NASA/TM--2002-211834 IECEC-2002-20155

Multi-Junction Thin-Film Solar Cells on

Flexible Substrates for Space Power

Aloysius E Hepp

Glerm Research Center, Cleveland, Ohio

Mark Smitt-t

Ohio Aerospace Institute, Brook Park, Ohio

Jotm 1--t.Scofield

Oberlin College, Oberlin, Ohio

John E. Dickman

Glerm Research Center, Cleveland, Ohio

Gregory B. Lush

University of Texas at E1 Paso, E]. Paso, Texas

Donald L. Morel and Christos Ferekides

University of South Florida, Tampa, Florida

Neelkanth G. Dhere

University of Central Florida, Orlando, Florida

October 2002
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NASA/TM--2002-211834 IECEC-2002-20155

Multi-Junction Thin-Film Solar Cells on

Flexible Substrates for Space Power

Aloysius E Hepp

Gleru_ Research Center, Cleveland, Ohio

Mark Smith

Ohio Aerospace Institute, Brook Park, Ohio

Jotm H. Scofield

Oberlin College, Oberlin, Ohio

Joh_ E. Dickman

Gleru_ Research Center, Cleveland, Ohio

Gregory B. Lush

University of Texas at E1 Paso, El. Paso, Texas

Donald L. Morel and Christos Ferekides

University of South Florida, Tampa, Florida

Neelkanth G. Dhere

University of Central FI.orida, Orlando, Florida

Prepared for the

37fl_ In.tersociety Energy Conversion Engin.eering Conference

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

Washington, DC, July 28-August 2, 2002

N afional Aeronautics and

Spa ce Adminis tration

Glelm Research Center

October 2002
Acknowledgments

Funding from NASA GRC is acknowledged for NCC3-800 (OAI), NCC3---7] 2 (FSEC), and NAG3---26] 4 (USF).

This report contains preliminary

findings, subject to revision as

analysis proceeds.

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.

NASA Center for Aerospace Information

71121Standard Drive

Hanover, MD 211076

Available frorn

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5285 Port Royal Road

Springfield, VA 22100

Available electronically at http://gltrs.grc.nasa.gov
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

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. Kapton TM) 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 (q) may be required. 3 The

leading thin-film materials - amorphous Si, CulnSe2

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

NASA/TM--2002-211834 l
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 1.6-

2.0 eV, an efficiency of 16-18%, 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

GRC's multi junction 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 l_m.

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.

-2.0 ..................................................................................................

-2.5 _.....................w::::::::::::::::::::::::::: ......................................................................

-30 :::::::::::::::::::::::::::

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-5.0 ...................................................................................................

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-5.5 i_ _

w.w.w.w.w.w.w.w.w.w.w.w.w.w.w.w.w.w.w.w_.._ --- ::::::::::::::::::::::::::: :::::::::::::

-6.0

-6.5 -,

0 0.2 0.4 0.6

Microns

Figure 2. Equilibrium band diagram for a

ZnSe/CdSe MIS structure.

CdSe, while lx104 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/cm 2 (assumed 5% reflection loss at front),

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

that it indicates that a MIS structure can meet our

'_ work peformance is finding requirements. external contactsA keyat theto getting right energies. this to

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

CdSe conduction band, and the front contact at

.................................................................... ...._ ....................................................... ............................................................... 0.3eV from the CdSe valence band. Thus the

:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: achieved Voc is essentially the band gap of the CdSe

i iS#eidddwi_,di (1.7) minus the sum of these two contact voltages.

................................................................ Experimentally the rear contact should not pose a

EdSe _iEd{n e _R._ problem, while finding a front contact with such a high

energy will be difficult. The highest Voc's reported for

_ran_parent COnde_O_ /z_E2 these devices were indeed for Pt and Au contacts (up

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

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. _ 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 lx10 _8 and lx10 _9, respectively. A donor

doping concentration of lx10 _4 was used for the

25-______

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_._:___ _::_.............

5

0

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_:_:_:_:_:_:_:_:_:_:_:_:_:_:_ _:_:_:_:_:_:_:_:_:_:_:_:_:_ :_:_:_:_:_:_:_:_:_:_:_:_:_:_:_ :_:_:_:_:_:_:_:_:_:_:_:_:_:_:_ :::::::::::::::::::::::::::N

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Va(Volts)

Figure 3. Simulated light IV for the ZnSe/CdSe

MIS structure with 17.4% efficiency.

NASA/TM--2002-211834 2
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.

,4 ............................................................................................................

1.2

1

0.8

0.6

0.4

_ii!;_iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii

i

3.5 3.7 3.9 4.1 4.3 4.5

Affi n ity

Figure 4. Voc and FF dependence on ZnSe affinity.

1.2

1

0.8

0.6

0.4

iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii

I+ voc

iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii;i;i;i;i_i I,,,,,_,,,,,FF

.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.4>×-

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::::::::::::::::::::::::::::::::::::::::::::::::

] ] i i

5 7 9 11 13 15

Thickness(nm)

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 IV's 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 lx10 -_2 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.

::::::::::::::::::::::::::::::::::::::::::: _:_:_:_:_:_:

;_;_......................... _;:;:;:;:;:;:;:;:;:;:;:i ===============================================================

_:_:_:_:_:_:_:_:_:_:_:_ ::_:_'T_ _:_:_:_: :::::::::::::::::::::::::::

:::::::::::::::::::::::::::::::::::::::

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:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

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Va(Volts)

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

of 3.37 eV.

1 .E+05

1 .E+02

1 .E-01

1.E-04

1.E-07

1.E-10

1.E-13

1 .E-16

-0.5

x+x+x+x+x+x+x, +x+x+x+x+x+x+x, x+x+x.

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:::::::::::::::::::::::::::::::::::::::: ::::::::::::::::::::::::::::::::::::::::::: ::::::::::::::::::::::::

w.w.w.w.w.w.... +x+x+x+x+x+x+x .w.w...

::::::::::::::::::::::::::::::::::::::: +x+x+x+x+x+x+x .w.w...

:::::::::::::::::::::::::::::::::::::::: ::::::::::::::::::::::::::::::::::::::::::: ::::::::::::::::::::: _:_:_:_:_:_:_:_:

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0.0 0.5 1.0

Va(Volts)

Figure 7. Dependence of dark current saturation on

the energy gap of the I layer

NASA/TM--2002-211834 3
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 Eft. ('q)

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 "M" 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 contactabsorberntransparent 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 CulnxGal_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 Cdl_xZnxTe (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.

V(volts)

0.00

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 Culm_xGaxS2 (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.

NASA/TM--2002-211834 4
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.

90 ..........................................................................................

8O

7O

6O

5O

4O

3O

2O

10

0 i i

600 700 800 900

Wavelength(nm)

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 CulZnSelCdSelTC. The

CdSe substrate temperature was varied over the

450-600 °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 Voc'S

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 Jsc'S 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/cm 2. This is the actually measured Jsc of the

device.

1

0 9 iiiiiiiiiiiiiiiiiiiiiiiiiiiii_::__ _iiiiii _, ililililililililililililili_i_i_i_i_i_i_i_

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0.6

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400 500 600 700 80£

Wavelen_h(nm)

Figure 10. External and internal QE of a

CulZnSelCdSelTC 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/cm 2.

This is within one mA/cm 2 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 Voc'S will come from;

higher Voc'S will increase the internal fields that should

further tweak Jsc values as well as FF's.

ClGS2 Thin Films on Metal Foil

Substrates used in this study included 127 #m

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

foils and 25 _Lm thick titanium foils. The results

discussed herein are for samples on 127 _Lmthick 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.

Culnl_xGaxS2 (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 Culno.7Gao.3S2 phase with a = 5.67

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

NASA/TM--2002-211834 5
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:AI window bilayer

by RF sputtering, and vacuum deposition of Ni/AI

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 1-3 #m with a

roughness of 0.19 #m (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 cm -3 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/cm 2, FF = 60.06%, and q =

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 metallZnSelCdSelTC

structure as our device format for evaluating CdSe.

External and internal Jsc'S of 14.7 and 18.3 mA/cm 2

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 1-3 pm with a roughness of 0.19 pm.

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.

REFERENCES

1. S.G. Bailey and D.J. Flood, "Space

Photovoltaics," Prog. Photovolt. Res. Appl. Vol. 6,

pp. 1-14 (1998).

2. P.A. lies, "Evolution of Space Solar Cells," Solar

Energy Materials and Solar Ceils Vol. 68, pp. 1-13

(2001).

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

M.K. Jacobs, and D. Ponnusamy, "Thin-Film

Photovoltaic Solar Array Parametric Assessment,"

Proceedings 35 th IECEC, AIAA, Washington, D.C.,

Vol. 1, pp. 670-680, AIAA-00-2919 (2000).

4. H.W. Schock, and R. Noufl, "CIGS-based Solar

Cells for the Next Millennium," Prog. Photovolt. Res.

Appl., Vol. 8, pp. 151-160 (2000).

5. H. Richter, "Development of p,n Heterojunctions

Based on Thin Polycrystalline CdSe Films,"

Proceedings of the 20 th IEEE PV Specialist

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

1541 (1988).

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

7. D. Bonnet and E. Rickus, "The CdSe Thin-Film

Solar Cell," Proceedings of the 14 th IEEE PV

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

632 (1980).

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

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

Solar Cells on Polymers with Lift-off Processes,"

th Proceedings of the 16 European PVSEC, James

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

Vol. 1, pp. 298-301(2000).

NASA/TM--2002-211834 6
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October 2002 Technical Memorandum

4, TITLE AND SUBTITLE 5. FUNDING NUMBERS

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

& AUTHOR(S)

Aloysius F. tlepp, Mark Smith, John El. Scofield, John E. Dickman,

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

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

John El. Glenn Resem'ch Center at Lewis Field

Cleveland, Ohio 44135 - 3191

WU-755-A4-00-10

8. PERFORMING ORGANIZATION

REPORT NUMBER

E---13528

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

National Aeronautics and Space Administration

Washington, DC 20546-0(101 NASA TM------2002-21 1834

IECEC---2002----20155

11. SUPPLEMENTARY NOTES

Prepared for the 37th Intersociety Energ'/Converskm Engineering Conference sponsored by the l[nstitute of Electrical and Electronics Engineers,

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

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

Umversity of l?xas at E1 Paso, Department of Electrical Eagineering, E1 Paso, 'Te×a.% Donald L. Morel and Christos Ferekides, I Tniversity of South

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

Orlando. Florida. Responsible person, Alo_,sius E Hepp or_aJ_ization code 54.10, 216 433 3835.

12a, DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE

Unclassified - Unlimited

Subject Category: 2.5 Distribution: Nonstandard

Av_dlable electronically at b.ttp://gltrs._rc.nasa.e, ov

"[his publication is available from the NASA Center for AeroSpace Im'ormation, 301-621-0390.

13. ABSTRACT (Maximum 200 words)

The ultimate objective of the thin-tilm 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 su bstrates such as polyimides

(i.e., Kapton sM) 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 wilh go()(] 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 oudined.

14. SUBJECT TERMS

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

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Unclassified

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12

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