NASA/TMm97-206322

Electrodeposited CuInSe2 Thin Film Junctions

R.P. Raffaelle and J.G. Mantovani

Florida Institute of Technology, Melbourne, Florida

S.G. Bailey and A.F. Hepp

Lewis Research Center, Cleveland, Ohio

E.M. Gordon and R. Haraway

Wilberforce University, Wilberforce, Ohio

Prepared for the

1997 Fall Meeting

sponsored by the Materials Research Society

Boston, Massachusetts, December 1-5, 1997

National Aeronautics and

Space Administration

Lewis Research Center

November 1997

https://ntrs.nasa.gov/search.jsp?R=19980013900 2018-05-11T16:44:25+00:00Z
Acknowledgments

We wish to thank E Montegani, the Ohio Aerospace Institute (OAI), and the NASA-ASEE program for their

support of R.P. RaffaeUe; NASA Cooperative Agreement NCC3-535 for support of E.M. Gordon; and National

Renewable Energy Laboratory's HBCU Photovoltaic Research Associates Program for support of R. Haraway.

This report contains preliminary

findings, subject to revision as

analysis proceeds.

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ELECTRODEPOSITED CuInSe2 THIN FILM JUNCTIONS

R.P. RAFFAELLE*, J.G. MANTOVANI*, S.G. BAILEY**, A.F. HEPP**, E.M. GORDON***,

and R. HARAWAY***

*Florida Institute of Technology, Melbourne, FL 32901, rpr@pss.fit.edu

**NASA Lewis Research Center, Cleveland, OH 44135

***Wilberforce University, Wilberforce, OH 45384

ABSTRACT

We have investigated thin films and junctions based on copper indium diselenide (CIS)

which have been grown by electrochemical deposition. CIS is a leading candidate for use in

polycrystalline thin film photovoltaic solar cells. Electrodeposition is a cost-effective method for

producing thin-film CIS. We have produced both p and n type CIS thin films from the same

aqueous solution by simply varying the deposition potential. A CIS pn junction was deposited

using a step-function potential. Stoichiometry of the single layer films was determined by energy

dispersive spectroscopy. Carder densities of these films increased with deviation from

stoichiometry, as determined by the capacitance versus voltage dependence of Schottky contacts.

Optical bandgaps for the single layer films as determined by transmission spectrocopy were also

found to increase with deviation from stoichiometry. Rectifying current versus voltage

characteristics were demonstrated for the Schottky barriers and for the pn junction.

INTRODUCTION

Copper indium diselenide (CIS) is one of the best optical absorber materials used in

polycrystalline thin film photovoltaic solar cells [1]. This is due to its favorable electrical and

optical properties, stability, and inexpensive means of production. Conversion efficiencies of

17.8% have been achieved for a vapor deposited CIS based solar cell [2]. Electrochemical

deposition is an inexpensive alternative to the standard vapor deposition techniques for producing

thin-film CIS [3].

Stoichiometric variation in polycrystalline CIS leads to a high concentration of electrically

active native defects. Small variations from a Cu-to-In ratio of unity have been shown to result in

large changes in the carrier density [4]. In fact, the semiconductor type changes from n to p as the

Cu to In ratio changes from less than one to greater than one. However, CIS has been shown to be

electrically and structurally stable over a wide range of stoichiometries and their associated native

defect concentrations [5]. This behavior has been recently explained in terms of electrically

inactive and ordered defect pairs [6].

The stoichiometry of our electrodeposited thin films is potentiostatically controlled. We

have previously shown that the Cu-to-In ratio will demonstrate a linear dependance with

deposition potential over a fairly wide range [7]. Therefore, we are able to deposit thin films with

different electrical, optical properties, and semiconductor type from the same aqueous solution by

simply changing the deposition voltage [8].

The optical band gap of CIS can be calculated from its absorption spectrum assuming

a direct energy gap [9]. CIS has been shown to have a low electron effective mass

NASA TM-206322 1
(me*--0.09me)[10]. Therefore,thesematerialsbecomedegenerateat low electroncarder

densitiesandwewould expectaBurstein-Mossshift in theopticalbandgap[11]. Thiseffecthas

beenobservedin CISsinglecrystalsgrownby theverticalBridgmanmethod[12].

Thecurrentversusvoltage(I-V) characteristicscanbeusedto determinethebarrier

heightsof metalto CISjunctions. For anidealSchottkycontactwith ann-typesemiconductor,

thebarrierheightq_ is the difference in the metal work function qt_m and the semiconductor

electron affinity qZ.

q_s = q(_m - Z) (1)

When the metal work function is comparable to or smaller than the semiconductor's electron

affinity we would expect an ohmic contact for an n-type semiconductor. In the case of a p-type

semiconductor, the barrier height is the difference between the metal work function and the sum

of the electron affinity and the bandgap. Assuming ideal junctions and an A1 work function of

4.28 eV [13], electron affinity of 4.48 eV and a CIS bandgap of leV [14], we would expect A1 to

make an ohmic contact on n-type CIS and a Schottky contact to p-type CIS, with a barrier height

of around 1.2 eV. However, if there is a large density of surface states, the barrier height is

determined by the semiconductor surface and is independent of the metal work function [13].

Schottky barrier junctions have been produced on both n and p type CIS (Au and A1,

respectively) [14].

The current density versus voltage for a Schottky barrier can be expressed as

J = Js e(qV/"kr) for V>> kT/q (2)

where Js is the saturation current density and n is the ideality factor. The ideality factor is very

close to unity at low dopings and high temperature. However, it can depart substantially from

unity when doping is increased or the temperature is lowered [15]. The barrier height can be

determined using

0 : n(A q T21 (3)

where A** is the effective Richardson constant which can be estimated based on the effective

mass [13].

The capacitance of an ideal Schottky barrier as a function of reverse bias voltage can be

expressed as

(4)

where A is the junction area, e0 is the dielectric constant, Nd is the semiconductor doping density,

and Vbi is the built-in voltage [13]. Therefore, the slope of 1/C 2 vs. V can be used to determine

the doping density.

The Shockley ideal pn junction diode equation follows the same form as equation (2)

above. The ideality factor is determined for the forward bias current and equals 2 when the

NASA TM-206322 2
recombinationcurrentdominatesandequals1whenthediffusioncurrentdominates. When both

currents are comparable, n has a value between 1 and 2 [13].

EXPERIMENT

A series of one micron thick films were electrodeposited on mechanically polished Mo

substrates using deposition potentials ranging from -1.0 to -1.4 V versus a saturated calomel

electrode (SCE). An analogous set of films was also deposited on indium tin oxide (ITO) coated

glass. The deposition solutions consisted of lmM CuSO 4, 10 mM In2(SO4)3, 5 mM Set2, and

25 mM Na-citrate.

The deposition potentials used to deposit our thin films were based upon the results of

cyclic voltammetry. These cyclic voltammograms and depositions were generated and monitored

by a Keithley 236 Source/Measure Unit interfaced to a personal computer and a EG&G 362

Scanning Potentiostat. The composition of the resulting films were characterized by energy

dispersive spectroscopy (EDS).

The thickness of the films was calculated using the following equation based on

Faraday' s law

m __

n F A p (5)

where n is the number of electrons transferred, F is Faraday's number, A is the electrode area, i is

the applied current, t is the deposition time, M is the formula weight, and p is the density [16].

3

For our calculations, we used the formula weight (336.28 g/mol) and density (5.77 g/cm ) of

intrinsic CIS [17]. This is an approximation, since the formula weight and density vary with

composition. The number of electrons transferred was taken as 13 according to the total

electrode reaction:

Cu 2++In 3+ + 2seO_- +12H ++13e _ CulnSe 2+6H20 (6)

The absorption coefficient versus photon energy of the films deposited on ITO was

determined from transmittance measurements in a Perkin Elmer Lambda 19 spectrophotometer.

Optical bandgaps were detemined from the absorption coefficient data using linear least-squares

analysis and assuming a direct bandgap dependence [9].

Schottky barriers were made on the series of films on Mo by thermal evaporation of

aluminum using a positive photo-resist mask of 0.00095 cm 2 pads. Current versus voltage (l-V)

measurements were performed on these junctions to determine their barrier heights. Capacitance

versus voltage (C-V) measurements were used to determine the carrier density of the films.

Based on our EDS results, we attempted to deposit pn junction from a single aqueous

solution using a step-function potential. A -1.2 V vs. SCE potential was applied for 900 s and

then changed to -1.3 V for 90 s. This was done in an attempt to produce an approximately

0.1 _n n-type CIS layer on a 1.0 grn p-type CIS layer. A similar procedure was used to grow a

pn junction with slightly different characteristics (e.g., majority carder densities) using voltages

of -1.1 and -1.3 V vs. SCE. Ohmic contacts were made to the top n-side of the junctions using

thermal evaporation of aluminum and a shadow mask.

NASA TM-206322 3
The I-V characteristics of the Schottky barriers and pn junctions were measured using a

signatone wafer probing station and a Keithley 236 Source/Measure Unit interfaced to a 486 PC.

The Keithley 236 is replaced by a Keithley 590 CV Analyzer in this setup to measure the

C-V behavior of the Schottky barriers. The C-V measurements were performed at lkHz.

RESULTS

The cyclic voltammogram of the deposition bath identified the deposition potentials for

the various atomic constituents. These atomic deposition potentials are indicated by the increases

in the deposition current: Cu @ -0.4 V, Se @ -0.8 V, In @ -1.0 V (see Fig. 1). Therefore, films

deposited at less negative potentials should have a larger Cu to In ratio. Comparing cyclic

voltammetry of ITO working electrodes to those of Mo showed a shift of -1.0V.

SEM micrographs show that the series of thin films on Mo were polycrystaUine and dense

with a sub-micron grain size and a uniform thickness (see Fig. 2). However, surface roughness

increased with more negative deposition potentials.

EDS analysis showed that the atomic percent of Se was between 50 and 53% for

deposition potentials from -1.0 to -1.4 V vs. SCE. However, there is a large degree of variation

in the Cu to In ratio in the films as a function of deposition potential (see Fig. 3). Based on these

results, we would expect that those films deposited at potentials less negative than -1.2 V vs.

SCE would be p-type, and those deposited at more negative potentials would be n-type [15].

6.0

5.0

4.0

"_ 3.0

N 2.0

_ 1.0

_ 0.0

J /

-1.0

-2.0

p_

jF0._ "'_ 0.5 1.0 1.5

i

Voltage (-V vs. SCE)

Figure 1. Cyclic voltammogram of the deposition

solution used in this study.

Figure 2. SEM micrograph of an -1.2 V

as-deposited 1.0 _ thick CIS film.

Analysis of the transmission spectra of the series of films deposited on ITO revealed a

2

linear dependence of (txhv) versus photon energy, indicating a direct energy gap. Non-linear

behavior occurred below 0.95 eV and was attributed to phonon related mechanisms. The optical

band gap was found to increase for potentials less than and greater than -2.2 V vs. SCE (see

Fig. 4). This change in optical bandgap is consistent with a Burstein-Moss shift, and indicates

an increasing carder density with deviation from stoichiometry. However, the linearity of

2

(cthv) versus hv was also reduced as the Cu/In ratio deviated from unity, most likely due to the

introduction of second phases.

NASA TM-206322 4
1.5

1[ • •

.-q _ • •

0.5 •

cL) i

0 " "

1.2 1.4 1.6

Deposition Voltage (-V vs. SCE)

Figure 3. Cu to In ratio versus deposition voltage of

the as-deposited CIS films as determined by EDS.

1.2

o

"-" 1.1 " *

t_

0.9 , *

O __--_ ± ..... 0.8

0.7 0.8 0.9 1.0 1.1 1.2 1.3

Cu to In Ratio

Figure 4. Optical bandgap versus deposition

potential.

The I-V behavior of the A1 contacts on CIS films deposited at less negative potentials

than -1.2 V vs. SCE showed the rectifying behavior of a Shottky barrier. This implies that these

films were p-type. AI contacts on the films deposited at more negative potential that -1.2 V vs.

SCE showed ohmic behavior indicative of n-type films. The semilog plots of current density

versus voltage showed the anticipated linear behavior and yielded barrier heights between 0.55

and 0.64 eV, with ideality factors much greater than one (see Table I). A value for the hole

effective mass of 0.71 was used in our calculations [1]. The measured barrier heights are much

smaller than the theoretical prediction of 1.2 eV. The measured barrier heights of the Schottky

barriers were found to decrease with increasing carrier density.

The capacitance versus voltage measurements of the Schottky barriers demonstrated

linear 1/C 2 vs. V behavior. The doping densities based on the least-square slopes were in good

agreement with previous studies and our EDS results (see Table I). A value of 8.1 was used for

the high frequency dielectric constant in our calculations [1 ].

Table I. Measured material properties versus deposition voltage

based on I-V and C-V results usin$ A1 contacts

Deposition

Voltage

(-V vs. SCE)

Semiconductor

Type

Barrier Height

(eV)

Carrier Density

1.00 P 0.56 1.1xl021

1.05 P 0.60 7.3x 1020

1.10 P 0.64 2.8x I 02°

1.20 N 0.0

1.30 N 0.0

1.40 N 0.0

The I-V results of the (-1.0/-1.3V)pn junction showed the anticipated rectifying behavior

(see Figure 5). The immediate flow of current in the reverse bias direction is indicative of a

"backward" diode [14]. This current is due to tunneling between degenerate or nearly degenerate

semiconductors. This degeneracy is consistent with the above optical and Schottky barrier

results. The (-1.1/-1.3V)pn junction showed less degeneracy as expected, with a more

characteristic Schockley diode behavior [13].

NASA TM-206322 5
[ 25.0-

I

i 20.0- *

<" 15.o

10.0 5.0- ******

r

-3.0 -2.0 -1.0-5.00,0 1.0 2.0 3.0

Voltage (V)

15.0

10.0 -

5.0 •

-3.0 -2.0 -1.0 0.0 1.0 2.0 3.0

-5.0-

Voltage (V)

Figure 5. I-V behavior of a -1.0/-1.3 V

CIS-based pn junction

Figure 6. I-V behavior of a-1.1/-1.3 V

CIS-based pn junction.

CONCLUSIONS

The electrodeposited CuxIn2.xSe 2 thin films were shown to have stoichiometries which

varied linearly with the range of deposition potentials used in this study. The optical band gaps

were found to be between 0.88 and 0.98 eV and increase with deviation from stoichiometry.

Schottky barrier behavior was observed for A1 contacts to films deposited at potentials less

negative than -1.2 V. The results are in agreement with these films being p-type as indicated by

our EDS results and previous studies on native defects in CIS. Current versus voltage

measurements confirmed the ability to deposit a pn junction from a single aqueous solution using

a step-function potential.

REFERENCES

1. A. Rockett and R.W. Birkmire, J. Appl. Phys. 70, R81(1991).

2. K. Zweibel, H. Ullal, and B. von Roedern, 25th IEEE PVSC (IEEE, New York, 1996).

3. F.J. Pern, J. Goral, R.J. Matson, T.A. Gessert, and R. Noufi, Solar Cells 24, 81 (1988)

4. S.P. Grindle, A.H. Clark, S. Rezaie-Serej, E. Falconer, J. Mcneily, and L.L. Kazmerski, J.

AppL Phys. 51, 5464 (1980).

5. Copper Indium Diselenide for Photovoltaic Applications, edited by T.J. Couts, L.L.

Kazmerski, and S. Wagner (Elsevier, Amsterdam, 1986).

6. S.B. Zhang, S.H. Wei, A. Zunger, Phys. Rev. Lett. 78, 21 (1997).

7. R.P. Raffaelle, J.G. Mantovani, and R.B. Friedfeld, SoLEner.Mat. & Sol.Cells 46, 201

(1997).

8. R.P. Raffaelle, J.G. Mantovani, R.B. Friedfeld, S.G. Bailey, and S.M. Hubbard, 26 th IEEE

Photovolt. Spec. Conf. (IEEE, New York, 1997).

9. E.J. Johnson, Semicond. and Semimet. 3, 153 (1967).

10. T. Irie, S. Endo, and S. Kimura, Jpn. J. Appl. Phys. 18, 1303 (1979).

11. E. Burstein, Phys. Rev. 93, 632 (1954); S. Moss, Proc. Phys. Soc. B 67, 775 (1954).

12. C. Rincon and G. Sanchez Perez, Solar Cells 16, 363 (1986).

13. S.M. Sze, Physics of Semiconductor Devices (John Wiley & Sons, Inc., New York, 1981).

14. H.M. Moiler, Semiconductors for Solar Cells (Artech House, Boston, 1993).

15. C.Y. Chang and S.M. Sze, Solid State Electron., 13, 727 (1970).

16. D.S. Lashmore and M.P. Dariel, J. Electrochem. Soc. 135, 1218 (1988).

17. R.C. Weast, ed., CRC Handbook of Chem. and Phys. (CRC, Boca Raton, FL, 1980).

NASA TM-206322 6

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November 1997 Technical Memorandum

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Electrodeposited CulnSe 2 Thin Film Junctions

6. AUTHOR(S)

R.P. Raffaelle, J.G. Mantovani, S.G. Bailey, A.F. Hepp, E.M. Gordon, R. Haraway

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

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135-3191

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

National Aeronautics and Space Administration

Washington, DC 20546-0001

WU-632-1A-I A-00

8. PERFORMING ORGANIZATION "

REPORT NUMBER

E-11014

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM--97-206322

11. SUPPLEMENTARY NOTES

1997 Fall Meeting sponsored by the Materials Research Society, Boston, Massachusetts, December 1-5, 1997. R.P.

Raffaelle and J.G. Mantovani, Florida Institute of Technology, Melbourne, Florida 32901; S.G. Bailey and A.F. Hepp,

NASA Lewis Research Center; E.M. Gordon and R. Haraway, Wilberforce University, Wilberforce, Ohio 45384. Responsible person, A.E Hepp, organization code 5410, (216) 433-3835.

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13. ABSTRACT (Maximum 200 words)

We have investigated thin films and junctions based on copper indium diselenide (CIS) which have been grown by electrochemical deposition. CIS is a leading candidate for use in polycrystalline thin film photovoltaic solar cells. Electrodeposition is a cost-effective method for producing thin-film CIS. We have produced both p and n type CIS thin films from the

same aqueous solution by simply varying the deposition potential. A CIS pn junction was deposited using a step-function

potential. Stoichiometry of the single layer films was determined by energy dispersive spectroscopy. Carrier densities of

these films increased with deviation from stoichiometry, as determined by the capacitance versus voltage dependence of

Schottky contacts. Optical bandgaps for the single layer films as determined by transmission spectroscopy were also found

to increase with deviation from stoichiometry. Rectifying current versus voltage characteristics were demonstrated for the

Schottky barriers and for the pn junction.

14. SUBJECT TERMS

CIS; Electrodeposition; Thin films

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