Aloysius F. Hepp

Glenn Research Center, Cleveland, Ohio

Ryne P. Raffaelle

Rochester Institute of Technology, Rochester, New York

Kulbinder K. Banger, Michael H. Jin, and Janice E. Lau

Ohio Aerospace Institute, Brook Park, Ohio

Jerry D. Harris, Jonathan E. Cowen, and Stan A. Duraj

Cleveland State University, Cleveland, Ohio

Chemical Vapor Deposition for

Ultra-Lightweight Thin-Film

Solar Arrays for Space

NASA/TM—2002-211835

October 2002

IECEC–2002–20156
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Aloysius F. Hepp

Glenn Research Center, Cleveland, Ohio

Ryne P. Raffaelle

Rochester Institute of Technology, Rochester, New York

Kulbinder K. Banger, Michael H. Jin, and Janice E. Lau

Ohio Aerospace Institute, Brook Park, Ohio

Jerry D. Harris, Jonathan E. Cowen, and Stan A. Duraj

Cleveland State University, Cleveland Ohio

Chemical Vapor Deposition for

Ultra-Lightweight Thin-Film

Solar Arrays for Space

NASA/TM—2002-211835

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–2002–20156
Acknowledgments

We gratefully acknowledge the National Aeronautics and Space Administration for its support through grants

NCC3–817, NCC3–869, and NAG3–2484, and the NASA Glenn Research Center Director’s Discretionary Fund.

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identification only. This usage does not constitute an official

endorsement, either expressed or implied, by the National

Aeronautics and Space Administration.

Available electronically at http://gltrs.grc.nasa.gov
NASA/TM2002-211835 1

ABSTRACT

The development of thin-film solar cells on flexible,

lightweight, space-qualified substrates provides an

attractive cost solution to fabricating solar arrays with

high specific power, (W/kg).The use of a

polycrystalline chalcopyrite absorber layer for thin film

solar cells is considered as the next generation

photovoltaic devices.A key technical issues outlined

in the 2001 U.S. Photovoltaic Roadmap, is the need

to develop low cost, high throughput manufacturing

for high-efficiency thin film solar cells.At NASA GRC

we have focused on the development of new singlesource-precursors (SSP’s) and their utility to deposit

the chalcopyrite semi-conducting layer (CIS) onto

flexible substrates for solar cell fabrication.The

syntheses and thermal modulation of SSP’s via

molecular engineering is described.Thin-film

fabrication studies demonstrate the SSP’s can be

used in a spray CVD process, for depositing CIS at

reduced temperatures, which display good electrical

properties, suitable for PV devices.

INTRODUCTION

One of the key technical issues outlined in

the U.S. Photovoltaic roadmap is the need to develop

low cost, high throughput manufacturing for highefficiency thin film solar cells.Thus, a key step for

device fabrication for thin film solar cells is the

deposition onto flexible, lightweight substrates such

as polyimides.The National Aeronautics and Space

Administration is interested in developing low

temperature deposition techniques (Hepp 2001 et al.),

for producing thin-film photovoltaics.Low temperature

routes (<400 °C) allow devices to be deposited onto

lightweight polymer substrates such as Kapton or

polybenzobisoxazole (PBO).These lightweight

devices would not only offer cost savings over present

technologies, but in many cases would be mission

enabling (Hepp 2001, Hoffman 2000 et al., Raffaelle,

2000 et al.).One of the most promising family of

materials for photovoltaic applications are the

chalcopyrite Cu(Ga,In)(S,Se)2 alloys.These

materials exhibit high absorption coefficients in the

visible to near IR spectrum range, are generally

prepared as p-type but can also be prepared n-type

by adjusting the stoichiometry of the material, have

good electrical characteristics and terrestrial stability,

and in the case of CuInSe2, Rockett (1991) et al. have

demonstrated these to be more radiation tolerant than

crystalline silicon and gallium arsenide.Tests on

laboratory scale by Ullal (1997) et al. show that

CuInSe2 based devices have achieved efficiencies

approaching 18%.Likewise, Dzionk (1997) et al.

have shown CuInS2 based devices to achieve

conversion efficiencies over 11%, and its direct band

gap of 1.5 eV is near optimal for solar radiation

utilization.In addition, from an environmental

standpoint, CuInS2 is free from the toxicity concerns

associated with the selenium analog, and thus may

be a more suitable material for terrestrial applications.

To facilitate low temperature deposition,

organometallic molecules are used because of their

low decomposition temperatures.To avoid premature

decomposition of the precursor, atmospheric pressure

spray chemical vapor deposition (CVD) was used.

Spray CVD combines the benefits of traditional metalorganic chemical vapor deposition (MOCVD) with

those of spray pyrolysis, while avoiding the

disadvantages of each.

CHEMICAL VAPOR DEPOSITION FOR ULTRA-LIGHTWEIGHT THIN-FILM SOLAR

ARRAYS FOR SPACE

Aloysius F.Hepp

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Ryne P.Raffaelle

Rochester Institute of Technology

Rochester, New York 14623

Kulbinder K.Banger, Michael H.Jin, and Janice.E.Lau

Ohio Aerospace Institute

Brook Park, Ohio 44142

Jerry D.Harris, Jonathan E.Cowen, and Stan A.Duraj

Cleveland State University

Cleveland, Ohio 44115
NASA/TM2002-211835 2

EXPERIMENTAL

All operations of moisture- and air-sensitive

materials were performed under an argon atmosphere

employing standard Schlenk techniques and a

double-manifold vacuum line.Solids were

manipulated in an argon filled glovebox.Solvents

were freshly distilled from appropriate drying agents

under Ar prior to use.The single source precursor,

(PPh3)2CuIn(SEt)4 was prepared using a procedure

reported elsewhere by Hirpo (1993) et al.,

Hollingsworth (1998) et al. and Banger (2001) et al.

Precursor purity was monitored by nuclear magnetic

resonance spectroscopy (NMR) and elemental

analysis.

Using a spray CVD reactor similar to Hollingworth's

(1999) and Harris (2000) et al. 1 μm thick films of

CuInS2 were deposited in 70–90 minutes from 150 ml

of 0.01 M solution of (PPh3)2CuIn(SEt)4 in toluene.

The solution was atomized by a 2.5 MHz nebulizer

and swept into a two-zone hot-wall reactor by argon

carrier gas (4 l/min) that was presaturated with the

solvent.Zone one (evaporation zone) of the reactor

was held at 128±1 °C, and zone two (deposition zone)

was held at 390±1 °C during depositions.For films

given a post-deposition anneal at higher

temperatures, the carrier gas was reduced to a

minimal flow and both zones of the furnace were

heated to 600±1 °C in 4 minutes, held isothermal for 8

minutes and then allowed to cool to room

temperature.

Films were characterized by transmission

spectroscopy (Perkin Elmer, Lambda-19), scanning

electron microscopy (SEM) (Hitachi S-3000N), Energy

Dispersive Spectroscopy (SEM-EDS) (EDAX),

profilometry (KLA-Tencor HRP 75), X-ray diffraction

(Philips) and photoelectrochemical analysis.Diode

curves were measured for Schottky barriers prepared

by thermally evaporating aluminum contacts onto the

CuInS2 films.Complete cells with the (top down)

composition of Al/ZnO/CdS/CuInS2/Mo/glass were

also prepared using the spray CVD deposited films.

Films were etched in a 1.5 M KCN solution for one

minute, prior to chemical bath deposition of CdS to

form the heterojunction.A 1 μm thick layer of

fluorine-doped ZnO2 was sputtered on top of the CdS,

followed by thermally evaporated aluminum contacts.

Complete cells were characterized under a simulated

AM0 solar spectrum.

RESULTS AND DISCUSSION

The SSP are prepared by the reaction of a

stabilized Cu(I) cation, with an indium(III), or

gallium(III) chalcogenide anion prepared in situ in

methanol.

[{Ph P} Cu(MeCN) ] [In(ER) ] [{Ph3P} 2Cu( -ER)2 In(ER)2 ] 2MeCN -

3 2 2 4

MeOH +  →  + + μ

The versatility of this synthetic pathway can be

illustrated by ability to modulate the physical

properties of the precursor and composition at any of

the intermediate synthetic steps by either;

• Adjusting the Lewis acid-base interaction (L→M,

M = Group I Metal)

• Adjusting the accessibility of the lone pair of

electrons on the neutral donor ligand by variation

of R

• Adjusting the bond strength between the

chalcogenide with either In/Ga and Cu metal

centers

• The ability to prepare analogues of group 16, (S,

Se, Te)

• The ability to prepare either indium or gallium

derivatives

The Lewis acid-base interaction is a valuable

component to the overall stability of the molecule,

given that the ability of the Lewis base to dissociate

from the cation at lower energies is pertinent to the

degradation of the precursor at reduced

temperatures.Hence, the Drago-Wayland

approximation (1987), can be used for ternary single

source precursor design, to quantitatively estimate the

strength of the Lewis acid-base interaction between

the copper center and the neutral donor.The

Single Source Precursors TGA DSC

Extrap.Onset

°C

MRW

°C

Residue

%

M.P.

°C

Decomp.

°C

[{PPh3}2Cu(SEt)2In(SEt)2] (1) 236 269 25 122 266

[{AsPh3}2Cu(SEt)2In(SEt)2] (2) 205 233 18 47 276

[{SbPh3}2Cu(SEt)2In(SEt)2] (3) 212 239 26 45 271

[{PPh3}2Cu(SPri

)2In(SPri)2] (4) 215 254 29 163 260

[{PPh3}2Cu(SPh)2In(SPh)2] (5) 261 325 22 117 280

[{PPh3}2Cu(SePh)2In(SePh)2] (6) 223 253 22 53 219

[{P(Bun

)3}2Cu(SEt)2In(SEt)2] (7) 189 238 27 - 264

[{P(Bun

)3}2Cu(S(Prn))2In(SPrn)2] (8) 171 225 22 - 239

TABLE 1 THERMAL DATA FOR TERNARY SINGLE SOURCE PRECURSORS.
NASA/TM2002-211835 3

cleavage of chalcogenide-R’ bond also plays an

important role, since this allows the chalcogenide to

be released for incorporation into the ternary

chalcopyrite matrix.Thus, the use of sterically

demanding R’ group’s of good “leaving ability”, would

promote the facile release of the chalcogenide.

Characterization

Initial studies focused on basic modification of the

SSP’s, and their influence on precursor stability.

Multinuclear NMR data demonstrated that the

precursors were free from any starting reagents.

Thermogravimetric analyses (TGA) were performed at

ambient pressure in platinum pans on samples of the

precursors, heated at a rate of 10 °C/min under a

dinitrogen atmosphere.Weight loss was associated

with decomposition of the complexes.Calculation of

the derivative maximum rate of weight loss (%/°C),

listed as MRW in Table I, shows a range from a low of

225 °C for 8 to a high of 325 °C for 5.Calculation of

the precursor efficiency to afford CIS/Se as the final

product, based on the residual material from the TGA

experiments found the samples to be within 5%.

A further example of the flexibility of the

[{LR3}2Cu(YR’)2M(ER’)2] architecture to direct

adjustment of these precursors are the SSP 7 and 8,

which represent the first liquid single source

precursors for the deposition of CuInS2.Low

temperature Differential Scanning Calorimetry (DSC)

was used to investigate the liquid phase for 7 and 8

(Figure 1).In separate studies, samples 7 and 8 were

subjected to both quench cooling and slow controlled

cooling before being heated at 10 and 5 °C/min.In

low temperature DSC experiments using controlled

and quench cooling, both samples 7 and 8 were

found not to show an endotherm assignable to a

melting phase transition thus confirming their liquid

phase at ambient temperatures.Remarkably both

liquid precursors show excellent solubility in both

polar and non-polar solvents, which can be attributed

to their ionic structure and the non-polar alkyl groups

resident on the tertiary phosphine.

FIGURE 1 LOW TEMP.DSC FOR SSP’s 7 and 8.

Thin-film Study

As deposited, SSP 1 afforded well adhering CuInS2

films, which were dark blue to black, depending on

film thickness.(Figure 2).The thickness of a typical

film ranged from 0.75 to 1.0 mm, with grain size for

the films of less than 0.5 mm As evident from the

SEM images, grain growth appears dense and

columnar, despite the small grain size (Figure 3).The

film thickness varies along the sample, with the

thinner portion of the film growing in the "up stream"

end of the substrate, and the thick portion of the film

growing on the "down stream" end.Thickness

uniformity of CVD deposited films has been shown by

Rossi et al. (1988) to be controlled by the mass

transfer rate of species in the gas phase to the

substrate surface, and the rate of reaction of the

species at the surface.It is believed the observed

FIGURE 2 SEM OF DEPOSITED CIS FILM USING

[{P(PPh)3}2Cu(SEt)2In(SEt)2] 1.

FIGURE 3 SEM EDGE-ON VIEW OF A CuInS2 FILM

SHOWING FILM THICKNESS AND COLUMNAR

GRAIN GROWTH.

gradient is a product of temperature inhomogeneities,

carrier gas flow rate and incline angle of the substrate

holder.Experiments are in progress to minimize

variation in film thickness.
NASA/TM2002-211835 4

Along with the thickness gradient, there is also a

gradient in the crystallographic orientation of the films.

As deposited, the majority of a typical film is highly

(112) oriented.Over the length of the 76 mm long

substrate, the leading 1/3 of the film is (220) oriented,

where as the down stream 2/3 of the film are strictly

(112) oriented (Figure 4).The observed slight shift in

diffraction lines from the front to the back is from the

glass substrate warping during annealing causing

slight shifts in alignment with the X-ray beam.Siemer

(2000) et al. have demonstrated that devices

prepared from preferred (112) oriented CuInS2 films

have better performance than photovoltaic devices

fabricated from other oriented films due mainly to a

lower series resistance.The reflection at 2Θ = 27° in

the front film, labeled with an * has been identified as

arising from In2S3 and In2Se3 in films of CuInS2 and

CuInSe2, respectively, prepared by spray pyrolysis.

However, spray pyrolyzed films only yield the

spurious reflection when indium rich solutions were

sprayed.For films grown by spray CVD, the reflection

is only observed on the leading 1/3 of the film, yet as

will be discussed later, composition of the films

remain constant (as determined by SEM-EDS) from

front to back.Films deposited by spray CVD also lack

the (101) reflection at 2Θ = 17.914°.It is unknown

whether this is a product of preferred orientation or

lack of long range order since chalcopyrite has the

zinc-blend structure with a doubled c-axis and

alternating Cu and In atoms replacing the Zn atoms.

Using a similar spray CVD reactor, Hollingsworth

(1999) demonstrated that crystal orientation is a

function of both carrier gas flow rate and solution

concentration.It is anticipated that experiments to

reduce the thickness gradient will improve the

orientation gradient as well, since it is the thinner

portion of the film that has the (220) crystal

orientation.

FIGURE 4 X-RAY POWDER DIFFRACTION

SPECTRA OF CuInS2 FILM ON MO USING SSP 1.

Films were deposited onto a variety of substrates,

including Ti, Mo and Ni foils, KaptonTM, PBO, SiO2,

and Mo coated glass.In all cases, SEM-EDS

measurements revealed the films were nearly

stoichiometric CuInS2, or only slightly indium and

sulfur rich, (with atomic percents for Cu, In and S as

23%, 24% and 53%, respectively).Films deposited

on all substrates, except nickel, show no evidence of

phosphorous or carbon by SEM-EDS, indicating that

the precursor molecules decompose cleanly.Within

experimental error of SEM-EDS, the stoichiometry of

the films remain constant along their length.SEMEDS data for films deposited on nickel substrates

revealed large quantities of phosphorous (16%) in the

material.SEM-EDS measurements of as deposited

films on nickel gave atomic percents for Cu, In, S and

P as 24%, 17%, 43% and 16%, respectively.Nickel

substrates must promote the cracking of the

phosphine during deposition, and would thus be

unacceptable for device preparation without a

passivating layer.

The electrical properties of the films were probed

using several techniques.The sheet resistance of

several films was measured in the van der Pauw

configuration and yielded resistivities ranging from 1

to 30 Ω•cm, which approach values obtained from

multi-source sputtered CuInS2 by Scheer (1997) et al.

These resistivities are also lower than those

previously obtained for CuInS2 films deposited using

the same single source precursor by Hollingsworth

(1999).

To further evaluate the electrical properties of the

deposited films, current verses voltage (IV)

measurements were recorded for the films using

thermally evaporated aluminum point contacts

(10 mm2

) to make Schottky barrier diodes.Many of

the Schottky barriers were excellent diodes on films

annealed at 600 °C, with "turn on" voltages of 0.6–0.8

volts and little leakage when reverse biased.

However, many of the contacts on the as-deposited

films gave large reverse bias currents and nearly

ohmic response (Figure 5).This behavior is indicative

of degeneracy of the semiconductor due to a high

carrier density resulting from native defects.The

improvement in the diode behavior of the annealed

films is attributed to enhanced crystallinity and

reduction of defects, as considerable narrowing of the

diffraction lines was observed in the annealed films.

-10

0

10

20

30

40

50

60

70

80

90

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

Voltage (V)

m A /cm2

FIGURE 5 SCHOTTKY BARRIER MADE FROM

EVAPORATED AL ON CuInS2 ON MO FOIL USING

SSP 1.

annealed

* as deposited
NASA/TM2002-211835 5

Complete solar cells were prepared from annealed

films deposited on Mo coated glass substrates.The

10 mm × 76 mm cell was mechanically scribed into

many smaller cells, with an IV curve for each

measured under a simulated AM0 light source.The

maximum efficiency achieved by one of the small

area cells was 0.68% (Figure 6), which was located

approximately midway along the 76 cm substrate and

had a total area of 0.5 cm2

.The most obvious feature

of the IV curve is the lack of a "knee," given that the

curve is nearly linear.From the slope of the curve

near the open circuit voltage (Voc), the device

appears to have high series resistance.At this time it

is unknown whether the series resistance is from the

absorber layer, from the interfaces between the

different layers or both.For an ideal diode, the curve

would intersect normal to the y-axis.From the slope

of the line near the short circuit current (Isc), the film

also has high shunt resistance.The mechanical

scribing of the small area cells could have caused the

shunting.

FIGURE 6 CURRENT-VOLTAGE CURVE FOR A 0.5

CM2 CuInS2 CELL ON Mo COATED GLASS.

SUMMARY AND OUTLOOK

Working photovoltaic devices have been prepared

from atmospheric pressure spray CVD deposited

CuInS2 thin films using the single source precursor

(PPh3)2CuIn(SEt)4.Although the Voc, Isc and fill

factor are low, it is anticipated that these will increase

as deposition parameters are optimized.X-ray

diffraction of the films reveal there is a preferred

orientation gradient along the substrate, with only the

back two thirds of the film having the desired (112)

crystal orientation.The films also range in thickness,

with the leading edge being thinner than the trailing

edge.Spray CVD is a promising technique for

depositing CuInS2 on to low temperature substrates

such as KaptonTM and PBO at temperatures below

400 °C.However, the films with the best electrical

properties were annealed at 600 °C following

deposition at 390 °C.The versatility of the

[{LR3}2Cu(ER’)2M(ER’)2] architecture is clearly

demonstrated by the preparation of SSP’s to multiternary semiconductors.Thermal analysis data

substantiates that steric and electronic molecular

modification on either the neutral donor, or

chalcogenide, permits directed adjustment of the

solid-state phase and stability of the precursors.

Evolved gas analysis, confirms the mechanism of

decomposition for the SSP’s proceeds “cleanly” by

the loss of the chalcogenide moiety, followed by loss

of the neutral donor.Spray CVD using SSP’s is a

mild, simple, clean, and scalable technique for

depositing CuInE2 or CuGaE2 (E= S, Se) thin-films on

flexible polymer substrates at reduced temperatures.

Although tests for the deposition of the wide bandgap

alloy Cu(Ga:In)S2, led to a non-homogenous film

composition, it is evident the use of two SSP’s with

similar thermal profiles, consistent film stoichiometry

can be achieved.

Clearly the full potential of CIS PV devices has not

been fully exploited since the combination of group IIII-VI2 elements can result in a variety of end products.

Therefore standards need to be defined which can

associate device processing, fabrication, film

composition etc.to cell band-gap and efficiency.

Spray CVD in conjunction with SSP design provides a

proof-of-concept for a reproducible high

manufacturability process.An outlook for further

investigation that needs to be undertaken is; (1)

Precursor Design: Development of more

volatile/thermally labile systems.This can be

achieved by the incorporation of fluorinated, or

silylated function groups.Importantly due to the high

propensity of fluorine by silicon, incorporation of both

elements in the molecule can serve not only to

increase volatility, but also as a “self-cleaning”

mechanism should not precursor decompose in

undesired pathway.(2) Processing Parameters:

Spray CVD has a number of tunable variables, such

as droplet size, flow-rate, concentration, solvent

polarity, which are advantageous to achieve the

desired film characteristics.Thus an in-dept study

needs to address these parameters to film

composition.(3) Device fabrication: Working devices

from deposited films need to be tested to aide SSP

design and spray CVD process parameters.The

work reported here on the molecular design of SSP’s

for their use in a spray CVD process although still in

its infancy, undoubtedly shows it as a mass

producible, cost effective method for fabricating

commercial thin film PV devices.

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materials”.Thin Solid Films (2002), 403–404

390–395.
NASA/TM2002-211835 6

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Bolko.“Current status of polycrystalline thin-film PV

technologies.” Conf. Record of the IEEE Photovoltaic

Specialists Conference (1997), 26th 301–305.
This publication is available from the NASA Center for AeroSpace Information, 301–621–0390.

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12

Chemical Vapor Deposition for Ultra-Lightweight Thin-Film

Solar Arrays for Space

Aloysius F. Hepp, Ryne P. Raffaelle, Kulbinder K. Banger, Michael H. Jin,

Janice E. Lau, Jerry D. Harris, Jonathan E. Cowen, and Stan A. Duraj

Solar cells; Indium compounds; Copper compounds; Thin films; Vapor deposition

Unclassified -Unlimited

Subject Category: 25 Distribution: Nonstandard

The development of thin-film solar cells on flexible, lightweight, space-qualified substrates provides an attractive cost

solution to fabricating solar arrays with high specific power, (W/kg). The use of a polycrystalline chalcopyrite absorber

layer for thin film solar cells is considered as the next generation photovoltaic devices. A key technical issues outlined in

the 2001 U.S. Photovoltaic Roadmap, is the need to develop low cost, high throughput manufacturing for high-efficiency

thin film solar cells. At NASA GRC we have focused on the development of new single-source-precursors (SSPs) and their

utility to deposit the chalcopyrite semi-conducting layer (CIS) onto flexible substrates for solar cell fabrication. The

syntheses and thermal modulation of SSPs via molecular engineering is described. Thin-film fabrication studies demonstrate the SSPs can be used in a spray CVD process, for depositing CIS at reduced temperatures, which display good

electrical properties, suitable for PV devices.

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, NASA Glenn Research Center; Ryne P. Raffaelle, Rochester

Institute of Technology, Rochester, New York; Kulbinder K. Banger, Michael H. Jin, and Janice E. Lau, Ohio Aerospace Institute, Brook Park, Ohio;

and Jerry D. Harris, Jonathan E. Cowen, and Stan A. Duraj, Cleveland State University, Department of Chemistry, Cleveland, Ohio. Responsible

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