Aloysius F. Hepp

Glenn Research Center, Cleveland, Ohio

Mark Smith

Ohio Aerospace Institute, Brook Park, Ohio

John H. Scofield

Oberlin College, Oberlin, Ohio

John E. Dickman

Glenn Research Center, Cleveland, Ohio

Gregory B. Lush

University of Texas at El Paso, El Paso, Texas

Donald L. Morel and Christos Ferekides

University of South Florida, Tampa, Florida

Neelkanth G. Dhere

University of Central Florida, Orlando, Florida

Multi-Junction Thin-Film Solar Cells on

Flexible Substrates for Space Power

NASA/TM—2002-211834

October 2002

IECEC–2002–20155
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Aloysius F. Hepp

Glenn Research Center, Cleveland, Ohio

Mark Smith

Ohio Aerospace Institute, Brook Park, Ohio

John H. Scofield

Oberlin College, Oberlin, Ohio

John E. Dickman

Glenn Research Center, Cleveland, Ohio

Gregory B. Lush

University of Texas at El Paso, El Paso, Texas

Donald L. Morel and Christos Ferekides

University of South Florida, Tampa, Florida

Neelkanth G. Dhere

University of Central Florida, Orlando, Florida

Multi-Junction Thin-Film Solar Cells on

Flexible Substrates for Space Power

NASA/TM—2002-211834

October 2002

National Aeronautics and

Space Administration

Glenn Research Center

Prepared for the

37th Intersociety Energy Conversion Engineering Conference

sponsored by the Institute of Electrical and Electronics Engineers, Electron Devices Society

Washington, DC, July 28–August 2, 2002

IECEC–2002–20155
Acknowledgments

Funding from NASA GRC is acknowledged for NCC3–800 (OAI), NCC3–712 (FSEC), and NAG3–2614 (USF).

Available from

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22100

Trade names or manufacturers’ names are used in this report for

identification only. This usage does not constitute an official

endorsement, either expressed or implied, by the National

Aeronautics and Space Administration.

This report contains preliminary

findings, subject to revision as

analysis proceeds.

Available electronically at http://gltrs.grc.nasa.gov
NASA/TM2002-211834 1 

ABSTRACT 

The ultimate objective of the thin-film program at 

NASA GRC is development of a 20% AM0 thin-film 

device technology with high power/weight ratio. 

Several approaches are outlined to improve overall 

device efficiency and power/weight ratio. One 

approach involves the use of very lightweight flexible 

substrates such as polyimides (i.e. KaptonTM) or metal 

foil. Also, a compound semiconductor tandem device 

structure that can meet this objective is proposed and 

simulated using Analysis of Microelectronic and 

Photonic Structures (AMPS). AMPS modeling of 

current devices in tandem format indicate that AM0 

efficiencies near 20 % can be achieved. And with 

improvements in materials, efficiencies approaching 

25% are achievable. Several important technical 

issues need to be resolved to realize these complex 

devices: development of a wide bandgap material 

with good electronic properties, development of 

transparent contacts, and targeting a 2-terminal 

device structure (with more complicated processing 

and tunnel junction) or 4-terminal device. Recent 

progress in the NASA GRC program is outlined. 

INTRODUCTION 

Concepts for future space platforms include very 

large satellites such as solar power satellite (SPS) 

down to very small satellites;1,2 long-term plans 

envisage swarms of distributed, autonomous, small 

satellites termed microsats or even nanosats. Solar 

electric propulsion (SEP) technology that uses 

propulsion by accelerated ions has been successfully 

demonstrated in Deep Space 1.2

 

Photovoltaic (PV) arrays will continue to be a 

key source for power generation in space. While Si 

and GaAs PV technologies continue to play a vital 

role in space, there are two areas in which 

improvement is sought: mass-specific power (MSP) or 

power/weight ratio and radiation hardness. Both 

materials suffer in this regard because they are based 

upon bulk materials, and Si is limited to single junction 

efficiencies. Solar cells based on thin-film materials 

offer the promise of much higher MSP and much 

lower cost. However, for space applications, a 15% 

or greater AM0 efficiency (η) may be required.3

 The 

leading thin-film materials - amorphous Si, CuInSe2

and CdTe have seen significant advances in 

efficiency over the last decade but may not achieve 

the required efficiency in the foreseeable future. The 

ultimate objective of the thin-film program at NASA 

GRC is development of a 20% AM0 thin-film device 

technology with high power/weight ratio. 

While current thin-film single-junction 

efficiencies rival those of silicon, tandem devices that 

offer significantly higher efficiencies are in production. 

These tandem devices more effectively utilize solar 

radiation by converting shorter wavelength radiation in 

the upper wide-bandgap layer and passing through 

MULTI-JUNCTION THIN-FILM SOLAR CELLS 

ON FLEXIBLE SUBSTRATES FOR SPACE POWER

Aloysius F. Hepp 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Mark A. Smith 

Ohio Aerospace Institute 

Brook Park, Ohio 44142 

John H. Scofield 

Oberlin College 

Oberlin, Ohio 44074 

John E. Dickman 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Gregory B. Lush 

University of Texas at El Paso 

El Paso, Texas 79968 

Donald L. Morel and Christos Ferekides 

University of South Florida 

Tampa, Florida 33620 

Neelkanth G. Dhere 

University of Central Florida 

Orlando, Florida 32816 
NASA/TM2002-211834 2 

non-absorbed longer wavelength light to a narrowbandgap bottom cell material. MSP can be further 

enhanced by use of thin, lightweight, metal foils, or 

even high-temperature plastics such as polyimides. 

Thin-film devices are fundamentally more radiation 

hard simply because of their reduced thickness, this 

should also apply to thin-film tandems. 

We have proposed and simulated a thin-film tandem 

device structure that can meet program objectives. 

The bottom cell is a 1.0 eV CIGS device (efficiency 

requirements for this device have been 

demonstrated).4

 However, there is no known device 

that meets the requirements of the top cell. The key 

requirements are: a band gap in the range 1.6ñ

2.0 eV, an efficiency of 16ñ18%, and transparency to 

transfer longer wavelengths to the bottom cell. An 

immediate objective is development of such a cell. 

Modeling and device-processing results of NASA 

GRCís multijunction thin-film program are highlighted. 

AMPS MODELLING 

Specifically the proposed structure is a 4-

terminal tandem as shown in figure 1. The thickness 

of the active layers is expected to be about 10 μm. 

The bottom polyimide can be as thick as needed for 

structural support, and the top polyimide (or other) 

layer will be designed to minimize radiation damage. 

Figure 1. 4-terminal tandem device schematic. 

The best CdSe cells to date have been in the 

MIS configuration with ZnSe as the insulating (I) 

layer.5

 AMPS was used to model this structure.6 

Typical values for the key parameters were taken 

from the literature, or by analogy to similar materials. 

An example of the latter is the density of states (DOS) 

for the conduction and valence bands that were 

chosen to be 1x1018 and 1x1019, respectively. A donor 

doping concentration of 1x1014 was used for the 

Figure 2. Equilibrium band diagram for a 

ZnSe/CdSe MIS structure. 

CdSe, while 1x104

 was used for ZnSe to make it an 

insulator. No defects in either layer were included. 

The equilibrium band diagram is shown in figure 2. As 

expected, the CdSe layer is completely depleted. The 

electron affinities used for ZnSe and CdSe were 3.67 

and 4.56, respectively. The conduction band offset 

acts as a good electron mirror, while the offset in the 

valence band does not impede hole flow. 

The light IV curve is shown in figure 3 and is 

near ideal. The parameters are Voc: 1.11 volts, Jsc: 

19.3 mA/cm2

 (assumed 5% reflection loss at front), 

FF: 0.81, and η: 17.4%. This is highly encouraging in 

that it indicates that a MIS structure can meet our 

performance requirements. A key to getting this to 

work is finding external contacts at the right energies. 

In this case the rear contact was at 0.3 eV from the 

CdSe conduction band, and the front contact at 

0.3 eV from the CdSe valence band. Thus the 

achieved Voc is essentially the band gap of the CdSe 

(1.7) minus the sum of these two contact voltages. 

Experimentally the rear contact should not pose a 

problem, while finding a front contact with such a high 

energy will be difficult. The highest Vocís reported for 

these devices were indeed for Pt and Au contacts (up 

to 0.85 volts, but with low Jsc and FF) which have the 

Figure 3. Simulated light IV for the ZnSe/CdSe 

MIS structure with 17.4% efficiency. 

0

5

10

15

20

25

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Va(Volts)

Ip(m A)

CdSe

-6.5

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

0 0.2 0.4 0.6

Microns

Energy(eV)
NASA/TM2002-211834 3 

highest work functions. Also reported was a 

dependence of Voc on work function.5

 Post-deposition 

treatment of the interface with metal was critical to 

achieving this; getting higher efficiencies with these 

structures has yet to be proven. 

To further our understanding of these structures, 

several parameters were varied. Using electron 

affinities for ZnSe and CdSe of 3.67 and 4.56 

respectively, the following observations were made. 

Changing the thickness of the ZnSe layer has no 

effect on performance. There is some disagreement 

in the literature on the values of electron affinities, so 

it is appropriate to vary this parameter. Jsc is found to 

be independent of the ZnSe affinity, while the Voc and 

FF dependence is shown in figure 4. 

Figure 4. Voc and FF dependence on ZnSe affinity. 

Figure 5. Dependence of Voc and FF on ZnSe 

thickness for a ZnSe affinity of 4.27. 

As ZnSe affinity is increased, increasing valence 

band offset occurs which results in poor hole transport 

across the interface and declining FF. This results in a 

small increase in Voc. With ZnSe affinity in the 

transition range (4.27) (see figure 5), there is now a 

strong ZnSe thickness correlation with FF and a weak 

dependence for Voc. At a ZnSe affinity of 4.27 the 

valence and conduction bands each have an offset of 

about 0.3 eV; interference with collection begins. It is 

apparent that having the right band gap and electron 

affinity for the I layer is important. It is noteworthy that 

there is quite a large range of acceptable values. 

It is interesting that in none of the above 

simulations was there a reduction in Jsc. To find the 

limits here we go back to the base case and 

systematically increase the band gap of the I layer 

while keeping everything else constant. Not until the I 

layer band gap reaches 3.37 eV is there an effect on 

Jsc. The light and dark IVís for this case are shown in 

figure 6. As can be seen, forward dark current is 

severely restricted. Jsc has just dropped down a bit to 

18.6, but the curve shape has plummeted. Any further 

increase in the band gap will result in similar losses in 

Jsc. The saturation of the dark current at 1x10ñ12 in 

figure 6 is due to the large band gap of the ZnSe. In 

figure 7 is shown the effect of the I layer band gap on 

dark current. 

Figure 6. Light and dark IV for an I layer bandgap 

of 3.37 eV. 

Figure 7. Dependence of dark current saturation on 

the energy gap of the I layer 

0.4

0.6

0.8

1

1.2

1.4

3.5 3.7 3.9 4.1 4.3 4.5

Affinity

Voc

FF

0.4

0.6

0.8

1

1.2

5 7 9 11 13 15

Thickness(nm)

Voc

FF

1.E-16

1.E-14

1.E-12

1.E-10

1.E-08

1.E-06

1.E-04

1.E-02

1.E+00

1.E+02

-0.5 0 0.5 1

Va(Volts)

I(mA)

Jl

Jd

1.E-16

1.E-13

1.E-10

1.E-07

1.E-04

1.E-01

1.E+02

1.E+05

-0.5 0.0 0.5 1.0

Va(Volts)

I(mA)

2.47

2.67

3.17
NASA/TM2002-211834 4 

As can be seen, dark current flow at just over 

one volt is affected for the ZnSe band gap of 2.67 eV. 

While this does not affect Jsc, FF is affected (Table1). 

The small loss in FF for the 2.67 eV band gap of 

ZnSe is due to the band gap itself and not on the 

details of band alignment. As seen in figure 4, it takes 

a large change in affinity to start affecting FF. Modest 

increases in band gap, however, result in more 

substantial losses. While none of this suggests that 

ZnSe is not a good choice, the implications are that a 

smaller band gap material would be somewhat better. 

This would have to be weighed against the favorable 

interface defect control reported for ZnSe.7

 In this 

regard ZnTe would seem to have a band gap 

advantage over ZnSe as an I layer, although a 

disadvantage because of the toxicity of Te. 

Table 1. MIS device properties as a function of 

Insulator-layer bandgap. 

Eg Jsc Voc FF Eff. (η) 

2.47 19.3 1.11 .89 19.0 

2.67 19.3 1.11 .81 17.4 

3.17 19.3 1.11 .37 7.9 

While there remain issues on details to be 

resolved for the MIS structure we are satisfied that 

good performance can be achieved under the right 

circumstances. At the outset we were concerned that 

sensitivity to the strong thickness dependence of 

tunneling for a true MIS structure would be 

troublesome, but these devices do not require 

tunneling to function well. ZnSe is not behaving as an 

I layer, but rather as a type of heterojunction contact 

to CdSe. Up to this point, we have ignored optical 

losses due to the metal contact. Metals have not been 

used as transparent conductors in solar cells for this 

reason. While this is a concern here, there may be 

mitigating circumstances that relax that concern. As 

will be discussed below, the ìMî in the MIS structure 

may also be a misnomer. That is, the metal, 

particularly Cu may be reacting with the 

semiconductor layers. This may be its most important 

role, and once this is accomplished by a small 

quantity of metal, the top of the device could be 

finished with a standard transparent conductor such 

as ZnO with proper opto-electronic properties. 

In fact this leads us back to our concept of a 

device structure p-transparent contact/absorber/ntransparent contact. Following up on what we have 

learned about the MIS structure, a metal/ZnSe 

structure would serve as our p-transparent contact. 

This should serve as a valid starting point for 

developing our devices, particularly the absorbers. 

Eventually, however, we will have to replace this p 

contact with one with larger effective contact energy 

to achieve high Voc. 

The CuInxGa1-xSe2 (CIGS) bottom cell 

technology already exists. This is certainly the case 

on glass substrates where efficiencies in excess of 

15% are routine. Some effort has gone into 

developing CIGS on metal foil and polyimide,8

 but 

performance is somewhat poorer. Nevertheless this 

can be improved upon with further effort, and we thus 

will focus our efforts under this project on the top cell. 

This is where innovation is needed because there is 

no known thin-film device with a high band gap and 

high efficiency. Below we report progress on 

development of this top cell. In addition to requiring 

high efficiency and light-weight there is the added 

requirement for a tandem device structure that it be 

transparent to longer wavelengths. Our approach is to 

break these down into individual components and 

address each technical issue in turn. 

As shown in figure 8, performing these 

simulations at AM0 results in projected efficiencies in 

excess of 20%13.8% is contributed by a top cell 

with band gap of 1.55 eV, and 6.3% is contributed by 

the underlying CIGS cell. In this case we argued that 

the top cell could be Cd1ñxZnxTe (CZT) with a small 

amount of zinc that should not change the electronic 

properties significantly from those of CdTe that were 

used in the simulation. As the band gap of the top cell 

is increased beyond 1.55 by adding more Zn, or using 

CdSe with its 1.7 eV band gap, the expected 

efficiency moves above 20%. Thus our primary 

objective is to demonstrate a top cell from either of 

these material options with an AM0 efficiency of 13% 

or more. 

Figure 8. Simulated AM0 IV curves for a tandem 

device consisting of a 1.55 EV top cell and 

0.95 eV bottom CIGS cell. Tandem 

efficiency is 20.1%. 

PROCESSING OF TOP-CELL MATERIALS 

We are pursuing CuIn1ñxGaxS2 (CIGS2) (FSEC), 

CdSe (USF), and CZT (USF) as candidate absorbers 

for the top cell. (CIGS2) thin films were prepared by 

sulfurization of DC magnetron-sputtered CuGa/In 

precursor on stainless steel foil substrates. The 

modeling and fabrication effort at USF is focused on 

CdSe and CZT as primary candidates. The progress 

that we discuss below will be for CdSe. Much of the 

effort thus far on CZT has been on material 

processing and characterization. 

0.00

5.00

10.00

15.00

20.00

25.00

30.00

-1.00 -0.80 -0.60 -0.40 -0.20 0.00

V(volts)

I(mA/cm^2)

1.55 eV CdZnTe

0.95 eV CIGS
NASA/TM2002-211834 5 

CdSe Fabrication

To check the optical performance of our CdSe 

films we deposited a 1600 nm thick film on glass and 

measured its transmission profile. As seen in figure 9, 

~80% is transmitted in the wavelength range above 

750 nm that is the domain of the underlying CIGS cell. 

This is only 5% lower than the 85% figure assumed in 

the simulations for efficiency. This is very encouraging 

because this film is thicker than the CdSe film we 

expect to use; no attempt was made to reduce 

reflection losses. Much of the 20% loss is expected 

to be reflection that will be reduced when top layers 

are present. When suitable AR layers are determined, 

a further reduction of losses is expected. 

Figure 9. Transmission profile of a 1600 nm 

CdSe film on glass. 

Our initial fabrication efforts centered upon the 

metal-insulator-semiconductor (MIS) structure. We 

used a transparent conductor as the growth surface. 

The device structure is Cu/ZnSe/CdSe/TC. The 

CdSe substrate temperature was varied over the 

450ñ600 °C range while the substrate temperature for 

ZnSe deposition was 200 °C. The ZnSe and Cu 

layers are deposited sequentially by PVD without 

breaking vacuum between. However, they are 

deposited in a different system from the CdSe, and 

thus the CdSe surface is exposed to ambient prior to 

deposition of the MI contact layers. The ZnSe is a 

continuous layer, while the metal is deposited through 

a shadow mask. 

Since our primary concern is to demonstrate 

high electronic quality CdSe, we have focused our 

efforts on demonstrating high short circuit currents in 

devices. Jsc is typically an indicator of bulk properties, 

while Voc and FF in thin film devices are often more 

affected by interface properties and contact 

properties. For example, we expect that the low Vocís 

in these devices are primarily the result of the 

effective contact energies of the contacts and have 

little to do with the CdSe. This is more likely to be the 

case if we are seeing good Jscís indicating good 

prope`rties for the CdSe bulk. Using Jsc as our 

figure-of-merit we proceeded to vary the CdSe 

deposition parameters to optimize its electronic 

properties, specifically Jsc. In figure 10 we show QE 

spectral response of a device with the highest Jsc that 

we have achieved to date. Calibration of QE is 

accomplished by calibrating our light source against a 

Si reference cell. Jsc is determined by integrating the 

external QE spectrum and is found to be 14.7 

mA/cm2

. This is the actually measured Jsc of the 

device. 

Figure 10. External and internal QE of a 

Cu/ZnSe/CdSe/TC device. 

When the external QE spectrum in figure 10 is 

normalized against the transmission of the Cu/ZnSe 

layer, the internal QE spectrum shown in figure 10 

results. Integration of this spectrum results in an 

equivalent internally generated Jsc of 18.3 mA/cm2

. 

This is within one mA/cm2

 of the Jsc for the simulated 

17.4% MIS device in Table 1. As such this result is a 

strong indication that we are making very good 

electronic quality CdSe. Since the CdSe is on a 

transparent conductor this represents a big step 

forward toward achieving our overall objectives. The 

next focus of our efforts is development of top contact 

layers. This is where higher Vocís will come from; 

higher Vocís will increase the internal fields that should 

further tweak Jsc values as well as FFís. 

CIGS2 Thin Films on Metal Foil

Substrates used in this study included 127 μm 

and 20 μm thick bright-annealed stainless steel (SS) 

foils and 25 μm thick titanium foils. The results 

discussed herein are for samples on 127 μm thick SS 

foils. CuGa and In precursor layers were DC 

magnetron-sputter deposited on Mo-coated SS foils 

with elemental ratio Cu/(In+Ga) of ~1.4. Metallic 

precursors were homogenized by heating the 

samples to 135 °C for 25 minutes in argon gas flow. 

CuIn1ñxGaxS2 (CIGS2) thin films were prepared by 

sulfurization of the CuGa/In precursor films in Ar:H2S 

(4%) gas mixture at 475 °C. CIGS2 thin films grew 

with a chalcopyrite CuIn0.7Ga0.3S2 phase with a = 5.67 

Å and c = 11.34 Å and preferred {112} orientation. 

0

10

20

30

40

50

60

70

80

90

600 700 800 900

Wavelength(nm)

% Transmission

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

400 500 600 700 800

Wavelength(nm)

QE

external

internal
NASA/TM2002-211834 6 

CuxS phase segregating at the surface was 

etched away with a 10% aqueous KCN solution. 

CIGS2 solar cells were completed by deposition of 

CdS heterojunctions by chemical bath deposition, 

transparent-conducting ZnO/ZnO:Al window bilayer 

by RF sputtering, and vacuum deposition of Ni/Al 

contact fingers through metal mask. CIGS2 thin films 

and solar cells were characterized by atomic force 

microscopy (AFM), scanning electron microscopy 

(SEM), X-ray diffraction (XRD), C-V, I-V, microphotoluminescence (micro-PL) and QE techniques. 

Non-contact AFM image showed the unetched 

CIGS2 films to consist of compactly packed, well 

faceted grains having size ranging from 1ñ3 μm with a 

roughness of 0.19 μm (Fig. 11). SEM image of 

unetched sample also showed large, well faceted 

grains. Thus the Cu-rich stoichiometry during the 

growth of CIGS2 films resulted in an improved 

morphology. 

Figure 11. Non-contact AFM image of an 

unetched CIGS2 film. 

SEM of etched CIGS2 films display a 

considerably rougher surface with protrusions and 

valleys. XRD and EPMA analysis do not show 

evidence of compositional or structural fluctuations. 

Inhomogeneous broadening, and variation of peak 

position and amplitude of micro-PL signal variations 

may therefore be attributed to the rough morphology 

of the etched films. The CIGS2 carrier density 

extracted from the CV data was found to vary in the 

mid-1016 cmñ3 range and relatively constant between 

80 and 180 nm from the junction. PV parameters of 

the best CIGS2 solar cell on SS flexible foil measured 

at NASA GRC under AM0 conditions were Voc = 

802.9 mV, Jsc = 25.07 mA/cm2

, FF = 60.06%, and η = 

8.84%. Quantum efficiency characteristics gave a 

band gap of 1.50 eV and only a modest CdS loss. 

CONCLUSIONS 

Considerable progress has been made with 

CdSe using a MIS structure. We have modeled this 

structure and demonstrated that this structure can 

meet our efficiency objectives. Guided by these 

results we have utilized a metal/ZnSe/CdSe/TC 

structure as our device format for evaluating CdSe. 

External and internal Jscís of 14.7 and 18.3 mA/cm2

respectively have been demonstrated. This is a 

significant result in that it is on a transparent contact 

and indicates that we are able to deposit high 

electronic-quality CdSe on transparent conductors. 

We have demonstrated 80% transmission of light for 

the bottom cell through 1600 nm CdSe films. This is 

within reach of our target of 85% through use of AR 

coatings. 

Unetched (CIGS2) thin films, prepared by 

sulfurization of DC magnetron-sputtered CuGa/In 

precursor on stainless steel foil substrates, consisted 

of compactly packed, well faceted grains having size 

ranging from 1ñ3 μm with a roughness of 0.19 μm. 

The etched CIGS2 films showed a considerably 

rougher surface with protrusions and valleys. The 

best efficiency of CIGS2 solar cell on SS flexible foil 

measured at NASA GRC was 8.84% AM0. 

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pp. 1ñ14 (1998). 

2. P. A. Iles, ìEvolution of Space Solar Cells,î Solar 

Energy Materials and Solar Cells Vol. 68, pp. 1ñ13 

(2001). 

3. D.J. Hoffman, T.W. Kerslake, A.F. Hepp, 

M.K. Jacobs, and D. Ponnusamy, ìThin-Film 

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Vol. 1, pp. 670ñ680, AIAAñ00ñ2919 (2000). 

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5. H. Richter, ìDevelopment of p,n Heterojunctions 

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Conference, IEEE, New York, NY, Vol. 2, pp. 1537ñ

1541 (1988). 

6. S.J. Fonash, www.psu.edu/dept/AMPS/, (2000). 

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Specialist Conference, IEEE, New York, NY, pp. 629ñ

632 (1980). 

8. D. Rudmann, F.-J. Haug, M. Krejci, H. Zogg and 

A. Tiwari, ìDevelopment of Flexible Cu(In,Ga)Se2

Solar Cells on Polymers with Lift-off Processes,î 

Proceedings of the 16th European PVSEC, James 

and James (Science Publishers) Ltd., London, UK, 

Vol. 1, pp. 298ñ301(2000). 
This publication is available from the NASA Center for AeroSpace Information, 301–621–0390.

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12

Multi-Junction Thin-Film Solar Cells On Flexible Substrates For Space Power

Aloysius F. Hepp, Mark Smith, John H. Scofield, John E. Dickman,

Gregory B. Lush, Donald L. Morel, Christos Ferekides, and Neelkanth G. Dhere

Tellurides; Cadmium selenides; thin films; solar cells; MIS semiconductors

Unclassified -Unlimited

Subject Category: 25 Distribution: Nonstandard

The ultimate objective of the thin-film program at NASA GRC is development of a 20 percent AM0 thin-film device

technology with high power/weight ratio. Several approaches are outlined to improve overall device efficiency and

power/weight ratio. One approach involves the use of very lightweight flexible substrates such as polyimides

(i.e., KaptonTM) or metal foil. Also, a compound semiconductor tandem device structure that can meet this objective is

proposed and simulated using Analysis of Microelectronic and Photonic Structures (AMPS). AMPS modeling of current

devices in tandem format indicate that AM0 efficiencies near 20 percent can be achieved. And with improvements in

materials, efficiencies approaching 25 percent are achievable. Several important technical issues need to be resolved to

realize these complex devices: development of a wide bandgap material with good electronic properties, development of

transparent contacts, and targeting a 2-terminal device structure (with more complicated processing and tunnel junction)

or 4-terminal device. Recent progress in the NASA GRC program is outlined.

Prepared for the 37th Intersociety Energy Conversion Engineering Conference sponsored by the Institute of Electrical and Electronics Engineers,

Electron Devices Society, Washington, DC, July 28–August 2, 2002. Aloysius F. Hepp and John E. Dickman, NASA Glenn Research Center; Mark

Smith, Ohio Aerospace Institute, Brook Park, Ohio; John H. Scofield, Oberlin College, Physics Department, Oberlin, Ohio; Gregory B. Lush,

University of Texas at El Paso, Department of Electrical Engineering, El Paso, Texas; Donald L. Morel and Christos Ferekides, University of South

Florida, Department of Electrical Engineering, Tampa, Florida; and Neelkanth G. Dhere, University of Central Florida, Florida Solar Energy Center,

Orlando, Florida. Responsible person, Aloysius F. Hepp, organization code 5410, 216–433–3835.