin-Film 

PV Devices 

Oregon State Novel Materials Development 

for Polycrystalline Thin-Film 

Solar Cells 

Light Spin 

Technologies 

Novel Polycrystalline ThinFilm Solar Cells 

Table 1. Phase IB, “Exploring and Accelerating Ultimate 

Pathways” Subcontractor Awards

PROJECT GOALS AND R&D FOCUS

Goals

To address HiPerf PV R&D long-term goals of 

bringing polycrystalline thin-film tandem cells (combining 

high-bandgap and low-bandgap single junctions) and 

modules toward 25% and 20% efficiencies, respectively, 

the project investigates a wide range of complex issues 

and provides initial modeling and baseline experiments of 

several advanced concepts. Recent work by Coutts et al. 

[3] modeling state-of-the-art thin-film devices has provided 

critical guidance for the project. A near-term milestone 

chart of the R&D thin-film polycrystalline tandems is shown 

by year and will be described here (see Table 2). 

Throughout the project’s term, there will be opportunities 

to reach established program goals by both disruptive 

technology advances and/or multiple incremental 

improvements. 

Date Milestone

2002 10%-Efficient, 1.5 <Egap<1.8 eV Cell 

(Completed) 

2003 Compare Device Design in Terms of 

Monolithic/Mechanical Structure 

(Completed) 

2004 Assess Research on Exploring 

Pathways

2005 14%-Efficient Polycrystalline ThinFilm Tandem 

2006 15%-Efficient PolycrystallineThin-Film

Tandem 

Table 2. Near-term Milestones, High-Performance PV 

Project, Polycrystalline Thin-Film Tandems. 

R&D Focus and Advancements

The wide-bandgap top cell material of the tandem is 

critical; it is anticipated that two-thirds of the tandem cell 

efficiency originates here. Therefore, R&D is focused on a 

top cell, which is integrated with the bottom cell via an 

interconnect junction. Transmission through the top cell 

is a challenge, requiring an optical bandgap (Eg) in the 

range of 1.5<Eg<1.8 eV and minimal sub-bandgap 

absorption. Process compatability to maintain the 

performance of the bottom cell is essential. 

Integration of the thin-film interconnect with the top 

cell—optically, electrically, and with an eye toward process 

compatibility is being investigated; this includes the role of 

defects and how they affect the transport properties of this 

junction, as well as diffusion of impurities into the bulk. 

Transparent conducting oxides (TCOs) are able to form a 

one-sided p/n+ interconnect (shorting/tunneling junction) 

between the TCO and a non-degenerate p-type absorber 

[4], playing a strong role in the tandem cell. 

The design in terms of a monolithic or mechanical 

stack is primarily determined by the choice of the highbandgap top cell material. There are pros and cons to both 

approaches. For example, with the monolithic approach, 

only one thick TCO, one grid, and one anti-reflection 

coating (ARC) would be needed. However, currentmatching and temperature-stability issues arise, as well as 

the necessity of a close thickness tolerance with the tunnel 

junction. Whereas the mechanical stack design may

appear at first glance much simpler than the monolithic 

design, other issues are involved. For example, more 

materials (ARCs, TCOs, and glass) would be needed for 

the overall structure. Regardless of the designs, both 

structures are being pursued during the project. 

HIGH-BANDGAP MATERIALS 

Several high-bandgap top cell materials have been 

identified under the project, but they still need further 

exploration (see Table 3). The table lists several materials 

that have been highly successful in terms of the operating 

parameters for the tandem structure. 

The Polycrystalline Thin Film PV Group at NREL has 

demonstrated that a surface-modified CGS cell exhibits 

the following NREL-confirmed device operating 

parameters: Voc = 0.823 volts, Jsc = 18.61 mA/cm2

, fill 

factor = 66.8%, and total-area efficiency = 10.2%. CGS is 

a candidate top cell absorber material. Its bandgap is ideal 

at 1.68 eV. This particular device had a bandgap of 1.64 

eV. Improving CGS device efficiency has proven to be a 

challenge over the past several years. The recent 

understanding of the differences in structural and 

electronic properties between CuIn(Ga)Se2 and CGS thin 

films and devices has led to varying the growth process in 

a way that is likely to make the CGS surface region similar 

to that of CI(G)S and to minimize defects in the material. 

The University of Delaware, Institute of Energy

Conversion (IEC), is investigating Cu(InGa)(SeS)2 films 

and Cd1-xZnxTe films of varying compositions and on 

specific substrates for the top cell of the tandem [5]. Solar 

cells were fabricated at IEC using the structure glass/ 

Mo/Cu(InGa)(SeS)2/CdS/ZnO/ITO with Ni-Al collection 

2 
grids and total area, defined by mechanical scribes, of 

0.47–0.51 cm2

. Current-voltage measurements were 

completed at NREL on devices from two different 

depositions. The best cell from one run had efficiency =

10.9 % with VOC = 0.826 V, JSC = 20.4 mA/cm2

, and fill 

factor = 64.5. From the other run, the best cell had 

efficiency = 10.9 % with VOC = 0.836 V, JSC=20.4 mA/cm2

, 

and fill factor = 64. Understanding the growth mechanisms 

of Cu(InGa)(SeS)2 films, particularly the incorporation of 

the chalcogen (S, Se) species, is fundamental to their application in wide-bandgap solar cells. Using deposition 

conditions that yield uniform through-film Cu(InGa)Se2

composition and high-efficiency devices, the growth of 

Cu(InGa)(SeS)2 or CuIn(SeS)2 has been shown to form a 

bi-layer film comprising two different chalcopyrite compositions. A layer at the back of the film is relatively rich in S, 

and a layer near the surface is S-poor relative to the flux 

during growth. This results in poor device behavior for the 

mixed chalcogen (S+Se) films. Although the growth 

mechanisms that produce this behavior are still under 

study, this phenomenon appears to be limited by growth 

kinetics rather than fundamental thermodynamics. 

Oregon State has worked on several new compositions, and films are being developed for use as conductive 

windows, tunnel junctions, and absorbers in tandem solar 

cells. The material BaCuTeF has been identified as a new

p-type semiconductor with a bandgap of 2.3 eV. 

Table 3. High-Bandgap Top Cell Structures and their 

Operating Parameters (NREL verified). 

Measurements on pressed pellets reveal an electrical 

conductivity of 8 S/cm and a Seebeck coefficient of +25 

μV/K; the temperature dependence of the electrical conductivity is consistent with degenerate behavior. Thin films 

of the material are being processed via pulsed laser deposition; these films should interface well with CdTe. In the 

family of compounds MCuSnQ4 (M = Ba, Sr; Q = S, Se), 

compositions have been systematically varied to identify

the material BaCuSnSe2S2 (direct Eg = 1.77 eV) as a candidate absorber for top cell applications 

PROGRESS IN TANDEM SOLAR CELLS 

Several polycrystalline thin-film tandem designs have 

been developed and demonstrated under the HiPerf project, but they still need further exploration. Table 4 lists 

several of these structures, which include both mechanical 

and monolithic designs, in terms of the operating parameters. Several of these novel devices will be described below. 

Researchers at the Georgia Institute of Technology

are looking to make Si-based tandem cells compatible to 

the processing conditions required to fabricate CuInSe2-

based solar cells using conventional crystalline Si cell 

structures. They have shown that c-Si devices remain intact after exposure to Cu(InGa)Se2 growth. However, Ag 

diffusion from the metallization of the Si cell was observed 

on the Cu(InGa)Se2 surface. This increased the resistance 

of the Si cell and reduced its fill factor. 

Glass/Cd2SnO4/ZnSnOx/nan .806 24.97 69.22 13.9 60%-40% transmission

o-CdS:O/CdTe/CuxTe(1.5

eV)

NREL

Glass/Mo/Cu(InGa)Se .826 20.4 64.5 10.9 2/CdS/

ZnO/ITO (1.5 eV)

Universit

y of

Delawar

e (IEC)

Glass/Mo/CGS/CdS/ZnO .905 14.8 70.9 9.53

(1.68 eV)

NREL

Glass/SnO2/CGS/CdS/ .864 15.36 51.25 6.8 60%-70% transmission

ZnO (1.68 eV)

NREL

66.8

Fill Factor

(%)

18.61

Jsc

(mA/cm2)

10.2

Efficiency

(%)

.823

Voc

(V)

Glass/Mo/CGS/CdS/ZnO Surface modified CGS

(1.64 eV)

NREL

High Band-Gap Top Cell Comments

Structure (eV)

Organiza

tion

Glass/Cd2SnO4/ZnSnOx/nan .806 24.97 69.22 13.9 60%-40% transmission

o-CdS:O/CdTe/CuxTe(1.5

eV)

NREL

Glass/Mo/Cu(InGa)Se .826 20.4 64.5 10.9 2/CdS/

ZnO/ITO (1.5 eV)

Universit

y of

Delawar

e (IEC)

Glass/Mo/CGS/CdS/ZnO .905 14.8 70.9 9.53

(1.68 eV)

NREL

Glass/SnO2/CGS/CdS/ .864 15.36 51.25 6.8 60%-70% transmission

ZnO (1.68 eV)

NREL

66.8

Fill Factor

(%)

18.61

Jsc

(mA/cm2)

10.2

Efficiency

(%)

.823

Voc

(V)

Glass/Mo/CGS/CdS/ZnO Surface modified CGS

(1.64 eV)

NREL

High Band-Gap Top Cell Comments

Structure (eV)

Organiza

tion

3 
Monolithic structure: .688 10.4 52.8 3.8

ZnO/ITO/CdS/ Cu(InGa)Se2

/ZnO/CdS/CIS/Mo/glass

University

of Delaware

(IEC)

4-

terminal

device

13.8

1.47

15.3

68.9

68.01

25.5

6.059

.786

.357

1.14

Topcell:Glass/Cd2SnO4/ZnSnOx/nanoCdS:O/CdTe/CuxTe

Bottom cell: glass/Mo/CIS/CdS/ZnO

Mechanical stack

NREL

62

51.25

69.17

Fill

Factor

(%)

2

15.36

12.46

Jsc 

(mA/cm2)

1.2

6.8

3.9

9.7

Efficiency

(%)

.960

.864

.456

1.29

Voc (V)

Monolithic structure:

SnO2:F/CdS/CdTe/ZnTe:N/ZnO:Al/CdS

/HgCdTe

University

of Toledo

4-

terminal

device

Top cell: glass/SnO2/CGS/CdS/ZnO

Bottom cell: glass/Mo/CIS/CdS/ZnO

Mechanical stack

NREL

Organization Tandem Structure Comment

Monolithic structure: .688 10.4 52.8 3.8

ZnO/ITO/CdS/ Cu(InGa)Se2

/ZnO/CdS/CIS/Mo/glass

University

of Delaware

(IEC)

4-

terminal

device

13.8

1.47

15.3

68.9

68.01

25.5

6.059

.786

.357

1.14

Topcell:Glass/Cd2SnO4/ZnSnOx/nanoCdS:O/CdTe/CuxTe

Bottom cell: glass/Mo/CIS/CdS/ZnO

Mechanical stack

NREL

62

51.25

69.17

Fill

Factor

(%)

2

15.36

12.46

Jsc 

(mA/cm2)

1.2

6.8

3.9

9.7

Efficiency

(%)

.960

.864

.456

1.29

Voc (V)

Monolithic structure:

SnO2:F/CdS/CdTe/ZnTe:N/ZnO:Al/CdS

/HgCdTe

University

of Toledo

4-

terminal

device

Top cell: glass/SnO2/CGS/CdS/ZnO

Bottom cell: glass/Mo/CIS/CdS/ZnO

Mechanical stack

NREL

Organization Tandem Structure Comment

Table 4. Tandem Structures and their Operating 

Parameters. 

The NREL Polycrystalline Thin-Film Group recently

demonstrated a 4-terminal polycrystalline thin-film tandem 

cell consisting of a CdTe-based top cell and a CIS-based 

bottom cell, officially measured at NREL with efficiency of 

15.3%. The top CdTe-based cell structure is: Corning 

glass/Cd-stannate/Zn-stannate/CdS:O/CdTe/CuxTe/ITO. 

The transparency of this structure is about 50%. The 

device parameter of this cell measured at: Voc=0.786 volts, 

Jsc=25.55 ma/cm2

, FF=68.9, efficiency = 13.8%. The CIS 

bottom cell structure is: glass/Mo/CIS/CdS/ZnO. Its 

efficiency as measured under the transparent CdTe cell is 

1.5%. The performance of the CIS cell degraded 

somewhat during the processing of the stacks into a 

measurable structure. This achievement represents a 

benchmark and a first step for this device; work is ongoing 

to design material improvements and develop a more 

optimal structure.

CONCLUSIONS 

Both monolithic and mechanical tandems have been 

developed under the project; they are listed in Table 4. 

These devices used high-bandgap alloys based on I-III-VI2

and II-VI compounds. To date, the highest efficiency

reported on a 4-terminal structure is 15.3%; thin CdTe is 

used for the high-bandgap top cell. 

The developments under the High-Performance PV 

Project reported here are progress toward achieving longterm DOE-goals [1]. The project is focused to assure that 

tandem thin-film polycrystalline modules reach efficiency

levels consistent with cost-competitive goals. 

ACKNOWLEDGMENTS 

This work is supported under DOE Contract No. DEAC36-99GO10337 with NREL. Many people have 

contributed to the development and implementation of the 

High-Performance PV project and to the R&D efforts 

carried out in this program. 

The author thanks the NREL High-Performance 

Polycrystalline Thin-Film Tandem Group: Rommel Noufi, 

Tim Coutts, Anna Duda, David Young, Tim Gessert, Scott 

Ward, Miguel Contreras, Xianzhu Wu, Dave Albin, 

Ramesh Dhere, Kannan Ramanathan, Fallah Hasoon, 

Jehad Abushami, Dean Levi, Sally Asher, Manuel 

Romero, and Yanfa Yan. The author also thanks all HighPerformance PV subcontractors listed in Table 1. 

REFERENCES 

[1] U.S. Photovoltaic Industry Roadmap, Chicago, Illinois,

Facilitated by the NCPV for the U.S. PV Industry,

Prepared by Energetics, Incorporated (Columbia, 

Maryland), May, 2001, 32 pp., URL: 

www.nrel.gov/ncpv/pdfs/30150/pdf 

[2] E.D. Jackson, Rand Conf, Use of Solar Energy, p. 122. 

Tuscon, Arizona (1955). 

[3] T. J. Coutts, S. Ward, D. Young, K. Emery, T. Gessert, 

and R. Noufi, Progress in Photovoltaics Research and 

Applications, 2003; 11-359-375. 

[4] D. L. Young, M. Contreras, M. Romero, S. Asher, C. 

Perkins, T. Gessert,J. Keane, T. Coutts, R. Noufi, 

International PVSEC-14, Bangkok, Thailand, 2004. 

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Status of High Performance PV: Polycrystalline Thin-Film Tandems 

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14. ABSTRACT (Maximum 200 Words)

The High-Performance Photovoltaic (HiPerf PV) Project was initiated by the U.S. Department of Energy to substantially 

increase the viability of photovoltaics (PV) for cost-competitive applications so that PV can contribute significantly to our 

energy supply and our environment. The HiPerf PV Project aims at exploring the ultimate performance limits of existing 

PV technologies, approximately doubling their sunlight-to-electricity conversion efficiencies during its course. This work 

includes bringing thin-film cells and modules toward 25% and 20% efficiencies, respectively, and developing 

multijunction concentrator cells and modules able to convert more than one-third of the sun’s energy to electricity (i.e., 

33% efficiency). This paper will address recent accomplishments of the NREL in-house research effort involving 

polycrystalline thin-film tandems, as well as the research efforts under way in the subcontracted area. 

15. SUBJECT TERMS

PV; high performance; conversion efficiency; thin film; solar cells; module; polycrystalline; tandems; 

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